Iridoids pharmacological activities

Galvez M, Martin-Cordero C, Ayuso MJ. Pharmacological Activities of Iridoids Bio-S3mthesized by Route II. Studies in Natural Products Chemistry 2005 32 365- 394. 

Tundis R, Loizzo MR, Menichini F, Statti GA, Menichini F. Biological and Pharmacological Activities of Iridoids Recent Developments. Mini-Reviews in Medicinal Chemistry 2008 8 399-420. 

ABSTRACT This paper attempts to present a review on the study of phytochemical and pharmacological activities of plants from the genus Hedyotis (Rubiaceae) in the last seven decades, which include our work on Malaysian Hedyotis species. The structure-activity relationships of compounds isolated from this genus are compiled and discussed. Finally, there is also a brief discussion on the biosynthesis of anthraquinones, iridoid glycosides and alkaloids, which are the common constituents of Hedyotis species. 

Iridoids and secoiridoids show a wide variety of biological and pharmacological activities which have been reviewed by Ghisalbert [6] up to 1998. We report here a review of the most relevant compounds of this type isolated from Oleaceae. 

The relevance of these compounds is in their properties, namely antiinflamatory, muscular relaxing, sedative, antihistaminic hypoglucemical and hypotensive, along with those attributed to iridoids. Therefore, the hydrolysis reaction does not mean the loss of pharmacological activities but it originates a dialdehylde-hemiketal intermediate with much more bioactive strength. The aldehyde may be considered as an amine acceptor in the formation of a Shiff base. 

It seems that the pharmacological activities showed by different iridoids are due to their genine, like aucubin. The structural similarity between the aglycone and glutaraldehyde suggests a similar mechanism of enzyme inhibition through protein cross-linking by Schiff reactions. 

A number of the pharmacological activities of iridoids are due to the aglycone, because the activity appears only after treatment with p-glucosidase, or only when the pure aglycone is tested (Stermitz, 1988). 

Contents L. Jaenicke and F.-J. Marner The Irones and Their Precursors. — M. Lounasmaa and P. Somersalo The Condylocarpine Group of Indole Alkaloids. — U. Sequin The Antibiotics of the Pluramycin Group (4//-Anthra [l,2- ]pyran Antibiotics). — R. M. Wenger Cyclosporine and Analogues — Isolation and Synthesis — Mechanism of Action and Structural Requirements for Pharmacological Activity. — H. Inouye and S. Uesato Biosynthesis of Iridoids and Secoiridoids. 

Iridoids have been isolated from several folk medicinal plants which are used as bitter tonics, sedatives, antipyretic, cough medicine, remedies for arthritis, lumbago and muscular pains, wounds and skin disorders, etc. A good number of research papers on the bioactive iridoids isolated from several folk medicinal plants have been published in the last few decades. We have summarized here only some of the fundamental pharmacological activities of iridoids to create an interest oti the importance of this class of natural products and for their wide applications in the formulations of several prospective herbal drugs. 

Ghisalberti, E.L. (1998) Biological and pharmacological activity of naturally occurring iridoids and secoiridoids. Phytomedicine, 5,147 163. 

Numerous iridoids are involved in plant-animal interactions e.g. the defensive function of this type of compounds in ants. From the pharmacological standpoint, the applications of this class of compounds are rather limited. Some iridoids have anti-inflammatory activity, which is weak by the oral route and stronger by topical application e.g. 1 mg of aucubin, verbenalin, or loganin have an activity almost similar to that of 0.5 mg of indomethacin on the TPA-induced mouse ear edema. Some are ingredients in various forms of allopathic medications (valerian), others are typically, phytotherapeutic products (devil s claw, olive tree). Others receive attention for their non-pharmaceutical applications (yellow gentian). The hepatoprotective effects of picrosides I and II from kutkin, the crude active fraction in Picrorhiza kurroa is well known and documented. 

The pharmacological assays and activities of natural iridoids biosynthesized by route II, extracted from a variety of plants are summarized in this review, such as antiinflammatory, antitumoral-chemopreventive, hepatoprotective and healing. Structure-activity relationships are also discussed. 

A current compilation of Mexican medicinal plants indicates that no pharmacological studies have been carried out to test the presumed properties of the decoction [13]. Chemical studies of the aerial parts of Penstemon barbatus have afforded four aucubin iridoid glycosides catalpol (53), scutellarioside (54), globularin (55) and barbatoside (56) "Fig. (15)." The two former compounds were isolated as their acetate derivatives [101]. Several iridoid glycosides have shown anti-inflammatory activity in experimentally induced edema in mice best results were obtained when administered topically. The most active compounds were two catalpol derivatives, besides aucubine, verbalin and loganin these compounds induced a 72 to 80% edema inhibition when applied at a dose of 1 mg/ear [102]. 

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