04:47 PM EDT, 04/01/2024 (MT Newswires) -- Gritstone Bio ( GRTS ) said Monday it had "positive" preliminary results from a phase 2 portion of a phase 2/3 trial assessing cancer vaccine Granite in metastatic microsatellite stable colorectal cancer.

Overall progression-free survival results showed "an early trend in benefit" for Granite patients and extended progression-free survival benefit in high-risk patients, the company said.

"Circulating tumor DNA (ctDNA) analysis over several months of treatment shows the expected relationship with disease progression and favors Granite, while short-term ctDNA response analysis (molecular response as defined per protocol) did not demonstrate a difference between study arms," Gritstone Bio ( GRTS ) said.

"The overall trend of PFS improvement in Granite recipients is great to see," Chief Executive Officer Andrew Allen said in a statement. "Pioneering new spaces carries inherent risks, and with regard to defining molecular response, we simply got it wrong. ctDNA levels in both arms decreased on chemotherapy for longer than we anticipated, generating similar short-term molecular response rates across arms and rendering our protocol measure of ctDNA change uninformative."

The company said that mature progression free survival data is expected in Q3, and overall survival data is slated for H1 2025.

The company's shares fell 31% in recent after-hours trading.

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