The novel analgesic, F 13640, produces intra- and postoperative analgesia in a rat model of surgical pain

Translating biased agonists from molecules to medications: Serotonin 5-HT1A receptor functional selectivity for CNS disorders

Biased agonism (or "functional selectivity") at G-protein-coupled receptors has attracted rapidly increasing interest as a means to improve discovery of more efficacious and safer pharmacotherapeutics. However, most studies are limited to in vitro tests of cellular signaling and few biased agonists have progressed to in vivo testing. As concerns 5-HT_1A receptors, which exert a major control of serotonergic signaling in diverse CNS regions, study of biased agonism has previously been limited by the poor target selectivity and/or partial agonism of classically available ligands. However, a new generation of highly selective, efficacious and druggable agonists has advanced the study of biased agonism at this receptor and created new therapeutic opportunities. These novel agonists show differential properties for G-protein signaling, cellular signaling (particularly pERK), electrophysiological effects, neurotransmitter release, neuroimaging by PET and pharmacoMRI, and behavioral tests of mood, motor activity and side effects. Overall, NLX-101 (a.k.a. F15599) exhibits preferential activation of cortical and brain stem 5-HT_1A receptors, whereas NLX-112 (a.k.a. befiradol or F13640) shows prominent activation of 5-HT_1A autoreceptors in Raphe nuclei and in regions associated with motor control. Accordingly, NLX-101 is potently active in rodent models of depression and respiratory control, whereas NLX-112 shows promising activity in models of Parkinson's disease across several species - rat, marmoset and macaque. Moreover, NLX-112 has also been labeled with ¹⁸F to produce the first agonist PET radiopharmaceutical (known as [¹⁸F]-F13640) for investigation of the active state of 5-HT_1A receptors in rodent, primate and human. The structure-functional activity relationships of biased agonists have been investigated by receptor modeling and novel compounds have been identified which exhibit increased affinity at 5-HT_1A receptors and new profiles of cellular signaling bias, notably for β-arrestin recruitment versus pERK. Taken together, the data suggest that 5-HT_1A receptor biased agonists constitute potentially superior pharmacological agents for treatment of CNS disorders involving serotonergic mechanisms.

N-Demethylsinomenine, an active metabolite of sinomenine, attenuates chronic neuropathic and inflammatory pain in mice

Chronic pain is a significant public health problem that afflicts nearly 30% of the global population, but current pharmacotherapies are insufficient. Previous report indicated that N-demethylsinomenine, an active metabolite of sinomenine, is efficacious against postoperative pain. The present study investigated whether N-demethylsinomenine is effective for chronic painful conditions or whether repeated treatment alters its effect. Both chronic constriction injury (CCI) surgery and complete Freund’s adjuvant (CFA) intraplantar injection induced significant and reliable mechanical allodynia at least for 7 days. Acute treatment with N-demethylsinomenine (10–40 mg/kg, i.p.) dose-dependently attenuated the mechanical allodynia both in CCI-induced neuropathic pain and CFA-induced inflammatory pain in mice. The potency of N-demethylsinomenine for reducing CFA-induced mechanical allodynia was slightly higher than sinomenine. During the period of repeated treatment, N-demethylsinomenine maintained its anti-allodynic effect against both neuropathic and inflammatory pain without producing carry-over effect. Pretreatment with bicuculline, a selective γ-aminobutyric acid type A (GABA_A) receptor antagonist, almost completely blocked the anti-allodynia of N-demethylsinomenine (40 mg/kg) both in CCI and CFA-treated mice. Our findings indicated that N-demethylsinomenine exhibits GABA_A receptor-mediated anti-allodynic effects in mouse models of neuropathic and inflammatory pain, suggesting it may be a useful novel pharmacotherapy for the control of chronic pain.

Antinociceptive, antiallodynic and antihyperalgesic effects of the 5-HT1A receptor selective agonist, NLX-112 in mouse models of pain

BACKGROUND AND PURPOSE

NLX-112 (a.k.a. befiradol, F13640) is a drug candidate intended for the treatment of l-DOPA-induced dyskinesia. It is a highly selective serotonin 5-HT_1A receptor full agonist which has been previously tested in a variety of models of CNS effects including analgesic activity in rat. Its activity in mouse models of pain has not been previously investigated.

EXPERIMENTAL APPROACH

The activity of NLX-112 was tested in mouse models of acute pain (hot plate), tonic pain (intraplantar formalin test), in the oxaliplatin-induced neuropathic pain model of chemotherapy-induced peripheral neuropathy and in the streptozotocin (STZ)-induced model of painful diabetic neuropathy.

KEY RESULTS

The main findings indicate that (i) NLX-112 was markedly active in the formalin test with potent reduction of paw licking in both phases of the test (minimal effective dose (MED) 0.5 mg/kg i.p. and p.o. in acute phase, and 0.1 mg/kg i.p. and 1 mg/kg p.o. in late phase). The effects of NLX-112 in this test were completely abolished by the selective 5-HT_1A receptor antagonist, WAY100635; (ii) NLX-112 was active in the hot plate test and in the oxaliplatin-induced neuropathic pain model of chemotherapy-induced peripheral neuropathy, but at markedly higher doses (MED 2.5 mg/kg i.p.); (iii) NLX-112 was least active in the STZ-induced model of painful diabetic neuropathy (MED 5 mg/kg i.p.); (iv) NLX-112 did not affect locomotor activity.

CONCLUSIONS AND IMPLICATIONS

NLX-112 may have significant potential for treatment of tonic pain but may be less promising as a candidate for treatment of chemotherapy-induced or diabetic neuropathic pain.

Antinociceptive effects of vitexin in a mouse model of postoperative pain

Vitexin, a C-glycosylated flavone present in several medicinal herbs, has showed various pharmacological activities including antinociception. The present study investigated the antinociceptive effects of vitexin in a mouse model of postoperative pain. This model was prepared by making a surgical incision on the right hindpaw and von Frey filament test was used to assess mechanical hyperalgesia. Isobolographical analysis method was used to examine the interaction between vitexin and acetaminophen. A reliable mechanical hyperalgesia was observed at 2 h post-surgery and lasted for 4 days. Acute vitexin administration (3–10 mg/kg, i.p.) dose-dependently relieved this hyperalgesia, which was also observed from 1 to 3 days post-surgery during repeated daily treatment. However, repeated vitexin administration prior to surgery had no preventive value. The 10 mg/kg vitexin-induced antinociception was blocked by the opioid receptor antagonist naltrexone or the GABA_A receptor antagonist bicuculline. The doses of vitexin used did not significantly suppress the locomotor activity. In addition, the combination of vitexin and acetaminophen produced an infra-additive effect in postoperative pain. Together, though vitexin-acetaminophen combination may not be useful for treating postoperative pain, vitexin exerts behaviorally-specific antinociception against postoperative pain mediated through opioid receptors and GABA_A receptors, suggesting that vitexin may be useful for the control of postoperative pain.

Study on the Antinociceptive Effects of Herba Epimedium in Mice

The present study was conducted to investigate the antinociceptive action of relationship between Herba Epimedium (HE) and 5-HT_1A receptor, between Herba Epimedium (HE) and 5-HT_2A receptor. We used the hot-plate method and the writhing assay in mice by the intracerebroventricular (i.c.v.) injection and observed the analgesic effect of HE. Furthermore, through the i.c.v. injection, 5-HT_1A receptor partial agonist Buspirone, antagonist Propranolol, the adrenaline β₁-receptor selective blocking agent Metoprolol, and 5-HT_2A receptor agonist hydrochloride DOI and antagonist Ketanserin were used, and, 5 min later, HE was used to investigate the impacts of drugs on the analgesic effect in the same way. Results showed that HE had fast and significant antinociception in nervous system, and the effects can persist for a long time. Buspirone and Hydrochloride DOI can remarkably increase the antinociception of HE in nervous system. Ketanserin leads to a significant decrease in its antinociception in nervous system; Metoprolol also has antinociceptive action in nervous system, but it can inhibit the antinociceptive effect of Herba Epimediumin peripheral region. These results suggest that HE has significant antinociception effect and its mechanism is related with 5-HT_1A receptor, 5-HT_2A receptor, and adrenaline β₁-receptor.

5-HT1A receptor agonist Befiradol reduces fentanyl-induced respiratory depression, analgesia, and sedation in rats

BACKGROUND

There is an unmet clinical need to develop a pharmacological therapy to counter opioid-induced respiratory depression without interfering with analgesia or behavior. Several studies have demonstrated that 5-HT1A receptor agonists alleviate opioid-induced respiratory depression in rodent models. However, there are conflicting reports regarding their effects on analgesia due in part to varied agonist receptor selectivity and presence of anesthesia. Therefore the authors performed a study in rats with befiradol (F13640 and NLX-112), a highly selective 5-HT1A receptor agonist without anesthesia.

METHODS

Respiratory neural discharge was measured using in vitro preparations. Plethysmographic recording, nociception testing, and righting reflex were used to examine respiratory ventilation, analgesia, and sedation, respectively.

RESULTS

Befiradol (0.2 mg/kg, n = 6) reduced fentanyl-induced respiratory depression (53.7 ± 5.7% of control minute ventilation 4 min after befiradol vs. saline 18.7 ± 2.2% of control, n = 9; P < 0.001), duration of analgesia (90.4 ± 11.6 min vs. saline 130.5 ± 7.8 min; P = 0.011), duration of sedation (39.8 ± 4 min vs. saline 58 ± 4.4 min; P = 0.013); and induced baseline hyperventilation, hyperalgesia, and "behavioral syndrome" in nonsedated rats. Further, the befiradol-induced alleviation of opioid-induced respiratory depression involves sites or mechanisms not functioning in vitro brainstem-spinal cord and medullary slice preparations.

CONCLUSIONS

The reversal of opioid-induced respiratory depression and sedation by befiradol in adult rats was robust, whereas involved mechanisms are unclear. However, there were adverse concomitant decreases in fentanyl-induced analgesia and altered baseline ventilation, nociception, and behavior.

Design, synthesis and biological activity evaluation of arylpiperazine derivatives for the treatment of neuropathic pain

In this work, a series of arylpiperazine derivatives were synthesized and screened by in vivo pharmacological trials. Among the tested compounds, 2-(4-(3-(trifluoromethyl)phenyl)piperazin-1-yl)-1-phenylethanone (18) and 2-(4-(2,3-dimethylphenyl)piperazin-1-yl)-1-phenylethanone (19) exhibited potent analgesic activities in both the mice writhing and mice hot plate tests. They showed more than 70% inhibition relative to controls in the writhing test, and increased latency by 116.0% and 134.4%, respectively, in the hot plate test. Furthermore, compound 18 was also active in the models of formalin pain and neuropathic pain without sedative side effects.

Effect of buspirone on thermal sensory and pain thresholds in human volunteers

Background

Buspirone is a partial 5-HT_1A receptor agonist. Animal studies have shown that modulation of serotoninergic transmission at the 5-HT_1A receptor can induce analgesia in acute pain models. However, no studies have been published so far on the effects of serotonin receptor agonists on pain perception in humans.

Methods

The effects of buspirone (30 mg p.o.) on thermal sensory and pain thresholds were investigated in twelve female volunteers (26 ± 2 yrs) in a prospective, randomized, double-blind, double-dummy, placebo-controlled study with morphine (10 mg i.v.) as positive control.

Results

Morphine significantly increased the heat pain detection threshold (ΔT: placebo 1.0°C and 1.3°C, p < 0.05) at 60 minutes. Buspirone caused mild sedation in six participants at 60 minutes, but was without effect on any of the measured parameters.

Conclusion

Buspirone in the maximal recommended dose was without significant effect on thermal pain. However, as it is only a partial agonist at the 5-HT_1A receptor and also acts on other receptor types, the negative results of the present study do not rule out a possible analgesic effect of more specific 5-HT_1A receptor agonists.

Curative-like analgesia in a neuropathic pain model: Parametric analysis of the dose and the duration of treatment with a high-efficacy 5-HT1A receptor agonist

High-efficacy activation of central 5-HT(1A) receptors by means of the recently discovered, selective 5-HT(1A) receptor ligand, F 13640 [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-{[(5-methyl-pyridin-2-ylmethyl)-amino]methyl}piperidin-1-yl]methanone, fumaric acid salt] causes an unprecedented, broad-spectrum analgesia in rat models of acute and chronic pain of nociceptive and neuropathic origin; it also is effective in conditions where opioids either are ineffective, induce analgesic tolerance, or elicit persistent hyperalgesia/allodynia. Inversely mirroring morphine's actions, F 13640's ("curative-like") analgesic effects persist after the discontinuation of treatment. Here, we examined the relationships, if any, between the dose and the duration of F 13640 treatment on the one hand, and the duration of persistent analgesia on the other. Rats received unilateral infraorbital nerve injury and developed allodynia - as assessed by an increased response to von Frey filament stimulation - within 24 days; thereafter, using osmotic pumps, rats were subcutaneously infused with F 13640 in two experiments. In one, a one-week infusion was instituted at 0.04-10-mg/day doses; in a second experiment, a 0.63-mg/day dose was implemented for a duration ranging from 1 to 56 days. These 250- and 56-fold variations of the dose and duration of treatment caused post-treatment, persistent analgesia for about 10 and 40 days, respectively. At least as much as dose, the duration of F 13640 treatment determines F 13640-induced persistent analgesia. Neuroadaptive modulations at pre- and postsynaptic, brain and spinal cord 5-HT(1A) receptors may be involved in the dynamical, dose- and time-dependent, pre-treatment rise and post-treatment decay of the analgesia induced by high-efficacy 5-HT(1A) receptor activation.