Proliferation of Human Tumor Cells in Vitro and in Vivo BN80927 : A Novel Homocamptothecin That Inhibits

BN80927 belongs to a novel family of camptothecin analogs, the homocamptothecins, developed on the concept of topoisomerase I (Topo I) inhibition and characterized by a stable seven-membered -hydroxylactone ring. Preclinical data reported here show that BN80927 retains Topo I poisoning activity in cell-free assay (DNA relaxation) as well as in living cells, in which in vivo complexes of topoisomerase experiments and quantification of DNA-protein-complexes stabilization, have confirmed the higher potency of BN80927 as compared with the Topo I inhibitor SN38. In addition, BN80927 inhibits Topo II-mediated DNA relaxation in vitro but without cleavable-complex stabilization, thus indicating catalytic inhibition. Moreover, a Topo I-altered cell line (KBSTP2), resistant to SN38, remains sensitive to BN80927, suggesting that a part of the antiproliferative effects of BN80927 are mediated by a Topo I-independent pathway. This hypothesis is also supported by in vitro data showing an antiproliferative activity of BN80927 on a model of resistance related to the noncycling state of cells (G0-G1 synchronized). In cell growth assays, BN80927 is a very potent antiproliferative agent as shown by IC50 values consistently lower than those of SN38 in tumor cell lines as well as in their related drug-resistant lines. BN80927 shows high efficiency in vivo in tumor xenograft studies using human androgen-independent prostate tumors PC3 and DU145. Altogether, these data strongly support the clinical development of BN80927.