This invention is in the area of improved compounds, compositions and methods of transiently protecting healthy cells, and in particular hematopoietic stem and progenitor cells (HSPC) as well as renal cells, from damage associated with DNA damaging chemotherapeutic agents. In one aspect, improved protection of healthy cells is disclosed using disclosed compounds that act as highly selective and short, transiently-acting cyclin-dependent kinase 4/6 (CDK 4/6) inhibitors when administered to subjects undergoing DNA damaging chemotherapeutic regimens for the treatment of proliferative disorders.