Indirubin 3′-oxime (Indox (1b)) suppresses cancer cell growth (IC50: 15 μM 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 (1.7 μM) 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- (24 h) (IC50: 0.67 μM) and extended-duration (72 h) 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.Download high-res image (166KB)Download full-size image

Indirubin is a potent inhibitor of cell cycle-related protein kinases by binding to the ATP-binding site and thus is a promising compound for development as an antitumor drug. We prepared indirubin 3′-(O-oxiran-2-ylmethyl)oxime (Epox/Ind), in which the ATP-binding site orientated part was attached by non-specific alkylating group. The IC50 value of Epox/Ind at 1.7 μM in HepG2 cells is comparable to that of cisplatin (4.0 μM). Furthermore, Epox/Ind was shown to be metabolized by a HepG2 cell lysate into indirubin 3′-(O-2,3-dihydroxypropyl)oxime (E804), the sole extractable metabolite. The lower toxicity of this metabolite may explain the lack of cytotoxicity of 1 μM Epox/Ind observed in HepG2 cells beyond an initial loss of viability in the first 24 h of treatment.Indirubin 3′-(O-oxiran-2-ylmethyl)oxime (Epox/Ind).

Ammonium ylides generated in situ from phenyldiazoacetates and anilines undergo asymmetric N−H insertion reactions catalyzed by cinchona alkaloids (1 mol %), affording phenylglycines up to 74% ee.

This paper reports the synthesis of methoxy- and bromo-indirubins, and their antiproliferative activities in human neuroblastoma. Among 20 compounds, 5′-methoxyindirubin induced cell death in human neuroblastoma cells (IMR-32, SK-N-SH and NB-39) without inhibiting normal cells (NHDF and HUVEC). Typical morphologic features of apoptosis were observed in 5′-methoxyindirubin-treated cells by Hoechst 33342 staining. Additional studies by flow cytometry support apoptosis induction. These data suggest that 5′-methoxyindirubin might be an effective drug for treatment of neuroblastoma.