Analgesic diterpenoids with diverse carbon skeletons from the leaves of Rhododendron auriculatum

Discovery of ent-kaurane diterpenoid glucosides as potent analgesics from the leaves of Pieris formosa

To search for structurally novel analgesics from Ericaceae plants, the leaves of Pieris formosa collected at Yichang, Hubei, China, were phytochemically investigated for the first time. A total of fifteen ent-kaurane diterpene glucosides (1-15) including twelve new ones, named forminosides A-L (1-12), were isolated. Their structures were elucidated by comprehensive spectroscopic data analyses, quantum chemical calculations (13C NMR and ECD calculations and DP4+ analysis), and chemical methods. The absolute configures of 1-3, 5-8, 11, and 13 were further determined by single-crystal X-ray diffraction analysis. Forminoside A (1) represents the first 3α-(β-d-glucopyranosyloxy)-11,16-epoxy-ent-kaurane diterpenoid bearing a unique 12-oxa-pentacyclo[9.3.3.01,10.04,9.013,16]heptadecane core. Forminoside J (10) is the first 17-nor-ent-kaurane type diterpenoid from Ericaceae family, while forminoside L (12) represents the first example of 4,5-seco-ent-kaurane diterpenoid glycoside bearing an unusual α-hydroxyl-α,β-unsaturated ketone block. Notably, the structure of mollisside A was revised to 3β-(β-d-glucopyranosyloxy)-16β,17-dihydroxy-ent-kaurane based on the NMR and single-crystal X-ray diffraction data analysis of forminoside C (3). All the isolates 1-15 showed potent analgesic activity in the HOAc-induced writhing test in mice. Among them, compounds 1-3, 5-12, and 15 exhibited significant analgesic effects at a dose of 5.0 mg/kg with the inhibition rates over 50%. Compounds 1, 5, 7, and 9-12 still displayed significant analgesic effects with the inhibition rates exceeding 50% at a lower dose of 1.0 mg/kg. Forminosides J (10) and L (12) still showed significant analgesic potency even at a lower dose of 0.2 mg/kg, comparable to that of the positive control, morphine. This is first report of the analgesic activity of 11,16-epoxy-ent-kaurane diterpenoid. A preliminary structure-activity relationship was explored, providing new clues to design novel analgesics based on the ent-kaurane and related diterpenoids.

Recent Advances in Grayanane Diterpenes: Isolation, Structural Diversity, and Bioactivities from Ericaceae Family (2018-2024)

Diterpenes represent one of the most diverse and structurally complex families of natural products. Among the myriad of diterpenoids, grayanane diterpenes are particularly notable. These terpenes are characterized by their unique 5/7/6/5 tetracyclic system and are exclusive to the Ericaceae family of plants. Renowned for their complex structures and broad spectrum of bioactivities, grayanane diterpenes have become a primary focus in extensive phytochemical and pharmacological research. Recent studies, spanning from 2018 to January 2024, have reported a series of new grayanane diterpenes with unprecedented carbon skeletons. These compounds exhibit various biological properties, including analgesic, antifeedant, anti-inflammatory, and inhibition of protein tyrosine phosphatase 1B (PTP1B). This paper delves into the discovery of 193 newly identified grayanoids, representing 15 distinct carbon skeletons within the Ericaceae family. The study of grayanane diterpenes is not only a deep dive into the complexities of natural product chemistry but also an investigation into potential therapeutic applications. Their unique structures and diverse biological actions make them promising candidates for drug discovery and medicinal applications. The review encompasses their occurrence, distribution, structural features, and biological activities, providing invaluable insights for future pharmacological explorations and research.

An updated review of the genus Rhododendron since 2010: Traditional uses, phytochemistry, and pharmacology

Rhododendron, the largest genus of Ericaceae, consists of approximately 1000 species that are widely distributed in Europe, Asia, and North America but mainly exist in Asia. Rhododendron plants have not only good ornamental and economic value but also significant medicinal potential. In China, many Rhododendron plants are used as traditional Chinese medicine or ethnic medicine for the treatment of respiratory diseases, pain, bleeding and inflammation. Rhododendron is known for its abundant metabolites, especially diterpenoids. In the past 13 years, a total of 610 chemical constituents were reported from Rhododendron plants, including 222 diterpenoids, 122 triterpenoids, 103 meroterpenoids, 71 flavonoids and 92 other constituents (lignans, phenylpropanoids, phenolic acids, monoterpenoids, sesquiterpenoids, coumarins, steroids, fatty acids). Moreover, the bioactivities of various extracts and isolates, both in vitro and in vivo, were also investigated. Our review summarized the research progress of Rhododendron regarding traditional uses, phytochemistry and pharmacology in the past 13 years (2010 to December 2022), which will provide new insight for prompting further research on Rhododendron application and drug development.

The efficacy and toxicity of grayanoids as analgesics: A systematic review

ETHNOPHARMACOLOGICAL RELEVANCE

Grayanoids are natural diterpenoids that are mostly found in the Ericaceae family, such as Rhododendron molle (Blume) G. Don (Relevant herb: nao yang hua), Rhododendron micranthum Turcz (also known as: zhao shan bai), which have traditionally been used to treat abdominal pain, cephalgia, and rheumatoid arthritis.

AIMS OF THE REVIEW

The review investigated advancements in notable anti-nociception, toxicity, and probable mechanisms of grayanoids. Meanwhile some binding sites of these compounds on voltage-gated sodium channels (VSGCs) were also analyzed and evaluated.

MATERIALS AND METHODS

The substantial grayanoids literature published before 2022, in SCI Finder, PubMed, Science Direct, Springer, Scopus, Wiley Online Library, J-Stage, and other literature databases had been exhaustively consulted and thoroughly screened.

RESULTS

More than 50 compounds in grayanoids exhibited exceptionally significant anti-nociception (intraperitoneal injection, less than 1 mg/kg), and the alteration of several substituents that were closely associated to the change in activity were investigated. Multiple possible mechanisms of analgesic action and toxicity had been proposed, with VSGCs playing a key part in both. As a result, the binding locations of these compounds on VGSCs (mostly grayanotoxin I and III) had been summarized.

CONCLUSIONS

The considerable anti-nociception, toxicity, and probable mechanisms of grayanoids, as well as the investigation of the binding sites on VSGCs, were discussed in this review. Furthermore, the homology of toxicity and anti-nociception of these substances was considered, as well as the possibility of grayanoids being developed as analgesics.

Gelstriamine A, a Triamino Monoterpene Indole Alkaloid with a Caged 6/5/7/6/6/5 Scaffold and Analgesic Alkaloids from Gelsemium elegans Stems

A novel triamino monoterpene indole alkaloid with an unprecedented skeleton, gelstriamine A (1), four new monoterpene indole alkaloids (2-5), and 12 known analogues (6-17) were isolated from Gelsemium elegans. The structures of 1-5 were established using extensive spectroscopic techniques, NMR calculations with iJ/dJ-DP4 and 2D C-H COSY ANNs analysis, ECD calculations, chemical methods, and single crystal X-ray diffraction analysis. Gelstriamine A (1) possesses an unprecedented 6/5/7/6/6/5 heterohexacyclic scaffold bearing a unique hexahydrooxazolo[4,5-b]pyridin-2(3H)-one motif, and a plausible biosynthetic pathway was proposed. All the isolated alkaloids 1-17 showed discernible analgesic activities in an acetic acid-induced writhing test in mice, and N-desmethoxyhumantenine N₄-oxide (3) exhibited more potent analgesic activities than those of morphine at doses of 0.04 and 0.2 mg/kg.

Structurally diverse diterpenoids with eight carbon skeletons from Rhododendron micranthum and their antinociceptive effects

Seventeen diterpenoids (1-17), classified into eight diverse carbon skeleton types, grayanane (1, 2, and 12), micranthane (3, 4, and 13), mollane (5-7 and 14), 1,5-seco-grayanane (8), kalmane (9-11), 1,5-seco-kalmane (15), A-homo-B-nor-ent-kaurane (16), and leucothane (17), respectively, were isolated from the leaves extract of Rhododendron micranthum. Among them, diterpenoids 1-9 are new compounds and their structures were elucidated via extensive spectroscopic methods, quantum chemical calculations including the ¹³C NMR-DP4+ analysis and electronic circular dichroism (ECD) calculations, and the single-crystal X-ray diffraction analysis. Micranthanol A (1) represents the first example of a 5αH,9αH-grayanane diterpenoid and a 6-hydroxy-6,10-epoxygrayanane diterpenoid, and micranthanone B (3) is the first 6,10-epoxymicranthane and the 5α-hydroxy-micranthane diterpenoids. 14-epi-Mollanol A (5) and mollanol B (6) represent the first examples of 14β-hydroxymollane diterpenoids. It is the first time to report mollane, 1,5-seco-kalmane, and A-homo-B-nor-ent-kaurane type diterpenoids from Rhododendron micranthum. All the seventeen diterpenoids showed significant antinociceptive activities at a dose of 5.0 mg/kg, and it is the first time to evaluate the antinociceptive activity of 1,5-seco-kalmane diterpenoid. Among them, compounds 3, 11, 14, and 15 exhibited significant antinociceptive activities even at a lower dose of 1.0 mg/kg. A preliminary structure-activity relationship for the antinociceptive effects of diterpenoids 1-17 is discussed, which provided a new basis to develop novel potent analgesics.

Structurally diverse diterpenoids from Pieris japonica as potent analgesics

Sixteen diterpenoids (1-16) including 10 new ones, pierisjaponins A-J (1-10), were isolated and identified from Pieris japonica, and their structures were classified into eight diverse carbon skeletons. Pierisjaponins A (1) and B (2) represent the first 1,5-seco-grayanane diterpenoid glucosides and only showed 17 carbon resonances instead of 26 carbons in the ¹³C NMR spectra, their structures were finally defined by single-crystal X-ray diffraction, and the unusual NMR phenomena were explained. Pierisjaponin E (5) is the first mollane diterpene glucoside. This is the first time to report ent-labdane (3, 4, and 11) and ent-rosane (15) type diterpenoids from the Ericaceae plants, which provided the precursors of the Ericaceae diterpenoids and enlarged the chemical diversity of Ericaceae diterpenoids. All the 16 isolates showed potent analgesic activities, and this is the first time to describe the analgesic activities of 1,5-seco-grayanane, ent-labdane, mollane, and ent-rosane type diterpenoids. A preliminary structure-activity relationship is discussed, which provided new clues to design novel analgesics based on the Ericaceae diterpenoids.

Grayanane diterpenoid glucosides as potent analgesics from Pieris japonica

A total of fifteen grayanane diterpenoid glucosides including eight undescribed ones, pierisjaponosides A-H, were isolated from the leaves of Pieris japonica (Thunb.) D. Don ex G. Don (Ericaceae). Their structures were established by extensive spectros copic techniques including HRESIMS and NMR, as well as chemical methods. The absolute configurations of pierisjaponosides A, B, and D were finally established by single-crystal X-ray diffraction with Cu Kα radiation. This is the first time to report the crystal structure of a 5,9-epoxygrayanane diterpenoid glucoside. Pierisjaponoside E represents the first example of a 9β-hydroxygrayan-1(10)-ene diterpenoid. All the isolated grayanane diterpenoid glucosides were evaluated for their analgesic activities in the acetic acid-induced writhing models in mice, and showed significant analgesic effects. Pierisjaponosides A and C-H, micranthanoside A, pieroside A, and craiobiosides A and B displayed significant analgesic effects with the writhe inhibition rates over 50% at a dose of 5.0 mg/kg. Pierisjaponoside E exhibited significant analgesic activities with the percentage inhibitions of 81.7%, 70.4%, and 52.1% at the doses of 5.0, 1.0, and 0.2 mg/kg, respectively. The preliminary structure-activity relationships of grayanane diterpenoid glucosides as potent analgesics were discussed, giving some clues to design novel analgesics.

Antinociceptive Grayanane Diterpenoids from the Leaves of Pieris japonica

Thirteen new grayanane diterpenoids (1-13) and 15 known analogues (14-28) were isolated from a leaf extract of Pieris japonica. Their structures were determined by spectrometric and spectroscopic methods, including HRESIMS, NMR, IR, and UV. The absolute configurations of 1, 3, 7-9, and 16 were defined by single-crystal X-ray diffraction analysis. 17-Hydroxygrayanotoxin XIX (1) represents the first example of a 17-hydroxygrayan-15(16)-ene diterpenoid. Diterpenoids 1-28 were evaluated for their antinociceptive activities, and 4, 9, 13, 21, and 26-28 displayed significant antinociceptive activities at a dose of 5.0 mg/kg (ip) in the HOAc-induced writhing test in mice. 17-Hydroxygrayanotoxin XIX (1) exhibited potent antinociceptive effects with writhe inhibition rates of 56.3% and 64.8% at doses of 0.04 and 0.2 mg/kg, respectively, which were almost equivalent to the positive control, morphine. Rhodomollein X (26) and rhodojaponin VI (27) showed more potent antinociceptive effects than morphine at doses of 0.04 and 0.2 mg/kg. A preliminary structure-activity relationship for the antinociceptive effects of diterpenoids 1-28 is discussed.