Daptomycin-oxacillin combinations in treatment of experimental endocarditis caused by daptomycin-nonsusceptible strains of methicillin-resistant Staphylococcus aureus with evolving oxacillin susceptibility (the "seesaw effect")

Abstract

In vivo development of daptomycin resistance (DAPr) among Staphylococcus aureus strains, especially methicillin-resistant S. aureus (MRSA) strains, in conjunction with clinical treatment failures, has emerged as a major problem. This has raised the question of DAP-based combination regimens to enhance efficacy against such strains. We studied five recent DAP-susceptible (DAPs)/DAPr clinical MRSA strain pairs obtained from patients who failed DAP monotherapy regimens, as well as one DAPs/DAPr MRSA strain pair in which the resistant strain was generated by in vitro passage in DAP. Of note, we identified a DAP-oxacillin (OX) "seesaw" phenomenon in vitro in which development of DAPr was accompanied by a concomitant fall in OX resistance, as demonstrated by 3- to 4-fold decreases in the OX MIC, a susceptibility shift by population analyses, and enhanced early killing by OX in time-kill assays. In addition, the combination of DAP and OX exerted modest improvement in in vitro bactericidal effects. Using an experimental model of infective endocarditis and two DAPs/DAPr strain pairs, we demonstrated that (i) OX monotherapy was ineffective at clearing DAPr strains from any target tissue in this model (heart valve, kidneys, or spleen) and (ii) DAP-OX combination therapy was highly effective in DAPr strain clearances from these organs. The mechanism(s) of the seesaw effect remains to be defined but does not appear to involve excision of the staphylococcal cassette chromosome mec (SCCmec) that carries mecA.

FIG. 1.

Population analyses of study strains upon exposure to a range of DAP (left panel) or OX (right panel) concentrations. These data represent the means (± SD) for two separate assays. conc., concentration.

FIG. 2.

OX time-kill analyses. Six DAP^s and DAP^r strain pairs were used for these experiments. (A) CB1118-CB2205; (B) MRSA 11/11-REF2145; (C) CB1482-CB184; (D) BMC1001-BMC1002; (E) CB5053-CB5054; (F) CB5035-CB5036. Time-kill experiments were performed using Mueller-Hinton broth with a 10⁶-CFU/ml inoculum in the presence of 0 to 256 μg/ml OX.

FIG. 2.

OX time-kill analyses. Six DAP^s and DAP^r strain pairs were used for these experiments. (A) CB1118-CB2205; (B) MRSA 11/11-REF2145; (C) CB1482-CB184; (D) BMC1001-BMC1002; (E) CB5053-CB5054; (F) CB5035-CB5036. Time-kill experiments were performed using Mueller-Hinton broth with a 10⁶-CFU/ml inoculum in the presence of 0 to 256 μg/ml OX.

FIG. 3.

In vitro synergy kill curves for DAP^r REF2145 (A) and CB5054 (B) strains. These two strain sets were used in the experimental IE studies detailed in the text. The growth curve for the no-antibiotic growth control for these experiments is not shown but was identical to the growth curve for both strains with one-quarter of the MIC of DAP.