Pharmacokinetic studies of passively administered ovine anti-testosterone antibodies given to cattle by the subcutaneous and intravenous routes.

Passive immunization of cattle with ovine anti-testosterone antiserum can result in an increased ovulation rate, but the effect is variable and may be influenced by the route of administration. Investigations were made into the pharmacokinetics of these antibodies in cattle when given intravenously (i.v.), subcutaneously (s.c.) or via a combination of these two routes. Serum levels of free residual binding sites were measured by testosterone radioimmunoassay, whilst total circulating ovine IgG was determined using a specific enzyme-linked immunosorbent assay which shows no cross-reactivity with bovine IgG. The biological half-life of the administered antibodies was longer when it was calculated by measuring titre than when it was calculated by measuring IgG. Subcutaneous injection resulted in a significantly longer half-life of IgG than intravenous injection or the combined route, with a concomitant increase in the area under the curve. No significant differences between the half-lives as measured by titre were noted following the various routes of administration, but the mean value following s.c. injection was longest. The choice of route of administration of antiserum for passive immunization can be used to control the timing and duration of effective antibody levels. The results of the present study suggest that the s.c. or combined i.v. and s.c. routes are the preferred methods of passive immunization if an effect of long duration is required. It may be that it is the period over which the maximum level is maintained, rather than the absolute maximum level, which is important for successful immunomodulation of ovulation rate.