Updated on Aug. 28 with new information below.

BETHESDA, Md. (TheStreet) -- Northwest Biotherapeutics (NWBO) confirmed on Friday that it has temporarily halted a phase III clinical trial involving its brain tumor vaccine DCVax-L.

Enrollment of new patients into the DCVax-L study has been suspended until the company "submits certain information from the trial for regulatory review," Northwest Bio said in a statement.

Northwest Bio didn't say what prompted regulators to suspend enrollment in the DCVax-L study, nor did the company offer specifics about the type of information requested. Patients previously enrolled in the study are continuing treatment, the company said.

Investors picked up on the temporary halt of the DCVax-L study before trading opened Friday from a notice posted on the European Union Clinical Trials Register web site. Northwest Bio didn't acknowledge the halt publicly until late Friday afternoon, but by then, the damage was done. Northwest Bio shares closed Friday down 22% to $6.96.

DCVax-L is a patient-specific vaccine designed to train immune cells to identify and kill brain tumor cells. Northwest Bio started the DCVax-L study in 2005 but numerous changes and setbacks over the years have delayed its completion.

Northwest Bio last tinkered significantly with the design and statistical powering of the DCVax-L study one year ago. At that time, CEO Linda Powers promised to have the study fully enrolled with 348 patients before the end of the current quarter.

On Friday, the company said that just over 300 patients had been "recruited" for the study to date. Northwest Bio spokesman Les Goldman didn't respond to an email asking how many of the "recruited" patients have actually been enrolled and treated in the study. The suspension of new patient enrollment makes meeting the 348-patient enrollment goal of the study by the end of September highly unlikely.

Cancer immunotherapy -- the technology by which drugs activate a patient's immune system to find and kill cancer cells -- is the most exciting, new chapter in cancer research in years. Recently approved immunotherapies like Bristol-Myers Squibb's (BMY) - Get Report Opdivo and Merck's (MRK) - Get Report Keytruda aren't cancer cures, but they have demonstrated the ability to greatly shrink tumors and prolong survival beyond what's been possible with standard therapies.

The scientific and investor euphoria for cancer immunotherapy has sparked a new biotech Gold Rush. It's almost impossible to count the number of new cancer drugs in development and clinical trials underway.

Northwest Bio has tried to ride the cancer immunotherapy coattails without success. Taking 10 years and counting to find 300 patients for the DCVax-L phase III study is a troubling sign that doctors have their doubts about the vaccine's potential. By comparison, Celldex Therapeutics (CLDX) - Get Report took just three years to enroll 745 brain tumor patients in a phase III study of its immunotherapy.

A week doesn't go by these days without large and small biopharma companies announcing partnerships and collaborations to investigate combinations of cancer immunotherapies. To date, Northwest Bio hasn't announced a single DCVax-L clinical development partnership.

With the concomitant sell-off across the biotech sector, Northwest Bio has plummeted 43% from its 52-week high reached in July.

Aug. 28 update: A European Union Clinical Trials Register web site still lists Northwest Biotherapeutics' DCVax-L phase III study as being "temporarily halted" in Germany seven days after the company said the study is still ongoing. 

In its statement, Northwest Bio said brain tumor patients previously enrolled in the DCVax-L study continue to be treated, therefore the study is not halted. However, the company also says screening for new patients to be enrolled into the study is suspended because "certain information" must be submitted for regulatory review. The phase III study has not yet completed patient enrollment, the company also said.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.