Fig. 6: Heteroresistance threatens the β-lactam/β-lactamase inhibitor development pipeline.

In vitro β-lactamase enzymatic assays determined that cefiderocol was resistant to extended-spectrum β-lactamases (ESBL) and most carbapenemases tested. Broth microdilution (BMD) antimicrobial susceptibility testing assigned low cefiderocol minimum inhibitory concentrations (MIC) to both susceptible (S) and heteroresistant (HR) isolates, the latter of which harbored such a low frequency of resistant cells (e.g., 1 in 100,000) that they did not alter the overall MIC. BMD could detect only conventional resistance (R) in which 100% of the cells exhibit phenotypic resistance. HR is expected to cause treatment failure in patients because cefiderocol treatment selects for the resistant subpopulations, which have amplifications of ESBL genes. These resistant cells become predominant during cefiderocol therapy and then can mediate treatment failure. In agreement with this model, unexpectedly high rates of treatment failure were observed in Phase III testing of cefiderocol, which correlated with rates of cefiderocol HR in surveillance studies. Taken together, the inability of the tests employed by the current antibiotic development pipeline (enzymatic assays and BMD) to detect HR leads drugs to which there are high rates of HR to progress to clinical testing, where treatment failures may be observed. Incorporating testing for HR in the antibiotic development pipeline could potentially make the process more efficient and efficacious.