Mobilization and Elimination of FLT3-ITD+ Acute Myelogenous Leukemia (AML) Stem/Progenitor Cells by Plerixafor/G-CSF/Sorafenib: Results From a Phase I Trial in Relapsed/Refractory AML Patients

G (10 ug/kg) and P(240 ug/kg) were given s.c. QOD on days 1 – 13, S (400–600mg) on d 1 – 28(one cycle). G/P was held when blasts exceeded 5×10^4/uL. Cell populations expressing CD34, 38, 123, CXCR4 (1D9, 12G5), VLA4, CD44 and phospho-proteins were assessed at baseline and at multiple time points during treatment by flow cytometry of up to 10 parameters and by flow cytometric mass spec using CyTOF.

13 patients have been treated so far; responses are as follows: 1 CR, 3 CRp, 6 PR and 4 failed (NR), for an overall response rate of 10/13 (77%); One patients achieved 2 CRp. Six/13 patients, including 3/6 responders and 3/4 NR were previously treated with and considered refractory to FLT3 inhibitors. Four patients had additional D835 mutations: 2 failed and 2 achieved PRs, none of the CR/p patients carried this mutation.

Side effects included hyperleukocytosis in 3/10 pts.(who missed 1 to 5 doses of G/P), skin rash (5 pts.), hand foot syndrome (3 pts.) hypertension (7 patients), diarrhea (10 pts.), nausea (8 pts.), headache (6 pts.), muscle weakness (3 pts.) and anorexia (5 pts.). Analysis of cells mobilized in 22 treatment cycles revealed massive mobilization: a 29-fold increase in WBC, 41-fold in absolute blasts and 77-fold in granulocytes. Increases in the numbers of circulating stem/progenitor cells: CD34+: 231-fold, CD34+/38-: 90-, CD34+/38-/123+(LSC): 148-, CXCR4+: 139-, VLA-4+: 68- and CD44+: 82-fold. Increase in circulating LSC was positively correlated with baseline blasts and VLA4 levels, but not with baseline CXCR4. Serial FISH analyses confirmed the preferential mobilization of leukemic vs. non-leukemic cells and 10-color flow cytometry demonstrated altered levels of pERK and pAKT but not of pSTAT3 in mobilized cells. Surprisingly, CXCR4 levels in mobilized cells were increased. CyTOF analysis of up to 29 parameters documented mobilization of primitive LSC.

The combination of G-CSF+Plerixafor appears superior in increasing the number of circulating leukemic blasts and stem/progenitor cells in FLT3-ITD AML, as compared to Plerixafor alone in R/R AML(blast increase 2.1-fold; Uy et al. Blood, 2012). Treatment resulted in 4/13 CR and CRp and 6/13 PRs, for an overall response rate of 77%. Mobilized stem/progenitor cells displayed altered MAPK/AKT signaling and increased CXCR4 expression. This is the first clinical study of G-CSF/Plerixafor for the “mobilization” of AML cells, aimed at removing them from their protective bone marrow microenvironment and the initial results are providing proof-of–concept and encouraging clinical responses.

Off Label Use: Clofarabine in AML. Burger:Pharmacyclics: Consultancy, Research Funding. Kantarjian:Genzyme: Research Funding.