123I-Iododexetimide Preferentially Binds to the Muscarinic Receptor Subtype M1 In Vivo

The muscarinic M-1 receptor (M1R) is highly involved in cognition, and selective M-1 agonists have procognitive properties. Loss of M1R has been found in postmortem brain tissue for several neuropsychiatric disorders and may be related to symptoms of cognitive dysfunction. I-123-iododexetimide is used for imaging muscarinic acetylcholine receptors (mAchRs). Considering its high brain uptake and intense binding in M1R-rich brain areas, I-123-iododexetimide may be an attractive radiopharmaceutical to image M1R. To date, the binding affinity and selectivity of I-123-iododexetimide for the mAchR subtypes has not been characterized, nor has its brain distribution been studied intensively. Therefore, this study aimed to address these topics. Methods: The in vitro affinity and selectivity of I-127-iododexetimide (cold-labeled iododexetimide), as well as its functional antagonist properties (guanosine 5'-[gamma-35S-thio] triphosphate [GTP gamma S-35] assay), were assessed on recombinant human M1R-M5R. Distributions of I-127-iododexetimide and I-123-iododexetimide in the brain were evaluated using liquid chromatography-mass spectrometry and storage phosphor imaging, respectively, ex vivo in rats, wild-type mice, and M-1-M-5 knock-out (KO) mice. Inhibition of I-127-iododexetimide and I-123-iododexetimide binding in M1R-rich brain areas by the M1R/M4R agonist xanomeline, or the antipsychotics olanzapine (M1R antagonist) and haloperidol (low M1R affinity), was assessed in rats ex vivo. Results: In vitro, I-127-iododexetimide displayed high affinity for M1R (pM range), with modest selectivity over other mAchRs. In bio-distribution studies on rats, ex vivo I-127-iododexetimide binding was much higher in M1R-rich brain areas, such as the cortex and striatum, than in cerebellum (devoid of M(1)Rs). In M-1 KO mice, but not M-2-M-5 KO mice, I-127-iododexetimide binding was strongly reduced in the frontal cortex compared with wild-type mice. Finally, acute administration of both an M1R/M4R agonist xanomeline and the M1R antagonist olanzapine was able to inhibit I-123-iododexetimide ex vivo, and I-123-iododexetimide binding in M-1-rich brain areas in rats, whereas administration of haloperidol had no effect. Conclusion: The current results suggest that I-123-iododexetimide preferentially binds to M1R in vivo and can be displaced by M1R ligands. I-123-iododexetimide may therefore be a useful imaging tool as a way to further evaluate M1R changes in neuropsychiatric disorders, as a potential stratifying biomarker, or as a clinical target engagement biomarker to assess M1R.