Cancer—a major global public health issue
Alarmingly 8.8million people are dying each year of cancer, accounting to one out of six deaths and more than the number of deaths from HIV/AIDS, malaria, and tuberculosis combined. Particularly, in low-income and middle-income countries (LMICs), the disease management remains strenuous and the burden is greatest, where almost 75% of cancer deaths occur and the number of cancer cases is increasing steadily. It is estimated that cancer incidence will double by the year 2035 [1].
On an estimate, around 50% to 60% of patients will require radiotherapy at some time during their treatment; the International Atomic Energy Agency (IAEA) reported that in developing countries due to a shortage of 5000 machines, nearly 70% of cancer patients could not benefit from curative or palliative radiotherapy (International Atomic Energy Agency 2016). The IAEA are currently focusing to introduce radiotherapy to many of these countries. Moreover, it could be seen that investing in radiotherapy will save lives as well as have positive impact over economic benefits [2].
Liver cancer—global context
Liver cancer is the seventh most frequently occurring cancer in the world and the fourth most common cause of cancer mortality [3]. Its incidence rates have been gradually rising in many countries. The highest incidence rates of liver cancer in the world happen to be in Asia and Africa. A majority of cause lies within the chronic infections with hepatitis B virus (HBV) and hepatitis C virus (HCV) for almost three-quarters (73.4%) of hepatic cellular carcinoma in the world [4].
Barriers in current cancer therapy
Current cancer therapy involves three major approaches such as surgical oncology, chemotherapy, and radiotherapy [5]. The major obstacles in the current anticancer agents are the development of drug resistance [6]. Multidrug resistance (MDR) is the simultaneous resistance to a number of structurally and functionally unrelated chemotherapeutic drugs. During cancer treatment, many cancers initially respond well to chemotherapy but subsequently lead to acquired resistance such that more than 90% of patients with metastatic cancer either fail to respond or relapse from chemotherapeutics. To address all these issues, various efforts have been invested to elucidate both on inherent and acquired MDR mechanisms [7].
Inorganic compounds in cancer therapy
Selenite is one of the most studied inorganic compounds because of its outstanding chemopreventive and anticancer features. It effectively inhibits cell proliferation of various types of cancer cells. Of different human cancer cell lines tested, lung cancer cells appeared to be more sensitive to selenite. A study reported that selenite cytotoxicity was correlated with Se uptake of three lung cancer cell lines (H157, H611, and U2020) and high concentrations (> 1 mM) of selenate were non-toxic for these cell lines. Se plays a role as natural killer (NK) in cell-based anticancer immunotherapy where it increases the susceptibility of cancer cells to CD94/NK group, and has potential clinical applications in lung cancer patients. The combined effect of selenite and thioredoxin reductase inhibitors was detected in human ovarian and lung cancer cell lines. These results indicated the capacities of Se compounds to enhance the activity and reduce the toxicity of anticancer drugs. Additionally, these compounds appear to be more effective in inhibiting the growth of anticancer drug-resistant cancer cells compared with drug-sensitive cancer cells by means of deactivating various resistance mechanisms adapted by the cancer cells. Chemotherapeutic drug-resistant lung cancer cells were more sensitive to selenite compared to drug-sensitive cancer cells [8].
Microorganisms in cancer treatment
Microbes offer a feasible means of inducing anti-tumor immunity. Oncolytic viruses were among the first successfully used microbes in cancer therapy. An intratumoral injection of herpes virus, T-VEC received approval from the US Food and Drug Administration for metastatic melanoma in 2015. The authors used Escherichia coli, which are non-pathogenic, grow extracellularly, and persist for nearly 6 days after intratumoral injection. After the delivery of bacteria into the tumors, several rounds of bacterial lysis continues, a constant, high local concentration of CD47nb. It showed robust tumor-antigen-specific CD8 T cell responses that targeted non-injected tumors with established immunologic memory. These results strongly suggested the induction of adaptive immunity as the major therapeutic mechanism by which CD47 blockade operates in immune competent animals. Successful induction of T cell responses requires the presence of antigen as well as adjuvant to be present at the same time. The quorum-sensing bacteria achieve it through three major strategies. Initially, bacteria provide an abundance of adjuvant and persist at high levels for about a week in tumors. Followed by local delivery and quorum lysis prevent sepsis and leakage of therapeutics cargo into circulation, decreasing systemic exposure of the drug and generation of anti-drug antibodies. And finally, bacteria have a remarkable ability for genetic cargo and can be explored further with more therapeutics in a strand [9].
Cancer nanotechnology
Nanoparticles (NPs) targeted for cancer therapies basically comprise of nanocarrier and the active drug, few NPs formulations of the drug by itself also exist. The nanodrug systems are advantageous with higher metabolic stability, enhanced membrane permeability, improved bioavailability, and prolonged activity. Nano transporters allow specialized targeted drug delivery of the anti-cancer agents to the tissue as well as at the cellular level. The key factors that affect the mucosal or transdermal absorption are size, surface charge, and hydrophobicity. The size of the particles plays a crucial role; smaller NPs are inducing higher transcellular uptake than the larger particles. But NPs larger than 300 nm could not be absorbed by intestinal cells. And only smaller NPs less than 500 nm can penetrate the bloodstream [10].
Nanotechnology in biomedicine
There are an increasing numbers of nanoparticle-based treatments being approved for clinical use which involves mostly spherical liposomes. However, scientists are constantly working on a wider range of NPs designs with different core materials bearing magnetic, optical, and biochemical properties and NPs of varying size, shape, hardness, and surface properties. Adding on to their therapeutic applications, NPs as well enable effective capabilities in imaging [11].
Microbial-mediated synthesis of nanomaterials
The microbial-mediated synthesis of nanomaterials (NMs) has been considered due to the following advantages like synthetized NMs have definitive chemical composition, size, and morphology; biosynthesis is carried out under at mild physicochemical conditions; ease of handling and cultivation of microbial cells; and possibility of tunable NMs with required characteristics by varying key parameters of cell culture set up. One of the greatest challenges faced in microbial nano biosynthesis is the control of dispersity of nanostructure materials, which primarily influence electronic and optical properties. When going for large-scale productions, costs of culture media for microbial growth affects to a greater extent. For instance, currently, bacterial nanocellulose applications are much limited to a few biomedical devices, mainly due to the costs of culture medium [12].
Conventional chemical method of synthesis
The synthesis of gold nanoparticles (AuNPs) was first reported in 1940 upon reacting gold chloride (HAuCl4) and trisodium citrate (Na3C6H5O7 or NaCt) which resulted in the formation of colloidal gold. Further on, the detailed work carried out by Turkevich and his coworkers has become one of the milestones of AuNPs synthesis. From then onwards, the method has been modified and improvised in order to meet the requirements of diverse area such as development of chemical sensors for water quality analysis using surface enhanced Raman spectroscopy (SERS), surface-induced catalytic activities, and drug delivery in biological systems and nano-toxicology studies. There are certain significant features of AuNPs that make them more attractive to employ in wide applications like surface plasmon resonance (SPR), a size- and shape-dependent property, and biocompatibility. Even though of all the distinctive properties of AuNPs, there is a major challenge residing over its usage, which is the control of the particle size and size distribution due to the fact of batch-to-batch variation affected by three factors, local temperature gradient, the efficiency of reagent mixing, and the resulting local concentration gradient [13].
Green synthesis of nanomaterials
Different physical and chemical methods such as solvothermal method, hydrothermal route, pulsed laser ablation technique, sonochemical process, and microwave irradiation has been exploited for the synthesis of NPs [14]. Conventional methods have limitations such as expensiveness, undesirable end products, and hazardous toxic chemicals, involving high temperature and pressure, etc. Whereas green synthesis is an environment-friendly, cost-effective method to synthesize nanostructural material to have tunable structures, morphologies, and particle size distributions. Green synthesis methods employ organisms like plant, bacteria, fungi, etc. for the synthesis of NMs, and it has gained much interest in the rising area of research due to their less toxic or nontoxic nature, eco-friendly nature, and low cost of production. Plant extract are produced from leaves, stems, flower, and seeds of various plants. The extracts are rich in protein, amino acid, enzymes, vitamins, terpenoids, flavonoids, alkaloids, phenolic acids, etc., that act as capping and reducing agents that reduces metal ions during the bioreduction process to produce NPs or nanostructures in varied dimensions and morphologies [15].
Limitations in the application of nanomaterials
Nanotechnology has entered cancer therapy in the recent years. Upon then, many medical products have been approved for clinical use like albumin-bound-paclitaxel, liposomal doxorubicin, and liposomal irinotecan. At the moment, many anti-cancer nanodrugs are into various phases of clinical trials and preclinical research. But the major drawback is the increased cost of nanodrugs, compared with free drugs. Another limitation is the uptake of nanodrugs, which occurs through reticuloendothelial system (RES). Nanodrugs should avoid clearance through uptake by the (RES) which occurs primarily for particle size larger than 100 nm. On the other aspect, localization of nanodrugs in tumor-associated macrophages will definitely increase the concentration of the drug at the tumor sites [10].
AuNP’s—as most preferred metallic nanoparticles
Significantly, AuNPs have become versatile candidates used for cancer detection and therapy due to their futuristic physicochemical properties. But still, the use of toxic chemicals in developing AuNPs is of great concern since it may create environmental issues. To combat with this, biological methods of fabricating AuNPs are the much preferred choice. Hence, synthesis of AuNPs using plants, seaweeds, and microbes are advantageous over physical and chemical methods. On this context, the phytocomponents present in the natural source contribute to added bio-compatibility and cytotoxicity by stabilizing the AuNPs. There have been numerous studies stating on the anticancer properties of AuNPs synthesized through biological approach. More studies about the comparative evaluation of AuNPs against cancer and normal cells are needed for further extended research [16].
Toxicity studies of metallic nanomaterials
Though there are several advantages of metallic nanoparticles, there is much concern regarding their safety on biological cells as reported in in-vitro/in-vivo toxicity studies. In higher metallic form/bulk form, gold is inert/non-toxic, but at the nano dimensions based on the manner in which these NPs are developed using different synthetic procedures and the nature of capping/stabilizing agents may lead to toxicity. Though green synthesized NPs using of various plant extracts containing polysaccharides, biomass, phytochemicals, and synthetic molecules are less toxic, there is a major drawback at the polydispersity, yield, purity, and metabolite formation aspects. There are many inconclusive data in the literature discussing whether AuNPs are toxic or non-toxic. The toxicity of NPs is also influenced by the size, shape, and surface chemistry of NPs. Some NPs have demonstrated size-dependent toxicity, whereas others have shown surface chemistry-dependent toxicity. Therefore, predicting toxicity becomes difficult since similar sizes of NPs results in differential toxicity on the same cell line [17].
Anticancer effects of Annona muricata’s Fruit—Graviola
Annona muricata is a tropical tree widely distributed in the regions of Africa, South America, and Southeast Asia. It belongs to the family Annonaceae. It is commonly referred as soursop, graviola, and guanabana. The fruit is edible in green heart-shaped and the tree comprises of large, glossy, dark green leaves. It has been reported in the literature that nearly 74 bioactive compounds of the plant exhibit a variety of anticancer effects in in vitro and in vivo animal model systems. A. muricata-derived preparations and formulations have been used for the treatment of different disease conditions, thereby proving its prominence in Ethnomedicine [18]. The bark, leaves, and roots of Annona muricata have been extensively applied globally in the disease management of various ailments, including inflammatory conditions, rheumatism, diabetes, hypertension, insomnia, cystitis, parasitic infections, and cancer [19].
Traditional medicines—treatment of liver disorders
Herbal and traditional medicines have been used to treat liver disorders for centuries. A. muricata is commonly used to treat various liver disorders, particularly jaundice. In a recent study, performed on phenyl hydrazine-induced jaundice in adult rats, treated with A. muricata aqueous extract at different doses, found a substantial reduction in elevated bilirubin. It has also been reported to protect against increased serum glutamic-oxaloacetic transaminase (SGOT), serum glutamic-pyruvic transaminase (SGPT), alkaline phosphatase (ALP), and liver and brain lipid peroxidation. In addition to the hepatoprotective properties, the aqueous extract was reported to protect against liver damage induced by CCl4 and acetaminophen. The glucosides of A. muricata mechanistically reduced the bilirubin levels by converting to glucuronic acid that conjugates with bilirubin for excretion. Apparently, the phytochemicals in the extract may act as regulators increasing the activity of enzymes and transporters, and thereby enhancing the overall activity involved in clearing bilirubin. Combining all these outcomes, the potential hepato-protective function A. muricata is clearly emphasized and its prominent role in liver function [19].
Hence, A. muricata was used for the synthesis of AuNPs in this study. Further, the synthesized nanoparticles were characterized and checked for the in vitro cytotoxicity in VERO cell line and anticancer activity in Hep2 liver cancer cell line.