Synthesis and Pharmacology of Benzoxazines as Highly Selective Antagonists at M4 Muscarinic Receptors

Previously, we reported on PD 102807 (41) as being the most selective synthetic M4 muscarinic antagonist identified to date. Synthesized analogues of 41 showed no improvement in affinity and selectivity at that time. However, several newly synthesized compounds exhibit a 7-fold higher affinity at M4 receptors and demonstrate a selectivity of at least 100-fold over all other muscarinic receptor subtypes. For example, compound 28 showed an affinity of pK i = 9.00 at M4 receptors and a selectivity of M1/M4 = 13 183-fold, M2/M4 = 339-fold, M3/M4 = 151-fold, and M5/M4 = 11 220-fold. This high selectivity along with high affinity has not been reported for any synthetic muscarinic antagonist, nor for natural occurring M4 antagonists such as the M4 selective Eastern Green Mamba venom MT3 (M4 pK b = 8.7, M1/M4 = 40-fold, M2/M4 ≥ 500-fold, M3/M4 ≥ 500-fold, and M5/M4 ≥ 500-fold). Derivative 24, a compound with a high selectivity pattern as well, has been tested for in vivo efficacy. It was able to block the l-3,4-dihydroxyphenylalanine accumulation produced by CI-1017, an M1/M4 selective muscarinic agonist, in the mesolimbic region and striatum, which suggests that 24 is capable of crossing the blood−brain barrier and confirms the pharmacokinetic data obtained on this compound. This is evidence that suggests that agonist-induced increase in catecholamine synthesis observed in these regions is mediated by M4 receptors.