RET kinase inhibitors

A compound of the formula:

• wherein
• A is C6-C10 aryl or C5-C6 heteroaryl;
• each R1 is independently hydrogen, halogen, C1-C6 alkyl, C1-C6 heteroalkyl, alkyl)(C5-C6 heteroalkyl), —(C0-C4 alkyl)(C3-C7 cycloalkyl), —(C0-C4 heteroalkyl)(C3-C7 cycloalkyl), —(C0-C4 alkyl)(C3-C7 cycloheteroalkyl), —(C0-C4 heteroalkyl)(C3-C7 cycloheteroalkyl), —(C0-C4 alkyl)(C4-C10 bicyclyl), —(C0-C4 alkyl)(C5-C6 aryl), —(C0-C4 alkyl)(C5-C6 heteroaryl), —(C0-C4 alkyl)(C4-C10 heterobicyclyl), C5-C12 spiranyl, C5-C12 heterospiranyl, dimethylphosphoryl, adamantyl, C alkoxy, alkyl)-S02-(C1-C4 alkyl), difluoromethylsulfanyl, or pentafluorosulfanyl, wherein each R1 is unsubstituted or when it is capable of being substituted, it is substituted with one or more substituents that are independently selected from the group consisting of halogen, cyano, hydroxyl, oxo, methyl, methoxy, hydroxymethyl, ethyl, ethoxy, hydroxyethyl, methylamine, N,N-dimethylmethylamine, mono-, di, or trihalomethoxy, mono-, di-, or tri-halomethyl, and C0-C3 alkyl-pyrrolidinyl, where the pyrrolidinyl group is unsubstituted or substituted with one, two or 3 independently selected halogen atoms, and wherein two R1 groups can fuse to form a ring structure that includes a portion of A and is optionally aromatic, and n is 1, 2, 3, 4, 5, or 6;
• X1, X2, X3, and X4 are each independently N, CH, C—CH3, C—CH2—OH, C—OCH3, C—CH2—OCH3, or C-halogen;
• and
• R2 is C1-C6 alkyl, —(C0-C4 alkyl)(C3-C7 cycloalkyl), —(C0-C4 alkyl)(C4-C7 heterocycloalkyl), —(C0-C4 alkyl)(C4-C10 bicyclic) each optionally substituted with one or more of halogen, cyano, hydroxyl, oxo, methyl, methoxy, hydroxymethyl, ethyl, ethoxy, hydroxyethyl, cyclopropyl, or mono-, di-, or tri-halomethyl;
• or a pharmaceutically acceptable salt thereof.