Comparative aspirin absorption kinetics after administration of sodium- and potassium-containing buffered solutions

Twelve fasting normal volunteers received three aspirin dosage forms in a single 325-mg dose in a complete crossover study; the plasma aspirin and salicylic acid levels and the urine salicylic acid and salicyluric acid levels were measured over 10 h. The three dosage forms included an unbuffered tablet and two effervescent solutions, one with sodium bicarbonate-citrate buffer and the other with potassium bicarbonate-citrate buffer. A significantly faster absorption rate was observed with the sodium bicarbonate-citrate buffer, when compared with the potassium bicarbonate-citrate buffer and the unbuffered tablets, which were equivalent. These differences were attributed primarily to gastric emptying rate differences. Urine pH and salicylate renal clearance were significantly affected by the single dose of antacid buffer. The area under the curve and urine accumulation comparisons suggested that approximately 25% more aspirin reaches the general circulation intact after administration of the unbuffered tablet than the two solutions, but that the total salicylate absorbed is equivalent for all three dosage forms. This difference in aspirin bioavailability is probably due to the fact that the two buffered solutions are predominantly absorbed through the intestine, in which presystemic hydrolysis occurs, whereas a significant portion of the tablet dose is absorbed through the gastric mucosa.