Flavopiridol (L86 8275; NSC 649890), a new kinase inhibitor for tumor therapy

Flavopiridol is a new synthetic flavone, structurally related to a natural alkaloid, originally purified from Dysoxylum binectariferum, a plant indigenous to India and used in Indian folk medicine. Flavopiridol was detected by a tandem screening system consisting in inhibition of the EGF-receptor Tyrosine phosphokinase and cytotoxicity. As a cytostatic mechanism, however, Flavopiridol strongly inhibits the cyclin-dependent kinases (cdk1, cdk2, cdk4, cdk7), with the potential to cause inhibition of cell cycle progression in G(1) and G(2) by multiple mechanisms relatable to cdk inhibition. In certain cell types, Flavopiridol induces apoptosis. The antitumor activity of that compound on human xenograft tumors is similar to standard cytostatic drugs and superior to them at least in prostate carcinoma. The dose limiting toxicity is diarrhea. Compared with other flavonoids or other kinase inhibitors Flavopiridol can be regarded as unique as no other compound is yet known that as specifically and potently inhibits nearly all the main cyclin dependent kinases and by that mechanisms can arrest cell cycle progression in G(1) as well as in G(2) and no other specific kinase inhibitor is known, which after i.v. or oral application reduces the growth of subcutaneous or subrenal xenografts of human tumors of different types. Initial results of a phase I study at the National Cancer Institute (NCI), USA, (Investigational New Drug Application no. 46211) provided some clinical and laboratory evidence for antineoplastic effect at nontoxic doses (no grade IV toxicities encountered). Thus, Flavopiridol is clearly in need of further clinical evaluation of its tumor therapeutic potential. In this review the chemical profile, tumorpharmacology (in vitro activity, inhibition of cdk's and preclinical in vivo activity), preclinical toxicology and pharmacokinetic of Flavopiridol are reviewed to provide a comprehensive source to aid further developmental efforts.

Reduction of Cdc25A contributes to cyclin E1-Cdk2 inhibition at senescence in human mammary epithelial cells

Replicative senescence may be an important tumor suppressive mechanism for human cells. We investigated the mechanism of cell cycle arrest at senescence in human mammary epithelial cells (HMECs) that have undergone a period of 'self-selection', and as a consequence exhibit diminished p16INK4A levels. As HMECs approached senescence, the proportion of cells with a 2N DNA content increased and that in S phase decreased progressively. Cyclin D1-cdk4, cyclin E-cdk2 and cyclin A-cdk2 activities were not abruptly inhibited, but rather diminished steadily with increasing population age. In contrast to observations in fibroblast, p21Cip1 was not increased at senescence in HMECs. There was no increase in p27Kip1 levels nor in KIP association with targets cdks. While p15INK4B and its binding to both cdk4 and cdk6 increased with increasing passage, some cyclin D1-bound cdk4 and cdk6 persisted in senescent cells, whose inhibition could not be attributed to p15INK4B. The inhibition of cyclin E-cdk2 in senescent HMECs was accompanied by increased inhibitory phosphorylation of cdk2, in association with a progressive loss of Cdc25A. Recombinant Cdc25A strongly reactivated cyclin E-cdk2 from senescent HMECs suggesting that reduction of Cdc25A contributes to cyclin E-cdk2 inhibition and G1 arrest at senescence. Although ectopic expression of Cdc25A failed to extend the lifespan of HMECs, the exogenous Cdc25A appeared to lack activity in these cells, since it neither shortened the G1-to-S phase interval nor activated cyclin E-cdk2. In contrast, in the breast cancer-derived MCF-7 line, Cdc25A overexpression increased both cyclin E-cdk2 activity and the S phase fraction. Thus, mechanisms leading to HMEC immortalization may involve not only the re-induction of Cdc25A expression, but also activation of this phosphatase.

Fanconi anemia group A and D cell lines respond normally to inhibitors of cell cycle regulation

Cells from patients with Fanconi anemia (FA) show decreased viability and decreased chromosome stability after treatment with DNA cross-linking agents, compared to normal cells. FA cells also show a relative accumulation at the G2/M transition after such treatment. This has suggested a possible checkpoint abnormality. In the studies presented here, treatment with hydroxyurea, caffeine or inhibitors of cell cycle kinases did not reveal abnormalities in survival or chromosome stability in FA-A or FA-D cells. Chromosomal breaks introduced by hydrogen peroxide or methyl methanesulfonate accumulated to the same extent in FA-A or FA-D cells as in normal cells. We conclude that FA-A and FA-D cells respond normally to agents known to alter the cell cycle or introduce DNA strand breaks. FA cells process strand breaks and a variety of DNA monoadducts normally. Our results are compatible with repair of DNA crosslinks being slower in FA than in normal cells and FA cells having normal cell cycle checkpoints.

The cell cycle inhibitor p27 is an independent prognostic marker in small (T1a,b) invasive breast carcinomas

Breast carcinomas < or = 1 cm in size (T1a,b) are being detected more frequently as a result of screening. Because traditional prognostic parameters are either lacking (tumor size) or rare (nodal metastases), a marker(s) is needed to identify the subset of patients who could benefit from adjuvant therapy. A retrospective series of 202 patients with stage T1a,b invasive breast carcinomas was evaluated. The clinicopathological features (age, histological grade, extensive in situ carcinoma, hormone receptor status, and nodal metastasis) as well as microvessel density and the expression of c-erb-B2, p53, MIB-1/Ki-67, and cdc25B were assessed. In addition, expression of the cell cycle inhibitor p27 was evaluated. Nineteen patients (18% of patients who had axillary dissection) had locoregional lymph node metastases. Forty-two % of them died of disease (median survival, 112 months), whereas mortality was 11% in node-negative patients (median survival, 168 months; P = 0.0055). Patients with low p27 expression had a median survival of 139 months (17% mortality) versus 174 months (9% mortality) in the group with high p27 expression (P = 0.0233). Lack of p27 was associated with poor prognosis when node-positive patients were excluded (P = 0.0252). Nodal status and low p27 were found to be the only independent prognostic parameters by both univariate and multivariate analysis, with relative risks of dying of disease of 4.9 (P = 0.001) and 3.4 (P = 0.0306), respectively. Assessment of p27, which yields prognostic information in node-negative patients, could be useful to identify patients with small, invasive breast carcinomas who might benefit from adjuvant therapy.

Selective effects of mastoparan analogs: separation of G-protein-directed and membrane-perturbing activities

Mastoparan (MP), a wasp venom peptide, is known both to interact with G-proteins and to alter membrane structure and function. To determine the structural requirements for these two aspects of MP action, we constructed several analogs of the peptide and characterized them using Swiss 3T3 cell membranes. The effects of these peptides were measured on: i) G-protein-mediated stimulation of phospholipase-C activity by GTP gamma S and bombesin and ii) the membrane enzyme activities, calcium-activated phospholipase-C and Na,K-ATPase. MP strongly inhibited all the above activities and caused membrane permeabilization. Substitution of one Lys residue by Gly at either the N- or C-terminal of the MP molecule resulted in peptides which selectively inhibited G-protein stimulated phospholipase-C with no or very slight membrane-perturbing effects. Introduction of additional Lys residues to MP led to the opposite effect. Thus, G-protein modulating and membrane disrupting actions of MP appear to be not necessarily linked, and may be separated.

Growth inhibition with reversible cell cycle arrest of carcinoma cells by flavone L86-8275

BACKGROUND

Previous studies have shown that polyhydroxylated flavonoids such as quercetin and genistein can inhibit tumor cell growth in vitro, and preliminary in vivo studies of the flavone L86-8275 have shown growth inhibition of LX529 and A549 lung carcinomas. L86-8275 [(-)cis-5,7-dihydroxy-2-(2-chlorophenyl)-8[4-(3-hydroxy-1-methyl)- piperidinyl]-4H-1-benzopyran-4-one] is a flavone of novel structure.

PURPOSE

The purpose of this study was to determine in vitro whether L86-8275 is a more potent inhibitor of growth in breast carcinoma and lung carcinoma cells than quercetin or genistein.

METHODS

We studied the effects of L86-8275 on cell growth in seven breast carcinoma cell lines and five lung carcinoma cell lines. MDA468 breast carcinoma was then selected for further study. Cell proliferation was measured by a colorimetric dye reduction assay; synthesis of DNA, RNA, and protein by incorporation of the radioactive metabolic precursors thymidine, uridine, or leucine, respectively; adenosine triphosphate (ATP) content by a luciferase-mediated bioluminescence reaction; and cell cycle progression by the use of cell-synchronizing drugs (aphidicolin and nocodazole) and flow cytometry.

RESULTS

L86-8275 was not cytotoxic to stationary-phase cells but reversibly inhibited the growth of cells in exponential growth phase. At concentrations of 25-160 nM, L86-8275 inhibited growth of human breast and lung carcinoma cell lines by 50%. MDA468 breast carcinoma cells were 60-fold and 400-fold more sensitive to L86-8275 than to quercetin and genistein, respectively. By 24 hours after addition of L86-8275, DNA synthesis in MDA468 cells was inhibited by greater than 95%, protein synthesis by 80%, and RNA synthesis by 40%-60%, under conditions that preserved cellular ATP levels at approximately 80%-90% of control values. When MDA468 cells released from aphidicolin-induced cell cycle arrest were exposed to 200 nM L86-8275, they completed the S phase but arrested in G2. When cells released from nocodazole-induced cell cycle arrest were exposed to 200 nM L86-8275, they completed mitosis but arrested in G1.

CONCLUSIONS

L86-8275 is a potent, yet reversible, growth-inhibitory flavone that can selectively block cell cycle progression in vitro at more than one point in the cell cycle.

IMPLICATIONS

These findings suggest that L86-8275 is a candidate for further preclinical development, as well as a model for the synthesis of other flavonoids that might potently delay cell cycle progression to achieve inhibition of tumor growth. Future studies need to address optimal schedules for antiproliferative activity in vivo and inhibition of clonogenic activity.

Loss of epithelial markers and acquisition of vimentin expression in adriamycin- and vinblastine-resistant human breast cancer cell lines

We have previously observed that breast cancer cell lines could exhibit either epithelial or fibroblastic phenotypes as reflected by their morphologies and intermediate filament protein expression (C. L. Sommers, D. Walker-Jones, S. E. Heckford, P. Worland, E. Valverius, R. Clark, M. Stampfer, and E. P. Gelmann, Cancer Res., 49:4258-4263, 1989). Fibroblastoid, vimentin-expressing breast cancer cell lines are more invasive in vitro and in vivo (E. W. Thompson, S. Paik, N. Brunner, C. L. Sommers, G. Zugmaier, R. Clarke, T. B. Shima, J. Torri, S. Donahue, M. E. Lippman, G. R. Martin, and R. B. Dickson, J. Cell. Physiol., 150: 534-544, 1992). We hypothesized that a breast cancer cell with an epithelial phenotype could undergo a transition to a fibroblastic phenotype, possibly resulting in more invasive capacity. We now show that two Adriamycin-resistant MCF-7 cell lines and a vinblastine-resistant ZR-75-B cell line have undergone such a transition. Adriamycin-resistant MCF-7 cells express vimentin, have diminished keratin 19 expression, have lost cell adhesion molecule uvomorulin expression, and have reduced formation of desmosomes and tight junctions as determined by reduced immunodetection of their components desmoplakins I and II and zonula occludens (ZO)-1. Other MCF-7 cell lines selected for resistance to vinblastine and to Adriamycin and verapamil did not have these characteristics, indicating that drug selection does not invariably cause these phenotypic changes. In addition, to determine if vimentin expression in MCF-7 cells alone could manifest a fibroblastic phenotype, we transfected the full-length human vimentin complementary DNA into MCF-7 cells. Although vimentin expression was achieved in MCF-7 cells, it did not affect the phenotype of the cells in terms of the distribution of keratins, desmoplakins I and II, ZO-1, or uvomorulin or in terms of in vitro invasiveness. We conclude that vimentin expression is a marker for a fibroblastic and invasive phenotype in breast cancer cells but does not by itself give rise to this phenotype.

Single-dose and steady-state pharmacokinetics and pharmacodynamics of perindopril in hypertensive subjects

In a double-blind, placebo-controlled, parallel group study, 24 essential hypertensive subjects were randomised to receive either placebo or 2, 4, or 8 mg perindopril. Perindopril, its deesterified metabolite, perindoprilat, and perindoprilat glucuronide were separated with an ion-exchange resin and determined by a radioimmunoassay (RIA). Pharmacokinetic and pharmacodynamic parameters were estimated for 96 h after the first dose and after 4-week once-daily treatment. Perindopril peak levels were achieved in less than or equal to 2 h after dosing with an elimination t1/2 of 1-2 h. Peak levels of perindoprilat were achieved more slowly, reaching a maximum level 5-8 h after dosing, and had an elimination t1/2 of 40 h. Levels of the perindopril glucuronide peaked approximately 0.5 h later than perindopril, with an elimination t1/2 of approximately 2 h. Perindopril, perindoprilat, and its glucuronide conjugate followed linear kinetics in the dose range of 2-8 mg, and there was no evidence of accumulation with chronic dosing. Perindopril 4 and 8 mg produced significant decreases in predose blood pressure (BP) with chronic dosing, with maximal decreases occurring 5-7 h after dosing. Perindopril also produced a prolonged dose-dependent inhibition of plasma angiotensin-converting enzyme (ACE) activity that was maximum after 4 h and had not fully recovered by 48 h after a single dose.

Receptor-induced phosphorylation by mammary-derived growth factor 1 in mammary epithelial cell lines

Previous work has shown that a mammary-derived growth factor (MDGF1), a human milk-derived, acidic, 62-kDa, N-glycosylated growth factor binds to cell surface receptors and stimulates proliferation of mammary epithelial cells. An 18-amino acid N-terminal partial sequence of the factor did not show any homology to other known growth factors or proteins. Using polyclonal antiserum raised against the synthetic peptide, we demonstrated that conditioned medium prepared from human breast cancer cell lines contains the factor. The antibody could adsorb the biological activity of the factor present in the conditioned medium. Earlier experiments on receptor cross-linking indicated that the receptor was approximately 120-140 kDa. Since tyrosine phosphorylation plays a crucial role in cell proliferation and cell transformation, experiments were conducted to find out whether MDGF1 induces the appearance of phosphotyrosine in MDGF1-receptor-positive MDA-MB 468, MCF-7, and 184A1N4 cell lines compared to receptor-negative lines. Western blot analysis using monoclonal antiphosphotyrosine indicated that MDGF1 induces phosphotyrosine in a 180-185-kDa protein in MDGF1 receptor-positive cell lines. Phosphorylation was not blocked and phosphorylated proteins were not immunoprecipitated by an antibody directed against the binding site of the EGF receptor. Cell membrane fractionation demonstrated that phosphorylation induced by MDGF1 was membrane-associated. The nature of this 180-185-kDa protein and its possible relationship to the MDGF1 receptor are under investigation.

Two distinct bombesin receptor subtypes are expressed and functional in human lung carcinoma cells

Bombesin-like peptides have been implicated as autocrine growth factors influencing the pathogenesis and progression of some human lung carcinoma cells. To determine the pharmacologic and structural properties of the bombesin receptors expressed in human lung carcinoma cells, cDNA clones encoding a human gastrin-releasing peptide receptor (GRP-R) and a pharmacologically distinct neuromedin-B preferring bombesin-receptor (NMB-R) were isolated from a human small cell lung carcinoma cell line (NCI-H345). After expression in Xenopus oocytes, a GRP-R-specific antagonist was effective in blocking responses elicited from the cloned GRP-R, but not the NMB-R. Both GRP-R and NMB-R mRNA expression was detected at varying levels in a panel of human lung cancer cell lines. These results indicate heterogeneity of bombesin receptor subtypes exists in human lung carcinoma cells and should be considered in the design of bombesin receptor antagonists intended to inhibit tumor cell growth.

Vimentin rather than keratin expression in some hormone-independent breast cancer cell lines and in oncogene-transformed mammary epithelial cells

To characterize differences in gene expression between hormone-dependent and hormone-independent mammary carcinoma, we cloned complementary DNAs of genes expressed in a hormone-independent breast carcinoma cell line that were not expressed in a hormone-dependent line. One clone, which was isolated in many copies, coded for the intermediate filament protein vimentin. A complementary DNA clone 1.8 kilobases long included the entire protein-coding region for vimentin. Vimentin was expressed by more than one-half of the hormone-independent breast carcinoma cell lines tested but not by the hormone-dependent cell lines. The cell lines which expressed vimentin expressed only low levels of cytokeratins. The correlation between vimentin expression and more advanced stages of mammary cell transformation was tested in a model system in which immortal, nontumorigenic human mammary epithelial cells or derivative lines transformed with v-ras-H or SV40 T-antigen were found not to express vimentin, whereas a derivative highly tumorigenic cell line transformed by both v-ras-H and T-antigen did express vimentin. Analysis of several other kinds of epithelial carcinoma cell lines showed only rare examples of vimentin expression.