Development of a pharmacological target profile for muscarinic agonists

The purpose of these studies was to determine the levels of intrinsic activity in vitro at muscarinic receptor pharmacological subtypes (M₁, M₂, and M₃) that would optimize cognitive activity while minimizing acute adverse cholinergic effects in vivo. These levels of intrinsic efficacy at M₁, M₂, and M₃ receptor subtypes, the target profile, could then be used to assess novel compounds for antidementia potential. To accomplish this, a series of muscarinic agonists (spirofuranone analogues) was prepared, and compounds with variable intrinsic activity at M₁, M₂, and M₃ receptor subtypes were selected for characterization in a series of in vivo tests (reversal of scopolamine-induced memory impairment, and agonist-induced salivation, lacrimation, diarrhea, hypothermia, and bradycardia). Memory enhancement was found to correspond best to M₁ activity with an apparent threshold of intrinsic activity of approximately 50% in human M₁-transfected CHO cells. Surprisingly, a preferential M₃ agonist (ARL 16037) produced a partial reversal of scopolamine-induced memory impairment, but only at high doses that resulted in severe diarrhea. However, it was unclear whether this memory effect was due to the weak M₁ or strong M₃ agonist activity of this compound. Three muscarinic agonists (ARL 14995, ARL 15467, and ARL 15424) reversed the scopolamine-induced memory deficits, as well as the cholinesterase inhibitor tacrine, suggesting that muscarinic agonists may be at least as clinically effective as cholinesterase inhibitors. Acute cholinergic side effects (diarrheal salivation, lacrimation) corresponded best to M₃ activity (contraction of guinea pig trachea) and were greatly diminished with in vitro intrinsic M₃ activity of 50% or less. Agonist-induced heart rate reduction corresponded best to M₃ receptor efficacy (inhibition of rat heart adenylyl cyclase). Identification of compounds without detectable in vitro intrinsic activity but with measurable heart rate effects suggested that signal amplification at M₁ receptors in vitro limited detection of weak partial agonists. Development of the target profile has proven useful in rapid evaluation of compounds for optimization of muscarinic pharmacological properties.