Activities and Molecular Mechanisms of Diterpenes, Diterpenoids, and Their Derivatives in Rheumatoid Arthritis

Diterpenes and their derivatives have many biological activities, including anti-inflammatory and immunomodulatory effects. To date, several diterpenes, diterpenoids, and their laboratory-derived products have been demonstrated for antiarthritic activities. This study summarizes the literature about diterpenes and their derivatives acting against rheumatoid arthritis (RA) depending on the database reports until 31 August 2021. For this, we have conducted an extensive search in databases such as PubMed, Science Direct, Google Scholar, and Clinicaltrials.gov using specific relevant keywords. The search yielded 2708 published records, among which 48 have been included in this study. The findings offer several potential diterpenes and their derivatives as anti-RA in various test models. Among the diterpenes and their derivatives, andrographolide, triptolide, and tanshinone IIA have been found to exhibit anti-RA activity through diverse pathways. In addition, some important derivatives of triptolide and tanshinone IIA have also been shown to have anti-RA effects. Overall, findings suggest that these substances could reduce arthritis score, downregulate oxidative, proinflammatory, and inflammatory biomarkers, modulate various arthritis pathways, and improve joint destruction and clinical arthritic conditions, signs, symptoms, and physical functions in humans and numerous experimental animals, mainly through cytokine and chemokine as well as several physiological protein interaction pathways. Taken all together, diterpenes, diterpenoids, and their derivatives may be promising tools for RA management.


Introduction
Arthritis is a long-term musculoskeletal illness marked by inflammation of the joints. Rheumatoid arthritis (RA) is one of the most common kinds of arthritis [1]. It is a long-term condition marked by inflammatory synovitis. Joint asymmetry and invasive inflammation are common symptoms of RA, which can lead to joint deformity, dysfunction, and even loss of function. Adults in rich countries have a prevalence of 0.5-1.0%, with 5-50 new cases per 100,000 persons each year. Women and the elderly, on the other hand, are the ones who suffer the most [2]. Although the exact origin of RA is unknown, medicinal therapy is a common and effective treatment option for RA patients.
Treatments for RA include nonsteroidal anti-inflammatory medications (NSAIDs), corticosteroids, diseasemodifying antirheumatic medicines (DMARDs), and biological response modifiers [3]. All of these anti-RA drugs, unfortunately, have numerous negative effects. NSAIDs may endanger patients' lives by increasing the risk of upper gastrointestinal (GI) haemorrhage, liver, and kidney damage [4,5]. Furthermore, headaches, cognitive impairments, and allergic reactions are common reasons for patients to stop taking NSAIDs, limiting their usage. Infection, hypersplenism, hypertension, osteoporosis, and fractures are all possible side effects of long-term corticosteroid usage [6,7]. Vomiting, diarrhea, rashes, low white blood cell (WBC) counts, and impaired liver and renal function are also side effects of DMARDs [8,9].
Biological therapies with high pharmacological selectivity and fewer side effects provide novel RA treatment alternatives [10]. Regrettably, these are pricey. As a result, many patients may be unable to afford these drugs [11]. As a result, it is critical to seek out treatments that have a positive therapeutic benefit, few side effects, and are affordable. Many ailments are treated according to conventional medical principles. Many major studies on therapeutic items with natural origins have been conducted by modern scientists.
Plants or their derivatives, marine items, and so forth are examples. ese natural items have been discovered as a promising treatment option for RA [12]. Aside from that, several conventional pharmaceutical formulae for RA care have been difficult. Two significant features of this method are the use of nutraceuticals and polyherbal approaches [13,14]. However, when these preparations are used in combination with other drugs, they may cause health problems. As a consequence, researchers devised a novel strategy for extracting active chemicals from some of these things. Terpenes, flavonoids, catechins, quinones, alkaloids, anthocyanins, and anthoxanthins are just a few of the plant-derived phytochemicals that can alter T cell development, inflammatory signaling pathways, and synoviocyte death. As a result, they can be utilized to treat rheumatoid arthritis [15].
Diterpenes are a diverse group of structurally diverse natural chemicals abundant in nature [16,17]. ese are C20 compounds containing four isoprene (C 5 H 8 ) units that may be found in both terrestrial and marine settings in plants, fungi, bacteria, and animals [18,19]. Several diterpenes are potential pharmaceutical candidates due to their exceptional pharmacological effect [20][21][22][23]. Some diterpenes are considered to be the defining traits of a genus, making them taxonomically significant [24].
Natural diterpenoid compounds come in a wide range of chemical forms and contain many medicinal and economically relevant molecules. All diterpenoids are made from the same substrate, (E, E, E)-geranylgeranyl diphosphate, which is then cyclized into one of the multiple scaffolds by a diterpene synthase [25]. Secondary metabolites with 20 carbon atoms result from the condensation of four isoprene units.
Diterpenoids are divided into approximately 45 distinct categories, and they are also present in marine animals, where their skeletons are fascinating [26]. Based on their skeletal nucleus, diterpenes are classified as linear, bicyclic, tricyclic, tetracyclic, pentacyclic, or macrocyclic. ey are usually found in nature polyoxygenated, with keto and hydroxyl groups that are commonly esterified by tiny aliphatic or aromatic acids [27].
Diterpenoids have a variety of biological functions, including antioxidant [23,28], anti-inflammatory [29,30], and immune-modulatory action [31]. Given the significance, the goal of this study is to outline the effects of diterpenes, diterpenoids, and their derivatives on RA based on current understanding.

Review Methodology
A search with the keywords "Diterpene AND Rheumatoid arthritis," "Diterpenoid AND Rheumatoid arthritis," "Diterpene AND Arthritis," "Diterpenoid AND Arthritis," "Diterpene derivative AND Rheumatoid arthritis," and "Diterpene derivative AND Arthritis" was conducted in the PubMed, Science Direct, Google Scholar, and Clinicaltrials. gov databases. A total of 2708 records were found. After screening, among them, this study used 48 published records that are related to its aim.
is study includes only the records of having antiarthritic or anti-RA along with anti-inflammatory activities of the diterpenes, diterpenoids, and their derivatives obtained from various sources (e.g., medicinal plants and marine origins) on various test systems (e.g., humans), laboratory animals (e.g., mice, rats, and rabbits), and their derivatives (e.g., cells, tissues or organs).
Most of the diterpenes, diterpenoids, and their derivatives have antioxidant and anti-inflammatory properties, and for this reason, this study does not include them. Reports on crude extracts or fractions without chemical characteristics having antiarthritic or anti-RA effects were also excluded in this study.
is study mainly focuses on the anti-RA activities of diterpenes, diterpenoids, and their derivatives. However, it will also focus on the antiarthritic effects of these substances based on updated database records (till 31 August 2021).
Chemokines have a role in the underlying pathophysiology of RA by attracting leukocytes and influencing angiogenesis. Published research indicated that XC chemokines and their receptors (such as XCL1 and XCR1) and CX3C chemokines and their receptors (e.g., CX3CL1 and CX3CR1) are upregulated in RA patients' mononuclear cells (MNCs) and FLS, respectively [47,48]. Numerous inflammatory chemokines are mostly generated in the joints of RA patients by synovial macrophages and FLS, while CX3CL1 is produced by synovial endothelial cells. e chemokines XC and CX3C are linked to the recruitment of T lymphocytes and synovial fibroblasts. Furthermore, CX3CL1 and XCL1 stimulate the migration of monocytes and subchondral mesenchymal progenitor cells into the RA synovium, respectively [49]. CC chemokines including CCL2-5, CCL7, CCL13, CCL14, CCL16, CCL18-21, and CCL-25 are differentially expressed in RA plasma and synovium [50]. An upregulated CC chemokine CCL5 is significantly correlated with swollen joints, erythrocyte sedimentation rate (ESR), and c-reactive protein (CRP) in RA patients [51]. On the other hand, CXC chemokines, such as CXCL1, CXCL2, CXCL5, CXCL8, CXCR1, and CXCR2, are generally involved in neutrophil chemotaxis [52]. e chemokine CXCL10 promotes effector T cells into the joint [53]. e expression of peroxisome proliferator-activated receptor-gamma (PPARc or PPARG) in human monocytes/ MDMs may be an indication of disease activity and treatment effectiveness in RA. Several studies have shown that key cell types in the joints [54,55] express PPARc at both the mRNA and protein levels.
Long noncoding RNAs (lncRNAs) are more than 200 nucleotides in length and are extensively expressed in many organs of the human body. Several researchers have shown that lncRNA could be used to diagnose RA [56,57].
In RA patients with active synovitis, osteoprotegerin (OPG) expression on macrophage type synovial lining cells and also endothelial cells is low. As a result, addressing OPG expression in RA patients' inflamed joints may be an essential approach for the treatment of RA in humans [58]. e RANKL/OPG pathway is the connecting factor between bone production and bone resorption in the complicated system of bone remodeling. RANKL promotes the activation and differentiation of preosteoclasts and mature osteoclasts by binding to their receptors (RANK).
Certain hormones, growth factors, and cytokines affect the synthesis of RANKL and OPG by osteoblasts in various ways. us, the level of proliferation and activity of osteoclasts are determined by the balance of RANKL and OPG. Bone erosions in RA are caused by osteoclastic bone resorption in synovitis sites, in which RANKL expression is also observed [59].
NSAIDs work by reducing the enzymatic activity of the cyclooxygenase (COX) enzymes, which are involved in the production of prostaglandins (PGs). NSAIDs inhibit COX-2, which limits PG synthesis at sites of inflammation; however, inhibiting COX-1 in other tissues (e.g., platelets and mucosa) results in classic NSAID side effects such as bleeding and GI ulcers [66]. Summarized scheme of the physiopathology of RA is shown in Figure 1.

Anti-RA Activities of Diterpenoids: Actions and Molecular Mechanisms
Diterpenoids are the most prominent source of anti-RA agents with potential pharmacological effects.

Cytokine Targeting Diterpenes and eir Derivatives.
A recent study has been claimed that diterpenes isolated from Caesalpinia minax (Hance) substantially reduced the change in paw swelling perimeter, arthritic score, and increased bodyweight loss in vivo study [67]. Furthermore, the primary components of the extract were 14 cassane derivatives, such as caesalpins A-H, caesalminaxin A-L, and others, which exhibit a promising effect on the expression of mRNA of the cytokines IL-1β and IL-6 and TNF-α generated by macrophage cells. Moreover, some other diterpenoids (rhodojaponin III, rhodojaponin VI, 2-O-methylrhodojaponin, and 5′-β-D-glucopyranosyloxyjasmonic acid) in Rhododendron molle fruits at 0.6 mg/kg dose dramatically reduced RA symptoms in CIA rats [68] by strongly preventing aberrant T and B lymphocyte proliferation and substantially decreased levels of the proinflammatory cytokines IL-1β and IL-6, as well as TNF-α. It has been seen that (5R)-5-hydroxytriptolide can inhibit IL-1β, IL-6, and IL-21 secretion and elevated IL-10 secretion in peripheral blood and synovial fluid of RA patients [69]. Andrographolide in bone marrow macrophages cells and mice inhibited RANKL-stimulated osteoclastogenesis via downregulating NF-κB and ERK/MAPK expression, thereby averting bone loss [70]. Cryptotanshinone at 6 and 18 mg/kg (p.o., for 16 days) in type II collagen-induced arthritis in female Wistar rats, and 5 and 20 μM concentration inhibits the degradation of NF-κB (IκB)-α blocker [71].
Research findings indicated that triptolide (1-4 nM/L) inhibited RANKL-induced NF-κB activation and RANKL and tumor cell-induced osteoclastogenesis [72]. Additionally, a derivative of triptolide (5R)-5-hydroxytriptolide downregulated the expression of (p)-IκB, a major regulator of the RANKL-signaling pathway in RA patients' peripheral blood and synovial fluid [69]. Another study suggested that triptolide (2.5-40 nM) enhanced the inhibitory effects of TREGS on osteoclast differentiation and bone resorption through an increase in the secretion of IL-10 and transforming growth factor-beta 1 (TGF-β1) in mice bone marrow macrophages [73]. Table 1 provides the list of various diterpenes, diterpenoids, and their derivatives, which act in various RA models.
Triptolide in collagen-induced arthritis rats significantly inhibited triggering receptors expressed on myeloid cells (TREM)-1 mRNA and DAP12 mRNA expression and activation of JAK2 and STAT3 in the ankles of test animals and in LPS-stimulated U937 cells [82].
In animal studies, WB2086, a human PAF receptor antagonist, reduces PAF-induced platelet aggregation [101]. Findings showed that sclareol exhibits significant anti-inflammatory effects in experimental animals; it inhibits NO production and upregulates inducible nitric oxide synthase (iNOS) and COX-2 expression in lipopolysaccharide (LPS)stimulated macrophages [102]. It also decreased paw edema and neutrophil infiltration in the λ carrageenan-induced paw edema animal model. On the other hand, aphamines A-C isolated from Aphanamixis polystachya exhibited inhibitory effects on NO production (IC 50 : 6.71-15.36 μmol/L) and reduced the expression of iNOS in LPS-induced RAW 264.7 macrophages [103].
Serralabdanes A-E isolated from the whole plant of Chloranthus serratus also showed inhibitory effects on LPSinduced NO production in RAW264.7 cells [104]. Other compounds such as tripterycoside A-C, 11-O-β-d-glucopyranosyl-neotritophenolide, and wilfordoside A at 10 μM exerted substantial inhibition of IL-1β secretion in LPSinduced rat primary synovial fibroblasts [105]. Similarly, researchers found that secoferruginol isolated from the heartwood of Cryptomeria japonica modulates human DC function in a fashion that favors 2 cell polarization [106], whereas songorine, a C 20 diterpenoid alkaloid and 12-keto analogue of napelline, isolated from Aconitum soongaricum, exhibited anti-inflammatory and antiarthritis activities [107].
Some of the important proteins involved in RA include JAK, p38 mitogen-activated protein kinase (MAPK), extracellular receptor kinase (ERK), JNK, IL-1 receptor-associated kinase (IRAK)-4, MMPs, toll-like receptor 4 (TLR-4), G protein-coupled receptor kinase (GRK)-2, Bruton's tyrosine kinase (BTK), CD3, CD11a, CD19, CD20, and CD80. JAK is a component of the JAK/STAT signaling  [108]. Published research showed that excavatolide B (2.5 and 5 mg/kg, s.c.) in adjuvant (AIA) and type II collagen-induced arthritis in rats attenuate the protein expression of CD11b and nuclear factor of activated T cells 1 (NFATc1) in ankle tissues [75]. Giannelli et al. [109] reported that evaluation of synovial fluid concentrations of TIMPs (e.g., TIMP-1 and TIMP-2) is more reliable than that determined in serum when remodeling cartilage ECM proteins, besides MMPs evaluated. ese researchers suggested that both TIMPs and MMP inhibitors might be a potential target for novel RA treatments administered directly into the joint area. In this respect, triptolide (10, 30, and 50 nM) in RA FLS from 7 RA patients reduced the TNFα-induced expression of phosphorylated JNK [83]. Additionally, it has been demonstrated that a JNK-specific inhibitor reduces the migration and invasion of RA FLS. Research by Zhong and colleagues showed that sclareol exhibits antiosteoarthritic properties in IL-1β-induced rabbit chondrocytes and a rabbit model of osteoarthritis induced by ACLT [110]. Sclareol also inhibited MMP, iNOS, and COX-2 expression and increased TIMP-1 expression and ameliorated cartilage degradation in the test systems. Similarly, 11-epi-sinulariolide acetate significantly inhibited the expression of the proinflammatory proteins iNOS and COX-2 in LPS-stimulated murine macrophages [89]. Moreover, oridonin (2-10 μg/mL for 24-72 h) increased apoptosis, protein levels of Bax, and cleaved caspase-3 in RA FLS. However, it significantly decreased IL-1β levels in the test system [52]. Meanwhile, excavatolide B at 10 μM inhibited multinucleated cell and actin ring formation and also tartrate-resistant acid phosphatase (TRAP or TRAPase), MMP-9, and cathepsin K expression in LPS-stimulated RAW 264.7 cells [75].
Researchers also demonstrated that the soft coral-derived diterpene at 2.5 and 5 mg/kg (s.c.) significantly attenuated the characteristics of RA, improved histopathological features, decreased the number of TRAP-positive multinucleated cells, and attenuated the protein expression of cathepsin K, MMP-2, and MMP-9 in ankle tissues as well as the level of IL-17A and macrophage colony-stimulating factor in adjuvant (AIA) and type II collagen-induced arthritis in rats. Figure 2 shows the possible mechanisms of diterpenes and their derivatives targeting cytokines.

Chemokines Targeting Diterpenes and eir Derivatives.
A clinical trial of E6011 (an anti-CX3CL1 mAb) is currently underway, and it has been shown to have a potential function in active RA patients [111]. In LPS-induced FLS, ginkgolide B (5-80 μM) remarkably inhibited RA FLS viability in a concentration-dependent fashion. It also reduced the apoptotic ratio and enhanced the expression of cleaved caspase-3 and Bax. Furthermore, it reduced Bcl-2 expression in RA FLS, decreased the development of inflammation by regulating inflammatory cytokine secretion and MMP gene expression, and reduced expression levels of Wnt5a, (p)-JNK, and p-P65 in synovial tissues and RA FLS [88].

Diterpenes and eir Derivatives Acting on Other Proteins.
Triptolide (0.01-10 μM) downregulated PPAR-c activation and induced DNA fragmentation in RSF in rheumatoid synovial fibroblasts from RA patients [79]. It also decreased arthritis scores and significantly reduced capillaries, small, medium, and large vessel density in the synovial membrane tissues of inflamed joints in bovine type II collagen-induced arthritis DA rats [81]. Moreover, triptolide inhibited Matrigel-induced cell adhesion of HFLS-RA, and HUVEC as well as disrupted tube formation of HUVEC on Matrigel, and suppressed the VEGF-induced chemotactic migration of HFLS-RA and HUVEC, respectively, in arthritis rats.
(5R)-5-Hydroxytriptolide increased the rate of osteoprotegerin (OPG) expression in CD3 + T leukomonocytes in peripheral blood and the ratio of OPG/RANKL in both peripheral blood and synovial fluid in peripheral blood and synovial fluid of RA patients [69]. It also inhibited IL-23 secretion in the supernatants of PBMCs and SFMCs in peripheral blood and synovial fluid of RA patients [69]. It additionally prevented collagen-induced arthritis via inhibiting OPG/RANK/RANKL signaling in osteoclastogenesis and IFN-c signaling in T cells [113,114]. Recently, Zhou et al. [63] demonstrated that it exerts an anti-RA effect through the WAKMAR2/miR-4478/E2F1/p53 dependent pathway in RA FLS. MEG3 lncRNA overexpression reduces inflammation by affecting the AKT/mTOR signaling pathway [115].
Tanshinone IIA (1-80 μM) exerted cytotoxically apoptosis effects through upregulating lncRNA GAS5, possibly with an increase in cleaved caspase-3/9 expression and inhibiting the PI3K/AKT signaling pathway in FLS from RA patients [93]. In addition, numerous studies indicated that PGs play an important role in physiological immune responses and in pathological diseases related to inflammation and tissue damage.
In murine macrophages (RAW264.7 cells) and human chondrocytes, carnosol, carnosic acid, carnosic acid-12methyl ether, 20-deoxocarnosol, and abieta-8,11,13-triene-11,12,20-triol reduced NO and PGE 2 production in a concentration-dependent manner. ey also significantly reduced iNOS and cytokine (IL-1α and IL-6) gene expression levels in the test systems [112]. Additionally, these substances altered the expression of catabolic and anabolic Evidence-Based Complementary and Alternative Medicine genes in the chondrosarcoma cell line SW1353 and primary human chondrocytes, stimulated by IL-1β, where catabolic genes such as MMP-13 and ADAM metallopeptidase with thrombospondin type 1 motif 4 (ADAMTS-4) that contribute to cartilage erosion were downregulated, whereas anabolic gene expression, particularly Col2A1 and aggrecan, was moved towards prepathophysiological equilibrium. Furthermore, carnosol exhibited the greatest overall impact on inflammatory mediators as well as macrophage and chondrocyte gene expression. It significantly inhibited IL-1β-induced nuclear translocation of NF-κB-p65, suggesting that it is primarily regulated through the NF-κB signaling pathway. Lobolide, a cembrane diterpene, also acts through the NF-κB signaling pathway [116]. Moreover, andrographolide attenuated mouse cortical chemokine levels from the CC and CXC subfamilies in LPS-induced chemokine upregulation in a mouse model [117]. Table 2 provides the list of diterpenes, diterpenoids, and their derivatives that interact with various proteins involved in inflammatory and RA processes. for 28 days) significantly delayed the onset of arthritis. In addition, the arthritis incidence, clinical arthritis severity score, histopathological arthritis severity score, and in vivo cell-mediated immunity to collagen were all reduced [78]. In bovine collagen type II and complete Freund's adjuvant-induced arthritis in DBA/1 mice, cryptotanshinone (20 and 60 mg/kg, p.o., for 6 weeks) ameliorated the inflammation and joint destruction [98]. It also suppressed p300-mediated STAT3 acetylation in test animals. Similarly, carnosic acid at 30 and 60 mg/kg (i.p., 4 weeks) in collagen-induced arthritis in male C57BL/KsJ-db/db mice and at 10 or 20 μM in mouse bone marrow cells reduced osteoclast formation and bone loss through suppression of inflammation by regulating the ROSdependent p38 pathway [99]. On the other hand, xylopic acid nanoformulation showed anti-inflammatory and anti-RA effects in RAW 264.7 cells and complete Freund's adjuvantinduced arthritis in male Sprague-Dawley rats [97].
Andrographolide (25 μM, for 16 h) in LPS-stimulated neutrophils accelerated apoptosis and inhibited autophagydependent extracellular TRAPs formation [77]. It also reduced neutrophil infiltration and NETosis in the ankle joints and relieved the systematic inflammation in adjuvant-induced arthritis C57BL/6 mice. Tanshinone IIA inhibited the NET formation of neutrophils in adjuvant-induced arthritis in female C57BL/6 mice [92]. On the other hand, triptolide (0.01-10 μM) reduced viability and proliferation and induced apoptosis of RSF in a concentration-dependent manner in FLS from RA patients [79]. It also upregulated caspase-3 activity in the test system, whereas retinoic acidplatinum (II) complex (0.25-12 μM) in MH7A cells induced apoptosis and caused the arrest of the cell cycle [90].

Improvement of Physiological Functions in RA Animals.
Andrographolide (50 mg/kg/day) combined with methotrexate (2 mg/kg/week) for 35 days (injection) in complete Freund's adjuvant-induced arthritis in Wistar rats improved the serum marker. is may be attributed to the antioxidant activity of this compound, as evidenced by histological alterations in the liver [76]. Andrographolide when combined with methotrexate in complete Freund's adjuvant-induced arthritis in Wistar rats strengthened the antiarthritic capacity of methotrexate, reduced the inflammatory symptoms  Evidence-Based Complementary and Alternative Medicine in animals, showed hepatoprotective activity, and significantly reduced serum TNF-α, IL-6, and IL-1β levels [76]. Triptolide loaded by a poly-c-glutamic acid-grafted l-phenylalanine ethyl ester copolymer at 6.25-200 nM reduced the damaging effects on the liver, kidney, and spleen of mice [85]. In addition, triptolide (100 μg/kg, i.p., 21 days) improved clinical arthritic conditions and joint destruction in collagen-induced arthritis in male DBA/1 mice [83], whereas triptolide-loaded poly(D,L-lactic acid) nanoparticles (0.05-0.2 mg/kg, p.o., for 14 days) in complete Freund's adjuvant-induced arthritis in male Wistar rats significantly inhibited arthritis and exerted a preferable antiinflammatory effect with long-time administration [86].
In another study, triptolide loaded with miR-30-5p inhibitor significantly inhibited RA synovial fibroblast proliferation and increased apoptosis in collagen-induced arthritis female Sprague-Dawley rats [84].
is nanopreparation also downregulated immune system activation in rats.
Phytol (acyclic diterpene alcohol derived from chlorophyll) at 200 μL (injection in the tail, for 10 days) was found to restore the oxidative-burst effect and induce a strikingly similar IFN-β-dependent pathway in DA rats [95].
Researchers suggested that it may be effective against naturally occurring genetic polymorphisms in the Ncf-1 gene that modulated the activity of the NADPH oxidase complex, which is potentially regulated in the severity of arthritis, whereas 11-epi-sinulariolide acetate significantly inhibited RA characteristics in adjuvant-induced arthritis in female Lewis rats [89].
Resiniferatoxin (10 μL of 0.001 or 0.003%, injection) significantly improved arthritis with monoarticular inflammatory arthritis in evoked pain scores in arthritic male C57BL6 mice [96]. Similarly, tanshinone IIA (30 mg/kg, i.p., for 30 days) alleviated cartilage erosion and neutrophil infiltration in the ankle joints and reduced proinflammatory cytokine expression levels in sera in adjuvant-induced arthritis in female C57BL/6 mice [92]. In the complete Freund's adjuvant-induced arthritis rat model, phlomisoside F (5, 10, and 20 mg/kg, p.o., for 28 days) markedly offset the body weight loss, inhibited the paw edema, and reduced the arthritis scores and indices of the thymus and spleen [92]. Leflunomide (20 mg once daily) in combination with methotrexate improved signs, symptoms, and physical function in RA patients [91], while ginkgolide B (10, 20, and 40 μM, i.p., for 43 days) in collagen II-induced arthritis male    14 Evidence-Based Complementary and Alternative Medicine DBA/1J mice significantly decreased arthritis scores, synovial hyperplasia, and cartilage and bone destruction [88]. e chemical structures of some anti-RA diterpenes and their derivatives are shown in Figure 4.

Discussion
Diterpenes and their derivatives are gaining popularity due to their intriguing biological and pharmacological properties.
us far, hundreds of natural diterpene compounds from terrestrial and marine species have been described. Many of these compounds have become clinically effective.
Plants are an important source of diterpenes. Diterpenes can be linear, bicyclic, tricyclic, tetracyclic, pentacyclic, or macrocyclic. ey are typically found in nature in a polyoxygenated form with keto and hydroxyl groups, which are frequently esterified by small-sized aliphatic or aromatic acids. For example, the anticancer drug taxol is used as a promising anticancer agent for ovarian, breast, and lung cancers. In addition, many of its synthetic derivatives are also examples of medicinal agents in the management of various diseases in humans. Docetaxel, sold under the brand name Taxotere ® , is a taxoid antineoplastic drug used to treat a variety of malignancies, including locally advanced or metastatic breast cancer, metastatic prostate cancer, gastric adenocarcinoma, and head and neck cancer. Moreover, carboplatin, when combined with this drug, was found to mask RA in an ovarian carcinoma patient [119]. Similarly, ginkgolides are other promising diterpenes that have strong and specific antagonistic action against platelet-activating factors rising in shock, burns, ulceration, and inflammatory skin disorders [120]. Additionally, ginkgolide B exhibits multiedge-like anti-RA effects in in vitro and in vivo test models [88]. Meanwhile, the anti-RA diterpene resiniferatoxin (an ultrapotent vanilloid derived from the latex of Euphorbia resinifera) is promising for bladder hyperreflexia and diabetic neuropathy [120]. In short, diterpenes, diterpenoids, and their derivatives might be promising tools to manage RA and its consequences.
According to current knowledge [50], the most promising therapeutic targets in RA include the following: It appears that diterpenes and their derivatives have multiedge-like actions on different RA models. ese compounds exerted anti-RA effects through the cytokine, chemokine, inflammatory/noninflammatory proteins, and small molecular metabolites pathways. Among the diterpenes, triptolide and its derivative (5R)-5-hydroxytriptolide have been found to display promising anti-RA activity in various test systems.
Other hopeful anti-RA diterpenes and diterpenoids found in this updated review include carnosol and carnosic acid and their derivatives, excavatolide B, Kirenol,  Evidence-Based Complementary and Alternative Medicine ginkgolide B, 11-epi-sinulariolide acetate, oridonin, phlomisoside F, phytol, retinoic acid, resiniferatoxin, sclareol, and xylopic acid among others. A novel triptolide derivative (also known as LLDT-8), which exhibited anti-RA therapeutic properties, is currently in phase II clinical studies in China [63]. Diterpenes and their derivatives act through multidimensional pathways in different RA animal models. Moreover, triptolide-loaded nanocomplexes also improved anti-RA potential in experimental modalities. Besides these compounds/formulations, andrographolide, tanshinone IIA, and its derived compound cryptotanshinone also displayed promising anti-RA effects in test systems.

Conclusion
To date, many natural products that have the anti-RA capacity, including those obtained from medicinal plants and marine organisms, have been identified. e sources of diterpenes and diterpenoids are widely distributed. Natural products, including medicinal plant-derived chemicals, are a prominent source of semisynthetic and synthetic derivatives. Hence, nature and modern medicinal science are capable of providing new and more effective diterpene derivatives. Diterpenes and their derivatives have been shown to possess promising immunomodulatory properties in various experimental models; therefore, these natural bioactive compounds are a promising adjuvant pharmacotherapy in RA.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request and are cited within the article as references.