When it rains in the forests of central Chile, soap suds can form on exposed areas of Quillaja saponaria, the soapbark tree – an evergreen tree that grows abundantly there. Chileans call it the “quillay” tree, meaning “to wash,” because its bark is rich in natural surfactants called saponins. Traditionally, those saponins have been used industrially as foaming agents and emulsifiers in the food and beverage industry. But since 2017, the soapbark tree has had a new role: delivering adjuvants – the hidden helpers of vaccines.
These immune-boosting compounds are crucial for vaccines that rely on microbial protein subunits, rather than those that use live pathogens, virus vectors, or mRNA. Adjuvants stimulate a stronger immune response than the vaccine alone, leading to better overall immunity. Among the range of excipients currently being developed or in use, Quillaja saponins emerge as one of the most effective. Saponin-based adjuvants are part of Novavax’s Covid-19 vaccine candidate, GSK’s blockbuster shingles vaccine, a breakthrough malaria vaccine and a promising vaccine candidate for tuberculosis from GSK. These successes are driving companies to secure supplies of soap bark, raising questions about the sustainability of the trees, while others hope to take advantage of the opportunities offered by synthetic saponins.
Emerging excipients
Saponins have been used as adjuvants in veterinary medicine for nearly a century, but for decades only a single adjuvant – alum – was approved for human vaccines. A breakthrough came in 2017 when: the FDA-approved Shingrix† This highly effective shingles vaccine contains a GSK adjuvant called AS01, a liposome-based formula that contains monophosphoryl lipid A (MLA – a derivative of a Salmonella cell wall molecule) and a saponin, QS-21. MLA and QS-21 act synergistically to induce strong antibody and helper T cell responses.
‘Developing adjuvants is not easy, because of the many variables you get in the system and it is difficult to define the contribution of each component,’ says immunologist Egil Lien at the University of Massachusetts, USA, who says GSK is reaping the benefits of years of investment in excipients. AS01 is better at stimulating frontline (innate) immune cells, promoting a superior T cell response, especially compared to alum. T cells are especially useful against pathogens that hide in our cells, such as malaria and tuberculosis. Shingrix’s turnover reached £2 billion in 2020 and GSK allegedly believes it could double sales in the next five years.
It’s been a long way from isolating the Quillaja saponins and identifying QS-21 as the most promising potential excipient for the approval of AS01 in 2017, with the FDA remaining wary of new excipients for much of this time. “It took 20 years from that original discovery to the first commercial product,” said Damian Hiley, chief of admixtures at Desert King, a US company that supplies most of the soap bark used to manufacture excipients.
The motivation behind the cost of QS-21 is that the [purification] is expensive
Damian Hiley, Desert King
As well as QS-21, others Quillaja saponins QS-7, QS-17, QS-18 and QS-19 also exhibit immunostimulatory effects. Their mechanism of action is not exactly understood, but they share structural similarities: a triterpene core, adorned with carbohydrates and fatty acids. QS-21 has a branched trisaccharide extending on one side of the triterpene and a linear tetrasaccharide on the other. The other saponin species, says: Tyler Martin, ‘differ in how they are decorated with sugars and fatty acids’. Until recently, Martin was chief executive of Adjuvant technologiesa company in Lincoln, Nebraska, with a synthetic QS-21.
GSK’s manufacturing process uses the bark of branches obtained from Desert King to make a hot tea – specific fractions are then extracted and purified to obtain QS-21. It costs $100,000 (£81,000) for a single gram, by one estimate, which provides approximately 20,000 human doses. This is largely due to the cost of purifying QS-21 to about 98%, admits Hiley: “The driving force behind the cost of QS-21 is not a shortage of raw materials, but that the chromatography is expensive.” He estimates that QS-21 costs $3 to 4 per vaccine dose, fine for Shingrix, which retails for $300. But very challenging for a public health vaccine like: GSK . malaria vaccine†
The adjuvant in the Novavax vaccine, Matrix-M, ties in with others Quillaja saponins – it is especially rich in QS-7 and QS-17. Desert King built a new $8 million plant in Chile to produce the precursors to Matrix-M and QS21. Because it is less purified, Matrix-M adjuvant is an order of magnitude less expensive than QS-21, Hiley estimates. Matrix-M is also in a malaria vaccine of: Adrian Hill’s lab in Oxford, which last year reported promising phase 2 results† Novavax said it has a manufacturing and supply network to support two billion doses of its Covid-19 vaccine containing Matrix-M annually. An FDA Advisory Committee in June recommended emergency authorization to use the Novavax vaccine for Covid-19.
As we move towards safe subunit vaccines, we need adjuvants. Alum won’t make it
Wolfgang Leitner, National Institute of Allergy and Infectious Diseases
The FDA approval of Shingrix in 2017 was a leap forward. “It really opened the floodgates in terms of interest in saponins,” Hiley recalls. Over the past five years, Desert King has ramped up production of saponin-based excipients from tens to thousands of pounds, and excipients now account for 65% of its sales, an increase of just one-tenth. Rising demand has raised major concerns about the sustainability of Quillaja bark for excipients. Still, Hiley is optimistic; Quillaja trees are not rare. ‘The trees we cut are probably 10 to 20 years old. They are quite large, with many branches,” Hiley says. The raw material for adjuvants is low in volume, and in the unlikely event of a shortage, ‘we would prioritize pharmaceutical production.’
New Paths to QS-21
Others bet on variants of QS-21. ‘QS-21 comes from the inner bark of the bark and is only found in practical concentrations in trees 10 years or older,’ says Martin, who will remain on the board of Adjuvance. ‘That is a very serious limitation of the offer.’
We think we can make public health vaccines more available, for example malaria or TB vaccines
Tyler Martin
Adjuvance was founded in 2009 by cancer vaccine pioneer Phil Livingston at Memorial Sloan Kettering Cancer Center in the US and medicinal chemist David Gin† Adjuvance’s QS-21 still relies on soap bark as raw material – the method hydrolyses Quillaja bark mixture up to the triterpene core, yielding 100 times higher concentrations than for QS-21, then adds a fat chain and a modified linear saccharide.
The branched-chain trisaccharide is left out, Martin says, because it contributes little to the adjuvant effect, but greatly to QS-21’s tolerability issues. ‘QS-21 lyses cell membranes and we think that this is an important contributor to local intolerance,’ says Martin. They also replaced an easily hydrolyzable ester bond to make it more stable. The compound is called TQL-1055.
“If you immunize a mouse with QS-21, even at a 10 mg dose, it will lose about 50% of its body weight on day two or three,” says Martin. “We think that translates into the fever, malaise and flu-like symptoms associated with QS-21.” TQL-1055 does not cause this weight loss, he adds. The company reported on a phase 1 safety trial given TQL-1055 with a commercial pertussis vaccine last August.
Adjuvant views utilizing the saponins used in food and beverages give them a huge advantage in terms of production volumes, allowing them to open the vaccine taps. “We think we can make public health vaccines more available, for example malaria or TB vaccines,” said Martin, who will remain on the company’s board.
Meanwhile, a British research group at the John Innes Center is investigating whether tobacco plants can do that produce saponins such as QS-21 as a replacement for: Quillaja bark. Desert King started Quillaja plantations in 2010 and clonal plantations rich in QS-21 in 2016, the extraction of which is to start in 2023. And there’s plenty of room to find other excipients, says Wolfgang Leitner, head of excipients at the National Institute of Allergy and Infectious Diseases† “There are a lot of alternatives to QS-21 in nature just waiting to be discovered, as well as alternative sources of QS-21. What remains is to address the reactogenicity of saponins through formulations or extensive studies of structure-activity relationships.”
New adjuvants are also now more likely thanks to huge advances in our understanding of the innate immune system. ‘There has been an explosion of publications in the past twenty years,’ says Lien. ‘Historically, many vaccines produced antibodies, but apart from intracellular infections, that is often not enough. We need T cells to attack infected cells.’ For some pathogens, specific T cell subtypes or a localized T cell response are sought, and this is something that saponin-containing adjuvants can help with.
“As we move towards safe subunit vaccines, we need adjuvants. Alum isn’t going to make it,” Leitner says. ‘There are enormous opportunities for new adjuvants.’
