Time-dependent inhibition of human drug metabolizing cytochromes P450 by tricyclic antidepressants.
OBJECTIVE
To investigate time-dependent inhibition (TDI) of human drug metabolizing CYP enzymes by tricyclic antidepressants (TCAs).


METHODS
CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A/CYP3A4 activities were investigated following co- and preincubation with TCAs using human liver microsomes (HLM) and human recombinant CYP proteins (expressed in Escherichia coli) as the enzyme sources. A two-step incubation method was employed to examine the in vitro mechanism-based inactivation (MBI) criteria. Potential metabolite-intermediate complex (MIC) formation was studied by spectral analysis.


RESULTS
TCAs generally exhibited significant TDI of recombinant CYP1A2, CYP2C19 and CYP2D6 (>10% positive inhibition differences between co- and preincubation conditions). TDI of recombinant CYP2C9 was minor (<10%), and was minor or absent in experiments utilizing recombinant CYP3A4 or HLM as the enzyme sources. Where observed, TDI of recombinant CYP occurred via alkylamine MIC formation, but evidence to support similar behaviour in HLM was limited. Indeed, only secondary amine TCAs reduced the apparent P450 content of HLM (3-6%) consistent with complexation. As a representative TCA, nortriptyline fulfilled the in vitro MBI criteria using recombinant CYP2C19 and CYP3A4 (K(I) and k(inact) values of 4 microm and 0.19 min(-1), and 70 microm and 0.06 min(-1)), but not with the human liver microsomal enzymes.


CONCLUSIONS
TCAs appear to have minimal potential for MBI of human liver microsomal CYP enzymes involved in drug metabolism. HLM and recombinant CYP (expressed in E. coli) are not equivalent enzyme sources for evaluating the TDI associated with some drugs.
Laboratory, Department of Clinical Pharmacology, Flinders University and Flinders Medical Centre, Adelaide, Australia. tom.polasek@flinders.edu.au
Thomas M
TM
Polasek
John O
JO
Miners
D000929:Antidepressive Agents, Tricyclic / Q000494:pharmacology*; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D006801:Humans; D008862:Microsomes, Liver / Q000187:drug effects; D008954:Models, Biological; D009661:Nortriptyline / Q000494:pharmacology*; D013997:Time Factors; D046509:Tissue Culture Techniques
D000929:Antidepressive Agents, Tricyclic; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D009661:Nortriptyline