Historical Review: Problematic Malaria Prophylaxis with Quinine.

Quinine, a bitter-tasting, short-acting alkaloid drug extracted from cinchona bark, was the first drug used widely for malaria chemoprophylaxis from the 19th century. Compliance was difficult to enforce even in organized groups such as the military, and its prophylaxis potential was often questioned. Severe adverse events such as blackwater fever occurred rarely, but its relationship to quinine remains uncertain. Quinine prophylaxis was often counterproductive from a public health viewpoint as it left large numbers of persons with suppressed infections producing gametocytes infective for mosquitoes. Quinine was supplied by the first global pharmaceutical cartel which discouraged competition resulting in a near monopoly of cinchona plantations on the island of Java which were closed to Allied use when the Japanese Imperial Army captured Indonesia in 1942. The problems with quinine as a chemoprophylactic drug illustrate the difficulties with medications used for prevention and the acute need for improved compounds.

Chemoprevention--history and general principles.

Our current understanding of tumourigenesis suggests that cancer develops as a series of cumulative genetic and epigenetic derangements across time culminating in a clone of cells differing from its population of origin in terms of cellular identity, growth control, and its contextual relationship to its environment. Our increasing knowledge of the timing, sequence, frequency, and specific implications of these changes provides unique opportunities for earlier identification of aberrations and preventive interventions. Here we discuss the fundamentals of cancer prevention including the targets, cohorts, agents, endpoints, mechanistic biomarkers, designs, and strategies employed in preventive drug development. There have been many notable successes in this field such as the identification and development of tamoxifen and raloxifene for breast cancer risk reduction, instillational BCG and valrubicin for treatment of preinvasive bladder cancer, and a variety of topical and systemic agents that effectively treat preinvasive neoplastic lesions of the skin. A variety of null or negative developmental endeavours have occurred as well, including trials of beta-carotene for lung cancer prevention, nutritional modifications for colorectal adenoma prevention, and most recently, selenium and alpha-tocopherol for prostate cancer prevention. A third category of prevention trials can be summarized as investigationally successful, but not achieving regulatory success. The development of finasteride and dutasteride for prostate cancer prevention, and celecoxib for colorectal neoplasia prevention fall into this category. In less than four decades, cancer chemoprevention has transformed from a concept to an achievable reality.

Cancer chemoprevention by natural products: how far have we come?

Since ancient times, natural products, herbs and spices have been used for preventing several diseases, including cancer. The term chemoprevention was coined in the late 1970s and referred to the prevention of cancer by selective use of phytochemicals or their analogs. The field utilizes experimental carcinogenesis models to examine the efficacy of chemopreventive agents in a stage-specific manner. The concept of using naturally derived chemicals as potential chemopreventive agents has advanced the field dramatically. Throughout the years, a vast number of chemopreventive agents present in natural products have been evaluated using various experimental models. A number of them have progressed to early clinical trials. More recently, the focus has been directed towards molecular targeting of chemopreventive agents to identify mechanism(s) of action of these newly discovered bioactive compounds. Moreover, it has been recognized that single agents may not always be sufficient to provide chemopreventive efficacy, and, therefore, the new concept of combination chemoprevention by multiple agents or by the consumption of "whole foods" has become an increasingly attractive area of study. Novel technologies, such as nanotechnology, along with a better understanding of cancer stem cells, are certain to continue the advancement of the field of cancer chemoprevention in years to come.

Tamoxifen: catalyst for the change to targeted therapy.

In the early 1970s, a failed post-coital contraceptive, ICI 46,474, was reinvented as tamoxifen, the first targeted therapy for breast cancer. A cluster of papers published in the European Journal of Cancer described the idea of targeting tamoxifen to patients with oestrogen receptor positive tumours, and proposed the strategic value of using long-term tamoxifen therapy in an adjuvant setting with a consideration of the antitumour properties of the hydroxylated metabolites of tamoxifen. At the time, these laboratory results were slow to be embraced by the clinical community. Today, it is estimated that hundreds of thousands of breast cancer patients are alive today because of targeted long-term adjuvant tamoxifen therapy. Additionally, the first laboratory studies for the use of tamoxifen as a chemopreventive were published. Eventually, the worth of tamoxifen was tested as a chemopreventive and the drug is now known to have an excellent risk benefit ratio in high risk pre-menopausal women. Overall, the rigorous investigation of the pharmacology of tamoxifen facilitated tamoxifen's ubiquitous use for the targeted treatment of breast cancer, chemoprevention and pioneered the exploration of selective oestrogen receptor modulators (SERMs). This new concept subsequently heralded the development of raloxifene, a failed breast cancer drug, for the prevention of osteoporosis and breast cancer without the troublesome side-effect of endometrial cancer noted in post-menopausal women who take tamoxifen. Currently, the pharmaceutical industry is exploiting the SERM concept for all members of the nuclear receptor superfamily so that medicines can now be developed for diseases once thought impossible.