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Nortriptyline E-10-hydroxylation in vitro is mediated by human CYP2D6 (high affinity) and CYP3A4 (low affinity): implications for interactions with enzyme-inducing drugs.
J Clin Pharmacol. 1999 Jun; 39(6):567-77.JC

Abstract

The human cytochrome P450 (CYP) isoforms mediating nortriptyline 10-hydroxylation have been identified using kinetic studies on heterologously expressed human CYPs and chemical inhibition studies on human liver microsomes. Nortriptyline was metabolized to E-10-hydroxynortriptyline by human lymphoblast-expressed CYPs 2D6 (Km 2.1 microM) and 3A4 (Km 37.4 microM) with high and low affinity, respectively, whereas CYPs 1A2, 2A6, 2B6, 2C9, 2C19, and 2E1 had no detectable activity. Human liver microsomal nortriptyline E-10-hydroxylation displayed biphasic kinetics. The high-affinity component (Km 1.3 +/- 0.4 microM, n = 11 livers) was selectively inhibited by the CYP 2D6 inhibitor quinidine, whereas the CYP3A4 inhibitor ketoconazole selectively inhibited the low-affinity component (K(m) 24.4 +/- 7 microM, n = 11 livers). Inhibition by ketoconazole increased with increasing substrate concentration, whereas the reverse was true for quinidine. The Vmax of the low-affinity component in human liver microsomes was significantly correlated (r2 = 0.84) with the relative activity factor for CYP3A4, a measure of the amount of catalytically active enzyme. A simulation of the relative contribution of CYPs 2D6 and 3A4 to net nortriptyline hydroxylation rate suggested that the relative contribution of CYP3A4 is only 20% even at the higher end of the therapeutic range. Induction of CYP3A4 will increase its importance and increase the net metabolic rate, whereas inhibition of CYP3A4 will be of little importance due to its minimal relative contribution under uninduced conditions. The identification of CYP3A4 as a low-affinity nortriptyline E-10-hydroxylase explains the ability of poor metabolizers of debrisoquin to hydroxylate nortriptyline, as well as the increased in vivo clearance via this pathway caused by CYP3A4-inducing drugs such as pentobarbital, carbamazepine, and rifampin.

Authors+Show Affiliations

Department of Pharmacology and Experimental Therapeutics, Tufts University School of Medicine, Boston, MA 02111, USA.No affiliation info availableNo affiliation info available

Pub Type(s)

Journal Article
Research Support, U.S. Gov't, P.H.S.

Language

eng

PubMed ID

10354960

Citation

Venkatakrishnan, K, et al. "Nortriptyline E-10-hydroxylation in Vitro Is Mediated By Human CYP2D6 (high Affinity) and CYP3A4 (low Affinity): Implications for Interactions With Enzyme-inducing Drugs." Journal of Clinical Pharmacology, vol. 39, no. 6, 1999, pp. 567-77.
Venkatakrishnan K, von Moltke LL, Greenblatt DJ. Nortriptyline E-10-hydroxylation in vitro is mediated by human CYP2D6 (high affinity) and CYP3A4 (low affinity): implications for interactions with enzyme-inducing drugs. J Clin Pharmacol. 1999;39(6):567-77.
Venkatakrishnan, K., von Moltke, L. L., & Greenblatt, D. J. (1999). Nortriptyline E-10-hydroxylation in vitro is mediated by human CYP2D6 (high affinity) and CYP3A4 (low affinity): implications for interactions with enzyme-inducing drugs. Journal of Clinical Pharmacology, 39(6), 567-77.
Venkatakrishnan K, von Moltke LL, Greenblatt DJ. Nortriptyline E-10-hydroxylation in Vitro Is Mediated By Human CYP2D6 (high Affinity) and CYP3A4 (low Affinity): Implications for Interactions With Enzyme-inducing Drugs. J Clin Pharmacol. 1999;39(6):567-77. PubMed PMID: 10354960.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Nortriptyline E-10-hydroxylation in vitro is mediated by human CYP2D6 (high affinity) and CYP3A4 (low affinity): implications for interactions with enzyme-inducing drugs. AU - Venkatakrishnan,K, AU - von Moltke,L L, AU - Greenblatt,D J, PY - 1999/6/4/pubmed PY - 1999/6/4/medline PY - 1999/6/4/entrez SP - 567 EP - 77 JF - Journal of clinical pharmacology JO - J Clin Pharmacol VL - 39 IS - 6 N2 - The human cytochrome P450 (CYP) isoforms mediating nortriptyline 10-hydroxylation have been identified using kinetic studies on heterologously expressed human CYPs and chemical inhibition studies on human liver microsomes. Nortriptyline was metabolized to E-10-hydroxynortriptyline by human lymphoblast-expressed CYPs 2D6 (Km 2.1 microM) and 3A4 (Km 37.4 microM) with high and low affinity, respectively, whereas CYPs 1A2, 2A6, 2B6, 2C9, 2C19, and 2E1 had no detectable activity. Human liver microsomal nortriptyline E-10-hydroxylation displayed biphasic kinetics. The high-affinity component (Km 1.3 +/- 0.4 microM, n = 11 livers) was selectively inhibited by the CYP 2D6 inhibitor quinidine, whereas the CYP3A4 inhibitor ketoconazole selectively inhibited the low-affinity component (K(m) 24.4 +/- 7 microM, n = 11 livers). Inhibition by ketoconazole increased with increasing substrate concentration, whereas the reverse was true for quinidine. The Vmax of the low-affinity component in human liver microsomes was significantly correlated (r2 = 0.84) with the relative activity factor for CYP3A4, a measure of the amount of catalytically active enzyme. A simulation of the relative contribution of CYPs 2D6 and 3A4 to net nortriptyline hydroxylation rate suggested that the relative contribution of CYP3A4 is only 20% even at the higher end of the therapeutic range. Induction of CYP3A4 will increase its importance and increase the net metabolic rate, whereas inhibition of CYP3A4 will be of little importance due to its minimal relative contribution under uninduced conditions. The identification of CYP3A4 as a low-affinity nortriptyline E-10-hydroxylase explains the ability of poor metabolizers of debrisoquin to hydroxylate nortriptyline, as well as the increased in vivo clearance via this pathway caused by CYP3A4-inducing drugs such as pentobarbital, carbamazepine, and rifampin. SN - 0091-2700 UR - https://www.unboundmedicine.com/medline/citation/10354960/Nortriptyline_E_10_hydroxylation_in_vitro_is_mediated_by_human_CYP2D6__high_affinity__and_CYP3A4__low_affinity_:_implications_for_interactions_with_enzyme_inducing_drugs_ DB - PRIME DP - Unbound Medicine ER -