DEVELOPMENT OF A SENSITIVE RADIOIMMUNOASSAY FOR FAB FRAGMENTS - APPLICATION TO FAB PHARMACOKINETICS IN HUMANS

Anti-sheep Fab fragment antisera were produced in rabbits using sheep digoxin-specific Fab fragments (Digidot) as immunogen. These antisera were used for the development of a radioimmunoassay (RIA) of sheep Fab fragments in human plasma and urine using I-125-labeled Fab fragments . Interference in the assays by digoxin, human proteins, and antibodie s from different species was insignificant, but cross-reactivity betwe en anti-sheep Fab antisera and goat IgG or Fab fragments was 22 to 67% . The limit of detection was 0.1 mug/mL and the assay was linear over a 0.6-28 mug/mL range of Fab fragments. Intra- and interassay coeffici ents of variation were less than 6.9 and 10.5%, respectively. Accuracy of plasma and urine assays at various Fab fragment levels ranged from % to 106%. RIA was applied to the pharmacokinetic study of sheep digo xin-specific Fab fragments in one patient acutely intoxicated by digit oxin and treated with Digidot. The Fab elimination half-life was 12.1 hr. Steady-state volume of distribution and total-body clearance were 10.8 L and 23.4 mL/min, respectively. Unchanged Fab fragments (50 kD) and degradation products (25 kD) isolated by gel filtration chromatogr aphy of a urine sample cross-reacted with the anti-Fab antiserum.