Efficacious Intermittent Dosing of a Novel JAK2 Inhibitor in Mouse Models of Polycythemia Vera

A high percentage of patients with the myeloproliferative disorder polycythemia vera (PV) harbor a Val617→Phe activating mutation in the Janus kinase 2 (JAK2) gene, and both cell culture and mouse models have established a functional role for this mutation in the development of this disease. We describe the properties of MRLB-11055, a highly potent inhibitor of both the WT and V617F forms of JAK2, that has therapeutic efficacy in erythropoietin (EPO)-driven and JAK2V617F-driven mouse models of PV. In cultured cells, MRLB-11055 blocked proliferation and induced apoptosis in a manner consistent with JAK2 pathway inhibition. MRLB-11055 effectively prevented EPO-induced STAT5 activation in the peripheral blood of acutely dosed mice, and could prevent EPO-induced splenomegaly and erythrocytosis in chronically dosed mice. In a bone marrow reconstituted JAK2V617F-luciferase murine PV model, MRLB-11055 rapidly reduced the burden of JAK2V617F-expressing cells from both the spleen and the bone marrow. Using real-time in vivo imaging, we examined the kinetics of disease regression and resurgence, enabling the development of an intermittent dosing schedule that achieved significant reductions in both erythroid and myeloid populations with minimal impact on lymphoid cells. Our studies provide a rationale for the use of non-continuous treatment to provide optimal therapy for PV patients

Decreased 5'-nucleotidase activity in a T lymphocyte subpopulation from patients with B cell chronic lymphocytic leukemia

Abstract The activity of the ectoenzyme 5′-nucleotidase (5′N) was determined in the T lymphocyte subpopulations from patients with chronic lymphocytic leukemia (CLL). 5′N could be detected only in the T cells from patients whose B cells also had enzyme activity. The specific activity of CLL T4 cells was 0.17 +/- 0.02 micron/h/mg protein, similar to that of normal T4 cells, which was 0.13 +/- 0.08 micron/h/mg. The CLL T8 cells, however, had a significantly lower 5′N activity (0.17 +/- 0.02 micron/h/mg) than normal T8 cells (0.41 +/- 0.11 micron/h/mg) (P = .003). Normal null cells had very low activity, while much higher levels were found in the null cells of CLL patients whose B cells had activity. These findings document a difference in activity of an enzyme between the T8 cell population of patients with CLL and that of normal subjects.</jats:p

Decreased 5'-nucleotidase activity in a T lymphocyte subpopulation from patients with B cell chronic lymphocytic leukemia

The activity of the ectoenzyme 5′-nucleotidase (5′N) was determined in the T lymphocyte subpopulations from patients with chronic lymphocytic leukemia (CLL). 5′N could be detected only in the T cells from patients whose B cells also had enzyme activity. The specific activity of CLL T4 cells was 0.17 +/- 0.02 micron/h/mg protein, similar to that of normal T4 cells, which was 0.13 +/- 0.08 micron/h/mg. The CLL T8 cells, however, had a significantly lower 5′N activity (0.17 +/- 0.02 micron/h/mg) than normal T8 cells (0.41 +/- 0.11 micron/h/mg) (P = .003). Normal null cells had very low activity, while much higher levels were found in the null cells of CLL patients whose B cells had activity. These findings document a difference in activity of an enzyme between the T8 cell population of patients with CLL and that of normal subjects.</jats:p

Human lymphocytes: 5'-nucleotidase-positive and -negative subpopulations.

The enzyme, 5'-nucleotidase (5'N) (E.C.-3.1.3.5) is present in lymphocytes isolated from the blood of normal subjects. This activity is markedly decreased or not detectable in the cells from three-quarters of patients with chronic lymphocytic leukemia (CLL), while supranormal levels are found in less than 10% of the cases. To determine whether the decreased 5'N value in CLL represents a lower activity per cell or fewer enzyme-containing cells than in the normal, conditions were established for the histochemical measurement of 5'N in human lymphocytes. It was found that the cells isolated from the blood of normal subjects or patients with CLL consist of 5'N-positive and 5'N-negative subpopulations. Normal subjects who had high 5'N specific activity were shown to have a greater percentage of 5'N-positive cells than individuals with low 5'N activity. Patients with CLL who had no activity by standard chemical assay had no 5'N-positive cells, while the exceptional patient with CLL with a higher than normal specific activity showed an percentage of 5'N-positive cells. It is suggested that the selective proliferation of 5'N-positive and 5'N-negative populations may account for the heterogeneity of 5'N in CLL

Decreased actin content of lymphocytes from patients with chronic lymphocytic leukemia

Abstract Actin, a major cytoskeletal protein, was quantitated in normal and chronic lymphocytic leukemia lymphocytes. The actin content of normal human blood lymphocytes was 2.2 +/- 0.4 mg/10(9) cells and represented 6.6% +/- 1.8% of the total cellular protein. A significant decrease (p less than 0.001) was noted in chronic lymphocytic leukemia lymphocytes that contained 1.4 +/- 0.3 mg actin/10(9) cells, constituting 4.3% +/- 1.1% of the total protein. Normal T and B cells did not differ in actin content. Reduced actin levels were found in the T as well as in the B lymphocytes of “B-cell” chronic lymphocytic leukemia. The possible importance of the decreased actin level in the anomalous capping response and motility of chronic lymphocytic leukemia lymphocytes is discussed.</jats:p

Decreased actin content of lymphocytes from patients with chronic lymphocytic leukemia

Actin, a major cytoskeletal protein, was quantitated in normal and chronic lymphocytic leukemia lymphocytes. The actin content of normal human blood lymphocytes was 2.2 +/- 0.4 mg/10(9) cells and represented 6.6% +/- 1.8% of the total cellular protein. A significant decrease (p less than 0.001) was noted in chronic lymphocytic leukemia lymphocytes that contained 1.4 +/- 0.3 mg actin/10(9) cells, constituting 4.3% +/- 1.1% of the total protein. Normal T and B cells did not differ in actin content. Reduced actin levels were found in the T as well as in the B lymphocytes of “B-cell” chronic lymphocytic leukemia. The possible importance of the decreased actin level in the anomalous capping response and motility of chronic lymphocytic leukemia lymphocytes is discussed.</jats:p