A type of common bacteria re-engineered by Aduro Biotech (ADRO) - Get Report to engage the immune system to kill pancreatic cancer failed a mid-stage study, the company announced Monday.

The negative trial results send Aduro shares down 25% to $8.01. Advaxis (ADXS) - Get Report , another company developing similar bacteria-based cancer immunotherapies, is flat at  $7.29.

In the phase IIb study of 303 advanced pancreatic cancer patients, the combination of Aduro's CRS-207 plus the cancer vaccine GVAX Pancreas yielded a median overall survival of 3.8 months. Patients treated with CRS-207 alone reported a median overall survival of 5.4 months, while a third arm of patients administered standard chemotherapy showed a median overall survival of 4.6 months.

Aduro was aiming to demonstrate the combination of CRS-207 and GVAX Pancreas would induce the longest survival for these heavily pre-treated pancreatic cancer patients. The actual results showed exactly the opposite.

"This is an unexpected outcome, and we are disappointed particularly for the pancreatic cancer patients who are in need of additional treatment options," said Aduro CEO Stephen Isaacs.

CRS-207 starts as listeria, a bacteria which can cause serious infectious from contaminated food. However, Aduro scientists remove two genes from listeria to render it incapable of replication. Then, the attenuated listeria is altered again to express mesothelia, an antigen found in pancreatic cancer, to signal a patient's immune system to identify and kill the pancreatic cancer cells.

GVAX Pancreas, another type of off-the-shelf cancer vaccine made from pancreatic cancer cells, has been shown previously to be ineffective on its own, but Aduro believed it could be used to boost the efficacy of CRS-207.

Monday's failed study proved Aduro's combination immunotherapy hypothesis wrong, at least for now. The company is still conducting a second, phase II study in which pancreatic cancer patients are being treated with a triple combination of CRS-207, GVAX Pancreas and Opdivo, the checkpoint inhibitor from Bristol-Myers Squibb (BMY) - Get Report , compared against patients treated with CRS-207 and GVAX Pancreas.

Aduro is working on other cancer immunotherapies which don't involve re-engineering listeria, but none have been tested in humans yet.

Advaxis' technology also uses an engineered form of listeria 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.

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