Dendreon Corp. said Thursday that sales of its prostate cancer therapy Provenge fell in the first quarter because of increased competition from two newer drugs.

Net revenue from Provenge fell 18 percent to $67.6 million, Dendreon said. The company faced tougher competition because regulators approved Medication Inc.’s drug Xtandi in August, and Johnson & Johnson received approval to market its drug Zytiga for use in prostate cancer patients who have not yet received chemotherapy. That put Zytiga in more direct competition with Provenge, which is approved in men who haven’t had chemotherapy.

Xtandi is approved for patients who haven’t been helped by chemotherapy. Zytiga and Xtandi are pills, while Provenge is given in three infusions over the course of four weeks. Provenge is approved for men whose cancer has spread elsewhere in the body and has not responded to hormone therapy or radiation. It is designed to train a patient’s immune system to fight prostate cancer and a round of treatment costs $93,000.

Shares of Dendreon lost 57 cents, or 12 percent, to $4.17 in afternoon trading.

Dendreon said it lost $72 million, or 48 cents per share, in the three months ended March 31. That’s down from a loss of $103.9 million, or 70 cents per share, a year ago. Revenue fell 18 percent, to $67.6 million from $82.1 million.

Analysts were expecting a loss of 48 cents per share and $79.6 million in revenue, according to FactSet.

Dendreon said it still expects sales of Provenge to grow in 2013. The company reported $321.5 million in net sales in 2012, and analysts, on average, expect $347 million in revenue for the year.

Provenge was approved in 2010 and analysts initially expected sales to reach the billions of dollars per year. However sales have been hurt by the high cost of the drug and because some doctors are not convinced that it extends patients’ lives a great deal compared with chemotherapy.