Doxorubicinol Concentrations Research Articles

Population pharmacokinetic modelling of doxorubicin and doxorubicinol in children with cancer: is there a relationship with cardiac troponin profiles?

Anthracyclines are a mainstay of the treatment of several childhood malignancies, but their utility is limited by dose-related cardiotoxicity. This study is aimed to explore the link between exposure of paediatric cancer patients to doxorubicin and its metabolite doxorubicinol, and cardiac troponin I (cTnI). In a prospective pilot study plasma doxorubicin, doxorubicinol, and cTnI concentrations were measured in samples from children undergoing cancer chemotherapy. A mixed-effects population pharmacokinetic model for doxorubicin and doxorubicinol and in combination with a turn-over model for cTnI were developed. Seventeen patients, aged 3.4-14.7year, treated for a variety of cancers had 99 doxorubicin and 119 doxorubicinol concentrations analysed from samples drawn between 0.5 and 336h after the start of the infusion. Eleven patients had received previous doses of anthracyclines, with a median cumulative prior dose of 90mg/m2 (range 0-225mg/m2). The median administered doxorubicin dose was 30mg/m2 (range 25-75mg/m2). Doxorubicin disposition was described by a three-compartment model with first-order elimination and metabolism to doxorubicinol. Body surface area was related to all clearance and distribution parameters and age further influenced clearance (CL, 58.7L/h/1.8m2 for an average 8.4-year-old patient). Combined doxorubicin and metabolite exposure stimulated a temporary increase in cTnI in plasma, with a concentration of 11.8µg/L required to achieve half-maximal effect. Prior cumulative anthracycline dosage received by patients was predictive of an increased cTnI baseline prior to a new doxorubicin dose. Prior anthracycline exposure increased baseline cTnI in a dose-dependent manner, consistent with the known cumulative risk of anthracycline exposure-induced cardiotoxicity.

A Model-Based Approach To Assessing the Importance of Intracellular Binding Sites in Doxorubicin Disposition.

Doxorubicin is an anticancer agent, which binds reversibly to topoisomerase I and II, intercalates to DNA base pairs, and generates free radicals. Doxorubicin has a high tissue:plasma partition coefficient and high intracellular binding to the nucleus and other subcellular compartments. The metabolite doxorubicinol has an extensive tissue distribution. This porcine study investigated whether the traditional implementation of tissue binding, described by the tissue:plasma partition coefficient (Kp,t), could be used to appropriately analyze and/or simulate tissue doxorubicin and doxorubicinol concentrations in healthy pigs, when applying a physiologically based pharmacokinetic (PBPK) model approach, or whether intracellular binding is required in the semi-PBPK model. Two semi-PBPK models were developed and evaluated using doxorubicin and doxorubicinol concentrations in healthy pig blood, bile, and urine and kidney and liver tissues. In the generic semi-PBPK model, tissue binding was described using the conventional Kp,t approach. In the binding-specific semi-PBPK model, tissue binding was described using intracellular binding sites. The best semi-PBPK model was validated against a second data set of healthy pig blood and bile concentrations. Both models could be used for analysis and simulations of biliary and urinary excretion of doxorubicin and doxorubicinol and plasma doxorubicinol concentrations in pigs, but the binding-specific model was better at describing plasma doxorubicin concentrations. Porcine tissue concentrations were 400- to 1250-fold better captured by the binding-specific model. This model adequately predicted plasma doxorubicin concentration-time and biliary doxorubicin excretion profiles against the validation data set. The semi-PBPK models applied were similarly effective for analysis of plasma concentrations and biliary and urinary excretion of doxorubicin and doxorubicinol in healthy pigs. Inclusion of intracellular binding in the doxorubicin semi-PBPK models was important to accurately describe tissue concentrations during in vivo conditions.

A comparison of Doxorubicin and Doxorubicinol concentrations in rat heart and liver tissue following anthracycline administration

Doxorubicin (DOX) is an anti‐cancer chemotherapeutic and although it is widely used, its accumulation over time in different tissues has not been thoroughly investigated. The purpose of this study was to examine the concentration of both DOX and its metabolite (Doxorubicinol, DOXol) in the heart and liver of male Sprague‐Dawley rats. Two doses (1.5 mg/kg and 4.5 mg/kg) were injected intraperitoneally and tissues were collected at 24, 48, 72 and 96 hrs post‐injection. In the heart, at all time points, DOX concentrations were higher (p<0.05) following the 4.5 vs 1.5 mg/kg injection (0.78±0.07 vs 1.81±0.26 nmol/g at 24 hrs, 0.37±0.06 nmol/g vs 0.96±0.06 nmol/g at 96 hrs). It was observed that DOX heart concentrations steadily decreased (p<0.05) over time for the 4.5 mg/kg dose, whereas for the 1.5 mg/kg dose, concentrations remained constant for the first 72 hrs and then decreased (p<0.05) at 96 hrs. In the liver, at all time points and for both doses, DOX concentrations were higher (p<0.05) as compared to the heart (3.85±1.1 and 5.29±0.87 nmol/g in the liver vs 0.78±0.07 and 1.81±0.26 nmol/g in the heart, at 24 hrs). While DOXol was detected in small concentrations in both heart and liver tissue, no accumulation was observed over the 96 hrs. These data suggest that in both heart and liver that DOX accumulates in a dose dependent fashion. In contrast, DOXol does not accumulate to any degree in either tissue, suggesting that it is rapidly degraded or eliminated from the tissues.Supported by NSERC.

Abstract No. 189: Drug pharmacokinetics after hepatic arterial delivery of doxorubicin loaded superabsorbent polymeric microspheres in an animal model

Abstract No. 189: Drug pharmacokinetics after hepatic arterial delivery of doxorubicin loaded superabsorbent polymeric microspheres in an animal model

Doxorubicin-Loaded QuadraSphere Microspheres: Plasma Pharmacokinetics and Intratumoral Drug Concentration in an Animal Model of Liver Cancer

The purpose of this study was to evaluate, in vitro and in vivo, doxorubicin-loaded poly (vinyl alcohol-sodium acrylate) copolymer microspheres [QuadraSphere microspheres (QSMs)] for transcatheter arterial delivery in an animal model of liver cancer. Doxorubicin loading efficiency and release profile were first tested in vitro. In vivo, 15 rabbits, implanted with a Vx-2 tumor in the liver, were divided into three groups of five rabbits each, based on the time of euthanasia. Twenty-five milligrams of QSMs was diluted in 10 ml of a 10 mg/ml doxorubicin solution and 10 ml of nonionic contrast medium for a total volume of 20 ml. One milliliter of a drug-loaded QSM solution containing 5 mg of doxorubicin was injected into the tumor feeding artery. Plasma doxorubicin and doxorubicinol concentrations, and intratumoral and peritumoral doxorubicin tissue concentrations, were measured. Tumor specimens were pathologically evaluated to record tumor necrosis. As a control, one animal was blandly embolized with plain QSMs in each group. In vitro testing of QSM doxorubicin loadability and release over time showed 82-94% doxorubicin loadability within 2 h and 6% release within the first 6 h after loading, followed by a slow release pattern. In vivo, the doxorubicin plasma concentration declined at 40 min. The peak doxorubicin intratumoral concentration was observed at 3 days and remained detectable till the study's end point (7 days). Mean percentage tumor cell death in the doxorubicin QSM group was 90% at 7 days and 60% in the bland QSM embolization group. In conclusion, QSMs can be efficiently loaded with doxorubicin. Initial experiments with doxorubicin-loaded QSMs show a safe pharmacokinetic profile and effective tumor killing in an animal model of liver cancer.

Abstract CN13-03: Intraductal route of breast cancer prevention and therapy: from preclinical studies to Phase I trials

Abstract CN13-03 We hypothesized that it is possible to prevent and treat breast cancer by accessing the mammary ductal network through the teat. This intraductal approach, which delivers a high dose of drug locally to the tumors but a low dose systemically, could be an alternative approach to therapy and prevention of breast cancer. Our previous work has demonstrated the effectiveness of intraductal administration of pegylated liposomal doxorubicin, (PLD) and 4-hydroxytamoxifen (4-OHT) using the rat N-methyl N’-nitrosourea (MNU)-induced and the spontaneous, HER2/neu transgenic mouse (neu-N) models of breast cancer. 4-OHT had a preventive effect in the rat MNU-induced models, and PLD had a preventive and therapeutic effect in both the rat and Her2/neu transgenic mouse models (Murata et al, Cancer Res. 2006). Recently we evaluated the effectiveness of other drugs such as carboplatin, methotrexate, Abraxane, 5-Fluorouracil and PLD by whole mount analysis of the treated mammary glands, histopathology following H&E staining, and immunohistochemistry for the proliferation marker, Ki67. We found that carboplatin, 5FU and PLD-treated groups showed significant protection in the prevention setting. PLD had long term effects on the mammary gland structure and function in treated rats and mice. In response to pregnancy, significantly reduced proliferation and milk production was observed in PLD treated Her2/neu mice compared to vehicle treated mice. The litter size and weight at birth were normal in the PLD treated group. Notably, unlike the control group, the PLD-treated Her2/neu transgenic mice remained protected from spontaneous tumor development despite exposure to vast hormonal modulations inherent to pregnancy. Based on the preclinical findings, we initiated a clinical trial to determine the feasibility, safety, and maximum tolerated dose of PLD administered into one duct of women with breast cancer awaiting a mastectomy. PLD was administered using a dose escalation schema based on our preclinical pharmacokinetic and safety data. Serial doxorubicin and doxorubicinol concentrations were determined in plasma and tissue by LC/MS/MS. At the time of mastectomy, blue dye was injected into the treated duct and tissue was obtained for pharmacokinetic and biomarker analysis. To date 14 women entered the study, and doses of 0 to 10 mg PLD per were tested without serious adverse events. Intraductal PLD was associated with a dose-dependent increase in both systemic and local exposure to doxorubicin and doxorubicinol. Neither doxorubicin nor the metabolite was detected in contralateral breast tissue. Doxorubicin and doxorubicinol were detectable in the breast tissue and plasma of women receiving the highest dose (10 mg/duct). Systemic exposure (in plasma) was lower than with conventionally administered PLD. No inflammatory or morphological changes were seen in the breast tissue. The study established safety and feasibility of administering PLD in an outpatient setting. Higher doses of PLD will be tested to achieve the maximum tolerated dose. Future studies will evaluate other agents administered to one or more ducts. In summary, these approaches may find particular use in treatment of DCIS as a safer alternative to surgery, and prevention of breast cancer in women at high risk of developing breast cancer due to inherited mutations or other factors. High local, but low circulating levels of drug, and thereby lower systemic toxicity are the major advantages of the ductal access to breast cancer treatment. This study demonstrates that intraductal injection provides more direct access to breast lesions, and has potential in the prevention and neo-adjuvant therapy of breast cancer. Citation Information: Cancer Prev Res 2008;1(7 Suppl):CN13-03.

Influence of ABCB1 and ABCG2 polymorphisms on doxorubicin disposition in Asian breast cancer patients

The influence of three high frequency ABCB1 polymorphisms (c.1236C>T, c.2677G>A/T, and c.3435C>T) and the ABCG2 c.421C>A polymorphism on the disposition of doxorubicin in Asian breast cancer patients receiving adjuvant chemotherapy was investigated in the present study. The allelic frequency of the ABCB1 c.1236T, c.2677T, c.2677A, and c.3435T variants were 60%, 38%, 7%, and 22%, respectively, and the frequency of the ABCG2 c.421A allele was 23%. Pairwise analysis showed increased exposure levels to doxorubicin in patients harboring at least one ABCB1 c.1236T allele (P = 0.03). Patients homozygous for the CC-GG-CC genotype had significantly lower doxorubicin exposure levels compared to the patients who had CT-GT-CT (P = 0.02) and TT-TT-TT genotypes (P = 0.03). Significantly increased clearance of doxorubicin was also observed in patients harboring CC-GG-CC genotypes when compared to patients harboring the CT-GT-CT genotype (P = 0.01). Patients harboring the CC-GG-CC genotypes had significantly lower peak plasma concentrations of doxorubicinol compared to patients who had TT-TT-TT genotypes (P = 0.03). No significant influences on doxorubicin pharmacokinetic parameters were observed in relation to the ABCG2 c.421C>A polymorphism. In conclusion, the present exploratory study suggests that the three high frequency linked polymorphisms in the ABCB1 gene might be functionally important with regards to the altered pharmacokinetics of doxorubicin in Asian breast cancer patients, resulting in significantly increased exposure levels and reduced clearance.

Toxicokinetics of the active doxorubicin metabolite, doxorubicinol, in sulphur-crested cockatoos ( Cacatua galerita)

The pharmacokinetics of doxorubicinol, a cytotoxic metabolite of the anticancer drug, doxorubicin, were studied in four healthy sulphur-crested cockatoos ( Cacatua galerita) after a 20 min intravenous infusion of 2 mg/kg. Plasma doxorubicinol concentrations were measured by HPLC. The pharmacokinetic parameters were estimated using a non-compartmental method. The mean (±SD) peak concentration was 8341 ± 3132 μg/L at 17.5 ± 5.0 min after the start of the infusion, and doxorubicinol concentrations declined biexponentially to 154.3 ± 34.5 μg/L, 40 min after the end of the infusion. Systemic clearance was 0.940 ± 0.473 L/h/kg, mean residence time was 0.165 ± 0.133 h, and steady-state volume of distribution was 0.123 ± 0.0526 L/kg. The terminal half-life was 0.660 ± 0.611 h. Detectible but unquantifiable concentrations of doxorubicinol were present in the plasma ultrafiltrate of two birds during the infusion, indicating very extensive plasma protein binding. Physiological, haematological and biochemical monitoring over 3 weeks showed that doxorubicinol at a single infused dose of 2 mg/kg caused no toxicities of major concern.

Evaluation of the effects of dietary n-3 fatty acid supplementation on the pharmacokinetics of doxorubicin in dogs with lymphoma

To determine the effect of dietary n-3 fatty acids on the pharmacokinetics of doxorubicin in dogs with lymphoma. 23 dogs with lymphoma in stages IIIa, IVa, and Va. Dogs receiving doxorubicin chemotherapy were randomly allocated to receive food with a high (test group) or low (control group) content of n-3 fatty acids. Serum doxorubicin and doxorubicinol concentrations were measured via high-performance liquid chromatography before and 6 to 9 weeks after initiation of the diets. Lymph node concentrations of doxorubicin were assessed 6 hours after the initial treatment. Dogs' body composition was assessed by means of dual-energy x-ray absorptiometry scans. No significant differences in doxorubicin pharmacokinetics were detected between treatment groups. Significant differences existed between the first and second sampling times among all dogs for area under the curve, maximum serum concentration, and clearance. Differences in body composition did not affect measured pharmacokinetic variables. The terminal elimination half-life was longer in dogs in which a long-term remission was achieved than in dogs that did not have remission. Dietary supplementation of n-3 fatty acids is common in veterinary patients with neoplasia, but supplementation did not affect doxorubicin pharmacokinetics in this population of dogs. Explanations for the beneficial effects of n-3 fatty acids other than alterations in the pharmacokinetics of chemotherapy drugs should be investigated. Dogs may metabolize drugs differently prior to remission of lymphoma than when in remission. The pharmacokinetics of doxorubicin at the time of the first administration may predict response to treatment.

Effect of dexrazoxane on doxorubicin pharmacokinetics in young and old rats.

Although dexrazoxane (ICRF-187) is used clinically to protect against doxorubicin cardiotoxicity, the age-related effect of dexrazoxane on doxorubicin pharmacokinetics has not been well studied. We therefore examined the effect of pretreatment with dexrazoxane (50 mg kg(-1) i.p. 1 h prior to administration of doxorubicin 2 mg kg(-1) i.v. bolus) on doxorubicin and doxorubicinol pharmacokinetics in Fischer 344 rats at 5 months of age (young adult) and 22 months of age (old). Dexrazoxane had no major effects on doxorubicin or doxorubicinol pharmacokinetics in plasma or heart in either young or old rats. However, age had significant effects on anthracycline pharmacokinetics. Early plasma concentrations were increased and systemic clearance of doxorubicin was decreased in old compared with young rats. Cardiac concentrations of doxorubicin (AUC) were significantly increased in old rats. In addition cardiac doxorubicinol concentrations (AUC 0-72 h) were increased by over 80% in old compared to young rats. The results suggest that dexrazoxane does not alter doxorubicin pharmacokinetics. In contrast, aging in the rat model is associated with altered doxorubicin and doxorubicinol pharmacokinetics, in particular in the heart. These changes could increase the risk of anthracycline cardiotoxicity with age.

Time‐related increases in cardiac concentrations of doxorubicinol could interact with doxorubicin to depress myocardial contractile function

1. The present study evaluated the time-dependency of acute anthracycline cardiotoxicity by varying the duration of exposure of rabbit isolated atria to doxorubicin and determining changes (1) in contraction and relaxation and (2) in atrial concentrations of doxorubicin and its C-13 hydroxy metabolite, doxorubicinol. 2. Following addition of doxorubicin (175 microM) to atria, contractility (dF/dt), muscle stiffness (resting force, RF) and relaxation (90% relaxation time, 90% RT) were monitored for a 3.5 h period. 3. Doxorubicin (175 microM) progressively diminished mechanical function (decreased dF/dt, increased RF and prolonged 90% RT) over 3 h. Doxorubicinol (1.8 microM), however, failed to produce time-related cardiac dysfunction; it depressed contractile function and increased muscle stiffness during the first 30 min without causing additional cardiac dysfunction during the remaining 3 h of observation. Doxorubicinol had no effect on 90% RT. 4. During treatment with doxorubicin, atria contained considerably more doxorubicin than doxorubicinol (ratio of doxorubicin to doxorubicinol ranged from 778 to 74 at 0.5 and 3 h, respectively). Elevations of doxorubicin and doxorubicinol in atria paralleled the degree of dysfunction of both contraction and relaxation; increases in muscle stiffness, however, were more closely associated with increases of doxorubicinol than doxorubicin. 5. To probe the relation between cardiac doxorubicinol and myocardial dysfunction further, without confounding effects of cardiac doxorubicin, concentration-response experiments with doxorubicinol (0.9-7.2 microM) were conducted. 6. Plots of doxorubicinol concentrations in atria vs contractility indicated that the cardiac concentration of doxorubicinol, at which contractility is reduced by 50%, is five fold lower in doxorubicin-treated than in doxorubicinol-treated preparations. Thus, doxorubicin and doxorubicinol appear to interact to depress contractile function.7. Cardiac concentrations of both doxorubicin and doxorubicinol, as observed in these studies, were found to stimulate markedly Ca2+ release from isolated SR vesicles, but 3 microM doxorubicinol promoted a 15 fold greater release rate than 3 microM doxorubicin.8. Our observations coupled with the previously reported finding that doxorubicinol inhibits Ca2+loading of SR, suggests that doxorubicinol accumulation in heart contributes to the time-dependent component of doxorubicin cardiotoxicity, through a mechanism that could involve perturbations of Ca2+ homeostasis.

The Disposition of Doxorubicin on Repeated Dosing

Twelve cancer patients (aged 49-74 years) receiving doxorubicin (66 +/- 8 mg/m2) as a 1-hour intravenous infusion had serial serum samples (0-48 hours) obtained after the first and second courses of therapy. Mean number of days between courses was 24.3, and all patients had normal liver function. Patients received the same concomitant antineoplastic agents and doses in both courses. Doxorubicin and doxorubicinol concentrations were assayed by high-performance liquid chromatography and fitted to a two- or three-compartment infusion model. White blood cell and platelet toxicity were evaluated as (initial--nadir/initial)* 100. Differences in pharmacokinetic parameters were determined by a paired t test. Wide intrapatient and interpatient variability was seen between therapeutic courses. A significant decrease in the apparent volume of distribution of the central compartment (Vc = 16.6 versus 10.4 L/m2; P < .05), and a nonsignificant decrease in clearance (CL = 748 versus 658 mL/min/m2) was observed on the second course of therapy. Doxorubicinol area under the curve and elimination half-life were similar between courses. Extensive chemotherapy-induced changes in white blood cell and platelet counts were observed but were similar in degree for courses 1 and 2. These data suggest that higher initial doxorubicin concentrations on the second course of therapy are secondary to an alteration in distribution volume (Vc). In this subset of patients, however, these changes were not associated with an increase in hematologic toxicity.

Doxorubicin and doxorubicinol pharmacokinetics and tissue concentrations following bolus injection and continuous infusion of doxorubicin in the rabbit

Cumulative dose-related, chronic cardiotoxicity is a serious clinical complication of anthracycline therapy. Clinical and animal studies have demonstrated that continuous infusion, compared to bolus injection of doxorubicin, decreases the risk of cardiotoxicity. Continuous infusion of doxorubicin may result in decreased cardiac tissue concentrations of anthracyclines, including the primary metabolite doxorubicinol, which may also be an important contributor to cardiotoxicity. In this study, doxorubicin and doxorubicinol plasma pharmacokinetics and tissue concentrations were compared in New Zealand white rabbits following intravenous administration of doxorubicin (5 mg.kg-1) by bolus and continuous infusion. Blood samples were obtained over a 72-h period after doxorubicin administration to determine plasma doxorubicin and doxorubicinol concentrations. Rabbits were killed 7 days after the completion of doxorubicin administration and tissue concentrations of doxorubicin and doxorubicinol in heart, kidney, liver, and skeletal muscle were measured. In further experiments, rabbits were killed 1 h after bolus injection of doxorubicin and at the completion of a 24-h doxorubicin infusion (anticipated times of maximum heart anthracycline concentrations) to compare cardiac concentrations of doxorubicin and doxorubicinol following both methods of administration. Peak plasma concentrations of doxorubicin (1739 +/- 265 vs 100 +/- 10 ng.ml-1) and doxorubicinol (78 +/- 3 vs 16 +/- 3 ng.ml-1) were significantly higher following bolus than infusion dosing. In addition, elimination half-life of doxorubicinol was increased following infusion. However, other plasma pharmacokinetic parameters for doxorubicin and doxorubicinol, including AUC infinity, were similar following both methods of doxorubicin administration. Peak left ventricular tissue concentrations of doxorubicin (16.92 +/- 0.9 vs 3.59 +/- 0.72 micrograms.g-1 tissue; P < 0.001) and doxorubicinol (0.24 +/- 0.02 vs 0.09 +/- 0.01 micrograms.g-1 tissue; P < 0.01) following bolus injection of doxorubicin were significantly higher than those following infusion administration. Tissue concentrations of parent drug and metabolite in bolus and infusion groups were similar 7 days after dosing. The results suggest that cardioprotection following doxorubicin infusion may be related to attenuation of the peak plasma or cardiac concentrations of doxorubicin and/or doxorubicinol.

Alteration in Doxorubicin and Doxorubicinol Plasma Concentrations with Repeated Courses to Patients

Fifteen cancer patients were studied following repeated courses of doxorubicin (12-44 mg/m2) (together with other anticancer agents) to consider the possibility of enhanced metabolism as a cause of the previously reported reduction in doxorubicin plasma concentrations with repeated courses. Plasma doxorubicin and doxorubicinol concentrations were measured by a modified high-performance liquid chromatography/fluorescence method. The results presented confirmed the significant decline in doxorubicin plasma concentration-dose ratios measured 3 h after the 1-h infusion. Although the degree of this reduction varied markedly between patients, it was shown not to be associated with a rise in the plasma concentration-dose ratio of the major metabolite doxorubicinol or with altered renal and/or hepatic function, which may have influenced disposition. Alternative mechanisms that might explain the reduction in doxorubicin concentrations, such as increased doxorubicin clearance or volume of distribution, were not considered in the present study.

Effect of phenytoin on the pharmacokinetics of doxorubicin and doxorubicinol in the rabbit.

Doxorubicin is metabolized extensively to doxorubicinol by the ubiquitous aldoketoreductase enzymes. The extent of conversion to this alcohol metabolite is important since doxorubicinol may be the major contributor to cardiotoxicity. Aldoketoreductases are inhibited in vitro by phenytoin. The present study was conducted to examine the effect of phenytoin on doxorubicin pharmacokinetics. Doxorubicin single-dose pharmacokinetic studies were performed in 10 New Zealand White rabbits after pretreatment with phenytoin or phenytoin vehicle (control) infusions in crossover fashion with 4-6 weeks between studies. Infusions were commenced 16 h before and during the course of the doxorubicin pharmacokinetic studies. Phenytoin infusion was guided by plasma phenytoin estimation to maintain total plasma concentrations between 20 and 30 micrograms/ml. Following doxorubicin 5 mg/kg by i.v. bolus, blood samples were obtained at intervals over 32 h. Plasma doxorubicin and doxorubicinol concentrations were measured by HPLC. The mean plasma phenytoin concentrations ranged from 17.4 to 33.9 micrograms/ml. Phenytoin infusion did not alter doxorubicin pharmacokinetics. The elimination half-life and volume of distribution were almost identical to control. Clearance of doxorubicin during phenytoin administration (60.9 +/- 5.8 ml/min per kg, mean +/- SE) was similar to that during vehicle infusion (67.5 +/- 5.4 ml/min per kg). Phenytoin administration was associated with a significant decrease in doxorubicinol elimination half-life from 41.0 +/- 4.8 to 25.6 +/- 2.8 h. The area under the plasma concentration/time curve (AUC) for doxorubicinol decreased significantly from 666.8 +/- 100.4 to 491.5 +/- 65.7 n.h.ml-1. These data suggest that phenytoin at clinically relevant concentrations does not alter the conversion of doxorubicin to doxorubicinol in the rabbit. The reduction in the AUC for doxorubicinol caused by phenytoin appears to be due to an increased rate of doxorubicinol elimination. Phenytoin or similar agents may have the effect of modifying doxorubicinol plasma concentrations by induction of doxorubicinol metabolism rather than by inhibition of aldoketoreductase enzymes.