Terrestrial plant-derived anticancer agents and plant species used in anticancer research

Cancer is a major cause of death and the number of new cases, as well as the number of individuals living with cancer; is expanding continuously. Due to the enormous propensity of plants that synthesize mixtures of structurally diverse bioactive compounds, the plant kingdom is potentially a very diverse source of chemical constituents with tumor cytotoxic activity. Despite the successful utilization of few phytochemicals, such as vincristine and taxol, into mainstream cancer chemotherapy, commercial plant-derived anticancer formulations represent only one-fourth of the total repertoire of the available treatment options. Though significant progress has been made towards the characterization of isolated compounds and their structure-related activities, the complex composition of plant extracts, along with the lack of reproducibility of activity and the synergy between different, even unidentified, components of an extract, prohibits the full utilization of plants in pharmaceutical research. In this review, the results of an extensive literature survey on the anticancer properties of terrestrial plants, covering a thirty-five-year-long span (1970-2005) are presented. A total of 187 plant species, belonging to 102 genera and 61 families have been identified as an active or promising source of phytochemicals with antitumor properties, corresponding to a 41 percent increase during the last five years. Among them, only 15 species (belonging to ten genera and nine families) have been utilized in cancer chemotherapy at a clinical level, whereas the rest of the identified species are either active against cancer cell lines or exhibit chemotherapeutic properties on tumor-bearing animals under experimental conditions. Phenylpropanoids are the most widely distributed compounds (18 families), followed by terpenoids (14 families), and alkaloids (13 families). Analytical, species-specific information on bioactive constituents and target cancers is provided. The outlook of phytochemistry-based cancer therapy is discussed, particularly in the perspective of identifying immunomodulatory anticancer agents with minimal toxicity on healthy tissues.