J . Med. Chem. zyxwvu 1995,38, 2239-2243 2239 zyxwvutsrqp Identification of 3,5-Dihydro-2-aryl-~-pyrazolo[3,4-c]quinoline-1,4(2H)-diones as Novel High-Affinity Glycine Site N-Methyl-D-aspartate Antagonists zyxwvutsrqponmlkjihgfedcba Angus M. MacLeod," Sarah Grimwood, Cheryl Barton, Linda Bristow, Kay L. Saywell, George R. Marshall, and Richard G. Ball Merck, Sharp and Dohme Research Laboratories, Neuroscience Research Centre, Terlings Park, Eastwick Road, Harlow, Essex CM20 2QR, U.K., and Merck Research Laboratories, Rahway, New Jersey 07065 Received February 21, 1995@ Almost all of the exisiting known antagonists at the glycine site of the N-methyl-D-aspartate (NMDA) receptor have a low propensity for crossing the blood-brain barrier. It has been suggested that in many cases this may be due to the presence of a carboxylic acid which is a common feature of most of the potent full antagonists at this receptor. In this study, 2-aryl- VI-pyrazolo[3,4-clquinoline-l,4(fL'J-n-diones were found to have high-affinity binding at the glycine receptor. In particular, structure-activity studies identified 7-chloro-3,5-dihydro-2- ~4-methoxyphenyl)-V-l-pyrazolo~3,4-clquinoline-1,4(~-dione as the most potent of a series of analogues with an 1% of 3.3 zyxwvu nM. "he measured pKa values in this class of compounds (typically 4.0) indicate they are of equivalent acidity to carboxylic acids. Functional antagonism was demonstrated by inhibition of NMDA-evoked responses in rat cortical slices. Anticonvulsant activity in DBN2 mice was achieved afier dosing by direct injection into the cerebral ventricles, but no activity was seen after systemic administration, suggesting low brain penetration with this class of antagonists. Introduction Antagonists of the N-methyl-D-aspartate (NMDA) receptor have attracted considerable attention in recent years as potential therapeutic agents for the treatment of a variety of neurological and neurodegenerative disorders. Many groups working in this area have focused on the neuroprotective activity of NMDA an- tagonists which may be clinically useful in reducing brain damage in victims of stroke. The NMDA receptor controls activity of a ligand-gated ion channel which, for activation, requires simultaneous binding of the endogenous ligand (L-glutamate) and glycine, which acts as a coagonist. Several different approaches have been taken to inhibit activity of this receptor, and chemical entities have been described which achieve this by (a) competitively antagonizing binding of L-glutamate;l (b) noncompetitively inhibiting the receptor by blocking the open ion channel;2 (c) antagonism of the coagonist gly~ine;~ and (d) modulating binding of the endogenous ligands by action at allosteric sites on the receptor pr~tein.~ Compounds from each of these categories have now entered clinical trials5 for the treatment of stroke, and the results of these studies are awaited with interest. Our interest in glycine site NMDA antagonists has been prompted by the observation that the low-efficacy partial agonist L-687,414 (la) is neuroprotective6 in zyxwvut vivo but does not cause the adverse side effects seen with other types of NMDA receptor antagonists such as the channel blocker dizocilpine (MK801). This partial ago- nist has relatively weak (IC5,, 1.4 pM) affinity at the glycine binding site, and although compounds with high receptor affinity have been described, such as 5-iodo-7- chlorokynurenic acid (2),7 L-689,560 (3),8 and MDL 29,- 951 (4),9 the central nervous system (CNS) activity of these highly polar carboxylic acids after systemic dosing is extremely weak because of their limited blood-brain @Abstract published in Advance ACS Abstracts, May 15, 1995. QQ22-2623/95/1838-2239$Q9.QQlQ barrier penetration. A recent reportlo from our labo- ratories described a novel series of 3-substituted 4-hy- droxyquinolin-2( W)-ones (e.g., 5) as selective glycine antagonists which possess potent centrally mediated in vivo activity after oral administration. As a continua- tion of this work, we sought to find other glycine site ligands with high receptor aEnity but which do not contain a carboxylic acid residue. In this paper we describe the identification of a novel class of high- affinity pyrazoloquinolone glycine antagonists and dis- cuss the results of in vivo anticonvulsant experiments. R,. HzNa**q-oH ! z ~ zyxwvutsrqponml 0 WNQ &y COzH CI ''°C02H H CI H C02H H CI H (4) (5) Biology Minities of the compounds in this study for the glycine site on the NMDA receptor were determined by displacement of binding of the glycine site antagonist [3H]-3 to rat cortical membranes.ll IC50 values were zyxwvutsrqp 0 1995 American Chemical Society