Background & Aims: The incidence of isoniazid (INH)- and rifampicin (RIF)-induced abnormal liver enzyme activity is 27%
but only 19% with INH alone. Cytochrome P450 2E1 (CYP2E1) is thought to contribute to the synergistic effects of RIF and INH. Pharmaceutical
excipients are inactive ingredients that are added to a pharmaceutical compound. The purpose of this study was to screen excipients
for CYP2E1 inhibition and identify whether the screened excipients prevented INH/RIF-induced hepatotoxicity.
Methods: Fifty-five known pharmaceutical excipients were screened for CYP2E1 inhibition. The hepatotoxic doses of INH and RIF were
50 and 100 mg/kg/day, respectively. Hepatotoxicity was assessed by the galactose single point (GSP) method (a US Food and Drug Administration
(FDA) recommended quantitative liver function test), liver histopathology, malondialdehyde (MDA) assay, and measurement
of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activity. We chose the CYP2E1-specific substrate chlorzoxazone
to assess CYP2E1 activity in animal and human.
Results: Mannitol inhibited CYP2E1 activity by 54% in mice with INH/RIF-induced hepatotoxicity (p < 0.005). Serum AST, ALT and
GSP levels were significantly increased 3.8- to 7.8-fold in these mice (p < 0.005), and these levels could be lowered by mannitol. Mannitol
significantly alleviated the depletion of hepatic glutathione (GSH) and partially reversed the increase in MDA formation in mice
treated with INH/RIF (p < 0.005). Mannitol also decreased CYP2E1 activity by 58% in humans (p < 0.005). Furthermore, an antituberculosis
(TB) efficacy assay revealed that mannitol did not affect the anti-TB effects of INH/RIF.
Conclusions: Mannitol, an FDA-approved excipient, was found to be a CYP2E1 inhibitor. Mannitol may be a useful adjuvant for drugs
that induce hepatotoxicity through CYP2E1, such as INH and RIF.