Moderna Therapeutics Inc. has closed the largest biotechnology venture capital financing on record, securing $450 million to advance new medicines called messenger RNA therapies into clinical trials.

Moderna drugs stimulate the body’s ability to produce proteins that could treat cancer, rare diseases, heart conditions and other illnesses. This promise has enabled it to raise more than $600 million in equity since forming in 2010. The fundraising surpasses a $273.7 million round completed by Reliant Pharmaceuticals Inc. in 2003, according to Dow Jones VentureSource.

Strong interest in biotech from institutional and public-market investors is enabling Moderna and some other companies to raise large sums without going public. In 2014, for example, diabetes drug-maker Intarcia Therapeutics Inc. raised $200 million and Juno Therapeutics Inc. rounded up $134 million to develop cell therapies for cancer. Juno went public in December.

Moderna tapped into existing investors and new backers such as Invus, RA Capital Management, Viking Global Investors and Wellington Management Co. to raise this round, which closed in December. Existing investors AstraZeneca PLC and Alexion Pharmaceuticals Inc. also participated. Additional, undisclosed prior investors also participated.

The Cambridge, Mass., company has $800 million in cash and is now valued at more than $1 billion, according to Chief Financial Officer Lorence Kim.