Serine/threonine kinase inhibitors

A compound of formula I:

• or a pharmaceutically acceptable salt thereof;
• wherein
• X1 is N or CH;
• X2 is NRa;
• X3 is C(═O);
• X4 is NR3;
• R1 is (i) a 4 to 7 membered saturated or partially unsaturated heterocyclyl containing an oxygen atom;
• or, (ii) a heteroaryl selected from the group consisting of 1-methyl-H-pyrazol-4-yl, 1-methyl-1H-pyrazol-3-yl, 2-ethyl-2H-pyrazol-3-yl, 2-methyl-2H-pyrazol-3-yl, 2-isopropyl-2H-pyrazol-3-yl, 1-methyl-1H-pyrazol-5-yl, 1-ethyl-1H-pyrazol-5-yl, 4-methylthiazol-2-yl, 1-methyl-1H-[1,2,4]triazol-3-yl, 2-methyl-2H-[1,2,3]-triazol-4-yl, 1-methyl-H-[1,2,4]-triazol-5-yl, 1,3-dimethyl-1H-pyrazol-4-yl, 1,3,5-trimethyl-1H-pyrazol-4-yl, 1-methyl-1H-tetrazol-5-yl, 2-methyl-2H-tetrazol-5-yl, 5-methyl-1,3,4-thiadizol-2-yl, oxetan-3-yl, 3-methyloxetan-3-yl, tetrahydropyran-4-yl, tetrahydro-2H-pyran-4-yl, tetrahydropyran-3-yl, 2-methyl-tetrahydropyran-4-yl, 2,2-dimethyl-tetrahydropyran-4-yl, 2-hydroxymethyltetrahydropyran-4-yl, 3-fluorotetrahydropyran-4-yl and tetrahydrofuran-3-yl;
• Ra is hydrogen or C1-3 alkyl;
• and
• R3 is benzyl, optionally substituted with one or more groups selected from fluoro, chloro, cyclopropyl, C1-3 alkoxy, and C1-3 alkyl.