Pharmacology/Drug Development

In the pain matrix, the insular cortex (IC) is mainly involved in discriminative sensory and motivative emotion. Abnormal signal transmission from injury site causes neuropathic pain, which generates enhanced synaptic plasticity. The mammalian target of rapamycin (mTOR) complex is the key regulator of protein synthesis; it is involved in the modulation of synaptic plasticity. To date, there has been no report on the changes in optical signals in the IC under neuropathic condition after treatment with mTOR inhibitors, such as Torin1 and XL388. Therefore, we aimed to determine the pain-relieving effect of mTOR inhibitors (Torin1 and XL388) and observe the changes in optical signals in the IC after treatment. Mechanical threshold was measured in adult male Sprague-Dawley rats after neuropathic surgery, and therapeutic effect of inhibitors was assessed on post-operative day 7 following the microinjection of Torin1 or XL388 into the IC. Optical signals were acquired to observe the neuronal activity of the IC in response to peripheral stimulation before and after treatment with mTOR inhibitors. Consequently, the inhibitors showed the most effective alleviation 4 h after microinjection into the IC. In optical imaging, peak amplitudes of optical signals and areas of activated regions were reduced after treatment with Torin1 and XL388. However, there were no significant optical signal changes in the IC before and after vehicle application. These findings suggested that Torin1 and XL388 are associated with the alleviation of neuronal activity that is excessively manifested in the IC, and is assumed to diminish synaptic plasticity.

G protein-coupled receptors can signal through both G proteins and ß-arrestin2. For the μ-opioid receptor, early experimental evidence from a single study suggested that G protein signalling mediates analgesia whereas ß-arrestin2 signalling mediates respiratory depression and constipation. Consequently, for more than a decade much research effort has been focused on developing biased μ-opioid agonists that preferentially target G protein signalling over β-arrestin signalling as it was believed that such drugs would be analgesics devoid of respiratory depressant activity. However, the prototypical compounds that have been developed based on this concept have so far failed in clinical and preclinical development.

There exists a limited number of studies investigating the correlation between spinal adenosine A1 receptors and Vincristine-induced peripheral neuropathy (VIPN). This study explored the role of intrathecal N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA) in neuropathic pain induced in rats by administering vincristine (100 μg/kg i.p.) for 10 days (two 5-day cycles with a 2-day pause).

The role of immune mediators, including pro-inflammatory cytokines in chemotherapy-induced peripheral neuropathy (CIPN), remains unclear. Here we studied the contribution of interleukin-20 (IL-20) to the development of paclitaxel-induced peripheral neuropathy. Increased serum levels of IL-20 in cancer patients with chemotherapy were accompanied by increased CIPN risk. In mouse models, proinflammatory IL-20 levels in serum and dorsal root ganglia fluctuated with paclitaxel treatment. Blocking IL-20 with the neutralizing antibody or genetic deletion of its receptors prevented CIPN, alleviated peripheral nerve damage, and dampened inflammatory responses, including macrophage infiltration and cytokine release. Mechanistically, paclitaxel up-regulated IL-20 via dysregulated Ca homeostasis, which augmented chemotherapy-induced neurotoxicity. Importantly, IL-20 suppression did not alter paclitaxel efficacy on cancer treatment both in vitro and in vivo. Together, targeting IL-20 ameliorates paclitaxel-induced peripheral neuropathy by suppressing neuroinflammation and restoring Ca homeostasis. Therefore, the anti-IL-20 monoclonal antibody is a promising therapeutic for the prevention and treatment of paclitaxel-induced neuropathy.