DEFINING THE MINIMAL STRUCTURAL REQUIREMENTS FOR PARTIAL AGONISM AT THE TYPE-I MYOINOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR

The novel synthetic analogues D-3-fluoro-myo-inositol 1,5-bisphosphate -4-phosphorothioate, [3F-Ins(1,5)P-2-4PS], D-3-fluoro-myo-inositol 1,4 -bisphosphate-5-phosphorothioate [3F-Ins(1,4)P-2-5PS], and D-3-fluoro- myo-inositol 1-phosphate-4,5-bisphosphorothioate [3F-Ins(1)P-(4,5)PS2] were utilised to define the structure-activity relationships,which co uld produce partial agonism at the Ca2+ mobilising myo-inositol 1,4,5- trisphosphate [Ins(1,4,5)P-3] receptor. Based on prior structure-activ ity data we hypothesised that the minimal structural requirements for Ins(1,4,5)P-3 receptor partial agonism, mere phosphorothioate substitu tion of the crucial vicinal 4,5-bisphosphate pair accompanied by anoth er structural perturbation, such fluorination of 3-position of the myo -inositol ring. All the analogues fully displaced [H-3]Ins(1,4,5)P-3 f rom a single Ins(1,4,5)P-3 binding site in pig cerebellar membranes [3 F-Ins(1,5)P-2-4PS (IC50 = 26 nM), 3F-Ins(1,4)P-2-5PS (IC50 = 80 nM) an d 3F-Ins(1)P(4,5)PS2 (IC50 = 109 nM) cf. Ins(1,4,5)P-3 (IC50 = 11 nM)] . In contrast, 3F-Ins(1,5)P-2-4PS (IC50 = 424 nM) and 3F-Ins(1,4)P-2+- 5PS (IC50 = 3579 nM) were weak full agonists at the Ca2+ mobilising In s(1,4,5)P-3 receptor of permeabilised SH-SY5Y neuroblastoma cells, bei ng respectively 4- and 36-fold less potent than Ins(1,4,5)P-3 (EC(50) = 99 nM). While 3F-Ins(1)P-(4,5)PS2 (EC(50) 11345 nM) was a partial ag onist releasing only 64.3 +/- 1.9% of the Ins(1,4,5)P-3-sensitive intr acellular Ca2+ pools. 3F-Ins(1)P-(4,5)PS2 was unique among the Ins(1,4 ,5)P-3 receptor partial agonists so far identified in having a relativ ely high affinity for the Ins(1,4,5)P-3 binding site, accompanied by a significant loss of intrinsic activity for Ca2+ mobilisation. This im proved affinity was probably due to the retention of the 1-position ph osphate, which enhances interaction with the Ins(1,4,5)P-3 receptor. 3 F-Ins(1)P-(4,5)PS2 may be an important lead compound for the developme nt of efficient Ins(1,4,5)P-3 receptor antagonists.