Stepwise optimization of tumor-targeted dual-action platinum(iv)-gemcitabine prodrugs.

While platinum-based chemotherapeutic agents have established themselves as indispensable components of anticancer therapy, they are accompanied by a variety of side effects and the rapid occurrence of drug resistance. A promising strategy to address these challenges is the use of platinum(iv) prodrugs, which remain inert until they reach the tumor tissue, thereby mitigating detrimental effects on healthy cells. Typically, platinum drugs are part of combination therapy settings. Consequently, a very elegant strategy is the development of platinum(iv) prodrugs bearing a second, clinically relevant therapeutic in axial position. In the present study, we focused on gemcitabine as an approved antimetabolite, which is highly synergistic with platinum drugs. In addition, to increase plasma half-life and facilitate tumor-specific accumulation, an albumin-binding maleimide moiety was attached. Our investigations revealed that maleimide-cisplatin(iv)-gemcitabine complexes cannot carry sufficient amounts of gemcitabine to induce a significant effect in vivo. Consequently, we designed a carboplatin(iv) analog, that can be applied at much higher doses. Remarkably, this novel analog demonstrated impressive in vivo results, characterized by significant improvements in overall survival. Notably, these encouraging results could also be transferred to an in vivo xenograft model with acquired gemcitabine resistance, indicating the high potential of this approach.

Platinum(IV)-Estramustine Multiaction Prodrugs Are Effective Antiproliferative Agents against Prostate Cancer Cells.

Herein, we describe the synthesis, characterization, and biological properties of Pt(IV) derivatives of cisplatin with estramustine at the first axial position, which is known to disrupt the microtubule assembly and act as an androgen antagonist, and varying the second axial position using an innocent ligand (acetate or hydroxyl) to prepare dual-action and triple-action prodrugs with known inhibitors of histone deacetylase, cyclooxygenase, and pyruvate dehydrogenase kinase. We demonstrate superior antiproliferative activity at submicromolar concentrations of the prodrugs against a panel of cancer cell lines, particularly against prostate cancer cell lines. The results obtained in this study exemplify the complex mode of action of "multiaction" Pt(IV) prodrugs. Interestingly, changing the second axial ligand in the Pt-estramustine complex has a significant effect on the mode of action, suggesting that all three components of the Pt(IV) prodrugs (platinum moiety and axial ligands) contribute to the killing of cells and not just one dominant component.

Multiaction Pt(IV) Carbamate Complexes Can Codeliver Pt(II) Drugs and Amine Containing Bioactive Molecules.

Multiaction Pt(IV) prodrugs can overcome resistance associated with the FDA approved Pt(II) drugs like cisplatin. Intracellular reduction of the octahedral Pt(IV) derivatives of cisplatin releases cisplatin and the two axial ligands. When the released axial ligands act synergistically with cisplatin to kill the cancer cells, we have multiaction prodrugs. Most Pt(IV) multiaction prodrugs have bioactive ligands possessing a carboxylate that is conjugated to the Pt(IV) because breaking the Pt(IV)-ligand bond releases the active moiety. As many drugs that act synergistically with cisplatin do not have carboxylates, a major challenge is to prepare multiaction Pt(IV) complexes with drugs that have amino groups or hydroxyl groups such that following reduction, the drugs are released in their active form. Our objective was to prepare multiaction Pt(IV) prodrugs that release bioactive molecules having amino groups. Because we cannot conjugate amino groups to the axial position of Pt(IV), we developed a novel and efficient approach for the synthesis of Pt(IV)-carbamato complexes and demonstrated that following reduction of the Pt(IV), the released carbamates undergo rapid decarboxylation, releasing the free amine, as in the case of the PARP-1 inhibitor 3-aminobenzamide and the amino derivative of the HDAC inhibitor SAHA. Pt(IV)-carbamato complexes are stable in cell culture medium and are reduced by ascorbate. They are reduced slower than their carboxylato and carbonato analogues. We believe that this approach paves the way for preparing novel classes of multiaction Pt(IV) prodrugs with amino containing bioactive molecules that up to now were not accessible.

Oxamusplatin: a cytotoxic Pt(ii) complex of a nitrogen mustard with resistance to thiol based sequestration displays enhanced selectivity towards cancer.

Pt(ii) drugs and nitrogen mustards show severe side effects, poor tumour selectivity and face growing resistance by cancer cells due to sequestration by thiol-containing molecules (viz. glutathione (GSH) and copper ATPases like ATP7A/7B). ATP7A and ATP7B-sequestered Pt(ii) complexes show dose inefficacy and resistance. The incorporation of bulky ligands and chelating leaving groups may prevent deactivation by thiols. In this work, we have synthesised four new Pt(ii) complexes (3-6) of two carrier ligands, bis(2-hydroxyethyl)pyridylmethylamine (L1) and bis(2-chloroethyl)pyridylmethylamine (L2) with oxalato and cyclobutanedicarboxylato leaving groups. Among these four new complexes, the Pt(ii) complex of L2 with the oxalato leaving group (5, termed "oxamusplatin") is cytotoxic. Oxamusplatin is more resistant than cisplatin or oxaliplatin towards hydrolysis, thiol binding and sequestration by ATP7B. It targets cellular DNA and is capable of disrupting the microtubule network in the cytoskeleton. Oxamusplatin demonstrates better selectivity than oxaliplatin towards cancerous cells. It is ca. 4-10 times more cytotoxic towards metastatic prostate carcinoma (DU-145, IC50 = 21 ± 1 µM) and ca. 10-24 times more cytotoxic towards breast adenocarcinoma (MCF-7, IC50 = 8.1 ± 0.8 µM) compared to the three noncancerous cells investigated.

A trans-dichloridoplatinum(II) complex of a monodentate nitrogen mustard: Synthesis, stability and cytotoxicity studies.

A trans-dichloridoplatinum(II) complex, trans-[PtIICl2(L)(DMSO)] (1) of a monodentate nitrogen mustard, bis(2-chloroethyl)amine (L), was synthesized by the reaction of cis-[PtIICl2(DMSO)2] &L.HCl in presence of Et3N. 1 was characterised by NMR, FT-IR and elemental analysis. L is unstable in aqueous solution while 1 displayed moderate stability. In aqueous buffer solution of pD 7.4, 1 starts to loose L slowly upon dissolution and even after 48 h there is still intact/aquated complex present in solution. 1 interacts with the model nucleobase 9-ethyl guanine. The ligand L was non-toxic against MCF-7, A549, HepG2 & MIA PaCa-2 up to 200 µM. In contrast, the Pt(II) complex 1 showed an excellent IC50 (ca. 600 nM) against MIA PaCa-2 and also displayed good IC50 value (3-7 µM) against the other cancer cell lines probed. The in vitro cytotoxicity of 1 is better than cisplatin against each of the treated cancer cell lines and it is not affected by hypoxia as per the in vitro studies. Complex 1 displays higher cellular accumulation than cisplatin and arrests the cell cycle in both S & G2/M phase inducing apoptotic cell death. The G2/M phase arrest is dominant at higher concentrations. The depolarisation of mitochondria by 1 combined with activation of caspase-7 indicates apoptotic cell death. Complex 1 induces low hemolysis of human blood signifying excellent blood compatibility.

Synthesis and Cytotoxicity of Water-Soluble Dual- and Triple-Action Satraplatin Derivatives: Replacement of Equatorial Chlorides of Satraplatin by Acetates.

Pt(II) complexes, such as cisplatin and oxaliplatin, are in widespread use as anticancer drugs. Their use is limited by the toxic side effects and the ability of tumors to develop resistance to the drugs. A popular approach to overcome these drawbacks is to use their kinetically inert octahedral Pt(IV) derivatives that act as prodrugs. The most successful Pt(IV) complex in clinical trials to date is satraplatin, cct-[Pt(NH3)(c-hexylamine)Cl2(OAc)2], that upon cellular reduction releases the cytotoxic cis-[Pt(NH3)(c-hexylamine)Cl2]. In an attempt to obtain water-soluble and more effective cytotoxic Pt(IV) complexes, we prepared a series of dual- and triple-action satraplatin analogues, where the equatorial chlorido ligands were replaced with acetates and the axial ligands include innocent and bioactive ligands. Replacement of the chlorides with acetates enhanced the water solubility of the compounds and, with one exception, all of the compounds were very stable in buffer. In general, compounds with one or two axial hydroxido ligands were reduced by ascorbate significantly more quickly than compounds with two axial carboxylates. While replacement of the chlorides with acetates in satraplatin led to a reduction in cytotoxicity, the dual- and triple-action analogues with equatorial acetates had low- to sub-micromolar IC50 values in a panel of eight cancer cells. The triple-action compound cct-[Pt(NH3)(c-hexylamine)(OAc)2(PhB)(DCA)] was active in all cell lines, causing DNA damage that induced cell cycle inhibition and apoptosis. Its good activity against CT26 cells in vitro translated into good in vivo efficacy against the CT26 allograft, an in vivo model with intrinsic satraplatin resistance. This indicates that multiaction Pt(IV) derivatives of diamine dicarboxylates are interesting anticancer drug candidates.

Expanding the Arsenal of PtIV Anticancer Agents: Multi-action PtIV Anticancer Agents with Bioactive Ligands Possessing a Hydroxy Functional Group.

Most multi-action PtIV prodrugs have bioactive ligands containing carboxylates. This is probably due to the ease of carboxylating the OH axial ligands and because following reduction, the active drug is released. A major challenge is to expand the arsenal of bioactive ligands to include those without carboxylates. We describe a general approach for synthesis of PtIV prodrugs that release drugs with OH groups. We linked the OH groups of gemcitabine (Gem), paclitaxel (Tax), and estramustine (EM) to the PtIV derivative of cisplatin by a carbonate bridge. Following reduction, the axial ligands lost CO2 , rapidly generating the active drugs. In contrast, succinate-linked drugs did not readily release the free drugs. The carbonate-bridged ctc-[Pt(NH3 )2 (PhB)(Gem-Carb)Cl2 ] was significantly more cytotoxic than the succinate-bridged ctc-[Pt(NH3 )2 (PhB)(Gem-Suc)Cl2 ], and more potent and less toxic than gemcitabine, cisplatin, and co-administration of cisplatin and gemcitabine.

Modulation of the reactivity of nitrogen mustards by metal complexation: approaches to modify their therapeutic properties.

The alkylating agents bearing the -N(CH2CH2Cl)2 moiety, commonly known as 'the nitrogen mustards,' are among the first chemotherapeutic agents against cancer. They form covalent alkyl linkages due to reaction with nucleophilic entities viz. N7 of guanine in DNA. The reactivity of nitrogen mustards may be controlled in various ways, which include metabolic activation, reductive or hypoxic activation and metal complexation. This review discusses how the metal complexation of nitrogen mustards affects their reactivity and mechanistic pathways. The discussion encompasses those transition metal complexes for which the structure has been well characterized and cytotoxicity studies have been performed. This review discusses how the binding of the metal centre along with its oxidation state helps to control the reactivity of the nitrogen mustards. The discussion emphasizes the effect of the reduction potential of the coordinated metal center on the reactivity of the respective mustard under specific conditions, the dependence of efficiency and specificity on the stability of the reduced species and the importance of steric hindrance around the metal center. The insight into the mechanism of action is expected to provide a better understanding to overcome the existing lacunae and design better molecules of this class which have excellent potential, given the ever growing need for therapeutics against cancer.

Anticancer activity of a chelating nitrogen mustard bearing tetrachloridoplatinum(iv) complex: better stability yet equipotent to the Pt(ii) analogue.

Two Pt(iv) complexes cis,cis,trans-[Pt(IV)(L1)Cl4] (1a) & cis,cis,trans-[Pt(IV)(L2)Cl4] (2a) containing the nitrogen mustard moieties -N(CH2CH2Cl)2 & -NHCH2CH2Cl, were prepared in a single step from the Pt(ii) complexes containing -N(CH2CH2OH)2 (1) & -NHCH2CH2OH (2) moieties respectively using only thionyl chloride. The characterization of both the Pt(iv) complexes was performed by NMR, IR, UV and elemental analysis. Complex 1a was also characterized by single crystal X-ray diffraction. 1a crystallized in the I2/a space group. 1a exhibited much higher solution stability than 2a in kinetic studies by (1)H NMR. 1a shows a prodrug like activity as it converts to its Pt(ii) congener, [Pt(II)(L1)Cl2] (3) after 2 days in buffered solution. The binding experiment of 1a with model nucleobase 9-ethylguanine (9-EtG), showed that 1a converts to 3 and forms mono-adducts with 9-EtG. In the presence of reduced glutathione (GSH), the formation of 3 from 1a is quicker and upon the formation of 3 it binds almost instantaneously to GSH to form cis-[PtCl(L1)SG] (3c). Complex 3c transformed within a day to give a free aziridinium ion of L1 (3b) by dissociation. The in vitro cytotoxicity of the complexes and the clinical anticancer drug cisplatin show that 1a is potent against MCF-7, A549, HepG2 and MIA PaCa-2. The potency is highest against MIA PaCa-2 exhibiting an IC50 value of 4.4 ± 0.5 µM. The in vitro cytotoxicity data also showed that between the two complexes only 1a is active against MCF-7, A549 and MIA PaCa-2 in normoxia and hypoxia, both in the presence and absence of added GSH. Even in the presence of excess GSH in hypoxia, 1a exhibits significant cytotoxicity against MIA PaCa-2 and MCF-7 with IC50 values of 4.5 ± 0.3 and 11.2 ± 1.8 µM respectively. Platinum accumulation studies by ICP-MS display greater internalization of 1a, than 2a, 3 and cisplatin inside MCF-7 cells. 1a arrests cell cycle at the G2/M phase in MCF-7, exhibits capability to inhibit metastasis, induces apoptotic cell death and displays blood compatibility with human blood.

Alteration of steric hindrance modulates glutathione resistance and cytotoxicity of three structurally related Ru(II)-p-cymene complexes.

The effect of steric hindrance on reactivity towards biomolecules while designing Ru(II)-η(6)-p-cymene based anticancer agents seems to be an important parameter in improving the activity and inducing resistance against glutathione (GSH) deactivation. Herein we present the structure, hydrolysis, anticancer activity and the effect of steric hindrance on deactivation by glutathione for three complexes, [Ru(II)(η(6)-p-cym)(L1)(Cl)](PF6) (1), [Ru(II)(η(6)-p-cym)(L2)(Cl)](PF6) (2) and [Ru(II)(η(6)-p-cym)(L3)(Cl)](PF6) (3). The ligands L1-L3 are Schiff bases which show increasing substitution in a benzene ring, such that two ortho hydrogens are replaced by -methyl in 2 and by -isopropyl in 3. The cytotoxicity results strongly suggest that controlling the rate of hydrolysis through tuning of steric hindrance may be a feasible pathway to derive GSH resistant anticancer agents. The cellular studies show that all the three complexes show good blood compatibility (haemolysis <3%) and induce cellular death through caspase activation via the mitochondrial pathway. They have anti-angiogenic activity and prevent the healing of treated cells.

Anticancer activity of a cis-dichloridoplatinum(ii) complex of a chelating nitrogen mustard: insight into unusual guanine binding mode and low deactivation by glutathione.

A pyridine ring containing a chelating nitrogen mustard ligand bis(2-chloroethyl)pyridylmethylamine hydrochloride (L2·HCl) was synthesized from bis(2-hydroxyethyl)pyridylmethylamine (L1) on reaction with thionyl chloride. Both the ligands upon reaction with cis-[PtCl2(DMSO)2] afforded square planar complexes cis-[PtCl2(L1)] (1) and cis-[PtCl2(L2)] (2) respectively. Both the complexes were characterized by NMR, IR, UV and elemental analysis. 2 crystallized in the P21/c space group. 2 shows greater solution stability than 1 in kinetic studies by 1H NMR. Both 1 and 2 bind the model nucleobase 9-ethylguanine (9-EtG) and form multiple mono-adducts. Existence of unusual N7,O6 chelated guanine bound 2 (2e) was traced. Binding studies of 2 with glutathione (GSH) show formation of a mono-adduct cis-[PtCl(L2)SG] (2c), which transformed within a day to give an aziridinium ion of L2 (2b) after loss of L2. In vitro cytotoxicity of ligands, complexes and the clinical anticancer drug cisplatin show that 2 is the most potent against MCF-7, A549 and MIA PaCa2 exhibiting IC50 values of 12.6 ± 0.8, 18.2 ± 1.8 and 4.2 ± 1.0 µM respectively. The in vitro cytotoxicity of 2 against MCF-7, A549 and MIA PaCa2 was also probed in hypoxia and in the presence and absence of added GSH. Even in the presence of excess GSH in hypoxia, 2 exhibits significant cytotoxicity against MIA PaCa2 and MCF-7 with IC50 of 4.4 ± 0.8 and 12.5 ± 1.1 µM respectively. Metal accumulation studies by ICP-MS display greater cellular internalization of 2, than 1 and cisplatin in MCF-7 cells. 2 arrests the cell cycle at sub G1 and G2/M phases in MCF-7 whereas cisplatin exhibits S phase arrest to be dominant with increase in concentration. Complex 2 exhibits a change in mitochondrial membrane potential, caspase activity and suggests apoptotic cell death through the intrinsic pathway. Moreover it is encouraging to find that 2 also restricts angiogenesis in chick embryo.

A hypoxia efficient imidazole-based Ru(II) arene anticancer agent resistant to deactivation by glutathione.

A slow hydrolyzing imidazole-based Ru(II)-arene complex [(L)Ru(II)(η(6)-p-cym)(Cl)](PF6) (1) with excellent stability in the extracellular chloride concentration shows better activity under hypoxia and strong resistance to glutathione (GSH) in vitro under hypoxic conditions. 1 arrests the cell cycle in sub G1 and G2/M phases and leads to apoptosis.