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)2Cl}2-μ-spermidine- N1,N8]Cl3 (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)2Cl{)2-μ-N4- R-spermidine-N1,N8]Cl2 where N4-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 1H 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 1 were calculated. Release of cytotoxic 1 at physiologically relevant pH followed the order 4 > 2 > 3. The calculated values for 4 (pH 5, 6.0(±3.9) × 10-10 s-1; pH 6, 6.5(±0.2) × 10-9 s-1; pH 7, 6.0(±0.2) × 10-8 s-1; pH 8, 1.6(±O.1) × 10-7 s-1) show a more pronounced pH dependence compared to 2 (pH 5, 4.6(±O.1) × 10-8 s-1; pH 6, 4.2(±O.1) × 10-8 s-1; pH 7, 3.2(±O.1) × 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.