The adrenergic α ₂ agonist lofexidine has been approved to treat opioid dependence while kratom has been used to mitigate opioid dependence in South Asia. The primary kratom alkaloid mitragynine has received attention due to its opioid pharmacology. However, the effects of mitragynine, including its discriminative stimulus and rate‐decreasing effects, have not been consistently blocked by the opioid antagonists naloxone and naltrexone. In contrast, the antinociceptive effects of mitragynine were blocked by the α ₂ antagonist idazoxan. The present study examined the opioid and α ₂ pharmacology of mitragynine with radioligand binding in cell membranes and drug discrimination in rats. The affinity of mitragynine at the μ‐opioid receptor (Ki=706nM) was 2‐, 10‐, 9‐ and 5‐fold higher than its affinity at the k‐ and d‐opioid, and a _2A and a _2C receptors, respectively, labelled with [ ³ H]DAMGO, [ ³ H]U69593, [ ³ H]DADLE, [ ³ H]RX821002 and [ ³ H]RX821002. In rats discriminating mitragynine (32mg/kg, i.p.), its ED ₅₀ value to produce mitragynine‐lever responding was 16mg/kg. The d‐, k‐, and μ‐opioid receptor agonists SNC80, U69593, and morphine, respectively, produced no greater than 60% mitragynine‐lever responding. The a ₂ agonists lofexidine and clonidine produced 74% and 45% mitragynine‐lever responding, respectively; the a ₂ antagonists yohimbine and atipamezole produced 40% and 46% mitragynine‐lever responding, respectively. Yohimbine (3.2mg/kg) and naltrexone (0.032mg/kg) antagonized the mitragynine discriminative stimulus, i.e., increased its ED ₅₀ by 2‐ and 5‐fold, respectively. Lofexidine (0.1mg/kg) and clonidine (0.32mg/kg) reduced the ED ₅₀ of the mitragynine discriminative stimulus by 9‐ and 5‐fold, respectively. In separate groups of rats discriminating either morphine (3.2mg/kg) or lofexidine (0.032mg/kg), mitragynine produced 72% morphine‐ and 100% lofexidine‐lever responding (ED ₅₀ s=11 and 15mg/kg). In the morphine‐trained rats, 0.032mg/kg naltrexone produced an 8‐fold rightward shift in the morphine discrimination dose‐effect function. In contrast, yohimbine, lofexidine and clonidine produced no more than a 2‐fold change in the morphine discrimination ED ₅₀ . The pharmacological selectivity of the effects of mitragynine as a test drug was further assessed using separate groups of rats discriminating either the adrenergic a ₁ agonist phenylephrine (3.2mg/kg) or the cannabinoid agonist THC (3.2mg/kg). Mitragynine produced 100% phenylephrine‐lever responding (ED ₅₀ =19mg/kg), but only 36% THC‐lever responding. Mitragynine had comparable binding affinity at opioid and a ₂ receptors in the micromolar range in vitro , and appeared to exhibit comparable μ‐opioid and a ₂ receptor agonist activity in vivo . By combining two separate pharmacologies in a single drug entity which otherwise requires treatment with two or more FDA approved drugs (e.g., methadone and lofexidine), mitragynine and its analogs may represent a useful approach for treating opioid use disorders. Support or Funding Information: NIDA grants DA23205 and DA48353.

The dual inhibitor of the 5-lipoxygenase-activating protein (FLAP) and the microsomal prostaglandin E₂ synthase-1 (mPGES-1), named BRP-187, represents a promising drug candidate due to its improved anti-inflammatory efficacy along with potentially reduced side effects in comparison to non-steroidal anti-inflammatory drugs (NSAIDs). However, BRP-187 is an acidic lipophilic drug and reveals only poor water solubility along with a strong tendency for plasma protein binding. Therefore, encapsulation in polymeric nanoparticles is a promising approach to enable its therapeutic use. With the aim to optimize the encapsulation of BRP-187 into poly(lactic-co-glycolic acid) (PLGA) nanoparticles, a single-phase herringbone microfluidic mixer was used for the particle preparation. Various formulation parameters, such as total flow rates, flow rate ratio, the concentration of the poly(vinyl alcohol) (PVA) as a surfactant, initial polymer concentration, as well as presence of a co-solvent on the final particle size distribution and drug loading, were screened for best particle characteristics and highest drug loading capacities. While the size of the particles remained in the targeted region between 121 and 259 nm with low polydispersities (0.05 to 0.2), large differences were found in the BRP-187 loading capacities (LC = 0.5 to 7.29%) and drug crystal formation during the various formulations.

Mitragyna speciosa (kratom) has been used as a treatment for opioid use disorder in Southeast Asia. The primary kratom alkaloid mitragynine has received much attention due to its opioid pharmacology. One of the in vivo metabolites of mitragynine, 7‐hydroxymitragynine, is a more potent μ‐opioid receptor agonist than mitragynine; this has led to the hypothesis that mitragynine is a prodrug that exerts opioid agonist activity, in part, through metabolic conversion to 7‐hydroxymitragynine. Here, we investigated this hypothesis by comparing mitragynine and 7‐hydroxymitragynine for 1) in vitro binding affinity ([ ³ H]DAMGO) and efficacy (GTPgS) at the μ‐opioid receptor, 2) plasma levels following p.o. mitragynine using a UPLC‐MS/MS method, and 3) potency and effectiveness to produce acute antinociception in the hotplate assay at 52° C. In the hotplate assay, the i.v. route of administration was used to minimize the conversion of mitragynine to 7‐hydroxymitragynine in order to directly compare the pharmacology of the two alkaloids. The binding affinity of 7‐hydroxymitragynine at the human μ‐opioid receptor (Ki=78 nM) was 22‐fold lower than morphine and 9.0‐fold higher than mitragynine. The % maximum GTPgS stimulations of 7‐hydroxymitragynine and morphine at the μ‐opioid receptor were 45% (up to 30 μM) and 90% (up to 30 μM), respectively. In contrast, mitragynine (up to 100 μM) did not significantly stimulate GTPgS. Following p.o. administration of mitragynine (HCl salt, 55 mg/kg), the Cmax value of 7‐hydroxymitragynine (85 ng/mL) was 14‐fold less than that of mitragynine. The Tmax values of 7‐hydroxymitragynine and mitragynine were 30 and 84 minutes, respectively. The % metabolite ratio [(AUC _{7‐hydroxymitragynine} /AUC _mitragynine )*100] was 8%. The % maximum possible effects (MPE, ED ₅₀ values in mg/kg, i.v.) of 7‐hydroxymitragynine (up to 0.1 mg/kg), morphine (up to 1.0 mg/kg) and mitragynine (up to 17.8 mg/kg) were 100% (1.9), 100% (5.1) and 34% (>17.8). In addition, 32 mg/kg mitragynine was lethal. Finally, drug discrimination was used as a pharmacologically selective measure of μ‐opioid receptor agonism in vivo . In rats discriminating morphine (3.2 mg/kg, i.p.) from vehicle, the discriminative stimulus effects of mitragynine were assessed 90 minutes after p.o. administration to correspond to its Tmax. Mitragynine (up to 178 mg/kg) produced 76% morphine‐lever responding (ED ₅₀ =51 mg/kg). Though the conversion rate of 7‐hydroxymitragynine from p.o. mitragynine is low, 7‐hydroxymitragynine is a more potent and efficacious μ‐opioid receptor agonist than mitragynine, suggesting that conversion to this metabolite may contribute to the in vivo μ‐opioid activity of mitragynine. Support or Funding Information: Supported by NIDA grants DA23205 and DA48353.

Opioid overdose is one of the leading causes of death in the United States in people under 50 years, which has resulted in a decline in life expectancy in the United States. Thus, there is a need for an alternative to prescription mu‐opioid agonists for pain treatment. Delta (9)‐tetrahydrocannabinol (THC), a cannabinoid (CB) agonist, has been shown to produce antinociceptive effects albeit less effectively than mu‐opioid receptor agonists. Sigma ₁ receptor (σ ₁ R) antagonists in combination with THC may provide a viable and safe pharmacological alternative to prescription opioids for pain treatment. The present study compared the pharmacological effects of the σ ₁ R antagonist CM304 alone and in combination with THC in Sprague Dawley rats. Hotplate latency was determined at 52°C followed by rectal temperature measurement and the drugs were administered intravenously (i.v.) and in cumulative doses every 5 min. THC dose‐dependently increased maximum possible effects (MPEs) up to 84% which were reversed by 10 mg/kg rimonabant (cannabinoid CB ₁ receptor antagonist). CM304 alone up to a dose of 10 mg/kg did not produce significant antinociceptive or hypothermic effects, but CM304 (1.0 and 3.2 mg/kg, i.v.) dose‐dependently left‐shifted the dose‐effect functions of the antinociceptive (7.3‐fold at 3.2 mg/kg CM304) and hypothermic effects of THC. In rats discriminating 3.2 mg/kg THC i.p. from vehicle under a fixed ratio (FR) 10 schedule of food delivery, the cannabinoid CB ₁ receptor agonist CP55,940 fully substituted for THC, whereas CM304 i.p. produced a maximum of 27 % THC‐lever responding at 17.8 mg/kg. CM304, at doses of 32 and 56 mg/kg, decreased response rate to 24 and 9 %, respectively. No dose of BD1063 (σ ₁ R antagonist), rimonabant, SR144528 (cannabinoid CB ₂ receptor antagonist), morphine (μ‐opioid receptor agonist), or naltrexone (opioid antagonist) produced greater than 30% THC‐lever responding. Pretreatment with rimonabant (1.78 mg/kg) produced a rightward shift in the dose effect curve of THC (4.5‐fold) and CP55940 (4.5‐fold) while pretreatment with CM304, BD1063, SR144528, and naltrexone had no significant effect on the dose effect curve of THC. The present results may support the development of a σ ₁ R antagonist as an adjunct to cannabinoids for treatment of acute pain without enhancing an undesirable side effect of THC. Support or Funding Information: This work was supported by National Institute on Drug Abuse grants DA23205 and DA48353.

There is an overdose epidemic associated with the use of illicitly manufactured as well as prescription mu‐opioid receptor agonists. Sigma ₁ receptor (σ ₁ R) antagonists in combination with other drugs may provide a viable, safer pharmacological option than opioids alone for treating pain. The present study compared pharmacological effects of the σ ₁ R antagonist CM304 alone and in combination with high (fentanyl and morphine) and low (buprenorphine) efficacy opioid agonists and the cannabinoid CB (CP55,940 and THC) receptor agonists in C57BL/6J mice. Rectal temperature, tail withdrawal latency (10‐seconds cutoff) from warm water of various temperatures (45°C, 50°C and 55°C) and counts of unhabituated locomotor activity were measured in this order. Basal latency for tail withdrawal systematically decreased from 10 seconds at 45°C to 1.3 seconds at 55°C. Morphine, fentanyl and buprenorphine dose‐dependently increased maximum possible effects (MPEs) up to 100% at 55°C (ED ₅₀ values: 7.50, 0.068, and 1.13 mg/kg, respectively), whereas CP55,940 and THC were less effective at 55°C (E _max values: 56.6% and 58.2%, respectively). CM304 (56 mg/kg, i.p.) produced an upward shift in the CP55,940 dose‐effect function such that 100% MPE was achieved at 3.2 mg/kg CP55,940, while CM304 (56 mg/kg) produced a 3‐fold leftward shift in the dose‐effect curve of THC. On the other hand, CM304 did not enhance the antinociceptive effects of morphine and fentanyl but enhanced the effects of buprenorphine. In contrast to tail withdrawal latency, there were no consistent effects of CM304 on cannabinoid‐induced decreases in rectal temperature or locomotor activity. The present results may support the development of a σ ₁ R antagonist as an adjunct to cannabinoid and low efficacy μ‐opioid receptor agonists for the treatment of acute pain. Support or Funding Information: This work was supported by National Institute on Drug Abuse grants DA23205 and DA48353.