In Vitro and In Vivo Pharmacological Profiles of LENART01, a Dermorphin-Ranatensin Hybrid Peptide

Diverse chemical and pharmacological strategies are currently being explored to minimize the unwanted side effects of currently used opioid analgesics while achieving effective pain relief. The use of multitarget ligands with activity at more than one receptor represents a promising therapeutic approach. We recently reported a bifunctional peptide-based hybrid LENART01 combining dermorphin and ranatensin pharmacophores, which displays activity to the mu-opioid receptor (MOR) and dopamine D2 receptor (D2R) in rat brains and spinal cords. In this study, we investigated the in vitro binding and functional activities to the human MOR and the in vivo pharmacology of LENART01 in mice after subcutaneous administration. In vitro binding assays showed LENART01 to bind and be selective to the human MOR over the other opioid receptor subtypes and delta, kappa and nociceptin receptors. In the [35S]GTPγS binding assay, LENART01 acted as a potent and full agonist to the human MOR. In mice, LENART01 produced dose-dependent antinociceptive effects in formalin-induced inflammatory pain, with increased potency than morphine. Antinociceptive effects were reversed by naloxone, indicating MOR activation in vivo. Behavioral studies also demonstrated LENART01's properties to induce less adverse effects without locomotor dysfunction and withdrawal syndrome compared to conventional opioid analgesics, such as morphine. LENART01 is the first peptide-based MOR-D2R ligand known to date and the first dual MOR-dopamine D2R ligand for which in vivo pharmacology is reported with antinociceptive efficacy and reduced opioid-related side effects. Our current findings may pave the way to new pain therapeutics with limited side effects in acute and chronic use.

New Insights into the Opioid Analgesic Profile of cis-(-)-N-Normetazocine-derived Ligands

In this work, we report on the in vitro and in vivo pharmacological properties of LP1 analogs to complete the series of structural modifications aimed to generate compounds with improved analgesia. To do that, the phenyl ring in the N-substituent of our lead compound LP1 was replaced by an electron-rich or electron-deficient ring and linked through a propanamide or butyramide spacer at the basic nitrogen of the (-)-cis-N-normetazocine skeleton. In radioligand binding assays, compounds 3 and 7 were found to display nanomolar binding affinity for the μ opioid receptor (MOR) (K_i = 5.96 ± 0.08 nM and 1.49 ± 0.24 nM, respectively). In the mouse vas deferens (MVD) assay, compound 3 showed an antagonist effect against DAMGO ([D-Ala², N-MePhe⁴, Gly-ol]-enkephalin), a highly selective MOR prototype agonist, whereas compound 7 produced naloxone reversible effect at MOR. Moreover, compound 7, as potent as LP1 and DAMGO at MOR, was able to reduce thermal and inflammatory pain assessed by the mouse tail-flick test and rat paw pressure thresholds (PPTs) measured by a Randall-Selitto test.

The effects of Lavandula angustifolia essential oil on analgesic effects and percutaneous absorption of naproxen sodium gel; an in vivo and in vitro study

The percutaneous bioavailability of naproxen is low and several technologies have been utilized to overcome the problem. Although, some studies have reported the permeation-enhancing properties of natural essential oils, no research has reflected the effectiveness of Lavandula angustifolia essential oil (LAEO) on increasing the percutaneous absorption of naproxen sodium from a topical gel. Therefore, the present study was designed to investigate whether LAEO increased the percutaneous absorption and the analgesic effects of naproxen sodium topical gel. In the present study, naproxen topical gel was formulated using carbopol 940 (a gelling agent) and several vehicles such as PEG 400, ethanol, and water and the properties of gels were measured. Percutaneous absorption-enhancing properties of LAEO were measured too. Based on our data, the essential oil-containing formulation of naproxen represented greater penetration into (222.19 ± 24.87 vs. 107.65 ± 6.38 μg/cm² ), and also across (22.07 ± 4.42 vs. 13.14 ± 2.87 μg/cm² ) skin layers compared to the naproxen gel. Additionally, a significant analgesic property was observed in the naproxen topical gel containing 0.5% essential oil during both first and late phases of formalin test, as well as the late phase of tail-flick test. It could be concluded that LAEO significantly enhanced naproxen percutaneous absorption and also its analgesic effects.

Modulatory role of the orexin system in stress-induced analgesia: Involvement of the ventral tegmental area

BACKGROUND

Exposure to stressful experiences is often accompanied by suppressing pain perception, referred to as stress-induced analgesia. The neuropeptides orexins are essential in regulating the mechanism that responds to stressful and painful stimuli. Meanwhile, the ventral tegmental area (VTA), as a part of descending pain inhibitory system, responds to noxious stimuli. This study aimed to investigate the role of intra-VTA administration of orexin receptor antagonists on stress-induced antinociceptive responses in the animal model of acute pain.

METHOD

Ninety-three adult Wistar rats weighing 230-250 g were unilaterally implanted by a cannulae above the VTA. Animals were pretreated with different doses (1, 3, 10 and 30 nM/0.3 μl) of SB334867 as the orexin-1 receptor antagonist and TCS OX2 29 as the orexin-2 receptor antagonist into the VTA, just 5 min before 6 min exposure to forced swim stress (FSS). Nociceptive threshold was measured using the tail-flick test as a model of acute pain.

RESULTS

The results showed that exposure to FSS could significantly increase analgesic responses. Moreover, intra-VTA administration of SB334768 and TCS OX2 29 blocked the antinociceptive effect of FSS in the tail-flick test.

CONCLUSION

The findings suggest that OX1 and OX2 receptors in the VTA might modulate the antinociceptive behaviours induced by FSS in part.

SIGNIFICANCE

Acute exposure to physical stress suppresses pain-related behaviors in the animal model of acute pain. Blockade of the OX1 and OX2 receptors in the VTA attenuates antinociceptive responses induced by FSS. The contribution of the OX2 receptors in the VTA is more predominant than OX1 receptors in stress-induced analgesia.

Novel 1,3,4-Oxadiazole Derivatives of Pyrrolo[3,4-d]pyridazinone Exert Antinociceptive Activity in the Tail-Flick and Formalin Test in Rodents and Reveal Reduced Gastrotoxicity

Despite the availability of the current drug arsenal for pain management, there is still a clinical need to identify new, more effective, and safer analgesics. Based on our earlier study, newly synthesized 1,3,4-oxadiazole derivatives of pyrrolo[3,4-d]pyridazinone, especially 10b and 13b, seem to be promising as potential analgesics. The current study was designed to investigate whether novel derivatives attenuate nociceptive response in animals subjected to thermal or chemical noxious stimulus, and to compare this effect to reference drugs. The antinociceptive effect of novel compounds was studied using the tail-flick and formalin test. Pretreatment with novel compounds at all studied doses increased the latency time in the tail-flick test and decreased the licking time during the early phase of the formalin test. New derivatives given at the medium and high doses also reduced the late phase of the formalin test. The achieved results indicate that new derivatives dose-dependently attenuate nociceptive response in both models of pain and exert a lack of gastrotoxicity. Both studied compounds act more efficiently than indomethacin, but not morphine. Compound 13b at the high dose exerts the greatest antinociceptive effect. It may be due to the reduction of nociceptor sensitization via prostaglandin E₂ and myeloperoxidase levels decrease.

Co-administration of Pregabalin and Curcumin Synergistically Decreases Pain-Like Behaviors in Acute Nociceptive Pain Murine Models

Analgesic drugs in a combination-form can achieve greater efficacy with lesser side effects compared to either drug alone. The combination of drugs acting at different targets or mechanisms of action has been recognized as an alternative approach for achieving optimal analgesia. In this study, the analgesic effects of pregabalin (30, 60, 100, 200 mg/kg), curcumin (15, 30, 60, 100, 120 mg/kg), and 1:1 fixed-dose ratio of the pregabalin-curcumin combination were assessed using two acute nociceptive pain models, the acetic acid-induced writhing and tail-flick tests in mice. The pregabalin-curcumin combination produced a dose-dependent decrease in mean of writhes and an increase in the percentage of antinociception by the acetic acid-induced writhing test. In the tail-flick test, the combination also showed an improvement in antinociception indicated by the tail-flick latency, % antinociception, and area under the curve (AUC). Isobolographic analysis of interactions demonstrated a significant synergistic interaction effect between pregabalin and curcumin in both acute nociceptive pain models with the experimental ED₅₀ below the predicted additive line and the combination index < 1. These findings demonstrate that the combination of pregabalin and curcumin exhibits a synergistic interaction in mouse models of acute nociceptive pain.

Design and Development of Lidocaine Microemulsions for Transdermal Delivery

Topical administration is a preferable choice for local anesthetic delivery. Microemulsions have shown great effectiveness for transdermal transport of lidocaine. However, fabrication of microemulsions containing highly concentrated lidocaine (10%) to provide an extended local anesthetic effect is still a challenge. This study investigated the feasibility of using microemulsions for transdermal delivery of a high dosage of lidocaine (10%). At first, eutectic mixtures by kneading lidocaine with thymol were tailored to form a lipophilic solution, then the mixtures were readily incorporated into the oil phase of microemulsions after addition of proper surfactants and cosurfactants. The physicochemical properties, the skin permeation, local anesthetic efficacy, and the irritation experiment of the developed microemulsions were evaluated. The optimum composition was as follows: 12% of ethyl oleate as oil phase, 28% of the mixed surfactant, and cosurfactant (polyoxyl 15 hydroxystearate and ethanol) and 60% of the aqueous phase. The average particle size was about 13 nm. The transmission electron microscope (TEM) studies revealed almost homogeneous spherical globules without aggregation. The Fourier-transform infrared spectroscopy (FTIR) results highlighted the drugs homogeneously dispersed in the microemulsions. In vitro skin permeation and in vivo anesthesia effect evaluation indicated that microemulsions can enhance and extend the anesthetic effect of lidocaine. The irritable results indicated that the microemulsions had the better biocompatibility and the negligible influence on the dermal. Therefore, incorporating the eutectic mixtures into microemulsions could be proposed as an attractive choice and a promising transdermal delivery strategy for the future topical anesthetic therapy.

Pharmacokinetic and pharmacodynamic studies of pregabalin suppositories based on pharmacological research

OBJECTIVES

As commercially available pregabalin preparations are limited to oral administration, it is impossible to use it as an adjuvant analgesic for neuropathic cancer-related pain in terminally ill cancer patients with oral feeding difficulties. The objective of this study was to develop a pregabalin suppository to be available at hospitals.

METHODS

Pregabalin suppositories were prepared using bases comprising six different compositions of Witepsol H-15, Witepsol S-55, and Witepsol E-75. The suppository release test and stability test were performed in vitro. The pharmacokinetics and pharmacodynamics of the suppositories were assessed in rats.

KEY FINDINGS

In the in vitro releasing test, the pregabalin suppositories with H-15, H-15 : S-55 = 1 : 1, H-15 : S-55 = 2 : 1, H-15 : S-55 = 1 : 2 released approximately 100% of the pregabalin within 180 min. Among these pregabalin suppositories, only the suppository with H-15 : S-55 = 2 : 1 demonstrated an equivalent AUC_0-∞ with the oral administration group. Consistent with the results of the pharmacokinetic study, the pregabalin suppository with H-15 : S-55 = 2 : 1 exhibited antinociceptive effects. In addition, the pregabalin suppository with H-15 : S-55 = 2 : 1 was stable for 12 weeks when refrigerated with light shielding.

CONCLUSIONS

The pregabalin suppositories prepared in this study may be applicable for pain control for terminally cancer ill patients with oral feeding difficulties.

Activation of spinal nociceptin receptors induces cardiovascular depression and antinociception in an independent manner in mice

Purpose

The nociceptin receptor (NOP) was discovered in 1994 and was designated opioid-like receptor; activation of NOP leads to reduced neuronal excitability. Although suggested by the anatomical localization of NOP in brain or spinal cord, the cardiovascular or nociceptive effects of its endogenous ligand, nociceptin, are equivocal. Taking advantage from intrathecal application of nociceptin to simultaneously activate NOP on sympathetic preganglionic neurons in the intermediolateral column (IML) and superficial laminae of dorsal horn, we investigated whether the nociceptin-induced cardiovascular effects engage the participation of baroreflex, and whether the concurrently elicited changes in blood pressure and pain responses are interrelated.

Methods

NOPs in the thoracic spinal cord of ICR or C57BL/6 mice were identified with immunofluorescence staining and were activated through intrathecal administration of nocicetpin. The elicited changes in cardiovascular parameters and tail-flick nociceptive responses were measured.

Results

Positive immunoreactivity against NOP colocalized with neurons in the IML and superficial dorsal horn layers of thoracic spinal cord. Intrathecal administration of nociceptin (1, 2, or 5 nmol) elicited a significant and dose-dependent decrease in blood pressure or heart rate that was paralleled by reduced baroreflex-mediated sympathetic vasomotor tone and mirrored by augmented cardiac vagal baroreflex, alongside prolonged tail-flick latency with an efficacy of hypotension <<< antinociception. Coadministration of the specific NOP antagonist, UFP101 (10 nmol), blunted all nociceptin-elicited responses. However, restoring blood pressure to baseline level failed to affect the antinociceptive actions of nociceptin.

Conclusion

Activation of thoracic spinal NOP in ICR and C57BL/6 mice induces blood pressure and heart rate by decreasing the sympathetic outflow of both arms of the baroreflex arc to the blood vessels and the heart, and the antinociceptive responses to nociceptin are independent of and disproportional to its cardiovascular actions.

Synthesis and Structure-Activity Relationships of LP1 Derivatives: N-Methyl-N-phenylethylamino Analogues as Novel MOR Agonists

The opioid pharmacological profile of cis-(−)-N-normetazocine derivatives is deeply affected by the nature of their N-substituents. Here, our efforts were focused on the synthesis and pharmacological evaluation of novel derivatives of the lead LP1, a multitarget opioid analgesic compound featuring an N-phenylpropanamido substituent. LP1 derivatives 5a–d and 6a–d were characterized by flexible groups at the N-substituent that allow them to reposition themselves relative to cis-(−)-N-normetazocine nucleus, thus producing different pharmacological profiles at the mu, delta and kappa opioid receptors (MOR, DOR and KOR) in in vitro and in vivo assays. Among the series, compound 5c, with the best in vitro and in vivo profile, resulted a MOR agonist which displays a K_i^MOR of 6.1 nM in a competitive binding assay, and an IC₅₀ value of 11.5 nM and an I_max of 72% in measurement of cAMP accumulation in HEK293 cells stably expressing MOR, with a slight lower efficacy than LP1. Moreover, in a mouse model of acute thermal nociception, compound 5c, intraperitoneally administered, exhibits naloxone-reversed antinociceptive properties with an ED₅₀ of 4.33 mg/kg. These results expand our understanding of the importance of N-substituent structural variations in the opioid receptor profile of cis-(−)-N-normetazocine derivatives and identify a new MOR agonist useful for the development of novel opioid analgesics for pain treatment.

Effects of salt-loading hypertension on nociception in rats

Background

There is on going controversy on the effect of experimentally induced hypertension on nociception. The effect of salt-loading-induced hypertension on pain was studied in male rats.

Method

Twenty-four male Sprague-Dawley rats (160–280 g) were divided into two groups. Group A (n = 12) was treated with normal-feed diet (control), while group B (n = 12) was treated with 8% salt-loaded diet for 10 weeks. After 10 weeks of the treatment, six rats each from groups A and B were used for blood pressure measurement, while the remaining six rats were used for both the tail-flick and formalin tests. Thermal and chemical pain test were assessed using tail immersion test (tail flick) and formalin test pain paradigms at onset of salt-loading diet and after 10 weeks of salt loading.

Results

Chronic administration of salt-loading diet caused significant increases (P < 0.001) in systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure. Moreover, salt-loading-induced hypertension was found to significantly reduce pain sensitivity in the tail-immersion test (P < 0.001) and in the early and late phase of the formalin test (P < 0.01). However, the hypoalgesia was higher in the late phase (94.8%) than in the early phase (56.8%) of the formalin test.

Conclusion

The present study suggests that high salt-loading-induced hypertension causes hypoalgesia in rats, which might be due more to reduction in inflammatory response.

Zimelidine attenuates the development of tolerance to morphine-induced antinociception

OBJECTIVES

The aim of this study was to investigate effect of zimelidine (a serotonin reuptake inhibitor) on morphine-induced tolerance in rats.

MATERIALS AND METHODS

Male Wistar albino rats weighing 160-180 g were used in these experiments (n=72). A 3-day cumulative dosing regimen was used for the induction of morphine tolerance. To constitute of morphine tolerance, animals received morphine twice daily for 3 days. After the last dose morphine was injected on the fourth day, morphine tolerance was evaluated. The analgesic effects of zimelidine (15 mg/kg; i.p.) and morphine (5 mg/kg) were considered at 30-min time intervals (0, 30, 60, 90 and 120 min) by tail-flick and hot-plate analgesiometer (n=6 in each experimental group).

RESULTS

The results showed that zimelidine significantly attenuated the development and expression of morphine tolerance. The maximal antinociceptive effect of zimelidine was obtained at the 60 minutes measurements in the zimelidine group and at the 30 minutes measurements in the morphine tolerant group by the tail-flick and hot-plate tests. Administration of zimelidine with morphine showed additive analgesic effect.

CONCLUSION

In conclusion, our results show that zimelidine reduces the development of tolerance to morphine-induced antinociception in rats.

Exercise alleviates hypoalgesia and increases the level of calcitonin gene-related peptide in the dorsal horn of the spinal cord of diabetic rats

OBJECTIVE

The aim of this study was to evaluate the effects of treadmill training on nociceptive sensitivity and immunoreactivity to calcitonin gene-related peptide in the dorsal horn of the spinal cord of diabetic rats.

METHODS

Male Wistar rats were divided into three groups: control, diabetic and trained diabetic. Treadmill training was performed for 8 weeks. The blood glucose concentrations and body weight were evaluated 48 h after diabetes induction and every 30 days thereafter. The nociceptive sensitivity was evaluated using the tail-flick apparatus. The animals were then transcardially perfused, and the spinal cords were post-fixed, cryoprotected and sectioned in a cryostat. Immunohistochemistry for calcitonin gene-related peptide analysis was performed on the dorsal horn of the spinal cord.

RESULTS

The nociceptive sensitivity analysis revealed that, compared with the control and trained diabetic animals, the latency to tail deflection on the apparatus was longer for the diabetic animals. Optical densitometry demonstrated decreased calcitonin gene-related peptide immunoreactivity in the dorsal horn of the spinal cord in diabetic animals, which was reversed by treadmill training.

CONCLUSION

We concluded that treadmill training can alleviate nociceptive hypoalgesia and reverse decreased calcitonin gene-related peptide immunoreactivity in the dorsal horn of the spinal cord of diabetic animals without pharmacological treatment.