Background: Carbon-carbon bond cleavage of a saturated aliphatic moiety is rarely seen in
xenobiotic metabolism. Olanexidine (Olanedine®), containing an n-octyl (C8) side chain, was mainly
metabolized to various shortened side chain (C4 to C6) acid-containing metabolites in vivo in preclinical
species. In liver microsomes and S9, the major metabolites of olanexidine were from multi-oxidation
on its n-octyl (C8) side chain. However, the carbon-carbon bond cleavage mechanism of n-octyl (C8)
side chain, and enzyme(s) responsible for its metabolism in human remained unknown.
Methods: A pair of regioisomers of α-ketol-containing C8 side chain olanexidine analogs (3,2-ketol
olanexidine and 2,3-ketol olanexidine) were synthesized, followed by incubation in human liver microsomes,
recombinant human cytochrome P450 enzymes or human hepatocytes, and subsequent metabolite
identification using LC/UV/MS.
Results: Multiple shortened side chain (C4 to C6) metabolites were identified, including C4, C5 and C6-
acid and C6-hydroxyl metabolites. Among 19 cytochrome P450 enzymes tested, CYP2D6, CYP3A4
and CYP3A5 were identified to catalyze carbon-carbon bond cleavage.
Conclusion: 3,2-ketol olanexidine and 2,3-ketol olanexidine were confirmed as the key intermediates in
carbon-carbon bond cleavage. Its mechanism is proposed that a nucleophilic addition of iron-peroxo species,
generated by CYP2D6 and CYP3A4/5, to the carbonyl group caused the carbon-carbon bond cleavage
between the adjacent hydroxyl and ketone groups. As results, 2,3-ketol olanexidine formed a C6 side
chain acid metabolite. While, 3,2-ketol olanexidine formed a C6 side chain aldehyde intermediate, which
was either oxidized to a C6 side chain acid metabolite or reduced to a C6 side chain hydroxyl metabolite.