RPR112378 and RPR115781: Two representatives of a new family of microtubule assembly inhibitors

A screening program aimed at the discovery of new antimicrotubule agents yi elded RPR112378 and RPR115781, two natural compounds extracted from the Ind ian plant Ottelia alismoides. We report their isolation, structural determi nation, and mechanisms of action. RPR112378 is an efficient inhibitor of tu bulin polymerization (IC50 = 1.2 mu M) and is able to disassemble preformed microtubules. Regarding tubulin activity, RPR115781 is 5-fold less active than RPR112378. Tubulin-RPR112378 complexes, when isolated by gel filtratio n, were able to block further tubulin addition to growing microtubules, a m echanism that accounts for the substoichiometric effect of the drug. RPR112 378 was found to prevent colchicine binding but not vinblastine binding to tubulin. Although colchicine binding is known to induce an increase of tubu lin GTPase activity, no such increase was observed with RPR112378. We show that RPR112378 is a highly cytotoxic compound and that RPR115781 is 10,000- fold less active as an inhibitor of KB cell growth. Part of the cytotoxicit y of RPR112378 is probably caused by a reaction of addition with sulfhydryl groups, an observation that has not been made with RPR115781. In conclusio n, these molecules represent a new class of inhibitors of microtubule assem bly with potential therapeutic value.