REDUCED ALBUMIN-BINDING PROMOTES THE STABILITY AND ACTIVITY OF TOPOTECAN IN HUMAN BLOOD

Topotecan, a semisynthetic water-soluble analogue of camptothecin, is the first topoisomerase I targeting anticancer agent to enter comparat ive phase III clinical trials, Here we elucidate the biophysical facto rs underlying the markedly improved bloodstream stability and cytotoxi c activity of topotecan relative to camptothecin. Each agent contains an alpha-hydroxy-delta-lactone ring that hydrolyzes under physiologica l pH to yield a biologically-inactive carboxylate form. In human plasm a, camptothecin lactone converts rapidly and completely to its carboxy late form due to a 200-fold binding preference by serum albumin (HSA) for the latter [Mi, Z., & Burke, T. G. (1994) Biochemistry 33, 10540-1 2545], Time-resolved fluorescence anisotropy measurements reveal that neither topotecan lactone nor carboxylate associates with HSA, thereby resulting in a significantly higher level of lactone stability in pla sma for topotecan (t(1/2) = 23.1 min, percent lactone at equilibrium o f 17.6) relative to camptothecin (t(1/2) = 10.6 min, percent lactone a t equilibrium of < 0.2). Moreover, studies with HL-60 human promyelocy tic leukemia cells reveal that a physiologically-relevant level (40 mg /mL) of HSA dramatically attenuates the cytotoxic activity of camptoth ecin in excess of 2600-fold (for a 72 h exposure, the IC50 value of 1. 5 nM in the absence of HSA increased to 4 mu M in the presence of HSA) . The activities of other clinically relevant anticancer analogues, 9- aminocamptothecin and SN-38, were also strongly modulated by the prese nce of 40 mg/mL HSA. In marked contrast, the presence of HSA effected no change on the cytotoxic activity of topotecan (IC50 = 12 nM both in the absence and in presence of HSA). Our data characterize important differences in the chemical dynamics between camptothecin and topoteca n in human blood which correlate with the favorable anticancer activit y of the latter agent observed in the presence of HSA.