Design of specific Molecular Probes for a Prostacyclin Receptor

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Isocarbacyclin, a stable carbocyclic analog of prostacyclin, has efficiently been synthesized by combination of rationally designed organic reactions starting a universal intermediate readily obtained by our three-component coupling process. An efficient photoaffinity probe, APNIC,for the PGI_2 receptor (IP_1) has been elaborated by modification of isocarbacyclin. APNIC has proven to exhibit a strong and high specific affinity with the PGI_2 receptor protein (IC_<50>=3nM) in mastocytoma P-815 cells and act as a PGI_2 receptor agonist. The photoaffinity labeling experiment allowed for the first time the idenification of the PGI_2 receptor proteins of approximately 43 kDa (mastocytoma cells), 45 kDa (porcine platelets), and 52 kDa (human platelets) as glycoproteins. We discovered a novel prostacyclin (PGI_2) receptor (IP_2) expressed in the central nervous system (CNS) and succeeded in creating a stable ligand, (15R) -16-m-tolyl-17,18,19,20-tetranorisocarba- cyclin (15R-TIC), with high b … More inding affinity and selectivity for this receptor. 15R-TIC exhibited high binding affinity (IC_<50>=30nM) for a novel PGI_2 receptor (IP_2) in the thalamus but showed very weak binding (IC_<50>=1.2muM) for the PGI_2 receptor (IP_1) in the NTS (nucleus tractus solitarius). We examined the reaction of methyl iodide and tributylphenylstannane (excess), giving toluene, with the focus on the realization of rapid coupling applicable to short-lived ^<11>C radionuclide (t_<1/2>=20min). Although methylation by usual Stille reaction conditions failed. We accomplished this methylation in >90% yield using 40 equiv of tributylphenylstannane within 5 min at 60 ﾟC in the presence of bis(tri-o-tolylphosphine) palladium (0), Cu (I) salt, and K_2CO_3 in DMF.This new protocol provided a firm chemical basis for the synthesis of ^<11>CH_3-incorporated PET tracers and, actually, it was successfully applied to the synthesis of [^<11>C] 15R-TIC which realized clear imaging of the IP_2 receptor in the living monkey brain. Less