Shot in the arm for Affinivax as it challenges Pfizer's Prevnar

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Already facing one rival to its blockbuster pneumococcal vaccine Prevnar, Pfizer could have another to contend with, after Affinivax reported positive phase 2 results with its ASP3772 candidate – plus a new breakthrough designation from the FDA.

Last year, Pfizer made almost $6 billion from its Prevnar 13 vaccine, which covers 13 common serotypes of Streptococcus pneumoniae – a bacteria which causes non-invasive illnesses like pneumonia, sinusitis and middle ear infections, as well as invasive diseases like meningitis. That is equivalent to 14% of Pfizer's total revenues.

The bulk of Prevnar 13 sales come from use in adults, and with Merck & Co in the final stages of regulatory review of its rival vaccine V114 – covering 15 serotypes and filed for adult use – the franchise is facing some serious competition for the first time.

Pfizer is fighting back with the approval of its new Prevnar 20 shot last month, covering 20 serotypes, but ASP3772 is attempting to go even further with coverage of 24 serotypes thanks to Affinivax' multiple antigen presenting system, or MAPS.

The shot also includes two "conserved" pneumococcal proteins – antigens that seem always to be present in the bacterium regardless of its serotype, so could potentially provide protection against an even broader range.

There's another key difference as well, according to the biotech, which is developing the new shot with big pharma partner Astellas under the terms of an agreement signed in 2017.

The MAP technology combines polysaccharide and protein antigens – joined with a biotin-rhizavidin bond – that can stimulate both B cell (antibody) and T cell immune responses with a single shot.

Conventional conjugate vaccines use covalent bonding to join the polysaccharide and protein antigens, which affects the way both are presented to the immune system.

The phase 2 trial involved more than 500 subjects aged 50 or over and showed that Affinivax was able to stimulate an antibody response to each of the 24 polysaccharides as well the conserved pneumococcal proteins.

The study also made a direct comparison with Prevnar 13, and found that for the 13 serotypes the two shots had in common, ASP3772 demonstrated a "similar or better" immune response measured by the level of antibody generation and their functional activity.

Affinivax' shot performed better than Prevnar 13 against serotype 3 – arguably the most problematic variant in terms of clinical severity – at all doses, and against serotypes 5 and 19F at the highest dose.

ASP3772 was also pitted against the combination of Prevnar 13 and Merck's older polysaccharide-based Pneumovax 23 vaccine. It achieved statistically higher immune responses for serotypes 3, 4, 5, 6A, 7F, 9V, and 18C at the highest dose tested, and at least similar – and in most cases statistically higher – for the remaining 11 serotypes.

"We believe that ASP3772 has the potential to offer broader protection than any pneumococcal vaccine currently on the market or in clinical testing today , and we look forward to continuing to advance this important vaccine candidate through clinical trials in both adults and infants," said Affinivax' chief executive Steve Brugger.

Preparation for a phase 3 programme is underway, and in the meantime the pneumococcal vaccine space looks set for a shake-up, as Pfizer tries to transition to Prevnar 20 and Merck tries to carve a slice of the market for V114.

Also emerging from the wings is Vaxcyte, which has a 24-valent shot in early development and hopes to have initial clinical data from that programme in late 2022/early 2023.