Evidence of significant absorption of antipyrine from urinary bladder of rats.

We examined the in vivo absorption and pharmacokinetics of antipyrine, a base that is unionized at physiologic pH, from the urinary bladder of adult female Fischer rats. The clearance of an i.v. dose of antipyrine (25 mg/kg) was found to vary considerably between animals, which is consistent with the literature data. Therefore, it was necessary to simultaneously determine drug clearance and bladder absorption in the same animal. This was accomplished by giving concomitantly an i.v. dose of 14C]antipyrine (2.5 muCi, about 90 micrograms/kg) via a jugular vein catheter and an intravesical dose of unlabeled antipyrine (33 mg/kg) via a urethral catheter. Unlabeled antipyrine was detected in plasma, indicating the absorption of antipyrine into systemic circulation. The bioavailability of the intravesical dose was calculated using the clearance of 14C]antipyrine and the plasma concentrations of unlabeled antipyrine. The intravesical dose was withdrawn through the urethral catheter after 90 min. To minimize mechanical manipulation and damage, the bladder was not rinsed. This may have caused the incomplete recovery of the unabsorbed dose; about 65 +/- 18% (mean +/- S.D.) of the dose was recovered at 90 min. Maximal plasma concentrations were achieved at 10 to 48 min after removal of the intravesical dose, which is consistent with a continued absorption of the residual dose. The intravesical bioavailability was variable between animals, with an average of 11.6 +/- 6.4% (mean +/- S.D.; range, 4.1-19.2%). In conclusion, these data demonstrate that neutral drugs such as antipyrine are absorbed from the bladder, that the extent of absorption is variable and that the urinary bladder may be a site of significant re-entry of drugs into the systemic circulation.