IMMUNOASSAYS IN MONITORING BIOTECHNOLOGICAL DRUGS

For the evaluation and interpretation of pharmacokinetic data reliable quantitative determinations are a requirement that can only be met by well-characterized and fully validated analytical methods. To cope wi th these requirements a method is being established that is based on a n integrated and automated fiber-optic biospecific interaction analysi s system (FOBIA) for immunoassays. Performance characteristics of this system used in monitoring of recombinant hirudin (CGP 39 393) are pre sented. Recombinant hirudin is a highly potent and selective inhibitor of human thrombin. Owing to its size and charge, recombinant hirudin is mainly eliminated by glomerular filtration. But only a fraction of the hirudin dose seems to be reabsorbed at the proximal tubule by lumi nal endocytosis and hydrolyzed by lysosomal enzymes, leaving similar t o 50% of the dose to be extracted in the urine. Thus, renal clearance of recombinant hirudin in the absence of renal insufficiency appears t o depend primarily on the glomerular filtration rate. During a 3-month i.v. tolerability study in dogs, some of the dogs developed antibodie s against recombinant hirudin. The hirudin-antibody complex accumulate d in plasma and apparent hirudin plasma concentrations were therefore much higher than expected from single-dose kinetics. Hirudin captured by antibodies showed an extended half-life and the hirudin-antibody co mplex is still pharmacologically active, as demonstrated by the observ ed increase in thrombin time. In conclusion, only appropriate analytic al methods allow adequate monitoring and pharmacokinetic characterizat ion of biotechnology drugs in biological materials.