Older age does not influence allogeneic peripheral blood stem cell mobilization in a donor population of mostly white ethnic origin

To the editor:

Several factors that affect peripheral blood stem cell (PBSC) mobilization were recently identified in a large cohort of healthy donors; factors included ethnic origin, weight, and the total dose of rhG-CSF received for mobilization.¹  However, the median age of this population was 40 years, which does not necessarily reflect current practices in allogeneic stem cell transplantation since the decrease of transplant-related mortality associated with the use of reduced-intensity conditioning (RIC) regimens has led to offering this therapeutic modality to older patients, and in turn soliciting older related donors. The question whether age has a negative impact on mobilization remains controversial.^1-5 

We retrospectively evaluated 129 consecutive related adult donors who underwent PBSC mobilization and collection at the Institut Paoli-Calmettes (Marseille, France) between January 2005 and December 2007. Median age was 51 years (range, 19-70 years) among 44 donors aged 55 years or older. All donors received 4 to 5 days of G-CSF (filgrastim; Amgen, Thousand Oaks, CA) given once daily in the evening, with the first leukapheresis initiated on the morning of day 5. Although the recommended dose for rhG-CSF is 10 μg/kg per day for stem cell mobilization, most donors received a total dose of rhG-CSF that was rounded to the lowest multiple of 300 μg, thus allowing the daily use of a whole number of vials: as a consequence, the median received dose per kilogram of body weight of rhG-CSF was 8.9 μg, which may contribute to the lower number of circulating CD34⁺ cells observed in our cohort of patients (59.5/μL), compared with the recently published cohort of North American donors (84/μL).¹ 

To identify factors affecting circulating CD34⁺ cell counts as a surrogate maker for stem cell mobilization, we first performed a univariate analysis, including age, sex, weight, height, total G-CSF dose, and G-CSF dose per kilogram of body weight. Ethnicity was not tested as an explanatory factor for stem-cell mobilization, because our cohort of donors was smaller and more homogeneous, mostly with individuals of white descent. Variables associated with higher post-G-CSF CD34⁺ cell counts were donor weight (P < .003) and the total dose of G-CSF received (P < .001), thus reproducing recently published results.¹  By multivariate analysis, only the total dose of G-CSF received remained significant (P < .001).

Interestingly, our group of recent donors was more heterogeneous in terms of age, and therefore might more accurately reflect current practices in allogeneic transplantation from related donors. It is important to stress that age did not significantly influence CD34⁺ cell counts (P = .344). When looking at subgroups according to age, no significant difference was found between the mobilization procedure and endpoints for older donors (age ≥ 55, n = 44, of whom 10 were older than 70 years) compared with younger donors (age < 55, n = 85; Table 1).

Our results contrast with the modest negative effect reported in the previously mentioned study. The absence of deleterious effect of older age on progenitor cell mobilization is important to consider in view of the increasing use of older donors, in the context of RIC allo-SCT.