Potential Therapeutic Applications of Jak2 Inhibitors in Beta-Thalassemia and Sickle Cell Disease,

Abstract 3187

ß-Thalassemia and sickle cell disease (SCD) are the most common genetic red blood cell (RBC) disorders characterized respectively by limited production of aberrant ß-globin chains. In both cases, chronic transfusions and iron chelation are required to treat the anemia and/or formation of abnormal RBC. In ß-thalassemia, anemia stimulates erythropoietin (Epo) synthesis, which in turn leads to increased erythropoiesis and development of hepatosplenomegaly, often resulting in the need for splenectomy. Recently, we demonstrated that erythroid cells from ß-thalassemic mice have a hyper-activation of Jak2, a kinase that mediates the signaling triggered by the binding of Epo to the Epo receptor. This led us to hypothesize that Jak2 inhibitors could be utilized to minimize erythroid expansion in this disorder, limiting splenomegaly.

A Jak2 inhibitor (Tg101209 or Tg) was first tested in mice affected by ß-thalassemia intermedia (th3/+). Two doses of Tg (150 and 100 mg/Kg/day) were given orally for 10 days. Tg administration induced a mild decrease of hemoglobin levels (8.8±0.2, 8±0.2 and 7.8±0.2g/dL for placebo, Tg 100 mg/Kg and Tg 150 mg/Kg treated mice, respectively. p<0.05) and in the number of reticulocytes (approximately 75% of the levels seen in controls, p<0.05). Splenomegaly was also reduced in Tg-treated mice (up to 60%; p<0.05), the extent of this effect correlating with the dosage used. Reduction of splenomegaly was associated with a decrease in the number of erythroid progenitors in this organ (p<0.05) and trend toward normalization of the splenic architecture. These data support our hypothesis that, in ß-thalassemia, splenomegaly is associated with increased erythroid proliferation and it can be alleviated by administration of Jak2 inhibitors, with only a mild increase in anemia. We further tested the effect of Tg in other anemias associated with extramedulary hematopoiesis (EMH) and splenomegaly, including SCD. Administration of the drug to mice affected by SCD led to a significant worsening of anemia (more pronounced than that seen in th3/+ mice) and a proportional reduction of splenomegaly and EMH.

We then evaluated the outcome of combining Tg with blood transfusion, a common therapy in b-thalassemia and SCD. In b-thalassemia, massively enlarged spleens are believed to sequester a significant proportion of circulating RBC, thereby limiting their lifespan and the efficacy of transfusion regimens. We hypothesize that decreasing splenomegaly by administration of Jak2 inhibitors could increase the efficacy of transfusion. This was first tested in th3/+ animals. In this case, transfusion alone was sufficient to increase the hemoglobin (Hb) levels approximately 3 g/dL and reduce the spleen size to 65% of that seen in non-transfused controls. In this model, the combined effect of transfusion and administration of Tg was more effective, the spleen size been 50% of non-transfused controls (p<0.05). We further tested this approach in mice affected by ß-thalassemia-major (th3/th3), for which transfusion is required for survival and massive splenomegaly develops rapidly. Administration of Tg together with transfusion led to a greater increase in Hb levels compared to transfusion alone (9.3±0.4 vs 7.3±0.5g/dL, p<0.05). This was likely a consequence of reduced splenomegaly and decreased sequestration of RBCs in Tg/transfused mice.

Lastly, we tested combination therapy in a mouse model of SCD. Mice treated with Tg and transfusion exhibited slightly lower levels of Hb than transfused controls (Hb=9.7±0.2g/dL versus Hb=10.9±0.2g/dL). However, compared to the control, mice receiving combination therapy exhibited a larger percentage of donor RBCs, while endogenous erythropoiesis was markedly suppressed along with the production of sickle RBCs (1.3±0.3×10⁶ RBC/ul compared to transfused-controls exhibiting 2.7±0.3×10⁶ RBC/ul).

In summary, administration of Jak2 inhibitors might reduce the production of pathological cells that, together with preservation of the splenic architecture, could minimize the propensity of patients to thrombotic events. Furthermore, suppression of endogenous erythropoiesis and reduction of the transfusion regimen would be expected to also reduce iron accumulation, making it easier to prevent its toxic effects through chelation therapy.