Cannabinoid CB2 Receptor Activation Attenuates Fentanyl-Induced Respiratory Depression
Carmen A. Zavala, Ana C. Thomaz, Vishakh Iyer, Ken Mackie, and Andrea G. Hohmann
Cannabis and Cannabinoid Research, 2021, Volume 6, Number 5, 389-400.
Introduction : Overdose fatalities associated with the opioid epidemic are predictably attributable to druginduced respiratory depression. In terms of illicit opioid abuse, fentanyl is the synthetic opioid responsible for the largest number of overdose deaths. There is, therefore, an urgent need to identify safe and effective therapeutics that can attenuate fentanyl-induced respiratory depression. Identification of effective alternate analgesic strategies that lessen the respiratory depression associated with narcotics would also help improve current strategies for pain management. Our laboratory recently reported that the G protein-biased CB2 cannabinoid receptor agonist LY2828360 suppressed chemotherapy-induced neuropathic nociception and attenuated both morphine tolerance and physical dependence in paclitaxel-treated mice. However, the impact of LY2828360 on other undesirable side effects of opioids, such as opioid-induced respiratory depression, remains unknown.
Materials and Methods : We used whole-body plethysmography to assess the impact of the CB2 cannabinoid agonist LY2828360 on fentanyl-induced respiratory depression using wild-type (WT) and CB2 knockout (CB2KO) mice.
Results : Fentanyl reduced minute ventilation and respiratory frequency without altering tidal volume in both WT and CB2KO mice. In WT mice, the high dose of fentanyl (0.2 mg/kg intraperitoneal [i.p.]) produced a greater suppression of respiratory parameters compared with the low dose of fentanyl (0.1 mg/kg i.p.). Coadministration of a behaviorally active dose of LY2828360 (3 mg/kg i.p.) with fentanyl (0.2 mg/kg i.p.) attenuated fentanyl induced respiratory depression in WT mice. Notably, LY2828360 (3mg/kg i.p.) did not attenuate fentanyl-induced respiratory depression in CB2KO mice, consistent with mediation by CB2 receptors. Moreover, LY2828360 (3 mg/kg i.p.) alone lacked intrinsic effects on respiratory parameters in either WT or CB2KO mice.
Conclusion : The combination of a CB2 agonist with fentanyl may represent a safer adjunctive therapeutic strategy compared with a narcotic analgesic alone by attenuating the development of opioid-induced respiratory depression. Moreover, the CB2 agonist, administered alone, did not alter respiration. Our findings suggest that the CB2 cannabinoid agonist LY2828360 may provide CB2-mediated protection against fentanyl-induced respiratory depression, a detrimental and unwanted side effect of opioid use and abuse.
Keywords : CB2 receptor; endocannabinoid; fentanyl; opioid overdose; respiratory depression
Opioid-related overdose deaths have been driven primarily by the synthetic opioid fentanyl and its analogs. 1,2 Fentanyl, originally developed as a rapidly acting analgesic, is much more potent than natural opioids such as morphine.3 Accidental contamination or intentional mixing of street drugs with fentanyl also creates an opioid mix of unpredictable and variable potency and increases fatal overdose risk.4,5 Fentanyl readily enters the central nervous system where it decreases respiratory drive and increases mechanical resistance to breathing (by decreasing thoracic compliance), potentially inducing fatality within minutes of administration. 6–8 Naloxone, a nonselective and competitive opioid receptor antagonist, is the only pharmacological treatment available to reverse such overdoses. However, due to the high potency of fentanyl and its longer duration of action,multiple naloxone doses are often required to provide sustained overdose reversal, increasing the risk of unsuccessful treatment and death.9,10 Reports of unsuccessful attempts to revive patients with naloxone persist despite administration of multiple or escalating doses due to the short window between drug intake and overdose.9,11–14 Thus, identification of safe and effective alternative/supplementary therapeutic strategies to attenuate opioid-induced respiratory depression remains an unmet clinical need.
The endocannabinoid system consists of CB1 and CB2 cannabinoid receptors, their endogenous lipid signaling molecules (endocannabinoids), and enzymes that control their synthesis and degradation.15,16 This neuromodulatory system can mitigate unwanted side effects of opioids.17 Endocannabinoids depress respiration in a CB1-mediated manner.18,19 Furthermore, selective CB1 agonists induce respiratory depression in a manner blocked by CB1 antagonists.20–23 By contrast, the CB2 receptor has been postulated as a potential therapeutic target to overcome the side effects of direct acting CB1 agonists or opioids. CB2 receptors are predominantly expressed in cells within the immune system24– 27 and are upregulated in response to injury or inflammation.28–30 Functional CB2 receptors are also present in neuroanatomical regions controlling respiration, including the brainstem and are activated by elevated levels of endocannabinoids.27,31,32 Furthermore, low levels of CB2 receptors are also reported in respiratory tissues such as the lungs.24,33,34 Although CB2 mechanisms help maintain opioid-induced analgesic efficacy through synergistic interactions with l-opioid receptors,35–38 their effects on opioid-induced respiratory depression, remain unexplored.
Our laboratories recently reported that the G proteinbiased cannabinoid CB2 receptor agonist LY2828360 attenuated morphine tolerance and physical dependence while also attenuating neuropathic nociception in a paclitaxel model of chemotherapy induced neuropathy.35
This LY2828360-induced attenuation of morphine tolerance was absent in paclitaxel-treated CB2 knockout (CB2KO) mice, consistent with CB2-receptor mediation. 35 Moreover, morphine-dependent CB2KO mice treated with paclitaxel showed higher levels of naloxoneprecipitated opioid withdrawal compared with their wild type (WT) counterparts.35 LY2828360 was previously evaluated in a phase-2 clinical trial for osteoarthritic pain where it failed for lack of efficacy but was otherwise safe in humans (www.clinicaltrials.gov identifier: NCT01319929).39 These observations raise the possibility that CB2 activation by LY2828360 may safely restore homeostasis and counter the impact of aberrant challenges such as opioid-induced side effects.
In this study, we used whole-body plethysmography to examine the impact of the CB2 agonist LY2828360 on respiratory depression induced by the synthetic opioid fentanyl. We characterized the impact of acute fentanyl administration on distinct respiratory parameters (i.e., minute ventilation, respiratory frequency, and tidal volume) in otherwise naive mice. We also assessed the impact of LY2828360 treatment on fentanyl-induced changes in respiratory parameters using both WT and CB2KO mice, to confirm CB2 receptor mediation.