Caramiphen, iodocaramiphen and nitrocaramiphen are potent, competitive, muscarinic M1 receptor-selective agents

Caramiphen, iodocaramiphen and nitrocaramiphen were examined for affinity at the muscarinic M1, M2 and M3 receptor subtypes in radioligand binding assays. Caramiphen binds with high affinity at the M1 site labeled by [3H]pirenzepine in rat cortex (Ki = 1.2 nM) and displays a 27-fold greater preference for the M1 than the M2 site labeled by [3H](-)-quinuclidinyl benzilate in rat heart, and a 6-fold greater preference for the M1 than the M3 site labeled by [3H]N-methylscopolamine in rat submaxillary gland. Iodocaramiphen binds with high affinity (Ki = 2.1 nM) and selectivity (59-fold) for the M1 vs. M2 subtype, and is 4-fold more selective for the M1 vs. M3 site. Nitrocaramiphen binds with high affinity for M1 sites (Ki = 5.5 nM) and with a 71-fold selectivity over M2, and a 10-fold selectivity for the M1 over the M3 subtype. All three compounds interacted with the M1 binding site in a competitive manner. Nitrocaramiphen and iodocaramiphen are as potent and showed a comparable selectivity for binding to the M1 over the M2 site than the prototypical agent pirenzepine (M1; Ki = 5.2 nM, 51-fold selectivity). Additionally, nitrocaramiphen demonstrates at least a 10-fold selectivity for the M1 over the M3 site. These ester-type antimuscarinics may be better ligands for the study of M1 receptors in brain than the hydrophilic agent pirenzepine.