Abstract
Background: Almost all cases of polycythemia vera (PV) and more than half of those with either essential thrombocythemia (ET) or primary myelofibrosis (PMF) harbor JAK2V617F. It has been previously shown that V617F-positive ET patients, as opposed to their mutation-negative counterparts, require lower doses of hydroxyurea (HU) for control of their platelet count. In the current study we looked for a similar scenario in PMF and PV.
Methods: Study patients were identified by database inquiry based on the presence of treatment history with HU and adequate follow-up. Qualitative (PMF) and quantitative (PV) analysis of JAK2V617F was performed according to previously published allele specific PCR-based methods. Mutation analysis was performed on bone marrow samples collected at time of initial diagnosis or within one year of diagnosis. HU treatment response assignment was according to criteria set by the International Working Group for Myelofibrosis Research and Treatment. In PV, a complete response (CR) required normalization of blood counts associated with a more than 50% benefit in both phlebotomy requirement and splenomegaly. Partial response (PR) required either becoming phlebotomy independent or normalization of blood counts or a greater than 50% improvement in splenomegaly.
Results: The study cohort consisted of 69 patients with PMF (median age 62 years; 36% females) and 56 patients with PV (median age 67 years; 45% females). In PMF, by univariate analysis, HU response was associated with a shorter disease duration (p=0.008), absence of previous therapy (p=0.01), older age at diagnosis (p=0.009), and presence of V617F (p=0.02). On multivariable analysis, only the latter retained its significance; HU response rate was 48% in V617F-positive patients (11/21 patients) and 8% in mutation-negative cases (1/12 patients). In PV, V617F allele burden correlated directly with HU response (p=0.05) and, in responding patients, inversely with HU dose (p=0.01; Figure). All 9 patients with upper quartile V617F allele burden responded (8 with CR) whereas 6 of the 11 (55%) HU non-responders displayed V617F allele burden in the lower quartile range. Among the 45 HU responders, the maximum daily dose of HU employed in all 9 patients in the upper quartile allele burden range and 4 of the 5 in the middle quartiles range was below 1500 mg. In contrast, 8 of the 31 patients (26%) in the lower allele burden quartile range required doses at 1.5 gm or more per day.
Conclusion: The current study suggests that response to HU therapy in both PMF and PV is more robust in the presence of JAK2V617F; the particular information is useful in identifying PMF patients who are likely to benefit from HU therapy and in individualizing HU starting dose in PV patients.
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