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The stock of Dendreon, maker of the prostate-cancer drug Provenge, fell the most in nine months after the Seattle company fell short of analysts’ revenue estimates.

Dendreon dropped 71 cents, or 15 percent, to $4.03 Thursday, its biggest decline since July 31. The stock has fallen 55 percent in the past 12 months.

First-quarter revenue fell 18 percent to $67.6 million, the company reported. Analysts expected sales of $79.7 million, the average of 19 estimates in a Bloomberg survey.

Provenge, Dendreon’s only product, began facing competition from Johnson & Johnson’s Zytiga in December.

“I am not satisfied with these results,” Dendreon CEO John Johnson said on a conference call with analysts and investors.

Second-quarter sales will grow to the “mid-70s range,” and the fourth quarter will be Dendreon’s strongest, he said.

Dendreon said its net loss for the first quarter narrowed to $72 million, or 48 cents a share, from $103.9 million, or 70 cents, a year earlier.

Provenge was approved in April 2010 as the first therapy in the U.S. that trains the body’s immune system to attack cancer cells as if they were a virus.

The treatment, which costs $93,000, was cleared for patients with advanced cases of the disease after the company’s three-year effort to persuade the Food and Drug Administration to back the medicine.

The first television commercial for Provenge ran March 7 as part of a direct-to-consumer advertising campaign.

Dendreon said it plans to spend $5 million each quarter on the effort.