MODULATION OF CYTOTOXICITY AND CELLULAR PHARMACOLOGY OF 1,2-DIAMINOCYCLOHEXANE PLATINUM(IV) COMPLEXES MEDIATED BY AXIAL AND EQUATORIAL LIGANDS

Isomers (RR-, SS-, and cis-) of 1,2-diaminocyclohexane (DACH) platinum (IV) complexes with selected axial and equatorial ligands were synthes ized and evaluated for in vitro antitumor activity, cellular uptake, a nd total DNA-Pt adducts. L1210 cells, sensitive to cis-diamminedichlor o-platinum(II) (CDDP) and tetraplatin (L1210/0), 160-fold resistant to CDDP [L1210/diamminedichloroplatinum (DDP)], or 70-fold resistant to tetraplatin (L1210/DACH), were used in conjunction with compounds havi ng the general structure DACH-Pt(IV)-X2Y2, where X and Y are axial and equatorial ligands and X2Y2 are specifically Cl2Cl2, Ac2Cl2, (TFA)2Cl 2, (OH)2Cl2, and Cl2CBDCA (Cl, chloro; Ac, acetato; TFA, trifluoroacet ato; OH, hydroxo; CBDCA, 1,1-cyclobutanedicarboxylato). Comparison of cytotoxicities between isomers of Cl2Cl2, Ac2Cl2, or Cl2CBDCA indicate d that R,R-isomers were the most effective against all three cell line s. The relatively lower activity of the S,S- and cis-isomers was cell line dependent: against L1210/DACH, both isomers of Cl2Cl2 were only 2 - to 3-fold less effective, and this contrasted with 7- and 26-fold lo wer cytotoxicities, respectively, against L1210/DDP. Cross-resistance factors in the L1210/DDP and L1210/DACH lines depended on both isomeri c form and the nature of axial or equatorial ligand. The L1210/DDP cel ls were 6- to 9-fold cross-resistant to the R,R-isomer, 8- to 15-fold to S,S-isomer, and 13- to 38-fold to cis-isomer. The L1210/DACH line w as only 4- to 7-fold cross-resistant to the three isomers of Ac2Cl2, b ut cross-resistance to the isomers was 47- to 79-fold for Cl2Cl2 and 2 2- to 56-fold for Cl2CBDCA complexes. Compared with CDDP, accumulation (2 h at 100 muM drug concentration) of Ac2Cl2 in the three L1210 cell lines was 26-50%, while uptake of Cl2Cl2 and (TFA)2Cl2 was 100-170% a nd 320-570%, respectively. The greatest DNA binding was seen with Cl2C l2 in all cell lines, followed by (TFA)2Cl2, CDDP, and Ac2Cl2. DNA bin ding correlated directly with potency (1/concentration producing 50% i nhibition) in the L1210/0 model (r = 0.973, P < 0.016) but not in the L1210/DDP and L1210/DACH models. Accumulation and DNA-binding studies indicated that binding efficiency to DNA was: Cl2Cl2 > Ac2Cl2 > CDDP > (TFA)2Cl2. In a nonreducing environment, the Pt(IV) complexes (20 muM ) did not react with salmon sperm DNA. Reduced glutathione (100 muM), as a reducing agent, rendered full binding capacity to Cl2Cl2; binding was 25-30% of the expected maximum for (TFA)2Cl2, while Ac2Cl2 remain ed inert. These data indicate Profound effects of axial and equatorial ligands in Pt(IV) complexes on the antitumor activity and cellular ph armacology of the compounds. The investigations also identified Ac2Cl2 as of Particular interest because of its low cross-resistance in the L1210/DDP and L1210/DACH cells.