Cellular pharmacology of azatoxins (topoisomerase‐II and tubulin inhibitors) in P‐glycoprotein‐positive and ‐negative cell lines

Azatoxin (NSC 640737), a synthetic molecule, was rationally designed as a topoisomerase‐ll inhibitor and was shown to be a potent cytotoxic agent that inhibits both tubulin and topoisom‐erase II. A structure‐activity relationship study allowed to select 3 derivatives that inhibit either tubulin (methylazatoxin) only or topoisomerase II (fluoroanilinoazatoxin and nitroanilino‐azatoxin) in MTT assays performed on K562 and K562/ADM cells; the latter, expressing P‐glycoprotein, indicated cross‐resistance of K562/ADM cells to all 4 compounds. DNA double‐strand breaks induced by the 3 azatoxins that inhibit topoisomerase II in vitro were decreased in K562/ADM as compared with K562 cells. Nitroanilino‐azatoxin was the only compound for which resistance and reduced DNA damage observed in K562/ADM cells was partially reversed by verapamil, suggesting that nitroanilinoazatoxin was a substrate for P‐glycoprotein. These results were confirmed by testing the cytotoxic activity of azatoxins on P‐glycoprotein‐expressing rat colon‐carcinoma DHDKI2/TRb cells in the absence and the presence of verapamil. Cell‐cycle and mitotic‐index studies indicated that azatoxin‐ and methylazatoxin‐induced M‐phase arrest was less in K562/ADM than in KS62 cells. The G2 block induced by fluoro‐ and nitroanilino‐azatoxins was delayed in K562/ADM cells. Verapamil increased cell‐cycle inhibition induced by nitroanilinoazatoxin in K562/ ADM cells without modifying cell‐cycle effects of fluoroanilinoazatoxin. These results (i) are consistent with the specific inhibition of topoisomerase II or tubulin by azatoxin derivatives in cells; (ii) indicate that the nitro group of nitroanilinoazatoxin allows recognition and efflux by the P‐glycoprotein; and (iii) suggest that cross‐resistance of K562/ADM cells to other azatoxin derivatives is not mediated by P‐glycoprotein.