Evaluation of Kratom Opioid Derivatives as Potential Treatment Option for Alcohol Use Disorder

Abstract

Background and Purpose: Mitragyna speciosa extract and kratom alkaloids decrease alcohol consumption in mice at least in part through actions at the δ-opioid receptor (δOR). However, the most potent opioidergic kratom alkaloid, 7-hydroxymitragynine, exhibits rewarding properties and hyperlocomotion presumably due to preferred affinity for the mu opioid receptor (µOR). We hypothesized that opioidergic kratom alkaloids like paynantheine and speciogynine with reduced µOR potency could provide a starting point for developing opioids with an improved therapeutic window to treat alcohol use disorder. Experimental Approach: We characterized paynantheine, speciociliatine, and four novel kratom-derived analogs for their ability to bind and activate δOR, µOR, and κOR. Select opioids were assessed in behavioral assays in male C57BL/6N WT and δOR knockout mice. Key Results: Paynantheine (10 mg∙kg^-1, i.p.) produced aversion in a limited conditioned place preference (CPP) paradigm but did not produce CPP with additional conditioning sessions. Paynantheine did not produce robust antinociception but did block morphine-induced antinociception and hyperlocomotion. Yet, at 10 and 30 mg∙kg^-1 doses (i.p.), paynantheine did not counteract morphine CPP. 7-hydroxypaynantheine and 7-hydroxyspeciogynine displayed potency at δOR but limited µOR potency relative to 7-hydroxymitragynine in vitro, and dose-dependently decreased voluntary alcohol consumption in WT but not δOR in KO mice. 7-hydroxyspeciogynine has a maximally tolerated dose of at least 10 mg∙kg^-1 (s.c.) at which it did not produce significant CPP neither alter general locomotion nor induce noticeable seizures. Conclusion and Implications: Derivatizing kratom alkaloids with the goal of enhancing δOR potency and reducing off-target effects could provide a pathway to develop novel lead compounds to treat alcohol use disorder with an improved therapeutic window.

Keywords: alcohol use disorder; biased signaling; delta opioid receptor; kratom; nociception; reward; seizures.

FIGURE 1

Blocking μOR attenuates 7-hydroxymitragynine (7OHM) induced hyperlocomotion. (A) 90-min ambulation time course of wild-type, C57Bl/6 male mice (n = 5 per group) treated with 7-hydroxymitragynine (3 mg∙kg⁻¹, i.p.) after pretreatment with the vehicle (s.c.) or naloxone (1 mg∙kg⁻¹, s.c., NLX) injection (10 min prior to 7-hydroxymitragynine injection). (B) Total ambulation (area under the curve) for the same data set. ***p < 0.001 (for details, see Supplementary Table S2).

FIGURE 2

Antagonistic action of paynantheine in vivo. The agonistic and antagonistic actions of kratom alkaloid paynantheine were further investigated in C57Bl/6 mice. Paynantheine (10 mg∙kg⁻¹, i.p. PAYN) was evaluated in a (A) 4-day and (B) 10-day model of conditioned place preference (CPP, two vs. four drug conditioning sessions, respectively, n = 8 each). (C) Seizure activity induced by paynantheine (30 mg∙kg⁻¹, i.p.) was evaluated in male δOR KO and WT mice (n = 5 per group). (D) Paynantheine (10 and 30 mg∙kg⁻¹, i.p.) was tested for agonist and antagonistic properties in male mice (n = 10 per dose) via the tail flick thermal nociception assay. For the antagonist assays, morphine (6 mg∙kg⁻¹, s.c., MOR) was administered 10 min following a dose of paynantheine (10 or 30 mg∙kg⁻¹, i.p.). Nociception data are expressed as maximum possible effect (%MPE) normalized to a saline baseline (treatment–saline baseline). (E) Paynantheine (10 and 30 mg∙kg⁻¹, i.p.) was evaluated for agonist and antagonist activity in an acute model of conditioned place preference by administering 10 min prior to morphine (6 mg∙kg⁻¹) or the vehicle (n = 8 for 10 mg∙kg⁻¹ doses, n = 6 for 30 mg∙kg⁻¹ dose). Locomotor data were extracted from the conditioning sessions of the CPP experiments in (A,E) and is shown as (F) ambulation over time and (G) total ambulation (total area under curve). For comparison in (F,G), locomotor data for morphine (6 mg∙kg⁻¹ morphine) was extracted from a previous CPP experiment with 30-min conditioning sessions. The vehicle locomotor data were extracted from the non–drug-paired side conditioning session for the 10 mg∙kg⁻¹ paynantheine + vehicle group. For locomotor data in (G), statistical significance of drug treatment vs. vehicle (VEH + VEH) is shown with stars; statistical significance between paynantheine + morphine treatments and morphine-only treatment (MOR) is shown with carets. *p < 0.05, **p < 0.01, ^^^p < 0.001, *** or ^^^^p < 0.0001 (for details, see Supplemental Tables S2–S5).

FIGURE 3

Synthesis and characterization of kratom alkaloid analogs. Structures of naturally occurring kratom alkaloids paynantheine and speciogynine were used as scaffolds for analog synthesis. Analogs with pseudo-indoxyl (PI) rearrangements or hydroxyl group additions were made for both compounds, and a naturally occurring minor kratom alkaloid and speciogynine isomer, speciociliatine, was also synthesized for testing. (A) Chemical structures of selected indole-based kratom alkaloids; (B) synthesis of 7-hydroxypaynantheine (7) and paynantheine pseudoindoxyl (8); and (C) synthesis of 7-hydroxyspeciogynine (9) and speciogynine pseudoindoxyl (10). 7-hydroxyspeciogynine (7OH SPG, 9), 7-hydroxypaynantheine (7OH PAYN, 7), and speciociliatine (SPECIO, 4) are compared to kratom alkaloids (dashed lines; 7-hydroxymitragynine (7OH MITRA), paynantheine (PAYN), and speciogynine (SPG) for inhibition of forskolin-induced cAMP in a GloSensor assay in transfected HEK-293 cells at δOR (D) and μOR (E). For additional in vitro characterization, see Supplemental Figure S4.

FIGURE 4

Speciociliatine decreases voluntary ethanol consumption and impairs motor coordination in wild-type and δOR knockout mice. 10% ethanol consumption, water consumption and ethanol preference in male C57BL/6 (A–C, respectively) (n = 11) and δOR KO (D–F, respectively) mice (n = 10) in a voluntary twobottle choice, limited access, drinking-in-the-dark paradigm, following treatment with speciociliatine (3, 10, and 30 mg⋅kg⁻¹, i.p.) (G) 150-minute duration rotarod assessment of motor incoordination inWTmice (n = 8) and δOR KO mice (n = 7), immediately followed by a 30 mg⋅kg⁻¹ dose of speciociliatine (i.p.); significance for WT mice and δOR KO mice is denoted with stars and carets, respectively. Open circles are the average intake/preference on the preceding 3 days (baseline), and closed circles are the intake on Fridays following drug exposure. * or^p < 0.05, ** or^p < 0.01, ***p < 0.001, ****p < 0.0001 (for details, see Supplemental Tables S6–S8).

FIGURE 5

Kratom analogs decrease voluntary ethanol consumption in mechanisms partially dependent on δOR. 10% ethanol consumption (left column), water consumption (middle column), and ethanol preference (right column) in male C57Bl/6 wild-type mice following treatment with (A–C) 7-hydroxyspeciogynine (3 and 10 mg⋅kg⁻¹, s.c., n = 12, 7OH SPG; 7OHS), (D–F) 7-hydroxypaynantheine (10 and/or 30 mg⋅kg^{− 1}, s.c., n = 8, 7OH PAYN; 7OHP), and in (G–I) male δOR KO mice (n = 9), following treatment with effective doses of both analogs in a voluntary two-bottle choice, limited access, drinking-in-the-dark paradigm. Open circles are the average intake/preference on the preceding 3 days (baseline), and closed circles are the intake on Fridays following drug exposure. *p < 0.05, **p < 0.01, ****p < 0.0001 (for details, see Supplemental Tables S6–S8).

FIGURE 6

Alcohol-modulating effects of 3 mg⋅kg⁻¹ 7-hydroxyspeciogynine are not sex specific. In WT female mice (n = 10), effects of 3 mg⋅kg⁻¹ 7-hydroxyspeciogynine (s.c., 7OG SPG) on 10% ethanol consumption (A), water consumption (B), and ethanol preference (C) were evaluated in a voluntary two-bottle choice, limited access, drinking-in-the-dark paradigm. Male and female responses to 7-hydroxyspeciogynine (3 and 10 mg⋅kg⁻¹, s.c.) in the two-bottle choice paradigm were pooled and are shown as (D) change (Δ) in 10% ethanol consumption, (E) change (Δ) in water consumption, and (F) change (Δ) in ethanol preference. In panels (A–C), open circles are the average intake/preference on the preceding 3 days (baseline), and closed circles are the intake on Fridays following drug exposure. In panel (D–F), female and male mice are depicted with blue and orange symbols, respectively. *p < 0.05, **p < 0.01 (for details, see Supplemental Tables S6–S9).

FIGURE 7

Side effect profile of 10 mg⋅kg⁻¹ 7-hydroxyspeciogynine. (A) In a 10-day conditioned place preference (CPP) paradigm, the rewarding effects of 7-hydroxyspeciogynine (s.c.) were evaluated in male, WT mice (n = 8). (B) Locomotor data were extracted from the CPP experiment in (A) and averaged across all vehicle/ drug treatment days (n = 7). (C) The highest Racine score collected every 3 min for 30 min following administration of 7-hydroxyspeciogynine was evaluated for 30 min after drug administration (n = 9). (D) 7-hydroxyspeciogynine was tested for agonist and analgesic properties in male mice via the tail flick thermal nociception assay (n = 10). In the same paradigm, antagonistic effects were evaluated after administering 7-hydroxypeciogynine, followed by morphine (6 mg⋅kg⁻¹, s.c.) 10 min later (n = 6), and were compared to vehicle plus morphine administration (n = 5) (for statistical details, see Supplemental Tables S2–S5).