LONDON, Aug 10 (Reuters) - AstraZeneca has invested another $140 million in Moderna Therapeutics, the U.S. biotech "unicorn" which already has a cash pile of around $1 billion and is developing drugs based on a molecule known as messenger RNA.

The British drugmaker said on Wednesday that the new investment, part of a preferred-stock financing, lifted its stake in Moderna to 9 percent. AstraZeneca first invested in Moderna in 2013.

Messenger RNA (mRNA) carries the recipe for making proteins inside the body. Using it as a medicine could offer a new way to tackle many hard-to-treat diseases, from cancer to infections to heart and kidney disorders.

In effect mRNA serves as software that can be injected into the body to instruct ribosomes, the "3D-printers" found inside cells, to churn out the desired proteins.

Moderna's work is still at an early stage. It has two Phase I studies underway for mRNA-based infectious disease vaccines, and last month Moderna and AstraZeneca filed for approval to run another Phase I study of a vascular disease treatment.

Moderna also has strategic agreements with Alexion Pharmaceuticals, Merck and Vertex Pharmaceuticals.

(Reporting by Ben Hirschler; Editing by Greg Mahlich)