A Pilot Study of Eltrombopag Plus G-CSF for Human CD34+ Cell Mobilization in Patients with Multiple Myeloma Undergoing Autologous Stem Cell Transplant
Authors: Laubach Jacob P. , Freedman Revital , Redd Robert A , Tippy Mason , Cummings Kristen , Dipietro Heidi , Naik Nivisha Upendra , Nicol Kelly , Richardson Paul G. , Najita Julie , Berliner Nancy 
Published: November 2019

Abstract
Background: Administration of recombinant human thrombopoietin (rhTPO) with G-CSF for stem cell mobilization is associated with high CD34+ stem cell yield, rapid neutrophil recovery following autologus transplantation (ASCT), and decreased red blood cell (rbc) and platelet (plt) transfusions (Solomo, G. et al. Blood 1999). However, clinical development of rhTPO was complicated by the formation of neutralizing anti-TPO antibodies (Li, J. et al. Blood 2001), prompting discontinuation of further clinical development of recombinant TPO. Eltrombopag (Elt) is an orally bioavailable small molecule thrombopoietin receptor (TPO-R) agonist approved by the FDA for treatment of chronic immune thrombocytopenic purpura (ITP). In vitro studies have demonstrated that Elt promotes megakaryocyte proliferation and differentiation of CD34+ bone marrow progenitor cells (Erickson-Miller CL Stem Cells 2009), suggesting that Elt might be a surrogate for rhTPO for stem cell mobilization. In this pilot trial, we evaluated the combination of Elt plus standard G-CSF and cyclophosphamide (C) for stem cell mobilization in patients (pts) with multiple myeloma (MM), a disease for which ASCT remains a standard of care (Blade et al. Blood 2010).
Methods: Primary objectives included determination of the median number of CD34+ cells/kg mobilized and the maximum tolerated dose (MTD) of Elt. Pts had MM that was stable or responsive to at least two cycles of chemotherapy with plans for stem cell mobilization and ASCT. Four pts were to be enrolled in each of four dose escalation arms in which they received 0 (Arm D), 50 (Arm A), 100 (Arm B), or 150 mg (Arm C) of eltrombopag in combination with standard C + G-CSF. Adverse events (AEs) were graded by NCI-CTCAE v4.
Results: 17 pts have been screened and enrolled to date. Two patients withdrew consent prior to receiving Elt and were excluded from statistical analysis. 15 patients have completed participation in the study to date and two patients remain to be enrolled in Arm C. The first subject in Arm A experienced delayed engraftment that was determined to be unrelated to ELT; rather, the event was attributed to administration of a one-time high dose of corticosteroid for management of a severe hypersensitivity reaction to DMSO that occurred during stem cell infusion. A second subject in Arm A had undergone mobilization with Elt prior to the previously described delayed engraftment event, and to ensure safety underwent a second mobilization with G-CSF and plerixafor. During ASCT, this patient received cells from the second mobilization procedure. While neither event met criteria for a dose-limiting toxicity, the protocol was amended such that three additional patients enrolled in Arm A underwent two rounds of mobilization - the first with Elt plus C and G-CSF and a second with G-CSF plus plerixafor - and received as part of ASCT cells mobilized with Elt. Each of these patients engrafted successfully. The median number of CD34+ cells/kg collected during the first collection day of apheresis in Arms D, A, B, and C was 8.0, 11.0, 15.3, and 26.4. The median total number of CD34+ stem cells collected following mobilization with Elt plus C and G-CSF in Arms D, A, B, and C was 13.2, 12.7, 15.4, and 26.4. The percentage of patients in Arms D, A, B, and C who achieved a target collection of 8 x 10^6 CD34+ stem cells in one collection day was 50, 60, 75, and 100%. There have been no severe adverse events related to Elt .
Conclusions: Administration of Elt in combination with C plus G-CSF for stem cell mobilization in pts with MM undergoing ASCT was safe and well tolerated, with no DLTs or severe AEs attributable to Elt. The small size of this pilot study precludes formal statistical comparison of outcomes across treatment Arms, but there appears to be a trend toward increase in yield of CD34+ cells and decrease in apheresis procedures required with increasing doses of Elt.

Disclosures
Richardson: Jazz Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees.