A new series of 4′-selenoadenosine-5′-N,N-dimethyluronamide derivatives as highly potent and selective human A₃ adenosine receptor (hA₃AR) antagonists, is described. The highly selective A₃AR agonists, 4′-selenoadenosine-5′-N-methyluronamides were successfully converted into selective antagonists by adding a second N-methyl group to the 5′-uronamide position. All the synthesized compounds showed medium to high binding affinity at the hA₃AR. Among the synthesized compounds, 2-H-N⁶-3-iodobenzylamine derivative 9f exhibited the highest binding affinity at hA₃AR. (K_i = 22.7 nM). The 2-H analogues generally showed better binding affinity than the 2-Cl analogues. The cAMP functional assay with 2-Cl-N⁶-3-iodobenzylamine derivative 9l demonstrated hA₃AR antagonist activity. A molecular modelling study suggests an important role of the hydrogen of 5′-uronamide as an essential hydrogen bonding donor for hA₃AR activation.

Background/Objectives: Fibroblast growth factor receptors (FGFRs) are frequently dysregulated in diverse cancers and represent important therapeutic targets. Here, we report the design and synthesis of a novel nucleoside-based scaffold which enables irreversible pan-FGFR inhibition as a potential anticancer strategy. Methods: A series of nucleoside analogues was synthesized and assessed through structure–activity relationship studies. Structural analyses, including X-ray co-crystallography and molecular dynamics simulations, were performed to define key determinants of potency and selectivity. Biochemical assays against FGFR1–4 proteins, cellular antiproliferative assays in HCT116 (FGFR1 amplification) and RT4 (FGFR3-TACC3) models, metabolic stability evaluations and covalent bonding confirmation were conducted to characterize representative compounds. Results: SAR studies revealed that fused aromatic substituents and 4′-thio ribose enhanced FGFR potency, whereas enantiomeric inversion of ribose reduced activity. X-ray co-crystallography further demonstrated that two hydroxyl groups form a key water-mediated hydrogen bond network, uniquely stabilizing the ligand and enhancing potency of inhibitors compared to reference compounds. The 7-methoxy-5-methylbenzo[b]thiophene scaffold and ribose moiety emerged as critical features. Compounds 13f, 19e, and 22f demonstrated potent inhibition of FGFR1-4 and dose-dependent suppression of FGFR1-mediated signaling, with strong antiproliferative activity in both FGFR-driven and wild-type cancer models. Compound 22f showed efficient irreversible covalent engagement of FGFRs, confirmed at the protein and cellular levels, and exhibited improved metabolic stability. Conclusions: Nucleoside analogues represent a privileged scaffold for covalent pan-FGFR inhibition. The findings highlight their potential as promising therapeutic candidates for targeting FGFR-driven malignancies. Future efforts will focus on further improving stability and optimizing physicochemical properties to advance these compounds toward translational development.