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Inhaled vaccines must resist degradation and penetrate the mucosal lining in the airways and lungs.
The COVID-19 pandemic remains a global threat due to continued transmission of the SARS-CoV-2 virus despite the availability of several intramuscular vaccines. Vaccines administered intramuscularly via injection can elicit strong systemic responses and thus prevent the occurrence of severe disease. These vaccines don’t generally, however, lead to the development of mucosal immunity, which neutralizes the pathogen at the point of entry into the body. Mucosal immunity reduces viral replication at the level of the mucosa, which results in lower carriage by the vaccinated individual, hence reducing transmission.
Effective vaccines delivered via inhalation can lead to both strong mucosal and systemic immunity. The first such product against COVID-19 (Convidecia Air)—a collaborative effort between pharma company CanSino Biologics (CanSinoBIO, Tianjin, China) and respiratory device developer Aerogen (Galway, Ireland)—received regulatory approval in China in early September 2022 (1).
However, developing these vaccines can be challenging. Active substance formulation and delivery-device development must be pursued simultaneously. Once delivered, the vaccination agent must penetrate protective mucosal barriers and survive long enough to stimulate the desired immune response.
Mucosal membranes line the respiratory, digestive, and urogenital tracts and form an interconnected network of mucosal compartments (2–4). Vaccination that leads to mucosal immunity at one of these mucosal sites thus leads to immunity at distant mucosal sites. Mucosal immune responses involve activation of antigen-presenting cells (APCs) and the production of many different types of immune cells, antibodies, and inflammatory cytokines/chemokines.
Targeting mucosal layers in the upper and lower respiratory tract and lungs is an attractive approach to vaccination because most airborne pathogens enter the body through this route and infect these tissues (5). “Generation of mucosal immunity results in activation of many parts of the immune system, not just antibodies. In addition, replication of viruses is prevented in the respiratory system, enabling interruption of the transmission cycle,” explains J. Robert Coleman, CEO of Codagenix.
Vaccines delivered via inhalation enable targeting of the respiratory tract mucosa and generation of both humoral and cell-mediated immunity, according to Pierre A. Morgon, executive vice president, Portfolio Strategy and Supranational Affairs and managing director, CanSinoBIO Europe. It is also possible to achieve good drug absorption, fast onset of action, and high bioavailability with inhaled vaccines (6).
In addition, they eliminate the need for needle-based administration, reducing the risk of needle-stick injuries and allowing avoidance of invasive injections, fear of which may keep some people from getting vaccinated (7).
In fact, inhaled vaccines delivered as liquids using nebulizers can potentially be administered with lower requirement for extensive healthcare infrastructure or as many trained healthcare personnel within an immunization center (2). Delivery by inhalation should, therefore, make large-scale mass vaccination easier to conduct, according to Ronan MacLoughlin, associate director of R&D, science and emerging technologies at Aerogen.
Importantly, less vaccine is needed for delivery via inhalation—typically less than 0.2 mL vs. 0.5 mL for injection, says Morgon. “Lower doses means more doses can be produced per batch, and mass vaccination can thus be achieved more cost-efficiently,” he says.
Furthermore, administration via inhalation can be theoretically achieved through the mouth or nose. The preferred route is, says MacLoughlin, entirely dependent on the target receptors for the vaccine. “If nasally expressed, then nasal should be the preferred route. Inhalation into the lung via the nose would allow for both nasal and lung deposition. Oral inhalation allows for a larger dose to be delivered to the lung, given that avoiding the nasal passages will increase the available fraction of aerosol,” he comments.Aerogen’s proprietary aerosol-generating technology converts the liquid vaccine into fine aerosol droplets that are suitable for delivery of the vaccine deep within the lung.
Some are of the opinion that vaccination through the mouth may result in enhanced induction of mucosal immunity. Researchers at McMaster University, for instance, have shown inhalation of a tuberculosis vaccine to be more effective than delivery via nasal sprays, because the vaccine penetrates much deeper into the airway (8).
Because inhaled vaccines provide local immunity to the respiratory tract, they are seen to be ideal solutions for interrupting the spread of viruses with high transmission rates and the potential to lead to global pandemics (9).
The specific types of vaccines that can be formulated for inhalation delivery is not fully understood yet, according to Jean-Denis Shu, SVP of Medical Affairs with CanSinoBIO. “The critical factors are determined by the target receptor, the vaccine’s stability in the lung, and whether sufficient dose can be given to illicit the immune response,” he observes. CanSinoBIO’s inhaled COVID-19 vaccine, now approved in China, is based on an adenovirus-vectored vaccine (10).
“Inhalation delivery offers the potential advantage of bypassing anti-vector immunity for viral vector vaccines for which people may have already developed some immunity through, for instance, exposure to cold viruses,” Morgon adds. The formulation of CanSinoBIO’s inhaled vaccine (Convidecia Air) is the exact same as that of its intramuscular vaccine (Convidecia) and there is no need for an adjuvant to boost immunogenicity.
Codagenix’s inhaled COVID-19 vaccine candidate is a live, attenuated virus that, according to Coleman, safely and stably brings not only the SARS-CoV-2 spike protein, but many other proteins of the virus to the immune system, allowing for broad protection.
Beyond these two approaches, there are inhalation vaccines under development based on attenuated influenza virus, parainfluenza virus (PIV) 5, lentiviruses, Newcastle disease virus (NDV), and vesicular stomatitis virus (VSV); bacterium vectors, nucleic acids (messenger RNA, DNA), and protein subunits (3). Unlike the CanSinoBIO vaccine, the latter require formulation with adjuvants to ensure sufficient immunogenicity (4,11). A range of adjuvants have been explored, such as polyethyleneimine, N-[1-(2,3-Dioleoyloxy) propyl]-N,N,N-trimethylammonium chloride, chitosan, toxoids, cytokines, saponins, lipids, and toll-like receptor agonists.
The need for effective delivery technologies remains the main challenge for realizing successful mucosal vaccines, including those administered via inhalation (12). Not only are strong adjuvants required for many types of vaccines, it can be difficult to maintain the integrity of the vaccines. The vaccination agent must resist degradation in the harsh mucosal environment in order to have time to reach the immune inductive tissue and elicit the desired immune response while also ensuring that oral tolerance does not develop (11). Overall, therefore, appropriate formulation of inhalation vaccines is crucial for their success (13).
Inhalation vaccines can be formulated as liquids or powders. Dry powders have the potential for long-term storage without cold-chain requirements, while liquid formulations are generally simpler to develop and implement, notes MacLoughlin. Dry powder formulations can be generated by various methods, such as spray drying and thin-film freezing (4,6). It is crucial to control the particle size and particle size distribution to achieve the desired release profiles and distribution in the lung.
The aqueous nature of liquid formulations, meanwhile, allows for rapid interaction with the airway epithelium. “Such interactions may be subdued for dry powders due to local conditions in the airway surface lining fluid,” Morgon observes. He also points out that the need for cold-chain management is not an automatic disadvantage for liquid vaccines, because storage and handling capabilities at 3 to 8 °C are generally available at most locations.
The physicochemical attributes of vaccine actives delivered intranasally must allow the vaccine to mimic the wild type natural virus that comes in through the airway, according to Coleman. “Such behavior is essential to induce very broad immunity and stop onward transmission,” he contends. He adds that Codagenix chose to use an attenuated live virus rather than a viral vector or protein subunit for this reason. “Using an attenuated virus allows for presentation of multiple viral proteins and avoids any underlying immunity issues, affording increased vaccine uptake,” states Coleman.
A consideration equally important to the design of the vaccine active and formulation is the development of an optimized vaccine/device combination, according to Shu. “Picking any delivery device off-the-shelf and applying that increases the risk that the immunization is not effective. A solid understanding of how the vaccine and device interact are critical,” Shu avers.
Morgon agrees. “Understanding formulation, device performance, delivered dose, distribution of dose within the airways, etc. all impact the
likelihood of success.”
“Co-development, such as that conducted by Aerogen and CanSinoBio, has enabled an effective immunization approach that uses a fraction of the injected dose by volume or mg of drug substance,” MacLoughlin concludes.
Devices use for the delivery of inhalation vaccines should, ideally, be patient agnostic. “Taking the variability out of the patient input in the process is a key requirement in order to ensure consistent, reproducible, and reliable dose across a population,” MacLoughlin observes. To this end, Aerogen has focused on development of devices that require the patient to make only a single inhalation for their entire vaccine dose. “This way, there is no requirement for coordination of breath with aerosol generation, which is a key failing of pressurized metered-dose inhalers, and every patient has the same dose available to them for inhalation,” he notes.
Development of mucosal vaccines, including inhalation vaccines, faces an additional hurdle that vaccines administered by other routes generally do not. “The historic body of work in vaccinology has focused on the production of antibodies in the bloodstream. Successful vaccines are determined to be so because they result in high levels of antibodies,” Coleman says.
Some mucosal vaccines, however, may generate lower levels of antibodies in the blood. That does not mean they are less effective, but regulators may not be aware of the differences and thus still compare the data for mucosal vaccines to those for injectable versions, Coleman adds. In the case of CanSinoBIO’s Convidecia Air, the humoral and cellular responses are higher with the inhaled booster than with the injectable one, Morgon comments.
One program designed to address this concern has been established by the World Health Organization (WHO). Its Solidarity Trial Vaccines program supports the development of second-generation COVID-19 vaccines with greater efficacy, greater protection against variants of concern, longer duration of protection, and improved storage and/or simplified delivery with needle-free administration (14). In the Phase III trial of Codagenix’s CoviLiv vaccine candidate, both antibody generation and actual efficacy (determined by comparing vaccinated and placebo groups) are being evaluated, so the actual efficacy rate will be measured. CanSinoBIO has also agreed to the request from WHO to include Convidecia Air in the Solidarity trial.
The same group of researchers at McMaster University that compared the delivery of a tuberculosis vaccine via nasal spray and inhalation also have an inhaled, adenoviral-vectored, aerosol COVID-19 candidate in a Phase I clinical trial (8,15).
Scientists at MIT have shown that peptide vaccines with albumin-binding lipid tails when formulated with the adjuvant CpG and administered via inhalation generate long-lasting immune responses (5). A group at University of North Carolina-Chapel Hill, meanwhile, used lung-derived exosomes, or nanoscale vesicles secreted from lung spheroid cells (LSC-Exo), to effectively deliver nucleotide and protein-based vaccines to the lungs via an inhaler (8,16).
In addition to the above inhalation vaccines under development for COVID-19, several groups are investigating the potential of intranasal vaccines intended to induce mucosal immunity. Some of these have been found wanting, most recently the intranasal version of the AstraZeneca viral-vector vaccine (17). These disappointing results, says Morgon, may drive a heightened focus toward the inhalation route.
At the same time, the approval of the CanSinoBIO inhaled COVID-19 vaccine provides evidence that inhalation delivery can be successful at generating mucosal immunity. “We now have significant data on the safety and efficacy of an inhaled vaccine. These data should give others confidence to proceed to trial with what is known to be a broad pipeline of vaccines that may have utility if inhaled,” Morgon says. “Indeed,” he adds, “significant activity has begun/restarted in the area, given that this new route of administration has been shown to be feasible, and on a global mass immunization scale.” Furthermore, if Convidecia Air is convincingly shown to materially reduce viral carriage in vaccinated individuals and therefore curb transmission, CanSinoBIO believes it will turn out to be a blockbuster product.
“The future of inhaled vaccines has just received a significant boost of more useful rapid methods, although further development is required to improve accuracy and repeatability,” he highlights. “Of the different ‘PAT [Process Analytical Technologies] methods’, ATP bioluminescence technology offers a higher sensitivity compared with colorimetric methods.”
In terms of management of cleaning validation processes, software developments are piquing Sandle’s interest, including some that are employing artificial intelligence. “Such packages can help to sort equipment into types where a matrix approach is preferred, as well as enabling scheduling and for setting re-validation targets,” he states.
At its very essence, cleaning validation will require greater consideration, simply as a result of the evolving therapeutic landscape, Kolbert asserts. “The increased use of biological therapeutic agents in pharmaceutical manufacturing will require increased complexity of analytical techniques as activity of the residue is as important as chemical presence,” he summarizes.
Cynthia A. Challener, PhD has been a freelance technical writer for over 20 years, leveraging her education from Stanford University (BS) and University of Chicago (PhD) and 10+ years of industry experience. She currently focuses on pharma/biopharma topics, writing technical articles, white papers, blogs, and other content for a variety of clients in addition to contributing regularly to BioPharm International and Pharmaceutical Technology.
Vol. 47, No. 1
Page: 26–29, 39
When referring to this article, please cite it as C. Challener. Inhalation Vaccine Development. Pharmaceutical Technology 2023 47 (1).