Synthesis and pharmacological profile of a series of 1-substituted-2-carbonyl derivatives of Diphenidol: novel M4 muscarinic receptor antagonists.

Novel 2-carbonyl analogues of diphenidol (1) - bearing lipophylic 1-substituents (2) - were synthesized starting from previously investigated diphenidol derivatives acting as M(2)-selective muscarinic antagonists. These compounds were tested for receptor binding affinity versus human muscarinic M(1)-M(5) receptors stably expressed in CHO-K1 cells. Their activity in functional assays carried out on CHO-K1 cells expressing human M(4) receptors (CHO-hM(4)) and on classical models of M(1)-M(3) receptors, in guinea pig and rabbit tissue preparations, was also evaluated. Compound 2d showed an affinity of pK(i) = 7.73 at the human M(4)-receptor subtype with selectivity ratios ranging from 31-fold (M(4)/M(5)) to 60-fold (M(4)/M(2)). Interestingly this compound, in CHO-hM(4) cells, blocked the inhibition of forskolin-activated cAMP accumulation produced by carbachol (IC(50)= 61 nM) whereas it was a weak muscarinic antagonist in functional tests carried out in guinea-pig and rabbit tissue expressing M(1) (pK(b) = 5.96), M(2) (pK(b) = 6.43) and M(3) (pK(b) = 6.09) receptors. In conclusion, the modifications performed in this work on reference compounds led us to obtain surprisingly a M(4) selective antagonist. Considering the therapeutic indications for M(4) selective antagonists, compound 2d may serve as a novel lead compound for further optimization.