Which is TRUE regarding anticholinergic medications?A. Oxybutynin causes direct smooth muscle relaxation via calcium channel blockade
M2 receptors are more abundant in the detrusor but M3 receptors are functionally more important. Anticholinergics work at muscarinic receptors to reduce the response to cholinergic stimulation, thus reducing detrusor contractions and detrusor pressure during filling. Oxybutynin has some selectivity for M1 and M3 receptor subtypes as well as direct smooth muscle relaxant affect, probably via calcium channel blockade. Randomised trials have confirmed the efficacy of oxybutynin at the expense of compliance due to side effect profiles. ER oxybutynin has comparable efficacy to immediate release but improved tolerability.
Tolterodine is not receptor specific but has greater affinity for the bladder compared to other organs. IR tolterodine has equivalent efficacy to oxybutynin with fewer side effects.
Solifenacin is an M3 receptor antagonist. The STAR trial was a prospective, doubleblind, 12-week study to compare efficacy and safety of solifenacin 5 or 10 mg and tolterodine ER 4 mg once daily in OAB patients. The study concluded that solifenacin had greater efficacy in decreasing urgency episodes, incontinence, urgency incontinence and pad usage and increasing the volume voided per micturition. Discontinuations were comparable and low in both groups.
Fesoterodine is a prodrug that is hydrolysed to the same active metabolite as tolterodine. [ER] = extended release; [IR] = immediate release; [OAB] = overactive bladder