A RADIORECEPTOR ASSAY FOR THE ANALYSIS OF AT(1)-RECEPTOR ANTAGONISTS - CORRELATION WITH COMPLEMENTARY LC DATA REVEALS A POTENTIAL CONTRIBUTION OF ACTIVE METABOLITES

A reliable and sensitive radioreceptor assay based on rat lung homogen ate as receptor preparation was developed to determine the angiotensin -II antagonistic profile of losartan and its main active metabolite EX P 3174 as well as its congeners exemplified by UP 269-6 and SL 91.0102 -90 DL. This method proved to De precise with an intra-and interday va riability of less than 10% and a limit of quantification less than or equal to 1 ng ml(-1). The analysis of the K-i values in protein-free H epes-buffer versus blank human or rat plasma revealed the distinct hig h plasma-protein binding of EXP 3174 which consequently caused a drama tic drop of potency from 10-15-fold in the buffer to only about 2-fold in control plasma, when compared to the parent compound losartan and the two congeners investigated. Upon evaluation of clinical samples by both the reported radioreceptor assay (RRA) and the established high- performance liquid chromatography (HPLC), the correlation of the norma lized data pairs (concentration equivalents) suggested the contributio n of active metabolites to the angiotensin-II antagonistic effect of S L 91.0102-90 DL, but not to the effect of UP 269-6. In the context of an extended preclinical study in rats, the correlation of RRA with the respective HPLC concentration equivalents of losartan and its main ac tive metabolite EXP 3174 confirmed previous findings that only losarta n and EXP 3174 exert the angiotensin-II-AT(1) receptor blockade withou t the contribution of other metabolites (P.C. Wong, W.A. Price, A.T. C hiu et al., J. Pharmacol. Exp. Ther. 255 (1990) 211-217). Published by Elsevier Science B.V. All rights reserved.