High Rates of Early Donor Chimerism in Intermediate and Poor Risk Patients Undergoing Stem Cell Transplantation Using Reduced Toxicity Ablative Conditioning Regimen Incorporating Busulfan Pharmacokinetics

Abstract Reduced toxicity ablative conditioning regimens are increasingly used for allogeneic stem cell transplantation (allo-SCT). The impact of early donor cell chimerism on outcomes of T cell–replete reduced-intensity conditioning SCT in myeloid disorders is ill defined. To explore the impact of measuring busulfan pharmacokinetics in conditioning regimens on early donor chimerism, we undertook a retrospective analysis of patients with myeloid disorders who received four days of fludarabine and busulfan with or without measuring busulfan pharmacokinetics at our center in the last 10 years. Methods: Chimerism assay was performed using a quantitative fluorescence-based short tandem repeat– polymerase chain reaction (STR-PCR) with capillary electrophoresis for PCR product resolution. Results: Thirty patients were identified and included in the analysis. All patients were conditioned with fludarabine (40 mg/m2/day x 4 doses), busulfan (3.2mg/kg/dose IV x 4 doses). Of these thirty patients, 7 had busulfan pharmacokinetics measured. There were 21 male and 9 female patients with a median age of 62 years (range 48–72yrs). Median time to follow up was 13.3 months. Diagnoses included AML (N=17), MDS (N=10), MPN (N=1), CMML (N=2). Disease risk was considered advanced in 17 patients, intermediate in 3 and early in 10. All patients in the busulfan pharmacokinetic group had advanced disease except one had early disease. Regarding cytogenetics, all patients in the busulfan pharmacokinetics group had high or intermediate risk cytogenetics. Median blast number at time of SCT was 5%. Stem cell source included peripheral blood in all patients. Eleven patients received matched related donor (MRD) SCT, 15 had matched unrelated donor (MUD) SCT and 4 had mismatched unrelated donor (MMUD) SCT. Prophylaxis of GVHD consisted of tacrolimus and methotrexate in MRD, and tacrolimus, methotrexate and anti-thymocyte globulin in MUD and MMUD. All patients engrafted neutrophils and platelets promptly (median 13 and 14 days, respectively). There were no primary graft failures. Total Donor cell Chimerism analysis in the Busulfan pharmacokinetics group showed 100% donor at both time points (days 30, 100) in all patients except in one who relapsed at day 30 (85.7%). While in the non-pharmacokinetics group only 7 out of 23 (30%) patients had complete chimerism at day 30 and day 100. Complete donor chimerism at day 100 in the non-pharmacokinetics group was 47.8% compared to 85.7% in the pharmacokinetic group (p=0.18). Ten out of 23 patients (43.5%) in the non-pharmacokinetics group had decreasing donor chimerism by day 100, while in the pharmacokinetic group only one patient (14%) who relapsed had decreasing donor chimerism by day 100 with an odds ratio of 0.241 (95% Confidence Interval =0.025-2.357; p-value=0.22). None developed sinusoidal obstructive syndrome. Conclusion: In this small cohort of consecutive patients from a single center, we found that patients with myeloid disorders who received fludarabine busulfan for 4 days incorporating busulfan pharmacokinetics had a trend for higher rates of early complete donor chimerism and less decreasing donor chimerism by day100, although not statistically significant, despite having intermediate or high risk disease at time of SCT. Longer follow up is needed for our patients to see if there is effect on relapse or survival but previous studies have showed that low or decreasing donor chimerism early after SCT is an independent risk factor for relapse and impaired survival. This is especially important in myeloid disorders. Busulfan pharmacokinetics may help target better level for inducing early total donor chimerism and donor chimerism may identify high-risk patient cohorts who may benefit from additional therapeutic interventions. These results should be interpreted with caution given the retrospective design of the study and the small number of patients. Disclosures No relevant conflicts of interest to declare.