An emerging type of vaccine that has risen to prominence in the COVID-19 pandemic could be applied to cancer treatment, research suggests.

Traditionally, vaccines consist of small or inactive doses of the proteins produced by the disease-causing organism – or the organism itself. They are designed to provoke an immune response.

But messenger RNA (mRNA) vaccines effectively trick the body into producing some of the viral protein itself; by harnessing molecules involved in putting DNA instructions into action.

Messenger RNA (mRNA) vaccines to prevent COVID-19 have made headlines around the world recently, but scientists have also been working on mRNA vaccines to treat or prevent other diseases, including some forms of cancer.

Now, researchers reporting in ACS’ Nano Lettershave developed a hydrogel that, when injected into mice with melanoma, slowly released RNA nanovaccines that shrank tumours and kept them from spreading to other parts of the body.

Cancer immunotherapy vaccines work similarly to mRNA vaccines for COVID-19, except they activate the immune system to attack tumours instead of a virus.

These vaccines contain mRNA that encodes proteins made specifically by tumour cells. When the mRNA enters antigen-presenting cells, they begin making the tumour protein and displaying it on their surfaces, triggering other immune cells to seek and destroy tumours that also make this protein.

However, mRNA is an unstable molecule that is quickly degraded by enzymes in the body. For cancer immunotherapy, researchers have tried using nanoparticles to protect and deliver mRNA, but they are typically cleared from the body within 1-2 days after injection.

Guangjun Nie, Hai Wang and colleagues wanted to develop a hydrogel that, when injected under the skin, would slowly release mRNA nanoparticles, along with an adjuvant – a molecule that helps activate the immune system.

To develop their system, the researchers used ovalbumin (a protein found in chicken egg whites) as a model antigen.

The team mixed ovalbumin mRNA and an adjuvant with other compounds to form a hydrogel.

When injected under the skin of mice with melanoma tumours engineered to express ovalbumin, the hydrogel slowly released mRNA and adjuvant nanoparticles over a 30-day period.

The mRNA vaccine activated T cells and stimulated antibody production, causing tumours to shrink in the treated mice. Also, in contrast to untreated mice, the vaccinated mice did not show any metastasis to the lung.

These results demonstrate that the hydrogel has great potential for achieving long-lasting and efficient cancer immunotherapy with only a single treatment, the researchers say.