Development of a High Yielding Bioprocess for a Pre-fusion RSV Subunit Vaccine

Respiratory syncytial virus (RSV) is a highly contagious virus causing severe infection in infants and the elderly. Various approaches are being used to develop an effective RSV vaccine. The RSV fusion (F) subunit, particularly the cleaved trimeric pre-fusion F, is one of the most promising vaccine candidates under development. The pre-fusion conformation elicits the majority of neutralizing antibodies during natural infection. However, this pre-fusion conformation is metastable and prone to conversion to a post-fusion conformation, thus hindering the potential of this construct as a vaccine antigen. The Vaccine Research Center (VRC) at the National Institutes of Health (NIH) designed a structurally stabilized pre-fusion F glycoprotein, DS-Cav1, that showed high immunogenicity and induced a neutralizing response in animal studies. To advance this candidate to clinical manufacturing, a production process that maintained product quality (i.e. a cleaved trimer with pre-fusion conformation) and delivered high protein expression levels was required. This report describes the development of the vaccine candidate including vector design and cell culture process development to meet these challenges. Co-transfection of individual plasmids to express DS-Cav1 and furin (for DS-Cav1 cleavage and activation) demonstrated a superior protein product expression and pre-fusion conformation compared to co-expression with a double gene vector. A top clone was selected based on these measurements. Protein expression levels were further increased by seeding density optimization and a biphasic hypothermia temperature downshift. The combined efforts led to a high-yield fed-batch production of approximately 1,500 mg/L (or up to 15,000 doses per liter) at harvest. The process was scaled up and demonstrated to be reproducible at 50 L-scale for toxicity and Phase I clinical trial use. Preliminary phase I data indicate the pre-fusion antigen has a promising efficacy (Crank et al., 2019).