Plants Used as Anticancer Agents in the Ethiopian Traditional Medical Practices: A Systematic Review

Background This systematic review aimed at examining the use patterns of Ethiopian anticancer traditional medicinal plants (MPs) in view of recommending further validation studies. Methods The information was retrieved from PubMed according to the PRISMA guideline. The electronic library of Addis Ababa University and relevant church-based religious books were also inspected for additional data. Results From 34 studies meeting specific inclusion/exclusion criteria, 119 anticancer MP species distributed in 98 genera and 57 families were recorded. Fabaceae (10.92 %) and Euphorbiaceae (10.08 %) were the most widely used families. Plumbago zeylanica (Plumbaginaceae) was the most frequently used anticancer MP species. Shrubs (42.02 %) and herbs (35.29 %) were dominant growth forms. About 89.08 % of the MPs were collected from wild habitats. Leaves (33.73 %) and roots (27.11 %) were the most frequently harvested parts. The most favored routes were dermal (33.33 %) and oral (29.25 %). About 87.07 % preparations were made from fresh plant materials. Breast cancer and skin cancer were treated with 14 % and 10.76 % of anticancer MPs, respectively. Conclusion The review showed that anticancer MPs are widely used across the regions of Ethiopia. Most of the MPs are not scientifically experimented and yet are at a higher vulnerability to loss mainly by human activities. Calling for conservation measures, we recommend experimental validation of the frequently used anticancer MPs. This augments global anticancer drug researches.


Cancer.
Cancer is the name given to a group of diseases comprising a combination of genetic, metabolic, and signaling pathway aberrations [1]. It is usually a fatal disease, which constitutes an enormous burden on society in both economically developed and underdeveloped nations alike [2]. It is the second leading cause of death globally next to cardiovascular diseases; and available estimates by GLOBOCAN (an International Agency for Research on Cancer-IARC) show that about 14.1 million new cancer cases and 8.8 million deaths occurred in 2012 worldwide. Furthermore, by 2012 32.6 million people were living with cancer, within 5 years of diagnosis [3,4]. Overall, 57 % of new cancer cases, 65 % of the cancer deaths, and 48 % of the 5-year prevalent cancer cases occurred in the less developed regions [3]. According to WHO estimates in 2015, about 200 known types of cancer exist and the most common causes of cancer deaths include cancer of the lung (1.69 million), liver (788, 000), colorectal (774, 000), stomach (754, 000), and breast (571, 000) [4]. The prevalence of cancer is increasing because of the growth and aging of the population, as well as an increasing prevalence of established risk factors such as smoking, overweight, physical inactivity, and changing reproductive patterns associated with urbanization and economic development [3,5].
In Africa, cancer is alarmingly becoming a critical public health problem, with cancer forms attributed to infectious agents (e.g., cervical cancer, liver cancer, Kaposi sarcoma, and urinary bladder cancer) being the dominant types [6]. According to the IARC, about 715,000 new cancer cases and 542,000 cancer deaths occurred in 2008 in Africa [6,7]. The numbers are projected to double by 2030 due to the aging and fast growth of the population. The potential 2 Evidence-Based Complementary and Alternative Medicine of being even higher has been imagined because of the adoption of behaviors and lifestyles associated with economic development [6][7][8][9]. Thus, prostate cancer in men and breast cancer in women have now become the most commonly diagnosed cancers in some parts of Africa [3,10].
Reports on the prevalence of cancer in Ethiopia are scanty and difficult to verify because oncology services are inadequate and national registry centers do not exist; there are no control and prevention programmes against the disease; diagnosis and treatment services are very limited [11][12][13]. However, the limited reports show that Kaposi sarcoma, liver, prostate, cervical, and breast cancer are the most common cancers [14] in Ethiopia. According to the cancer country profiles of WHO, most prevalent forms among males are colorectal, Kaposi sarcoma, leukemia, lymphomas (non-Hodgkin), and prostate cancers in the order of listing, whereas cancer of the breast, cervix-uteri, ovary, colorectal, and leukemia, in the order given, are topping the list among females [15]. A study at the radiotherapy center of Tikur Anbessa specialized hospital (currently the only one in the country) showed that breast cancer had a high prevalence (27.8 %) [16]. On the scale of mortality leukemia (12.7 %) in males and breast cancer (24.4 %) in females were reported to be the highest in 2014 [15].

Traditional Medicines and Cancer.
Herbal medicine (phytomedicine, phytotherapy, or botanical medicine) is the oldest system of complementary and alternative medicine (CAM) in the world with a history of more than 2,000 years [17]. It is made exclusively from plants and is used by all societies and are common to numerous cultures [18]. There are, however, variations in the preparation and treatment procedures throughout the world [19]. Certain herbal medicines defend the body from malignancy by augmenting detoxification or cleaning the body. Some biological response modifiers derived from herbs are known to hinder the growth of cancer by modifying the activities of hormones and enzymes, while others diminish lethal side effects and complications of chemotherapy and radiotherapy [20].
The use of CAM is common among cancer patients in general and breast cancer patients, in particular [21]. They are perceived by the general public to be safe, cause less complications, and are less likely to cause dependency [19]. Most cancer patients combine herbal remedies with conventional therapy in the hope of boosting the effect of conventional medicine [22]. Nevertheless, cancer patients who use CAM report outcomes including improvement of clinical symptom, quality of life, reduction of chemo/radiotherapy induced side effects, and reduced tumor size [23][24][25]. Supporting conventional cancer treatment, preventing recurrence and eventual prolonging of survival were also reported [26]. In addition to these claims, anticancer agents derived from plants were shown to inhibit angiogenesis, suppress cell proliferation, inhibit or reverse tumor development, and show general antioncogenic effects [27]. A number of phytoconstituents resulting from the herbs were also reported to assist the body's immune system to combat cancer more efficiently [20].

Ethiopian Flora and Medicinal Plants.
Historical accounts substantiate age-long usage of traditional medicines (TMs) in Ethiopia. For instance, collections of medicoreligious manuscripts of the Axumite kingdom, medical textbooks that have been written in Ge' ez and Arabic languages between mid-17 th and beginning of the 18 th century are few among long-standing sources [28,29]. It is widely reported that over seventy percent of the Ethiopian people depend on TM for their healthcare, where more than 95 % of traditional medicines are sourced from plants [30,31]. Ethiopia's location in a tropical area, its huge landmass, and incredible variation in altitude ranging from about 120 meters below sea level to 4,543 meters above sea level have strongly influenced the range of its ecosystem and have contributed to the high diversity and rate of endemism in the flora [32][33][34]. Thus, the flora of Ethiopia has about 6,027 species of vascular plants with 10 % endemism [35][36][37][38]. According to recent reports, over 1000 species are used as traditional medicinal plants (MPs) of which about 33 species are endemic to Ethiopia [39,40].

The Rationale for the Systematic Review on the Anticancer MPs of Ethiopia
It is evident that the incidence of cancer is on the rise globally. The disparity in cancer survival between high and low economic settings mainly lies on two factors: the stage of its diagnosis and availability and access to treatments [6,81,82]. Although surgery is the existing standard curative therapy for most cancer forms, its therapeutic efficacy is compromised as most tumors are often diagnosed at an advanced stage, particularly in the underdeveloped part of the world [83,84]. Therefore, treatments such as radiotherapy and chemotherapy are next ladder therapies although they achieve only very modest results [85]. The everincreasing occurrence of cancer and the severe side effects and limited efficacy of current cancer chemotherapy based on synthetic drugs shift the attention toward drugs of plant origin [86,87]. In Ethiopia, environmental degradation, deforestation, intermittent drought, high rate of cultural and habitat loss, and various anthropogenic activities are threatening the MPs. However, recent ethnobotanical studies from different parts of the country have shown that TM is widely practiced in the country. It is also claimed that cancer patients prefer TMs to conventional therapeutic approaches mainly due to its cultural acceptance and ease of access [37,39,51,52,56,59,72,73,76,77,[88][89][90][91][92][93][94][95][96][97][98][99]. The Ethiopian flora largely remained untouched with tremendous potential that could offer a lot in the fight against cancer. Therefore, it is believed that the systematic review of ethnobotanical studies reported from Ethiopia would help identify potential anticancer MPs. This will serve as a basis for initiating rigorous scientific investigations for the chemopreventive and anticancer attributes of the MP species most frequently used across the regions of Ethiopia. Such investigation would contribute to the global anticancer drug discovery effort.  [100]. Electronic repository of the Addis Ababa University (AAU) (http://etd.aau.edu.et) and sources that make references to the TM knowledge of church-based religious teachers ('debteras') were examined as additional sources ( Figure 1). In the search process, keywords including, "ethnobiology", "ethnobotany", "ethnobotanical" "ethnomedicine", "ethnomedicinal", "medicinal plants", "traditional healer", "traditional medicine practitioner", "traditional medicine", "traditional herbal medicine "herbal medicine", "food supplements" and "medico-cultural", all with the term "Ethiopia" were used. After completing identification, screening and checking the eligibility of the literature for the systematic review, only those reporting forms of malignancies/cancer, neoplasms, 'nekersa' , tumor and swelling due to 'nekersa' were analyzed. Throughout the review paper the term "cancer" or 'nekersa' or "malignancy" is used to define a condition where cells of neoplastic features divide without control and can invade nearby tissues. and (x) ethnobotanical information with no reports of MP use for the treatment of malignancies/cancer and/or any neoplasm were excluded.

Data Extraction and Review Process.
After the retrieval from the electronic databases, the research articles were imported to the ENDNOTE software version X7 (Thomson Reuters, USA) and the duplicates were removed. After the removal of the duplicates, all the imported articles and the additional files were checked against the inclusion/exclusion criteria. Following the confirmation of the eligibility for the systematic review, each document was carefully examined at a time. We extracted the following data from each eligible document: (i) taxonomic diversity (the scientific name of the species and family name), (ii) vernacular/local name of the plant, (iii) habitat of the plant, (iv) the growth form (habit of the MP), (v) parts of the MPs used for the remedy preparation and routes of administration, (vi) the forms of malignancies treated, and (vii) reports on the adverse effect(s); contraindications and antidotes were carefully extracted and analyzed. The citation frequency and mapping of their geospatial distribution were done. Data on the distribution of MPs in the Ethiopian flora region and the altitudinal range the plants grow was generated. Data extraction was carried out twice independently and the data sheet was further checked for methodological conformity and correction of any discordance. Descriptive statistics was used to summarize the findings.

Taxonomic Diversity and Growth form and Distribution
of the Anticancer MPs of Ethiopia. One hundred nineteen (119) species of anticancer MPs distributed in 98 genera and 57 families were retrieved from thirty-four (34) reports that

Geospatial Distribution of the Anticancer MPs.
According to the present systematic review, the eligible studies on anticancer MPs were reported mainly from the Oromia Regional State (37.04 %), Amhara Regional State (33.33 %), and Southern Nations, Nationalities and Peoples Regional (SNNPR) State (18.52 %) ( Table 3). The frequency of reports across the regions and distribution in the Ethiopian Flora Region are shown in Figure 2 and Appendix.  cancer reported include breast cancer (14.0 %) and skin cancer/'Minshiro nekersa' (10.67 %). The remaining list includes bone cancer, brain cancer, cervical cancer, rectal cancer, anal cancer, invasive cancer/ 'kaysi nekersa' , swelling with wound/'kemenzina nekersa' , leukemia, and lung cancer among others ( Figure 3).

Parts of the MPs Used for the Remedy Preparation and
Routes of Administration. Analysis of the eligible ethnobotanical findings showed that various parts of the anticancer MPs were used for the remedy preparation. The most frequently harvested parts of the anticancer MPs are leaves (33.73 %) and roots (27.11 %). Debarking constituted 10.84 % whereas the whole parts of the MPs were used in as high as 4.22 % of the preparations. The stem and shoots were rarely collected (only 1.81 %, each) for the remedy preparation ( Figure 4).
From the various formulations and application procedures reported, the most preferred administration route of the TMs was dermal application (33.33 %) followed by oral (29.25 %). About 4.08 % of the applications recommend both oral and dermal routes of applications and in 26.53 % of the cases, the route of administration has not been specified ( Figure 5). About 12.93 % of the preparations were made from dried plant materials whereas 87.07 % were prepared from fresh plant materials and water is the solvent that is mostly used to prepare anticancer TMs. About 4.76 % of preparations included honey as an additive to the remedy. Butter, flour of 'teff' , spicy stew, and sulfur (locally called 'digne' in Amharic) among others were found to be mixed in about 4.08 % of the remedies. Common methods of dermal application included dressing the swelling with warm plant material, smearing on the wound, rubbing the affected area with the TM, tying the residue on the swelling, and giving a message with the fresh plant material preparation, as in the cases of breast and skin cancer.

Adverse Effects, Contraindications Implicated, and Antidotes.
The present finding revealed that nine eligible studies discussed the existence and management approaches used by the TM practitioners in case the anticancer remedies cause any possible side effect(s). Besides, contraindications were also noted, pregnant women being the most commonly at risk. Out of 119 MP species compiled, eleven (11) species were found to have adverse side effects and/or contraindications. The most commonly implicated species was Phytolacca dodecandra, followed by Croton macrostachyus. Two studies have reported that any MP could result in mild adverse effects especially among the risk groups or in case of overdosing of the remedy. During such circumstances, drinking coffee, local beer and flax, and eating local food like 'SHIRO' alleviates the symptoms [41,69]. The findings are summarized along with the recommended antidote(s) ( Table 4).

Discussion
These days, there is a higher level of dynamics in sociocultural transformations in Ethiopia. It is evident, however, that the communities by and large retained valuable knowledge of the MPs and their uses with regard to traditional healthcare. In the present investigation, 119 anticancer MPs were documented. This implies a wider usage of CAM among cancer patients in Ethiopia. This could be related to lack of full access to healthcare facilities. In addition, CAM usage is culturally accepted among Ethiopians as an effective cure and safe and is affordable. It is also perceived more efficacious against certain types of diseases including cancer [103][104][105]. However, the rapid population growth has resulted in an alarmingly high demand for agricultural land which in turn seriously threatens forest cover and hence the MPs. This makes designing and implementing of an insightful conservation program mandatory [29,30,106]. Consistent with various ethnobotanical studies, shrubs made up a larger proportion of the anticancer MPs (42.02 %) followed by herbs (35.29 %) [31,44,97,[107][108][109]. This may be explained by the fact that shrubs are perennial in the arid or subarid environments and may be available for use as MPs [64]. The dominance of certain life forms of MPs in different study areas might be explained by their availability and adaptation in the particular ecological setting and the dynamics involved due to bushy vegetation being left behind when the forest recedes.
Dominant families from which anticancer MPs are prepared were found to be Fabaceae ( [50,57] ["SHIRO WOT" (sauce made of pulse grains) and powder of Linum usitatissimum infusion in water] [60] attributed to the fact that these families are among the most widely distributed ones in the Ethiopian Flora Regions [31,67,110]. Since Plumbago zeylanica (Plumbaginaceae) is the anticancer MP species most widely used across the country, it may have a better healing potential over other anticancer MPs. This finding would make it a prime candidate for further in-depth experimental investigations.
The present review also revealed that roots (27.11 %), bark (10.84 %), and the whole part of the MPs (4.22 %) accounted for about 42.17 % usage in TM. This will significantly affect the sustainability of the MPs unlike use of aerial parts, such as leaves [91,106,111]. It was revealed that most remedies are prepared from fresh plant materials (87.07 %). This would result in the extensive exploitation of the MPs and in a long Evidence-Based Complementary and Alternative Medicine 9 run and will compromise the sustainability of the MPs [55,58,111].
The finding that a significant proportion of the remedies were given orally (29.25 %) implies that the remedies are safe for systemic applications. However, in the present report over ten MP species used as anticancer agents across the country had contraindications and/or adverse effects calls for cautionary usage. In addition, there are experimental evidences on the toxicity of some anticancer MPs (e.g., Hagenia abyssinica and the species from genus Vernonia) reported in the present study [112][113][114]. This indicates the need for acute and chronic toxicity investigations on the most commonly used traditional anticancer MPs.
The majority of anticancer MPs reported in the present study were from wild habitats (89.08 %). These habitats are vulnerable to demise by anthropogenic activities [57,115]. As intermittent drought and wide spreading climatic changes are posing additional threats to the anticancer MPs of the country, they are limited to inaccessible areas. Therefore, as d' Avigdor and the coworkers suggested, the community must be encouraged to grow them in and around home gardens and backyard [46].
One of the greatest challenges in ethnomedicinal researches is the issue of access and benefit sharing (ABS) and that of intellectual property rights. WHO states that "agreements on the return of immediate and/or long-term benefits and compensation for the use of MP materials and associated community knowledge must be discussed and concluded" [116]. In the year 1986, there were over 6,000 TM practitioners in the country registered with the Ethiopian Ministry of Health [117]. However, there is no national guideline to address the regulatory situation [118]. The proclamation, which was issued based on the National Drug Policy in 1999 in Article 6, Sub-Article 8 states, "the Drug Administration and Control Authority shall prepare standards of safety, efficacy, and quality of TMs and shall evaluate laboratory and clinical studies in order to ensure that these standards are met". It states that the authority shall also issue licenses for the use of TMs in the official health services. However, there were no official education and training systems to strengthen the TM practitioners in addition to the lack of regulation to address the issue of property rights and benefit sharing [117].
In Ethiopia, there have been attempts to implement ABS system of the Convention on Biological Diversity (CBD) according to the provisions in the Nagoya Protocol and other international agreements [119][120][121]. The country has acceded to it and developed a code of conduct to administer the ABS regime. However, there are several unmet issues requiring resolution [119,120].

Conclusion
From the present study, it can be concluded that CAM practice is an integral part of the primary healthcare system of Ethiopians, where the traditional anticancer MPs are reported from different corners of the country. The majority of anticancer MPs are found in wild habitats. This shows that most MPs are vulnerable to destructive anthropogenic activities directed against forests and other environmental factors including climate variability. Thus, due attention should be given to conserving these valuable resources in addition to raising awareness of the community on how to use these plants sustainably . We also call for the proper enforcement of the Nagoya Protocol with all its international recommendations and in accordance with the national setups in order to protect the TM knowledge and associated rights of indigenous communities. This will further not only grant an opportunity to salvage the indigenous knowledge held by the communities, but also help the scientific endeavor in plant-based anticancer and other diseases drug discoveries.
The efficacies of most Ethiopian traditional anticancer treatment claims with the MPs are not validated scientifically. Besides, only a small proportion of the MPs were reported to have side effects and/or contraindications. To avoid such overlooked health risks, we recommend further assessment on the safety of the anticancer MPs. Scientific investigation on the MP's potential toxicity and anticancer efficacy must also be made. This would possibly provide a lead material to a more thorough anticancer drug development researches.

Appendix
See Table 5.

AAU:
Addis Ababa University ABS: Access and benefit sharing CAM: Complementary and alternative medicine IARC: International Agency for Research on Cancer MP/MPs: Medicinal plant(s) SNNP: Southern Nations, Nationalities and Peoples TM/TMs: Traditional medicine(s) WHO: World Health Organization.

Disclosure
This work did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of Interest
The authors declare that they have no conflicts of interest.

Authors' Contributions
Nigatu Tuasha has incepted and designed this systematic review, searched the literature, screened the literature for eligibility against inclusion/exclusion criteria, extracted the data, carried out the analysis of the results, and prepared the draft manuscript. Beyene Petros and Zemede Asfaw independently checked for the methodological conformity, corrected the discordance, provided consistent comments during the analysis and write-up, and rigorously reviewed the        GD, GJ, SU, AR, BA, HA (3000-4350) [61] 18 Evidence-Based Complementary and Alternative Medicine      GD, GJ, WG, KF (800-1650) [61] 24 Evidence-Based Complementary and Alternative Medicine EW, WU, SU, AR, SD, HA (sea level up to 2300) [65] Note: PU § = part used manuscript. All authors have read, approved, and agreed on submission of the final manuscript.