Cancer chemoprevention with natural agents: Where we are now?
Chemoprevention is defined as a means of cancer control in which the occurrence of clinical cancer can be prevented, slowed or reversed by the administration of one or more synthetic or naturally occurring biologic compounds [1-5]. In contrast to cancer treatment, the goal of chemoprevention is to reduce the incidence of clinical cancer. Preventing the onset of clinical cancer is desirable because therapy for established cancer is not uniformly effective. Chemoprevention also implies the prevention of precancerous lesions, which are called pre-invasive neoplasia, dysplasia, or intraepithelial neoplasia, depending on the organ system [5, 6]. The process of carcinogenesis involves the accumulation of multiple sequential genetic alterations and mutations that transforms a normal cell to a malignant cell capable of stromal invasion and ultimately metastasis. Carcinogenesis is traditionally separated into three phases: initiation, promotion and progression [6]. The initiation stage is relatively rapid and involves carcinogen binding and damage to DNA. The promotion phase is reversible, as tumor promoters act as mitogens to induce clones of initiated cells to expand. The promotion phase is a consequence of the functional loss of regulatory proteins and cellular checkpoints important for cellular proliferation and apoptosis. Progression is a stage in which phenotypically and genotypically altered cells develop irreversible macroscopic and microscopic changes. Because initiation and progression are irreversible, the promotion phase of carcinogenesis may be the best target for chemoprevention.