A radioimmunoassay for the measurement of plasma prednisolone has been developed and validated. It has been used to investigate the effect of several gastrointestinal disorders on the absoption and bioavailability of orally and rectally administered prednisolone.

A weight-related dose of prednisolone was administered orally under similar conditions to healthy subjects and to patients suffering from. Crohn's disease and coeliac disease. Intra- and inter-subject variation in all pharmacokinetic parameters were observed, but no differences were found in the pattern of absorption or plasma prednisolone levels between either group with bowel disease (irrespective of activity) and the healthy subjects.

Administration of antacids or substitution with an enteric-coated preparation, are sometimes tried in patients who suffer from dyspepsia whilst receiving plain prednisolone. Antacid mixtures containing either magnesium trisilicate or aluminium hydroxide did not impair the absorption or bioavailability of orally administered prednisolone.

The bioavailability of the most widely used preparation of enteric-coated prednisolone was found to be similar to that of the plain form. The irregular absorption previously reported, was considered to be due to the formulation of the enteric-coating.

The amount of prednisolone appearing in the peripheral circulation following administration of prednisolone-21-phosphate as a retention enema to patients suffering from proctocolitis, was calculated to be approximately 44% of that after the same dose of the drug had been given by mouth. However, even in patients who had received prolonged therapy, the adrenocortical response to ACTH was within normal limits. After 20 mg of prednisolone had been given as the metasulphobenzoate ester in the form of a retention enema, only small quantities of the steroid were detectable in the peripheral circulation. Nevertheless, it seemed to be as effective as prednisolone-21-phosphate for the treatment of proctocolitis. The beneficial effect of rectally administered prednisolone is probably due predominantly to a local rather than systemic action.