Prolonged toxicity after amitriptyline overdose in a patient deficient in CYP2D6 activity.

INTRODUCTION: Amitriptyline and its metabolite, nortriptyline, are metabolized, in part, by CYP2D6, a polymorphic enzyme. About 8% of Caucasians are deficient in CYP2D6 activity.

CASE REPORT: We present the case of a comatose woman who intentionally overdosed on amitriptyline and displayed rising serum total tricyclic antidepressant concentrations for at least 6 days after admission. Serial immunoassay total tricyclic antidepressant concentrations in our patient showed gradual decline beginning day 7, although the patient did not regain normal mental status until day 12. Genotyping revealed the patient to be homozygous for the CYP2D6*4 allele, the most common explanation of CYP2D6 enzymatic deficiency among Caucasians. Patients taking tricyclic antidepressants who are homozygous for CYP2D6*4 demonstrate >3 times concentration-time curve (AUCs) and prolonged elimination half-lives, especially of secondary amines such as nortriptyline.

DISCUSSION: We believe this is the first report describing toxicokinetics after tricyclic antidepressant overdose in a CYP2D6-deficient patient.