Tricyclic pyrone analogs: a new synthetic class of bifunctional anticancer drugs that inhibit nucleoside transport, microtubule assembly, the viability of leukemic cells in vitro and the growth of solid tumors in vivo.

Tricyclic pyrones (TPs) may represent a novel synthetic class of microtubule (MT) de-stabilizing anticancer drugs previously shown by us to inhibit macromolecule synthesis, tubulin polymerization, and the proliferation of leukemic and mammary tumor cells in vitro. A linear skeleton with a N-containing aromatic ring attached at C3 of the top A-ring, a central pyran B-ring and a six-membered bottom C-ring with no alkylation at C7 are required for the antitumor activities of the lead compounds, a 3-pyridyl benzopyran (code name H10) and its somewhat weaker 2-pyridyl regioisomer (code name H19). Increasing concentrations of H10 do not alter the binding of [3H]vinblastine and [3H]GTP to tubulin but mimic the ability of unlabeled colchicine (CLC) to reduce the amount of [3H]CLC bound to tubulin, suggesting that TPs may interact with the CLC binding site to inhibit tubulin polymerization. Exogenous Mg2+ cations absolutely required for the binding of GTP to tubulin and MT assembly cannot overcome the antitubulin action of H10. H10 reduces the viability of L1210 cells in vitro (IC50: 0.5 microM) but its antitumor activity may be related to its ability to inhibit tubulin polymerization and rapidly increase the mitotic index rather than to induce DNA cleavage and apoptosis. The anticancer potential of TPs in vivo is demonstrated by the fact that i.p. injections of the water-soluble H10-HCl decrease the growth of solid tumors in mice inoculated s.c. with Lewis lung carcinoma. A critical finding is that the antimitotic H10 is a bifunctional anticancer drug, which also blocks the cellular transport of nucleosides (IC50: 6 microM) to inhibit DNA synthesis. Since few CLC site-binding antimitotic agents are active in solid tumor models in vivo, the ability of these new MT destabilizing TPs to totally block nucleoside transport might be valuable in polychemotherapy to arrest tumor cells at several phases of their cycle, potentiate the action of antimetabolites and sensitize multidrug-resistant tumor cells.