Metabolism and pharmacokinetics of oxazaphosphorines

The 2 most commonly used oxazaphosphorines are cyclophosphamide and ifosfamide, although other bifunctional mustard analogues continue to be investigated. The pharmacology of these agents is determined by their metabolism, since the parent drug is relatively inactive. For cyclophosphamide, elimination of the parent compound is by activation to the 4-hydroxy metabolite, although other minor pathways of inactivation also play a role. Ifosfamide is inactivated to a greater degree by dechloroethylation reactions. More robust assay methods for the 4-hydroxy metabolites may reveal more about the clinical pharmacology of these drugs, but at present the best pharmacodynamic data indicate an inverse relationship between plasma concentration of parent drug and either toxicity or antitumour effect. The metabolism of cyclophosphamide is of particular relevance in the application of high dose chemotherapy. The activation pathway of metabolism is saturable, such that at higher doses (greater than 2 to 4 g/m2) a greater proportion of the drug is eliminated as inactive metabolites. However, both cyclophosphamide and ifosfamide also act to induce their own metabolism. Since most high dose regimens require a continuous infusion or divided doses over several days, saturation of metabolism may be compensated for, in part, by auto-induction. Although a quantitative distinction may be made between the cytochrome P450 isoforms responsible for the activating 4-hydroxylation reaction and those which mediate the dechloroethylation reactions, selective induction of the activation pathway, or inhibition of the inactivating pathway, has not been demonstrated clinically. Mathematical models to describe and predict the relative contributions of saturation and autoinduction to the net activation of cyclophosphamide have been developed. However, these require careful validation and may not be applicable outside the exact regimen in which they were derived. A further complication is the chiral nature of these 2 drugs, with some suggestion that one enantiomer may have a favourable profile of metabolism over the other. That the oxazaphosphorines continue to be the subject of intensive investigation over 30 years after their introduction into clinical practice is partly because of their antitumour activity. Further advances in analytical and molecular pharmacological techniques may further optimise their use and allow rational design of more selective analogues.

Influence of pharmacogenetics on response and toxicity in breast cancer patients treated with doxorubicin and cyclophosphamide

Background:

Doxorubicin and cyclophosphamide (AC) therapy is an effective treatment for early-stage breast cancer. Doxorubicin is a substrate for ABCB1 and SLC22A16 transporters. Cyclophosphamide is a prodrug that requires oxidation to 4-hydroxycyclophosphamide, which yields a cytotoxic alkylating agent. The initial oxidation is catalysed by cytochrome P450 enzymes including CYP2B6, CYP2C9, CYP2C19 and CYP3A5. Polymorphic variants of the genes coding for these enzymes and transporters have been identified, which may influence the systemic pharmacology of the two drugs. It is not known whether this genetic variation has an impact on the efficacy or toxicity of AC therapy.

Methods:

Germ line DNA samples from 230 patients with breast cancer on AC therapy were genotyped for the following SNPs: ABCB1 C1236T, G2677T/A and C3435T, SLC22A16 A146G, T312C, T755C and T1226C, CYP2B6^*2, ^*8, ^*9, ^*3, ^*4 and ^*5, CYP2C9^*2 and ^*3, CYP3A5^*3 and CYP2C19^*2. Clinical data on survival, toxicity, demographics and pathology were collated.

Results:

A lower incidence of dose delay, indicative of less toxicity, was seen in carriers of the SLC22A16 A146G, T312C, T755C variants. In contrast, a higher incidence of dose delay was seen in carriers of the SLC22A16 1226C, CYP2B6^*2 and CYP2B6^*5 alleles. The ABCB1 2677A, CYP2B6^*2, CYP 2B6^*8, CYP 2B6^*9, CYP 2B6^*4 alleles were associated with a worse outcome.

Conclusion:

Variant alleles in the ABCB1, SLC22A16 and CYP2B6 genes are associated with response to AC therapy in the treatment of breast cancer.

Estimation of glomerular filtration rate in cancer patients

The frequent need to obtain an estimate of renal function in cancer patients, not least for targeting carboplatin dose, has led to a number of approaches to estimate glomerular filtration rate (GFR). This study aimed to develop a simple and reliable method to estimate GFR using readily-available patient characteristics. Data from 62 patients with estimates of 51Cr-EDTA clearance were analysed to determine the most appropriate formula relating this method of measuring GFR to patient characteristics. The population pharmacokinetics of 51Cr-EDTA were analysed using NONMEM to evaluate the influence of each covariate. The formulae derived were then validated using a further 38 patients and compared with those obtained using existing formulae. 51Cr-EDTA clearance (GFR) was positively related to Dubois surface area, negatively related to age, and inversely related to serum creatinine (SCr). Females had lower 51Cr-EDTA clearance than males. The enzymatic method of SCr assay gave more reliable results than the Jaffe colorimetric method. A measure of creatine kinase significantly improved the estimation of GFR. The new formula produced estimates of GFR which were less biased (Mean Prediction Error = -3%) and more precise (Mean Absolute Prediction Error = 12%) than Cockcroft and Gault (-8% and 16%) or Jelliffe (-15% and 19%) estimates. The formulae developed here can be used to provide reliable estimates of GFR, particularly in regard to targeted dosing of carboplatin.

Phase I clinical and pharmacokinetic study of pemetrexed and carboplatin in patients with malignant pleural mesothelioma

PURPOSE

To determine the maximum tolerated dose (MTD) of pemetrexed and carboplatin given in combination, to derive a recommended dose for phase II studies, and to explore its efficacy. We assessed toxicities and explored the activity of the drug combination exclusively in patients with malignant pleural mesothelioma (MPM). The pharmacokinetics of both agents was investigated.

PATIENTS AND METHODS

Twenty-seven patients (23 male, four female) with MPM were treated on five escalating dose levels. Doses ranged from pemetrexed 400 mg/m(2) (as a 10-minute intravenous infusion), followed by carboplatin area under the plasma concentration-time curve (AUC) 4 mg/mL.min (as a 30-minute intravenous infusion) to pemetrexed 500 mg/m(2), carboplatin AUC 6 mg/mL.min. All patients had a World Health Organization performance status of 1. A total of 163 courses of treatment were administered (median, six; range, one to 10).

RESULTS

The main toxicity was hematologic, particularly neutropenia, although this was characteristically short-lived and caused few clinical problems. The MTD was pemetrexed 500 mg/m(2), carboplatin AUC 6, because three of the five patients treated at this dose level experienced a dose-limiting toxicity. Eight partial responses (in 25 assessable patients) were observed for a response rate of 32%. Seventy percent of patients noticed an improvement in symptoms, usually (84%) after only two courses. Median time to progression was 305 days, and median survival time was 451 days.

CONCLUSION

The MTD was pemetrexed 500 mg/m(2) and carboplatin AUC 6 mg/mL.min. The recommended phase II dose of the combination is pemetrexed 500 mg/m(2) and carboplatin AUC 5 mg/mL.min. The combination is both active and well tolerated in MPM and deserves further exploration.

Curcumin as a clinically-promising anti-cancer agent: pharmacokinetics and drug interactions

INTRODUCTION

Curcumin has been extensively studied for its anti-cancer properties. While a diverse array of in vitro and preclinical research support the prospect of curcumin use as an anti-cancer therapeutic, most human studies have failed to meet the intended clinical expectation. Poor systemic availability of orally-administered curcumin may account for this disparity. Areas covered: This descriptive review aims to concisely summarise available clinical studies investigating curcumin pharmacokinetics when administered in different formulations. A critical analysis of pharmacokinetic- and pharmacodynamic-based interactions of curcumin with concomitantly administered drugs is also provided. Expert opinion: The encouraging clinical results of curcumin administration are currently limited to people with colorectal cancer, given that sufficient curcumin concentrations persist in colonic mucosa. Higher parent curcumin systemic exposure, which can be achieved by several newer formulations, has important implications for optimal treatment of cancers other than those in gastrointestinal tract. Curcumin-drug pharmacokinetic interactions are also almost exclusively in the enterocytes, owing to extensive first pass metabolism and poor curcumin bioavailability. Greater scope of these interactions, i.e. modulation of the systemic elimination of co-administered drugs, may be expected from more-bioavailable curcumin formulations. Further studies are still warranted, especially with newer formulations to support the inclusion of curcumin in cancer therapy regimens.

A phase I and pharmacokinetic study of paclitaxel poliglumex (XYOTAX), investigating both 3-weekly and 2-weekly schedules

PURPOSE

To determine the safety, maximum tolerated dose, pharmacokinetics, and toxicities associated with administration of paclitaxel poliglumex (PPX, XYOTAX, Cell Therapeutics, Inc., Bresso, Italy) given on either 3-weekly or 2-weekly schedule.

EXPERIMENTAL DESIGN

Nineteen patients were investigated on the 3-weekly phase Ia study and 11 patients on the 2-weekly phase Ib study. Dose escalation starting with 100% increments and one patient per dose level was modulated in accordance with the observed toxicities. Conjugated and unconjugated paclitaxel were measured in plasma.

RESULTS

Dose-limiting toxicity of neutropenia was encountered at 266 mg/m(2) (paclitaxel equivalents) in phase Ia and the maximum tolerated dose was 233 mg/m(2). Neuropathy was dose-limiting in phase Ib with a maximum tolerated dose of 177 mg/m(2). Pharmacokinetic investigations indicated a prolonged half-life of >100 hours for conjugated taxanes. Plasma concentrations of unconjugated paclitaxel were similar to those following administration of an equivalent dose of Taxol. Two partial responses were observed, one in a patient with mesothelioma at 177 mg/m(2) in phase Ia and one in a patient with gastric carcinoma at 175 mg/m(2) in phase Ib.

CONCLUSION

PPX is a water-soluble paclitaxel-polymer conjugate with a prolonged half-life and limited volume of distribution. Dose-limiting toxicities were neutropenia and neuropathy. PPX showed activity in this patient population.

Imagestream detection and characterisation of circulating tumour cells - A liquid biopsy for hepatocellular carcinoma?

BACKGROUND & AIMS

The lack of progress in developing and delivering new therapies for hepatocellular carcinoma (HCC) is in part attributed to the risk related avoidance of tumour biopsy at diagnosis. Circulating tumour cells (CTCs) are a potential source of tumour tissue that could aid biological or biomarker research, treatment stratification and monitoring.

METHODS

An imaging flow cytometry method, using immunofluorescence of cytokeratin, EpCAM, AFP, glypican-3 and DNA-PK together with analysis of size, morphology and DNA content, for detection of HCC CTCs was developed and applied to 69 patient and 31 control samples. The presence of CTCs as a prognostic indicator was assessed in multivariate analyses encompassing recognised prognostic parameters.

RESULTS

Between 1 and 1642 CTCs were detected in blood samples from 45/69 HCC patients compared to 0/31 controls. CTCs positive for the epithelial markers cytokeratin and EpCAM were detected in 29% and 18% of patients respectively, while an additional 28% of patients had CTCs negative for all markers other than size and evidence of hyperploidy. CTC number correlated significantly with tumour size and portal vein thrombosis (PVT). The median survival of patients with >1 CTC was 7.5months versus >34months for patients with <1 CTC (p<0.001, log-rank), with significance retained in a multivariate analysis (HR 2.34, 95% CI 1.005-5.425, p=0.049) including tumour size and PVT.

CONCLUSIONS

The use of multiple parameters enhanced HCC CTC detection sensitivity, revealing biological associations and predictive biomarker potential that may be able to guide stratified medicine decisions and future research.

LAY SUMMARY

Characteristics of tumour tissues can be used to predict outcomes for individual patients with cancer, as well as help to choose their best treatment. Biopsy of liver cancers carries risks, however, and is usually avoided. Some cancer cells enter the blood, and although they are very rare, we have developed a method of finding and characterising them in patients with liver cancer, which we hope will provide a low risk means of guiding treatment.

Population pharmacokinetics and pharmacodynamics of paclitaxel and carboplatin in ovarian cancer patients: a study by the European organization for research and treatment of cancer-pharmacology and molecular mechanisms group and new drug development group

PURPOSE

Paclitaxel and carboplatin are frequently used in advanced ovarian cancer following cytoreductive surgery. Threshold models have been used to predict paclitaxel pharmacokinetic-pharmacodynamics, whereas the time above paclitaxel plasma concentration of 0.05 to 0.2 micromol/L (t(C > 0.05-0.2)) predicts neutropenia. The objective of this study was to build a population pharmacokinetic-pharmacodynamic model of paclitaxel/carboplatin in ovarian cancer patients.

EXPERIMENTAL DESIGN

One hundred thirty-nine ovarian cancer patients received paclitaxel (175 mg/m(2)) over 3 h followed by carboplatin area under the concentration-time curve 5 mg/mL*min over 30 min. Plasma concentration-time data were measured, and data were processed using nonlinear mixed-effect modeling. Semiphysiologic models with linear or sigmoidal maximum response and threshold models were adapted to the data.

RESULTS

One hundred five patients had complete pharmacokinetic and toxicity data. In 34 patients with measurable disease, objective response rate was 76%. Neutrophil and thrombocyte counts were adequately described by an inhibitory linear response model. Paclitaxel t(C > 0.05) was significantly higher in patients with a complete (91.8 h) or partial (76.3 h) response compared with patients with progressive disease (31.5 h; P = 0.02 and 0.05, respectively). Patients with paclitaxel t(C > 0.05) > 61.4 h (mean value) had a longer time to disease progression compared with patients with paclitaxel t(C > 0.05) < 61.4 h (89.0 versus 61.9 weeks; P = 0.05). Paclitaxel t(C > 0.05) was a good predictor for severe neutropenia (P = 0.01), whereas carboplatin exposure (C(max) and area under the concentration-time curve) was the best predictor for thrombocytopenia (P < 10(-4)).

CONCLUSIONS

In this group of patients, paclitaxel t(C > 0.05) is a good predictive marker for severe neutropenia and clinical outcome, whereas carboplatin exposure is a good predictive marker for thrombocytopenia.

Temozolomide Pharmacodynamics in Patients with Metastatic Melanoma: DNA Damage and Activity of Repair EnzymesO6-Alkylguanine Alkyltransferase and Poly(ADP-Ribose) Polymerase-1

PURPOSE

Temozolomide, a DNA methylating agent used to treat melanoma, induces DNA damage, which is repaired by O6-alkylguanine alkyltransferase (ATase) and poly(ADP-ribose) polymerase-1 (PARP-1)-dependent base excision repair. The current study was done to define the effect of temozolomide on DNA integrity and relevant repair enzymes as a prelude to a phase I trial of the combination of temozolomide with a PARP inhibitor.

EXPERIMENTAL DESIGN

Temozolomide (200 mg/m2 oral administration) was given to 12 patients with metastatic malignant melanoma. Peripheral blood lymphocytes (PBL) were analyzed for PARP activity, DNA single-strand breakage, ATase levels, and DNA methylation. PARP activity was also measured in tumor biopsies from 9 of 12 patients and in PBLs from healthy volunteers.

RESULTS

Temozolomide pharmacokinetics were consistent with previous reports. Temozolomide therapy caused a substantial and sustained elevation of N7-methylguanine levels, a modest and sustained reduction in ATase activity, and a modest and transient increase in DNA strand breaks and PARP activity in PBLs. PARP-1 activity in tumor homogenates was variable (828 +/- 599 pmol PAR monomer/mg protein) and was not consistently affected by temozolomide treatment.

CONCLUSIONS

The effect of temozolomide reported here are consistent with those documented in previous studies with temozolomide and similar drug, dacarbazine, demonstrating that a representative patient population was investigated. Furthermore, PARP activity was not inhibited by temozolomide treatment and this newly validated pharmacodynamic assay is therefore suitable for use in a proof-of-principle phase I trial a PARP-1 inhibitor in combination with temozolomide.

Phase I study of MG98, an oligonucleotide antisense inhibitor of human DNA methyltransferase 1, given as a 7-day infusion in patients with advanced solid tumors

PURPOSE

To assess the safety and tolerability, pharmacokinetics, and early evidence of antitumor activity of escalating doses of MG98, an antisense oligonucleotide to DNA methyltransferase 1 (DNMT1), which has been shown to reduce CpG island methylation and allow reexpression of tumor suppressor genes in vitro.

EXPERIMENTAL DESIGN

In this phase I, open-label study, patients with advanced solid malignancies were treated with escalating doses of MG98 administered as a continuous i.v. infusion over 7 days repeated every 14 days. Cohorts of three patients, which could be expanded to six patients, were studied. The maximum tolerated dose was defined as the highest dose at which no more than 33% of subjects experienced dose-limiting toxicity. Pharmacokinetic and pharmacodynamic parameters of MG98 were also characterized.

RESULTS

Thirty-three patients were treated at doses of 100 to 250 mg/m(2)/d MG98. MG98 was well tolerated with mild fatigue and myalgia, dose-limiting toxicity was asymptomatic transaminitis, and the maximum tolerated dose was 200 mg/m(2)/d. One patient achieved a partial response and another prolonged disease stabilization. Plasma half-life of MG98 was short (2 hours), drug concentrations reaching a dose-dependent steady state during infusion with a volume of distribution equivalent to plasma volume. Suppression of DNMT1 expression was observed in 26 of 32 patients studied.

CONCLUSIONS

MG98 was well tolerated with early evidence of clinical activity. Proof of mechanism was observed and measurement of DNMT1 expression in peripheral blood mononuclear cells may be useful in future phase II development.

Body surface area estimation in children using weight alone: application in paediatric oncology

The majority of chemotherapy regimens and trials specify doses of cytotoxic drugs normalized to body surface area. Estimation of BSA in paediatric patients is particularly problematic, as conventional nomograms require accurate determination of both height and weight. The chemotherapy standards group of the UKCCSG (United Kingdom Children's Cancer Study Group) has evaluated a method for calculation of body surface area (BSA) estimation, based solely on patient weight. In comparison with BSA estimations using 2 commonly used methods, which require both weight and height measurements, deviation in the estimate of BSA was less than 10%. This method may be extended to the dosing of chemotherapeutic agents in infants of body weight less than 10 kg, with appropriate recommendations for dose modification. Until better correlates of drug clearance, such as GFR for carboplatin, are identified BSA is used to standardize doses for most chemotherapeutic agents. The formula presented here provides a more robust and reliable method of calculation of BSA from weight alone. Although this approach has been shown to be equivalent to other currently used methods, care should be taken extending this calculation of BSA to children less than 10 kg, to obese patients and to those with cachexia.

Identification of the major human hepatic cytochrome P450 involved in activation and N-dechloroethylation of ifosfamide

Two NADPH-dependent metabolic routes for the anticancer drug ifosfamide, 4-hydroxylation (activation) and N-dechloroethylation (a detoxication pathway), were studied in human liver microsomes to identify the cytochrome P450 enzymes involved. Naringenin, a grapefruit aglycone and an inhibitor of cytochrome P450 3A4 (CYP3A4)-catalysed reactions, was found to inhibit ifosfamide activation and N-dechloroethylation by human liver microsomes. IC50 for both reactions was of the order of 70 microM. The CYP3A4-specific inhibitor triacetyloleandomycin inhibited ifosfamide N-dechloroethylation by human liver microsomes with an IC50 of approximately 10 microM. Furthermore, anti-human CYP3A4 antiserum inhibited by about 80% N-dechloroethylation of ifosfamide by human liver microsomes. The relative levels of cytochromes P450 1A, 2C, 2E and 3A4 in 12 human livers were determined by western blotting analysis. A strong correlation (P < 0.001) was observed between CYP3A4 expression and both activation and N-dechloroethylation of ifosfamide. A role for human CYP3A4 in both pathways of ifosfamide metabolism was thus demonstrated. This was substantiated by the observation that the nifedipine oxidase activities of the 12 samples of human liver microsomes correlated with ifosfamide activation (P < 0.009) and N-dechloroethylation (P < 0.001). These findings have important clinical implications. The involvement of the same key cytochrome P450 enzyme in both reactions prohibits selective inhibition of the N-dechloroethylation pathway, as might be desirable to reduce toxic side effects. They also demonstrate the need to consider interaction with co-administered drugs that are CYP3A4 substrates.

Efficacy of PARP Inhibitor Rucaparib in Orthotopic Glioblastoma Xenografts Is Limited by Ineffective Drug Penetration into the Central Nervous System

PARP inhibition can enhance the efficacy of temozolomide and prolong survival in orthotopic glioblastoma (GBM) xenografts. The aim of this study was to evaluate the combination of the PARP inhibitor rucaparib with temozolomide and to correlate pharmacokinetic and pharmacodynamic studies with efficacy in patient-derived GBM xenograft models. The combination of rucaparib with temozolomide was highly effective in vitro in short-term explant cultures derived from GBM12, and, similarly, the combination of rucaparib and temozolomide (dosed for 5 days every 28 days for 3 cycles) significantly prolonged the time to tumor regrowth by 40% in heterotopic xenografts. In contrast, the addition of rucaparib had no impact on the efficacy of temozolomide in GBM12 or GBM39 orthotopic models. Using Madin-Darby canine kidney (MDCK) II cells stably expressing murine BCRP1 or human MDR1, cell accumulation studies demonstrated that rucaparib is transported by both transporters. Consistent with the influence of these efflux pumps on central nervous system drug distribution, Mdr1a/b(-/-)Bcrp1(-/-) knockout mice had a significantly higher brain to plasma ratio for rucaparib (1.61 ± 0.25) than wild-type mice (0.11 ± 0.08). A pharmacokinetic and pharmacodynamic evaluation after a single dose confirmed limited accumulation of rucaparib in the brain is associated with substantial residual PARP enzymatic activity. Similarly, matrix-assisted laser desorption/ionization mass spectrometric imaging demonstrated significantly enhanced accumulation of drug in flank tumor compared with normal brain or orthotopic tumors. Collectively, these results suggest that limited drug delivery into brain tumors may significantly limit the efficacy of rucaparib combined with temozolomide in GBM.

Cyclophosphamide pharmacokinetics in children

1. Cyclophosphamide pharmacokinetics were measured in 38 children with cancer. 2. A high degree of inter-patient variation was seen in all pharmacokinetic parameters. Cyclophosphamide half-life varied between 1.1 and 16.8 h, clearance varied between 1.2 and 10.61 h-1 m-2 and volume of distribution varied between 0.26 and 1.48 1 kg-1. 3. The half-life of cyclophosphamide was prolonged at high dose levels (P = 0.008). 4. Children who had received prior treatment with dexamethasone showed a mean increase in clearance of 2.51 h-1 m-2 (P = 0.001) presumably as a result of CYP450 enzyme induction. 5. Treatment with allopurinol or chlorpromazine was associated with a significant increase in cyclophosphamide half-life (P < 0.001 in both cases). 6. Dose and concurrent treatment may influence cyclophosphamide metabolism in vivo and thus potentially alter the drugs therapeutic effect.

A phase I study in paediatric patients to evaluate the safety and pharmacokinetics of SPI-77, a liposome encapsulated formulation of cisplatin

Pre-clinical studies indicate that cisplatin encapsulated in STEALTH((R))liposomes (SPI-77) retains anti-tumour activity, but has a much reduced toxicity, compared to native cisplatin. A phase I study was conducted to determine the toxicity and pharmacokinetics of SPI-77 administered to children with advanced cancer not amenable to other treatment. Paediatric patients were treated at doses ranging from 40 to 320 mg m(-2)by intravenous infusion every 4 weeks. Blood samples taken during, and up to 3 weeks after, administration and plasma and ultrafiltrate were prepared immediately. Urine was collected, when possible, for 3 days after administration. SPI-77 administration was well tolerated with the major toxicity being an infusion reaction which responded to modification of the initial infusion rate of SPI-77. Limited haematological toxicity and no nephrotoxicity were observed. No responses to treatment were seen during the course of this phase I study. Measurement of total plasma platinum showed that cisplatin was retained in the circulation with a half life of up to 134 h, with maximum plasma concentrations approximately 100-fold higher than those reported following comparable doses of cisplatin. Comparison of plasma and whole blood indicated that cisplatin was retained in the liposomes and there was no free platinum measurable in the ultrafiltrate. Urine recovery was less than 4% of the dose administered over 72 h. Results from this phase I study indicate that high doses of liposomal cisplatin can safely be given to patients, but further studies are required to address the issue of reformulation of liposomally bound cisplatin.

2-[11C]thymidine positron emission tomography as an indicator of thymidylate synthase inhibition in patients treated with AG337

BACKGROUND

Some anticancer drugs inhibit thymidylate synthase (TS), a key enzyme for thymidine nucleotide biosynthesis. Cells can compensate for depleted thymidine levels by taking up extracellular thymidine via a salvage pathway. We investigated the use of 2-[11C]thymidine positron emission tomography (PET) to measure thymidine salvage kinetics in vivo in humans.

METHODS

Five patients with advanced gastrointestinal cancer were PET scanned both before and 1 hour after oral administration of the TS inhibitor AG337 (THYMITAQ [nolatrexed]); seven control patients were scanned twice but not treated with AG337. Thymidine salvage kinetics were measured in vivo using 2-[11C]thymidine PET and spectral analysis to obtain the standardized uptake values (SUV), the area under the time-activity curve (AUC), and the fractional retention of thymidine (FRT). Changes in PET parameters between scans in the AG337-treated and control groups were compared using the Mann-Whitney U test. The relationship between AG337 exposure and AG337-induced changes in tumor FRT and in plasma deoxyuridine levels (a conventional pharmacodynamic systemic measure of TS inhibition) was examined using Spearman's regression analysis. Statistical tests were two-sided.

RESULTS

The between-scan change in FRT in patients treated with AG337 (38% increase, 95% confidence interval [CI] = 8% to 68%) was higher than that in control patients (3% increase, 95% CI = -11% to 17%) (P =.028). The level of AG337-induced increase in both 2-[11C]thymidine FRT and plasma deoxyuridine levels was statistically significantly correlated with AG337 exposure (r = 1.00, P =.01 for both).

CONCLUSIONS

AG337 administration was associated with increased tumor tracer retention that was consistent with tumor cell uptake of exogenous 2-[11C]thymidine as a result of TS inhibition. 2-[11C]Thymidine PET can be used to measure thymidine salvage kinetics directly in the tissue of interest.

Pharmacokinetics of dactinomycin in a pediatric patient population: a United Kingdom Children's Cancer Study Group Study

PURPOSE

Dactinomycin (actinomycin D) is an antitumor antibiotic used routinely to treat certain pediatric and adult cancers. Despite concerns over the incidence of toxicity, little is known about the pharmacology of dactinomycin. A study was done to investigate dactinomycin pharmacokinetics in children.

EXPERIMENTAL DESIGN

Dactinomycin was administered to 31 patients by bolus i.v. infusion, at doses of 0.70 to 1.50 mg/m2. Plasma concentrations were determined by liquid chromatography-mass spectrometry up to 24 hours after drug administration and National Cancer Institute Common Toxicity Criteria was assessed.

RESULTS

Pharmacokinetic data analysis suggested that a three-compartment model most accurately reflected dactinomycin pharmacokinetics. However, there was insufficient data available to fully characterize this model. A median peak plasma concentration (Cmax) of 25.1 ng/mL (range, 3.2-99.2 ng/mL) was observed at 15 minutes after administration. The median exposure (AUC0-6), determined in 16 patients with sampling to 6 hours, was 2.67 mg/L.min (range, 1.12-4.90 mg/L.min). After adjusting for body size, AUC0-6 and Cmax were positively related to dose (P = 0.03 and P = 0.04, respectively). Patients who experienced any level of Common Toxicity Criteria grade had a 1.46-fold higher AUC0-6, 95% confidence interval (1.02-2.09). AUC0-6 was higher in patients <40 kg, possibly indicating a greater toxicity risk.

CONCLUSIONS

Data presented suggest that dosing of dactinomycin based on surface area is not optimal, either in younger patients in whom the risk of toxicity is greater, or in older patients where doses are capped.

Stereoselectivity in pharmacokinetics: a general theory

Stereoselectivity in pharmacokinetics may be characterized by a measurable difference between enantiomers in a pharmacokinetic parameter. We propose that pharmacokinetic parameters may be classified according to three levels of organization in the body and that the hybrid character of parameters increases with the level of organization that they represent. At the molecular level are intrinsic metabolite formation clearances and fraction of drug unbound in plasma, reflecting the selectivity of an endogenous macromolecule for the enantiomers of a chiral drug molecule. At the organ level, pharmacokinetic parameters represent the combined effects of stereoselectivity in each of their component parameters within an organ. As a result, these parameters are of intermediate hybrid character. Parameters with the highest degree of hybrid character describe the pharmacokinetic behavior of a drug in the whole body. The stereoselectivity associated with each of the component parameters could either amplify or dampen the resultant stereoselectivity in hybrid parameters. The hypothesis that kinetic differences between enantiomers are inversely correlated with the degree of hybrid character was examined for four drugs: warfarin, verapamil, mephenytoin, and propranolol. By classifying pharmacokinetic parameters according to both the level of organization that they characterize and their hybrid nature, it becomes possible to account for stereoselectivity in drug distribution and elimination.

Central nervous system penetration and enhancement of temozolomide activity in childhood medulloblastoma models by poly(ADP-ribose) polymerase inhibitor AG-014699

BACKGROUND

Temozolomide shows activity against medulloblastoma, the most common malignant paediatric brain tumour. Poly(ADP-ribose) polymerase (PARP) inhibitors enhance temozolomide activity in extracranial adult and paediatric human malignancies.

METHODS

We assessed the effect of AG-014699, a clinically active PARP inhibitor, on temozolomide-induced growth inhibition in human medulloblastoma models. Pharmacokinetic, pharmacodynamic and toxicity assays were performed in tumour-bearing mice.

RESULTS

Sensitivity to temozolomide in vitro was consistent with methylguanine methyltransferase (MGMT) and DNA mismatch repair (MMR) status; MGMT(+) MMR(+) D384Med cells (temozolomide GI(50)=220 μM), representative of most primary medulloblastomas, were sensitised fourfold by AG-014699; MGMT⁻ MMR(+) D425Med cells were hypersensitive (GI(50)=9 μM) and not sensitised by AG-014699, whereas MGMT(+) MMR⁻ temozolomide-resistant D283Med cells (GI₅₀=807 μM) were sensitised 20-fold. In xenograft models, co-administration of AG-014699 produced an increase in temozolomide-induced tumour growth delay in D384Med xenografts. Consistent with the in vitro data, temozolomide caused complete tumour regressions of D425Med xenografts, whereas D283Med xenografts were relatively resistant. AG-014699 was not toxic, accumulated and reduced PARP activity ≥75% in xenograft and brain tissues.

CONCLUSION

We show for the first time central nervous system penetration and inhibition of brain PARP activity by AG-014699. Taken together with our in vitro chemosensitisation and toxicity data, these findings support further evaluation of the clinical potential of AG-014699-temozolomide combinations in intra-cranial malignancies.

Prospective validation of renal function-based carboplatin dosing in children with cancer: A United Kingdom Children's Cancer Study Group Trial

PURPOSE

Carboplatin dosing in adults with cancer is based on renal function. The purpose of the current study was to validate a previously developed pediatric carboplatin-dosing formula.

PATIENTS AND METHODS

Thirty-eight pediatric patients were randomized to receive a carboplatin dose calculated according to surface area or a renal function-based dosing formula. On the next course of therapy, the alternative dosing method was used for each patient. Carboplatin pharmacokinetics (based on free plasma platinum concentrations) were measured after both courses.

RESULTS

The mean observed areas under the carboplatin concentration-versus-time curve (AUCs) after renal function- and surface area-based dosing were 98% and 95% of the target AUCs, respectively. The variation in the observed AUC was significantly less after renal function-based dosing (F test, P =.02), such that 74% of courses had an observed AUC within +/- 20% of the target value, versus 49% for courses after dosing according to surface area. Only one of 22 courses at the center with the most experience with renal function-based dosing was associated with an AUC outside +/- 20% of the target value, versus nine of 22 courses after surface area-based dosing in the same center. There was a relationship (r(2) =.71) between carboplatin AUC and thrombocytopenia in 10 neuroblastoma patients treated with a combination of carboplatin, vincristine, etoposide, and cyclophosphamide.

CONCLUSION

Renal function-based carboplatin dosing in children results in more consistent drug exposure than surface area-based drug administration.

The kinetics of the auto-induction of ifosfamide metabolism during continuous infusion

It has often been reported that the oxazaphosphorines ifosfamide and cyclophosphamide induce their own metabolism. This phenomenon was studied in 21 paediatric patients over 35 courses of therapy. All patients received 9 gm-2 of ifosfamide as a continuous infusion over 72 h. Plasma concentrations of parent drug and of the major metabolite in plasma, 3-dechloroethylifosfamide (3DC) were determined, using a quantitative thin-layer chromatography (TLC) technique. A one-compartment model was fitted simultaneously to both ifosfamide and 3DC data. The model included a time-dependent clearance term, increasing asymptotically from an initial value to a final induced clearance and characterised by a first-order rate constant. A time lag, before induction of clearance began, was determined empirically. Metabolite kinetics were characterised by an elimination rate constant for the metabolite and a composite parameter comprising a formation clearance, proportional to the time-dependent clearance of parent drug, divided by the volume of distribution of the metabolite. Thus, the parameters to estimate were the volume of distribution of parent drug (V), initial clearance (Cli), final clearance (Cls), the rate constant for changing clearance (Kc), the elimination rate constant for the metabolite (Km) and Vm/fm, the metabolite volume of distribution divided by the fractional clearance to 3DC. The model of drug and metabolite kinetics produced a good fit to the data in 22 of 31 courses. In a further 4 courses an auto-inductive model for parent drug alone could be used. In the remaining courses, auto-induction could be demonstrated, but there were insufficient data to fit the model. For some patients this was due to a long time lag (up to 54 h) relative to the infusion time. The time lag varied from 6 to 54 (median, 12)h and values for the other parameters were Cli, 3.27 +/- 2.52 lh-1 m-2, Cls, 7.50 +/- 3.03 lh-1 m-2, V, 22.0 +/- 11.0 1 m-2, Kc, 0.086 +/- 0.074 h-1; Km, 0.159 +/- 0.077 h-1 and Vm/fm, 104 +/- 82 1m-2. The values of Kc correspond to a half-life of change in clearance ranging from 2 to 157 h, although for the majority of the patients the half-life was less than 7 h and a new steady-state level was achieved during the 72 h infusion period. This model provides insight into the time course of enzyme induction during ifosfamide administration, which may continue for up to 10 days in some protocols.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacokinetics and metabolism of 13-cis-retinoic acid (isotretinoin) in children with high-risk neuroblastoma - a study of the United Kingdom Children's Cancer Study Group

The administration of 13-cis-retinoic acid (13-cisRA), following myeloablative therapy improves 3-year event-free survival rates in children with high-risk neuroblastoma. This study aimed to determine the degree of inter-patient pharmacokinetic variation and extent of metabolism in children treated with 13-cisRA. 13-cis-retinoic acid (80 mg m⁻² b.d.) was administered orally and plasma concentrations of parent drug and metabolites determined on days 1 and 14 of courses 2, 4 and 6 of treatment. Twenty-eight children were studied. The pharmacokinetics of 13-cisRA were best described by a modified one-compartment, zero-order absorption model combined with lag time. Mean population pharmacokinetic parameters included an apparent clearance of 15.9 l h⁻¹, apparent volume of distribution of 85 l and absorption lag time of 40 min with a large inter-individual variability associated with all parameters (coefficients of variation greater than 50%). Day 1 peak 13-cisRA levels and exposure (AUC) were correlated with method of administration (P<0.02), with 2.44- and 1.95-fold higher parameter values respectively, when 13-cisRA capsules were swallowed as opposed to being opened and the contents mixed with food before administration. Extensive accumulation of 4-oxo-13-cisRA occurred during each course of treatment with plasma concentrations (mean±s.d. 4.67±3.17 μM) higher than those of 13-cisRA (2.83±1.44 μM) in 16 out of 23 patients on day 14 of course 2. Extensive metabolism to 4-oxo-13-cisRA may influence pharmacological activity of 13-cisRA.

A phase I trial of AT9283 (a selective inhibitor of aurora kinases) in children and adolescents with solid tumors: a Cancer Research UK study

PURPOSE

A phase I trial of AT9283 (a multitargeted inhibitor of Aurora kinases A and B) was conducted in children and adolescents with solid tumors, to identify maximum-tolerated dose (MTD), safety, efficacy, pharmacokinetics, and pharmacodynamic (PD) activity.

EXPERIMENTAL DESIGN

AT9283 was administered as a 72-hour continuous intravenous infusion every 3 weeks. A rolling-six design, explored six dose levels (7, 9, 11.5, 14.5, 18.5, and 23 mg/m(2)/d). Pharmacokinetic and PD assessments, included inhibition of phospho-histone 3 (pHH3) in paired skin punch biopsies.

RESULTS

Thirty-three patients were evaluable for toxicity. There were six dose-limiting toxicities and the MTD was 18.5 mg/m(2)/d. Most common drug-related toxicities were hematologic (neutropenia, anemia, and thrombocytopenia in 36.4%, 18.2%, and 21.2% of patients), which were grade ≥3 in 30.3%, 6.1%, and 3% of patients. Nonhematologic toxicities included fatigue, infections, febrile neutropenia and ALT elevation. One patient with central nervous system-primitive neuroectodermal tumor (CNS-PNET) achieved a partial response after 16 cycles and 3 cases were stable for four or more cycles. Plasma concentrations were comparable with those in adults at the same dose level, clearance was similar although half-life was shorter (4.9 ± 1.5 hours, compared with 8.4 ± 3.7 hours in adults). Inhibition of Aurora kinase B was shown by reduction in pHH3 in 17 of 18 patients treated at ≥11.5 mg/m(2)/d.

CONCLUSION

AT9283 was well tolerated in children and adolescents with solid tumors with manageable hematologic toxicity. Target inhibition was demonstrated. Disease stabilization was documented in intracranial and extracranial pediatric solid tumors and a phase II dose determined.

Thiothymidine combined with UVA as a potential novel therapy for bladder cancer

BACKGROUND

Thiothymidine (S(4)TdR) can be incorporated into DNA and sensitise cells to DNA damage and cell death following exposure to UVA light. Studies were performed to determine if the combination of S(4)TdR and UVA could be an effective treatment for bladder cancer.

METHODS

Uptake and incorporation of S(4)TdR was determined in rat and human bladder tumour cell lines. Measures of DNA crosslinking and apoptosis were also performed. In vivo activity of the combination of S(4)TdR and UVA was investigated in an orthotopic model of bladder cancer in rats.

RESULTS

Thiothymidine (200 μM) replaced up to 0.63% of thymidine in rat and tumour bladder cancer cells. The combination of S(4)TdR (10-200 μM) and UVA (1-5 kJ m(-2)) caused apoptosis and cell death at doses that were not toxic alone. Addition of raltitrexed (Astra Zeneca, Alderley Edge, Cheshire, UK) increased the incorporation of S(4)TdR into DNA (up to 20-fold at IC(5)) and further sensitised cells to UVA. Cytotoxic effect was associated with crosslinking of DNA, at least partially to protein. Intravenous administration of S(4)TdR, in combination with UVA delivered directly to the bladder, resulted in an antitumour effect in three of five animals treated.

CONCLUSION

These data indicate that the combination of S(4)TdR and UVA has potential as a treatment for bladder cancer, and give some insight into the mechanism of action. Further work is necessary to optimise the delivery of the two components.