Structure-activity relationship of neplanocin A analogues for the identification of novel anti-HBV leads

Drug-resistant hepatitis B virus (HBV) strains necessitate the development of novel antiviral agents. In our previous report, we identified a 7-deazaadenine-based neplanocin analogue II (NepA) as a potent anti-HBV compound (EC₅₀ = 0.77 µM, CC₅₀ > 100 µM) with unique therapeutic potential. This pseudonucleoside analogue (NA) II reduced HBsAg production and promoted pregenomic RNA (pgRNA) degradation in the HepG2.2.15.7 cell line. Notably, its mechanism of action is distinct from those of nucleoside polymerase inhibitors and operates independently of S-adenosylhomocysteine hydrolase (SAHase) inhibition. To further explore the structure-activity relationship (SAR), we synthesized a series of NepA analogues with selected modifications at the 5'- and 2'-positions of the carbasugar. This series includes 5'- and 2'-hydroxy masking (O-Me, 4a-4g and 13), 5'-OH truncation (7a and 7b), and steric interference at the 5'-OH position (10a and 10b). All analogues were synthesized through a concise synthetic route from a common intermediate (2a-2g). Among these, two derivatives exhibited notable antiviral activity: one is 5'-hydroxy masked (4a, EC₅₀ = 3.0 µM, CC₅₀ > 100 µM) and the other is 5'-hydroxy truncated (7a, EC₅₀ = 3.6 µM, CC₅₀ > 100 µM). These findings suggest that HBV polymerase inhibition may not be the sole antiviral mechanism of pseudo-NA (II), highlighting the need for further mechanistic investigations. This study provides valuable insights for the development of next-generation HBV therapeutics.