What happened

After updating investors earlier this week on the timeline for filing a new drug application for its lead drug candidate, maralixibat, in Alagille syndrome (ALGS), Mirum Pharmaceuticals (NASDAQ:MIRM) saw its shares rally 44.1% on Wednesday.

So what

Mirum is a relatively new company dedicated to developing drugs for rare liver diseases. One of its founders, executive chairman Michael Grey, was previously behind the development of maralixibat at Lumena, which was acquired by Shire (NASDAQ: SHPG) in June 2014.

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In November 2018, Mirum Pharmaceuticals licensed the rights to maralixibat (formerly SHPG625) and a second drug, volixibat, from Shire in exchange for $7.5 million in up-front cash, 1,859,151 shares (worth $7 million), future development and sales milestones, and up to mid-teens royalties on eventual sales.

On Monday, Mirum unveiled plans to submit a rolling filing for approval as early as Q3 2020, following discussions with the FDA. A phase 3 trial evaluating maralixibat's ability to reduce itching in Alagille syndrome patients is planned to begin early next year.  

ALGS, a rare liver disease, is an early onset genetic disorder characterized by a toxic buildup of bile in the liver that causes severe itching and often leads to liver failure and a transplant. Mirum Pharmaceuticals estimates there are 9,000 patients with ALGS in the United States and approximately 14,000 patients in Europe.

Now what

A timeline for a filing for ALGS coincides with its timeline for the development of maralixibat to reduce itching in patients with progressive familial intrahepatic cholestasis, another disorder that results in too much bile building up in the liver. Mirum began enrolling patients in its phase 3 clinical trial in July 2019, and it expects data from that trial will be available by the end of 2020.

There's no guarantee these trials will succeed. At Shire, maralixibat failed to deliver on the primary endpoint -- reducing serum bile acids -- in a phase 2 ALGS trial. However, it did significantly relieve itching in mid-stage trials, and that's the primary endpoint Mirum Pharmaceuticals is pursuing.