Discovery and Biological Characterization of 1-[4-(2-Difluoromethoxyphenyl)-5-methylthiazol-2-yl]-3-(naphthalen-1-yl)urea (DFMNTU): A Novel Thiazolyl-Urea Chemotype Targeting Yellow Fever Virus and Host Casein Kinase 2 (CK2)

Yellow fever virus (YFV) is a re- emerging mosquito- borne flavivirus that causes severe hemorrhagic fever with high case- fatality rates. Due to vaccine contraindications, incomplete immunization coverage, and the absence of approved specific antivirals, host- directed drug discovery targeting cellular machinery exploited by YFV is a critical therapeutic priority. Here, 1-[4-(2- difluoromethoxyphenyl)- 5- methylthiazol- 2- yl]- 3-(naphthalen- 1- yl)urea (DFMNTU) was synthesized, characterized by NMR, MS, and HPLC, and evaluated for antiviral efficacy against YFV strain 17 D in Vero cells. DFMNTU potently inhibited virus- induced cytopathic effects, with EC 50 values of 4.3 µg/mL (neutral red uptake) and 2. 4 µg/mL (visual CPE assessment), and corresponding cytotoxicity (CC 50) values of 22 and 13 µg/mL, yielding selectivity indices (SI) of 5. 5.2 and 5. 5.5, respectively. Machine- learning target prediction via SuperPred identified 23 putative human protein kinase targets, with casein kinase 2 (CK 2) showing the highest probability (95. 82%). Biochemical kinase profiling confirmed concentration- dependent inhibition of recombinant human CK 2 α (IC 50 = 22 ± 8 µM), whereas non- receptor tyrosine kinase ABL 1 was only weakly inhibited (20% at 20 µM). Computational molecular docking into the ATP- binding cleft of human CK 2 α (PDB ID: 4 UBA) revealed key hydrogen- bonding interactions with Lys 68 and Asp 175, along with optimal steric accommodation of the 2- difluoromethoxyphenyl ring in the adenine pocket. In silico ADMET profiling revealed high lipophilicity (MolLogP = 5. 431), low predicted aqueous solubility (MolLogS =- 7. 225), and low passive gastrointestinal absorption (SwissADME BOILED- Egg model). Extensive Structure–Activity Relationship (SAR) analysis identified the urea linker and difluoromethoxy substituent as key pharmacophores for target binding, while highlighting the hydrophobic 1- naphthyl moiety as the main driver of biopharmaceutical liabilities. DFMNTU serves as a viable host- directed antiviral hit compound, providing a clear computational and structural foundation for hit- to- lead optimization.