The present invention relates to compounds of formula I that function as inhibitors of RET (rearranged during transfection) kinase enzyme activity: wherein HET, X1, X2, X3, X4, integer a and R3 are each as defined herein. The present invention also relates to processes for the preparation of these compounds, to pharmaceutical compositions comprising them, and to their use in the treatment of proliferative disorders, such as cancer, as well as other diseases or conditions in which RET kinase activity is implicated.