Selectivity profile of the novel muscarinic antagonist UH‐AH 37 determined by the use of cloned receptors and isolated tissue preparations

Abstract

1 Functional in vitro experiments were carried out to determine the antimuscarinic potencies of the pirenzepine derivative UH-AH 37 (6-chloro-5,10-dihydro-5-[(1-methyl-4-piperidinyl)acetyl]-11H-dibenzo-[b,e] [1,4] diazepine-11-one hydrochloride) at M₁ muscarinic receptors of rabbit vas deferens, M₂ receptors of rat left atria and M₃ receptors of rat ileum. Furthermore, N-[³H]-methylscopolamine competition binding experiments were performed to obtain its affinities for the five cloned human muscarinic receptors (m1-m5) stably expressed in CHO-K1 cells. Pirenzepine served as a reference drug throughout all experiments. 2 In all preparations used, UH-AH 37 interacted with muscarinic receptors in a fashion characteristic of a simple competitive antagonist. 3 In the functional studies, UH-AH 37, like pirenzepine, showed high affinity for M₁ (pA₂ 8.49) and low affinity for M₂ muscarinic receptors (pA₂ 6.63). In contrast to pirenzepine, UH-AH 37 also displayed high affinity for M₃ receptors (pA₂ 8.04). 4 In agreement with its functional profile, UH-AH 37 bound with highest affinity to m1 (pK_i 8.74) and with lowest affinity to m2 receptors (pK_i 7.35). Moreover, it showed a 7 fold higher affinity for m3 (pK_i 8.19) than for m2 receptors, whereas pirenzepine bound to both receptors with low affinities. 5 The binding affinity of UH-AH 37 for m4 and m5 receptors (pK_i 8.32 for both receptors) was only ca. 2.5 fold lower than that for m1 receptors, while the corresponding affinity differences were 6 and 13 fold in case of pirenzepine. 6 In conclusion, the receptor selectivity profile of UH-AH 37 differs clearly from that of its parent compound, pirenzepine, in both functional and radioligand binding studies, the major characteristics being its pronounced M₂ (m2)/M₃ (m3) selectivity. UH-AH 37 thus represents a useful tool for the further pharmacological characterization of muscarinic receptor subtypes.