Cyproheptadine arrests cell lines in the G₁ phase of the cell cycle. LP1 cells (A) and OCI-AML2 cells (B) (2 × 10⁶) were treated with 5 and 20 μM of cyproheptadine. Twenty-four hours after treatment, the percentage of cells in each phase of the cell cycle was determined by PI staining and flow cytometry. A representative histogram is shown. (C) LP1 cells (2 × 10⁶) were treated with 20 μM of cyproheptadine. At increasing times of incubation, the percentage of cells in the G₀/G₁ phase of the cell cycle was determined by PI staining and flow cytometry. Data represent the mean plus or minus SD percentage of cells of in the G₀/G₁ phase. (D) OCI-AML2 cells (1.5 × 10⁶) were pretreated with 10 and 20 μM of cyproheptadine overnight, followed by incubation with 20 μM of BrdU for 1 hour. Labeled cells were analyzed on a flow cytometer as described in “Cell-cycle analysis.” A representative histogram is shown. (E) MM1.S and MM1.R myeloma cells (2 × 10⁶) were treated with increasing concentrations of cyproheptadine. Twenty-four hours after treatment, cell lysates were prepared, normalized for total protein, and analyzed by SDS-PAGE/immunoblotting using antibodies specific for p53, p21, and anti–β-actin. (F) KMS11 and LP1 myeloma cells and OCI-AML2 and Jurkat leukemia cells (2 × 10⁶) were treated with increasing concentrations of cyproheptadine. Twenty-four hours after treatment, cell lysates were prepared, normalized for total protein, and analyzed by SDS-PAGE/immunoblotting using antibodies specific for SP1, APA2, C/EBPA, cyclin D2 (CCND2), cyclin D3 (CCND3), and anti–β-actin. APA2, CCND2, and CCND3 ratios represent (density of the protein)/(density of β-actin) relative to control cells. (G) LP1 myeloma and OCI-AML2 leukemia (2 × 10⁶) were treated with 20 μM of cyproheptadine. At increasing times after incubation, cell lysates were prepared, normalized for total protein, and analyzed by SDS-PAGE/immunoblotting using antibodies specific for APA2 and anti–β-actin.