Multi-platinum anti-cancer agents. Substitution-inert compounds for tumor selectivity and new targets.

This tutorial review summarizes chemical, biophysical and cellular biological properties of formally substitution-inert "non-covalent" polynuclear platinum complexes (PPCs). We demonstrate how modulation of the pharmacological factors affecting platinum compound cytotoxicity such as cellular accumulation, reactivity toward extracellular and intracellular sulfur-ligand nucleophiles and consequences of DNA binding is achieved to afford a profile of biological activity distinct from that of covalently-binding agents. The DNA binding of substitution-inert complexes is achieved by molecular recognition through minor groove spanning and backbone tracking of the phosphate clamp. In this situation, the square-planar tetra-am(m)ine Pt(ii) coordination units hydrogen bond to phosphate oxygen OP atoms to form bidentate N-O-N motifs. The modular nature of the polynuclear compounds results in high-affinity binding to DNA and very efficient nuclear condensation. These combined effects distinguish the phosphate clamp as a third mode of ligand-DNA binding, discrete from intercalation and minor-groove binding. The cellular consequences mirror those of the biophysical studies and a significant portion of nuclear DNA is compacted, a unique effect different from mitosis, senescence or apoptosis. Substitution-inert PPCs display cytotoxicity similar to cisplatin in a wide range of cell lines, and sensitivity is indifferent to p53 status. Cellular accumulation is mediated through binding to heparan sulfate proteoglycans (HSPG) allowing for possibilities of tumor selectivity as well as disruption of HSPG function, opening new targets for platinum antitumor agents. The combined properties show that covalently-binding chemotypes are not the unique arbiters of cytotoxicity and antitumor activity and meaningful antitumor profiles can be achieved even in the absence of Pt-DNA bond formation. These dual properties make the substitution-inert compounds a unique class of inherently dual-action anti-cancer agents.

Translesion DNA synthesis across double-base lesions derived from cross-links of an antitumor trinuclear platinum compound: primer extension, conformational and thermodynamic studies.

Polynuclear platinum complexes represent a unique structural class of DNA-binding agents of biological significance. They contain at least two platinum coordinating units bridged by a linker, which means that the formation of double-base lesions (cross-links) in DNA is possible. Here, we show that the lead compound, bifunctional [{trans-PtCl(NH3)2}2µ-trans-Pt(NH3)2{H2N(CH2)6NH2}2]4+ (Triplatin or BBR3464), forms in DNA specific double-base lesions which affect the biophysical and biochemical properties of DNA in a way fundamentally different compared to the analogous double-base lesions formed by two adducts of monofunctional chlorodiethylenetriamineplatinum(ii) chloride (dienPt). We find concomitantly that translesion DNA synthesis by the model A-family polymerase, the exonuclease deficient Klenow fragment, across the double-base lesions derived from the intrastrand CLs of Triplatin was markedly less extensive than that across the two analogous monofunctional adducts of dienPt. Collectively, these data provide convincing support for the hypothesis that the central noncovalent tetraamine platinum linker of Triplatin, capable of hydrogen-bonding and electrostatic interactions with DNA and bridging the two platinum adducts, represents an important factor responsible for the markedly lowered tolerance of DNA double-base adducts of Triplatin by DNA polymerases.

Enhancement of the physicochemical properties of [Pt(dien)(nucleobase)]2+ for HIVNCp7 targeting.

Physicochemical properties of coordination compounds can be exploited for molecular recognition of biomolecules. The inherent π-π stacking properties of [Pt(chelate)(N-donor)]2+ ([PtN4]) complexes were modulated by systematic variation of the chelate (diethylenetriamine and substituted derivatives) and N-donor (nucleobase or nucleoside) in the formally substitution-inert PtN4 coordination sphere. Approaches to target the HIV nucleocapsid protein HIVNCp7 are summarized building on (i) assessment of stacking interactions with simple tryptophan or tryptophan derivatives to (ii) the tryptophan-containing C-terminal zinc finger and (iii) to the full two-zinc finger peptide and its interactions with RNA and DNA. The xanthosine nucleoside was identified as having significantly enhanced stacking capability over guanosine. Correlation of the LUMO energies of the modified nucleobases with the DFT π-stacking energies shows that frontier orbital energies of the individual monomers can be used as a first estimate of the π-stacking strength to Trp. Cellular accumulation studies showed no significant correlation with lipophilicity of the compounds, but all compounds had very low cytotoxicity suggesting the potential for antiviral selectivity. The conceptual similarities between nucleobase alkylation and platination validates the design of formally substitution-inert coordination complexes as weak Lewis acid electrophiles for selective peptide targeting.

Effects of coordination mode of 2-mercaptothiazoline on reactivity of Au(I) compounds with thiols and sulfur-containing proteins.

Gold(I) based drugs are interesting for their potential medical use. The relatively facile ligand substitution in linear gold(I) compounds makes the identification of active species complicated. Ligands such as PR3 and CN- are likely to be carrier ligands due to their strong trans-directing properties and will dictate the nature of substitution reactions. The 2-mercaptothiazoline (mtz) ligand is an N,S-heterocyclic compound which presents an exocyclic thiol sulfur as well as a heterocyclic nitrogen. The coordination of mtz to transition metals can be modulated by the trans ligand and complexes with metal bound through the nitrogen and/or the exocyclic sulfur are known. Therefore, the complexes [NCAu(N-mtz)] (N-coordinated) and [(Ph3P)Au(S-mtz)] (S-coordinated) were investigated to compare the influence of CN- and PR3 as well as the coordination mode of the mtz ligand on reactivity with thiols and sulfur-containing proteins. As a further comparison the compound [(Ph3P)AuCN] was also studied. Human serum albumin, egg white lysozyme and, principally, the C-terminal zinc finger (ZF2) of the nucleocapsid NCp7protein of HIV-1 were studied. Results from zinc finger studies show that the coordination structure can determine the reactivity toward biomolecules. Due to ligand scrambling, the complex [NCAu(N-mtz)] forms very reactive species in solution generating [NCyAux-biomolecule] adducts, where x,y≤3. The observation by mass spectrometry of [(CN)Au]-ZF confirms the ability of Au(I) compounds to form [(Ligand)Au] adducts on zinc fingers, in contrast to Au(III), where all ligands are lost upon reaction with the zinc finger. On the other hand, [(Ph3P)Au(S-mtz)] also generates the [(Ph3P)2Au]+ species due to ligand scrambling, that showed lower reactivity, probably due to steric hindrance. For this complex [(Ph3P)Au-biomolecule] and [Au-biomolecule] adducts are dominant. The results corroborate the hypothesis of modulation through coordination as the reactivity clearly depends on not only the ligand, but also the coordination mode.

Solution studies on DNA interactions of substitution-inert platinum complexes mediated via the phosphate clamp.

The phosphate clamp is a distinct mode of ligand-DNA binding where the molecular recognition is manifested through ("non-covalent") hydrogen-bonding from am(m)ines of polynuclear platinum complexes to the phosphate oxygens on the oligonucleotide backbone. This third mode of DNA binding is unique to the "classical" DNA intercalators and minor groove binding agents and even the closely related covalently binding mononuclear and polynuclear drugs. 2D (1)H NMR studies on the Dickerson-Drew dodecamer (DDD, d(CGCGAATTCGCG)2) showed significant A-T contacts mainly on nucleotides A6, T7 and T8 implying a selective bridging from C9G10 in the 3' direction to C9G10 of the opposite strand. {(1)H, (15)N} HSQC NMR spectroscopy using the fully (15)N-labelled compound [{trans-Pt(NH2)3(H2N(CH2)6NH3}2µ-(H2N(CH2)6NH2)2(Pt(NH3)2](8+) (TriplatinNC) showed at pH 6 significant chemical shifts and (1)J((195)Pt-(15)N) coupling constants for the free drug and DDD-TriplatinNC at pH 7 indicative of formation of the phosphate clamp. (31)P NMR results are also reported for the hexamer d(CGTACG)2 showing changes in (31)P NMR chemical shifts indicative of changes around the phosphorus center. The studies confirm the DNA binding modes by substitution-inert (non-covalent) polynuclear platinum complexes and help in further establishing the chemotype as a new class of potential anti-tumour agents in their own right with a distinct profile of biological activity.

A new ruthenium radiosensitizer: RuCl2(DMSO)2(4-nitroimidazole)2.

Earlier studies on Ru complexes, such as, cis-RuCl2(DMSO)4, as antineoplastic agents, have suggested a DNA binding mechanism similar to that of the clinically successful platinum complex, cis-diammine(dichloro)platinum(II), (cis-DDP). As part of our study on metal-radiosensitizer complexes, cis-RuCl2(DMSO)4 is used as a precursor for synthesis of Ru(II)-nitroimidazole complexes. These complexes were identified and characterized and their toxicity and radiosensitizing abilities were examined in vitro. In the series of Ru(II)-nitroimidazole complexes synthesized, RuCl2(DMSO)2(4-nitroimidazole)2, "Ru-(4-nitro)," was the most effective radiosensitizer. At 200 microM, Ru-(4-nitro) produced a sensitizer enhancement ratio (SER) of 1.6 in hypoxic Chinese hamster ovary (CHO) cells, but did not sensitize oxic CHO cells. Other Ru-nitroimidazole complexes gave SER values of 1.2-1.4 at 200 microM. These Ru(II)-nitroimidazole complexes also showed lower toxicity than the free nitroimidazoles alone at equimolar concentration. The enhanced radiosensitizing effect of Ru-(4-nitro) may be due to the metal's ability to target the sensitizer to DNA, or may be related to changes in reduction potential: from -685 mV for the free ligand to -355 mV and -615 mV for the complex. This complex did not deplete non-protein sulfhydryls (NPSH) at the time intervals and concentrations used. DNA binding was studied using inhibition of restriction enzyme activity on plasmid DNA.

Anticancer activity in murine and human tumor cell lines of bis(platinum) complexes incorporating straight-chain aliphatic diamine linker groups.

The biological activity of a series of dinuclear bis(platinum) complexes of formula [(cis-PtX2-(NH3)]2(NH2(CH2)nNH2)] (X = Cl, n = 4-9, compounds 6-11; X2 = malonate, n = 5 or 6, compounds 12 and 13) is described in selected murine leukemia, murine solid tumor, and human tumor cell lines and in murine leukemia cell lines rendered resistant to cisplatin (cis-[Pt(NH3)2Cl2]). The bis(platinum) compounds showed greater activity in vitro against murine tumor cell lines resistant to either cisplatin or DACH ([Pt(DACH)Cl2]). The resistance factor is dependent on chain length of the diamine, and the structural feature of a dinuclear complex is of general use in reducing cross-resistance with cisplatin. In vivo [(cis-PtCl2(NH3)]2(NH2(CH2)5NH2)] (7) showed a % T/C of 204 against murine L1210 leukemia resistant to cisplatin compared to a % T/C of 104 for cisplatin itself at optimal doses. The complex [(Pt(mal)(NH3)]2(NH2(CH2)6NH2)] (13) was highly active in the colon 26 tumor line with 3/10 tumor-free survivors (dose of 186 mg/kg, ip D1,5,9); however, 13 was subject to substantial cross-resistance in the cisplatin resistant L1210 leukemia (% T/C 139 versus % T/C of 223 in the sensitive line). In four selected human tumor lines in vitro, compounds 6-11 were uniformly more potent than cisplatin. In the corresponding xenografts, compound 7 showed greater activity in the HCT-8 (coloadenocarcinoma) and H23 (nonsmall cell lung), but diminished potency in AH125 and H520 (both nonsmall cell lung) lines in comparison to cisplatin. Retention of activity against cisplatin-resistant cell lines and a different spectrum of activity compared to cisplatin in some human tumor cell lines suggest that this class of complexes is mechanistically different from mononuclear complexes and worthy of further development toward clinical trials.

Trypanocidal and antitumour activity of platinum-metal and platinum-metal-drug dual-function complexes.

A number of antitumour platinum-metal complexes related to cis-platin showed trypanocidal activity against Trypanosoma rhodesiense in vitro but not in vivo. New platinum- and rhodium-metal complexes of diamidine and plenanthridinium trypanocides are described which showed higher therapeutic indices than the parent drugs, due to increased activity in the former drug type and decreased toxicity in the latter. Some evidence of potentiation of antitumour activity was noted in these drug complexes. At the ultrastructural level, complex-treated trypanosomes showed a number of nuclear effects and other lesions specifically attributable to platinum-metal action. Some of the lesions were similar to those induced by cis-platin in tumour cells.

Platinum complexes with one radiosensitizing ligand [PtCl2(NH3) (sensitizer)]: radiosensitization and toxicity studies in vitro.

Complexes of general formula [PtCl2(NH3)L] with one radiosensitizing ligand per platinum are compared with ligand L alone, complexes with two radiosensitizers per platinum [PtCl2L2], and their analogs with NH3 ligands, with respect to radiosensitizing properties and toxicity in CHO cells. Radiosensitizing ligands, L, were misonidazole, metronidazole, 4(5)-nitroimidazole, and 2-amino-5-nitrothiazole, and the ammine analogs were cis- and trans-DDP [diamminedichloroplatinum(II)] and the monoammine, K[PtCl3(NH3)]. Results are related to a previous study on plasmid DNA binding by these series. The toxicity of the mono series [PtCl2(NH3)L], attributable to DNA binding, is much higher than the corresponding bis complexes, [PtCl2L2]. For L = misonidazole, toxicity is similar to the monoammine, but higher in hypoxic than in aerobic cells. trans-[PtCl2(NH3)-(misonidazole)] is more toxic than the cis isomer. Except for L = 4(5)-nitroimidazole, the complexes [PtCl2(NH3)L] are more toxic than L in air and hypoxia. Hypoxic radiosensitization by the mono complexes is comparable to the monoammine and is not better than free sensitizers, again except for L = 4(5)-nitroimidazole. Significantly lower sensitization is observed in oxic cells. The bis complexes [PtCl2L2], which do not bind to DNA as well as the mono complexes, are less effective radiosensitizers and less toxic than the [PtCl2(NH3)L] series.

Potentiation of radiation-induced bacterial cell killing by binuclear rhodium(II) carboxylates.

A series of binuclear rhodium(II) tetracarboxylate complexes was examined for potentiation of radiation-induced killing of Salmonella typhimurium cells. Carboxylate bridging ligands were varied as formate, acetate, trifluoroacetate, and propionate. All complexes caused hypoxic non-dose-modifying radiation potentiation in that variable thresholds were obtained with the radiation dose response. In phosphate-buffered saline (PBS), decreasing threshold doses, i.e., increasing potentiating efficiencies, were seen in the order of acetate = trifluoroacetate less than propionate less than formate. Beyond the threshold dose, the degree of potentiation for all complexes in PBS approximated 12 times the degree of radiation sensitivity seen for the N2 baseline of the radiation dose-response curve. No radiation potentiation by Rh2 carboxylates was seen for fully oxic suspensions. Irradiation of cells in the absence of phosphate increased the efficiency as well as the degree of radiation potentiation. It is hypothesized that bacterial radiation potentiation is initiated by one-electron reduction of the Rh2 carboxylates, most likely involving the hydrated electron, followed then by formation of an active product. These events likely occur outside the bacterial cell.

Potentiation of radiation-induced bacterial cell killing by binuclear rhodium(II) complexes and their amines.

Rhodium(II) binuclear complexes were surveyed for potentiation of radiation-induced cell killing of hypoxic and fully oxic Salmonella typhimurium cells. The Rh2 tetracarbonate ion substantially potentiated hypoxic cell radiation sensitivity. Phosphate interfered with this potentiation. In the latter two respects, radiation potentiation by Rh2 tetracarbonate is similar to that found for Rh2 tetracarboxylates. Amines such as ammonia, methylamine, ethylamine, n-propylamine, and n-butylamine were examined with both Rh2 tetracarbonate and tetraacetate complexes. With Rh2 tetraacetate in phosphate-buffered saline, these amines variably increased radiation potentiation to a maximum of nearly that seen by Rh2 tetraacetate alone in the absence of phosphate. With Rh2 tetracarbonate, particular amines were found to either enhance or restrict radiation potentiation. Results as a whole support the hypothesis that a radiolytic Rh species initiated in a one-electron reduction process external to the cell is responsible for the potentiation by Rh2 complexes in bacteria. Phosphate interference of potentiation by Rh2 tetracetate appears to be limited competitively by amines, suggesting that axial associations of phosphate with the Rh2 center may be involved in the inhibition of radiation potentiation. Of interest in this regard is the finding that 5'-adenosinemonophosphate eliminates the potentiation seen with Rh2 tetraacetate.

Radiosensitization by metal complexes of 4(5)-nitroimidazole.

Four closely-related cis-platinum (Pt) complexes of 4(5)-nitroimidazole have been examined with respect to properties of radiobiological interest, to test the hypothesis that targeting a nitroimidazole (NO2Im) to DNA could enhance its radiosensitizing ability: I [PtCl2(5-NO2Im)2]; II [PtCl2(4-NO2Im)2]; III [PtCl2(NH3)(5-NO2Im)]; IV [PtCl2(NH3)(4-NO2Im)]. The reduction potential was affected to the same extent on metal binding in all of the complexes (delta E1/2 = +200 mV, cf. ligand measured polographically). Higher sensitization by 5-NO2 complexes I, III (cf. II, IV) was found. Only the mono complexes III and IV bind to DNA (in an assay using inhibition of restriction endonuclease activity); these radiosensitize as well as, or better than, free ligand in hypoxic CHO cells, and better than the bis complexes (I and II). The toxicity of the mono complexes is higher than ligand, and parallels the binding (III, IV, mono bis analogues). The complexes are compared with 4-nitroimidazole complexes of ruthenium, with respect to toxicity, binding and radiosensitization.

Radiosensitization by nickel lapachol.

The properties of interest in the radiosensitization of a metal complex, nickel lapachol, are compared with those of the 2-nitroimidazole, misonidazole. These very different compounds were found to be surprisingly similar in terms of their reduction potential (-370 mV), enhancement ratios for killing of hypoxic Chinese hamster ovary cells by X-irradiation, and enhancement of DNA breaks in hypoxia. For nitroimidazoles, the sensitization depends on 'electron affinity', reduction of the nitro group; for nickel lapachol it is the metal which is necessary for reduction, yet the sensitization efficiencies are remarkably close. However, the metal complex has additional activities (some sensitization in aerobic cells; increased sensitization with preincubation) which are as yet unexplained but are assumed to be related to the nature of the naphthoquinone ligand, rather than to the reduction of the metal.

Assessment of DNA binding of platinum-radiosensitizer complexes by inhibition of restriction enzymes.

A simple and rapid method has been used to compare the binding of platinum complexes to DNA, in a relatively qualitative manner. A compound bound at or near the restriction site inhibits enzymatic cleavage of DNA; inhibition of BamHI and EcoRI activity by complexes was assessed in this study using linearized pSV2-gpt plasmid. Our particular interest was in DNA binding by complexes of platinum (Pt) with known organic radiosensitizers (RS), to determine whether the Pt was able to target the RS to the DNA. Although the Pt-RS complexes investigated themselves have moderate radiosensitizing ability (like the inorganic complexes, cis- or trans-diamminedichloroplatinum(II), c- or t-DDP) none of the Pt-RS inhibit to the same extent as c- or t-DDP. However, there appears to be some correlation between enhanced radiosensitization by Pt-RS over Pt(RS)2, with the degree of Pt binding (as assessed by our assay). Our results using isolated DNA suggest that not all complexes bind well (e.g. Pt with two RS ligands), but that in certain cases (e.g. Pt with only one RS), it is possible to target the drug to the DNA. An ammine or amine ligand may be required in order to target a radiosensitizer to DNA using platinum.

Chromosome-damaging activity of a ruthenium radiosensitizer, RuCl2 (DMSO)2 (4-nitroimidazole)2, in Chinese hamster ovary cells in vitro.

The metal complex, RuCl2 (DMSO)2 (4-nitroimidazole)2, 1, which has hypoxic radiosensitizing properties, was examined for genotoxic activity, as measured by the in vitro induction of chromosome aberrations (chromatid breaks and chromatid exchanges) in Chinese hamster ovary (CHO) cells. A dose-dependent increase in the frequencies of metaphases with chromatid aberrations was observed for 1. Addition of S9 liver microsomal mixture and 1 to the cultured CHO cells did not alter the clastogenic activity noted for the complex itself. The clastogenic (chromosome damaging) activity of a precursor complex, cis-RuCl2(DMSO)4 and the ligand, 4-nitroimidazole (4-NO2-Im) were found to be less than that of 1 at corresponding concentrations. A comparison with two drugs used clinically with radiation, cis-dichlorodiammineplatinum(II) (cis-DDP) and misonidazole (miso), indicated that the clastogenic activity of 1 was similar to miso and much less than that of cis-DDP.

Zinc metalloproteins as medicinal targets.

Zinc bioinorganic chemistry has emphasized the role of the metal ion on the structure and function of the protein. There is, more recently, an increasing appreciation of the role of zinc proteins in a variety of human diseases. This critical review, aimed at both bioinorganic and medicinal chemists, shows how apparently widely-diverging diseases share the common mechanistic approaches of targeting the essential function of the metal ion to inhibit activity. Protein structure and function is briefly summarized in the context of its clinical relevance. The status of current and potential inhibitors is discussed along with the prospects for future developments (162 references).

Binding mode of 2-amino-5-nitrothiazole (ANT) in platinum complexes, trans-[PtCl2(ANT)2], affects DNA binding, toxicity and radiosensitizing ability.

The radiosensitizer 2-amino-5-nitrothiazole (ANT) can react with platinum to form many products because of the availability of two potential nitrogen donors as ligands for metals. Two of these complexes, both with two ANT molecules in the trans configuration, differ because of their linkages. In the first, Pt is bound to ANT via the amine group (A) and in the second via the thiazole ring nitrogen (R). Isomer R is a better radiosensitizer than A in hypoxic Chinese hamster ovary cells, giving enhancement ratios of 1.6 versus 1.15 with 100 mumol dm-3 complex. The ring-bound isomer also exhibits higher toxicity than the amine bound in air and under conditions of hypoxia. Sensitization by isomer R is much higher under conditions of hypoxia than in air. In an assay to assess DNA binding, isomer R inhibited restriction enzyme cleavage of DNA but isomer A did not (up to 6 h at 300 mumol dm-3). It appears from this study on two very similar complexes that the complex which exhibits stronger binding may be targetting the radiosensitizer to the DNA, resulting in greatly improved sensitization.

Interaction of novel bis(platinum) complexes with DNA.

Bis(platinum) complexes [[cis-PtCl2(NH3)]2H2N(CH2)nNH2] are a novel series of potential anticancer agents in which two cis-diamine(platinum) groups are linked by an alkyldiamine of variable length. These complexes are potentially tetrafunctional, a unique feature in comparison with known anticancer agents. Studies of DNA interactions of bis(platinum) complexes in comparison with cisplatin demonstrate significant differences. Investigations of interstrand crosslink formation in which crosslinking of a short DNA fragment is detected by gel electrophoresis under denaturing conditions demonstrate that interstrand crosslinks are 250 fold more frequent among bis(platinum) adducts than among cisplatin-derived adducts under the conditions examined. These investigations indicate that bis(platinum) adducts contain a high frequency of structurally novel interstrand crosslinks formed through binding of the two platinum centers to opposite DNA strands. Unlike cisplatin, bis(platinum) complex binding does not unwind supercoiled DNA. Studies with the E. coli UvrABC nuclease complex demonstrate that both linear and supercoiled DNA containing bis(platinum) adducts are subject to incision by the repair enzyme complex. Initial studies using UvrABC nuclease as a probe to define the base and sequence specificity for bis(platinum) complex binding suggest that the specificity of the bis(platinum)s is similar, but not identical, to that of cisplatin.

Assessment of toxicity of bis-platinum complexes in hypoxic and aerobic cells.

There is considerable interest in the bis-platinum series of complexes as potential chemotherapeutic agents, due to their activity in cisplatin-resistant lines and in various tumor types. Our interest in their hypoxic selectivity stems from the fact that cisplatin exhibits greater cytotoxicity in hypoxic than aerobic cells. Unlike nitroaromatics, quinones, tirapazamine and many other hypoxia selective agents, a 'bioreductive' moiety cannot explain these observations. We hypothesized that DNA-protein cross-links (D-P) might play a role in the mechanism. Bis-platinum complexes have variable cross-linking potentials, and their toxicities were assessed in air or hypoxia in CHO cells. Of the three classes examined, only those from the 2,2/cis,cis series show greater hypoxic selectivity than cisplatin. These have greater potential for cross-links than cisplatin, being potentially bifunctional at each platinum, with the two leaving groups (X) in the cis position, and with variable distance (n) between the platinum centers: cis-[(PtX2(NH3))2H2N(CH2)nNH2]. Cellular platinum accumulation and DNA binding were also measured, and like cisplatin, results are consistent with a more toxic lesion formed in hypoxia. Lower hypoxic selectivity in the UV20 cell line may reflect an inability to excise the relevant lesion. These results support the D-P hypothesis. Further support comes from a 1,1/trans,trans complex which does not form D-P and which exhibited the reverse behavior to 2,2/cis,cis or cisplatin, i.e. higher toxicity in aerobic than in hypoxic cells. This study examines the possibility of an additional mechanism of selection for hypoxic toxicity involving DNA-protein cross-links.

Toxicity of [PtCl2(NH3)L] in hypoxia; L = misonidazole or metronidazole.

There is increasing interest in compounds which show selective toxicity to the resistant hypoxic portions of tumors. Cisplatin does not generally show preferential toxicity in hypoxic cells, as do nitroimidazoles. It is proposed that attachment of a nitroimidazole could add a degree of hypoxic selectivity to Pt agents. Platinum complexes containing one nitroimidazole ligand bind to DNA and show higher toxicity in hypoxic than aerobic CHO cells. Cis and trans isomers of complexes with misonidazole (a 2-nitroimidazole) and metronidazole (a 5-nitroimidazole) are compared with respect to binding to DNA (approximately the same), reduction potential (trans miso greater than cis miso greater than cis metro greater than trans metro), and toxicity (trans greater than cis miso, cis greater than trans metro, with trans miso approximately cis metro in hypoxia, despite significantly different reduction potentials). The effect of platination on nitroimidazole toxicity is not entirely explained by DNA binding and increased reduction potential. These compounds do not exhibit cross resistance with cisplatin in L1210 resistant cells. This factor, their selectivity for hypoxia, and preliminary results in vivo indicating potentiation of anti-tumor activity by the vasoactive compound, hydralazine, which increases tumor hypoxia, suggest further development of these compounds for use in tumors with resistant hypoxic portions.