Treatment of childhood acute myelogenous leukemia with an intensive regimen (AML-87) that individualizes etoposide and cytarabine dosages: short- and long-term effects

The purpose of this study was to assess the feasibility and efficacy of a treatment regimen for pediatric acute myelogenous leukemia (AML) that uses four rotating drug pairs and adjusts dosages of etoposide and cytarabine to target specific plasma concentrations. Thirty-one girls and 27 boys (median age, 9.7 years) with de novo AML were treated on the protocol. Six cycles of chemotherapy were planned. Cycles 1 to 4 comprised the drug combinations cytarabine plus etoposide, cytarabine plus daunomycin, etoposide plus amsacrine, and etoposide plus azacitidine, respectively. For cycles 5 and 6, the first two combinations were repeated. Dosages were adjusted to achieve plasma concentrations of 1.0 microM +/- 0.1 microM cytarabine and 30 microM +/- 0.3 microM etoposide. Forty-four patients (76%) entered complete remission. Of those, 24 have had relapses; 23 remain alive in first or subsequent remission. The 5-year event-free survival (EFS) estimate was 31.0% +/- 5.9%; the 5-year survival estimate was 41.4% +/- 6.3%. Six patients (10%) died of the toxic effects of therapy. Severe neutropenia occurred in all cycles. Long-term complications of therapy included hepatitis C, cardiac insufficiency, and hearing loss. Adjustment of cytarabine and etoposide dosage was feasible for achieving targeted plasma drug concentrations; however, the potential clinical efficacy of this approach was offset by substantial acute and long-term toxicity.

Conventional compared with individualized chemotherapy for childhood acute lymphoblastic leukemia

BACKGROUND

The rate of clearance of antileukemic agents differs by a factor of 3 to 10 among children with acute lymphoblastic leukemia. We hypothesized that the outcome of treatment would be improved if doses were individualized to prevent low systemic exposure to the drugs in patients with fast drug clearance.

METHODS

We stratified and randomly assigned 182 children with newly diagnosed acute lymphoblastic leukemia to postremission regimens that included high-dose methotrexate and teniposide plus cytarabine. The doses of these drugs were based on body-surface area (in the conventional-therapy group) or the rates of clearance of the three medications in each patient (in the individualized-treatment group). In the individualized-treatment group, doses were increased in patients with rapid clearance and decreased in patients with very slow clearance.

RESULTS

Patients who received individualized doses had significantly fewer courses of treatment with systemic exposures below the target range than did patients who received conventional doses (P<0.001 for each medication). Among the patients with B-lineage leukemia, those who received individualized therapy had a significantly better outcome than those given conventional therapy (P=0.02); the mean (+/-SE) rates of continuous complete remission at five years were 76+/-6 percent and 66+/-7 percent, respectively. There was no significant difference between treatments for patients with T-lineage leukemia (P=0.54). In a proportional-hazards model, the time-dependent systemic exposure to methotrexate, but not to teniposide or cytarabine, was significantly related to the risk of early relapse in children with B-lineage leukemia.

CONCLUSIONS

Adjusting the dose of methotrexate to account for the patient's ability to clear the drug can improve the outcome in children with B-lineage acute lymphoblastic leukemia.

Probenecid alters topotecan systemic and renal disposition by inhibiting renal tubular secretion

Topotecan is primarily eliminated by the kidneys, with 60 to 70% of the dose recovered as topotecan total in the urine. To elucidate the mechanisms of topotecan renal clearance, we evaluated the effect of probenecid on topotecan renal and systemic disposition in mice. Topotecan lactone or hydroxy acid (1.25 mg/kg i.v.) was administered alone or in combination with probenecid (600 or 1,200 mg/kg) given by oral gavage 30 min before and 3 hr after topotecan. Serial blood samples (three mice per time point) and urine samples (five mice per treatment arm) were collected during a 6-hr period. Compared with topotecan alone, coadministration of topotecan lactone or hydroxy acid with probenecid (600 mg/kg) decreased topotecan lactone, total, and hydroxy acid systemic clearance, and total renal clearance. The predominant effect of probenecid was to increase hydroxy acid area under the plasma concentration time curve after administration of topotecan lactone (238.8 vs. 109.9 ng.hr/ml alone, P < .05), or hydroxy acid (1297.2 vs. 355.0 ng.hr/ml alone, P < .05). By inhibiting renal tubular secretion, probenecid decreased renal and systemic clearance which led to an increase in topotecan systemic exposure. These data suggest that probenecid primarily inhibited secretion of the anionic hydroxy acid form, and by direct or indirect mechanisms increased topotecan lactone systemic exposure. Topotecan elimination through renal tubular secretion may have clinical relevance for the use of topotecan in patients with altered renal function.

A phase II trial of high-dose methotrexate in previously untreated children and adolescents with high-risk unresectable or metastatic rhabdomyosarcoma

PURPOSE

The outcome for children with advanced-stage rhabdomyosarcoma remains poor with contemporary treatment regimens. Evaluation of new drugs is important to improve clinical outcome. Because methotrexate has shown promising activity in the treatment of patients with recurrent rhabdomyosarcoma, we conducted a phase II trial in untreated children with advanced-stage disease to evaluate the efficacy and safety of this agent.

PATIENTS AND METHODS

Fifteen patients received 1 to 4 courses of high-dose methotrexate (HDMTX, 12 g/m2). Patients then received standard multiagent chemotherapy (vincristine, dactinomycin, cyclophosphamide, ifosfamide, mesna) with cytokine support and local radiotherapy. Patients who responded to HDMTX received four additional courses of this drug during continuation therapy.

RESULTS

Twelve patients were evaluable for response after 2 or more courses of HDMTX; 4 achieved a partial response (33.3%). After administration of standard chemotherapy and radiation, the estimated 2-year progression-free survival for all patients was 56% (SD 15%). The drug was well-tolerated and the most common side effects included mucositis, transient elevation of transaminases, and neutropenia. The four patients who received additional courses of HDMTX during continuation therapy had limited toxicity which included mucositis, anemia, and thrombocytopenia.

CONCLUSIONS

About one-third of children with previously untreated advanced-stage rhabdomyosarcoma responded to HDMTX. Its different mechanism of action and non-overlapping toxicity with other agents make HDMTX an attractive candidate for incorporation into front-line treatment regimens for rhabdomyosarcoma.

Disposition of irinotecan and SN-38 following oral and intravenous irinotecan dosing in mice

The present study was conducted to quantitate the disposition of irinotecan lactone and its active metabolite SN-38 lactone in mice following oral and intravenous administration, and to evaluate the systemic exposure of irinotecan lactone and SN-38 lactone associated with antitumor doses of irinotecan lactone in mice bearing human tumor xenografts. Nontumor-bearing mice were given a single oral or intravenous irinotecan dose (5, 10, 40, or 75 mg/kg), and serial plasma samples were subsequently obtained. Irinotecan and SN-38 lactone plasma concentrations were measured using an isocratic HPLC assay with fluorescence detection. The disposition of intravenous irinotecan lactone was modeled using a two-compartment pharmacokinetic model, and the disposition of oral irinotecan and SN-38 lactone was modeled with noncompartmental methods. Irinotecan lactone showed biphasic plasma disposition following intravenous dosing with a terminal half-life ranging between 1.1 to 3 h. Irinotecan lactone disposition was linear at lower doses (5 and 10 mg/kg), but at 40 mg/kg irinotecan lactone clearance decreased and a nonlinear increase in irinotecan lactone AUC was observed. The steady-state volume of distribution ranged from 19.1 to 48.1 l/m2. After oral dosing, peak irinotecan and SN-38 lactone concentrations occurred within 1 h, and the irinotecan lactone bioavailability was 0.12 at 10 mg/kg and 0.21 at 40 mg/kg. The percent unbound SN-38 lactone in murine plasma at 1000 ng/ml was 3.4 +/- 0.67%, whereas at 100 ng/ml the percent unbound was 1.18 +/- 0.14%. Irinotecan and SN-38 lactone AUCs in micebearing human neuroblastoma xenografts were greater than in nontumor-bearing animals. Systemic exposure to unbound SN-38 lactone in nontumor-bearing animals after a single oral irinotecan dose of 40, 10, and 5 mg/kg was 28.3, 8.6, and 2.9 ng h/ml, respectively. Data from the present study provide important information for the design of phase I studies of oral irinotecan.

Hepatic drug clearance in patients with mild cystic fibrosis

The plasma disposition of three model substrates (lorazepam, indocyanine green, and antipyrine) and the formation clearance of antipyrine metabolites (3-hydroxymethylantipyrine, norantipyrine, and 4-hydroxyantipyrine) were evaluated in 15 subjects with mild cystic fibrosis and in 15 healthy control subjects. Plasma clearance was significantly greater in patients with cystic fibrosis for both lorazepam (1.7 +/- 0.4 versus 1.2 +/- 0.5 ml/min/kg) and indocyanine green (14.2 +/- 6.1 versus 9.1 +/- 3.0 ml/min/kg). In contrast, the clearance of antipyrine was not significantly different (1.0 +/- 0.7 versus 0.8 +/- 0.3 ml/min/kg), but the formation clearance for 3-hydroxymethylantipyrine was significantly greater in patients with cystic fibrosis. Lorazepam and antipyrine apparent steady-state volume of distribution were not different between groups. These results suggest that clearance of drugs that undergo conjugation (e.g., lorazepam) or biliary excretion (e.g., indocyanine green) is increased in patients with mild cystic fibrosis. In contrast, the increased formation clearance of only one antipyrine metabolite suggests that alterations in clearance of drugs metabolized by cytochrome P450 enzymes are substrate specific and isoform specific in patients with cystic fibrosis.

Topoisomerase I interactive drugs in children with cancer

Topotecan, irinotecan, and 9-aminocamptothecin (9-AC) are analogs of the plant alkaloid 20(S)-camptothecin (CMT), the prototypical DNA topoisomerase I interactive agent. These agents interact with the topoisomerase I-DNA complex and prevent resealing topoisomerase I-mediated DNA single-strand breaks. This eventual leads to double-strand DNA breaks and apoptosis or cell death. Topotecan, irinotecan, and 9-AC have shown significant activity in mice bearing pediatric solid tumor xenografts; the greatest antitumor responses were found with protracted continuous schedules. Preclinical data also suggest that maintenance of an exposure-duration threshold (EDT) may be required to achieve optimal cytotoxicity. Pediatric Phase I trials have evaluated the toxicity and safety to camptothecin analogs in children with relapsed solid tumors and relapsed acute leukemia. The primary dose-limiting toxicity (DLT) for the CMT analogs in children has been myelosuppression, except for mucositis observed with the 120-hr continuous topotecan infusion schedule. Pharmacodynamic relationships with these analogs have been reported between systemic exposure, and myelosuppression and mucositis. Although not a primary objective of the early Phase I studies, antitumor responses have been reported. In this review, the pharmacokinetic and pharmacodynamics of the CMT analogs studied in children are summarized, and future studies of these agents are discussed.

Liver volume as a determinant of drug clearance in children and adolescents

Many drugs eliminated by the liver exhibit age-related differences in systemic clearance, necessitating different dosage requirements in children and adults. However, the physiological basis for these age-related changes is not well defined, including the importance of liver size in determining systemic clearance. Therefore, magnetic resonance imaging was used to determine liver volume in pediatric and adolescent patients, in whom systemic clearance of three model substrates [lorazepam (0.03 mg/kg), antipyrine (10 mg/kg), and indocyanine green (ICG; 0.5 mg/kg)] was also determined. In 16 children (ages 3.3-18.8 years; 8 boys), liver volume ranged from 469 to 1640 ml (median 937), and was significantly related to age, body weight, and body surface area (BSA). Younger children had larger liver normalized to body weight (ml/kg), but there was no difference when liver volume was normalized to BSA (ml/m2). Unnormalized lorazepam and ICG clearances (ml/min) were significantly related to absolute liver volume (r2 = 50.2% and 31.4%, respectively), whereas unnormalized antipyrine clearance was not. Lorazepam, ICG, and antipyrine clearance normalized to BSA did not exhibit age-related changes, nor did lorazepam or ICG clearance normalized to body weight decreased significantly with increasing age (r2 = 36.9%, p=0.012), as did antipyrine clearance relative to liver volume. Thus, age-related changes in drug clearance and the importance of liver volume may differ based on the principal hepatic mechanisms involved in drug elimination.

Cerebrospinal fluid pharmacokinetics and penetration of continuous infusion topotecan in children with central nervous system tumors

The purpose of this study was to describe the cerebrospinal fluid (CSF) penetration of topotecan in humans, to generate a pharmacokinetic model to simultaneously describe topotecan lactone and total concentrations in the plasma and CSF, and to characterize the CSF and plasma pharmacokinetics of topotecan administered as a continuous infusion (CI). Plasma and CSF samples were collected from 17 patients receiving 5.5 or 7.5 mg/m2 per day as a 24-h CI (5 patients, 7 courses), or 0.5 to 1.25 mg/m2 per day as a 72-h CI (12 patients, 12 courses). CSF samples were obtained from either a ventricular reservoir (VR) or a lumbar puncture (LP). Topotecan lactone and total (lactone plus hydroxy acid) concentrations were determined by HPLC and fluorescence detection. Using MAP-Bayesian modelling, a three-compartment model was fitted simultaneously to topotecan lactone and total concentrations in the plasma and CSF. The penetration of topotecan into the CSF was determined from the ratio of the CSF to the plasma area under the concentration-time curve. The median CSF ventricular lactone concentrations, obtained prior to the end of infusion (EOI), were 0.86, 1.4, 0.73, 5.3, and 4.6 ng/ml for patients receiving 0.5, 1.0, 1.25, 5.5, and 7.5 mg/m2 per day, respectively. EOI CSF lumbar lactone concentrations measured in three patients were 0.44, 1.1, and 1.7 ng/ml for topotecan doses of 1.0, 5.5, and 7.5 mg/m2 per day, respectively. In two patients receiving 1.25 mg/m2 per day, EOI CSF concentrations were obtained simultaneously from a VR and LP; the lumbar lactone concentrations were 30% and 49% lower than the ventricular concentrations. During a 24-h and a 72-h CI, the median CSF penetration of topotecan lactone was 0.29 (range 0.10 to 0.59) and 0.42 (range 0.11 to 0.86), respectively. A three-compartment model adequately described topotecan lactone and total concentrations in the plasma and CSF. Topotecan was therefore found to significantly penetrate into the CSF in humans. The pharmacokinetic model presented may be useful in the design of clinical studies of topotecan to treat CNS tumors.

Pharmacokinetics in the child

Pharmacokinetic studies have made many significant contributions to rational therapeutics in children. Pharmacokinetic data have helped distinguish between differences in drug disposition and drug sensitivity in children as compared to adults and led to the establishment of age-specific dosage guidelines. Factors influencing the observed differences between drug disposition in children and adults are reviewed. Specific examples utilizing anticancer drugs are presented. The use of model substrates to study hepatic drug metabolism and renal excretion in children is described and some results are discussed. The significance of genetic polymorphic drug metabolism is presented and the use of model substrates to determine individual metabolic phenotypes is described. The use of pharmacokinetic data to define the maximum-tolerated systemic exposure rather than the maximum-tolerated dosage of anticancer drugs in children is presented.

Pharmacokinetics of vincristine in children and adolescents with acute lymphocytic leukemia

We studied the pharmacokinetics of vincristine in children with acute lymphocytic leukemia by means of a specific high-performance liquid chromatographic assay with ultraviolet and electrochemical detection and a limited sampling strategy. Our objectives were to characterize the disposition of vincristine in pediatric patients, to determine clinical, demographic, or biochemical variables related to variability in vincristine pharmacokinetic parameters, and to assess the relationship between pharmacokinetic parameters and vincristine neurotoxicity. Plasma samples were collected at 5 and 30 minutes, and 1, 3, and 24 hours after a rapid intravenous injection during 3 minutes. Vincristine-induced neurotoxicity was retrospectively evaluated by chart review. Pharmacokinetic studies were completed for 64 doses in 54 children between 2 months and 18 years of age (median, 4.3 years), including 2-month-old monozygous twin girls. Vincristine clearance, estimated by Bayesian methods, was highly variable, with a mean (SD) clearance of 19.9 (14.9) ml/min per kilogram or 482 (342) ml/min per square meter. Mean clearance for all subjects was faster than in published studies of adults, which may be related in part to the greater specificity of the assay used in our study, as well as to age-related differences in drug disposition. Vincristine-associated neurotoxicity was frequent but mild and was not predicted by vincristine systemic exposure; however, neurotoxicity may have been underestimated. Clearance in one patient who received concomitant treatment with pentobarbital exceeded the 75th percentile for all patients, and four of five patients receiving concomitant histamine2 antagonists had clearances below the 25th percentile for all subjects, suggesting that drugs that induce or inhibit hepatic cytochrome P-450 enzymes may affect vincristine disposition. Further studies are needed to identify the factors responsible for interpatient variability in vincristine disposition and to develop improved dosing guidelines.

Clinical pharmacodynamics of continuous infusion topotecan in children: systemic exposure predicts hematologic toxicity

PURPOSE

Topotecan pharmacokinetics and pharmacodynamics were studied following a 72-hour continuous infusion in 20 children with cancer (median age, 8 years; range, 3.5 to 18).

METHODS

Serial plasma and urine samples were collected during the infusion and for up to 6 hours following the end of infusion. Topotecan (lactone) and total (lactone plus hydroxy acid) concentrations were determined by a sensitive and specific high-performance liquid chromatography (HPLC) assay with fluorescence detection. Using maximum a posteriori-Bayesian modeling, lactone and total plasma concentrations were described separately by a two-compartment model. Hematologic toxicity was expressed as the percent decrease in absolute neutrophil count (ANC) and platelet count. The relation between systemic exposure (SE) and hematologic toxicity was modeled using a sigmoid maximum-effect model.

RESULTS

Systemic clearance rates for lactone and total topotecan were (mean +/- SD) 18.5 +/- 7.0 and 6.5 +/- 2.4 L/h/m2, respectively. Urinary recovery of total topotecan was (mean +/- SD) 67.5% +/- 25.2% (n = 12 patients). SE (area under the concentration-time curve from zero to infinity [AUC] or steady-state plasma concentration [Cpss]) to either topotecan lactone or total topotecan was significantly correlated to hematologic toxicity (P < .05). Overall, patients with a higher SE to topotecan experienced greater hematologic toxicity.

CONCLUSION

These data demonstrate a relation between systemic exposure to topotecan and clinical effect (myelosuppression). Moreover, these data provide the basis for development of individualized topotecan administration schedules.

Complete hematologic remissions induced by 2-chlorodeoxyadenosine in children with newly diagnosed acute myeloid leukemia

The majority of children with acute myeloid leukemia (AML) who are treated exclusively with chemotherapy die of progressive disease. Improvement in outcome will likely require new active drugs capable of eradicating resistant blast cells early in the clinical course. We therefore assessed the cytoreductive potential of 2-chlorodeoxyadenosine (2-CdA), a halogenated purine analogue, in 22 consecutive children with newly diagnosed AML. The drug was administered as a single 120-hour continuous infusion (8.9 mg/m2 of body surface area per day) before the introduction of standard remission induction therapy. Six patients (27%) had complete hematologic remissions by a median of 21 days after treatment with the nucleoside (range, 14 to 33 days). Seven others had partial responses, yielding a total response rate of 59%. The drug also eliminated leukemic cells from cerebrospinal fluid in 4 of the 6 patients tested. Concentrations of 2-CdA in cerebrospinal fluid on day 5 after the initiation of treatment ranged from 12.4% to 38.0% (mean, 22.7%) of the steady-state plasma concentrations. Severe but reversible myelosuppression and thrombocytopenia developed in all patients. Analysis of factors that may have influenced the complete remission rate suggested a better outcome in patients with myeloblastic leukemia (M0-M2 subtypes in the revised French-American-British classification system). These results demonstrate clinically significant activity by 2-CdA against previously untreated AML in children, including leukemic blast cells in the central nervous system. Its use in combination chemotherapy may improve the outlook for patients with this often fatal hematologic cancer.

Patient characteristics associated with high-risk methotrexate concentrations and toxicity

PURPOSE

Following high-dose methotrexate (HD-MTX) treatment, delayed MTX elimination is an important problem because it necessitates increased leucovorin rescue and additional hospitalization for hydration and urinary alkalinization. Our purpose was to identify factors associated with high-risk MTX plasma concentrations (defined by plasma concentration > or = 1.0 mumol/L at 42 hours from the start of MTX) and with toxicity.

PATIENTS AND METHODS

Variables associated with MTX concentrations and toxicity were assessed in 134 children treated with one to five courses of HD-MTX (900 to 3,700 mg/m2 intravenously [i.v.] over 24 hours for a total of 481 courses) for acute lymphoblastic leukemia (ALL).

RESULTS

High-risk MTX concentrations, toxicity (usually mild mucositis), and delay in resuming continuation chemotherapy occurred in 106 (22%), 123 (26%), and 66 (14%) of 481 courses, respectively. Using a mixed effects model for repeated measures, high-risk MTX concentrations were significantly associated with a higher MTX area-under-the-concentration-time curve (AUC), low urine pH, emesis, low MTX clearance, low urine output relative to intake, use of antiemetics during the MTX infusion, and concurrent intrathecal therapy (all p values < .01). Clinical toxicities and delay in resumption of continuation chemotherapy due to myelosuppression were more common in those with high 42-hour MTX concentrations, despite increased leucovorin rescue for all patients with high-risk MTX concentrations. However, with individualized rescue, no patient developed life-threatening toxicity. A more aggressive hydration and alkalinization regimen for subsequent courses reduced the frequency of high-risk MTX concentrations to 7% of courses (13 of 183) (P = .0001), and the frequency of toxicity decreased to 11% of courses (P = .0074).

CONCLUSION

This study identified several clinical variables that influence MTX disposition that, when modified, can reduce the frequency of high-risk MTX concentrations and toxicity.

Pharmacokinetics of cladribine (2-chlorodeoxyadenosine) in children with acute leukemia

Cladribine is a synthetic purine nucleoside with demonstrated activity in hairy cell leukemia and acute myeloid leukemia. We have studied the pharmacokinetics of this drug in 25 pediatric patients with acute leukemia treated with cladribine as a single agent, 8.9 mg/m2/24 h, for 5 days by continuous i.v. infusion. Twelve patients were in relapse, and acute myeloid leukemia was newly diagnosed in 13 patients. Plasma, urine, and cerebrospinal fluid cladribine concentrations were determined by a radioimmunoassay with a limit of detection of 1 nM. An open two-compartment model was fit to the plasma concentration data. The mean (SD) clearance was 39.4 (12.4) liters/h/m2 and ranged from 14.4-55.4 liters/h/m2. When clearance was normalized to body weight (liters/h/kg) it was negatively correlated with age, with older patients having slower clearances per unit of body weight. However, when clearance was normalized to body surface area, no significant correlation with age was observed. The mean (SD) steady-state plasma concentration (predicted 120-h concentration) was 37.7 (17.3) nM and ranged from 23.2-84.5 nM. The terminal phase half-life in 22 patients ranged from 14.3-25.8 h, with a mean (SD) of 19.7 (3.4) h. The volume of distribution at steady state was highly variable, with a mean (SD) of 356.6 (225.2) liters/m2. None of these parameters was significantly different between patients in relapse and patients with newly diagnosed disease. Renal clearance was determined in 7 patients and ranged from 34.6-643.6 ml/min/m2, with a mean (SD) of 317.9 (208.7) ml/min/m2. Renal clearance as a percentage of total systemic clearance ranged from 11.0-85.1%, with a mean of 51.0%. In 11 patients, the mean (SD) cerebrospinal fluid concentration was 6.1 (3.97) nM, a mean of 18.2% of the steady-state plasma concentration. The CSF:plasma concentration ratio was significantly higher on day 5 (22.7% in 7 patients) than on day 4 (7.6% in 3 patients; P = 0.03). Additional studies are needed to further define the metabolic fate of cladribine. In this paper we provide the first comprehensive description of the pharmacokinetics of this drug in children and provide data which suggest that cladribine may be useful in the treatment of patients with meningeal leukemia or malignancies of the central nervous system.

Saturable elimination and saturable protein binding account for flavone acetic acid pharmacokinetics

Flavone acetic acid (FAA) is an antineoplastic agent that has undergone extensive study in Phase I trials. Concentration-dependent plasma protein binding has been demonstrated in vitro at concentrations of total drug that are achieved in vivo. Moreover, dose-dependent total systemic clearance has been described when FAA has been administered as a short iv infusion. When administered as a prolonged 24-hr infusion, total FAA (bound plus unbound) plasma pharmacokinetics are well described with a first-order two-compartment model. However, measurement of unbound FAA intra- and post-intravenous infusion in eight patients revealed a twofold increase in fraction of FAA unbound in plasma intrainfusion. We attempted to fit pharmacokinetic structural models of varying complexity to the unbound concentrations alone and simultaneously to the unbound and bound FAA plasma concentrations. The data were adequately described only by a model that incorporated simultaneous saturable plasma protein binding and a Michaelis-Menten process for elimination. A comparison among models is presented, as well as pharmacokinetic parameter estimates for FAA in children. These clinical data are consistent with predictions of the clearance model in which both saturable protein binding (resulting in a dynamically increasing unbound fraction) and saturable elimination (resulting in gradually decreasing unbound intrinsic clearance) are operative.

Clinical pharmacokinetics and pharmacodynamics of anticancer drugs in children

Pharmacokinetic variability in children with cancer is substantial and confounds drawing conclusions regarding optimal therapy based only on dose-response relationships. Careful pharmacokinetic studies performed during drug development in conjunction with an assessment of patient characteristics, such as age, renal and hepatic function, and concomitant therapy, is essential for defining those factors that may alter drug disposition. By integrating pharmacokinetic studies with measures of efficacy and toxicity, a pharmacodynamic framework can be established for guiding therapy to minimize differences in systemic exposure among subpopulations of patients (eg, impaired renal function and neonates). In selected instances when pharmacokinetic variability cannot be predicted by patient covariates, the potential for individualizing dosages based on patient-specific pharmacokinetic parameters is now a clinically feasible option. The need for and benefits of incorporating such strategies into routine therapy represents an exciting area for further clinical research.

Effect of chirality on pharmacokinetics and pharmacodynamics

The clinical importance of individual pharmacokinetic and pharmacodynamic differences among enantiomers is discussed. A number of mechanisms in the body can be stereoselective, among them first-pass metabolism, metabolic clearance, renal clearance, and protein and tissue binding. Differences in first-pass metabolism may cause differences in the ratio of plasma concentrations of enantiomers when a drug is given by the intravenous route compared with the oral route. Stereoisomers of a drug may be metabolized by two different enzyme systems, resulting in different rates of metabolic clearance; age and sex may also affect the rates of enzymatic metabolism of stereoisomers. Substantial differences in the protein binding of two enantiomers may result in a difference in their glomerular filtration rates. Two enantiomers may bind differently to protein or to other tissue receptor sites, resulting in differences in drug effects or distribution. There are no simple answers to questions regarding the pharmacokinetics of racemic drug mixtures and single enantiomers. The properties of enantiomers in each drug will have to be evaluated for their pharmacokinetic disposition and their therapeutic index.

Disposition of antineoplastic agents in the very young child

Maturation of physiologic process which govern the disposition of pharmacologic agents can yield significant changes in absorption, distribution, metabolism, and elimination of drugs in neonates, infants and children. However, there are very little data concerning the disposition of anticancer drugs in young children. Pharmacokinetic data for six anticancer agents were compared in infants less than 1 year of age and children greater than 1 year of age treated at St Jude Children's Research Hospital. No pharmacokinetic data were available for infants less than 2 months of age. Median methotrexate clearance tended to be lower in four infants (0.26-0.99 years) vs 108 children (1-19 years): 80 vs 103 ml min-1 m-2, respectively (P = 0.01). There was no difference in the median 42 h methotrexate concentration. Teniposide systemic clearance and terminal half-life and cytarabine systemic clearance were not different between the two groups. There was no significant difference in etoposide systemic clearance when normalised to body surface area (ml min-1 m-2), however a significantly lower systemic clearance relative to body weight (ml min-1 kg-1) was observed in two infants, 0.5 to 1 year of age, vs 23 children, 3-18 years of age. Doxorubicin systemic clearance was not significantly different between the two groups when systemic clearance was expressed in ml min-1 kg-1. However, there was a trend toward a lower rate of systemic clearance in ml min-1 m-2 in infants.(ABSTRACT TRUNCATED AT 250 WORDS)

Raised plasma methotrexate concentrations following intrathecal administration in children with renal dysfunction

Plasma methotrexate (MTX) concentrations were measured following intrathecal (IT) MTX treatment in four patients with acute lymphocytic leukemia and acute renal dysfunction. All four patients had raised serum MTX concentrations to potentially cytotoxic concentrations for a prolonged period of time, (96-120 h). In contrast, serum MTX concentrations after the same dosage of IT treatment ranged from undetectable to 0.11 microM by 15-24 h in seven control patients with normal renal function, and were undetectable by 48 h in all controls. The terminal MTX T1/2 was 19-44 h in the patients with renal dysfunction. Decreased renal clearance or a rapid efflux of MTX from cerebrospinal fluid, or both, could account for the high and sustained concentrations. Plasma MTX concentrations after IT treatment were normal in two patients treated after their renal function returned to normal. Patients with renal dysfunction should be carefully monitored for plasma MTX concentrations and may require leucovorin to prevent systemic side-effects after IT MTX treatment.

Age-related differences in hepatic drug clearance in children: studies with lorazepam and antipyrine

The disposition of intravenous antipyrine and lorazepam, administered as model substrates for hepatic oxidative metabolism and conjugation, was evaluated in 50 children (mean age, 7.8 years; range, 2.3 to 17.8 years) with acute lymphocytic leukemia in complete remission and compared with a group of ten healthy adults. Antipyrine clearance normalized to body weight was significantly greater in children than in adults (0.91 versus 0.59 ml/min/kg; p = 0.012), but was not different when normalized to body surface area. In contrast, lorazepam total clearance (CL) and unbound clearance (CLu) normalized to body weight were not significantly different between children and adults but were smaller in children when normalized to body surface area (CL = 31.9 versus 40.6 ml/min/m2, p = 0.036; CLu = 352 versus 485 ml/min/m2, p = 0.010). The mean lorazepam fraction unbound in children was 0.087, which was not different from adult volunteers (0.084). This study has identified significant differences between children and adults in the disposition of these two compounds, with higher milliliter per minute per kilogram clearance for antipyrine but not lorazepam.

Concept of maximum tolerated systemic exposure and its application to phase I-II studies of anticancer drugs

The traditional approach to conducting Phase I studies of anticancer drugs is to select a starting dosage for humans based on preclinical data (e.g., mg equivalent of 1/10 LD10 in mice), then empirically escalate dosages in cohorts of patients until the maximum tolerated dosage (MTD) is established. More recently, NCl and EORTC investigators have advocated the use of pharmacokinetic data from preclinical studies to facilitate more rapid dose escalation (e.g., double the dose until the area under the concentration-time curve [AUC] in humans equals the AUC in mice at the LD10). The present paper describes a strategy which builds on the above approach, by extending the application of pharmacokinetic principles to systematically escalate systemic exposure (AUC) instead of dosage in Phase I trials. Human trials are initiated at whatever patient-specific dosage is required to achieve an AUC equal to 1/10 the AUC in mice at the LD10, such that three patients at the first treatment level might receive three different dosages. If no dose-limiting toxicity is observed, the next cohort of patients receives whatever dosage is required to achieve 2 x AUC of the first dosage level, with AUC escalation continuing until the maximum tolerate systemic exposure (MTSE) is reached. By escalating systemic exposure instead of dosage, one adjusts for interpatient pharmacokinetic variability. This strategy will permit more rapid and precise dosage escalations and, more importantly, it should more precisely establish the maximum level of treatment intensity for future Phase II trials.

Enhanced hepatic drug clearance in patients with cystic fibrosis

To examine whether hepatic drug metabolism is altered in patients with cystic fibrosis (CF), we evaluated the pharmacokinetics of three model pharmacologic substrates (antipyrine, a marker of hepatic oxidative metabolism; lorazepam, a marker of hepatic glucuronosyltransferase activity; and indocyanine green (ICG), a marker of hepatic blood flow and biliary secretion) in 14 patients with CF (14.6 to 29.2 years of age) and in 12 children and adolescents with cancer (7.2 to 19.4 years of age), which was treated with only surgery and radiation. Each study subject received a single intravenous dose of the combined model substrates (0.03 mg/kg lorazepam, 10 mg/kg antipyrine, and 0.5 mg/kg ICG) for 5 minutes, followed by repeated blood sampling (n = 10) during a 24-hour postinfusion period. Patients with CF had a significantly greater plasma clearance of lorazepam (56.5 +/- 5.2 vs 25.9 +/- 1.9 ml/min/m2) and ICG (892.5 +/- 176.4 vs 256.5 +/- 41.7 ml/min/m2) but not of antipyrine (27.2 +/- 3.8 vs 20.7 +/- 2.0 ml/min/m2) in comparison with control subjects. The apparent steady-state volume of distribution for lorazepam, ICG, and antipyrine was significantly higher in the patients with CF (2.0-, 3.1-, and 1.4-fold, respectively) than in control subjects. Clearance of the model substrates did not correlate with standard biochemical markers of hepatic function. Similarly, no significant relationships were observed between the clearance or steady-state volume of distribution of the compounds and the National Institutes of Health prognostic scores for the patients with CF. These data demonstrate that the plasma clearance of lorazepam and ICG is increased in patients with CF and suggest that hepatic glucuronosyltransferase activity and biliary secretory capacity are enhanced in this disease.

Evaluation of aminoglycoside disposition in patients previously treated with cisplatin

The alterations in aminoglycoside disposition in patients previously treated with cisplatin were determined by reviewing the medical records of 37 cancer patients. The patients received 44 courses of an aminoglycoside antibiotic (gentamicin, n = 27; amikacin, n = 14; and tobramycin, n = 3). The mean (SD) half-life of 171 (120) min was greater than our previously published mean aminoglycoside half-life in children with cancer who were not receiving cisplatin. Twenty-five of 44 courses were completed without an aminoglycoside dosage reduction and only 5 courses were discontinued because of delayed aminoglycoside elimination. There was no significant difference in the duration of aminoglycoside therapy between the group that had a dosage reduction and the group that did not [6.6 (2.3) versus 5.8 (2.9) days, p = 0.42, respectively]. Multiple linear regression analysis of patient variables identified serum creatinine and cumulative cisplatin dose as the best predictors of aminoglycoside half-life (r2 = 46.0%, p less than 0.001). The only predictor of aminoglycoside clearance was serum creatinine (r2 = 35.2%, p less than 0.001). Patients previously treated with cisplatin are at greater risk for delayed aminoglycoside elimination. Prior administration of cisplatin is not an absolute contraindication to the use of aminoglycoside antibiotics. When clinically indicated, patients who have previously received cisplatin and have apparently normal renal function should be treated cautiously with standard doses of aminoglycoside antibiotics, and pharmacokinetic monitoring should be routinely performed.

Clinical pharmacology of cancer chemotherapy in children

The pharmacokinetics of most anticancer drugs are highly variable in children, and are commonly different when children are compared to adults. Several recent studies have demonstrated that variability in systemic exposure due to interpatient pharmacokinetic variability, may be related to the probability of oncolytic effects or toxicity for some anticancer drugs. This review has exemplified differences in the clinical pharmacology of several anticancer drugs, when children are compared to adults. Such age-related differences in the pharmacokinetics and pharmacodynamics of these drugs, together with biologic differences between pediatric and adult cancers, provide the rationale for systematically conducting pediatric phase I through IV studies of anticancer drugs and denote the risks of relying on adult trials to identify new therapeutic strategies for childhood cancers.

Hepatic drug clearance in children: studies with indocyanine green as a model substrate

For several drugs metabolized by the liver, higher dosages (mg/kg body weight) are required in children to attain serum concentrations comparable to those in adults. Indocyanine green (ICG), a commonly used model substrate for hepatic elimination of high intrinsic clearance drugs, has been extensively evaluated in adults but not in children. We evaluated the disposition of ICG in 115 children with leukemia and nine healthy adult volunteers. The mean (SD) ICG plasma clearance (CLp) for all 115 children (age 0.9-17.8 years) was significantly greater (p = 0.0006) than for adults [14.8 (7.8) versus 10.6 (2.4) mL/min/kg]. When clearances from only children less than 10 years of age (N = 85) were compared with those from adults, the difference was even greater [15.6 (7.3) versus 10.6 (2.4) mL/min/kg; p = 0.0001]. However, when ICG CLp was normalized to body surface area, values for children did not differ significantly from adults [378 (204) versus 422 (102) mL/min/m2]. These data provide insight as to why dosage (mg/kg) requirements of certain drugs are higher in children.

Dextromethorphan and caffeine as probes for simultaneous determination of debrisoquin-oxidation and N-acetylation phenotypes in children

The feasibility and reliability of simultaneously determining debrisoquin oxidation and N-acetylation phenotypes was assessed in children with use of two innocuous substrate probes given by mouth, 30 mg dextromethorphan (Pertussin ES) and 25 to 46 mg caffeine (Coca-Cola beverage). Twenty-six children and adolescents (aged 3 to 21 years) were studied three times, once with each substrate given alone and once with the two substrates given together. Urine was collected for 4 hours, and the molar urinary metabolic ratios for dextromethorphan:dextrorphan and for two caffeine metabolites (AFMU:1X) were determined by HPLC ultraviolet assays. The urinary metabolic ratios for both substrates were not significantly different when the substrates were given alone compared with when they were given together. There also was no difference in either the oxidation or acetylation phenotype assignments when the two substrates were given alone and when they were given together. No adverse effects were observed. We conclude that dextromethorphan and caffeine can be given together to simultaneously determine oxidation and acetylation phenotypes and can thereby provide an innocuous, noninvasive method for the assessment of polymorphic drug metabolism in various pediatric populations.

Anticancer therapy as a pediatric pharmacodynamic paradigm

A series of clinical pharmacokinetic and pharmacodynamic studies of anticancer drugs has been conducted in children with cancer. Since these drugs typically have narrow therapeutic indices, frequently producing toxicities at dosages required for therapeutic effects, they represent an exemplary class of drugs for pediatric pharmacodynamic studies. Reviewed herein are pediatric pharmacokinetic studies of teniposide and high-dose methotrexate, which demonstrate age-related differences in the disposition of these two highly active anticancer drugs, and pharmacodynamic studies which demonstrate a relation between the disposition of these drugs and their clinical effects (efficacy and toxicity) in children with acute lymphocytic leukemia.

Removal of methotrexate, leucovorin, and their metabolites by combined hemodialysis and hemoperfusion

This article documents the case of a patient with severe renal failure immediately after having been given high-dose methotrexate; the patient was effectively treated with repeated hemodialysis, charcoal hemoperfusion, leucovorin, and thymidine. The methotrexate plasma concentration was reduced from 390 mumol/L to 7 mumol/L as a result of 24.5 hours of hemodialysis along with 39.5 hours of hemoperfusion. Although a rebound in the plasma methotrexate concentration occurred the first three times that hemodialysis and/or hemoperfusion was stopped, reinstitution of the procedure was always effective in further lowering methotrexate concentrations. The patient was subsequently managed with leucovorin and thymidine rescue. Simultaneous measurements before and after the hemodialysis-hemoperfusion apparatus and before and after the hemoperfusion device alone revealed a percent decrease in the concentration of d-leucovorin of 36% and 79%; 1-leucovorin, 82% and 75%; 5-methyltetrahydrofolate, 52% and 64%; methotrexate, 73% and 37%; and 7-hydroxymethotrexate, 21% and 24%, respectively. Gastrointestinal and hematologic toxicities were completely prevented, and serum creatinine normalized within 24 days.

Effect of hydration on methotrexate plasma concentrations in children with acute lymphocytic leukemia

Hydration and urinary alkalinization are used with high-dose methotrexate (HDMTX) to minimize renal toxicity resulting from methotrexate (MTX) precipitation in the kidney tubules. The effect of two hydration and alkalinization schedules on MTX plasma concentrations were evaluated in 100 children with acute lymphocytic leukemia (ALL) following two courses of MTX, 2 g/m2. The mean 21- and 44-hour MTX plasma concentrations were significantly lower in the group receiving the greater hydration and alkalinization schedule: 0.79 (0.90 SD) v 1.39 (1.99 SD) mumol/L for 21-hour MTX plasma concentrations, P = .01; and 0.18 (0.38 SD) v 0.25 (0.50 SD) mumol/L for 44-hour MTX plasma concentrations, P = .01. Although the overall incidence of toxic events was similar in both groups, the incidence of severe toxicity was reduced in the group that received the greater hydration and alkalinization, 6% v 16%. This study demonstrated that the amount of hydration and alkalinization can affect MTX plasma concentrations. Optimizing the hydration and alkalinization schedule is important for minimizing the incidence of severe toxicity associated with HDMTX.

Pharmacokinetics and toxicity of methotrexate in children with Down syndrome and acute lymphocytic leukemia

Children with Down syndrome and acute lymphocytic leukemia (ALL) have poor tolerance to antineoplastic drugs, including methotrexate (MTX). We evaluated MTX pharmacokinetics and toxicity in five patients with Down syndrome and ALL who had received multiple high doses of MTX (1 g/m2). Three control patients without Down syndrome were matched to each case according to sex, race, age, and initial leukocyte count. Median MTX plasma concentrations, measured 42 hours after infusion, were significantly higher in patients with Down syndrome versus control patients (average 0.47 vs 0.24 mumol/L, respectively, P = 0.03). When a 42-hour MTX concentration of 0.5 mumol/L was used to identify patients at risk for toxicity, more courses were considered at high risk for toxicity among patients with Down syndrome (31 of 62, 50%) than in control patients (13 of 214, 6.1%, P less than 0.0001). The average MTX clearance was 64.1 mL/min/m2 in Down syndrome vs an average control value of 80.6 mL/min/m2 (P = 0.13). Toxicity after each high-dose MTX course was graded according to standardized criteria. Grades 2 through 4 gastrointestinal toxicity and grades 3 and 4 hematologic toxicity occurred more frequently in the patients with Down syndrome (36% and 13.4% of courses, respectively) vs the control patients (3.6% and 0.9% respectively, P less than 0.0001 for both). This higher frequency of toxicity occurred despite higher doses and prolonged duration of leucovorin given to all patients with Down syndrome. We conclude that altered MTX pharmacokinetics may contribute to the higher incidence of MTX-induced toxicity seen in patients with Down syndrome.

Simultaneous administration of multiple model substrates to assess hepatic drug clearance

We have evaluated a method to simultaneously assess three major processes involved in hepatic drug clearance using three model substrates administered simultaneously as a 5-minute intravenous injection. Lorazepam, indocyanine green, and antipyrine are used to assess conjugation, liver blood flow, and microsomal oxidative metabolism, respectively. These substrates were administered individually and as a mixture to 10 healthy adult male volunteers to determine if clearances of any of the compounds were affected by simultaneous administration. Mean clearances of the substrates were not different when administered alone (9.97, 0.78, and 0.53 ml/min/kg) vs. together (11.5, 0.89, and 0.52 ml/min/kg), using a paired t test. Since we were using this method to assess hepatic drug clearance in children with leukemia, the effect of short-term allopurinol was assessed. The three model substrates were administered to the volunteers after 0, 1, 8, and 22 days of treatment with allopurinol, 200 mg t.i.d. There was no change in mean clearance of any of the three compounds at any point during allopurinol treatment (repeated-measures ANOVA). We conclude that this technique is a simple and valid method to simultaneously assess three major processes involved in hepatic drug clearance and is not affected by up to 22 days of oral allopurinol treatment. This simple technique, requiring a single set of blood samples, has potential applications in the assessment of developmental changes in hepatic drug clearance, as well as the effects of environmental, therapeutic, and pathophysiologic factors on three major processes involved in hepatic drug clearance.

Hepatic drug clearance in children with leukemia: changes in clearance of model substrates during remission-induction therapy

We administered a "cocktail" of three model substrates for hepatic processes: antipyrine for oxidation, lorazepam for glucuronidation, and indocyanine green for hepatic blood flow, to children with acute lymphocytic leukemia (ALL). The plasma clearance of these substrates was determined before and after remission-induction therapy in 14 children with ALL. We found an improvement in clearance of antipyrine (0.65 to 0.95 ml/min/kg; P less than 0.01) and lorazepam (0.93 to 1.20 ml/min/kg; P less than 0.05) in 12 of 14 patients, with a mean increase of 67% and 52%, respectively, from before to after remission. There was no significant difference in mean indocyanine green clearance from before to after induction (14.8 vs. 14.4 ml/min/kg). There were no significant differences in liver function test results (SGOT, prothrombin time, or serum bilirubin) from before to after induction. Plasma concentrations of albumin, alpha 1-acid glycoprotein, and apolipoprotein A changed by a mean of +11.1%, -38.2%, and +68.6%, respectively, from before to after remission. However, these changes did not account for the changes in total plasma clearance of lorazepam, because lorazepam free fraction did not change and lorazepam free clearance increased by a mean of 83%. Our hypothesis is that eradication of hepatic leukemic infiltration by ALL remission therapy resulted in an improvement in microsomal metabolism of antipyrine and lorazepam.

Methotrexate bioavailability after oral and intramuscular administration in children

Although methotrexate is one of the most commonly used drugs for maintenance therapy in childhood acute lymphocytic leukemia (ALL), its oral absorption is highly variable and its intramuscular bioavailability at dosages used for ALL therapy has not been assessed in children. We therefore determined the absolute bioavailability of orally and intramuscularly administered methotrexate in 12 pediatric patients receiving 13 to 120 mg/m2 methotrexate every week as maintenance therapy for ALL. Mean bioavailability, as determined by comparing the area under the concentration-time curve after oral or intramuscular administration with that produced by the same dosage given intravenously, was 33% (range 13% to 76%) for oral (n = 11) and 76% (54% to 112%) for intramuscular (n = 7) administration (P less than 0.01). Median bioavailability (with orally administered dosages less than or equal to 40 mg/m2 (range 13 to 40 mg/m2) was 42% (19% to 76%); at dosages greater than 40 mg/m2 (43 to 76 mg/m2), bioavailability was significantly lower, 17.5% (12.7% to 22.3%, p less than 0.02). Conversely, there was no significant relationship between dosage and bioavailability with intramuscularly administered drug. The substantially higher bioavailability for intramuscularly injected methotrexate may warrant its consideration as an alternative to oral administration, especially for dosages greater than 40 mg/m2.

Urinary N-acetyl-beta-D-glucosaminidase and serum creatinine concentrations predict impaired excretion of methotrexate

We determined the risk of impaired excretion of methotrexate (MTX) in children with osteosarcoma, who also were receiving cisplatin, by analyzing urinary markers of renal tubular damage, as well as serum creatinine measured before each dose of MTX. MTX clearance was impaired in seven of the ten patients studied after cisplatin therapy. Patients with a urinary N-acetyl-beta-D-glucosaminidase (NAG) concentration of greater than 1.5 U/mmol creatinine or a greater than 50% increase in serum creatinine relative to the pretherapy level were approximately 30 times more likely to have MTX half-lives greater than 3.5 hours than were patients with lower values for these markers; MTX clearance was always impaired if both markers were elevated. If neither urinary NAG nor serum creatinine concentrations increased, the risk of impaired MTX excretion was negligible. Our findings demonstrate that urinary NAG and serum creatinine levels, measured before MTX administration, can be used to identify patients who will have difficulty in clearing the drug.

Pharmacokinetics of anticancer drugs in children

Interpatient pharmacokinetic variability normally observed in adults is often of even greater magnitude in paediatric patients because of age-related maturation of physiological processes responsible for drug disposition. Several antineoplastic agents have shown age-related changes, including alterations in volume of distribution, hepatic (doxorubicin, cyclophosphamide), and renal (bleomycin, methotrexate) clearances. These differences in pharmacokinetics as a function of age alter systemic exposure to chemotherapy, and may alter the efficacy and toxicity profile for standard doses of antineoplastic drugs. The relationship of systemic exposure to toxicity has been most clearly defined for methotrexate. Clinical monitoring of methotrexate serum concentrations, and adjustment of folinic acid dosages and duration of rescue based on methotrexate disposition is now routine. More recently, pharmacodynamic data have been published for high-dose methotrexate, epipodophyllotoxins, cisplatin, and cytarabine (cytosine arabinoside), indicating a relation between drug disposition and toxicity or efficacy. Collectively, these data suggest that the pharmacokinetics of many anticancer drugs in children is different from adults, and that variability in drug disposition may have an important influence on toxicity or efficacy.

Etoposide pharmacokinetics in patients with normal and abnormal organ function

Precise guidelines for dose modification of etoposide in patients with hepatic dysfunction have not been determined. Etoposide pharmacokinetics were determined in 17 patients. Nine patients had bilirubin less than or equal to 1 mg/dL and eight had bilirubin ranging from 1.9 to 23 mg/dL. Twelve patients received etoposide 100 mg/m2 days 1, 3, and 5, in combination with cisplatin 70 mg/m2 or iproplatin 225 mg/m2 on day 1. Five patients received only one dose of etoposide. Etoposide was measured using a published high pressure liquid chromatography (HPLC) method which also quantitates picro etoposide and its hydroxy acid. Systemic clearance, Vdss and t1/2 beta averaged (+/- SD) 21.4 (+/- 7.4) mL/min/m2, 10.7 (+/- 4.1) L/m2, and 8.1 (+/- 2.8) hours in the nine patients with bilirubin less than or equal to 1 mg/dL, and 22.4 (+/- 9.6) mL/min/m2, 13.6 (+/- 11.3) L/m2, and 8.4 (+/- 3.9) hours in the eight patients with bilirubin 1.9 to 23.0 mg/dL. Stepwise multiple linear regression analysis of liver and renal function tests and other patient-specific variables identified creatinine clearance as the strongest predictor of etoposide systemic clearance (r2 = 40.8). Serum albumin was identified as the next strongest predictor, improving the r2 to 57.3%. Cumulative biliary excretion of unchanged etoposide and glucuronide or sulfate conjugates over 48 hours accounted for less than 3% of the dose in six patients studied. Toxicity occurred in patients with normal and abnormal bilirubin and was unrelated to etoposide clearance. Patients with total bilirubin 1.9 to 23 mg/dL, but creatinine clearance greater than 30 mL/min/m2 had etoposide clearance within the range for patients with normal liver function (16.8 to 35 mL/min/m2). Although these patients did not have reduced etoposide clearance, the major routes of etoposide non-renal elimination remain to be clearly defined. Additional patients should be evaluated to establish more precise guidelines for dosing etoposide in patients with abnormal liver function.

Use of the automatic interaction detector method to identify patient characteristics related to methotrexate clearance

Systemic methotrexate (MTX) clearance was determined in 108 children who received 15 courses of MTX, 1000 mg/m2 iv over 24 hours, as a component of therapy for acute lymphocytic leukemia. The median MTX clearance was used as the representative value for each patient, and these values ranged from 44.7 to 132.1 ml/min/m2 (X = 78.4 ml/min/m2). The automatic interaction detector approach was used to determine the patient characteristics that correlated with MTX clearance. Characteristics examined were sex, age, estimated creatinine clearance, SGPT, and body surface area. The initial splits were based on creatinine clearance, and mean MTX clearances in three subgroups (50 to 100, 100 to 150, and greater than 150 ml/min/m2) were 73.1, 78.3, and 90.5 ml/min/m2, respectively. For patients with the slowest creatinine clearance, abnormal SGPT concentrations (greater than 35 IU/L) were associated with slower MTX clearance (77.6 vs. 67.8 ml/min/m2). In the latter subgroup, boys had faster clearance than girls (77.4 vs. 60.9 ml/min/m2). These results demonstrate that for children with normal serum creatinine concentrations, interpatient variability in MTX clearance can partly be explained by measures of renal and hepatic function, which indicates that the observed variability in MTX clearance is not totally random.

Clinical pharmacodynamics of high-dose methotrexate in acute lymphocytic leukemia. Identification of a relation between concentration and effect

High-dose methotrexate (500 to 33,600 mg per square meter of body-surface area) with leucovorin rescue is a common component of therapy for acute lymphocytic leukemia. To increase understanding of the relation between the serum concentration and the effect of methotrexate, we conducted a randomized, prospective study of 108 children with "standard-risk" acute lymphocytic leukemia who were treated with 15 doses of methotrexate (1000 mg per square meter) that were infused over 24 hours. The median length of follow-up was 3.5 years from diagnosis for patients still in remission. Variability between patients in methotrexate clearance produced steady-state serum concentrations that ranged from 9.3 to 25.4 microM. Patients with median methotrexate concentrations of less than 16 microM (n = 59) had a lower probability of remaining in remission (P less than 0.05) than patients with concentrations of 16 microM or more (n = 49). Multivariate analyses indicated that patients with methotrexate concentrations of less than 16 microM were 3 times more likely to have any kind of relapse during therapy (P = 0.01) and 7 times more likely to have a hematologic relapse during therapy (P = 0.001). Stepwise Cox's regression identified leukemic-cell DNA content, methotrexate concentration, and hemoglobin as significant prognostic variables for hematologic relapse (P = 0.0005). We conclude that there is a concentration-effect relation for high-dose methotrexate in acute lymphocytic leukemia and that 1000 mg per square meter infused over a period of 24 hours may not be optimal for patients with relatively fast drug clearance.

Loop-column extraction and liquid chromatographic analysis of doxorubicin and three metabolites in plasma

A sensitive and specific high performance liquid chromatographic method for the separation and quantitation of doxorubicin and three metabolites is described. The new method involves isocratic reversed-phase separation of these compounds, with quantitation by either electrochemical or fluorescence methods. A novel loop-column method was used to extract the compounds of interest from plasma, eliminating the need for organic extraction of biological samples before injection into the chromatographic system. The limit of sensitivity with the fluorescence method was about 2.3 ng on column. Sensitivity with electrochemical detection was better, although there was more interference by early eluting plasma components. The between-run CV (%) for replicate analysis of doxorubicin in identical plasma samples was 2.0 at 112.5 ng/ml, 5.4 at 40.0 ng/ml, and 9.9 at 14.0 ng/ml. Precision for each of the three metabolites was comparable. Accuracy for doxorubicin and each metabolite ranged from 93 to 107% at concentrations of 14.0, 40.0, 65.0, 112.5, and 140.0 ng/ml. The new loop-column extraction method, with injection of plasma samples directly onto the chromatograph, is simpler and more efficient than previous methods requiring organic extraction, making it more feasible for routine processing of clinical samples.

The effect of prior cisplatin therapy on the pharmacokinetics of high-dose methotrexate

The pharmacokinetics of high-dose methotrexate (MTX, 5-15 g/m2) were evaluated in 11 children and adolescents who had previously received two to eight doses of cisplatin (90 mg/m2) in the treatment of malignant solid tumors. The half-life for disappearance of MTX from serum during the first 24 hours after infusion was determined from serum samples obtained at the end of a six-hour infusion and six, 12, and 24 hours after infusion. These values were compared to a mean half-life of 2.83 (+/- 0.34) hours following 489 courses administered to 71 patients who had not received cisplatin. Stepwise multiple linear regression analysis of patient variables revealed cumulative cisplatin dosage and time from last cisplatin dose as the best predictors of MTX half-life (r2 = 65.4%, p less than 0.001). The best predictors of 24-hour serum concentration were cumulative cisplatin dosage and MTX dosage (r2 = 54.2%, p less than 0.001) in the multiple linear regression model. Patients with delayed MTX clearance received additional leucovorin and experienced no severe toxicity. Patients receiving up to 270 mg/m2 of cisplatin appear to have minimal increases in MTX half-life, while the likelihood of delayed clearance increases in patients who have received 360 mg/m2 or more of cisplatin. All patients who have previously received cisplatin should be treated cautiously with high-dose MTX and prospective pharmacokinetic monitoring should be routinely performed.

Teniposide (VM26) disposition in children with leukemia

The clinical pharmacokinetics of teniposide (VM-26, NSC 122819) has been studied in 21 children (median age, 4.7 years) with acute lymphocytic leukemia. Teniposide was administered at a dosage of 165 mg/sq m as a 30- to 60-min i.v. infusion. Patients were studied either on the first or second dosage of the drug. Plasma samples were assayed for teniposide and metabolites by high-performance liquid chromatography with electro-chemical detection. Both compartmental and noncompartmental pharmacokinetic analyses were performed. Systemic clearance and apparent volume of distribution of steady state averaged 13.82 +/- 6.0 ml/min/sq m (S.D.) and 7.9 +/- 4.0 liter/sq m, respectively. Univariate and multivariate stepwise regression analyses were used to construct mathematical models to describe the relationships between certain patient-specific demographic and laboratory values and the pharmacokinetic parameters, systemic clearance, elimination rate constant, and area under the concentration-time curve. A significant relationship between serum alkaline phosphatase and systemic clearance, elimination rate constant, and area under the concentration-time curve was found, suggesting that liver function influences the disposition of this anticancer drug in humans.

Methotrexate systemic clearance influences probability of relapse in children with standard-risk acute lymphocytic leukaemia

108 children with standard-risk acute lymphocytic leukaemia (ALL) were randomised to a post-induction treatment protocol including 15 doses of intermediate-dose methotrexate (1000 mg/m2) in addition to conventional oral therapy of mercaptopurine and low-dose methotrexate. After median follow-up of 26 months, 22 patients have had relapses. Among the 108 patients, rates of methotrexate systemic clearance ranged from 44.7 to 132 ml/min/m2. When the group was divided into three subgroups according to the patients' rates of methotrexate clearance, statistical analysis of the Kaplan-Meier curves estimating the probability of complete remission showed significant differences (p = 0.016) among the subgroups, patients with faster clearance having higher probability of relapse. Multivariate Cox's regression analysis incorporating other potential prognostic variables identified three significant variables influencing the risk of relapse--methotrexate clearance and white-blood-cell count and haemoglobin level at diagnosis (p = 0.0015). This study has demonstrated the potential clinical importance of the rate of drug clearance in children with ALL.

Cisplatin-induced changes in bleomycin elimination

Bleomycin disposition was studied in two children who received at least six courses of vinblastine, bleomycin, and cisplatin. Total plasma clearance of bleomycin decreased from 39 to 18 ml/min/m2, while the terminal-phase half-life increased from 4.4 to 6.0 hrs since the cumulative cisplatin dose was greater than 300 mg/m2. In one patient, renal clearance of bleomycin decreased from 30 to 8.2 ml/min/m2. Changes in serum creatinine or BUN were not predictive of changes in bleomycin elimination.

Bleomycin disposition in children with cancer

Bleomycin kinetics were determined in 14 children after intravenous bolus and prolonged infusion doses. Plasma and urine bleomycin concentrations were determined by radioimmunoassay. After intravenous bolus, bleomycin concentrations were adequately described by a two-compartment open model with a mean t1/2 alpha and t1/2 beta of 0.3 +/- 0.1 and 3.2 +/- 0.7 hr (mean +/- SEM). Volume of the central compartment and volume of distribution at steady-state (Vss) were 4.3 +/- 0.5 and 9.9 +/- 1.1 l/m2. Total plasma (CLT) and renal (CLR) clearance were 51.8 +/- 6.1 and 33.5 +/- 2.4 ml/min/m2. Three intravenous bolus courses were given to two patients who received more than four courses of cisplatin (greater than 300 mg/m2); CLT and CLR for these courses were 18.0 +/- 3.3 and 8.2 ml/min/m2. Conversely, children under 3 yr old eliminated bleomycin more rapidly than older children. Decline in bleomycin concentrations after seven 24- or 48-hr intravenous infusions was described by a one-compartment model. Mean values for plasma t1/2, Vss, CLT, and CLR were 2.1 +/- 0.1 hr, 11.0 +/- 2.6 l/m2, 57.1 +/- 13.5 ml/min/m2, and 33.2 +/- 6.4 ml/min/m2. One patient received his bleomycin infusion when ureteral obstruction was present; CLT and CLR for this course were 4.8 and 4.1 ml/min/m2. These data indicate that young children eliminate bleomycin more rapidly than older children and that children with impaired renal function may have prolonged elevations in plasma concentration due to reduced bleomycin clearance. Bleomycin disposition in older children is as in adults.

Methotrexate cerebrospinal fluid and serum concentrations after intermediate-dose methotrexate infusion

Twenty-nine children with acute lymphocytic leukemia were given 24-hr infusions of intermediate-dose methotrexate (MTX, 1000 mg/m2) with and without intrathecal (IT) MTX (12 mg/m2), followed by leucovorin rescue. There was substantial interpatient variability in MTX systemic clearance (98.3 +/- 51 ml/min/m2), inducing total steady-state serum MTX concentrations ranging from 5.4 to 33.7 microM. The cerebrospinal fluid (CSF) concentration at the end of the infusion was 0.27 (+/- 0.1) microM when no IT-MTX was given and correlated with total steady-state (24-hr) serum concentration of MTX. By stepwise regression, the CSF MTX concentration correlated better with the nonprotein bound (free) steady-state serum MTX concentration (r = 0.66, P less than 0.01) than with total steady-state serum MTX concentration. Mean CSF: serum MTX concentration ratio was 0.023 (+/- 0.04) when no IT MTX was given. When an IT MTX dose (12 mg/m2) was given at the start of the MTX infusion, the steady-state CSF MTX concentration was 1.1 (+/- 0.4) microM, leading to a mean CSF: serum ratio of 0.073 (+/- 0.05). Despite 7-hydroxy-MTX serum concentrations exceeding MTX concentrations immediately after infusion, 7-hydroxy-MTX was not detectable in CSF of most patients (21 of 29), and was less than 50% of the concurrent MTX concentration when detectable. These data establish the substantial interpatient variability in CSF distribution of MTX after intermediate-dose MTX infusions and establish a significant correlation between steady-state free concentration of MTX in serum and CSF MTX concentration.