Bone marrow evaluation according to the PVSG and WHO criteria in 90 essential thrombocythemia (ET) patients treated with PEG interferon alpha-2b. Preliminary results

Ninety ET patients diagnosed according to the PVSG criteria were enrolled in a phase II study (sponsored by the Schering-Plough Company) designed to evaluate the efficacy, safety and tolerability of a two years treatment with PEG Interferon alpha-2 b (PEG Intron). The patients, observed in 16 Italian Centres belonging to the GIMEMA Cooperative Group and judged at high risk, had been previously treated with cytoreductive (97%) and antiplatelet (91%) drugs. At the study start the patients, 60 F and 30 M, mean age 45 years, showed splenomegaly in 22% of cases. The Hematological Response (HR: PLT<500 x109/L) was observed in 64/81 (79%) and 48/55 (87%) of the patients on PEG Intron treatment at the end of the first and second year, respectively; the spleen enlargement disappeared in 75% of cases. The Bone Marrow data reported in the study CRFs document that the baseline increase of cellularity, of granulopoyesis, of megakaryocytes (MK) number, size and ploidy significantly decreased during PEG Intron treatment, while the MK displasia and fibrosis rate globally increased. In these patients a revision of the Bone Marrow biopsy slides was blindly performed by an Expert Pathologist Panel by applying the WHO criteria. The evaluation of two-thirds of cases showed the following distribution at the baseline: true ET (23%), IMF-0 (pre-MF 17%), IMF-1 (early MF 40%), IMF-2/3 (classical MF 3%), MPD-U (17%). Interestingly, the true ET patients never showed spleen enlargement that, otherwise, was present in IMF-0 (20%) and IMF-1/2/3 (42%) patients. The patients with true ET did not show any bone marrow evolution during the PEG Intron treatment, while the IMF-1/2/3 picture significantly increased. Moreover, the rate of the HR at the end of the first year was higher in true ET(93%) respect to the IMF-0 (60%) and IMF-1/2(71%) patients. These preliminary data show that the patients classified as true ET (WHO Criteria) are not splenomegalic, have no myelofibrotic evolution and reach a better response to the IFN treatment.