The fate of 4-hydroxycarbazole metabolite: metabolism and carcinogenicity assessment of a β-adrenergic receptor modulator containing carbazole structure

Abstract

1. LY377604 has a potential to form 4-hydroxycarbazole, which was reported in the literature as a mutagen. This safety concern led to our investigation of the metabolism and carcinogenicity of LY377604.

2. In in vitro studies with LY377604, 4-hydroxycarbazole was detected in the presence of liver microsomes prepared from different species. When incubated with liver slices, only the conjugate of 4-hydroxycarbazole was detected.

3. Subsequent in vivo radio-labelled studies were conducted to characterise the formation of 4-hydroxycarbazole from LY377604. Free 4-hydroxycarbazole was not detected in vivo, but the O-glucuronide conjugate was identified as a minor metabolite in urine samples, representing 0.2% and 0.9% of the radioactive dose in rats and monkeys. The low level of circulating 4-hydroxycarbazole glucuronide conjugate was also detected in plasma.

4. LY377604 was negative in all genetic toxicology assays and was not associated with tumour induction in a 6-month carcinogenicity study using RasH2+/− mouse model. The exposure to free 4-hydroxycarbazole was not measurable after one dose and was about 0.1%−0.2% of the parent exposure at the end of the 6-month study.

5. These data suggested that 4-hydroxycarbazole was formed as a minor metabolite in vivo, but it was primarily conjugated and excreted in urine as the glucuronide conjugate. The absence of tumours in the carcinogenicity study combined with the exposure data suggested that the low level of free 4-hydroxycarbazole did not represent a carcinogenic risk.

Keywords::

• 4-hydroxycarbazole
• metabolism
• carcinogenicity

Acknowledgment

We would like to thank Tom Lindsay for his involvement of early metabolism work. We want to thank Randall J. Press, Madhu Sanga and Diana M. Doherty at Covance for conducting ¹⁴C animal studies, Boris Czeskis for coordinating the synthesis of radio-labelled compounds in ABC labs, and James Ivett, Jo-Anne Burlew and Richard Cardy at Covance for conducting the rasH2 mouse study. We are also grateful to Shelby Anderson, May Pat Knadler, Thomas Raub, Everett Perkins, Gerald Long and James Stevens for valuable discussions during the preparation of this manuscript.

Declaration of interest

Authors declare no conflict of interest.