On Wednesday, Moderna (NASDAQ:MRNA) announced encouraging results from a clinical trial with an experimental cancer vaccine. Treatment with the messenger RNA-based candidate, mRNA-4157, shrank tumors for five out of ten patients with head and neck squamous cell carcinoma (HNSCC) when given in combination with Keytruda. Two of those patients achieved complete remission.

Keytruda's a successful cancer immunotherapy from Merck (NYSE:MRK) that's approved to treat HNSCC patients as a single agent. Moderna didn't include a control group for comparison, but we know the FDA approved Keytruda to treat advanced-stage HNSCC patients as a monotherapy after it shrank tumors for just 16% of patients, 5% of whom achieved complete remission.

Smiling medical person in a laboratory holding a small vial.

Image source: Getty Images.

Unfortunately, treatment with mRNA-4157 plus Keytruda didn't shrink any tumors in a group of 17 patients with advanced-stage colon cancer not driven by microsatellite instability mutations. Keytruda is approved to treat patients with these mutations, but it hasn't succeeded among the larger population of colon-cancer patients who don't fall in this category.

Some encouraging responses in the larger group of colon-cancer patients would have helped support the idea that mRNA-4157 is an effective therapy. Before assuming Moderna's cancer-vaccine candidate is responsible for the early observed responses, it's probably best to wait and see if the impressive response rate holds up as more HNSCC patients are added to the study.

Since Keytruda's approval in 2014, hardly a month has gone by without at least one clinical-stage biotech company reporting impressive early-response rate data produced by their experimental therapies in combination with Keytruda or similar drugs. Such combinations take the brakes off immune systems so they can fight cancer. So far, none have led to FDA approvals.

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