NR1I2 and tuberculosis: RIF, a first line anti-TB drug, is a human specific activator of pregnane X receptor (PXR), a xenobiotic nuclear receptor that regulates the expression of drug-metabolizing enzymes including cytochromes P450 (CYP).27, 28, 29 In patients receiving INH, RIF appears to promote the formation of toxic INH metabolites and potentiate INH hepatotoxicity.18