Kinetic and equilibria studies of the aquation of the trinuclear platinum phase II anticancer agent [{trans-PtCl(NH3)2}2{μ-trans-Pt(NH3)2(NH2(CH2)6NH2)2}]4+ (BBR3464)

Abstract

The hydrolysis profile of the bifunctional trinuclear phase II clinical agent (trans-PtCl(NH3)2}2{μ-trans-Pt(NH3)2(NH2(CH2)6NH2)2}]4+ (BBR3464, 1) has been examined using [1H,15N] heteronuclear single quantum coherence (HSQC) 2D NMR spectroscopy. Reported are estimates of the rate and equilibrium constants for the first and second aquation steps, together with the acid dissociation constant (pKa1 ≈ pKa2 ≈ pKa3). The equilibrium constants for the aquation determined by NMR at 298 and 310 K (I = 0.1 M, pH 5.3) are similar, pK1 = pK2 = 3.35 ± 0.04 and 3.42 ± 0.04, respectively. At lower ionic strength (I = 0.015 M, pH 5.3) the values at 288, 293, and 298 K are pK1 = pK2 = 3.63 ± 0.05. This indicates that the equilibrium is not strongly ionic strength or temperature dependent. The aquation and anation rate constants for the two-step aquation model at 298 K in 0.1 M NaClO4 (pH 5.3) are k1 = (7.1 ± 0.2) × 10-5 s-1, k-1 = 0.158 ± 0.013 M-1 s-1, k2 = (7.1 ± 1.5) × 10-5 s-1, and k-2 = 0.16 ± 0.05 M-1 s-1. The rate constants in both directions increase 2-fold with an increase in temperature of 5 K, and rate constants increase with a decrease in solution ionic strength. A pKa value of 5.62 ± 0.04 was determined for the diaqua species [{trans-Pt(NH3)2(OH2)}2{μ-trans-Pt(NH3)2(NH2(CH2)6−NH2)2}]6+ (3). The speciation profile of 1 under physiological conditions is explored and suggests that the dichloro form predominates. The aquation of 1 in 15 mM phosphate was also examined. No slowing of the initial aquation was observed, but reversible reaction between aquated species and phosphate does occur.