PRINCETON, N.J. (TheStreet) -- The U.S. Food and Drug Administration placed a halt on clinical trials involving a cancer immunotherapy from Advaxis (ADXS) - Get Report because of concerns a bacteria used to deliver the drug may have contributed to a patient death.

Advaxis believes the patient died due to the progression of cervical cancer, but FDA is asking for additional information to confirm the company's experimental therapy, axalimogene filolisbac, played no role. Until the FDA's concerns are resolved, four clinical trials involving Advaxis' axalimogene filolisbac are on clinical hold.

Shares of Advaxis fell 25% to $7.63 in after-hours trading on news of the clinical trials halt. The bad news couldn't come at a worse time as investors are already in a sour mood over a prolonged and painful retrenchment in biotech stocks.

Advaxis' cancer immunotherapy technology uses an engineered form of listeria, a common bacteria, to trick the immune system into viewing tumors as bacterial infections requiring elimination. Advaxis is developing three immunotherapy products which pair listeria with different cancer targets -- human papillomavirus (HPV), prostate-specific antigen (PSA) and HER-2.

Tuesday's safety issue cropped up with the HPV-targeted therapy. The cervical cancer patient was last treated with axalimogene filolisbac in early 2013. In July 2015, the patient was hospitalized and tests discovered the presence of the Advaxis-engineered listeria bacteria in her bloodstream. The patient died in August.

The listeria bacteria used to make axalimogene filolisbac is modified to prevent harmful infection. But the presence of the Advaxis listeria in the patient's bloodstream two years after treatment with axalimogene filolisbac caused enough alarm bells at the FDA to warrant a closer look.

Advaxis was informed by FDA about the clinical hold last Thursday but didn't disclose the news to investors until Tuesday night. On a conference call Tuesday night, Advaxis called the case an isolated incident. An investigation into the patient's death has not found any evidence that the listeria played a harmful role. The company has already provided FDA with the information requested.

Advaxis has ongoing cancer immunotherapy collaborations with the Medimmune arm of AstraZeneca (AZN) - Get Report and Merck (MRK) - Get Report .

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.