The effect of drug-specific active immunization on digoxin and benzylpenicillin disposition in the bile duct-cannulated rat.

Rats were immunized with a digoxin-human serum albumin conjugate i.m. This resulted in a several hundred-fold increase in plasma radioactivity and a 90% reduction in biliary drug elimination when [3H]digoxin (10 micrograms kg-1, i.v.) was subsequently injected into anaesthetized bile duct-cannulated rats. It was calculated that about 90% of the drug dose remained antibody-bound within the plasma compartment, with essentially no drug distributing into organs such as the heart and liver. Digoxin-specific antibody levels, determined by equilibrium dialysis, were high in the plasma but at least an order of magnitude lower in the bile. Immunization via Peyer's patches did not increase antibody levels in the bile. Immunization (i.m.) with a benzylpenicillin-human serum albumin conjugate gave specific antibody plasma titres with values less than 10% of those obtained after immunization with a digoxin-protein conjugate. However, although subsequent injection of the hapten (40 micrograms kg-1, [14C]benzylpenicillin, i.v.) was associated with much lower increases and decreases in plasma and biliary radioactivity, respectively, they were still statistically significant. It appears that endogenously-formed drug-specific antibodies, when present in the blood, will inhibit drug distribution and elimination. It is unlikely that their secretion in the bile plays a significant role in mediating biliary drug hapten elimination.