Novel Antimicrobial 8-Hydroxyquinoline-Based Agents: Current Development, Structure-Activity Relationships, and Perspectives

The search for new antimicrobials is imperative due to the emergent resistance of new microorganism strains. In this context, revisiting known classes like 8-hydroxyquinolines could be an interesting strategy to discover new agents. The 8-hydroxyquinoline derivatives nitroxoline and clioquinol are used to treat microbial infections; however, these drugs are underused, being available in few countries or limited to topical use. After years of few advances, in the last two decades, the potent activity of clioquinol and nitroxoline against several targets and the privileged structure of 8-hydroxyquinoline nucleus have prompted an increased interest in the design of novel antimicrobial, anticancer, and anti-Alzheimer agents based on this class. Herein, we discuss the current development and antimicrobial structure-activity relationships of this class in the perspective of using the 8-hydroxyquinoline nucleus for the search for novel antimicrobial agents. Furthermore, the most investigated molecular targets concerning 8-hydroxyquinoline derivatives are explored in the final section.

Insights of 8-hydroxyquinolines: A novel target in medicinal chemistry

8-Hydroxyquinoline (8-HQ) is a significant heterocyclic scaffold in organic and analytical chemistry because of the properties of chromophore and is used to detect various metal ions and anions. But from the last 2 decades, this moiety has been drawn great attention of medicinal chemists due to its significant biological activities. Synthetic modification of 8-hydroxyquinoline is under exploration on large scale to develop more potent target-based broad spectrum drug molecules for the treatment of several life-threatening diseases such as anti-cancer, HIV, neurodegenerative disorders, etc. Metal chelation properties of 8-hydroxyquinoline and its derivatives also make these potent drug candidates for the treatment of various diseases. This review comprises 8-hydroxyquinoline derivatives reported in the literature in last five years (2016-2020) and we anticipate that it will assist medicinal chemists in the synthesis of novel and pharmacologically potent agents for various therapeutic targets, mainly anti-proliferative, anti-microbial, anti-fungal and anti-viral as well as for the treatment of neurodegenerative disorders.

Carnoquinolines Target Copper Dyshomeostasis, Aberrant Protein-Protein Interactions, and Oxidative Stress

Metal dysregulation, oxidative stress, protein modification, and aggregation are factors strictly interrelated and associated with neurodegenerative pathologies. As such, all of these aspects represent valid targets to counteract neurodegeneration and, therefore, the development of metal-binding compounds with other properties to combat multifactorial disorders is definitely on the rise. Herein, the synthesis and in-depth analysis of the first hybrids of carnosine and 8-hydroxyquinoline, carnoquinolines (CarHQs), which combine the properties of the dipeptide with those of 8-hydroxyquinoline, are reported. CarHQs and their copper complexes were characterized through several techniques, such as ESI-MS and NMR, UV/Vis, and circular dichroism spectroscopy. CarHQs can modulate self- and copper-induced amyloid-β aggregation. These hybrids combine the antioxidant activity of their parent compounds. Therefore, they can simultaneously scavenge free radicals and reactive carbonyl species, thanks to the phenolic group and imidazole ring. These results indicate that CarHQs are promising multifunctional candidates for neurodegenerative disorders and they are worthy of further studies.

Recent update on antibacterial and antifungal activity of quinoline scaffolds

Although most of the heterocycles have been reported to possess a significant pharmacological activity, only a few of them, namely quinoline derivatives, have exhibited the finest biological activities. Despite the few medicinal properties of the plain quinoline molecule, its derivatives exhibit diverse pharmacological properties such as anticancer, anti-inflammatory, antibacterial, antiviral, antifungal, antiprotozoal activities, and so on. The potential antimicrobial properties of the quinoline derivatives are evident from the decades of research on these derivatives. Owing to limitations like drug resistance, high cost, severe side effects, and less bioavailability of previously synthesized antimicrobial agents, these drugs have become obsolete in recent years. Hence, the design of more efficient antimicrobial drugs must be given topmost priority. A breakthrough in drug discovery is a must to prevent malevolent microbial diseases. Addressing all these issues, researchers have been continuously contributing to antimicrobial drug discovery. Herein, a short description of the pharmacology of antimicrobial agents such as antibacterials and antifungals synthesized recently is provided. The versatile derivatization of the quinoline moiety leading to significant antimicrobial potencies is discussed, considering the structure-activity relationship.