Stereoselective synthesis, in vitro, and initial in vivo evaluation of 1-methylpiperidin-4-yl α-hydroxy-α-(1-iodo-1-propen-3-yl)-α-phenylacetate (IPIP): A novel radioiodinated molecular probe with high affinity for the muscarinic receptor

1-Methylpiperidin-4-yl α-hydroxy-α-(1-iodo-1-propen-3-yl)-α-phenylacetate (IPIP, Fig. 1) was investigated as a potential radioiodinated molecular probe targeted to the muscarinic receptor complex. The IPIP stereoisomers were synthesized via a chiral intermediate in >95% enantiomeric excess. The R-isomers demonstrated a M₁ to M₂ subtype selectivity of approximately 3 to 1 and the S-isomers demonstrated non-subtype selective binding in vitro. IPIP was radiolabeled with iodide-125 with an average radiochemical yield of 74.4% (±14.8, n = 5), specific activities >800 mCi/μmol, and radiochemical purities >97%. In vivo the Z-isomers demonstrated high uniform cerebral uptake suggesting non-subtype selective binding. In contrast, E-R-IPIP, after allowing a low uptake in M₂ rich areas to clear, demonstrated a retention of activity in M₁ and M₄ rich cerebral regions. In addition, the cerebral uptake of E-R-IPIP and Z-S-IPIP were inhibited by 70-90% via pretreatment with R-QNB, an established muscarinic antagonist. An ex vivo metabolism study demonstrated Z-S-IPIP was stable at the receptor site with an absence of radiolabeled metabolites.