Estimates of the number of fungal species on Earth range widely, from around half a million to 10 million. Recent estimates by Hawksworth and Lucking indicate 2.2–3.8 million from which only 120,000 species have been named so far [1]. Thus, only about 8% of the estimated number of species is presently known to mycologists [2]. The estimated number of mushroom species on Earth is 150,000–160,000 [3]; however, so far, only ~14,000 species are identified, of which ~7000 have varying degrees of edibility, with 3000 species mainly edible and falling within 21 genera [4]. Moreover, ~2000 species are estimated to be useful medicinally, while only 270 species are reported to possess therapeutic potential for human health [4]. Therefore, mushrooms have already proved themselves as a potential source of drugs against both communicable and non-communicable diseases based on clinical reports. In addition, they supplement primary food in daily life and contain several unique secondary metabolites, polysaccharides, essential minerals, proteins, and vitamins ^[5][6][5,6]. However, only 10% of existing mushroom species are known to science so far, while <1% is exploited for therapeutic uses. From this perspective, mushrooms appear to be a neglected natural source, whose therapeutic potential deserves to be explored in a scientific manner for the discovery of new drugs.

At present, cancer is a leading cause of death worldwide. Nearly 10 million deaths were recorded globally from different cancers in 2020 [https://www.who.int/news-room/fact-sheets/detail/cancer; assessed on 1 January 2022]. Cancer arises through several external factors or agents, such as physical carcinogens by ultraviolet or ionizing radiation, chemical carcinogens by the consumption of contaminated water, food, transition metals, asbestos, aflatoxin, tobacco smoke, etc., and biological carcinogens, such as certain viruses, bacteria, and parasites. It arises through a transformation of normal cells into tumor cells by a multi-stage process from a pre-cancerous lesion to a malignant tumor. According to WHO reports, in 2020, 2.2 million cases of breast cancer were recorded, leading to 685,000 deaths, 2.21 million cases of lung cancer were recorded, with 1.18 million deaths, 1.93 million cases of colon and rectum cancer were recorded, with 935,000 deaths, and 1.09 million cases of stomach cancer were recorded, with 769,000 deaths. “Despite the growing success of conventional personalized cancer therapies, recurrence and metastases remain common, depending on the type of cancer and the stage of disease” [6]. Although early detection and appropriate diagnosis play a crucial role in cancer management, the development of anticancer treatments through synthetic chemicals, or by exploring unique metabolites extracted from mushrooms or other natural sources, is a promising approach to help clinical oncology in developing new cancer drugs.

Over 60% of anticancer drugs can be traced to a natural products, but none so far originate from a mushroom [7]. This is surprising, since mushrooms have long been claimed to have anticancer effects. Traditionally, mushrooms have been used for treating cancers. “Over the past three decades, scientific and medical research in Japan, China, and Korea and recently in the USA have confirmed the properties and unique compounds extracted of mushrooms for prevention and treatment of cancer and other chronic diseases” [8]. A few of these species include: Agaricus, Albatrellus, Antrodia, Calvatia, Clitocybe, Cordyceps, Flammulina, Fomes, Funlia, Ganoderma, Inocybe, Inonotus, Lactarius, Phellinus, Pleurotus, Russula, Schizophyllum, Suillus, Trametes, and Xerocomus, etc. They show promising anticancer activity and may contain potent anticancer compounds. Dunneram et al. have suggested inclusionding of more mushrooms in theiour diet as a protective measure against cancer [5]. As such, the market for dietary supplements containing mushrooms is rapidly growing, with a market size of over 18 billion USD. This represents about 10% of the overall market for dietary supplements. Fungal genera, such as Ganoderma, Ophiocordyceps, and Cordyceps, have a prominent share [6].

Mushroom-derived polysaccharides exhibit potent antitumor activity against several tumor metastasis cells. Moreover, they showed better activity when used in conjunction with chemotherapy. Mechanistically, the antitumor action is facilitated through a thymus-dependent immune mechanism, which necessitates an intact T cell component. Polysaccharides class components mainly trigger cytotoxic macrophages, natural killer cells, dendritic cells, monocytes, neutrophils, and chemical messengers that activate complementary and acute phase responses. In addition, these polysaccharides act as multi-cytokine inducers, capable of stimulating gene expression of many immunomodulating cytokines and their receptors ^{[7][8][9][10]}[7,8,9,10]. Terpenes are another class of compounds, well known for their bioactivity, and many mushroom-derived terpenes have shown potential anticancer properties. Terpenes can modulate the immune system by inducing the expression of genes coding for proteins engaged in the immune response. Mushrooms are also a rich source of carbohydrate-binding proteins known as lectins, and exhibit cytotoxicity/anticancer properties with different mechanisms of action. Several lectins are known to have antitumor and antiproliferative properties. Other important metabolites include phenolic compounds, well known as antioxidants with different mechanisms of action. “Overall, mushroom treatment in oncology studies appeared safe and devoid of side effects. Changes in chemical parameters or clinical signs suggest that mushrooms do affect body physiology, but clinical benefits were more on quality of life than on hard endpoints like disease-free survival or mortality” [9].

2. Summary Results of Literature Analysis

Types and stages of cancer, study parameters (such as sample sizes, dose, treatment duration), and outcomes were noted for each trial with a particular mushroom species. The literature search was performed in PubMed, combining the terms “mushroom” and “cancer”, and limiting the results to clinical trials (https://pubmed.ncbi.nlm.nih.gov/?term=mushrooms+and+cancer&filter=pubt.clinicaltrial&filter=pubt.randomizedcontrolledtrial; assessed on 21 September 2021). This yielded clinical trials on the following medicinal mushrooms (MM): Agaricus bisporus (single trials, [10]); A. blazei (three trials, ^[11][12][13][11,12,13]); A. sylvaticus (two trials; ^[14][15][14,15]); Antrodia cinnamomea (single trial, [16]); Coriolus versicolor (two trials; ^[17][18][17,18]); Ganoderma lucidum (single trial, [19]); Grifola frondosa (three trials; ^[20][21][22][20,21,22]); Lentinus edodes (four trials; ^{[23][24][25][26]}[23,24,25,26]); Phellinus rimosus (single trial; [27]); Poria cocos (single trial; [28]). In parallel, reswearchers searched clinical trial databases to record clinical trial information and added some additional trials on Agaricus bisporus (one randomized phase II trial for prostate cancer, NCT04519879; one interventional clinical trial for breast cancer, NCT007090200), Coriolus versicolor (one randomized, parallel, double-blind, placebo-controlled trial for breast cancer, NCT00647075), Grifola frondosa (one randomized, interventional clinical trial for lung neoplasms and breast carcinoma, NCT02603016), and Trametes versicolor (one phase I clinical trial for breast cancer, NCT02568787). Interestingly, four published reports on Agaricus blazei were based on one trial, but presented different results ^{[16][18][29][30]}[16,18,29,30]. ThWey found that the majority of clinical studies were carried out with just 3 species: Lentinula edodes (22.2%), Coriolus versicolor, and Ganoderma lucidum (both 13.9%); followed by Agaricus bisporus and Grifola frondosa (both 11.1%) (Figure 1). There were 2 other species of Agaricus which were also well studied, including A. blazei (8.3%) and A. sylvaticus (5.6%). Most clinical studies were conducted in humans, except one in dogs (whose results are not impressive, as Maitake^@ treatment could not reduce lymph node size by more than 50%, while two dogs developed adverse effects [21]). The distribution of different types of cancer across the reviewed clinical studies is shown in Figure 2. Most studies were treating breast cancer (18.6%), followed by colorectal (14%) and prostate cancer (11.6%) (Figure 2). Other treated cancer conditions included liver, lung (both 6.98%), cervical, and ovarian cancer (both 4.65%) (Figure 2). Few of these studies were placebo-controlled, double-blind randomized trials (RCTs). The administration of mushrooms was largely oral. Several clinical trials studied a combination with chemotherapy to reduce side effects and improve quality of life (QOL), and observe changes in hematologic parameters (HP), overall survival (OS), antitumor activity, or immunomodulation. As only a few mushroom species (11) were evaluated in clinical studies, and the total number of studies was small (only 36), rwesearchers continued theiour literature search for finding preclinical oncology evidence on mushroom species (database source: https://pubmed.ncbi.nlm.nih.gov/; assessed on 11 October 2021). The anticancer properties from these are summarized for individual species including the type of extracts/fraction/active compounds, type of cancer study, in vitro/in vivo, the dose of treatment, the mechanism involved, etc. Figure 3 and Figure 4 show the distribution of mushrooms with various cancer types for in vitro and in vivo studies, respectively. The most interesting clinical trials with their major outcomes are summarized in Table 1. wey summarized all mushrooms species per type of cancer in Table 2, listing in vitro vs. in vivo as well as clinical trials. For in vitro studies with cell lines, most used breast cancer cell lines (43.9%), followed by lung (14%) and colorectal (13.1%), as well as (8.41%) liver cancer cell lines (Figure 3). For in vivo studies, most papers pertained to antitumor effects in mice (58.7%), which may be due to the common availability of tumor mouse models (Figure 4). Moreover, among the cancer types once more breast cancer is most studied (23.9%) followed by liver cancer (10.9%) (Figure 4). To facilitate interpretation, th For more detailed interpretation, a summary table was prepared listing the name of the mushroom species aimed at different types of cancer. After analyzing the types of studies, each category, such as in vitro, in vivo, in silico, isolation of active constituents, and clinical study, was rated with scores—ideal (excellent, >5 studies-***), (good, 3–5 studies-**), (poor, 1–2 studies-*), or no study (-)—and the overall strength of recommendation score was given based on the available literature (Table 3).