Phytochemistry and Pharmacological Activities of the Diterpenoids from the Genus Daphne

There are abundant natural diterpenoids in the plants of the genus Daphne from the Thymelaeaceae family, featuring a 5/7/6-tricyclic ring system and usually with an orthoester group. So far, a total of 135 diterpenoids has been isolated from the species of the genus Daphne, which could be further classified into three main types according to the substitution pattern of ring A and oxygen-containing functions at ring B. A variety of studies have demonstrated that these compounds exert a wide range of bioactivities both in vitro and in vivo including anticancer, anti-inflammatory, anti-HIV, antifertility, neurotrophic, and cholesterol-lowering effects, which is reviewed herein. Meanwhile, the fascinating structure–activity relationship is also concluded in this review in the hope of providing an easy access to available information for the synthesis and optimization of efficient drugs.


Introduction
The genus Daphne Linn., with its ca. 95 species, is the most diverse genus in the Thymelaeaceae family [1]. Some of the species from the genus Daphne have been applied for a long history in traditional treatments for aches, rheumatism, inflammation, and abortion in Asia, Africa, and Europe [2]. Yet, none of the principles of these bioactivities had been identified until daphnetoxin was isolated as a major toxic principle from commercial "mezeron" bark which was made from Daphne mezereum L. and other Daphne species in 1970. Subsequently, an increasing number of diterpenoids have been discovered from Daphne species.
The diterpenoids are believed to be representative components of the genus Daphne, and the genus Daphne itself also acts as an important role in the discovery of phytochemical and bioactive properties of diterpenoids. The archetype of one class, daphnetoxin, was first isolated from D. mezereum, which was named after the genus Daphne. Then, daphnetoxin and its analogues have been collectively known as the daphnetoxin class. Similarly, genkwanine A from D. genkwa is the archetypical diterpenoid of genkwanines.
The diterpenoids from the Daphne genus also contribute to the pharmacological study of diterpenoids and have been demonstrated to possess a variety of important biological activities including anticancer, anti-inflammatory, anti-HIV, antifertility, neurotrophic, and cholesterol-lowering effects [3], and some of them are undoubtedly efficient agents which have the potential to be developed as new drugs, such as yuanhuacine and genkwadaphnin.
The current review provides a comprehensive coverage of all natural diterpenoids in the genus Daphne. The occurrence and distribution of these diterpenoids are also discussed thoroughly, including the source species of every diterpenoid listed in a chronological order of discovery and the parts of plants which they were isolated from. Besides, detailed information on every class of diterpenoids is provided in this review. When the adequate information is given, the structure-activity relationship (SAR) is discussed.
Although a large amount of diterpenoids were isolated and identified, they were reported to occur mainly in Thymelaeaceae and Euphorbiaceae and to mainly distribute in the genus Daphne, Wikstroemia and Stellera in the Thymelaeaceae family, as well as Excoecaria and Euphorbia in the Euphorbiaceae family [3]. In terms of the genus Daphne, diterpenoids are reported to be obtained from fifteen Diterpenes are abundant in several Daphne species including D. odora, D. acutiloba, and D. tangutica, especially in D. genkwa. It is intuitively demonstrated that diterpenoids from the Daphne genera are mainly of the 6-epoxy daphnane-type. In D. genkwa, diterpenoids of every class discussed in this review have been isolated, which may suggest a variety of diterpenoids in D. genkwa. Interestingly, it is also observed that the quantity of 12hydrodaphnetoxins is much larger than that of daphnetoxins in D. odora while the amounts of these two types are nearly equivalent in other species, and the biogenetic mechanism behind this remains unclear.

6-Epoxy Daphnane Diterpenoids
The diterpenoids from this class share the common features of a 6α,7α-epoxy and 4β,5β-dihydroxy in the seven-membered ring B. The 6-epoxy daphnane diterpenoids could be further divided into two classes, 12-hydroxydaphnetoxins (1-54) and daphnetoxins , based on the oxygenated substituent at C-12 in the ring C.

54
Gnidilatimonoein D. mucronata (leaves) [59,60] 1 The fine lines in the table are used to separate the diterpenoids with slightly different skeletons (same in the tables below). 2 For the diterpenoids with multiple sources, the source species are listed in the chronological order in which the diterpenoids were isolated from them (same in the tables below).

Genkwanines
The highly oxygenated diterpenoids genkwanines has a 6,7-dihydroxyl in the sevenmembered ring B instead of a 6α,7α-epoxy in 6-epoxy daphnane diterpenoids or a 6,7double bond possessed by resiniferonoids and also a saturated ring A, which could be almost viewed as 1,2-dihydro-3-hydroxy-daphnetoxin derivatives. The archetypal diterpenoid of this class is genkwanine A (84). Among these genkwanines, only genkwanine L (95) possesses a ketone function at C-3 and an oxygen-containing function at C-12 ( Figure 4). Up to now, all compounds of this class were reported to occur in D. genkwa, and these compounds were isolated only from the flower (mostly buds) of D. genkwa [43,54,55,57,66,73] ( Table 3).

Resiniferonoids
The resiniferonoids are a group of 5-deoxy-6,7-double bond daphnetoxin derivatives, in which the A/B ring system possesses the pattern of phorbol. One of the most representative resiniferonoids, resiniferatoxin (RTX), was isolated from the dried latex of E. resinifera, showing significant transient receptor potential vanilloid 1 (TRVP1) activating activity and strong irritant effect [3,5], and compounds discovered in this class so far contain the same skeleton as RTX ( Figure 5). Resiniferonoids have a quite narrow distribution, mostly limited to several Euphorbia genus in the Euphorbiaceae [3,5]. Two novel resiniferonoids daphneresiniferins A and B (96 and 97) obtained from the Daphne species [43] are practically identical to each other, except for the variation of oxygenated functions at C-12, which is quite rare and noteworthy (Table 4).

Lathyrane-Type Diterpenoids
Lathyranes, named after Euphorbia lathyris, were more often found in species from the Euphorbiaceae other than those in the Thymelaeaceae, especially in the Euphorbia species [86,93]. The archetypal compound of lathyrane is 17-hydrojolkinol obtained as a derivative product of 'ester 7 from the seeds of E. lathyris [94]. As for the skeleton with an additional 4,15-bond possessed by 134 and 135, it was believed to be a precursor for crotofolin from Croton corlifolious [86] (Figure 9). So far, these are the only two lathyrane diterpenoids reported to be isolated from the Daphne species [66] (Table 8).

Biological Activities
Several Daphne plants have been used as traditional medicines for the treatment of cancer, inflammation, and rheumatism in Asia, North Africa, and Europe, and some of these plants were also regarded as virulent poisons. The flower bud of D. genkwa, a Chinese traditional medicine, has been used as a diuretic, antitussive, and pesticide, and one of its synonyms is "yu-du", which means "a fish poison" [1,2,95].
However, the poisonous principles of any Daphne species have never been identified until daphnetoxin (56) was isolated from commercial "mezeron" bark (D. mezereum, D. laureola, and D. gnidium) in 1970 and identified as a major toxic component possessing both a similar skeleton and similar sites of functions as phorbol, which was a known toxic principle in Croton species [61,96] at that time. Subsequently, mezerein (24) with the same skeleton as daphnetoxin (56) was identified as another toxic principle of D. mezereum [33].
With a considerable number of modern pharmacological and chemical studies, it was demonstrated that diterpenoids from the Daphne genus possess a wide range of pharmacological activities including anticancer, anti-inflammatory, anti-HIV, cholesterollowering, neurotrophic, antifertility, skin irritant, nematicidal, and pesticidal activities. Among these biological activities, the anticancer ones have received the most attention since there remains a need for both efficient and safe anticancer drugs with novel structures.

Anticancer Activity
Previous studies have shown that diterpenoids from Daphne species exhibit potent anticancer activities against various types of cancers both in vitro and in vivo [2,83,97]. As for anticancer bioactivities in vitro, more than a half of the diterpenoids have been testified to exhibit cytotoxicity with IC 50 values ranging from 10 −6 -98.46 µM (Table S1)   The cytotoxicity of 51 diterpenoids against certain cell lines are intuitively shown in Scheme 2. As for the cell lines sensitive to very few diterpenoids and the diterpenoids only cytotoxic to a very limited number of cell lines, they will be additionally discussed below.

Structure-Activity Relationship (SAR)
For an intuitive demonstration of the structure-anticancer activity relationship of diterpenoids from the genus Daphne, the structure and functions requirements are illustrated with a daphnane diterpenoid skeleton ( Figure 10).

SAR in the Ring A
It is generally accepted that the 1,2-double bond could be a possible alkylating functionality in a diterpenoid and thus the reduction of it might make it a less active compound [98]. Genkwadaphnin (15), yuanhuatine (47), and yuanhuaoate B (112) share the similar structure with the only exception of an unsaturated bond in the ring A, and yuanhuatine (47) among them is the least active anticancer compound [8,14]. The site of an unsaturated carbon bond in the ring A would also have certain affect. For genkwadane A (111) and yuanhuaoate B (112), it appears that the change from the 1,10-to 1,2-double bond enhances the anticancer activity [14].
The 20-acyloxy group in the ring B might have an important role in affecting the anticancer activity in diterpenoids. One type of 20-acylocy groups that could favor the bioactivity is a 20-palmitate ester. Yuanhuacine (30) showed no inhibitory activity against P-388 in vivo, while its 20-palmitate derivative gnidilatidin 20-palmitate (19) exhibited a strong inhibitory activity at dosages of 0.5-2.0 mg/kg/d, and the 20-plamitate derivative of gnidilatin was also observed to be more active than gnidilatin itself against P-388 [100]. This fact might also suggest the positive impact of a long-chain fatty ester on determining the anticancer activity.
Interestingly, for a more statured skeleton, for instance, genkwanine, the caged 9,13,14orthoester seems to reversely have a negative impact on anticancer activity. Genkwanine J (92) showed potent inhibition against P388 and A549 (IC 50 [14]. Evidence showed that the presence of a long-chain fatty could enhance the inhibitory activity against carcinoma cells. Yuanhuacine (30) and yuanhuadine (31) showed promising inhibition against HepG2 cell line with the IC 50 values in the range of 5.56 to 17.06 µM, while yuanhuafine (32) showed more limited activity at the IC 50 value of 42.37 µM [14].
The absence of a 12-acyl group generally causes the reduction of anticancer bioactivity. Mezerein (24), a major toxic principle of D. mezereum, showed significant inhibitory activity in vivo against P-388 and L-1210 leukemia cell lines at dosage of 50 µg/kg in mice [34]. Gnidicin (16), gnididin (18), and gniditrin (20) with the closely related skeleton isolated from Gnidia lamprantha Gilg were also proved to be substantial antileukemia agents. Huratoxin (59) and simplexin (60) exhibited similar inhibitory activities against L1210 and K562 in vitro, but less active than an esterified derivative subtoxin [77]. By comparison, 12hydroxydaphnetoxin (39) bearing a hydroxyl at C-12 in the ring C showed no antileukemia activity and esterification of it could establish the bioactivity [101]. Genkwadaphnin (15), which is in essence the benzoyl derivative of 12-hydroxydaphnin (39), was found to exert both in vitro and in vivo antileukemia activity against P-388 cells [16]. Although all the above together suggests that an acyl function at C-12 might be a prerequisite for the in vivo antileukemia bioactivity, this does not represent that a 12-acyloxyl group is necessary for antileukemia activities regarding the fact that some daphnetoxins were reported to show a quite promising inhibition against a variety of cell lines including HL-60 and K562 as well (Scheme 2).
It is noteworthy that gnidimacrin (101) is one of the most potent antileukemia agents both in vitro (IC 50 in the range of 0.16-0.28 nM) to HL-60, K562, and CCRF-CEM cells [78] and in vivo at dosages of 0.02-0.03 mg/kg/d [78,79], which is also much more active than other 1-alkyldaphnanes isolated from Daphne species [57]. It is generally considered that the antineoplastic activity is related to the 18-benzoyloxy substituent at C-18 [3,102].

Anticancer Activity and Involved Mechanisms Leukemia
It is also revealed that genkwadaphnin (15) could oppose the protein and DNA synthesis of P-388 cells to exert both in vitro and in vivo antileukemia activities [103,104]. Genkwadaphnin (15, IC 50 : 11.8 µM) and yuanhuacine (30, IC 50 : 10.8 µM) were determined to suppress Bcl-2 and Bcl-X L in a dose-dependent manner to induce apoptosis in human myelocytic HL-60 cells [39].
Gnidimacrin (101) showed a significant antiproliferative effect against K562 cell lines at the IC 50 of 1.2 nM by activating protein kinase C (PKC) and arresting the cell cycle at G 1 phase [79].
Yuanhualine (64), yuanhuadine (63), and yuanhuagine (33) showed notable inhibitory effects in drug-resistant cell lines including gemcitabine-resistant A549, gefitinib-and erlotinib-resistant H292. Further research indicated that these diterpenoids were able to arrest cell cycle in the G 0 /G 1 and G 2 /M phase in A549 cells by upregulating the expression of cyclin dependent kinase inhibitors P21 and P53 as well as downregulating cell-cycle regulators, for example, c-Myc and cyclin B1/cell division cycle 2 (CDC2) complex and suppress Akt/STAT/Src signaling pathway [106]. Additionally, yuanhualine (64) was observed to have synergistic effects with certain chemotherapeutic agents (gemcitabine, gefitinib and erlotinib) in the treatment of A549 cell line [106].

Wu et al. evaluated the effects of genkwadaphnin (15) on hepatocellular carcinoma (HCC) cells both in vitro and in vivo with
Hep3B and PLC/PRF/5 cell lines and BALB/c nude mice, respectively, the results showed that genkwadaphnin (15) suppressed growth and invasion of HCC cells both in vitro and in vivo by blocking DHCR24-mediated cholesterol biosynthesis and lipid rafts formation [44]. Evidence also showed that yuanhuacine (30) and genkwadaphnin (15) were hepatotoxic on normal human liver cells HL-7702 in a dose-and time-dependent manner; meanwhile, the change of cell morphology and increased AST and ALT were observed as well [40].

Breast Carcinoma
Yuanhuacine (30) was found to be an active inhibitor in both MCF-7 and MDA-MB-231 cell lines, and the preliminary mechanism of strong cytotoxicity of yuanhuacine (30) against MCF-7 was investigated further by using Western blot and flow cytometry analysis; the results suggested that yuanhuacine (30) induced apoptosis via the regulation of Bcl-2, Bax, and cleavage of PARP expression in MCF-7 cells [84]. Yuanhuatine (47) was also observed to inhibit the growth of estrogen receptor alpha (ERα)-positive cells MCF-7 (IC 50 : 0.62 µM) significantly compared to tamoxifen (IC 50 : 14.43 µM) through mitochondrial dysfunction and apoptosis in ERα-positive breast cancer cells MCF-7 caused by ERα-downregulation [107]; for ERα-negative cells MDA-MB-231, either cytotoxicity or apoptosis was observed [107].
Besides, the antiproliferative activity of the ethyl acetate and aqueous extract from the leaves of D. gnidium was observed in B16-F0 and B16-F10 cell lines inducing G 2 /M cell cycle arrest and the ethyl acetate extract was also capable of enhancing melanogenesis stimulation activity in a concentration-dependent manner in B16-F10 cells [109]; furthermore, the aqueous extract of D. gnidium exerted in vitro and in vivo antimelanoma effects on B16-F10 by activating natural killer (NK) cell and cytotoxic T lymphocyte (CTL) [110], hopefully these findings might lead to the discovery of more potential compounds affecting the melanogenesis and cell cycle of melanoma cells.

Fibrosarcoma
The alcohol-water extract of the aerial parts of D. mucronata was evaluated to possess anticancer property both in vitro and in vivo and its mechanism, similar to the natural anticancer drug Taxol, was probably related to the downregulation of human tumor necrosis factor alpha receptors (TNF-αR) [111]; the probable principle of anticancer activity, gnidilatimonoein (54), was isolated from D. mucronata afterwards and showed a strong antiproliferation effect on WEHI-164 by mediating the progress of DNA synthesis [60].

Colon Carcinoma
The mechanism of anticancer effect of genkwadaphnin (15) was revealed when it was found that it enhanced the p21 expression and simultaneously suppressed the c-Myc expression in a PRDM1-denpendent manner to arrest the cell-cycle progression in the human colon cancer SW620 cell line [112].
Yuanhuacine (30) and yuanhuadine (31) were revealed to exhibit more significant inhibitory effects on the proliferation of the COLO250 (IC 50 : 2-3 µM) than HT-29 cell line (IC 50 : 13-23 µM) [84], suggesting the selectivity of the antineoplastic activity. A further study indicated that yuanhuacine (30) inhibited the HCT116 cell line by upregulating p21 expression and transcription via a p53 protein independent cascade [113]. Daphgenkin A (108), along with yuanhuacine (30) and yuanhuadine (31) obtained from the petroleum ether extract from D. genkwa, showed definite cytotoxic effects on both SW620 and RKO cell lines with the respective IC 50 value of 3.0 and 6.5 µM; then, the results of further research revealed that daphgenkin A (108) inhibited SW620 cell proliferation by stalling the cell cycle at G 0 /G 1 phase, causing cell death by apoptosis as well as inducing cell cycle arrest via regulating the PI3K/Akt/mTOR signaling pathway [10].

Others
Yuanhuacine (30) and yuanhuajine (7) presented more obvious inhibitory activity against DNA topoisomerase I (DNA topo I) at the IC 50 levels of 40.0 and 38.3 µM than a known topo I inhibitor hydroxycamptothecin (hCPT, IC 50 : 48.0 µM), and further study with the prepared derivatives suggested that less electron-withdrawing groups at C-5, C-12, and C-20 facilitated the combination between the compounds and DNA topo I [47].
The chloroform extract from D. altaica was also found to significantly suppress the proliferation of esophageal squamous carcinoma Eca-109 cell line at the IC 50 level of 10.6 µM and in a dose-dependent manner [114], and the ethyl acetate extract of D. altaica was reported to function by inducing apoptosis and cell cycle arrest in the S phase in the Eca-109 cell line via the PPARγ-mediated pathway [115]. However, whether its anticancer effect is related to diterpenes or not requires detailed studies.

Anti-HIV Activity
Both daphnane-and tigliane-type diterpenoids from Daphne species were demonstrated to possess an anti-HIV activity even stronger than some anti-HIV agents such as 3 -azido-3 -deoxythymidine (AZT), and structure-activity relationship in anti-HIV bioactivity is quite similar to that in the anticancer one ( Figure 10).
The substituent at C-12 in the ring C acts as an important role in anti-HIV activity as well. A tigliane diterpenoid 12-O-benzoylphorbol-13-octanoate (125) from D. aurantiaca showed definite anti-HIV-1 activity against C8166 cell line with EC 50 value of 0.282 nM and SI value of 65177.305, while phorbol 13-monoacetate (130) possessing a 12-hydroxyl showed limited activity, which suggests an acyl group at C-12 might favor anti-HIV-1 bioactivity [87].

Anti-HIV Activity and Involved Mechanism
Prostratin (129) is known as a potent anti-HIV agent [117] and its mechanism is the protection of CD4+ cells by downregulation of the HIV receptor CD4 and co-receptors and the interaction with PKC to stimulate viral replication in infected cells [6]. Wikstroelide E (98), a HIV-latency-reversing compound that is strikingly 2500-fold more potent that prostratin (129), functioned by regulating various signaling pathways including the MAPK, PI3K-Akt, JAK-Stat, TNF, and NF-κB ones [116].  [8].
Based on the discovery of the antiretroviral activity of the dichloromethane extract of D. gnidium without displaying cytotoxicity, daphnetoxin (56), gnidicin (16), gniditrin (20), and excoecariatoxin (58) were determined to be the principles of anti-HIV bioactivity according to the HPLC-based profiling; meanwhile, a more detailed study showed that daphnetoxin (56) selectively interfered with the expression of two key cell-surface factors CXCR4 and CCR5 for HIV-1 entry [106].
It is worth noting that wikstroelide M (77) inhibited not only HIV-1 but HIV-2 strains in a concentration-dependent manner with high SI values and low cytotoxicity, and its mechanism might be related to the inhibition of HIV-1 reverse transcription and integrase nuclear translocation [71].
As for another inflammatory inhibitor genkwadaphnin (15), the mechanism includes the activation of PKD1/NF-κB signaling to induce CD44 expression in a time-and concentration-dependent manner and thus promoting the migration of K562 cells, resulting in an innate immune response [39]. Genkwadaphnin (15) was also observed to restore exhausted LCMV-specific CD4 + and CD8 + T cells by downregulating negative regulatory molecule Tim-3 [118].

Cholesterol-Lowering Activity
Gniditrin (20) and daphnetoxin (56) extracted from D. giraldii were found to present potent cholesterol-lowering activity in vitro at the EC 50 levels of 0.59 and 4.3 µM, respectively, by up-regulating the low-density lipoprotein receptor (LDLR) level and consequently promoting LDLR expression [119]. Gniditrin (20) had a lower EC 50 for activating LDLRpromoter than that of daphnetoxin (56), which might indicate that the acyl group at C-12 might improve the cholesterol-lowering bioactivity as well.

Neurotrophic Activity
Yuanhuacine (30) and genkwanine N (69) significantly enhanced the function of the orphan nuclear receptor Nurr1 at a concentration of 0.3 µM and inhibited LPS-induced neuroinflammation in vitro as well as improved behavioral deficits in a hydroxydopamine (6-OHDA) -induced rat model of Parkinson's disease [17].
Dapholosericin A (133), a tigliane diterpene from the EtOAc extract of D. holosericea, was discovered to be a moderate acetylcholinesterase (AChE) inhibitor at a concentration of 100 µM, which suggested its potential usage in the mediation of Alzheimer's disease [65].

Antifertility Activity
The flowers of D. genkwa are also used as a traditional Chinese herbal remedy for abortion. Both yuanhuacine (30) and yuanhuadine (31) have already been used clinically as labor-induced drugs at the per capita dose of 70-80 and 60 µg, respectively [42]. Yuanhuatine (47) [56] and tanguticacine (26) [25] also showed antifertility activity in Rhesus monkeys at the dosage levels of 50 and 300 µg/monkey, respectively. Further experiments suggested that the hydroxyl group at C-15 and C-20 in diterpenoid orthoesters favored antifertility activity and esterification of the hydroxyl group at C-20 by long-chain fatty acid would decrease the toxicity of the diterpenoids [120], and the underlying mechanism was preliminarily established that these diterpenoids induced the release of endogenous prostaglandin by damaging decidual cells [13].

Discussion
Back to 1970, the very first diterpenoid from Daphne species, daphnetoxin (56), was isolated as a toxic principle [61]. A total of 135 diterpenoids have been discovered from the genus Daphne after decades. These diterpenoids could be classified as daphnane-, tigliane-, and lathyrane-types according to the oxygen-containing functions and substitution pattern, and daphnane-type could be subdivided into 6-epoxy, genkwanine, 1-alkyldaphnane, and resiniferonoid. Within more and more novel diterpenoids isolated from Daphne genera, some of them could not fall into any classification. For example, genkwanine L (95) with a 3-ketone and 12-acetyl group does not completely satisfy the definition of genkwanines. As for compound 108-123, one or more suitable classifications have not been established yet.
Although the genus of Daphne embraces more than ninety species distributed in Asia, Africa, and Europe [2], the origins of these diterpenoids are limited to fifteen Daphne species and diterpenoids have been reported to occur mainly in D. genkwa. For D. mucronata, only two isolated diterpenoids but one of those, gnidilatimonoein (54), showed potent anticancer activity [59,60,122,123], which suggests the potential in the diterpenoids from the genus Daphne. The distributions of every diterpenoid in different parts of plants are available in the hope that it would be a guide for isolation and identification of diterpenoids.
Furthermore, some Daphne species have been assayed and found to possess pharmacological activities which might be closely related to diterpenoids. The methanol extract of D. malyana showed significant antimicrobial potential [124] and the identification of the antibacterial or antifungal principles has not been reported yet. D. linearifolia, of which the stem bark was used to treat inflammation and rheumatism such as some Daphne species mentioned above, showed affinity towards a promising anticancer target Hsp90 [125]. The D. cneorum extract exhibits potent antimicrobial and antioxidant activities, which makes it a possible source of new agents [126]. For D. alpina, its antioxidant and antimicrobial bioactivities have been investigated [127,128] and gniditrin (20) has been isolated from this species [29]. These suggest a prospect of further phytochemistry and pharmacological study in Daphne species.
For the structure-activity relationship (SAR) summarized in this review, despite the fact that some of the SAR remains unclear and needs to be further investigated, it showed that the pharmacological effect, to a large extent, depends on the substituents at C-3, C-12, C-20, and C-1 and the saturation of the ring A in these molecules. The SAR study is believed to provide very useful information for the optimization and synthesis of diterpenoids and lead to the development of novel agents; however, the SAR of diterpenoids has not been completely clarified.

Conclusions
In this review, a total of 135 natural diterpenoids in the past decades from the plants of the genus Daphne were covered. Many of the Daphne species are bioactive and thus have been used as traditional treatments for various diseases. Both the source species and parts from which diterpenoids were isolated have been provided as detailed reference information. The natural diterpenoids from Daphne species present interesting structures with complicated stereochemistry, which are closely related to their abundant bioactivities. The biological activities and the structure-activity relationship of certain classes of diterpenoids were reviewed in the hope of providing an easier way for researchers to understand the general situation of phytochemical and pharmacological properties in diterpenoids from the genus Daphne.

Conflicts of Interest:
The authors declare no conflict of interest.