Opioid-sparing Effects of the Nrf2 Activator Diroximel Fumarate in Mice

Abstract

Opioids are still considered as a gold standard for the treatment of acute and chronic pain. However, their long-term administration can result in tolerance and paradoxical opioid-induced hyperalgesia (OIH). We hypothesized that activating the antioxidant transcription factor Nrf2 would alleviate morphine tolerance and OIH. Diroximel fumarate (DRF), an FDA approved drug, was used to activate Nrf2. For the OIH experiment, naïve mice were implanted with subcutaneous osmotic minipumps to infuse morphine dose for 7 days (10 mg/kg/day). Oral DRF (100 mg/kg/day) or vehicle was administered the day before minipump implantation, and then daily for the 7-day morphine infusion. For the tolerance experiment, naïve mice were administered with subcutaneous morphine (5 mg/kg/twice daily) or saline along with oral DRF (100 mg/kg/day) or vehicle for 5 days. On day 5, cumulative doses of morphine (1.8, 3.2, 5.6, 10.0 and 18.0 mg/kg) were injected after every 20 min. Withdrawal thresholds to heat and mechanical stimuli were assessed using hot plate tests and Von Freys, respectively. Male and female mice treated with morphine alone had decreased withdrawal thresholds to heat and mechanical stimuli by day 7, indicating development of OIH. However, DRF co-treatment blunted development of OIH. Mice treated with DRF alone did not show any change in withdrawal thresholds. Similarly, cumulative doses of morphine on day 5 caused a rightward shift in the dose response curve of morphine treated mice when compared to saline, showing development of tolerance. DRF co-treatment partially prevented development of tolerance. Expression of antioxidant genes/proteins in the spinal cord and brain will also be presented and Nrf2 positive cells identified. DRF has the potential to partially prevent the development of OIH and tolerance, thereby suggesting that DRF has opioid-sparing properties. Future studies will investigate the effects of DRF on other adverse effects of opioids. Grant support from NIH 1RF1NS113840-01. Opioids are still considered as a gold standard for the treatment of acute and chronic pain. However, their long-term administration can result in tolerance and paradoxical opioid-induced hyperalgesia (OIH). We hypothesized that activating the antioxidant transcription factor Nrf2 would alleviate morphine tolerance and OIH. Diroximel fumarate (DRF), an FDA approved drug, was used to activate Nrf2. For the OIH experiment, naïve mice were implanted with subcutaneous osmotic minipumps to infuse morphine dose for 7 days (10 mg/kg/day). Oral DRF (100 mg/kg/day) or vehicle was administered the day before minipump implantation, and then daily for the 7-day morphine infusion. For the tolerance experiment, naïve mice were administered with subcutaneous morphine (5 mg/kg/twice daily) or saline along with oral DRF (100 mg/kg/day) or vehicle for 5 days. On day 5, cumulative doses of morphine (1.8, 3.2, 5.6, 10.0 and 18.0 mg/kg) were injected after every 20 min. Withdrawal thresholds to heat and mechanical stimuli were assessed using hot plate tests and Von Freys, respectively. Male and female mice treated with morphine alone had decreased withdrawal thresholds to heat and mechanical stimuli by day 7, indicating development of OIH. However, DRF co-treatment blunted development of OIH. Mice treated with DRF alone did not show any change in withdrawal thresholds. Similarly, cumulative doses of morphine on day 5 caused a rightward shift in the dose response curve of morphine treated mice when compared to saline, showing development of tolerance. DRF co-treatment partially prevented development of tolerance. Expression of antioxidant genes/proteins in the spinal cord and brain will also be presented and Nrf2 positive cells identified. DRF has the potential to partially prevent the development of OIH and tolerance, thereby suggesting that DRF has opioid-sparing properties. Future studies will investigate the effects of DRF on other adverse effects of opioids. Grant support from NIH 1RF1NS113840-01.