5-Bromoindirubin 3'-(O-oxiran-2-ylmethyl)oxime: A long-acting anticancer agent and a suicide inhibitor for epoxide hydrolase

Abstract

Indirubin 3'-oxime (Indox (1b)) suppresses cancer cell growth (IC₅₀: 15μM towards HepG2 cells) and inhibits cell cycle-related kinases such as cyclin-dependent kinases and glycogen synthase kinase-3β. We have previously reported that the conjugation of 1b with oxirane, a protein-reactive component, enhanced the cytotoxic activity of Indox as determined from the IC₅₀ value (1.7μM) of indirubin 3'-(O-oxiran-2-ylmethyl)oxime (Epox/Ind (1c)). Here we prepared Epox/Ind derivatives with one or two halogen atoms or a methoxy group on the aromatic ring(s) of an Indox moiety and studied the structure-activity relationships of the substituent(s). We found that bromine-substitution at the 5-position on 1c or any Epox/Ind derivative(s) having bromine on the aromatic ring except Epox/6'-Br-Ind was efficient to improving anticancer activity. Of the 22 Epox/Ind derivatives, 5-bromoindirubin 3'-(O-oxiran-2-ylmethyl)oxime (Epox/5-Br-Ind (2c)) was the best anticancer agent in both short- (24h) (IC₅₀: 0.67μM) and extended-duration (72h) cultures. The high anticancer activity of 2c was partly due to it being a poor substrate and a suicide inhibitor for epoxide hydrolase as epoxide hydrolase was identified as the enzyme primarily responsible for the metabolism of 2c.

Keywords: 5-Bromoindirubin 3′-(O-oxiran-2-ylmethyl)oxime; Epoxide hydrolase; Long-acting anticancer agent; Structure-activity relationships; Suicide inhibition.