Page 44 - Haematologica-5

776
F. Boulad et al.
normal healthy donors, in that fewer HPC may be mobi- lized by plerixafor than by G-CSF, where up to 50% of G- CSF-mobilized CD34+ cells are CD38-negative.13,40
Because we enrolled only one patient on chronic trans- fusion, we cannot assess any correlation between transfu- sion and CD34 mobilization, although notably this patient had the second highest baseline and highest peak CD34+ cell counts in our study. Other studies of plerixafor in SCD4,32 have initiated chronic transfusion based on the hypothesis that the inflammatory nature of SCD affects the bone marrow and transfusion assuages bone marrow inflammation and stress erythropoiesis. Although replica- tive and oxidative stress of HPC in bone marrow may occur,41-44 there is limited evidence that HPC are damaged in SCD.36 Five of our patients had HbF-associated increas- es in hemoglobin concentration and hematocrit to more than 10 g/dL and 30%, respectively (similar to values in chronically transfused patients) but HbF levels did not cor- relate with CD34 cell mobilization.
Based on our data, it is possible that continued dose esca- lation could result in greater efficacy of mobilization, since we observed a dose-related response in the median CD34+ cell fold increase (P=0.01), as also observed in healthy donors.45 Patient 3, a repeat enrollment who had never been on hydroxyurea and was clinically stable, is instruc- tive in that his 12 h peak CD34+ cell count following a pler- ixafor dose of 80 μg/kg was only 8/μL whereas at the dose of 240 μg/kg it was 40/μL, even though his baseline CD34+ cell concentrations (1/μL and then 2/μL) were similar, sug- gesting a dose-response to plerixafor. Notably, his two periods in the study were separated by 19 months, sug- gesting that, as with healthy donors,46 intra-individual CD34+ cell counts in stable SCD patients not on hydrox- yurea may remain stable over time. Based on these data and given the safety and continued dose response between 240 μg/kg and 480 μg/kg observed in healthy donors,20 we plan to continue dose escalation in SCD patients through to the 480 μg/kg dose, barring significant adverse events. Adding the CXCR2 agonist, GROβ, might be useful.47
Only two of 15 patients (13%) developed serious adverse events as compared to three of seven patients (43%) in the study of plerixafor mobilization in SCD by Tisdale et al.,4 although this must be qualified by the fact that the patients in the study by Tisdale et al. also underwent leukapheresis. Our low rate of serious adverse events could, however, also be due to chronic hydroxyurea therapy and the subsequent lower WBC and ANC peaks. As the fraction of activated neutrophils did not increase significantly with plerixafor, our low serious adverse event rate may be related to mod- eration of ANC elevations by hydroxyurea, reducing the absolute number of activated cells. Given the still uncertain risks of morbidity, as seen with G-CSF, the use of plerixafor in SCD requires further evaluation.
In summary, our present data suggest that, with regards the efficacy of CD34 mobilization, red blood cell transfu-
sion may be more effective than continuing standard of care. Whether red blood cell transfusion will remain more effective as we escalate the plerixafor dose (as safety allows) to 480 μg/kg, with protocol revisions for hydrox- yurea-treated patients, is unknown. Finally, potential can- didates for SCD gene therapy may not be able to receive regular red blood cell transfusions (e.g. if they have red cell alloimmunization or a history of hyperhemolysis) or may not be willing to do so (e.g. Jehovah Witnesses), even for the relatively short duration of 2-3 months.
This study has several limitations. Firstly, despite this study being the largest study to date of plerixafor adminis- tration in SCD patients, overall the number of patients involved remains small; thus comparisons, for example, between hydroxyurea-treated and non-hydroxyurea-treat- ed patients, may not be representative of the actual popu- lations. Secondly, we measured WBC, ANC and CD34 mobilization in this study only at ~12 and ~20 h after pler- ixafor administration. It is possible that an initial peak could have occurred at an earlier time (6-9 h) after plerix- afor and could, therefore, have been missed. Nevertheless, CD34 cell concentrations remain at ~70% of peak levels at 12 h.20,45,48 Our current study will be amended to include the addition of earlier post-plerixafor assessments. Thirdly, we determined peripheral blood CD34 cell mobilization in the 15 patients treated with plerixafor, without performing apheresis. However, there is a well-described correlation between peripheral blood CD34 cell concentration and the ultimate CD34 cell dose obtained after apheresis. It is pos- sible that technical adjustments may be required for this equation in the context of SCD. Finally, other than enu- merating CD34+CD38- cells, we did not further character- ize CD34+ cells to study “stemness”, for example by deter- mining glycophorin A positivity and CD34 dimness.4 CD34+ or CD34+CD38- enumeration is not specific for HSC49 and it is, therefore, unclear whether patients had a true increase in HSC, as opposed to more mature lineage- committed CD34+ progenitors, which are either mobilized or present in bone marrow.42 We plan to characterize the CD34+ cells further as we move forward in the study, which is currently enrolling at the 320 μg/kg dose level. Despite mobilization of HSC possibly being less efficient with plerixafor than with G-CSF, plerixafor-mobilized HSC may have an engraftment advantage over G-CSF- mobilized HSC with regard to better retention of CXCR4, which facilitates homing.32
Acknowledgments
The authors would like to thank our study subjects for their participation; the Doris Duke Charitable Foundation for a 2011 Innovation in Clinical Research Award for trial support (to PAS and MS); Sanofi-Genzyme for provision of plerixafor; Jena Simon for referring one study patient; W. Beau Mitchell for assis- tance with platelet activation studies; and Henny Billett, Narla Mohandas, and Beth Shaz for departmental support.
References
1. Mansilla-Soto J, Riviere I, Sadelain M. Genetic strategies for the treatment of sickle cell anaemia. Br J Haematol. 2011;154(6): 715-727.
2. Ribeil JA, Hacein-Bey-Abina S, Payen E, et al. Gene therapy in a patient with sickle cell disease. N Engl J Med. 2017;376 (9):848-855.
3. Sadelain M, Boulad F, Lisowki L, Moi P, Riviere I. Stem cell engineering for the treat- ment of severe hemoglobinopathies. Curr Mol Med. 2008;8(7):690-697.
4. Tisdale JF, Pierciey Jr. JF, Kanter J, et al.
Successful Plerixafor-mediated mobilization, apheresis, and lentiviral vector transduction of hematopoietic stem cells in patients with severe sickle cell disease. Blood. 2017;130 (Suppl 1):990.
5. Fitzhugh CD, Hsieh MM, Bolan CD, Saenz C, Tisdale JF. Granulocyte colony-stimulat- ing factor (G-CSF) administration in individ-
haematologica | 2018; 103(5)