Radioimmunoassay of acipimox in human plasma and urine

A radioimmunoassay was developed for the determination of acipimox (5-methylpyrazinecarboxylic acid 4-oxide) in human plasma and urine. Acipimox was conjugated to bovine serum albumin through a spacer with 4 carbon atoms, and repeatedly injected into rabbits. Antisera raised in these animals were highly specific and virtually no cross-reaction was observed with 5-methylpyrazinecarboxylic acid and nicotinic acid. Despite the low specific activity of the labelled antigen used, concentrations of acipimox in human plasma and urine as low as 40 ng/ml could be determined. The intra- and inter-assay coefficients of variation ranged between 4.32–6.25% and 6.19–11.55% respectively, and mean recovery of the compound spiked to plasma was 100–103%. The method was applied to determine plasma levels and urinary excretion of acipimox after oral doses of 150 mg and 250 mg to four volunteers.