The Use of Enzyme-linked Immunosorbent Assays to Study the Plasma Disposition of Sheep Polyclonal and Rat Monoclonal Digoxin-specific Fab Fragments in the Rabbit

The plasma disposition of sheep polyclonal digoxin-specific Fab (fragment antigen-binding) fragments has been studied in rabbits after their intravenous injection (1 mg kg-1) using enzyme-linked immunosorbent assays exploiting both the species-specificity (ELISA1) and the digoxin-specificity (ELISA2) of digoxin-specific Fab fragments. The log concentration versus time profiles were best described by a biexponential plasma disposition when either assay was used. Although the plasma concentrations determined by ELISA1 and ELISA2 at each sampling time were not significantly different, there was a tendency for certain ELISA2 values to be higher. This resulted in the ELISA2-derived data giving a significantly longer distribution half-life (t1/2 alpha), but similar values for elimination half-life (t1/2 beta), apparent volume of distribution at steady state (Vdss), and clearance. Using ELISA2, which was generally the more sensitive assay, to compare the plasma disposition of the sheep polyclonal digoxin-specific Fab fragments with rat monoclonal digoxin-specific Fab fragments, it was shown that the rat product had a shorter t1/2 alpha (11 vs 22 min), a t1/2 beta which was not significantly different (253 vs 168 min), but a faster clearance (1.2 vs 0.7 mL kg-1 min-1), associated with a much larger Vdss (321 vs 108 mL kg-1). The extracellular fluid volume, using thiocyanate as a marker, was about 216 mL kg-1 for the nine rabbits used. This suggests that the rat preparation penetrates more extensively into the extracellular space and may indicate that some degree of extracellular binding or cell penetration is occurring.