Pharmacology/Drug Development

The safety and efficacy of opioids are compromised as analgesic tolerance develops. Opioids are also ineffective against neuropathic pain. Recent reports have suggested that inhibitors of the epidermal growth factor receptor (EGFR), a receptor tyrosine kinase (RTK), may have analgesic effects in cancer patients suffering from neuropathic pain. It has been shown that the platelet-derived growth factor receptor-beta (PDGFR-↓), an RTK that has been shown to interact with the EGFR, mediates opioid tolerance but does not induce analgesia. Therefore, we sought to determine whether EGFR signaling was involved in opioid tolerance and if EGFR and PDGFR signaling could induce pain in rats.We found that gefitinib, an EGFR antagonist, eliminated morphine tolerance. In addition, repeated epidermal growth factor (EGF) administration rendered animals unresponsive to subsequent analgesic doses of morphine, a phenomenon we call 'pre-tolerance'. Using a nerve injury model, we found that gefitinib alone was not analgesic. Rather, it reversed insensitivity to morphine analgesia ('pre-tolerance') caused by the release of EGF by injured nerves. We also showed that repeated, but not acute EGF or platelet-derived growth factor-BB (PDGF-BB) administration induced mechanical hypersensitivity in rats. EGFR and PDGFR-↓ signaling interacted to produce this sensitization. EGFR was widely expressed in primary sensory afferent cell bodies, demonstrating a neuroanatomical substrate for our findings.Taken together, our results suggest a direct mechanistic link between opioid tolerance and mechanical sensitization. EGFR antagonism could eventually play an important clinical role in the treatment of opioid tolerance and neuropathic pain that is refractory to opioid treatment. Opioid tolerance and associated reduced effectiveness of opioids against neuropathic pain are two major clinical problems that are prime contributors to the opioid epidemic. However, the mechanisms underlying these phenomena are not clearly understood. Here we show that EGFR antagonism not only blocks morphine tolerance but also restores the effectiveness of opioids against neuropathic pain. Chronic EGF or PDGF administration induces mechanical sensitization, a prominent component of neuropathic pain, and renders animals 'pre-tolerant' to subsequent analgesic doses of morphine. Taken together, these results suggest a direct mechanistic link between opioid tolerance and neuropathic pain. EGFR antagonism could eventually play an important role in the treatment of opioid tolerance and severe neuropathic pain that requires ever increasing doses of opioids.

Pharmacological evaluation of the mu-opioid receptor (MOR) agonist properties of NKTR-181 in rodent models.

Endometriosis is an estrogen-dependent inflammatory disease that affects approximately 10% of women. Debilitating pelvic or abdominal pain is one of its major clinical features. Current animal models of endometriosis-associated pain require surgery either to implant tissue or to remove the ovaries. Moreover, existing models do not induce spontaneous pain, which is the primary symptom of patients with chronic pain, including endometriosis. A lack of models that accurately recapitulate the disease phenotype must contribute to the high failure rate of clinical trials for analgesic drugs directed at chronic pain, including those for endometriosis. We set out to establish a murine model of endometriosis-associated pain. Endometriosis was induced non-surgically by injecting a dissociated uterine horn into a recipient mouse. The induced lesions exhibited histological features that resemble human lesions along with an increase in pro-inflammatory cytokines and recruitment of immune cells. We also observed the presence of CGRP-, TRPA1-, and TRPV1-expressing nerve fibers in the lesions. This model induced mechanical allodynia, spontaneous abdominal pain, and changes in thermal selection behavior that indicate discomfort. These behavioral changes were reduced by drugs used clinically for endometriosis, specifically letrozole (aromatase inhibitor) and danazol (androgen). Endometriosis also induced neuronal changes as evidenced by activation of the NF-κB signaling pathway in TRPA1- and TRPV1-expressing DRG neurons. In conclusion, we have established a model of endometriosis-associated pain that responds to clinically active drugs and can, therefore, be used to identify novel therapies.

IL-4 is a pleiotropic antiinflammatory cytokine, which can be neuroprotective after nervous system injury. The beneficial actions of IL-4 are thought to result from the blunting of action of inflammatory mediators, such as proinflammatory cytokines. Here, we demonstrate that IL-4 induces M2 macrophages to continuously produce opioid peptides and ameliorate pain. IL-4 application at injured nerves in mice shifted F4/80+ macrophages from the proinflammatory M1 to the antiinflammatory M2 phenotype, which synthesized opioid peptides (Met-enkephalin, β-endorphin, and dynorphin A 1-17). These effects were accompanied by a long-lasting attenuation of neuropathy-induced mechanical hypersensitivity, beyond the IL-4 treatment. This IL-4-induced analgesia was decreased by opioid peptide antibodies and opioid receptor (δ, μ, κ) antagonists applied at injured nerves, which confirms the involvement of the local opioid system. The participation of M2 macrophages was supported by analgesia in recipient mice injected at injured nerves with F4/80+ macrophages from IL-4-treated donors. Together, IL-4-induced M2 macrophages at injured nerves produced opioid peptides, which activated peripheral opioid receptors to diminish pain. Fostering the opioid-mediated actions of intrinsic M2 macrophages may be a strategy to tackle pathological pain.

Morphine-3-glucuronide (M3G), the main metabolite of morphine, has been implicated in the development of tolerance and of opioid-induced hyperalgesia, both limiting the analgesic use of morphine. We evaluated the acute and chronic effects of M3G and morphine as well as development of antinociceptive cross-tolerance between morphine and M3G after intrathecal administration and assessed the expression of pain-associated neurotransmitter substance P in the spinal cord. Sprague-Dawley rats received intrathecal M3G or morphine twice daily for 6 days. Nociception and tactile allodynia were measured with von Frey filaments after acute and chronic treatments. Substance P levels in the dorsal horn of the spinal cord were determined by immunohistochemistry after 4-day treatments. Acute morphine caused antinociception as expected, whereas acute M3G caused tactile allodynia, as did both chronic M3G and morphine. Chronic M3G also induced antinociceptive cross-tolerance to morphine. M3G and morphine increased substance P levels similarly in the nociceptive laminae of the spinal cord. This study shows that chronic intrathecal M3G sensitises animals to mechanical stimulation and elevates substance P levels in the nociceptive laminae of the spinal cord. Chronic M3G also induces antinociceptive cross-tolerance to morphine. Thus, chronic M3G exposure might contribute to morphine-induced tolerance and opioid-induced hyperalgesia.