DRUG DELIVERY SYSTEMS BASED ON ENDOPEROXIDES USEFUL IN DIAGNOSIS AND THERAPY, AND METHODS THEREOF Technical domain of the invention The present invention relates to novel drug delivery systems based on endoperoxide moieties such as 1,2,4,5-tetraoxanes, namely compounds of formula I, appropriately modified to release active pharmaceutical ingredients (API) in the presence of higher levels of Fe(II), in a subject, wherein Formula I Iron metabolism dysregulation occurs in diseases such as malaria and cancer. Therefore, the present invention also relates to biomarkers comprising said compounds of formula I. Another aspect, the present invention relates to a process of synthesis of compounds of formula I, and respective intermediaries. Further, the present invention also relates to a process for labelling, detection, and identification of tumours in a tissue sample. The present invention is thus applicable to the medical and pharmacological areas, in particular the ones related to cancer detection and treatment, and malaria treatment. The present invention discloses a drug delivery system with 2 different vias, based on a 1,2,4,5-tetraoxane scaffold that selectively react with Fe(II): while the first drug delivery system spontaneously releases the active pharmaceutical ingredient (API) after reaction with Fe(II), the second drug delivery system, in alternative to the first slightly modified, needs to react with Fe(II) and glutathione to release the active pharmaceutical ingredient (API). Background of the invention Iron is a cofactor for enzymes involved in crucial biological processes such as mitochondrial respiration and cell cycle control. Its role depends on the redox cycle between ferric (Fe(III)) and ferrous iron (Fe(II)). However, it can also lead to reactive oxygen species (ROS) production by Fe(II) reaction with hydrogen peroxide. Despite being extremely important to several biological processes, iron can also be potentially harmful. Iron homeostasis in cells maintains rigorous levels of iron that are adequate to assure its functions, while reducing its availability in the redox active ferrous form. An equilibrium between iron import, storage and export ensures iron homeostasis in cells. The mechanisms of iron metabolism are related by ferrous iron (redox active iron) that is named labile iron pool, mainly present in the cytosol. Iron metabolism dysregulation occurs in diseases such as inflammation, infectious diseases, cancer, cardiovascular disease, and neurodegenerative diseases. Endoperoxides such as artemisinin and its derivatives are used in the clinic for the treatment of malaria. Endoperoxides such as 1,2,4-trioxanes, 1,2,4-trioxolanes and 1,2,4,5-tetraoxanes are selectively activated by Fe(II)-rich parasite infected erythrocytes to form carbon-centred radicals. It appears that infected red blood cells contain elevated levels of Fe(II) when compared to those detected in healthy tissues. Antimalarial drugs have harmful side effects and increasing resistance to the first line of treatment for malaria has been reported. Several studies report a link between iron and cancer, showing that within cancer cells there are increased levels of the labile iron pool to support their biological needs. Moreover, many cancers modify iron import and export processes, leading to augmented levels of the labile iron pool, when compared with normal cells, which is a phenomenon named “iron addiction”. Currently, chemotherapeutic drugs are not selective towards their site of action. In consequence, they confer severe and harmful side effects. Iron metabolism dysregulation represents a novel strategy for selective drug delivery by preparing two systems based on tetraoxanes. The drug delivery systems described in this invention can modify drug efficacy and drug resistance appearance since they are inactively administered and selectively activated towards the chosen target, in this case, the ferrous iron. Document WO2015123595A1 describes compounds based on 1,2,4-trioxane or 1,2,4-trioxolane ring system for the treatment of parasitic and neoplastic diseases associated with conditions presenting higher level of ferrous iron. However, document WO2015123595A1 is silent in what regards to delivery systems comprising compounds based on 1,2,4,5-tetraoxane. Moreover, as it is widely recognized in the literature, the 1,2,4- trioxolane ring system does not present good metabolic stability, which remains a major liability to their clinical development. The 1,2,4,5-tetraoxane ring system is thermodynamically and metabolically more stable than the 1,2,4-trioxolane ring system. Besides, these compounds are very toxic to the organism. Finally, the second drug delivery system disclosed is not amenable to contain the 1,2,4-trioxolane ring system due to limitations inherent to the preparation of the 1,2,4-trioxolane scaffold. The present invention aims to overcome the problems of the prior art by providing new 1,2,4,5-tetraoxane compounds and two drug delivery systems based in said compounds, which are selective to the targeted tissues, can contribute to the treatment of malaria and cancer, and further, can be successfully used as biomarkers. Description of the Figures Figure 1 shows the effect of cell death (Y axis) in colorectal cancer cells (HCT116) in the presence of compound IId alone, represented by column #3), of compound IId together with an iron chelator (deferoxamine mesylate - DFO), represented by column #4), of DFO alone, represented by column #2, and of dimethyl sulfoxide (DMSO) as a control compound, represented by column #1. On the Y axis is represented cell viability determined by the (3-(4,5- dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfop henyl)- 2H-tetrazolium (MTS) metabolism assay. Metabolically active cells (viable and alive) will metabolize the MTS into formazan and this molecule is quantified by absorbance. Higher amounts of formazan mean that the cells can survive when exposed to the compounds while lower amounts of formazan mean that the compounds are able to induce cell death. From this figure it is possible to observe that compound IId induces cell death and that the co-treatment of compound IId with DFO promotes a lower induction of the cell death. Thus, cell death exerted by compound IId is iron-dependent. Data represent mean values ± standard error of the mean (SEM) of three independent experiments against to untreated control (DMSO). Ordinary One-Way ANOVA and Tukey´s multiple comparisons test. Versus DMSO: δ p < 0.0001; Versus compound IId without DFO: ** p < 0.01. Figure 2 shows the effect of cell death (Y axis) in colorectal cancer cells (HCT116) in the presence of compound IIe alone, represented by column #3), of compound IIe together with an iron chelator (deferoxamine mesylate - DFO), represented by column #4), of DFO alone, represented by column #2, and of DMSO as a control compound, represented by column #1. On the Y axis is represented cell viability determined by the (3-(4,5-dimethylthiazol-2-yl)-5- (3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) metabolism assay. Metabolically active cells (viable and alive) will metabolize the MTS into formazan and this molecule is quantified by absorbance. Higher amounts of formazan means that the cells can survive when exposed to the compounds while lower amounts of formazan means that the compounds are able to induce cell death. From this figure it is possible to observe that compound IIe induces cell death. Also, the co-treatment of compound IIe with DFO promotes a small reduction in the induction of the cell death by compound IIe. Thus, cell death exerted by compound IIe can be iron- dependent because ferrous iron (Fe(II)) activates this compound in cells. Data represent mean values ± SEM of three independent experiments normalized to untreated control (DMSO). Ordinary One- Way ANOVA. Versus DMSO: δ p < 0.0001. Figure 3 presents images of HCT116 cells after 24 hours incubation with system control (DMSO), doxorubicin and compound IId. Fluorescence emission was detected using a 460 nm emission filter (blue channel) and a 560 nm emission filter (red channel). Hoechst dye 33258, is a fluorescent stain used to stain the cell nuclei and whose fluorescence emission can be detected by the blue channel. Doxorubicin has intrinsic fluorescence, and its emission wavelength is detected by the red channel. Merge is the overlap of fluorescence detected by the blue and red channels given by Hoechst dye 33258 and doxorubicin, respectively. AA represents system control (DMSO) and fluorescence emission in blue channel, AB is system control (DMSO) and fluorescence emission in red channel, AC represents the system control (DMSO) and merge of fluorescence emission from both channels (blue and red), AD represents the zoom- in field for the system control (DMSO) and merge of fluorescence emission from both channels (blue and red), BA is doxorubicin (1 µM) and fluorescence emission in blue channel, BB represents doxorubicin (1 µM) and fluorescence emission in red channel, BC represents doxorubicin (1 µM) and merge of fluorescence emission from both channels (blue and red), BD represents the zoom-in field for doxorubicin (1 µM) and merge of fluorescence emission from both channels (blue and red), CA is compound IId (1 µM) and fluorescence emission in blue channel, CB represents compound IId (1 µM) and fluorescence emission in red channel, CC is compound IId (1 µM) and merge of fluorescence emission from both channels (blue and red), CD represents the zoom-in field for compound IId (1 µM) and merge of fluorescence emission from both channels (blue and red). Objective: 63x. Scale bar = 50 µm. From these images it is possible to observe that fluorescence in system control (DMSO) is only present in the blue channel meaning that only nuclei can be detected (images AA, AC and AD). It can be observed that doxorubicin accumulates in the nucleus through the co-localization of fluorescence of the blue and red channels (images BC and BD). In turn, compound IId presents fluorescence around the nuclei (images CC and CD). As compound IId has no intrinsic fluorescence, this means that compound IId is activated in the cytosol, where ferrous iron (Fe(II)) levels are elevated, and releases doxorubicin. Figure 4 shows the relative tumour volume (Y axis) for the four groups of mice injected with colorectal cancer cells (HT-29) during the days of treatment (X axis), for the mice treated with compound IId, represented by an unfilled circle, and doxorubicin, represented by an unfilled square. Mice without any treatment (negative control group) are represented by an unfilled and inverted triangle and the mice that received cremophor in phosphate buffered saline solution (vehicle control), which is the solvent to dissolve compound IId for administration, is represented by a filled circle. On the Y axis is represented the relative tumour volume in mm ³ determined by the ratio between volumes at the indicated day and volume at the beginning of treatment for each group of mice. From this figure it is possible to observe a reduction on the progression of the tumour mass for the mice treated with compound IId and doxorubicin when compared with the negative control group and the vehicle control. Moreover, compound IId was more efficient in reducing the tumour mass than doxorubicin. Mixed-effects analysis (two-way anova like) and Tukey´s multiple comparisons test. At day 12, mice treated with compound IId presented a significant reduction in tumour volume versus vehicle control: * p < 0.05. Description of the invention The present invention relates to novel drug delivery systems based on endoperoxide moieties such as 1,2,4,5-tetraoxanes, namely compounds of formula I, appropriately modified to release active pharmaceutical ingredients (API) in the presence of higher levels of Fe(II), in a subject. 1. Novel tetraoxane compounds Tetraoxane compounds according to the present invention are represented by formula I, wherein: Formula I R ¹ and R ² are independently hydrogen, substituted or unsubstituted alkyl, substituted or unsubstituted heteroalkyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted heterocycloalkyl, substituted or unsubstituted aryl and substituted or unsubstituted heteroaryl. R ¹ and R ² may be joined to form a substituted or unsubstituted cycloalkyl and substituted or unsubstituted heterocycloalkyl. X is -CH ₂ O-, -OCH ₂ - or -O-O-. R ³ is -C(R ⁴ )(R ⁵ )-, -CH(R ⁴ )-CH(R ⁵ )- or -C(R ⁴ )=C(R ⁵ )-. R ⁴ is hydrogen, -OH, -CH ₂ OH, -CH ₂ O(C=O)A or -CH ₂ O(C=S)A, wherein A is independently one or more APIs, proteins and antibodies for inflammation, diabetes, cardiovascular disease, malaria, orphan diseases, neurodegenerative disorders, infectious diseases and cancer (diagnosis and treatment). R ⁵ and R ⁶ are independently hydrogen, -OH, -O(C=O)A or -O(C=S)A, wherein A is independently one or more APIs, proteins and antibodies for inflammation, diabetes, cardiovascular disease, malaria, orphan diseases, neurodegenerative disorders, infectious diseases and cancer (diagnosis and treatment). Compounds of formula I may be further subdivided in compounds of formulae IIa to IIe to exploit the tetraoxanes of the invention as drug delivery systems in diagnostic and treatment. 1.1. Characteristics and properties of the compounds of the invention for malaria treatment (formula IIa and IIb)

Formula IIa Formula IIb Compound IIa and IIb are coupled with drugs for malaria treatment defining a drug delivery system IIa and IIb. In a preferred embodiment, compound IIa is coupled with primaquine, an antimalarial drug used in the clinic. In another embodiment, compound IIb is coupled with boc-piperazine. These compounds exhibit a half maximal inhibitory concentration (IC ₅₀ ) in the nanomolar range against chloroquine-sensitive 3D7 strain of Plasmodium falciparum. Therefore, these compounds can be applied in malaria treatment. Compounds IIa and IIb also include respective racemic mixture or respective enantiomers. 1.2. Characteristics and properties of the compounds of the invention for cancer treatment (formulae IIc, IId and IIe) Further compounds are in the scope of the present invention, such as compound of formula IIc, IId and IIe. Formula IIc Formula IId Formula IIe Compounds IIc, IId and IIe are coupled with an anticancer drug typically used in the clinic thus defining drug delivery systems IIc, IId and IIe. In a preferred embodiment, the compounds of the invention are coupled with doxorubicin. These compounds are able to induce cell death in vitro, in the micromolar range, for at least four types of cancer with iron metabolism instability, wherein the specific cell lines are identified between brackets: glioblastoma (U251), breast adenocarcinoma (MCF7), prostate adenocarcinoma (PC3) and colorectal carcinoma (HCT116). Thus, these compounds can be applied in cancer treatment. Compounds IIc, IId and IIe also include respective racemic mixture or respective enantiomers. 1.3. Characteristics and properties of the compounds of the invention for cancer diagnostic (formulae IIc, IId and IIe) Besides its ability to induce cell death in cancer cells, some other compounds having intrinsic fluorescence, such as doxorubicin are useful to be used as tumour markers, when coupled with compounds of the invention. This fluorescence is lost when said compounds are linked to drug compounds IIc, IId and IIe. When these fluorescent compounds are activated in cancer cells, upon reaction with ferrous iron, said fluorescent compound is released from IIc, IId and IIe, thus, recovering its fluorescence properties, which is detected by fluorescence microscopy. Therefore, these compounds can also be employed as drug delivery systems for tumour markers. In a preferred embodiment doxorubicin is linked to compounds IIc, IId and IIe to be used as tumour markers. 2. Process of preparation of compounds of formula I In an embodiment of the present invention, the process for preparation of compounds of formula I (see Example 1) and of compounds of formulae IIa, IIb, IIc and IId (see Example 2), as described in the previous section is represented by scheme 1 below: Scheme 1. with the following steps: (i) Sodium dichromate dihydrate, water, sulfuric acid; (ii) Rhenium (VII) oxide, hydrogen peroxide 50%, acetonitrile; (iii) 2-Adamantanone or 5-hydroxy-2-adamantanone, rhenium (VII) oxide, dichloromethane:acetonitrile (1:1), inert atmosphere; (iv) 4-Nitrophenyl chloroformate, N,N-diisopropylethylamine, 4- dimethylaminopyridine, dichloromethane; (v) API, 1-hydroxybenzotriazole hydrate, triethylamine, N,N- dimethylformamide. In step i) there is the oxidation of 1,3-cyclohexanediol (Compound 0) generating rac-3-hydroxycyclohexanone (Compound 1). In step ii), Compound 1 reacts with hydrogen peroxide in the presence of rhenium(VII) oxide to produce the dihydroperoxide (Compound 2). In step iii) the dihydroperoxide (Compound 2) reacts with a carbonyl reagent, employing again a catalytic amount of rhenium(VII) oxide. 2-Adamantanone (Compound 3) and 5-hydroxy-2- adamanatone (Compound 4) can be used as carbonyl for the formation of the 1,2,4,5-tetraoxane scaffold of the first drug delivery system. The alcohol group on position 3″ of the cyclohexane ring generates unsymmetrical tetraoxanes (compounds 3 and 4) with a stereogenic center, obtaining 2 enantiomers. Thus, this step may also employ a racemic mixture of tetraoxanes to be applied as a drug delivery system instead of performing a stereocontrolled synthesis of the 1,2,4,5-tetraoxane system. Next, in step iv) compounds (3) and (4) were activated with 4- nitrophenyl chloroformate together with excess amine bases (N,N- diisopropylethylamine and 4-dimethylaminopyridine) to generate compounds (5) and (6), respectively. Finally, compound (5) was coupled with primaquine.diphosphate (IIa) and boc-piperazine (IIb) with 55% and 33% yields, respectively. Compound (5) and (6) were coupled with doxorubicin to obtain the carbamates with 60% (IIc) and 75% (IId), producing the correspondent drug delivery system. Compounds 1 to 6 with the above-described formulae, and enantiomers of compounds 3 to 6 are included in the present invention as intermediaries in the process of production of compounds IIa, IIb, IIc and IId and respective drug delivery systems. Finally, considering that the intermediary compounds 3 to 6 are used in the process of preparing compounds IIa, IIb, IIc and IId, this results that said compounds and respective drug delivery systems include a racemic mixture or respective enantiomers. Accordingly, the present invention also relates to racemic mixture or respective enantiomers of compounds IIa, IIb, IIc and IId and correspondent drug delivery systems. In another embodiment of the invention, the process for preparation of compounds of formula I (see Example 1), and of compounds of formulae IIe (see Example 2), is described in scheme 2 below: with the following steps: (i) Formaldehyde, N-methylpyrrolidine, barium hydroxide octahydrate, water:methanol (5:1); (ii) Rhenium (VII) oxide, hydrogen peroxide 50%, acetonitrile; (iii) 5-Hydroxy-2-adamantanone, rhenium (VII) oxide, dichloromethane:acetonitrile (1:1), inert atmosphere; (iv) 4-Nitrophenyl chloroformate, N,N-diisopropylethylamine, 4-dimethylaminopyridine, dichloromethane; (v) API, 1-hydroxybenzotriazole hydrate, triethylamine, N,N- dimethylformamide. In step i) the hydroxymethyl group was installed on the position 2 of 2-cyclohexen-1-one (compound 7) by Morita-Baylis-Hillman chemistry, generating compound 8. In step ii), Compound 8 reacts with hydrogen peroxide in the presence of rhenium(VII) oxide to produce the dihydroperoxide (Compound 9). In step iii) the dihydroperoxide (Compound 9) reacts with a carbonyl reagent, employing again a catalytic amount of rhenium(VII) oxide. 5-hydroxy-2-adamanatone (Compound 10) can be used as carbonyl for the formation of the 1,2,4,5-tetraoxane scaffold of the second drug delivery system. The alcohol group on position 5 of the adamantyl group generates an unsymmetrical tetraoxane (compound 10) with a stereogenic center, obtaining 2 enantiomers. This step may also employ a racemic mixture of tetraoxanes to be applied as a drug delivery system instead of performing a stereocontrolled synthesis of this drug delivery system. Next, in step iv) compound (10) was activated with 4-nitrophenyl chloroformate together with excess amine bases (N,N- diisopropylethylamine and 4-dimethylaminopyridine) to generate compound (11). Finally, compound (11) was coupled with doxorubicin (IIe) with 73% yield. Compounds 8 to 11 with the above-described formulae, and enantiomers of compounds 10 and 11 are included in the present invention as intermediaries in the process of production of compounds of formula I and of formula IIe. Finally, considering that the intermediary compounds 10 and 11 are used in the process of preparing the above-mentioned compounds, this results that said compounds include a racemic mixture or respective enantiomers. Accordingly, the present invention also relates to racemic mixture or respective enantiomers of compounds I and IIe. 3. Mechanism of the first drug delivery system according to the invention The first drug delivery system contains the 1,2,4,5-tetraoxane scaffold that reacts with Fe(II) and forms a cyclohexanone intermediate (Scheme 3). This intermediate spontaneously releases the API by retro-Michael reaction (β-elimination). Scheme 3 presents the mechanism of drug delivery via Fe(II)- activation of tetraoxane compounds of the present invention and subsequent β-elimination, wherein A represents one or more APIs that can be coupled to the tetraoxane. The reaction of the tetraoxane present in compounds IIa, IIb, IIc and IId with a ferrous iron source will produce a cyclohexanone derivative, which spontaneously releases the API by β-elimination (Scheme 3). In the scope of the present invention, there are several suitable APIs that can be used alone or in combination for different purposes of treatment and/or diagnosis being some of them mentioned below. In this scheme, compounds IIa and IIb can be coupled with one or more APIs against the malaria parasite such as primaquine and boc- piperazine. Other compounds suitable to couple with this system for malaria are the following and their derivatives: atovaquone, quinine sulphate, sulphonamides (sulphadoxine and sulphamethoxypyridazine) and falcipain inhibitors (dipeptidyl vinyl sulfones and peptidomimetic pyrimidine nitriles). Compounds IIc and IId can be coupled preferably with doxorubicin, an API against cancer. Other compounds suitable to couple with this system for cancer either alone or in combination are the following and their derivatives: alectinib, afatinib, abiraterone acetate, azacitidine, axitinib, bendamustine, brigatinib, bosutinib, bicalutamide, calicheamicin, capecitabine, carfilzomib, combretastatin, ceritinib, cladribine, clofarabine, cobimetinib, crizotinib, cryptophycin, cytarabine, dabrafenib, dasatinib, daunorubicin, decitabine, duocarmycin, enasidenib, epirubicin, eribulin mesylate, erlotinib, everolimus, etoposide, exatecan, fludarabine, fulvestrant, gemcitabine, hemiasterlin, histone deacetylase inhibitors (abexinostat, belinostat, entinostat, givinostat, mocetinostat, oxamflatin, panobinostat, pyroxamide, quisinostat, vorinostat, trichostatin A, tacedinaline, tucidinostat, tubastatin A), hydroxyurea, ibrutinib, idelalisib, imatinib, irinotecan, ixabepilone, lapatinib, lenalidomide, maytansine, mitomycin C, monomethyl auristatins (MMA-E, MMA-F, dolastatin 10 and dolastatinol), methotrexate, neratinib, nilotinib, niraparib, olaparib, octreotide, osimertinib, paclitaxel, palbociclib, pazopanib, pyrrolobenzodiazepine, pemetrexed, pomalidomide, ponatinib, ribociclib, rucaparib, sorafenib, tamoxifen, triazenes, trifluridine, topotecan, tubulysin, vandetanib. Other compounds suitable to couple with this system as tumour markers includes fluorophores 4,4-difluoro-4-bora-3a,4a-diaza-s- indacene (BODIPY), cyanines, dicyanomethines, doxorubicin, fluorescein, nitrobenzofurazan, rhodamine and derivatives thereof. 3.1. Mechanistic proof-of-concept of the first drug delivery system The activation of the 1,2,4,5-tetraoxane scaffold after reaction with Fe(II) causes the production of carbon-centred radicals. This reaction mechanism involves Fe(II) coordination with two possible oxygens of the tetraoxane ring. The first drug delivery system contain a tetraoxane scaffold that, in the presence of Fe(II), is expected to produce carbon-centred radicals and a cyclohexanone derivative. This derivative will spontaneously release the API by β-elimination. To confirm the proposed mechanism of drug delivery, compound IIc was exposed to an inorganic salt of Fe(II) and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO), as described in O´Neill et. al, J. Med. Chem. 2011, 54, 6443. TEMPO was used in this experiment to validate the formation of carbon-centred radicals by the drug delivery system. TEMPO reacts with the radicals generated in this reaction and allows their detection by ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS). Fe(II)-triggered tetraoxane activation and fragmentation of compound IIc detected TEMPO adducts by UHPLC-MS consistent with the formation of carbon-centred radicals (see Example 3). Moreover, the cyclohexanone derivative and doxorubicin were also identified by UHPLC-MS (see Example 3), which confirms the API release by the proposed mechanism of drug delivery. 4. Mechanism of the second drug delivery system disclosed in this invention The alternative second drug delivery system, which is a particular embodiment of the first system described above, also contains the 1,2,4,5-tetraoxane scaffold, which after reaction with Fe(II), produces a cyclohex-2-enone derivative. This derivative does not release the API spontaneously. This derivative possesses an alkene group that after reaction with glutathione (Michael addition) allows the release of the API by elimination (Scheme 4). Considering that API release only occurs after reaction with Fe(II) and glutathione, which is elevated in many types of cancer, this can lead to improved selectivity when compared with the first drug delivery system. Scheme 4 presents the mechanism of drug delivery via Fe(II)- activation, Michael addition of glutathione and elimination of tetraoxane compounds of the present invention, wherein A represents an API that can be coupled to the tetraoxane.

The reaction of the tetraoxane present in compound IIe with a ferrous iron source will produce a cyclohex-2-enone derivative, which releases the API by elimination after reacting with glutathione (Scheme 4). In the scope of the present invention, there are several suitable APIs that can be used alone or in combination for different purposes of treatment and/or diagnosis being some of them mentioned below. In this scheme, compound IIe is preferably coupled with doxorubicin, an API against cancer. Other compounds suitable to couple with this system for cancer alone or in combination are the following and their derivatives: alectinib, afatinib, abiraterone acetate, azacitidine, axitinib, bendamustine, brigatinib, bosutinib, bicalutamide, calicheamicin, capecitabine, carfilzomib, combretastatin, ceritinib, cladribine, clofarabine, cobimetinib, crizotinib, cryptophycin, cytarabine, dabrafenib, dasatinib, daunorubicin, decitabine, duocarmycin, enasidenib, epirubicin, eribulin mesylate, erlotinib, everolimus, etoposide, exatecan, fludarabine, fulvestrant, gemcitabine, hemiasterlin, histone deacetylase inhibitors (abexinostat, belinostat, entinostat, givinostat, mocetinostat, oxamflatin, panobinostat, pyroxamide, quisinostat, vorinostat, trichostatin A, tacedinaline, tucidinostat, tubastatin A), hydroxyurea, ibrutinib, idelalisib, imatinib, irinotecan, ixabepilone, lapatinib, lenalidomide, maytansine, mitomycin C, monomethyl auristatins (MMA-E, MMA-F, dolastatin 10 and dolastatinol), methotrexate, neratinib, nilotinib, niraparib, olaparib, octreotide, osimertinib, paclitaxel, palbociclib, pazopanib, pyrrolobenzodiazepine, pemetrexed, pomalidomide, ponatinib, ribociclib, rucaparib, sorafenib, tamoxifen, triazenes, trifluridine, topotecan, tubulysin, vandetanib. Other compounds suitable to couple with this system as tumour markers includes fluorophores 4,4-difluoro-4-bora-3a,4a-diaza-s- indacene (BODIPY), cyanines, dicyanomethines, doxorubicin, fluorescein, nitrobenzofurazan, rhodamine and derivatives thereof. 4.1. Mechanistic proof-of-concept of the second drug delivery system As described for the first drug delivery system, the tetraoxane reaction with a ferrous iron source of the second drug delivery system also produces carbon-centred radicals, by Fe(II) coordination with two oxygens of the tetraoxane scaffold. The second drug delivery system generate a cyclohex-2-enone derivative upon activation by Fe(II), which then undergo a Michael addition by cellular glutathione and subsequent departure of the API by elimination. To confirm the first step of the proposed mechanism of drug delivery, compound IIe was exposed to an inorganic salt of Fe(II) and TEMPO, as described in O´Neill et. al, J. Med. Chem. 2011, 54, 6443. TEMPO was used in this experiment to validate the formation of carbon-centred radicals by the drug delivery system. TEMPO reacts with the radicals generated in this reaction and allows their detection by UHPLC-MS. Moreover, this experiment was performed to confirm tetraoxane fragmentation of compound IIe producing the cyclohex-2-enone derivative. Finally, doxorubicin is not released under these conditions because of the absence of glutathione or similar agents, therefore, the Michael addition to the cyclohex-2-enone derivative cannot occur. Fe(II)-triggered tetraoxane activation and fragmentation of compound IIe detected TEMPO adducts by UHPLC-MS (see Example 4), for the two possibilities of coordination with Fe(II) and consist with the formation of carbon-centred radicals. Moreover, the cyclohex-2-enone derivative was also identified by UHPLC-MS (see Example 4), in accordance with the proposed mechanism. Finally, in this experiment doxorubicin was not detected. This drug delivery system produces a cyclohex-2-enone derivative that after activation by Fe(II) can undergo a Michael addition by cellular glutathione and, subsequent release of the API. To confirm this mechanism of drug delivery, compound IIe reacted with a ferrous iron source and N-acetylcysteine (NAC), to simulate the Michael addition of glutathione, which relies on a thiol group (- SH). In this example, to assure that this reaction occurs, a pH over 8 is necessary for the thiol group to be deprotonated (-S-). Fe(II)-triggered tetraoxane activation and fragmentation of compound IIe detected a carbon-centred radical by UHPLC-MS (see Example 4). Additionally, the cyclohex-2-enone derivative was also detected (see Example 4), in accordance with the first step of activation of this drug delivery system. Finally, release of doxorubicin was detected (see Example 4), validating the mechanism of action for this drug delivery system because Michael addition of NAC to the cyclohex-2-enone derivative occurred. 5. Biological activity assessment The biological activity of the compounds and correspondent drug delivery systems of the invention can be accessed by measurement of different parameters, in line with the known processes by the skilled person in the art. 5.1. Evaluation of the antimalarial effect Antimalarial activity of compounds IIa and IIb can be accessed by exposing a P. falciparum strain (see Example 5), such as strain 3D7, which is chloroquine and mefloquine sensitive, to said compounds, and by using the SYBR Green I assay as described in Machado et. al, Ann. Clin. Med. Microbio. 2016, 2, 1010. Asynchronous parasites are cultivated for 72 hours in the presence of each compound. Fluorescence intensity was measured with a multi- mode microplate reader (Triad, Dynex Technologies). Compounds IIa and IIb were active against the malaria parasite (see Example 5, Table 1). 5.2. Evaluation of the antitumoral effect Typically, the antitumoral effect against cancer cells growth is accessed by measuring the half maximal inhibitory concentration (IC ₅₀ ) in a cell culture exposed to the compounds to be evaluated, according to the described in Florindo et. al, Dalton Trans 2016, 45, 11926. The IC ₅₀ values can be calculated by dose-response curves, such as by using GraphPad Prism 8.0.2. software followed by statistical analysis. For this purpose, tumour cell lines are exposed to a range of concentrations of suitable compounds IIc, IId and IIe and correspondent drug delivery systems being cells metabolic activity evaluated after 72 hours by CellTiter 96 ^® Aqueous Non-Radioactive Cell Proliferation Assay (3-(4,5-dimethylthiazol-2-yl)-5-(3- carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium - MTS; Promega, Madison, WI, USA) and absorbance signal recorded using a GloMax ^® -MultiDetection System (Promega). Compounds IIc, IId and IIe were able to induce cell death in all the tested tumour cell lines (see Example 6, Table 2). Moreover, compounds IIc, IId and IIe were less potent than doxorubicin, which is the API against cancer coupled in these compounds. 5.3. Evaluation of the effect of compounds in toxicity Compounds IIc, IId and IIe and doxorubicin were tested on a non- tumorigenic cell line (AML12) to assess the toxicity of the compounds described in this invention. The toxicity can be measured by calculating the IC ₅₀ of the compounds in this non-tumoral cell line. The conditions of the assay and the software can be the same as described previously for compounds IIc, IId and IIe against the four tumour cell lines. Compounds IIc, IId and IIe were able to ameliorate the toxicity associated with doxorubicin (see Example 7). 5.4. Evaluation of biological activity dependent on iron Compound IId and IIe were incubated alone and together with the iron chelator desferoxamine mesylate (DFO) in colorectal cancer cells (HCT116), as described in Mariani et. al, Chem. Commun. 2016, 52, 1358 (see Example 8). The cellular metabolic activity can be determined again with the MTS assay and absorbance signal detection, as described above for the evaluation of the antitumoral effect. The IC ₅₀ calculations and statistical analysis can also be performed by GraphPad Prism 8.0.2. The induction of cell death by compound IId relies on iron because the cytotoxicity of this compound was reduced in the presence of the iron chelator DFO (Figure 1). Therefore, the presence of Fe(II) is essential for the activity of the first drug delivery system in cells. The second drug delivery system contains an alkene group as an additional element of selectivity when compared with the first drug delivery system. Compound IIe is activated in cells by iron, but the major contributor of the cytotoxicity is related with the Michael addition by cellular glutathione. The induction of cell death by compound IIe showed a small reduction in the cytotoxicity of this compound in the presence of the iron chelator DFO (Figure 2). Therefore, in this example the presence of Fe(II) activates the second drug delivery system in cells. This activation generates the cyclohex-2-enone derivative, which contains an alkene group, that after reaction with cellular glutathione allows the release of the API, which is further described in example 4. 5.5. Compounds of formula II as tumour markers Evaluation of drug delivery systems of the invention IIc, IId and IIe as tumour markers was performed using fluorescence microscopy to detect their cellular localization. Compound IId and doxorubicin were incubated in colorectal cancer cells (HCT116) and after 24 hours of incubation images were acquired with a Zeiss Axio Scope.A1 microscope coupled with an AxioCam HR R3 camera and images analysed with Zen Software 2012 Blue Edition. Compound IId was present in a different cellular localization when compared with doxorubicin (see Example 9 and Figure 3). 5.6. In vivo validation for the first drug delivery system Compound IId and doxorubicin were tested for colorectal cancer using HT-29 cells in a xenograft murine model. During the treatment, the tumour size was analysed and used to determine the tumour volume and the relative tumour volume. Twenty immunocompromised Foxn1 ^nu/nu mice were divided into four groups: negative control, vehicle control, compound IId and doxorubicin. The negative control group is composed of five mice that did not receive treatment. The vehicle control is composed of five mice that received the solvent used to dissolve compound IId. Finally, five mice were treated with compound IId and five mice were treated with doxorubicin. Compound IId and doxorubicin were more efficient in reducing the relative tumour mass while comparing with the control groups (Figure 4). The reduction of the relative tumour mass was higher for mice treated with compound IId when compared with mice treated with doxorubicin (Figure 4). EXAMPLES Example 1. Synthesis of 1,2,4,5-tetraoxane compounds of formula I Cyclocondensation of dihydroperoxide (2) in the presence of a carbonyl (2-adamantanone or 5-hydroxy-2-adamantanone) produced 1,2,4,5-tetraoxane scaffolds (3) and (4). Alcohol activation in position 3 with 4-nitrophenyl chloroformate generated compounds (5) and (6). Finally, API coupling with compounds (5) and (6) produced the four compounds (IIa to IId) described in this invention for the first drug delivery system. Cyclocondensation of dihydroperoxide (9) in the presence of a carbonyl reagent, in this case 5-hydroxy-2-adamantanone, produced the 1,2,4,5-tetraoxane scaffold (10). Alcohol activation of compound (10) with 4-nitrophenyl chloroformate generated compound (11). Finally, API coupling with compound (11) produced the compound IIe described in this invention for the second drug delivery system. For this purpose, several intermediary compounds (1 to 6 and 8 to 11) were produced, according to the described below. 1.1. Synthesis of compound (1): rac-3-hydroxycyclohexanone A sodium dichromate solution (3.01 g, 10.3 mmol) in concentrated sulfuric acid/water (1.7 mL/12 mL) was added to a 1,3- cyclohexanediol (3.45 g, 29.7 mmol) solution in diethyl ether (12 mL) dropwise. After complete addition, the reaction was left to stir at 25°C for 3.5 hours. The reaction mixture was extracted with ethyl acetate (3x30 mL), the organic fractions combined, dried over anhydrous sodium sulphate, filtered, and evaporated. Purification by flash chromatography using Hexane:Ethyl acetate(2:8) as eluent isolated compound 1 as a colourless oil (1.35 g, 11.8 mmol) and 1,3-cyclohexanediol (1.26 g, 10.8 mmol). Yield: 63%; ¹ H NMR (300 MHz, CDCl ₃ ): δ (ppm) 4.18-4.10 (m, 1H), 2.93 (br s, 1H), 2.60 (dd, J = 14.1, 4.2 Hz, 1H), 2.37 (dd, J = 13.8, 7.5 Hz, 1H), 2.27 (t, J = 6.6 Hz, 2H), 2.09-1.88 (m, 2H), 1.80-1.55 (m, 2H); ¹³ C NMR (75 MHz, CDCl ₃ ): δ (ppm) 210.4, 69.8, 50.5, 41.0, 32.8, 20.8; MS (ES ⁺ ): [M+H] ⁺ 114.9 (12). 1.2. Synthesis of compound (2): 3,3-dihydroperoxycyclohexan-1-ol A 25 mL round-bottom flask was charged with rac-3- hydroxycyclohexanone (0.219 g, 1.92 mmol) and rhenium(VII) oxide (0.044 g, 0.092 mmol) in acetonitrile (10,5 mL). Then, hydrogen peroxide 50% (0.40 mL, 7.0 mmol) was added at room temperature and left to stir for 40 minutes. The mixture was filtered under vacuum through a silica plug and washed with ethyl acetate. The solution was concentrated, and compound 2 was isolated as a colourless oil (0.313 g, 1.91 mmol). Yield: 100%; ¹ H NMR (300 MHz, (CD ₃ ) ₂ CO): δ (ppm) δ 9.99 (s, 1H), 9.67 (s, 1H), 3.91-3.63 (m, 2H), 2.46-2.36 (m, 1H), 2.13-2.07 (m, 1H), 1.91-1.82 (m, 1H), 1.69-1.59 (m, 1H), 1.53-1.14 (m, 4H); ¹³ C NMR (75 MHz, (CD ₃ ) ₂ CO): δ (ppm) 110.7, 67.5, 39.6, 39.4, 35.6, 20.4; 1.3. Synthesis of compound (3): dispiro[adamantane-2,3'- [1,2,4,5]tetraoxane-6',1''-cyclohexan]-3''-ol A 25 mL round-bottom flask was charged with 2-adamantanone (0.433 g, 2.88 mmol), rhenium(VII) oxide (0.020 g, 0.041 mmol) in dichloromethane (5.7 mL) under inert atmosphere. Then, 3,3- dihydroperoxycyclohexan-1-ol (0.313 g, 1.91 mmol) in acetonitrile (5.7 mL) was added at room temperature and left to stir for 45 minutes. The mixture was filtered under vacuum through a silica plug and washed with ethyl acetate. The solution was concentrated and purification by flash chromatography in Hexane:Ethyl acetate (7:3) isolated compound 3 as a white solid (0.315 g, 1.06 mmol). Yield: 56%; ¹ H NMR (300 MHz, CDCl ₃ ): δ (ppm) 4.52-4.45 (m, 1H), 3.11-3.03 (m, 1H), 2.16-1.67 (m, 21H); ¹³ C NMR (75 MHz, CDCl ₃ ): δ (ppm) 110.7, 73.2, 41.3, 37.0, 35.8, 33.8, 33.2, 31.0, 27.1, 25.9; MS (ES ⁺ ): [M+H] ⁺ 297.2 (10); [M-Tetraoxane cleavage] ⁺ 167.0 (100). 1.4. Synthesis of compound (4): dispiro[adamantane-2,3'- [1,2,4,5]tetraoxane-6',1''-cyclohexane]-3'',5-diol For the synthesis of compound 4 the same method was used with 3,3- dihydroperoxycyclohexan-1-ol (0.141 g, 0.861 mmol) as starting material; however, 2-adamantanone was replaced by 5-hydroxy-2- adamantanone (0.216 g, 1.30 mmol). Purification by flash chromatography in Hexane:Ethyl acetate (2:8) isolated compound 4 as a white solid (0.145 g, 0.464 mmol). Yield: 54%; ¹ H NMR (300 MHz, CDCl ₃ ): δ (ppm) 4.63-4.59 (m, 1H), 3.18-3.12 (m, 1H), 2.59-2.39 (m, 2H), 2.16-1.50 (m, 18H); ¹³ C NMR (100 MHz, CDCl ₃ ): δ (ppm) 108.6, 74.6, 66.7, 43.7, 42.1, 41.5, 38.8, 34.7, 30.4, 29.9; MS (ES ⁺ ): [M-Tetraoxane cleavage] ⁺ 183.0 (100). Elemental Analysis: Found: C, 61.46; H, 7.60. C ₁₆ H ₂₄ O ₆ requires C, 61.52; H, 7.74. 1.5. Synthesis of compound (5): dispiro[adamantane-2,3'- [1,2,4,5]tetraoxane-6',1''-cyclohexan]-3''-yl (4-nitrophenyl) carbonate Compound 3 (0.120 g, 0.404 mmol) was dissolved in dichloromethane (1.6 mL) and, afterwards, N,N-diisopropylethylamine (0.21 mL, 1.21 mmol) and 4-dimethylaminopyridine (0.0520 g, 0.421 mmol) were added to the solution. At 0°C, 4-nitrophenyl chloroformate (0.168 g, 0.814 mmol) was added and the reaction was left to stir under inert atmosphere for 15 minutes, 0°C to room temperature for 10 minutes and at room temperature for 20 minutes. The reaction mixture was diluted in diethyl ether (30 mL), washed with 5% potassium bisulphate (10 mL) and sodium bicarbonate saturated solution (5x10 mL). The organic phase was washed with brine (1x25 mL), dried with anhydrous sodium sulphate, and afterward was filtered and concentrated. Purification by flash chromatography in Hexane:Ethyl acetate (9:1) isolated compound 5 as a white solid (0.0416 g, 0.090 mmol). Yield: 22%; ¹ H NMR (300 MHz, CDCl ₃ ): δ (ppm) 8.28 (dd, J = 7.2, 2.4 Hz, 2H), 7.39 (dd, J = 6.9, 2.1 Hz, 2H), 4.93 (s, 1H), 3.32 (br s, 1H), 3.13 (br s, 1H) 2.23-1.57 (m, 20H); ¹³ C NMR (75 MHz, CDCl ₃ ): δ (ppm) 155.7, 151.7, 145.5, 125.4, 122.0, 111.0, 75.6, 37.0, 33.2, 30.6, 27.1; MS (ES ⁺ ): [M-NO ₂ ] ⁺ 415.2 (20). 1.6. Synthesis of compound (6): 5-hydroxydispiro[adamantane-2,3'- [1,2,4,5]tetraoxane-6',1''-cyclohexan]-3''-yl (4-nitrophenyl) carbonate Compound 6 was prepared using the same method, but the starting material was compound 4 (0.151 g, 0.483 mmol). Purification by flash chromatography in Hexane:Ethyl acetate (1:1) isolated compound 6 as a white solid (0.0996 g, 0.208 mmol). Yield: 43%; ¹ H NMR (300 MHz, CDCl ₃ ): δ (ppm) 8.28 (dd, J = 7.2, 2.4 Hz, 2H), 7.36 (dd, J = 6.9, 2.4 Hz, 2H), 4.76-4.69 (m, 1H), 3.34- 3.26 (m, 1H), 2.68-1.74 (m, 20H); ¹³ C NMR (75 MHz, CDCl ₃ ): δ (ppm) 155.4, 150.0, 145.5, 125.4, 121.9, 81.9, 73.9, 41.3, 39.8, 38.2, 34.7, 30.6, 29.9; 1.7. Synthesis of compound (8): 2-(hydroxymethyl)cyclohex-2-en-1- one A 10 mL round-bottom flask was charged with barium hydroxide octahydrate (0.0079 g, 0.0250 mmol), dissolved in water/methanol (5:1; 2:0.5 mL) and N-methylpyrrolidine (5.5 µL, 0.0520 mmol) was added. Afterwards, formaldehyde (0.12 mL, 1.56 mmol) and compound 7 (0.10 mL, 1.04 mmol) were added successively to the previous solution and the reaction was left to stir at 0°C for 21 hours. The reaction mixture was quenched with a solution of hydrochloric acid (concentration: 1 mol/L) until pH = 3. Then, sodium bicarbonate saturated solution was added until pH around 7 and the resulting aqueous phase was extracted with dichloromethane (3x20 mL). The organic fractions were combined, dried over anhydrous sodium sulphate, filtered, and evaporated. Purification by flash chromatography using Hexane:Ethyl acetate (4:6) as eluent isolated compound 8 as a colourless oil (0.0706 g, 0.560 mmol). Yield: 54%; ¹ H NMR (300 MHz, CDCl ₃ ): δ (ppm) 6.93 (t, J = 4.2 Hz, 1H), 4.24 (q, J = 1.2 Hz, 2H), 2.48-2.35 (m, 4H), 2.32-2.12 (br s, 1H), 2.08-1.95 (m, 2H); ¹³ C NMR (75 MHz, CDCl ₃ ): δ (ppm) 200.6, 147.9, 138.2, 61.3, 38.2, 25.6, 22.7; MS (ES ⁺ ): [M+H] ⁺ 127 (75). 1.8. Synthesis of compound (9): (6,6-dihydroperoxycyclohex-1-en-1- yl)methanol A 25 mL round-bottom flask was charged with compound 8 (0.0774 g, 0.614 mmol) and rhenium(VII) oxide (0.0171 g, 0.0353 mmol) in acetonitrile (3.7 mL). Then, hydrogen peroxide 50% (0.14 mL, 2.46 mmol) was added at room temperature and left to stir for 50 minutes. The mixture was filtered under vacuum through a silica plug and washed with ethyl acetate. The solution was concentrated, and compound 9 was isolated as a colourless oil (0.105 g, 0.598 mmol). Yield: 97%; ¹ H NMR (300 MHz, (CD ₃ ) ₂ CO): δ (ppm) 9.97 (s, 1H), 9.65 (s, 1H), 4.26-4.13 (m, 1H), 4.02-3.89 (m, 1H), 3.71-3.62 (m, 1H), 2.49-1.25 (m, 2H), 2.01-1.62 (m, 4H); ¹³ C NMR (75 MHz, (CD ₃ ) ₂ CO): δ (ppm) 145.1, 59.5, 37.8, 26.1, 23.8. 1.9. Synthesis of compound (10): 2''- (hydroxymethyl)dispiro[adamantane-2,3'-[1,2,4,5]tetraoxane-6 ',1''- cyclohexan]-2''-en-5-ol A 25 mL round-bottom flask was charged with 5-hydroxy-2- adamantanone (0.338 g, 2.03 mmol), rhenium(VII) oxide (0.0165 g, 0.034 mmol) in dichloromethane (4.0 mL) under inert atmosphere. Then, compound 9 (0.234 g, 1.33 mmol) in acetonitrile (4.0 mL) was added at room temperature and left to stir for 1 hour. The mixture was filtered under vacuum through a silica plug and washed with ethyl acetate. The solution was concentrated and purification by flash chromatography in Hexane:Ethyl acetate (2:8) isolated compound 10 as a white solid (0.132 g, 0.406 mmol). Yield: 31%; ¹ H NMR (300 MHz, CDCl ₃ ): δ (ppm) 4.68-4.59 (m, 1H), 3.24-3.15 (m, 1H), 2.49-2.39 (m, 2H), 2.16-1.68 (m, 18H); MS (ES ⁺ ): [M+ACN+H] ⁺ 366 (30), [M+Na] ⁺ 347 (30). 1.10. Synthesis of compound (11): 5-hydroxydispiro[adamantane-2,3'- [1,2,4,5]tetraoxane-6',1''-cyclohexan]-2''-en-2''-yl)methyl (4- nitrophenyl) carbonate Compound 10 (0.126 g, 0.388 mmol) was dissolved in dichloromethane (1.53 mL) and, afterwards, N,N-diisopropylethylamine (0.20 mL, 1.17 mmol) and 4-dimethylaminopyridine (0.0510 g, 0.413 mmol) were added to the solution. At 0°C, 4-nitrophenyl chloroformate (0.162 g, 0.788 mmol) was added and the reaction was left to stir under inert atmosphere for 15 minutes, 0°C to room temperature for 10 minutes and at room temperature for 30 minutes. The reaction mixture was diluted in diethyl ether (30 mL), washed with sodium bicarbonate saturated solution (5x30 mL). The organic phase was washed with brine (1x25 mL), dried with anhydrous sodium sulphate, and afterward was filtered and concentrated. Purification by flash chromatography in Hexane:Ethyl acetate (1:1) isolated compound 11 as a white solid (0.0565 g, 0.115 mmol). Yield: 30%; ¹ H NMR (300 MHz, CDCl ₃ ): δ (ppm) 8.28 (dd, J = 7.0, 2.4 Hz, 2H), 7.36 (dd, J = 7.0, 2.4 Hz, 2H), 4.76-4.69 (m, 1H), 3.34- 3.26 (m, 1H) 2.66-1.77 (m, 20H). MS (ES ⁺ ): [M+ACN+Na] ⁺ 553 (15), [M+NH ₄ ] ⁺ 507 (100). Example 2. Synthesis of compounds of the invention (IIa, IIb, IIc, IId and IIe) 2.1. Synthesis of compound (IIa): dispiro[adamantane-2,3'- [1,2,4,5]tetraoxane-6',1''-cyclohexan]-3''-yl (4-((7- methoxyquinolin-5-yl)amino)pentyl)carbamate To a primaquine.diphosphate (0.042 g, 0.0922 mmol) solution in N,N- dimethylformamide (0.34 mL), triethylamine (12.0 µL, 0.0866 mmol) was added. After 30 minutes, 1-hydroxybenzotriazole hydrate (0.012 g, 0.0888 mmol) was added to the previous solution and compound 5 (0.0205 g, 0.0444 mmol) in N,N-dimethylformamide (0.34 mL). The mixture was kept stirring at room temperature overnight. Then, brine (20 mL) was added to the mixture and extracted with ethyl acetate (3x20 mL). The organic fractions were combined, washed with distilled water (1x20 mL), dried over anhydrous sodium sulphate, filtered and the solvent was removed under reduced pressure. The obtained residue was purified by preparative thin layer chromatography using Hexane:Ethyl acetate (7:3) as eluent. Compound IIa was isolated (0.0143 g, 0.024 mmol). Yield: 55%; ¹ H NMR (300 MHz, CDCl ₃ ): δ (ppm) 8.52 (dd, J = 4.2, 1.5 Hz, 1H), 7.91 (dd, J = 8.4, 1.5 Hz, 1H), 7.30 (dd, J = 8.4, 4.2 Hz, 1H), 6.32 (dd, J = 17.4, 2.4 Hz, 2H), 6.00 (br s, 1H), 4.82 (br s, 1H), 4.70 (br s, 1H), 3.89 (s, 3H), 3.62 (br s, 1H), 3.18 (br s, 3H), 2.81 (br s, 1H), 2.15-1.62 (m, 24H), 1.30 (d, J = 6.3 Hz, 3H); MS (ES ⁺ ): [M+H] ⁺ 582 (100). 2.2. Synthesis of compound (IIb): 1-(tert-butyl) 4- dispiro[adamantane-2,3'-[1,2,4,5]tetraoxane-6',1''-cyclohexa n]-3''- yl) piperazine-1,4-dicarboxylate To a boc-piperazine (0.014 g, 0.0752 mmol) solution in N,N- dimethylformamide (0.27 mL), triethylamine (10.0 µL, 0.0684 mmol) was added. After 30 minutes, 1-hydroxybenzotriazole hydrate (0.0098 g, 0.0704 mmol) was added to the previous solution and compound 5 (0.0158 g, 0.0342 mmol) in N,N-dimethylformamide (0.27 mL). The mixture was kept stirring at room temperature overnight. Then, brine (20 mL) was added to the mixture and extracted with ethyl acetate (3x20 mL). The organic fractions were combined, washed with distilled water (1x20 mL), dried over anhydrous sodium sulphate, filtered and the solvent was removed under reduced pressure. The obtained residue was purified by preparative thin layer chromatography using Hexane:Ethyl acetate (8:2) as eluent. Compound IIb was isolated (0.0057 g, 0.011 mmol). Yield: 33%; ¹ H NMR (300 MHz, CDCl ₃ ): δ (ppm) 4.91 (br s, 1H), 3.41 (m, 8H), 3.12 (br s, 1H), 3.00-2.56 (m, 1H), 2.31-1.55 (m, 20H), 1.46 (s, 9H). 2.3. Synthesis of compound (IIc): dispiro[adamantane-2,3'- [1,2,4,5]tetraoxane-6',1''-cyclohexan]-3''-yl (3-hydroxy-2-methyl- 6-((2S,4S)-4,5,12-trihydroxy-4-(2-hydroxyacetyl)-10-methoxy- 6,11- dioxo-1,2,3,4,6,11-hexahydrotetracen-2-yl)oxy)tetrahydro-2H- pyran- 4-yl)carbamate To a doxorubicin.hydrochloride (0.112 g, 0.193 mmol) solution in N,N-dimethylformamide (0.76 mL), triethylamine (27.0 µL, 0.193 mmol) was added. After 30 minutes, 1-hydroxybenzotriazole hydrate (0.027 g, 0.194 mmol) was added to the previous solution and compound 5 (0.0445 g, 0.0964 mmol) in N,N-dimethylformamide (0.76 mL). The mixture was kept stirring at room temperature for 3 hours. Then, brine (20 mL) was added to the mixture and extracted with ethyl acetate (3x20 mL). The organic fractions were combined, washed with distilled water (1x20 mL), dried over anhydrous sodium sulphate, filtered and the solvent was removed under reduced pressure. The obtained residue was purified by preparative thin layer chromatography using Dichloromethane:Methanol 4% as eluent. A red powder matching with the compound IIc was isolated (0.050 g, 0.058 mmol). Yield: 60%; ¹ H NMR (400 MHz, CDCl ₃ ): δ (ppm) 13.94 (d, J = 4.4 Hz, 1H), 13.18 (s, 1H), 8.00 (d, J = 7.6 Hz, 1H), 7.76 (t, J = 8.0 Hz, 1H), 7.37 (d, J = 8.4 Hz, 1H), 5.49 (br s, 1H), 5.25 (br s, 1H), 5.13-5.04 (m, 1H), 4.75 (s, 3H), 4.59-4.46 (m, 1H), 4.15-4.09 (m, 1H), 4.06 (s, 3H), 3.83 (br s, 1H), 3.67 (br s, 1H), 3.28-2.85 (m, 4H), 2.77 (br s, 1H), 2.68-2.37 (m, 1H), 2.36-2.26 (m, 1H), 2.24- 2.08 (m, 2H), 2.04-1.57 (m, 21H), 1.28 (d, J = 6.8 Hz, 3H); ¹³ C NMR (101 MHz, CDCl ₃ ): δ (ppm) 214.0, 187.1, 186.7, 161.1, 156.3, 155.7, 155.1, 135.9, 135.5, 133.7, 120.9, 119.9, 118.6, 111.6, 111.5, 110.8, 110.7, 100.9, 76.7, 69.8, 69.6, 67.4, 65.7, 56.8, 47.0, 37.0, 35.8, 34.1, 33.2, 31.1, 31.0, 30.3, 30.2, 29.8, 27.1, 17.0; MS (ES ⁺ ): [M+Na] ⁺ 888 (90); HRMS (ESI): m/z [M-H]- calculated for C ₄₄ H ₅₀ NO ₁₇ : 864.3084, found: 864.3062. 2.4. Synthesis of compound (IId): 5-hydroxydispiro[adamantane-2,3'- [1,2,4,5]tetraoxane-6',1''-cyclohexan]-3''-yl (3-hydroxy-2-methyl- 6-(((2S,4S)-4,5,12-trihydroxy-4-(2-hydroxyacetyl)-10-methoxy -6,11- dioxo-1,2,3,4,6,11-hexahydrotetracen-2-yl)oxy)tetrahydro-2H- pyran- 4-yl)carbamate Compound IId was prepared using the same method, but the starting material was compound 6 (0.0310 g, 0.0649 mmol). Purification by preparative thin layer chromatography in Dichloromethane:Methanol 4% isolated compound IId as a red powder (0.0429 g, 0.049 mmol). Yield: 75%; ¹ H NMR (400 MHz, DMSO): δ (ppm) 13.97 (s, 1H), 13.21 (s, 1H), 7.89-7.80 (m, 2H), 7.59 (d, J = 7.2 Hz, 1H), 6.56 (d, J = 8.0 Hz, 1H), 5.39 (s, 1H), 5.17 (s, 1H), 4.86 (d, J = 6.0 Hz, 2H), 4.84 (br s, 1H), 4.66 (d, J = 5.6 Hz, 1H), 4.55 (d, J = 5.6 Hz, 2H), 4.51 (br s, 1H), 4.11 (d, J = 6.8 Hz, 2H), 3.95 (s, 3H), 3.62 (br s, 2H), 3.44-3.37 (m, 2H), 3.02-2.82 (m, 4H), 2.76-2.60 (m, 1H), 2.45-1.51 (m, 19H), 1.09 (d, J = 6.4 Hz, 3H); ¹³ C NMR (101 MHz, DMSO): δ (ppm) 213.9, 186.5, 186.4, 176.7, 160.8, 156.1, 154.5, 154.1, 136.2, 135.5, 134.6, 134.1, 119.9, 119.7, 119.0, 110.8, 110.6, 100.4, 75.9, 75.0, 73.6, 69.8, 68.1, 66.7, 63.7, 56.6, 46.7, 40.9, 38.5, 36.5, 34.8, 34.7, 34.2, 32.1, 29.8, 29.7, 29.2, 17.0; MS (ES ⁺ ): [M+2ACN+H] ⁺ 964 (25), [M+CH3OH+H] ⁺ 914 (45), [M+Na] ⁺ 904 (30); MS (ES-): [M-H]- 880 (35), [M+HCOOH-H]- 925 (20). 2.5. Synthesis of compound (IIe): 5-hydroxydispiro[adamantane-2,3'- [1,2,4,5]tetraoxane-6',1''-cyclohexan]-2''-en-2''-yl)methyl (3- hydroxy-2-methyl-6-(((1R,3R)-3,5,12-trihydroxy-3-(2-hydroxya cetyl)- 10-methoxy-6,11-dioxo-1,2,3,4,6,11-hexahydrotetracen-1- yl)oxy)tetrahydro-2H-pyran-4-yl)carbamate To a doxorubicin.hydrochloride (0.0743 g, 0.128 mmol) solution in N,N-dimethylformamide (0.50 mL), triethylamine (18.0 µL, 0.128 mmol) was added. After 30 minutes, 1-hydroxybenzotriazole hydrate (0.0183 g, 0.135 mmol) was added to the previous solution and compound 11 (0.0312 g, 0.064 mmol) in N,N-dimethylformamide (0.50 mL). The mixture was kept stirring at room temperature for 7 hours. Then, brine (20 mL) was added to the mixture and extracted with ethyl acetate (3x20 mL). The organic fractions were combined, washed with distilled water (1x20 mL), dried over anhydrous sodium sulphate, filtered and the solvent was removed under reduced pressure. The obtained residue was purified by preparative thin layer chromatography (TLC) using Dichloromethane:Methanol 4% as eluent. Compound IIe was isolated as a red powder (0.0415 g, 0.046 mmol). Yield: 73%; ¹ H NMR (400 MHz, DMSO): δ (ppm) 13.95 (s, 1H), 13.20 (s, 1H), 7.90-7.76 (m, 2H), 7.58 (d, J = 7.6 Hz, 1H), 6.56 (d, J = 8.0 Hz, 1H), 5.37 (s, 1H), 5.16 (s, 1H), 4.91-4.81 (m, 2H), 4.73- 4.61 (m, 1H), 4.60-4.45 (m, 4H), 4.11 (d, J = 7.2 Hz, 1H), 3.94 (s, 3H), 3.62 (br s, 2H), 3.39 (br s, 2H), 3.04-2.75 (m, 4H), 2.40-1.50 (m, 20H), 1.41 (d, J = 12 Hz, 1H), 1.09 (d, J = 6.4 Hz, 3H); ¹³ C NMR (101 MHz, DMSO): δ (ppm) 213.9, 186.5, 186.4, 176.7, 160.8, 156.1, 154.5, 154.1, 136.2, 135.5, 134.6, 134.0, 119.9, 119.7, 119.0, 110.7, 110.6, 100.4, 75.9, 75.0, 73.5, 69.8, 68.1, 66.7, 63.7, 56.6, 46.7, 40.9, 38.5, 36.5, 34.8, 34.7, 34.2, 32.1, 29.8, 29.7, 29.2, 17.0; MS (ES ⁺ ): [M+H] ⁺ 894 (35), [M+Na] ⁺ 916 (70); MS (ES-): [M-H]- 892 (40). Example 3 - Mechanistic proof-of-concept of the first drug delivery system A solution of compound IIc (0.009 mmol) in dichloromethane was added to a schlenk tube, under inert atmosphere, charged with iron(II) bromide (2 equivalents) and TEMPO (2 equivalents) in acetonitrile. The reaction was left to stir at room temperature for 24 hours. The reaction mixture was diluted in ethyl acetate (10 mL), washed with distilled water (1x10 mL) and brine (1x10 mL). The organic fraction was dried over anhydrous sodium sulphate, filtered and the solvent was removed under reduced pressure. The residue was analysed by UHPLC-MS. The mobile phase consisted of millipore water containing 0.1% formic acid (A) and acetonitrile (B), at a flow rate of 0.30 mL min ^-1 . A gradient elution was applied, consisting of 10% B for 1 minutes; from 10 to 95% B for 5 minutes and 95% B for 4 minutes, finally returning to the initial conditions (10% B) for 5 minutes. Tetraoxane reaction with a Fe(II) source and TEMPO was performed for compound IIc to confirm tetraoxane fragmentation and, more importantly, that spontaneous β-elimination of the cyclohexanone derivative occurs to liberate doxorubicin. The UHPLC-MS analysis of this reaction detected two TEMPO adducts on positive ([M+H] ⁺ = 324) and negative mode ([M-H]-= 322; [M-H]-= 855 and [M+CN-H]-= 881), for the two possibilities of coordination with Fe(II), which confirms tetraoxane fragmentation with formation of carbon-centred radicals (Scheme 5). Additionally, 2-adamantanone ([M+K] ⁺ = 189) and the cyclohexanone derivative ([M-H]-= 682) were also identified, in accordance with the proposed mechanism. Importantly, doxorubicin was also detected ([M+H] ⁺ = 544; [M-H]-= 542), confirming API release by β-elimination of the cyclohexanone derivative (Scheme 6). Scheme 5 presents the Fe(II) activation and TEMPO reaction of compound IIc generating TEMPO adducts. Scheme 6 presents compound IIc activation and doxorubicin release by the cyclohexanone derivative. Example 4 – Mechanistic proof-of-concept of the second drug delivery system To validate the ferrous iron activation and TEMPO trapping of the second drug delivery system, the following protocol was performed. A solution of compound IIe (0.0077 g, 0.009 mmol) in dichloromethane was added to a schlenk tube, under inert atmosphere, charged with iron(II) bromide (2 equivalents) and TEMPO (2 equivalents) in tetrahydrofuran. The reaction was left to stir at room temperature for 24 hours. The reaction mixture was diluted in ethyl acetate (10 mL), washed with distilled water (1x10 mL) and brine (1x10 mL). The organic fraction was dried over anhydrous sodium sulphate, filtered and the solvent was removed under reduced pressure. The residue was analysed by UHPLC-MS. The mobile phase consisted of millipore water containing 0.1% formic acid (A) and acetonitrile (B), at a flow rate of 0.30 mL min ^-1 . A gradient elution was applied, consisting of 30% B for 1 minutes; from 30 to 95% B for 5 minutes and 95% B for 4 minutes, finally returning to the initial conditions (30% B) for 5 minutes. To validate the ferrous iron activation and N-acetylcysteine (NAC) Michael addition of the second drug delivery system, the following protocol was performed. 250 µL of a 10 mM stock solution of compound IIe in tetrahydrofuran was added to a schlenk tube, under inert atmosphere, charged with iron(II) bromide (2 equivalents) and NAC (1.5 equivalents) in 250 µL of millipore water. Then, 100 µL of sodium hydroxide (concentration: 6.25 mol/L) was added to assure basic pH. The reaction was left to stir at room temperature for 24 hours. The reaction mixture was diluted in distilled water (10 mL) and extracted with ethyl acetate (10 mL). The aqueous fraction was acidified with hydrochloric acid (concentration: 1 mol/L) and extracted with ethyl acetate (2x10 mL). The aqueous phase was neutralized with sodium hydroxide (concentration: 6.25 mol/L) and extracted with ethyl acetate (2x10 mL). The organic fractions were combined, dried over anhydrous sodium sulphate, filtered and the solvent was removed under reduced pressure. The residue was analysed by UHPLC-MS. The mobile phase consisted of millipore water containing 0.1% formic acid (A) and acetonitrile (B), at a flow rate of 0.30 mL min ^-1 . A gradient elution was applied, consisting of 30% B for 1 minutes; from 30 to 95% B for 5 minutes and 95% B for 4 minutes, finally returning to the initial conditions (30% B) for 5 minutes. Compound IIe reacted with an iron source and TEMPO trapping to confirm tetraoxane fragmentation and, more importantly, that doxorubicin was not released under these conditions. The UHPLC-MS analysis of this reaction detected two TEMPO adducts on positive mode ([M+H] ⁺ = 340; [M+K] ⁺ = 907), for the two possibilities of coordination with Fe(II), which confirms tetraoxane fragmentation with formation of carbon-centred radicals (Scheme 7). Scheme 7 presents the Fe(II) activation and TEMPO trapping of compound IIe generating TEMPO adducts. Additionally, 5-hydroxy-2-adamantanone ([M+NH4] ⁺ = 184; [M+DMSO+H] ⁺ = 245) and the cyclohex-2-enone derivative ([M+K] ⁺ = 734; [M+DMSO+H] ⁺ = 774) were also identified, in accordance with the proposed mechanism (Scheme 7 and 8). Finally, in this experiment doxorubicin was not detected. Scheme 8 presents the Fe(II) activation of compound IIe generating the cyclohex-2-enone derivative. Reagents and conditions: (i) iron(II) bromide, TEMPO, tetrahydrofuran. Compound IIe reacted with a Fe(II) source and N-acetylcysteine (NAC) to confirm the mechanism of the second drug delivery system. The UHPLC-MS analysis of this reaction detected one carbon-centred radical (Scheme 9) on positive mode ([M+H] ⁺ = 713 and [M+Na] ⁺ = 735), confirming tetraoxane fragmentation upon reaction with Fe(II). Scheme 9 presents the Fe(II) activation of compound IIe generating a carbon-centred radical. Additionally, the cyclohex-2-enone derivative ([M+Na] ⁺ = 718 and [M+K] ⁺ = 734) was also detected, in accordance with the first step of activation of the second drug delivery system. Finally, release of doxorubicin was detected, validating the mechanism of action for the second drug delivery system because Michael addition of NAC to the cyclohex-2-enone derivative occurred (Scheme 10). Scheme 10 presents the mechanism of drug delivery of compound IIe validated after detecting doxorubicin. Example 5. Evaluation of biological activity of compounds of formulae II against malaria Sample preparation Compounds were dissolved in RPMI-1640 with L-glutamine (Biowest™) supplemented with AlbuMAXII (Gibco™), herein after referred to as RPMIc, to obtain an intermediate solution of 100 µM. P. falciparum in vitro culture Laboratory-adapted P. falciparum strain 3D7, a chloroquine and mefloquine sensitive strain, was continuously cultured as described elsewhere (Machado et. al, Ann. Clin. Med. Microbio. 2016, 2, 1010). Parasites were cultivated in 5% haematocrit, 37°C and atmosphere with 5% of CO ₂ . AlbuMAXII (Gibco™) at 0,5% was used as a substitute of human serum in the culture medium. Antimalarial activity determined using the Whole-Cell SYBR Green I assay Staging and parasitemia were determined by light microscopy of Giemsa-stained thin blood smears. Anti-malarial activity was determined using the SYBR Green I assay (Machado et. al, Ann. Clin. Med. Microbio. 2016, 2, 1010 and Lobo et. al, Malar. J. 2018, 17, 1). Briefly, asynchronous parasites were cultivated for 72 hours in the presence of a 1:3 serial dilution of each compound. Fluorescence intensity was measured with a multi-mode microplate reader (Triad, Dynex Technologies), with excitation and emission wavelengths of 485 and 535 nm, respectively, and analysed by nonlinear regression using GraphPad Prism to determine IC ₅₀ values. Compounds IIa and IIb were potent against chloroquine-sensitive 3D7 strain of Plasmodium falciparum. Compound IIa and IIb have a IC ₅₀ of 32.82 nM and 11.38 nM, respectively (Table 1). Both compounds were as potent as chloroquine (CQ), an antimalarial used in the clinic (Table 1). Table 1 – Antimalarial activity (IC ₅₀ ) of the compounds against Plasmodium falciparum. *Results in nanomolar (nM) from at least 3 independent assays, each in triplicate. SD, standard deviation. CQ, chloroquine. Example 6. Evaluation of biological activity of compounds of formulae II in cancer treatment PC3 human prostate adenocarcinoma, MCF7 human breast adenocarcinoma, HCT116 human colorectal carcinoma and U251 human glioblastoma were obtained from American Type Culture Collection (ATCC). Cell lines were cultured under adherent conditions in Roswell Park Memorial Institute (RPMI) 1640 (PC3), Dulbecco’s modified Eagle’s medium (DMEM; MCF7 and U251), McCoy’s 5A (HCT116), all supplemented with 10% (v/v) heat-inactivated fetal bovine serum (FBS) and 1% (v/v) antibiotic/antimycotic solution (all from Gibco, ThermoFisher Scientific, Paisley, UK). Additionally, MCF7 media was supplemented with 1% (v/v) glutamax (Gibco); U251 supplemented with 1% (v/v) glutamax and 1% (v/v) 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer (both from Gibco). All cell lines were cultured at 37ºC under a humidified atmosphere of 5% CO ₂ (HeraCell 150i CO ₂ incubator, ThermoFisher Scientific). To quantitatively assess the compounds inhibitory potency, their half maximal inhibitory concentration (IC ₅₀ ) was determined through dose-response curves. Cell lines were plated in 96-well plates at a cellular density of 5000 cells/well, and 24 hours after, cells were treated with compounds IIc, IId, IIe, doxorubicin, compound 3 and compound 10 (concentrations ranging from 1.5x10 ^-3 ρM to 1 mM). Following 72 hours of incubation, cell metabolic activity assessed by determination of MTS metabolism. Viable cells (metabolically active cells) will metabolize the MTS into formazan and this molecule is quantified by absorbance. The higher the amount of formazan more cells survived after exposure to the compounds while the lower the amount of formazan more cells are not metabolically active. For all assays, cellular metabolic activity was measured using CellTiter 96 ^® Aqueous Non-Radioactive Cell Proliferation Assay (MTS; Promega, Madison, WI, USA) and absorbance signal recorded using a GloMax ^® -MultiDetection System (Promega). IC ₅₀ determination and all statistical analysis was preformed using GraphPad Prism 8.0.2. software (San Diego, CA, USA). All data are expressed as mean ± SEM from at least three independent experiments. According to the normality of values distribution, assessed with Shapiro-Wilk test, Ordinary One-Way ANOVA or Kruskal-Wallis test, followed by Bonferroni or Dunn’s multiple comparison test were used to evaluate differences among sample groups. A p-value inferior to 0.05 was considered significant. All compounds were tested alone and in suitable doses on a panel of cancer cell lines with the results shown in Table 2. As it is possible to observe from Table 2, compound IIc was potent in all cell lines in the micromolar range, except for the U251 cell line, which was in the submicromolar range. Moreover, compound IIc was less potent than doxorubicin alone. Compound IId was also tested on the cancer cell lines used previously (U251, PC3, MCF7 and HCT116). Despite their difference in terms of lipophilicity, compound IId was also potent in the micromolar range and less potent than doxorubicin. Compound (3) was tested as a control to demonstrate that the potency of compounds IIc and IId on cancer cells relies on the API coupled to compound (3). This compound was chosen since compound (3) represents the central structure of compounds IIc and IId. Compound (3) was not active on the tested cell lines. These results show that the potency of compounds IIc and IId relies on the API coupled in the system and not on the tetraoxane structure. Compound IIe was potent in the micromolar range in all cancer cell lines. Compound IIe was less potent than doxorubicin in all cancer cell lines evaluated. Compound (10) was tested as a control to demonstrate that the potency of compound IIe on cancer cells relies on the API coupled to compound (10). This compound was chosen since compound 10 represents the central structure of compound IIe. Compound (10) was not active in all the cell lines tested (Table 2). Hence, these results prove that the tetraoxane scaffold does not possess cytotoxicity and the contributor of the activity of compound IIe is the API coupled in the system, in this case, doxorubicin. Table 2 - IC ₅₀ of the tested compounds in all cell lines. Data represent IC ₅₀ values of three independent experiments and respective confidence interval Results in micromolar (µM) from at least 3 independent assays. C. I.: confidence interval; N. D.: not determined; N. A.: not active. Example 7. Evaluation of the effect of compounds of formulae II in toxicity for non-tumoral cells To evaluate the toxicity effect of doxorubicin, compounds IIc, IId, IIe and doxorubicin were tested on a non-tumorigenic cell line (AML12) and the results are shown in Table 3. AML12 murine immortalized hepatocyte cell line was obtained from European Collection of Authenticated Cell Cultures (Porton Down, UK). AML12 cell line was cultured under adherent conditions in DMEM/Nutrient mixture F-12 (Gibco, ThermoFisher Scientific, Paisley, UK), supplemented with 10% (v/v) heat-inactivated fetal bovine serum (FBS; Gibco), 1% (v/v) antibiotic/antimycotic solution (Gibco) 1% (v/v) insulin-transferrin-selenium (Gibco) and 40 ng/mL of dexamethasone (Sigma-Aldrich, St. Louis, MO, USA). This cell line was cultured at 37ºC under a humidified atmosphere of 5% CO ₂ (HeraCell 150i CO ₂ incubator, ThermoFisher Scientific). AML12 cells were plated in 96-well plates at a cellular density of 5000 cells/well, and 24 hours after, cells were treated with compound IIc, IId, IIe and doxorubicin (concentrations ranging from 1.5x10 ^-3 ρM to 1 mM). Cellular metabolic activity was measured after 72 hours using CellTiter 96 ^® Aqueous Non-Radioactive Cell Proliferation Assay (MTS; Promega, Madison, WI, USA) and absorbance signal recorded using a GloMax ^® -MultiDetection System (Promega). IC ₅₀ determination and all statistical analysis was preformed using GraphPad Prism 8.0.2. software (San Diego, CA, USA). All data are expressed as mean ± SEM from at least three independent experiments. According to the normality of values distribution, assessed with Shapiro-Wilk test, Ordinary One-Way ANOVA or Kruskal- Wallis test, followed by Bonferroni or Dunn’s multiple comparison test were used to evaluate differences among sample groups. A p- value < 0.05 was considered significant. From Table 3 it is possible to observe that both compounds IIc and IId were able to induce reduced toxicity than doxorubicin. Interestingly, compound IIc was 10 times less toxic when compared with doxorubicin. However, AML12 cells were highly resistant to compound IId, with a IC ₅₀ ratio of 4294. In conclusion, the first drug delivery system according to the present invention is able to reduce the toxicity of doxorubicin. From Table 3 it is possible to observe that compound IIe was less toxic in non-tumorigenic cells than doxorubicin. Interestingly, compound IIe was 180-fold less toxic when compared with doxorubicin. So, the second drug delivery system can reduce the toxicity associated with doxorubicin. Table 3 - IC ₅₀ of the tested compounds in AML12 cell line. Data represent IC ₅₀ values of three independent experiments and respective confidence interval Results in micromolar (µM) from at least 3 independent assays. C. I.: confidence interval; N. D.: not determined. Example 8. Evaluation of biological activity dependent on iron For iron competition assays, HCT116 cells were incubated with 1 µM DFO (Figure 1 column #2)), 10 µM compound IId (Figure 1 column #3)) and 10 µM compound IId together with 1 µM DFO (Figure 1 column #4)). Moreover, HCT116 cells were incubated with 1 µM DFO (Figure 2 column #2)), 10 µM compound IIe (Figure 2 column #3)) and 10 µM compound IIe together with 1 µM DFO (Figure 2 column #4)). Following 24 hours of incubation, cellular metabolic activity was determined. Cellular metabolic activity was measured using CellTiter 96 ^® Aqueous Non-Radioactive Cell Proliferation Assay (MTS; Promega, Madison, WI, USA) and absorbance signal recorded using a GloMax ^® -MultiDetection System (Promega). IC ₅₀ determination and all statistical analysis was performed using GraphPad Prism 8.0.2. software (San Diego, CA, USA). All data are expressed as mean ± SEM from at least three independent experiments. According to the normality of values distribution, assessed with Shapiro-Wilk test, Ordinary One-Way ANOVA or Kruskal-Wallis test, followed by Bonferroni or Dunn’s multiple comparison test were used to evaluate differences among sample groups. A p-value < 0.05 was considered significant. Compound IId was incubated with an iron chelator (deferoxamine mesylate - DFO) of ferrous iron in HCT116 cells. A significant reduction was detected in the potency of compound IId (Figure 1 column #4)) when compared with compound IId alone (Figure 1 column #3)). Thus, cell death by compound IId is iron-dependent. Compound IIe was incubated with the iron chelator DFO in HCT116 cells. A small reduction (7%) was detected in the potency of compound IIe (Figure 2 column #4)) when compared with compound IIe alone (Figure 2 column #3)). Probably, compound IIe is activated by Fe(II) as described, but the major contributor of the cytotoxicity is related with the reaction of the cyclohex-2-enone derivative with glutathione (Michael addition), allowing the release of the API by elimination (see Example 4). The cyclohex-2-enone derivative possesses an alkene group that only becomes available after tetraoxane activation by Fe(II), which allows to improve the selectivity when compared with the first drug delivery system. Example 9. Evaluation of biological activity of compounds of formulae II as tumour markers Quantum yields determination Ultraviolet (UV) spectra were traced in Thermo Scientific Evolution 201 UV-visible spectrophotometer. Fluorescence measurements were done on a SHIMADZU Spectro fluorophotometer RF-6000 instrument. Fluorescence quantum yields were measured with Fluorescein in 0.1 M sodium hydroxide as a standard (Φf = 0.95). All solvents were of analytical reagent grade and were purchased from Alfa Aesar or Sigma-Aldrich. Doxorubicin has fluorescence properties. Dilute solutions of compounds IIc, IId and doxorubicin in water with 30% dimethyl sulfoxide (DMSO) were prepared. The quantum yield of doxorubicin was 2.8% and the quantum yield of compounds IIc and IId was below 1%. Fluorescence imaging for compound cellular localization HCT116 cells were plated in 35 mm dishes at 2.5x10 ⁵ cells/ml cellular density over a glass coverslip. After 24 hours of plating, cells were incubated for another 24 hours with system control (DMSO) or 1 µM of compounds IId or doxorubicin. Cells were gently washed with Phosphate Buffered Saline (PBS) 1x and fixed with 4% paraformaldehyde (PFA), pH 7.4 at room temperature. Fixed cells were incubated with Hoechst dye 33258 (5 µg/mL) for 10 minutes in the dark. Cells were washed 3x with PBS 1x and mounted into a glass slide with 7 µL Mowiol ^® . Images were acquired with a Zeiss Axio Scope.A1 microscope coupled with an AxioCam HR R3 camera and images analysed with Zen Software 2012 Blue Edition (Carl Zeiss Microscopy, GmbH). Doxorubicin photoluminescence properties are ablated in the drug delivery system of formula II (quantum yield below 1%). Therefore, next we explored the cellular localization of compound IId in cells. Interestingly, fluorescence imaging in HCT116 cells showed that after 24 hours of incubation, doxorubicin accumulated in the nucleus (Figure 3 BC and BD) while compound IId was activated and released doxorubicin in the cytosol (Figure 3 CC and CD), where the labile iron pool is elevated. Example 10 - In vivo validation of the first drug delivery system The antitumor effect of compound IId was tested in a xenograft murine model. Immunocompromised Foxn1 ^nu/nu mice received a subcutaneous injection of a colorectal cancer cell line (2x10 ⁶ HT-29 cells) suspended in PBS (phosphate buffered saline) solution (100 µL) in the right flank. When tumour masses became palpable, treatment started. Four groups, with five mice per group, were established: negative control group - mice that did not receive any treatment; vehicle control – mice that received cremophor in PBS (phosphate buffered saline) solution (the solvent used for solubilizing compound IId); IId – mice that received the compound under study; doxorubicin – mice that received doxorubicin solubilized in PBS (phosphate buffered saline) solution. All tested formulations were intravenously administered in the tail vein in a total number of eight injections with a therapeutic dose of 2 mg/kg of body weight. The doses administered to mice were established by a ratio between the mass of compound and body weight of the animal. Tumour size was regularly monitored using a digital calliper. Respective tumour volumes were calculated according to the formula: V (mm ³ ) = (L×W ² )/2, where L and W represent the longest and shortest axis of the tumour, respectively. Relative tumour volume was also determined for each animal: ratio between volumes at the indicated day and volume at the beginning of treatment. Body weight was also regularly evaluated. Mice treated with compound IId and doxorubicin had a higher reduction of the tumour mass when compared with negative control group and the vehicle control (Figure 4). Also, the reduction of the tumour mass was higher for mice treated with compound IId (Figure 4). The molecular weight of compound IId is 30% higher than the molecular weight of doxorubicin. Therefore, the number of moles of doxorubicin released from compound IId was 30% lower when compared with the number of moles of the administration of doxorubicin alone. Possibly, an experiment where the animals are administered with the same number of moles of both compounds can lead to a higher difference in the reduction of the tumour mass between compound IId and doxorubicin.