Current Pharmaceutical Design - Volume 24, Issue 36, 2018

The study of natural sources such as plants, microorganisms and marine organisms has developed interest among the scientific community in recent years for their extensive and diverse chemical composition and consequent biological potential. The search for antitumor compounds is among the lead causes that justify phytochemical studies. Although some natural products have served as FDA approved chemotherapeutic agents, there is still a demand for the search of compounds with those characteristics. The Plectranthus genus has long been used in traditional medicine, and scientific studies have already proven its undeniable value as a source of bioactive compounds. Diterpenes are the most prominent biologically active group of secondary metabolites present in this genus. In particular, abietane diterpenes have long been studied for their biological activities, namely their anti-tumoral potential. In this review, abietane diterpenes isolated from Plectranthus genus with antiproliferative, antitumoral or cytotoxic potential are reported. In addition, a correlation between this subclass of diterpenes with their mechanisms of cell death has been discussed.

Diterpenes present complex structure and due to their unique carbon skeleton and interesting biological activities, have been the focus of continuous studies for the development of new anticancer agents. Phorbol esters have been known for their activity against skin malignancies since ancient times. Taxol was first studied in melanoma cells, and recently, ingenol mebutate has been approved for the chemoprevention of melanoma in actinic keratosis patients. Therefore, there is a scope for research on this class of compounds. We here aim to review the relevant original research reporting on isolated diterpenes with cytotoxic and/or antitumoral activities upon melanoma cells. By collating and discussing the implications of past and current developments on diterpenes, we hope to steer further interest on this field and facilitate the drug discovery activities of the scientific community towards finding potential alternatives to current melanoma chemotherapy.

It is nowadays widely accepted that some tumors have a niche of cells endowed with stemness features, which may cause resistance to conventional anticancer therapies and relapse/recurrence of the malignancy. These cells are usually referred to as cancer stem cells (CSCs) and, different from normal cancer cells, are rather quiescent. Targeting CSCs is thus a highly challenging but promising strategy to counteract tumor growth, and to develop innovative anticancer agents. Here, we review the chemical, biological and multidisciplinary efforts that have been spent in targeting CSCsspecific signaling pathways Notch and Hedgehog (Hh) for anticancer drug discovery. In particular, the use of natural products as a valuable source of lead compounds or chemical biology tools is emphasized. Examples of natural products functionalization through semi-synthetic transformations or total syntheses, aimed at improving pharmacokinetics and/or pharmacodynamics properties of natural products in Notch or Hh inhibition, are provided as well.

The growing incidence of cancer, the toxic side-effects associated with conventional chemotherapeutic agents and the development of multidrug resistance (MDR) drive the search for novel and more effective drugs with multi-target activity and selectivity towards cancer cells. Stilbenes are a group of naturally occurring phenolic compounds of plant origin derived from the phenylpropanoid pathway that may exist as cis- or trans-isomers. Although the trans-isomer is the more common and stable configuration, resveratrol being a representative compound, cis-stilbenes are potent cytotoxic agents that bind to and inhibit tubulin polymerization, destabilizing microtubules. This review summarizes the chemistry and biological evaluation of cytotoxic stilbenes and their synthetic derivatives as promising antimitotic leads for cancer therapy, focusing on the most potent compounds, the combretastatins. Combretastatins isolated from the South African bushwillow Combretum caffrum are among the most potent antimitotic and vascular disrupting agents (VDAs) of natural origin. Preclinical studies have demonstrated their potent antitumor effects in a wide variety of tumors, both in vitro and in vivo, being currently under evaluation in phase 2 and phase 3 clinical trials for several types of solid tumors. Topics covered herein include synthetic medicinal chemistry, modes of action, structure-activity relationships (SAR), preclinical and clinical studies as VDAs in cancer therapy, either as single agents or in combination with cytotoxic anticancer drugs, antiangiogenic agents, or radiation therapy, and development of appropriate formulations based on nanocarriers (e.g., liposomes, nanoemulsions, polymeric, lipid and ceramic nanoparticles, carbon nanotubes) for improved bioavailability and targeted delivery of combretastatins to the tumor vasculature.

The exponential growth of cancer cases worldwide together with recent advances concerning the pathophysiological mechanisms of the disease at the molecular level led to a paradigm shift in chemotherapy, from monotherapy to targeted drug combination regimens. However, adverse effects and the emergence of multidrug resistance (MDR) limit the effectiveness of these therapies. In this context, hybrid combinations mixing anticancer drugs and bioactive phytochemical components from medicinal plants, or even plant extracts, that can act synergistically on multiple targets and signaling pathways represent a promising approach with the potential to expand the current therapeutic arsenal. This review aims to provide a synopsis on anticancer hybrid combinations based on their multi-target mechanisms and synergistic effects from an extensive literature search focusing mainly on publications from the last ten years. In most of these combinations, the phytochemical component was shown to enhance the anticancer activity of the chemotherapeutic agent and to sensitize chemoresistant tumors in several types of cancer. Hybrid combinations, due to synergistic interactions, are also associated with less severe adverse events since lower doses can be used to achieve the same therapeutic effect. Further preclinical and clinical studies are needed, as well as the development of an adequate regulatory framework, before hybrid combination therapy can be translated into clinical practice.

Medicinal value of natural products comes from symbiotic and competitive evolution in Earth’s complex biosphere. Billions of years of co-evolutionary interactions among millions of species have produced a large repertoire of defense molecules effective in fighting bacteria, viral, and fungal pathogens. Each species contains millions of different and useful molecules and new research technologies enabled the screening of molecules and complex mixtures from diverse biological sources. Traditional use of plants and other species led to the discovery of many bioactive compounds with various properties. In the last four decades, a large number of them were evaluated for their potential to treat cancer. Penetration of drugs into the cancer cell is necessary for their lethal pharmacological effect through interaction with intracellular target molecules. Increased activity of membrane efflux pumps reduces the intracellular drug accumulation, thereby preventing drug-target interactions. The discovery of the efflux transporter P-glycoprotein (P-gp) in multidrug resistant (MDR) cancer cells prompted the efforts in overcoming drug resistance by P-gp inhibition. The search for nontoxic anticancer agents from natural sources able to overcome MDR has been imperative in the field of drug design and discovery. Herein, we review various natural compounds from diverse sources emphasizing their potential to inhibit P-gp activity and/or expression.

Sesquiterpene lactones are naturally occurring compounds that have attracted considerable attention because to their vast array of biological activities. These plant-derived compounds contain the α-methylene-γ- butyrolactone functional group, which is the structural requirement for their pharmacological activities. Many of them exhibit cancer cell cytotoxicity and are promising anticancer agents through multiple mechanisms of action. Sesquiterpene lactones are alkylating agents that form covalent adducts in vivo and inhibit enzymes and key proteins. They are also potent apoptotic inducers in several cancer cells. This kind of cell death is recognized as a property that is useful for identifying anticancer drugs. This review focuses on the advances on cytotoxic sesquiterpene lactones and specifically the signal transduction pathways of cell death triggered in cancer cells.

Cancer is a major cause of morbidity and mortality worldwide. Chemotherapeutic agents currently used in cancer treatment are associated with severe side effects and development of resistance. Thus, there is a pressing need for novel and more potent anticancer drugs with high selectivity for tumor cells and reduced toxicity to normal tissue. Natural products remain an important source of bioactive compounds and drug prototypes that can lead to new and more effective antitumor agents. Coniferous plants are rich in abietane diterpenoids with a wide range of biological activities that provide useful templates for synthetic modification. Abietic acid and dehydroabietic acid (DHA), the major diterpenic resin acids from Pinus rosin, and dehydroabietylamine found in commercial disproportionated rosin amine, display antibacterial and antitumor properties. These compounds and their synthetic derivatives have been reported as promising anticancer agents with potent growth inhibitory activity against several types of human cancer cell lines, including breast, ovarian, prostate, colon, liver, lung and cervical carcinoma cells. Their mechanisms of action are diverse and include DNA binding, induction of apoptosis or oncosis, tubulin polymerization inhibition and disruption of intracellular cholesterol transport. This review covers the main aspects of natural rosin abietane diterpenoids (abietic acid, DHA and DHAA) and synthetic derivatives concerning their anti-proliferative, cytotoxic and antitumor activities, mechanisms of action and structure- activity relationships relevant for the development of novel anticancer agents for cancer chemotherapy.