Purinone compounds and their use in treating cancer

The specification generally relates to compounds of Formula (I): (I) and pharmaceutically acceptable salts thereof, where R, A1, A2 and A3 have any of the meanings defined herein. The specification also relates to the use of such compounds and salts thereof to treat or prevent DNA-PK mediated disease, including cancer. The specification further relates to pharmaceutical compositions comprising such compounds and salts; kits comprising such compounds and salts; methods of manufacture of such compounds and salts; intermediates useful in the manufacture of such compounds and salts; and to methods of treating DNA-PK mediated disease, including cancer, using such compounds and salts.