*Purpose: Ceftolozane/Tazobactam (CT) is a potent anti-pseudomonal agent with broad gram-negative activity. Our academic medical center recommends the use of CT for patients with a current or recent multidrug resistant (MDR) infection. This study aimed to establish outcomes of solid organ transplant (SOT) patients with Pseudomonas infections treated with beta-lactam (BL) antibiotics and evaluate the use of CT at our institution.

*Methods: Adult SOT patients who received > 24 h of BL for Pseudomonas from December 2012 to May 2018 and CT since May 2015 were reviewed. Clinical success was the absence of pre-treatment signs/symptoms and/or no escalated antibiotic treatment within 48 h of completing therapy.

*Results: 15 patients received 26 courses of CT for a median 10 d (IQR 5,15) and 20 received 28 courses of other BL (15 pip/tazo, 8 meropenem, 5 cefepime) for a median 7 d (6,11.5) of Pseudomoans treatment. Overall 83.3% were male, median 45 y (31.5,62.8) while 42.6% of CT patients were male, 49 y (34.3,60.5). The median time from transplant was 1 y (1,7.75) for CT and 10 y (1,11) for other BL (p<0.05). All patients had an infectious diseases consult and everyone that received CT had current MDR pathogens or a recent history. In contrast, 7 of the 28 courses (25%) of other beta-lactams were for MDR Pseudomonas (resistant to two or more beta-lactams). 44% of CT patients were in the ICU compared to 32% with other BL (p>0.05). CT was directed therapy 19 times. Of 7 empiric uses, it was de-escalated in 5. Other BLs were empiric for 20 courses and de-escalated in 9. The most common indications overall & for CT were bacteremia (27.8, 30.8%), pneumonia (24, 30.8%) and urinary tract infections (26, 27%). Overall clinical cure was 64.8% with 30-day mortality of 14.8%. CT was successful in 54% of courses and 30-day mortality was 38%. (p>0.05).

*Conclusions: CT was used to treat SOT recipients at our institution who were often in the ICU with MDR Pseudomonas. Initially it was directed therapy after resistance to other BLs was reported, but later it was used empirically in patients with MDR history. Patients were more likely to have a recent transplant and resistant infection than those receiving other BLs for Pseudomonas. Despite the complexity of these patients, clinical success was seen for the majority. Further study of CT among the immunocompromised is warranted with adjustment for severity of illness when compared to other traditional BLs.

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