Plant Iridoid Glycosides: Biological Properties and Pharmacological Potential

Iridoid glycosides are a diverse group of plant secondary metabolites with broad biological activity and promising pharmacological potential. This review summarizes their structural diversity, occurrence in plants, biosynthetic pathways, and biological properties. Particular attention is given to the biosynthesis derived from IPP and DMAPP, including the well-characterized secoiridoid pathway and the less understood pathway leading to compounds such as aucubin and catalpol. The most important biological activities of iridoids, including antibacterial, anti-inflammatory, neuroprotective, gastroprotective, and anticancer effects, are discussed with clear distinction between in vitro studies, animal models, and limited clinical evidence. The review emphasizes that most available data remain preclinical, while clinical studies are still scarce. Major limitations affecting the therapeutic application of iridoids include low bioavailability, variable chemical stability, lack of extract standardization, and difficulties in translating experimental findings into clinical practice.