Muscarinic receptor binding of fesoterodine, 5-hydroxymethyl tolterodine, and tolterodine in rat tissues after the oral, intravenous, or intravesical administration

Brain distribution and muscarinic receptor occupancy of an active metabolite of fesoterodine in rats

Several large-scale clinical trials have shown that long-term use of antimuscarinic agents for overactive bladder treatment increases the risk of dementia and that this risk varies between agents. Fesoterodine, an overactive bladder antimuscarinic, reportedly has no significant effect on cognitive function. Differences in the brain distribution of antimuscarinic agents and blockade of muscarinic receptors could be the reasons for the differences in central side effects among antimuscarinics. In this study, we assessed the brain distribution of antimuscarinic agents and muscarinic receptor occupancy using brain microdialysis and in vivo receptor-binding analysis in rats. The test drugs were 5-hydroxymethyl tolterodine (5-HMT), an active metabolite of fesoterodine, oxybutynin, and trihexyphenidyl, a central antimuscarinic agent. The brain/plasma unbound concentration ratio at steady state (Kpuu) of 5-HMT was much lower than that of oxybutynin and trihexyphenidyl, indicating very low 5-HMT distribution in the brain. The muscarinic receptor occupancy in the rat brain at clinical plasma concentrations of 5-HMT was rarely observed, whereas the occupancy by oxybutynin and trihexyphenidyl was 20% and 67%, respectively. The brain muscarinic receptor occupancy, reconstituted using Kpuu values and muscarinic receptor affinity retrieved from literature, revealed a significant correlation between the calculated and measured values. These results indicate that 5-HMT has low brain distribution and muscarinic receptor occupancy, which might be the reasons for the insignificant effect of fesoterodine on cognitive function. SIGNIFICANCE STATEMENT: This study reports that 5-hydroxymethyl tolterodine, the active metabolite of fesoterodine, has much lower brain distribution and muscarinic receptor occupancy compared with oxybutynin and trihexyphenidyl based on brain microdialysis and in vivo receptor-binding analysis with unlabeled muscarinic tracer in rats. The low brain distribution and muscarinic receptor occupancy of 5-hydroxymethyl tolterodine might be the reasons for the insignificant effect of fesoterodine on cognitive function.

Anticholinergic medication: Related dry mouth and effects on the salivary glands

OBJECTIVE

Salivary glands are among the most sensitive target organs of medications with anticholinergic (AC) properties, interrupting the neural stimulation of saliva secretion and reducing saliva flow. Hyposalivation results in dry mouth, leading to dental caries, intraoral infection, orofacial pain, problems with speaking and swallowing, and diminished oral health--related quality of life. Current understanding of the pharmacokinetics of AC medications and their effect on muscarinic receptors in the salivary glands were reviewed to assist clinicians in predicting salivary damage in patients with AC medication-induced dry mouth.

STUDY DESIGN

We summarized the literature related to the mechanisms and properties of AC medications, anticholinergic adverse effects, and their effect on salivary function and management strategies to prevent oral health damage.

RESULTS

Although a large number of studies reported on the frequencies of medication-induced dry mouth, we found very limited data on predicting individual susceptibility to AC medication--caused hyposalivation and no prospective clinical studies addressing this issue.

CONCLUSION

Dry mouth is most frequently caused by medications with AC properties, which interrupt the neural stimulation of saliva secretion. Interdisciplinary care should guide pharmacotherapeutics and dental interventions should aim in preventing AC salivary adverse effects and reducing the oral health burden from AC medication-induced dry mouth.

Direct in vitro and in vivo demonstration of muscarinic receptor binding by the novel radioligand, [3H]5-hydroxymethyltolterodine, in the bladder and other tissues of rats

In vitro and in vivo binding sites of [³H]-labeled 5-hydroxymethyltolterodine (5-HMT), a new radioligand for labeling muscarinic receptors in rat tissues were characterized. Specific [³H]5-HMT binding in rat tissues was saturable and of high affinity in each tissue. The dissociation constant (K_d) was significantly lower in bladder and heart than in submaxillary gland. Significant levels of in vivo specific [³H]5-HMT binding by intravenous injection of the radioligand were detected in tissues, except for cerebral cortex. Thus, [³H]5-HMT was shown to specifically label muscarinic receptors in rat tissues, suggesting a useful radioligand for labeling muscarinic receptors with high affinity.