Axial Ligands and Alicyclic Ring Size Modulate the Activity and Biochemical Pharmacology of Ammine/Cycloalkylamine-Platinum(IV) Complexes in Tumor Cells Resistant to a's-Diarnminedichloroplatinum(II) or trans- l/^/MS^S-DiaminocyclohexanetetrachloroplatinumilV)1

A platinum(II) and 3 platinum! l\ i ammine/cycloalkylamine ho mologous series were evaluated for cytotoxicity and biochemical pharmacology in murine leukemia L1210/0, cis-diamminedichloroplatinum(H)-resistant 1.1210/1)1)1', and diaminocyclohexaneplatinum-resistant L1210/l,2-diaminocyclohexane (DACH) cells. Within each se ries, which contained 4 homologues with differing alicyclic (cycloalkyl) ring size (cyclopropane, cyclobutane, cyclopentane, or cyclohexane), cytotoxicity increased with increasing ring size. This appeared to be due to an increase in partition coefficient, and the resulting increase in drug accumulation and intracellular DNA adducts in ascending each of the series. There were exceptions to this generalization, predominantly in I 1210/1)\( II cells, where the biochemical pharmacology was not entirely consistent with the cytotoxic response and suggested that other factors may be at play. The relationship between structure and ability to circumvent m-diamminedichloroplatinum(II) and/or trans-lR,2R15,25-diaminocyclohexanetetrachloroplatinum(IV) resistance was complex. Ascending the platinami 111series caused resistance factors to decrease in 1,1210/1)1)1'cells but increase in L1210/DACH cells. An increase in resistance factors was also observed in ascending the axial chloroplatinum(IV) series in the 1,1210/1)\( II line. In contrast, as cending the axial chloroplatinum(IV) series in the I 1210/1)1)1'line and axial acetatoplatinum(IV) and axial hydroxoplatinum(IV) series in both cell lines resulted in increases in resistance factors for the first stepwise increase in the cycloalkylamine ring size, but resistance fac tors then decreased progressively with further increases in ring size. Reduction of the platinum(IV) analogues to the platinum(II) congener appears to be necessary for binding to DNA. The similarity in biolog ical actions between the platinum! Ill and axial chloroplatinum(IV) series is likely due to the rapid reduction of tetravalent members to platinum! II>forms. The axial acetatoplatinum(IV) and axial hydroxoplatinum(IV) complexes were reduced more slowly, which may explain their lower potency, but not the ability of the higher member to circumvent both cis-diamminedichloroplatinum(II) and trans-lR,2RlS,25-diaminocyclohexanetetrachloroplatinum(IV) resistances. Expla nation for this will require additional studies. The results have dem onstrated high dependencies on ring size of the carrier amine ligand, valence state of platinum, and the nature of the axial ligand for modulation of potency, cross-resistance property, and biochemical pharmacology of ammine/cycloalkylamine complexes. Received 12/22/93; accepted 6/22/94. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1This work was supported by Grants ROI CA50380 and ROI CA41581 from the National Cancer Institute. 2 Present address: Department of Surgery II, Faculty of Medicine, Kyushu University, Fukuoka 812, Japan. 1 To whom requests for reprints should be addressed, at Department of Clinical Investigation, Box 52, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston. TX 77030. INTRODUCTION The inorganic coordination complex cisplatin4 has potent activity against several human cancers, particularly those of the ovary, testes, bladder, and the head and neck (1). A major drawback of cisplatin relates to its severe side effects, which include renal toxicity, peripheral neuropathy, ototoxicity, and nausea and vomiting (1). The dose-limiting nephrotoxicity has been severe enough to require intervention with such procedures as hypertonic saline and sulfhydryl-containing compounds (2). However, the most significant impact on this toxicity has come from the development of the highly successful carboplatin, which not only lacks the ability to induce significant nephrotox icity but also is free of ncurotoxic and ototoxic effects at standard doses (3). The second major limitation of cisplatin is acquired resistance. The mechanisms proposed for the development of this impediment include decreased cisplatin accumulation, increased cellular GSH or metallothionein, and increased DNA repair of platinum-induced interstrand and intrastrand DNA cross-links (4). These mechanisms have gener ated substantial interest in experimental procedures designed to re duce or eliminate them (2, 5), but only the toxic regimen of high-dose cisplatin has demonstrated that circumvention of clinical resistance is feasible (2). Specific analogues of cisplatin have been identified in an attempt to also bypass cisplatin resistance mechanisms. The first analogue to demonstrate this ability contained DACH as a prerequisite carrier ligand coordinated to the central platinum atom (6). Tetraplatin (ormaplatin) and r/wi.c-l/?,2/?-diaminocyclohexaneoxalatoplatinum(II) are 2 examples of this structural class that are presently under clinical evaluation for their potential efficacies against such cancers (7). Recently, a new class of ammine/amine-platinum(IV) congeners with axial carboxylato ligands has also demonstrated activity against cisplatin-resistant cells in vitro (8). Kelland et al. (8) examined several such analogues to establish structure-activity relationships and re ported that complexes with alicyclic amines (cycloalkylamines) were superior to those with aliphatic or aromatic amine substitutions. They further demonstrated that increasing the number of carbon atoms in the alicyclic ring of the ammine/cycloalkylamine series increased potency against ovarian tumor cells for each stepwise increase in ring size. However, similar structure-activity relationships within homol ogous series in pairs of sensitive and acquired-resistant cell lines are lacking. Information on structure-activity relationships for ammine/ cycloalkylamine-platinum(IV) complexes with alternative axial li gands, such as chloro or hydroxo, is also scant. Platinum analogues containing the DACH ligand have been a consistent focus of our attention (9-12). However, the overlapping 4 The abbreviations used are: cisplatin, ci.v-diamminedichloroplatinum(II); Ictraplatin, /r«H.c-lA.2/?-IS,2S-diaminocyclohexane-tetrachloroplatinutn(IV); MTT, 3-(4,5-diniethylthiazol-2-yl)-2,5-diphenyltctrazolium bromide: 1C5(1,concentration of drug required to inhibit cell growth to 50% of control; FAAS, flameless atomic absorption speclrophotometry; GSH. reduced glutathione; C3, ri.v-ammine/cyclopropylamine-dichloroplatinum complex: C4. r/'-v-ammine/cyclobutylamine-dichloroplatinum complex; C5. r/.v-ammine/ cyclopentylamine-dichloroplatinum complex; C6, c/j-ammine/cyclohexylamine-dichloroplatinum complex; DACH, 1.2-diaminocyclohexane.