Novel tyrosine kinase inhibitors in chronic myelogenous leukemia

Leukemia Stem Cells in Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder characterized by a chromosome translocation that generates the BCR-ABL oncogene encoding a constitutively activated tyrosine kinase. Although BCR-ABL tyrosine kinase inhibitors (TKIs) are highly effective in treating CML at chronic phase, a number of patients develop drug resistance due to the inability of TKIs to kill leukemia stem cells (LSCs). Similar to other types of hematopoietic malignancies, LSCs in CML are believed to be a rare cell population responsible for leukemia initiation, disease progression, and drug resistance. Therefore, a full understanding of the biology of LSCs will help to develop novel therapeutic strategies for effective treatment of CML to possibly reach a cure. In recent years, a significant progress has been made in studying the biology of LSCs in both animal models and human patients at cellular and molecular levels, providing a basis for designing and testing potential molecular targets for eradicating LSCs in CML.

Optic neuropathy secondary to dasatinib in the treatment of a chronic myeloid leukemia case

The drug dasatinib is a new therapeutic option for patients with chronic myeloid leukemia (CML) as well as acute lymphocytic lymphoblastic leukemia (ALL). However, the scientific literature has not reached a consensus regarding the types of secondary ophthalmologic effects that this drug may have. In this study, we present the case of a 36-year-old male patient who was treated with dasatinib. Two and a half months later, this patient began to experience progressive visual loss in the superior visual field of both eyes.

After ruling out various diagnostic options and performing extensive complementary tests, the suspected diagnosis was compatible with optic neuropathy secondary to dasatinib. The patient partially improved after stopping this medication and receiving oral corticosteroid treatment.

Although secondary ophthalmological effects related to dasatinib are practically non-existent, our case is the first to report optic neuropathy secondary to this drug.

SB365, Pulsatilla saponin D, targets c-Met and exerts antiangiogenic and antitumor activities

SB365, Pulsatilla saponin D isolated from the root of Pulsatilla koreana, has exhibited potential beneficial effects as a chemopreventive agent for critical health conditions including cancer. However, the molecular mechanisms underlying the activity of SB365 remain unknown. Here, we examined anticancer efficacy of SB365 against gastric cancer and its mechanism of action. SB365 effectively inhibited the growth of gastric cancer cells. Its apoptotic effect was accompanied by increased evidence of cleaved caspase-3 and poly(ADP ribose) polymerase. To elucidate the anticancer mechanism of SB365, we used an array of 42 different receptor tyrosine kinases (RTKs). Of the 42 different phospho-RTKs, SB365 strongly inhibited expression of activated c-mesenchymal-epithelial transition factor (c-Met) in gastric cancer cells. Also, the activation of the c-Met signal cascade components, including Akt and mammalian target of rapamycin, was inhibited by SB365 in a dose-dependent manner. In angiogenesis studies, SB365 inhibited tube formation in hepatocyte growth factor (HGF)-induced human umbilical vein endothelial cells and suppressed microvessel sprouting from the rat aortic ring, ex vivo, and blood vessel formation in the Matrigel plug assay in mice. In xenograft animal models, SB365 significantly delayed tumor growth in a dose-dependent manner. In tumor tissue, SB365 suppressed c-Met signaling, proliferation and angiogenesis and induced apoptosis. These findings suggest that SB365 docks at an allosteric site on c-Met and thereby targets c-Met signaling pathway, cell growth/angiogenesis inhibition and apoptosis induction. Therefore, SB365 may be a novel drug candidate for the treatment of gastric cancer.

Allosteric interactions between the myristate- and ATP-site of the Abl kinase

Abl kinase inhibitors targeting the ATP binding pocket are currently employed as potent anti-leukemogenic agents but drug resistance has become a significant clinical limitation. Recently, a compound that binds to the myristate pocket of Abl (GNF-5) was shown to act cooperatively with nilotinib, an ATP-competitive inhibitor to target the recalcitrant “T315I” gatekeeper mutant of Bcr-Abl. To uncover an explanation for how drug binding at a distance from the kinase active site could lead to inhibition and how inhibitors could combine their effects, hydrogen exchange mass spectrometry (HX MS) was employed to monitor conformational effects in the presence of both dasatinib, a clinically approved ATP-site inhibitor, and GNF-5. While dasatinib binding to wild type Abl clearly influenced Abl conformation, no binding was detected between dasatinib and T315I. GNF-5, however, elicited the same conformational changes in both wild type and T315I, including changes to dynamics within the ATP site located approximately 25 Å from the site of GNF-5 interaction. Simultaneous binding of dasatinib and GNF-5 to T315I caused conformational and/or dynamics changes in Abl such that effects of dasatinib on T315I were the same as when it bound to wild type Abl. These results provide strong biophysical evidence that allosteric interactions play a role in Abl kinase downregulation and that targeting sites outside the ATP binding site can provide an important pharmacological tool to overcome mutations that cause resistance to ATP-competitive inhibitors.

Conformational disturbance in Abl kinase upon mutation and deregulation

Protein dynamics are inextricably linked to protein function but there are few techniques that allow protein dynamics to be conveniently interrogated. For example, mutations and translocations give rise to aberrant proteins such as Bcr-Abl where changes in protein conformation and dynamics are believed to result in deregulated kinase activity that provides the oncogenic signal in chronic myelogeous leukemia. Although crystal structures of the down-regulated c-Abl kinase core have been reported, the conformational impact of mutations that render Abl resistant to small-molecule kinase inhibitors are largely unknown as is the allosteric interplay of the various regulatory elements of the protein. Hydrogen exchange mass spectrometry (HX MS) was used to compare the conformations of wild-type Abl with a nonmyristoylated form and with 3 clinically relevant imatinib resistance mutants (T315I, Y253H and E255V). A HX-resistant core localized to the interface between the SH2 and kinase domains, a region known to be important for maintaining the down-regulated state. Conformational differences upon demyristoylation were consistent with the SH2 domain moving to the top of the small lobe of the kinase domain as a function of activation. There were conformational changes in the T315I mutant but, surprisingly, no major changes in conformation were detected in either the Y253H or the E255V mutants. Taken together, these results provide evidence that allosteric interactions and conformational changes play a major role in Abl kinase regulation in solution. Similar analyses could be performed on any protein to provide mechanistic details about conformational changes and protein function.

Phosphorylation regulates transcriptional activity of PAX3/FKHR and reveals novel therapeutic possibilities

Inhibition of constitutive active signaling pathways, which are a characteristic phenomenon for many tumors, can be an effective therapeutic strategy. In contrast, oncogenic transcription factors, often activated by mutational events, are in general less amenable to small-molecule inhibition despite their obvious importance as therapeutic targets. One example of this is alveolar rhabdomyosarcoma (aRMS), in which specific translocations lead to the formation of the chimeric transcription factor PAX3/FKHR. Here, we found unexpectedly that the transcriptional activity of PAX3/FKHR can be inhibited by the kinase inhibitor PKC412. This occurs via specific phosphorylation sites in the PAX3 domain, phosphorylation of which is required for efficient DNA-binding and subsequent transcriptional activity. Consequently, we show that PKC412 exerts a potent antitumorigenic potential for aRMS treatment both in vitro and in vivo. Our study suggests that posttranscriptional modifications of oncogenic transcription factors can be explored as a promising avenue for targeted cancer therapy.

Active FKHRL1 overcomes imatinib resistance in chronic myelogenous leukemia-derived cell lines via the production of tumor necrosis factor-related apoptosis-inducing ligand

FKHRL1 (also called FOXO3a) is a member of the Forkhead Box, class O (FOXO) subfamily of forkhead transcription factors and functions downstream of Bcr-Abl tyrosine kinase as a phosphorylated inactive form in chronic myelogenous leukemia (CML). The Bcr-Abl tyrosine kinase inhibitor imatinib induces cell cycle arrest and subsequent apoptosis via the conversion of FKHRL1 from the phosphorylated inactive form to the dephosphorylated active form in CML-derived cell lines. In the present study, we examined whether active FKHRL1 can overcome resistance to imatinib. To this end, we generated a 4-hydroxytamoxifen-inducible active FKHRL1 (FKHRL1-TM; a triple mutant of FKHRL1 in which all three Akt phosphorylation sites have been mutated)-estrogen receptor fusion protein expression system in CML-derived imatinib-resistant cell lines. 4-Hydroxytamoxifen inhibited cell growth and cell cycle progression, and subsequently induced apoptosis, accompanied by upregulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Thus, active FKHRL1 antagonized deregulated proliferation and induced apoptosis in these cell lines. In addition, imatinib-resistant cells underwent apoptosis after transfection with full-length TRAIL cDNA. Collectively, our results suggest that active FKHRL1 can overcome imatinib resistance in CML cells, in part via TRAIL production.

Platelet-derived growth factor receptors and ligands are up-regulated in paediatric fibromatoses

AIMS

Platelet-derived growth factors (PDGF) and their receptors (PDGFR) play an essential role in pathways involved in the regulation of cell proliferation, growth and function. Overexpression of PDGF/R is reported in a wide range of solid tumours. The aim was to determine levels of PDGF/R expression in paediatric fibromatoses and myofibromatosis.

METHODS AND RESULTS

Quantitative real-time polymerase chain reaction was used to examine the expression level of alpha and beta isoforms of PDGF/R in 17 fibromatoses, four myofibromatoses and three dermatofibrosarcoma protuberans (DFSP) in children. Fifteen of 17 (88%) fibromatoses and all myofibromatoses and DFSPs demonstrated increased expression of PDGFalpha and beta compared with a panel of normal tissues. In terms of the cognate receptors, 13/17 (76%) fibromatoses demonstrated increased expression for PDGFRalpha and Rbeta, whereas 3/4 myofibromatoses demonstrated increased expression of PDGFRalpha and all four had increased PDGFRbeta expression. All DFSPs were associated with increased expression of both PDGFRalpha and PDGFRbeta compared with normal control tissues.

CONCLUSIONS

Increased expression of PDGF/Ralphabeta may play an important role in the mechanism of growth of these paediatric fibromatous lesions and warrants further investigation, since novel therapeutic interventions could potentially be developed in the light of the expression patterns.

Quantitative assessment of the BCR-ABL transcript using the Cepheid Xpert BCR-ABL Monitor assay

CONTEXT

Chronic myelogenous leukemia (CML) and the assessment of the BCR-ABL transcript has become a new paradigm. Novel tyrosine kinase inhibitors as mainstream therapeutic options for the CML patient warrant routine quantification of the BCR-ABL transcript. The Xpert BCR-ABL Monitor assay is a nested reverse transcriptase polymerase chain reaction that greatly reduces technical time by using a single cartridge to isolate RNA and run a quantitative reverse transcriptase polymerase chain reaction.

OBJECTIVE

To evaluate the Xpert BCR-ABL Monitor assay for quantitative assessment of the BCR-ABL transcript in CML patients.

DESIGN

A standard curve of K-562 cells diluted in normal peripheral blood was used to test the sensitivity, linearity, and percent coefficient of variation of the assay. Specimen stability was tested by running standard curves immediately and after 24 hours or 96 hours of storage at 4 degrees C. Specimens from normal controls, patients known to have CML, or patients suspected of having CML were also tested.

RESULTS

The sensitivity of the assay was sufficient to detect 1 K-562 cell in 10(5) normal cells. The R2 of the standard curve was 0.98 and the percent coefficient of variation for each data point was 15% to 24%. Eleven of 14 patients with known CML on imatinib treatment tested positive for the BCR-ABL transcript, whereas 10 normal controls tested negative.

CONCLUSIONS

The Xpert BCR-ABL Monitor assay is a rapid, sensitive method for monitoring the presence of the BCR-ABL transcript in CML patients. The single-use cartridge minimizes hands-on technical time, minimizes the potential for contamination, and allows quantitative BCR-ABL testing to be performed in a random access fashion.

Emerging therapeutic options for Philadelphia-positive acute lymphocytic leukemia

Acute lymphocytic leukemia (ALL) is a heterogeneous group of disorders that are associated with a cure rate of > 80% in children. The prognosis in adults is considerably inferior, with age, disease bulk, leukemia karyotype and immune phenotype being prognostically relevant. Adult ALL treatment programs include induction, intensified consolidation and maintenance phases with CNS prophylaxis. The addition of imatinib in patients with BCR-ABL-positive ALL has improved the prognosis of this subgroup, but their survival is still poor. Initial data on the second-generation BCR-ABL inhibitors, dasatinib and nilotinib, indicate a potentially greater efficacy than imatinib, but the improvement is likely to be modest. The overall efforts in terms of developmental therapeutics in ALL are very modest and not in keeping with the urgent need for improvement. Most agents being investigated have mechanisms of action similar to those of existing agents for ALL therapy and thus represent modest opportunities to improve results. Of such agents, data on BCR-ABL inhibitors, sphingosomal vincristine, pemetrexed, talotrexin, annamycin and ABT-751 are reviewed.

Umbilical cord blood transplantation for myeloid malignancies

PURPOSE OF REVIEW

To review the available data on the outcomes of pediatric and adult patients with acute myeloid leukemia, myelodysplastic syndrome and chronic myelogenous leukemia after umbilical cord blood transplantation.

RECENT FINDINGS

The literature shows that after umbilical cord blood transplantation the relapse rate, disease-free survival and overall survival of patients with myeloid malignancies is similar to other hematopoietic stem cells sources. Disease status at the time of transplantation is found in several studies to be a very important determinant of long-term outcome. Newer strategies such as double umbilical cord blood transplant and utilization of nonmyeloblative conditioning regimens show promising results.

SUMMARY

Umbilical cord blood is a valuable alternative source of hematopoietic stem cells for transplantation of patients with myeloid malignancies who need an allogeneic transplant, but lack a suitable sibling donor.

Management of Patients with Newly Diagnosed Chronic Myeloid Leukemia: Opportunities and Challenges

Chronic myelogenous leukemia (CML) is a progressive and often fatal hematopoietic neoplasm characterized by the presence of the Philadelphia chromosome. This arises from a balanced translocation between chromosomes 9 and 22, creating the bcr-abl fusion gene. It is often stated that the only proven curative option is allogeneic stem cell transplantation, which is indicated for only a limited subset of patients. The Bcr-Abl tyrosine kinase inhibitor imatinib represented a major advance over conventional CML therapy. After imatinib treatment, > 90% of patients had a complete hematologic response, and 70%-80% had a complete cytogenetic response. With 5 years of follow-up, the data are very encouraging and exhibit a major change in the natural history of the disease. The understanding of some of the mechanisms of resistance to imatinib has led to a rapid development of new agents that might overcome this resistance. The outlook today for patients with CML is much brighter than that of a few years ago.

The Abl SH2-kinase linker naturally adopts a conformation competent for SH3 domain binding

The core of the Abelson tyrosine kinase (c-Abl) is structurally similar to Src-family kinases where SH3 and SH2 domains pack against the backside of the kinase domain in the down-regulated conformation. Both kinase families depend upon intramolecular association of SH3 with the linker joining the SH2 and kinase domains for suppression of kinase activity. Hydrogen deuterium exchange (HX) and mass spectrometry (MS) were used to probe intramolecular interaction of the c-Abl SH3 domain with the linker in recombinant constructs lacking the kinase domain. Under physiological conditions, the c-Abl SH3 domain undergoes partial unfolding, which is stabilized by ligand binding, providing a unique assay for SH3:linker interaction in solution. Using this approach, we observed dynamic association of the SH3 domain with the linker in the absence of the kinase domain. Truncation of the linker before W254 completely prevented cis-interaction with SH3, while constructs containing amino acids past this point showed SH3:linker interactions. The observation that the Abl linker sequence exhibits SH3-binding activity in the absence of the kinase domain is unique to Abl and was not observed with Src-family kinases. These results suggest that SH3:linker interactions may have a more prominent role in Abl regulation than in Src kinases, where the down-regulated conformation is further stabilized by a second intramolecular interaction between the C-terminal tail and the SH2 domain.

New insights into the pathobiology and treatment of chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is one of the most frequently diagnosed forms of leukemia. Recent advances in the understanding of the molecular mechanisms involved in the pathogenesis of this disorder led to new diagnostic and therapeutic approaches. From a disease that used to be diagnosed purely on morphologic grounds, CML is now an entity that cannot be diagnosed in the absence of relevant cytogenetic or molecular data. Identification of the characteristic t(9;22) cytogenetic abnormality or of the BCR-ABL molecular abnormality is mandatory, as current treatment approaches rely on the use of agents active against this specific molecular target. This article presents the evolution in the understanding of the pathogenic mechanisms involved in CML, as well as the methods and the criteria currently in use in its diagnosis and treatment.