title: Relative contribution of CYP3A to amitriptyline clearance in humans: in vitro and in vivo studies.
abstract: The relative contribution of cytochrome P450 3A (CYP3A) to the oral clearance of amitriptyline in humans has been assessed using a combination of in vitro approaches together with a clinical pharmacokinetic interaction study using the CYP3A-selective inhibitor ketoconazole. Lymphoblast-expressed CYPs were used to study amitriptyline N-demethylation and E-10 hydroxylation in vitro. The relative activity factor (RAF) approach was used to predict the relative contribution of each CYP isoform to the net hepatic intrinsic clearance (sum of N-demethylation and E-10 hydroxylation). Assuming no extrahepatic metabolism, the model-predicted contribution of CYP3A to net intrinsic clearance should equal the fractional decrement in apparent oral clearance of amitriptyline upon complete inhibition of the enzyme. This hypothesis was tested in a clinical study of amitriptyline (50 mg, p.o.) with ketoconazole (three 200 mg doses spaced 12 hours apart) in 8 healthy volunteers. The RAF approach predicted CYP2C19 to be the dominant contributor (34%), with a mean 21% contribution of CYP3A (range: 8%-42% in a panel of 12 human livers). The mean apparent oral clearance of amitriptyline in 8 human volunteers was decreased from 2791 ml/min in the control condition to 2069 ml/min with ketoconazole. The average 21% decrement (range: 2%-40%) was identical to the mean value predicted in vitro using the RAF approach. The central nervous system (CNS) sedative effects of amitriptyline were slightly greater when ketoconazole was coadministered, but the differences were not statistically significant. In conclusion, CYP3A plays a relatively minor role in amitriptyline clearance in vivo, which is consistent with in vitro predictions using the RAF approach.
authors.affiliation: Department of Pharmacology and Experimental Therapeutics, Tufts University School of Medicine and New England Medical Center, Boston, MA 02111, USA.
authors.forename: K
authors.initials: K
authors.lastname: Venkatakrishnan
authors.forename: J
authors.initials: J
authors.lastname: Schmider
authors.forename: J S
authors.initials: JS
authors.lastname: Harmatz
authors.forename: B L
authors.initials: BL
authors.lastname: Ehrenberg
authors.forename: L L
authors.initials: LL
authors.lastname: von Moltke
authors.forename: J A
authors.initials: JA
authors.lastname: Graf
authors.forename: P
authors.initials: P
authors.lastname: Mertzanis
authors.forename: K E
authors.initials: KE
authors.lastname: Corbett
authors.forename: M C
authors.initials: MC
authors.lastname: Rodriguez
authors.forename: R I
authors.initials: RI
authors.lastname: Shader
authors.forename: D J
authors.initials: DJ
authors.lastname: Greenblatt
mesh_terms: D000328:Adult; D000639:Amitriptyline / Q000097:blood / Q000493:pharmacokinetics*; D000929:Antidepressive Agents, Tricyclic / Q000097:blood / Q000493:pharmacokinetics*; D000935:Antifungal Agents / Q000097:blood / Q000493:pharmacokinetics; D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D004311:Double-Blind Method; D004347:Drug Interactions / Q000235:genetics; D005260:Female; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D007654:Ketoconazole / Q000097:blood / Q000493:pharmacokinetics; D008297:Male; D008657:Metabolic Clearance Rate / Q000235:genetics; D008862:Microsomes, Liver / Q000201:enzymology; D008875:Middle Aged; D006899:Mixed Function Oxygenases / Q000235:genetics / Q000378:metabolism*; D009661:Nortriptyline / Q000097:blood / Q000493:pharmacokinetics; D010089:Oxidoreductases, N-Demethylating / Q000235:genetics / Q000378:metabolism*; D010641:Phenotype
chemical_list: D000929:Antidepressive Agents, Tricyclic; D000935:Antifungal Agents; D007527:Isoenzymes; D000639:Amitriptyline; D003577:Cytochrome P-450 Enzyme System; D009661:Nortriptyline; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; D010089:Oxidoreductases, N-Demethylating; D007654:Ketoconazole