Massive induction of apoptosis of multicellular tumor spheroids by a novel compound with a calmodulin inhibitor-like mechanism

Background: A number of anticancer drug candidates have been identified by cell-based screens utilizing tumor cells grown in monolayer culture.Since such cultures are poor models of 3-D tumor micro-environments, we here aimed to identify novel compounds showing anti-proliferative activity on multicellular spheroids.Methods: A chemical library was used to screen for compounds capable of reducing viability and inducing apoptosis of colon carcinoma cells grown as multicellular spheroids.Assessment of possible mechanism of action was performed using gene expression profiling.Results: The screen identified NSC647889 as a potent apoptotic compound.NSC647889 induced dramatic increases in tumor cell apoptosis in multicellular spheroids compared to standard antineoplastic agents.Interestingly, quiescent cells in spheroid cores were resistant to NSC647889-induced apoptosis and appeared to die by other mechanisms.The cellular phenotypic response to NSC647889 was similar to that of known calmodulin inhibitors and the compound stimulated rapid increases of intracellular calcium levels.NSC647889 was, however, not a direct inhibitor of calmodulin-dependent calcineurin activity.Finally, NSC647889 induced tumor apoptosis in a xenograft tumor model.Conclusions: Novel drugs that show considerably stronger cytotoxic activity in 3-D culture compared to standard agents can be identified.Further development of such drugs may be a useful strategy for improved treatment of solid tumors.