Dendreon said that it no longer expects sales of its prostate cancer therapy Provenge to grow in 2013, and the company’s shares slumped 13 percent in aftermarket trading.

Net sales of Provenge decreased 8 percent in the second quarter and Dendreon’s revenue from the drug fell 13 percent in the first half of the year. The company said sales should improve in the third and fourth quarters, year, but based on sales in July and early August, Dendreon does not expect overall revenue from Provenge to grow from the $321.5 million it reported in 2012.

FactSet says analysts currently expect $314.1 million in Provenge revenue in 2013.

The company’s stock dropped 61 cents to $3.98 in aftermarket trading.

Provenge is designed to train a patient’s immune system to fight prostate cancer. The drug was approved in 2010, and a round of treatment costs $93,000. Analysts initially expected sales to reach the billions of dollars per year. However sales have been hurt because of the drug’s high price tag and because some doctors are not convinced that it extends patients’ lives a great deal compared with chemotherapy.

Provenge is also now competing with newer drugs like Medication Inc.’s Xtandi and Johnson & Johnson’s Zytiga. Both of those medicines are pills, while Provenge is given in three infusions over four weeks.

The Seattle company said it lost $68.8 million, or 45 cents per share, in the latest quarter. Dendreon reported a loss of $96.1 million, or 65 cents per share, in the second quarter of 2012. Revenue slipped to $73.3 million from $80 million.

Analysts were expecting a loss of 42 cents per share and $74.6 million in revenue, according to FactSet.

Shares of Dendreon lost 7 cents to $4.59 in Thursday’s regular trading session. The shares have traded between $7.22 and $3.69 in the last year.