Aegle marmelos (L.) Correa (Bael) and Its Phytochemicals in the Treatment and Prevention of Cancer

Abstract

Aegle marmelos, commonly known as Bael and belonging to the family Rutaceae is an important medicinal plant in the traditional Indian system of medicine, the Ayurveda. The extract prepared by boiling the bark, leaves or roots in water is useful as laxative, febrifuge, and expectorant. The extract is also useful in ophthalmia, deafness, inflammations, catarrh, diabetes, and asthmatic complaints. The fruits are used in treating diarrhea, dysentery, stomach ache, and cardiac ailments. Scientific studies have validated many of Bael’s ethnomedicinal properties and its potential antimicrobial effects, hypoglycemic, astringent, antidiarrheal, antidysenteric, demulcent, analgesic, anti-inflammatory, antipyretic, wound-healing, insecticidal, and gastroprotective properties. In addition, studies have also shown that Bael and some of the Bael phytochemicals possess antineoplastic, radioprotective, chemoprotective, and chemopreventive effects, properties efficacious in the treatment and prevention of cancer. For the first time, the current review summarizes the results related to these properties and emphasizes aspects that require further investigation for Bael’s safe and effective use in the near future.

Introduction

The International Agency for Cancer Research recently reported that cancer is the second leading cause of death globally and in 2008, approximately 12.7 million new cancer cases (56% of which were in developing regions of the world) and 7.6 million cancer deaths (63% in less developed regions) occurred. ¹ By the year 2020, predictions report the incidence of cancer will increase 3-fold, with a disproportionate rise in cancer cases and deaths in developing countries with limited resources to tackle the problem. ²

Depending on the stage, tumor location, and the health of the patient, cancer may be treated with surgery, ionizing radiation, or chemotherapy. However, when the prognosis is grim, for effective control and palliation, combining 2 or all treatment modalities is frequent. ³ Unfortunately, the use of chemotherapy, ionizing radiation, and their combination cause deleterious effects from their lack of specificity and resulting in cytotoxic effects on healthy normal cells. ³ These conventional treatments are very expensive and because of this a large number of patients living in the developing countries prefer to use complementary and alternative medicine for treating and managing cancer symptoms and pain. ⁴

Ayurveda, the traditional Indian system of medicine, is one of the oldest systems of medicine. ⁴ Emphasis in Ayurveda is on disease prevention and promotion of good health by adopting proper lifestyle and following therapeutic measures that will rejuvenate the body. ⁵ The Ayurvedic remedies, which are both preventive and therapeutic, are mostly of plant origin. ⁴ Preclinical studies with experimental animals have shown that many of the commonly used Ayurvedic plants are effective and have the potential to be of human use in future. ⁴ One such plant that has been extensively studied is the medium-sized deciduous Aegle marmelos (Rutaceae family; Figure 1). In colloquial terms, it is known as the stone apple or golden yellow apple tree in English, as Bilva, Sriphal, or Shivadruma (the tree of Shiva) in Sanskrit, and Bael in Hindi. ⁶

Figure 1.

Photographs of (a) Bael plant with the fruits, (b) leaf, and (c) fruit

Bael is one of the most important medicinal plants in Ayurveda. According to Charaka (1500 bc), the high priest of Ayurveda, Bael is one of the most important medicinal plants in Ayurveda, which has been in existence for a long time and is extensively used by inhabitants of India. ⁷ Bael is referred to as an emblem of fertility and is considered to be a healing tree that strengthens the body. It is also used in various folk medicines in India, Nepal, Sri Lanka, Bangladesh, and Burma (now Myanmar). ⁶ A sweet drink prepared from the ripe fruit pulp is supposed to be effective against bacillary dysentery. The unripe fruits of Bael are reported to be useful in treating diarrhea, dysentery with spells of constipation, and stomach ache. ⁶ The roots are also one of the important ingredients of the Ayurvedic traditional drug Dashamula, a panacea for colitis, dysentery, diarrhea, flatulence, and fever. ⁶ The leaves are supposed to reduce bowel complaints, bleeding piles, dropsy (edema), diarrhea, and dysentery. ⁶

Chemical analysis suggests that Bael contains tannins, skimmianin, essential oils like caryophyllene, cineole (Figure 2), citral (Figure 2), cuminaldehyde, citronella, p-cymene, d-limonene (Figure 2), and eugenol (Figure 2), sterols and/or triterpenoids, including lupeol (Figure 2), β− and γ-sitosterol, α− and β-amyrin, flavonoids like rutin (Figure 2) and coumarins, including aegeline, marmesin, umbelliferone marmelosine, marmelin (Figure 2), o-methyl halfordinol, alloimperatorin methyl ether, o-isopentenyl halfordinol, phlobatannins, flavon-3-ols, leucoanthocyanins, and anthocyanins.^8-10 Most of these chemicals possess beneficial health effects and exhibit diverse pharmacological effects.

Figure 2.

Phytochemicals of Bael

Pharmacological studies have validated most of the ethnomedicinal uses of Bael such as antimicrobial effects, antimicrofilarial, antifungal, hypoglycemic, astringent, antidiarrheal, antidysenteric, demulcent, analgesic, anti-inflammatory, antipyretic, hypoglycemic, antidyslipidemic, immunomodulatory, antiproliferative, wound-healing, insecticidal, anticancer, antidiabetic, and cardioprotective properties. ¹⁰ Here, we analyze the role of Bael in the treatment and prevention of cancer.

Bael as an Anticancer Agent

Chemotherapy has been the mainstay in cancer treatment for nearly 6 decades. However, most of the clinically used chemotherapeutic agents possess inherent normal tissue toxicity, thereby compromising the therapeutic advantage. ¹¹ Preclinical studies have shown that Bael leaf extracts were effective in inhibiting the growth of leukemic K562, T-lymphoid Jurkat, B-lymphoid Raji, erythroleukemic HEL, melanoma Colo38, and breast cancer cell lines MCF7 and MDA-MB-231.^12,13 The hydroalcoholic extract of the Bael leaves is also shown to possess antineoplastic effects on the Ehrlich ascites carcinoma in Swiss albino mice. ¹⁴ The ethanolic extract of the fruit is also shown to possess cytotoxic effect on SKBR3 (human breast adenocarcinoma cells) in vitro. ¹⁵ Studies have also shown that treatment with the Bael extract did not increase ERα mRNA levels in MCF7 cells and MDA-MB-231. ¹⁶ However, when added in combination with the decoy molecule, the extract and one of its phytochemicals, lupeol, stimulated the decoy effect of RA4 DNA sequence, increased ERa gene expression in MDA-MB-231 (ERα-negative breast cancer cells) and inhibited cell proliferation. ¹⁶

Activity-guided fractionation studies with the Bael leaf extracts have also shown that butyl p-tolyl sulfide, 6-methyl-4-chromanone, and butylated hydroxyanisole were effective, whereas 5,6-dimethoxy-1-indanone, palmitic acid, methyl linoleate, and 5-methoxypsoralen were not effective when compared with the clinically used drugs such as cisplatin, chromomycin, 5-fluorouracil, and cytosine arabinoside. The efficacy of the various phytochemicals and the standard drugs was observed to be as follows: cytosine arabinoside (IC₅₀ = 0.25 µM)> cisplatin (IC₅₀ = 5 µM) = chromomycin (IC₅₀ = 5 µM) > butyl p-tolyl (IC₅₀ = 7 µM) > 6-methyl-4-chromanone (IC₅₀ = 15 µM) > butylated hydroxyanisole (IC₅₀ = 35 µM) > 5-fluorouracil (IC₅₀ = 50 µM) > 5,6-dimethoxy-1-indanone (IC₅₀ = 100 μM) > palmitic acid (IC₅₀ = 100 μM) > methyl linoleate (IC₅₀ = 200 μM). ¹² The antiproliferative activity of butyl-p-tolyl sulfide, 6-methyl-4-chromanone, and 5-methoxypsolaren was observed to be associated with the activation of the differentiation pattern of K562 cells. ¹²

Parallel studies by Subramaniam et al ¹⁷ have also shown that the various fractions (hexane, dichloromethane, ethyl acetate, and n-butanol soluble fractions) of ethanolic extract of the Bael leaf possess cytotoxic effect in HEp-2 cells (alveolar epithelial carcinoma cells) with the best effect being observed in the ethyl acetate fraction. Activity-guided fractionation studies of the ethyl acetate fraction showed that the observed cytotoxic effects were primarily due to marmelin (discussed in detail in the next paragraph). ¹⁷ Additionally, experiments have also shown that the other phytochemicals such as lupeol,^18-25 eugenol,^26-28 citral,^29,30 cineole, ³¹ and d-limonene ³¹ present in Bael possess antineoplastic effects. In the following paragraphs, the antineoplastic effects of these phytochemicals are discussed in detail.

Marmelin

Marmelin, chemically 1-hydroxy-5,7-dimethoxy-2- naphthalene-carboxaldehyde, is a novel compound present in Bael. Seminal studies by Subramaniam et al ¹⁷ have shown that at equivalent doses, marmelin was effective at inhibiting the growth of epithelial cancer cells (HCT-116 colon and HEp-2, alveolar epithelial carcinoma cells), but not normal cells (mouse embryo fibroblasts), suggesting it to be noncytotoxic. Mechanistic studies showed that marmelin activated apoptosis through tumor necrosis factor-α (TNF-α), TNF receptor–associated death domain (TRADD), and caspases. ¹⁷ It induced TNF-α, TNFR1, and TRADD mRNA and protein expression, G1 cell cycle arrest, and mediated apoptosis through activated caspase-3, which was abrogated when pretreated with caspase-3 inhibitors. Marmelin also caused activation of caspase-8 and Bid, with release of cytochrome C was also observed, suggesting the existence of a cross-talk between death receptor and the mitochondrial pathways. ¹⁷

Studies showed that marmelin significantly suppressed TNF-α-mediated activation and translocation of nuclear factor-κB (NF-κB). ¹⁷ NF-κB is a transcriptional factor that regulates a battery of genes critical to innate and adaptive immunity, cell proliferation, inflammation, and tumor development. When the cells are unstimulated, NF-κB is sequestered in the cytoplasm by IκB (inhibitor of κB). Activation of the NF-κB is initiated by the signal-induced degradation of IκB proteins by IκB kinase (IKK). With the degradation of IκB, the NF-κB complex is released to enter the nucleus. In the nucleus, NF-κB initiates the expression of specific genes that have DNA-binding sites for NF-κB. The activation of these genes by NF-κB then leads to various physiological responses. By suppressing the activation and translocation of NF-κB, marmelin mediates the protective effects. ¹⁷

Treatment with marmelin inhibited the growth of HCT-116 colon cancer tumor xenografts in vivo. Immunostaining for CD31 showed that there was a significant reduction in microvessels in the marmelin-treated animals, coupled with decreased cyclooxygenase-2 (COX 2), interleukin-8 (IL-8), and vascular endothelial growth factor (VEGF) mRNA. ¹⁷ IL-8, VEGF, and COX 2 are shown to be important in tumor growth, angiogenesis, and metastasis. Therefore, reduction in their levels reduces/prevents both angiogenesis and metastasis. ¹⁷ Marmelin also inhibited AKT and extracellular signal–regulated kinase phosphorylation both in cells in culture and in tumor xenografts. ¹⁷ AKT, which plays a key role in tumor cell survival, proliferation, and invasiveness, is frequently altered in certain cancers. Tumor cells that have constantly active AKT may depend on it for survival. By reducing the AKT levels, marmelin decreases the cell survival, proliferation, and invasiveness. ¹⁷

Lupeol

Lupeol, a triterpene, is shown to possess antineoplastic effects on various human neoplastic cell lines, for example, human melanoma 451Lu cells, ¹⁸ WM35 cells, ¹⁸ and B162F2 cells ¹⁹ ; human pancreatic adenocarcinoma cells AsPC-1 ²⁰ ; human epidermoid carcinoma A431 cells ²¹ ; hepatocellular carcinoma SMMC7721 cells ²² ; and prostate carcinoma cell lines LNCaP, ²³ CWR22Rγ1, ²³ and PC-3. ²⁴ Its cytotoxicity was selective for neoplastic cells.^18,20,23

Animal studies have validated the in vitro observations. Experiments have shown that feeding lupeol to athymic nude mice reduced the growth of 451Lu and CWR22Rnu1 tumors by modulating the expression of proliferation markers, apoptotic markers, and cell cycle regulatory molecules in tumor xenografts.^18,23 Lupeol caused G1-S phase cell cycle arrest and decreased expression of cyclin D1, cyclin D2, and cdk2 with concomitant increase in expression of p21 protein in PC-3 cells. ²⁴ The cell cycle is aberrant in neoplastic cells; by decreasing the molecules (cyclin D1, cyclin D2, cdk2) mediating the deregulation, and by increasing p21, a cyclin-dependent kinase inhibitor, involved in regulation of cell cycle progression, lupeol mediates the cell cycle arrest. ²⁴

Studies with cultured human AsPC-1 have shown that when compared with the controls, lupeol inhibited cell growth and proliferation, and increased the levels of apoptosis by increasing the expression of Bax protein and activating caspases. Lupeol reduced the expression of Ras, PKCa, and ODC proteins. It reduced the expression of PI3K/Akt, MAPK proteins p38 and Erk1/2, and phosphorylation of IkBa and NF-κB/p65. ²⁰ Experiments with SMMC7721 cells have also shown that lupeol also decreased the expression of death receptor 3 (DR3), an activator of NF-κB, and increased the expression of Fas associated protein with death domain (FADD mRNA). ²²

Lupeol is also reported to modulate the expression of ErbB2, tissue inhibitor of metalloproteinases-3, cyclin D1, and matrix metalloproteinase (MMP)-2 genes, which are associated with proliferation and survival in LNCaP cells. ²⁵ The ErbB2 (Her2) proto-oncogene encodes a receptor tyrosine kinase, which is frequently amplified and overexpressed in many human tumors, such as breast, ovarian, prostate, non–small cell lung cancers, and cancers of the head and neck. ErbB2 is a cell membrane surface–bound receptor with intrinsic tyrosine kinase activity and is involved in the signal transduction pathways. Its overexpression promotes aberrant cell growth, differentiation and metastasis. ²⁵

Patients with ErbB2-overexpressing cancer have substantially lower overall survival rates and shorter disease-free intervals than patients whose cancer does not overexpress ErbB2. ²⁵ Thus, the HER-2/Neu/ErbB-2 receptor tyrosine kinase is a validated therapeutic target, especially in breast cancer. Studies by Saleem et al ²⁵ have shown that treatment with lupeol caused a concentration-dependent decrease in the expression level of ErbB2 in prostate cancer cells indicating its usefulness in the near future. ²⁵

Lupeol also inhibited the expression of cyclin B, cdc25C, and plk1 but induced the expression of 14-3-3σ genes in PC-3 cells. ²⁴ Lupeol is also shown to induce apoptosis by downregulating Bcl2 (an anti-apoptotic protein), upregulating Bax (a pro-apoptotic protein), activating caspase-3, caspase-9, and apaf1 genes, and inducing poly(ADP)ribose polymerase cleavage in the CWR22Rnu1 and PC-3 neoplastic cells.^18,24 Lupeol treatment increases reactive oxygen species, causes loss of mitochondrial membrane potential, and induces DNA fragmentation in PC-3 cells. ²⁴ Lupeol attenuated stress fiber assembly, decreased phosphocofilin, and inhibited the haptotaxis of B16 2F2 melanoma cells to fibronectin. ¹⁹

Eugenol

Eugenol is an allyl chain–substituted guaiacol (2-methoxyphenol) and a member of the phenylpropanoid class of chemical compound in aromatic plants. Cell culture studies have shown that eugenol possess cytotoxic effects against salivary gland tumor cell line (HSG) and normal human gingival fibroblast (HGF) in vitro. ²⁶ Eugenol possesses cytotoxic effects on the malignant HepG2 hepatoma cells, malignant Caco-2 colon cells, and the nonmalignant human VH10 fibroblasts. ²⁷ Studies with the human malignant melanoma cell line, WM1205Lu, have also shown that eugenol arrested cells in the S phase of the cell cycle, induced apoptosis and that the deregulation of E2F1 may be a key factor in eugenol-mediated melanoma growth inhibition both in vitro and in vivo. ²⁸ Studies with B16 melanoma bearing mice showed that eugenol treatment inhibited cell proliferation, retarded tumor growth kinetics, reduced the tumor size by nearly 40%, increased the median survival time by nearly 20%, and inhibited invasion and metastasis in nearly 50% of the animals in comparison with the control group. Eugenol was also observed to be well tolerated as the body weights of the mice treated were not affected. ²⁸

Citral

Citral (3,7-dimethyl-2,6-octadien-1-al), a key component of Bael, has been recently shown to induce apoptosis in several hematopoietic cancer cell lines and the apoptotic activity was comparable to that of staurosporine, a Streptomyces staurosporeus–derived antineoplastic antibiotic with potent effects. ²⁹ Recently, Chaouki et al ³⁰ have also reported that citral possessed antiproliferative effects, inhibited cell cycle progression in G2/M phase, induced apoptosis of the human breast cancer cell line MCF-7, and decreased the prostaglandin E(2) synthesis.

Cineole

Cineole (1,8-cineole), also known as eucalyptol, is a terpene present in many aromatic plants such as eucalyptus, mugwort, sweet basil, rosemary, sage, and cardamom. In vitro studies have also shown that cineole induces apoptosis in the human leukemia cell lines Molt 4B and HL-60 cells, but not in human stomach cancer KATO III cells. The authors observed a concentration- and time-dependent apoptosis (as evaluated by the DNA fragments) in both Molt 4B and HL-60 cells, confirming that the antineoplastic effects of 1,8-cineole are mediated through induction of apoptosis and that the action is cell specific. ³¹

Limonene

Limonene is a monoterpene found in the peel from citrus fruits, dill, caraway, fennel, and celery. Studies have shown d-limonene possess chemotherapeutic activity against pancreatic, mammary, and prostatic tumors. Mechanistic studies indicate that several pathways are responsible for the observed effects. However, the most important is the observation that monoterpenes inhibit the posttranslational isoprenylation of cell growth-regulatory proteins such as Ras. ³²

Isoprenylation of Ras initiates a series of posttranslational processing events required for protein interaction with cell membrane and for expression of oncogenic activity. By inhibiting this process d-limonene prevents the molecular signaling of Ras and its activities. Monoterperpenes are also observed to regress rat mammary tumors by increasing the expression of transforming growth factor-β (TGFβ), which regulates mammary epithelial cell division and also causes tumor remodeling/redifferentiation to a more benign phenotype. ³² Together, all these reports indicate that Bael and its phytochemicals possess antineoplastic effects and could be useful.

Protective Effects of Bael and Its Phytochemicals Against the Cytotoxic Effects of Ionizing Radiation, Doxorubicin, and Cyclophosphamide-Induced Toxicity

The use of ionizing radiation and chemotherapeutic drugs are associated with deleterious effects resulting from the normal tissue damage. ³³ In such situations, an agent that can differentiate between cancer and normal cells will be highly beneficial to protect against unwanted cytotoxicity and alterations in normal cell functioning. ³³ Therapeutic differential may be achieved with chemical compounds that may selectively enhance the chemotherapeutic drugs or radiation’s antineoplastice effects (chemosensetizers and radiation sensitizers) or by selectively protecting the normal cells from the deleterious effects of radiation and chemotherapeutic agents (radioprotectors and chemoprotectors).^3,34

Studies have shown that the hydroalcoholic extracts of both fruit and leaf were effective in ameliorating radiation-induced sickness (eg, alopecia, dermatitis, diarrhea, redness, irritation) and mortality in the Swiss albino mice when administered through the intraperitoneal route.^35,36 Comparatively, the leaf was observed to be better than fruit extract in ameliorating the radiation-induced sickness and mortality, and was also better than 2-mercaptopropionyl glycine, a synthetic radioprotective thiol compound, used as a positive control.^35,36 Detailed investigations also showed that the leaf extract when administered orally for 5 consecutive days before exposure to lethal doses of radiation effectively ameliorated the radiation-induced sickness and mortality in mice. ³⁷ The optimal dose of 250 mg/kg body weight was not toxic and offered significant protection to both hemopoetic and gastrointestinal cells. ³⁸ The leaf extract prevented radiation clastogenesis in the cultured human peripheral blood lymphocytes and the bone marrow cells of mice, indicating its effectiveness as an antimutagen.^39,40 The phytochemical eugenol ⁴¹ is also reported to possess radioprotective effects and its presence in the extract may have contributed to these results.

Studies have shown that the oral administration of the hydroalcoholic extract of the Bael leaf possess anticlastogenic effects and reduced the doxorubicin-induced micronuceli in the mice bone marrow cells. ⁴² Additionally, studies have also shown that the phytochemical cardenolide present in the leaves protected mice against doxorubicin-induced cardiotoxicity and hepatotoxicity by cytoprotective, antioxidant, and hypolipidemic action. ⁴³ Together, these observations clearly suggest the usefulness of Bael as a chemoprotective agent against doxorubicin-induced ill effects.

The phytochemical lupeol is also shown to be effective in preventing cyclophosphamide-induced ill effects by modulating oxidative stress, serum lipoproteins, lipids, and the activity of lipid-metabolizing enzymes in the cardiac tissues of rats.^44,45 Lupeol alleviated the cyclophosphamide-induced increase in the activities of lysosomal hydrolases in serum and heart, and concomitantly increased the levels of cellular thiols. ⁴⁶ Lupeol increased the activities of TCA cycle enzymes and mitochondrial complexes of electron transport chain in the cardiac tissue ⁴⁷ and normalized the activities of ATPases, urea, uric acid, and creatinine. ⁴⁸ Electron microscopic studies showed that the degree of damage to the myofibres, characterized by the swollen and loss of myofilaments, mitochondrial swelling, and damaged cristae were also decreased.^46,47 Together, all these observations indicate that the hydroalcoholic extract of Bael leaf and its phytochemicals lupeol and cardenolide possess cardioprotective effects

Bael as a Chemopreventive Agent

Preclinical studies have shown that the ethanolic extract of the fruit to possess chemopreventive effects against the 7,12-dimethylbenz(a) anthracene (DMBA)–induced skin papillomagenesis in Swiss albino mice. Oral administration of the Bael extract (50 mg/kg body weight) in the peri-initiational phase (7 days before and 7 days after DMBA application) caused a 70% decrease whereas during the post-initiational phase (from the day of croton oil treatment till the end of the experiment) caused 50% decrease in tumor incidence. Bael also reduced the cumulative number of tumors, the tumor burden per animal and tumor yield suggesting its usefulness as a chemopreventive agent. ⁵⁰

Very recently, Khan and Sultana ⁵¹ have also reported that the methanolic extract of Bael (25 and 50 mg/kg body weight) was effective in inhibiting the diethylnitrosamine initiated and 2-acetyl aminofluorene promoted hepatocarcinogenesis in Wistar rats. When compared with the carcinogen alone (without Bael) treated controls, co-administering Bael caused a decrease in the incidence of liver tumors. Mechanistic studies clearly showed that pretreatment with Bael extract (25 and 50 mg/kg body weight) prevented 2-acetyl aminofluorene–induced oxidative stress by restoring the levels of antioxidant enzymes and detoxification enzymes and by concomitantly decreasing the activity of ornithine decarboxylase and synthesis of DNA. ⁵¹

Studies have also shown that some of the phytochemicals present in Bael, such as lupeol, ⁵² eugenol, ⁵³ limonene,^54,55, citral, ⁵⁶ rutin, ⁵⁷ and anthocyanins^58,59 have been reported to possess chemopreventive effects. The presence of these compounds in the extract may have contributed to the observed effects.

Mechanism(s) Responsible for the Chemopreventive and Radioprotective Action

Mechanistic studies suggest that the radioprotective effects of Bael are multifactorial and are due to the free radical scavenging, antioxidant, immunomodulatory increase in the levels of glutathione and decrease in lipid peroxidation. The leaf extract is observed to be a potent scavenger of both reactive oxygen species and reactive nitrogen species and a good iron chelator in the in vitro systems of studies.^35,39,60-62

The methanol and acetone extracts of the fruits are also observed to be effective in reducing the hydrogen peroxide and aflatoxin B1–induced SOS response in chromotest, suggesting they possess antimutagenic effects and prevent the mutagenesis, which is the antecedent for carcinogenesis. ⁶³ Bael leaf has been reported to increase the activities of antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase) in mice, ⁶⁴ irradiated mice, ⁶⁰ and in alloxan-treated diabetic rats. ⁶⁵

The leaf and fruit extracts also prevented the radiation-induced lipid peroxidation in the liver, kidney, intestine, and spleen of mice with a concomitant increase in the levels of glutathione.^35-37,60 The leaf extract stimulated the immune response by acting on macrophages from the systemic immune compartment in particular with the peritoneal cavity. The effect that has been observed played a significant role in view of the fact that macrophages form the first line against microbial invasion and neoplastic diseases. ⁶⁰

Conclusion

Information accrued from preclinical studies suggests that Bael is useful in the treatment and prevention of cancer. However, gaps in the studies conducted are apparent, which need to be bridged in order to exploit the full medicinal potential of Bael. With regard to antineoplastic activities, studies have clearly shown that both Bael extracts and some of its phytochemicals such as marmelin, butyl p-tolyl sulfide, 6-methyl-4-chromanone, butylated hydroxyanisole, lupeol, citral, cineole (1,8 cineole), d-limonene, and eugenol are observed to be effective in selectively inhibiting proliferation of neoplastic cells. While most studies have been done with cultured cells and affirm the antineoplastic activities, studies with tumor-bearing animals of different histological, and more important, with highly metastatic cells should be performed. Only when these studies are performed will the efficacy of these compounds in cancer control be realized.

With regard to chemoprotection, radiation protection, and chemoprevention, all published observations have been with experimental animals and help in validating the applicability on the human system. A drug selective in protecting normal tissue will always be beneficial in cancer treatment and cure. Therefore, studies should be performed to understand the selective radioprotective and chemoprotective effects with tumor-bearing animals of different histological and metastatic characteristics. In vitro studies should also be planned to decipher the underlying protective mechanism with both whole extract and the phytochemicals.

Apart from applications in the clinic, Bael can be used as a radiation countermeasure in the management of radiological/nuclear incidents. Studies aimed at understanding the efficacy of Bael in ameliorating the radiation-induced damage with individuals occupationally exposed to radiation are required. As there are no existing safe and effective treatments for radiation injury, these studies seem worthwhile. Bael also possesses chemopreventive effects against chemical-induced skin carcinogenesis and hepatocarcinogenesis. Future studies should be aimed at understanding its efficacy in other models of carcinogens especially prostate, lung, colon, and breast models of carcinogenesis and also on understanding the mechanisms responsible for the chemopreventive effects.

From the phytochemical perspective, there is considerable variation in the composition among various samples of Bael. A quality control should be established for the authenticity of the plant and the presence of active phytochemicals in the required levels. In this regard, the availability of authentic metabolite standards for quantification of the phytochemicals will make the scientific observations more reliable and reproducible. Studies should also be done on understanding which of the phytochemicals are responsible for the observed beneficially effects and their mechanism of action.

Because of its abundance, low cost, and safety in consumption, Bael remains a species with tremendous potential and countless possibilities for further investigation. As human beings have been consuming Bael since time immemorial, the major advantage lies in its easy acceptability, nontoxic nature, and easy affordability. Additionally, preclinical studies have also confirmed that the Bael leaf extract has a high margin of drug safety and does not possess systemic, teratogenic, or genotoxic effects at optimal protective concentrations.^35,40,66,67 When these observations are considered along with its beneficial effects in the prevention and treatment of cancer, it is safe to assume that Bael remains a species with tremendous potential but only when the lacunae in the existing knowledge are bridged. The outcomes of such studies may be useful for the clinical applications of Bael in humans and may open up a new therapeutic avenue.