Summary of Studies Showing Age-Differences in Pharmacokinetics: Bladder Anticholinergics
The potently anticholinergic drug oxybutynin follows the trend of increasing peak plasma levels and bioavailability with increasing age and frailty119. This effect is so significant that study authors suggested halving the dose of oxybutynin so older adults to achieve the same plasma levels as younger adults. AUC and CMax are increased 20% and 16% respectively when an older population is given the same dose of oxybutynin as a younger population. Moreover, solifenacin, a newer bladder anticholinergic, has a longer t1/2 due to slower elimination and longer time to reach CMax in older adults. This can be explained by the slowed absorption of solifenacin in older adults which increases their exposure to solifenacin by about 1.2-fold99. In a study of 16 young men, 16 older men and 16 older women, receiving either 8 mg of fesoterodine extended release or matching placebo, the renal clearance of fesoterodine was 28% lower in older men and women than younger men97 (table 3). This increased exposure to fesoterodine in older adults may predict increased exposure of tolterodine in older adults as well, as fesoterodine and tolterodine are related compounds, with both being metabolized to the same active ingredient.