Toxic Animal-Based Medicinal Materials Can Be Effective in Treating Endometriosis: A Scoping Review

Animal toxins and venoms have recently been developed as cancer treatments possessing tumor cell growth-inhibitory, antiangiogenesis, and proapoptotic effects. Endometriosis is a common benign gynecological disorder in reproductive-age women, and no definite treatment for this disorder is without severe side effects. As endometriosis and malignant tumors share similar characteristics (progressive, invasive, estrogen-dependent growth, and recurrence), animal toxins and venoms are thought to be effective against endometriosis. The objective of this study was to outline studies using toxic animal-based medicinal materials (TMM) as endometriosis treatment and to explore its clinical applicability. Preclinical and clinical studies using TMM were searched for in four databases from inception to October 2020. A total of 20 studies of TMM on endometriosis were included. In eight clinical studies, herbal medicines containing TMM were effective in relieving symptoms of endometriosis, with no side effects. In twelve experimental studies, the main therapeutic mechanisms of TMM against endometriosis were proapoptotic, antiangiogenesis, estrogen level-reducing, and possible anti-inflammatory effects. TMM are thus considered promising sources for the development of an effective treatment method for endometriosis. Further studies are needed to clarify the therapeutic mechanism of TMM against endometriosis and to provide sufficient grounds for clinical application.


Introduction
Endometriosis is a common estrogen-dependent, chronic inflammatory disease experienced by 10-15% of reproductive-age women [1][2][3]. It is defined as a disease in which endometrium-like tissue exists outside the uterus [2]. Endometriosis negatively affects the quality of life and well-being of women of childbearing age owing to distressing and debilitating symptoms and complications [4]. The most common clinical symptoms are dysmenorrhea, pelvic pain, infertility, and various clinical symptoms, such as dyspareunia, anal pain, and abdominal and adnexal masses [5,6]. Hypotheses on the origin of endometriosis occurrence include endometrial origin due to retrograde menstruation and non-endometrial origin, such as coelomic metaplasia and lymphatic and vascular metastasis [4,7]. The interaction between endocrine, immunologic, pro-inflammatory, and

Literature Search Strategy
We searched the following databases from inception to October 2020: MEDLINE, EMBASE, one Chinese database (China National Knowledge Infrastructure (CNKI)), and one Korean database (Oriental Medicine Advanced Searching Integrated System (OASIS)). The  Bufonis OR Hirudo OR Leech OR Scolopendra OR Scolopendrae OR Eupolyphaga OR Steleophaga OR Scorpio OR Gecko OR Tokay" AND "endometriosis". There were no language limitations in this study.

Inclusion and Exclusion Criteria
Type of studies: Experimental studies, such as animal or cell studies, and clinical studies using toxic animal-based medicinal materials for treating endometriosis were included in this review. All types of clinical studies, including randomized controlled trials (RCTs), observational studies, cohort studies, case reports, and case series were included in this review. Literature studies, qualitative studies, and surveys were excluded.
Subjects: Clinical studies with female patients with endometriosis were included. Experimental studies on animal endometriosis models or cell culture studies were considered for inclusion.
Types of intervention: Studies using 10 types of toxic animal-based medicinal materials in the treatment of endometriosis patients, endometriosis animal models, and endometriosis cells were included. In the case of toxic animal-based medicinal materials alone, a combination of toxic animal-based medicinal materials and other toxic medicinal materials was also included.

Study Selection and Data Extraction
Two independent authors (SIH and SHS) selected the studies according to the predefined criteria. Any disagreements were discussed by a third author (JKP). Two authors (SIH, GTH) independently extracted the following data from the selected experimental studies: author, year, study design, toxic animal-based medicinal materials used in intervention, target cell (in vitro)/animal model (in vivo), dosage, treatment period, outcome measures, results, and components of herbal medicine. Two authors (SHS, YJY) independently extracted the following data from clinical studies: author, study design, sample size, age, main symptoms, toxic animal-based medicinal materials used in intervention, control intervention, treatment duration, outcome measures, main results, adverse events, components of herbal medicine, dosage, frequency, and dosing period. We resolved any disagreements by discussing with a third author (JKP).

Results
After literature searching, we identified a total of 153 studies (6 in English database, 146 in Chinese database, and 1 in Korean database), from which 74 duplicate studies were excluded. By screening the titles and abstracts of 79 studies, 11 studies were found to be irrelevant due to the study type (10 reviews and 1 survey), 25 studies irrelevant due to the subject, and 13 studies irrelevant due to the intervention were excluded. After assessing the full text of the remaining 30 studies for eligibility, 20 studies were finally included, excluding one study due to irrelevance in the study type, one study due to irrelevance in the subject, three studies due to irrelevance in the intervention, and five studies due to insufficient details about the composition of the herbal medicine. Of the 20 studies finally selected, 8 were experimental studies and 12 were clinical studies. A flow diagram of the study selection process is shown in Figure 1.
The medicinal interventions used in the eight studies were largely divided into three categories, namely medicinal compounds extracted from animal toxins in two studies [28,29], individual medicinal extracts in two studies [27,34], and medicinal decoctions in four studies [30][31][32][33]. The composition of the medicinal formulae used in the four studies is summarized in Table 1.
According to the eight studies, the underlying mechanisms of animal toxins against endometriosis were as follows: induction of apoptosis [28][29][30], inhibition of angiogenesis [31,32], reduction of estrogen level [27], suppression of matrix metalloproteinase (MMP)-9 expression [34], and anti-inflammatory effect [33] (Table 2). Flow chart of the study selection process. After literature searching, we identified a total of 153 studies, from which 74 duplicate studies were excluded. We screened 79 studies' titles and abstracts. As a result, 49 studies were excluded, and 30 studies passed the first round of the selection process. Of these 30 studies, 10 irrelevant studies were excluded again, and the final 20 studies were selected for this study. Of the 20 studies finally selected, 8 were experimental studies and 12 were clinical studies.
The medicinal interventions used in the eight studies were largely divided into three categories, namely medicinal compounds extracted from animal toxins in two studies [28,29], individual medicinal extracts in two studies [27,34], and medicinal decoctions in four studies [30][31][32][33]. The composition of the medicinal formulae used in the four studies is summarized in Table 1. Among the toxic animal-based medical materials, Hirudo was the most commonly used in medicinal decoctions [30][31][32][33], followed by bufalin twice [28,29].
All included clinical studies reported improvement of symptoms related to endometriosis, and three studies [37,42,45] reported that no side effects were observed ( Table 3). The dosing regimens of herbal medicines containing toxic animal-based medicinal materials in the included clinical studies are presented in Table 4.

Discussion
In this scoping review, we outlined preclinical and clinical studies of toxic animalbased medicinal materials for the treatment of endometriosis. By reviewing the experimental and clinical studies included in this study, we identified the potential of toxic animal-based medicinal materials as a treatment of endometriosis. However, the current evidence is insufficient for establishing a standard for endometriosis treatment with toxic animal-based medicinal materials. High-quality studies are needed to investigate the various action mechanisms of toxic animal-based medical materials against endometriosis.
Even though we did not impose any restrictions in the literature search, we found more studies published in China than in any other country. Traditional Korean medicine and traditional Chinese medicine were alike as both are recognized by the healthcare systems of those counties. However, in the healthcare systems of the two countries, traditional Chinese medicine has a bigger share than that of traditional Korean medicine. Furthermore, traditional Chinese medicine spans a broader range, as it includes not only traditional Chinese medicine but also the integration of Chinese and Western medicine [47]. Therefore, it is believed that there would be more cases of using toxic animal-based medicinal materials in clinical contexts in China, with more reports for that matter.
The toxic animal-based medical materials investigated in the preclinical and clinical studies included in this analysis were somewhat different. Hirudo and Scolopendra were used in both preclinical and clinical studies. In contrast, Eupolyphaga and Scorpio were used only in clinical studies, and bufalin, bee venom, and snake venom were used only in preclinical studies. This is because Bufonis Venenum, bee venom, and snake venom are animal-derived toxins that can cause serious side effects, including death [22,[48][49][50][51], and thus have not been traditionally used to treat endometriosis and benign disease. Although these three animal venoms have recently been reported to exert proapoptotic [52][53][54] and anti-inflammatory effects [55,56], there has been no clinical evidence of their benefit in endometriosis treatment. These three venoms are thought to be effective against endometriosis because they have been reported to induce apoptosis, inhibit angiogenesis, reduce estrogen levels, suppress MMP-9 expression, and exert anti-inflammatory effects in preclinical studies. However, considering the serious side effects that may occur, the in vitro and in vivo toxicity and action mechanisms of bufo venom, bee venom, and snake venom must be elucidated before their efficacy against endometriosis can be investigated in clinical trials.
The 12 clinical studies included in this study reported improvement of various endometriosis symptoms, such as dysmenorrhea, pelvic pain, dyspareunia, mittelschmerz, anal pain, as well as increased pregnancy rate after administration of a decoction containing Hirudo, Eupolyphaga, Scolopendra, and Scorpio. However, as there were differences in the symptoms, dosage, and dosing period of the toxic animal-based medicinal materials used, the evidence was considered insufficient for establishing a standard for the application of toxic animal-based medicinal materials in the treatment of endometriosis. In future studies, the optimal dosage of the medicines should be established with consideration for both efficacy and toxicity, and clinical studies must be conducted to standardize the dosing period and dosage.
The most widely used toxic animal-based medicinal material among the included clinical studies was Hirudo Figure 2. Hirudo is the dried entire body of Whitmania pigra Whitman, Hirudo nipponica Whitman, or W. acranulata Whitman, and is a representative toxic animal-based medicinal material that has been used since the beginning of civilization. It has the efficacy of "breaking blood and expelling stasis", and there is historical evidence of its use in the treatment of endometriosis [22,57,58]. However, it is difficult to confirm the effectiveness of Hirudo against endometriosis, as there were no studies examining only Hirudo. Nevertheless, a medicinal decoction containing Hirudo was shown to exert therapeutic effect against endometriosis in both preclinical and clinical studies. The most widely used toxic animal-based medicinal material among the included clinical studies was Hirudo. Hirudo is the dried entire body of Whitmania pigra Whitman, Hirudo nipponica Whitman, or W. acranulata Whitman, and is a representative toxic animalbased medicinal material that has been used since the beginning of civilization. It has the efficacy of "breaking blood and expelling stasis", and there is historical evidence of its use in the treatment of endometriosis [22,57,58]. However, it is difficult to confirm the effectiveness of Hirudo against endometriosis, as there were no studies examining only Hirudo. Nevertheless, a medicinal decoction containing Hirudo was shown to exert therapeutic effect against endometriosis in both preclinical and clinical studies.
In the studies included in this scoping review, medicinal decoctions containing Hirudo was shown to downregulate the expression of hypoxia-inducible factor 1-alpha (HIF-1α) and reduce the level of vascular endothelial growth factor (VEGF), thereby suppressing the progression of endometriosis [32]. They also reduced the levels of basic fibroblast growth factor, platelet-derived growth factor, and VEGF, and were involved in the angiogenesis pathway by downregulating TLR4 and NF-κB, which participate in the signaling process of angiogenesis, thus improving endometriosis [31]. It was also suggested that a medical decoction containing Hirudo exerted anti-inflammatory effect [33]. These findings are supported by other experimental studies. Hirudo has been revealed to have anticoagulant, anti-inflammatory, bacteriostatic, and analgesic effects [22,59]. Hirudo is also known to inhibit tumor angiogenesis by improving the tumor hypoxia microenvironment, reducing the mRNA and protein expression of HIF-1α, and contributing to the downregulation of VEGF mRNA expression [60]. In addition, Hirudo induces apoptosis and inhibits cell proliferation in Hep G2 human liver cancer cells and HL-60 leukemic cells, resulting in an antitumor effect [61,62]. Although the use of Hirudo is controversial, Hirudo exerts anti-inflammatory effect by inhibiting carboxypeptidases In the studies included in this scoping review, medicinal decoctions containing Hirudo was shown to downregulate the expression of hypoxia-inducible factor 1-alpha (HIF-1α) and reduce the level of vascular endothelial growth factor (VEGF), thereby suppressing the progression of endometriosis [32]. They also reduced the levels of basic fibroblast growth factor, platelet-derived growth factor, and VEGF, and were involved in the angiogenesis pathway by downregulating TLR4 and NF-κB, which participate in the signaling process of angiogenesis, thus improving endometriosis [31]. It was also suggested that a medical decoction containing Hirudo exerted anti-inflammatory effect [33]. These findings are supported by other experimental studies. Hirudo has been revealed to have anticoagulant, anti-inflammatory, bacteriostatic, and analgesic effects [22,59]. Hirudo is also known to inhibit tumor angiogenesis by improving the tumor hypoxia microenvironment, reducing the mRNA and protein expression of HIF-1α, and contributing to the downregulation of VEGF mRNA expression [60]. In addition, Hirudo induces apoptosis and inhibits cell proliferation in Hep G2 human liver cancer cells and HL-60 leukemic cells, resulting in an antitumor effect [61,62]. Although the use of Hirudo is controversial, Hirudo exerts anti-inflammatory effect by inhibiting carboxypeptidases (kininase 1), inhibits platelet function, exerts anticoagulant activity, and increases blood flow [63].
Scolopendra was the second-most widely used toxic animal-based medicinal material in the included clinical studies. Scolopendra is the dried body of Scolopendra subspinipes mutilans L. Koch, which is poisonous. It has the efficacy of "extinguishing wind and suppressing convulsion, unblocking collaterals and relieving pain, counteracting toxins, and dissipating masses" and is used for "spasm and convulsion due to internal stirring of liver wind, etc." [57]. Medicinal decoctions containing Scolopendra were shown to be effective in treating endometriosis in both clinical and preclinical studies included in the present review.
Experimental studies included in this study suggested that medical decoctions containing Scolopendra may exert therapeutic effect against endometriosis, which is a chronic inflammatory condition, through anti-inflammatory action [33]. This is supported by other studies, in which Scolopendra was reported to have anticoagulation, antiseptic, and antiinflammatory effects [64][65][66]. Scolopendra is also known to induce the downregulation of matrix metallopeptidase-2 (MMP-2) and -9 in tumor cells, suppress the migration and invasion of tumor cells, and inhibit cell proliferation [67]. In addition, Scolopendra inhibits the proliferation of EGFR-overexpressing cells by inducing apoptosis and modulating the EGFR pathway [68,69]. It also exerts anti-inflammatory activity partially through inhibition of the NF-κB signaling pathway [70].
Eupolyphaga, the third-most commonly used toxic animal-based medicinal material in this review, is the dried body of female Eupolyphaga sinensis Walker or Steleophaga plancyi (Boleny), which is slightly poisonous. It has the efficacy of "breaking blood and expelling stasis" and has been used clinically for "aggregation-accumulation due to static blood obstruction" [57]. Clinical studies in this review revealed that medicinal decoctions containing Eupolyphaga were effective in alleviating endometriosis symptoms. However, there have been no experimental studies that elucidate the anti-endometriosis mechanism of Eupolyphaga. Nevertheless, in other studies, Eupolyphaga was shown to inhibit cell adhesion to fibronectin and collagen IV as well as cell migration and invasion in A549 human lung cancer cell [71]. It also inhibits cell proliferation and reduces MMP-2 and -9 expression in hepatocellular carcinoma [72], and reduces cell invasive ability by downregulating MMP-2 and -9 protein expression in breast cancer [73]. Furthermore, it induces the detachment and apoptosis of A549 human lung cancer cells [71], inhibits tumor cell growth in hepatocarcinoma, promotes TNF-α and IFN-γ expression, increases the Bax/Bcl-2 ratio, and activates caspases-3 to induce apoptosis [74]. Moreover, Eupolyphaga is known to activate the immune function by modulating oxidative systems, enhancing the phagocytic function of macrophages, and elevating serum IL-2 level [75].
Lastly, the dried body of Buthus martensii Karsch scorpion is known to be poisonous and exert liver-pacifying and wind-extinguishing effects. Scorpio has pharmacological properties, such as pain-reducing, anti-inflammatory, and anticoagulant properties [76,77], but no experimental studies have determined the therapeutic mechanism of Scorpio against endometriosis. Scorpio exerts an antitumor effect by inhibiting cell proliferation, inducing apoptosis, and decreasing migration and invasive functions in Hepa 1-6 cells and HepG2 cells [78,79]. They also inhibit MMP activity in breast and colorectal cancers, thereby reducing the motility and invasion of tumor cells, and induce apoptosis by decreasing the expression of antiapoptotic proteins and increasing that of proapoptotic proteins [80]. In addition, chlorotoxin derived from scorpion toxin inhibits the expression of ERα in breast cancer cells by inhibiting the ERα signaling pathway through direct binding to ERα, thus changing the protein secondary structure of its LBD domain [81].
As outlined above, toxic animal-based medicinal materials are composed of several molecules that exhibit a wide range of pharmacological activities. Considering that endometriosis has several possible mechanisms of pathogenesis, multi-target medicines have the potential to treat endometriosis through multiple pathways. Further research is needed to elucidate the specific therapeutic mechanisms of individual toxic animal-based medicinal materials against endometriosis. Although the mechanisms underlying the progression of endometriosis are still unclear, endocrine, immunologic, pro-inflammatory, and pro-angiogenic processes are known to be involved in the development of endometriosis. Considering the pharmacological effects of toxic animal-based medicinal materials on endometriosis and other diseases, the possible action mechanisms of toxic animal-based medicinal materials against endometriosis can be summarized as follows: induction of apoptosis, antiangiogenesis effect, reduction of estrogen level, anti-inflammatory action, and suppression of cell adhesion and invasion (Figure 3). development of endometriosis. Considering the pharmacological effects of toxic animalbased medicinal materials on endometriosis and other diseases, the possible action mechanisms of toxic animal-based medicinal materials against endometriosis can be summarized as follows: induction of apoptosis, antiangiogenesis effect, reduction of estrogen level, anti-inflammatory action, and suppression of cell adhesion and invasion (Figure 3). Toxic animal-based medicinal materials may cause unwanted side effects due to their toxic effects. Minimizing adverse events is an essential part of drug development. Even with the medicines that have been used based on historical experience, the types and frequency of adverse events must be identified. However, among the studies included in this review, 75% of the studies had no reports on side-effects, and three studies [37,42,45] reported that no side effects were observed after administration of herbal medicines containing Hirudo and Scolopendra [37], Scolopendra [45], and Scorpio [42]. However, it was not possible to find any information on what kind of events they were monitoring.
The number of participants who participated in the studies that reported no side effects was small. Since the known occurrence of adverse events was 5% to 10% [82,83], the findings of this study are not sufficient to support a conclusion that the administration of toxic animal-based medicinal materials upon the patients with endometriosis would result in no side effects. Even if the toxic animal-based medicinal materials have historical evidence, it is necessary to identify the existence, occurrence rate, and severity of adverse events through real data collection. A clinical study must be designed to report any adverse events, no matter how insignificant they may look.
In addition, in order to identify the possible adverse events that can be caused by toxic animal-based medicinal materials, we searched other studies that reported adverse events after administering toxic animal-based medicinal materials. Among the studies included in this review, three studies [37,42,45] reported that no side effects were observed after administration of herbal medicines containing Hirudo and Scolopendra [37], Scolopendra [45], and Scorpio [42], and that no adverse effects were reported following Toxic animal-based medicinal materials may cause unwanted side effects due to their toxic effects. Minimizing adverse events is an essential part of drug development. Even with the medicines that have been used based on historical experience, the types and frequency of adverse events must be identified. However, among the studies included in this review, 75% of the studies had no reports on side-effects, and three studies [37,42,45] reported that no side effects were observed after administration of herbal medicines containing Hirudo and Scolopendra [37], Scolopendra [45], and Scorpio [42]. However, it was not possible to find any information on what kind of events they were monitoring.
The number of participants who participated in the studies that reported no side effects was small. Since the known occurrence of adverse events was 5% to 10% [82,83], the findings of this study are not sufficient to support a conclusion that the administration of toxic animal-based medicinal materials upon the patients with endometriosis would result in no side effects. Even if the toxic animal-based medicinal materials have historical evidence, it is necessary to identify the existence, occurrence rate, and severity of adverse events through real data collection. A clinical study must be designed to report any adverse events, no matter how insignificant they may look.
In addition, in order to identify the possible adverse events that can be caused by toxic animal-based medicinal materials, we searched other studies that reported adverse events after administering toxic animal-based medicinal materials. Among the studies included in this review, three studies [37,42,45] reported that no side effects were observed after administration of herbal medicines containing Hirudo and Scolopendra [37], Scolopendra [45], and Scorpio [42], and that no adverse effects were reported following the administration of herbal medicines containing Eupolyphaga. However, these findings are inconsistent with those of other studies. Hirudo is prohibited for pregnant women, patients with profuse menstruation or hemorrhagic tendency, patients with no blood stasis, and weak patients because Hirudo is highly potent in breaking blood and expelling stasis, unblocking the meridian, and eliminating mass [26,57]. Zeng et al. reported gastrointestinal bleeding (1.9%) and cerebral hernia (0.9%) after the administration of herbal medicines containing Hirudo and Tabanus bivittatus Matsumura in patients with acute intracerebral hemorrhage [84]. Historically, owing to its toxicity, Scolopendra has not been used at a high dosage, as its excessive use can damage healthy qi and cause early abortion [26]; therefore, it is prohibited for use in pregnant women and patients with blood deficiency generating wind [57]. In one patient with chronic hepatitis B infection, hepatotoxicity was reported after the administration of herbal medicine containing Scolopendra [85]. How-ever, this herbal medicine contained not only Scolopendra but also the hepatotoxic Meliae Fructus and Rhei Radix et Rhizoma [85]; these toxic effects cannot be attributed solely to Scolopendra, but caution is needed when subjecting patients with chronic hepatitis B to Scolopendra treatment. In traditional Chinese medicine, Eupolyphaga is contraindicated for pregnant women because it may induce early abortion owing to its blood-breaking and stasis-expelling efficacies, and it should be used with caution in patients with no blood stasis or patients with both blood deficiency and blood stasis [26]. Several studies have reported the side effects of Eupolyphaga. However, in a study where 21 HIV/AIDS patients were treated with a drug containing 20 g of Eupolyphaga twice a day for 4 months, no side effects or toxic reactions were reported [86]. Scorpion should be prohibited in patients with blood deficiency generating wind, and should be used at a dose range of 3-5 g owing to its toxicity [26,57]. There was one case of hepatotoxicity after administration of herbal medicine containing Scorpio in a patient with chronic hepatitis B infection [85]. Taken together, the patient's conditions should be considered when administering toxic animal-based medicinal materials, and attention should be paid to the menstrual cycle during treatment with these materials, as they can cause excessive bleeding in patients with menorrhagia. In addition, when administering these medicinal materials to women of childbearing age, the possibility of pregnancy should be assessed. Moreover, overuse of these medicinal materials should be avoided because it has been reported to be associated with hepatitis. This is the first study to review both preclinical and clinical studies investigating the efficacy of toxic animal-based medicinal materials in the treatment of endometriosis. Our analysis results suggested the possible therapeutic mechanisms of toxic animal-based medical materials against endometriosis and provided data as a reference for future translational studies. In addition, we included studies from major databases, such as PubMed and Embase, and searched without language and date restrictions. However, there were some limitations in this review. First, as we used only the biopharmaceutical names as search terms, there is a possibility that potentially relevant articles not containing our search key words and phrases in the titles or abstracts were not included in our analysis. Second, the number of included studies was small. Third, it was difficult to determine whether the reported therapeutic effects were exerted by the toxic animal-based medicinal materials, as these materials were used concomitantly with other medicinal materials. Fourth, there were some factors that required attention in interpreting the study findings. Five of the clinical studies we reviewed were case series, and therefore the level of evidence was somewhat low. Seven RCTs did not provide their protocols and were without any description of their randomization or blinding methods. Furthermore, some studies used mifepristone or different types of herbal medicines rather than the conventional treatment of endometriosis as control, so caution is needed as one interprets the results of these studies. Some studies did not report side effects or what kind of important harms or unintended effects they observed. Therefore, even if some studies did not report side effects, it does not necessarily support the conclusion that toxic animal-based medicinal materials are safe to use. Lastly, endometriosis is diagnosed by ascertaining symptoms, undertaking examination, and performing laparoscopic surgery [87]. Of the studies that were included in this review, only three [32,34,35] used laparoscopes to diagnose endometriosis. Other studies all relied on presumed diagnosis based on symptoms, examination, ultrasonography, and lab test results, without any classification of endometriosis, which were the limitations of these studies.
Through this scoping review, we showed that toxic animal-based medicinal materials might be effective as a clinical treatment of endometriosis. However, because the number of clinical studies included was small and the intervention and treatment goals were different, the findings of this review are insufficient for establishing a standard for the application of toxic animal-based medicinal materials as a treatment of endometriosis. In addition, although toxic animal-based medicinal materials have been shown to be effective against endometriosis in experimental studies, research on their specific therapeutic mechanisms is still lacking. Therefore, evidence supporting the use of toxic animal-based medicinal materials as a treatment of endometriosis is still limited, and further research is necessary.
The preclinical studies and clinical studies included in this review showed that the most promising toxic animal-based medicinal material for endometriosis is likely to be Hirudo. As shown in Figure 3, Hirudo, Scolopendra, and Scorpio meet three categories of action mechanisms related to the progression of endometriosis. Eupolyphaga meets two categories. In addition, in the clinical studies shown in Table 3, Hirudo was included in eight studies. Scolopendra, Eupolyphaga, and Scoprpio were included in five, four, and one study, respectively. Thus, we assume that Hirudo might be the most prominent candidate for treating endometriosis, among these toxic animal-based medicinal materials. The preclinical studies demonstrated that Hirudo has activities inducing apoptosis and inhibiting angiogenesis and inflammation. Although these molecular mechanisms are closely related to the progression of endometriosis, the anti-endometriotic efficacy of Hirudo was not fully demonstrated by a clinical study. In addition, there were no studies that used Hirudo only, so further study is needed to examine the treatment effect of Hirudo upon endometriosis. Scorpio, Scolopendra, and Eupholyphaga have been in clinical use, but their preclinical study is not sufficient. Bee venom, Bufonis Venenum, and snake venom have been used in preclinical contexts, without any report of toxicity. As bee venom, Bufonis Venenum, and snake venom may cause a serious adverse event, it is necessary to identify the adverse events and appropriate doses via preclinical studies.
In the future, the following steps would have to be taken in order to develop a new drug for endometriosis based on toxic animal-based medicinal materials. First, classical literature and medical records should be searched to obtain additional evidence supporting the use of toxic animal-based medicinal materials in clinical setting. Next, extensive preclinical studies are needed to elucidate the mechanisms of endometriosis treatment that are frequently used in clinical practice. Research is needed not only on the therapeutic mechanism of a single toxic animal-based medicinal material against endometriosis but also on the interaction between medicinal materials and decoctions that are traditionally used in combinations, including toxic animal-based medicinal materials. As they have toxic properties, quality control of single toxic animal-based medicinal materials is necessary, and it is important to assess their safety and toxicity in future studies. Additional clinical trials should be conducted to establish the standardized as well as optimal dosage and dosing period of these medicinal materials.

Conclusions
In this scoping review, preclinical and clinical studies investigating the efficacy of toxic animal-based medicinal materials against endometriosis did not provide sufficient evidence to allow standardization of these materials as a treatment of endometriosis. However, the included studies have identified toxic animal-based medical materials as potential treatments for endometriosis. In clinical studies, the administration of herbal medicines containing toxic animal-based medicinal materials alleviated symptoms such as dysmenorrhea, infertility, endometrioma, and menstrual irregularities in patients with endometriosis, with no side effects. In experimental studies, the main action mechanisms of toxic animalbased medicinal materials against endometriosis were identified as induction of apoptosis, antiangiogenesis effect, reduction of estrogen levels, and possibly, anti-inflammatory action. Taken together, toxic animal-based medicinal materials are considered promising to be developed as an effective treatment of endometriosis; however, additional studies are needed.