Neplanocin A (NpcA), a cytotoxic cyclopentenyl adenosine analogue, is a naturally occurring antitumor antibiotic with in vivo activity against murine L1210 leukemia and in vitro antiviral activity against vaccinia virus, herpes simplex-1, herpes simplex-2, and vesicular stomatitis virus. It is a potent irreversible inhibitor of bovine liver and Alcaleigenes faecalis S-adenosylhomocysteine (AdoHcy) hydrolase—the sole enzyme for AdoHcy removal in eukaryotes—with inhibition leading to elevated AdoHcy levels and suppressed S-adenosylmethionine (AdoMet)-dependent methylations, correlating with antiviral activity. However, NpcA’s utility is limited by metabolism: rapid deamination by adenosine deaminase to inactive neplanocin D (NpcD) and phosphorylation by adenosine kinase to cytotoxic NpcATP. To address these limitations, we synthesized analogue 1 (lacking the 5'-hydroxymethyl group) to retain AdoHcy hydrolase inhibition while reducing phosphorylation/deamination, and analogue 2 (a 3-deazaadenine derivative) to further diminish interactions with adenosine kinase/deaminase. We report their in vitro effects on bovine liver AdoHcy hydrolase and adenosine deaminase substrate activity. Compounds 1 and 2 are time- and concentration-dependent inhibitors with Ki values of 41 and 35 nM (vs. 3.8 nM for NpcA) and Kcat values of 0.5 min⁻¹ each (vs. 1.98 min⁻¹ for NpcA). Unlike NpcA (half-life <1 min), 1 has a half-life of 15 hours with adenosine deaminase, and 2 shows no detectable deamination. Their inhibition mechanism involves reducing enzyme-bound NAD⁺ to NADH, as evidenced by decreased NAD⁺ content and reactivation with NAD⁺; no adenine release was detected. Compounds 1 and 2 also exhibit potent antiviral activity against vaccinia virus with reduced cytotoxicity, supporting AdoHcy hydrolase as the antiviral target and adenosine kinase as the mediator of NpcA’s cytotoxicity. Additionally, we report the synthesis and biological activity of 7-substituted 5-amino-1-cyclopropyl-6,8-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acids—a new class of quinolone antibacterials. Building on structure-activity studies showing amino-substituted pyrrolidines as efficient piperazine mimics (enhancing Gram-positive potency) and C8 fluorine as optimal for in vivo efficacy, these compounds address gaps in 5-substituted quinolone design, with evaluation of antibacterial activity and DNA-gyrase inhibition.