Discussion
The concept of biosimilar products essentially refers to copies of biologic products whose patent protection has expired. Given the complexity of manufacturing biologics, these products are obtained differently from generics (copies of small-molecule drugs) through a specific regulatory process. In this context, obtaining a biosimilar status requires evidence of physico-chemical, pharmacological and clinical similarity, including comparative trials for at least one ”sensitive” indication for which the reference product is licensed. The most important benefits of biosimilars include reduced healthcare expenditures and ease of access.9
G-CSF is primarily produced by fibroblasts, bone marrow endothelial cells, monocytes and macrophages. G-CSF exerts its main effect by ensuring the production, maturation, mobilization and survival of neutrophils.10 It also plays a role in chemotaxis by stimulating the release of arachidonic acid from neutrophils, production of leukocyte alkaline phosphatase (LAP), myeloperoxidase, and superoxide anions.11 In addition to being used to prevent or shorten the duration of severe neutropenia after chemotherapy, especially in the pediatric age group, G-CSF is also commonly used for stem cell mobilization. Currently, while PBSC mobilized with G-CSF is widely used as a stem cell source, stem cells are rarely obtained from BM, and G-CSF is rarely used to provide mobilization in this setting.12
Although PBSC is the more commonly preferred source of stem cells for adult patients, BM remains the main stem cell source for standard allogeneic HSCT performed in children due to a variety of conditions, including certain malignancies, hemoglobinopathies, bone marrow failure syndromes, immunodeficiency, and congenital metabolic diseases.13
Compared to BM, a PBSC graft contains roughly 3-10 times as many CD34+ and CD3+ cells. Preparation by using G-CSF for stem cell collection from BM substantially alters the growth and proliferative properties of the bone marrow cells. After G-CSF stimulation to healthy donors, the number of CD34+ cells increase by 26-fold on average in peripheral blood, while this increase is only 1.4-1.7-fold in the bone marrow. These results suggest that G-BM stem cells may acquire proliferative properties leading to a faster hematopoietic recovery after HSCT. Furthermore, modulating the expression of certain adhesion molecules involved in the pathogenesis of GvHD, such as VLA-4, ICAM-1, L-selectin, and LFA-1 as well as the migration and homing of T-cells, significantly reduces the development of GvHD in G-BM. Selective inhibition of these adhesion molecules may explain the reduced incidence of GvHD after G-BM transplantation.14
The objective of G-BM transplantation is to ensure faster engraftment without increasing the risk of GvHD and allow the patient’s recovery in a shorter period of time. When the outcomes in patients receiving G-BM are compared to historic records or case-control cohort studies of patients receiving BM only, study results reveal that G-BM patients achieve higher numbers of total nucleated cells, CD34+ cells, CFU-GM, and faster neutrophil and platelet engraftment. 7,15
BM prepared with G-CSF is an alternative stem cell source that can provide a higher number of stem cells without increasing the risk of GvHD. Kim et. al. reported faster engraftment in their study including 33 patients who underwent G-BM transplantation (median time: 13 days for neutrophils and 18 days for platelets) as well as a lower incidence of acute GvHD (> grade II, 12%) and chronic GvHD (34%), and the cumulative incidence of TRM was 15%. The probability of 10-year overall and event-free survival (EFS) was 68% and 62%, respectively.16 Similarly, Isola et. al. observed faster neutrophil and platelet engraftment in 10 patients who received G-CSF 10 g/kg two days before allogeneic BM harvesting compared to the control group who underwent BM transplantation without G-CSF.15
In a prospective randomized study comparing G-CSF-stimulated BM and PBSC in adult patients with malignant disease, there was no significant difference in the duration of neutrophil and platelet engraftment, but when compared in terms of GvHD risk, PBSC recipients had a much higher risk of acute (17% vs. 46%) and chronic (27% vs. 77%) GvHD.3
While the ideal G-CSF dose for PBSC is well-established in studies, the ideal G-CSF dose for preparation before BM harvesting appears to be less understood. A prospective multicenter study in children who received allogeneic BM from HLA-identical siblings showed that G-CSF administration in the pediatric age group was well tolerated and resulted in a high dose of nucleated and CD34+ cells after using G-CSF at a dose of 5 μg/kg for five days.2,3
In a single-center study of a small number of pediatric patients, the investigators used a dose of 5 μg/kg for three days before BM harvesting and compared the results with similar patients who did not receive G-CSF stimulation. Patients who received G-CSF prior to BM harvesting had higher doses of TNC and CD34+ cells; however, faster engraftment was not achieved although all patients received post-transplant G-CSF. In a different prospective study involving 57 patients, after receiving 10 μg/kg of G-CSF daily for 5 days, patients were randomized to G-CSF-stimulated BM (28 patients) and G-CSF-stimulated PBSC (G-PBSC) (29 patients). Patients in the G-PBSC group received three-fold CD34+ cells and nine-fold CD3+ cells than the patients in the G-BM group. Median times to neutrophil and platelet engraftment were similar. The cumulative incidences of refractory aGvHD and extensive cGvHD were significantly higher for the G-PB group. It was concluded by the authors that G-BM results in less severe aGvHD and less cGvHD compared to G-PB graft. Post-transplant recurrence rate and OS were similar.8 A comprehensive review of the literature shows numerous studies demonstrating that the use of BM prepared with G-CSF provides faster neutrophil and platelet engraftment in recipients, with lower rates of GvHD and TRM.
There are many studies of PBSC that involve different G-CSF products applied for stem cell mobilization in HSCT and compare the results obtained with these products. Similarly, while there are studies that compare the doses of G-CSF used for cell mobilization from BM or test whether this approach should be adopted, there is no study comparing different G-CSF products administered and investigating the effect of G-CSF products on the collected BM product. When we evaluated the results of our patients in light of the available information, we found that lenograstim, which was administered as a single dose 24 hours before transplantation, resulted in obtaining a higher level of CD34/UL compared to the control group and the groups that received filgrastim or biosimilar filgrastim. In a PBSC study which compared lenograstim, filgrastim and biosimilar filgrastim, similar to our study, no difference was found in terms of CD34/UL. However, the target CD34 cell count was reached with an apheresis procedure in 87% of the donors receiving lenograstim and 93% of the donors receiving biosimilar filgrastim. 17 In a different multicenter study involving a large number of adult patients who underwent autologous transplantation, lenograstim, filgrastim and biosimilar filgrastim were compared, and no difference was found between the groups in terms of CD34+ progenitor cells (×106/kg). 18 In another study involving 243 cases of allogeneic PBSC by Sivgin et al., no difference was found in terms of CD34/UL obtained from healthy donors using lenograstim, filgrastim or biosimilar filgrastim, and biosimilar filgrastim (Leucostim®) was found to be comparable with original Filgrastim (Neupogen®) and lenograstim (Granocyte®) for PBSC mobilization in donors of patients undergoing allo-HSCT.19
The limitation of our study is that it was not conducted with a large number of subjects in the donor and control groups, especially in the pediatric age group. We acknowledge that the content of the post-transplantation product should have been evaluated in a high number of donors and included in this study. We also believe the late effects of biosimilar filgrastim (Leucostim®), original filgrastim (Neupogen®) and lenograstim (Granocyte®) should be evaluated. The strength of our study is that it is the first to use different G-CSF molecules for the mobilization of stem cells from the bone marrow, and the first to compare the results of different G-CSFs with one another as well as with a control group.
In conclusion, biosimilar filgrastim (Leucostim®), original filgrastim (Neupogen®) and lenograstim (Granocyte®) can be safely used for BM mobilization in donors of patients undergoing allo-HSCT. However, lenograstim (Granocyte®) should be preferred with regard to median CD34/UL count. Further studies are required to determine the ideal dose and number of administrations in this setting. Our results warrant being supported by prospective studies with larger case series.