Synthesis of diketopiperazine-based carboline homodimers and in vitro growth inhibition of human carcinomas

Starting from d- or l-tryptophan, we have synthesized and characterized six compounds 2.29-2.31a and b that belong to a class of nitrogen heterocycles: the carboline-based homodimers. Each individual homodimer features a 1,3-trans relationship on each side of the central diketopiperazine core, but differs in absolute stereochemistry and also in substitution on the 4' and 4'' oxygens (-Bn, -CH(3), or -H). The in vitro cytotoxicity of the six compounds was evaluated by measuring the growth inhibition in NCI-H520 and PC-3 human carcinoma cells. Phenol 2.30a inhibited Cancer cell growth approximately three times better than its enantiomer 2.30b and possessed a GI(50) comparable to the clinically used agent etoposide in both cell lines. We have concluded that both the stereochemistry imparted by l-tryptophan and the presence of hydroxy substituents at the 4' and 4'' positions are necessary to generate cytotoxic properties in the homodimer class. We are now employing 2.30a as a new lead compound in our efforts to discover improved indole-based Cancer chemotherapeutics.