The use of molecular markers in farnesyltransferase inhibitor (FTI) therapy of breast cancer

Structure-activity relationship study to improve cytotoxicity and selectivity of lonafarnib against breast cancer cells

Lonafarnib is designed as a farnesyltransferase (FTase) inhibitor and displays inhibitory activities against a wide range of tumor cells. However, a major disadvantage is its unselective activity and high cytotoxicity against nonmalignant cells. Therefore, we structurally modified the terminal 4-methylpiperidine-1-carboxamide residue of lonafarnib and evaluated the antiproliferative effects of the resulting derivatives in Michigan Cancer Foundation - 7 (MCF-7) breast cancer cells as well as simian virus 80 (SV-80) fibroblasts. The highest cytotoxicity against both cell lines (IC₅₀ about 2 µM) was shown by the piperidin-4-yl carbamate 15i and the S-(piperidin-4-yl) carbamothioate 15j. Selectivity for tumor cells was realized in the case of the 1-cyclohexyl-1-methylurea derivative 15b. It reduced the growth of MCF-7 cells with an IC₅₀ of 11.4 µM (lonafarnib: IC₅₀  = 10.8 µM) without influence on the growth of SV-80 cells (IC₅₀  > 50 µM; lonafarnib: IC₅₀  = 14.0 µM). Molecular modeling studies were performed to correlate the cytotoxicity with possible FTase interactions. The theoretical investigations, however, documented a comparable attachment of active, less active, and inactive compounds and did not allow an interpretation of the biological results based on these theoretical considerations.

Ibandronate increases the expression of the pro-apoptotic gene FAS by epigenetic mechanisms in tumor cells

There is growing evidence that aminobisphosphonates like ibandronate show anticancer activity by an unknown mechanism. Biochemically, they prevent posttranslational isoprenylation of small GTPases, thus inhibiting their activity. In tumor cells, activated RAS-GTPase, the founding member of the gene family, down-regulates the expression of the pro-apoptotic gene FAS via epigenetic DNA-methylation by DNMT1. We compared ibandronate treatment in neoplastic human U-2 osteosarcoma and in mouse CCL-51 breast cancer cells as well as in the immortalized non-neoplastic MC3T3-E1 osteoblastic cells. Ibandronate attenuated cell proliferation in all cell lines tested. In the neoplastic cells we found up-regulation of caspases suggesting apoptosis. Further we found stimulation of FAS-expression as a result of epigenetic DNA demethylation that was due to down-regulation of DNMT1, which was rescued by re-isoprenylation by both geranylgeranyl-pyrophosphate and farnesylpyrophosphate. In contrast, ibandronate did not affect FAS and DNMT1 expression in MC3T3-E1 non-neoplastic cells. Data suggest that bisphosphonates via modulation of the activity of small-GTPases induce apoptosis in neoplastic cells by DNA-CpG-demethylation and stimulation of FAS-expression. In conclusion the shown epigenetic mechanism underlying the anti-neoplastic activity of farnesyl-transferase-inhibition, also explains the clinical success of other drugs, which target this pathway.

Combining the farnesyltransferase inhibitor lonafarnib with paclitaxel results in enhanced growth inhibitory effects on human ovarian cancer models in vitro and in vivo

OBJECTIVES

To determine the effects of combining lonafarnib with paclitaxel on the growth of human ovarian cancer cells and tumor xenografts as well as to monitor a pharmacodynamic marker of farnesyltransferase inhibition (HDJ-2) in peripheral blood mononuclear cells (PBMCs) isolated from tumor-bearing animals after treatment with this combination.

METHODS

Proliferation of A2780, PA-1, IGROV-1, and TOV-112D cells was assessed after treatment with lonafarnib and paclitaxel. Cell cycle progression was determined by flow cytometry, and apoptosis was evaluated by assaying for caspase-3 and cleaved PARP. The effects of lonafarnib and paclitaxel on the tumor growth of each model were determined in immunocompromised mice. Proteins extracted from cells, tumors, and PBMCs were assayed for HDJ-2 mobility shifts by Western blotting as well as for farnesyl protein transferase (FTase) enzyme activity by biochemical analyses.

RESULTS

In A2780, PA-1, IGROV-1, and TOV-112D cells lonafarnib potentiated the growth inhibitory effects of paclitaxel. In each of the models lonafarnib enhanced paclitaxel-induced mitotic arrest and apoptosis. The combination of lonafarnib plus paclitaxel resulted in marked tumor regressions in A2780, TOV-112D, PA-1, and IGROV-1 tumor xenografts. Western blotting demonstrated that in PBMCs isolated from the animals, paclitaxel treatment suppressed lonafarnib-induced HDJ-2 mobility shifts. Paclitaxel did not affect lonafarnib inhibition of FTase enzyme activity levels in these PBMCs.

CONCLUSIONS

Lonafarnib enhances the antiproliferative effects of paclitaxel on ovarian cancer cells in vitro and ovarian tumor xenografts in vivo. Measuring FTase enzyme activity levels rather than HDJ-2 shifts in PBMCs may be a more accurate biomarker to predict levels of farnesyltransferase inhibition in patients who are also receiving paclitaxel chemotherapy.

Enhancement of the antitumor activity of tamoxifen and anastrozole by the farnesyltransferase inhibitor lonafarnib (SCH66336)

Lonafarnib is an orally bioavailable farnesyltransferase inhibitor. Originally developed to block the membrane localization of Ras, subsequent work suggested that farnesyltransferase inhibitors mediate their antitumor activities by altering the biological activities of additional farnesylated proteins. Breast tumor models that express wild-type Ras have been shown to be sensitive to farnesyltransferase inhibitors. We have determined the effects of combining lonafarnib with the antiestrogen 4-hydroxy tamoxifen on hormone-dependent breast cancer cell lines in vitro. The effects of combining lonafarnib with tamoxifen or the aromatase inhibitor anastrozole on the growth of two different MCF-7 breast tumor xenograft models were also evaluated. In four of five human breast cancer cell lines, lonafarnib enhanced the antiproliferative effects of 4-hydroxy tamoxifen. The combination prevented MCF-7 cells from transitioning through the G1 to S phase of the cell cycle and augmented apoptosis. This was associated with reduced expression of E2F-1 and a reduction in hyperphosphorylated retinoblastoma protein. Lonafarnib plus 4-hydroxy tamoxifen also inhibited the mammalian target of rapamycin signal transduction pathway. In nude mice bearing parental MCF-7 or aromatase-transfected MCF-7Ca breast tumor xenografts, lonafarnib enhanced the antitumor activity of both tamoxifen and anastrozole. These studies indicate that lonafarnib enhances the efficacy of endocrine agents clinically used for treating hormone-dependent breast cancer.

Dissecting the senescence-like program in tumor cells activated by Ras signaling

Activated Ras signaling can induce a permanent growth arrest in osteosarcoma cells. Here, we report that a senescence-like growth inhibition is also achieved in human carcinoma cells upon the transduction of H-Ras(V12). Ras-induced tumor senescence can be recapitulated by the transduction of activated, but not wild-type, MEK. The ability for H-Ras(V12) to suppress tumor cell growth is drastically compromised in cells that harbor endogenous activating ras mutations. Notably, growth inhibition of tumor cells containing ras mutations can be achieved through the introduction of activated MEK. Tumor senescence induced by Ras signaling can occur in the absence of p16 or Rb and is not interrupted by the inactivation of Rb, p107, or p130 via short hairpin RNA or the transduction with HPV16 E7. In contrast, inactivation of p21 via short hairpin RNA disrupts Ras-induced tumor senescence. In summary, this study uncovers a senescence-like program activated by Ras signaling to inhibit cancer cell growth. This program appears to be intact in cancer cells that do not harbor ras mutations. Moreover, cancer cells that carry ras mutations remain susceptible to tumor senescence induced by activated MEK. These novel findings can potentially lead to the development of innovative cancer intervention.

Effect of a farnesyl transferase inhibitor (R115777) on ductal carcinoma in situ of the breast in a human xenograft model and on breast and ovarian cancer cell growth in vitro and in vivo

Introduction

The ras pathway is essential for cell growth and proliferation. The effects of R115777, a farnesyl transferase inhibitor, were investigated in cancer cell lines expressing varying levels of growth factor receptors and with differing ras status. Effects on tumour xenografts and human ductal carcinoma in situ (DCIS) of the breast in a xenograft mouse model were also tested.

Method

In vitro, the concentrations required to reduce cell numbers by 50% (50% inhibitory concentration) were established (MDA-MB231, MCF-7, MCF-7/HER2-18, BT-474, SK-BR3 and SKOV3). Human DCIS was implanted in nude mice or, in separate experiments, cultured cells were injected (MDA-MB231, MCF-7/HER2-18, SKOV3) and allowed to form tumours. Proliferation and apoptosis were determined by immunohistochemistry in xenografts and cell tumours.

Results

The 50% inhibitory concentrations varied a hundred-fold, from 39 nmol/l (± 26 nmol/l) for SKBR3 to 5.9 μmol/l(± 0.8 μmol/l) for MDA-MB231. In MCF-7/HER2-18 and SKOV3 cells the levels of tumour growth inhibition were approximately 85% and 40%, respectively. There was a significant decrease in the cell turnover index (CTI; proliferation/apoptosis). In MDA-MB 231 with activated k-ras no inhibition was observed. In treated DCIS xenografts proliferation decreased and apoptosis increased. The CTI ratio between the start and 1 and 2 weeks of treatment were 1.99 and 1.50, respectively, for controls and 0.85 (P = 0.005) and 0.75 (P = 0.08) for treated xenografts.

Conclusion

Treatment with the farnesyl transferase inhibitor reduced cell growth in vitro and cell tumour growth in vivo. In DCIS treatment resulted in a reduced CTI. R115777 is a promising treatment for breast cancer but the relation between effect and growth factor receptor and ras status has to be established.

Synthesis, Pharmacological Evaluation, and Molecular Modeling Studies of Novel Peptidic CAAX Analogues as Farnesyl-Protein-Transferase Inhibitors

Fifteen analogues of the C-terminal CA1A2X motif were synthesized and evaluated for their inhibition potency against farnesyltransferase (FTase). Replacement of the A2 residue by phenylalanine or tyrosine-derived analogues, in which a different number of methyl groups were introduced on the aromatic ring, resulted in compounds less active than the reference compound CVFM against FTase except for compounds I and VI (IC50=1 microM and 2.5 microM, respectively) that were comparable to CVFM and compound IV (IC50=0.1 microM), which was 6-fold more active than the reference compound. Because pseudopeptidic derivatives I-IX were inactive in the cellular assays, the N-formyl- and methyl-ester derivatives (compounds X-XV) were synthesized and tested on different cell lines, showing, in some cases, activity and appreciable selectivity against transformed cells. To rationalize the obtained results, molecular modeling experiments were carried out suggesting the molecular basis of FTase inhibition by these products.

Thematic review series: Lipid Posttranslational Modifications. Farnesyl transferase inhibitors

Some proteins undergo posttranslational modification by the addition of an isoprenyl lipid (farnesyl- or geranylgeranyl-isoprenoid) to a cysteine residue proximal to the C terminus. Protein isoprenylation promotes membrane association and contributes to protein-protein interactions. Farnesylated proteins include small GTPases, tyrosine phosphatases, nuclear lamina, cochaperones, and centromere-associated proteins. Prenylation is required for the transforming activity of Ras. Because of the high frequency of Ras mutations in cancer, farnesyl transferase inhibitors (FTIs) were investigated as a means to antagonize Ras function. Evaluation of FTIs led to the finding that both K- and N-Ras are alternatively modified by geranylgeranyl prenyltransferase-1 in FTI-treated cells. Geranylgeranylated forms of Ras retain the ability to associate with the plasma membrane and activate substrates. Despite this, FTIs are effective at inhibiting the growth of human tumor cells in vitro, suggesting that activity is dependent on blocking the farnesylation of other proteins. FTIs also inhibit the in vivo growth of human tumor xenografts and sensitize these models to chemotherapeutics, most notably taxanes. Several FTIs have entered clinical trials for various cancer indications. In some clinical settings, primarily hematologic malignancies, FTIs have displayed evidence of single-agent activity. Clinical studies in progress are exploring the antitumor activity of FTIs as single agents and in combination. This review will summarize the basic biology of FTIs, their antitumor activity in preclinical models, and the current status of clinical studies with these agents.

The synergistic combination of the farnesyl transferase inhibitor lonafarnib and paclitaxel enhances tubulin acetylation and requires a functional tubulin deacetylase

Farnesyl transferase (FT) inhibitors (FTI) are anticancer agents developed to target oncogenic Ras proteins by inhibiting Ras farnesylation. FTIs potently synergize with paclitaxel and other microtubule-stabilizing drugs; however, the mechanistic basis underlying this synergistic interaction remains elusive. Here we show that the FTI lonafarnib affects the microtubule cytoskeleton resulting in microtubule bundle formation, increased microtubule stabilization and acetylation, and suppression of microtubule dynamics. Notably, treatment with the combination of low doses of lonafarnib with paclitaxel markedly enhanced tubulin acetylation (a marker of microtubule stability) as compared with either drug alone. This synergistic effect correlated with FT inhibition and was accompanied by a synergistic increase in mitotic arrest and cell death. Mechanistically, we show that the combination of lonafarnib and paclitaxel inhibits the in vitro deacetylating activity of the only known tubulin deacetylase, histone deacetylase 6 (HDAC6). In addition, the lonafarnib/taxane combination is synergistic only in cells lines expressing the wild-type HDAC6, but not a catalytic-mutant HDAC6, revealing that functional HDAC6 is required for the synergy of lonafarnib with taxanes. Furthermore, tubacin, a specific HDAC6 inhibitor, synergistically enhanced tubulin acetylation in combination with paclitaxel, similar to the combination of lonafarnib and paclitaxel. Taken together, these data suggest a relationship between FT inhibition, HDAC6 function, and cell death, providing insight into the putative molecular basis of the lonafarnib/taxane synergistic antiproliferative combination.

Farnesyl transferase inhibitors in the treatment of breast cancer

Until recently, the therapeutic treatment of breast cancer has been dominated by endocrine-based drugs (oestrogen receptor antagonists, aromatase inhibitors etc.) and conventional cytotoxics (doxorubicin, cyclophosphamide, 5-fluorouracil etc.). However, the advent of new generation signal transduction inhibitor drugs targeted against the molecular abnormalities of breast cancer (e.g., the antibody trastuzumab, directed against the cERBB2 receptor) has the promise of providing a new era of more tumour selective therapy. Inhibitors of the enzyme farnesyl transferase (FTIs) are now undergoing early-stage clinical trials, including in patients with advanced breast cancer. Although originally developed as inhibitors of RAS signal transduction pathways, it is now apparent that these drugs are better described as prenylation inhibitors; the addition of a 15-carbon prenyl or farnesyl moiety by farnesyl transferase being critical to the function of a number of proteins, including RAS. At least three FTIs are currently undergoing clinical evaluation; R115777 (tipifarnib, Zarnestra), SCH66336 (lonafarnib, Sarasar) and BMS-214662. In terms of their potential use in the chemotherapeutic treatment of advanced breast cancer, a Phase II trial of R115777 (using either continuous or intermittent twice-daily oral dosing) has demonstrated promising activity (approximately 10% partial response rate). Overall, however, the single agent activity of FTIs in various Phase II trials has been rather modest (as well as the above mentioned breast cancer trial, some responses have been seen in patients with acute and chronic myeloid leukaemias). The main dose-limiting toxicities that have been reported are myelosuppression and fatigue and neurotoxicity (with R115777). Two Phase III trials of R115777 in colorectal (versus placebo) and pancreatic (with gemcitabine versus placebo) cancer have failed to show a survival benefit. It is likely that the future clinical direction of FTIs will be as combination therapy, especially with the taxanes, where synergy has been seen in a variety of preclinical studies.

Inhibiting Ras signaling in the therapy of breast cancer

Ras is a small guanosine triphosphate-binding protein that plays an important role in signal transduction pathways that influence cellular proliferation, apoptosis, cytoskeletal organization, and other important biological processes. Prenylation of Ras proteins by the enzyme farnesyltransferase renders the protein hydrophobic, causing localization to the inner surface of the cell membrane, where it exerts its biological effects. Ras mutations that result in constitutive activation of the Ras pathway are common in certain human cancers, and transfection of cell lines with mutant Ras renders them tumorigenic. Farnesyltransferase inhibitors (FTIs) were initially developed to inhibit growth of cancers harboring Ras mutations, but preclinical data suggests that they also have antiproliferative effects in cell lines with wild-type Ras. Preclinical data suggest that FTIs have antiproliferative and antitumor effects in breast cancer cell lines, but the precise target(s) remain to be defined. One phase II trial has demonstrated that one orally administered FTI has significant antitumor activity in metastatic breast cancer. In addition, preclinical evidence suggests that FTIs may augment the activity of cytotoxic agents and hormonal therapy. Clinical trials are currently underway evaluating whether these agents have a useful role in the management of advanced breast cancer.