SM-1, a novel PAC-1 derivative, activates procaspase-3 and causes cancer cell apoptosis

To develop more potent procaspase-3 activator, 7 novel derivatives of PAC-1 were synthesized and evaluated. Among them, SM-1 stood out for its promising activity and good pharmacokinetics properties. The purpose of this study is to elucidate the pharmacological mechanism of SM-1 and evaluate its efficacy and toxicity in-depth.

To reveal the effects of SM-1 on caspase-3 activity, both in vitro activation assay and in cells fluorometric assay were tested. The protein levels and distributions of procaspase-3 and cleaved caspase-3 were also measured by western blot and immunostaining. MTT assay, apoptosis assay and mouse xenograft model were applied to evaluate the efficacy of SM-1. Preliminary safety assessments also tested the acute toxicity and tissue distribution of SM-1.

Compared to PAC-1, SM-1 showed higher cytotoxicity in cancer cells. Further investigation demonstrated that SM-1 relieved zinc-mediated inhibition of procaspase-3 and activated the caspase-3 activity both in tube test and in cells. Efficacy evaluation showed SM-1-induced cell apoptosis mainly via activation of caspase-3 and reduced tumor size in mouse xenograft model. Its apoptosis induction efficacy was higher than PAC-1. The preliminary safety assessment demonstrated that the overall LD50 of SM-1 lied between 500 and 1000 mg/kg and the distribution of SM-1 in brain was low.

We identified SM-1 as a promising antitumor candidate, which displayed enhanced procaspase-3 activating activity and potent cytotoxicity for cancer cells but low toxicity for normal cells.