Steric Determinants of Pt/DNA Interactions and Anticancer Activity.

Studies directed at establishing the structural features that control Pt/DNA interactions and the anticancer activity of Pt drugs are described. [(1)H, (15)N]-HSQC 2D NMR spectroscopic studies of the reactions of cisplatin with oligonucleotides containing ApG and GpA binding sites reveal dramatic differences in the rates of formation of monofunctional adducts at the two sites. When the reactant is cis-[Pt(NH(3))(2)(OH(2))(2)](2+) no such differences are observed suggesting that outer-sphere interactions between the reactant and the oligonucleotide may play a substantial role in determining the rates. Rates of closure to the bifunctional adducts are similar to those observed for cisplatin. Studies of the adduct profiles formed by sterically bulky and/or optically active complexes reveal that steric interactions play a major role in mediating the binding of Pt(ll) to DNA but that hydrogen bonds play less of a role. In vitro cytotoxic activities for these complexes do not always follow the trends that would be expected on the basis of the adduct profiles.

Preparation, characterization, DNA binding, and in vitro cytotoxicity of the enantiomers of the platinum(II) complexes N-methyl-, N-ethyl- and N,N-dimethyl-(R)- and -(S)-3-aminohexahydroazepinedichloroplatinum(II).

A series of chiral diaminedichloroplatinum(II) complexes derived from [Pt(ahaz)Cl2] (ahaz = 3-aminohexahydroazepine) have been synthesized and tested for cytotoxic activity. Novel synthetic pathways were developed to produce the structural derivatives of the ahaz ligand, with alkyl substituents on the exocyclic nitrogen atom. The platinum(II) complexes of these ligands were synthesized and characterized by NMR and CD spectroscopy, confirming isomeric and enantiomeric purity. The crystal structures of three of these complexes, [Pt(S-meahaz)-Cl2], [Pt(R-etahaz)Cl2], and [Pt(S-dimeahaz)Cl2] (meahaz = N-methylahaz, etahaz = N-ethylahaz, dimeahaz = N,N-dimethylahaz), have been determined to establish the orientation of the protons and alkyl substituents on the nitrogen donor atoms. In vitro assays of the cytotoxic activity of the complexes have revealed a significant and reproducible enantioselective trend with the R-enantiomers more active than the S-enantiomers in all cell lines. Increasing the steric bulk on the amine groups was found to have only a modest effect on activity. No enantioselectivity was observed in the binding of R- and S-[Pt(etahaz)Cl2] to calf-thymus DNA.

Preparation, DNA binding, and in vitro cytotoxicity of a pair of enantiomeric platinum(II) complexes, [(R)- and (S)-3-aminohexahydroazepine]dichloroplatinum(II). Crystal structure of the S enantiomer.

A pair of enantiomeric Pt(II) complexes, [Pt(R-ahaz)Cl2] and [Pt(S-ahaz)Cl2] (ahaz = 3-aminohexahydroazepine), has been investigated for their ability to bind enantioselectively to DNA. Improved synthetic procedures were developed for preparing both the ligands and the Pt complexes. The structure of the complex of the S enantiomer was determined by X-ray crystallographic methods. Crystals of [Pt(S-ahaz)Cl2] are orthorhombic, space group P2(1)2(1)2(1), with a = 6.917(1) A, b = 11.167(1) A, c = 12.373(2) A, Z = 4, and the structure was refined to R = 0.023 (1505F). Molecular modeling techniques were used to investigate the role of steric interactions between the ligand and DNA in influencing the bifunctional binding of the two enantiomers, and it was found that the S enantiomer should bind more readily. The binding of the S enantiomer, to calf thymus DNA, was indeed found to be slightly greater than that for the R enantiomer though slightly less than that for cis-DDP. Assays of the proportion of monofunctional adducts showed that a substantially greater proportion of monofunctional adducts remained for the R enantiomer and cisplatin than for the S enantiomer. Each of the enantiomers was subjected to in vitro cytotoxicity assays using cultures of human bladder (BL13/0), lung and resistant lung (PC9 and PC9cisR), and prostate (DU145) cancer cells. The R enantiomer was found to be slightly more cytotoxic in the bladder cell line and may be less cytotoxic in the lung cell line but there were no significant differences in the resistant cell line nor in the prostate cell line. The two enantiomers were taken up equally by the bladder cancer cells.

Preparation, characterization, cytotoxicity, and mutagenicity of a pair of enantiomeric platinum(II) complexes with the potential to bind enantioselectively to DNA.

The synthesis of a pair of enantiomeric Pt(II) complexes, [Pt(R,R-eap)Cl2] and [Pt(S,S-eap)Cl2] (eap = N,N-diethyl-2,4-pentanediamine), designed to bind enantioselectively to GpG and ApG sequences of DNA is described. The in vitro cytotoxicity of each of the enantiomers toward murine leukemia and human bladder tumor cells has been measured. The R,R enantiomer was found to be more active in the leukemia cells, but the difference was not as great as expected (IC50; R,R 14 microM, S,S 33 microM). In the bladder tumor cell line, no significant difference in activity was found. The two enantiomers had similar mutagenicity in the Salmonella reversion assay, but the R,R enantiomer was more cytotoxic in the bacterial cells. A structural analysis of the R,R enantiomer revealed that the ligand adopted an unexpected configuration, and a strain energy minimization analysis showed that this was a consequence of interactions between the diamine ligand and the dichloro ligands. The significance of the structural preferences with respect to the lower than expected enantiospecificity is discussed. Crystals of [Pt(R,R-eap)Cl2] are monoclinic; space group, P2(1)2(1)2(1); a = 7.909(5), b = 12.972(9), and c = 13.269(12) A; Z = 4; and the structure was refined to R = 0.025 (1657F).