The era of cancer chemotherapy began in the 1940s with the first use of nitrogen mustards and folic acid antagonist drugs. The targeted therapy revolution has arrived, but many of the principles and limitations of chemotherapy discovered by the early researchers still apply.
Beginnings The beginnings of the modern era of cancer chemotherapy can be traced directly to the German introduction of chemical warfare during World War I. Among the chemical agents used, mustard gas was particularly devastating. Although banned by the Geneva Protocol in 1925, the advent of World War II caused concerns over the possible re-introduction of chemical warfare. Such concerns led to the discovery of nitrogen mustard, a chemical warfare agent, as an effective treatment for some types of cancer.
In 1925, Marie Wreschner and Laurence Farmer Loeb at the Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry in Germany patented a method for treating cancer through the encapsulation of radioactive materials. In their patent, granted by the U.S. Patent and Trademark Office on 1 April 1930, Wreschner described the adsorption of uranyl compounds suspended in ferric hydroxide capable of emitting beta particles as a possible method for treating carcinoma. German chemist Herbert Freundlich later wrote to British colleagues that Wreschner's experiments saw the successful destruction of malignant tumors in animal tests. Wreschner had wished to continue to human trials, but was dismissed from the institute after the Nazis purged Jews from academia in 1933.
Two pharmacologists from the Yale School of Medicine, Louis S. Goodman and Alfred Gilman, were recruited by the US Department of Defense to investigate potential therapeutic applications of chemical warfare agents. Goodman and Gilman observed that mustard gas was too volatile an agent to be suitable for laboratory experiments. They exchanged a sulfur molecule for nitrogen and had a more stable compound in nitrogen mustard. Goodman and Gilman hypothetized that this agent could be used to treat lymphoma, a tumor of lymphoid cells. They first set up an animal model by establishing lymphomas in mice and demonstrated they could treat them with mustard agents. Next, in collaboration with a thoracic surgeon, Gustaf Lindskog, they injected a related agent, mustine (the prototype nitrogen mustard anticancer chemotherapeutic), into a patient with non-Hodgkin's lymphoma. The patient, a Polish immigrant to Connecticut known in literature only as JD, received his first injections on August 27, 1942 at 10 a.m. The doctors observed a dramatic reduction in the patient's tumor masses. Although the effect lasted only a few weeks, and the patient had to return for another set of treatment, that was the first step to the realization that cancer could be treated by pharmacological agents. The patient ultimately died of cancer on December 1, 1942, 96 days after his first dose. Publication of the first clinical trials was reported in 1946 in The New York Times.
A year into the start of their research, in December 1943, a German air raid in Bari, Italy led to the exposure of more than 1000 people to the SS John Harvey's secret cargo composed of mustard gas bombs. Dr. Stewart Francis Alexander, a lieutenant colonel who was an expert in chemical warfare, was subsequently deployed to investigate the aftermath. Autopsies of the victims suggested that profound lymphoid and myeloid suppression had occurred after exposure. In his report, Dr. Alexander theorized that since mustard gas all but ceased the division of certain types of somatic cells whose nature was to divide fast, it could also potentially be put to use in helping to suppress the division of certain types of cancerous cells.
After World War II was over, the Yale group's studies and the Bari incident eventually converged prompting a search for other similar compounds.
Antifolates
Shortly after World War II, a second approach to drug therapy of cancer began. Sidney Farber, a pathologist at Harvard Medical School, studied the effects of folic acid on leukemia patients. Folic acid, a vitamin crucial for DNA metabolism (the significance of DNA was not known at that time), had been discovered by Lucy Wills, when she was working in India, in 1937. It seemed to stimulate the proliferation of acute lymphoblastic leukemia (ALL) cells when administered to children with this cancer. In one of the first examples of rational drug design (rather than accidental discovery), Farber used folate analogues synthesized by Harriett Kiltie and Yellapragada Subbarow of Lederle Laboratories. These analogues — first aminopterin and then amethopterin (now methotrexate) were antagonistic to folic acid, and blocked the function of folate-requiring enzymes. When administered to children with ALL in 1948, these agents became the first drugs to induce remission in children with ALL. Remissions were brief, but the principle was clear — antifolates could suppress proliferation of malignant cells, and could thereby re-establish normal bone-marrow function. Farber met resistance to conducting his studies at a time when the commonly held medical belief was that leukemia was incurable, and that the children should be allowed to die in peace. Afterwards, Farber's 1948 report in the New England Journal of Medicine was met with incredulity and ridicule.
In 1947, Major League Baseball Hall of Famer Babe Ruth, who was battling nasopharyngeal cancer, became one of the first human subjects of pteroyl triglutamate (also known by its brand name Teropterin, and similar to aminopterin) treatment. Dr. Richard Lewisohn of Mount Sinai Hospital in New York administered the drug, and over the course of several months, Ruth's condition began to improve. However, Ruth died the following year.
In 1951, Jane C. Wright demonstrated the use of methotrexate in solid tumors, showing remission in breast cancer. Wright's group was the first to demonstrate use of the drug in solid tumors, as opposed to leukemias, which are cancers of the marrow. Several years later at the National Cancer Institute, Roy Hertz and Min Chiu Li then demonstrated complete remission in women with choriocarcinoma and chorioadenoma in 1956, discovering that methotrexate alone could cure choriocarcinoma (1958), a germ-cell malignancy that originates in trophoblastic cells of the placenta. In 1960 Wright et al. produced remissions in mycosis fungoides.
6-MP Joseph Burchenal, at Memorial Sloan-Kettering Cancer Center in New York, with Farber's help, started his own methotrexate study and found the same effects. He then decided to try to develop anti-metabolites in the same way as Farber, by making small changes in a metabolite needed by a cell to divide. With the help of George Hitchings and Gertrude Elion, two pharmaceutical chemists who were working at the Burroughs Wellcome Co. in Tuckahoe, many purine analogues were tested, culminating in the discovery of 6-mercaptopurine (6-MP), which was subsequently shown to be a highly active antileukemic drug.
Vinca Alkaloids The Eli Lilly natural products group found that alkaloids of the Madagascar periwinkle (Vinca rosea), originally discovered in a screen for anti-diabetic drugs, blocked proliferation of tumour cells. The antitumour effect of the vinca alkaloids (e.g. vincristine) was later shown to be due to their ability to inhibit microtubule polymerization alkaloys, and therefore cell division.
Cancer Chemotherapy National Service Center
The NCI, headed by Dr. John R. Heller Jr., lobbied the United States Congress for financial support for second-generation chemotherapy research. In response, Congress created a Cancer Chemotherapy National Service Center (CCNSC) at the NCI in 1955. This was the first federal programme to promote drug discovery for cancer – unlike now, most pharmaceutical companies were not yet interested in developing anticancer drugs. The CCNSC developed the methodologies and crucial tools (like cell lines and animal models) for chemotherapeutic development.
Combination chemotherapy In 1965, a major breakthrough in cancer therapy occurred. James F. Holland, Emil Freireich, and Emil Frei hypothesized that cancer chemotherapy should follow the strategy of antibiotic therapy for tuberculosis with combinations of drugs, each with a different mechanism of action. Cancer cells could conceivably mutate to become resistant to a single agent, but by using different drugs concurrently it would be more difficult for the tumor to develop resistance to the combination. Holland, Freireich, and Frei simultaneously administered methotrexate (an antifolate), vincristine (a Vinca alkaloid), 6-mercaptopurine (6-MP) and prednisone — together referred to as the POMP regimen — and induced long-term remissions in children with acute lymphoblastic leukaemia (ALL). With incremental refinements of original regimens, using randomized clinical studies by St. Jude Children's Research Hospital, the Medical Research Council in the UK (UKALL protocols) and German Berlin-Frankfurt-Münster clinical trials group (ALL-BFM protocols), ALL in children has become a largely curable disease.
This approach was extended to the lymphomas in 1963 by Vincent T. DeVita and George Canellos at the NCI, who ultimately proved in the late 1960s that nitrogen mustard, vincristine, procarbazine and prednisone — known as the MOPP regimen — could cure patients with Hodgkin's and non-Hodgkin's lymphoma.
Currently, nearly all successful cancer chemotherapy regimens use this paradigm of multiple drugs given simultaneously, called combination chemotherapy or polychemotherapy.
Adjuvant therapy
As predicted by studies in animal models, drugs were most effective when used in patients with tumours of smaller volume. Another important strategy developed from this — if the tumour burden could be reduced first by surgery, then chemotherapy may be able to clear away any remaining malignant cells, even if it would not have been potent enough to destroy the tumor in its entirety. This approach was termed "adjuvant therapy".
Emil Frei first demonstrated this effect — high doses of methotrexate prevented recurrence of osteosarcoma following surgical removal of the primary tumour. 5-fluorouracil, which inhibits thymidylate synthase, was later shown to improve survival when used as an adjuvant to surgery in treating patients with colon cancer. Similarly, the landmark trials of Bernard Fisher, chair of the National Surgical Adjuvant Breast and Bowel Project, and of Gianni Bonadonna, working in the Istituto Nazionale Tumori di Milano, Italy, proved that adjuvant chemotherapy after complete surgical resection of breast tumours significantly extended survival — particularly in more advanced cancer.
Drug discovery at the NCI and elsewhere
Zubrod's initiatives In 1956, C. Gordon Zubrod, who had formerly led the development of antimalarial agents for the United States Army, took over the Division of Cancer Treatment of the NCI and guided development of new drugs. In the two decades that followed the establishment of the NCCSC, a large network of cooperative clinical trial groups evolved under the auspices of the NCI to test anticancer agents. Zubrod had a particular interest in natural products, and established a broad programme for collecting and testing plant and marine sources, a controversial programme that led to the discovery of taxanes (in 1964) and camptothecins (in 1966). Both classes of drug were isolated and characterized by the laboratory of Monroe Wall at the Research Triangle Institute.
Taxanes