The first patient was an infant born by spontaneous vaginal delivery at 23 weeks gestational age with a birth weight of 630 grams. At age 2 months, the infant developed MRSA bacteremia. The organism was also resistant to clindamycin, erythromycin, and trimethoprim/sulfamethoxazole, and it was susceptible to rifampin. Susceptibility to gentamicin was unknown. She had persistently positive blood cultures on day 1, 2, and 6 (on day 4 the blood culture was sterile) despite treatment with vancomycin (trough concentrations of 3.3 mcg/ml, 5.8 mcg/ml, 7.9 mcg/ml, 13 mcg/ml, and 19.3 mcg/ml on therapy days 2, 4, 6, 7, and 7, respectively) and central line removal on day three of bacteremia. A transthoracic echocardiogram on day 4 of bacteremia revealed a patent foramen ovale with left-to-right shunt and no vegetations. Because of persistent bacteremia, on day 10 she was started on daptomycin 6mg/kg IV every 12h. Blood cultures drawn 48 hours after and immediately before starting daptomycin were sterile. She received a total of 14 days of daptomycin after negative cultures and 7 days of gentamicin as adjunct therapy. Cerebrospinal fluid was not obtained for evaluation of meningitis and chest radiographs obtained throughout the course of therapy did not reveal evidence of pneumonia. Daptomycin peak and trough plasma samples were collected around dose 22. The peak concentration at the end of the 1-hour infusion was 41.7 mcg/ml and the 12-hour trough concentration was 12.7 mcg/ml. At the time of sample collection the infant had normal creatinine and blood urea nitrogen values (0.5 mg/dl and 3 mg/dl, respectively).
The second patient was a large for gestational age neonate with 11q terminal deletion syndrome born by spontaneous vaginal delivery at 32 weeks with a birth weight of 2670 grams. At 3 weeks of age, she developed signs and symptoms consistent with sepsis, however blood culture was sterile. She was given empirical therapy with vancomycin and gentamicin. After 48 hours of vancomycin therapy, she developed a cervical abscess located in the right anterior submandibular space and piperacillin/tazobactam was started. The abscess was excised on day 4 of vancomycin therapy, and MRSA was isolated. The organism was resistant to clindamycin and erythromycin, but was susceptible to trimethoprim/sulfamethoxazole. This neonate was not evaluated for meningitis and did not have evidence of pneumonia on chest radiograph. She was treated with daptomycin 6 mg/kg IV every 12h for 10 days with resolution of the abscess. After the third dose of daptomycin, the end of the 1-hour infusion peak was 36.7 mcg/ml and 12-hour trough was 16.3 mcg/ml. At the time of sample collection the infant had normal creatinine and blood urea nitrogen values (0.5 mg/dl and 15 mg/dl, respectively). Creatine phosphokinase (CK) measurements were not obtained for these infants.