Results of triple therapy with interferon-alpha, cytarabine, and homoharringtonine, and the impact of adding imatinib to the treatment sequence in patients with Philadelphia chromosome-positive chronic myelogenous leukemia in early chronic phase

High efficacy of azacitidine combined with homoharringtonine, idarubicin, and cytarabine in newly diagnosed patients with AML: A single arm, phase 2 trial

Introduction

This study aims to evaluate the efficacy and safety of the novel combination of Aza and HIA as the frontline induction therapy in newly diagnosed AML patients eligible for intensive chemotherapy (IC) (registered on ClinicalTrials.gov, number NCT04248595).

Methods

Aza (75mg/m2/d on days1-5 subcutaneous) is administered in combination with HIA [HHT 2mg/m2/d on days 4-8 intravenous over 3 hours, idarubicin 6mg/m2/d on days 4-6 intravenous, and cytarabine 100mg/m2/d on days 4-10 intravenous]. The primary endpoint was complete remission (CR) or CR with incomplete blood count recovery (CRi). Secondary endpoints were overall survival (OS), relapse-free survival (RFS), and adverse events (AEs).

Results

A total of 20 AML patients (aged 18-70 years) were enrolled between Jan 2020 and Sep 2022. 95% (19/20) of patients achieved CR/CRi, and 89.5% (17/19) had undetectable MRD, in which 94.7% (18/19) reached CR/CRi, and 88.9% (16/18) obtained MRD negative after the 1st cycle of induction therapy. Median OS and RFS were both not reached during the follow-up. The estimated 2-year OS and RFS were 87.5% (95%CI, 58.6% to 96.7%) and 87.1% (95%CI, 57.3% to 96.6%), respectively. No patient discontinued the treatment for AEs.

Discussion

This study provides preliminary evidence for this novel combination therapy as the first-line induction therapy for young or older AML patients fit for IC.

Imatinib Mesylate

Imatinib mesylate (Gleevec, Glivec [Novartis, Basel, Switzerland], formerly referred to as STI571 or CGP57148B) represents the paradigm of a new class of anticancer agents, so-called small molecules. They have a high selectivity against a specific molecular target known to be the cause for the establishment and maintenance of the malignant phenotype. Imatinib is a rationally designed oral signal transduction inhibitor that specifically targets several protein tyrosine kinases, Abl, Arg (Abl-related gene), the stem cell factor receptor (c-KIT), platelet-derived growth factor receptor (PDGF-R), and their oncogenic forms, most notably BCR-ABL. Imatinib has been shown to have remarkable clinical activity in patients with chronic myeloid leukemia (CML) and malignant gastrointestinal stroma tumors (GIST) leading to its approval for treatment of these diseases. Treatment with imatinib is generally well tolerated with a low incidence of severe side effects. The most common adverse events include mild to moderate edema, muscle cramps, diarrhea, nausea, skin rashes, and myelosuppression. Several mechanisms of resistance have been identified. Clonal evolution, amplification, or overexpression of BCR-ABL as well as mutations in the catalytic domain, P-loop, and other mutations have been demonstrated to play a role in primary and secondary resistance to imatinib, respectively. Understanding of the underlying mechanisms of resistance has led to the development of new second- and third-generation tyrosine kinase inhibitors (see chapters on dasatinib, nilotinib, bosutinib, and ponatinib).

A Review of Omacetaxine: A Chronic Myeloid Leukemia Treatment Resurrected

The paradigm of targeted therapy was pioneered for chronic myeloid leukemia (CML). The advent of tyrosine kinase inhibitors (TKIs) has led to marked improvements in responses and overall survival; however, there is still a subset of patients that are either resistant through a multitude of mechanisms or intolerant to standard TKI therapy. Omacetaxine mepesuccinate (omacetaxine), a semisynthetic purified homoharringtonine compound, has been studied for over 40 years and was approved in 2012 by the Food and Drug Administration (FDA) for patients with CML refractory or intolerant to two or more TKIs. Omacetaxine has a novel mechanism of action-inhibition of protein synthesis, which does not overlap with kinase inhibition. Multiple studies have demonstrated that omacetaxine can achieve responses in heavily treated patients with either chronic-phase or accelerated-phase CML, regardless of the presence of mutations in the tyrosine kinase domain. This review will outline the tortuous story of omacetaxine, including preclinical and clinical studies of homoharringtonine, current indications, and management guidelines.

Role of Non Receptor Tyrosine Kinases in Hematological Malignances and its Targeting by Natural Products

Tyrosine kinases belong to a family of enzymes that mediate the movement of the phosphate group to tyrosine residues of target protein, thus transmitting signals from the cell surface to cytoplasmic proteins and the nucleus to regulate physiological processes. Non-receptor tyrosine kinases (NRTK) are a sub-group of tyrosine kinases, which can relay intracellular signals originating from extracellular receptor. NRTKs can regulate a huge array of cellular functions such as cell survival, division/propagation and adhesion, gene expression, immune response, etc. NRTKs exhibit considerable variability in their structural make up, having a shared kinase domain and commonly possessing many other domains such as SH2, SH3 which are protein-protein interacting domains. Recent studies show that NRTKs are mutated in several hematological malignancies, including lymphomas, leukemias and myelomas, leading to aberrant activation. It can be due to point mutations which are intragenic changes or by fusion of genes leading to chromosome translocation. Mutations that lead to constitutive kinase activity result in the formation of oncogenes, such as Abl, Fes, Src, etc. Therefore, specific kinase inhibitors have been sought after to target mutated kinases. A number of compounds have since been discovered, which have shown to inhibit the activity of NRTKs, which are remarkably well tolerated. This review covers the role of various NRTKs in the development of hematological cancers, including their deregulation, genetic alterations, aberrant activation and associated mutations. In addition, it also looks at the recent advances in the development of novel natural compounds that can target NRTKs and perhaps in combination with other forms of therapy can show great promise for the treatment of hematological malignancies.

Cephalotaxus Alkaloids

Cephalotaxus alkaloids represent a family of plant secondary metabolites known for 60 years. Significant activity against leukemia in mice was demonstrated for extracts of Cephalotaxus. Cephalotaxine (CET) (1), the major alkaloid of this series was isolated from Cephalotaxus drupacea species by Paudler in 1963. The subsequent discovery of promising antitumor activity among new Cephalotaxus derivatives reported by Chinese, Japanese, and American teams triggered extensive structure elucidation and biological studies in this family. The structural feature of this cephalotaxane family relies mainly on its tetracyclic alkaloid backbone, which comprises an azaspiranic 1-azaspiro[4.4]nonane unit (rings C and D) and a benzazepine ring system (rings A and B), which is linked by its C3 alcohol function to a chiral oxygenated side chain by a carboxylic function alpha to a tetrasubstituted carbon center. The botanical distribution of these alkaloids is limited to the Cephalotaxus genus (Cephalotaxaceae). The scope of biological activities of the Cephalotaxus alkaloids is mainly centered on the antileukemic activity of homoharringtonine (HHT) (2), which in particular demonstrated marked benefits in the treatment of orphan myeloid leukemia and was approved as soon as 2009 by European Medicine Agency and by US Food and Drug Administration in 2012. Its exact mechanism of action was partly elucidated and it was early recognized that HHT (2) inhibited protein synthesis at the level of the ribosome machinery. Interestingly, after a latency period of two decades, the topic of Cephalotaxus alkaloids reemerged as a prolific source of new natural structures. To date, more than 70 compounds have been identified and characterized. Synthetic studies also regained attention during the past two decades, and numerous methodologies were developed to access the first semisynthetic HHT (2) of high purity suitable for clinical studies, and then high grade enantiomerically pure CET (1), HHT (2), and analogs.

Omacetaxine mepesuccinate in chronic myeloid leukemia

INTRODUCTION

Homoharringtonine (HHT) and other alkaloid esters were originally isolated from the Cephalotaxus evergreen tree and have been used in traditional Chinese medicine since the 1970s to treat a variety of malignancies. Although HHT was investigated for the treatment of chronic myeloid leukemia (CML) in the 1990s with good results, the advent of BCR-ABL1 tyrosine kinase inhibitors (TKIs) at that time rapidly established a new standard of care for CML. Omacetaxine mepesuccinate is a semisynthetic derivative of HHT with known clinical activity in relapsed or refractory CML following TKI therapy.

AREAS COVERED

In this review, we summarize the biologic effects of HHT and its derivative, omacetaxine, in CML. Additionally, we analyze the concepts learned from the early trials using these drugs. Data from clinical trials resulting in drug approval are also reviewed.

EXPERT OPINION

Omacetaxine has a clear role in the CML armamentarium for patients in chronic and accelerated phase who have failed or were intolerant to two or more TKIs.

Omacetaxine: a protein translation inhibitor for treatment of chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is driven by the Bcr-Abl fusion protein, which is a result of a (9;22) chromosomal translocation. Imatinib, dasatinib, and nilotinib (tyrosine kinase inhibitors, TKI) have revolutionized how CML is treated. Although the majority of patients respond to these kinase inhibitors, a subset becomes resistant to these therapeutics. Synribo (omacetaxine mepesuccinate) was recently approved by the U.S. Food and Drug Administration for Philadelphia-positive CML either in the chronic or the accelerated phase whose disease failed two prior TKIs. With omacetaxine 1.25 mg/m(2) twice daily for 14 days during induction and for 7 days during maintenance, a major cytogenetic response occurred in 20% of patients in the chronic phase and major hematologic response in 27% of patients in the accelerated phase. Laboratory investigations unraveled the mechanism of action and effectiveness of this agent. Bcr-Abl protein is intrinsically programmed to turn over with a short half-life that makes it susceptible to protein translation inhibitors. Omacetaxine (homoharringtonine) inhibits total protein biosynthesis by binding to A-site cleft of ribosomes. As a corollary to this action, there is a diminution of short-lived proteins, such as Bcr-Abl, followed by cell death. Approval of this first-in-class protein translation inhibitor opens up new avenues for its use in other diseases as well as mechanism-based combinations.

Omacetaxine mepesuccinate in the treatment of intractable chronic myeloid leukemia

In a significant proportion of patients with chronic myeloid leukemia, resistance to BCR-ABL tyrosine kinase inhibitors develops due to acquisition of BCR-ABL kinase domain mutations and insensitivity of leukemia stem cells to tyrosine kinase inhibitors. Omacetaxine mepesuccinate (formerly called homoharringtonine) is a natural alkaloid that inhibits protein synthesis and induces cell death. Omacetaxine mepesuccinate has been recently approved by the US Food and Drug Administration to treat patients with chronic myeloid leukemia who failed to respond to multiple tyrosine kinase inhibitors and/or acquired the BCR-ABL-T315I mutation. In this review, we discuss the use and effectiveness of omacetaxine mepesuccinate in the treatment of chronic myeloid leukemia, with coverage of its pharmacology, mode of action, and pharmacokinetics. We believe that omacetaxine mepesuccinate will be beneficial to many patients with chronic myeloid leukemia who do not respond well to tyrosine kinase inhibitors.

Novel therapies for patients with chronic myeloid leukemia

The most immediate issues that will have a major impact on the long-term survival of patients with chronic myeloid leukemia is the optimal use of imatinib mesylate (Gleevec, Novartis) and the development of effective therapies for those patients who are intolerant of, or become resistant to, optimal doses of this agent. Of the multiple new agents that are currently being developed for patients with chronic myeloid leukemia, most are being investigated in patients who have developed resistance to imatinib, which is a confounding factor in itself. The mechanisms of action of novel agents are diverse and they may have a variably synergistic therapeutic relationship with imatinib. The complete blockade of the intracellular pathways that are triggered by Bcr-Abl, combined with successful reversal of apoptotic and/or angiogenic abnormalities in chronic myeloid leukemia, may well lead to a cure for the majority of patients.

Homoharringtonine and omacetaxine for myeloid hematological malignancies

Homoharringtonine (HHT), a plant alkaloid with antitumor properties originally identified nearly 40 years ago, has a unique mechanism of action by preventing the initial elongation step of protein synthesis. HHT has been used widely in China for the treatment of chronic myeloid leukemia (CML), acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). Omacetaxine, a semisynthetic form of HHT, with excellent bioavailability by the subcutaneous route, has recently been approved by FDA of the United States for the treatment of CML refractory to tyrosine kinase inhibitors. This review summarized preclinical and clinical development of HHT and omacetaxine for myeloid hematological malignancies.

Imatinib mesylate

Imatinib mesylate (Gleevec, Glivec [Novartis, Basel, Switzerland], formerly referred to as STI571 or CGP57148B) represents the paradigm of a new class of anticancer agents, so-called small molecules. They have a high selectivity against a specific molecular target known to be the cause for the establishment and maintenance of the malignant phenotype. Imatinib is a rationally designed oral signal transduction inhibitor that specifically targets several protein tyrosine kinases, Abl, Arg (Abl-related gene), the stem cell factor receptor (c-KIT), platelet-derived growth factor receptor (PDGF-R), and their oncogenic forms, most notably BCR-ABL. Imatinib has been shown to have remarkable clinical activity in patients with chronic myeloid leukemia (CML) and malignant gastrointestinal stroma tumors (GIST) leading to its approval for treatment of these diseases. Treatment with imatinib is generally well tolerated with a low incidence of severe side effects. The most common adverse events include mild to moderate edema, muscle cramps, diarrhea, nausea, skin rashes, and myelosuppression. Several mechanisms of resistance have been identified. Clonal evolution, amplification, or overexpression of BCR-ABL as well as mutations in the catalytic domain, P-loop, and other mutations have been demonstrated to play a role in primary and secondary resistance to imatinib, respectively. Understanding of the underlying mechanisms of resistance has led to the development of new second- and third-generation tyrosine kinase inhibitors (see chapters on dasatinib, nilotinib, bosutinib, and ponatinib).

Homoharringtonine/omacetaxine mepesuccinate: the long and winding road to food and drug administration approval

Homoharringtonine/omacetaxine is a unique agent with a long history of research development. It has been recently approved by the Food and Drug Administration for the treatment of chronic myeloid leukemia after failure of 2 or more tyrosine kinase inhibitors. Research with this agent has spanned over 40 years, with many instructive lessons to cancer research, which are summarized in this review.

Omacetaxine mepesuccinate (synribo) - newly launched in chronic myeloid leukemia

INTRODUCTION

Omacetaxine mepesuccinate (formerly known as homoharringtonine [HHT]) is a natural alkaloid with significant anticancer activity partly through inhibition of protein synthesis and induction of apoptosis. Prior to the development of tyrosine kinase inhibitors (TKIs), HHT was the most active therapy in chronic myeloid leukemia (CML) after interferon failure. Subsequent trials showed that HHT and omacetaxine are active in patients failing several TKIs or carrying the T315I mutation.

AREAS COVERED

This review will discuss the preclinical development of HHT and omacetaxine mepesuccinate in CML and the clinical studies leading to its approval by the Food and Drug Administration (FDA).

EXPERT OPINION

A sizable number of patients with CML will develop TKI resistance, frequently through the acquisition of BCR-ABL1 kinase domain mutations. Omacetaxine is active in patients with CML after failure to multiple TKIs and in those carrying the T315I mutation, which is highly resistant to all FDA-approved TKIs except for ponatinib. Both ponatinib and omacetaxine have been recently approved by the FDA and represent useful treatment options for patients with CML who failed several TKIs and/or acquired the T315I mutation. The development of an oral formulation of omacetaxine would greatly facilitate its use and provide an attractive option for TKI-based combinatorial strategies.

Omacetaxine mepesuccinate for the treatment of leukemia

INTRODUCTION

Omacetaxine mepesuccinate, formerly known as homoharringtonine, is a first-in-class cephalotaxine that has experienced phases of increasing and waning interest since its first use in traditional Chinese medicine. With activity being reported in patients with chronic myeloid leukemia (CML) resistant to currently available tyrosine kinase inhibitors, renewed interest has recently been generated.

AREAS COVERED

The development of omacetaxine mepesuccinate, with emphasis on synthesis and mode of administration, is addressed. An overview on current clinical results as a single agent or within combination regimens in patients with acute myeloid leukemia (AML) and CML is given.

EXPERT OPINION

Omacetaxine mepesuccinate has a unique mode of action and appreciable activity in AML and CML with generally mild nonhematologic toxicity. In patients with AML, results indicate a role within combination regimens in selected, possibly elderly patient populations. In CML, patients with resistance to tyrosine kinase inhibitors, especially due to the T315I mutation, are the most intensively studied. Despite successful results in some patients, single-agent therapy with omacetaxine mepesuccinate has resulted in modest results. However, upfront combination with tyrosine kinase inhibitor represents an attractive option due their differing mechanisms of action.

Dasatinib combined with interferon-alfa induces a complete cytogenetic response and major molecular response in a patient with chronic myelogenous leukemia harboring the T315I BCR-ABL1 mutation

Mutations of BCR-ABL1 are observed in 50% of patients with imatinib-resistant chronic myeloid leukemia (CML). The T315I mutation is resistant to imatinib and second-generation tyrosine kinase inhibitors (TKIs). We report the case of a 57-year-old man diagnosed with CML in 2003 in whom imatinib therapy failed after which he acquired the T315I mutation. He was treated sequentially with an anti-T315I-specific agent, KW-2449, that led to eradication of the mutation without any further improvement. Subsequent introduction of combination therapy that included dasatinib and pegylated interferon led to the achievement of a sustained complete cytogenetic and major molecular response (MMR). This case illustrates the benefit of combination therapy that includes a TKI and a second agent with a different mechanism of action, either sequentially (TKI followed by KW-2449) or concomitantly (TKI + interferon), in eradicating resistant disease with the T315I clone.

Homoharringtonine, omacetaxine mepesuccinate, and chronic myeloid leukemia circa 2009

Homoharringtonine (HHT) is a natural alkaloid that is obtained from various Cephalotaxus species. The mechanism of action by which HHT exerts its antitumor activity is through inhibition of protein synthesis and promotion of apoptosis. In the 1990s, HHT proved to be significantly active as salvage therapy for patients with chronic myeloid leukemia (CML) after failure on interferon-alpha therapy. However, the remarkable success of imatinib mesylate in the treatment of CML relegated HHT to oblivion. The development of omacetaxine mepesuccinate, a subcutaneously bioavailable semisynthetic form of HHT, and its activity in imatinib-resistant CML has established this agent for the second time as a valuable option in the management of this disease. Preliminary results appear to support the use of this agent for patients who have imatinib-resistant CML, including those who carry the tyrosine kinase inhibitor-insensitive mutation that exchanges the amino acids threonine and isoleucine at position 315 (the T315I mutation). In this article, the authors discuss the current data on omacetaxine and the prospects of this agent to be integrated into the state-of-the-art treatment algorithms for CML.

Advances in the biology and therapy of patients with chronic myeloid leukaemia

Chronic myelogenous leukaemia (CML) is a progressive and often fatal haematopoietic neoplasm. The Bcr-Abl tyrosine kinase inhibitor imatinib mesylate represented a major therapeutic advance over conventional CML therapy, with more than 90% of patients obtaining complete haematologic response and 70-80% of patients achieving a complete cytogenetic response. Resistance to imatinib represents a clinical challenge and is often a result of point mutations causing a conformation change in Bcr-Abl, which impairs imatinib binding. Novel targeted agents designed to overcome imatinib resistance include dasatinib, nilotinib, bosutinib 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.

Second-line therapy and beyond resistance for the treatment of patients with chronic myeloid leukemia post imatinib failure

Chronic myeloid leukemia (CML) is characterized at the molecular level by the presence of the Philadelphia chromosome (Ph) and the resultant oncogenic signaling by the BCR-ABL fusion protein. The treatment and outlook for CML were revolutionized by the introduction of imatinib, but resistance is a substantial barrier to successful treatment in many patients. Introduction of the second-generation tyrosine kinase inhibitors (TKI) dasatinib and nilotinib has provided effective therapeutic options for many patients with resistance to front-line imatinib. However, the T315I mutation remains a significant clinical issue because it is insensitive to all currently available agents. A number of new agents are in development and many hold the promise of activity in T315I-mutated disease. Successful treatment of patients with disease harboring T315I might lie in the effective combination or sequencing of these new agents with existing TKI therapies.

Imatinib-resistant CML cells have low ENT activity but maintain sensitivity to gemcitabine

Philadelphia chromosome-positive chronic myelogenus leukemia (CML) is widely treated with imatinib mesylate (imatinib), a potent inhibitor of the Bcr-Abl tyrosine kinase. However, resistance to this compound remains a concern. Current treatment approaches include combinations of imatinib with nucleoside analogs such as gemcitabine, which requires equilibrative nucleoside transporters (ENTs) for uptake, to overcome this resistance. Here we report that imatinib treatment decreased ENT1-dependent activity and mRNA expression. Although, imatinib-resistant cells showed decreased levels of both ENT1 and ENT2 activity and expression, these cells remained sensitive to gemcitabine, suggesting that nucleoside analogs can be used as adjunctive therapy.

NS-187 (INNO-406), a Bcr-Abl/Lyn Dual Tyrosine Kinase Inhibitor

Protein kinases catalyze the transfer of the γ-phosphoryl group of adenosine triphosphate (ATP) to the hydroxyl groups of protein side chains, and they play critical roles in regulating cellular signal transduction and other biochemical processes. They are attractive targets for today's drug discovery and development, and many pharmaceutical companies are intensively developing various kinds of protein kinase inhibitors. A good example is the recent success with the Bcr-Abl tyrosine kinase inhibitor imatinib mesylate (Gleevec™) in the treatment of chronic myeloid leukemia. Though imatinib has dramatically improved the treatment of Bcr-Abl-positive chronic myeloid leukemia, resistance is often found in patients with advanced-stage disease. Several mechanisms have been proposed to explain this resistance, including point mutations within the Abl kinase domain, amplification of the bcr-abl gene, overexpression of the corresponding mRNA, increased drug efflux mediated by P-glycoprotein, and activation of the Src-family kinase (SFK) Lyn. We set out to develop a novel drug whose affinity for Abl is higher than that of imatinib and whose specificity in inhibiting Lyn is higher than that of SFK/Abl inhibitors such as dasatinib (Sprycel™) or bosutinib (SKI-606). Our work has led to the development of NS-187 (INNO-406), a novel Abl/Lyn dual tyrosine kinase inhibitor with clinical prospects. To provide an overview of how a selective kinase inhibitor has been developed, this review presents chemical-modification studies carried out with the guidance of molecular modeling, the structural basis for the high potency and selectivity of NS-187 based on the X-ray structure of the NS-187/Abl complex, and the biological profiling of NS-187, including site-directed mutagenesis experiments.

The Bcr-Abl tyrosine kinase inhibitor imatinib and promising new agents against Philadelphia chromosome-positive leukemias

Chronic myeloid leukemia (CML) was the first human malignant disease to be linked to a single, acquired genetic abnormality. Identification of the Bcr-Abl kinase fusion protein and its pivotal role in the pathogenesis of CML provided new opportunities to develop molecular-targeted therapies. Imatinib mesylate (IM, Gleevec, Novartis Pharmaceuticals, Basel, Switzerland), which specifically inhibits the autophosphorylation of the Abl TK, has improved the treatment of CML. However, resistance is often reported in patients with advanced-stage disease. Several novel TK inhibitors have been developed that override IM resistance mechanisms caused by point mutations within the Abl kinase domain. Inhibitors of Abl TK are divided into two main groups, namely, ATP-competitive and ATP noncompetitive inhibitors. The ATP-competitive inhibitors fall into two subclasses, the Src/Abl inhibitors, and the 2-phenylaminopyrimidine-based compounds. Dasatinib (formerly BMS-354825), AP23464, SKI-606, and PD166326 are classified as Src/Abl inhibitors, while nilotinib (AMN107) and INNO-406 (NS-187) belong to the latter subclass of inhibitors. Of these agents, dasatinib and nilotinib underwent clinical trials earlier than the others and favorable results are now accumulating. Clinical studies of the other compounds, including SKI-606 and INNO-406, have been performed in rapid succession. Because of their strong affinities for the ATP-binding site compared to IM, most ATP-competitive inhibitors may be effective in IM-resistant patients. However, an ATP-competitive inhibitor that can inhibit the phosphorylation of T315I Bcr-Abl has not yet been developed. Instead, ATP noncompetitive inhibitors, such as ON012380, Aurora kinase inhibitor MK0457 (VX-680), and p38 MAP kinase inhibitor BIRB-796, have been developed to address this problem. This review provides an update on the underlying pathophysiologies of disease progression and IM resistance, and discusses the development of new targeted TK inhibitors for managing CML and the importance of future strategies targeting CML stem cells.

From traditional Chinese medicine to rational cancer therapy

Many natural products and derivatives thereof belong to the standard repertoire of cancer chemotherapy. Examples are Vinca alkaloids, taxanes and camptothecins. In recent years, the potential of natural products from plants, notably from medicinal plants used in traditional Chinese medicine (TCM), has been recognized by the scientific community in the Western world. To provide an example of the most recent developments in this field, we have selected several compounds, namely artesunate, homoharringtonine, arsenic trioxide and cantharidin, that are found in natural TCM products and that have the potential for use in cancer therapy. Controlled clinical studies have shown that homoharringtonine and arsenic trioxide can exert profound activity against leukaemia. Increased knowledge of the molecular mechanisms of TCM-derived drugs and recent developments in their applications demonstrate that the combination of TCM with modern cutting-edge technologies provides an attractive strategy for the development of novel and improved cancer therapeutics.

Dual inhibition of ras and bcr-abl signalling pathways in chronic myeloid leukaemia: a phase I/II study in patients in complete haematological remission

Zoledronic acid inhibits the prenylation of ras-related proteins downstream of bcr-abl and preclinical studies have shown augmentation of the inhibitory effects of imatinib in BCR-ABL expressing cells. A Phase I/II study was designed to assess the safety and efficacy of the addition of zoledronic acid to imatinib in patients with chronic myeloid leukaemia (CML) with a suboptimal response to imatinib alone. Ten patients with CML who had been treated with imatinib for at least 2 years and had achieved and maintained a complete haematological response were included. Zoledronic acid was administered intravenously on one occasion every 28 d. The initial dose of 4 mg was given for three consecutive months; in the absence of significant toxicity and/or response the dose was escalated to 8 mg for an additional 3 months. Efficacy was assessed by serial monitoring of blood levels of BCR-ABL transcripts and bone marrow cytogenetics. Addition of zoledronic acid to imatinib caused no haematological toxicity. There were no grade III or IV non-haematological adverse effects. Grade I fatigue, hypocalcaemia and fever were common side effects. No responses were demonstrated after 6 months on the combination.

Phase I/II study of subcutaneous homoharringtonine in patients with chronic myeloid leukemia who have failed prior therapy

BACKGROUND

Homoharringtonine (HHT) is a cephalotaxus alkaloid that inhibits the synthesis of proteins leading to apoptosis. Intravenous HHT has demonstrated activity in patients with chronic myeloid leukemia (CML) after failure with interferon.

METHODS

A Phase I study was completed of subcutaneous (s.c.) HHT in patients with CML in accelerated or blast phases and demonstrated efficacy and good tolerance at the same doses used by intravenous (i.v.) administration. The maximal tolerated dose (MTD) was 1.25 mg/m(2) s.c. twice daily. The cohort was then expanded to treated at the MTD to include patients in late chronic phase CML after imatinib failure. Therapy consisted of an i.v. loading dose of HHT 2.5 mg/m(2) over 24 hours, followed by 1.25 mg/m(2) s.c. twice daily for 14 days every 28 days until remission, then for 7 days every 28 days. Six patients (median age, 53 years) who had failed imatinib were treated and 5 were evaluable. Patients received a median of 4.5 courses of s.c. HHT.

RESULTS

Complete hematologic remission was obtained in all 5 evaluable patients and 3 had cytogenetic (CG) responses: 1 complete and 2 minor. The 2 patients with BCR-ABL kinase domain mutations at the start of therapy with HHT had a CG response and in both instances the mutations became undetectable. All patients developed myelosuppression and 3 had their HHT dose reduced due to prolonged neutropenia. Nonhematologic toxicity was mild and manageable.

CONCLUSIONS

Subcutaneous HHT is well tolerated and may have clinical activity in patients with CML after imatinib failure.

Overcoming drug resistance in chronic myeloid leukemia

PURPOSE OF REVIEW

Despite the excellent clinical results with imatinib in chronic myeloid leukemia, most patients have minimal residual disease and others will develop resistance and may eventually progress. Thus there is a need for developing approaches to overcome and prevent resistance to imatinib.

RECENT FINDINGS

Several new agents have been developed with significant activity in imatinib-resistant chronic myeloid leukemia. A second generation of more potent tyrosine kinase inhibitors, some with dual activity against Abl and Src, have shown very impressive results. Other agents, such as hypomethylating agents, farnesyl transferase inhibitors and homoharringtonine, have also shown preclinical and clinical promise. The use of vaccines as a way of providing an immunomodulatory approach to chronic myeloid leukemia is starting to develop as a major strategy to achieve eradication of the disease.

SUMMARY

Multiple effective agents are being developed to overcome resistance to imatinib. The challenge for the future is to incorporate them into effective strategies that can eliminate the disease and cure all patients with chronic myeloid leukemia.

A phase I dose-finding and pharmacokinetic study of subcutaneous semisynthetic homoharringtonine (ssHHT) in patients with advanced acute myeloid leukaemia

To determine the maximum-tolerated dose (MTD), dose-limiting toxicities and pharmacokinetic of semisynthetic homoharringtonine (ssHHT), given as a twice daily subcutaneous (s.c.) injections for 9 days, in patients with advanced acute leukaemia, 18 patients with advanced acute myeloid leukaemia were included in this sequential Bayesian phase I dose-finding trial. A starting dose of 0.5 mg m⁻² day⁻¹ was explored with subsequent dose escalations of 1, 3, 5 and 6 mg m⁻² day⁻¹. Myelosuppression was constant. The MTD was estimated as the dose level of 5 mg m⁻² day⁻¹ for 9 consecutive days by s.c. route. Dose-limiting toxicities were hyperglycaemia with hyperosmolar coma at 3 mg m⁻², and (i) one anasarque and haematemesis, (ii) one life-threatening pulmonary aspergillosis, (iii) one skin rash and (iv) one scalp pain at dose level of 5 mg m⁻² day⁻¹. The mean half-life of ssHHT was 11.01±3.4 h, the volume of distribution at steady state was 2±1.4 l kg⁻¹ and the plasma clearance was 11.6±10.4 l h⁻¹. Eleven of the 12 patients with circulating leukaemic cells had blood blast clearance, two achieved complete remission and one with blast crisis of CMML returned in chronic phase. The recommended daily dose of ssHHT on the 9-day schedule is 5 mg m⁻² day⁻¹.

Semisynthetic homoharringtonine induces apoptosis via inhibition of protein synthesis and triggers rapid myeloid cell leukemia-1 down-regulation in myeloid leukemia cells

Semisynthetic homoharringtonine (ssHHT) is now being evaluated in phase II clinical trials for the treatment of chronic myelogenous leukemia and acute myelogenous leukemia patients. Here, we examined the mechanism of the apoptosis induced by ssHHT in myeloid leukemia cells. First, we have shown that ssHHT induces apoptosis in HL60 and HL60/MRP cell lines in a time- and dose-dependent manner, and independently of the expression of Bax. The decrease of mitochondrial membrane potential and the release of cytochrome c were observed in the apoptotic cells induced by ssHHT. To unveil the relationship between ssHHT and the mitochondrial disruption, we have shown that ssHHT decreased myeloid cell leukemia-1 (Mcl-1) expression and induced Bcl-2 cleavage in HL60 and HL60/MRP cell lines. The Bcl-2 cleavage could be inhibited by the Z-VAD.fmk caspase inhibitor. However, Mcl-1 turnover was very rapid and occurred before caspase activation. The Mcl-1 turnover was only induced by ssHHT and cycloheximide, but not by daunorubicin and cytosine arabinoside, and could be restored by proteasome inhibitors. Second, we confirmed that ssHHT rapidly induced massive apoptosis in acute myelogenous leukemia patient cells. We have also confirmed the release of cytochrome c and a rapid turnover of Mcl-1 in these patient cells, taking place only in apoptotic cells induced by ssHHT but not in cells undergoing spontaneous apoptosis. Finally, we have shown that ssHHT inhibits protein synthesis in both cell line and patient cells. We suggest that the inhibition of protein synthesis and resulting Mcl-1 turnover play a key role in the apoptosis induced by ssHHT. Our results encourage further clinical trials for the use of ssHHT in acute myelogenous leukemia.

New Strategies in Chronic Myeloid Leukemia

Most patients with chronic myeloid leukemia (CML) achieve clinically relevant hematologic and cytogenetic responses to imatinib. Patients who show resistance to imatinib need new therapeutic options. A range of options are being developed to treat imatinib-resistant patients who have CML. Promising results of early-phase clinical trials have been reported for new tyrosine kinase inhibitors, farnesyl transferase inhibitors, decitabine, homoharringtonine, and vaccines. Further clinical trials are needed to characterize the efficacy and safety profile of these new agents and to determine which agents improve the long-term prognosis for patients with CML who have shown resistance to imatinib.

NS-187, a potent and selective dual Bcr-Abl/Lyn tyrosine kinase inhibitor, is a novel agent for imatinib-resistant leukemia

Although the Abelson (Abl) tyrosine kinase inhibitor imatinib mesylate has improved the treatment of breakpoint cluster region–Abl (Bcr-Abl)–positive leukemia, resistance is often reported in patients with advanced-stage disease. Although several Src inhibitors are more effective than imatinib and simultaneously inhibit Lyn, whose overexpression is associated with imatinib resistance, these inhibitors are less specific than imatinib. We have identified a specific dual Abl-Lyn inhibitor, NS-187 (elsewhere described as CNS-9), which is 25 to 55 times more potent than imatinib in vitro. NS-187 is also at least 10 times as effective as imatinib in suppressing the growth of Bcr-Abl–bearing tumors and markedly extends the survival of mice bearing such tumors. The inhibitory effect of NS-187 extends to 12 of 13 Bcr-Abl proteins with mutations in their kinase domain but not to T315I. NS-187 also inhibits Lyn without affecting the phosphorylation of Src, Blk, or Yes. These results suggest that NS-187 may be a potentially valuable novel agent to combat imatinib-resistant Philadelphia-positive (Ph+) leukemia.

New Targeted Approaches in Chronic Myeloid Leukemia

The treatment of chronic myeloid leukemia has changed dramatically in the last few years. Stem-cell transplantation and the use of interferon alfa had already offered the possibility of complete and durable cytogenetic responses, improving the survival over that expected with conventional chemotherapy. The introduction of imatinib mesylate has started the era of molecular therapy with remarkable results including complete cytogenetic responses in up to 90% of patients and major molecular responses in most. However, some patients, particularly those treated in the advanced stages, may develop resistance to imatinib. Thus there has been interest in developing new agents that would not only help patients for whom imatinib is ineffective or intolerable, but that could also be combined with the intention of eliminating all evidence of disease. Several approaches are being pursued. These include new and more potent tyrosine kinase inhibitors that may not be affected by the most common mutations seen in the clinic. Some of these agents also inhibit Src-related kinases that may play a role in the development of resistance to imatinib. Other agents are directed at downstream or alternative pathways in leukemic cells, exploring potential synergy with imatinib. Another approach is to pursue an immune modulation that might eliminate small amounts of residual disease. Many of these agents are already showing promising results in the clinic. This manuscript reviews some of these agents, particularly those for which clinical data are already available.

Identification of mTOR as a novel bifunctional target in chronic myeloid leukemia: dissection of growth‐inhibitory and VEGF‐suppressive effects of rapamycin in leukemic cells

The mammalian target of rapamycin (mTOR) has recently been described to be constitutively activated in Bcr-Abl-transformed cells and to mediate rapamycin-induced inhibition of growth in respective cell lines. We have recently shown that rapamycin down-regulates expression of vascular endothelial growth factor (VEGF), a mediator of leukemia-associated angiogenesis, in primary CML cells. In the present study, we analyzed growth-inhibitory in vitro and in vivo effects of rapamycin on primary CML cells and asked whether rapamycin-induced suppression of VEGF in leukemic cells is related to growth inhibition. Rapamycin dose dependently inhibited growth of primary CML cells obtained from patients with imatinib-responsive or imatinib-resistant disease as well as growth of Bcr-Abl-transformed imatinib-resistant cell lines. Moreover, we observed potent cytoreductive effects of rapamycin in a patient with imatinib-resistant Bcr-Abl+ leukemia. The growth-inhibitory effects of rapamycin on CML cells were found to be associated with G1 cell cycle arrest and with induction of apoptosis. In all cell types tested, rapamycin was found to down-regulate expression of VEGF. However, exogenously added VEGF did not counteract the rapamycin-induced decrease in proliferation. In conclusion, rapamycin inhibits growth of CML cells in vitro and in vivo and, in addition, down-regulates expression of VEGF. Both effects may contribute to the antileukemic activity of the drug in CML.

Homoharringtonine: a new treatment option for myeloid leukemia

HHT, one of the alkaloids from a Chinese natural plant, Cephalotaxus, has shown its potential in leukemia treatment. This compound demonstrated strong growth-inhibiting activities in vitro and in animal experiments, and obtained encouraging results in some clonal proliferative disease such as in chronic myeloid leukemia (CML) and in polycythemia vera. Evidences also confirmed HHT as an apoptosis inducer in tumor cell lines and fresh cells from cancer patients. The CR rate reported with HHT-based regimen in acute nonlymphocytic leukemia showed no statistic differences from that with DNR-based regimen, although the case number was limited. While used in clinical trial, the drug often cause noticeably cardiovascular disturbances if be given rapidly by intravenous infusion. Myelosuppression is the common complication in HHT-based chemotherapy. Although with the anti-growth activity in vitro and praisable achievement in acute and chronic myeloid leukemia treatment, the drug shows no beneficial effect in lymphocytic leukemia and solid tumors. The underlying mechanism for the discrepancy of efficacy keeps unknown. This review will present with the preclinical research data including the action mechanism, pharmacokinetics and drug resistance of HHT as well as the result from the clinical trial with HHT in China and the United States.

Macrophage colony-stimulating factor receptor c-fms is a novel target of imatinib

Imatinib is a tyrosine kinase inhibitor that suppresses the growth of bcr-abl-expressing chronic myeloid leukemia (CML) progenitor cells by blockade of the adenosine triphosphate (ATP)-binding site of the kinase domain of bcr-abl. Imatinib also inhibits the c-abl, platelet-derived growth factor (PDGF) receptor, abl-related gene (ARG) and stem-cell factor (SCF) receptor tyrosine kinases, and has been used clinically to inhibit the growth of malignant cells in patients with CML and gastrointestinal stromal tumors (GISTs). Although initially considered to have minimal effects of normal hematopoiesis, recent studies show that imatinib also inhibits the growth of some nonmalignant hematopoietic cells, including monocyte/macrophages. This inhibition could not be attributed to the known activity profile of imatinib. Here, we demonstrate for the first time that imatinib targets the macrophage colony-stimulating factor (M-CSF) receptor c-fms. Phosphorylation of c-fms was inhibited by therapeutic concentrations of imatinib, and this was not due to down-regulation in c-fms expression. Imatinib was also found to inhibit M-CSF-induced proliferation of a cytokine-dependent cell line, further supporting the hypothesis that imatinib affects the growth and development of monocyte and/or macrophages through inhibition of c-fms signaling. Importantly, these results identify an additional biologic target to those already defined for imatinib. Imatinib should now be assessed for activity in diseases where c-fms activation is implicated, including breast and ovarian cancer and inflammatory conditions.

Phase I/II trial of adding semisynthetic homoharringtonine in chronic myeloid leukemia patients who have achieved partial or complete cytogenetic response on imatinib

BACKGROUND

A Phase I/II study was designed to show whether the addition of semisynthetic homoharringtonine (sHHT) would reduce the level of residual disease in patients with Ph-positive chronic myeloid leukemia who appeared to have achieved a suboptimal response to imatinib alone.

METHODS

Patients with CML who had achieved >/= 35% Ph-negativity on imatinib were included. All patients had been treated with imatinib at >/= 400 mg/day for at least 2 years and had achieved a plateau in BCR-ABL transcripts defined by measuring BCR-ABL transcripts on at least 4 occasions over a minimum period of 1 year with the latest value not lower than the previous minimum value. Initially sHHT was given subcutaneously at a dose of 1.25 mg/m(2) twice daily for 1 day. Courses were repeated every 28 days. The dosage of sHHT was escalated by adding one day of treatment every two days. Efficacy was assessed by serial monitoring of blood levels of BCR-ABL transcripts.

RESULTS

Of 10 evaluable patients, 7 had an appreciable decline in BCR-ABL transcript levels; in 5 cases the reduction was greater than 1 log. Asthenia (n = 10) and cytopenias (n = 3) were prominent side-effects, but the drug was generally well tolerated. Mutations in the P-loop of the BCR-ABL kinase domain were found in 2 of the patients who responded to the addition of sHHT.

CONCLUSIONS

The addition of sHHT should be considered for patients on imatinib who fail to obtain low levels of minimal residual disease.

Two different point mutations in ABL gene ATP-binding domain conferring Primary Imatinib resistance in a Chronic Myeloid Leukemia (CML) patient: A case report

Imatinib (Gleevec) is the effective therapy for BCR-ABL positive CML patients. Point mutations have been detected in ATP-binding domain of ABL gene which disturbs the binding of Gleevec to this target leading to resistance. Detection of mutations is helpful in clinical management of imatinib resistance. We established a very sensitive (ASO) PCR to detect mutations in an imatinib-resistant CML patient. Mutations C944T and T1052C were detected which cause complete partial imatinib resistance, respectively. This is the first report of multiple point mutations conferring primary imatinib resistance in same patient at the same time. Understanding the biological reasons of primary imatinib resistance is one of the emerging issues of pharmacogenomics and will be helpful in understanding primary resistance of molecularly-targeted cancer therapies. It will also be of great utilization in clinical management of imatinib resistance. Moreover, this ASO-PCR assay is very effective in detecting mutations related to imatinib resistance.

Investigational strategies in chronic myelogenous leukemia

Imatinib is the cornerstone of therapy in chronic myelogenous leukemia (CML) and a model for the development of novel agents directed at specific targets. The results of imatinib therapy continue to improve with approaches such as higher doses of imatinib and, possibly, with combinations of imatinib and interferon-alpha with or without cytarabine. There are multiple targets with agents directed to them that may prove to be synergistic with imatinib. These approaches are attractive, particularly when dealing with imatinib resistant CML, to prevent resistance and improve the probability of cure. The continued understanding of the biology of CML and mechanisms of resistance to imatinib and the ability to develop target-specific therapies should lead to the increased probability of cure for most patients who have CML.

Accelerated and blastic phases of chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) may have a biphasic or triphasic course, whereby patients who were initially diagnosed in the chronic phase (CP) develop more aggressive disease, frequently pass through an intermediate or accelerated phase (AP), and finally evolve into an acute leukemia like blastic phase (BP). A slowing in the rate of development of AP or BP has accompanied successive improvements in therapy for patients who have CP CML. Variable diagnostic criteria for AP and BP are used in the literature, rendering comparisons difficult. The management of patients in AP or BP consistently has been less effective than the management of those inCP for all modalities of therapy. This article reviews the current diagnostic criteria, therapeutic strategies, outcomes, and investigational therapies for AP and BP CML.