Synthesis and biodistribution of iodine-125-labeled 1-azabicyclo[2.2.2]oct-3-yl alpha-hydroxy-alpha-(1-iodo-1-propen-3-yl)-alpha-phenylacetate. A new ligand for the potential imaging of muscarinic receptors by single photon emission computed tomography.

1-Azabicyclo[2.2.2]oct-3-yl alpha-hydroxy-alpha-(1-iodo-1-propen-3-yl)- alpha-phenylacetate (IQNP, 3), an analogue of QNB in which a phenyl ring has been replaced with an iodopropenyl substituent, was prepared and evaluated in vitro and in vivo for m-AChR selectivity and specificity. High specific activity [125]IQNP ([125I]-3) was synthesized in greater than 60% yield utilizing an electrophilic iododestannylation reaction with hydrogen peroxide for the oxidation of iodide. In in vitro receptor binding studies, 3 demonstrated high affinity for M1 (Ki = 0.78 nM), M2 (Ki = 1.06 nM), and M3 (Ki = 0.27 nM) subtypes. In vivo biodistribution studies in female rats [125I]-3 demonstrated high uptake in areas rich in muscarinic receptors such as the brain (cortex and striatum) and the heart. Blocking studies were performed with a series of receptor specific agents and demonstrated that the uptake of [125I]-3 was selective and specific for cerebral muscarinic receptor rich areas and that the binding to m-AChR is reversible. The high-yield preparation and specificity and selectivity of high specific activity [125I]IQNP for muscarinic receptors suggest that this is an attractive new agent for potential imaging of cerebral receptors using single photon tomographic imaging (SPECT).