It is desirable to develop a fast method for quantification of melphalan due to its instability. Here we report a method for quantification of melphalan (MPL) in human plasma using a UPLC-PDA system. Briefly, 50 µL plasma sample was mixed with 25 µL internal standard (2500 ng/mL acetylmelphalan in methanol) and 25 µL 20% trichloroacetic acid, and centrifuged at 21,000 g (15,000 rpm) at 4 °C for 3 min. The supernatant (5 µL) was injected onto an Acquity™ BEH C18 LC column (2.1 × 50 mm, 1.7 µm) and eluted with 25 mM NH₄AC (pH 4.7)-acetonitrile in a gradient mode at a flow rate of 0.6 mL/min. The column kept at 40 ± 5 °C and the autosampler kept at 4 ± 5 °C. The detector set at 261 nm, and sampling rate was 40points/sec. The retention times were typically 2.11 min for melphalan and 2.38 min for the internal standard. Total run time is 4 min per sample. Calibration range was 100-40,000 ng/mL. The lower limit of quantification was 100 ng/mL. The method was validated based on the FDA guidelines, and applied to a clinical pharmacokinetic study in pediatric patients.

A prospective multicenter study was conducted to characterize the pharmacokinetics (PK) and pharmacodynamics (PD) of fludarabine plasma (f-ara-a) and intracellular triphosphate (f-ara-ATP) in children undergoing hematopoietic cell transplantation (HCT) and receiving fludarabine with conditioning. Plasma and peripheral blood mononuclear cells (PBMCs) were collected over the course of therapy for quantitation of f-ara-a and f-ara-ATP. Nonlinear mixed-effects modeling was used to develop the PK model, including identification of covariates impacting drug disposition. Data from a total of 133 children (median age, 5 years; range, .2 to 17.9) undergoing HCT for a variety of malignant and nonmalignant disorders were available for PK-PD modeling. The implementation of allometric scaling of PK parameters alone was insufficient to describe drug clearance, particularly in very young children. Renal impairment was predicted to increase drug exposure across all ages. The rate of f-ara-a entry into PBMCs (expressed in pmoles per million cells) decreased over the course of therapy, resulting in 78% lower f-ara-ATP after the fourth dose (1.7 pmoles/million cells [range, .2 to 7.2]) compared with first dose (7.9 pmoles/million cells [range, .7 to 18.2]). The overall incidence of treatment-related mortality (TRM) was low at 3% and 8% at days 60 and 360, respectively, and no association with f-ara-a exposure and TRM was found. In the setting of malignancy, disease-free survival was highest at 1 year after HCT in subjects achieving a systemic f-ara-a cumulative area under the curve (cAUC) greater than 15 mg*hour/L compared to patients with a cAUC less than 15 mg*hour/L (82.6% versus 52.8% P = .04). These results suggest that individualized model-based dosing of fludarabine in infants and young children may reduce morbidity and mortality through improved rates of disease-free survival and limiting drug-related toxicity. ClinicalTrials.gov Identifier: NCT01316549.