Background Lefamulin (LEF) is a pleuromutilin antibiotic approved by the United States (US) FDA for the treatment of community-acquired bacterial pneumonia in adults. In addition to the potent antibacterial activity, LEF has demonstrated anti-inflammatory activity in an LPS induced lung neutrophilia model in mice. We investigated the anti-inflammatory activity of lefamulin in the H1N1 influenza mouse model in comparison to oseltamivir (OTV) and azithromycin (AZM). Methods Infection was performed in BALB/c mice by intranasal challenge ∼70 pfu influenza virus H1N1 A/PR/8/1934 (Day 0). Treatment with drugs at clinically relevant doses started on Day -1 (LEF 70 mg/kg/day and 110/140 mg/kg/day, s.c., AZM 30 mg/kg/day, i,p. and OTV 20 mg/kg/day p.o.) to Day 6. On Days 3 and 6, bronchioalveolar lavage fluid (BALF) was collected to measure infiltrating lung leukocytes and cytokines. Lung immunopathology following infection was evaluated on Day 6 by gross pathology at termination together with hematoxylin and eosin histopathology. Results In untreated vehicle control animals, the influenza infection progressed as expected with bodyweight loss, increased cell infiltration into the lung and increased levels of TNF-α, IL-6 at day 3 and 6. Treatment with LEF significantly decreased the total immune cell infiltration into the lung by day 6 at both doses tested (Figure Left). Cytokine levels in the BALF were significantly reduced on day 3 when the viral load peaked. Furthermore, LEF showed positive effects on lung gross pathology and survival. Oseltamivir and LEF, at both doses, appeared efficacious in the suppression of the development of influenza-induced bronchi-interstitial pneumonia, whereas azithromycin didn’t show reduced pathology (Figure Right). Anti-inflammatory effects of LEF, AZM and OTV in BALF of H1N1 infected mice infected with influenza virus H1N1. Left: Total lung leukocyte infiltrate following infection on day 0. Bars represent mean ± SEM. Right: Day 6 lung inflammation and degeneration histopathology score. Data are presented as mean histopathology score + SEM (n=10). One-way ANOVA, with Dunnett’s multiple comparisons against the vehicle control treatment was run for each timepoint. * represents significance level of p<0.05. ** indicates p<0.01. Conclusion Lefamulin showed anti-inflammatory treatment following acute influenza virus infection. Following influenza infection LEF was able to significantly reduce lung immunopathology and improve clinical outcome in mice. Results from this experiment were consistent with that observed in the LPS induced lung neutrophilia mouse model. Further studies are warranted to evaluate the immunomodulatory potential of Lefamulin. Disclosures Wolfgang W. Wicha, MSc, Nabriva Therapeutics: Grant/Research Support|Nabriva Therapeutics: Inventor|Nabriva Therapeutics: Employee|Nabriva Therapeutics: Stocks/Bonds Sandy Kimber, PhD, Nabriva Therapeutics: Grant/Research Support Charlotte Cumper, BSc, Nabriva Therapeutics: Grant/Research Support Hon S. Lam, MSc, Nabriva Therapeutics: Grant/Research Support Lorena S. Ballesteros, MSc, Nabriva Therapeutics: Grant/Research Support Christopher Kirkham, PhD, Nabriva Therapeutics: Grant/Research Support Claire Richards, PhD, Nabriva Therapeutics: Grant/Research Support Steven P. Gelone, PharmD, Nabriva Therapeutics: Board Member|Nabriva Therapeutics: Inventor|Nabriva Therapeutics: Employee|Nabriva Therapeutics: Stocks/Bonds|Nabriva Therapeutics: Stocks/Bonds Susanne Paukner, PhD, Nabriva Therapeutics: Inventor|Nabriva Therapeutics: Employee|Nabriva Therapeutics: Stocks/Bonds.