Drug-protein conjugates--XVIII. Detection of antibodies towards the antimalarial amodiaquine and its quinone imine metabolite in man and the rat.

A specific enzyme-linked immunosorbent assay (ELISA) was developed for the detection and characterisation of antibodies directed against amodiaquine (AQ), an anti-malarial drug associated with agranulocytosis and liver damage in man. The assay incorporated an antigen which was produced by the reaction of amodiaquine quinone imine (AQQI), a protein reactive product produced from AQ by silver oxide oxidation, and metallothionein. The protein-conjugate (AQ-MT) had a ratio of AQ to protein of 5.2:1. Specific anti-drug antibody was defined as the differential binding to AQ-MT and unconjugated MT which was inhibitable by AQ-mercapturate (5 microM). Following administration of AQ (0.27 mmol/kg; for 4 days) to male Wistar rats there was a significant increase in the IgG anti-AQ activity on day 18 (P less than 0.05, 0.596 +/- 0.410, N = 7) compared to pre-injection levels (0.111 +/- 0.074, N = 7). This activity was shown to be specific for the AQ determinant by hapten inhibition with AQ (IC50 250 nM) and AQ-mercapturate (IC50 310 nM). Following administration of AQQI (27 mumol/kg; i.m.; 4 days) there was a significant increase in IgG anti-AQ antibody activities on day 18 (0.584 +/- 0.161, N = 7) compared to pre-injection levels (0.078 +/- 0.048, N = 7). This activity was inhibited by AQ (IC50 150 nM) and AQ-mercapturate (IC50 180 nM). In addition IgG anti-AQ antibodies were detected in four patients who exhibited agranulocytosis and one patient who exhibited hepatitis (range 0.017-0.842) whilst receiving AQ at a dose of 400 mg weekly for several weeks, but not in individuals who had not received the drug (-0.014 +/- 0.022, N = 7). There was no increase in IgG anti-AQ antibody activities in patients who had not exhibited an adverse reaction whilst receiving the drug for the treatment of malaria (-0.059 +/- 0.074 on day 0 and -0.053 +/- 0.068 on day 7, N = 13). Thus, we have shown that AQ is immunogenic in the rat and that the formation of a chemically reactive metabolite (AQQI) is involved in the generation of the antibody response. Furthermore, drug-specific antibodies were detected in sera from five patients with severe adverse reactions to the drug.