Unique biochemical, cytotoxic, and antitumor activity of camptothecin and 4β-amino-4'-O-demethylepipodophyllotoxin conjugates

Two compounds having a camptothecin (CPT) analog conjugated to the 4β-amino-4'-O-demethylepipodophyllotoxin analog were evaluated for their biochemical and biological activities. W1 [camptothecin-(para)-4β-amino-4'-O-demethylepipodophyllotoxin] had no activity against topoisomerase II (TOP II), but inhibited topoisomerase I (TOP I) with an IC₅₀ value 2-fold higher than CPT. W2 [camptothecin-(ortho)-4β-amino-4'-O-demethylepipodophyllotoxin] had inhibitory activity against TOP I and TOP II with IC₅₀ values 1.5-fold higher than either CPT or etoposide (VP-16). Both conjugates had similar cytotoxicity against the KB cell line, although the protein-linked DNA breaks (PLDBs) generated by W2 in KB cells were about 4-fold more than those of W1. No cross-resistance with the two conjugates was seen in a VP-16-resistant KB subline, which showed down-regulation of TOP II and overexpression of the multiple drug resistance-associated protein, or in a vincristine-resistant KB subline with overexpression of gp-170/mdr-1. The CPT-resistant KB variant (KB CPT 100), which has a reduction in TOP I content and another mechanism that occurs post-PLDB formation, was partially resistant to both compounds. W1 was not affected by this post-PLDB resistance mechanism. Cell cycle analysis demonstrated that W1 and W2 had similar cell cycle effects on KB and KB CPT 100 cells, which accumulated in S-phase upon drug treatment. These results suggested that W1 and W2 exerted their cytotoxicity through TOP I. In CPT-resistant cells, however, an unidentified target also may be involved in the cytotoxic action of W1, and TOP II may still be a target for W2. In vivo, W1 was more effective against the growth of human prostate cancer cells in nude mice than VP-16, CPT, or W2. Given its antitumor activity and unique biochemical mechanism of action, W1 warrants exploration as an antitumor compound. Copyright (C) 2000 Elsevier Science Inc.