Efficacy of the farnesyl transferase inhibitor R115777 in chronic myeloid leukemia and other hematologic malignancies

Targeting Harvey rat sarcoma viral oncogene homolog in head and neck cancer: how to move forward?

PURPOSE OF REVIEW

Despite recent advances, treatment personalization remains an issue for recurrent metastatic head and neck squamous cell carcinoma (RM HNSCC) patients. After human papilloma virus (HPV) and programmed death ligand 1 (PDL1) expression, Harvey rat sarcoma viral oncogene homolog (HRAS) appears as an emerging target in this field. In this review, we summarize the features of HRAS -mutated HNSCC and its targeting by farnesyl transferase inhibitors.

RECENT FINDINGS

HRAS mutations define a small subgroup of RM HNSCC patients with a poor prognosis and often refractory to the standard treatments. Posttranslational processing of HRAS being dependent on farnesylation, farnesyl transferase inhibitors have been evaluated in HRAS -mutated tumors. Tipifarnib, a first in class farnesyl transferase inhibitor, has shown efficacy in phase 2 trials with HRAS -mutated tumors. Despite reported high response rates in selected population, the efficacy of Tipifarnib is inconsistent and always transient, probably because of limiting hematological toxicities leading to dose reduction and occurrence of secondary resistance mutations.

SUMMARY

Tipifarnib is the first in the class of farnesyl transferase inhibitors to show efficacy in HRAS -mutated RM HNSCC. The understanding of mechanisms of resistance will pave the way for the design of second-generation farnesyl transferases inhibitors.

RAS pathway regulation in melanoma

Activating mutations in RAS genes are the most common genetic driver of human cancers. Yet, drugging this small GTPase has proven extremely challenging and therapeutic strategies targeting these recurrent alterations have long had limited success. To circumvent this difficulty, research has focused on the molecular dissection of the RAS pathway to gain a more-precise mechanistic understanding of its regulation, with the hope to identify new pharmacological approaches. Here, we review the current knowledge on the (dys)regulation of the RAS pathway, using melanoma as a paradigm. We first present a map of the main proteins involved in the RAS pathway, highlighting recent insights into their molecular roles and diverse mechanisms of regulation. We then overview genetic data pertaining to RAS pathway alterations in melanoma, along with insight into other cancers, that inform the biological function of members of the pathway. Finally, we describe the clinical implications of RAS pathway dysregulation in melanoma, discuss past and current approaches aimed at drugging the RAS pathway, and outline future opportunities for therapeutic development.

Summary: This Review describes the molecular regulation of the RAS pathway, presents the clinical consequences of its pathological activation in human cancer, and highlights recent advances towards its therapeutic inhibition, using melanoma as an example.

Potential Approaches Versus Approved or Developing Chronic Myeloid Leukemia Therapy

Tyrosine kinase inhibitors (TKIs) have revolutionized the treatment of patients with chronic myeloid leukemia (CML). However, continued use of these inhibitors has contributed to the increase in clinical resistance and the persistence of resistant leukemic stem cells (LSCs). So, there is an urgent need to introduce additional targeted and selective therapies to eradicate quiescent LSCs, and to avoid the relapse and disease progression. Here, we focused on emerging BCR-ABL targeted and non-BCR-ABL targeted drugs employed in clinical trials and on alternative CML treatments, including antioxidants, oncolytic virus, engineered exosomes, and natural products obtained from marine organisms that could pave the way for new therapeutic approaches for CML patients.

Examination of clinically-derived p210 BCR/ABL1 RhoGEF mutations in a murine bone marrow transplantation model of CML

Expression of the p210 BCR/ABL1 fusion protein has been described in virtually all patients with chronic myelogenous leukemia (CML). Previous studies have identified a guanine nucleotide exchange factor (RhoGEF) domain within BCR that is retained in p210 BCR/ABL1. Missense mutations at residues T654 (T654K) and F547 (F547L) within this domain have been reported in a CML patient in blast crisis (BC). In this study, we have evaluated p210 BCR/ABL1 constructs that contain these substitutions in a murine bone marrow transplantation (BMT) model of CML. The mutants exhibit normal expression and tyrosine kinase activity but altered signaling. When examined in the BMT assay, mice that express the mutants exhibit earlier onset of disease but have significantly extended lifespans relative to mice that express unmodified p210 BCR/ABL1. While mice that express p210 BCR/ABL1 exhibit neutrophilia that progresses to a less differentiated phenotype at death, disease in the mutant mice is characterized by eosinophilia with no maturation arrest. This observation was confirmed in vitro using myeloid cells and was associated with enhanced p53 phosphorylation and G1/S arrest. These results suggest that residues within the RhoGEF domain of p210 BCR/ABL1 can influence disease progression.

Biology, pathology, and therapeutic targeting of RAS

RAS was identified as a human oncogene in the early 1980s and subsequently found to be mutated in nearly 30% of all human cancers. More importantly, RAS plays a central role in driving tumor development and maintenance. Despite decades of effort, there remain no FDA approved drugs that directly inhibit RAS. The prevalence of RAS mutations in cancer and the lack of effective anti-RAS therapies stem from RAS' core role in growth factor signaling, unique structural features, and biochemistry. However, recent advances have brought promising new drugs to clinical trials and shone a ray of hope in the field. Here, we will exposit the details of RAS biology that illustrate its key role in cell signaling and shed light on the difficulties in therapeutically targeting RAS. Furthermore, past and current efforts to develop RAS inhibitors will be discussed in depth.

Novel therapeutic approaches in chronic myeloid leukemia

The tyrosine kinase inhibitors (TKIs) have revolutionized the management of chronic myeloid leukemia (CML) and BCR-ABL1 inhibitors form the mainstay of CML treatment. Although patients with CML generally do well under TKI therapy, there is a subgroup of patients who are resistant and/or intolerant to TKIs. In these group of patients, there is the need of additional treatment strategies. In this review, we provide an update on the current knowledge of these novel treatment approaches that can be used alone and/or in combination with TKIs.

Non ABL-directed inhibitors as alternative treatment strategies for chronic myeloid leukemia

The introduction of ABL Tyrosine Kinase Inhibitors (TKIs) has significantly improved the outcome of Chronic Myeloid Leukemia (CML) patients that, in large part, achieve satisfactory hematological, cytogenetic and molecular remissions. However, approximately 15-20% fail to obtain optimal responses according to the current European Leukemia Network recommendation because of drug intolerance or resistance.Moreover, a plethora of evidence suggests that Leukemic Stem Cells (LSCs) show BCR-ABL1-independent survival. Hence, they are unresponsive to TKIs, leading to disease relapse if pharmacological treatment is discontinued.All together, these biological events generate a subpopulation of CML patients in need of alternative therapeutic strategies to overcome TKI resistance or to eradicate LSCs in order to allow cure of the disease.In this review we update the role of "non ABL-directed inhibitors" targeting signaling pathways downstream of the BCR-ABL1 oncoprotein and describe immunological approaches activating specific T cell responses against CML cells.

Molecular-Targeted Agents in Pancreatic Cancer

Background

Despite the acceptance of gemcitabine as the standard first-line agent for the treatment of advanced pancreatic cancer as well as the improved response rates seen with gemcitabine combinations, novel therapies are needed for this disease, which has one of the lowest survival rates. The growing understanding of the molecular basis of pancreatic cancer and the recent introduction of targeted therapeutic agents have initiated novel studies that have the potential to improve on existing treatments.

Methods

We review the rationale and the clinical studies of therapeutic agents that target some of the molecular abnormalities commonly found in pancreatic cancer.

Results

Matrix metalloproteinase inhibitors (MMPIs), farnesyltransferase inhibitors (FTIs), and tyrosine-kinase inhibitors and monoclonal antibodies against growth factors or their receptors are novel agents that have undergone phase II or III trials. Phase III studies of MMPIs, alone or in combination with gemcitabine, and phase III studies of FTIs have produced disappointing results. Other agents in earlier phases of clinical development remain promising.

Conclusions

Despite the negative studies of MMPIs and FTIs, the results of phase II trials of other drugs are encouraging. Targeted agents may improve the prognosis of pancreatic cancer.

Novel targeted drug therapies for the treatment of childhood acute leukemia

The cure rates for childhood acute leukemia have dramatically improved to approximately 70% overal, with treatments that include intensive cytotoxic chemotherapy and, in some cases, hematopoietic stem cell transplantation. However, many children still die of their disease or of treatment-related toxicities. Even in patients that are cured, there can be significant and, not uncommonly debilitating, acute and late complications of treatment. Improved understanding of the molecular and cellular biology of leukemia and the increasing availability of high-throughput genomic techniques have facilitated the development of molecularly targeted therapies that have the potential to be more effective and less toxic than the standard approaches. In this article, we review the progress to date with agents that are showing promise in the treatment of childhood acute leukemia, including monoclonal antibodies, inhibitors of kinases and other signaling molecules (e.g., BCR-ABL, FLT3, farnesyltransferase, mTOR and γ-secretase), agents that target epigenetic regulation of gene expression (DNA methyltransferase inhibitors and histone deacetylase inhibitors) and proteasome inhibitors. For the specific agents in each of these classes, we summarize the published preclinical data and the clinical trials that have been completed, are in progress or are being planned for children with acute leukemia. Finally, we discuss potential challenges to the success of molecularly targeted therapy, including proper target identification, adequate targeting of leukemia stem cells, developing synergistic and tolerable combinations of agents and designing adequately powered clinical trials to test efficacy in molecularly defined subsets of patients.

Targeting mutant KRAS for anticancer therapeutics: a review of novel small molecule modulators

The RAS proteins play a role in cell differentiation, proliferation, and survival. Aberrant RAS signaling has been found to play a role in 30% of all cancers. KRAS, a key member of the RAS protein family, is an attractive cancer target, as frequent point mutations in the KRAS gene render the protein constitutively active. A number of attempts have been made to target aberrant KRAS signaling by identifying small molecule compounds that (1) are synthetic lethal to mutant KRAS, (2) block KRAS/GEF interactions, (3) inhibit downstream KRAS effectors, or (4) inhibit the post-translational processing of RAS proteins. In addition, inhibition of novel targets outside the main KRAS signaling pathway, specifically the cell cycle related kinase PLK1, has been shown have an effect in cells that harbor mutant KRAS. Herein we review the use of various high-throughput screening assays utilized to identify new small-molecule compounds capable of targeting mutant KRAS-driven cancers.

Ras and Ras mutations in cancer

Ras genes are pre-eminent genes that are frequently linked with cancer biology. The functional loss of ras protein caused by various point mutations within the gene, is established as a prognostic factor for the genesis of a constitutively active Ras-MAPK pathway leading to cancer. Ras signaling circuit follows a complex pathway, which connects many signaling molecules and cells. Several strategies have come up for targeting mutant ras proteins for cancer therapy, however, the clinical benefits remain insignificant. Targeting the Ras-MAPK pathway is extremely complicated due its intricate networks involving several upstream and downstream regulators. Blocking oncogenic Ras is still in latent stage and requires alternative approaches to screen the genes involved in Ras transformation. Understanding the mechanism of Ras induced tumorigenesis in diverse cancers and signaling networks will open a path for drug development and other therapeutic approaches.

Cumulative clinical experience from a decade of use: imatinib as first-line treatment of chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a malignant disease that originates in the bone marrow and is designated by the presence of the Philadelphia (Ph+) chromosome, a translocation between chromosomes 9 and 22. Targeted therapy against CML commenced with the development of small-molecule tyrosine kinase inhibitors (TKIs) exerting their effect against the oncogenic breakpoint cluster region (BCR)-ABL fusion protein. Imatinib emerged as the first successful example of a TKI used for the treatment of chronic-phase CML patients and resulted in significant improvements in response rate and overall survival compared with previous treatments. However, a significant portion of patients failed to respond to the therapy and developed resistance against imatinib. Second-generation TKIs nilotinib and dasatinib were to have higher efficiency in clinical trials in imatinib- resistant or intolerant CML patients compared with imatinib. Identification of novel strategies such as dose escalation, drug combination therapy, and use of novel BCR-ABL inhibitors may eventually overcome resistance against BCR-ABL TKIs. This article reviews the history of CML, including the treatment strategies used prediscovery of TKIs and the preclinical and clinical data obtained after the use of imatinib, and the second-generation TKIs developed for the treatment of CML.

Targeting of CD34+CD38- cells using Gemtuzumab ozogamicin (Mylotarg) in combination with tipifarnib (Zarnestra) in Acute Myeloid Leukaemia

BACKGROUND

The CD34+CD38- subset of AML cells is enriched for resistance to current chemotherapeutic agents and considered to contribute to disease progression and relapse in Acute Myeloid Leukaemia (AML) patients following initial treatment.

METHODS

Chemosensitivity in phenotypically defined subsets from 34 primary AML samples was measured by flow cytometry following 48 hr in vitro treatment with gemtuzumab ozogamicin (GO, Mylotarg) and the farnesyltransferase inhibitor tipifarnib/zarnestra. The DNA damage response was measured using flow cytometry, immunofluorescence and immunohistochemistry.

RESULTS

Using a previously validated in vitro minimal residual disease model, we now show that the combination of GO (10 ng/ml) and tipifarnib (5 μM) targets the CD34+CD38- subset resulting in 65% median cell loss compared to 28% and 13% CD34+CD38- cell loss in GO-treated and tipifarnib-treated cells, respectively. Using phosphokinome profiling and immunofluorescence in the TF-1a cell line, we demonstrate that the drug combination is characterised by the activation of a DNA damage response (induction of γH2A.X and thr68 phosphorylation of chk2). Higher induction of γH2AX was found in CD34+CD38- than in CD34+CD38+ patient cells. In a model system, we show that dormancy impairs damage resolution, allowing accumulation of γH2AX foci.

CONCLUSIONS

The chemosensitivity of the CD34+CD38- subset, combined with enhanced damage indicators, suggest that this subset is primed to favour programmed cell death as opposed to repairing damage. This interaction between tipifarnib and GO suggests a potential role in the treatment of AML.

The role of protein phosphorylation in therapy resistance and disease progression in chronic myelogenous leukemia

This review focuses on the central role that protein phosphorylation plays in the pathogenesis of chronic myelogenous leukemia (CML). It will cover the signaling pathways that are dysregulated by the oncogenic tyrosine kinase, BCR-ABL1, which both defines and drives the disease, and the barriers to disease control. These will include the mechanisms that underlie drug resistance, as well as the features of CML that prevent its cure by tyrosine kinase inhibitors. In the second section, we will cover the proteins and pathways that lead to the transformation of early chronic-phase CML to the more advanced blast phase of the disease. Here, we will outline the key pathophysiologic differences between the chronic and the blast phase, the mechanisms that contribute to these differences, and how these might be therapeutically targeted in patients. In the final section, we will summarize the major lessons learnt from the CML clinic. We will focus on how these observations have impacted our understanding of the therapeutic potential of modulating protein phosphorylation in human diseases and areas in which future research in CML pathophysiology may be important.

Tipifarnib and tanespimycin show synergic proapoptotic activity in U937 cells

BACKGROUND

Farnesyltransferase inhibitor tipifarnib (R115777) has been used for treatment of hematological malignancies; however, its observed anticancer effect was limited. This prompted us to search for inhibitors that would show synergic, proapoptotic effect when combined with R115777. We decided to study LY294002, which inhibits PI-3 kinase, and tanespimycin (17AAG), which inhibits Hsp90--a chaperone for a number of proteins, including Akt kinase.

METHODS

The effect of drugs, used alone or in combination, was tested in U937 cells (human leukemic monocyte lymphoma), which are often used as a model for liquid tumor. The number of viable cells was evaluated with trypan blue staining, while apoptosis was assessed by presence of active caspase-3 and terminal dUTP nick-end labeling of DNA (TUNEL).

RESULTS

At concentrations in which R115777, LY294002 and 17AAG were only slowing down the proliferation rate, when used separately, the combination of R115777 + LY294002 and R115777 + 17AAG significantly reduced the number of cells and induced cellular apoptosis.

CONCLUSIONS

Our results suggest that the combination of R115777 + 17AAG could be useful in treating some of the hematological malignancies.

Tipifarnib in the treatment of newly diagnosed acute myelogenous leukemia

Farnesyltransferase inhibitors (FTIs) represent a new class of signal transduction inhibitors that block the processing of cellular polypeptides that have cysteine terminal residues and, by so doing, interdict multiple pathways involved in proliferation and survival of diverse malignant cell types. Tipifarnib is an orally bioavailable, nonpeptidomimetic methylquinolone FTI that has exhibited clinical activity in patients with myeloid malignancies including elderly adults with acute myelogenous leukemia (AML) who are not candidates for traditional cytotoxic chemotherapy, patients with high-risk myelodysplasia, myeloproliferative disorders, and imatinib-resistant chronic myelogenous leukemia. Because of its relatively low toxicity profile, tipifarnib provides an important alternative to traditional cytotoxic approaches for elderly patients who are not likely to tolerate or even benefit from aggressive chemotherapy. In this review, we will focus on the clinical development of tipifarnib for treatment of newly diagnosed AML, both as induction therapy for elderly adults with poor-risk AML and as maintenance therapy following achievement of first complete remission following induction and consolidation therapies for poor-risk AML. As with all other malignancies, the optimal approach is likely to lie in rational combinations of tipifarnib with cytotoxic, biologic and/or immunomodulatory agents with non-cross-resistant mechanisms of action. Gene expression profiling has identified networks of differentially expressed genes and gene combinations capable of predicting response to single agent tipifarnib. The clinical and correlative laboratory trials in progress and under development will provide the critical foundations for defining the optimal roles of tipifarnib and in patients with AMl and other hematologic malignancies.

Danazol therapy combined with intermittent application of chemotherapy induces lasting remission in myeloproliferative disorder (MPD): an alternative for the elderly with advanced MPD

There is no good alternative therapy available for elderly patients with advanced myeloproliferative disorders (MPD) who failed on conventional therapies and are not candidates for bone marrow transplant. We report here an effective therapy that induced exceptionally long-lasting remissions and improved quality of life. Eighteen elderly patients (mean age: 70·6 years) (16 myelofibrosis and 2 thrombocythemia) who had failed on conventional therapies were treated. Danazol was administered daily at 200-800 mg throughout the study. Chemotherapy was applied intermittently as needed to reduce spleen size and blood counts. Busulfan (2-4 mg/day) was used most often and 6-mercaptopurine (6-MP) (50-100 mg/day) and/or cytarabine (100-200 mg/m(2)) if the white blood cell (WBC) count rose rapidly. When MPD stabilized, chemotherapy was discontinued and dosage of danazol was reduced. Therapy was well tolerated. Overall, 61% of patients responded with unexpectedly long-lasting remissions and improved quality of life. Three (17%) had excellent (E) response, defined by normalization of blood counts and non-palpable spleen, while eight (44%) had good (G) response, defined by rise of Hct by ≥7% and ≥50% reduction of spleen. Mean duration of remission was 45 months (10-78 months) in E responders and 11 months in G responders (2-22 months). This regimen offers a safe and effective alternative for advanced MPD in the elderly.

A phase 1-2 study of a farnesyltransferase inhibitor, tipifarnib, combined with idarubicin and cytarabine for patients with newly diagnosed acute myeloid leukemia and high-risk myelodysplastic syndrome

BACKGROUND

The authors conducted a phase 1/2 study of tipifarnib in combination with idarubicin and cytarabine (IA) in 95 patients with previously untreated acute myeloid leukemia (AML) or high-risk myelodysplastic syndrome.

METHODS

Induction consisted of idarubicin 12 mg/m(2) a day on days 1-3, cytarabine 1.5 g/m(2) intravenously continuously daily on days 1-4 (days 1-3 if age ≥60 years), and tipifarnib, with the first cohort (n = 6) receiving 200 mg orally twice a day and all others receiving 300 mg twice a day for 21 days every 28 days. Consolidation consisted of 5 courses of idarubicin 8 mg/m(2) a day on days 1-2, cytarabine 0.75 g/m(2) a day on days 1-3, and tipifarnib 300 mg twice a day for 14 days every 4-6 weeks. Maintenance with tipifarnib 300 mg twice a day for 21 days every 4-6 weeks was continued for 6 months.

RESULTS

With a median follow-up of 33 months, 61 patients achieved complete remission (CR) (64%), and 9 achieved complete remission with incomplete platelet recovery (CRp) (9%). The median duration of CR was not reached. Median overall survival was 17 months. The most common grade 3 adverse events were gastrointestinal toxicities, liver dysfunction, and skin rash. Compared with historical IA, IA and tipifarnib showed a better CR duration (P = .04) and a trend toward a higher CR rate in patients with chromosome 5/7 abnormalities.

CONCLUSIONS

The combination of IA and tipifarnib is safe and active. Further studies exploring different dosages and schedules are warranted, particularly in patients with poor-risk AML.

Suppression of farnesyltransferase activity in acute myeloid leukemia and myelodysplastic syndrome: current understanding and recommended use of tipifarnib

IMPORTANCE OF THE FIELD

Acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) incidence in the United States increases with age. Given the progressive ageing of the general population, incidence of these diseases is likely to continue to rise in the future. There is an acute need for therapeutic developments because of the poor prognosis of these diseases. Since the knowledge of molecular genetics in AML and MDS has expanded recently, targeted therapeutics should offer an exciting new frontier for advancement. Of all the targeted inhibitors developed, tipifarnib represents one of the few compounds with some activity as a single agent.

AREAS COVERED IN THIS REVIEW

Described in this review are the molecular targets of tipifarnib, safety and tolerability of the drug, chemistry, and clinical efficacy in AML.

WHAT THE READER WILL GAIN

The reader will gain a thorough understanding of tipifarnib as it relates to the current and future use of the drug in AML.

TAKE HOME MESSAGE

The future of tipifarnib, along with other molecularly-targeted drugs, lies in achieving a better understanding of leukemia biology and harnessing the activity of this agent using predictive biomarkers for improved patient selection.

Enlarging the scope of cell-penetrating prenylated peptides to include farnesylated 'CAAX' box sequences and diverse cell types

Protein prenylation is a posttranslational modification that is present in a large number of proteins; it has been proposed to be responsible for membrane association and protein-protein interactions, which contribute to its role in signal transduction pathways. Research has been aimed at inhibiting prenylation with farnesyltransferase inhibitors based on the finding that the farnesylated protein Ras is implicated in 30% of human cancers. Despite numerous studies on the enzymology of prenylation in vitro, many questions remain about the process of prenylation as it occurs in living cells. Here we describe the preparation of a series of farnesylated peptides that contain sequences recognized by protein farnesyltransferase. Using a combination of flow cytometry and confocal microscopy, we show that these peptides enter a variety of different cell types. A related peptide where the farnesyl group has been replaced by a disulfide-linked decyl group is also shown to be able to efficiently enter cells. These results highlight the applicability of these peptides as a platform for further study of protein prenylation and subsequent processing in live cells.

Therapeutic modulation of k-ras signaling in colorectal cancer

KRAS has an important role in colorectal carcinogenesis and mutant KRAS leads to a permanently activated k-ras protein. To exert its biological activity, k-ras requires post-translational modification by prenylation. K-ras modulation has become a promising concept for new therapies, mostly by interference with the mevalonate pathway and subsequently by the prenylation of k-ras. Clinical data of agents interfering with the mevalonate pathway and the prenylation of ras are summarized and suggest that these agents might be effective when administered in combination with anticancer drugs that target k-ras. Here, we discuss the novel concept that modulation of k-ras might potentiate EGFR therapy by altering the KRAS phenotype.

The bone marrow microenvironment as a sanctuary for minimal residual disease in CML

Bcr-abl kinase inhibitors have provided proof of principal that targeted therapy holds great promise for the treatment of cancer. However, despite the success of these agents in treating chronic myelogenous leukemia (CML), the majority of patients continue to present with minimal residual disease contained within the bone marrow microenvironment. These clinical observations suggest that the bone marrow microenvironment may provide survival signals that contribute to the failure to eliminate minimal residual disease. The bone marrow microenvironment is comprised of multiple sub-domains which vary in cellular composition and gradients of soluble factors and matrix composition. Experimental evidence indicate that exposure of tumor cells to either bone marrow derived soluble factors or the extracellular matrix can confer a multi-drug resistance phenotype. Together, these data indicate that targeting such pathways may be a viable approach for increasing the efficacy of chemotherapy. Moreover, we propose that personalized medicine must go beyond understanding predictive models inherent to tumors but rather build predictive models that consider diversity in response due to interactions with the tumor microenvironment. Although review will focus on CML, understanding the contribution of the bone marrow microenvironment could contribute to rationale combination therapy in other types of leukemia, multiple myeloma and solid tumors which metastasize to the bone.

Combination of tipifarnib and rapamycin synergistically inhibits the growth of leukemia cells and overcomes resistance to tipifarnib via alteration of cellular signaling pathways

Small molecules are attractive agents for the treatment of leukemia. We found that a combination of a farnesyltransferase inhibitor, tipifarnib, and an mTOR inhibitor, rapamycin, synergistically inhibited the growth of myeloid leukemia cell lines and primary leukemia cells by inducing apoptosis and cell-cycle blockage. The combined agents reduced the level of phospho-ERK1/2, suggesting that they altered the network of signaling pathways. They also showed synergistic effects in tipifarnib-resistant K562/RR cells. The results support the utility of this combination as a potential therapy for leukemia. The combination might also be effective in overcoming resistance to tipifarnib.

Standard management of patients with chronic myeloid leukemia

The successful introduction of the tyrosine kinase inhibitors (TKIs) has revolutionized the treatment of patients with chronic myeloid leukemia (CML). Imatinib therapy induces high rates of complete cytogenetic and major molecular responses, and improves survival in CML. Following imatinib treatment, more than 90% of patients obtain complete hematologic response, and over 80% achieve a complete cytogenetic response. With 7 years of follow-up, the results are still very favorable, resulting in a major change in the natural history of the disease. Resistance to imatinib represents a clinical challenge. Although some clinical and biologic features have been found to be associated with a lower probability of response to imatinib, at present no precise markers allowing for the prediction of outcome for individual patients exist. The most common mechanisms of resistance to imatinib include BCR-ABL kinase domain mutations, amplification, and overexpression of the BCR-ABL oncogene, and clonal evolution with activation of additional transformation pathways. These mechanisms are eventually caused by the genomic instability, which characterizes the Philadelphia chromosome-positive clone. Several approaches to overcome resistance have been proposed. The understanding of at least some of the mechanisms of resistance to imatinib has led to a rapid development of new therapeutic agents that might overcome this resistance. Novel targeted agents designed to overcome imatinib resistance include second-generation TKIs such as dasatinib, nilotinib, bosutinib, bafetinib, and others. Other approaches are exploring combination therapy, with agents affecting different oncogenic pathways, and immune modulation. Herein, we review some of these targeted therapies, particularly those for which clinical data are already available.

Farnesyl Transferase Inhibitors for Patients with Lung Cancer

The ras family of genes have been identified as potential targets for therapeutic intervention because of somatic mutations in different human cancers. They are mutated in non-small cell lung cancer (NSCLC) approximately 20% of the time. The enzyme farnesyl transferase is involved in posttranslational modification of the ras proteins by covalently linking a farnesyl group to the ras protein. This permits the ras protein to be translocated to the surface membrane, allowing the protein to be involved in signaling for increased proliferation and inhibition of apoptosis. The class of farnesyl transferase inhibitors is designed to block farnesylation and prevent the mature ras signaling and thus inhibit cell proliferation and facilitate apoptosis. Multiple agents that inhibit farnesylation have been developed, and two farnesyl transferase inhibitors have been tested in patients with lung cancer in three Phase II trials. R115777 has been studied in patients with NSCLC and in patients with relapsed small cell lung cancer (SCLC) after chemotherapy. There has been a single trial of L-778,123 in patients with untreated NSCLC. No objective tumor responses in patients with stage IIIB/IV NSCLC were seen in these studies. There were also no objective responses among the 22 patients with relapsed SCLC treated with R115777. The median survival for the 44 patients with NSCLC treated with R115777 was approximately 8 months, whereas it was 11 months for the 23 patients treated with L-778,123. R115777 and L-778,123 were well tolerated in these studies but showed no significant activity as single-agent therapy in relapsed SCLC or untreated NSLC.

Clinical Activity of Farnesyl Transferase Inhibitors in Hematologic Malignancies: Possible Mechanisms of Action

Farnesyl transferase inhibitors (FTIs) are a novel class of anti-cancer agents that competitively inhibit farnesyl protein transferase (FTase). Initially developed to inhibit the prenylation necessary for Ras activation, their mechanism of action seems to be more complex, involving other proteins unrelated to Ras. FTIs have been developed and tested across a wide range of human cancers. At least 3 agents within this family have been investigated in hematologic malignancies. These are tipifarnib (R115777, Zarnestra), lonafarnib (SCH66336, Sarasar), both of which are orally administered, and BMS-214662, which is given intravenously. Preliminary results from clinical trials demonstrate enzyme target inhibition, a favorable toxicity profile and promising efficacy. Ongoing studies will better determine their mechanism of action and the role of combination with other agents, defining their place in the therapeutic arsenal of hematologic disorders.

Angiogenesis and Anti-angiogenic Therapy in Myelofibrosis with Myeloid Metaplasia

Myelofibrosis with myeloid metaplasia (MMM) is a clonal stem cell disorder that is characterized by florid bone marrow stromal reaction including collagen fibrosis, osteosclerosis, and angiogenesis. Almost all patients with MMM display increased bone marrow microvessel density (MVD) and the extent is among the highest in hematological malignancies. This particular information has encouraged the therapeutic use of anti-angiogenic drugs in MMM. In the current review, we summarize the general concepts regarding angiogenesis, assessment of angiogenesis in hematological malignancies and then the current literature on angiogenesis and anti-angiogenic therapy in MMM.

Nitrogen-containing bisphosphonate incadronate augments the inhibitory effect of farnesyl transferase inhibitor tipifarnib on the growth of fresh and cloned myeloma cellsin vitro

RAS gene mutations occur in 30 - 40% of multiple myeloma (MM) patients. Farnesylation is the first step in the post-translational modification of RAS proteins. Tipifarnib is a potent farnesyl transferase inhibitor, and incadronate prevents post-translational prenylation of GTP-binding proteins such as RAS proteins. We examined the effect of tipifarnib in combination with incadronate on the growth of fresh and cloned myeloma cells in vitro. Tipifarnib inhibited the growth of myeloma cells, and this inhibition was intensified when tipifarnib was combined with incadronate. Tipifarnib, in combination with incadronate, may have some benefits in MM patients.

Emerging therapies for multiple myeloma

Multiple myeloma (MM) is a clonal plasma cell malignancy clinically characterized by osteolytic lesions, immunodeficiency, and renal disease. There are an estimated 750,000 people diagnosed with MM worldwide, with a median overall survival of 3 - 5 years. Besides chromosomal aberrations, translocations, and mutations in essential growth and tumor-suppressor genes, accumulating data strongly highlight the pathophysiologic role of the bone marrow (BM) microenvironment in MM pathogenesis. Based on this knowledge, several novel agents have been identified, and treatment options in MM have fundamentally changed during the last decade. Thalidomide, bortezomib, and lenalidomide have been incorporated into conventional cytotoxic and transplantation regimens, first in relapsed and refractory and now also in newly diagnosed MM. Despite these significant advances, there remains an urgent need for more efficacious and tolerable drugs. Indeed, a plethora of preclinical agents awaits translation from the bench to the bedside. This article reviews the scientific rationale of new therapy regimens and newly identified therapeutic agents - small molecules as well as therapeutic antibodies - that hold promise to further improve outcome in MM.

Suboptimal response to or failure of imatinib treatment for chronic myeloid leukemia: what is the optimal strategy?

Treatment responses to imatinib vary among patients with chronic myeloid leukemia (CML), and definitions of treatment failure and suboptimal response have been published. This article discusses monitoring and treatment of patients with CML after failure of or suboptimal response to imatinib therapy. We reviewed articles listed on PubMed from January 1, 2002, to July 31, 2008, and abstracts from the 2007 Annual Meeting of the American Society of Hematology. Search terms used were chronic myeloid/myelogenous leukemia, imatinib, and BCR-ABL. To enable early recognition of suboptimal responses, patients should be frequently monitored according to published guidelines, including cytogenetic analysis every 6 months until a complete response is achieved and molecular monitoring every 3 months from the start of therapy or monthly if an increasing BCR-ABL1 transcript level is detected. Mutational analysis of BCR-ABL1 may assist with treatment selection. A recent survey suggests that a notable proportion of physicians do not follow treatment guidelines and that broader communication is required. Recent recommendations state that, in patients whose response to imatinib at 400 mg/d is suboptimal, the dose should be increased, whereas alternative therapies, such as dasatinib, nilotinib, and allogeneic stem cell transplant (in eligible patients), and imatinib dose escalation should be considered after imatinib failure. However, clinical data are lacking to confirm this sequence of treatments, and introducing alternative therapies at an earlier stage of treatment, for example, after a suboptimal response, may produce better long-term outcomes in a higher proportion of patients. Patient and disease characteristics should be carefully considered to optimize treatment strategy for CML.

Suboptimal response to or failure of imatinib treatment for chronic myeloid leukemia: what is the optimal strategy?

Treatment responses to imatinib vary among patients with chronic myeloid leukemia (CML), and definitions of treatment failure and suboptimal response have been published. This article discusses monitoring and treatment of patients with CML after failure of or suboptimal response to imatinib therapy. We reviewed articles listed on PubMed from January 1, 2002, to July 31, 2008, and abstracts from the 2007 Annual Meeting of the American Society of Hematology. Search terms used were chronic myeloid/myelogenous leukemia, imatinib, and BCR-ABL. To enable early recognition of suboptimal responses, patients should be frequently monitored according to published guidelines, including cytogenetic analysis every 6 months until a complete response is achieved and molecular monitoring every 3 months from the start of therapy or monthly if an increasing BCR-ABL1 transcript level is detected. Mutational analysis of BCR-ABL1 may assist with treatment selection. A recent survey suggests that a notable proportion of physicians do not follow treatment guidelines and that broader communication is required. Recent recommendations state that, in patients whose response to imatinib at 400 mg/d is suboptimal, the dose should be increased, whereas alternative therapies, such as dasatinib, nilotinib, and allogeneic stem cell transplant (in eligible patients), and imatinib dose escalation should be considered after imatinib failure. However, clinical data are lacking to confirm this sequence of treatments, and introducing alternative therapies at an earlier stage of treatment, for example, after a suboptimal response, may produce better long-term outcomes in a higher proportion of patients. Patient and disease characteristics should be carefully considered to optimize treatment strategy for CML.

Active oral regimen for elderly adults with newly diagnosed acute myelogenous leukemia: a preclinical and phase 1 trial of the farnesyltransferase inhibitor tipifarnib (R115777, Zarnestra) combined with etoposide

The farnesyltransferase inhibitor tipifarnib exhibits modest activity against acute myelogenous leukemia. To build on these results, we examined the effect of combining tipifarnib with other agents. Tipifarnib inhibited signaling downstream of the farnesylated small G protein Rheb and synergistically enhanced etoposide-induced antiproliferative effects in lymphohematopoietic cell lines and acute myelogenous leukemia isolates. We subsequently conducted a phase 1 trial of tipifarnib plus etoposide in adults over 70 years of age who were not candidates for conventional therapy. A total of 84 patients (median age, 77 years) received 224 cycles of oral tipifarnib (300-600 mg twice daily for 14 or 21 days) plus oral etoposide (100-200 mg daily on days 1-3 and 8-10). Dose-limiting toxicities occurred with 21-day tipifarnib. Complete remissions were achieved in 16 of 54 (30%) receiving 14-day tipifarnib versus 5 of 30 (17%) receiving 21-day tipifarnib. Complete remissions occurred in 50% of two 14-day tipifarnib cohorts: 3A (tipifarnib 600, etoposide 100) and 8A (tipifarnib 400, etoposide 200). In vivo, tipifarnib plus etoposide decreased ribosomal S6 protein phosphorylation and increased histone H2AX phosphorylation and apoptosis. Tipifarnib plus etoposide is a promising orally bioavailable regimen that warrants further evaluation in elderly adults who are not candidates for conventional induction chemotherapy. These clinical studies are registered at www.clinicaltrials.gov as #NCT00112853.

Biologic therapies for advanced pancreatic cancer

Patients with metastatic pancreatic cancer have poor prognosis and short survival due to lack of effective therapy and aggressiveness of the disease. Pancreatic cancer has widespread chromosomal instability, including a high rate of translocations and deletions. Upregulated EGF signaling and mutation of K-RAS are found in most pancreatic cancers. Therefore, inhibitors that target EGF receptor, K-RAS, RAF, MEK, mTOR, VEGF and PDGF, for example, have been evaluated in patients with pancreatic cancer. Although significant activities of these inhibitors have not been observed in the majority of pancreatic cancer patients, an enormous amount of experience and knowledge has been obtained from recent clinical trials. With a better inhibitor or combination of inhibitors, and improvement in the selection of patients for available inhibitors, better therapy for pancreatic cancer is on the horizon.

Therapeutic advances in leukemia and myelodysplastic syndrome over the past 40 years

Major therapeutic progress has been accomplished in leukemia and myelodysplastic syndrome (MDS) over the past 40 years, which may not be fully appreciated by the larger medical community. The objective of this review was to briefly highlight the treatment breakthroughs in leukemia and MDS. Therapeutic progress happened through better understanding of disease pathophysiologies and rational development of targeted agents, like imatinib mesylate in chronic myeloid leukemia (CML), and through astute, empirical discoveries in the clinic, like all-trans retinoic acid and arsenic trioxide in acute promyelocytic leukemia (APL) and chlorodeoxyadenosine in hairy cell leukemia (HCL). Today, the 5- to 10-year survival rates in patients with APL and HCL exceed 80%. In patients with CML, imatinib therapy has been associated with estimated 5- to 7-year survival rates from 85% to 90%. In patients with adult acute lymphocytic leukemia, modern intensive regimens have improved the 5-year survival rates from 20% up to 40%. In patients with chronic lymphocytic leukemia, chemoimmunotherapy recently produced high rates of quality responses and improved long-term outcome. In younger patients with acute myeloid leukemia (AML), the 5-year survival rates range from 40% to 50%, although elderly AML remains a therapeutic challenge. In patients with MDS, it was recently demonstrated that epigenetic therapy with hypomethylating agents improved survival. Much therapeutic progress has been witnessed in leukemia and MDS, and much more is expected to occur soon.

Phase II trial of tipifarnib as maintenance therapy in first complete remission in adults with acute myelogenous leukemia and poor-risk features

PURPOSE

Acute myelogenous leukemia (AML) does not have a high cure rate, particularly in patients with poor-risk features. Such patients might benefit from additional therapy in complete remission (CR). Tipifarnib is an oral farnesyltransferase inhibitor with activity in AML. We conducted a phase II trial of maintenance tipifarnib monotherapy for 48 adults with poor-risk AML in first CR.

EXPERIMENTAL DESIGN

Tipifarnib 400 mg twice daily for 14 of 21 days was initiated after recovery from consolidation chemotherapy, for a maximum of 16 cycles (48 weeks).

RESULTS

Twenty (42%) patients completed 16 cycles, 24 (50%) were removed from study for relapse, and 4 (8%) discontinued drug prematurely for intolerance. Nonhematologic toxicities were rare, but tipifarnib dose was reduced in 58% for myelosuppression. Median disease-free survival (DFS) was 13.5 months (range, 3.5-59+ months), with 30% having DFS >2 years. Comparison of CR durations for 25 patients who received two-cycle timed sequential therapy followed by tipifarnib maintenance with 23 historically similar patients who did not receive tipifarnib showed that tipifarnib was associated with DFS prolongation for patients with secondary AML and adverse cytogenetics.

CONCLUSIONS

This study suggests that some patients with poor-risk AML, including patients with secondary AML and adverse cytogenetics, may benefit from tipifarnib maintenance therapy. Future studies are warranted to examine alternative tipifarnib dosing and continuation beyond 16 cycles.

Molecularly targeted therapies for pediatric acute myeloid leukemia: progress to date

While acute myeloid leukemia (AML) is significantly less common than acute lymphoblastic leukemia (ALL) in childhood, it is significantly more deadly with only half as many children likely to be cured with standard therapy. In addition, the typical treatment for AML is among the most toxic of treatments for pediatric cancer; it includes intensive multiagent chemotherapy and, often, hematopoietic stem cell transplantation. Given the poor prognosis of pediatric AML and the significant toxicity of standard AML therapy, novel therapies are needed. Improved understanding of the molecular and cellular biology of leukemia has facilitated the development of molecularly targeted therapies. In this article, we review progress to date with agents that are showing promise in the treatment of pediatric AML including targeted immunoconjugates, inhibitors of signaling molecules (e.g. FMS-like tyrosine kinase 3 [FLT3], farnesyltransferase, and mammalian target of rapamycin [mTOR]), agents that target epigenetic regulation of gene expression (DNA methyltransferase inhibitors and histone deacetylase inhibitors), and proteasome inhibitors. For the specific agents in each of these classes, we summarize the published preclinical data and the clinical trials that have been completed, are in progress, or are being planned for children with AML. Finally, we discuss potential challenges to the success of molecularly targeted therapy including demonstrating adequate targeting of leukemia stem cells, developing synergistic and tolerable combinations of agents, and designing adequately powered clinical trials to test efficacy in molecularly defined subsets of patients.

Phase I study of alternate-week administration of tipifarnib in patients with myelodysplastic syndrome

PURPOSE

To determine the safety and describe the antitumor activity of tipifarnib in patients with myelodysplastic syndrome (MDS) using an alternate-week schedule.

EXPERIMENTAL DESIGN

Patients with MDS were given tipifarnib, escalating from 100 mg orally twice daily until the maximum tolerated dose for 8 weeks followed by maintenance therapy (same dose/schedule) for patients with stable disease or better.

RESULTS

Sixty-three patients were treated. The most common toxicity was myelosuppression (60% of patients). Twenty percent of patients had no side effects. Nonhematologic toxicities included fatigue (20%), skin rash (9%), diarrhea (16%), increase in liver transaminases (14%) and bilirubin (11%), and nausea (11%). Dose-limiting toxicities of ataxia (n = 1), fatigue (n = 1), nausea (n = 1), and neutropenic fever (n = 2) occurred at tipifarnib doses above 1,200 mg/d. Sixteen of 61 (26%) evaluable patients responded (3 complete remissions and 13 hematologic improvements) with major platelet responses being most common (11 of 16 responders). There was no obvious dose-response relationship. Four of the 16 responders (25%; including a complete responder) were treated at the lowest dose level (100 mg twice daily). Only one responder had a Ras mutation. Giving tipifarnib resulted in potent inhibition of farnesyl transferase (usually more than 75%) in peripheral blood mononuclear cells regardless of dose. Partial farnesyl transferase inhibition persisted during the week off.

CONCLUSIONS

Alternate-week tipifarnib is active and well tolerated in patients with MDS at doses up to and including 600 mg orally twice daily. The biological activity of tipifarnib is not dependent on dose.

Targeted therapy in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is characterized by the formation of the Philadelphia chromosome and oncogenic signaling by the resulting Bcr-Abl fusion protein. Understanding the molecular basis of CML has led to the development of highly effective targeted therapies that block Bcr-Abl tyrosine kinase activity. Imatinib, the current first-line therapy for CML, induces durable treatment responses in most patients. However, patients may develop imatinib resistance, which is often due to BCR-ABL mutations. With the availability of second generation tyrosine kinase inhibitors, an effective therapeutic option other than stem cell transplantation is available following imatinib failure. Randomized trial data suggest that dasatinib treatment is superior to imatinib dose escalation in patients with imatinib resistance. Nilotinib, a recently approved analogue of imatinib, has also demonstrated encouraging treatment responses in patients with imatinib-resistant CML. Other agents (including bosutinib and INNO-406) are in clinical development. With the potential availability of multiple treatment options for patients with CML, it may be possible to tailor treatment according to individual patient or disease characteristics, for example, BCR-ABL mutations. Future CML treatment may involve combination strategies. Overall, targeted agents have significantly improved the prognosis of patients diagnosed with CML.

Novel agents in CML therapy: tyrosine kinase inhibitors and beyond

The emergence of resistance to imatinib has become a significant problem despite the remarkable clinical results achieved with this tyrosine kinase inhibitor in the treatment of chronic myeloid leukaemia. The most common cause of imatinib resistance is the selection of leukemic clones with point mutations in the Abl kinase domain. These mutations lead to amino acid substitutions and prevent the appropriate binding of imatinib. Genomic amplification of BCR-ABL, modulation of drug efflux or influx transporters, and Bcr-Abl-independent mechanisms also play important roles in the development of resistance. Persistent disease is another therapeutic challenge and may in part, be due to the inability of imatinib to eradicate primitive stem cell progenitors. A multitude of novel agents have been developed and have shown in vitro and in vivo efficacy in overcoming imatinib resistance. In this review, we will discuss the current status of the ATP-competitive and non-ATP-competitive Bcr-Abl tyrosine kinase inhibitors. We will also describe inhibitors acting on targets found in signaling pathways downstream of Bcr-Abl, such as the Ras-Raf-mitogen-activated protein kinase and phosphatidylinositol-3 kinase-Akt-mammalian target of rapamycin pathways, and targets without established links with Bcr-Abl.