SX-3228

Effect of SX-3228, a selective ligand for the BZ1 receptor, on sleep and waking during the light-dark cycle in the rat

The effects of the benzodiazepine1 (BZ1) receptor agonist SX-3228 were studied in rats (N = 12) implanted for chronic sleep procedures. Administration of 0.5, 1.0 and 2.5 mg/kg SX-3228, sc, to rats 1 h after the beginning of the light phase of the light-dark cycle induced a significant reduction of rapid-eye-movement sleep (REMS) during the third recording hour. Moreover, slow wave sleep (SWS) was increased during the fourth recording hour after the two largest doses of the compound. Administration of 0.5, 1.0 and 2.5 mg/kg SX-3228 one hour after the beginning of the dark period of the light-dark cycle caused a significant and maintained (6-h recording period) reduction of waking (W), whereas SWS and light sleep (LS) were increased. REMS values tended to increase during the entire recording period; however, the increase was statistically significant only for the 1.0 mg/kg dose during the first recording hour. In addition, a significant and dose-related increase of power density in the delta and the theta regions was found during nonREM sleep (LS and SWS) in the dark period. Our results indicate that SX-3228 is a potent hypnotic when given to the rat during the dark period of the light-dark cycle. Moreover, the sleep induced by SX-3228 during the dark phase closely resembles the physiological sleep of the rat.

Limited anxiolytic-like effects of non-benzodiazepine hypnotics in rodents

The present experiments compared the anxiolytic-like effects of the benzodiazepine (BZD) hypnotic triazolam with those of four non-BZD hypnotics including one non-selective (zopiclone) and three omega1-BZD selective (zolpidem, zaleplon and SX-3228) receptor ligands, in classical animal models including conflict tests (punished lever pressing and punished drinking tests in rats) and exploratory models (elevated plus-maze test in rats and light/dark choice test in mice), and a recently developed mouse defence test battery (MDTB) which has been validated for the screening of anxiolytic drugs. Results from both conflict procedures showed that zopiclone (0.3-10 mg/kg) produced anxiolytic-like effects comparable to those of triazolam (0.1-3 mg/kg), whereas the selective omega1-BZD receptor hypnotics zolpidem (0.3-3 mg/kg), zaleplon (0.1-3 mg/kg) and SX-3228 (0.1-1 mg/kg) displayed weaker and/or non-specific anxiolytic-like effects. Similarly, in the light/dark test in mice, zolpidem (0.1-1 mg/kg), zaleplon (0.3-10 mg/kg) and SX-3228 (0.03-0.3 mg/kg) showed a reduced potential to produce anxiolytic-like effects as compared to the non-selective omega-BZD receptor hypnotics triazolam (0.03-1 mg/kg) and zopiclone (1-30 mg/kg). In the elevated plus-maze test, zopiclone (1-10 mg/kg), zolpidem (0.1-1 mg/kg), zaleplon (0.3-3 mg/kg) and SX-3228 (0.1-1 mg/kg) displayed anxiolytic-like activity at doses close to those producing behavioural impairment, whereas triazolam (0.03-1 mg/kg) exhibited anxiolytic-like effects over a wide dose range in the absence of decreases in general activity. In the MDTB, zaleplon (0.3-10 mg/kg) decreased all defensive responses, a profile which was similar to that of triazolam (0.03-1 mg/kg), while zopiclone (1-30 mg/kg), zolpidem (0.3-10 mg/kg) and SX-3228 (0.03-1 mg/kg) had fewer effects on defensive behaviours with several effects occurring only at motor-impairing doses. Taken together, these results demonstrate that, although selective omega1-BZD receptor hypnotics display anxiolytic-like activity, the effects are generally weaker than those observed with non-selective omega-BZD receptor selective hypnotics such as triazolam or zopiclone. In particular, the anxiety-reducing potential of the omega1-BZD receptor selective compounds is limited to certain anxiety measures and may be confounded and/or masked by behavioural suppression.

Comparison of the pharmacological properties of classical and novel BZ-omega receptor ligands

The experiments in this study compared the pharmacological properties of several BZ-omega receptor ligands, including the imidazobenzodiazepine imidazenil, the beta-carboline abecarnil, the pyridazinone Y-23684, the pyrido [1,2-a]benzimidazole RWJ 46771 and the 1,6-naphthyridin-2(1H)-one derivative SX-3228, with the prototypical BZs diazepam, clobazam and bretazenil. In in vitro experiments diazepam, bretazenil, imidazenil and Y-23684 displaced [3H]flumazenil binding non-selectively in membranes from rat cerebellum and spinal cord, two brain areas enriched in the BZ-omega 1 and BZ-omega 2 receptor subtypes, respectively. In contrast, abecarnil, RWJ 46771 and SX-3228 were more potent in displacing [3H]flumazenil binding to membranes from rat cerebellum than from spinal cord or hippocampus, indicating selectivity for the BZ-omega 1 receptor subtype. The in vivo experiments showed that all compounds increased the latency to clonic seizures produced by isoniazid. However, the maximal increase in latency induced by diazepam, clobazam, abecarnil, RWJ 46771 and SX-3228 was greater than that of bretazenil, imidazenil and Y-23684, thereby indicating that these latter compounds have low intrinsic efficacy. In the punished drinking, the punished lever pressing and the elevated plus-maze tests in rats, three models of anxiety, diazepam, clobazam and imidazenil elicited clear anxiolytic-like effects but at doses which were close to those producing hypolocomotion, ataxia and myorelaxation as measured in activity cages, the rotarod and the loaded grid tests, respectively. In contrast, bretazenil and Y-23684 induced anxiolytic-like activity at much lower doses than those which impaired motor performances. The magnitude of the positive effects of Y-23684 was similar to that of the reference BZs, suggesting that it may become a valuable alternative to currently used agents for the treatment of anxiety disorders. Abecarnil, RWJ 46771 and SX-3228 produced weaker or non-specific anxiolytic-like effects as they decreased anxiety-related behaviours at doses similar or close to those impairing motor performance. However, unlike the other compounds they induced myorelaxation at doses which were 3-10 times higher than those needed to produce decrease in exploratory activity. It is suggested that the behavioural profiles of abecarnil, RWJ 46771 and SX-3228 may be attributed to their selectivity for the BZ-omega 1 receptor subtype which may account for their sedative activity, thereby masking other effects including anxiolytic-like activity. This suggests that BZ receptor modulation of anxiety may involve BZ receptor subtypes other than BZ-omega 1.

Effects of compression and grinding on chemical stability of a benzodiazepine receptor agonist

SX-3228, 6-benzyl-3-(5-methoxy-1,3,4-oxadiazol-2-yl)-5,6,7,8-tetrahydro-1,6-naphthyridin-2(1H)-one, is a newly synthesized benzodiazepine receptor agonist intended to be developed as a tablet preparation. However, it was found that the drug substance was remarkably chemically unstable in tablet form compared to the powder mixture for tableting. Chemical destabilization due to compression also occurred in the drug substance alone. After investigating the cause of the destabilization, powder X-ray diffraction analysis showed that the crystallinity of the drug substance decreased depending on the extent of mechanical treatments such as compression and grinding. In thermal analysis it became evident that the exothermic peaks due to degradation clearly broadened and shifted toward lower temperatures by these mechanical treatments. It was then revealed that the degradation temperature decreased and the amount of degradation products after storage increased with decreasing crystallinity, even though there was little change in the amount of degradation products shortly after mechanical treatments. These results demonstrated that the drug substance became chemically unstable with decreasing crystallinity. It was proved that chemical instability of the drug substance in the tablet preparation was due to decreasing crystallinity caused by compression. It would therefore be difficult to produce chemically stable tablets containing this compound using a conventional manufacturing process. Tablets for this compound should be prepared without mechanical treatments such as compression and grinding.

Stabilization study on a wet-granule tableting method for a compression-sensitive benzodiazepine receptor agonist

SX-3228, 6-benzyl-3-(5-methoxy-1,3,4-oxadiazol-2-yl)-5,6,7,8-tetrahydro-1,6-naphthyridin-2(1H)-one, is a newly-synthesized benzodiazepine receptor agonist intended to be developed as a tablet preparation. This compound, however, becomes chemically unstable due to decreased crystallinity when it undergoes mechanical treatments such as grinding and compression. A wet-granule tableting method, where wet granules are compressed before being dried, was therefore investigated as it has the advantage of producing tablets of sufficient hardness at quite low compression pressures. The results of the stability testing showed that the drug substance was chemically considerably more stable in wet-granule compression tablets compared to conventional tablets. Furthermore, the drug substance was found to be relatively chemically stable in wet-granule compression tablets even when high compression pressure was used and the effect of this pressure was small. After investigating the reason for this excellent stability, it became evident that near-isotropic pressure was exerted on the crystals of the drug substance because almost all the empty spaces in the tablets were occupied with water during the wet-granule compression process. Decreases in crystallinity of the drug substance were thus small, making the drug substance chemically stable in the wet-granule compression tablets. We believe that this novel approach could be useful for many other compounds that are destabilized by mechanical treatments.