Quinolinones, also called quinolones, are a group of heterocyclic compounds with a broad spectrum of biological activities. These compounds occur naturally in plants and microorganisms but can also be obtained synthetically. The first synthesis of quinolinones took place at the end of the 19th century, and the most recent methods were published just a few years ago. They allow for obtaining an unlimited number of analogs differing in biological properties. In this review, we described the plethora of methods leading to quinolin-4-ones. Several of these compounds have been used as antibiotics for over four decades, but recently, their antiproliferative effects have been of particular interest to researchers. This review summarizes the experimental progress made in the synthetic development of various routes leading to quinoline-4-ones and presents an overview of the structures, their evolution, and their relation to activity.

Opioid peptide hybrids are multifunctional molecules in which an opioid pharmacophore is covalently integrated with a second opioid or non-opioid pharmacophore within a single molecular entity. Their development is driven by a major challenge in analgesic pharmacotherapy: although μ-opioid receptor (MOR) agonists remain among the most effective agents for the treatment of moderate-to-severe pain, their therapeutic use is limited by adverse effects and risks associated particularly with prolonged or high-dose exposure, including tolerance, constipation, respiratory depression, physical dependence, and abuse liability. Especially, chronic and neuropathic pain involves complex interactions among multiple receptor systems, neurotransmitter networks, and intracellular signaling pathways. Accordingly, hybrid peptide ligands provide a rational multitarget strategy aimed at achieving analgesia through the coordinated modulation of complementary pharmacological targets rather than selective modulation of a single receptor system. This approach may enhance analgesic efficacy and potentially improve the therapeutic profile of opioid-based treatments by reducing some of the dose-limiting adverse effects associated with conventional opioid analgesics. In this review, we discuss the functional and pharmacological interactions between opioid receptors and other receptor systems involved in pain perception and modulation, with emphasis on those for which peptide-based hybrid ligands with demonstrated antinociceptive activity have been developed.