Kinetics of Aspirin, Salicylic Acid, and Salicyluric Acid following Oral Administration of Aspirin as a Tablet and Two Buffered Solutions

Abstract

Twelve fasting normal volunteers received three aspirin dosage forms in a single-dose, complete crossover study; the plasma and urine levels of aspirin, salicylic acid, and salicyluric acid were measured for 10hr. The three dosage forms were an unbuffered tablet, an effervescent solution with 16mEq of buffer, and an effervescent solution with 34mEq of buffer. Significant differences in the absorption rate were observed, with the solution having 16mEq of buffer being fastest, the solution having 34mEq of buffer being intermediate, and the tablet being slowest. These differences are attributed to gastric emptying rates and tablet dissolution. Urine pH and renal clearance for all three acid compounds are influenced by the buffer during the first 2hr following dosing but not later. Area under the curve comparisons suggest that ~20% more aspirin reaches the general circulation intact following the tablet but that the total amount of salicylate absorbed is not different. Further studies are required to select the optimal buffer content to provide rapid absorption with minimal sodium dose and urine alkalinization.