The use of an enzyme-linked immunosorbent assay to study the disposition of sheep digoxin-specific immunoglobulin G and Fab fragments in the rat.

An enzyme-linked immunosorbent assay was developed for the measurement of sheep digoxin-specific immunoglobulin G and Fab fragments. With the latter, two preparations were examined, one available commercially (Digibind, Wellcome) and one prepared by ourselves (DSFab). The assay exhibited a greater sensitivity towards immunoglobulin G compared with Digibind and DSFab, presumably because the Fab preparations lacked some of the sheep-specific antigens present on the whole antibody molecule. The assay was used subsequently to examine the disposition of the antibody preparations after injection of 1 mg/kg i.v. into anaesthetised bile duct-cannulated rats. The plasma distribution half-lives (1.8-3.3 min) were similar for all three preparations, but while plasma elimination half-life values for Digibind and DSFab were much the same (110-115 min), that for immunoglobulin G was longer (425 min). The shorter half-life values for Fab fragments were linked to a rate of urinary elimination 10-20 fold faster. No antibody excretion in the bile was detected. The apparent volume of distribution of immunoglobulin G was 35 ml/kg, indicating that the whole antibody was largely confined to the plasma space. The volume of distribution for Digibind or DSFab (about 46 ml/kg) was not significantly larger than that for immunoglobulin G and much smaller than the extracellular fluid volume, which was measured as 305 ml/kg. Thus the distribution of sheep Fab fragments in the rat markedly differs from that in the baboon (Smith et al., 1979) where the apparent volume of distribution approximates to the extracellular fluid volume.