Post-transplant cyclophosphamide (PTCy) and mycophenolate mofetil (MMF) are commonly used in combination with tacrolimus or sirolimus as standard-of-care prophylaxis of graft-versus-host disease (GVHD) for allogeneic hematopoietic cell transplant (alloHCT). However, both Cy and MMF are prodrugs that undergo complex metabolism in vivo, leading to interindividual variability in pharmacokinetic (PK) exposure and, consequently, variation in pharmacological effects. However, whether the interindividual variation in pharmacokinetic (PK) exposure impact treatment outcomes remains elusive.

We conducted a pilot observational study to estimate the interindividual variation in PK exposure and investigated its association with immune reconstitution. We assessed PK biomarkers that are associated with Cy and MMF in patients undergoing HLA-haploidentical alloHCT with standard high-dose PTCy (50mg/kg/day on days +3/+4). The plasma concentration profiles of 6 PK biomarkers were captured, including Cy, Carboxycyclophosphamide (Cepm), N-Dechloroethyl Cyclophosphamide (Dccy), 4-ketocyclophosphamide (Ketocy), mycophenolic acid (MPA), and mycophenolic acid glucuronide (MPAG). The exposure to each metabolite was calculated as area under the curve (AUCs) using the trapezoidal method.

The PK data showed large interindividual variability across all PK biomarkers, with 1 study subject who experienced profoundly higher exposure to Cepm, MPA, and MPAG. Overall, interindividual variability in drug exposure, as reflected by fold differences in AUCs (including the noted outlier), was 2.6-fold for Cy, 19.1-fold for Cepm, 12.5-fold for Dccy, 2.9-fold for Ketocy, 3.9-fold for total MPA, and 12.3-fold for total MPAG.

To investigate the association between PK biomarkers and immune reconstitution, we monitored the dynamic recovery of lymphocytes and T-cell subsets, and subsequently performed correlation analysis between immune recovery data and AUCs of the six metabolites. The data showed that three patients who showed slow lymphocyte recovery (less than 100 count/ul on day +30) also had significantly higher exposure to all metabolites. Correlation analysis confirmed the consistent inverse relation between the AUCs of Cepm, MPAG, and the absolute lymphocyte concentration up to day +60 post HCT. This finding indicates that high exposure to Cy and MMF metabolites negatively impact lymphocyte recovery. Furthermore, serum creatinine and blood urea nitrogen levels were found to be positively correlated with AUCs of Cepm, MPA, and MPAG, suggesting that higher exposure to metabolites is related to kidney function.

This study revealed substantial inter-patient variability in drug exposure to PTCy and MMF. Furthermore, it demonstrated that elevated exposure to Cy and MMF metabolites is associated with delayed lymphocyte recovery. The findings also suggest that renal dysfunction may increase exposure to Cy and MMF metabolites. Our data supported the need for further research in larger cohorts to investigate the relationship between pharmacokinetic biomarkers and clinical outcomes.

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2025
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