The pharmacokinetics of aspirin in rats and the effect of buffer

Aspirin (acetylsalicyclic acid) was administered to rats intravenously, orally, and intraintestinally at different doses or in different dosage forms. The distribution and elimination kinetics of aspirin in rats following intravenous administration were best described by a two-compartmental open system and were dose independent up to 15 mg/kg. The terminal elimination half-life following intravenous dosing (10 mg/kg) was 3.36 +/- 0.85 min (n = 15) with the clearance being 8.40 +/- 1.24 L/(kg.hr). Intravenous distribution and elimination kinetics of aspirin in rats were not influenced by an orally administered buffered solution with a buffer capacity of 0.933 mEq ANC (acid neutralizing capacity) per kg of body weight. However, this orally buffered solution did change the gastrointestinal absorption kinetics of aspirin in rats. The absolute bioavailable dose of aspirin was 56.6 +/- 10.4% (n = 6) following its administration in an unbuffered solution while it was only 31.8 +/- 8.0% (n = 6) following administration in the buffered solution. The corresponding values of the absolute bioavailable doses were 43.4 +/- 3.7% and 25.5 +/- 1.8% following intraintestinal administration. The lower systemic availability of aspirin in the presence of buffer is attributed to a greater fraction of the administered dose becoming available for absorption from the intestine where the extraction efficiency is higher than that in the stomach.