While this (MSI-H) is not one of the highest burden cancers in the US, it is worth noting that this indication had been granted both (i) a priority review, and (ii) breakthrough status by FDA, in late 2015.
Now the FDA staffers are asking for more data. The would-be FDA complete response date should have been March 8, 2017. That date miss would more than occasionally suggest some smallish off-target effect was emerging, in the data -- or there were questions about the efficacy, in this patient population. I suspect that it will all sort itself out, but this has to be disappointing news, to Kenilworth. From OncLive, then:
. . . .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). . . .
As ever, very rare is the biologic that progresses in a straight line -- this we all know, from painful experience. Onward.