OBJECTIVES To evaluate the in vitro activity and in vivo efficacy of delafloxacin against Bacillus anthracis, the causative agent of anthrax. METHODS MICs were obtained according to CLSI guidelines for 30 virulent isolates and 14 attenuated antibiotic-resistant strains. For the in vivo efficacy study, mice were administered delafloxacin (30-62.5 mg/kg) subcutaneously, or ciprofloxacin (30 mg/kg) intraperitoneally beginning at either 24 or 48 ± 1 h post-challenge (post-exposure prophylaxis) and continued every 12 h for 14 days with study termination on day 30. The mean inhaled dose in the study was approximately 103 × LD50 equivalents, and the range was 87-120 × LD50. RESULTS Delafloxacin (MIC90 = 0.004 mg/L) was 16-fold more potent than ciprofloxacin (MIC90 = 0.06 mg/L) against a 30-strain set of virulent B. anthracis. Against a panel of attenuated antibiotic-resistant strains, delafloxacin demonstrated potency ≥128-fold over that observed with ciprofloxacin. When evaluated in vivo, mice treated with all delafloxacin doses tested at 24 h post-challenge demonstrated equivalent survival compared with mice treated with the positive control ciprofloxacin. Because of the high challenge dose of spores, mice treated at 48 h showed rapid and high mortality in all groups including the positive control. Surviving animals in all delafloxacin- and ciprofloxacin-treated groups (24 and 48 h) showed complete splenic clearance of infection and <2.2 × 103 cfu/g lung tissue. CONCLUSIONS Given the high bar set by the 100 × LD50 challenge dose in this study, the results from delafloxacin treatment are promising for the treatment of inhaled anthrax.

Abstract Background The worldwide spread of serine carbapenemases threatens carbapenems. VABOMERE (combination of meropenem and vaborbactam (MV), is a new class of beta-lactamase inhibitors including inhibition of Klebsiella pneumoniae. The PK, safety, and efficacy data of MV in adults support study in pediatrics. Methods Phase 1, open-label, dose-finding study. Pediatric subjects, 3 mos. to < 18 yrs already on antibiotics received a single-dose weight based infusion of MV over 3 hr. Blood samples were drawn pre-dose, 3,4, and 6 hr. for PK parameters. Population PK model included weight as a covariate. Safety assessments included AEs, vital signs, safety labs, physical exams, infusion tolerance, ECG. A DSMB reviewed safety and PK data before enrolling next cohort. Results MV was well-tolerated and safe in all age groups. Clearance per kg body weight of both M and V increased with decreasing body weight over all age groups suggesting an allometric relationship. Conclusion Targeted AUC0-8 efficacious in adult trials ranges of 91.7-513 mg•hr/L (meropenem) and 131-494 mg•hr/L (vaborbactam) were achieved in all pediatric cohorts. Disclosures Paula M. Bokesch, MD, Melinta Therapeutics: Advisor/Consultant Antonio C. Arrieta, MD, FIDSA, FPIDS, Astellas Pharma Global Development, Inc.: Advisor/Consultant|Astellas Pharma Global Development, Inc.: Grant/Research Support|Astellas Pharma Global Development, Inc.: Honoraria|Cumberland Pharmaceutical: Grant/Research Support|IDbyDNA: Advisor/Consultant|IDbyDNA: Grant/Research Support|Melinta: Grant/Research Support|Merck: Advisor/Consultant|Merck: Grant/Research Support|Nabriva: Grant/Research Support|Paratek Pharmaceuticals: Grant/Research Support|Pfizer, Inc: Advisor/Consultant|Pfizer, Inc: Grant/Research Support|Roche/Genentech: Grant/Research Support|The Medicine Company: Grant/Research Support Randall K. Hoover, Ph.D., Melinta Therapeutics: Advisor/Consultant Sandra McCurdy, MS, Melinta Therapeutics: Employee

In vitro activities of delafloxacin and ciprofloxacin were evaluated against Burkholderia pseudomallei mutants expressing or lacking defined resistance-nodulation-cell division (RND) efflux pumps using CLSI methodology at pHs of 5.8 and 7.2. Delafloxacin MIC values were as much as 8-fold lower at pH 5.8 than those at pH 7.2, while ciprofloxacin MICs increased as much as 8-fold. The data from this study suggest that compared to ciprofloxacin, delafloxacin may have improved efflux avoidance, notably at acidic pH. In contrast to ciprofloxacin, delafloxacin may thus retain its therapeutic potential, even in BpeEF-OprC efflux-pump-expressing B. pseudomallei strains that compromise the use of fluoroquinolones, such as ciprofloxacin. IMPORTANCE Resistance-nodulation-cell division (RND) efflux pumps play a major role in intrinsic and acquired antibiotic resistance in Burkholderia pseudomallei, and these pumps are its only known multidrug resistance determinants. Fluoroquinolones have performed poorly in clinical settings and are currently not recommended for treatment of B. pseudomallei infections. While the reasons for the poor clinical performance of this pathogen remain unclear, efflux may be partially responsible since fluoroquinolones like ciprofloxacin are prone to efflux by RND pumps, notably BpeEF-OprC. In vitro efficacy testing using a panel of efflux-proficient and efflux-deficient strains allows identification of fluoroquinolones that compared to ciprofloxacin are less prone to efflux.

The in vitro activity and in vivo efficacy of delafloxacin were evaluated against the causative pathogen of melioidosis, Burkholderia pseudomallei. Delafloxacin MICs were determined by broth microdilution according to CLSI guidelines for 30 isolates of B. pseudomallei. ABSTRACT The in vitro activity and in vivo efficacy of delafloxacin were evaluated against the causative pathogen of melioidosis, Burkholderia pseudomallei. Delafloxacin MICs were determined by broth microdilution according to CLSI guidelines for 30 isolates of B. pseudomallei. The in vivo efficacy of delafloxacin was studied at a range of doses in a postexposure prophylaxis (PEP) murine model of melioidosis. Delafloxacin was active in vitro against B. pseudomallei (MIC90, 1 μg/ml). When the mice were dosed with 50 mg/kg body weight and 80 mg/kg body weight delafloxacin at both 16 and 24 h, greater survival was observed (90% to 100% survival) than with the 30-mg/kg-dosed mice (70% survival). All delafloxacin-treated cohorts contained no detectable B. pseudomallei in the spleens at the end of the study. This contrasts with ceftazidime 16- and 24-h administration, which had 40% and 20% survival, respectively. Complete clearance of infection was observed for most but not all surviving cohorts administered ceftazidime. In the mouse model of infection, survival curves for delafloxacin- and ceftazidime-treated animals at treatment start times of 16 and 24 h were statistically significant (P values of <0.0001). Estimated daily delafloxacin exposures in the B. pseudomallei murine aerosol study were similar to daily human exposures with the approved twice a day (BID) intravenous (i.v.) (300 mg) or oral (450 mg) dosing regimens. Based on its in vitro and in vivo activity, its safety, and its tolerability profile, delafloxacin may offer an attractive treatment option as PEP or eradication therapy for B. pseudomallei. Evaluation in other in vivo infection models for B. pseudomallei should be considered.

Abstract Objectives To report Streptococcus pneumoniae serotyping and susceptibility data from a recent clinical trial (ML-3341-306) comparing delafloxacin with moxifloxacin in the treatment of adults with community-acquired bacterial pneumonia (CABP). Methods Serotyping and susceptibility testing were conducted on 142 baseline S. pneumoniae isolates recovered from subjects participating in a CABP clinical trial. Results Overall, 113/142 (79.6%) isolates were vaccine serotypes. 76.8% (109/142) of serotyped isolates were PPSV23 serotypes and 59.9% (85/142) of isolates were PCV13 serotypes. 15.5% (22/142) of serotyped isolates were serotypes not covered by either vaccine; 4.9% (7/142) of tested isolates were non-typeable. The most common serotypes were serotypes 3 (19.0%; 27/142), 19F (9.9%; 14/142) and 23F (7.0%; 10/142). All of the 142 isolates were susceptible to delafloxacin and moxifloxacin, 76.1% were susceptible to azithromycin and 71.8% were susceptible to penicillin. Multidrug resistance was found among 19A (4/5; 80%), 6A (1/4; 25%), 6B (1/4; 25%), 14 (1/4; 25%), 19F (1/14; 7.1%), and 23F serotypes (2/10; 20%), and among non-typeable S. pneumoniae isolates (1/7; 14.3%). Conclusions S. pneumoniae vaccine-targeted serotypes were the main cause of CABP in this Phase 3 CABP study. Fluoroquinolones including delafloxacin remain a good treatment option for CABP in adults caused by S. pneumoniae.

ObjectivesTo report atypical pathogens from clinical trial data comparing delafloxacin to moxifloxacin in the treatment of adults with community-acquired bacterial pneumonia (CABP).MethodsMultiple diagnostic methods were employed to diagnose atypical infections including culture, serology, and urinary antigen.ResultsThe microbiological intent-to-treat (MITT) population included 520 patients; 30% had an atypical bacterial pathogen identified (156/520). Overall, 13.1% (68/520) had a monomicrobial atypical infection and 2.3% (12/520) had polymicrobial all-atypical infections. Among patients with polymicrobial infections, Streptococcus pneumoniae was the most frequently occurring co-infecting organism and Chlamydia pneumoniae was the most frequently occurring co-infecting atypical organism. For Mycoplasma pneumoniae and Legionella pneumophila, serology yielded the highest number of diagnoses. Delafloxacin and moxifloxacin had similar in vitro activity against M. pneumoniae and delafloxacin had greater activity against L. pneumophila. Two macrolide-resistant M. pneumoniae isolates were recovered. No fluoroquinolone-resistant M. pneumoniae were isolated. The rates of microbiological success (documented or presumed eradication) at test-of-cure were similar between the delafloxacin and moxifloxacin groups. There was no evidence of a correlation between minimum inhibitory concentration (MIC) and outcome; a high proportion of favorable outcomes was observed across all delafloxacin baseline MICs.ConclusionsDelafloxacin may be considered a treatment option as monotherapy for CABP in adults, where broad-spectrum coverage including atypical activity is desirable.

Abstract Background DLX is a novel fluoroquinolone (FQ) antibiotic with Gram-positive/MRSA, Gram-negative and atypical activity. It offers IV and oral treatment with no QT restrictions. In a Phase 3 study in CABP patients, DLX was non-inferior to moxifloxacin (MOX) in the primary endpoint, early clinical response at 96 ± 24 hours (88.9 vs. 89.0; 95% CI: −4.4, 4.1) in the intent-to-treat (ITT) population. A detailed microbiological analysis was conducted. Methods CABP pathogens were identified by culture/non-culture methods. Pathogens identified by non-culture methods included Streptococcus pneumoniae (Sp; culture, urinary antigen [UA], nasopharyngeal [NP] swab lytA PCR), Legionella pneumophila (Lp) (culture, UA, serology), Mycoplasma pneumoniae (Mp; culture, serology), and Chlamydia pneumoniae (Cp; serology). All other pathogens were identified using culture only. For Sp cultured from NP, a concomitant lytA PCR value of ≥ 1000 gene copies/mL was required. All isolates underwent susceptibility testing, and a subset of isolates underwent molecular or phenotypic characterization including whole-genome sequencing for FQ resistance mechanisms, PCR for PVL/mecA genes (S. aureus), β-lactamases (Haemophilus/Moraxella spp), and serotyping (Sp). Results Included in submitted image. Conclusion DLX demonstrated potent in vitro and clinical activity against CABP pathogens, including Sp [MRSP, MDRSP, PRSP], MRSA, Haemophilus species, Enterobacteriaceae, P. aeruginosa, and the atypical pathogens Cp, Mp, and Lp. Disclosures All authors: No reported disclosures.

SESSION TITLE: Monday Electronic Posters 1 SESSION TYPE: Original Inv Poster Discussion PRESENTED ON: 10/21/2019 02:30 PM - 03:15 PM PURPOSE: An etiological agent is often not identified at the time of diagnosis of community-acquired bacterial pneumonia (CABP) and as such, the Infectious Diseases Society of America/American Thoracic Society Guidelines recommend that empirical treatment cover atypical pathogens. Fluoroquinolones (FQ) have a spectrum of activity that covers gram-positive, gram-negative and atypical pathogens common in CABP. Delafloxacin (DLX) is a new FQ with activity against many Gram-positive FQ-resistant strains. A Phase 3 global, randomized, double-blind, active-controlled study was recently completed to evaluate IV-to-oral DLX compared with moxifloxacin (MOX) in the treatment of CABP. We report the clinical outcomes among patients with atypical pathogens. METHODS: CABP patients were randomized to receive 5-10 days of either DLX or MOX and could switch from IV to oral dosing after Day 3. Serologic diagnosis of Legionella pneumophila (Lp), Mycoplasma pneumoniae (Mp), and Chlamydophila pneumoniae (Cp) was made using paired acute and convalescent sera showing at least a 4-fold increase in titer. Additionally, Lp could be diagnosed by sputum culture or urinary antigen and Mp could be diagnosed by culture of oropharyngeal swabs. Susceptibility testing was performed on Lp and Mp isolates. Efficacy assessments were performed at 96 (±24) hours for Early Clinical Response (ECR) and at the Test of Cure (TOC) visit 5-10 days after the end of treatment. RESULTS: 859 patients were enrolled from sites in 18 countries, and a microbiological diagnosis (all pathogens) was established in 60.5% of patients. In the patients with baseline pathogens, Lp, Mp and Cp were detected in 11.9, 12.5 and 7.9% of these patients, respectively. Overall, 13.1% of microbial diagnoses were monomicrobial infections with atypical pathogens. DLX had potent activity against Mp and Lp. The DLX MIC90 was 0.5 μg/mL for Mp and the MICs ranged from 0.00025-0.001 μg/mL (n=4) for Lp. Overall DLX was non-inferior to MOX for the treatment of CABP in the primary outcome of ECR and by-pathogen success rates were high. In the evaluable population, ECR (DLX vs. MOX) was 93.1% vs. 87.5% for Lp, 85.7% vs. 96.7% for Mp, and 96.0% vs 87.5% for Cp. At the TOC visit, clinical success was 93.1% vs. 100% for Lp, 96.7% vs. 100% for Mp, and 100% vs 100% for Cp. CONCLUSIONS: Atypical pathogens were frequently identified in this Phase 3 study in CABP, including as monomicrobial infections. DLX, which is approved for treatment of acute bacterial skin and skin structure infections in the US, was confirmed as efficacious in monotherapy for CABP and provided comparable outcomes to MOX for the treatment of patients with atypical pathogens. CLINICAL IMPLICATIONS: DLX may be a useful treatment option for monotherapy in CABP on either an inpatient or outpatient basis when empirical coverage of atypical pathogens is desired. DISCLOSURES: Employee relationship with Melinta Please note: >$100000 Added 03/03/2019 by Sue Cammarata, source=Web Response, value=Salary Employee relationship with Melinta Therapeutics, Inc. Please note: >$100000 Added 03/04/2019 by Kara Keedy, source=Web Response, value=Salary Stock holder relationship with Melinta Therapeutics, Inc. Please note: $1001 - $5000 Added 03/12/2019 by Kara Keedy, source=Web Response, value=stock Employee relationship with Melinta Therapeutics Please note: >$100000 Added 03/12/2019 by Laura LAWRENCE, source=Web Response, value=Salary No relevant relationships by Yang Li, source=Web Response Employee $100000 Added 03/12/2019 by Sandra McCurdy, source=Web Response, value=Salary $100000 Added 03/12/2019 by Ashley Nenninger, source=Web Response, value=salary and stock Employee relationship with Melinta Therapeutics, Inc. Please note: >$100000 Added 03/12/2019 by Megan Quintas, source=Web Response, value=Salary Employee relationship with Melinta Therapeutics Inc Please note: >$100000 Added 03/12/2019 by Amanda Sheets, source=Web Response, value=Salary Stock holder relationship with Melinta Therapeutics Inc Please note: $1-$1000 Added 03/12/2019 by Amanda Sheets, source=Web Response, value=Consulting fee Removed 03/12/2019 by Amanda Sheets, source=Web Response Stock holder relationship with Melinta Therapeutics Inc Please note: $1001 - $5000 Added 03/12/2019 by Amanda Sheets, source=Web Response, value=Stock

Delafloxacin (DLX) is a broad-spectrum fluoroquinolone (FQ) antibacterial; approved in 2017 by the Food and Drug Administration for treatment of acute bacterial skin and skin structure infections (ABSSSIs). DLX is in clinical development for community-acquired bacterial pneumonia (CABP). In this study, in vitro susceptibility (S) for DLX and comparator agents for Gram-negative (GN) and Gram-positive (GP) anaerobic isolates from Phase 3 ABSSSI clinical trials were determined and compared with the microbiologic response for evaluable isolates. A total of 84 anaerobic isolates were collected during Phase 3 ABSSSI clinical trials and 9 additional Bacteroides fragilis (BF) were collected as part of the 2017 SENTRY surveillance program. The isolates tested included 11 BF, 13 Clostridium perfringens (CP), and other species with <10 isolates (table). Isolate identifications were confirmed by molecular methods. Susceptibility testing was performed according to CLSI agar dilution methodology (M11, 2012). Other antimicrobials tested included clindamycin (CD), metronidazole (MTZ), and moxifloxacin (MXF). In addition, the activity of DLX and MXF were compared at standard pH 7.0 and at pH 6.0. DLX had the lowest MIC50/90 values against both GP and GN species and was 32-fold more active than MXF for all organisms. For BF, DLX was 4- to 16-fold more active than the other comparators. For CP, DLX was 32- to 64-fold more active than the 3 comparators. When comparing the activity of DLX and MXF at pH 6 vs. pH 7, DLX had the same MIC50/90 values while MXF MIC50/90 values were 2-fold less active at the lower pH (Table 1). Of the 84 clinical trial isolates, 21 were recovered from subjects in the microbiologically evaluable at follow-up (MEFU) population. All of the subjects had a favorable microbiological response (presumed eradication) at FU. DLX demonstrated potent in vitro antibacterial activity against anaerobic isolates tested, including BF and CP and was more active than MXF. For all isolates combined, DLX activity was unchanged at lower pH while MXF MIC values increased 2-fold. These data suggest that DLX activity remains potent at a lower pH common at sites of infection. D. Shortridge, Melinta Therapeutics: Research Contractor, Research support. S. P. McCurdy, Melinta Therapeutics: Employee, Salary. P. R. Rhomberg, Melinta Therapeutics: Research Contractor, Research support. M. D. Huband, Melinta Therapeutics: Research Contractor, Research support. R. K. Flamm, Melinta Therapeutics: Research Contractor, Research support.

Delafloxacin, a recently approved anionic fluoroquinolone, was tested within an international resistance surveillance program. The in vitro susceptibilities of 7,914 indicated pathogens causing acute bacterial skin and skin structure infections (ABSSSI) were determined using Clinical and Laboratory Standards Institute (CLSI) broth microdilution MIC testing methods.

DLX, an anionic fluoroquinolone antibiotic is in development for treatment of ABSSSI. This analysis focuses on patients with Gram-negative pathogens from 2 phase 3 ABSSSI trials. 2 double-blind trials of adults with ABSSSI randomized patients 1:1 to receive either IV/oral DLX monotherapy or VAN 15 mg/kg with AZ for 5 – 14 days, conducted in US, Europe, Latin America and Asia. Key endpoints were objective response at 48–72 hours and investigator assessment of outcome at Follow-up (FU day 14) and Late Follow-up (LFU day 21–28). In the 2 studies, 1042/1510 patients enrolled had a pathogen at baseline (Micro Intent to Treat [MITT]). 197 patients (19%) had a GN pathogen isolated at baseline; most were part of a mixed infection. 66% were male, mean age 52 yrs. Median digital erythema area at baseline was ~227 cm2. 25% had cellulitis, 27% abscesses, and 47% wound infections.The most common location was the lower extremities. K. pneumoniae was the most frequent GN isolate. The MIC50, MIC90 and MIC range were 0.12, 0.25, 0.03–4 µg/mL for K. pneumoniae; 0.12, 2, 0.03->8 µg/mL for E. cloacae; 0.06, 4, 0.008–4 µg/mL for E. coli; and 0.25, 4, 0.12->8 µg/mL for P. aeruginosa. Key endpoints are shown below: In the overall population, the proportion of patients with at least one treatment-emergent AE (TEAE) was similar for DLX (45.1%) compared with VAN/AZ (47.7%). The most frequent treatment-related adverse events were gastrointestinal in nature including nausea seen in 6.1% and 4.3% and diarrhea seen in 6.1% and 2% of DLX and VAN/AZ patients respectively. There were 0.8% of DLX patients and 2.4% VAN/AZ patients who discontinued treatment due to treatment related AEs. GN pathogens are a consideration in ABSSSI antibiotic selection. Fixed dose DLX monotherapy was comparable to VAN/AZ in effects in patients with GN pathogens based on the early and investigator assessed response and micro response. DLX appears effective and well tolerated in patients with GN pathogens in ABSSSI. W. O’Riordan, Melinta: Investigator, Research grant; S. Overcash, Melinta: Investigator, Research grant; L. Lawrence, Melinta: Employee, Salary; S. P. McCurdy, Melinta Therapeutics: Employee, Salary; C. Tseng, Melinta: Consultant and Research Contractor, Consulting fee; S. K. Cammarata, Melinta Therapeutics: Employee, Salary

Novel (non-fluoroquinolone) inhibitors of bacterial type II topoisomerases (NBTIs) are an emerging class of antibacterial agents. We report an optimized series of cyclobutylaryl-substituted NBTIs. Compound 14 demonstrated excellent activity both in vitro (S. aureus MIC90=0.125μg/mL) and in vivo (systemic and tissue infections). Enhanced inhibition of Topoisomerase IV correlated with improved activity in S. aureus strains with mutations conferring resistance to NBTIs. Compound 14 also displayed an improved hERG IC50 of 85.9μM and a favorable profile in the anesthetized guinea pig model.

Abstract Background Delafloxacin (DLX) is an investigational anionic fluoroquinolone with an NDA that is under US FDA review to treat acute bacterial skin and skin structure infections and is undergoing Phase 3 studies to treat community-acquired bacterial pneumonia. Methods A total of 36,683 Gram-positive (GP) and -negative (GN) bacteria isolated during 2014–2016 were selected from medical centers in the US and Europe. Susceptibility testing (S) was performed by frozen-form broth microdilution methods for DLX and comparators. Results DLX was very active against Staphylococcus aureus (SA, n = 9,355; MIC50/90, 0.008/0.5 mg/L) while the levofloxacin (LEV) MIC50/90 was 0.25/>4 mg/L (67.9%S). The MIC50/90 for methicillin-resistant SA (MRSA) was 0.12/1 mg/L. For MRSA, all isolates were S to vancomycin and daptomycin (DAP), linezolid and tigecycline (TGC) S was ≥99.9%. Decreased rates of S were noted for LEV (29.8%), clindamycin (72.9%), and erythromycin (17.3/17.8%; CLSI/EUCAST). Minocycline (MIC50/90, 0.12/0.25 mg/L), ceftaroline (MIC50/90, 0.25/0.5 mg/L), DAP (MIC50/90, 0.5/0.5 mg/L), and DLX (MIC50/90, 0.015/0.5 mg/L) were the most active agents tested against coagulase-negative staphylococci. Against Streptococcus pneumoniae (SPN), the MIC50/90 for DLX (0.015/0.03 mg/L) and TGC (0.03/0.06 mg/L) were the lowest among the agents tested. The DLX MIC50/90 values did not vary among the penicillin-S, -intermediate, and -R subgroups of SPN. The MIC50/90 values for DLX against S. pyogenes and S. agalactiae were 0.015/0.03 mg/L. DLX was highly active against Haemophilus influenzae. The DLX MIC50/90 (≤0.001/0.004 mg/L) was the same for β-lactamase positive and negative H. influenzae. Against Enterobacteriaceae, 76.0% of DLX MIC values were ≤1 mg/L. Susceptibility to LEV was 80.8%, and S to ceftriaxone, ceftazidime (CAZ), and cefepime ranged from 78.5–86.3%. A total of 72.6% of Pseudomonas aeruginosa isolates exhibited DLX MIC values ≤1 mg/L, while LEV S was 73.2% and CAZ was 81.6%. The MIC50/90 for both DLX and LEV were 0.5/>4 mg/L, respectively. Conclusion DLX was active against a broad range of GP and GN bacteria, including MRSA and multidrug-resistant SPN. DLX merits further study as therapy in infections in which these organisms may occur. Disclosures R. K. Flamm, Melinta Therapeutics: Research Contractor, Research grant D. Shortridge, Melinta Therapeutics: Research Contractor, Research grant M. D. Huband, Melinta Therapeutics: Research Contractor, Research grant S. McCurdy, Melinta Therapeutics: Employee, Salary M. A. Pfaller, Melinta Therapeutics: Research Contractor, Research grant

ABSTRACT Delafloxacin is an investigational anionic fluoroquinolone antibiotic with broad-spectrum in vitro activity, including activity against Gram-positive organisms, Gram-negative organisms, atypical organisms, and anaerobes. The in vitro activity of delafloxacin and the percent microbiological response in subjects infected with fluoroquinolone-susceptible and nonsusceptible Staphylococcus aureus isolates were determined from two global phase 3 studies of delafloxacin versus vancomycin plus aztreonam in patients with acute bacterial skin and skin structure infections (ABSSSI). Patients from 23 countries, predominately the United States but also Europe, South America, and Asia, were enrolled. The microbiological intent-to-treat (MITT) population included 1,042 patients from which 685 S. aureus isolates were submitted for identification and susceptibility testing per CLSI guidelines at the central laboratory (JMI Laboratories, North Liberty, IA). The comparator fluoroquinolone antibiotics included levofloxacin and ciprofloxacin. Nonsusceptibility to these antibiotics was determined using CLSI breakpoints. S. aureus isolates were 33.7% levofloxacin nonsusceptible (LVX-NS). The delafloxacin MIC 90 values against levofloxacin-nonsusceptible S. aureus , methicillin-resistant S. aureus (MRSA), and methicillin-susceptible S. aureus isolates were all 0.25 μg/ml. Delafloxacin demonstrated high rates of microbiological response against LVX-NS isolates as well as isolates with documented mutations in the quinolone resistance-determining region (QRDR). S. aureus was eradicated or presumed eradicated in 98.4% (245/249) of delafloxacin-treated patients. Similar eradication rates were observed for delafloxacin-treated subjects with levofloxacin-nonsusceptible S. aureus isolates (80/81; 98.8%) and MRSA isolates (70/71; 98.6%). Microbiological response rates of 98.6% were observed with delafloxacin-treated subjects with S. aureus isolates with the S84L mutation in gyrA and the S80Y mutation in parC , the most commonly observed mutations in global phase 3 studies. The data suggest that delafloxacin could be a good option for the treatment of infections caused by S. aureus isolates causing ABSSSI, including MRSA isolates, where high rates of ciprofloxacin and levofloxacin nonsusceptibility are observed. (The phase 3 studies described in this paper have been registered at ClinicalTrials.gov under identifiers NCT01984684 and NCT01811732.)

Antimicrobial susceptibility testing (AST) is performed to assess the in vitro activity of antimicrobial agents against various bacteria. The AST results, which are expressed as minimum inhibitory concentrations (MICs) are used in research for antimicrobial development and monitoring of resistance development and in the clinical setting for antimicrobial therapy guidance. Dalbavancin is a semi-synthetic lipoglycopeptide antimicrobial agent that was approved in May 2014 by the Food and Drug Administration (FDA) for the treatment of acute bacterial skin and skin structure infections caused by Gram-positive organisms. The advantage of dalbavancin over current anti-staphylococcal therapies is its long half-life, which allows for once-weekly dosing. Dalbavancin has activity against Staphylococcus aureus (including both methicillin-susceptible S. aureus [MSSA] and methicillin-resistant S. aureus [MRSA]), coagulase-negative staphylococci, Streptococcus pneumoniae, Streptococcus anginosus group, β-hemolytic streptococci and vancomycin susceptible enterococci. Similar to other recent lipoglycopeptide agents, optimization of CLSI and ISO broth susceptibility test methods includes the use of dimethyl sulfoxide (DMSO) as a solvent when preparing stock solutions and polysorbate 80 (P80) to alleviate adherence of the agent to plastic. Prior to the clinical studies and during the initial development of dalbavancin, susceptibility studies were not performed with the use of P-80 and MIC results tended to be 2-4 fold higher and similarly higher MIC results were obtained with the agar dilution susceptibility method. Dalbavancin was first included in CLSI broth microdilution methodology tables in 2005 and amended in 2006 to clarify use of DMSO and P-80. The broth microdilution (BMD) procedure shown here is specific to dalbavancin and is in accordance with the CLSI and ISO methods, with step-by-step detail and focus on the critical steps added for clarity.

In over a decade (2002 to 2012) of Staphylococcus aureus surveillance testing on 62,195 isolates, dalbavancin was demonstrated to be active against isolates that were either susceptible or nonsusceptible to daptomycin, linezolid, or tigecycline. Nearly all (99.8%) multidrug-resistant methicillin-resistant S. aureus isolates were inhibited by dalbavancin at ≤0.12 μg/ml (MIC50/90, 0.06/0.06 μg/ml), the current U.S. Food and Drug Administration (U.S. FDA) breakpoint. Overall, only 0.35% of the monitored S. aureus isolates had a dalbavancin MIC of either 0.25 or 0.5 μg/ml (i.e., were nonsusceptible).