Abstract LB-294: Determination of JAK2V617F allele burden in a phase II study of patients with myelofibrosis treated with SAR302503 using a sensitive and robust allele-specific qPCR assay.

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Abstract Janus kinase-2 (JAK2)/STAT pathway activation is commonly observed in myeloproliferative neoplasms and plays an important role in disease development and progression. The V617F activating mutation in the JAK2 pseudokinase domain is found in 95% of patients with polycythemia vera (PV) and in 50-70% of patients with primary myelofibrosis (MF) or essential thrombocythemia (ET). Studies have shown the clinical efficacy of JAK2 inhibitors in treating MF in terms of a reduction in splenomegaly and relief of constitutional symptoms. A reduction in the JAK2V617F allele burden after treatment with a JAK2-targeted agent could be a surrogate for a disease-modifying effect of this therapy. To demonstrate this, a robust method that can accurately measure the JAK2V617F allele burden is required. Here, we report the development of a sensitive quantitative PCR (qPCR) assay to determine the JAK2V617F allele burden in patients treated with SAR302503, the validation of the assay in a Phase 0 clinical study and implementation in a phase II study using samples from patients with MF. The assay demonstrated robust performance, with high sensitivity (LLOD=0.05% and LLOQ=0.5%) and accuracy. A phase 0 study in MF, PV, and ET patients showed that (1) there was minimal intra-subject sampling variability in JAK2V617F allele burden, and (2) whole-blood samples are stable for at least 48 hours at ambient conditions for DNA preparation without a significant impact on JAK2V617F allele burden measurement.Of the 31 patients with intermediate-2 or high-risk MF enrolled in a phase II clinical study (NCT01420770), baseline samples were available for 29 patients and of these, 26 were JAK2V617F-positive using this assay. Of the 19/26 patients for whom samples were available at all 3 time points, the median allele burden was 93% at baseline, 87% at the end of Cycle 3, and 78% at the end of Cycle 6. Among the 24 patients who were JAK2V617F-positive and for whom spleen measurements were available at the end of Cycle 6, a total of 15 (63%) had a spleen response (≥ 35% reduction in spleen volume by MRI versus baseline). In contrast, two JAK2V617F-negative patients did not have a spleen response. Patients with baseline allele burden levels of 0 to <25%, 25% to 75%, and ≥75% showed spleen response rates of 50%, 44%, and 69%, respectively, at the end of Cycle 6. JAK2V617F allele burden will continue to be assessed every 3 cycles. Longer follow up is needed to determine the clinical effect of allele burden reduction. Citation Format: Feng Liu, Moshe Talpaz, Animesh Pardanani, Catriona Jamieson, Nashat Gabrail, Ayalew Tefferi, Tianlei Lei, Rita Greco, Francisco Adrian, Nikki Daskalakis, Claudia Lebedinsky, Pamela Cohen, Donald Bergstrom. Determination of JAK2V617F allele burden in a phase II study of patients with myelofibrosis treated with SAR302503 using a sensitive and robust allele-specific qPCR assay. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-294. doi:10.1158/1538-7445.AM2013-LB-294

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