From today, the Australian government will expand Pharmaceutical Benefit Scheme (PBS) access to the drug Keytruda for three additional types of cancer.

This is part of a broader shift away from cancer treatment with traditional chemotherapy drugs that can be less effective and have more intense side effects.

So what is Keytruda? How does it work? And which cancers can it treat?

The problems with standard chemo

Most chemotherapy drugs developed in the 1950s to 1990s are small molecules that scientists refer to as cytotoxics. This means they mess with the internal processes of cancer cells so much that the cancers essentially shut themselves down.

But cytotoxics can’t tell the difference between cancerous and healthy cells. So they attack all fast-growing cells. Cancers are comprised of fast-growing cells, but so is the lining of the gastrointestinal tract, bone marrow and hair.

Damage to these healthy cells leads to the typical chemotherapy side effects many patients experience. Damage to the stomach and intestines causes nausea, vomiting and diarrhoea.

Bone marrow damage leads to lethargy because of low red blood cell numbers. Low levels of........

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