Transient Protection of Hematopoietic Stem and Progenitor Cells Against Ionizing Radiation

A method for reducing the effect of ionizing radiation exposure on cyclin-dependent kinase 4 (CDK4) replication-dependent hematopoietic stem cells and/or progenitor cells (HSPCs) in a subject exposed to ionizing radiation, the method comprising administering to the subject an effective amount of a CDK4/6 inhibitor compound having the formula:

• wherein R is C(H)X, NX, C(H)Y, or C(X)2,
• wherein X is a straight, branched or cyclic C1 to C5 alkyl group, including methyl, ethyl, propyl, cyclopropyl, isopropyl, butyl, sec-butyl, tert-butyl, isobutyl, cyclobutyl, pentyl, isopentyl, neopentyl, tert-pentyl, sec-pentyl, and cyclopentyl;
• wherein Y is NR1R2 wherein R1 and R2 are independently X, or wherein R1 and R2 are alkyl groups that together form a bridge that includes one or two heteroatoms (N, O, or S);
• and
• wherein two X groups can together form an alkyl bridge or a bridge that includes one or two heteroatoms (N, S, or O) to form a spiro compound;
• or its pharmaceutically acceptable salt thereof;
• optionally in a pharmaceutically acceptable carrier.