A novel transient receptor potential C3/C6 selective activator induces the cellular uptake of antisense oligonucleotides.

Antisense oligonucleotide (ASO) therapy is a novel therapeutic approach in which ASO specifically binds target mRNA, resulting in mRNA degradation; however, cellular uptake of ASOs remains critically low, warranting improvement. Transient receptor potential canonical (TRPC) channels regulate Ca2+ influx and are activated upon stimulation by phospholipase C-generated diacylglycerol. Herein, we report that a novel TRPC3/C6/C7 activator, L687, can induce cellular ASO uptake. L687-induced ASO uptake was enhanced in a dose- and incubation-time-dependent manner. L687 enhanced the knockdown activity of various ASOs both in vitro and in vivo. Notably, suppression of TRPC3/C6 by specific siRNAs reduced ASO uptake in A549 cells. Application of BAPTA-AM, a Ca2+ chelator, and SKF96365, a TRPC3/C6 inhibitor, suppressed Ca2+ influx via TRPC3/C6, resulting in reduced ASO uptake, thereby suggesting that Ca2+ influx via TRPC3/C6 is critical for L687-mediated increased ASO uptake. L687 also induced dextran uptake, indicating that L687 increased endocytosis. Adding ASO to L687 resulted in endosome accumulation; however, the endosomal membrane disruptor UNC7938 facilitated endosomal escape and enhanced knockdown activity. We discovered a new function for TRPC activators regarding ASO trafficking in target cells. Our findings provide an opportunity to formulate an innovative drug delivery system for the therapeutic development of ASO.

Pharmacological Activation of TRPC6 Channel Prevents Colitis Progression.

We recently reported that transient receptor potential canonical (TRPC) 6 channel activity contributes to intracellular Zn2+ homeostasis in the heart. Zn2+ has also been implicated in the regulation of intestinal redox and microbial homeostasis. This study aims to investigate the role of TRPC6-mediated Zn2+ influx in the stress resistance of the intestine. The expression profile of TRPC1-C7 mRNAs in the actively inflamed mucosa from inflammatory bowel disease (IBD) patients was analyzed using the GEO database. Systemic TRPC3 knockout (KO) and TRPC6 KO mice were treated with dextran sulfate sodium (DSS) to induce colitis. The Zn2+ concentration and the mRNA expression levels of oxidative/inflammatory markers in colon tissues were quantitatively analyzed, and gut microbiota profiles were compared. TRPC6 mRNA expression level was increased in IBD patients and DSS-treated mouse colon tissues. DSS-treated TRPC6 KO mice, but not TRPC3 KO mice, showed severe weight loss and increased disease activity index compared with DSS-treated WT mice. The mRNA abundances of antioxidant proteins were basically increased in the TRPC6 KO colon, with changes in gut microbiota profiles. Treatment with TRPC6 activator prevented the DSS-induced colitis progression accompanied by increasing Zn2+ concentration. We suggest that TRPC6-mediated Zn2+ influx activity plays a key role in stress resistance against IBD, providing a new strategy for treating colitis.

Myocardial TRPC6-mediated Zn2+ influx induces beneficial positive inotropy through ß-adrenoceptors.

Baroreflex control of cardiac contraction (positive inotropy) through sympathetic nerve activation is important for cardiocirculatory homeostasis. Transient receptor potential canonical subfamily (TRPC) channels are responsible for α1-adrenoceptor (α1AR)-stimulated cation entry and their upregulation is associated with pathological cardiac remodeling. Whether TRPC channels participate in physiological pump functions remains unclear. We demonstrate that TRPC6-specific Zn2+ influx potentiates ß-adrenoceptor (ßAR)-stimulated positive inotropy in rodent cardiomyocytes. Deletion of trpc6 impairs sympathetic nerve-activated positive inotropy but not chronotropy in mice. TRPC6-mediated Zn2+ influx boosts α1AR-stimulated ßAR/Gs-dependent signaling in rat cardiomyocytes by inhibiting ß-arrestin-mediated ßAR internalization. Replacing two TRPC6-specific amino acids in the pore region with TRPC3 residues diminishes the α1AR-stimulated Zn2+ influx and positive inotropic response. Pharmacological enhancement of TRPC6-mediated Zn2+ influx prevents chronic heart failure progression in mice. Our data demonstrate that TRPC6-mediated Zn2+ influx with α1AR stimulation enhances baroreflex-induced positive inotropy, which may be a new therapeutic strategy for chronic heart failure.

Deletion of TRPC3 or TRPC6 Fails to Attenuate the Formation of Inflammation and Fibrosis in Non-alcoholic Steatohepatitis.

Non-alcoholic steatohepatitis (NASH) is a disease that has progressed from non-alcoholic fatty liver disease (NAFLD) and is characterized by inflammation and fibrosis. Two transient receptor potential canonical (TRPC) subfamily members, TRPC3 and TRPC6 (TRPC3/6), reportedly participate in the development of fibrosis in cardiovascular and renal systems. We hypothesized that TRPC3/6 may also participate in NASH fibrosis. We evaluated the effects of TRPC3 or TRPC6 functional deficiency in a NASH mouse model using choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD). Wild-type (WT) and TRPC3 or TRPC6 gene-deficient (KO) mice were fed with CDAHFD or standard diet for 6 weeks. The CDAHFD-induced body weight loss in TRPC6 KO mice was significantly lower compared with WT mice with CDAHFD. CDAHFD treatment significantly increased TRPC3 mRNA expression level and tissue weight in WT liver, which were suppressed in TRPC3 KO mice. However, either systemic deletion of TRPC3 or TRPC6 failed to attenuate liver steatosis, inflammation and fibrosis. These results imply that TRPC3 and TRPC6 are unlikely to be involved in liver dysfunction and fibrosis of NASH model mice.

Screening of Transient Receptor Potential Canonical Channel Activators Identifies Novel Neurotrophic Piperazine Compounds.

Transient receptor potential canonical (TRPC) proteins form Ca(2+)-permeable cation channels activated upon stimulation of metabotropic receptors coupled to phospholipase C. Among the TRPC subfamily, TRPC3 and TRPC6 channels activated directly by diacylglycerol (DAG) play important roles in brain-derived neurotrophic factor (BDNF) signaling, promoting neuronal development and survival. In various disease models, BDNF restores neurologic deficits, but its therapeutic potential is limited by its poor pharmacokinetic profile. Elucidation of a framework for designing small molecules, which elicit BDNF-like activity via TRPC3 and TRPC6, establishes a solid basis to overcome this limitation. We discovered, through library screening, a group of piperazine-derived compounds that activate DAG-activated TRPC3/TRPC6/TRPC7 channels. The compounds [4-(5-chloro-2-methylphenyl)piperazin-1-yl](3-fluorophenyl)methanone (PPZ1) and 2-[4-(2,3-dimethylphenyl)piperazin-1-yl]-N-(2-ethoxyphenyl)acetamide (PPZ2) activated, in a dose-dependent manner, recombinant TRPC3/TRPC6/TRPC7 channels, but not other TRPCs, in human embryonic kidney cells. PPZ2 activated native TRPC6-like channels in smooth muscle cells isolated from rabbit portal vein. Also, PPZ2 evoked cation currents and Ca(2+) influx in rat cultured central neurons. Strikingly, both compounds induced BDNF-like neurite growth and neuroprotection, which were abolished by a knockdown or inhibition of TRPC3/TRPC6/TRPC7 in cultured neurons. Inhibitors of Ca(2+) signaling pathways, except calcineurin, impaired neurite outgrowth promotion induced by PPZ compounds. PPZ2 increased activation of the Ca(2+)-dependent transcription factor, cAMP response element-binding protein. These findings suggest that Ca(2+) signaling mediated by activation of DAG-activated TRPC channels underlies neurotrophic effects of PPZ compounds. Thus, piperazine-derived activators of DAG-activated TRPC channels provide important insights for future development of a new class of synthetic neurotrophic drugs.

Structural insight into receptor-selectivity for lurasidone.

Lurasidone is a novel antipsychotic agent with high affinity for dopamine D(2), 5-hydroxyltryptamine 5-HT(2A), and 5-HT(7) receptors. Lurasidone has negligible affinity for histamine H(1) and muscarinic M(1) receptors, which are thought to contribute to side effects such as weight gain, sedation, and worsening of cognitive deficits. Our interests focus on why lurasidone has such high selectivity for only a part of these aminergic G-protein coupled receptors (GPCRs) and the different binding profile from ziprasidone, which has the same benzisothiazolylpiperazine moiety as lurasidone. In order to address these issues, we constructed structural models of lurasidone-GPCR complexes by homology modeling of receptors, exhaustive docking of ligand, and molecular dynamics simulation-based refinement of complexes. This computational study gave reliable structural models for D(2), 5-HT(2A), and 5-HT(7), which had overall structural complementarities with a salt bridge anchor at the center of the lurasidone molecule, but not for H(1) and M(1) owing to steric hindrance between the norbornane-2,3-dicarboximide and/or cyclohexane part of lurasidone and both receptors. By comparison with the structural models of olanzapine-GPCRs and ziprasidone-GPCRs constructed using the same computational protocols, it was suggested that the bulkiness of the norbornane-2,3-dicarboximide part and the rigidity and the bulkiness of the cyclohexyl linker gave lurasidone high selectivity for the desired aminergic GPCRs. Finally, this structural insight was validated by a binding experiment of the novel benzisothiazolylpiperazine derivatives. This knowledge on the structural mechanism behind the receptor selectivity should help to design new antipsychotic agents with preferable binding profiles, and the established computational protocols realize virtual screening and structure-based drug design for other central nervous system drugs with desired selectivity for multiple targets.

Synthesis and pharmacological evaluation of novel benzoylazole-based PPAR α/γ activators.

In our search for new PPARα/γ agonists, we designed and synthesized a series of benzoylazole-based carboxylic acids. Compound 9 showed potent PPARγ partial agonistic activity with modest PPARα agonistic activity. The sodium salt of 9 (9Na) demonstrated potent efficacy in lowering both blood glucose and lipids in an animal model without causing significant body weight gain, a well-known side effect associated with PPARγ full agonists.

Development of a new class of benzoylpyrrole-based PPARα/γ activators.

Starting with a subtle blood glucose-lowering effect of a TGF-ß inhibitor, we designed and synthesized a series of benzoylpyrrole-based carboxylic acids as PPARs activators. Among these compounds, 10sNa exhibited favorable blood glucose-lowering effect without body weight gain. We assume that the beneficial effect of 10sNa is attributed to not only its compound PPARα agonistic activity but also its PPARγ partial agonistic activity.

Combination therapy with SMP-534 and an angiotensin-converting enzyme inhibitor provides additional renoprotection in 5/6 nephrectomized rats.

The number of patients with chronic kidney disease (CKD) has continuously grown worldwide. Treatment with antihypertensive agents reduces the rate of progression of CKD, however, there is still a large unmet need to develop strategies for the treatment of CKD. Although we have previously reported that the antifibrotic agent, SMP-534 inhibits the progression of CKD, it is unknown whether combination therapy with SMP-534 and antihypertensive agent shows additive effects on CKD. In present study, we examined whether combination therapy with SMP-534 and the antihypertensive agent, lisinopril is more effective than single therapy with SMP-534 or lisinopril on five-sixths nephrectomized (5/6Nx) rat model. Combination therapy with SMP-534 (50 mg/kg) and lisinopril (5 mg/kg) significantly decreased urinary albumin excretion, blood urea nitrogen (BUN) and serum creatinine and increased creatinine clearance in 5/6Nx rats. On the other hands, single treatment with SMP-534 or lisinopril did not improve renal function at this dose. In addition, combination therapy with SMP-534 and lisinopril significantly decreased extracellular matrix (ECM) accumulation in renal glomeruli and tubulointerstitial injury. These data suggest that combination therapy with an antifibrotic agent and an antihypertensive agent may offer a new therapeutic option for suppressing the progression of CKD.

The anti-fibrotic agent SMP-534 attenuates bleomycin-induced pulmonary fibrosis in hamsters.

Pulmonary fibrosis is a progressive and lethal lung disease characterized by accumulation of ECM and loss of pulmonary function. However, no cure exists for this disease, and current treatments often fail to slow its progression or relieve its symptoms. We have previously reported that the anti-fibrotic agent SMP-534 has beneficial effects on renal fibrosis in animal model of nephropathy. In this study, we examined whether SMP-534 has beneficial effects on pulmonary fibrosis in bleomycin-treated hamsters. Treatment with SMP-534 [low dose (70 mg/kg) or high dose (110 mg/kg)] counteracted inhibition of body weight increase induced by bleomycin. In addition, SMP-534 significantly inhibited bleomycin-induced increase in lung hydroxyproline level, an index of collagen formation. Moreover, SMP-534 significantly ameliorated histological pulmonary fibrotic changes induced by bleomycin. The results of this study indicate that the anti-fibrotic agent SMP-534 may offer a new therapeutic option for the treatment of pulmonary fibrosis.

Chronic administration of SMP-534 ameliorates renal dysfunction in 5/6 nephrectomized rats.

BACKGROUND/AIMS: Chronic kidney disease (CKD) is the common cause of end-stage renal disease. Antihypertensive agents are clinically used to inhibit the progression of CKD. However, these agents cannot completely prevent progression to renal failure. We have previously reported that 5-chloro-2-{(1E)-3-[2-(4-methoxybenzoyl)-4-methyl-1H- pyrrol-1-yl]prop-1en-1-yl}-N-(methylsulfonyl)benzamide (SMP-534) improves renal disease and prevents the production of extracellular matrix in vitro. Additionally, SMP-534 inhibits glomerular fibrosis and provides renoprotection in vivo. In the present study, we investigated the effect of SMP-534 on renal dysfunction in a 5/6 nephrectomized (5/6Nx) rat model. METHOD: Five groups of rats were studied: sham operated, 5/6Nx + vehicle, 5/6Nx + SMP-534 30 mg/kg, 5/6Nx + SMP-534 60 mg/kg and 5/6Nx + SMP-534 90 mg/kg. Treatment with SMP-534 began 13 weeks after surgery, when hypertension and renal insufficiency had developed. Serum creatinine, blood urea nitrogen levels, creatinine clearance and urinary albumin were measured at specific time points. RESULTS: Serum creatinine and blood urea nitrogen levels were significantly reduced in SMP-534-treated groups. In addition, SMP-534 dose-dependently suppressed the increase in urinary albumin excretion observed in 5/6Nx rats. Moreover, survival rates were improved in SMP-534-treated groups. CONCLUSION: We have shown in this study that chronic oral administration of SMP-534 improves renal dysfunction in 5/6Nx rats. These findings indicate that SMP-534 may be a new therapeutic agent for the treatment of CKD.

Amelioration of established diabetic nephropathy by combined treatment with SMP-534 (antifibrotic agent) and losartan in db/db mice.

BACKGROUND/AIMS: Diabetic nephropathy is the main cause of end-stage renal disease. Previously we have demonstrated that SMP-534 (an antifibrotic agent) prevents the development of diabetic nephropathy in db/db mouse and that combined treatment with SMP-534 and losartan (antihypertensive agents) markedly prevents the development of diabetic nephropathy compared with single treatment. SMP-534 or losartan was prophylactically administered to db/db mice before the onset of diabetic nephropathy. In the present study, we evaluated the efficacy of combined treatment when administration was started after the onset of diabetic nephropathy. METHODS: db/db mice were raised untreated until 17 weeks of age, by which time increase of urinary albumin was noted, and then treated with SMP-534 and/or losartan for another 8 weeks. Biochemical and histological analyses were performed at 25 weeks of age. RESULTS: Combined treatment with SMP-534 and losartan markedly prevented the increase of urinary albumin and ameliorated the progression of mesangial matrix expansion, even when administration was started long after the increase of urinary albumin. CONCLUSION: The study results indicate that a combination of SMP-534 and losartan might be a valuable therapeutic approach for the treatment of diabetic nephropathy even when administration is started after the onset of diabetic nephropathy.

Tricyclic quinoxalinediones, aza-kynurenic acids, and indole-2-carboxylic acids as in vivo active NMDA-glycine antagonists.

This review article describes the development of in vivo active antagonists for the glycine binding site of the N-Methyl-D-Aspartate (NMDA) receptor. There were several difficulties in identifying a class of antagonists with in vivo efficacy and only a few compounds succeeded in emerging with activity in vivo. A series of tricyclic quinoxalinediones was highly potent glycine antagonists in vitro and the derivatives having a zwitterionic moiety including SM-18400 indeed showed in vivo activity. Similarly, tricyclic indole-2-carboxylic acids having a zwitterionic moiety such as SM-31900 were also active in vivo. In fact, SM-18400 and SM-31900 exhibited efficacy in several animal stroke models using intravenous infusion protocols. The practical syntheses of SM-18400 and SM-31900 as well as the novel synthesis of moderately active glycine antagonists, tricyclic azakynurenic acids, were also developed.

Synthesis and pharmacological profile of an orally-active growth hormone secretagogue, SM-130686.

Hypothalamic hormones physiologically regulate pulsatile release of growth hormone (GH) from the anterior pituitary gland. Since the discovery of these hormones in the 1970s, several new chemically synthesized peptidyl and non-peptidyl derivatives have been proved to stimulate and amplify GH secretion, and this series of molecules has been named the growth hormone secretagogues (GHSs). One of these compounds led to the discovery of a GPCR-type receptor for GHSs (GHS-R), and subsequently the endogenous ligand for the receptor has been identified, and is referred to as ghrelin. The identification of GHSs as physiological regulators of GH secretion encouraged us to examine our GHSs pharmacologically. We previously reported that novel oxindole derivatives have been identified as GHS-R agonists from our internal chemical library. Among these derivatives, (+)-6-carbamoyl-3-(2-chlorophenyl)-(2-diethylaminoethyl)-4-trifluoromethyloxindole (SM-130686, 37S) was found to have potent activity in vitro with a good pharmacokinetic profile in rats (bioavailability of 28%). In this article, we review the synthesis and pharmacological evaluation of SM-130686. SM-130686 binds specifically to GHS-R and increases the Ca(2+) concentration in Chinese hamster ovary cells expressing recombinant GHS-R. Maximal enhancement of the intracellular Ca(2+) concentration induced by SM-130686 treatment was approximately 55% that induced by ghrelin, suggesting that SM-130686 may be a partial GHS-R agonist. Also, in in vivo studies, oral administration of SM-130686 increased body length and fat-free mass gain. We compare the pharmacological profile of SM-130686 with other GHSs, including GHRH and ghrelin, and discuss the therapeutic usefulness of GHSs against several disorders, as well as for treatment of GH deficiency.

Enhanced effect of combined treatment with SMP-534 (antifibrotic agent) and losartan in diabetic nephropathy.

BACKGROUND/AIMS: Diabetic nephropathy is now the most common cause of end-stage renal disease. It is also clear that the current therapy, angiotensin II blockage, cannot prevent the progression of diabetic nephropathy. We had previously demonstrated that an antifibrotic agent, SMP-534, reduced extracellular matrix production induced by transforming growth factor-beta in vitro, and that SMP-534 prevented renal fibrosis and urinary albumin in diabetic db/db mice via a nonantihypertensive mechanism. We expected that combined use of SMP-534 and losartan would produce a more highly renoprotective action. METHODS: We examined the effects of combined treatment with SMP-534 and losartan on urinary albumin and glomerular fibrosis in db/db mice. Diet containing these agents was provided from age 9 to 25 weeks. Blood and urine analyses were performed at 8, 17, and 25 weeks. At the end of the study, kidney tissues were histologically analyzed. RESULTS: SMP-534 significantly suppressed an increase in urinary albumin excretion and ameliorated the progression of glomerular fibrosis in db/db mice, whereas losartan did not. Combined treatment with SMP-534 and losartan markedly prevented the increase of urinary albumin excretion compared with treatment with either SMP-534 or losartan alone. In contrast, renal histological analysis revealed that combined treatment did not significantly prevent an increase of mesangial expansion in the kidney compared with treatment with SMP-534 alone. CONCLUSION: A combination of the two agents, SMP-534 and losartan, might be a valuable therapeutic approach for the treatment of diabetic nephropathy.

SMP-534 ameliorates progression of glomerular fibrosis and urinary albumin in diabetic db/db mice.

Diabetic nephropathy is currently the most common cause of end-stage renal disease. Diabetic nephropathy patients, whether insulin dependent or not, develop fibrotic changes in glomeruli that manifest as overt nephropathy. Previously, we demonstrated that 5-chloro-2-{(1E)-3-[2-(4-methoxybenzoyl)-4-methyl-1H-pyrrol-1-yl]prop-1-en-1-yl}-N-(methylsulfonyl)benzamide (SMP-534) reduces extracellular matrix (ECM) production induced by transforming growth factor-beta (TGF-beta) in vitro and prevents the accumulation of ECM in glomeruli in rat Thy-1 nephritis models. In this study, we examined the long-term effects of SMP-534 on renal insufficiency and glomerulosclerosis in db/db mice, which are models of type 2 diabetes. A diet containing SMP-534 was given to the mice from the age of 9 to 25 wk, and blood and urine analysis were performed at 8, 17, and 25 wk. At the end of study, kidney tissues were analyzed histologically. Treatment with SMP-534 dose dependently suppressed the increase of urinary albumin and type IV collagen excretion in db/db mice. The renal histological analysis showed that SMP-534 dose dependently suppressed the increase of mesangial expansion in the kidney. In the immunohistological analysis, fibronectin and type IV collagen expression were lower in SMP-534-treated db/db mice compared with vehicle-treated db/db mice. This study suggested that SMP-534 ameliorated the increase of ECM production in kidney of db/db mice, possibly through the inhibition of TGF-beta action. Hence, antifibrotic agents such as SMP-534 might be a new therapeutic option for the treatment of diabetic nephropathy.

SMP-534 inhibits TGF-beta-induced ECM production in fibroblast cells and reduces mesangial matrix accumulation in experimental glomerulonephritis.

Transforming growth factor-beta (TGF-beta) is a potent fibrotic factor responsible for the synthesis of extracellular matrix (ECM) and is implicated as the major determinant in pathogenesis of chronic fibroses, including kidney. The novel small compound SMP-534 reduced ECM production induced by TGF-beta in fibroblast cells. SMP-534 inhibited TGF-beta-induced p38 mitogen-activated protein kinase (p38) activation but did not inhibit epidermal growth factor (EGF)-induced extracellular signal-related kinase (ERK) activation. We also found that oral administration of SMP-534 dose dependently lowered hydroxyproline contents in the cortical region of the kidney in rat anti-Thy-1 nephritis models. In periodic acid-Schiff staining of kidney sections, ECM accumulation was reduced by SMP-534 treatment. These data indicate that SMP-534 has potential in therapy for fibrotic diseases, including nephropathy.

Structure-activity relationships of the oxindole growth hormone secretagogues.

A series of substituted oxindole derivatives of SM-130686 was synthesized and evaluated as ghrelin receptor agonists. Modification of the substituents on the C3-aromatic part of the oxindole led to compounds with subnanomolar binding affinities. Compound 4i (IC(50)=0.02 nM) was orally active at low doses and showed in vivo activity when orally administered, 2 mg/kg twice a day for 4 days, as evidenced by significant body weight gain.

Tricyclic indole-2-carboxylic acids: highly in vivo active and selective antagonists for the glycine binding site of the NMDA receptor.

A series of tricyclic indole-2-carboxylic acid derivatives were synthesized and evaluated by the radioligand binding assay and the anticonvulsant effects in the mouse NMDA-induced seizure model. Among them, derivatives of 3S-(-)-4 such as 3a, 3f, and 3g which had certain zwitterionic anilides showed high affinity to the NMDA-glycine binding site. The absolute configuration of 3S-(-)-4 was confirmed by X-ray crystallographic analysis. In particular, 3g (SM-31900) was found to be a highly active glycine antagonist for both in vitro and in vivo assays (K(i) = 1.0 +/- 0.1 nM, ED(50) = 2.3 mg/kg, iv) and also showed high selectivity for the glycine site. In addition, 3g was soluble enough in aqueous media (>10 mg/mL at pH 7.4) to use for medications by intravenous injection.

In vitro and in vivo antagonistic activities of SM-31900 for the NMDA receptor glycine-binding site.

The purpose of this study was to clarify the in vitro pharmacological profile and the in vivo activity of (3S)-7-chloro-3-[2-((1R)-1-carboxyethoxy)-4-aminomethylphenyl]aminocarbonylmethyl-1,3,4,5-tetrahydrobenz[c,d]indole-2-carboxylic acid hydrochloride (SM-31900). SM-31900 inhibited the binding of [3H]glycine and [3H]5,7-dichlorokynurenic acid, radioligands for the N-methyl-D-aspartate (NMDA) receptor glycine-binding site, to rat brain membranes in a competitive manner, with K(i) values of 11+/-2 and 1.0+/-0.1 nM, respectively, and completely prevented the binding of [3H]dizocilpine (MK-801), a radioligand for the NMDA receptor channel site. In cultures of rat cortical neurons, SM-31900 markedly prevented the neuronal cell death induced by transient exposure to glutamate, in a concentration-dependent manner. Its neuroprotective potency was much stronger than those of other glycine-binding site antagonists (4-trans-2-carboxy-5,7-dichloro-4-phenylaminocarbonylamino-1,2,3,4-tetrahydroquinoline (L-689,560), 5,7-dichlorokynurenic acid, and 7-chlorokynurenic acid). Furthermore, SM-31900 showed anticonvulsant activity when administered systemically, unlike other antagonists. These data indicate that SM-31900 is a systemically active antagonist with high affinity for the NMDA receptor glycine-binding site.