You will remember that Approvable Letters or Complete Response Letters [CRL] are issued when the division overseeing the approval process - the Center for Biologics Evaluation and Research - decides that after reviewing the application for approval, more information is needed.
That can mean something as simple as label language, a risk management program, a re-cut of existing data or new data in the form of new clinical trials. You will not find the content of the letter on the FDA Web site.
According to the company press release, "[t]he FDA has requested additional clinical data in support of the efficacy claim contained in the BLA. The Company is seeking a clarification from the FDA as to the nature of the data that is being requested. The FDA has also requested additional information with respect to the chemistry, manufacturing and controls [CMC] section of the BLA, which the Company believes it can supply to the FDA in a timely manner."
This past March, on the efficacy question, the Cellular, Tissue and Gene Therapies Advisory Committee had voted 13-4 in favor of efficacy - but the nature of such a committee is "advisory" and the FDA actions can depart from the recommendations of the committee.
There are many reasons for the anticipation of Provenge and it was on Fast Track consideration. This is an example, however, of the fact that Fast Track does not always indicate a product approval is imminent.
Mitchell H. Gold, MD, president and chief executive officer of Dendreon, said in the release,
We are committed to working closely with the FDA to resolve these questions in a timely and efficient manner to bring PROVENGE to patients with advanced prostate cancer who currently have few appealing treatment options.
For more on approvable letters, see my prior posting on What Do Approvable Letters Mean?