INCB018424, a Selective Inhibitor of JAK1 and JAK2, Downregulates the Expression of Leukocyte Alkaline Phosphatase (LAP) On Circulating Granulocytes in Patients with Polycythemia Vera and Essential Thrombocythemia.

Abstract 2905

Poster Board II-881

Vascular complications are the major causes of morbidity in myeloproliferative neoplasms (MPN) like essential thrombocythemia (ET) and polycythemia vera (PV), and their pathogenesis is likely multifactorial. Granulocyte activation may promote thrombogenesis through multiple interactions with both platelets and vessel wall, and granulocytes from patients with ET and PV were previously shown to be functionally activated as indicated by increased values of LAP expression. LAP is stored in intracellular secretory vesicles, which following granulocyte activation fuse into a tubular system communicating with the plasma membrane, thus exporting LAP to the cell surface. A direct relationship between granulocyte LAP expression and JAK2 (V617F) mutant allele burden has been previously documented in PV [Passamonti et al, Blood. 2006 May 1;107(9):3676-82]. The identification of the unique JAK2 (V617F) mutation in more than 70% of patients with MPN has prompted the development of JAK2 inhibitors for treatment of these disorders. INCB018424, a selective inhibitor of JAK1 and JAK2, is currently being evaluated in a phase II trial in patients with ET or PV refractory or intolerant to hydroxyurea (ClinicalTrials.gov Identifier: NCT00726232). The Department of Hematology Oncology, Fondazione IRCCS Policlinico San Matteo & University of Pavia, Pavia, Italy, has enrolled 23 patients in this clinical trial. To investigate the effect of INCB018424 on granulocyte activation, we measured LAP expression levels on circulating granulocytes of these patients (13 cases of ET and 10 cases of PV). LAP expression was evaluated by flow cytometry immunophenotyping (Beckman Coulter, Milan Italy) before study entry and then after one and three months of treatment. The anti-LAP monoclonal antibody was employed according to the manufacturer's instructions (PharMingen-Becton Dickinson, Milan Italy). Results were expressed as mean fluorescence intensity (MFI) ratio, defined as the ratio between the mean fluorescence of the specific marker tested to the mean fluorescence of the negative control included in the assay. Given that patients with ET and PV had different peripheral granulocyte counts, results were adjusted for this parameter. In the 23 patients with MPN enrolled in the clinical trial NCT00726232, the median value of granulocyte LAP expression was 11 MFI (range, 3.7-34.1) at baseline, 7.3 MFI (range, 4.3-24.1) after one month, and 8.6 MFI (range, 3.9-29.9) after three months of treatment with INCB018424. The Mann-Whitney U test indicated that there was a significant reduction in granulocyte LAP expression with INCB018424 treatment (P = .04). We further investigated the time course (slope) of LAP expression by applying general linear models for repeated measurements with diagnosis (ET vs PV), JAK2 status (V617F vs wild type), hematologic response to treatment with INCB018424 (complete or partial response vs no response), and aspirin treatment as covariates. This analysis showed that the reduction in granulocyte LAP expression was independent from diagnosis, JAK2 status, hematologic response to INCB018424, and aspirin administration (P = .00009). Interestingly, three patients who were temporarily interrupted from INCB018424 treatment because of anemia displayed an increase in LAP expression after interruption. In conclusion, our observations suggest that INCB018424 treatment is able to decrease LAP expression on circulating granulocytes in patients with ET or PV, which likely reflects a decrease in granulocyte activation.