Anti-tumor activity of noble indirubin derivatives in human solid tumor models in vitro

Indirubin has been identified as a component of a traditional Chinese medicine, Danggui Longhui Wan, which is used for the treatment of chronic myelogenous leukemia. Indirubin inhibits cyclin-dependent kinases (CDKs) and induces cell cycle arrest and apoptosis in cancer cells. Many indirubin derivatives have been studied for their potential anti-solid tumor activity. We have synthesized and evaluated many indirubin derivatives. In order to compare and confirm the potential of our major derivatives as anti-solid tumor agents, we examined their anti-proliferative activity in monolayers, as well as in multicellular spheroids (MCS) cultures of human colorectal cancer cells, DLD-1 and HT-29. The MCS model is an in vitro solid tumor model that is increasingly used for the evaluation of anti-solid tumor activity. 5-nitro-indirubin-3'-oxime (4c) and 5'-bromo-5-nitro-indirubin-3'-oxime (4l), compared to 5-trimethylacetamido-indirubin-3'-oxime (11) and 5-diphenylacetamido-indirubin-3'-oxime (33) showed greater anti-proliferative effects in monolayers, but lower anti-proliferative effects in MCS. Overall, our data suggest that compounds 11 and 33 may exert a significant anti-solid tumor activity via a mechanism other than CDK inhibition, different from that of 4c and 4l. These compounds are worth further investigation with respect to their anti-solid tumor activity and their mechanism of action in various solid tumor models.