FLT3 and KIT mutated pediatric acute myeloid leukemia (AML) samples are sensitive in vitro to the tyrosine kinase inhibitor SU11657.

New treatment strategies to improve the outcome of pediatric acute myeloid leukemia (AML) are required as 40% of children diagnosed with AML do not survive. Around 30% of pediatric AML patients harbour a mutation in the tyrosine kinases FLT3 (+/-20%) or KIT (+/-10%). In this study we investigated whether pediatric AML samples (N=61) were sensitive to the tyrosine kinase inhibitor SU11657 (similar to the clinically available drug sunitinib) in vitro, and whether sensitivity was related to expression of, and mutations in, FLT3 and KIT. Overall, SU11657 showed only moderate cytotoxicity. A FLT3 mutation was detected in 35% and a KIT mutation in 8% of the samples. FLT3 and KIT mutated samples were significantly more sensitive to SU11657 than WT KIT and FLT3 samples. Samples without KIT or FLT3 mutations, but with a high wild-type (WT) KIT expression were significantly more sensitive to SU11657 than samples with low KIT expression. Further clinical evaluation of SU11657 and sunitinib combined with chemotherapy would be of interest. Inclusion in clinical trials should not be restricted to patients with FLT3 or KIT mutations.

In vitro profiling of the sensitivity of pediatric leukemia cells to tipifarnib: identification of T-cell ALL and FAB M5 AML as the most sensitive subsets.

Although the prognosis of pediatric leukemias has improved considerably, many patients still have relapses. Tipifarnib, a farnesyl transferase inhibitor (FTI), was developed to target malignancies with activated RAS, including leukemia. We tested 52 pediatric acute myeloid leukemia (AML) and 36 pediatric acute lymphoblastic leukemia (ALL) samples for in vitro sensitivity to tipifarnib using a total cell-kill assay and compared these results to those obtained with normal bone marrow (N BM) samples (n = 25). AML samples were significantly more sensitive to tipifarnib compared to B-cell precursor ALL (BCP ALL) or N BM samples. Within AML, French-American-British (FAB) M5 samples were most sensitive to tipifarnib. T-cell ALL samples were significantly more sensitive than BCP ALL and N BM samples. In AML there was a marked correlation between tipifarnib resistance and daunorubicin or etoposide resistance, but not to cytarabine or 6-thioguanine. RAS mutations were present in 32% of AML and 18% of ALL samples, but there was no correlation between RAS mutational status and sensitivity to tipifarnib. Future studies are needed to identify biomarkers predictive of tipifarnib sensitivity. In addition, clinical studies, especially in T-cell ALL, seem warranted.

Immunophenotypic cell lineage and in vitro cellular drug resistance in childhood relapsed acute lymphoblastic leukaemia.

At relapse, T-cell acute lymphoblastic leukaemia (ALL) has a worse patient outcome than B-cell precursor (BCP-) ALL. To investigate this further, we compared in vitro cellular drug resistance profiles of T-cell and BCP-ALL samples obtained at relapse. We investigated 237 paediatric relapsed ALL cases, including 151 samples taken at first relapse, of which 30 were T-cell ALL. In vitro drug resistance was measured using the 4-day methyl-thiazol-tetrazolium (MTT) assay and cellular immunophenotype was determined at central reference laboratories. Similar results were found for first relapsed ALL samples and for the total group: T-cell ALL samples were more resistant to 4-HOO-ifosfamide (1.4-fold, P = 0.019) and cisplatin (3.7-fold, P = 0.005). The samples were more sensitive to thiopurines such as mercaptopurine (2.1-fold, P = 0.007) and thioguanine (1.7-fold, P = 0.003). Resistance/sensitivity to 16 other drugs did not differ significantly. These results do not explain the relatively poor prognosis of T-cell ALL at relapse, but do suggest that the more intensive use of thiopurines in relapsed T-cell ALL may be beneficial.

A genome-wide view of the in vitro response to l-asparaginase in acute lymphoblastic leukemia.

To investigate the effect of l-asparaginase on acute lymphoblastic leukemia (ALL), we used cDNA microarrays to obtain a genome-wide view of gene expression both at baseline and after in vitro exposure to l-asparaginase in cell lines and pediatric ALL samples. In 16 cell lines, a baseline gene expression pattern distinguished l-asparaginase sensitivity from resistance. However, for 28 pediatric ALL samples, no consistent baseline expression pattern was associated with sensitivity to l-asparaginase. In particular, baseline expression of asparagine synthetase (ASNS) was not predictive of response to l-asparaginase. After exposure to l-asparaginase, 5 cell lines and 10 clinical samples exhibited very similar changes in the expression of a large number of genes. However, the gene expression changes occurred more slowly in the clinical samples. These changes included a consistent increase in expression of tRNA synthetases and solute transporters and activating transcription factor and CCAAT/enhancer binding protein family members, a response similar to that observed with amino acid starvation. There was also a consistent decrease in many genes associated with proliferation. Taken together, the changes seem to reflect a consistent coordinated response to asparagine starvation in both cell lines and clinical samples. Importantly, in the clinical samples, increased expression of ASNS after l-asparaginase exposure was not associated with in vitro resistance to l-asparaginase, indicating that ASNS-independent mechanisms of in vitro l-asparaginase resistance are common in ALL. These results suggest that targeting particular genes involved in the response to amino acid starvation in ALL cells may provide a novel way to overcome l-asparaginase resistance.