Quinoline as a privileged scaffold in cancer drug discovery

Quinoline (1-azanaphthalene) is a heterocyclic aromatic nitrogen compound characterized by a double-ring structure that contains a benzene ring fused to pyridine at two adjacent carbon atoms. Quinoline compounds are widely used as "parental" compounds to synthesize molecules with medical benefits, especially with anti-malarial and anti-microbial activities. Certain quinoline-based compounds also show effective anticancer activity. This broad spectrum of biological and biochemical activities has been further facilitated by the synthetic versatility of quinoline, which allows the generation of a large number of structurally diverse derivatives. This includes numerous analogues derived from substitution of the quinoline ring system, and derivatization of quinoline ring structure. Quinoline and its analogs have recently been examined for their modes of function in the inhibition of tyrosine kinases, proteasome, tubulin polymerization and DNA repair. In this review, we have summarized our knowledge on quinoline compounds with respect to their anticancer activities, mechanisms of action, structure-activity relationship (SAR), and selective and specific activity against various cancer drug targets. In particular, we focus our review on in vitro and in vivo anticancer activities of quinoline and its analogs in the context of cancer drug development and refinement.