Advances in Structural Diversity, Pharmacological Activities, and Drug Development of β-Carboline Alkaloids

β-Carboline alkaloids are natural products built around an indole-pyridine tricyclic core. Their relatively simple structure and ease of modification have drawn lasting interest from medicinal chemists. This review organizes their pharmacological activities according to structural types: simple β-carbolines, those with substitutions at the C-1, N-2/N-9 or C-3 positions, cyclized derivatives, and bis-β-carboline dimers. For each type, we discuss reported mechanisms, such as inhibition of CDK4, HDAC, and topoisomerases. The available data point to promising applications in cancer, inflammatory disorders, and infectious diseases. Overall, the structural diversity of β-carbolines makes them a valuable source of lead compounds for drug discovery.