Analgesic and antidepressive effects of low-dose amitriptyline in relation to its metabolism in patients with chronic pain

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Abstract
The analgesic and antidepressive effects of amitriptyline (AT) in relation to its kinetics and metabolism were studied in 19 outpatients with chronic pain who received a daily dose of 75 mg AT for at least 6 weeks. Riboflavin was added to the medication to check compliance. On days 0, 4, and 8 and weeks 3, 6, 9, and 12 after the start of dosing, blood samples were drawn from the patients 10 .+-. 1 hours after the first morning dose and a sample of the first morning urine was taken to check riboflavin. Serum levels of AT and its metabolites, especially nortriptyline (NT) and E-10-hydroxy-nortriptyline (E-10-OH-NT), were measured by HPLC. On day 0 and at 3, 6, and 12 weeks the severity of depression was scored by means of a self-rating depression scale and pain intensity scores were measured. In addition, after 6, weeks of dosing patients estimated their percentage of pain in comparison with baseline. Mean (.+-. SD) steady-state concentrations of AT, NT, and E-10-OH-NT were 36 .+-. 23.5, 28 .+-. 14.9, and 52 .+-. 23.7 .mu.g/L, respectively, in male patients (n = 8; age 45 .+-. 7.4 years) and 34 .+-. 14.6, 45 .+-. 25.1, and 40 .+-. 15.6 .mu.g/L, respectively, in female patients (n = 11; age 46 .+-. 6.8 years). There was a significant sex-related difference in the NT/AT ratio, which was higher in women. After 6 weeks of treatment, neither the improvement in the depression score nor the improvement in pain intensity score in comparison with baseline scores correlated significantly with steady-state levels of AT, NT, and E-10-OH-NT and the NT/AT ratio.

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