Population pharmacokinetics of cyclophosphamide, doxorubicin and etoposide in 30 patients with BEACOPP chemotherapy

Population pharmacokinetics of doxorubicin: A systematic review

Because of the high interindividual pharmacokinetic variability, several population pharmacokinetic (PopPK) models of doxorubicin (DOX) were developed to characterize factors influencing such variability. However, significant predictors for DOX pharmacokinetics identified using PopPK models varied across studies. Thus, this review aims to summarize PopPK models of DOX and its metabolites (if any) as well as significant covariates influencing DOX (and its metabolites) pharmacokinetic variability. A systematic search from PubMed, CINAHL Complete, Science Direct, and SCOPUS databases identified 503 studies. Of these, 16 studies met the inclusion criteria and were included in this review. DOX pharmacokinetics was described with two- or three-compartment models. Most studies found a significant increase in DOX clearance with an increase in body surface area from the median value of 1.8 m² . Moreover, this review identified that while a 10-year increase in patient age resulted in a decrease in DOX clearance in adults and the elderly, younger children had lower DOX clearance compared to older children. Further, low DOX exposure was observed in pregnant women, and thus dosage adjustment is required. Concerning model applicability, predictive performance assessment of these published models should be performed before implementing such models in clinical practice.

Population pharmacokinetics of doxorubicin and doxorubicinol in patients diagnosed with non-Hodgkin's lymphoma

AIMS

The aims of the study were: (i) to characterize the pharmacokinetics (PK) of doxorubicin (DOX) and doxorubicinol (DOXol) in patients diagnosed with non-Hodgkin's lymphoma (NHL) using a population approach; (ii) to evaluate the influence of various covariates on the PK of DOX; and (iii) to evaluate the role of DOX and DOXol exposure in haematological toxicity.

METHODS

Population PK modelling (using NONMEM) was performed using DOX and DOXol plasma concentration-time data from 45 NHL patients treated with R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine and prednisone). The influence of drug exposure on haematological toxicity was analysed using the Mann-Whitney-Wilcoxon test.

RESULTS

A five-compartment model, three for DOX and two for DOXol, with first-order distribution and elimination for both entities best described the data. Population estimates for parent drug (CL) and metabolite (CL_m ) clearance were 62 l h^-1 and 27 l h^-1 , respectively. The fraction metabolized to DOXol (F_m ) was estimated at 0.22. While bilirubin and aspartate aminotransferase showed an influence on the CL and CL_m , the objective function value decrease was not statistically significant. A trend towards an association between the total area under the concentration-time curve (AUC_total ), the area under the concentration-time curve for DOX (AUC) plus the area under the concentration-time curve for DOXol (AUC_m ), and the neutropenia grade (P = 0.068) and the neutrophil counts (P = 0.089) was observed, according to an exponential relationship.

CONCLUSIONS

The PK of DOX and DOXol were well characterized by the model developed, which could be used as a helpful tool to optimize the dosage of this drug. The results suggest that the main active metabolite of DOX, DOXol, is involved in the haematological toxicity of the parent drug.

Population pharmacokinetics of doxorubicin: establishment of a NONMEM model for adults and children older than 3 years

PURPOSE

The aim of the current investigation was to develop a population pharmacokinetic model for doxorubicin and doxorubicinol that could provide improved estimated values for the pharmacokinetic parameters clearance of doxorubicin, volume of distribution of the central compartment, clearance of doxorubicinol and volume of distribution of the metabolite compartment for adults and children older than 3 years. A further aim was to investigate the potential influence of the covariates body surface area, body weight, body height, age, body mass index, sex and lean body mass on the pharmacokinetic parameters.

METHODS

Three different datasets, two containing data from adults and one containing data from adults and children, were merged and the combined dataset was analysed retrospectively. In total, the combined dataset contained 934 doxorubicin and 935 doxorubicinol plasma concentrations from 82 patients [64 adults and 18 children (<18 years)]. With this combined dataset, a population pharmacokinetic model was developed, using NONMEM(®) 7.2 and a predefined model-building strategy. Different structural models, error models and estimation methods were tested, and the inter-individual and the inter-occasion variability (variability between separate (two or three) doxorubicin infusions) were tested. Using a subset of 52 patients, the influence of different covariates on the pharmacokinetic parameters was investigated. The pharmacokinetic parameter estimates obtained from doxorubicin concentrations with the best model were fixed, and an additional compartment for doxorubicinol was added to the model. With the final model for both substances, a potential age dependency and body mass index dependency of the clearance of doxorubicin and doxorubicinol as well as of the volumes of distribution of the central and the metabolite compartment were evaluated.

RESULTS

A four-compartment model best described the doxorubicin and doxorubicinol data of the combined dataset. This model included a proportional residual error model and an inter-individual variability on the clearance of doxorubicin, on the inter-compartmental clearances of the peripheral compartments, on the clearance of doxorubicinol and on the volumes of distribution of the central, one peripheral and the metabolite compartment. Furthermore, the body surface area as covariate on all pharmacokinetic parameters and an inter-occasion variability for the clearance of doxorubicin and the volume of distribution of the central compartment were incorporated in the model. For a patient with the body surface area of 1.8 m², the clearance of doxorubicin was 53.3 L/h (inter-individual variability 31%, inter-occasion variability 13%) and the volume of distribution of the central compartment was 17.7 L (inter-individual variability 19%, inter-occasion variability 21%), respectively. The residual variability of the model was 22% for doxorubicin and 26% for doxorubicinol. The clearance of doxorubicinol was estimated at 44 L/h (inter-individual variability 50%) and the volume of distribution of the metabolite compartment at 1,150 L (inter-individual variability 57%). The evaluation of a possible age dependency and body mass index dependency showed a trend to a smaller volume of distribution of the central compartment (normalised to the body surface area) and a higher volume of distribution of the metabolite compartment (normalised to the body weight) in younger patients.

CONCLUSIONS

A four-compartment NONMEM(®) model for doxorubicin and doxorubicinol adequately described the plasma concentrations in adults and children (>3 years). No pronounced effects of age on the clearance of doxorubicin or doxorubicinol were found, and the analysis did not support the modification of the dosing strategies presently used in children and adults.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

Clinical pharmacokinetics of cyclophosphamide

Cyclophosphamide is an extensively used anticancer and immunosuppressive agent. It is a prodrug undergoing a complicated process of metabolic activation and inactivation. Technical difficulties in the accurate determination of the cyclophosphamide metabolites have long hampered the assessment of the clinical pharmacology of this drug. As these techniques are becoming increasingly available, adequate description of the pharmacokinetics of cyclophosphamide and its metabolites has become possible. There is incomplete understanding on the role of cyclophosphamide metabolites in the efficacy and toxicity of cyclophosphamide therapy. However, relationships between toxicity (cardiotoxicity, veno-occlusive disease) and exposure to cyclophosphamide and its metabolites have been established. Variations in the balance between metabolic activation and inactivation of cyclophosphamide owing to autoinduction, dose escalation, drug-drug interactions and individual differences have been reported, suggesting possibilities for optimisation of cyclophosphamide therapy. Knowledge of the pharmacokinetics of cyclophosphamide, and possibly monitoring the pharmacokinetics of cyclophosphamide in individuals, may be useful for improving its therapeutic index.