Keywords

10.1 Introduction

Modern medicine and therapies offer cure for most ailments in today’s world, but in some cases, they lead to numerous critical side effects, as seen in the case of cancer therapy (Yin et al. 2013). Nevertheless, much before the use of allopathic medicines, plant-based products were commonly used globally for improving health conditions. There are plenty of functional foods and herbs that are available today, and they possess phytocompounds, which play a vital role in curing a plethora of diseases like jaundice (Kamala et al. 2018), diabetes (Middha et al. 2014; 2019), inflammation (Prashanth Kumar et al. 2019), mouth ulcers, cancer, etc. (Lee et al. 2012). Despite the remarkable advances in diagnostics and therapeutics, cancer (malignancy) is branded to be the deadliest disease. The mortality associated with cancer can be reduced drastically by early diagnosis, intervention, and prevention measures. Epidemiological investigations have indicated that intake of natural products, such as vegetables and fruits, can reasonably decrease the risk of cancer incidence (Donaldson 2004; Syed et al. 2013). There is also a growing use of herbal medicine by cancer patients, due to their safety aspects and cost-effectiveness. Pomegranate (Punica granatum), which has been termed as a “Superfood” (Middha et al. 2013a), due to its multiple biological properties is one such plant that holds a great potential for treating cancers (Sharma et al. 2017).

Pomegranate is thought to have taken its root throughout the world, initially being cultivated in Iran, followed by the Himalayan regions of India and different microclimatic zones (Middha et al. 2016). It is a drought-resistant plant that has a long life span approximating up to 200 years with high and healthy fruit yield in the first 20 years (Zarfeshany et al. 2014). The flush, visual appearance, flavor, and antioxidant competence of the fruit are known to be influenced by the climatic circumstances in which the plant exists. It is a spiny deciduous plant belonging to the plant family, Punicaceae (new classification Lythraceae), which also houses another species Punica protopunica (Socotran pomegranate). Punicaceae is placed under the subclass, Rosidae belonging to the order of Myrtales. Pomegranate has small slender leaves, heterostylous funnel-shaped red flowers and the fruit having 5–12 cm diameter with a hexagonal rounded figure amid an upright crown (Singh et al. 2018). The fruiting body can be separated majorly into three parts, namely, rind/husk/pericarp/peel/skin, aril, and the seeds, as shown in Fig. 10.1. It is a plant rich in nutrients and phytochemicals. Be it any part of the pomegranate, i.e., peel, juice, root, bark, flowers, leaves, or seeds, all of them possess potent therapeutic properties. It also targets a range of diseases, including cancer, diabetes, cardiovascular disorders, aging, male infertility, Alzheimer’s disease, and Acquired Immune Deficiency Syndrome (AIDS) (Viuda-Martos et al. 2010; Middha et al. 2013a, b). The pomegranate fruit and the other nonedible parts are loaded with anthocyanidins, anthocyanins, flavanols, flavones, flavonones, phenolic acids, and tannins (punicalagin and punicalin). The pharmacological outcome of pomegranate extracts could be linked to their polyphenolic richness (Rummun et al. 2013). The whole genome data have further helped in elucidating genetics, evolution, and other interesting pharmacological effects of pomegranate (Qin et al. 2017).

Fig. 10.1
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Major parts of pomegranate

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Pomegranate peel, an affluent natural antioxidant and having varied chemical molecules, was accounted previously for its diverse pharmacological properties such as aging, Alzheimer’s disease, cancer, cardiovascular disorders, diabetes, and infertility disorders (Middha et al. 2013a, b; Sun et al. 2016; Singh et al. 2018). This systemic chapter benefits the readers in understanding the usage of pomegranate peel as a natural and alternative medicine in cancer prevention, as it emphasizes the antiproliferative, antimetastatic, and anti-invasive role of pomegranate peel and summarizes its mechanism using an array of cancer cell lines (in vitro), preclinical and clinical models.

10.2 History and Cultural Significance of Pomegranate Peel

Pomegranate is one among the only two species of the kind in Punica genus belonging to the family Lythraceae (previously, Punicaceae) (Qin et al. 2017). The common name, Punica, is the feminized Latin term for Carthage (the capital town of the prehistoric Carthaginian society, Tunisia). It is originally derived from the Greek Phoinix referring to the Phoenician settlers around Carthage. The precise epithet, granatum, implies seedy or grainy. Prior to its rechristening by Linnaeus in the eighteenth century, the plant was identified as Malum punicum, the apple of Carthage (Stover and Mercure 2017). Another representation by few historians stated that the “Tree of Life” in the holy Bible was a pomegranate tree.

The Arabic or Semitic (rumman) and Biblical Hebrew (rimmon) names used for pomegranate mean “fruit of paradise” and has been known to be a symbol of love since ancient times. Pomegranate has been associated with fertility, abundance, immortality, invincibility, blessings, prosperity, posterity, and the endurance of marriage (Stover and Mercure 2017). The Greek physician Soranus documented five prescriptions for the seeds or rind of pomegranate to be used as oral contraceptives or vaginal (douche) suppositories (Foster and Johnson 2006). The contraceptive use of pomegranate seeds or rinds has also been elucidated by Hippocrates (468–377 BCE), Dioscorides (40–90 CE), and Ibn Sina (Avicenna, 980–1037 CE). Historically, pomegranate has been significant in numerous cultures for its food and medicinal uses, as well as for its spiritual and artistic symbolism. The Ebers Papyrus from Egypt (one of the oldest preserved medical documents, ca. 1500 BCE) prescribed it as a remedy for roundworm (Ebbell 1937). Dried fruit rind and pulp have been used commonly for upset stomachs and diarrhea, prepared as infusions (teas) or tinctures (alcoholic extractions) (Van-Wyk and Wink 2004).

Most parts of the pomegranate, including the leaves, fruits, flowers, rind, dried seeds and fresh seeds, trunk bark or root bark, fresh fruits, and preparations thereof (e.g., juice), have defined therapeutic applications in the traditional medicinal systems, such as Ayurveda, Siddha (Traditional Indian), gSo-ba Rig-pa (Traditional Bhutanese), Sowa-Rigpa (Traditional Tibetan), Traditional Chinese Medicine (TCM), Traditional Iranian Medicine, Unani (Perso-Arabic traditional medicine), and European Homeopathy. The fresh fruits and fruit juice are used widely as food, and the juice is also used as a rich source of polyphenols. Essential oils and extracts from different parts of the fruit, as well as isolates and derivatives, such as pomegranate fruit peel extract octenylsuccinate, seed oil hydroxyphenethyl esters, and sterols obtained from the seed oil are used as cosmetic ingredients (Van-Wyk and Wink 2004). In TCM, pomegranate husk/rind/peel (called shi liu pi) is utilized to cure dysentery, diarrhea, rectal prolapse, spermatorrhea, premature ejaculation, uterine bleeding, and vaginal discharge due to kidney instability, and to kill and expel parasites. It is also used topically for ringworm (Bensky 1993) and in combination with other herbs for the conditions mentioned above.

10.3 Characteristics of Pomegranate Peel

The peel is tough and leathery, about 2–5 inches in width, and its color ranges from yellow to deep pink/red. The peel makes almost 50% of the entire mass of the product (fruit) (Fawole et al. 2012). This part of the pomegranate is exceptionally rich in astringent properties and possesses many ethnomedical applications, due to the presence of numerous phytocompounds, but is usually disregarded as agricultural waste (Middha et al. 2013a).

10.3.1 Physicochemical Composition of Pomegranate Peel

The peel of pomegranate is said to be rich in lot of nutrients. The occurrence of total solid, total sugars, reducing sugars, proteins, crude fiber, fat content, and ash is shown to occur in the peels of pomegranate. Apart from these, it is rich in calcium, complex polysaccharides, minerals, nitrogen, magnesium, potassium, phosphorus, and sodium (Middha et al. 2013b).

10.3.2 Phytochemicals of Pomegranate Peel and their Medicinal Properties

Previous studies have shown that the peel is rich in tannins, flavonoids, alkaloids, and organic acids. Tannins, high-molecular-weight polyphenols, are organically and chemically divided into three divergent groups: condensed tannins (also called proanthocyanidins), hydrolyzable tannins or ellagitannins (ETs), and gallotannins (GTs). Pomegranate peel is found to be rich in hydrolyzable tannins, such as punicalin, punicalagin, and other tannins like pedunculagin, gallic acid, and casuarinin are predominantly present. Hence, it possesses superior antioxidative property. Isolariciresinol (10.5 mg/kg) is one of the predominant lignins present in the peel (Syed et al. 2013; Rahmani et al. 2017). Numerous flavonoids, such as catechin, epicatechin, epigallocatechin-3-gallate, flavan-3-ol, kaempferol (Al-Rawahi et al. 2014), kaempferol-3-O-glucoside, kaempferol-3-O-rhamnoglycoside (Akhtar et al. 2015; Moradian et al. 2017), luteolin, luteolin 7-O-glucoside, naringin, pelargonidin, prodelphindin, quercetin (Middha et al. 2013a, b), and rutin, are observed to be present in the peel, as reported from different experimental works (Wang et al. 2004). These phytochemicals attribute to the antibacterial, antioxidant, anti-inflammatory, antiviral, and antineoplastic effects of the peel. Also, there is the existence of gallic acid, gallagyldilacton, and granatin B, which provide anti-inflammatory activity to the peel (Satomi et al. 1993). A plethora of evidence suggests that the presence of these polyphenolics has rendered the peel with the property of anticarcinogenic activity. A total of 108 compounds (Table 10.1) have been reported to be present in the peel through various studies using High Performance Liquid Chromatography (HPLC) and Gas Chromatography–Mass Spectrometry (GC–MS) methods (Syed et al. 2013; Barathikannan et al. 2016; Middha et al. 2013a, b).

Table 10.1 Chemical constituents present in the pomegranate peel
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10.4 Anticancer Activity of Pomegranate Peel

The World Health Organization (WHO) and the American Cancer Society (ACS) reported that globally the second growing fatal disease is cancer, claiming approximately 9.6 million mortality in 2018. Around 1 in 6 demises worldwide is caused by malignancy and roughly 70% of these mortalities transpire in low- and middle-revenue nations. According to the ACS 2018 statistics in the United States (USA), an approximated 1,735,350 latest cancer cases in various hospitals/institutions and 609,640 cancer demises are witnessed (http://www.who.int/news-room/ fact-sheets/detail/cancer).

The currently available cancer treatments include radiotherapy, surgery, and systemic cures comprising cytotoxic chemotherapy, hormonal remedy, immunotherapy, and intentioned or targeted therapies such as hydrogels and magnetogels (Veloso et al. 2018). Although numerous therapies are available, all of them have their own side effects and some of them even create a financial burden on the patient. So, there is an urgent need for definitive preventive measures and complete cure for cancer, which will have lesser systemic toxicity as well as one that will not drill a hole in the patient’s pocket (Thomford et al. 2018). Recently, the advances in novel cancer remedies have become a major issue owing to the cells developing resistance to modern chemotherapy. Alternatively, in the present time, an herbal therapy is in focus owing to its less costs and toxicity. The current chapter attempts to summarize the literature available in PubMed and Google Scholar on the various model systems (in silico, in vitro, and in vivo) utilized to examine the anticancerous activities of peel and different molecular effects exhibited by the pomegranate peel (Table 10.2). A number of studies have centered on the anticancerous properties of pomegranate peel using in silico, in vitro, and in vivo model systems. The early scientific evidences majorly focused on the antidiarrheal activity of pomegranate peel and its genotoxicity studies. For the first time, Settheetham and Ishida (1995) reported that the administration of pomegranate aqueous extract encouraged DNA fragmentation during apoptosis in Raji and P3HR-1 cells. Though there was a huge gap until 2004, the later years have witnessed a drastic increase and interest in the anticancer and other pharmacological properties of pomegranate peel. This chapter also focuses on the demand for pomegranate peel and its derived compounds and their anticancer properties. Interestingly, the extracts of pomegranate peel have shown a selective inhibition against various types of cancer cells with no or less visible toxicity in normal cells.

Table 10.2 Summary of molecular targets and anticancer potential of the pomegranate peel
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10.4.1 Pomegranate Peel Against Breast Cancer

Breast cancer is a very commonly occurring cancer type in females around the world with an impact on more than 1.5 million women every year. It is also a reason for the massive number of cancer-related fatalities in women. A plethora of scientific evidence suggests that the consumption of phytochemicals-rich food can trim down the threat of cancer disease. Pomegranate peel extract (PPE), the biowaste material, is rich in polyphenols. Ricci et al. (2006) reported that the fruit juice has a higher polyphenolic content (0.063–0.0003 mg/g dry weight) than different varieties of the pomegranate peel (1.892–0.1070 mg/g dry weight) (Dikmen et al. 2011). In vitro and in vivo explorations have divulged that PPE, rich in polyphenols, act as potent antioxidants, which help in the inhibition of cell growth in cancer. Jeune et al. (2005) performed a study that demonstrated the in vitro anticancer effects of combining pomegranate extracts and genistein. They also proposed that the combination of both is more efficacious, as compared to single treatments in a time- and dose-dependent manner. They used lactate dehydrogenase, 3-(4,5-Dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium bioassays, acridine orange–ethidium bromide, and terminal deoxyribonucleotidyl transferase-mediated dUTP nick-end labeling to study the cytotoxic and growth inhibition effects of pomegranate extracts and genistein on MCF-7 cancer cells. Similar studies were also carried out by Dikmen et al. (2011) to demonstrate that the methanolic extract of pomegranate peel at varied concentrations (25, 50, 100, 200, and 300 μg/ml) decreased cell proliferation and stimulated apoptosis in MCF-7 cancer cells. This was evident from the enhanced expression of pre-apoptotic gene Bax and decrease in the expression of anti-apoptotic gene Bcl-2. The effect of PPE was proportional to the dose and the incubation interval. Bcl-2/Bax plays a vital function in regulating caspase-dependent and caspase-independent apoptosis mediated by the mitochondrial pathway. The possible anticancer and apoptotic effects of pomegranate peel can be credited to ellagic acid, ellagic tannin, and gallic acid. The data gathered from both these studies aid to invent new chemotherapeutic and chemopreventive agents of pomegranate peel to treat breast cancer.

10.4.2 Pomegranate Peel Against Colorectal Cancer

Colorectal cancer (CRC) is the anomalous division of cells that occur in the colon or rectum or colorectum, also called the large intestine. As per the Cancer Statistics 2018 report, colorectal cancer is the third most familiar cancer to be detected equally in both men and women in the United States. The WHO reports approximately 862,000 CRC deaths across the globe in 2018 (https://www.who.int/news-room/fact-sheets/detail/cancer). Siegel et al. (2016) reported that roughly 4.6% of man (1 in 22) and 4.2% of women (1 in 24) will be identified with CRC in their life span. Waly et al. (2012) observed an improvement in the redox status and decrease in preneoplastic lesions of the colonic cells in azoxymethane (AOM)-induced in vivo colon tumors treated with PPE. The proposition that PPE extracts might prevent colon cancer was based on in vitro studies that showed the high antioxidant properties of pomegranate peel. Increasing evidences have implicated that the augment in reactive oxygen species (ROS) is one of the causes of cell damage and cancer. In this context, Negi et al. (2003) used two strains of Salmonella typhimurium (S. typhimurium), i.e., TA100 and TA1535, to test the efficacy of PPE extracts against sodium azide mutagenicity. The study revealed that strong antimutagenicity of soxhlet extracts of water, methanol, and ethyl acetate pomegranate peel at 2500 μg decreased mutagenicity in both strains of Salmonella species. The antioxidant and antimutagenic effects could be the result of the polyphenols, such as catechins, chlorogenic, caffeic, ellagic acid, and ferulic acids, present in the peel.

10.4.3 Pomegranate Peel Against Prostate Cancer

Prostate cancer (Pca) is accountable for the highest number of cancer cases reported in men. It accounts for every 1 in 5 new cancers diagnosed in men according to the American Cancer Society (Siegel et al. 2016). According to Cancer Statistics 2018 reports of the American Institute for Cancer Research, prostate cancer is the second most lethal cancer spotted in men with 1.3 million new cases in the United States (https://www.wcrf.org/dietandcancer/cancer-trends/prostate-cancer-statistics). Deng et al. (2017) examined the in vitro outcomes of Punica peel on Pca cells and further provided evidence for its application to inhibit the Pca growth and metastasis. The study was inspired by Venclexta, a drug used to treat chronic lymphocytic leukemia (CLL), approved by the US FDA on April 11, 2016. Venclexta is a Bcl-2 inhibitor that specifically targets the apoptosis pathway in prostate cancer cell lines, DU145, PC3, and the mouse prostate cancer cell TRAMP-C1 (Ng and Davids 2014; Deng et al. 2017). HPLC analysis showed the presence of two compounds, namely punicalagin (PG) and ellagic acid (EA), with a molecular weight of 1083.0 and 301.0, respectively, as determined by mass spectrophotometry. The viability of cells was evaluated by a routine 3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide (MTT) test. The morphological analysis of nuclei by Hoechst staining showed the inhibition of cell viability by the Punica peel extract attributed to apoptosis. The content of punicalagin (479.8 mg/g) and ellagic acid (7.5 mg/g) in Punica was recorded based on the regression equation and the relevant area under the curve (AUC) of each factor. The antiproliferative effect was significantly seen in TRAMP-C1, compared to DU145 and PC3 cells. The results also showed a reduced expression of anti-apoptotic Bcl-2 and amplified expression of cleaved caspase-3 and pro-apoptotic Bax posttreatment, indicating mitochondrial dysfunction causing apoptosis. Punica peel treatment also showed decreased mitochondrial transmembrane potential and ROS production. The results also indicated that after treatment with peel extract, the expression levels of matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9) in TRAMP-C1 were significantly suppressed and tissue inhibitor of metalloproteinases-2 (TIMP2) was upregulated indicating the inhibition and invasion, the two important steps in cancer metastasis. Therefore, the study clearly demonstrates that the Punica peel presents a clear inhibitory effect on the growth and viability of prostate cancer cell lines.

A subcutaneous xenograft of human prostate cancer cells (PC-3) in nude mouse models was established by Ma et al. (2015) to observe the antiproliferative and apoptotic effects of pomegranate peel polyphenols. Pomegranate peel helped in shrinking tumor dimensions and mass in tumor-bearing nude mice, and significantly enhanced the rate of apoptosis. In addition, tumor necrosis factor (TNF)-α was amplified and vascular endothelial growth factor (VEGF) in serum was decreased. The study showed the antitumor activity of three polyphenols ellagic acid, gallic acid, and punicalagin found in pomegranate peels.

A plethora of literature has described the association between inflammation and prostate carcinogenesis (Kohnen and Drach 1979). The nuclear factor-κB (NF-κB) pathway is one of the well-established signaling pathways that arbitrate cancer-related inflammatory responses (Baldwin Jr 2001). Contagious NF-κB activation has been learnt in breast, cervical, liver, melanoma, and prostate cancers. Importantly, constitutive activation of NF-κB in primary prostate cancer specimens represents an independent risk factor for tumor recurrence after surgery. Rettig et al. (2008) indicated the suppression of NF-κB signaling followed by Punica granatum peel extract treatment in both in vitro and in vivo prostate cancer models (Rettig et al. 2008). In vitro stimulation of apoptosis by peel was demonstrated and found to be dependent on the inhibition of NF-κB activity. For in vivo studies, SCID mice implanted with LAPC4 prostate cancer peel impedimented (delayed) the emergence of LAPC4 androgen-independent xenografts in castrated mice through cell proliferation inhibition and apoptosis induction.

10.4.4 Pomegranate Peel Against Skin Cancer

Skin cancer prevalence has increased among light-skinned populations and artificial tanning devices have significantly contributed to this increase over the last three decades (http://www.who.int/bulletin/volumes/95/12/17-021217/en/. Accessed 12 Dec 2018). One in five Americans is prone to skin cancer in their life span according to the American Skin Cancer Foundation Statistics (http://www.who.int/uv/faq/skincancer/en/index1.html. Accessed 12 Dec 2018). Increasing incidence of skin cancer provide a strong basis for chemoprevention with natural remedies. Aslam et al. (2006) first reported the ability of the aqueous fraction of pomegranate peel to advance propagation and procollagen synthesis and MMP-1 inhibition in organ cultures developed from punch biopsies of sun-confined hip skin attained from adults.

10.4.5 Pomegranate Peel Against Thyroid Cancer

Thyroid cancer is a cancer of the thyroid glands and is of four different types—papillary thyroid cancer (PTC), follicular thyroid cancer (FTC), medullary thyroid cancer (MTC), and anaplastic thyroid cancer (ATC). The thyroid cancer survival rate, especially PTC, is relatively higher than any other malignancy. Thyroid cancers are majorly treated with surgical procedures that profusely affect the patient’s quality of life and it also requires patients to adhere to stern indications and contraindications. All these abovementioned conditions call for latest clinical interventions in thyroid cancer treatment. The earliest evidence for the utilization of pomegranate peel in the prevention and treatment of thyroid cancer came from studies carried out by Li et al. (2016). They evaluated the effect of pomegranate peel on thyroid cancer cells, such as BCPAP (harboring BRAF V600E mutation), TPC-1 (harboring RET-PTC rearrangement), and Nthy-ori 3–1 (human thyroid follicular epithelial cell line). The outcome of the study showed a significant decrease in the propagation of thyroid cancer cells in a time- and dose-dependent manner.

The preliminary mechanism was identified to be the induction of intrinsic apoptosis by Bcl-2 and Bax proteins and a decrease in mitochondrial membrane potential by PG and EA. Similar studies were conducted by the same group in vivo in the BCPAP tumor model in BALB/c athymic mice. Punica peel (dosage of 125 mg/kg/d) showed antitumor activity with 69.8% inhibition of tumor growth, improved the expression of cleaved caspase-3 (CC-3), and suppressed the expression of Ki-67 (a protein) in cancerous tissues in comparison with the untreated groups. Thyroid cancer can also metastasize to the lymph nodes and lungs. Restraining the tumor cell from migrating and invading other organs is a useful approach to inhibit metastasis, and Punica peel significantly downregulates the expression of MMP-9, which is one of the important proteins in the focal adhesion kinase (FAK)/MMP pathway, a vital pathway in tumor assault/invasion and metastasis.

10.4.6 Pomegranate Peel Against Osteosarcoma

Osteosarcoma is a high-grade primary skeletal sarcoma and is characterized by the deposition of immature osteoid matrix by mesenchymal spindle cells. In a study by Li et al. (2014), a homogeneous acidic polysaccharide was isolated from the pomegranate peel and its antiproliferative activity against human U-2 osteosarcoma (U-2OS) cells examined. The same study also elucidated the chemical composition of a pomegranate peel. It was shown to contain total sugar (72.4%), uronic acid (19.5%), and a negligible quantity of protein (9.7%). Further, the pomegranate peel was effective in inducing the arrest of G2/M phase, encouraged apoptosis, and hindered the growth of U-2OS cells in a dose-dependent manner. Western blotting analysis displayed that the pomegranate peel elicited the mitochondrial-mediated apoptosis, which was evident from the elevated levels of Bax/Bcl-2 ratio, release of cytochrome c, triggering of caspase-3 and caspase-9, and cleavage of poly-(ADP-ribose)-polymerase (PARP) in U-2OS cells.

10.5 Punicalagin and Ellagic Acid Isolated from Pomegranate Peel with Anticancer Potential

Punicalagin (PG) and ellagic acid (EA) are the two major constituents of pomegranate peel. Studies by Zahin et al. (2014) have clearly demonstrated in vitro antimutagenic and antiproliferative effects of these compounds in human lung cancer cells. Ellagitannins are the abundant polyphenols present in the peels of pomegranate. Punicalagin, a unique ellagitannin of pomegranate, has the capability to be hydrolyzed to ellagic acid ending in prolonged release of ellagic acid into the blood after the ingestion of pomegranate peel. It is known to be the largest polyphenol, having a molecular weight greater than 1000 g/mol. Punicalagin is most abundant in the peel/rind, compared to seeds/fruit (Lu et al. 2007). Structurally similar to gallagic acid and ellagic acid moiety, punicalagins are attached to glucose and exist in two reversible anomer types, i.e., α and β forms. Punicalagin can inhibit sulfoconjugation (Saruwatari et al. 2008) and has antioxidant (Sun et al. 2016), antiproliferative, antigenotoxic, antiviral, antiplasmodial, and immunosuppressive activity (Aqil et al. 2012; Sharma et al. 2017). Ellagitannins are also metabolized into bioactive urolithins by gastrointestinal (gut) microbiota, which conjugate in the hepatic lobes and are urinated out. These urolithins are known to reduce prostate cancer intensification (Heber 2008).

Ellagic acid (EA) is a polyphenolic molecule present in a variety of vegetables and fruits, and pomegranate peel is one among them. It is usually believed as an antioxidant. Clinical tests with EA on cultured cell lines (Human) revealed the prevention of denaturation of the p53 gene. Further, another study proposed that one of the mechanisms by which EA hinders mutagenesis and carcinogenesis is by the development of adducts with DNA, camouflaging binding sites from the mutagen or carcinogen (Sharma et al. 2017). The advent of next-generation sequencing techniques has improvised the overall understanding of biosynthesis of secondary metabolites in different plant species. The whole genome sequence and transcriptome of pomegranate peel reveal the pathway of biosynthesis of ellagitannins in pomegranate, explained by Qin et al. (2017). As reviewed all through, researchers from different regions of the world have indicated that the pomegranate peel and its chemical constituents like punicalagin and ellagic acid are effectual in hindering vital pathways at various stages of carcinogenesis, as depicted in Fig. 10.2.

Fig. 10.2
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Overall mechanisms of action of the pomegranate peel and its constituents

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10.6 Toxicity and Stability of Pomegranate Peel

Previous in vivo as well as in vitro studies prove that there is no adverse toxicity pomegranate peel on mammals, and it was also shown that in Wistar mice the intraperitoneal (i.p.) LD50 of aqueous extract of peel is 1321 ± 15 mg/kg (Qnais et al. 2007). The comparative study of median lethal dose (LD50) for pomegranate peel extract revealed that the oral LD50 was more than 5 g/kg body weight (b.w.) and the i.p. LD50 in rodents, such as rats and mice, were 217 mg/kg b.w. and 187 mg/kg b.w., respectively. A dosage of 600 mg/kg/day of pomegranate peel extract was a NOAEL (no observed effect level) identified through a protocol of subchronic administration for a 90-day period (Patel et al. 2008; Eleonora et al. 2015). The first evidence of the possible genotoxicity of pomegranate peel used in folk medicine indicated no visible mutagenicity at the dose of 1–3 g/kg b.d. for 3 days in bone marrow cells (Sanchez-Lamar et al. 2008). The liquid extract of pomegranate peel was found to be stable at high temperatures of sterilization and at cold storage temperatures (Qu et al. 2013).

10.7 Clinical Studies

Around 13 clinical trials can be accessed from https://clinicaltrials.gov/ search page using the search term “pomegranate and cancer.” One of the clinical trials was terminated for different reasons, six have been completed, three of each have been inactive, nonrecruiting, and of unknown status. Clinical trials with pomegranate peel in cancer are greatly lacking, despite the impressive amount of in vitro and preclinical studies, revealing its anticancer activity with no visible toxicity.

10.8 Conclusions

In the present scenario, employing dietary agents or functional foods for the prevention of cancer is a promising arena of oncology. The complementary and alternative medicine has drawn the attention of both clinical scientists and the general public due to dietary agents like pomegranate fruit and its waste product peel, having verified with their ability to avert or restrain cancers, their low cost, and trouble-free availability. Nevertheless, the present challenge lies in establishing the key compound or constituent of these functional foods liable for the anticancer consequences and the systems through which they stifle cancers. Scientific explorations grant ample amount of substantiations related to the bioactivities of pomegranate peel and its derivatives (products) with a focus on their anticancer features. Reports suggest promising chemopreventive/chemotherapeutic agents in pomegranate peel by exerting antioxidant, antiproliferative, anti-apoptotic, antimutagenic, and antitumorigenic effects by mitochondrial signaling pathway modulations. A significant amount of research reveals the in vitro efficiency of pomegranate peel against malignancy and promotion of cancer; however, in vivo and human trials are essential to authenticate the independent or existing therapies combined with the use of pomegranate peel against various cancers, such as breast, hepatic, prostate, skin, and thyroid cancers. It is anticipated that the chapter will provide inputs for the scientific community on the ongoing and further experimentations on pomegranate peel in cancer studies.