Japan's Eisai Co. Ltd. has received U.S. approval for a therapy for previously untreated patients with a common form of liver cancer, paving the way for multibillion dollar sales.

The oral drug, Lenvima, was jointly developed with Merck & Co Inc to treat unresectable hepatocellular carcinoma and is the first U.S. approval born of the collaboration.

Under the deal signed in March, Merck is entitled to half of all global sales, even for Lenvima's already-approved uses for thyroid cancer and in combination with another drug for kidney cancer.

Lenvima is expected to bring in revenue of $3.46 billion by 2022, according to Thomson Reuters data.

Thursday's approval by the U.S. Food and Drug Administration also bodes well for Merck as investors remain concerned about its increasing reliance on blockbuster cancer immunotherapy Keytruda.

A wider adoption of Lenvima could help allay some of these fears by providing another "reasonable source of support" to the company, analysts have said.