Mechanism of resistance to the ABL tyrosine kinase inhibitor STI571 in BCR/ABL-transformed hematopoietic cell lines

The tyrosine kinase activity of the Bcr/Abl oncogene is required for transformation of hematopoietic cells. The tyrosine kinase inhibitor STI571 (formerly called CGP57148B, Novartis Pharmaceuticals) inhibits BCR/ABL, TEL/ABL, and v-ABL kinase activity and inhibits growth and viability of cells transformed by any of these ABL oncogenes. Here we report the generation of 2 BCR/ABL-positive cell lines that have developed partial resistance to STI571. BCR/ABL-transformed Ba/F3 hematopoietic cells and Philadelphia-positive human K562 cells were cultured in gradually increasing concentrations of STI571 over a period of several months to generate resistant lines. Resistant Ba/F3.p210 cells were found to have an increase in Bcr/Abl messenger RNA, amplification of the Bcr/Abl transgene, and a greater than tenfold increase in the level of BCR/ABL protein. In contrast to Ba/F3.p210 cells, drug-resistant K562 cells did not undergo detectable amplification of the BCR/ABL gene, although they displayed a 2-fold to 3-fold increase in p210BCR/ABL protein. The addition of STI571 to both resistant Ba/F3. p210 and K562 cells resulted in a rapid reduction of tyrosine phosphorylation of cellular proteins, similar to that observed for nonresistant cells. However, the inhibition of kinase activity was transient and partial and was not accompanied by apoptosis. The results suggest that resistance to STI571 may be multifactorial. Increased expression of the target protein BCR/ABL was observed in both lines, and resulted from oncogene amplification in one line. However, altered drug metabolism, transport, or other related mechanisms may also contribute to drug resistance.     

Ph(+) acute lymphoblastic leukemia resistant to the tyrosine kinase inhibitor STI571 has a unique BCR-ABL gene mutation

The tyrosine kinase inhibitor STI571 is a promising agent for the treatment of advanced Philadelphia chromosome positive (Ph(+)) acute lymphoblastic leukemia (ALL), but resistance develops rapidly in most patients after an initial response. To identify mechanisms of resistance to STI571, 30 complementary DNAs (including 9 matched samples) obtained from the bone marrow of individuals with Ph(+) ALL were analyzed by direct sequencing of a 714-base pair region of ABL encoding for the adenosine triphosphate (ATP)-binding site and the kinase activation loop. A single point mutation was found at nucleotide 1127 (GI6382056) resulting in Glu255Lys. This mutation occurred in 6 of 9 patients (67%) following their treatment with STI571 but not in the samples from patients before beginning treatment with STI571. Glu255Lys is within the motif important for forming the pocket of the ATP-binding site in ABL and it is highly conserved across species. In conclusion, Ph(+) ALL samples resistant to STI571 have a unique mutation Glu255Lys of BCR-ABL.     

Inhibition of c-kit receptor tyrosine kinase activity by STI 571, a selective tyrosine kinase inhibitor

STI 571 (formerly known as CGP 57148B) is a known inhibitor of the c-abl, bcr-abl, and platelet-derived growth-factor receptor (PDGFR) tyrosine kinases. This compound is being evaluated in clinical trials for the treatment of chronic myelogenous leukemia. We sought to extend the activity profile of STI 571 by testing its ability to inhibit the tyrosine kinase activity of c-kit, a receptor structurally similar to PDGFR. We treated a c-kit expressing a human myeloid leukemia cell line, M-07e, with STI 571 before stimulation with Steel factor (SLF). STI 571 inhibited c-kit autophosphorylation, activation of mitogen-activated protein (MAP) kinase, and activation of Akt without altering total protein levels of c-kit, MAP kinase, or Akt. The concentration that produced 50% inhibition for these effects was approximately 100 nmol/L. STI 571 also significantly decreased SLF-dependent growth of M-07e cells in a dose-dependent manner and blocked the antiapoptotic activity of SLF. In contrast, the compound had no effect on MAP kinase activation or cellular proliferation in response to granulocyte-macrophage colony-stimulating factor. We also tested the activity of STI 571 in a human mast cell leukemia cell line (HMC-1), which has an activated mutant form of c-kit. STI 571 had a more potent inhibitory effect on the kinase activity of this mutant receptor than it did on ligand-dependent activation of the wild-type receptor. These findings show that STI 571 selectively inhibits c-kit tyrosine kinase activity and downstream activation of target proteins involved in cellular proliferation and survival. This compound may be useful in treating cancers associated with increased c-kit kinase activity.     

BCR-ABL independence and LYN kinase overexpression in chronic myelogenous leukemia cells selected for resistance to STI571

Clinical studies have shown that the tyrosine kinase inhibitor STI571 effectively controls BCR-ABL-positive chronic myelogenous leukemia (CML). However, disease progression while on STI571 therapy has been reported, suggesting de novo or intrinsic resistance to BCR-ABL-targeted therapy. To investigate possible mediators of acquired STI571 resistance, K562 cells resistant to 5 microM STI571 (K562-R) were cloned and compared to the parental cell population. K562-R cells had reduced BCR-ABL expression and limited activation of BCR-ABL signaling cascades (Stat 5, CrkL, MAPK). STI571 failed to activate caspase cascades or to suppress expression of survival genes (bcl-xL) in resistant cells. Gene sequencing and tyrosine kinase activity measurements demonstrated that K562-R cells retained wild-type and active BCR-ABL tyrosine kinase that was inhibitable by in vitro incubation with STI571, suggesting that BCR-ABL was not coupled to proliferation or survival of K562-R cells. The src-related kinase LYN was highly overexpressed and activated in K562-R cells, and its inhibition reduced proliferation and survival of K562-R cells while having limited effects of K562 cells. Specimens taken from patients with advanced CML that progressed on STI571 therapy also were analyzed for LYN kinase expression, and they were found to be elevated to a level similar to that of K562-R cells. Comparison of samples from patients taken prior to and following STI571 failure suggested that expression and/or activation of LYN/HCK occurs during disease progression. Together, these results suggest that acquired STI571 resistance may be associated with BCR-ABL independence and mediated in part through overexpression of other tyrosine kinases.     

Overcoming STI571 resistance with the farnesyl transferase inhibitor SCH66336

The development of chronic myeloid leukemia (CML) is dependent on the deregulated tyrosine kinase of the oncoprotein BCR-ABL. STI571 (imatinib mesylate), an abl tyrosine kinase inhibitor, has proven remarkably effective for the treatment of CML. However, resistance to STI571 because of enhanced expression or mutation of the BCR-ABL gene has been detected in patients. In the current study we show that the farnesyl transferase inhibitor (FTI) SCH66336 (lonafarnib) inhibits the proliferation of STI571-resistant BCR-ABL-positive cell lines and hematopoietic colony formation from peripheral blood samples of STI571-resistant patients with CML. Moreover, SCH66336 enhances STI571-induced apoptosis in STI571-sensitive cells and, in patients with STI571 resistance from gene amplification, cooperates with STI571 to induce apoptosis. Our data provide a rationale for combination clinical trials of STI571 and SCH66336 in CML patients and suggest that combination therapy may be effective in patients with STI571 resistance.     

Molecular mechanisms of resistance to STI571 in chronic myeloid leukemia

Therapeutic use of the recently FDA-approved drug STI571 has been successful in the treatment of Philadelphia chromosome-positive leukemias. STI571 is a small molecule inhibitor with activity against BCR-ABL, the deregulated tyrosine kinase responsible for initiation and maintenance of the disease in the chronic phase of chronic myeloid leukemia (CML). Clinical trials demonstrated the ability of STI571 to induce remissions in patients with chronic phase CML with only rare relapses after 18 months of follow-up. However, in patients with more advanced stages of disease, responses to STI571 were less common and often transient. Studies investigating the molecular mechanisms of resistance to this novel compound have progressed rapidly and point to the continued importance of BCR-ABL in disease maintenance even at its latest stages. Here the authors review recent work aimed at elucidating the nature of STI51 resistance.     

Effects of tyrosine kinase inhibitor STI571 on human mast cells bearing wild-type or mutated c-kit

OBJECTIVE: STI571 is a tyrosine kinase inhibitor which inhibits the kinase activity of kit, the receptor for stem cell factor (SCF). Because activating mutations of c-kit affecting codon 816 are associated with human mast cell neoplasms, we determined whether STI571 exerted a similar cytotoxic effect on neoplastic and normal human mast cells. METHODS: We investigated the effect of addition of STI571 in increasing concentrations (0.01 to 10 micromolar) to two HMC-1 human mast cell leukemia cell lines carrying two different activating c-kit mutations in codons 816 or 560, as well as the effect of the drug on short-term bone marrow cultures obtained from patients who carry a mutated codon 816 or wild-type c-kit. RESULTS: STI571 failed to inhibit the growth of HMC-1(560,816) cells bearing a codon 816 mutation but effectively suppressed the proliferation of HMC-1(560) carrying c-kit with the wild-type codon 816. STI571 did not induce preferential killing of neoplastic bone marrow mast cells in short-term cultures from patients bearing a codon 816 c-kit mutation. In contrast, STI571 caused a dramatic reduction in mast cells in patients without codon 816 c-kit mutations. CONCLUSION: These results suggest that STI571, while effectively killing mast cells with wild-type c-kit, did not show preferential cytotoxicity to neoplastic human mast cells and thus may not be effective in the treatment of human systemic mastocytosis associated with codon 816 c-kit mutations.     

In vitro cytotoxic effects of a tyrosine kinase inhibitor STI571 in combination with commonly used antileukemic agents

The BCR/ABL tyrosine kinase has been implicated in the pathogenesis of chronic myelogenous leukemia (CML) and Philadelphia chromosome-positive (Ph(+)) acute lymphoblastic leukemia (ALL). STI571 is a novel anticancer agent that selectively inhibits the BCR/ABL tyrosine kinase. The cytotoxic effects of STI571 were studied in combination with antileukemic agents against Ph(+) leukemia cell lines, KU812, K-562, TCC-S, and TCC-Y. The cells were exposed to STI571 and to other agents simultaneously for 5 or 7 days. Cell growth inhibition was determined by MTT assay. The cytotoxic effects in combinations at the inhibitory concentration of 80% level were evaluated by the isobologram. STI571 produced synergistic effects with recombinant and natural alpha-interferons in 2 of 3 and 3 of 3 cell lines, respectively. STI571 produced additive effects with hydroxyurea, cytarabine, homoharringtonine, doxorubicin, and etoposide in all 4 cell lines. STI571 with 4-hydroperoxy-cyclophosphamide, methotrexate, or vincristine produced additive, antagonistic, and synergistic effects in 3 of 4 cell lines, respectively. These findings suggest that the simultaneous administration of STI571 with other agents except methotrexate would be advantageous for cytotoxic effects against Ph(+) leukemias. Among them, the simultaneous administration of STI571 and alpha-interferons or vincristine would be highly effective against Ph(+) leukemias and these combinations would be worthy of clinical trials. In contrast, the simultaneous administration of STI571 with methotrexate would have little therapeutic efficacy. Although there are gaps between in vitro studies and clinical trials, the present findings provide useful information for the establishment of clinical protocols involving STI571. (Blood. 2001;97:1999-2007)     

The selective tyrosine kinase inhibitor,STI571, inhibits growth of anaplastic thyroid cancer cells

To establish a molecular targeting therapy for anaplastic thyroid carcinomas, we studied the effect of the specific tyrosine kinase inhibitor, STI571, on anaplastic thyroid cancer cell lines highly expressing c-ABL ARO (mutated p53) and FRO (undetectable p53). These lines showed marked inhibition of cell growth after treatment with STI571. In contrast, the growth of papillary thyroid cancer cell lines that harbor wild-type p53 and have low levels of c-ABL was not affected by STI571. Fluorescent-activated cell sorting analysis revealed that STI571 treatment increased the fraction of FRO and ARO cells in S and G(2)/M phases, respectively, indicating induction of S and G(2)/M transition arrest. These changes were accompanied by inhibition of c-ABL phosphorylation/activation and increased expression of p21(cip1) in FRO and p27(kip1) in both FRO and ARO cells. Treatment with STI571 also led to reduction of cyclin A, B1, and CDC2 levels. The growth of FRO cells implanted into immunocompromised mice was significantly inhibited by STI571. Taken together, these results suggest that selective suppression of c-ABL activity by STI571 may represent a potential anticancer strategy for p53-mutated undifferentiated thyroid carcinomas.     

Establishment of a murine model for therapy-treated chronic myelogenous leukemia using the tyrosine kinase inhibitor STI571.

The murine bone marrow retroviral transduction and transplantation model of chronic myelogenous leukemia (CML) imperfectly mimics human CML because the murine CML-like disease causes death of all animals from an overwhelming granulocytosis within 3 to 4 weeks. In this report, mice reconstituted with P210(BCR/ABL)-transduced bone marrow cells received posttransplantation therapy with either the tyrosine kinase inhibitor STI571 or placebo. Compared with the rapidly fatal leukemia of placebo-treated animals, 80% of the STI571-treated mice were alive on day 74, with marked improvement in peripheral white blood counts and splenomegaly. There was decreased tyrosine phosphorylation of STAT5, Shc, and Crk-L in leukemic cells from STI571-treated animals, consistent with STI571-mediated inhibition of the Bcr/Abl tyrosine kinase in vivo. In some STI571-treated animals Bcr/Abl messenger RNA and protein expression were markedly increased. In contrast to the polyclonal leukemia of placebo-treated mice, STI571-treated murine CML was generally oligoclonal, suggesting that STI571 eliminated or severely suppressed certain leukemic clones. None of the STI571-treated mice were cured of the CML-like myeloproliferative disorder, however, and STI571-treated murine CML was transplanted to secondary recipients with high efficiency. These results demonstrate the utility of this murine model of CML in the evaluation of novel therapeutic agents against Bcr/Abl-induced leukemias. This improved murine chronic-phase CML model may be a useful tool for the study of STI571 resistance, CML progression, and the anti-CML immune response.     

Modification of the clonogenic capacity of bone marrow cells from normal individuals by the tyrosine kinase inhibitor STI571

STI571 shows clinical activity in the treatment of chronic myelogenous leukemia, Philadelphia positive acute lymphoblastic leukemia and gastrointestinal stromal tumors. Resistance of normal progenitor cells to STI571 is essential when determining the optimal therapeutic window for patients with cancer without bone marrow involvement, and for patients with chronic myeloid leukemia who achieved complete cytogenetic remission. The effect of graded concentrations of STI571 on the clonogenic potential of normal fresh bone marrow hematopoetic progenitor cells from 13 normal individuals was analyzed. It was shown that lower concentrations of STI571 (0.1 and 0.5 microM/l) may increase colony numbers, whereas higher concentrations (>1 microM/l) may reduce normal colony formation.     

Mutation in the ATP-binding site of BCR-ABL in a patient with chronic myeloid leukaemia with increasing resistance to STI571

The Abl kinase inhibitor STI571 (imatinib mesylate) induces haematological remissions in many patients with chronic myeloid leukaemia (CML) but advanced stage CML usually becomes resistant to STI571. We describe a patient in whom progressive resistance to STI571 correlated with the appearance of a mutation in the Bcr-Abl kinase domain. This was a G to A transition that resulted in a glutamic acid to lysine substitution at position 255 (E255K) in the Abl type 1a protein. We suggest that the acquisition of point-mutations in the tyrosine kinase domain of Bcr-Abl may cause progressive clinical resistance to STI571.     

Treatment with STI571, a tyrosine kinase inhibitor, for gastrointestinal stromal tumor with peritoneal dissemination and multiple liver metastases

Background. Gastrointestinal stromal tumors (GISTs) are usually refractory to standard chemotherapeutic agents. We successfully treated a patient with a tyrosine kinase inhibitor (STI571) for GIST with peritoneal dissemination and liver metastases. Methods. In a 32-year-old man presenting with abdominal pain from diffuse peritonitis, a GIST and associated perforated small intestine were resected. Multiple liver metastases were present. After therapies with microwave coagulation, ethanol injection, and local and systemic antineoplastic drugs (fluorouracil, cisplatin, tegafur-uracil, and tegafur) failed, investigational treatment with a tyrosine kinase inhibitor was initiated (STI571, 300mg, p.o. daily). Results. Anorexia and abdominal fullness resolved within a few days. At 24 days after initiation, positron emission tomography showed a remarkable decrease in the abdominal uptake of [¹⁸F] fluorodeoxyglucose. Adverse effects of STI571, including mild alopecia and anemia, were minimal. Conclusions. The tyrosine kinase inhibitor STI571 may be effective against GISTs.     

Treatment of chronic myelogenous leukemia with the tyrosine kinase inhibitor STI571 results in marked regression of bone marrow fibrosis.

Morphologic bone marrow changes in patients with BCR-ABL-positive chronic myelogenous leukemia (CML) were investigated during treatment with the tyrosine kinase inhibitor STI571. Bone marrow trephine biopsy specimens from 23 pretreated patients with CML were examined morphologically and by morphometry before and 6 weeks and 3 months after the initiation of STI571 therapy (Glivec, Novartis, Basel, Switzerland). Bone marrow changes during treatment showed a quantitative normalization of erythropoiesis, a marked reduction of granulopoiesis, and a significant decrease in megakaryocytes with the reappearance of normal-sized forms. Furthermore, a significant regression of bone marrow fibrosis was observed in patients with initial fibrosis (P <.000,000,001). These results may expand the profile of STI571 and may offer novel therapeutic possibilities in diseases with bone marrow fibrosis.     

Efficacy of STI571, an abl tyrosine kinase inhibitor, in conjunction with other antileukemic agents against bcr-abl-positive cells

Chronic myelogenous leukemia (CML), a malignancy of a hematopoietic stem cell, is caused by the Bcr-Abl tyrosine kinase. STI571(formerly CGP 57148B), an Abl tyrosine kinase inhibitor, has specific in vitro antileukemic activity against Bcr-Abl-positive cells and is currently in Phase II clinical trials. As it is likely that resistance to a single agent would be observed, combinations of STI571 with other antileukemic agents have been evaluated for activity against Bcr-Abl-positive cell lines and in colony-forming assays in vitro. The specific antileukemic agents tested included several agents currently used for the treatment of CML: interferon-alpha (IFN), hydroxyurea (HU), daunorubicin (DNR), and cytosine arabinoside (Ara-C). In proliferation assays that use Bcr-Abl-expressing cells lines, the combination of STI571 with IFN, DNR, and Ara-C showed additive or synergistic effects, whereas the combination of STI571 and HU demonstrated antagonistic effects. However, in colony-forming assays that use CML patient samples, all combinations showed increased antiproliferative effects as compared with STI571 alone. These data indicate that combinations of STI571 with IFN, DNR, or Ara-C may be more useful than STI571 alone in the treatment of CML and suggest consideration of clinical trials of these combinations.