Illustration of the thyroid cancer drug Pz-1 blocking its two target enzymes. Pz-1 is a tyrosine kinase inhibitor, a class of drugs that block the action of specific enzymes involved in the growth and spread of cancer cells. Unusually, Pz-1 is able to block two different tyrosine kinases, RET (rearranged during transfection) and VEGFR2 (vascular endothelial growth factor receptor 2), depending upon which part of the molecule is used. This has the benefit of reducing the chance of the tumour cells developing resistance to the drug.