There's a major roadblock to one of the most talked about new cancer therapies

T-cell immunity immune system immunotherapy NIAID / FlickrA healthy human t-cell is shown in this scanning electron micrograph image.

When it comes to treating difficult diseases like cancer, researchers have been leaning more and more toward the concept of precision medicine.

It means finding a unique course of treatment that is tailored toward a cancer tumour’s particular characteristics.

But, it turns out, there’s a big disincentive to drugmakers looking to developed precision medicine: The additional testing needed to identify a tumour’s specific characteristics has doctors favouring more general approaches.

A Wall Street Journal profile of two immuno-oncology drugs — which activate the immune system to attack cancer cells — illustrates this.

One of the drugs, Keytruda, made by Merck, requires a diagnostic test to scan for proteins in the tumour that indicate it might work, while the other, Opdivo, made by Bristol-Myers Squibb, is marketed more broadly and approved for use without the additional test.

There hasn’t been a trial demonstrating how well the drugs work in comparison to one another, though The Journal notes that doctors consider them similarly effective.

By leaving the diagnostic test out doctors run a greater risk of prescribing a drug that doesn’t work.

The one that requires less testing, Opdivo, had $2.1 billion in 2015 sales versus Keytruda’s $566 million.

The difference is in part because Bristol-Myers has taken a mass-marketing approach: Opdivo made the top 20 list of most-advertised drugs, joining the ranks of household names like Botox and Lyrica.

But it’s also notable that doctors are choosing to prescribe a drug that doesn’t come with the added steps of sending out a diagnostic test.

This is where the challenge for precision medicine comes in. Using diagnostic tests that ensure that the drug would work best for that particular patient are going to be the bedrock of precision medicine, which will be medicines that work best with specific genetic markers and mutations, as well as other lifestyle factors.

Right now, Opdivo and Keytruda are approved to treat cancer patients where other treatments have failed. The Journal reports that if Keytruda were to get approval to treat cancer patients who haven’t received any treatment for their cancer yet, the hope is that the ease of getting the drug’s diagnostic test added to other biopsy tests will make the more widely used in appropriate patients.

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