Anticancer α-, γ-, and δ-carboline derivatives: structures, mechanisms of action, and SARs

Carboline consists of a pyridine ring fused with an indole skeleton, and it serves as a potential drug scaffold. Depending on the relative position of the pyridine N to the indole N, carbolines can be categorized into α-, β-, γ-, and δ-subfamilies. They have diverse pharmacological activities, such as anticancer, antimalaria, antibacterial, antifungal, anti-Alzheimer's disease, etc. Among them, β-carbolines are the most widely studied and are also well reviewed. Herein, we review the anticancer activity of α-, γ-, and δ-carboline natural products and their synthetic derivatives, as they provide new inroads for cancer therapy. Particularly, we highlight the new derivatives of α-, γ-, and δ-carboline with anticancer activity, and their anticancer mechanisms, as well as structure-activity relationship. Meanwhile, we propose strategies for the development of new α-, γ-, and δ-carboline derivatives for cancer therapy.