Effects of P-glycoprotein modulators on etoposide elimination and central nervous system distribution

In this study, P-glycoprotein modulator effects on pharmacokinetics and central nervous system distribution of the chemotherapeutic agent etoposide were evaluated. The multidrug resistance transporter P-glycoprotein is expressed in normal tissues, and its physiological function is thought to be an excretory and/or protective one. To examine this further, we evaluated etoposide under steady-state and bolus dose conditions. In microdialysis infusion studies, etoposide 15 mg/kg/hr was administered to 12 rats. Rats received sodium cyanide (1 or 100 mM), trifluoperazine (30 mM) or cyclosporine (4.14 mM) via microdialysis probe at 3.5 hr after etoposide infusion initiation. High-dose sodium cyanide (100 mM) increased the etoposide BBR,corr from 0.09 +/- 0.03 to 0.85 +/- 0.35. Similarly, trifluoperazine significantly increased the BBR,corr (0.05 +/- 0.02 vs. 1.30 +/- 0.43), whereas cyclosporine had no effect. In bolus studies, etoposide (10-12 mg/kg) was given alone or concomitant to cyclosporine (5 mg/kg) or tamoxifen (13.5 mg/kg). Control etoposide total systemic clearance (ml/min/kg) was 29.3 +/- 13.0 vs. 16.0 +/- 1.9 and 22.6 +/- 5.3 for cyclosporine and tamoxifen treatments, respectively. Etoposide nonrenal clearance (ml/min/kg) values for cyclosporine (12.0 +/- 1.6) and tamoxifen (18.1 +/- 3.6) treatments was also decreased from controls (23.5 +/- 10.5). Etoposide renal clearance (ml/min/kg) values (5.7 +/- 2.5) were not significantly different from cyclosporine (4.0 +/- 0.7) or tamoxifen (4.6 +/- 1.7) treatments, respectively. In this study, the ability of sodium cyanide and trifluoperazine to alter etoposide BBR,corr, demonstrated that etoposide distribution into brain is partly controlled by an active transport process. Similarly, the results indicate cyclosporine inhibits etoposide transport at the canalicular membrane and/or etoposide P-450 metabolism.

Phase I and pharmacokinetic study of preirradiation chemotherapy with BCNU, cisplatin, etoposide, and accelerated radiation therapy in patients with high-grade glioma

PURPOSE

We conducted a Phase I study of bischloroethylnitrosourea (BCNU), cisplatin, and oral etoposide administered prior to and during accelerated hyperfractionated radiation therapy in newly diagnosed high-grade glioma. Pharmacokinetic studies of oral etoposide were also done.

METHODS AND MATERIALS

Patients started chemotherapy after surgery but prior to definitive radiation therapy (160 cGy twice daily x 15 days; 4800 cGy total). Initial chemotherapy consisted of BCNU 40 mg/m2 days 1-3, cisplatin 30 mg/m2 days 1-3 and 29-31, and etoposide 50 mg orally days 1-14 and 29-42, repeated in 8 weeks concurrent with radiation therapy. BCNU 200 mg/m2 every 8 weeks x 4 cycles was given after radiation therapy.

RESULTS

Sixteen patients, 5 with grade 3 anaplastic astrocytoma and 11 with glioblastoma were studied. Grade 3-4 leukopenia (38%) and thrombocytopenia (31%) were dose-limiting. Other toxicities were anorexia (81%), nausea (94%), emesis (56%), alopecia (88%), and ototoxicity (38%). The maximum tolerated dose was BCNU 40 mg/m2 days 1-3, cisplatin 20 mg/m2 days 1-3 and 29-31, and oral etoposide 50 mg days 1-21 and 29-49 prior to radiation therapy and repeated in 8 weeks with the start of radiation therapy followed by BCNU 200 mg/m2 every 8 weeks for 4 cycles. Median time to progression and survival were 13 and 14 months respectively. Responses occurred in 2 of 9 (22%) patients with evaluable disease. In pharmacokinetic studies, all patients achieved plasma concentrations of >0.1 microg/ml etoposide (the in vitro radiosensitizing threshold), following a 50 mg oral dose. The mean +/- SD 2 hr and 6 hr plasma concentrations were 0.92 +/- 0.43 microg/ml and 0.36 +/- 0.12 microg/ml, respectively. Estimated duration of exposure to >0.1 microg/ml etoposide was 10-17 hr.

CONCLUSIONS

Preirradiation chemotherapy with BCNU, cisplatin, and oral etoposide with accelerated hyperfractionated radiation therapy in high-grade gliomas is feasible and merits further investigation. Sustained radiosensitizing concentrations can be achieved with low oral doses of etoposide.

Recycling and redox cycling of phenolic antioxidants

Effectiveness of phenolic antioxidants in protecting against oxidative stress depends on their reactivity towards reactive oxygen species and the reactivity of the antioxidant phenoxyl radicals towards critical biomolecules. Reduction of phenoxyl radicals by intracellular reductant (ascorbate, thiols) as well as by enzymes or intermediates of electron transport (e.g., in mitochondria and the endoplasmic reticulum) recycles phenolic antioxidants, thus enhancing antioxidant protection. Several cascades may be involved in physiologically relevant recycling of vitamin E from its phenoxyl radicals. The two major ones are dihydrolipoic acid-->(GSH)-->ascorbate, and enzymes of electron transport-->coenzyme Q. Importantly, phenoxyl radicals of vitamin E are not directly reduced by intracellular thiols. By contrast, a number of natural phenolic compounds that act as very effective scavengers of reactive oxygen species and organic radicals, may generate reactive secondary radicals of antioxidants. These secondary radicals react and modify critical intracellular targets (lipids, proteins, and DNA). As a result, the role of these phenolic compounds as biological antioxidants may be limited because of their ability to cause cyto- and genotoxic effects. Typical examples are some estrogens and phenolic drugs (e.g., the antitumor drug, etoposide) that can protect lipids but oxidize GSH and protein sulfhydryls. Moreover, phenoxyl radicals produced in the course of radical scavenging by some phenolic compounds (e.g., phenol) are capable of oxidizing both proteins and lipids. Hence, reactivity of phenoxyl radicals should be considered as a critical factor in the development of new antioxidant protectants.

A randomised, concentration-controlled, comparison of standard (5-day) vs. prolonged (15-day) infusions of etoposide phosphate in small-cell lung cancer

PURPOSE

This randomised trial was designed to investigate the activity and toxicity of continuous infusion etoposide phosphate (EP), targeting a plasma etoposide concentration of either 3 micrograms/ml for five days (5d) or 1 microgram/ml for 15 days (15d), in previously untreated SCLC patients with extensive disease.

PATIENTS AND METHODS

EP was used as a single agent. Plasma etoposide concentration was monitored on days 2 and 4 in patients receiving 5d EP and on days 2, 5, 8 and 11 in patients receiving 15d EP, with infusion modification to ensure target concentrations were achieved. Treatment was repeated every 21 days for up to six cycles, with a 25% reduction in target concentration in patients with toxicity.

RESULTS

The study has closed early after entry of 29 patients (14 with 5d EP, 15 with 15d EP). Objective responses were seen in seven of 12 (58%, confidence interval (CI): 27%-85%) evaluable patients after 5d EP, and two of 14 (14%, CI: 4%-42%) evaluable patients after 15d EP (P = 0.038). Grade 3 or 4 neutropenia or leucopenia during the first cycle of treatment was observed in six of 12 patients after 5d EP and 0/14 patients after 15d EP (P = 0.004), with median nadir WBC count of 2.6 x 10(9)/1 after 5d and 5.0 x 10(9)/1 after 15d EP (P = 0.017). Only one of 49 cycles of 15d EP was associated with grade 3 or worse haematological toxicity, compared to 14 of 61 cycles of 5d EP.

CONCLUSIONS

Although the number of patients entered into this trial was small, the low activity seen at 1 microgram/ml in the 15d arm suggests that this concentration is below the therapeutic window in this setting. Further concentration-controlled studies with prolonged EP infusions are required.

Indirect-response model for the time course of leukopenia with anticancer drugs

BACKGROUND

Because both the nadir count and the duration of leukopenia after chemotherapy with anticancer drugs are important, a pharmacodynamic model describing the entire time course of leukopenia is valuable. In this study, a pharmacodynamic model was developed to simulate leukopenia.

METHODS

The model was developed with the 3-hour infusion data of paclitaxel. A concentration-time curve of paclitaxel for each patient estimated by a 3-compartment pharmacokinetic model was used as input to the model, which had 2 compartments corresponding to leukocytes in bone marrow and peripheral blood, respectively. Differentiation stages of myeloid cells sensitive to anticancer drugs were assumed, and exposure to a drug during their sensitive period as a function of time was used to inhibit the production of leukocytes in bone marrow. The model was validated by fitting the data of 24-hour infusion of paclitaxel or 14-day infusion of etoposide.

RESULTS

Successful fitting of the leukopenia after a 3-hour infusion of paclitaxel was achieved. The following parameters were estimated: lag-time, 58 +/- 38 (mean +/- SD) hours before the leukocyte count started to decline; exposure giving 50% inhibition of leukocyte production (IC), 12.1 +/- 6.1 microg x h/mL; and sensitive period, 288 +/- 64 hours. These estimations were within physiologic ranges. In validation, leukopenia after 24-hour infusion of paclitaxel or 14-day infusion of etoposide was also explained by the model. Age was significantly negatively correlated with IC of paclitaxel (P = .039).

CONCLUSIONS

This mechanistic model describes the time course of leukopenia and may provide a platform for pharmacodynamic analysis of anticancer drugs.

Relation between unbound plasma concentrations and toxicity in a prolonged oral etoposide schedule

OBJECTIVE

This study was undertaken in order to evaluate the impact of pharmacokinetics on the toxicity of oral etoposide administered daily for 21 days.

METHODS

The daily dose was 50 mg/m2. Thirty-two patients 24 males and eight females, 36 76 years old, treated for various tumour types), were evaluated. Blood samples were obtained on day 1 for all patients, and on day 21 for 16 patients. Plasma etoposide concentrations were determined by high-performance liquid chromatography, and etoposide plasma protein binding by equilibrium dialysis.

RESULTS

On day 1, the mean value (with coefficient of variation for interindividual variability) for the unbound fraction (fu), area under the concentration versus time curve (AUC), and unbound AUC was 9.8% (59%), 34 mg x h/l (39%), and 3.5 mg x h/l (92%), respectively. The ratio between AUC on day 1 and day 21 ranged between 0.5 and 1.8 (mean 0.9, with CV 33%). The plasma trough unbound concentrations and the unbound AUCs both corresponding to the first administration were significantly higher in the 11 patients who had a severe neutropenia than in the 21 patients who had no or moderate toxicity. However, total etoposide concentrations did not differ between these two groups. A limited sampling strategy using the NONMEM program and a database of 89 patients previously studied was performed. The optimal sampling schedule (i.e. 1, 4, and 24 h after oral etoposide administration) allowed to obtain the AUC accurately on day 1.

CONCLUSION

Individual adjustment of oral etoposide based on unbound pharmacokinetics after the first administration appears relevant and feasible.

Intraocular concentrations of chemotherapeutic agents after systemic or local administration

OBJECTIVES

To investigate the concentrations of carboplatin and etoposide achieved in the aqueous and vitreous humors after intravenous infusion in nonhuman primates, and to investigate whether local administration of carboplatin might result in higher concentrations in the vitreous humor.

METHODS

Macaca fascicularis primates were treated with 1 of 3 regimens: (1) intravenous carboplatin (18.7 mg/kg), etoposide (5 mg/kg), and vincristine sulfate (0.05 mg/kg), (2) peribulbar carboplatin (10 mg/mL), or (3) episcleral balloon carboplatin (10 mg/mL). Concentrations of chemotherapeutic agents were measured in the plasma and in the aqueous and vitreous humors.

RESULTS

No measurable amount of etoposide was detected in the aqueous or vitreous humor after intravenous administration. Mean measured peak vitreous concentration of carboplatin after intravenous administration was 0.31 microg/mL, which was 1% of the peak plasma value. Mean measured peak vitreous concentrations of carboplatin after peribulbar or episcleral balloon administration were 2.38 microg/mL and 2.95 microg/mL, respectively, which represent 7.68- and 9.52-fold increases over the concentration achieved after intravenous administration. No serious toxic effect was observed in any animal.

CONCLUSIONS

Peribulbar and episcleral balloon administration of carboplatin seemed to be safe and resulted in higher vitreous concentrations than intravenous administration in this model. These results suggest that these alternate routes of delivery should be explored in children with vitreous seeding of retinoblastoma.

Unexpected hepatotoxicities in patients with non-Hodgkin's lymphoma treated with irinotecan (CPT-11) and etoposide

BACKGROUND

Irinotecan (CPT-11) is a topoisomerase I inhibitor that has been confirmed to be active against a broad spectrum of neoplasms including non-Hodgkin's lymphoma (NHL). Because the combination of topoisomerase I and II inhibitors seemed to be an attractive therapeutic strategy owing to their complementary functions, we conducted a combination phase I study of CPT-11 and etoposide, a topoisomerase II inhibitor, in relapsed or refractory non-Hodgkin's lymphoma (NHL).

METHODS

The starting doses of CPT-11 and etoposide were 30 mg/m2/day (days 1-3 and 8-10) and 40 mg/m2 (days 1-3), respectively.

RESULTS

All three patients who received the starting dose developed dose-limiting toxicities including one case of grade 4 neutropenia lasting for > 7 days, one of grade 3 serum transaminase elevation and one of grade 3 hyperbilirubinemia. All three patients presented hepatotoxicity > or = grade 2. The starting dose level was judged to be the maximum tolerated dose (MTD) and further dose escalation of this combination was halted. The patient who developed grade 3 hyperbilirubinemia presented a second peak of plasma SN-38, an active metabolite of CPT-11, on the concentration-time curve for day 3, suggesting the possibility of the enterohepatic circulation of SN-38 and of a drug-to-drug interaction. No durable objective response was observed in the three patients treated at the starting dose.

CONCLUSIONS

We conclude that etoposide is not recommended for combination with CPT-11 in NHL patients because of unexpected frequent hepatotoxicities.

Cancer chemotherapy in the elderly

As the geriatric population is growing, it is increasingly important to be familiar with chemotherapy for the elderly. Age-related changes in pharmacokinetics are documented for doxorubicin, etoposide, ifosfamide, daunorubicin, mitomycin, cisplatin and methotrexate. The hematological toxicity of most standard-dose chemotherapy is not affected by age in patients with normal organic functions and good performance status, although increased toxicity with aging is suggested in the use of actinomycin-D, etoposide, vinblastin, methotrexate, methyl-CCNU, doxorubicin and mitomycin, and in dose-intensive chemotherapy. Among non-hematological toxicities, only doxorubicin-induced cardiomyopathy and bleomycin-induced pulmonary toxicity are demonstrated to be accelerated in the elderly. There is no evidence that advanced age decreases the efficacy of chemotherapy for tumors, except for Hodgkin's disease and acute leukemia. These results suggest that advanced chronological age alone is not always associated with severe toxicity and poor prognosis, and that many elderly patients with cancer will benefit from chemotherapy. To answer questions regarding the optimal chemotherapy regimen, dose and intensity in this population, the influence of age should be analyzed in a multivariate approach in future studies.

Tumor cell resistance to topoisomerase II poisons: role for intracellular free calcium in the sensitization by inhibitors or calcium-calmodulin-dependent enzymes

Tumor cell resistance to inhibitors of topoisomerase II (topo II) is associated frequently with the overexpression of P-glycoprotein (PGP), and strategies to overcome resistance are focused on restoring defects in drug accumulation. Inhibitors of calcium-calmodulin-dependent enzymes sensitize resistant tumor cells to the topo II poison etoposide (VP-16) by enhancing DNA damage and an apoptotic response. In the present study, we have investigated the consequences of buffering intracellular calcium with 1,2-bis(o-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid tetra(acetoxy-methyl) ester (BAPTA-AM) on the sensitizing effects of the calmodulin-dependent protein kinase II inhibitor 1-[N,O-bis(1,5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-piperazine (KN-62) in etoposide-resistant human leukemia HL-60 (HL-60/ADR0.05) cells. In cells pretreated with 20 microM BAPTA-AM for 2 hr, extracellular ATP failed to trigger intracellular calcium transients, and no effects on the accumulation of VP-16 were apparent. Also, the effect of KN-62 in significantly (P=0.002 to 0.042) enhancing the accumulation of VP-16 in HL-60/ADR0.05 cells was unaffected due to pretreatment with BAPTA-AM. In contrast, pretreatment with BAPTA-AM reduced the DNA damage induced by VP-16, and significantly (P=0.038) reversed the enhancement by KN-62 of VP-16-stabilized topo II-mediated DNA cleavable complex formation. The pretreatment of HL-60/ADR0.05 cells with BAPTA-AM was also associated with the hypophosphorylation of topo IIalpha. Consistent with the ability of BAPTA-AM to circumvent the potentiation by KN-62 of VP-16-induced DNA damage, survival of cells treated with 40 microM VP-16 in the absence of KN-62 and 10 microM VP-16 in the presence of KN-62 was significantly (P=0.026 to 0.031) higher due to BAPTA-AM pretreatment. Results demonstrate that intracellular calcium transients could play a key role in the sensitization of etoposide-resistant tumor cells by inhibitors of calcium-calmodulin-dependent enzymes.

Characterization of the regional intestinal kinetics of drug efflux in rat and human intestine and in Caco-2 cells

PURPOSE

The aim of the present study was to investigate the transport kinetics of intestinal secretory processes in the jejunum, ileum and colon of rats and humans and in Caco-2 cells, in vitro.

METHODS

Etoposide, vinblastine sulphate and verapamil hydrochloride were chosen as model substrates since they have been reported to undergo efflux in various other tissues. The concentration dependence, inhibition, directionality, temperature dependence, proton/sodium dependence, and ATP dependence of efflux were studied using side-by-side diffusion chambers and brush border membrane vesicles (BBMVs). Intestinal tissue from rats and humans and Caco-2 cells (passage no. 26) were used. Directional steady state effective permeabilities were calculated from drug appearance in the apical (AP) or basolateral (BL) chambers. Kinetic studies were carried out by investigating substrate efflux at concentrations ranging from 0.2 microns to 1000 microns. Since substrate efflux may be a result of more than one transporter, the hybrid efflux Km (Michaelis-constant), Pc (carrier-mediated permeability), and Pm (passive permeability) were determined as a function of intestinal region. Inhibitor studies were performed using quinidine (0.2mM), a mixed inhibitor of P-glycoprotein (Pgp) and Multidrug Resistance-Associated Protein (MRP), and Leukotriene C4 (100 nM), an inhibitor of MRP and the canalicular multispecific organic anion transporter (cMOAT). Temperature dependent efflux was determined by investigating the BL to AP transport at temperatures ranging from 3 degrees C to 37 degrees C. Energies of activation (Ea) were determined from an Arrhenius analysis. Sodium, proton, and ATP dependence were determined using BBMVs. Immunoquantitation of Pgp, MRP and Lung Resistance Protein (LRP) in Caco-2 cells were carried out using Western blot analysis.

RESULTS

Active efflux of all substrates was observed in all regions of rat and human intestine and in Caco-2 cells. Directionality was observed with BL to AP transport exceeding AP to BL transport. The BL to AP/AP to BL permeability ratio, the efflux ratio, ranged from 1.4 to 19.8. Ileal efflux was significantly higher (p < 0.001) than in other regions. Kinetic studies revealed that hybrid efflux Km values ranged from 4 to 350 microns. In some cases, efflux was not saturable due to the solubility limits of the compounds utilized in this study. In presence of inhibitors, efflux ratios approached 1. BL to AP transport was temperature dependent in rat ileum for all substrates. Each of the intestinal efflux was found to be 11.6, 8.3, and 15.8 kcal/mole for etoposide, vinblastine and verapamil, respectively, suggesting an active, energy-dependent efflux mechanism. Substrate efflux was not sodium or proton dependent but was dependent on ATP. Using Western blot analysis the presence of Pgp, MRP, and LRP was demonstrated in Caco-2 cells and the amount of each transport protein varied as a function of passage number.

CONCLUSIONS

Using multiple putative efflux substrates, the current results demonstrate that intestinal efflux was regionally dependent, mediated by multiple efflux transporters, the Km's were in the micro-molar range, and involved an energy dependent mechanism(s).

[Pharmacokinetic aspects of oral administration of etoposide]

BACKGROUND

Etoposide is a cytotoxic agent which is frequently employed in paediatric oncology and which is available for intravenous as well as oral application. Many advantages of the formulation for oral use have been opposed by concerns about its interindividually varying bioavailability. The influence of the dosage of etoposide on its activity and toxicity ("schedule dependency") has also been discussed. The present paper deals with the pharmacokinetics of oral etoposide focusing on the interindividual variability.

PATIENTS

Sixteen patients aged between 3 and 73 years received oral etoposide at a dosage of 28 mg/m2 to 149 mg/m2 in combination with oral trofosfamide for palliation.

METHOD

HPLC was used to measure total and free serum etoposide in 16 patients, and the etoposide concentration in several urine samples from 8 patients. Pharmacokinetic parameters were normalized to a dosage of 100 mg/m2.

RESULTS

The peak serum concentration, the time to peak concentration, the area under the concentration-time curve, the terminal half-life and the apparent clearance after oral application were calculated to be 6.7 micrograms/ml, 2.1 h, 51.8 (microgram.h/ml)/(100 mg/m2), 5.6 h, and 40.3 ml/min for total etoposide, and 0.23 microgram/ml, 1.9 h, 1.76 (microgram.h/ml)/ (100 mg/m2), 5.9 h, and 1172 ml/min for free serum etoposide, respectively. On an average, urinary recovery of etoposide was 21% of the oral dose. The fraction of free etoposide was calculated to be close to 4%. Regarding the systemic exposure to etoposide, a variation coefficient of 40% was determined. Additional studies showed that the interindividual variability mainly concerned the peak levels, while the duration for which intermediate etoposide levels were maintained varied less between individuals. On simulating different dosage schedules, it was seen that the duration of intermediate concentrations (0.5-2 micrograms/ml) may be extended significantly by dividing the daily dose of etoposide into two oral applications.

CONCLUSION

The total systemic exposure under oral etoposide treatment varies considerably between individuals. Extended intervals of intermediate etoposide concentration and less variation are, however, possible with oral therapy. Dividing the daily dose into two applications seems advisable. Future studies are warranted to test hypotheses on pharmacokinetic-pharmacodynamic aspects by pharmacological drug monitoring.

Therapeutic drug monitoring of 21-day oral etoposide in patients with advanced non-small cell lung cancer

The purpose of this study was to prospectively test a pharmacodynamic model for therapeutic drug monitoring of 21-day oral etoposide. In our previous studies, etoposide trough concentrations on this schedule were related to the hematological toxicity, expressed as WBC and neutrophil counts at the nadir. The following pharmacodynamic model estimated the absolute neutrophil count at the nadir (ANCn) based on the etoposide concentration (Ec) and the pretreatment count (ANCp): ANCn=0.32(1 + ANCp x e(-2.47 x Ec)). Patients were treated with 40 mg/m2/day etoposide p.o. x 21 days and 100 mg/m2 cisplatin i.v. on day 1. All patients had non-small cell lung cancer stage IIIB or IV, had a performance status of 0-2, and had a median age of 66 (range, 42-80). Etoposide was measured in the plasma on day 8 by high-performance liquid chromatography, and dosage adjustments were made for the remainder of the course. We targeted for grade 3 neutropenia (ANCn, 500 to 999/microl) and attempted to avoid grade 4 neutropenia (ANCn, <500/microl). Of 25 patients entered, 22 were evaluable for therapeutic drug monitoring in the first course. Three patients developed grade 3 neutropenia, and seven patients developed grade 4 neutropenia. Etoposide concentrations were significantly correlated with ANCn in the first course (r=-0.50, P < 0.02). For those patients whose dosages were not changed, the estimated correlation between predicted and actual ANCn was 0.77 (P < 0.01). No evidence of significant bias of the pharmacodynamic model was detected. The etoposide dosages were increased in 12 patients and were not changed in the remaining patients. The precision of the model was good in patients whose dosages were not changed but poor in patients whose dosages were increased. The actual observed ANCn was compared with the predicted ANCn based on the pharmacodynamic model. The prediction was considered accurate if the predicted and actual ANCn values were within 500/microl of each other. Using this margin, the ANCn was accurately predicted in 10 of 22 patients. Etoposide concentrations >0.3 microg/ml on this schedule were significantly correlated with combined grades 3 and 4 neutropenia (P < 0.0001). In conclusion, the pharmacodynamic model is statistically sound when applied to a population of patients. However, when applied to individual patients for therapeutic drug monitoring, the model lacks precision and accuracy.

[Cytotoxic mechanism and antineoplastic action of etoposide]

Etoposide differs from its parent podophyllotoxin--classical spindle poison; unlike this compound etoposide delays progression of the cell cycle through the late S or early G2 phase. Etoposide inhibits uptake and incorporation of nucleosides into tumour cells but at the concentrations higher than those which are cytotoxic. Single- and double-strand breaks caused by etoposide in cellular DNA are generally considered as the initial event which leads to cell death; the DNA strand breaks result from stabilization of the cleavable complex of topoisomerase II DNA with DNA by etoposide. On the other hand metabolites of etoposide are also likely to exert cytotoxic action. Finally, etoposide has been shown to induce apoptosis, active, energy-dependent process of cellular self-destruction, in human tumour cells.

A study of the feasibility and accuracy of pharmacokinetically guided etoposide dosing in children

Pharmacokinetically guided dosing was performed in nine paediatric patients receiving etoposide. Doses on day 2 of a 2- or 3-day schedule were adapted on the basis of the day-1 area under the plasma etoposide concentration vs time curve (AUC). The day-1 AUC was estimated using a limited sampling model and the day-2 target AUC defined by the etoposide dose-AUC relationship observed in 33 children. Target AUC values (4.6-8.2 mg ml(-1) x min) were achieved with a high degree of precision and with little bias (mean error 11% and root mean squared error 15% respectively). Pharmacokinetic parameters were similar to those reported previously in children, although interpatient pharmacokinetic variability was less than that observed previously: plasma clearance, 23 (18-26) ml min(-1) m(-2); volume of distribution at steady state (Vdss), 6.0 (3.9-8.9) l m(-2); t(1/2) 254 (127-550) min (median and range). This study has demonstrated that pharmacokinetically guided dosing with etoposide is feasible. However, pharmacokinetically guided dosing is likely to be of most benefit in patients with abnormalities of renal or hepatic function, or in children with prior exposure to cisplatin.

High-dose cyclosporin with etoposide--toxicity and pharmacokinetic interaction in children with solid tumours

The tolerability, anti-tumour activity and pharmacokinetic interaction of high-dose intravenous cyclosporin combined with intravenous etoposide was evaluated in children. Eighteen patients with recurrent or refractory tumours, all of whom had previously received etoposide, were treated with a combination of high-dose cyclosporin and etoposide. In 13, cyclosporin was given as a continuous infusion (15 mg kg(-1) per 24 h for 60 h) and in five a short 3-hour infusion of 30 mg kg(-1) day(-1) on three consecutive days. Pharmacokinetic profiles of etoposide were determined with and without cyclosporin. Cyclosporin levels ranged from 1359 to 4835 ng ml(-1) and cyclosporin increased the median area under the concentration time for etoposide curve from 7.2 to 12.5 mg ml(-1) min. The major toxicity was acute with varying forms of hypersensitivity reactions. In four cases this was severe. Hyperbilirubinaemia was present in 25 of 32 courses but was of short duration. In 14 courses, creatinine and/or urea was elevated, but was also transient. Significant hypertension was seen in six courses. Four of 17 patients evaluable for response obtained a partial response and one showed stable disease. It is concluded that in children given the combination of high-dose cyclosporin and etoposide, the etoposide dose should be halved in order to achieve an area under the drug concentration-time curve similar to that with etoposide alone. A continuous infusion schedule of cyclosporin is better tolerated during the period of administration but is associated with similar hepatic and renal dysfunction to a short schedule. The 24% response rate in children who had previously received etoposide suggests that this may be an effective method of enhancing drug sensitivity and further phase II evaluation is justified.

Etoposide phosphate: new preparation. Makes outpatient therapy easier

The clinical file on etoposide phosphate provides no new efficacy data. Pharmacokinetic studies have shown that non esterified etoposide can be replaced by etoposide phosphate; the latter is rapidly metabolised and yields the same amount of bioavailable etoposide. The only available comparative trial showed no difference in adverse effects between etoposide phosphate and non esterified etoposide. The small size of this trial rules out conclusions on infrequent adverse effects such as those possibly due to the solvents in non esterified etoposide. The main value of etoposide phosphate is its simplicity of use, which should make it more suitable for treatment at home: contrary to non esterified etoposide it can be administered as a short infusion, is compatible with all infusion equipment, and appears to be stable in various types of aqueous solution.

Etoposide and antimetabolite pharmacology in patients who develop secondary acute myeloid leukemia

Etoposide, an effective agent for acute lymphoblastic leukemia (ALL), can cause secondary acute myeloid leukemia (AML) in a subset of patients. Our objectives were to determine whether patients who develop secondary AML displayed altered etoposide pharmacokinetics or other pharmacologic characteristics compared to identically treated patients who did not develop AML. Children with newly diagnosed ALL were treated according to a protocol which included etoposide 300 mg/m2 given three times over 8 days during remission induction and once every 2-4 weeks during 120 weeks of continuation therapy. Characteristic 11q23 rearrangements were documented in seven of the eight patients with AML. Etoposide clearance, time that etoposide concentrations exceeded 10 microM, etoposide or etoposide catechol area-under-the-plasma-concentration vs time curve (AUC), serum albumin, and average methotrexate concentration did not differ significantly between the two groups; thiopurine methyltransferase (TPMT) activity tended to be lower in the eight children who did vs the 23 matched control children who did not develop AML (P=0.16). Further regression analyses likewise indicated that lower TPMT activity tended to be associated with shorter onset of secondary AML (P=0.11); it also tended to be lower among the eight index cases compared to the entire unmatched cohort of 105 identically treated children with ALL (P=0.10). We observed no statistically significant differences in etoposide disposition and antimetabolite pharmacology between patients who did and did not develop secondary AML.

Population pharmacokinetics of total and unbound etoposide

A population pharmacokinetics study using the NONMEM program was undertaken to determine the effects of different covariates on the pharmacokinetic parameters of etoposide. A total of 1,044 plasma etoposide concentrations were determined by high-performance liquid chromatography (HPLC) in 100 patients (pts; 75 men and 25 women aged 25-85 years) treated for various tumor types with i.v. (57 pts) or oral (43 pts) etoposide. For 67 pts, etoposide plasma protein binding was determined by equilibrium dialysis; the unbound fraction ranged from 4% to 24%. A linear two-compartment model with first-order absorption (for oral dosing) accurately described the concentration versus time data. The central and peripheral volumes of distribution were significantly correlated with the body surface area [Vc (L) = 5.5 x BSA (m2) and Vp = 4.1 x BSA], but even after BSA had been taken into account, the interindividual variability of the two volumes remained high (34% and 57%, respectively). The clearance (CL) was not correlated with the following covariates: age, BSA, sex, height, and levels of serum bilirubin and liver enzymes. The final regression model for CL was CL (ml/min) = 49.8 x (1 - 0.009 x PRO) x WT/Scr + 33.8 x (1 - 0.29 x META) x (1 - 0.012 x ALB), where ALB, PRO, WT, and Scr, respectively, were albuminemia, proteinemia (g/l), weight (kg), and serum creatinine (microM) and META = 1 if the patient had liver metastases (otherwise, META = 0). The interindividual variability in CL (mean value 30 ml/min) decreased only from 32% to 26% when these covariates were taken into account. The mean oral bioavailability was 66%, showing an interindividual variability of 37%. The plasma clearance of the unbound fraction was strongly and negatively correlated with Scr but was not dependent on either PRO or ALB. These data show that modifications in PRO levels do not directly affect plasma exposure to unbound etoposide. This analysis makes possible the rational consideration of modifications of covariates such as Scr in etoposide dosing. This population data base will constitute the prerequisite for adaptative control with feedback dosing for continuous oral administration of etoposide.

Evidence that the multidrug resistance protein (MRP) functions as a co-transporter of glutathione and natural product toxins

The MRP (multidrug resistance protein) gene, a member of the ubiquitous superfamily of ATP-binding cassette transporters, is associated with the multidrug resistance of mammalian cells to natural product anticancer agents. We have previously shown that abrogation of MRP expression by gene targeting leads to hypersensitivity to several drugs. In two independently produced MRP double knockout clones, the baseline export of glutathione (GSH) was one-half that of wild-type embryonic stem (ES) cells. The export of GSH from wild-type ES cells, but not from the MRP double knockout clones, increased in the presence of etoposide (VP-16) and sodium arsenite, accompanied by equivalent decreases in intracellular levels of GSH. In the two MRP double knockout clones, the intracellular steady-state concentration of etoposide was twofold greater than that in wild-type cells. Depletion of intracellular GSH by D,L-buthionine sulfoximine increased the intracellular accumulation of radiolabeled etoposide in parental ES cells up to the level present in the two MRP knockout clones but did not change etoposide levels in the MRP knockout clones. These observations provide evidence that: (a) MRP exports GSH physiologically, presumably in association with an endogenous compound(s); (b) baseline MRP expression protects cells from the toxic effects of xenobiotics by effluxing the xenobiotics and GSH from the intracellular compartment into the extracellular medium by a co-transport mechanism; and (c) disruption of the gene encoding MRP abrogates the cotransport of xenobiotics and GSH.

Enhancement and modification of etoposide release from crospovidone particles loaded with oil-surfactant blends

A novel solid formulation for oral delivery of pH-sensitive, scarcely water-soluble etoposide has been designed, characterized, and tested in vitro. The purpose of this study was to assess the performance of the new dosage forms, in comparison to marketed, liquid-filled capsules. The solid formulation was developed by grinding the drug with a cross-linked polymeric carrier (crospovidone) under controlled process conditions (mechano-physical drug activation), and subsequently incorporating selected oil/surfactant (o/s) blends into the polymer particles. Physicochemical characterization (thermal analysis, drug dissolution kinetics, drug o/w partition studies) provided information on drug-polymer interaction at the solid state, and on the formulation performance in vitro, resulting in the enhancement and modification of the etoposide solubilization process. DSC thermograms showed the amorphous or nanocrystalline state of etoposide within the carrier, as indicated by the shifting of DSC peaks (delta T > -10 degrees C). Solubility kinetics of etoposide in oversaturation conditions were strongly affected by the chemical nature of the vehicle used: short-chain triglycerides afforded drug concentrations well above 600 micrograms ml-1 for more than 3 hr, versus a drug equilibrium solubility of approximately 150 micrograms ml-1. Drug dissolution curves under sink conditions were superimposable to those of liquid-filled capsules available on the market (Vepesid 50, Bristol-Myers Squibb), yielding 100% drug release in 10 min. The oil phase/water partition coefficient of etoposide (P) was affected by the surfactant concentration. The biphasic trend observed in P values suggested a dual mechanism in drug release from polymeric particles: the presence of oily vehicles and surfactants in the formulation could create, upon release, a favorable environment to sustain etoposide dissolution, slowing down drug reprecipitation. Such solid formulation could be considered equivalent, in vitro, to the current marketed product.

Increased sensitivity to anticancer drugs and decreased inflammatory response in mice lacking the multidrug resistance-associated protein

The multidrug resistance-associated protein (MRP) mediates the cellular excretion of many drugs, glutathione S-conjugates (GS-X) of lipophilic xenobiotics and endogenous cysteinyl leukotrienes. Increased MRP levels in tumor cells can cause multidrug resistance (MDR) by decreasing the intracellular drug concentration. The physiological role or roles of MRP remain ill-defined, however. We have generated MRP-deficient mice by using embryonic stem cell technology. Mice homozygous for the mrp mutant allele, mrp-/-, are viable and fertile, but their response to an inflammatory stimulus is impaired. We attribute this defect to a decreased secretion of leukotriene C4 (LTC4) from leukotriene-synthesizing cells. Moreover, the mrp-/- mice are hypersensitive to the anticancer drug etoposide. The phenotype of mrp-/- mice is consistent with a role for MRP as the main LTC4-exporter in leukotriene-synthesizing cells, and as an important drug exporter in drug-sensitive cells. Our results suggest that this ubiquitous GS-X pump is dispensable in mice, making treatment of MDR with MRP-specific reversal agents potentially feasible.

Phase II study of 21 day schedule oral etoposide in children. New Agents Group of the United Kingdom Children's Cancer Study Group (UKCCSG)

We report a multicentre phase II study of orally administered prolonged schedule etoposide in children with refractory or relapsed malignancy. 83 children were entered into the study. The largest diagnostic groups were neuroblastoma (n = 20), rhabdomyosarcoma/soft tissue sarcoma (n = 16) and brain tumours (n = 16). Etoposide was administered twice daily at a dose of 50 mg/m2/day for 21 days using the intravenous preparation given orally. Disease reassessment was performed after the second course. Etoposide plasma concentrations were measured by HPLC, 2 and 6 h after administration of therapy on days 7 and 14 in 15 patients. 61 patients completed two courses and were evaluable for response. There was 1 complete response (CR), 5 partial responses (PR) 22 stable disease (SD) and 33 progressive disease (PD). Of the 6 with responses, 3 had a diagnosis of medulloblastoma/cerebral primitive neuroectodermal tumour. 24 of 26 patients with SD/PR/CR received further courses with excellent palliative effect. The main toxicity observed was myelosuppression, with 8% and 7% of evaluable courses complicated by grade III-IV neutropenia and thrombocytopenia, respectively. Severe infection (grade III-IV) was rare, complicating only 2/94 evaluable courses. Plasma etoposide median concentrations at 2 h after administration on day 7 of course 1 were 1.5 (range 0.6-2.4) micrograms/ml. Total course 1 area under the etoposide plasma concentration versus time curve (AUC) values were estimated using a limited sampling model. Grade > or = 2 leucopenia was only observed in patients with a day 72 h etoposide concentration of > 2 micrograms/ml or a course 1 AUC of > 35 mg/ml.min. It is concluded that given at a dose of 50 mg/m2/day in two doses for 21 day courses, oral etoposide is well tolerated in children. A correlation between drug concentrations and toxicity was observed. Overall, a low response rate was seen (approximately 10%), but disease stabilisation appears to occur, and useful palliative effect was frequently noted. The response in brain tumours was more encouraging (3/14 PR) and this group requires further evaluation.

P-glycoprotein-independent decrease in drug accumulation by phorbol ester treatment of tumor cells

The effect of a change in the phosphorylation state of the drug transporter P-glycoprotein (P-gp) on its drug transport activity was studied for the substrates daunorubicin (DNR), etoposide (VP-16), and calcein acetoxymethyl ester (Cal-AM). Phorbol ester (PMA), added to stimulate phosphorylation of P-gp by protein kinase C (PKC), caused a decrease in the cellular accumulation of DNR and VP-16, both in multidrug-resistant (MDR) P-gp-overexpressing cells and in wild-type cells. Since treatment of cells with kinase inhibitor staurosporine (ST) reversed this effect of PMA and the non-PKC-stimulating phorbol ester 4alpha-phorbol, 12,13-didecanoate (4alphaPDD) did not result in a decreased DNR accumulation, we conclude that this effect is the result of kinase activity. The concentration dependence of the inhibition of P-gp by verapamil (Vp) was not influenced by PMA. Accumulation of the P-gp substrate Cal-AM was not influenced by PMA in wild-type cells. Therefore, Cal-AM was used to study the effect of PMA-induced phosphorylation of P-gp on its transport activity. Activation of PKC with PMA or inhibition of protein phosphatase 1/2A (PP1/PP2A) with okadaic acid (OA) did not affect the accumulation of Cal-AM in the MDR cells or wild-type cells. The kinase inhibitor ST increased the Cal-AM accumulation only in the MDR cells. Neither stimulating PKC with PMA nor inhibiting PP1/PP2A with OA led to a decreased inhibition of P-gp by ST, indicating that ST inhibits P-gp directly. From these experiments, we conclude that PKC and PP1/PP2A activity do not regulate the drug transport activity of P-gp. However, these studies provide evidence that PMA-induced PKC activity decreases cellular drug accumulation in a P-gp-independent manner.