Normal tyrosine kinase enzymes operate an ‘on’ and ‘off’ switch mechanism for many of the functions in the body. The tyrosine kinase enzyme in the bone marrow is involved in controlling the growth and multiplication of white blood cells.

The BCR-ABL1 fusion gene in patients with CML results in an abnormal version of the tyrosine kinase enzyme. This allows the overproduction of white blood cells.

TKIs inhibit the action of the abnormal tyrosine kinase resulting from the BCR‑ABL1

In the UK, there are five TKIs approved for use in patients with CML and the Philadelphia chromosome. All are recommended by NICE.