Cdk4/6 inhibitor

A compound of formula (I), a pharmaceutically acceptable salt thereof or an isomer thereof,

• wherein,
• R1 is H, or selected from the group consisting of C1-3 alkyl, C1-3 heteroalkyl,
• each of which is optionally substituted by 1, 2 or 3 R;
• each of R2 is independently H, OH, CN, halogen, or selected from the group consisting of C1-5 alkyl, C1-5 heteroalkyl, C3-6 cycloalkyl and 3-6 membered heterocycloalkyl, each of which is optionally substituted by 1, 2 or 3 R;
• ring A is 4-11 membered heterocycloalkyl;
• ring B is selected from the group consisting of C3-6 cycloalkyl, 3-6 membered heterocycloalkyl, phenyl and 5-6 membered heteroaryl, each of which is optionally substituted by 1, 2 or 3 R;
• R is halogen, OH, CN, NH2, NO2, or selected from the group consisting of C1-3 alkyl and C1-3 heteroalkyl, each of which is optionally substituted by 1, 2 or 3 R′;
• R′ is selected from the group consisting of F, Cl, Br, I, OH, CN and NH2;
• each of the “hetero” in the C1-3 heteroalkyl, C1-5 heteroalkyl, 3-6 membered heterocycloalkyl, 4-11 membered heterocycloalkyl and 5-6 membered heteroaryl is independently selected from the group consisting of N, —O—, —S—, —NH—, —(C═O)—, —(S═O— and —(S═O)2—;
• in any of the above cases, the number of the heteroatom or the heteroatomic group is independently 1, 2 or 3.