Injectable mRNA-based vaccines were instrumental in halting the spread of COVID-19 infections. Now researchers from Yale University, Howard Hughes Medical Institute, and elsewhere are looking into developing safe inhalable mechanisms for delivering vaccines and other kinds of therapies into the body. In a paper published in Science Translational Medicine, the team described a nanoparticle-delivery-based system for carrying mRNA vaccines against the SARS-CoV-2 virus that can be delivered nasally or intratracheally. They used the system to deliver the vaccine to mice.

The primary goal of this study was to develop an inhalable or topically administered delivery vehicle that could “facilitate minimally invasive and lung-targeted therapies for pulmonary pathologies,” the researchers wrote. They also aimed to “provide a proof of concept for the translational potential of our vehicle as a platform for mucosal vaccination against a respiratory pathogen.”

For some people, getting vaccinated or receiving other kinds of injectable treatments is extremely difficult because of their aversion to needles. An inhalable vaccine would avoid this issue and that is certainly one viable reason to develop them. But there are biological reasons for why inhalable vaccines are preferable particularly for respiratory pathogens, according to Mark Saltzman, PhD, a professor of chemical and biomedical engineering and cellular and molecular physiology at Yale University and a senior author on the paper. He and his colleagues elaborated on the biological basis for inhalable vaccines in a separate paper published last year.

Simply put, “the kind of immune response depends on how the virus comes into your body,” he explained. Since respiratory pathogens typically enter the body through the nasal or tracheal pathway, the body’s immune response will be different from a pathogen that enters the body through another means such as from a cut. “By delivering the vaccine in the same way [as the pathogen], we are trying to induce the special characteristics of mucosal immunity which [provide] a more protective immune response.”

Developing inhalable vaccines is challenging for a number of reasons. For example, the vaccine must be able to slip through physiological barriers like the respiratory mucosal layer, and there has to be enough vaccine to get the desired therapeutic response. Furthermore, an ideal vehicle would deliver the vaccine without causing inflammation in the respiratory mucosa as seen with at least one other method that used liquid nanoparticles, according to the paper.

For their vaccine vehicle, the research team used a biodegradable poly(amine-co-ester) (PACE) polymer that forms so-called polyplexes with mRNA. Previous studies showed that PACE polymers can safely and securely encapsulate nucleic acids for delivery in vivo. An important benefit of these polymers is that their chemical composition can be modified during polymerization by using different components and various ratios of those components as well as by changing the synthesis conditions. “We capitalized on the highly tunable nature of PACE polyplexes by screening a library of delivery vehicles with different chemical end groups and [polyethylene glycol] content to optimize for high protein expression after local delivery to the respiratory tract,” they wrote.

Identifying the right chemistry for polymer was probably the most involved part of the research process. “We knew that we could change the chemistry of PACE in ways that could potentially change its effectiveness [but] we could not from first principles predict which ones were going to work the best, so we had to develop [various] screening methods,” he explained.

Once they ultimately found an optimal polymer preparation that was both safe and well-tolerated by mice, the team used it to deliver mRNA vaccines up to 4,000 nucleotides long to mice. Mice that were vaccinated with PACE-mRNA showed “de novo immunity to SARS-CoV-2 through both systemic and local induction of antibodies” when they were exposed to the virus. Furthermore, the polymer vehicle successfully protected the mRNAs from being degraded by enzymes in the mucosa as it made its way to the lungs. Imaging various organs from vaccinated mice confirmed that the delivery was confined to the lungs and high levels of protein expression were present. Other tests showed recruitment of antigen-specific B and T cells to the lungs further indicating that the vaccine successfully triggered an immune response.

They also observed greater survival rates among mice that were exposed to what should have been a lethal dose of virus. And there were no unusual side effects in the test subjects. Some mice did show signs of weight loss but no more than would be expected from other procedures in the lab such as administering anesthesia or a placebo injection.

Ultimately, the goal is to make inhalable vaccines and therapies work in humans. A topic for the team’s future studies will be how long a single dose lasts. Saltzman and his colleagues are currently planning experiments aimed at developing and testing inhalable therapies in nonhuman primates in collaboration with Xanadu Biosciences. The privately held nanoparticle delivery company is developing an intranasal SARS-CoV-2 vaccine booster and licensed the PACE technology last year. Those studies will also assess the vaccine’s safety and tolerance in primates.

“There is a lot of interest in next-generation vaccines now. If we can generate data in nonhuman primates fast enough, we will be able to take advantage of some of those opportunities,” Saltzman said.

In the future, PACE could be used to deliver COVID-19 boosters or to administer other kinds of routine vaccines such as annual flu shots. But that’s just one example of what’s possible. There are applications beyond vaccines that may benefit from the use of inhalable therapies. For example, it may be possible to use the same mechanisms to package up and deliver gene replacement therapies for respiratory diseases like cystic fibrosis.

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