Plant secondary metabolites in cancer chemotherapy: where are we?

Nowadays we have a number of chemical and biological agents at our disposal to treat chronic pathologies such as cancer. Although most drugs display significant activity, thus improving the clinical outcome, side-effects and emergence of resistances cannot be looked down. From an historical point of view, higher plants have been very important in the search of new therapeutic agents and they were in the origin of the first medicines used in human health. The contribute of plants to treat pathologies such as cancer is far from being over, mainly due to the high number of new drugs that are currently being evaluated in clinical trials. Metabolomics-based studies have rendered several new chemical entities, some of them with remarkable complex chemistry, which sometimes results in novel mechanisms of action, higher potency and lower toxicity. In this review, we will focus the most important plant-derived classes of compounds in clinical use, as well as those currently in clinical trials, with special focus on vinca alkaloids, taxanes, combretastatins, podophylotoxins and camptothecins. The molecular mechanism of action and spectrum of activity will also be discussed.