Eli Lilly's first-quarter earnings plunged 53 percent after the expiration of a key patent protecting its top-selling antidepressant cut into the drugmaker's revenue.

The Indianapolis company lost U.S. patent protection for Cymbalta last year, exposing the drug to generic competition. It also lost protection for the osteoporosis treatment Evista. Revenue from those drugs sank 64 percent and 38 percent, respectively, in the quarter.

Lilly, which is known for its portfolio of diabetes and cancer treatments, had already warned of sliding revenue and profit this year due to the patent expirations. It hopes to return to growth after 2014, with help from new drugs, existing products, cost cutting and the company's growing animal health business.

The company said earlier this week that it would spend $5.4 billion to acquire the animal health business owned by Swiss drugmaker Novartis.

In the first quarter, Lilly earned $727.9 million, or 68 cents per share. That compares with $1.55 billion, or $1.42 per share, from last year's first quarter. Results in 2013 were bolstered by a one-time payment of $495 million from former drug development partner Amylin Pharmaceuticals.

Adjusted earnings totaled 70 cents per share, and revenue dropped 16 percent to $4.68 billion.

Earnings were in line with analyst expectations, though revenue fell just shy, according to FactSet.

Shares of Eli Lilly and Co. climbed 11 cents to $59.67 before markets opened Thursday.