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FDA Review Continues for Pembrolizumab in MSI-H Cancer

Jason M. Broderick @jasoncology
Published: Friday, Mar 10, 2017

A supplemental biologics license application (sBLA) for pembrolizumab (Keytruda) for previously treated patients with advanced microsatellite instability-high (MSI-H) cancer remains under review by the FDA, according to a statement from Merck, the manufacturer of the anti–PD-1 agent.

UPDATE 5/23/2017: FDA Approves Pembrolizumab for Microsatellite Instability-High and Mismatch Repair Deficient Cancers

Merck announced in November 2016 that pembrolizumab had received a priority review from the FDA for use in this setting, with a final decision expected under the Prescription Drug User Fee Act (PDUFA) by March 8, 2017. The company now reports that it has submitted further data and analyses supporting the pending sBLA. The new PDUFA action date for the FDA to make its decision is June 9, 2017.

The sBLA for pembrolizumab is based on results from 5 open-label, multicohort phase I/II trials that evaluated pembrolizumab in patients with MSI-H tumors. The specific regimen would be a fixed dosed of 200 mg of pembrolizumab every 3 weeks.

In November 2015, the FDA granted a breakthrough therapy designation to pembrolizumab as a potential therapy for patients with MSI-H metastatic colorectal cancer (mCRC).

The designation was based on findings from an ongoing phase II study, which demonstrated high response rates with pembrolizumab in patients with heavily pretreated CRC with mismatch repair (MMR) deficiency, a condition that causes MSI. Findings from the analysis were published in The New England Journal of Medicine (NEJM); however, data that were simultaneously presented at the 2015 ASCO Annual Meeting were from a more up-to-date analysis.1,2

In the findings presented at ASCO, the objective response rate (ORR) was 62% with pembrolizumab in MMR-deficient mCRC compared with 0% in patients with MMR-proficient tumors. Median progression-free survival (PFS) and overall survival (OS) were not reached, with many patients responding to treatment for longer than 12 months in the MMR-deficient arm.

In the 3-arm study that was the basis for the new designation, pembrolizumab was administered at 10 mg/kg every 2 weeks to patients with CRC who were MMR-deficient (n = 13) and MMR-proficient (n = 25). Additionally, a separate arm looked at pembrolizumab in patients with MMR-deficient non-CRC malignancies (n = 10). MMR and microsatellite instability testing was conducted using PCR and IHC, which are standard tests conducted for patients with CRC in order to detect Lynch syndrome.

Defects in MMR commonly lead to microsatellite instability, which can be found in most cancers, including a majority of patients with hereditary nonpolyposis CRC (Lynch syndrome). Without this repair mechanism, the mutational burden is generally higher, suggesting a higher likelihood of developing cancer. In total, more than 80% of patients in the MMR-deficient arm were positive for Lynch syndrome.

The primary endpoint of the study was immune-related PFS and response rate at 20 weeks. Secondary endpoints focused on OS, PFS, and disease control rate (DCR; complete response, partial response, plus stable disease). Response and survival were assessed by RECIST criteria in addition to immune-related criteria.

In the 48 patients analyzed from the study for the ASCO presentation, those with MMR-deficient CRC experienced a DCR of 92% compared with 16% in MMR-proficient tumors. After a median treatment duration of 5.9 months, no patients in the MMR-deficient group who responded had progressed. In patients with MMR-deficient non-CRC tumors, the ORR was 60% and the DCR was 70%.

OS and PFS were not reached in the MMR-deficient group versus a median PFS of 2.3 months (HR, 0.10; 95% CI, 0.03-0.37; P <.001) and an OS of 7.6 months in the MMR-proficient group (HR, 0.22; 95% CI, 0.05-1.00; P = .05).

In the analysis published in NEJM, which contained data from fewer patients, the ORR with pembrolizumab was 40% in patients with MMR-deficient mCRC (n = 10). In this same group, the PFS rate with pembrolizumab at 20 weeks was 78%.

The adverse events (AEs) seen in the study were consistent with other studies of pembrolizumab. The most common side effects were rash/pruritus (17%), pancreatitis (15%), and thyroiditis/hypothyroidism (10%).

Interestingly, patients with Lynch syndrome (n = 11) were less likely to respond compared with those with other forms of MMR, according to the data published in NEJM. In those with Lynch syndrome, the ORR was 27% with pembrolizumab compared with 100% in those with MMR that was unrelated to Lynch syndrome (n = 6).

In total, 1782 somatic mutations were identified per patient in the MMR-deficient arm compared with 73 in those with MMR-proficient tumors. Predominately, these tumors were found to alter amino acids (63%) and 578 of the somatic mutations in the deficient arm were associated with the immune system.

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