Synthesis, Activity Evaluation, and Putative Mode of Action of 4,6-Dihydroxypyrimidine Hydrazones as Antimicrobial Agents Against Fungi, Bacteria, and Cyanobacteria

Broad-spectrum antimicrobial agents are often useful for the management of phytopathogenic fungi, bacteria, and cyanobacteria species. Twenty novel 4,6-dihydroxypyrimidine hydrazones (aka IV-C) were designed based on lead scaffold 5, synthesized, chemically characterized, and evaluated for inhibitory activity against 15 fungi, 5 bacteria, and 2 cyanobacteria. The in vitro tests indicated that these IV-C exhibited a much broader spectrum than 5, especially for the IV-C9. At 50 μg/mL, IV-C9 exhibited >80% control on 11 fungi, >85% control for 5 bacteria, and 2 cyanobacteria. In the in vivo test at 200 μg/mL, the protective activity of compound IV-C9 against Monilinia fructigena and Xanthomonas oryzae pv oryzae was 94 and 58%, respectively, which was much better than that of chlorothalonil (79%) and thiodiazole copper (31%). IV-C9 also inhibited cyanobacteria Microcystis aeruginosa FACHB905 and Synechocystis sp. PCC6803 with corresponding EC50 values of 0.30 and 0.82 μg/mL, being 5.8- and 3.6-fold more potent than the algicide prometryne (EC50 = 1.74 and 2.97 μg/mL, respectively) and matched the efficacy of the copper sulfate (EC50 = 0.29 and 0.37 μg/mL, respectively). Compared to IV-C9, IV-D1 with the hydroxyl group free shows almost 40-fold, 10-fold, and 160-fold less activity against fungi, bacteria, and cyanobacteria, respectively, confirming the importance of hydroxyl groups. Mechanistic studies showed that IV-C9 induced oxidative damage in pathogenic bacteria, affecting the morphology of bacteria and causing massive leakage of intracellular electrolytes and proteins. The results suggest that 4,6-dihydroxypyrimidine hydrazone IV-C9 has the potential to be a broad-spectrum microbial inhibitor.