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NEW YORK — Dendreon said it no longer expects sales of its prostate-cancer therapy Provenge to grow in 2013, and the company’s shares slumped 14.4 percent in after-hours trading Thursday

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 Seattle company said sales should improve in the third and fourth quarters, but that based on July and early August, it does not expect overall revenue from Provenge to grow from the $321.5 million reported in 2012.

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

Chairman and CEO John Johnson said a TV campaign aimed at prostate-cancer patients “continues to generate powerful and promising” response. Dendreon is spending about $5 million per quarter on the campaign, which began in March.

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 annual sales to reach billions of dollars. However, sales have been hurt because of the drug’s high price tag and because some doctors are not convinced Provenge extends patients’ lives a great deal compared with chemotherapy.

Provenge is also 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.

Dendreon said it lost $68.8 million, or 45 cents a share, in the latest quarter, compared with a loss of $96.1 million, or 65 cents a share, in the second quarter of 2012.

Revenue in the quarter slipped to $73.3 million from $80 million in 2012.

Analysts were expecting a loss of 42 cents a 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 and dropped 66 cents to $3.93 in after-hours trading.

The shares have traded between $7.22 and $3.69 in the last year.

Seattle Times business staff contributed to this report.