Cell Cycle Synchronization of Human Lymphoid Cells in Vitro by 2,3-Dihydro-1H-imidazo[1,2-b]pyrazole¹ Free

2,3-Dihydro-1H-imidazo[1,2-b]pyrazole (IMPY), a DNA synthesis-inhibitory drug, reversibly arrests growth of human lymphoblasts in vitro. DNA distribution histograms of cultures exposed to 0.5 to 2.0 mm IMPY show accumulation of cells with G₁-early S DNA content. On reincubation in fresh medium, cell cycle traverse is resumed by the blocked cells in a synchronized manner. Maximum incorporation of [H³]thymidine into DNA (174 to 220% of control) and labeling indexes (72 to 86%) are seen after 4 hr of incubation, and a major increase in cell number is seen between the 9th and 13th hr. DNA distribution histograms of cells reincubated in fresh medium (after double block), show an initial increase in the number of cells with S-G₂-M DNA content and a corresponding decrease in the number of G₁-early S cells. After 4 hr of reincubation, a gradual increase in the number of G₁-early S cells was seen as the earlier blocked cells completed cell cycle traverse and mitosis. Cells exposed to 2.0 mm IMPY took approximately 2 hr longer to traverse than did cells exposed to 0.5 mm IMPY.