Rectal pharmacokinetics of budesonide

Abstract

The pharmacokinetics and systemic availability of budesonide after rectal administration of two single enema doses (2 mg in 100 ml fluid of almost identical composition) were compared in 15 healthy volunteers. In 11 of these subjects, 2 mg oral budesonide in a gelatine capsule was given on a separate occasion. An intravenous administration (0.5 mg) was given as reference. With this design, individual hepatic bypass of the rectally administered budesonide dose could be estimated. The pharmacokinetics of the two enema formulations were similar, although not bioequivalent. Mean systemic availability was 16% (range 4.2–43%) and 15% (3.2–50%) after rectal administration and 6.3% (2.4–10%) after oral administration. The rectal data revealed a small intra- but a substantial inter-subject variability in systemic availability. C_max was 3.3 nmol·l^-1 (0.95–8.2), 3.0 nmol·l^-1 (0.64–8.9) and 1.3 nmol·l^-1 (0.61–3.0), respectively, for the three formulations. Absorption was rapid and essentially terminated within 3 h after rectal dosing [t_max=1.3 h for both formulations (range 0.5–2.0)], but was slower after oral dosing [t_max=2.1 h (1.0–6.0)]. If a complete absorption after oral and rectal dosing is assumed, the fraction of the rectal dose entering the liver at first pass can be calculated to be 88% (55–99%).

The higher systemic availability and intersubject variability after rectal dosing does not seem to be caused by differences in first-pass liver metabolism but rather by hepatic bypass of a varying portion of administered drug. This portion seems to be typical for an individual and might be explained by anatomical differences between subjects.