Anti-inflammatory properties of a novel N-phenyl pyridinone inhibitor of p38 mitogen-activated protein kinase: preclinical-to-clinical translation

Signal transduction through the p38 mitogen-activated protein (MAP) kinase pathway is central to the transcriptional and translational control of cytokine and inflammatory mediator production. p38 MAP kinase inhibition hence constitutes a promising therapeutic strategy for treatment of chronic inflammatory diseases, based upon its potential to inhibit key pathways driving the inflammatory and destructive processes in these debilitating diseases. The present study describes the pharmacological properties of the N-phenyl pyridinone p38 MAP kinase inhibitor benzamide [3- [3-bromo-4-[(2,4-difluorophenyl)methoxy]-6-methyl-2- oxo-1(2H)-pyridinyl]-N,4-dimethyl-, (-)-(9CI); PH-797804]. PH-797804 is an ATP-competitive, readily reversible inhibitor of the alpha isoform of human p38 MAP kinase, exhibiting a K(i) = 5.8 nM. In human monocyte and synovial fibroblast cell systems, PH-797804 blocks inflammation-induced production of cytokines and proinflammatory mediators, such as prostaglandin E(2), at concentrations that parallel inhibition of cell-associated p38 MAP kinase. After oral dosing, PH-797804 effectively inhibits acute inflammatory responses induced by systemically administered endotoxin in both rat and cynomolgus monkeys. Furthermore, PH-797804 demonstrates robust anti-inflammatory activity in chronic disease models, significantly reducing both joint inflammation and associated bone loss in streptococcal cell wall-induced arthritis in rats and mouse collagen-induced arthritis. Finally, PH-797804 reduced tumor necrosis factor-alpha and interleukin-6 production in clinical studies after endotoxin administration in a dose-dependent manner, paralleling inhibition of the target enzyme. Low-nanomolar biochemical enzyme inhibition potency correlated with p38 MAP kinase inhibition in human cells and in vivo studies. In addition, a direct correspondence between p38 MAP kinase inhibition and anti-inflammatory activity was observed with PH-797804, thus providing confidence in dose projections for further human studies in chronic inflammatory disease.

Structural bioinformatics-based prediction of exceptional selectivity of p38 MAP kinase inhibitor PH-797804

PH-797804 is a diarylpyridinone inhibitor of p38alpha mitogen-activated protein (MAP) kinase derived from a racemic mixture as the more potent atropisomer (aS), first proposed by molecular modeling and subsequently confirmed by experiments. On the basis of structural comparison with a different biaryl pyrazole template and supported by dozens of high-resolution crystal structures of p38alpha inhibitor complexes, PH-797804 is predicted to possess a high level of specificity across the broad human kinase genome. We used a structural bioinformatics approach to identify two selectivity elements encoded by the TXXXG sequence motif on the p38alpha kinase hinge: (i) Thr106 that serves as the gatekeeper to the buried hydrophobic pocket occupied by 2,4-difluorophenyl of PH-797804 and (ii) the bidentate hydrogen bonds formed by the pyridinone moiety with the kinase hinge requiring an induced 180 degrees rotation of the Met109-Gly110 peptide bond. The peptide flip occurs in p38alpha kinase due to the critical glycine residue marked by its conformational flexibility. Kinome-wide sequence mining revealed rare presentation of the selectivity motif. Corroboratively, PH-797804 exhibited exceptionally high specificity against MAP kinases and the related kinases. No cross-reactivity was observed in large panels of kinase screens (selectivity ratio of >500-fold). In cellular assays, PH-797804 demonstrated superior potency and selectivity consistent with the biochemical measurements. PH-797804 has met safety criteria in human phase I studies and is under clinical development for several inflammatory conditions. Understanding the rationale for selectivity at the molecular level helps elucidate the biological function and design of specific p38alpha kinase inhibitors.

Examination of the kinetic mechanism of mitogen-activated protein kinase activated protein kinase-2

The kinetic mechanism of mitogen-activated protein kinase activated protein kinase-2 (MAPKAPK2) was investigated using a peptide (LKRSLSEM) based on the phosphorylation site found in serum response factor (SRF). Initial velocity studies yielded a family of double-reciprocal lines that appear parallel and indicative of a ping-pong mechanism. The use of dead-end inhibition studies did not provide a definitive assignment of a reaction mechanism. However, product inhibition studies suggested that MAPKAPK2 follows an ordered bi-bi kinetic mechanism, where ATP must bind to the enzyme prior to the SRF-peptide and the phosphorylated product is released first, followed by ADP. In agreement with these latter results, surface plasmon resonance measurements demonstrate that the binding of the inhibitor peptide to MAPKAPK2 requires the presence of ATP. Furthermore, competitive inhibitors of ATP, adenosine 5'-(beta,gamma-imino)triphosphate (AMPPNP) and a staurosporine analog (K252a), can inhibit this ATP-dependent binding providing further evidence that the peptide substrate binds preferably to the E:ATP complex.

2,576 ultrasounds for blunt abdominal trauma

BACKGROUND

Determination of intra-abdominal injury following blunt abdominal trauma (BAT) continues to be a diagnostic challenge. Ultrasound (US) has been described as a potentially useful diagnostic tool in this setting and is being used with increasing frequency in trauma centers. We determined the diagnostic capability of US in the evaluation of BAT.

METHODS

A retrospective analysis of our trauma US database was performed over a 30-month period. Computed tomographic scan, diagnostic peritoneal lavage, or exploratory laparotomy confirmed the presence of intra-abdominal injury.

RESULTS

During the study period, 8,197 patients were evaluated at the Ryder Trauma Center. Of this group, 2,576 (31%) had US in the evaluation of BAT. Three hundred eleven (12%) US exams were considered positive. Forty-three patients (1.7%) had a false-negative US; of this group, 10 (33%) required exploratory laparotomy. US had a sensitivity of 86%, a specificity of 98%, and an accuracy of 97% for detection of intra-abdominal injuries. Positive predictive value was 87% and negative predictive value was 98%.

CONCLUSION

Emergency US is highly reliable and may replace computed tomographic scan and diagnostic peritoneal lavage as the initial diagnostic modality in the evaluation of most patients with BAT.

Subclavian arterial injury associated with blunt trauma

Blunt subclavian artery trauma is an uncommon but challenging surgical problem. The purpose of this study was to retrospectively review the management of blunt subclavian artery injuries treated by the Trauma and Vascular Surgery Services at the East Tennessee State University-affiliated hospitals between 1992 and 1998. Six patients with seven blunt subclavian artery injuries were identified. Physical signs indicating blunt subclavian artery injury were pain or contusion around the shoulder joint; fractures of the clavicle, scapula, or ribs; periclavicular hematomas; and ipsilateral pulse or neurologic deficits. Seven subclavian artery injuries were treated-two arterial transections, two pseudoaneurysms, and three intimal dissections. Associated injuries included four clavicle fractures, one humerus fracture, one combined rib and scapular fractures, and two pneumothoraxes. Vascular surgical treatment included three primary arterial repairs, two saphenous vein interposition grafts, and one polytetrafluoroethylene (PTFE) graft. One patient was treated nonoperatively with anticoagulation. No deaths occurred. Morbidity occurred in two patients with chronic upper extremity neuropathy producing prolonged disability from pain and weakness; one patient had reflex sympathetic dystrophy, and the other had a brachial plexus injury. In conclusion, blunt subclavian artery trauma can be successfully managed with early use of arteriography and prompt surgical correction by a variety of vascular techniques. Vascular morbidity is usually low, but long-term disability because of chronic neuropathy may result from associated brachial plexus nerve injury despite a successful arterial repair.

Fibrin glue eliminates the need for packing after complex liver injuries

Hemostasis after traumatic liver injury can be extremely difficult to obtain, particularly in coagulopathic patients who have suffered extensive liver damage. We determined the ability of a fibrin glue preparation (FG) to terminate ongoing bleeding using a new, clinically relevant porcine model of complex hepatic injury. Anesthetized swine (n = 6, 18 to 19 kg) received an external blast to the right upper abdomen and were immediately anticoagulated with intravenous heparin (200 u/kg). Uncontrolled hemorrhage from blast continued from time of injury (t = 0 minutes) to t = 15 minutes. Lactated Ringer's solution was infused to keep mean arterial pressure (MAP) > 80 mm Hg until the end of experiment (t = 90 minutes). Animals underwent routine surgical techniques to control bleeding, and FG was employed in the event these measures failed. Estimated blood loss and fluid resuscitation volume were measured. Serial MAP, arterial base excess, and temperature were recorded. Animals were severely injured with significant blood loss prior to laparotomy (26 +/- 6 cc/kg) and during routine surgical efforts to arrest hemorrhage (11 +/- 2 cc/kg). Bleeding could not be controlled with standard techniques in any animal. FG rapidly controlled hemorrhage and eliminated the need for packing. Re-bleeding was noted in only one animal (portal vein injury). FG can control severe hepatic hemorrhage when surgical techniques fail. Further work in the clinical arena is warranted to determine the potential benefits of FG in arresting hemorrhage in hemodynamically unstable coagulopathic patients with complex hepatic injuries.

Mesenteric panniculitis: case report and literature review

Mesenteric panniculitis is an extremely rare inflammatory condition of the adipose tissue of unknown etiology in which the mesentery is replaced with fibrosis. Knowledge of this rare syndrome should prevent any unwarranted aggressive therapy and help to use the clinical, radiological, and surgical sources to obtain the diagnosis. This paper is a review of symptomatology, pathology, treatment, and outcome of this disorder. A case report is described that presented with obstruction of the sigmoid colon.

Can surgeons evaluate emergency ultrasound scans for blunt abdominal trauma?

OBJECTIVE

To determine whether surgeons and residents with minimal training can evaluate accurately emergency ultrasound (US) examinations compared with radiologists for blunt abdominal trauma.

METHODS

Over 7 months, we conducted a prospective study comparing the evaluation of emergency US for blunt abdominal trauma by surgeons and attending radiologists. US readings from the surgical team and the radiologists were correlated with outcome.

RESULTS

One hundred-twelve patients were included in the study. Ninety-two patients had an US read as negative by the surgical and radiology services with no subsequent injuries identified. Eighteen patients had an US deemed positive by the surgical service and radiologists. Injuries were confirmed in this group by operation or computed tomography. One patient had an US deemed positive by the surgical team and subsequently negative by the radiologist. A diagnostic peritoneal lavage was performed which was negative. Another patient had an US interpreted as negative by the surgical evaluator and positive by the radiologist. Exploratory laparotomy was negative for intraabdominal hemorrhage or organ injury. Overall results reveal an accuracy on US reading of 99% for the surgical team and 99% for the attending radiologists.

CONCLUSION

Surgeons and surgical residents at different levels of training can accurately interpret emergency ultrasound examinations for blunt trauma from the real-time images, at a level comparable to attending radiologists.

Rapid and selective induction of blood eosinophilia in guinea pigs by recombinant human interleukin 5

Interleukin 5 (IL-5) has been implicated as a causal mediator in the development of pulmonary eosinophilia and airway hyperreactivity in human asthma. Supportive evidence for a pathogenic role of IL-5 in this disease has been provided by guinea pig models in which antigen-induced lung eosinophilia and bronchial hyperresponsiveness have been specifically attenuated with a neutralizing antibody to IL-5. In the present report, we describe a rapid mechanism-based model of IL-5-induced eosinophilia in the guinea pig. Our results show that intravenous injection of human IL-5 induced a 5-10-fold increase in the percentage and number of eosinophils in blood within 1 hour. In contrast, neutrophils and mononuclear cells were not recruited during this time. The increase in eosinophils was blocked by pretreatment of animals with an anti-IL5 monoclonal antibody (TRFK5) in doses similar to those which inhibit eosinophilia in guinea pig asthma models. Furthermore, dexamethasone, a potent inhibitor of eosinophilia in guinea pig asthma, profoundly suppressed the eosinophilia induced by human IL-5. This rapid model will be useful for elucidating the eosinophil activating properties of IL-5 in vivo and may potentially facilitate the development of IL-5 receptor antagonists for the specific blockade of the eosinophilic inflammation observed in human asthma.

Benefits of pulmonary artery catheter and transesophageal echocardiographic monitoring in laparoscopic cholecystectomy patients with cardiac disease

BACKGROUND

Because the abdominal insufflation and desufflation associated with laparoscopic procedures may adversely effect a compromised myocardium, patients with significant cardiopulmonary disease should be closely monitored during these procedures. The utility of intraoperative pulmonary artery catheter (PAC) and transesophageal echocardiography (TEE) monitoring was studied in 10 patients with moderate to severe cardiopulmonary disease to identify patients at greatest risk for cardiovascular complications during laparoscopic cholecystectomy.

METHODS

Ten patients were enrolled in this prospective study; 7 had suffered a previous myocardial infarction, 6 had undergone coronary artery bypass grafting, and 9 had disease classified as Goldman's class II or greater. The heart was monitored by TEE throughout the laparoscopic cholecystectomy by using real-time, two-dimensional mode to study the wall thickness and motion. Several PAC measurements were taken directly: cardiac output, systemic vascular resistance, pulmonary artery wedge pressure, and central venous pressure. Heart rate and blood pressure were also obtained at corresponding intervals. Cardiac index, stroke volume, and left and right ventricular stroke work were then calculated.

RESULTS

TEE demonstrated no significant changes in ventricular wall motion throughout laparoscopy. In patients who had postoperative cardiovascular complications, significant changes in cardiac index, left ventricular stroke work, and stroke volume were seen after pneumoperitoneum release. Compared to that of patients who did not develop complications, the cardiac index in those with complications dropped 42% (3.10 +/- 0.72 versus 1.80 +/- 0.10 L/min per m2, respectively; P < 0.01); left ventricular stroke work dropped 64% (139.00 +/- 11.36 versus 50.38 +/- 10.55 g x min/beat, respectively; P < 0.01); and stroke volume dropped 51% (86.90 +/- 12.68 versus 42.50 +/- 5.08 mL/beat, respectively; P < 0.01).

CONCLUSIONS

PCA monitoring in patients with compromised cardiac function is useful in identifying patients who may not tolerate hemodynamic changes after pneumoperitoneum release. Normalization of hemodynamic changes secondary to abdominal insufflation and desufflation in patients with compromised hearts may not occur in patients with compromised hearts may not occur for hours postoperatively. Abnormal hemodynamic changes occur within the first hour after desufflation in patients who later develop cardiovascular complications, which are heralded by significant drops in left ventricular stroke work, cardiac index, and stroke volume. TEE did not prove to be useful for intraoperative monitoring.

Characterization of indole-2-carboxylate derivatives as antagonists of N-methyl-D-aspartate receptor activity at the associated glycine recognition site

We have synthesized a series of indole-2-carboxylate derivatives and, with the use of radioligand binding, electrophysiological techniques and an in vivo transient bilateral carotid occlusion model of ischemic damage known to be sensitive to NMDA antagonists, have evaluated the indole-2-carboxylate derivatives ability to inhibit N-methyl-D-aspartate (NMDA) receptor activity through the associated glycine modulatory site. By using [3H]glycine to label this modulatory site, we found that the compounds with the highest affinity (Ki less than 1 microM) contained a chloro group at position C-6 and a polar, hydrogen-bond-accepting group at position C-3 of the indole ring. When these compounds were tested for their ability to modulate [3H]MK-801 [(+)-[3H]-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclophepten-5,10- imine maleate) binding, a functional assessment of NMDA receptor activation, binding was inhibited, indicative of NMDA receptor antagonist character. Schild regression analysis indicated that this antagonism was competitive with glycine. Next, several of these indole-2-carboxylate derivatives were analyzed electrophysiologically in rat cortex mRNA-injected Xenopus oocytes shown to express a functional NMDA receptor channel complex. These compounds inhibited NMDA receptor activity in a manner noncompetitive with NMDA. They also produced a parallel right-ward shift in the glycine dose response for potentiation of the NMDA responses in the oocytes and thus provided further evidence for a competitive interaction at the glycine site. Finally, in vivo transient bilateral carotid artery occlusion experiments revealed that these compounds were capable of reducing the damage typically associated with an ischemic insult in Mongolian gerbil hippocampal neurons.

Identification of a novel structural class of positive modulators of the N-methyl-D-aspartate receptor, with actions mediated through the glycine recognition site

This study describes a new structural class of compounds which interact at the N-methyl-D-aspartate (NMDA) receptor-associated glycine recognition site. These E-gamma-substituted vinylglycine derivatives were active in displacing [3H]glycine binding from the NMDA receptor-associated recognition site in rat forebrain synaptic plasma membranes, with Ki values in the range of 0.24-8.7 microM. Functional analyses of these compounds indicate that they positively modulate basal [3H](+)-5-methyl-10,11-dihydro-5H- [a,d]cyclohepaten-5,10-imine ([3H]MK-801) binding, consistent with their having agonist characteristics. Little stereospecificity is observed with the gamma-substituted methyl and propyl derivatives while the L-isomer of the hexyl analog is significantly more potent than the D-isomer. The D- and L-hydroxyethyl gamma-substituted vinylglycines were the most potent inhibitors of [3H]glycine binding with Ki values of 0.75 +/- 0.06 microM and 0.24 +/- 0.02 microM, respectively. The 3,4-double bond was necessary for activity in that the saturated hexyl derivative (2-aminodecanoate) was inactive. Based on the results reported herein, the hypothesis that there is a distinct size restriction for functional agonists which interact with the glycine recognition site, should be altered to include these larger extensions of vinylglycine.

Cis-2,4-methanoglutamate is a potent and selective N-methyl-D-aspartate receptor agonist

Cis- and trans-2,4-methanoglutamate were compared with L-glutamate as acidic amino acid ligands. Cis-2,4-methanoglutamate had a Ki of 0.052 microM against N-methyl-D-aspartate (NMDA)-specific L-[3H]glutamate binding compared with 0.050 microM for L-glutamate. Cis-2,4-methanoglutamate exhibited no significant affinity against [3H]kainate or [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate ([3H]AMPA) binding. Trans-2,4-methanoglutamate had no significant affinity for any of these sites. Cis-2,4-methanoglutamate increased [3H]N-1[2-thienyl]cyclohexyl-3,4-piperidine [( 3H]TCP) binding with EC50 of 0.35 +/- 0.14 microM. It produced an inward current in rat brain mRNA-injected Xenopus oocytes which was blocked by the NMDA antagonist, D-2-amino-7-phosphonoheptanoate (D-AP7). Cis-2,4-methanoglutamate (EC50 = 15.9 microM) was 100-fold more potent than L-glutamate (EC50 = 1,584 microM) in reducing the excitatory postsynaptic potential in CA1 of hippocampal slices. Cis-2,4-methanoglutamate is the most potent, selective NMDA agonist known.

Differential modulation of the associated glycine recognition site by competitive N-methyl-D-aspartate receptor antagonists

The competitive N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonopentanoate and two other five-atom linkage (C-5) omega-phosphono-alpha-amino acid analogs reduced [3H]glycine binding, in a dose-dependent manner, to a maximum of 45-55%, whereas seven-atom linkage (C-7) analogs had significantly less effect. The IC50 of the C-5 antagonists for the inhibition of [3H]glycine binding closely paralleled their potency both in displacing NMDA-selective L-[3H]glutamate binding and in negatively modulating (+)-[3H]5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate ([3H]MK-801) binding. Additionally, reduction of glycine binding by the C-5 antagonists was reversed by both NMDA receptor agonists and C-7 competitive NMDA antagonists, providing evidence that the site of action of these C-5 antagonists is the NMDA recognition site, resulting in indirect modulation of the glycine site. These data imply a functional coupling between the NMDA and associated glycine recognition sites and, furthermore, suggest a differential interaction of C-5 and C-7 competitive NMDA antagonists with the NMDA receptor complex.

Characterization of D-3,4-cyclopropylglutamates as N-methyl-D-aspartate receptor agonists

The 4 configurational isomers of D-3,4-cyclopropylglutamate (D-CGA) have been synthesized and analyzed for their interactions as excitatory amino acid recognition sites. Additionally, functional assessment of the action of these compounds at the N-methyl-D-aspartate (NMDA) receptor was performed. All 4 analogs function as agonists at the NMDA receptor as evidenced by their ability to stimulate [3H]MK-801 binding to the coupled PCP recognition site. Furthermore, the rank order of potency of these compounds in stimulating [3H]MK-801 binding corresponds with their Ki values for the displacement of NMDA-selective L-[3H]glutamate and [3H]CGS-19755 binding (D-CGA-C greater than D-CGA-B greater than D-CGA-D greater than D-CGA-A). The D-CGA-C isomer has affinity and potency at the NMDA receptor similar to the endogenous agonist, L-glutamate. This high potency coupled with greater specificity than L-glutamate, makes D-CGA-C a potentially useful pharmacological tool for the study of this receptor.

N-methyl-D-aspartate recognition site ligands modulate activity at the coupled glycine recognition site

In synaptic plasma membranes from rat forebrain, the potencies of glycine recognition site agonists and antagonists for modulating [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding and for displacing strychnine-insensitive [3H]glycine binding are altered in the presence of N-methyl-D-aspartate (NMDA) recognition site ligands. The NMDA competitive antagonist, cis-4-phosphonomethyl-2-piperidine carboxylate (CGS 19755), reduces [3H]glycine binding, and the reduction can be fully reversed by the NMDA recognition site agonist, L-glutamate. Scatchard analysis of [3H]glycine binding shows that in the presence of CGS 19755 there is no change in Bmax (8.81 vs. 8.79 pmol/mg of protein), but rather a decrease in the affinity of glycine (KD of 0.202 microM vs. 0.129 microM). Similar decreases in affinity are observed for the glycine site agonists, D-serine and 1-aminocyclopropane-1-carboxylate, in the presence of CGS 19755. In contrast, the affinity of glycine antagonists, 1-hydroxy-3-amino-2-pyrrolidone and 1-aminocyclobutane-1-carboxylate, at this [3H]glycine recognition site increases in the presence of CGS 19755. The functional consequence of this change in affinity was addressed using the modulation of [3H]TCP binding. In the presence of L-glutamate, the potency of glycine agonists for the stimulation of [3H]TCP binding increases, whereas the potency of glycine antagonists decreases. These data are consistent with NMDA recognition site ligands, through their interactions at the NMDA recognition site, modulating activity at the associated glycine recognition site.

Neuropharmacological characterization of 1-aminocyclopropane-1-carboxylate and 1-aminocyclobutane-1-carboxylate, ligands of the N-methyl-D-aspartate-associated glycine receptor

Following intravenous administration, 1-aminocyclobutane-1-carboxylate (ACBC, 100 mg/kg), a N-methyl-D-aspartate (NMDA)-associated glycine receptor antagonist, was eliminated with a T1/2 of 5 min in mouse brain and 4 min in rat cerebrospinal fluid (CSF). 1-Aminocyclopropane-1-carboxylate (ACC), a NMDA-associated glycine receptor agonist, was found to have a T1/2 of less than 5 min in mouse brain. ACC and ACBC did not alter basal cerebellar cGMP. Glycine and D-serine increased cGMP, and 1-hydroxy-3-aminopyrrolidone-2 (HA-966), a glycine antagonist, reversed the D-serine-induced increases in cGMP. In contrast, ACBC did not reverse the D-serine-induced increases in cGMP. These data suggest that despite their brain bioavailability and marked potency at the glycine receptor in vitro, ACC and ACBC are rapidly inactivated and thus have limited in vivo utility.

Evidence for a functional coupling of the NMDA and glycine recognition sites in synaptic plasma membranes

Activation of the N-methyl-D-aspartate (NMDA) receptor complex is subject to modulation via interactions at a coupled [3H]glycine recognition site in rat brain synaptic plasma membranes (SPM). We examined the effect of the potent and specific glycine site antagonists, 1-hydroxy-3-amino-2-pyrrolidone (HA-966) and 1-aminocyclobutane-1-carboxylate (ACBC), on the NMDA recognition site. These glycine analogs were found to significantly stimulate the binding of the competitive NMDA antagonist, [3H]3-(2-carboxypiperazin-4-y1)propyl-1-phosphonate ([3H]CPP) in a dose-dependent fashion, whereas both compounds inhibited NMDA-specific L-[3H]glutamate (agonist) binding. Additionally, both glycine antagonists reduced the binding of [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) to SPM, a functional assessment of activation of the NMDA receptor-channel complex. The glycine site agonists, glycine and serine reversed these effects in a dose-dependent manner, with the serine reversal being stereospecific for D-serine. The relative potencies of these compounds in reversing the glycine antagonist effects on the NMDA recognition site corresponded with their ability to competitively displace strychnine-insensitive [3H]glycine binding. These results provide evidence for a functional coupling between the glycine and NMDA recognition sites and further, may provide a mechanism by which compounds interacting at the glycine recognition site may modulate NMDA receptor activity.

Guanine nucleotide modulation of [3H]TCP binding to the NMDA receptor complex

Guanine nucleotides have been examined for their effect on [3H]1-[1-(2-thienyl)-cyclohexyl]-piperidine ([3H]TCP) binding to rat forebrain synaptic plasma membranes (SPM). We report that of the series of guanine nucleotides tested, GTP, GDP, 5'-guanylylimidodiphosphate (Gpp(NH)p) and 5'-guanylylmethylenediphosphate (Gpp(CH2)p) are significantly more potent at decreasing [3H]TCP binding than GMP, cyclic GMP, and guanosine. GTP, the most potent compound tested, inhibited basal [3H]TCP binding with an IC50 of 38.7 microM. Stimulation of [3H]TCP binding with either the N-methyl-D-aspartate (NMDA) agonist, L-glutamate, or Mg2+ was also inhibited by GTP. Addition of GTP resulted in a rightward shift in the glutamate dose-response curve and a decrease in the maximum level of stimulation. The Mg2+ stimulation of [3H]TCP binding was completely blocked by the addition of GTP. These results, coupled with the previous findings that guanine nucleotides inhibit the binding of L-[3H]glutamate to the NMDA recognition site (Monahan et al., 1988), indicate that guanine nucleotides antagonize NMDA receptor-mediated neurotransmission, at least in part, through their action (direct or indirect) on the NMDA recognition site and thus may be endogenous negative modulators of the NMDA receptor.

Characterization of a [3H]glycine recognition site as a modulatory site of the N-methyl-D-aspartate receptor complex

A [3H]glycine recognition site in rat brain synaptic plasma membranes (SPM) has been identified, having characteristics expected of a modulatory component of the N-methyl-D-aspartate receptor complex. Incubation of SPM with [3H]glycine for 10 min at 2 degrees C results in saturable, reversible binding with a KD of 0.234 microM and a Bmax of 9.18 pmol/mg. A pharmacological analysis of this binding site indicates that D-serine (Ki = 0.27 microM), D-alanine (Ki = 1.02 microM), and D-cycloserine (Ki = 2.33 microM) are potent inhibitors of binding, whereas the corresponding L isomers have significantly less activity (Ki = 25.4 microM, 15.9 microM, and greater than 100 microM, respectively). Inactive at concentrations of up to 100 microM were strychnine, L-valine, N,N-dimethylglycine, aminomethylphosphonate, and aminomethylsulfonate. The active compounds were analyzed further for their ability to stimulate [3H]1-[1-(2-thienyl)cyclohexyl]piperidine [( 3H]TCP) binding to Triton X-100-washed SPM. Results indicate that the affinity of the compounds for the [3H]glycine recognition site correlates with the ability of these analogues to stimulate [3H]TCP binding.

D-cycloserine: a ligand for the N-methyl-D-aspartate coupled glycine receptor has partial agonist characteristics

We have previously shown that D-cycloserine displaces [3H]glycine binding to a recognition site with properties consistent with an N-methyl-D-aspartate (NMDA) receptor modulatory site. Additionally, D-cycloserine positively modulates the NMDA receptor as evidenced by its dose-dependent enhancement of [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding to the NMDA receptor-coupled ionophore. Further evaluation of this compound indicates that the maximal stimulation of [3H]TCP binding induced by D-cycloserine is lower than that produced by other compounds acting at the NMDA receptor associated glycine modulatory site (glycine and D-serine). Moreover, the stimulation of [3H]TCP binding induced by D-cycloserine in the presence of various fixed concentrations of glycine results in a family of dose-response curves which asymptotically converge to 40-50% of the maximal stimulation induced by glycine alone. These results are consistent with D-cycloserine acting as a partial agonist of the NMDA receptor via its interaction with the coupled glycine modulatory site.

Effects of guanine nucleotides on N-methyl-D-aspartate receptor-ligand interactions

Guanine nucleotides have been examined as to their effects on subclass-specific excitatory amino acid receptor-ligand interactions. Guanine nucleotides selectively inhibit L-[3H]glutamate binding to the N-methyl-D-aspartate (NMDA) recognition site while showing a lesser effect on [3H]kainate, [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and sodium-dependent L-[3H]glutamate binding. Of the series of guanine nucleotides tested in the inhibition of NMDA-specific L-[3H]glutamate binding, GTP, GDP, 5'-guanylylimidodiphosphate and 5'-guanylylmethylenediphosphate were significantly more potent than GMP, cyclic GMP and guanosine. Scatchard analysis indicates that the GTP inhibition (IC50 = 28 microM) of this NMDA-specific L-[3H]glutamate binding results from a decrease in the affinity of L-glutamate for the NMDA receptor whereas no alteration in the number of binding sites is observed. A kinetic analysis indicates that this decrease in affinity may be attributed to a decrease in association rate whereas no change in dissociation rate is observed. GTP (25 microM) lowers the affinities of both NMDA agonists (NMDA, L-glutamate, L-aspartate, and L-homocysteate) and antagonists (D-2-amino-5-phosphonovalerate, D-2-amino-7-phosphonoheptanoate, and D-2-aminoadipate). Pretreatment of the synaptic plasma membranes with either pertussis or cholera toxin had no significant effect on the GTP inhibition of NMDA-specific L-[3H] glutamate binding. The data suggest that guanine nucleotides can negatively modulate the NMDA receptor; however, the mechanism of this modulation is unclear.

Determination of the pharmacokinetics of 2-amino-7-phosphonoheptanoate in rat plasma and cerebrospinal fluid

The pharmacokinetics of 2-amino-7-phosphonoheptanoate (AP7) in rat plasma and appearance in cerebrospinal fluid (CSF) are reported. Using high-performance liquid chromatography (HPLC) with fluorescence detection, concentrations of AP7 can be detected as low as 1.0 microM. Peak CSF concentrations (12-15 microM) for both the D-AP7 and D,L-AP7 are observed 10-15 min after i.v. administration and amount to approximately 0.1% of a 1 mmol/kg dose. Significant quantities (3 microM) are present in CSF at 2 h and no AP7 is detectable at 4 h. Following i.v. administration, a monoexponential clearance was observed for D-AP7 clearance from plasma, 15.4 +/- 0.8 S.E.M. ml/min/kg with a t1/2 of 38.9 +/- 0.8 S.E.M. min. However, a biexponential clearance from plasma was observed for D,L-AP7.

Excitatory amino acid receptor potency and subclass specificity of sulfur-containing amino acids

The sulfur-containing amino acids, L- and D-cysteate, L-cysteine, L- and D-cysteine sulfinate, L- and D-cysteine-S-sulfate, L-cystine, L- and D-homocysteate, L- and D-homocysteine sulfinate, L-homocysteine, L-serine-O-sulfate, and taurine were tested in two excitatory amino acid receptor functional assays and in receptor binding assays designed to label specifically the AA1/N-methyl-D-aspartate (NMDA), AA2/quisqualate, and AA3/kainate receptor recognition sites, as well as a CaCl2-dependent L-2-amino-4-phosphonobutanoate site, and a putative glutamate uptake site. Agonist efficacies were determined by chick retinal excitotoxicity and stimulated sodium efflux from rat brain slices. D-Homocysteine sulfinate, L-homocysteate, and L-serine-O-sulfate had affinities most selective for the NMDA binding site, whereas the binding affinities of D-cysteate, D-cysteine sulfinate, D-homocysteate, and L-homocysteine sulfinate were less selective. However, the correlation of agonist activity sensitive to blockade by D-2-amino-7-phosphonoheptanoate or D-2-amino-5-phosphonopentanoate in the functional assays with affinity in the NMDA binding assay (r = 0.87, p less than 0.005 and r = 0.98, p less than 0.005 for excitotoxicity and sodium efflux, respectively) allows characterization of these sulfur-containing amino acids as acting at NMDA subclass receptors. L-Homocysteate, which has been found in the brain, and L-serine-O-sulfate are selective agonists and could serve as endogenous neurotransmitters at the NMDA receptor.