Chemotherapeutics can combat cancer cells efficiently, however, this effect is not selective as it can damage normal cells as well. Moreover, the long use of chemotherapeutics induces severe side effects and drug resistance, and so a dose reduction or therapy interruption is often required. In the light of the increased incidence of tumor resistance among many cancer patients, there is an urgent need to develop new therapeutic agents or adjuvants that can tackle or reverse resistance mechanisms in different types of cancer, enhancing the success of chemotherapeutic outcomes and saving millions of dollars in the development of novel chemotherapeutic agents.

Many natural products extracted from medicinal plants, marine bioproducts, and arthropods have potent anticancer effects. Natural product-chemotherapeutic combined treatment has become broadly applied in cancer therapy aiming to accomplish a co-operative effect, reducing the dosage, side effects, and drug resistance of standard anticancer drugs. However, the interactions between different compounds could lead to either an agonistic or antagonistic effect, as they can be affected by several biological factors. This could be considered as most anticancer drugs act through the triggering of oxidative stress, which could be associated with the development of drug resistance and can damage healthy cells. On the other hand, most natural products have potent antioxidant activities favoring their use as adjuvants to anticancer drugs to minimize the effects of oxidative damage. The potent chemotherapeutic agent could not only kill cancer cells but also prevent their invasion and metastasis. Most mortality from cancer may be related to metastasis. However, several natural products have anti-migratory and anti-angiogenesis potential against a large variety of cancer cells. Generally, natural products can act as chemotherapeutic adjuvants through three mechanisms: direct enhancement of the anticancer potential through sensitizing cancer cells to be more responsive to standard anticancer agents, reduction of drug resistance through minimizing drug efflux and increasing the concentration of anticancer drugs within cancer cells, and alleviation of toxicity triggered by anticancer drugs.

The aim of this Special Issue is to focus on novel research exploring the potential anticancer activities of several natural products that have been reported to disturb the microenvironment of tumor cells. In cancer research, natural products have played an important role in the development of clinically relevant therapeutic agents, such as taxols, vinblastine, and vincristine. We welcome both original research and review articles.

Potential topics include but are not limited to the following:

• Synergistic effects of natural products when combined with chemotherapeutic agents
• Natural products as biological tools to study resistance mechanisms in different types of cancer
• Natural products and chemotherapeutic agents combined therapy to inhibit the metastasis of cancer cells
• Natural products acting as chemotherapeutic drug sensitizers
• Evidence-based traditional medicine in the combination of chemotherapeutic agents as new tools to manage cancer
• Natural products alleviating the toxicity of chemotherapeutic agents
• Clinical observations that support the effect of natural remedies on patients under chemotherapies
• Evaluation of the toxicity of natural product–anticancer drug interactions
• Effect of newly discovered natural products on cancer
• Novel approaches to minimize chemotherapeutic agent side effects