Novel approaches to polynuclear platinum pro-drugs. Selective release of cytotoxic platinum spermidine species through hydrolytic cleavage of carbamates

Abstract

BBR3464 is a novel trinuclear platinum drug currently in Phase II clinical trials. Polyamine-bridged dinuclear platinum compounds as represented by [trans-Pt(NH3)(2)Cl(2)-mu -spermidine-N-1,N-8]Cl-3 (1) are highly interesting second-generation analogues of BBR3464 because the hydrogen-bonding and electrostatic contributions of the central platinum-amine group in BBR3464 are replicated by the free, noncoordinated “central” quaternary nitrogens of the linear polyamine linker while the presence of two separate Pt-Cl bonds maintains the bifunctional binding mode on the DNA adducts. Preclinical investigations confirm the potency of these species with cytotoxicity in the nanomolar range. This remarkable potency results in a relatively narrow therapeutic index. To enhance the therapeutic index of these drugs, we investigated the potential for “pro-drug” delivery of less toxic and better tolerated derivatives such as the compounds [trans-Pt(NH3)(2)Cl(2)-mu -N-4-R-spermidine-N-1,N-8]Cl-2 where N-4-R represents BOC (tert-butyl), CBz (benzyl), and Fmoc (fluorenylmethyl) carbamate blocking groups, 2-4, respectively. The bulky Fmoc derivative showed evidence for conformational isomers by H-1 NMR spectroscopy due to the inequivalence of the two n-propyl and n-butyl side chains of the spermidine moiety. The rate constants for hydrolysis and release of I were calculated. Release of cytotoxic I at physiologically relevant PH followed the order 4 > 2 > 3. The calculated values for 4 (PH 5, 6.0(+/-3.9) x 10(-10) s(-1); PH 6, 6.5(+/-0.2) x 10(-9) s(-1); pH 7, 6.0(+/-0.2) x 10(-8) s(-1); pH 8, 1.6(+/-0.1) x 10(-7) s(-1)) show a more pronounced PH dependence compared to 2 (pH 5, 4.6(+/-0.1) x 10(-8) s(-1); pH 6, 4.2(+/-0.1) x 10(-8) s(-1); pH 7, 3.2(+/-0.1) x 10(-8) s(-1)). Preliminary biological assays of cellular uptake and cytotoxicity confirm the utility of the pro-drug concept. While blocked-polyamine compounds such as 2-4 are, in general, 2-3 orders of magnitude less cytotoxic than 1, there is significant cell type variability. Specifically, the Fmoc derivative 4 showed significantly enhanced cytotoxicity warranting further study of the pro-drug concept for greater selectivity and/or oral delivery.