Eltrombopag (epag), a thrombopoietin receptor agonist (TPO-RA), increases platelet counts in patients with chronic immune thrombocytopenia (cITP). TPO-RAs have been associated with varying degrees of increases in bone marrow reticulin (Brynes 2011; Ghanima 2011). Due to lack of pretreatment evaluations, the incidence and clinical significance of these findings have not been established. Inconsistencies in specimen preparation, staining, and analysis across institutions further confound conclusions. The purpose of this 2-year (y) study (NCT01098487) is to assess for bone marrow fibrosis (reticulin and/or collagen) in patients treated with epag for cITP. Baseline and 1y findings are presented.

Bone marrow biopsies are being collected at baseline (before treatment with epag) and at 1 and 2y of treatment. Specimens are centrally processed and stained for reticulin (silver) and collagen (trichrome) and undergo central independent pathology review of cellularity; megakaryocyte, erythroid, and myeloid quantity and appearance; trabecular bone quality; reticulin grade; and presence of collagen (European Consensus scale-MF; Thiele 2005).

Baseline and 1y (10–14 months) data are available for 101 patients. Median age is 42y (18–78); 70 patients are female; 50% are Caucasian/European, 22% are East Asian, and 29% are Central South Asian. Median time since ITP diagnosis is 4.2y (0.2–45.7). All patients had received prior ITP therapy, and 8 patients had received prior TPO-RA treatment (epag [7], romiplostim [1]), the last dose ≥6 months before enrollment.

At baseline, 91 patients had reticulin grade 0 (MF-0), 10 MF-1, and 0 MF≥2. At 1y, 59 patients had MF-0, 38 MF-1, 3 MF-2, and 1 MF-3 (Figure). Compared with baseline, there was no change at 1y in MF grade in 61 patients, a decrease by 1 grade in 3, an increase by 1 grade in 35, and an increase in 2 or 3 grades in 1 patient each (Table). Three patients had collagen at 1y (1 patient each with MF-1, MF-2, and MF-3). None of the 4 patients with MF≥2 had adverse events or hematologic abnormalities considered related to impaired bone marrow function, and none withdrew due to bone marrow findings. Among the 8 patients with prior TPO-RA treatment, all had baseline reticulin of MF-0 and none had collagen; at 1y, 6 remained MF-0, 1 was MF-1, and 1 MF-3 (collagen demonstrated).

Cellularity was normal in 83% and 80% of patients at baseline and 1y, respectively. Other than normalization of erythroid lineage numbers, no changes occurred in marrow cellular composition. In 3 of 4 patients with MF≥2, cellularity was increased at 1y.

Trabecular bone thinning was found at baseline in 28 patients (the majority with prior steroid use) and 51 patients at 1y.

10% of patients had MF-1 at baseline. After 1y of treatment, no increase or a mild increase in reticulin was observed in 63% and 35% of patients. No patient with MF≥2 (n=4) had clinical signs or symptoms indicative of bone marrow dysfunction and none withdrew from the study. Results were similar to those reported for EXTEND, an eltrombopag extension study (median treatment duration >2 years; Brynes 2011).

These data suggest that treatment with epag is generally not associated with clinically relevant increases in bone marrow reticulin or collagen. The potential association of TPO-RAs and increased bone marrow reticulin needs further study.

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Brynes:GlaxoSmithKline: Research Funding. Orazi:GlaxoSmithKline: Research Funding. Wong:Roche: Research Funding; MSD: Research Funding; Johnson & Johnson: Research Funding; Bayer: Consultancy, Research Funding; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Biogen-Idec: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Research Funding; GlaxoSmithKline: Research Funding; Bristol-Myers Squibb: Research Funding. Bakshi:GlaxoSmithKline: Employment, Equity Ownership. Bailey:GlaxoSmithKline: Employment, Equity Ownership. Brainsky:GlaxoSmithKline: Employment, Equity Ownership, Patents & Royalties.