Background: Quinoline-derived metabolites exhibit notable chemical complexity. What causes minor structural alterations to induce significant changes in disease outcomes? Historically, eclipsed by more straightforward scaffolds, these chemicals serve as a dynamic hub in tryptophan metabolism, linking immunomodulation, excitotoxicity, and cancer. However, many of these compounds struggle to cross the blood–brain barrier, and we still do not fully understand how certain structural changes affect their bioavailability or off-target effects. Thus, contemporary research highlights halogenation, esterification, and computational modeling to enhance structure–activity relationships. Summary: This narrative review emphasizes the integration of rational drug design, multi-target ligands, and prodrug methods in enhancing quinoline scaffolds. We explore each molecule’s therapeutic promise, refine each scaffold’s design, and develop each derivative to maximize clinical utility. Translating these laboratory findings into clinical practice, however, remains a formidable challenge. Conclusions: Through the synthesis of findings regarding NMDA receptor antagonism, improved oral bioavailability, and reduced metabolic instability, we demonstrate how single-site changes might modulate excitotoxicity and immunological signaling. Advancing quinoline-based medicines will yield significant advancements in neurology, psychiatry, and oncology. This enlarged framework fosters collaborative discovery, engages various audiences, and advances the field towards next-generation disease-modifying therapies. Robust preclinical validation, patient classification, and comprehensive toxicity evaluations are crucial stages for achieving these extensive endeavors and fostering future therapeutic discoveries globally.

The central nervous system (CNS) is the final frontier in drug delivery because of the blood–brain barrier (BBB), which poses significant barriers to the access of most drugs to their targets. Kynurenic acid (KYNA), a tryptophan (Trp) metabolite, plays an important role in behavioral functions, and abnormal KYNA levels have been observed in neuropsychiatric conditions. The current challenge lies in delivering KYNA to the CNS owing to its polar side chain. Recently, C-3 side chain-modified KYNA analogs have been shown to cross the BBB; however, it is unclear whether they retain the biological functions of the parent molecule. This study examined the impact of KYNA analogs, specifically, SZR-72, SZR-104, and the newly developed SZRG-21, on behavior. The analogs were administered intracerebroventricularly (i.c.v.), and their effects on the motor domain were compared with those of KYNA. Specifically, open-field (OF) and rotarod (RR) tests were employed to assess motor activity and skills. SZR-104 increased horizontal exploratory activity in the OF test at a dose of 0.04 μmol/4 μL, while SZR-72 decreased vertical activity at doses of 0.04 and 0.1 μmol/4 μL. In the RR test, however, neither KYNA nor its analogs showed any significant differences in motor skills at either dose. Side chain modification affects affective motor performance and exploratory behavior, as the results show for the first time. In this study, we showed that KYNA analogs alter emotional components such as motor-associated curiosity and emotions. Consequently, drug design necessitates the development of precise strategies to traverse the BBB while paying close attention to modifications in their effects on behavior.

Kynurenic acid (KYNA), a putative neuroprotective agent, modulates glutamatergic pathways in schizophrenia and Parkinson’s disease but is limited by acute motor activity impairments (e.g., ataxia). Research leveraging animal disease models explores its structure–activity relationship to enhance therapeutic efficacy while mitigating adverse effects, addressing global neuropsychiatric disorders affecting over 1 billion people. Structural analogs of KYNA (SZR-72, SZR-73, and SZR-81) were designed to uncouple therapeutic benefits from motor toxicity; yet, systematic comparisons of their acute behavioral profiles remain unexplored. Here, we assess the motor safety, time-dependent effects, and therapeutic potential of these analogs in mice. Using acute intracerebroventricular dosing, we evaluated motor coordination (rotarod), locomotor activity (open-field), and stereotypic behaviors. KYNA induced significant ataxia and stereotypic behaviors at 15 min, resolving by 45 min. In contrast, all analogs avoided acute motor deficits, with SZR-73 maintaining baseline rotarod performance and eliciting a delayed decrease in ambulation and inquisitiveness in open-field assays. These findings demonstrate that the structural optimization of KYNA successfully mitigates motor toxicity while retaining neuromodulatory activity. Here, we show that SZR-73 emerges as a lead candidate, combining transient therapeutic effects with preserved motor coordination. This study advances the development of safer neuroactive compounds, bridging a critical gap between preclinical innovation and clinical translation. Future work must validate chronic efficacy, disease relevance, and mechanistic targets to harness the full potential of KYNA analogs in treating complex neuropsychiatric disorders.

By being an antagonist of glutamate and other receptors, kynurenic acid serves as an endogenous neuroprotectant in several pathologies of the brain. Unfortunately, systemic administration of kynurenic acid is hindered by its low permeability through the blood–brain barrier. One possibility to overcome this problem is to use analogues with similar biological activity as kynurenic acid, but with an increased permeability through the blood–brain barrier. We synthesized six novel aminoalkylated amide derivatives of kynurenic acid, among which SZR-104 (N-(2-(dimethylamino)ethyl)-3-(morpholinomethyl)-4-hydroxyquinoline-2-carboxamide) proved to have the highest permeability through an in vitro blood–brain barrier model. In addition, permeability of SZR-104 was significantly higher than that of kynurenic acid, xanthurenic acid and 39B, a quinolone derivative/xanthurenic acid analogue. Since peripherally administered SZR-104 is able to inhibit epileptiform activity in the brain, we conclude that SZR-104 is a promising kynurenic acid analogue with good penetrability into the central nervous system.

This article provides an overview about specifically modified Mannich reactions where the process involves an ortho-quinone methide (o-QM) intermediate. The reactions are classified on the basis of the o-QM source followed by the reactant, e.g., the dienophile partner in cycloaddition reactions (C=C or C=N dienophiles) or by the formation of multicomponent Mannich adducts. Due to the important pharmacological activities of these reactive o-QM intermediates, special attention is paid to the biological activity of these compounds.

The binding affinity of Ru(η⁶-p-cymene) and Rh(η⁵-C₅Me₅) complexes of 8-hydroxyquinoline derivatives and oligopyridine ligands to albumin was studied by spectroscopic and separation techniques.