The objective of this study was to characterize the nature of isoniazid-mediated disruption of heme biosynthesis that is associated with hepatotoxicity.

In a mouse model, rifampicin and isoniazid combination treatment results in cholestatic liver injury that is associated with an increase in protoporphyrin IX (PPIX), the penultimate heme precursor. Both ferrochelatase (FECH/Fech) and aminolevulinic acid synthase 1 (ALAS1/Alas1) are crucial enzymes in regulating heme biosynthesis. Isoniazid has recently been reported to upregulate Alas1 but downregulate Fech protein levels in mice; however, the mechanism by which isoniazid mediates disruption of heme synthesis has been unclear. Interestingly, two metabolites of isoniazid, pyridoxal isonicotinoyl hydrazone (PIH, the isoniazid–vitamin B6 conjugate) and hydrazine, have been detected in the urine of humans treated with isoniazid.

qRT-PCR and Western blot analysis were used to quantify mRNA and protein expression changes, respectively. LC/MS/MS analysis was used to quantify isoniazid metabolite levels.