Ceftazidime-avibactam is a novel ␤-lactam/␤-lactamase inhibitor with activity against carbapenem-resistant Enterobacteriaceae (CRE) that produce Klebsiella pneumoniae carbapenemase (KPC). We report the first cases of ceftazidimeavibactam resistance to develop during treatment of CRE infections and identify resistance mechanisms. Ceftazidime-avibactam-resistant K. pneumoniae emerged in three patients after ceftazidime-avibactam treatment for 10 to 19 days. Wholegenome sequencing (WGS) of longitudinal ceftazidime-avibactam-susceptible and -resistant K. pneumoniae isolates was used to identify potential resistance mechanisms. WGS identified mutations in plasmid-borne bla KPC-3 , which were not present in baseline isolates. bla KPC-3 mutations emerged independently in isolates of a novel sequence type 258 sublineage and resulted in variant KPC-3 enzymes. The mutations were validated as resistance determinants by measuring MICs of ceftazidime-avibactam and other agents following targeted gene disruption in K. pneumoniae, plasmid transfer, and bla KPC cloning into competent Escherichia coli. In rank order, the impact of KPC-3 variants on ceftazidime-avibactam MICs was as follows: D179Y/T243M double substitution Ͼ D179Y Ͼ V240G. Remarkably, mutations reduced meropenem MICs Ն4-fold from baseline, restoring susceptibility in K. pneumoniae from two patients. Cefepime and ceftriaxone MICs were also reduced Ն4-fold against D179Y/T243M and D179Y variant isolates, but susceptibility was not restored. Reverse transcription-PCR revealed that expression of bla KPC-3 encoding D179Y/T243M and D179Y variants was diminished compared to bla KPC-3 expression in baseline isolates. In conclusion, the development of resistance-conferring bla KPC-3 mutations in K. pneumoniae within 10 to 19 days of ceftazidime-avibactam exposure is troubling, but clinical impact may be ameliorated if carbapenem susceptibility is restored in certain isolates.

Thirty-seven carbapenem-resistant Enterobacteriaceae (CRE)-infected patients were treated with ceftazidime-avibactam. Clinical success and survival rates at 30 days were 59% (22/37) and 76% (28/37), respectively. In 23% (5/22) of clinical successes, CRE infections recurred within 90 days. Microbiologic failure rate was 27% (10/37). Ceftazidime-avibactam resistance was detected in 30% (3/10) of microbiologic failures.Keywords. ceftazidime-avibactam resistance; carbapenemresistant Enterobacteriaceae; Klebsiella pneumoniae carbapenemase.Ceftazidime-avibactam is a novel β-lactam/β-lactamase inhibitor combination that was recently approved by the US Food and Drug Administration for the treatment of complicated intraabdominal and complicated urinary tract infections [1]. The agent demonstrates in vitro activity against carbapenem-resistant Enterobacteriaceae (CRE) that produce Klebsiella pneumoniae carbapenemase (KPC), but not metallo-β-lactamases such as New Delhi MBL (NDM), Verona integron-encoded MBL (VIM), or imipenemase (IMP) [2]. Ceftazidime-avibactam may offer a significant advance over previously developed antimicrobials with in vitro activity against CRE, such as colistin, gentamicin, and tigecycline, which are limited by concerns over efficacy and/or toxicity. Our objective in this study was to describe our initial clinical experience with ceftazidimeavibactam against CRE infections. METHODSWe conducted a retrospective study of patients with CRE infection who were treated with ceftazidime-avibactam at the University of Pittsburgh Medical Center between April 2015 and February 2016. CRE was defined by current Centers for Disease Control and Prevention criteria as resistance to any carbapenem [3]. A standard dosage of 2.5 g intravenously (IV) every 8 hours was used, with adjustments for renal impairment made according to manufacturer recommendations [1]. Types of CRE infection were classified according to National Healthcare Safety Network criteria [4]. Clinical success was defined as survival and absence of recurrence at 30 days following the onset of infection, resolution of signs and symptoms of infection, and sterilization of site-specific cultures within 7 days of treatment initiation. Microbiologic failure was defined as isolation of CRE following ≥7 days of ceftazidime-avibactam treatment. Recurrences within 90 days of onset were defined by microbiologic failure and concomitant signs of infection. Minimum inhibitory concentrations (MICs) were determined using reference Clinical and Laboratory Standards Institute broth microdilution methods; avibactam was tested at a fixed concentration of 4 µg/mL [2]. Isolates were tested for the presence of β-lactamases as described previously [5,6]. Comparisons between groups were made using Fisher exact test (categorical variables) and Mann-Whitney U (continuous variables). Significance was defined as P ≤ .05 (2-tailed). RESULTSThirty-seven consecutive patients treated for 3 days or longer with ceftazidime-avibactam were evaluated. Median age was 64 years (range, 26-...

The (-) enantiomers of 2',3'-dideoxy-5-fluoro-3'-thiacytidine [(-)-FTC] and 2',3'-dideoxy-3'-thiacytidine [(-)-BCH-189] were recently shown to inhibit selectively human immunodeficiency viruses (HIV) and hepatitis B virus in vitro. In the current study, the potential for HIV type 1 (HIV-1) resistance to these compounds was evaluated by serial passage of the virus in human peripheral blood mononuclear cells and MT-2 cells in the presence of increasing drug concentrations. Highly drug-resistant HIV-1 variants dominated the replicating virus population after two or more cycles of infection. The resistant variants were cross-resistant to (-)-FTC, (-)-BCH-189, and their (+) congeners but remained susceptible to 2',3'-dideoxycytidine, 3'-azido-3'-deoxythymidine, 3'-fluoro-3'-deoxythymidine, 2',3'-dideoxyinosine, phosphonoformate, the TIBO compound R82150, and the bis(heteroaryl)piperazine derivative U-87201E. Reverse transcriptase derived from drug-resistant viral particles was 15- to 50-fold less susceptible to the 5'-triphosphates of FTC and BCH-189 compared with enzyme from parental drug-susceptible virus. DNA sequence analysis of the reverse transcriptase gene amplified from resistant viruses consistently identified mutations at codon 184 from Met (ATG) to Val (GTG or GTA) or Ile (ATA). Sequence analysis of amplified reverse transcriptase from a patient who had received (-)-BCH-189 therapy for 4 months demonstrated a mixture of the Met-184-to-Val (GTG) mutation and the parental genotype, indicating that the Met-184 mutation can occur in vivo.

There are no data comparing outcomes of patients treated with ceftazidime-avibactam versus comparators for carbapenem-resistant Enterobacteriaceae infections. At our center, ceftazidime-avibactam treatment of carbapenem-resistant Klebsiella pneumoniae bacteremia was associated with higher rates of clinical success (P ϭ 0.006) and survival (P ϭ 0.01) than other regimens. Across treatment groups, there were no differences in underlying diseases, severity of illness, source of bacteremia, or strain characteristics (97% produced K. pneumoniae carbapenemase). Aminoglycoside-and colistin-containing regimens were associated with increased rates of nephrotoxicity (P ϭ 0.002).