Antibiotic therapy for infective endocarditis requires use of bactericidal agents. Vancomycin is generally considered the drug of choice for treatment of methicillin-resistant Staphylococcus aureus (MRSA) native valve endocarditis. However, slow clinical response to vancomycin has been documented, which may be related to slow bactericidal or bacteriostatic properties.1 Daptomycin (Cubicin) is a novel cyclic lipopeptide agent with concentration-dependent bactericidal activity against many gram-positive bacteria, including MRSA.2 Spontaneous resistance to daptomycin is reportedly rare,3 yet loss of bacterial susceptibility to this drug during clinical therapy has been reported.4-6 We report loss of daptomycin susceptibility in blood isolates of MRSA after in vivo exposure to this drug for only 3 days.
The patient was a 37-year-old woman with chronic myelogenous leukemia diagnosed in October 2001. After chemotherapy and blast transformation to acute myelogenous leukemia, she underwent allogeneic bone marrow transplantation in April 2002. Relapse in September 2003 was followed by further chemotherapeutic treatment. In May and June 2004, she was hospitalized twice for MRSA bacteremia from an unclear source and received vancomycin therapy for 2 to 3 weeks with negative blood cultures (BCs) after each of these episodes.
In July 2004, the patient presented in septic shock after 1 day of fever, malaise, and delirium. Physical examination revealed an ill-appearing, confused woman without clinical signs of heart failure, appreciable heart murmur, or peripheral stigmata of endocarditis. Her implantable venous access device Port-A-Cath (Smiths Medical, Inc, St. Paul, MN) site appeared uninfected. Admission laboratory results demonstrated white blood cell count of 3200/μL (70% polymorphonuclear leukocytes, 25% bands, 4% lymphocytes, and 1% metamyelocytes), hemoglobin of 11.5 g/dL, and platelet count of 8000/μL. Chemistries showed sodium, 153 mmol/L; potassium, 3.3 mmol/L; chloride, 106 mmol/L; blood urea nitrogen, 12 mg/dL; and creatinine, 2 mg/dL in a background of previously normal renal function. She had unremarkable liver function tests except for total bilirubin of 1.4 mg/dL. Portable chest x-ray revealed a small abnormal density in the right upper medial chest with elevation of the minor fissure and a vascular catheter correctly positioned. Her hospital course included emergent removal of her Portacath and transfer to the intensive care unit where she required pressors and aggressive fluid resuscitation. She was empirically treated with vancomycin 1 g intravenously twice daily, imipenem-cilastatin 500 mg intravenously every 6 hours, and clindamycin 900 mg/IV every 8 hours. Two sets of BC drawn on admission and the catheter tip submitted to the microbiology laboratory for semiquantitative culture grew MRSA (the catheter tip growing >15 colony-forming units) with a similar antibiotic susceptibility pattern of resistant to clindamycin, erythromycin, and levofloxacin, susceptible to rifampin, trimethoprim-sulfamethoxazole, and vancomycin (minimum inhibitory concentration [MIC], 2 μg/mL). By the fifth hospital day (HD), she was transferred out of the intensive care unit when she had hemodynamically stabilized, her mental status returned to her baseline, and her acute renal failure had resolved. Clindamycin was discontinued. However, fever and bacteremia persisted with numerous additional BCs growing MRSA. Vancomycin trough levels were 10.5 to 26.6 mg/L. On the ninth HD, a new apical systolic murmur was noted. Transthoracic echocardiography showed a mobile, pedunculated, 1.2 cm × 1.5-cm vegetation attached to the anterior leaflet of the mitral valve, with severe mitral valve regurgitation and an ejection fraction of 55% to 65%. Nonetheless, she remained hemodynamically stable without clinical signs of heart failure. The risk of surgical intervention was deemed prohibitively high because of her underlying disease, severe thrombocytopenia, and coagulopathy. On the 11th HD, antibiotic therapy was changed to daptomycin monotherapy at a dose of 6 mg/kg daily. She expired on the 19th HD with persistent bacteremia, 1 day after having a massive right frontoparietal hemorrhagic stroke. BC isolates of MRSA were tested for susceptibility to daptomycin.
Blood culture isolates of MRSA were studied for susceptibility to daptomycin using Kirby-Bauer (KB) disk diffusion (30-μg disks, Cubist Pharmaceuticals, Inc, Lexington, MA, for investigational use only). Clinical Laboratory Standards Institute as well as daptomycin package insert recommendations were followed (including interpretive criterion for "susceptible" ≥16 mm for S. aureus). Ca2+ content of Mueller-Hinton (MH) agar (BBL, Becton-Dickinson and Co, Sparks, MD) could not be directly measured but was indirectly assessed by noting that the result for S. aureus quality control strain (American Type Culture Collection, strain 25923) fell in the middle of the daptomycin target range.
Isolates were sent to Laboratory Specialists, Inc (Westlake, Ohio) for confirmatory testing by the broth microdilution reference method (using Food and Drug Administration-approved breakpoint of ≤1 μg/mL for susceptibility). E-test (AB Biodisk, Solna, Sweden) was performed on MH agar. Trypticase soy broth (TSB, BBL, Becton-Dickinson and Co) was used for serial subculture (35°C, room air) with intermittent purity assessment on Trypticase soy agar with 5% sheep blood (BAP, BBL, Becton-Dickinson and Co, Sparks, MD), 35°C, 5% CO2.
This patient's final MRSA isolate showed a zone size of 15 mm around the daptomycin disk, confirmed by repeat testing. Next, all 27 isolates were concurrently tested, revealing initial susceptibility and then progressive decrease in zone size after in vivo exposure to daptomycin (Fig. 1). After manufacturer notification, isolates were sent to a reference laboratory, where broth microdilution testing showed MIC of 0.5 μg/mL for initial isolates and MIC of 4 μg/mL for later isolates, confirming our findings. Subsequent to these studies, concerns emerged regarding the reliability of disk diffusion testing for detecting daptomycin nonsusceptibility,7 and E-test strips became available. Isolates with MI of 4 μg/mL by the reference method also showed MIC of 4 μg/mL by E-test in our laboratory.
We roughly assessed phenotypic stability of 2 nonsusceptible isolates by serial subculture in drug-free media (TSB) and repeat testing. After 17 passes through TSB over 18 days (with 3 periodic assessments of purity), isolates were reexamined by E-test. Both were found to have MIC of 1.0 μg/mL for daptomycin, indicating apparent reversion to a susceptible phenotype and suggesting instability of the resistance mechanism.
Use of a bactericidal agent is considered essential for successful treatment of infective endocarditis. Vancomycin is slowly bactericidal against MRSA, which is often clinically associated with slow clearance of bacteremia.1 Despite limited clinical data, the rapidly bactericidal drug daptomycin is a potentially attractive option for treating many MRSA infections, including infective endocarditis. Successful daptomycin treatment of MRSA bacteremia with or without endocarditis has been reported.8 In a recently concluded phase III trial, daptomycin monotherapy (6 mg/kg per day) was reported to be noninferior to vancomycin in the treatment of MRSA bacteremia.9
Nonsusceptibility of S. aureus to daptomycin is reportedly rare in clinical isolates. Only 3 (0.2%) of 1500 subjects showed nonsusceptible isolates during drug development studies (personal communication, Cubist Pharmaceuticals, Inc, Customer Service), including one case of MRSA endocarditis similar to ours. In our case, clinical isolates lost susceptibility to daptomycin after only 3 days of therapy, representing the shortest reported time interval to loss of daptomycin susceptibility after in vivo exposure. Two daptomycin nonsusceptible MRSA isolates from osteomyelitis cases (MIC, 4 μg/mL),4 one developing resistance on day 35 of therapy, have been previously reported. Isolates from 2 cases of MRSA bacteremia have also been reported to show increased MICs to daptomycin: one became nonsusceptible (MIC, 2 μg/mL) after 27 days of daptomycin therapy,5 and another showed a 4-fold increase in daptomycin MIC (from 0.25 to 1.0 μg/mL) occurring after 49 days of daptomycin treatment.6
We cannot conclude that nonsusceptibility to daptomycin was the cause of our patient's therapeutic failure. Her overall health condition was deemed prohibitive for surgical intervention. The presence of possible septic clot and paravalvular and/or tunnel abscess was not ruled out because of sudden deterioration of her clinical condition and institution of comfort measures only.
We successfully identified daptomycin nonsusceptibility using KB testing despite eventual reported unreliability of this method. Kirby-Bauer testing was found to have unacceptably high rates of very major errors when compared with the gold-standard method of broth microdilution performed at 50 μg/mL calcium, which is thought to be due to variable Ca2+ levels in MH agar formulations.10 Free calcium ions play an important role in the mechanism of action of daptomycin, as the drug appears to bind to the cytoplasmic membrane in Ca2+-dependent manner, causing a rapid cell depolarization leading to the arrest of various cellular functions.11 E-test strips that contain supplemental calcium have now become available for use in routine testing in clinical laboratories.
The mechanism of daptomycin resistance is currently unknown. Our isolates reverted back to a susceptible phenotype after several serial passages in drug-free media, suggesting instability of the mechanism.
We report a case of rapidly developing loss of susceptibility to daptomycin in MRSA isolated from BCs in a patient with native valve infective endocarditis. Currently, although not commonly reported, daptomycin nonsusceptibility should be suspected in persistently bacteremic patients on daptomycin therapy. The mechanism of drug resistance is unknown.
No financial support was utilized. R.B.B is on the speaker's bureau for Merck and Co, Inc; Ortho-McNeil, Inc; Roche, Pfizer, Inc; and Cubist Pharmaceuticals, Inc.
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