SPECIFIC RADIOIMMUNOASSAY FOR AMITRIPTYLINE AND NORTRIPTYLINE IN PLASMA

  • 1 January 1977
    • journal article
    • research article
    • Vol. 18  (1) , 125-136
Abstract
A specific radioimmunoassay (RIA) for determination in plasma of the widely used tricyclic antidepressant amitriptyline (AT) and its major metabolite nortriptyline (NT) was developed employing 3H-AT as the radioligand and a rabbit antiserum to a bovine serum albumin conjugate of N-succinyl-nortriptyline. Although the antiserum cross-reacted almost equally well with AT and NT, specificity was achieved by selective extraction of each compound from plasma at a different pH. At no time during the entire extraction procedure was the AT or NT taken out of solution. Both compounds were back extracted from the organic phase into 0.1 N HCl and the acid fraction subjected to RIA directly. The method had a limit of sensitivity of .apprx. 2 ng/ml using a 0.5-ml plasma sample. Satisfactory agreement was obtained for plasma levels of AT and NT when determined by the RIA and a specific GC/MS [gas chromatography/mass spectrometry] procedure. The correlation coefficients were 0.89 and 0.98 for AT and NT, respectively. The RIA was used to measure steady-state levels of AT and NT in man after chronic administration of AT and following a single oral 75 mg dose. The method lent itself for the specific determination of NT alone in patients receiving therapeutic doses of NT.