A novel mitragynine analog with low efficacy mu-opioid receptor agonism displays antinociception with attenuated adverse effects

Abstract

Dried kratom leaves are anecdotally used for the treatment of pain, opioid dependence, and alcohol use disorder. We have previously shown that kratom and its natural products (mitragynine) and semi-synthetic analogs (7-hydroxy mitragynine (7OH) and mitragynine pseudoindoxyl) are mu opioid receptor (MOR) agonists that show minimal beta-arrestin2 recruitment. To further investigate the structure activity relationships of G-protein potency, efficacy, and beta-arrestin2 recruitment, we diversified the mitragynine/7OH templates at the C9, -10 and -12 positions of the aromatic ring of the indole moiety. Three lead C9 analogs, synthesized by swapping the 9-methoxy group with varied substituents, namely phenyl (SC11), methyl (SC12), 3-furanyl (SC13), were further characterized using a panel of in vitro and ex vivo electrophysiology assays. All three compounds were partial agonists with lower efficacy than both DAMGO and morphine in heterologous G-protein assays and synaptic physiology. SC11-13 also showed lower recruitment of both β-arrestin subtypes compared to DAMGO, and in assays with limited MOR receptor reserve, the G-protein efficacy of SC11, SC12 and SC13 was comparable to buprenorphine. In mouse models, at equianalgesic doses SC13 showed MOR-dependent analgesia with potency similar to morphine without respiratory depression, hyperlocomotion, constipation, or place conditioning. Taken together, these results suggest that MOR agonists with a G-protein efficacy profile similar to buprenorphine can be developed into opioids that are effective analgesics with greatly reduced liabilities.

The respiratory depressant effects of mitragynine are limited by its conversion to 7‐OH mitragynine

“…The anti‐nociceptive effects of both mitragynine and 7‐OH mitragynine were abolished in μ receptor knockout mice, but not in mice lacking either the δ or the κ opioid receptors (Kruegel et al, 2019). The analgesic effects of kratom, together with the low rates of acute toxicity or overdose following its ingestion (Veltri & Grundmann, 2019), have attracted considerable attention in the search for novel, safer opioid‐based analgesics amid the current opioid crisis (Bhowmik et al, 2021; Chakraborty, Diberto, Faouzi, et al, 2021; Chakraborty, Uprety, Daibani, et al, 2021; Gutridge et al, 2020).…”

Modified Akuamma Alkaloids with Increased Potency at the Mu-opioid Receptor

“…To evaluate the G-protein signaling properties of 33, we employed the recently developed TRUPATH assay (Figure 3B-C). 8,40 The GloSensor assay measures the inhibition of cAMP production by adenylate cyclase as a downstream measure of activation of the Gi/o -signaling cascade induced by agonists activating a GPCR. By relying on the measurement of downstream events, these assays can overestimate the efficacy of ligands at the opioid receptors.…”

“…By relying on the measurement of downstream events, these assays can overestimate the efficacy of ligands at the opioid receptors. 41 Conversely, the TRUPATH assay system employs bioluminescence resonance energy transfer (BRET) biosensors to measure the dissociation of the G-protein heterotrimer that initiates the signaling cascade. Importantly, the TRUPATH system can interrogate activation of specific Gαi/o transducers, which in turn, could reveal signaling bias for different ligands.…”

“…Several promising strategies have emerged to develop safer opioid analgesics including the development of biased agonists, 6 mixed pharmacology, 7 partial agonism, [8][9][10] and investigation of delta and kappa opioid receptor (δOR and κOR) agonists. 11 The exploration of each of these strategies has been greatly aided by the discovery and design of novel opioid ligands, particularly those that lack the common pharmacophore shared by morphinan and fentanyl-derived analgesics.…”

“…12 More recently, of the structure-activity relationships (SAR) studies on the mitragynine alkaloids have revealed how even small changes to the scaffold allow for the fine-tuning of opioid receptor signaling. 8,[13][14][15] In addition to providing new chemical tools to probe the opioid receptor and its function, the development of these novel classes of opioids offers new opportunities to develop potential analgesics.…”