p276-00 and Multiple-Myeloma

P276-00 is a novel cyclin-dependent kinase inhibitor especially potent for Cdk9-T1, Cdk4-D1 and Cdk1-B. Multiple myeloma (MM) is a B-cell malignancy characterized by the accumulation of malignant plasma cells. Treatment of MM cell lines with P276-00 resulted in apoptosis that correlated with transcription inhibition and a significant decline in Mcl-1 protein levels with the appearance of cleaved PARP in these cells. In vivo studies of P276-00 confirmed antitumor activity in RPMI-8226 xenograft. These results suggest that P276-00 causes multiple myeloma cell death by disrupting the balance between cell survival and apoptosis through inhibition of transcription and downregulation of Mcl-1.

Topics: Animals; Apoptosis; Apoptosis Regulatory Proteins; Blotting, Western; Cell Cycle; Cell Cycle Proteins; Cell Line, Tumor; Cell Survival; Cyclin-Dependent Kinase 9; Cyclin-Dependent Kinases; Dose-Response Relationship, Drug; Flavones; Humans; Mice; Mice, SCID; Microscopy, Confocal; Multiple Myeloma; Myeloid Cell Leukemia Sequence 1 Protein; Proto-Oncogene Proteins c-bcl-2; Reverse Transcriptase Polymerase Chain Reaction; RNA Interference; RNA Polymerase II; Time Factors; Transcription, Genetic; Xenograft Model Antitumor Assays

Cyclin D dysregulation and overexpression is noted in the majority of multiple myeloma (MM) patients, suggesting its critical role in MM pathogenesis. Here, we sought to identify the effects of targeting cyclin D in MM. We first confirmed cyclin D mRNA overexpression in 42 of 64 (65%) patient plasma cells. Silencing cyclin D1 resulted in >50% apoptotic cell death suggesting its validity as a potential therapeutic target. We next evaluated P276-00, a clinical-grade small-molecule cyclin-dependent kinase inhibitor as a way to target the cyclins. P276-00 resulted in dose-dependent cytotoxicity in MM cells. Cell-cycle analysis confirmed either growth arrest or caspase-dependent apoptosis; this was preceded by inhibition of Rb-1 phosphorylation with associated downregulation of a range of cyclins suggesting a regulatory role of P276-00 in cell-cycle progression through broad activity. Proliferative stimuli such as interleukin-6, insulin-like growth factor-1 and bone-marrow stromal cell adherence induced cyclins; P276-00 overcame these growth, survival and drug resistance signals. Because the cyclins are substrates of proteasome degradation, combination studies with bortezomib resulted in synergism. Finally, in vivo efficacy of P276-00 was confirmed in an MM xenograft model. These studies form the basis of an ongoing phase I study in the treatment of relapsed/refractory MM.

Topics: Animals; Antineoplastic Agents; Apoptosis; Blotting, Western; Bone Marrow; Boronic Acids; Bortezomib; Caspases; Cell Adhesion; Cell Cycle; Cell Proliferation; Cyclin D1; Cyclin-Dependent Kinase Inhibitor Proteins; Down-Regulation; Drug Evaluation, Preclinical; Drug Resistance, Neoplasm; Drug Synergism; Flavones; Gene Expression Profiling; Humans; Insulin-Like Growth Factor I; Interleukin-6; Male; Mice; Mice, SCID; Multiple Myeloma; Oligonucleotide Array Sequence Analysis; Phosphorylation; Pyrazines; Retinoblastoma Protein; Stromal Cells; Transplantation, Heterologous; Tumor Cells, Cultured