What if tamoxifen (ICI 46,474) had been found to produce rat liver tumors in 1973? A personal perspective

Serendipity in the search for "morning-after pills" led to clomiphene for the induction of ovulation

The approval of the oral contraceptive on June 23, 1960, by the United States Food and Drug Administration (FDA), changed society forever. For the first time, a pill designed and tested by men, supported by influential women, allowed women to control their fertility. For the first time, the FDA approved a medicine to be taken by humans without a disease. The chance discovery of a new group of medicines called nonsteroidal antiestrogens, created an opportunity for the pharmaceutical industry. These compounds were shown to be postcoital antifertility agents in rats and mice. In the 1960s, the development of a "morning-after pill" would have had an enormous market. Numerous companies focused discovery efforts to evaluate the development of their patented nonsteroidal antiestrogens: Merrell (clomiphene), Upjohn (U-11,100A), and ICI Pharmaceutical Division (ICI46,474). However, the antifertility effects of antiestrogens in rats and mice does not mean that the new medicine would be an antifertility agent in women. In this case, clomiphene did exactly the opposite of what it was predicted to prevent. Clomiphene became the first medicine to induce ovulation in subfertile women. This article describes the twists and turns of drug discovery and development over the past half century. The conclusion emphasizes the evolution of drug development over decades, based on fashions in medical research and discoveries in clinical pharmacology. As a result, new uses for old molecules, that started life as "nonsteroidal antiestrogens," have revolutionized women's health as members of the new group of medicines called selective estrogen receptor modulators.

The evolution of nonsteroidal antiestrogens to become selective estrogen receptor modulators

The discovery of the first nonsteroidal antiestrogen ethamoxytriphetol (MER25) in 1958, opened the door to a wide range of clinical applications. However, the finding that ethamoxytriphetol was a "morning after" pill in laboratory animals, energized the pharmaceutical industry to discover more potent derivatives. In the wake of the enormous impact of the introduction of the oral contraceptive worldwide, contraceptive research was a central focus in the early 1960's. Numerous compounds were discovered e.g., clomiphene, nafoxidine, and tamoxifen, but the fact that clinical studies showed no contraceptive actions, but, in fact, induced ovulation, dampened enthusiasm for clinical development. Only clomiphene moved forward to pioneer an application to induce ovulation in subfertile women. The fact that all the compounds were antiestrogenic made an application in patients to treat estrogen responsive breast cancer, an obvious choice. However, toxicities and poor projected commercial returns severely retarded clinical development for two decades. In the 1970's a paradigm shift in the laboratory to advocate long term adjuvant tamoxifen treatment for early (non-metastatic) breast cancer changed medical care and dramatically increased survivorship. Tamoxifen pioneered that paradigm shift but it became the medicine of choice in a second paradigm shift for preventing breast cancer during the 1980's and 1990's. This was not surprising as it was the only medicine available and there was laboratory and clinical evidence for the eventual success of this application. Tamoxifen is the first medicine to be approved by the Food and Drug Administration (FDA) to reduce the risk of breast cancer in women at high risk. But it was the re-evaluation of the toxicology of tamoxifen in the 1980's and the finding that there was both carcinogenic potential and a significant, but small, risk of endometrial cancer in postmenopausal women that led to a third paradigm shift to identify applications for selective estrogen receptor (ER) modulation. This idea was to establish a new group of medicines now called selective ER modulators (SERMs). Today there are 5 SERMs FDA approved (one other in Europe) for applications ranging from the reduction of breast cancer risk and osteoporosis to the reduction of menopausal hot flashes and improvements in dyspareunia and vaginal lubrication. This article charts the origins of the current path for progress in women's health with SERMs.

New insights into the metabolism of tamoxifen and its role in the treatment and prevention of breast cancer

The metabolism of tamoxifen is being redefined in the light of several important pharmacological observations. Recent studies have identified 4-hydroxy N-desmethyltamoxifen (endoxifen) as an important metabolite of tamoxifen necessary for antitumor actions. The metabolite is formed through the enzymatic product of CYP2D6 which also interacts with specific selective serotonin reuptake inhibitors (SSRIs) used to prevent the hot flashes observed in up to 45% of patients taking tamoxifen. Additionally, the finding that enzyme variants of CYP2D6 do not promote the metabolism of tamoxifen to endoxifen means that significant numbers of women might not receive optimal benefit from tamoxifen treatment. Clearly these are particularly important issues not only for breast cancer treatment but also for selecting premenopausal women, at high risk for breast cancer, as candidates for chemoprevention using tamoxifen.

Tamoxifen (ICI46,474) as a targeted therapy to treat and prevent breast cancer

Antihormonal therapy targeted to the oestrogen receptor (OER) is recognized as a significant advance in the treatment and prevention of breast cancer. However, the research method used to achieve the current successes seen in the clinic was not linear but was based on the changing fashions in research and the application of appropriate testing models. The discovery and investigation of nonsteroidal antioestrogens by the pharmaceutical industry during the 1960s was initially an exciting prospect for clinical development. The drugs were superb antifertility agents in laboratory animals, so the prospect of marketing a 'morning after' pill was a high priority. Unfortunately, the reproductive endocrinology of the rat was found to be completely different from that of the human. Antioestrogens, in fact, improved fertility by inducing ovulation in subfertile women so much of the drug development was discontinued. The successful reinvention of ICI46,474 from its origins as a failed contraceptive to a pioneering breast cancer treatment targeted to the OER presaged the development of the current menu of medicines targeted to a range of different survival mechanisms in cancer cells.

Tamoxifen: a most unlikely pioneering medicine

For more than 25 years, tamoxifen has been the gold standard for the endocrine treatment of all stages of oestrogen-receptor-positive breast cancer, and the World Health Organization lists tamoxifen as an essential drug for the treatment of breast cancer. It is estimated that more than 400,000 women are alive today as a result of tamoxifen therapy, and millions more have benefited from palliation and extended disease-free survival. Interestingly, tamoxifen also became the first cancer chemopreventive approved by the Food and Drug Administration (FDA) for the reduction of breast-cancer incidence in both pre- and post-menopausal women at high risk. However, 40 years ago, it was hard to imagine that a non-toxic targeted treatment for breast cancer could be developed at all.

Immunoprevention of cancer poses a challenge to pharmacological research

The evolving understanding of the molecular mechanisms of carcinogenesis establishes that the long latency period would offer numerous opportunities for intervention before the final step of fully developed malignancy has been reached. Immunoprevention of cancer may be a new approach to cancer control, by eliminating the cellular minimal deviations that are seen in the early phases of carcinogenesis. Available results of immunotherapy of preneoplastic lesions and data on anticarcinogenesis with immunoregulators in experimental models are in good accordance with theoretical expectations. However, clinical research on immunoregulators still focuses on the treatment of advanced cancer. The major problem hampering a wider application of immunoregulators in cancer prevention is the possibility of potential adverse effects, which are largely unknown. The promotion of future research in this area is essential. Moreover, the ethical issues of intervention trials in cancer must be discussed, by considering the potential psychological and social consequences.

A realistic clinical perspective of tamoxifen and endometrial carcinogenesis

Tamoxifen has been the endocrine treatment of choice for all stages of breast cancer for nearly a decade. Millions of women are currently receiving tamoxifen worldwide, while large-scale randomised trials have been launched aiming to investigate the drug's merit as a preventive agent. However, there are now concerns about tamoxifen's potential carcinogenicity. The goal of this review is to address these concerns, re-evaluate the available data from laboratory biological models and those from clinical reports and put the whole issue into perspective. Our focus is the association between tamoxifen and the increased frequency of endometrial tumours, while key issues, such as the role of duration of tamoxifen therapy, are also addressed. Finally, we discuss the various monitoring strategies for early detection of endometrial lesions and pertinent problems most likely to be encountered by clinicians taking care of patients who are receiving tamoxifen.

Third annual William L. McGuire Memorial Lecture. "Studies on the estrogen receptor in breast cancer"--20 years as a target for the treatment and prevention of cancer

In 1973, McGuire and Chamness (In: O'Malley BW and Means AR (ed) Receptors for Reproductive Hormones, Plenum Press) summarized their work on the estrogen receptor in animal and human breast tumors, and in so doing described a target for therapeutic intervention. At that time there were no clinically useful antiestrogens, but the subsequent development of tamoxifen for breast cancer therapy has revolutionized the approach to treatment. Long-term adjuvant tamoxifen adjuvant therapy (i.e., greater than one year) has proven efficacy to enhance the survival of breast cancer patients. In addition, because there is an associated 40% decrease in contralateral breast cancer during adjuvant tamoxifen therapy and tamoxifen maintains bone density and reduces fatal myocardial infarction, clinical trials to test the worth of tamoxifen as a preventive for breast cancer in high risk women have started in the United States, United Kingdom, and Italy. Initial concerns that long-term tamoxifen causes endometrial cancer have been placed in perspective and analyzed by a review of the literature. Tamoxifen only doubles the normal risk of detecting endometrial cancer (i.e., to 2 per 1,000 tamoxifen-treated women per year), and 80% of these cases are early stage, good prognosis disease. Annual gynecological examinations and education are essential to provide reassurance for patients. The success of tamoxifen has encouraged the development of new antiestrogens to exploit the estrogen receptor as a therapeutic target. Droloxifene and TAT-59 mimic the metabolite 4-hydroxytamoxifen in having a high affinity for the estrogen receptor (Jordan et al, J Endocrinol 75:305, 1977). These drugs appear to have a pharmacological profile similar to tamoxifen. In contrast, the new pure antiestrogens have a distinct mechanism of action and will be valuable either as a first line therapy for advanced breast cancer or as a second line endocrine therapy after the failure of long-term adjuvant tamoxifen therapy. Finally, a new strategy is being developed to exploit the target site specific action of antiestrogens. Raloxifene, an antiestrogen with high affinity for the estrogen receptor but only weak estrogenicity for the uterus, prevents rat mammary tumorigenesis and maintains bone density. The drug is to be evaluated as a treatment for osteoporosis, but may also prevent the development of breast and endometrial cancer in a broad group of treated subjects. The identification of the estrogen receptor as a target for therapeutic opportunities has proved to be extremely beneficial for the control of breast cancer and has the added potential to control osteoporosis and coronary heart disease in women.