1047. Development of a Next Generation 30+ Valent Pneumococcal Conjugate Vaccine (VAX-XP) Using Site-Specific Carrier Protein Conjugation

Background

Due to the diversity of serotypes, exacerbated by the phenomenon of serotype replacement, there remains an unmet medical need for a pneumococcal conjugate vaccine (PCV) containing additional serotypes. Using a cell-free protein synthesis (CFPS) platform to produce an enhanced carrier protein (eCRM®) based on the CRM197 sequence, Vaxcyte is developing a PCV encompassing over 30 serotypes. The eCRM carrier protein contains multiple insertions of the non-native amino acid para-azidomethyl-L-phenylalanine (pAMF) that facilitates site-specific conjugation of the pneumococcal polysaccharides (PS) to eCRM. Unlike conventional methodologies, site-selective conjugation enhances process consistency and increases capacity for inclusion of additional serotypes in a PCV without promoting carrier suppression. Using this platform, the aim of the current study was to employ CFPS technology to construct a 31-valent PCV and evaluate its immunogenicity in New Zealand White (NZW) rabbits.

Methods

The eCRM carrier protein was individually conjugated to each of 31 selected pneumococcal PSs using copper-free click chemistry to produce 31 Conjugate Drug Substances (DS), which were then mixed with aluminum phosphate to produce the VAX-XP Drug Product. 24 of the DS conjugates in VAX-XP were generated at manufacturing scale. Two doses of VAX-XP were administered to NZW rabbits at 0 and 21 days to assess its ability to elicit anti-capsular IgG antibodies. Additionally, rabbits were also administered either Prevnar13 or a mixture of Pneumovax 23 and 8 incremental PS in isotonic saline, as comparators.

Results

VAX-XP showed conjugate-like immune responses for all 31 serotypes, as demonstrated by superior responses to PS-based vaccines and comparable responses to Prevnar13. IgG responses for VAX-XP compared with Prevnar13 and Pneumovax 23 at 14 days post dose 2

Conclusion

These results demonstrate that increasing the number of pneumococcal serotypes does not result in immunological attenuation in any of the serotypes contained in VAX-XP relative to the current standard of care. Furthermore, the data confirm the scalability and reproducibility of the CFPS platform in the production of VAX-XP conjugates, creating the foundation for a next generation broad-valency PCV.

Disclosures

Chris Behrens, PhD, Vaxcyte, Inc. (Employee) Jeff Fairman, PhD, Vaxcyte, Inc. (Employee) Paresh Agarwal, PhD, Vaxcyte, Inc. (Employee) Shylaja Arulkumar, MS, Vaxcyte, Inc. (Employee) Sandrine Barbanel, MS, Vaxcyte, Inc. (Employee) Leslie Bautista, n/a, Vaxcyte, Inc. (Employee) Aym Berges, PhD, Vaxcyte, Inc. (Employee) John Burky, BS, Vaxcyte, Inc. (Employee) Peter Davey, MS, Vaxcyte, Inc. (Employee) Chris Grainger, PhD, Vaxcyte, Inc. (Employee) Sherry Guo, PhD, Vaxcyte, Inc. (Employee) Sam Iki, MS, Vaxcyte, Inc. (Employee) Mark Iverson, BS, Vaxcyte, Inc. (Employee) Neeraj Kapoor, PhD, Vaxcyte, Inc. (Employee) Olivier Marcq, PhD, Vaxcyte, Inc. (Employee) Thi-Sau Migone, PhD, Vaxcyte, Inc. (Employee) Lucy Pill, MS, Vaxcyte, Inc. (Employee) Mohammed Sardar, n/a, Vaxcyte, Inc. (Employee) Paul Sauer, MBA, Vaxcyte, Inc. (Employee) James Wassil, MS MBA, Vaxcyte, Inc. (Employee)

Non-clinical immunological comparison of a Next-Generation 24-valent pneumococcal conjugate vaccine (VAX-24) using site-specific carrier protein conjugation to the current standard of care (PCV13 and PPV23)

Despite widespread utilization of pneumococcal conjugate vaccines (PCVs) and the resultant disease reduction, the development of PCVs containing additional serotypes remains a public health priority due to serotype replacement and the resultant shift to non-vaccine containing serotypes. However, incorporating additional serotypes to existing PCVs using conventional technologies has proven problematic. Immune responses to individual serotypes have consistently decreased as more polysaccharide-conjugates are added due to carrier suppression. Using our proprietary cell-free protein synthesis (CFPS) platform, we have successfully produced eCRM® based on the CRM₁₉₇ sequence for use as an enhanced carrier protein to develop a 24-valent PCV. The eCRM carrier protein contains multiple non-native amino acids (nnAAs) located outside of the primary T-cell epitope regions, thereby enabling site-specific covalent conjugation of the pneumococcal polysaccharides to the nnAAs to consistently expose the critical T-cell epitopes. eCRM also serves to reduce structural heterogeneity associated with classic reductive-amination conjugation while promoting formation of the conjugate matrix structures, the hallmark of PCVs. This process serves to increase the overall polysaccharide:protein ratio, enabling the inclusion of more serotypes while minimizing carrier-mediated immunological interference. The aim of this non-clinical study was to construct a 24-valent PCV and evaluate its immunogenicity. Using the XPressCF® CFPS platform, the eCRM carrier protein was separately conjugated through nnAAs to each of the 24 pneumococcal polysaccharides through click chemistry and mixed with aluminum phosphate to produce VAX-24, Vaxcyte's proprietary PCV preclinical candidate. VAX-24, Prevnar13® and Pneumovax®23 were administered to New Zealand White rabbits to compare the resulting opsonophagocytic activity (OPA) and anti-capsular IgG antibodies. VAX-24 showed conjugate-like immune responses to all 24 serotypes based on comparable OPA and IgG responses to Prevnar13 and higher responses than Pneumovax 23. This study demonstrates the utility of site-specific conjugation technology in a preclinical setting and the potential for a PCV with improved serotype coverage.