Proenkephalin-derived BAM8-22 peptide analogue alleviates neuropathic corneal pain in mice

Abstract

Purpose : Damage to the corneal sensory nerve endings contributes to the emergence of refractory pain in neuropathic corneal pain (NCP), for which there are currently no approved therapies available. Proenkephalin-derived endogenous peptide, BAM8-22, has been attributed with generating analgesia. In this study, we evaluated the effect of topical ophthalmic therapy with BAM8-22 peptide analogues on NCP, in a murine model of ciliary nerve ligation.

Methods : Eye-wipe response to topical hyperosmolar saline (HOS) was used as the primary outcome measure. Following the establishment of a baseline eye-wipe response to HOS [5M], 10–12 week-old male mice underwent ciliary nerve ligation for induction of NCP. Post-surgery, corneal fluorescein staining (CFS) and Cochet-Bonnet esthesiometry were used to assess ocular epithelial integrity and corneal sensitivity in operated mice that had a positive corneal pain outcome. Animals were randomized in treatment groups (n=5) and topically treated with lipidated BAM8-22, non-lipidated BAM8-22, and vehicle for 11 days. Systemic gabapentin was used as the positive control. Excised corneal whole mounts were immunostained for ßIII-Tubulin and CD45 to assess nerve and immune cell density alterations.

Results : NCP induction surgery generated a sustained pain response in all mice (24.46±1.32 vs.12.62±1.25 eye wipes/30 sec, p<0.05). Corneal sensitivity amongst all the treatment groups remained intact and no corneal epithelial defects were observed post-surgery. Topical application of both lipidated BAM8-22 analogue (24.2±1.59 vs.12.6±0.98, p=0.045) and non-lipidated BAM8-22 (25.17±1.66 vs.13.18±0.7, p=0.018) resulted in a decrease in pain response compared to vehicle treatment. Systemic gabapentin treated mice were used as the positive control and showed a lowered pain response (25.5±2.25 vs.12.5±2.6, p=0.011). Corneal nerve density decreased in all the NCP mice compared to sham (64884.53±4353.51 vs. 152348.87±6344.17 mm/mm², p<0.0001). Among all the different NCP treatment groups, significantly higher corneal nerve density was detected in non-lipidated BAM8-22 treated mice as compared to vehicle (104188.79±5622.31 vs. 54429.42±6343.29 mm/mm², p<0.0001) while no significant differences were observed in the CD45⁺ cell density among groups (P>0.05).

Conclusions : BAM8-22 analogues, administered topically, were effective in alleviating neuropathic corneal pain in our murine model of NCP.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.