Flavopiridol, a novel cyclin-dependent kinase inhibitor, suppresses the growth of head and neck squamous cell carcinomas by inducing apoptosis.

Flavopiridol (HMR 1275) has been identified recently as a novel antineoplastic agent in the primary screen conducted by the Developmental Therapeutics Program, National Cancer Institute. Flavopiridol inhibits most cyclin-dependent kinases (cdks) and displays unique anticancer properties. Here, we investigated whether this compound was effective against head and neck squamous cell carcinomas (HNSCC). Exposure of HNSCC cells to flavopiridol diminished cdc2 and cdk2 activity and potently inhibited cell proliferation (IC50 43-83 nM), which was concomitant with the appearance of cells with a sub-G1 DNA content. Moreover, DNA fragmentation and TUNEL (terminal deoxynucleotidyl transferase-mediated nick end labeling) reaction confirmed that flavopiridol induces apoptosis in all cell lines, even on certain HNSCC cells that are insensitive to apoptosis to DNA-damaging agents (gamma-irradiation and bleomycin). A tumorigenic HNSCC cell line was used to assess the effect of flavopiridol in vivo. Treatment (5 mg/kg per day, intraperitoneally) for 5 d led to the appearance of apoptotic cells in the tumor xenografts and caused a 60-70% reduction in tumor size, which was sustained over a period of 10 wk. Flavopiridol treatment also resulted in a remarkable reduction of cyclin D1 expression in HNSCC cells and tumor xenografts. Our data indicate that flavopiridol exerts antitumor activity in HNSCC, and thus it can be considered a suitable candidate drug for testing in the treatment of refractory carcinomas of the head and neck.

Preclinical and clinical development of cyclin-dependent kinase modulators.

In the last decade, the discovery and cloning of the cyclin-dependent kinases (cdks), key regulators of cell cycle progression, have led to the identification of novel modulators of cdk activity. Initial experimental results demonstrated that these cdk modulators are able to block cell cycle progression, induce apoptotic cell death, promote differentiation, inhibit angiogenesis, and modulate transcription. Alteration of cdk activity may occur indirectly by affecting upstream pathways that regulate cdk activity or directly by targeting the cdk holoenzyme. Two direct cdk modulators, flavopiridol and UCN-01, are showing promising results in early clinical trials, in which the drugs reach plasma concentrations that can alter cdk activity in vitro. Although modulation of cdk activity is a well-grounded concept and new cdk modulators are being assessed for clinical testing, important scientific questions remain to be addressed. These questions include whether one or more cdks should be inhibited, how cdk inhibitors should be combined with other chemotherapy agents, and which cdk substrates should be used to assess the biologic effects of these drugs in patients. Thus, modulation of cdk activity is an attractive target for cancer chemotherapy, and several agents that modulate cdk activity are in or are approaching entry into clinical trials.

Jasplakinolide's inhibition of the growth of prostate carcinoma cells in vitro with disruption of the actin cytoskeleton.

BACKGROUND: Jasplakinolide, a cyclodepsipeptide produced by an Indo-Pacific sponge, Jaspis johnstoni, has been reported to inhibit the growth of breast cancer cells. PURPOSE: The effects of jasplakinolide on the proliferation of three human immortalized prostate carcinoma cell lines (PC-3, LNCaP, and TSU-Pr1) were studied. The growth-inhibitory effect of jasplakinolide on the PC-3 cell line was studied in detail to elucidate its mechanism of action. METHODS: Cell counts were used to study growth inhibition. A protein-based microplate assay was used to assess the time of exposure needed to cause persistent growth inhibition and to study the effects of jasplakinolide analogues. Metabolic changes were assessed by following the incorporation of radiolabeled precursors. The effects of jasplakinolide on the cytoskeleton were studied by fluorescent microscopy, using rhodamine phalloidin (RP) and antibodies to cytoskeletal components. Changes in RP binding were quantified by extracting bound fluorescent material from fixed cells and measuring the amount of fluorescence in a spectrofluorometer. RESULTS: The growth of PC-3, LNCaP, and TSU-Pr1 cells was potently inhibited by exposure to jasplakinolide for 48 hours; doses of jasplakinolide that led to 50% growth inhibition were 65 nM for PC-3 cells, 41 nM for LNCaP cells, and 170 nM for TSU-Pr1 cells. In PC-3 cells, exposure to 160 nM for 48 hours led to total growth inhibition, which persisted for several days even after drug removal. Several jasplakinolide analogues also inhibited the growth of PC-3 cells, although analogues in which the rigidity of the macrolide ring was altered were ineffective. No early changes in the synthesis of DNA, RNA, or protein or in intracellular adenosine triphosphate levels were seen in the PC-3 cells after exposure to jasplakinolide. Growth inhibition by jasplakinolide was accompanied by striking morphologic changes. Exposure for several doublings led to multinucleated cells. Further investigation of these changes in the PC-3 cells revealed a dramatic and early disruption of the actin cytoskeleton and a statistically significant decrease in RP binding. The doses of jasplakinolide, the time of exposure, and the pattern of growth inhibition by structural analogues corresponded with the changes seen in actin distribution. CONCLUSIONS: Jasplakinolide represents a novel marine natural product with potent in vitro antiproliferative activity against human prostate carcinoma cell lines, and it appears to target the actin cytoskeleton. IMPLICATIONS: Jasplakinolide is a potential candidate for further preclinical development and a lead structure for a novel class of therapeutic agents that can disrupt the actin cytoskeleton in mammalian cells.

Flavopiridol, a protein kinase inhibitor, down-regulates hypoxic induction of vascular endothelial growth factor expression in human monocytes.

We have investigated the effects of flavopiridol, a novel protein kinase inhibitor that is selective for cyclin-dependent kinases, on hypoxia-induced vascular endothelial growth factor (VEGF) expression in human monocytes. We found that hypoxia induces a time-dependent increase of VEGF mRNA expression and protein levels in human monocytes. Flavopiridol showed a minimal effect on the constitutive levels of VEGF mRNA but completely blocked hypoxia-induced VEGF mRNA and protein expression. The inhibitory effects of flavopiridol on VEGF mRNA induction also occurred in the presence of cycloheximide. The transcriptional activation of either a VEGF promoter-luciferase construct or a hypoxia-inducible factor 1 reporter plasmid was not affected by addition of flavopiridol in transient transfection experiments. In contrast, actinomycin D experiments demonstrated that flavopiridol dramatically decreased VEGF mRNA stability. These data provide the first evidence that flavopiridol can affect gene expression by altering mRNA stability. We propose that flavopiridol may interfere with one or more signaling events, leading to hypoxia-induced, protein kinase-modulated, RNA protein binding activity. An important clinical implication of our results is that flavopiridol, presently under investigation in clinical trials, might have antiangiogenic as well as direct antiproliferative effects.

Down-regulation of cyclin D1 by transcriptional repression in MCF-7 human breast carcinoma cells induced by flavopiridol.

Flavopiridol is a novel flavonoid that induces cell cycle arrest at different stages of the cell cycle because of the inhibition of cyclin-dependent kinases (cdks). In previous studies from our laboratory, (B. A. Carlson et al., Cancer Res., 56: 2973-2978, 1996), we observed that exposure of the MCF-7 breast carcinoma cell line to flavopiridol resulted in G1-S arrest, which was associated with the loss of cdk4 and cdk2 activity by 24 h of exposure. Along with this inhibition, flavopiridol decreased total cyclin-D protein levels in this cell line. In this work, we demonstrate that using isoform-specific antibodies, flavopiridol induces an early (by 6 h) decrease in cyclin D1 protein levels. This decline is followed by a decline in cyclin D3 with no effect on cyclin D2 or cyclin E levels by 10 h. Furthermore, at early time points (up to 8 h), the activity of cdk4 and the expression of endogenous phosphorylated retinoblastoma species from intact cells exposed to flavopiridol are unchanged. Thus, the decline in cdk4 activity and the induction of retinoblastoma hypophosphorylation follows cyclin D1 decline. Turnover studies demonstrate that the half-life of cyclin D1 (approximately 30 min) is not shortened in flavopiridol-exposed cells, and that the turnover of cdk4-bound cyclin D1 is unaltered. However, steady-state levels of cyclin D1 mRNA display a significant decrease by 4 h of flavopiridol treatment, with total disappearance by 8 h. This mRNA decline is not abrogated by the presence of cycloheximide. Furthermore, we have found that flavopiridol specifically represses the activity of the full-length cyclin D1 promoter linked to a luciferase reporter gene. In summary, we have found that the flavopiridol-induced decline in cyclin D1 is an early event, specific and, at least in part, due to the transcriptional repression of the cyclin D1 promoter. These results extend our understanding of flavopiridol's action to include regulation of cyclin D1 transcription.

Discovery and initial characterization of the paullones, a novel class of small-molecule inhibitors of cyclin-dependent kinases.

Analysis of the National Cancer Institute Human Tumor Cell Line Anti-Cancer Drug Screen data using the COMPARE algorithm to detect similarities in the pattern of compound action to flavopiridol, a known inhibitor of cyclin-dependent kinases (CDKs), has suggested several possible novel CDK inhibitors. 9-Bromo-7,12-dihydro-indolo[3,2-d][1]benzazepin-6(5H)-one, NSC-664704 (kenpaullone), is reported here to be a potent inhibitor of CDK1/cyclin B (IC50, 0.4 microM). This compound also inhibited CDK2/cyclin A (IC50, 0.68 microM), CDK2/cyclin E (IC50, 7.5 microM), and CDK5/p25 (IC50, 0.85 microM) but had much less effect on other kinases; only c-src (IC50, 15 microM), casein kinase 2 (IC50, 20 microM), erk 1 (IC50, 20 microM), and erk 2 (IC50, 9 microM) were inhibited with IC50s less than 35 microM. Kenpaullone acts by competitive inhibition of ATP binding. Molecular modeling indicates that kenpaullone can bind in the ATP binding site of CDK2 with residue contacts similar to those observed in the crystal structures of other CDK2-bound inhibitors. Analogues of kenpaullone, in particular 10-bromopaullone (NSC-672234), also inhibited various protein kinases including CDKs. Cells exposed to kenpaullone and 10-bromopaullone display delayed cell cycle progression. Kenpaullone represents a novel chemotype for compounds that preferentially inhibit CDKs.

Unpredicted clinical pharmacology of UCN-01 caused by specific binding to human alpha1-acid glycoprotein.

The pharmacokinetics of UCN-01 after administration as a 72- or 3-h infusion to cancer patients in initial Phase I trials displayed distinctive features that could not have been predicted from preclinical data. The distribution volumes (0.0796-0.158 liters/kg) and the systemic clearance (0.0407-0.252 ml/h/kg) were extremely low, in contrast to large distribution volume and rapid systemic clearance in experimental animals. The elimination half-lives (253-1660 h) were unusually long. In vitro protein binding experiments demonstrated that UCN-01 was strongly bound to human alpha1-acid glycoprotein. The results suggest that unusual pharmacokinetics of UCN-01 in humans could be due, at least in part, to its specifically high binding to alpha1-acid glycoprotein.

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.

Phase II trial of suramin, leuprolide, and flutamide in previously untreated metastatic prostate cancer.

PURPOSE: To assess the efficacy and toxicity of suramin, hydrocortisone, leuprolide, and flutamide in previously untreated metastatic prostate cancer. PATIENTS AND METHODS: Patients with stage D2 and poor-prognosis stage D1 prostate cancer were given suramin on a pharmacokinetically derived dosing schedule to maintain suramin concentrations between 175 and 300 micrograms/mL. Additionally, all patients received flutamide 250 mg orally three times daily, initiated on day 1 and continued until disease progression; depot leuprolide 7.5 mg intramuscularly begun on day 5 and repeated every 4 weeks indefinitely; and replacement doses of hydrocortisone. RESULTS: Fifty patients were entered onto the study: 48 with stage D2 and two with stage D1 disease. The median age was 59 years (range, 42 to 79) and 31 patients had a Karnofsky performance status (KPS) of 100%. Forty-five patients had bone metastases and 25 had measurable soft tissue disease. Forty-one (82%) had severe disease. The overall response rate in 49 assessable patients was three complete responses (CRs) and 30 partial responses (PRs) for an overall response rate of 67%. Eighteen patients have died. The median survival time has not been reached, with a median potential follow-up duration of 44 months. Grade 3 to 4 toxicity was seen in 38% of patients and was predominantly hematologic and reversible. CONCLUSION: The high response rate and prolonged survival in a poor-prognosis group of patients with metastatic prostate cancer warrant a phase III randomized comparison of this regimen versus hormonal therapy alone. Toxicity was moderate and reversible.

Phase I trial of continuous infusion flavopiridol, a novel cyclin-dependent kinase inhibitor, in patients with refractory neoplasms.

PURPOSE: We conducted a phase I trial of the cyclin-dependent kinase inhibitor, flavopiridol (National Service Center [NSC] 649890), to determine the maximum-tolerated dose (MTD), toxicity profile, and pharmacology of flavopiridol given as a 72-hour infusion every 2 weeks. PATIENTS AND METHODS: Seventy-six patients with refractory malignancies with prior disease progression were treated with flavopiridol, with first-cycle pharmacokinetic sampling. RESULTS: Forty-nine patients defined our first MTD, 50 mg/m2/d x 3 with dose-limiting toxicity (DLT) of secretory diarrhea at 62.5 mg/kg/d x 3. Subsequent patients received antidiarrheal prophylaxis (ADP) to define a second MTD, 78 mg/m2/d x 3 with DLT of hypotension at 98 mg/m2/d x 3. Other toxicities included a proinflammatory syndrome with alterations in acute-phase reactants, particularly at doses >50 mg/ m2/d x 3, which in some patients prevented chronic therapy every 2 weeks. In some patients, ADP was not successful, requiring dose-deescalation. Although approximately 70% of patients displayed predictable flavopiridol pharmacology, we observed unexpected interpatient variability and postinfusion peaks in approximately 30% of cases. At the two MTDs, we achieved a mean plasma flavopiridol concentration of 271 nM (50 mg/m2/d x 3) and 344 nM (78 mg/m2/d x 3), respectively. One partial response in a patient with renal cancer and minor responses (n=3) in patients with non-Hodgkin's lymphoma, colon, and renal cancer occurred. CONCLUSION: The MTD of infusional flavopiridol is 50 mg/m2/d x 3 with dose-limiting secretory diarrhea at 62.5 mg/m2/d x 3. With ADP, 78 mg/m2/d x 3 was the MTD, with dose-limiting hypotension at 98 mg/m2/d x 3. Based on chronic tolerability, 50 mg/m2/d x 3 is the recommended phase II dose without ADP. Antitumor effect was observed in certain patients with renal, prostate, and colon cancer, and non-Hodgkin's lymphoma. Concentrations of flavopiridol (200 to 400 nM) needed for cyclin-dependent kinase inhibition in preclinical models were achieved safely.

Phase I trial of 72-hour continuous infusion UCN-01 in patients with refractory neoplasms.

PURPOSE: To define the maximum tolerated dose (MTD) and dose-limiting toxicity (DLT) of the novel protein kinase inhibitor, UCN-01 (7-hydroxystaurosporine), administered as a 72-hour continuous intravenous infusion (CIV). PATIENTS AND METHODS: Forty-seven patients with refractory neoplasms received UCN-01 during this phase I trial. Total, free plasma, and salivary concentrations were determined; the latter were used to address the influence of plasma protein binding on peripheral tissue distribution. The phosphorylation state of the protein kinase C (PKC) substrate alpha-adducin and the abrogation of DNA damage checkpoint also were assessed. RESULTS: The recommended phase II dose of UCN-01 as a 72-hour CIV is 42.5 mg/m(2)/d for 3 days. Avid plasma protein binding of UCN-01, as measured during the trial, dictated a change in dose escalation and administration schedules. Therefore, nine patients received drug on the initial 2-week schedule, and 38 received drug on the recommended 4-week schedule. DLTs at 53 mg/m(2)/d for 3 days included hyperglycemia with resultant metabolic acidosis, pulmonary dysfunction, nausea, vomiting, and hypotension. Pharmacokinetic determinations at the recommended dose of 42.5 mg/m(2)/d for 3 days included mean total plasma concentration of 36.4 microM (terminal elimination half-life range, 447 to 1176 hours), steady-state volume of distribution of 9.3 to 14.2 L, and clearances of 0.005 to 0.033 L/h. The mean total salivary concentration was 111 nmol/L of UCN-01. One partial response was observed in a patient with melanoma, and one protracted period ( > 2.5 years) of disease stability was observed in a patient with alk-positive anaplastic large-cell lymphoma. Preliminary evidence suggests UCN-01 modulation of both PKC substrate phosphorylation and the DNA damage-related G(2) checkpoint. CONCLUSION: UCN-01 can be administered safely as an initial 72-hour CIV with subsequent monthly doses administered as 36-hour infusions.

Development of cyclin-dependent kinase modulators as novel therapeutic approaches for hematological malignancies.

The majority of hematopoietic malignancies have aberrancies in the retinoblastoma (Rb) pathway. Loss in Rb function is, in most cases, a result of the phosphorylation and inactivation of Rb by the cyclin-dependent kinases (cdks), main regulators of cell cycle progression. Flavopiridol, the first cdk modulator tested in clinical trials, is a flavonoid that inhibits several cdks with evidence of cell cycle block. Other interesting preclinical features are the 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 non-Hodgkin's lymphoma, renal, prostate, colon and gastric carcinomas. Main side-effects were secretory diarrhea and a pro-inflammatory syndrome associated with hypotension. Phase 2 trials with infusional flavopiridol in CLL and mantle cell lymphoma, other schedules and combination with standard chemotherapies are ongoing. The second cdk modulator tested in clinical trials, UCN-01, is a potent protein kinase C inhibitor that inhibits cdk activity in vitro as well. UCN-01 blocks cell cycle progression and promotes apoptosis in hematopoietic models. Moreover, UCN-01 is able to abrogate checkpoints induced by genotoxic stress due to modulation in chk1 kinase. The first clinical trial of UCN-01 demonstrated very prolonged half-life (approximately 600 h), 100 times longer than the half-life observed in preclinical models. This effect is due to high binding affinity of UCN-01 to the human plasma protein alpha-1-acid glycoprotein. Main side-effects in this trial were headaches, nausea/vomiting, hypoxemia and hyperglycemia. Clinical activity was observed in patients with melanoma, non-Hodgkin's lymphoma and leiomyosarcoma. Of interest, a patient with anaplastic large cell lymphoma refractory to high-dose chemotherapy showed no evidence of disease after 3 years of UCN-01 therapy. Trials of infusional UCN-01 in combination with Ara-C or gemcitabine in patients with acute leukemia and CLL, respectively, have commenced. In conclusion, flavopiridol and UCN-01 are cdk modulators that reach biologically active concentrations effective in modulating CDK in vitro, and show encouraging results in early clinical trials in patients with refractory hematopoietic malignancies. Although important questions remain to be answered, these positive experiences will hopefully increase the therapeutic modalities in hematological malignancies.

Induction of apoptosis and inhibition of c-erbB-2 in breast cancer cells by flavopiridol.

Flavopiridol is a flavone that inhibits several cyclin-dependent kinases and exhibits potent growth-inhibitory activity against a number of human tumor cell lines, both in vitro and when grown as xenografts in mice. It is presently being investigated as a novel antineoplastic agent in the primary screen conducted by the Developmental Therapeutics Program, National Cancer Institute. Because breast cancer is the most common cancer and second leading cause of cancer-related deaths in women in the United States, we investigated whether flavopiridol could be an effective agent against a series of isogenic breast- cancer cell lines having different levels of erbB-2 expression and differential invasion and metastatic characteristics. Flavopiridol was found to inhibit the growth of MDA-MB-435 (parental) and 435.eB (stable transfectants) cells that were established by transfecting c-erbB-2 cDNA into MDA-MB-435. Induction of apoptosis was also observed in these cell lines when treated with flavopiridol, as measured by DNA laddering, PARP, and CPP32 cleavages. We also found modest up-regulation of Bax and down-regulation of Bcl-2, but there was a significant down-regulation of c-erbB-2 in flavopiridol-treated cells. Gelatin zymography showed that flavopiridol inhibits the secretion of matrix metalloproteinase (MMP; MMPs 2 and 9) in the breast cancer cells and that the inhibition of c-erbB-2 and MMPs may be responsible for the inhibition of cell invasion observed in flavopiridol-treated cells. Collectively, these molecular effects of flavopiridol, however, were found to be independent of c-erbB-2 overexpression, suggesting that flavopiridol may be effective in all breast cancer. From these results, we conclude that flavopiridol inhibits the growth of MDA-MB-435 breast cancer cells, induces apoptosis, regulates the expression of genes, and inhibits invasion and, thus, may inhibit metastasis of breast cancer cells. These findings suggest that flavopiridol may be an effective chemotherapeutic or preventive agent against breast cancer.

Overexpression of the ATP-binding cassette half-transporter, ABCG2 (Mxr/BCrp/ABCP1), in flavopiridol-resistant human breast cancer cells.

We sought to characterize the interactions of flavopiridol with members of the ATP-binding cassette (ABC) transporter family. Cells overexpressing multidrug resistance-1 (MDR-1) and multidrug resistance-associated protein (MRP) did not exhibit appreciable flavopiridol resistance, whereas cell lines overexpressing the ABC half-transporter, ABCG2 (MXR/BCRP/ABCP1), were found to be resistant to flavopiridol. Flavopiridol at a concentration of 10 microM was able to prevent MRP-mediated calcein efflux, whereas Pgp-mediated transport of rhodamine 123 was unaffected at flavopiridol concentrations of up to 100 microM. To determine putative mechanisms of resistance to flavopiridol, we exposed the human breast cancer cell line MCF-7 to incrementally increasing concentrations of flavopiridol. The resulting resistant subline, MCF-7 FLV1000, is maintained in 1,000 nM flavopiridol and was found to be 24-fold resistant to flavopiridol, as well as highly cross-resistant to mitoxantrone (675-fold), topotecan (423-fold), and SN-38 (950-fold), the active metabolite of irinotecan. Because this cross-resistance pattern is consistent with that reported for ABCG2-overexpressing cells, cytotoxicity studies were repeated in the presence of 5 microM of the ABCG2 inhibitor fumitremorgin C (FTC), and sensitivity of MCF-7 FLV1000 cells to flavopiridol, mitoxantrone, SN-38, and topotecan was restored. Mitoxantrone efflux studies were performed, and high levels of FTC-reversible mitoxantrone efflux were found. Northern blot and PCR analysis revealed overexpression of the ABCG2 gene. Western blot confirmed overexpression of ABCG2; neither P-glycoprotein nor MRP overexpression was detected. These results suggest that ABCG2 plays a role in resistance to flavopiridol.

Modulation of clinical drug resistance in a B cell lymphoma patient by the protein kinase inhibitor 7-hydroxystaurosporine: presentation of a novel therapeutic paradigm.

Emerging evidence suggests that apoptosis is an important mechanism of tumor cell death from antineoplastic therapy. 7-hydroxystaurosporine (UCN-01) is a novel protein kinase inhibitor that increases chemotherapy-induced apoptosis in vitro and is in early phases of clinical development. In this report, we present a 68-year-old patient with chemotherapy-resistant lymphoma treated with UCN-01 and chemotherapy. He had a stage IV plasmacytoid lymphoma that failed to enter remission with high-dose EPOCH II (etoposide, prednisone, vincristine, cyclophosphamide, doxorubicin) chemotherapy. Due to disease progression and transformation to large cell lymphoma in the liver and bone marrow, he received UCN-01. Four weeks later, he received "standard-dose" EPOCH because of progression, developed severe neutropenia for 9 days, and expired from Candida sepsis on day 23. At autopsy, there was no histological evidence of residual lymphoma, although PCR for immunoglobulin gene rearrangement analysis revealed a faint clonal band in two of six nodes but none in the liver. Significantly, no B cells were detected by immunohistochemistry in lymph nodes, and a polyclonal ladder pattern associated with the presence of normal B cells was not seen in the immunoglobulin gene rearrangement PCR assay. Profound peripheral lymphopenia (50 cells/microliter) was also observed. Pharmacokinetics showed UCN-01 salivary concentrations, a surrogate for free drug concentrations, to be within an effective range in vitro (45 nmol/L) as a modulator of DNA-damaging agent cytotoxicity. In vitro, UCN-01 is synergistic with multiple cytotoxic agents and increases fludarabine-induced apoptosis in a human breast cell line. These results suggest that UCN-01 sensitized the lymphoma to the cytotoxic effects of EPOCH, possibly by modulating the "threshold" for apoptosis, and may illustrate a new paradigm for reversal of drug resistance.

A phase I study of combination therapy with immunotoxins IgG-HD37-deglycosylated ricin A chain (dgA) and IgG-RFB4-dgA (Combotox) in patients with refractory CD19(+), CD22(+) B cell lymphoma.

This study used an 8-day continuous infusion regimen of a 1:1 mixture of two immunotoxins (ITs) prepared from deglycosylated ricin A chain (dgA) conjugated to monoclonal antibodies directed against CD22 (RFB4-dgA) and CD19 (HD37-dgA; Combotox) in a Phase I trial involving 22 patients with refractory B cell lymphoma to determine the maximum tolerated dose, clinical pharmacology, and toxicity profile and to characterize any clinical responses. Adult patients received a continuous infusion of Combotox at 10, 20, or 30 mg/m2/192 h. No intrapatient dose escalation was permitted. Patients with > or =50 circulating tumor cells (CTCs)/mm3 in peripheral blood tolerated all doses without major toxicity. The maximum level of serum IT (Cmax) achieved in this group was 345 ng/ml of RFB4-dgA and 660 ng/ml of HD37-dgA (1005 ng/ml of Combotox). In contrast, patients without CTCs (<50/mm3) had unpredictable clinical courses that included two deaths probably related to the IT. Additionally, patients exhibited a significant potential for association between mortality and a history of either autologous bone marrow or peripheral blood stem cell transplants (P2 = 0.003) and between mortality and a history of radiation therapy (P2 = 0.036). In patients with CTCs, prior therapies appeared to have little impact on toxicity. Subsequent evaluation of the ITs revealed biochemical heterogeneity between two lots of HD37-dgA. In addition, HD37-dgA thawed at the study site tended to contain significant particulates, which were not apparent in matched controls stored at the originating site. This suggests that a tendency to aggregate may have resulted from shipping, storage, and handling of the IT that occurred prior to preparation for administration. It is not clear to what extent, if any, the aggregation of HD37-dgA IT was related to the encountered clinical toxicities; however, the potential to aggregate does suggest one possible basis for problems in our clinical experience with HD37-dgA and leads us to the conclusion that non-aggregate-forming formulations for these ITs should be pursued prior to future clinical trials.

Identification of cytosolic aldehyde dehydrogenase 1 from non-small cell lung carcinomas as a flavopiridol-binding protein.

The synthetic flavone flavopiridol can be cytostatic or cytotoxic to mammalian cells, depending on the concentration of the drug and the duration of exposure. It has been shown to inhibit the cyclin-dependent kinase (CDK) family of cell cycle regulatory enzymes. However, the existence of additional potential targets for drug action remains a matter of interest to define. To identify cellular targets, flavopiridol was immobilized. CDKs, particularly CDK 4, bound weakly to immobilized flavopiridol when ATP was absent but not in its presence. Two proteins with molecular weights of 40 kDa and 120 kDa had high affinities to the immobilized flavopiridol independent of the presence of ATP. They were present in all cell lines analyzed: cervical (HeLa), prostate and non-small cell lung carcinoma (NSCLC) cell lines. A 60-kDa protein, which was present only in NSCLC cells and bound similarly well to immobilized flavopiridol, was identified as cytosolic aldehyde dehydrogenase class 1 (ALDH-1). The level of this protein correlated with the resistance of NSCLC cell lines to cytotoxicity caused by 500 nM flavopiridol but not higher flavopiridol concentrations. Despite binding to ALDH-1, there was no inhibition of dehydrogenase activity by flavopiridol concentrations as high as 20 microM and flavopiridol was not metabolized by ALDH-1. The results suggest that high cellular levels of ALDH-1 may reduce cytotoxicity of flavopiridol and contribute to relative resistance to the drug. This is the first report that flavopiridol binds to proteins other than CDKs.

Histone acetylation and the cell-cycle in cancer.

A number of distinct surveillance systems are found in mammalian cells that have the capacity to interrupt normal cell-cycle progression. These are referred to as cell cycle check points. Surveillance systems activated by DNA damage act at three stages, one at the G1/S phase boundary, one that monitors progression through S phase and one at the G2/M boundary. The initiation of DNA synthesis and irrevocable progression through G1 phase represents an additional checkpoint when the cell commits to DNA synthesis. Transition through the cell cycle is regulated by a family of protein kinase holoenzymes, the cyclin-dependent kinases (Cdks), and their heterodimeric cyclin partner. Orderly progression through the cell-cycle checkpoints involves coordinated activation of the Cdks that, in the presence of an associated Cdk-activating kinase (CAK), phosphorylate target substrates including members of the "pocket protein" family. One of these, the product of the retinoblastoma susceptibility gene (the pRB protein), is phosphorylated sequentially by both cyclin D/Cdk4 complexes and cyclin E/Cdk2 kinases. Recent studies have identified important cross talk between the cell-cycle regulatory apparatus and proteins regulating histone acetylation. pRB binds both E2F proteins and histone deacetylase (HDAC) complexes. HDAC plays an important role in pRB tumor suppression function and transcriptional repression. Histones are required for accurate assembly of chromatin and the induction of histone gene expression is tightly coordinated. Recent studies have identified an important alternate substrate of cyclin E/Cdk2, NPAT (nuclear protein mapped to the ATM locus) which plays a critical role in promoting cell-cycle progression in the absence of pRB, and contributes to cell-cycle regulated histone gene expression. The acetylation of histones by a number of histone acetyl transferases (HATs) also plays an important role in coordinating gene expression and cell-cycle progression. Components of the cell-cycle regulatory apparatus are both regulated by HATs and bind directly to HATs. Finally transcription factors have been identified as substrate for HATs. Mutations of these transcription factors at their sites of acetylation has been associated with constitutive activity and enhanced cellular proliferation, suggesting an important role for acetylation in transcriptional repression as well as activation. Together these studies provide a working model in which the cell-cycle regulatory kinases phosphorylate and inactivate HDACs, coordinate histone gene expression and bind to histone acetylases themselves. The recent evidence for cross-talk between the cyclin-dependent kinases and histone gene expression on the one hand and cyclin-dependent regulation of histone acetylases on the other, suggests chemotherapeutics targeting histone acetylation may have complex and possibly complementary effects with agents targeting Cdks.

Induction of growth inhibition and apoptosis in prostate cancer cells by flavopiridol.

Flavopiridol is an inhibitor of several cyclin-dependent kinases, and exhibits potent growth-inhibitory activity against a number of human tumor cell lines both in vitro, and when grown as xenografts in mice. It has shown promising antineoplastic activity and is currently undergoing clinical phase II testing. Prostate cancer (PCa) remains a leading cause of morbidity and mortality among males in the United States. There are no effective treatments for hormone and/or radiation refractory PCa, suggesting that novel and newer treatment strategy may be useful in the management of PCa. Our previous study showed that flavopiridol induces cell growth inhibition and apoptosis in breast cancer cells. Here, we investigated whether flavopiridol was effective against prostate cancer cells. Flavopiridol was found to inhibit growth of PC3 prostate cancer cells. Induction of apoptosis was also observed in PC3 cells treated with flavopiridol, as measured by DNA laddering and PARP cleavage. We also found a significant down-regulation of Bcl-2 in flavopiridol-treated cells. These findings suggest that down-regulation of Bcl-2 may be one of the molecular mechanisms through which flavopiridol induces apoptosis and inhibits cell growth, suggesting that flavopiridol may be an effective chemotherapeutic agent against prostate cancer.

Induction of differentiation accompanies inhibition of Cdk2 in a non-small cell lung cancer cell line.

Induction of differentiation in a variety of model systems is accompanied by cell cycle exit and inhibition of Cdk2 kinase activity. We asked whether inhibition of Cdk2 activity is sufficient to allow differentiation to occur in a non-small cell lung cancer cell line. Treatment of NCI-H358 with flavopiridol, an inhibitor of multiple Cdk's, resulted in growth arrest and induction of mucinous differentiation. The onset of differentiation coincided temporally with loss of Cdk2 kinase activity. Western analysis revealed that flavopiridol treatment resulted in depletion of both cyclin E and D1, suggesting that loss of the regulatory subunits is at least partially responsible for the loss of Cdk kinase activity. Similarly, roscovitine, an inhibitor of Cdk's 1, 2, and 5, but not Cdk4, also induced differentiation in NCI-H358, although the resulting pattern of expression of cell cycle regulatory genes differed from the pattern obtained with flavopiridol. Furthermore, stable expression of an antisense Cdk2 construct in NCI-H358 also resulted in the appearance of a marker of mucinous differentiation. These results show that the inhibition of activity of cyclin dependent kinases, particularly Cdk2, by multiple different mechanisms is accompanied by differentiation. Thus, induction of differentiation is one potential mechanism of action for agents that down-regulate Cdk activity.