2-SUBSTITUTED MINO-2-((1SR,2SR)-2-CARBOXYCYCLOPROP-1-YL)GLYCINES AS POTENT AND SELECTIVE ANTAGONISTS OF GROUP-II METABOTROPIC GLUTAMATE RECEPTORS - 2 - EFFECTS OF AROMATIC-SUBSTITUTION, PHARMACOLOGICAL CHARACTERIZATION, AND BIOAVAILABILITY

In this paper we describe the synthesis of a series of alpha-substitut ed analogues of the potent and selective group II metabotropic-glutama te receptor (mGluR) agonist (1S,1'S,2'S)-carboxycyclopropylglycine (2, L-CCG 1). Incorporation of a substituent on the amino acid carbon con verted the agonist 2 into an antagonist. All of the compounds were pre pared and tested as a series of four isomers, i.e., two racemic diaste reomers, On the basis of the improvement in affinity realized for the alpha-phenylethyl analogue 3, in this paper we explored the effects of substitution on the aromatic ring as a strategy to increase the affin ity of these compounds for group II mGluRs. Affinity for group II mGlu Rs was measured using [H-3]glutamic acid (Glu) binding in rat forebrai n membranes. Antagonist activity was confirmed for these compounds by measuring their ability to antagonize (1S,3R)-1-amino cyclopentane-1,3 -dicarboxylic acid-induced inhibition of forskolin stimulated cyclic-A MP in RGT cells transfected with human mGluR2 and mGluR3. Meta substit ution on the aromatic ring of 3 with a variety of substituents, both e lectron donating (e.g., methyl, hydroxy, amino, methoxy, phenyl, pheno xy) and electron withdrawing (e.g.,fluorine, chlorine, bromine, carbox y, trifluoromethyl) gave from 1.5- to 4.5-fold increases in affinity. Substitution with p-fluorine, as in 97 (IC50 = 0.022 +/- 0.002), was t he exception. Here, a greater increase in affinity was realized than f or either the ortho-or meta-substituted analogues; 97 was the most pot ent compound resulting from monosubstitution of tile aromatic. At best , only modest increases in affinity were realized for certain compound s bearing either two chlorines or two fluorines, and two methoxy group s gave no improvement in affinity (all examined in a variety of substi tution patterns). Three amino acids, 4, 5, and 104, were resolved into their four constituent; isomers, and affinity and functional activity for group II mGluRs was found to reside solely in the S,S,S-isomers o f each, consistent with I. With an IC50 = 2.9 +/- 0.6 nM, the resolved xanthylmethyl compound 168 was the most potent compound from this SAR . Amino acid 168 demonstrated high plasma levels following intraperito neal (ip) administration and readily penetrated into the brain, This c ompound, however, had only limited (similar to 5%) oral bioavailabilit y. Systemic administration of 168 protected mice from limbic seizures produced by the mGluR agonist 3,5-dihydroxyphenylglycine, with an ED50 = 31 mg/kg (ip, 60 min preinjection). Thus, 168 represents a valuable tool to study the role of group II mGluRs in disease.