Discovery of plant-origin anti-cancer agents

What is cancer? Cancer is a generic term for a large group of diseases characterized by the growth of abnormal cells beyond their usual boundaries that can then invade adjoining parts of the body and/or spread to other organs. Other common terms used are malignant tumours and neoplasms. Cancer, which is the uncontrollable multiplication of cells, has been in existence as long as animals have; evidence of cancers has been found in dinosaur bones. Cancers have also been found on mummies dating back 2,500 years. Hippocrates (460–370 b.c.) coined the word cancer, which means “crab” in Greek. There are over 110 types of cancer, which can be divided into four categories depending on the tissue involved: carcinoma, lymphoma, leukemia, and sarcoma. Cancer is the second leading cause of death globally and is estimated to account for 9.6 million deaths in 2018. Lung, prostate, colorectal, stomach and liver cancer are the most common types of cancer in men, while breast, colorectal, lung, cervix and thyroid cancer are the most common among women. Plants have been used as medicinal agents since the origin of mankind. The toxicity of chemotherapeutic drugs sometimes creates a significant problem in the treatment of cancer using allopathy or established medicine. Various therapies have been propounded for the treatment of cancer, many of which use plant-derived products. There are four classes of plant-derived anticancer agents in the market today, the vinca alkaloids (vinblastine, vincristine and vindesine), the epipodophyllotoxins (etoposide and teniposide), the taxanes (paclitaxel and docetaxel) and the camptothecin derivatives (camptotecin and irinotecan). Plants still have enormous potential to provide newer drugs and as such are a reservoir of natural chemicals that may provide chemoprotective potential against cancer. Cancer and Alkaloids! In 1958, during their pursuit of diabetes drugs, R. L. Noble and C. T. Beer at the University of Western Ontario in London, Ontario, Canada, isolated two important cancer drugs from Madagascar periwinkle: vincristine and vinblastine. As early as 1949, Noble had heard stories that Jamaicans were treating diabetes using Vinca rosea leaves. He and Beer procured some of the leaves and fed them to four rabbits but did not see any effects on the level of blood sugar. Having failed orally, they tried the injection route. After injection of the leaf extract, many rabbits died of an overwhelming infection induced by Pseudomonas bacterium. They found that rabbits treated with the plant extract developed critically low counts of white blood cells, leaving them with damaged bone marrow and defenseless against bacterial infections. Because one of the hallmarks of cancer is abnormal proliferation of white blood cells, they tried the plant extract on animals with transplanted tumors and saw tumor shrinkage. They then looked for the specific alkaloids in the extract that might slow or halt white blood cell production and identified two, vincristine and vinblastine, as the most potent. The Canadians presented their findings in 1958, and the chemotherapeutic effects of vinca alkaloids became known to the public. It was at that time that Noble learned that Gordon H. Svoboda at Eli Lilly had also made similar discoveries. In 1957, Svoboda submitted extract of the whole vinca plant for testing. It showed a 60–80% prolongation of life for mice infected with P-1534 leukemia, an acute lymphoblastic leukemia. The yield of vincristine was low from the dry periwinkle plants, one of the lowest of any medically important alkaloids. American drug firms contracted farmers in the hill country of eastern India, near the Chinese border, and promised to buy all the vinca they could grow. The supply of vinca leaves imported to the United States was plentiful and largely steady, except a brief interruption during the China-India border in October and November of 1962. Since 1964, Eli Lilly has marketed vincristine (trade name Oncovin) for acute childhood leukemia and vinblastine (trade name Velban) for lymphoma such as Hodgkin’s disease, for advanced testicular cancer, and for advanced breast cancer. Similar tocolchicine, the vinca alkaloids work by serving as a “spindle poison.” They bind to tubulin, one of the key constituents of microtubules, thus preventing the cell from making the spindles it needs to divide. Before the emergence of vincristine and vinblastine, the diagnosis of Hodgkin’s disease was virtually a death sentence. Now there is a 90% chance of survival with the treatment of vinca alkaloids and other chemotherapy. Madagascar periwinkle, the little ornamental plant, has been transformed into a lifesaving gift from Mother Nature. Paclitaxel, with the trade name Taxol, has had considerable success in treating ovarian and breast cancer since 1992. It was initially isolated from the Pacific yew tree as part of the National Cancer Institute-United States Department of Agriculture (NCI-USDA) plant-screening program. The NCI is one of the many divisions of the National Institutes of Health (NIH). Arthur Barclay, a Harvard-educated botanist, along with three graduate students, traveled to the Gifford Pinchot Forest in the state of Washington. The quartet found a little-known pacific yew tree, Taxus brevifolia. They collected samples of twigs, leaves, and fruits. Barclay labeled them as B-1046, because it was his 1,046th sample, and shipped them to the NCI. One of the NCI’s contractors was the Wisconsin Alumni Research Foundation (WARF, most famous for their important anticoagulant, warfarin). WARF tested the B-1046 extracts and found them to be cytotoxic. In 1966, after being rejected by many other laboratories for fear of toxicity, the stem bark found its way to the hands of Monroe E. Wall, chief chemist of the Fractionation and Isolation Laboratory at the Research Triangle Institute in North Carolina. Wall initially put the Pacific yew tree project on the back burner because he and his colleague, Mansukh C. Wani, were focusing on camptothecin, which was very promising as a chemotherapeutic agent. Camptothecin was isolated from the Chinese Camptotheca acuminate, whose Chinese name is Xi Su (Joy Tree). Although camptothecin did not become a major cancer chemotherapy agent, it served as the prototype for irinotecan (Camptosar) and topotecan (Hycamtin), two very important cancer drugs, which were more soluble. Wani described the isolation of camptothecin as “the most exciting scientific event in my life.” He once commented on Wall’s contribution to the Research Triangle Institute “the institute was nothing but four ‘walls.’ It was not until the fifth ‘Wall’ arrived that the chemistry, in the form of the Natural Products Laboratory, started moving.” To isolate the active principle from the Pacific yew, chemists in Wall’s laboratory ground the stem bark into a fine powder and extracted the active ingredients with ethanol. Guided by a process called “bioactivity directed fractionation,” they were able to purify the crude extract so that the cytotoxic potency increased 1,000-fold. In 1967, they isolated the active principle as white crystals in a 0.014% yield from the dry bark. The molecule was later determined to have a molecular formula of C47H51NO4 and a molecular weight of 839. Due to the complexity of the molecule, deciphering Taxol’s structure was a long and frustrating endeavor. Wall directed Wani to work on it with lower priority, tinkering with it only once in a while. In the end, Wall’s persistence and Wani’s patience paid off. Importantly, Wani’s expertise in recrystallization was crucial for obtaining a single crystal suitable for X-ray crystallography, a key technique for deciphering the structure of Taxol in combination with proton nuclear magnetic resonance (NMR) spectroscopy. Securing proof of concept from the Phase II trial was a triumph for the NCI, which had overseen the clinical development of Taxol from the beginning, more than 20 years previously. However, the NCI was not set up to take on the expensive and long Phase III trials that involved numerous disciplines such as oncology, pharmaceutical science, pharmacokinetics and drug metabolism, statistics, drug safety science, and more. Following a competitive selection process, Bristol-Myers Squibb (BMS), the only major U.S. pharmaceutical company to have made a bid, was awarded the molecule under the Cooperative Research and Development Agreement (CRADA) with the NCI in 1991. At the time, the commercial potential of Taxol had not fully manifested for the breast cancer indication. The NCI’s choice of BMS over the French company Rhône-Poulenc made sense because Rhône-Poulenc already had a competing drug, Taxotere, discovered by Frenchman Pierre Potier, who invented Taxotere by a minor modification of Taxol (replacing the benzoyl group on Taxol with a tert-butoxylcarbonyl group). Taxotere, more potent than Taxol, had annual sales of $1.54 billion in 2003. The FDA approved Taxol for use in refractory ovarian cancer in December 1992, for breast cancer in 1994, and later for non-small-cell lung cancer and Kaposi’s sarcoma. By 2000, Taxol was the best-selling cancer drug of all time, with annual sales of $1.6 billion. The raw material for isolating Taxol became an extremely contentious issue in the late 1980s and early 1990s. Because it takes 100 years for the Pacific yew trees to become useful in terms of Taxol content, harvesting the yew trees for stem bark meant destruction of the forest. To make matters worse, the forest that harbors the yew trees is home to the endangered spotted owl. The battle raged for many years between the environmentalists, who wanted to save the trees, and cancer patients and oncologists, who were eager to get access to the drug. It ended abruptly in early 1993, when BMS started to use a semisynthetic route to make Taxol that did not use the Pacific yew trees at all. Instead, they extracted 10-deacetylbacctin (10-DAB) from Taxus baccata, the English yew, a common ornamental plant. BMS then used the side-chain installation process patented by Robert Holton at Florida State University to make Taxol. More than 3 tons of English yew needles, a renewable source, must be collected and processed in order to produce 1 kilogram of 10-DAB. The switch was worthwhile, because Taxol had cost as much as $600,000 per kilogram. Even now, it is still nearly $400,000 per kilogram. The worldwide market for Taxol is about 400 kilograms per year. Currently, Taxol is produced in large fermentation tanks using plant cells. Taxol is one of the best weapons in an oncologist’s arsenal, and many even refer to times prior to 1994 as the “pre-Taxol” era. Interestingly, a major new use of Taxol is as a coating on stents to prevent restenosis, providing a larger market for its use than as just an anticancer drug. This was resulted in the mass exploration of the plants with anticancer potential. Till date, among all the therapeutic agents approved by FDA, 40% are linked to natural sources directly or indirectly. Isolation of podophyllotoxin and several other compounds (known as lignans) from the common mayapple (Podophyllum peltatum) ultimately led to the development of drugs used to treat testicular and small cell lung cancer. The National Cancer Institute (NCI) has screened approximately 35,000 plant species for potential anticancer activities. Among them, about 3,000 plant species have demonstrated reproducible anticancer activity. Many studies have focused on the chemoprotective properties of plants such as anticarcinogenic properties of Abrus precatorius on Yoshida sarcoma in rats, fibrosarcoma in mice and ascites tumor cells. Similarly, Dhar et al. have examined the anticancer properties of Albizzia lebbeck on sarcoma in mice and Alstonia scholaries on benzo[a]pyrene-induced forestomach carcinoma in humans. Other plants that have shown anticarcinogenic properties include Anacardium occidentale in hepatoma, Asparagus racemosa in human epidermoid carcinoma, Boswellia serrata in human epidermal carcinoma of the nasopharynx, Erthyrina suberosa in sarcoma, Euphorbia hirta in Freund virus leukemia, Gynandropis pentaphylla in hepatoma, Nigella sativa in Lewis lung carcinoma, Peaderia foetida in human epidermoid carcinoma of the nasopharynx, Picrorrhiza kurroa in hepatic cancers, and Withania somnifera in various tumors. The anticancer characteristics of a number of plants are still being actively researched and some have shown promising results. By Babayeva N. References: Li, Jie Jack. (2006). Laughing gas, viagra, and lipitor: the human stories behind the drugs we use: Oxford University Press https://www.who.int/cancer/en/ Arpita R., Shruti A., and Navneeta B. (2017). A Review on Medicinal Plants against Cancer. J Plant Sci Agric Res. Vol.2 No.1:008 Shakeel I., Naveed A., Muhammad S. K., Abdul H., Muhammad I., Muhammad A. A., Sajid A., Muhammad A. (2018). Plant derived anticancer agents: A green approach towards skin cancers. Biomedicine & Pharmacotherapy, 103 (2018) 1643–1651