The Novel Cyclin-Dependent Kinase Inhibitor Flavopiridol Downregulates Bcl-2 and Induces Growth Arrest and Apoptosis in Chronic B-Cell Leukemia Lines

Flavopiridol induces cellular FLICE-inhibitory protein degradation by the proteasome and promotes TRAIL-induced early signaling and apoptosis in breast tumor cells

The cyclin-dependent kinase inhibitor flavopiridol is undergoing clinical trials as an antitumor drug. We show here that pretreatment of different human breast cancer cell lines with flavopiridol facilitates tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. In breast tumor cells, apoptosis induction by TRAIL is blocked at the level of apical caspase-8 activation. Flavopiridol treatment enhances TRAIL-induced formation of death-inducing signaling complex and early processing of procaspase-8. Subsequently, a TRAIL-induced, mitochondria-operated pathway of apoptosis is activated in cells treated with flavopiridol. Down-regulation of cellular FLICE-inhibitory proteins (c-FLIP; c-FLIP(L) and c-FLIP(S)) is observed on flavopiridol treatment. c-FLIP loss and apoptosis sensitization by flavopiridol are both prevented in cells treated with an inhibitor of the ubiquitin-proteasome system. Furthermore, targeting c-FLIP directly with small interfering RNA oligonucleotides also sensitizes various human breast tumor cell lines to TRAIL-induced apoptosis. Our results indicate that flavopiridol sensitizes breast cancer cells to TRAIL-induced apoptosis by facilitating early events in the apoptotic pathway, and this combination treatment could be regarded as a potential therapeutic tool against breast tumors.

Flavopiridol administered using a pharmacologically derived schedule is associated with marked clinical efficacy in refractory, genetically high-risk chronic lymphocytic leukemia

Despite promising preclinical studies with the cyclin-dependent kinase inhibitor flavopiridol in chronic lymphocytic leukemia (CLL) and other diseases, previous clinical trials with this agent have been disappointing. The discovery of differential protein binding of flavopiridol in human and bovine serum contributed to an effective pharmacokinetic-derived schedule of administration of this agent. On the basis of pharmacokinetic modeling using our in vitro results and data from a previous trial, we initiated a phase 1 study using a 30-minute loading dose followed by 4 hours of infusion administered weekly for 4 of 6 weeks in patients with refractory CLL. A group of 42 patients were enrolled on 3 cohorts (cohort 1, 30 mg/m2 loading dose followed by 30 mg/m2 4-hour infusion; cohort 2, 40 mg/m2 loading dose followed by 40 mg/m2 4-hour infusion; and cohort 3, cohort 1 dose for treatments 1 to 4, then a 30 mg/m2 loading dose followed by a 50 mg/m2 4-hour infusion). The dose-limiting toxicity using this novel schedule was hyperacute tumor lysis syndrome. Aggressive prophylaxis and exclusion of patients with leukocyte counts greater than 200x10(9)/L have made this drug safe to administer at the cohort 3 dose. Of the 42 patients treated, 19 (45%) achieved a partial response with a median response duration that exceeds 12 months. Responses were noted in patients with genetically high-risk disease, including 5 (42%) of 12 patients with del(17p13.1) and 13 (72%) of 18 patients with del(11q22.3). Flavopiridol administered using this novel schedule has significant clinical activity in refractory CLL. Patients with bulky disease and high-risk genetic features have achieved durable responses, thereby justifying further study of flavopiridol in CLL and other diseases.

Flavopiridol disrupts STAT3/DNA interactions, attenuates STAT3-directed transcription, and combines with the Jak kinase inhibitor AG490 to achieve cytotoxic synergy

Up-regulated signal transducers and activators of transcription (STAT)-mediated signaling is believed to contribute to the pathogenesis of a variety of solid and hematologic cancers. Consequently, inhibition of STAT-mediated signaling has recently been proposed as a potential new therapeutic approach to the treatment of cancers. Having shown previously that the pan-cyclin-dependent kinase inhibitor flavopiridol binds to DNA and seems to kill cancer cells via that process in some circumstances, we evaluated the hypothesis that flavopiridol might consequently disrupt STAT3/DNA interactions, attenuate STAT3-directed transcription, and down-regulate STAT3 downstream polypeptides, including the antiapoptotic polypeptide Mcl-1. SDS-PAGE/immunoblotting and reverse transcription-PCR were used to assess RNA and polypeptide levels, respectively. DNA cellulose affinity chromatography and a nuclear elution assay were used to evaluate the ability of flavopiridol to disrupt STAT3/DNA interactions. A STAT3 luciferase reporter assay was used to examine the ability of flavopiridol to attenuate STAT3-directed transcription. Colony-forming assays were used to assess cytotoxic synergy between flavopiridol and AG490. Flavopiridol was found to (a) disrupt STAT3/DNA interactions (DNA cellulose affinity chromatography and nuclear elution assay), (b) attenuate STAT3-directed transcription (STAT3 luciferase reporter assay), and (c) down-regulate the STAT3 downstream antiapoptotic polypeptide Mcl-1 at the transcriptional level (reverse transcription-PCR and SDS-PAGE/immunoblotting). Furthermore, flavopiridol, but not the microtubule inhibitor paclitaxel, could be combined with the STAT3 pathway inhibitor AG490 to achieve cytotoxic synergy in A549 human non-small cell lung cancer cells. Collectively, these data suggest that flavopiridol can attenuate STAT3-directed transcription in a targeted fashion and may therefore be exploitable clinically in the development of chemotherapy regimens combining flavopiridol and other inhibitors of STAT3 signaling pathways.

Induction of apoptosis by flavopiridol in human neuroblastoma cells is enhanced under hypoxia and associated with N-myc proto-oncogene down-regulation

PURPOSE

Neuroblastoma is the most common extracranial solid tumor of children that arises from the sympathetic nervous system. Survival rates for neuroblastoma patients is low despite intensive therapeutic intervention, and the identification of new effective drugs remains a primary goal. The cyclin-dependent kinase inhibitor, flavopiridol, has demonstrated growth-inhibitory and cytotoxic activity against various tumor types. Our aim was to investigate flavopiridol effects on advanced-stage, N-myc proto-oncogene (MYCN)-amplified human neuroblastomas and the modulation of its activity by hypoxia, a critical determinant of tumor progression and a major challenge of therapy.

EXPERIMENTAL DESIGN

Cell viability was monitored by 3-(4,5 dimethyl-2 thiazolyl)-2,5 diphenyl-2H tetrazolium bromide (MTT) and trypan blue dye exclusion assays; DNA synthesis was assessed with the bromodeoxyuridine pulse-labeling technique; apoptosis was studied by Giemsa staining, DNA fragmentation, terminal deoxynucleotidyl-transferase-mediated dUTP nick end labeling reaction, flow cytometric determination of hypodiploid DNA content, and evaluation of caspase activity and cytochrome c (CytC) release; MYCN expression was determined by Northern and Western blotting.

RESULTS

Flavopiridol caused dose- and time-dependent decreases in neuroblastoma viability by inducing apoptosis, as confirmed by morphologic and biochemical criteria. Cell death was preceded by DNA synthesis inhibition and G1-G2 arrest, reversed by the pancaspase inhibitor, zVAD-fmk, and associated with caspase-3 and -2 activation and CytC increase. Moreover, flavopiridol strongly down-regulated MYCN mRNA and protein expression. Exposure to hypoxia enhanced both the extent of apoptosis and flavopiridol effects on CytC, caspase 3, and MYCN.

CONCLUSIONS

These results indicate that flavopiridol has growth-inhibitory and apoptotic activity against advanced-stage neuroblastomas in vitro and is worthy of further investigation for the treatment of this disease.

Phase 1 Trial of Flavopiridol Combined with Cisplatin or Carboplatin in Patients with Advanced Malignancies with the Assessment of Pharmacokinetic and Pharmacodynamic End Points

PURPOSE

Flavopiridol, a cyclin-dependent kinase inhibitor, transcription inhibitor, and DNA-interacting agent, was combined with cisplatin or carboplatin to establish toxicities, evaluate pharmacokinetics, and examine its effects on patient cancers and levels of selected polypeptides in patient peripheral blood mononuclear cells (PBMC).

EXPERIMENTAL DESIGN

Therapy was given every 3 weeks. Stage I: cisplatin was fixed at 30 mg/m2 with escalating flavopiridol. Stage II: flavopiridol was fixed at the stage I maximum tolerated dose (MTD) with escalation of cisplatin. Stage III: flavopiridol was fixed at the stage I MTD with escalation of carboplatin.

RESULTS

Thirty-nine patients were treated with 136 cycles of chemotherapy. Neutropenia was seen in only 11% of patients. Grade 3 flavopiridol/CDDP toxicities were nausea (30%), vomiting (19%), diarrhea (15%), dehydration (15%), and neutropenia (10%). Flavopiridol combined with carboplatin resulted in unexpectedly high toxicities and one treatment-related death. Stable disease (>3 months) was seen in 34% of treated patients, but there were no objective responses. The stage II MTD was 60 mg/m2 cisplatin and 100 mg/m2/24 hours flavopiridol. As given, CDDP did not alter flavopiridol pharmacokinetics. Flavopiridol induced increased p53 and pSTAT3 levels in patient PBMCs but had no effects on cyclin D1, phosphoRNA polymerase II, or Mcl-1.

CONCLUSIONS

Flavopiridol and cisplatin can be safely combined in the treatment of cancer patients. Unexpected toxicity in flavopiridol/carboplatin-treated patients attenuates enthusiasm for this alternative combination. Analysis of polypeptide levels in patient PBMCs suggests that flavopiridol may be affecting some, but not all, of its known in vitro molecular targets in vivo.

Flavopiridol sensitivity of cancer cells isolated from ascites and pleural fluids

PURPOSE

We examined the efficacy of flavopiridol, a cyclin-dependent kinase inhibitor that is undergoing clinical trials, on primary cancer cells isolated from the ascites or pleural fluids of patients with metastatic cancers.

EXPERIMENTAL DESIGN

Metastasized cancer cells were isolated from the pleural fluids (n = 20) or ascites (n = 15) of patients, most of whom were refractory to chemotherapy. These primary cancer cells were used within 2 weeks of isolation without selecting for proliferative capacities. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide viability assay was used to characterize the response of these cancer cells to commonly used chemotherapeutic agents, and their response to flavopiridol was compared with rapidly dividing cultured cell lines.

RESULTS

The primary cancer cells displayed phenotypes that were different from established cell lines; they had very low replication rates, dividing every 1 to 2 weeks, and underwent replicative senescence within five passages. These primary tumor cells retained their resistance to chemotherapeutic drugs exhibited by the respective patients but did not show cross-resistance to other agents. However, these cancer cells showed sensitivity to flavopiridol with an average LD50 of 50 nmol/L (range, 21.5-69 nmol/L), similar to the LD50 in established cell lines. Because senescent cells also showed similar sensitivity to flavopiridol, it suggests that the mechanism of action is not dependent on the activity of cyclin-dependent kinases that regulate the progression of the cell cycle.

CONCLUSION

Using cancer cells isolated from the ascites or pleural fluids, this study shows the potential of flavopiridol against cancer cells that have developed resistance to conventional chemotherapeutic agents.

Treatment of Relapsed Chronic Lymphocytic Leukemia by 72-Hour Continuous Infusion or 1-Hour Bolus Infusion of Flavopiridol: Results from Cancer and Leukemia Group B Study 19805

PURPOSE

Flavopiridol has in vitro activity in chronic lymphocytic leukemia (CLL) and promotes apoptosis independent of p53 function or prior fludarabine exposure. We sought to determine if flavopiridol has activity in previously treated CLL using two schedules of administration.

PATIENTS AND METHODS

Patients with previously treated CLL were enrolled in two sequentially done phase II studies. Patients in the first trial received flavopiridol (50 mg/m(2)/d) as a continuous infusion (CI) for 72 hours every 2 weeks. Patients in the second trial received flavopiridol 50 mg/m(2) as a 1-hour bolus (IVB) daily for 3 days repeated every 3 weeks. Patients received up to 12 (CI cohort) or 8 (IVB cohort) cycles of therapy.

RESULTS

Fifteen patients were enrolled in the 72-hour CI phase II trial; 6 (40%) had intermediate-risk (Rai stage I or II) and 9 (60%) had high-risk (Rai stage III and IV) stages. No responses were noted in this group; 27% had stable disease and 73% had progressive disease. Thirty-six patients were enrolled in the second IVB trial, with 13 (36%) having intermediate and 23 (64%) having high-risk disease. Four patients (11%) had partial responses, 19 (53%) had stable disease, and 13 (36%) had progressive disease. The progression-free survival for responders in the IVB trial was 3, 3, 9, and 19 months. The median progression-free survival was 2 months [95% confidence interval (95% CI), 1.8-3.8] for patients in the CI trial and 3 months (95% CI, 2.5-7.4) for the IVB trial. The median overall survival was 27 months (95% CI, 20-42) for the CI trial and 24 months (95% CI, 18-31) for the IVB trial. Toxicity was manageable and included mainly myelosuppression, infections, diarrhea, and fatigue.

CONCLUSIONS

Flavopiridol has modest, schedule-dependent clinical activity in relapsed CLL and warrants future investigation utilizing alternative schedules of administration.

Flavopiridol Induces p53 via Initial Inhibition of Mdm2 and p21 and, Independently of p53, Sensitizes Apoptosis-Reluctant Cells to Tumor Necrosis Factor

Flavopiridol (FP) inhibits gene expression and causes apoptosis, and these effects cannot be explained by inhibition of cyclin-dependent kinases that govern cell cycle. The simple and established notion that FP is an inhibitor of transcription predicts its effects. Because Mdm-2 targets p53 for degradation, FP, as predicted, dramatically induced p53 by inhibiting Mdm-2. Once p53 was induced, restoration of transcription (by removal of FP) resulted in superinduction of p21 and Mdm-2. Similarly, low concentrations of FP (50 nm) induced p21 and Mdm-2 because of their initial down-regulation. A sustained decrease of Mdm-2/p21 expression and accumulation of p53 coincided with near-maximal cytotoxicity of FP at concentrations >100 nm. Induction of p53 was a marker, not a cause, of cytotoxicity. FP caused rapid apoptosis (caspase-dependent cell death) in p53-null leukemia cells. In these cells, FP-induced apoptosis was converted to growth arrest by inhibitors of caspases. In apoptosis-reluctant A549 and PC3M cancer cells, FP inhibited cell proliferation but did not cause apoptosis. Like typical inhibitors of transcription, FP sensitized cells to apoptotic stimuli, allowing tumor necrosis factor to cause rapid and massive apoptosis in otherwise apoptosis-reluctant cells. We discuss that, as a reversible inhibitor of transcription, FP can be used clinically in novel rational drug combinations.

Small-molecule cyclin-dependent kinase modulators

Aberrations in cell cycle progression occur in the majority of human malignancies. The main pathway affected is the retinoblastoma (Rb) pathway. The tumor suppressor gene Rb is an important component in the G(1)/S transition and its function is abnormal in most human neoplasms. Loss in Rb function occurs by the hyperactivation of the cyclin-dependent kinases (cdk's). Therefore, modulation of cdk's may have an important use for the therapy and prevention of human neoplasms. Efforts to obtain small-molecule cdk modulators yielded two classes of modulators: direct and indirect modulators. Direct cdk modulators are small molecules that specifically target the ATP binding site of cdk's. Examples for this group include flavopiridol, roscovitine and BMS-387032. In contrast, indirect cdk modulators affect cdk function due to modulation of upstream pathways required for cdk activation. Some examples include perifosine, lovastatin, and UCN-01. The first example of a direct small-molecule cdk modulator tested in the clinic, flavopiridol, is a pan-cdk inhibitor that not only promotes cell cycle arrest but also halts transcriptional elongation, promotes apoptosis, induces differentiation, and has antiangiogenic properties. Clinical trials with this agent were performed with at least three different schedules of administration: 1-, 24- and 72-h infusions. The main toxicities for infusions >/=24-h are secretory diarrhea and proinflammatory syndrome. In addition, patients receiving shorter infusions have nausea/vomiting and neutropenia. A phase II trial of patients with advanced non-small-cell lung carcinoma using the 72-h infusion every 2 weeks was recently completed. The median overall survival for the 20 patients who received treatment was 7.5 months, a survival similar to that obtained in a randomized trial of four chemotherapy regimens containing platinum analogues in combination with taxanes or gemcitabine, or with gefitinib, a recently approved EGFR inhibitor for the treatment of advanced lung cancer. Based on these encouraging results, a phase III trial comparing standard combination chemotherapy versus combination chemotherapy plus flavopiridol is currently under investigation. The second example of direct small-molecule cdk modulator tested in clinical trials is UCN-01 (7-hydroxystaurosporine). UCN-01 has interesting preclinical features: it inhibits Ca(2+)-dependent PKCs, promotes apoptosis, arrests cell cycle progression at G(1)/S, and abrogates checkpoints upon DNA damage. The first phase I trial of UCN-01 demonstrated a very prolonged half-life. Based on this novel feature, UCN-01 is administered as a 72-h continuous infusion every 4 weeks (in second and subsequent cycles UCN-01 is administered as a 36-h infusion). Other shorter schedules (i.e. 3 h) are being tested. Dose-limiting toxicities include nausea/vomiting, hypoxemia, and insulin-resistant hyperglycemia. Combination trials with cisplatin and other DNA-damaging agents are being tested. Recently, phase I trials with two novel small-molecule cdk modulators, BMS 387032 and R-Roscovitine (CYC202), have commenced with good tolerability. In summary, novel small-molecule cdk modulators are being tested in the clinic with interesting results. Although these small molecules are directed towards a very prevalent cause of carcinogenesis, we need to test them in advanced clinical trials to determine the future of this class of agents for the prevention and therapy of human malignancies.

Damaged microtubules can inactivate BCL-2 by means of the mTOR kinase

Rapamycin, a specific inhibitor of the serine/threonine mTOR kinase, markedly inhibited both cell growth and apoptosis in human B-cell lines. Besides arresting cells in G(1) by increasing p27(kip1), rapamycin tripled the cellular level of the BCL-2 protein. The activity was dose-dependent and specific for the p27(kip1) and BCL-2 proteins. Rapamycin did not affect bcl-2 mRNA although it increased cellular BCL-2 concentration by inhibiting phosphorylation, a mechanism initiating the decay process. To add new insight, we combined rapamycin treatment with treatment by taxol, which, by damaging microtubules, can phosphorylate BCL-2 and activate apoptosis. It was found that the mTOR kinase was activated in cells treated with taxol or with nocodazole although it was inhibited in cells pre-treated with rapamycin. BCL-2 phosphorylation, apoptosis and hyperdiploidy were also inhibited by rapamycin. In contrast, taxol-induced microtubule stabilization or metaphase synchronization were not inhibited by rapamycin. Taken together, these findings indicate that mTOR belongs to the enzymatic cascade that, starting from damaged microtubules, phosphorylates BCL-2. By regulating apoptosis, in addition to the control of a multitude of growth-related pathways, mTOR plays a nodal role in signaling G(1) and G(2)-M events.

Small molecule modulators of cyclin-dependent kinases for cancer therapy

The majority of human malignancies have aberrancies in the Retinoblastoma (Rb) pathway. Loss in Rb function results from the phosphorylation and inactivation of Rb by the cyclin-dependent kinases (cdks), main regulators of cell cycle progression. Thus, modulators of cdks may have a role in the treatment of human malignancies. Flavopiridol, the first cdk modulator tested in clinical trials, demonstrates interesting preclinical features: cell cycle block, induction of apoptosis, promotion of differentiation, inhibition of angiogenic processes and modulation of transcriptional events. Initial clinical trials with infusional flavopiridol demonstrated activity in some patients with lymphomas and renal, colon gastric carcinomas. Main side effects were diarrhea and hypotension. Phase 2 trials with infusional flavopiridol, other schedules and combination with standard chemotherapies are ongoing. The second cdk modulator tested in clinical trials, UCN-01, is a PKC inhibitor that can also modulate cdk activity. Similar to flavopiridol, UCN-01 blocks cell cycle progression and promotes apoptosis. Moreover, UCN-01 may abrogate checkpoints induced by genotoxic stress due to inhibition of chk1 kinase. The first clinical trial of UCN-01 demonstrated very prolonged half-life (approximately 600 h), due to high binding affinity of UCN-01 to the human alpha-1-acid glycoprotein. Main side effects were headaches, vomiting, hypoxemia and hyperglycemia. Clinical activity was observed in some patients with melanoma and lymphoma. Trials of shorter infusions of UCN-01 or in combination with standard chemotherapeutic agents are ongoing. Although several important basic and clinical questions remain unanswered, development of cdk modulators is a reasonable strategy for cancer therapy.

Dysregulation of cyclin dependent kinase 6 expression in splenic marginal zone lymphoma through chromosome 7q translocations

The increased or inappropriate expression of genes with oncogenic properties through specific chromosome translocations is an important event in the pathogenesis of B-cell lymphoproliferative diseases. Recent studies have found deletions or translocations of chromosome 7q to be the most common cytogenetic abnormality observed in SLVL, a leukemic variant of SMZL, with the q21-q22 region being most frequently affected. In three patients with translocations between chromosomes 2 and 7, the cloning of the breakpoints at 7q21 revealed that each was located within a small region of DNA 3.6 kb upstream of the transcription start site of cyclin dependent kinase 6 (CDK6). In each case the translocation event was consistent with aberrant VJ recombination between the immunoglobulin light chain region (Ig kappa) on chromosome 2p12 and DNA sequences at 7q21, resembling the heptamer recombination site. The t(7;21) breakpoint in an additional patient with splenic marginal zone lymphoma (SMZL), resided 66 kb telomeric to the t(2;7) breakpoints juxtaposing CDK6 to an uncharacterized transcript. In two of the SLVL patient samples, the CDK6 protein was found to be markedly over expressed. These results suggest that dysregulation of CDK6 gene expression contributes to the pathogenesis of SLVL and SMZL.