Faculty Bibliography

The role of signal transducers and activators of transcription 5 (STAT5) in chronic myelogenous leukemia (CML) is controversial. To clarify the role of STAT5 signaling in P210(BCR/ABL) leukemogenesis, P210 was introduced into primary murine STAT5A-deficient (STAT5A(-/-)) bone marrow (BM) cells, which, unlike STAT5A/5B double knockout BM cells, have no major intrinsic hematopoietic defects. Interestingly, only 21% of mice reconstituted with P210-transduced STAT5A(-/-) BM cells developed classic CML, compared with 80% to 100% of P210/STAT5A(+/+) and P210/STAT5A(+/-)-reconstituted animals. The remainder of P210/STAT5A(-/-) animals died from an acute B-cell lymphoblastic leukemia (ALL)-like disease (32%) or a CML/ALL mix (47%), reflecting impairment in the induction and maintenance of CML, which normally predominates in this mouse model. Of mice that ultimately developed CML, P210/STAT5A(-/-) animals had prolonged survival and increased myeloid immaturity. Importantly, reconstitution of wild-type mice with BM cells coexpressing P210 and dominant-negative STAT5 also profoundly reduced the incidence of CML, without impairing the induction of ALL. Altogether, these findings indicate that STAT5 and STAT5A play an important role in the pathogenesis of the CML-like disease in mice. A greater understanding of the STAT5 target genes involved in CML induction may lead to new therapeutic targets that influence CML progenitor cell biology.

Ewing sarcoma is the second most common malignant pediatric bone tumor. Over 80% of Ewing sarcoma contain the oncogene EWS/FLI-1, which encodes the EWS/FLI-1 oncoprotein, a hybrid transcription factor comprised of NH2-terminal sequences from the RNA-binding protein EWS and the DNA-binding and COOH-terminal regions of the Ets transcription factor FLI-1. Although numerous genes are dysregulated by EWS/FLI-1, advances in Ewing sarcoma cancer biology have been hindered by the lack of an animal model because of EWS/FLI-1-mediated cytotoxicity. In this study, we have developed conditions for the isolation and propagation of murine primary bone-derived cells (mPBDC) that stably express EWS/FLI-1. Early-passage EWS/FLI-1 mPBDCs were immortalized in culture but inefficient at tumor induction, whereas later-passage cells formed sarcomatous tumors in immunocompetent syngeneic mice. Murine EWS/FLI-1 tumors contained morphologically primitive cells that lacked definitive lineage markers. Molecular characterization of murine EWS/FLI-1 tumors revealed that some but not all had acquired a novel, clonal in-frame p53 mutation associated with a constitutive loss of p21 expression. Despite indications that secondary events facilitated EWS/FLI-1 mPBDC tumorigenesis, cells remained highly dependent on EWS/FLI-1 for efficient transformation in clonogenic assays. This Ewing sarcoma animal model will be a useful tool for dissecting the molecular pathogenesis of Ewing sarcoma and provides rationale for the broader use of organ-specific progenitor cell populations for the study of human sarcoma.

Imatinib mesylate is highly effective in newly diagnosed chronic myeloid leukemia (CML), but BCR/ABL (breakpoint cluster region/abelson murine leukemia)-positive progenitors persist in most patients with CML treated with imatinib mesylate, indicating the need for novel therapeutic approaches. In this study, we have used the murine CML-like myeloproliferative disorder as a platform to characterize the pharmacokinetic, signal transduction, and antileukemic properties of PD166326, one of the most potent members of the pyridopyrimidine class of protein tyrosine kinase inhibitors. In mice with the CML-like disease, PD166326 rapidly inhibited Bcr/Abl kinase activity after a single oral dose and demonstrated marked antileukemic activity in vivo. Seventy percent of PD166326-treated mice achieved a white blood cell (WBC) count less than 20.0 x 10(9)/L (20,000/microL) at necropsy, compared with only 8% of imatinib mesylate-treated animals. Further, two thirds of PD166326-treated animals had complete resolution of splenomegaly, compared with none of the imatinib mesylate-treated animals. Consistent with its more potent antileukemic effect in vivo, PD166326 was also superior to imatinib mesylate in inhibiting the constitutive tyrosine phosphorylation of numerous leukemia-cell proteins, including the src family member Lyn. PD166326 also prolonged the survival of mice with imatinib mesylate-resistant CML induced by the Bcr/Abl mutants P210/H396P and P210/M351T. Altogether, these findings demonstrate the potential of more potent Bcr/Abl inhibitors to provide more effective antileukemic activity. Clinical development of PD166326 or a related analog may lead to more effective drugs for the treatment of de novo and imatinib mesylate-resistant CML.

Despite recent improvements in the treatment of early-stage disease, the blastic phase of chronic myeloid leukemia (CML) remains a therapeutic challenge. For imatinib-naïve patients, imatinib provided encouraging hematologic and cytogenetic benefits; however, the vast majority of CML blast crisis cases today arise in patients already on imatinib-based therapy. Clonal evolution and duplication of the Philadelphia chromosome continue to be associated with blastic phase transformation, but recent studies have identified BCR/ABL kinase domain mutations in 30%-40% of blast crisis patients. This implies that BCR-ABL-targeted therapy might have influenced the molecular road map to blastic transformation. In this review, we will examine the effect of imatinib on primitive CML progenitors and how this might influence the pathophysiology of blast crisis. A rational framework for deciding how best to integrate stem cell transplantation, traditional chemotherapy, imatinib, and other BCR-ABL kinase inhibitors in the care of blast crisis patients will also be discussed.

Of the current mouse chronic myelogenous leukemia (CML) models,the murine bone marrow (BM) transduction and transplantation model most efficiently mimics many of the central features of human CML. In this model, lethally irradiated mice are reconstituted with primary murine BM cells transduced with a P210BCR/ABL retrovirus. All recipient mice develop a fatal peripheral blood and BM granulocytosis and splenomegaly, a disease termed the murine CML-like myeloproliferative disorder. This model has been used to establish the causative role of Bcr/Abl in CML, identify those signaling pathways and regions of Bcr/Abl critical for leukemogenesis, and explore the limitations of targeted CML therapy. Future refinements in this CML mouse model will make it a more effective tool for studying imatinib-resistant CML, reproducing chronic- and blastic-phase human CML, and performing CML progenitor studies.

PURPOSE/OBJECTIVE:Despite recent advances in cancer therapy, long-term survival in small cell lung cancer (SCLC) remains uncommon, underscoring the need for novel therapeutic approaches. Previous studies have identified constitutive expression of the receptor tyrosine kinase, c-Kit, and its ligand, stem cell factor, in a substantial proportion of SCLC specimens. The purpose of this study was to determine whether imatinib mesylate, an inhibitor of c-Kit, could achieve therapeutic concentrations in tumors and in brain (a frequent site of SCLC metastasis) and interfere with SCLC tumor growth in vivo. EXPERIMENTAL DESIGN/METHODS:Human SCLC tumor cell lines with constitutive c-kit expression and tyrosine phosphorylation (NCI-H209, NCI-H526, and NCI-H1607) were used to establish SCLC tumor xenografts in NCr nude (nu/nu)-immunodeficient mice. SCLC tumor-bearing mice were randomly assigned to imatinib or control (water) administered twice a day by oral gavage. Imatinib concentrations in plasma, brain, and tumor were quantitated and correlated with tumor response. RESULTS:Therapeutic concentrations of imatinib were achieved in plasma and tumor xenografts but not in the brain. Imatinib blocked the constitutive activation of c-kit in SCLC tumor cell lines in vitro but had a negligible effect on SCLC xenograft growth in vivo. CONCLUSIONS:Orally administered imatinib rapidly reaches therapeutic concentrations in SCLC xenografts, suggesting the feasibility of combining imatinib with other novel or traditional chemotherapeutic agents in SCLC or other solid tumors. The c-Kit signaling pathway does not appear to play a critical role in SCLC proliferation and viability in vivo, however, suggesting that imatinib is unlikely to be effective as monotherapy for SCLC.

The chronic myelogenous leukemia (CML)-like myeloproliferative disorder observed in the BCR/ABL murine bone marrow transduction and transplantation model shares several features with the human disease, including a high response rate to the tyrosine kinase inhibitor imatinib mesylate (STI571). To study the impact of chronic imatinib mesylate treatment on the CML-like illness, mice were maintained on therapeutic doses of this drug and serially monitored. Unexpectedly, despite excellent systemic control of the CML-like illness, many of the mice developed progressive neurologic deficits after 2 to 4 months of imatinib mesylate therapy because of central nervous system (CNS) leukemia. Analysis of imatinib mesylate cerebral spinal fluid concentrations revealed levels 155- fold lower than in plasma. Thus, in the mouse, the limited ability of imatinib mesylate to cross the blood-brain barrier allowed the CNS to become a sanctuary for Bcr/Abl-induced leukemia. This model will be a useful tool for the future study of novel anti-CML drugs and in better defining the mechanisms for limited imatinib mesylate penetration into the CNS.

BACKGROUND:Although chimeric EWS gene and Ets gene fusions are pathognomonic of Ewing sarcoma (ES) and primitive neuroectodermal tumors (PNET), the molecular pathogenesis of these pediatric malignancies is poorly understood. Recently, the human epidermal growth factor (HER)-2 receptor, which plays an important role in the biology of certain epithelial cancers, has been implicated in ES tumor cell line growth and chemosensitivity. MATERIALS/METHODS:To investigate whether HER2 might be a rational target for ES/PNET therapy, five ES cell lines and 13 archival primary ES/PNET samples were examined by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) for evidence of HER2 overexpression. RESULTS:Although several ES cell lines demonstrated modest constitutive HER2 expression by immunoblot, none of the ES cell lines or primary tumor samples showed evidence of HER2 overexpression by IHC or HER2 gene amplification by FISH. Moreover, treatment of human ES cell lines with the HER2-targeted agent trastuzumab (Herceptin) had little effect on cell survival, proliferation, or growth in semi-solid medium. CONCLUSIONS:These results suggest that HER2 is not a biologically or therapeutically important pathway in ES/PNET.