Animal Studies

Functional interactions between the mu opioid receptor (MOR) and the metabotropic glutamate receptor 5 (mGluR5) in pain and analgesia have been well established. MMG22 is a bivalent ligand containing MOR agonist (oxymorphamine) and mGluR5 antagonist (MPEP) pharmacophores tethered by a 22-atom linker. MMG22 has been shown to produce potent analgesia in several models of chronic inflammatory and neuropathic pain. This study assessed the efficacy of systemic administration of MMG22 at reducing pain behavior in the spared nerve injury (SNI) model of neuropathic pain in mice, as well as its side effect profile and abuse potential. MMG22 reduced mechanical hyperalgesia and spontaneous ongoing pain after SNI, with greater potency early (10 days) as compared to late (30 days) after injury. Systemic administration of MMG22 did not induce place preference in naïve animals, suggesting absence of abuse liability when compared to traditional opioids. MMG22 also lacked the central locomotor, respiratory, and anxiolytic side effects of its monomeric pharmacophores. Evaluation of mRNA expression showed the transcripts for both receptors were co-localized in cells in the dorsal horn of the lumbar spinal cord and dorsal root ganglia. Thus, MMG22 reduces hyperalgesia after injury in the SNI model of neuropathic pain without the typical centrally mediated side effects associated with traditional opioids.

MT-45 is a synthetic opioid that was developed in the 1970s as an analgesic compound. However, in recent years MT-45 has been associated with multiple deaths in Europe and has been included in the class of novel psychoactive substances known as novel synthetic opioids (NSOs). Little is known about the pharmaco-toxicological effects of MT-45. Therefore, we used a dynamic mass redistribution (DMR) assay to investigate the pharmacodynamic profile of this NSO in vitro compared with morphine. We then used in vivo studies to investigate the effect of the acute systemic administration of MT-45 (0.01-15 mg/kg i.p.) on motor and sensorimotor (visual, acoustic and tactile) responses, mechanical and thermal analgesia, muscle strength and body temperature in CD-1 male mice. Higher doses of MT-45 (6-30 mg/kg i.p.) were used to investigate cardiorespiratory changes (heart rate, respiratory rate, SpO saturation and pulse distention). All effects of MT-45 were compared with those of morphine. In vitro DMR assay results demonstrated that at human recombinant opioid receptors MT-45 behaves as a potent selective mu agonist with a slightly higher efficacy than morphine. In vivo results showed that MT-45 progressively induces tail elevation at the lowest dose tested (0.01 mg/kg), increased mechanical and thermal antinociception (starting from 1 to 6 mg/kg), decreased visual sensorimotor responses (starting from 3 to 6 mg/kg) and reduced tactile responses, modulated motor performance and induced muscle rigidity at higher doses (15 mg/kg). In addition, at higher doses (15-30 mg/kg) MT-45 impaired the cardiorespiratory functions. All effects were prevented by the administration of the opioid receptor antagonist naloxone. These findings reveal the risks associated with the ingestion of opioids and the importance of studying these drugs and undertaking more clinical studies of the current molecules to better understand possible therapeutic interventions in the case of toxicity.

The long-term treatment of chronic pain by opioids is limited by tolerance and risk of addiction/dependence. Previously, we have shown that combination treatment of morphine with a dopamine D or D receptor modulator restored morphine analgesia in morphine-resistant neuropathic pain and decreased morphine's reward potential in an acute setting. Here, we investigated whether such adjunct therapy with a dopamine D receptor preferring antagonist (SCH 39166) or a dopamine D receptor preferring agonist (pramipexole) could prevent morphine tolerance and reduce withdrawal symptoms. Initially, tolerance to the combination of morphine + pramipexole was assessed in mice. Mice receiving intraperitoneal injections of morphine showed reduced thermal thresholds on Day 7 whereas those receiving morphine + pramipexole maintained analgesia at Day 7. Next, tolerance and withdrawal to both combinations were tested over 14 days in rats. Rats were assigned one of four drug conditions, (1) saline, 2) morphine, 3) morphine + SCH 39166, 4) morphine + pramipexole), for chronic administration via osmotic pumps. Chronic administration of morphine over 14 days resulted in a significant reduction of morphine analgesia. However, analgesia was maintained when morphine was administered with either the dopamine D receptor preferring antagonist or the D receptor preferring agonist. Withdrawal symptoms peaked at 48 h and were decreased in rats receiving either combination compared to morphine alone. The data suggests that adjunct therapy with dopamine D or D receptor preferring modulators prevents morphine tolerance and reduces the duration of morphine withdrawal symptoms, and thus this combination has potential for long-term pain management therapy.

The analgesic potency of morphine-6-glucuronide (M6G) has been shown to be 50-fold higher than morphine after intracerebral injection. However, the brain penetration of M6G is significantly lower than morphine, thus limiting its usefulness in pain management. Here, we created new entities by the conjugation of the Angiopep-2 peptide (An2) that crosses the blood-brain-barrier (BBB) by LRP1 receptor-mediated transcytosis, with either morphine or M6G. We demonstrated improvement of BBB permeability of these new entities compared with that of unconjugated M6G and morphine. Intravenous or subcutaneous administration of the An2-M6G conjugate exerted greater and more sustained analgesic activity than equivalent doses of either morphine or M6G. Likewise, subcutaneous An2-morphine induced a delayed but prolonged antinociceptive effect. The effects of these conjugates on the gastrointestinal tract motility were also evaluated. An2-morphine significantly reduced the intestinal transit time while An2-M6G exhibited a reduced constipation profile, as compared to morphine. In summary, we have developed new brain-penetrant opioid conjugates exhibiting improved analgesia to side-effect ratios. These results thus support the use of An2 carrier peptides as an innovative BBB targeting technology to deliver effective drugs such as M6G for the pain management. SIGNIFICANCE STATEMENT: The metabolite M6G does not cross efficiently the blood-brain barrier. The LRP1 peptide ligand Angiopep-2 may serve as an effective drug delivery system to the brain. Here, we demonstrated that the coupling of M6G to An2 improves its brain penetration and significantly increases its analgesic potency. The An2-M6G conjugate has a favorable side-effect profile that includes reduction of developing constipation. An2-M6G exhibits a unique pharmacodynamic profile with a better therapeutic window than morphine.

Muscarinic antagonists promote sensory neurite outgrowth in vitro and prevent and/or reverse multiple indices of peripheral neuropathy in rodent models of diabetes, chemotherapy-induced peripheral neuropathy and HIV protein-induced neuropathy when delivered systemically. We measured plasma concentrations of the M1 receptor selective muscarinic antagonist pirenzepine when delivered by sub-cutaneous injection, oral gavage or topical application to the skin and investigated efficacy of topically delivered pirenzepine against indices of peripheral neuropathy in diabetic mice. Topical application of 2% pirenzepine to the paw resulted in plasma concentrations 6hr post-delivery that approximated those previously shown to promote neurite outgrowth in vitro. Topical delivery of pirenzepine to the paw of streptozotocin-diabetic mice dose-dependently (0.1-10.0%) prevented tactile allodynia, thermal hypoalgesia and loss of epidermal nerve fibers in the treated paw and attenuated large fiber motor nerve conduction slowing in the ipsilateral limb. Efficacy against some indices of neuropathy was also noted in the contralateral limb, indicating systemic effects following local treatment. Topical pirenzepine also reversed established paw heat hypoalgesia while withdrawal of treatment resulted in a gradual decline in efficacy over 2-4 weeks. Efficacy of topical pirenzepine was muted when treatment was reduced from 5 to 3 or 1 days per week. Similar local effects were noted with the non-selective muscarinic receptor antagonist atropine when applied either to the paw or to the eye. Topical delivery of muscarinic antagonists may serve as a practical therapeutic approach to treating diabetic and other peripheral neuropathies. SIGNIFICANCE STATEMENT: Muscarinic antagonist pirenzepine alleviates diabetic peripheral neuropathy when applied topically in mice.