Molecular design of prostaglandin probes in brain research:: High, specific binding to a novel prostacyclin receptor in the central nervous system

Molecular design to develop a stable biochemical probe for a study of the role of prostacyclin (PGI(2)) in the brain led to the discovery of (15R)-16-m-tolyl-17, 18, 19,20-tetranorisocarbacylin (referred to as 15R-TIC), that selectively bind to a novel PGI(2) receptor, IP2, expressed in the central nervous system (CNS). This artificial prostaglandin with the 15R configuration exhibits high binding affinity for the IP2 receptor in the thalamus (IC50 = 32 nM) and weak affinity for the peripheral-type PGI(2) receptor, IP1, in the NTS (IC50 = 1.2 mu M). The length of the omega side-chain and the position of the methyl substituent on the aromatic ring strongly influence the binding characteristics. The features of the IP2 receptor were elucidated by quantitative mapping, specificity studies, and Scatchard analysis, as well as by a study using knockout mice with a tritium-labeled 15R-TIC and related radioligands. In order to conduct in vivo PET studies, a rapid methylation reaction using methyl iodide and an excess amount of an aryltributylstannane has been developed. This has successfully been applied to the synthesis of short-lived C-11-incorporated PET tracers, 15R-[C-11]TIC and its methyl eater. The PET experiments accomplished the imaging of the IP2 receptor in the brain of living rhesus monkeys through intravenous administration. The elimination of the C(15) chirality results in 15-deoxy-TIC with ten-fold higher affinity and selectivity for the IP2 receptor than original ISR-TIC. Neither 15R-TIC nor 15-deoxy-TIC inhibit platelets aggregation, up to 400 nM, while PGI(2) derivatives which bind with the IP1 receptor show a very potent inhibitory effect at a several nM level. Notably, these artificial CNS-specific PGI(2) ligands, like the unstable natural PGI(2) itself, prevent the apoptotic cell death of hippocampal neurons induced under high (50%) oxygen atmosphere and by xanthine and xanthine oxidase or serum deprivation. The difference in the binding potency between 15R-TIC and 15-deoxy-TIC for the IP2 receptor correlates well with the extent of the prevention of the neuronal cell death (IC50 values of 300 and 30 nM, respectively, under high oxygen atmosphere). 15R-TIC protects CA1 pyramidal neurons against ischemic damage in gerbils. Thus the designed TICs have neuronal survival-promoting activity both in vitro and in vivo, providing the possibility as a new type of chemotherapeutic agents for applications in neurodegeneration.