Objective: To inform the target activity profile of future movement disorder therapies, pre-clinical studies quantified the contribution of M4 receptor occupancy (M4RO) to reversal of muscle rigidity in the rat Haloperidol Induced Catalepsy (HIC) model. Activity of three M4 selective antagonists of various chemotypes were characterized at maximal levels of striatal M4RO and compared to a pan muscarinic antagonist at equivalent M4RO to quantify the M4 effect relative to that of multiple subtypes.

Background: Cholinergic hyperactivity is implicated in the pathophysiology of movement disorders and consistent with the effectiveness of muscarinic antagonists in treating dystonia and Parkinson’s Disease. Supported by efficacy in pre-clinical models, selective M4 antagonists are currently being explored with the goal of maintaining efficacy and improving tolerability in the treatment of movement disorders.  Multiple genetic and pharmacologic datasets, however, also attribute efficacy to M1, 2 and 3 receptors, suggesting inhibition beyond M4 may be required to completely reset the striatal neurochemical imbalance. These preclinical studies were designed to understand the relationship between M4RO and efficacy in the presence and absence of additional muscarinic receptor antagonism, with the goal of profiling target receptor activity(ies) required for new therapies.

Method: IC50 values for three M4 selective and a pan-muscarinic antagonist were calculated in rat striatum using in vitro displacement binding against [3H] ‐Pirenzepine (M1R) and [3H]‐AF‐DX‐384 (M4R) in brain slices. M1 and M4 RO were assessed ex vivo; brain and plasma exposure were determined using LC-MS/MS. Efficacy in the HIC model was evaluated at 3 concentrations per molecule in triplicate, using 8-10 rats per group.

Results: All compounds tested achieved a maximal receptor occupancy of ~ 55-70% at the M4 receptor. Despite relatively low M4RO, pan muscarinic antagonist achieved > 95% reversal of HIC, with contribution from M1RO, while maximal M4 selective reversal was <70%.

Conclusion: Muscarinic antagonism of M4R and M1R is superior to M4 selective antagonism in addressing neurochemical imbalance in models of dystonia and PD; therapies engaging multiple muscarinic subtypes are likely to provide broader efficacy than selective molecules if tolerability can be improved.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/muscarinic-receptor-antagonism-beyond-m4-is-required-for-efficacy-in-a-preclinical-model-of-dystonia-and-parkinsons-disease/