Provided herein are compounds of the Formula I: and stereoisomers and pharmaceutically acceptable salts or solvates thereof, in which A, B, D, E, X1, X2, X3 and X4 have the meanings given in the specification, which are inhibitors of RET kinase and are useful in the treatment and prevention of diseases which can be treated with a RET kinase inhibitor, including diseases or disorders mediated by a RET kinase.