Dual enkephalinase inhibitor PL265: a novel topical treatment to alleviate corneal pain and inflammation

Abstract

Ocular pain is a core symptom of inflammatory or traumatic disorders affecting the anterior segment. To date, the management of chronic ocular pain remains a therapeutic challenge in ophthalmology. The main endogenous opioids (enkephalins) play a key role in pain control but exhibit only transient analgesic effects due to their rapid degradation. The aim of this study was to explore the antinociceptive and anti-inflammatory effects of topical administration of PL265 (a dual enkephalinase inhibitor) on murine models of corneal pain. On healthy corneas, chronic PL265 topical administration did not alter corneal integrity nor modify corneal mechanical and chemical sensitivity. Then, on murine models of corneal pain, we showed that repeated instillations of PL265 (10 mM) significantly reduced corneal mechanical and chemical hypersensitivity. PL265-induced corneal analgesia was completely antagonized by naloxone methiodide, demonstrating that PL265 antinociceptive effects were mediated by peripheral corneal opioid receptors. Moreover, flow cytometry (quantification of CD11b+ cells) and in vivo confocal microscopy analysis revealed that instillations of PL265 significantly decreased corneal inflammation in a corneal inflammatory pain model. Chronic PL265 topical administration also decreased Iba1 and neuronal injury marker (ATF3) staining in the nucleus of primary sensory neurons of ipsilateral trigeminal ganglion. These results open a new avenue for ocular pain treatment based on the enhancement of endogenous opioid peptides' analgesic effects in tissues of the anterior segment of the eye. Dual enkephalinase inhibitor PL265 seems to be a promising topical treatment for safe and effective alleviation of ocular pain and inflammation.

Morphological and Functional Changes of Corneal Nerves and Their Contribution to Peripheral and Central Sensory Abnormalities

“…Monitoring c-Fos, ATF3, and c Jun immunopositive cells is a commonly used approach to studying the activation and damage of primary sensory neurons from dorsal root and trigeminal ganglia. cFos, c-Jun, and ATF3 protein expressions were shown to be increased in the ipsilateral TG from animals with corneal damage (De Felipe and Belmonte, 1999;Launay et al, 2016;Fakih et al, 2019;Reaux-Le Goazigo et al, 2019). Both CGRP and ATF3 protein expressions in TG cell bodies increased after injury and returned to a normal level by 1 week, paralleling the time course of changes in nociceptive responses (Hegarty et al, 2018).…”

“…Activation of SGCs is characterized by GFAP upregulation in the injured trigeminal nerve branch associated with the development of hyperalgesia (Vit et al, 2006;Katagiri et al, 2012), but this has not been observed under non-pathological conditions (Shinoda et al, 2019). Interestingly, increased spontaneous pain behavior and corneal allodynia have been associated with morphological changes (hypertrophy) and upregulation of GFAP protein expression in the SGCs at the level of the ipsilateral trigeminal nerve V1 branch in preclinical models of corneal injury (Launay et al, 2016;Fakih et al, 2019;Reaux-Le Goazigo et al, 2019).…”

“…In addition, accumulative evidence reports that immune cells as monocytes/macrophages infiltrate into the TG and become activated, following orofacial pathologies, including peripheral trigeminal nerve trauma and orofacial inflammation (Iwata and Shinoda, 2019). Fundamental studies performed in mice reported that corneal injury induced an increase and activated resident and proliferated macrophages in mouse TGs (Ferrari et al, 2014;Launay et al, 2016;Fakih et al, 2019;Reaux-Le Goazigo et al, 2019). The infiltrated monocytes/macrophages show larger soma and thicker ramifications; such morphological structural changes indicate their activation (Shinoda et al, 2019).…”

Chronic dry eye induced corneal hypersensitivity, neuroinflammatory responses, and synaptic plasticity in the mouse trigeminal brainstem

“…The microscope was calibrated with samples from the sham mice before acquisitions of those from the DED mice. For the quantitative analysis of GFAP, Iba1, and Piccolo immunoreactivity, TG and TBSC sections were analyzed under epifluorescence microscope using a × 20 objective and the same camera parameters (Axio Vision ImagerM1; Carl Zeiss) as previously described [35]. Five ipsilateral TBSC and TG sections per animal were used for the DED and sham animals.…”

Novel Analgesics with Peripheral Targets

“…On the other hand, inhibition of both enzymes has been successfully tested in animals [22], but only with intrathecal application in humans [23]. Progression of enkephalin inhibitors has been discussed [24], and there is sparse but ongoing development for analgesia [25]. Pharmacological intervention with opioids leads to analgesia via a reduction in neuronal excitability [26].…”