This is the first study to evaluate F-ara-A exposure and its relationship to stem cell engraftment, GVHD, TRM, and survival in nonmyeloablative allogeneic HCT. We showed that F-ara-A exposure is highly variable with 5.7 fold difference in AUC in patients receiving fludarabine at 40mg/m2. Our data also demonstrates that higher F-ara-A concentrations are associated with a greater risk of TRM and decreased survival. Finally, in preparative regimens that use 40mg/m2 of fludarabine, dose reductions greater than 20-25% should be considered in patients presenting for transplant with renal impairment.
The pharmacokinetic parameters in our study were consistent to those previously described in HCT patients.19-20
Studies outside the setting of HCT demonstrate F-ara-A clearance correlates with CrCl, with F-ara-A urinary excretion as high as 60% of the total administered dose.13
In phase I trials of fludarabine a decline in renal function was associated with a decrease in total body clearance and volume of distribution.18
As as result, fludarabine dose reductions of 20-25% are recommended by the manufacturer in patients with CrCl< 70 ml/min.27
In our study, the nine patients with pre-existing mild to moderate renal impairment (median CrCl 57.1ml/min), despite receiving a 20-25% dose reduction, had higher plasma concentrations, reduced clearance of F-ara-A, and longer half-life compared to patients not receiving dose reductions (median CrCl 85.9ml/min). Therefore, dose reductions >20-25% may be required in HCT patients with mild to moderate renal impairment in preparative regimens that use fludarabine 40mg/m2
Our analyses showed a strong relationship between high plasma F-ara-A exposure, TRM and overall survival. This is not surprising since higher exposure may result in greater tissue damage, and more potent and longer immunosuppression. A high comorbidity score and poor renal function may also be associated with greater TRM, however, they had no independent effect in the multivariate analyses. Patients also received cyclophosphamide and TBI. The metabolism and clearance of cyclophosphamide and its metabolites have been shown to be influenced by renal function and increased concentrations are associated increased risk of toxicity in HCT.28-29
Given that multiple factors influence TRM, accumulation of F-ara-A plasma concentrations may be a surrogate marker of overall preparative regimen intensity.
Higher doses of fludarabine (40-100 mg/m2
) have been associated with delayed onset severe neurotoxicity including progressive multifocal leukoencephalopathy (PML), coma and death. Early phase I/II trials conducted in relapsed leukemia estimated an 18% incidence of severe fludarabine neurotoxicity at these doses.30-31
In allogeneic HCT the incidence of neurotoxicity and risk factors are not well defined. Fludarabine has been suggested as a potential risk factor for neurotoxicity following umbilical cord transplantation.32
Patients with elevated systemic plasma concentrations of F-ara-A may experience higher drug concentrations in the central nervous system. However, several confounding variables must be considered. PML resulting from opportunistic infection with JC virus has been reported in chronic lymphocytic leukemia (CLL) patients treated with fludarabine.33-36
More recently MMF has been associated with cases reports of PML.37
Finally, all patients in this study were treated with cyclosporine. Neurotoxicity, including posterior reversible leukoencephalopathy, may occur in up to 30% of patients receiving cyclosporine.38
Irreversible encephalopathy with the use of cyclosporine in pediatric patients have also been reported.39
We found no influence of F-ara-A pharmacokinetic measures on neutrophil engraftment. However, our engraftment rate was high thereby reducing our ability to detect an effect. Studies in relapsed CLL patients comparing chemotherapy responders vs non-responders suggest that sufficient F-ara-A plasma concentrations are needed to maintain a minimum inhibitory F-ara-ATP concentration in tumor cells.12
It is possible that the higher fludarabine dose, long terminal half-life, combined with the 5 day dosing regimen leads to sufficient intracellular F-ara-ATP needed for engraftment. Interestingly, detectable plasma F-ara-A was still present on day of transplant in all patients. It is possible that patients receiving lower doses of fludarabine (25-30 mg/m2
) may not exceed these thresholds. Future studies in HCT should incorporate the evaluation of intracellular F-ara-ATP concentrations and the potential role of genetic polymorphisms involved in fludarabine pathway including cellular transporters, kinases and targets.40-42
Engraftment is a complex process and is influenced by several factors including cell dose, donor source, and MMF exposure and substantially larger sample sizes will be required to evaluate both genetic and non-genetic effects.43
We found a strong association between higher plasma concentrations of fludarabine and increased risk of TRM and reduced overall survival following nonmyeloablative HCT. Whether these relationships hold true for other preparative regimens, especially those containing lower doses of fludarabine have yet to be established. Renal insufficiency led to higher concentrations of fludarabine and dose reductions are warranted to avoid excessive systemic exposure. Future studies are needed to further define the minimal level of F-ara-A exposure required for stem cell engraftment so that toxicity is minimized.