Pharmacokinetic interaction between imipramine and antidepressant neuroleptics in rats

Antidepressant neuroleptics, perazine (PZ), levomepromazine (LMZ) and flupenthixol (FPX) given to rats jointly with imipramine (IMI) for 2 weeks affected the plasma concentration of IMI only slightly but markedly elevated the concentration of its metabolite, desipramine (DMI). In the brain PZ significantly elevated both IMI and DMI concentrations while LMZ and FPX showed a tendency to increase the concentration of IMI and decrease the concentration of DMI. All three neuroleptics markedly decreased the DMI/IMI ratio in the brain and (except FPX) increased it in the plasma. Given alone for two weeks PZ, LMZ, FPX did not affect the levels of cytochromes P-450 and b-5 in liver microsomes. Chronic treatment with IMI significantly elevated the concentration of cytochrome P-450 in the liver and had a tendency to increase the concentration of cytochrome b-5. FPX, but not PZ or LMZ abolished this effect. Neuroleptics coadministered with IMI to rats did not affect the activity of the enzymes responsible for the IMI biotransformation as compared with IMI-treated animals. The neuroleptics added to the incubation mixture in vitro inhibited IMI hydroxylation noncompetitively. The demethylation was inhibited competitively by LMZ but noncompetitively by PZ and FPX. The inhibitory effect of neuroleptics on the hydroxylation was much more marked than that on the demethylation. FPX was the weakest inhibitor of IMI metabolism among the neuroleptics studied.