The novel mu-opioid antagonist, GSK1521498, reduces ethanol consumption in C57BL/6J mice.

RATIONALE Using the drinking-in-the-dark (DID) model, we compared the effects of a novel mu-opioid receptor antagonist, GSK1521498, with naltrexone, a licensed treatment of alcohol dependence, on ethanol consumption in mice. OBJECTIVE We test the ability of GSK1521498 to reduce alcohol consumption and compare its intrinsic efficacy to that of naltrexone by comparing the two drugs at doses matched for equivalent receptor occupancy. METHODS Thirty-six C57BL/6J mice were tested in a DID procedure. In 2-day cycles, animals experienced one baseline, injection-free session, and one test session when they received two injections, one of test drug and one placebo. All animals received GSK1521498 (0, 0.1, 1 and 3 mg/kg, i.p., 30 min pre-treatment) and naltrexone (0, 0.1, 1 and 3 mg/kg, s.c. 10 min pre-treatment) in a cross-over design. Receptor occupancies following the same doses were determined ex vivo in separate groups by autoradiography, using [3H]DAMGO. Binding in the region of interest was measured integrally by computer-assisted microdensitometry and corrected for non-specific binding. RESULTS Both GSK1521498 and naltrexone dose-dependently decreased ethanol consumption. When drug doses were matched for 70-75 % receptor occupancy, GSK1521498 3 mg/kg, i.p., caused a 2.5-fold greater reduction in alcohol consumption than naltrexone 0.1 mg/kg, s.c. Both GSK1521498 and naltrexone significantly reduced sucrose consumption at a dose of 1 mg/kg but not 0.1 mg/kg. In a test of conditioned taste aversion, GSK1521498 (3 mg/kg) reduced sucrose consumption 24 h following exposure to a conditioning injection. CONCLUSIONS Both opioid receptor antagonists reduced alcohol consumption but GK1521498 has higher intrinsic efficacy than naltrexone

Allosteric modulation of [(3)H]-CGP39653 binding through the glycine site of the NMDA receptor: further studies in rat and human brain

1. Binding of D,L-(E)-2-amino-4-[(3)H]-propyl-5-phosphono-3-pentenoic acid ([(3)H]-CGP39653), a selective antagonist at the glutamate site of the NMDA receptor, is modulated by glycine in rat brain tissue. We have further investigated this phenomenon in rodent and human brain by means of receptor binding and quantitative autoradiography techniques. 2. In rat cerebral cortical membranes the glycine antagonist 3-[2-(Phenylaminocarbonyl)ethenyl]-4,6-dichloro-indole-2-carboxylic acid sodium salt (GV150526A) did not change basal [(3)H]-CGP39653 binding, but competitively reversed the high affinity component of [(3)H]-CGP39653 binding inhibition by glycine, with a pK(B) value of 8.38, in line with its affinity for the glycine site (pK(i)=8.49 vs [(3)H]-glycine). Glycine (10 μM) significantly decreased [(3)H]-CGP39653 affinity for the NMDA receptor (with no change in the B(max)), whereas enhanced L-glutamate affinity (P<0.05, paired-samples Student's t-test). 3. In rat brain sections the addition of GV150526A (30 μM) to the incubation medium increased [(3)H]-CGP39653 binding to 208% of control (average between areas), indicating the presence of endogenous glycine. The enhancement presented significant regional differences (P<0.05, two-way ANOVA), with striatum higher than cerebral cortex (282 and 187% of control, respectively; P<0.05, Fisher's LSD). On the contrary, there was not any significant variation in affinity values of [(3)H]-CGP39653, L-glutamate, glycine and GV150526A in striatal and cortical membranes. These results confirmed the existence of regionally distinct NMDA receptors subtypes with different glycine/glutamate allosteric modulation. 4. Whole brain autoradiography revealed an uneven distribution of [(3)H]-CGP39653 binding sites in human brain. High levels of binding were determined in hippocampus and in cingulate, frontoparietal and insular cortex. Intermediate to low levels of binding were found in diencephalic nuclei and basal ganglia. [(3)H]-CGP39653 binding was increased to 216% of control (mean between areas) by 30 μM GV150526A. The enhancement, however, did not present significant regional differences. 5. These results introduce GV150526A as a useful tool to identify NMDA receptor subtypes by means of receptor autoradiography; moreover, they demonstrate that the allosteric inhibition of [(3)H]-CGP39653 binding by glycine parallels an increase in receptor affinity to the endogenous ligand L-glutamate. Finally, this study provides the first detailed anatomical description of the regional distribution of [(3)H]-CGP39653 binding sites in human brain

Occupancy of brain dopamine d(3) receptors and drug craving: a translational approach

Selective dopamine D(3) receptor (D(3)R) antagonists prevent reinstatement of drug-seeking behavior and decrease the rewarding effects of contextual cues associated with drug intake preclinically, suggesting that they may reduce drug craving in humans. GSK598809 is a selective D(3)R antagonist recently progressed in Phase I trials. The aim of this study was to establish a model, based on the determination of the occupancy of brain D(3)Rs (O(D(3))(R)) across species, to predict the ability of GSK598809 to reduce nicotine-seeking behavior in humans, here assessed as cigarette craving in smokers. Using ex vivo [(125)I](R)-trans-7-hydroxy-2-[N-propyl-N-(3′-iodo-2′-propenyl)amino] tetralin ([(125)I]7OH-PIPAT) autoradiography and [(11)C]PHNO positron emission tomography, we demonstrated a dose-dependent occupancy of the D(3)Rs by GSK598809 in rat, baboon, and human brains. We also showed a direct relationship between O(D(3))(R) and pharmacokinetic exposure, and potencies in line with the in vitro binding affinity. Likewise, GSK598809 dose dependently reduced the expression of nicotine-induced conditioned place preference (CPP) in rats, with an effect proportional to the exposure and O(D(3))(R) at every time point, and 100% effect at O(D(3))(R) values ⩾72%. In humans, a single dose of GSK598809, giving submaximal levels (72–89%) of O(D(3))(R), transiently alleviated craving in smokers after overnight abstinence. These data suggest that either higher O(D(3))(R) is required for a full effect in humans or that nicotine-seeking behavior in CPP rats only partially translates into craving for cigarettes in short-term abstinent smokers. In addition, they provide the first clinical evidence of potential efficacy of a selective D(3)R antagonist for the treatment of substance-use disorders