AMPA RECEPTOR AGONISTS - SYNTHESIS, PROTOLYTIC PROPERTIES, AND PHARMACOLOGY OF 3-ISOTHIAZOLOL BIOISOSTERES OF GLUTAMIC-ACID

A number of 3-isothiazolol bioisosteres of glutamic acid (1) and analo gs of the AMPA receptor agonist, amino-3-(3-hydroxy-5-methylisoxazol-4 -yl)propionic acid(AMPA,2a), including ino-3-(3-hydroxy-5-methylisothi azol-4-yl)propionic acid (thio-AMPA, 2b), were synthesized. Comparativ e in vitro pharmacological studies on this series of 3-isothiazolol an d the corresponding 3-isoxazolol amino acids were performed using a se ries of receptor binding assays (IC50 values) and the electrophysiolog ical rat cortical slice model (EC(50) values). Whereas 2a (IC50 = 0.04 +/- 0.005 mu M, EC(50) = 3.5 +/- 0.2 mu M) is markedly more potent th an the tert-butyl analog ATPA (3a) (IC50 = 2.1 +/- 0.16 mu M, EC(50) = 34 +/- 2.4 mu M) in [H-3]AMPA binding and electrophysiological studie s, 2b (IC50 = 1.8 +/- 0.13 mu M, EC(50) = 15.0 +/- 2.4 mu M) was appro ximately equipotent with thio-ATPA (3b) (IC50 = 0.63 +/- 0.07 mu M, EC (50) = 14 +/- 1.3 mu M) (RS)-2-Amino-3-(3-hydroxyisoxazol-5-yl)propion ic acid (HIBO, 4a) was approximately equipotent with its thio analog 4 b, whereas 4-Br-HIBO (5a) (IC50 = 0.65 +/- 0.12 mu M, EC(50) = 22 +/- 0.6 mu M) turned out to be much more potent than the corresponding 3-i sothiazolol 5b (IC50 = 17 +/- 2.2 mu M, EC(50) = 500 +/- 23 mu M). 2b (ED(50) = 130 mu mol/kg) was more patent than 2a (220 mu mol/kg) as a convulsant after subcutaneous administration in mice. The protolytic p roperties of 2a,b-4a,b were determined using C-13 NMR spectroscopy. Fo r each pair of compounds, the a-amino acid groups showed similar proto lytic properties, whereas the 3-isoxazolol moieties typically showed p K(a) values 2 units lower than those of the 3-isothiazolols. According ly, calculations of ionic species distributions revealed pronounced di fferences between 3-isoxazolol and 3-isothiazolol amino acids. No simp le correlation between activity as AMPA agonists in vitro and pK(a) va lues of these compounds was apparent. On the other hand, the relative potencies of AMPA (2a) and thio-AMPA (2b) in vitro and in vivo may ref lect that these compounds predominantly penetrate the blood-brain barr ier as net uncharged diprotonated ionic species.