Zidovudine plus Fosfomycin: synergistic effect against clinical isolates of multidrug-resistant Enterobacterales. In vitro and in vivo evidence.

In vitro synergistic activity between zidovudine (AZT) and fosfomycin against clinical isolates of MDR Enterobacteriaceae Background: Multidrug-resistant (MDR) Enterobacteriaceae are a priority health issue with few therapeutic options. “Old” antibiotics alongside with non-antibiotic molecules with antibacterial properties are being evaluated as alternative antimicrobial options in experimental studies. Fosfomycin inhibits bacterial wall synthesis with a unique mechanism of action and has been revalued as a carbapenem-sparing agent. Zidovudine (AZT), a thymidine analogue licensed for HIV infection, has also known antibacterial properties. We evaluated the in vitro antimicrobial activity of the combination AZT + fosfomycin against clinical strains of MDR Enterobacteriaceae collected in different Italian hospitals. Materials/methods: We performed checkerboard assays on 33 MDR Enterobacterales (E. coli, K. pneumoniae, E. aerogenes, E. cloacae). Resistance mechanisms included ESBL, porin loss, KPC, OXA-48, NDM, VIM, mcr-1. Time-kill assays (0h, 4h, 8h, 24h) to confirm the most significant results are ongoing. Results: Among 33 strains tested with checkerboard assays, a synergistic effect between AZT and fosfomycin (FIC index ≤ 0.5) was observed in 20 (61%). In the remaining strains the combination of the two drugs showed an additive effect (0.5 < FIC index ≤1). According to EUCAST breakpoints, 18 strains were fosfomycin-resistant (MIC >32 µg/ml). In 14 (77.8%) of them AZT, given in combination, was able to restore fosfomycin-susceptibility, lowering fosfomycin MIC to ≤32 µg/ml. As shown in the figure below, preliminary time-kill assays, performed on 3 isolates exhibiting high resistance level towards fosfomycin, confirmed the results found with checkerboard assays. Conclusions: We demonstrated the synergistic activity of the combination AZT + fosfomycin in 61% of tested MDR strains and an additive effect was however observed in the remaining strains. AZT breakpoints for bacterial infections are unknown. Literature data suggest that AZT concentrations currently tested can be reached in human serum/urine using the actual or slightly increased licensed dosage. The potential mild side effects of such drug combination and the observed synergism (with reversal fosfomycin susceptibility) make this combination worthy of in vivo studies.