697. Pseudomonas aeruginosa PcrV and Psl, the Molecular Targets of Bispecific Monoclonal Antibody MEDI3902, Are Conserved Among Diverse Hospital Isolates Collected From an International Surveillance Study

Background Pseudomonas aeruginosa is a frequent cause of life-threatening infections in mechanically ventilated patients and is associated with high mortality rates. Bispecific monoclonal antibody MEDI3902 targeting Pa type-3-secretion system (PcrV) and the Psl exopolysaccharide is currently under phase 2b development for the prevention of pneumonia in mechanically ventilated subjects with Pa colonization in the lower respiratory tract. In this study, we sought to survey a vast collection of global Pa clinical isolates for presence of pcrV and psl loci and MEDI3902 epitope conservation to evaluate the magnitude of Pa strain coverage by MEDI3902. Methods 913 Pa clinical isolates were collected from diverse patients and geographical locations in 2004–2014. Whole genome sequencing of the full collection was performed via MiSeq 2 × 250 runs (Illumina®). PcrV and Psl expression was detected by immunoblotting and ELISA, respectively. The crystal structure of anti-PcrV fab and PcrV fragment complex-crystals was solved at 2.8 Å resolution. MEDI3902 activity against representative isolates was tested in cytotoxicity and opsonophagocytosis assays and in a murine pneumonia model. Results Whole-genome sequencing revealed intact pcrV and psl genetic elements in 99% and 94% of isolates, respectively. We identified 46 variants of PcrV that were all bound by the anti-PcrV moiety of MEDI3902 and confirmed through crystal structure analysis that antibody-antigen contact residues were preserved in all variants. Similarly, anti-Psl binding was confirmed for selected isolates containing the complete Psl operon and strains lacking non-essential psl genes. Importantly, 99.9% of isolates contained the full complement of either genetic element. Consistent with these results, we observed potent MEDI3902 activity against diverse strain types, including strains that expressed only a single target. Conclusion Our results indicate PcrV and Psl are highly prevalent in recent clinical isolates from around the world, suggesting that MEDI3902 can mediate broad coverage against Pa. Disclosures D. E. Tabor, Astra Zeneca: employee, Salary.