Effect of food on pharmacokinetics of chlorambucil and its main metabolite, phenylacetic acid mustard

Population pharmacokinetics identifies rapid gastrointestinal absorption and plasma clearance of oral chlorambucil administered to cats with indolent lymphoproliferative malignancies

OBJECTIVE

To establish the pharmacokinetics of a single 2-mg oral dose of chlorambucil in cats with indolent lymphoproliferative malignancies.

ANIMALS

24 client-owned cats.

PROCEDURES

Cats were assigned to 1 of 4 groups, with each group having a total of 3 sample collection time points over 12 hours after receiving a single 2-mg oral dose of chlorambucil. Each time point combined to generate 6 full patient plasma chlorambucil concentration-time curves from the 24 cats. Chlorambucil treatment was continued every other day and a single, variably timed sample collection was obtained on day 14. Population parameter estimates were obtained by nonlinear mixed-effects modeling. Covariates investigated included age, sex, baseline serum cobalamin, study location, weight, and body condition score.

RESULTS

Chlorambucil administered orally to cats was found to have a peak plasma concentration of approximately 170 ng/mL (SE, 31.1 ng/mL), percent coefficient of variation (%CV) of 18.4% within 15 minutes, and a terminal half-life of 1.8 hours (SE, 0.21 hour; %CV, 12.4). At the 4-hour mark, a smaller secondary peak in plasma chlorambucil was found. Day 14 samples were similar to those of the initial dose. No covariates showed a significant effect in the population model.

CLINICAL RELEVANCE

In these cats, chlorambucil at a 2-mg dose administered every other day undergoes rapid gastrointestinal absorption and plasma clearance with no drug accumulation between doses. These data are critical to inform future work investigating the association of chlorambucil drug exposure with adverse events and outcome of cats with lymphoproliferative diseases.

Effects of Food on the Clinical Pharmacokinetics of Anticancer Agents

Pharmacokinetic interactions between food and orally administered drugs involve changes mainly in the absorption and metabolism of a drug, and may have clinical implications. Such interactions, in particular, may be of major clinical significance for cancer chemotherapy since the majority of anticancer agents are toxic, have a low therapeutic index and are administered long term, most often in combination with other cytotoxic agents. The purpose of this review is to compare the pharmacokinetic profiles of various anticancer drugs, including chemopreventive agents that have been examined previously in fasted and fed conditions, and to discuss the underlying basis/mechanisms of food effect in light of a drug's physicochemical and pharmacokinetic properties. Clinical pharmacokinetic parameters such as maximum concentration, area under the concentration-time curve, time to maximum concentration and half-life for each drug are compared in fasted and fed states, and specific dietary recommendations are summarised accordingly. In addition, the effects of food on the metabolite kinetics and pharmacodynamic responses, and the potential role of food effect in the modulation of oral biovariability and multidrug resistance have been extensively discussed. Overall, this comprehensive pharmacokinetic analysis indicates that a broad spectrum of food effects is seen among anticancer agents because of diverse factors regulating each drug's oral bioavailability and its interactions with food. The consideration of such effects is important, as it could lead to more rational pharmacological monitoring and possibly improve the oral chemotherapy of cancer in children, adults and the elderly.

Food-drug interactions

Interactions between food and drugs may inadvertently reduce or increase the drug effect. The majority of clinically relevant food-drug interactions are caused by food-induced changes in the bioavailability of the drug. Since the bioavailability and clinical effect of most drugs are correlated, the bioavailability is an important pharmacokinetic effect parameter. However, in order to evaluate the clinical relevance of a food-drug interaction, the impact of food intake on the clinical effect of the drug has to be quantified as well. As a result of quality review in healthcare systems, healthcare providers are increasingly required to develop methods for identifying and preventing adverse food-drug interactions. In this review of original literature, we have tried to provide both pharmacokinetic and clinical effect parameters of clinically relevant food-drug interactions. The most important interactions are those associated with a high risk of treatment failure arising from a significantly reduced bioavailability in the fed state. Such interactions are frequently caused by chelation with components in food (as occurs with alendronic acid, clodronic acid, didanosine, etidronic acid, penicillamine and tetracycline) or dairy products (ciprofloxacin and norfloxacin), or by other direct interactions between the drug and certain food components (avitriptan, indinavir, itraconazole solution, levodopa, melphalan, mercaptopurine and perindopril). In addition, the physiological response to food intake, in particular gastric acid secretion, may reduce the bioavailability of certain drugs (ampicillin, azithromycin capsules, didanosine, erythromycin stearate or enteric coated, and isoniazid). For other drugs, concomitant food intake may result in an increase in drug bioavailability either because of a food-induced increase in drug solubility (albendazole, atovaquone, griseofulvin, isotretinoin, lovastatin, mefloquine, saquinavir and tacrolimus) or because of the secretion of gastric acid (itraconazole capsules) or bile (griseofulvin and halofantrine) in response to food intake. For most drugs, such an increase results in a desired increase in drug effect, but in others it may result in serious toxicity (halofantrine).

Chromatographic analysis of anticancer drugs

The present review on the methods for the analysis of anticancer drugs should be seen as an addition to the excellent work of Eksborg and Ehrsson published half a decade ago in this journal (Vol. 340, p.31). The style and format have been followed closely, with the focus again on chromatographic techniques. We felt it important to add a list of compound (group) structures as a service to the reader. Methods have been reviewed for alkylating agents, platinum compounds, antitumour antibiotics, antimetabolites, alkaloids, suramin, 1-hydroxy-3-amino-propylidene-1,1-bisphosphonate and tamoxifen.

Concentration and time-dependent inter-relationships for cytotoxicities of nitrogen mustard drugs against lymphoblasts in-vitro

Peripheral lymphoblasts were exposed either to different initial concentrations of the alkylating agents (melphalan, chlorambucil or phenylacetic acid mustard) using a fixed incubation time or a constant [3H]methylthymidine incorporation into the trichloracetic acid-insoluble fraction of the cells. The concentration-time relationships were evaluated by calculating the amount of drug which had chemically reacted in the incubation system. Melphalan showed lower cytotoxicity at short exposure times and high drug concentrations, while chlorambucil exhibited higher cytotoxicity at longer exposure times. In the latter case the effect could be accounted for by the cytotoxic activity of monohydroxy chlorambucil which was formed in the incubation system.

Inter- and intraindividual differences in oral chlorambucil pharmacokinetics

Inter- and intraindividual variation in the pharmacokinetics of oral chlorambucil was investigated in patients with chronic lymphocytic leukaemia. Five patients were given in randomized order 15, 40, 60 and 70 mg chlorambucil p.o. and plasma was analyzed for chlorambucil and its cytotoxic metabolite phenyl acetic acid mustard. The area under the plasma concentration-time curve of chlorambucil varied two- to fourfold between patients at each dose level. The AUC of the metabolite was higher and showed twofold interindividual variation. The intraindividual dose-corrected AUCcs also varied twofold. No dose dependency of the dose corrected AUC was seen either for chlorambucil or the metabolite. Elimination of both compounds form plasma was rapid, with half-lives of 1.01 and 1.94, respectively.

Studies on the mechanism of gastrointestinal absorption of melphalan and chlorambucil

The uptake of melphalan into tumour cells has been shown previously to involve active transport, while that of chlorambucil is by passive diffusion. In view of these findings, the mechanism of their gastrointestinal absorption was investigated using the in situ rat intestinal model. Segment lengths in all cases were (mean +/- SD) 47.2 +/- 4.7 cm. Drug absorption was monitored from control intestinal segments and from segments pretreated with the metabolic inhibitors 2,4 dinitrophenol (DNP) or carbonylcyanide-M-chlorophenyl-hydrazone (CCCP). Aliquots of gut-perfusing solution were removed at 5-min intervals over 30 min and assayed for drug using a high-performance liquid chromatography (HPLC) method selective for the alkylating agents. Absorption of melphalan by control animals was (mean +/- SD) 1.22% +/- 0.25% cm-1 gut length, as against 0.59% +/- 0.13% cm-1 in DNP- and 0.45% +/- 0.07% cm-1 in CCCP-treated animals. Absorption of chlorambucil was 1.47% +/- 0.17% cm-1 (control), 1.49% +/- 0.06 cm-1 (DNP), and 1.58% +/- 0.23% cm-1 (CCCP). It was clear, therefore, that pretreatment of intestinal segments with metabolic inhibitors led to a reduced absorption of melphalan (P less than 0.01) but did not influence that of chlorambucil. The experimental data suggest that melphalan uptake from the intestine involves an energy-dependent system whereas chlorambucil is passively absorbed.

Can food affect the bioavailability of chlorambucil in patients with haematological malignancies?

Pharmacokinetic studies in ten patients with haematological disorders were undertaken on the first and second days of one course of chemotherapy. Patients received chlorambucil under fasting and non-fasting conditions. Plasma concentrations of chlorambucil were determined by a reversed-phase high-performance liquid chromatography assay. Statistical analysis by the Wilcoxon signed rank test for non-parametric data indicated that food caused a significant reduction in peak plasma levels (P less than 0.01), elimination rate constants (P less than 0.01) and area under the plasma chlorambucil/time curve (P = 0.01). Food was also found to prolong the time taken to attain peak plasma levels (P less than 0.01). Regression analysis of renal function with elimination rate constants showed that chlorambucil elimination was independent of renal function (n = 8; r = -0.007; P = 0.72). In view of these results we suggest that chlorambucil is given on an empty stomach.

Drug level monitoring: cytostatics

The present review on the quantification of cytostatic drugs has mainly been focussed on chromatographic techniques. Special attention has been paid to the precautions that have to be taken into account to ensure the selectivity and accuracy of the various methods. The various cytostatics that have been dealt with are: alkylating agents, antimetabolites, vinca alkaloids, antibiotics, cis-diamminedichloroplatinum, podophyllotoxine derivatives, and nitrosoureas.