Pharmacological characterization of kratom alkaloids at opiate receptors: binding affinities, in vitro and in vivo functional assessments

Abstract

Kratom (Mitragyna speciosa) is consumed by 10‐15 million individuals in the US for its mood‐elevating effects, as an alternative to traditional opiates for pain relief and to attenuate opiate withdrawal symptoms. Complex natural products, such as kratom, contain a variety of chemicals whose combined effects can only be understood by the meticulous identification and characterization of the individual chemicals and their effects in preclinical model systems. Twenty‐five indole and oxindole kratom alkaloids were evaluated for mu (MOR), delta (DOR) and kappa (KOR) opiate receptors binding affinities, opiate receptor mediated G‐protein activation, inhibition of forskolin stimulated cAMP accumulation and b‐arrestin2 recruitment. Several indole and oxindole alkaloids exhibited partial agonism at the MOR (7‐hydroxymitragynine, corynoxine A > epiallo‐isopaynantheine, isospeciofoline, speciociliatine, corynoxine B > mitragynine, 3‐epicorynoxine B > paynantheine and speciogynine) – all of which also exhibited strong G protein bias. Mitraciliatine and isopaynantheine were weak MOR antagonists. Speciophylline exhibited positive allosteric modulatory activity at the MOR as evidenced by increased affinity of DAMGO binding and potentiation of [met]‐enkephalin‐induced inhibition of forskolin stimulated cAMP activity in the absence of intrinsic activity. Corynoxine A induced displayed similar affinity at the mu opiate receptor as 7 hydroxymitragynine but was less potent in the nociceptive hot plate assay and the mechanical allodynia von Frey test. Corynoxine did not significantly alter respiration or heart rate or induce a conditioned place preference. Results demonstrate the pharmacological complexity of kratom alkaloids and have important implications for determining the combined effects of less abundant indole and oxindole alkaloids with regard to the therapeutic as well as potential harmful effects of kratom.