Peri-Transplant Alemtuzumab Levels Dictate the Risks of Acute Gvhd and Mixed Chimerism

Introduction:  Alemtuzumab, fludarabine, and melphalan reduced intensity conditioning (RIC) regimens are increasingly used for pediatric patients with non-malignant diseases.  We have previously reported that the incidences of acute graft versus host disease (GVHD) and mixed chimerism are both influenced by the timing and dose of alemtuzumab.  We speculated that these effects were due to variations in peri-transplant levels of alemtuzumab and resultant differences in extent of graft T cell depletion. 

Hypothesis:  We hypothesized that low or absent levels of alemtuzumab would result in elevated risk for acute GVHD but low risk of mixed chimerism, and that high levels of alemtuzumab would result in low risk of acute GVHD but increased risk of mixed chimerism. 

Methods:  We measured alemtuzumab levels on the day of graft infusion +/- 3 days in 90 patients who received an alemtuzumab, fludarabine, and melphalan RIC regimen.  We created cumulative incidence curves to compare the incidences of acute GVHD grades II-IV and mixed chimerism in patients with alemtuzumab levels of 0.1ug/mL or less versus 0.2ug/mL or greater.  We used cause-specific Cox hazard regression to model the risks of acute grades II-IV GVHD or mixed chimerism (defined as whole blood donor chimerism of less than 95% on two or more consecutive occasions) taking into account the covariates of age, diagnosis, HLA Match, graft source, and alemtuzumab level (ug/mL, continuous variable).  All computations were done using R (The R Foundation for Statistical Computing, Vienna, Austria).  Final models were selected using stepwise selection.  Statistical significance was considered for p<0.05. 

Results:  The cumulative incidence of acute GVHD grades II-IV was 48% in patients with alemtuzumab levels of 0.1 ug/mL or less, and 18% in patients with levels of 0.2ug/mL or greater (p<0.001).  The cumulative incidence of mixed chimerism was 34% in patients with alemtuzumab levels of 0.1 ug/mL or less, and 48% in patients with levels of 0.2ug/mL or greater (p=0.24).  In multivariate analysis, we found that alemtuzumab level was the only significant factor associated with the risks of acute grades II-IV GVHD and mixed chimerism.  Increasing levels of alemtuzumab decreased the risk of grades II-IV acute GVHD (HR 0.495, CI 0.282-0.867, p=0.014).  Conversely, increasing levels of alemtuzumab increased the risk of mixed chimerism (HR 1.169, CI 1.033-1.324, p=0.014).  Receipt of a cord blood graft also influenced the risk (HR 4.134, p=0.068) but did not reach statistical significance, perhaps due to small sample size. 

Conclusion:  We conclude that peri-transplant levels of alemtuzumab dictate the risks for acute GVHD and mixed chimerism.  Detailed pharmacokinetic studies are needed to optimize and individualize alemtuzumab dosing for pediatric patients in order to minimize these complications.