Nearly half of all vaccines go to waste. This is due to the logistical obstacles involved in transporting them to diverse regions of the world. Most vaccines require strict temperature regulation from the manufacturing line to injection into a human arm. Maintaining a constant temperature along the cold (supply) chain is a challenging feat in the best of circumstances.
Rising to the challenge, scientists from ETH Zurich’s Macromolecular Engineering and Organic Chemistry Labs and entrepreneurs from Colorado-based Nanoly Bioscience worked together to develop a safe, versatile platform to increase the thermal stability of vaccines. Their aim? To vastly improve the distribution of viable vaccines and reduce the economic costs of the cold chain.
Like Tupperware for proteins
“Think of it like an egg,” explains Bruno Marco-Dufort, a doctoral researcher in Professor Mark Tibbitt’s Macromolecular Engineering lab. “At room temperature or in the refrigerator the egg maintains its viscous-like protein structure, but once it hits boiling water or the frying pan its structure changes permanently.” It is similar for the proteins in a vaccine – once exposed to certain temperatures they clump together. Cooling them down again will not reverse their denaturation – you cannot “un-cook” the egg.
So rather than altering mother nature, Marco-Dufort and the research team developed a new type of hydrogel, the details of which were just published in the journal Science Advances. The gel is based on a biocompatible, synthetic polymer known as PEG that serves as a protective cloaking device for very large – yet invisible to the naked eye – complex molecules such as the proteins found in vaccines, antibodies, or gene therapies.
The packaging works kind of like a molecular Tupperware, encapsulating the proteins and keeping them separated. It enables the proteins to withstand a higher range of temperature fluctuations. Instead of the traditional +2 to +8 degrees Celsius (35 to 45 degrees Fahrenheit) range for the cold chain, encapsulation allows for a range of 25 to 65 degrees Celsius (75 to 150 degrees Fahrenheit). Most importantly, the encapsulated cargo is simply released by adding a sugar solution, enabling easy on-demand recovery of the vaccines at their point of use.
Usage in cancer research
In addition to a higher rate of vaccine viability, the real game changer of this new biomedical hydrogel technology is the potential economic effect it could have on reducing costs and health risks associated with the cold chain. “In 2020, the overall market for cold chain services (from manufacturing to distribution) was $17.2 billion and forecasted to rise,” the researchers reported. Rising costs pose potentially dire consequences for public health and public trust if vaccines arrive via a compromised cold chain.
“Most vaccines are sensitive to hot and cold. This creates a large barrier for global immunization campaigns, because vaccine distribution and administrative costs often exceed the costs of production,” explains Marco-Dufort. While more investment will be needed to shore up the cold chain, encapsulation offers a cost saving solution that could be put towards production of more vaccines and thus, save more lives.
Yet, there is still a long way to go in terms of further research, safety studies, and clinical trials before the hydrogels can be implemented for vaccine distribution. Their more immediate use is for transporting heat sensitive enzymes used in cancer research, for example, or protein molecules for research in lab settings.
One step towards solving a global issue
While new biotechnologies and cost savings are a step in the right direction, there are still tremendous logistical, political, and socio-economic challenges in resolving the global issues surrounding equitable vaccine distribution and vaccine hesitancy. Marco-Dufort’s motivation is undeterred. His childhood experience living in the Democratic Republic of the Congo instilled a deep appreciation for the need for vaccines against infectious diseases, not just for COVID-19, but also for Polio, Meningitis, and Ebola. He, more than most, is aware of the tremendous challenges people living in sub-Saharan Africa face in terms of access to vaccines where infectious diseases are still prevalent.
Mark Tibbitt, Bruno Marco-Dufort, and the team’s work represent a substantive advancement in vaccine excipient development. Their work also offers a glimmer of hope for a positive societal impact. Even a small relief of the economic factors associated with the distribution of vaccines, medicines, and biomedical research could result in larger impacts down the road.
(A) The thermal stabilization platform is based on dynamic covalent boronic ester–based hydrogels, where boronic acids (yellow) and diols (purple) form reversible boronic ester cross-links (blue). After encapsulation within the reversible hydrogels, the stored biologics are protected from thermal stresses. On-demand recovery and subsequent use of the biologics are enabled by the addition of sugars, which triggers network dissolution. (B) The reversible boronic ester–based hydrogels were formed from four-arm PEG macromers (Mn ≈ 10,000 g mol−1) end-functionalized with a phenylboronic acid derivative (APBA) or a cis-1,2-diol containing moiety [gluconolactone (GL)]. (C) The fractional release of β-galactosidase over time, β-gal(t)/β-gal0, from the PEG-APBA/GL gels [7.5% (w/w)] is modulated by the addition of cis-diol containing sugars such as mannitol and glucose (100 mg ml−1, pH 7.5). The release rate depends on the magnitude of the equilibrium binding constant, Keq,D, between PEG-APBA and the added sugars (Keq,mannitol = 20 and Keq,glucose = 3) (37). (D) Reversible hydrogels are suitable for the thermal stabilization of multiple classes of biologics, including model enzymes, protein-based vaccines, whole viruses, and clinical diagnostic enzymes with sizes ranging from 10 to 100 nm. For comparison, the mesh size of the reversible hydrogels, ξ, is estimated to be 7.5 nm.
DISCUSSION
There is a pressing need to improve the thermal stability of biologics, including vaccines, to reduce the economic costs and health risks associated to the cold chain. In this work, we demonstrated that reversible hydrogels are suitable for the long-term thermal stabilization of biologics, up to 65°C. Protection was broad, covering multiple classes of biologics, including model and clinical diagnostic enzymes, protein-based vaccines, and whole viruses, with sizes ranging from 10 to 100 nm. Our data point toward a generalized protection mechanism for the thermal stabilization of diverse biologics using direct encapsulation in reversible hydrogels. We attributed protection to the formation of polymer networks around the biologics, which restricted their mobility and physically prevented them from interacting with each other, reducing aggregation and bioactivity loss after exposure to heat.
The platform has the potential to be a very user-friendly solution for the thermal stabilization of biologics. The rapid release enabled by the reversible hydrogel design allows facile recovery of the bioactive cargo. Furthermore, the reversible cross-linking will facilitate processing, as the hydrogels are moldable and self-healing. Another advantage of our platform is that protection was achieved using both dried and nondried formulations, which would reduce the amount of formulation work required to translate the platform as an off-the-shelf excipient in the biotechnology industry for the thermal stabilization of biologics. The technology is not limited by the gel volume or the biologic loading; we successfully encapsulated, thermally stabilized, and recovered functional biologics at therapeutically relevant concentrations (gel loading of 40 mg ml−1; 10 mg ml−1 after release) and volumes (0.5 ml of gels; 2 ml after release).
Nevertheless, limitations remain for the platform. Preliminary toxicology data suggest that the gel components are safe for in vivo use; however, one of the gel components could be found to be unsuitable for clinical translation. As the stabilization mechanism is generalized, this limitation could be overcome by another reversible cross-linking chemistry (or by physical cross-linking) or a different polymer backbone. We envision that the platform will be used in combination with other approaches, as an extra layer of protection or as part of a toolbox to improve the thermal stability of biologics. We anticipate that the ability to protect biologics from thermal stress with reversible hydrogels offers a useful and complementary approach to mitigate the costs and risks associated with reliance on a continuous cold chain for biologic transport and storage.
More information: Bruno Marco-Dufort et al, Thermal stabilization of diverse biologics using reversible hydrogels, Science Advances (2022). DOI: 10.1126/sciadv.abo0502. www.science.org/doi/10.1126/sciadv.abo0502
