Limitations of G-CSF Mobilization in Splenectomized Patients with Beta-Thalassemia Major: Implications for Thalassemia Gene Therapy.

Abstract 2150

Poster Board II-127

For gene therapy (GT) of thalassemia (TH), high numbers of genetically-modified hematopoietic stem cells (HSCs) are required to effectively compete for niche space in the hypercellular thalassemic bone marrow (bm). Mobilized peripheral blood is the preferable source of HSCs for thalassemia GT due to higher yields of CD34+cells compared to bm harvest. There is limited information on the mobilization efficacy of adult patients with major β-TH as well as on the safety of the procedure in a condition of splenomegaly and extramedullary hemopoiesis. Rare events of splenic rupture or thrombosis with G-CSF in normal donors raise safety concerns for its use in TH where chronic splenomegaly and hypercoagulability exist. Pretreatment of patients with hydroxyurea (HU) could reduce the risk of splenic rupture or thrombosis by decreasing the splenic hemopoiesis and thereby the spleen size in the non-splenectomized (non-SPL), and the circulating cells in the splenectomized (SPL) patients before G-CSF. In an on going mobilization study, we aim to assess the safety and efficacy of G-CSF mobilization with or without HU pretreatment in adult patients with β-TH major. Sufficient mobilization is considered to be the yield of ≥2×10⁶CD34+cells/kg/2aphereses. Sixteen patients have been enrolled so far, 9 SPL and 7 non-SPL. One non-SPL patient withdrew during the study. Six SPL and 4 non-SPL patients received HU pretreatment (20mg/kg/d the non-SPL, 25-30mg/kg/d the SPL) for 1 month before G-CSF. There was a 1-2 weeks' interval between HU cessation and G-CSF initiation. No severe adverse events were observed. In non-SPL patients, HU decreased the spleen volume over baseline (306cm³ vs 536cm³, p=0,03) resulting in 9% max increase during mobilization compared to 45% size increase in patients w/o HU pretreatment. In non-SPL patients, HU negatively affected the CD34+yield when the ‘wash-out' period before G-CSF was 8 days (mean CD34+cells 1,86±0,76×10⁶/kg/2aphereses, n=2). However, when the interval period from the HU stop to the G-CSF initiation increased up to 18 days allowing for bm recovery after the myelosuppressive effect of HU, mobilization was successful (P12:CD34+cells 6,5×10⁶/kg/2aph). Non-SPL patients w/o HU pretreatment (n=3) yielded adequate numbers of HSCs (CD34+cells:5,8±3,89×10⁶/kg/2aph). Surprisingly, CD34+cell yields were very low in the first 2 non-HU pretreated SPL patients (CD34+cells:0,98±0,14×10⁶/kg/2aph). This was due to the development of early excessive leukocytosis (mean max WBCs 81×10⁹/l, day 3) with the regular 10mcg/kg/d G-CSF dose, which necessitated dose hold or therapeutic leukapheresis and resulted in loss of the CD34+cell peak in blood. However, when mobilization started with lower (2,5mcg/kg/d) and adjusted to the WBCs doses of G-CSF (mean daily dose 3,21mcg/kg) and aphereses were initiated later (day6), CD34+cell yield markedly improved (P15:4,5×10⁶/kg/2aph) without inducing early excessive leukocytosis (max WBCs 67×10⁹/l, day 5). In SPL patients, HU was shown to decrease the high PLT and WBC numbers before G-CSF (PLTs:from 640±132×10⁹/l at baseline to 240±53×10⁹/l, p=0,0002 / WBCs:from 20,23±15,8×10⁹/l at baseline to 11,47±6,11×10⁹/l, p=0,23) potentially reducing the risk of thrombosis and partially preventing excessive leukocytosis during mobilization (max WBCs SPL-HU:51,68±29,37×10⁹/l vs SPL-no HU:74,77±8,78×10⁹/l, p=0,23). HU pretreatment negatively affected the yield in SPL patients when the ‘wash-out' period before G-CSF was 7-10 days (mean CD34+yield 0.62±0,41×10⁶/kg/2aph, n=5). However, when the interval period from HU stop to G-CSF initiation increased up to 12 days, mobilization was successful (P16:CD34+cells 3,8×10⁶/kg/2aphereses). G-CSF dose adjustment was also needed in HU-pretreated SPL patients with WBCs≥14,6×10⁹/l before G-CSF (P9,P16). Overall, it seems that mobilization of SPL thalassemic patients is challenging. Mobilization is not inherently inefficient in SPL patients but it results from mandatory G-CSF-dose modifications to avoid hyperleukocytosis. Patient-tailored schemes of G-CSF mobilization or alternative ways of mobilization (ie AMD 3100) will be required in order to obtain high numbers of HSCs from SPL patients. HU seems to play a safety role as pretreatment before mobilization, especially in the SPL patients, however the time to G-CSF initiation after HU cessation is critical for a sufficient CD34+cell yield.