An article in npj vaccine in February 2023 explores the role of squalene in vaccine development and how a synthetic alternative, squalane, opens new doors. The authors, from the US, suggest that although squalene is a useful addition to vaccine composition, the natural, highly concentrated source is under pressure. It is traditionally isolated from shark liver oil, yet overfishing and a reduction in shark populations highlight the need to seek an alternative.
What is squalene?
Squalene is a triterpene belonging to the “large natural product family of terpenoids”. There are over 55,000 known members of this sprawling family. Many of these are found in low concentrations in plants, but squalene is “unusual in that it occurs at high concentrations in shark liver oil”.
What does it do?
Shark-derived squalene has been used as a vaccine adjuvant in “hundreds of millions of influenza vaccine doses” and has a demonstrated “excellent safety profile” and “dose-sparing capability”. More recently, several COVID-19 vaccine candidates are including shark squalene-based formulations. These candidates are expected to contribute to the “global pandemic response” with improved stability profiles compared to mRNA vaccines.
Shark squalene is also being used in candidates for tuberculosis, malaria, schistosomiasis, and leishmaniasis.
“Indeed, next to aluminium salts, shark squalene-based emulsions are the most widely employed vaccine adjuvant formulation in licensed inactivated or protein-based vaccines.”
However, the “mechanisms of action” of shark squalene emulsion adjuvants are “not completely understood”.
“For instance, it has been shown that shark squalene-based emulsions increase vaccine antigen uptake, enhance recruitment and activation of various immune cells at the injection site and the draining lymph node, and cause production of danger-associated molecular patterns that result in proinflammatory signalling cascades; however, it is not clear how the structural properties of squalene relate to these mechanisms.”
Unfortunately, the study identifies a decline in oceanic sharks and rays, “by 71% since 1970”, attributed to overfishing. This has, understandably, led to calls for “prohibitions and precautionary catch limits” to prevent population collapse. Thus, a replacement with “equivalent or better” adjuvant properties is desired.
The authors suggest that a union between synthetic biology and metabolic engineering has resulted in “industrial-scale production of volumes of natural products” that were previously “subject to supply constraints” or unavailable in the “quantities and purities required for commercial usage”.
For squalene, a “sustainable, renewable” solution appears to be squalane, derived from “β-farnesene by semi-synthesis”. The researchers “leveraged the ready availability of fermentation-derived, isomerically pure β-farnesene to investigate the structure-activity relationship (SAR) of squalene analogues as vaccine adjuvant components”.
Squalane vs squalene
The compounds were investigated in blood assays to compare immune responses. Dr Christopher Fox led the study and told Chemistry World that he observed “enhanced adjuvant activity” in “four or five of the molecules”. They found that chain length and saturation was key. Although the results of the study were promising, this requires further study before “next generation vaccines” can be developed.
“Additional preclinical work in different animal models is needed to establish protective efficacy and safety.”