Preliminary progression-free survival and long-term circulating tumor DNA (ctDNA) data from phase II portion of ongoing phase II/III study of GRANITE vaccine, in front-line metastatic microsatellite stable colorectal cancer -Gritstone bio

The randomized, controlled, open-label study is designed to quantify the clinical benefit of maintenance therapy with GRANITE (GRT-C901/GRT-R902) in combination with immune checkpoint blockade in addition to fluoropyrimidine/bevacizumab versus fluoropyrimidine/bevacizumab alone. Overall progression free survival (PFS) data show an early trend in benefit for GRANITE patients (HR=0.82, [95% CI, 0.34-1.67]; 62% censored) and extended PFS benefit in high-risk patients (HR=0.52 [95% CI, 0.15-1.38]; 44% censored), in whom progression occurs faster. 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 successfully manufactured GRANITE product candidate for every eligible patient (i.e., 100% vaccine manufacturing success rate).

“Today's preliminary Phase II results are highly encouraging and represent the first randomized trial evidence, albeit early, that a personalized neoantigen-directed vaccine can potentially drive efficacy in a metastatic ‘cold’ tumor. The overall trend of PFS improvement in GRANITE recipients is great to see, and the exploratory PFS hazard ratio of 0.52 in the high-risk group, a more mature dataset, is a striking signal,” said Andrew Allen, MD, PhD, Co-founder, President & CEO of Gritstone bio. “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.

Fortunately, long-term analysis demonstrates the expected correlation of ctDNA with clinical benefit and favors GRANITE patients . We believe these preliminary findings put us in a strong position to share mature PFS data in the third quarter and then enter regulatory discussions regarding Phase III. The growing body of evidence favoring GRANITE in this trial, including positive PFS and long-term ctDNA trends in both high and low-risk populations, is exciting and suggests GRANITE is working in this notoriously underserved patient population.”

“Up to 97% of patients with metastatic colorectal cancer, the second most common cause of cancer death, are MSS. Unlike patients with melanoma and lung cancer, they have not benefited from standard immunotherapies such as checkpoint inhibitors. These preliminary results indicate that GRANITE is inducing a potentially significant immune response in a disease that has been felt to be immunologically cold,” said J. Randolph Hecht, MD, Professor of Clinical Medicine and Director of the UCLA GI Oncology Program and an investigator in the GRANITE Phase II/III study. “The PFS difference, particularly in a poor prognosis group of patients, indicates the potential for clinical benefit and provides the rationale for a confirmatory Phase III trial, about which I am very excited. Furthermore, we are learning how to better analyze ctDNA continuously to study the efficacy of this novel immunotherapy. Expanding the scope of immunotherapy to a broader spectrum of cancer patients is the ‘holy grail’ of oncology, especially for MSS colorectal cancer. While early, these promising results suggest GRANITE has potential to deliver clinically meaningful benefit in MSS-CRC and other cold tumors.”

Key Findings from Preliminary Phase II Data in Front-Line Metastatic MSS-CRC: Clinical data cut as of March 8, 2024; ctDNA data cut as of March 12, 2024: One hundred and four (104) patients were randomized (1:1) in the study: Sixty-seven (67) patients (39 GRANITE arm, 28 control arm) are included in the treated analysis below. Thirty-six patients have left the study prior to randomized treatment primarily due to early progressive disease or withdrawal of consent, and one patient has yet to begin study treatment start. Demographics and clinical characteristics were balanced between arms (stage, sidedness, presence of liver metastases), with approximately 75% of patients having liver metastases.

Progression Free Survival (PFS): i. Early trend in PFS benefit was observed for GRANITE recipients. ii. Hazard ratio of 0.82 ([95% CI, 0.34-1.67]; 62% censored) in all patients. iii. Hazard ratio of 0.52 ([95% CI, 0.15-1.38]; 44% censored) in high-risk patients1 (>90% have liver metastases). Median PFS of 12 months (GRANITE) vs. 7 months (control). a. High-risk subgroup defined as baseline ctDNA above the median value (2%) for the control group (ctDNA quantified as mean variant allele frequency [VAF] at time of study randomization). This analysis was performed on 44 patients who received study treatment (control and GRANITE arms) and have available baseline ctDNA data. iv. GRANITE and control arms begin separating 1-2 months after initiation of GRANITE treatment, consistent with expected kinetics of GRANITE-induced immune response.

Biomarker Results – Circulating Tumor DNA (ctDNA) : i. Short-term molecular response ( greater than 30% reduction in ctDNA using single time-point analysis, defined per protocol) is uninformative due to unanticipated continuation of ctDNA drop beyond induction chemotherapy. a. Molecular response, similar in both arms (30% [6/20] in vaccine arm; 42% [5/12] in control arm) ii. Long-term ctDNA responses align with PFS trends and favor GRANITE vs. control patients a. Analysis in the high-risk group1 shows that between first blood draw (time of randomization) and last blood draw (most recent study visit), ctDNA shifted from high ( greater than 2% VAF) to low ( less than 2% VAF) in 56% (9/16) of GRANITE patients vs 22% (2/9) of control patients. Progressive disease was observed in 44% (7/16) vs 78% (7/9), respectively, within this group .b. Analysis in patients whose ctDNA was negative after induction chemotherapy, a low-risk group, favors GRANITE. Sustained ctDNA negativity was observed in 67% (6/9) of GRANITE recipients vs 38% (3/8) control patients. Progressive disease was observed in 11% (1/9) and 38% (3/8) of these patients, respectively.

Safety and Tolerability : GRANITE demonstrated a favorable safety and tolerability profile; a. Vast majority of adverse events (AEs) were Grade 1/2. b. Common AEs were mild systemic and local effects typically associated with any potent vaccine (i.e., transient flu-like illness). c. No patients discontinued study treatment due to an AE.

Comment: While the company claimed its vaccine may be working, the effects weren’t conclusive nor were they correlated with a marker meant to indicate a treatment response.