Long Term Efficacy and Tolerance of Pegylated Interferon Alpha Therapy for Patients with High Risk Essential Thrombocythemia: An Observational Study of the French Intergroup of Myeloproliferative Disorders (FIM)

Background: Therapeutic guidelines in essential thrombocytemia (ET) are based on established criteria, ie. age more than 60 years, history of thrombosis, platelets more than 1000-1500 ×10⁹/L and to lesser degree cardio-vascular risk factors. Hydroxyurea and anagrelide are the most commonly used cytoreductive therapy for high risk ET. Despite a proven activity in Ph negative myeloproliferative disorders, generalized use and efficacy of Interferon-alpha (IFN-α) have been hampered by frequent side effects. Nonetheless it is a non leukemogenic therapeutic option for younger patients (pts) and during pregnancy. Two phase II trials using Pegylated (Peg) forms of IFN-α in pts with polycythemia vera (PV) or high risk ET have provided interesting results: i) an improved toxicity profile generally associated with a rapid hematolologic response, ii) a molecular response described in a subset of JAK2V617F positive PV or ET pts treated with PegIFN-α.

Aim: To estimate the rationale of PegIFN-α therapy selection in high risk ET patients, efficacy and tolerance.

Methods:Cases of high risk ET patients, who started PegIFN-α therapy between 2006 and 2011, were reported by centres of the French Intergroup of Myeloproliferative disorders and included in an observational study. We collected information regarding history of MPN, treatment, hematologic response at different timepoints, toxicities related to PegIFN-α, thrombo-hemorrhagic events and hematologic progression. The current analysis was performed on 103 consecutive pts.

Results: Median follow-up from ET diagnosis was 9 years (3-27). 74% of pts were female, median age was 37 years (range 16-67). Previous vascular event was observed in 36% of cases and JAK2V617F mutation was detected in 52% of pts. .

Median time from diagnosis to PegIFN-α was 40 months (1-255). PegIFN-α was administered after hydroxyurea and anagrelide in second or third line (56% and 27% respectively). No prior therapy has been used for 16% of pts. Reasons for starting PegIFN-α were lack of hematologic response (21%), toxicity to prior therapies (17%), pregnancy (15%), or medical decision for younger pts (47%). Pts received either PegIFNα-2a (n=91) or PegIFNα-2b (n=12), based on physician decision.

According to the 2009 European LeukemiaNet criteria and with a median exposure to PegIFN-α of 29 months (1-92), 81% of pts achieved a complete hematologic response (CHR). Cumulative incidence of CHR were 58 % (95% CI: 49-68), 73% (95% CI: 65-81) and 80% (95% CI: 71-87) at 6, 12 and 24 months respectively.

By analysing the response according to JAK2 status, cumulative incidence of CHR was better for the JAK2V617F positive subgroup (p=0.0183, overall). At 36 months estimated CHR rates were 89% (95% CI: 79-95) and 70% (95% CI: 57-82) for JAK2VF positive vs negative subgroup respectively. Median exposure to PegIFN-α was 29 months in both groups.

At last follow-up, with a median of 42 months since PegIFN-α initiation, 53% of the pts were still treated with PegIFN-α. Among them, 76% have maintained a CHR. Reasons for discontinuation of PegIFN-α (47% of pts) were consecutive to PegIFN-α toxicity (59%), -mainly chronic moderate non hematologic toxicity-, or hematologic responses were considered insufficient in16%. In addition, PegIFN-α has been stopped in a subset of pts who achieved a durable stable CHR.

In Conclusion, This study provides results of PegIFN-α therapy in a cohort of high risk ET pts not included in clinical trials. In this selected population of young pts, a durable efficacy of therapy was observed in half of patients. Characteristics of hematologic response according to molecular status will be presented. These results support the PegIFN-α as an alternative therapy to hydroxyurea and anagrelide in high risk ET, and warrant its investigation in further prospective randomized studies.