Pharmacogenetics and Therapeutic Drug Monitoring of Nortriptyline

Pharmacogenetic Considerations

• CYP2D6 genotyping is recommended when available because the enzyme is a major determinant of nortriptyline metabolism (Mayo Clinic guidelines) 1.
• The Mayo Clinic guidelines advise genetic testing in treatment‑resistant depression, anticipated high dosing, known family history of CYP2D6 poor metabolism, or polypharmacy 1.

Therapeutic Drug Monitoring (TDM)

• Nortriptyline has a therapeutic plasma concentration range of 70–170 ng/mL; routine TDM is a Level 1 (strong) recommendation (Dialogues in Clinical Neuroscience) 2, 3.
• Because of its narrow therapeutic index, nortriptyline is one of the few antidepressants for which TDM is considered standard of care (Mayo Clinic guidelines; Dialogues in Clinical Neuroscience) 1, 2.

Special Populations Requiring Adjusted Monitoring

Elderly Patients

• Pharmacokinetic alterations in older adults necessitate lower initial doses and more frequent plasma‑level monitoring (Dialogues in Clinical Neuroscience) 4, 5.

Pediatric Patients

• Developmental stage markedly influences nortriptyline pharmacokinetics, making adult dosing inappropriate for children and adolescents (Dialogues in Clinical Neuroscience) 4, 5.
• A linear relationship exists between dose and plasma level in pediatric patients (correlation r = 0.50, p < 0.0001); baseline ECG is essential due to predictable cardiac effects (Dialogues in Clinical Neuroscience) 4, 5.

Pregnant/Lactating Women

• Blood‑level monitoring is recommended during pregnancy and lactation to minimize fetal or infant drug exposure (Dialogues in Clinical Neuroscience) 4, 5.

Drug‑Interaction Considerations

• Co‑administration of strong CYP2D6 inhibitors (e.g., paroxetine, fluoxetine) markedly reduces nortriptyline clearance and can convert extensive metabolizers into phenotypic poor metabolizers (Mayo Clinic guidelines) 1.
• Clinicians should not assume therapeutic failure without checking plasma levels, because poor metabolizers may reach toxic concentrations at standard doses (Mayo Clinic guidelines) 1.