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.