Enhanced Cytarabine and Daunorubicin Population Pharmacokinetics When Administered As CPX-351: A Novel Liposomal Formulation Not Requiring Dose Reduction for Mild Renal or Hepatic Dysfunction

Introduction: CPX-351 (Vyxeos) is a novel liposomal formulation of cytarabine and daunorubicin in a fixed 5:1 molar ratio that has been studied in a phase III clinical trial for secondary acute myeloid leukemia. The current study investigated pharmacokinetic and pharmacodynamic relationships of this liposomal formulation.

Methods: CPX-351 pharmacokinetic data from a Phase I study of patients (median age: 62, range 24-81 years) with relapsed and refractory acute myeloid leukemia were used for the analysis. CPX-351 was given as a 90-minute infusion on days 1, 3, and 5 of induction therapy at doses ranging from 3 to 134 Units/m² (1 unit: 1 mg cytarabine/0.44 mg daunorubicin). Plasma samples were obtained between 45 minutes and 11 days after the start of the day 1 infusion. Cytarabine, daunorubicin, and the concentration of metabolites (uracil arabinoside, daunorubicinol) were determined using liquid chromatography-tandem mass spectrometry. Liposomal membranes were dissolved prior to cytarabine and daunorubicin measurements thus drug concentrations represented combined liposomal and non-liposomal drug. Population pharmacokinetic modeling was completed using nonlinear mixed effects modeling (NONMEM v. VII). The relationship between exposure (AUC) and response data including: 1. achievement of complete remission (logistic regression) and 2. time to recovery of WBC or platelets over the course of initial induction therapy (Cox proportional hazard regression) were explored using statistical software (SAS v. 9.4).

Results: 39 patients (3589 samples) were evaluated. Median (range) of laboratory values prior to treatment were: serum creatinine 0.90 (0.6-1.6) mg/dL, total bilirubin 0.60 (0.20 -1.8) mg/dL, AST 28 (12 - 124) U/L, and ALT 27 (15-151) U/L. Liposomal cytarabine and daunorubicin were evaluatedseparately as one compartment models with their respective metabolites (uracil arabinoside, daunorubicinol) added to each model and best fit as a two compartment metabolite model. Weight was an independent predictor of liposomal cytarabine and daunorubicin and metabolite volumes of distribution (Vd), however age, gender, AST, ALT, total bilirubin, and serum creatinine were not independent predictors of clearance (CL) or Vd. The final model demonstrated a liposomal CL of 0.094 and 0.108 L/hr and Vd of 4.6 and 3.5 L for cytarabine and daunorubicin, respectively. Inter-subject variability was 52% and 44% for CL and 117% and 146% for liposomal Vd for cytarabine and daunorubicin. Liposomal cytarabine and daunorubicin had terminal half-lives of 33.9 and 22.0 hr as compared to the published values of 3.0 and 18.5 hr for non-liposomal cytarabine and daunorubicin, respectively. The typical AUCs for the maximum tolerated dose of 101 U/m² were 1990.6 and 762.1 mcg*hr/mL for cytarabine and daunorubicin respectively, which were a thousand-fold greater than published non-liposomal values. Complete remission after first induction was achieved in 21% of patients (8 out of 39). Despite the large dose and AUC ranges (60.9 - 4901.6 cytarabine, 29.0 - 1572.9 daunorubicin mcg*hr/mL) identified, no significant differences were present between the achievement of complete remission and AUC for cytarabine (p=0.37) or daunorubicin (p=0.26). WBC recovery was achieved at a median of 28 days (n=15; range 21-53 days) while platelet recovery occurred at a median of 35 days (n= 13; range 14-42 days). The remaining patients did not achieve WBC or platelet recovery following initial induction due to residual disease and the majority received a second induction. AUC vs. time to WBC recovery (p=0.077 cytarabine, p=0.0531 daunorubicin) showed a trend toward significance, however no associations were seen with AUC and time to platelet recovery (p=0.60 cytarabine, p=0.81 daunorubicin).

Conclusions: The liposomal cytarabine-daunorubicin formulation led to a low clearance and small volume of distribution strongly supporting slow release with prolonged exposure and that no dose modification will be needed in this patient population based on renal or hepatic function. Increased exposure may lead to longer time to WBC recovery, but this will need to be confirmed with larger studies.

Support: Celator Pharmaceuticals, Inc., a subsidiary of Jazz Pharmaceuticals, plc.