Mechanistic Insights, Structure-activity Relationships, and Safety Profile of Mitragyna speciosa Alkaloids

Abstract

Mitragyna speciosa, better known as kratom, is a traditional medicinal plant from Southeast Asia. It contains mainly two indole alkaloids, mitragynine and 7- hydroxymitragynine, which differ from classical opioids by displaying a unique mode of opioid pharmacology. This review discusses the chemistry, Structure-Activity Relationships (SAR), receptor mechanisms, metabolism, and safety of Mitragyna speciosa alkaloids, with particular reference to biased opioid receptor signaling and their translational implications. Articles were identified through a literature search in PubMed, Scopus, and Web of Science from 2000 to January 2025. Peer-reviewed studies discussing the phytochemistry, pharmacology, pharmacokinetics, metabolism, toxicology, and human observational studies were considered. Non-peer-reviewed and poorly designed studies were excluded. Mitragynine and 7-hydroxymitragynine are partial agonists of μ-opioid receptors, promoting preferential G protein signaling with minimal β-arrestin recruitment. This may explain their reduced capacity to cause respiratory depression compared to standard opioids. Based on SAR studies, hydroxylation at C-7, the substitution at C-9, and the stereochemistry of the indole core constitute the major factors controlling receptor binding affinity, signaling bias, and functional selectivity. Besides opioid effects, kratom alkaloids also interact with adrenergic, serotonergic, dopaminergic, and NMDA receptors, resulting in dose-dependent polypharmacology. Alkaloids from Mitragyna speciosa have been identified as a unique class of plantderived small molecules. Integrating SAR, metabolic activation, and biased receptor signaling improves the mechanistic understanding of their therapeutic potential and safety, thereby emphasizing the need for standardized preparation and controlled clinical trials. However, its clinical applicability is limited due to an insufficient number of well-designed controlled human studies.