What happened

Shares of Mirum Pharmaceuticals (NASDAQ:MIRM), a clinical-stage biopharmaceutical company, soared 240.1% in December, according to data from S&P Global Market Intelligence. Investors were responding to important steps forward the company took, including with its experimental treatment for children with rare liver diseases, maralixibat.

So what 

Maralixibat is the only clinical-stage new drug candidate in Mirum Pharmaceuticals' pipeline. The stock surged last month because the company told investors the FDA is willing to review a rolling submission of a new drug application for maralixibat. 

Person in a suit holding a giant wad of cash money.

Image source: Getty Images.

The company has maralixibat aimed at a small population of children with severe chronic itching, or pruritus, associated with Alagille syndrome. The stock also rose because the FDA gave maralixibat for Alagille syndrome-associated pruritus a rare pediatric disease designation.

Now what

Mirum is conducting a phase 3 clinical trial that enrolled its first patient in July and will end up treating 30 patients for 26 weeks with maralixibat or a placebo. Results from the ongoing phase 3 trial will probably complete the rolling submission that the company will begin in the second half of 2020.

Maralixibat's rare pediatric disease designation makes Mirum eligible to receive a valuable priority review voucher if it earns approval. Since these vouchers are transferable, larger drugmakers that want to knock four months off the review time for an important new drug application regularly purchase them from companies like Mirum for nine-figure sums.