ANtIcANc Er tHErAPY Double boost for mRNa cancer vaccines€¦ · and the STING pathway, as APCs from Sting-knockout mice did not mature upon treatment with A18. Linear LNPs were

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the team led by Daniel Anderson has developed a new series of lipid nano-particles (LNPs) that mediate mRNA delivery and provide targeted immune stimulation via the stimulator of interferon genes (STING) pathway

Cancer vaccines are designed to target tumour-associated antigens that are preferentially expressed by, or are unique to, tumour cells. Although mRNA vaccines have several advantages over peptide or DNA vaccines, delivering mRNA intracellularly and to the right cells, and inducing a strong immune activa-tion as a result, can be challenging. Now, the team led by Daniel Anderson has developed a new series of lipid nanoparticles (LNPs) that mediate mRNA delivery and provide targeted immune stimulation via the stimulator of interferon genes (STING) pathway, improving the vaccine effectiveness against melanoma in mice.

Vaccine-induced cancer immunity is the result of antigenic peptide presentation by antigen presenting cells (APCs) to T cells. However, resting APCs need to be activated — in an interferon-mediated process called APC maturation — to express immunogenic factors. Previously developed mRNA vaccines have had limited clinical success, mainly because of insufficient antigen expression or insufficient APC maturation. Although administering adjuvants that stimulate interferon secretion can potentiate the immune response of vaccines, nonspecific activation of these immune pathways can result in inflammation.

Miao et al. were interested in synthe-sizing lipid structures that could simultane-ously mediate mRNA delivery while providing targeted adjuvant stimula-tion via the STING pathway. To that end, they developed a library of 1,080 lipids using a one-step three-component reaction (3-CR) that coupled various lipid structures to primary or secondary amines,

ketones and isocyanides or isocyanide derivatives.

First, the authors evaluated the efficacy of the lipids in the library to deliver mRNA to APCs in vitro by loading firefly luciferase (mLuc) on LNPs. They observed that 232 lipids increased luciferase expression to over 10,000 units compared with naked mRNA. These lipids all had longer, unsaturated alkyl chains and an ester group. Indeed, lipids contain-ing two amines in the polar head group, no hydroxyl group and at least one tertiary amine were optimal for mRNA delivery. Specifically, the candidates A2-Iso5-2DC18 (A2) and A12-Iso5-2DC18 (A12) were the most potent mRNA delivery vehicles. This was confirmed in vivo, in mice undergoing either subcutaneous or intramuscular injections.

Next, the authors tested the anti- tumour immune response of these candidates in a B16F10 melanoma mouse model that expressed ovalbumin (OVA). The A2 LNP vehicle delivering OVA mRNA (mOVA) showed significant tumour suppression compared with the A12 mOVA LNP, which showed

A N t I c A N c E r t H E r A P Y

Double boost for mRNa cancer vaccines

almost no antitumour efficacy. This antitumour activity was mediated by a high antigen-specific cytotoxic T lymphocyte (CTL) response and IFN-γ secretion, neither of which was induced by A12 LNPs. When A2 mOVA LNPs were administered with an anti-PD1 antibody, this combination therapy significantly delayed tumour growth and improved overall survival.

A2 contains a heterocyclic amine whereas A12 contains a linear amine, so the authors generated a second library in which they varied the amine components in the head group to investigate whether the cyclic nature of the lipids increased immunogenicity. LNPs containing cyclic lipids (A18 and A21) showed significantly increased IFN-γ secretion compared with linear lipids. They also induced a stronger and more durable CTL response.

Mechanistically, these cyclic second-generation LNPs activate type I IFN though activation of interferon regulatory factors (IRFs) and the STING pathway, as APCs from Sting-knockout mice did not mature upon treatment with A18. Linear LNPs were unable to induce a strong STING-type I IFN activation and APC maturation, so this adjuvant effect is specific to cyclic LNPs.

Finally, the authors tested the A18 LNP with other tumour-associated antigen mRNAs (tyrosinase-related protein 2, Trp2, and the viral oncogene-coding E7 mRNA) in the melanoma and TC-1 human papillomavirus-driven tumour model, respectively, and observed systemic and prolonged antitumour effects.

In addition to the anticancer application indicated above, this new vaccine formulation — integrating a targeted adjuvant in an efficient delivery system — may offer a generalized approach to vaccination.

M. Teresa Villanueva

OrIGINAl ArtIclE Miao, L. et al. Delivery of mRNA vaccines with heterocyclic lipids increases anti-tumor efficacy by STING-mediated immune cell activation. Nat. Biotechnol. 37, 1174–1185 (2019)FurtHEr rEAdING Pastor, F. et al. An RNA toolbox for cancer immunotherapy. Nat. Rev. Drug Discov. 17, 751–767 (2018) | Pardi, N. et al. mRNA vaccines — a new era in vaccinology. Nat. Rev. Drug Discov. 17, 261–279 (2018)

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