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    Figure 1—

    Plasma etoposide concentration-versus-time data for 8 tumor-bearing dogs administered a single dose of etoposide (50 mg/m2) IV during a 60-minute period (A) and administered a single dose of the parenteral formulation of etoposide (50 mg/m2) orally (B). The initial route of administration was randomly assigned, and the second dose was administered via the alternate route 3 to 7 days after the first dose. Each symbol represents results for 1 dog. Plasma etoposide concentrations after oral administration are reported for only 6 dogs because they were less than the limit of detection in 2 dogs. Time 0 was the onset of the infusion and time of administration of the etoposide for IV and oral administration, respectively.

  • 1.

    Montecucco A, Biamonti G. Cellular response to etoposide treatment. Cancer Lett 2007;252:918.

  • 2.

    Hande KR. Etoposide: four decades of development of a topoisomerase II inhibitor. Eur J Cancer 1998;34:15141521.

  • 3.

    Ross W, Rowe T, Glisson B, et al. Role of topoisomerase II in mediating epipodophyllotoxin-induced DNA cleavage. Cancer Res 1984;44:58575860.

    • Search Google Scholar
    • Export Citation
  • 4.

    Long BH, Musial ST, Brattain MG. Single- and double-strand DNA breakage and repair in human lung adenocarcinoma cells exposed to etoposide and teniposide. Cancer Res 1985;45:31063112.

    • Search Google Scholar
    • Export Citation
  • 5.

    Slevin ML, Clark PI, Joel SP, et al. A randomized trial to evaluate the effect of schedule on the activity of etoposide in small-cell lung cancer. J Clin Oncol 1989;7:13331340.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6.

    McLeod HL, Relling MV. Stability of etoposide solution for oral use. Am J Hosp Pharm 1992;49:27842785.

  • 7.

    Mathew P, Ribeiro RC, Sonnichsen D, et al. Phase I study of oral etoposide in children with refractory solid tumors. J Clin Oncol 1994;12:14521457.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8.

    Edick MJ, Gajjar A, Mahmoud HH, et al. Pharmacokinetics and pharmacodynamics of oral etoposide in children with relapsed or refractory acute lymphoblastic leukemia. J Clin Oncol 2003;21:13401346.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9.

    Hohenhaus AE, Matus RE. Etoposide (VP-16). Retrospective analysis of treatment in 13 dogs with lymphoma. J Vet Intern Med 1990;4:239241.

    • Search Google Scholar
    • Export Citation
  • 10.

    Ogilvie GK, Cockburn CA, Tranquilli WJ, et al. Hypotension and cutaneous reactions associated with intravenous administration of etoposide in the dog (Erratum published in Am J Vet Res 1988;49:1765). Am J Vet Res 1988;49:13671370.

    • Search Google Scholar
    • Export Citation
  • 11.

    Masini E, Planchenault J, Pezziardi F, et al. Histamine-releasing properties of polysorbate 80 in vitro and in vivo: correlation with its hypotensive action in the dog. Agents Actions 1985;16:470477.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12.

    Gough WB, Zeiler RH, Barreca P, et al. Hypotensive action of commercial intravenous amiodarone and polysorbate 80 in dogs. J Cardiovasc Pharmacol 1982;4:375380.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13.

    McEntee M, Silverman JA, Rassnick K, et al. Enhanced bioavailability of oral docetaxel by co-administration of cyclosporin A in dogs and rats. Vet Comp Oncol 2003;1:105112.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14.

    Pommier Y, Goldwater F. Topoisomerase inhibitors: the epipodophyllotoxins, acridines, and ellipticines. In: Chabner BA, Longo DL, eds. Cancer chemotherapy and biotherapy: principles and practice. 4th ed. Philadelphia: Lippincott Williams & Wilkins, 2005;451475.

    • Search Google Scholar
    • Export Citation
  • 15.

    Veterinary co-operative oncology group—common terminology criteria for adverse events (VCOG-CTCAE) following chemotherapy or biological antineoplastic therapy in dogs and cats v1.0. Vet Comp Oncol 2004;2:195213.

    • Search Google Scholar
    • Export Citation
  • 16.

    Robieux I, Aita P, Sorio R, et al. Determination of unbound etoposide concentration in ultrafiltered plasma by high-performance liquid chromatography with fluorimetric detection. J Chromatogr B Biomed Appl 1996;686:3541.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 17.

    Kato Y, Mawatari H, Nishimura S, et al. Determination of etoposide serum concentrations in small pediatric samples by an improved method of reversed-phase high-performance liquid chromatography. Acta Med Okayama 2003;57:2124.

    • Search Google Scholar
    • Export Citation
  • 18.

    Stewart DJ, Nundy D, Maroun JA, et al. Bioavailability, pharmacokinetics, and clinical effects of an oral preparation of etoposide. Cancer Treat Rep 1985;69:269273.

    • Search Google Scholar
    • Export Citation
  • 19.

    Cunningham D, McTaggart L, Soukop M, et al. Etoposide: a pharmacokinetic profile including an assessment of bioavailability. Med Oncol Tumor Pharmacother 1986;3:9599.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20.

    Slevin ML, Joel SP, Whomsley R, et al. The effect of dose on the bioavailability of oral etoposide: confirmation of a clinically relevant observation. Cancer Chemother Pharmacol 1989;24:329331.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 21.

    Hainsworth JD, Johnson DH, Frazier SR, et al. Chronic daily administration of oral etoposide—a phase I trial. J Clin Oncol 1989;7:396401.

  • 22.

    Hainsworth JD, Williams SD, Einhorn LH, et al. Successful treatment of resistant germinal neoplasms with VP-16 and cisplatin: results of a Southeastern Cancer Study Group trial. J Clin Oncol 1985;3:666671.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 23.

    Hande KR, Krozely MG, Greco FA, et al. Bioavailability of low-dose oral etoposide. J Clin Oncol 1993;11:374377.

  • 24.

    Igwemezie LN, Kaul S, Barbhaiya RH. Assessment of toxicokinetics and toxicodynamics following intravenous administration of etoposide phosphate in beagle dogs. Pharm Res 1995;12:117123.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 25.

    Eisenberg EJ, Eickhoff WM. Determination of etoposide in blood by liquid chromatography with electrochemical detection. J Chromatogr 1993;621:110114.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 26.

    Lana S, U'ren L, Plaza S, et al. Continuous low-dose oral chemotherapy for adjuvant therapy of splenic hemangiosarcoma in dogs. J Vet Intern Med 2007;21:764769.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 27.

    Wolff SN, Grosh WW, Prater K, et al. In vitro pharmacodynamic evaluation of VP-16–213 and implications for chemotherapy. Cancer Chemother Pharmacol 1987;19:246249.

    • Search Google Scholar
    • Export Citation
  • 28.

    Drewinko B, Barlogie B. Survival and cycle-progression delay of human lymphoma cells in vitro exposed to VP-16–213. Cancer Treat Rep 1976;60:12951306.

    • Search Google Scholar
    • Export Citation
  • 29.

    Heck MM, Hittelman WN, Earnshaw WC. Differential expression of DNA topoisomerases I and II during the eukaryotic cell cycle. Proc Natl Acad Sci U S A 1988;85:10861090.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 30.

    Fox ME, Smith PJ. Long-term inhibition of DNA synthesis and the persistence of trapped topoisomerase II complexes in determining the toxicity of the antitumor DNA intercalators mAMSA and mitoxantrone. Cancer Res 1990;50:58135818.

    • Search Google Scholar
    • Export Citation
  • 31.

    Allen JD, Brinkhuis RF, Wijnholds J, et al. The mouse Bcrp1/Mxr/Abcp gene: amplification and overexpression in cell lines selected for resistance to topotecan, mitoxantrone, or doxorubicin. Cancer Res 1999;59:42374241.

    • Search Google Scholar
    • Export Citation
  • 32.

    Leu BL, Huang JD. Inhibition of intestinal P-glycoprotein and effects on etoposide absorption. Cancer Chemother Pharmacol 1995;35:432436.

  • 33.

    Keller RP, Altermatt HJ, Donatsch P, et al. Pharmacologic interactions between the resistance-modifying cyclosporine SDZ PSC 833 and etoposide (VP 16–213) enhance in vivo cytostatic activity and toxicity. Int J Cancer 1992;51:433438.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 34.

    Allen JD, Van Dort SC, Buitelaar M, et al. Mouse breast cancer resistance protein (Bcrp1/Abcg2) mediates etoposide resistance and transport, but etoposide oral availability is limited primarily by P-glycoprotein. Cancer Res 2003;63:13391344.

    • Search Google Scholar
    • Export Citation
  • 35.

    Yahanda AM, Alder KM, Fisher GA, et al. Phase I trial of etoposide with cyclosporine as a modulator of multidrug resistance. J Clin Oncol 1992;10:16241634.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 36.

    Lum BL, Kaubisch S, Yahanda AM, et al. Alteration of etoposide pharmacokinetics and pharmacodynamics by cyclosporine in a phase I trial to modulate multidrug resistance. J Clin Oncol 1992;10:16351642.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 37.

    Shah JC, Chen JR, Chow D. Preformulation study of etoposide: identification of physicochemical characteristics responsible for the low and erratic oral bioavailability of etoposide. Pharm Res 1989;6:408412.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 38.

    Boothe DM. Veterinary compounding in small animals: a clinical pharmacologist's perspective. Vet Clin North Am Small Anim Pract 2006;36:11291173.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 39.

    Reif S, Nicolson MC, Bisset D, et al. Effect of grapefruit juice intake on etoposide bioavailability. Eur J Clin Pharmacol 2002;58:491494.

  • 40.

    Clark PI, Slevin ML, Joel SP, et al. A randomized trial of two etoposide schedules in small-cell lung cancer: the influence of pharmacokinetics on efficacy and toxicity. J Clin Oncol 1994;12:14271435.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 41.

    Toutain PL, Bousquet-Melou A. Bioavailability and its assessment. J Vet Pharmacol Ther 2004;27:455466.

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Oral bioavailability of etoposide after administration of a single dose to tumor-bearing dogs

Andrea B. FloryDepartment of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Kenneth M. RassnickDepartment of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Cheryl E. BalkmanDepartment of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Michael A. KiselowDepartment of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Karoliina AutioDepartment of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

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Bernard B. BeaulieuSection of Clinical Pharmacology, Department of Medicine, Dartmouth Medical School and Dartmouth-Hitchcock Medical Center, Dartmouth University, Lebanon, NH 03756.

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Lionel D. LewisSection of Clinical Pharmacology, Department of Medicine, Dartmouth Medical School and Dartmouth-Hitchcock Medical Center, Dartmouth University, Lebanon, NH 03756.

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Abstract

Objective—To characterize oral bioavailability and pharmacokinetic disposition of etoposide when the IV formulation was administered orally to dogs.

Animals—8 tumor-bearing dogs.

Procedures—An open-label, single-dose, 2-way crossover study was conducted. Dogs were randomly assigned to initially receive a single dose of etoposide (50 mg/m2) IV or PO. A second dose was administered via the alternate route 3 to 7 days later. Medications were administered before IV administration of etoposide to prevent hypersensitivity reactions. Oral administration of etoposide was prepared by reconstituting the parenteral formulation with 0.9% NaCl solution and further diluting the reconstituted mixture 1:1 with a sweetening agent. Plasma samples were obtained after both treatments. Etoposide concentrations were measured with a high-performance liquid chromatography assay, and plasma etoposide concentration–time profiles were analyzed by use of noncompartmental methods.

Results—4 dogs had hypersensitivity reactions during IV administration of etoposide. No adverse effects were detected after oral administration. Plasma etoposide concentrations were undetectable in 2 dogs after oral administration. Oral administration of etoposide resulted in significantly lower values for the maximum plasma concentration and the area under the plasma etoposide concentration-versus-time curve, compared with results for IV administration. Oral bioavailability of etoposide was low (median, 13.4%) and highly variable among dogs (range, 5.7% to 57.3%).

Conclusions and Clinical Relevance—Vehicle-related toxicosis can limit the IV administration of etoposide in dogs. The parenteral formulation of etoposide can be safely administered orally to dogs, but routine use was not supported because of low and variable oral bioavailability in this study.

Abstract

Objective—To characterize oral bioavailability and pharmacokinetic disposition of etoposide when the IV formulation was administered orally to dogs.

Animals—8 tumor-bearing dogs.

Procedures—An open-label, single-dose, 2-way crossover study was conducted. Dogs were randomly assigned to initially receive a single dose of etoposide (50 mg/m2) IV or PO. A second dose was administered via the alternate route 3 to 7 days later. Medications were administered before IV administration of etoposide to prevent hypersensitivity reactions. Oral administration of etoposide was prepared by reconstituting the parenteral formulation with 0.9% NaCl solution and further diluting the reconstituted mixture 1:1 with a sweetening agent. Plasma samples were obtained after both treatments. Etoposide concentrations were measured with a high-performance liquid chromatography assay, and plasma etoposide concentration–time profiles were analyzed by use of noncompartmental methods.

Results—4 dogs had hypersensitivity reactions during IV administration of etoposide. No adverse effects were detected after oral administration. Plasma etoposide concentrations were undetectable in 2 dogs after oral administration. Oral administration of etoposide resulted in significantly lower values for the maximum plasma concentration and the area under the plasma etoposide concentration-versus-time curve, compared with results for IV administration. Oral bioavailability of etoposide was low (median, 13.4%) and highly variable among dogs (range, 5.7% to 57.3%).

Conclusions and Clinical Relevance—Vehicle-related toxicosis can limit the IV administration of etoposide in dogs. The parenteral formulation of etoposide can be safely administered orally to dogs, but routine use was not supported because of low and variable oral bioavailability in this study.

Etoposide (VP-16) is a semisynthetic derivative of podophyllotoxin and is widely used in the treatment of adults and children with various solid and hematopoietic cancers.1 Etoposide is 1 component of several standard regimens used to treat people with germ-cell tumors and small-cell lung cancer. Etoposide also has important activity against Hodgkin's and non-Hodgkin's lymphomas and acute leukemia.2 The anticancer activity of etoposide appears to be primarily mediated by inhibition of topoisomerase II, which is an enzyme that introduces temporary double-stranded DNA breaks to regulate overwinding and underwinding of the double helix in DNA replication and resolves nucleic acid knots and tangles.1,2 The interaction of etoposide with topoisomerase II is reversible after cessation of etoposide,3,4 and antitumor effects may be related to exposure to relatively low etoposide concentrations for protracted periods instead of exposure to high concentrations for short periods.5 This has provided the rationale for prolonged administration of etoposide in many clinical treatment regimens.

Etoposide is commercially available in 2 FDA-approved formulations (a parenteral formulation for IV administration and capsules for oral administration). Capsules can be used for continuous, low-dose administration without the need for IV infusions and hospitalization. However, the oral formulation of etoposide is limited to 50-, 100-, and 500-mg gelatin capsules that cannot be divided, which precludes accurate dosing in many instances. The injectable formulation of etoposide can be diluted 1:2 with saline (0.9% NaCl) solution and will remain stable for at least 3 weeks,6 and diluted mixtures of the parenteral formulation of etoposide can be given orally to pediatric oncology patients.7,8 In a phase I study7 of children with refractory solid tumors, diluted injectable etoposide was suspended in juice or flavored syrup and then given orally 3 times daily for 21 consecutive days. Pharmacokinetic data revealed a dose-dependent increase in the duration of time that plasma etoposide concentrations remained > 1 μg/mL, which is a concentration that is purportedly associated with antitumor effects.7 Diarrhea was a dose-limiting effect, and myelosuppression was also detected.7

The IV administration of etoposide to dogs causes cutaneous reactions and hypotension characteristic of an acute hypersensitivity reaction.9,10 Polysorbate-80 is the solvent in the etoposide formulation used for parenteral administration; severe acute hypersensitivity reactions caused by histamine release have been reported in dogs treated with polysorbate-80 alone11 or with drugs containing this solubilizing vehicle (eg, docetaxel).12,13 Limited data exist on the use and pharmacokinetics of etoposide when administered orally to dogs. Because commercially available capsules for oral administration would be difficult to use to achieve accurate doses in dogs of various body weights, the purpose of the study reported here was to characterize the bioavailability and pharmacokinetic disposition of etoposide when the same doses of the parenteral formulation were administered IV and orally to tumor-bearing dogs.

Materials and Methods

Animals—Eight tumor-bearing client-owned dogs were used in an open-label, single-dose, randomized crossover study performed at the Cornell University College of Veterinary Medicine. Dogs were considered eligible to receive etoposide when they had a recurrent or metastatic naturally developing neoplasm not amenable to surgical resection or refractory to conventional treatments; an expected survival time of at least 4 weeks; a history of no chemotherapy, immunotherapy, or radiotherapy for at least 2 weeks; and adequate bone marrow (absolute neutrophil count ≥ 3,000 cells/ML and ≥ 100,000 platelets/ML), renal (serum creatinine concentration b 1.3 mg/dL), hepatic (serum bilirubin concentration b 0.3 mg/dL and serum alanine transaminase and aspartate transaminase activities b 2 times the upper limit of the respective reference ranges), and cardiac function. Written informed consent was obtained from all clients. The study protocol and consent form were approved by the Institutional Animal Care and Use Committee at Cornell University.

Experimental design—Etoposide was administered IV or orally in accordance with a randomized crossover design such that each dog received the drug by both routes of administration in a random order. There was a washout period of 3 to 7 days between administrations, which was based on the elimination half-life of etoposide in humans.14 The etoposide dose of 50 mg/m2 was based on reports2,9,10 that dogs tolerate etoposide administered IV at doses of 25 to 100 mg/m2 and on the clinical experience of one of the investigators (KMR) that dogs tolerate etoposide when administered orally as capsules at a daily dose of 50 mg/m2 for 21 days. Food and water were withheld for 12 hours before and 12 hours after drug administration. Clinical observations were recorded for at least 24 hours after administration, and toxic effects were graded in accordance with criteria reported in another study.15

Oral administration of etoposide—A commercially available parenteral formulation of etoposidea was used for oral administration. Each vial contained 20 mg of etoposide/mL and was diluted 1:2 to a final concentration of 6.67 mg/mL by the addition of saline solution. This concentration was selected to mimic oral administration of the parenteral formulation of etoposide to children.7 The dose for each dog (50 mg/m2) was then mixed 1:1 with a commercially available flavoring agentb and administered via an oral dosing syringe. After oral administration of etoposide, each dog received 10 mL of drinking water administered with a syringe. Time of capsule administration was designated as time 0.

IV administration of etoposide—The commercially available parenteral formulation of etoposidea was used for IV administration. The prescribed dose (50 mg/m2) was diluted with saline solution and then immediately administered as an IV infusion during a period of 60 minutes. Onset of the 60-minute infusion was designated as time 0. The volume of saline solution used for dilution was based on body weight (100 mL of saline solution for dogs weighing 10 to 35 kg and 250 mL of saline solution for dogs weighing > 35 kg). Injectable etoposide is complexed with polysorbate-80, and dogs can have hypersensitivity reactions to this vehicle.11 Therefore, beginning 24 hours before IV administration of etoposide, dogs were medicated with hydrocortisonec (4.4 mg/kg, PO, q 12 h), diphenhydramined (2.2 mg/kg, PO, q 8 h), and famotidinee (0.5 mg/kg, PO, q 12 h). Twenty minutes before IV administration of etoposide, dogs were given hydrocortisone sodium succinatef (4.4 mg/kg, IV), diphenhydramineg (4 mg/kg, IM), and famotidineh (0.5 mg/kg, IV). Etoposide and other medications were administered IV via an indwelling catheter inserted into a cephalic vein.

Collection of pharmacokinetic blood samples—An indwelling catheter was inserted in a jugular vein of each dog, and blood samples (2 mL) were collected into lithium-heparin tubes after oral and IV administration of etoposide. Samples were immediately centrifuged at 1,500 × g for 10 minutes; plasma was harvested and stored at −70°C until analyzed. For oral administration of etoposide, samples were obtained before administration (time 0) and 0.5, 1, 2, 4, 6, 8, and 24 hours after administration. For IV administration of etoposide, samples were obtained before administration (time 0) and 60, 65, 75, and 90 minutes and 2, 3, 4, 5, 6, 8, and 24 hours after the start of the IV infusion.

Measurement of etoposide concentrations—The HPLC assay used to determine etoposide concentrations in canine plasma in the study reported here was a combined modification of 2 HPLC assays described elsewhere.16,17 Podophyllotoxini (10 ML of a 1 mg/mL solution in methanol) was added as an internal standard along with 300 ML of chloroform to each 200-ML plasma sample in a 1.5-mL polypropylene microcentrifuge vial. Contents were mixed, and vials were then centrifugedj for 1 minute at 13,000 × g to separate the liquid phases. An aliquot of 150 ML was removed from the lower chloroform phase and transferred to a 12 × 75-mm borosilicate test tube. Tubes were evaporated to dryness under a nitrogen stream at 40°C. The residue was then resuspended in 200 ML of mobile phase and transferred to a 200-ML conical glass insert. To settle any remaining particulates, the glass insert was centrifuged inside a 500-ML microcentrifuge vial at 13,000 × g for 1 minute and then removed to an amber glass sample vial in an autosampler. The HPLC system consisted of dual-model pumps,k an autosampler with a 300-ML sample loop,l and a UV detector.m Data acquisition, handling, and reporting were performed via HPLC software.n Analyte separation was achieved by use of a reversed–phase octadecysilyl columno (4.6 × 150 mm) protected with a guard columnp (3.2 × 15 mm). The mobile phase (50% methanol and 50% acetonitrile-to-water mixture [840:60]) was pumped at a flow rate of 1 mL/min at ambient temperature. Retention time for etoposide and podophyllotoxin was 4.1 and 7.9 minutes, respectively. Analyte peak areas were corrected for variations in sample volume by means of the internal standard and were compared with a known standard regression curve to calculate concentrations.

The assay was conducted in accordance with the principles of good laboratory practice, which included standard curves at the beginning and end of the unknown samples and quality-assurance samples evaluated over the range of the standard curves provided at the beginning, mid point, and end of the unknown samples. The lower limit of quantification of the etoposide HPLC assay was 0.025 μg/mL. The etoposide standard curve was linear over the range of 25 to 10,000 ng/mL. Intraday assay variability ranged from 2.1% to 14.7%, and interday assay variability ranged from 6.6% to 12.1%. Mean accuracy at 0.25, 0.5, and 1 μg/mL was 116.6% (coefficient of variation, 12.0%), 114.6% (coefficient of variation, 12.1%), and 110.2% (coefficient of variation, 6.6%), respectively.

Pharmacokinetic analysis—Plasma etoposide concentrations after oral and IV administration were inspected on a semilogarithmic plot of concentration versus time for each dog. The etoposide Cmax and Tmax were the values from the raw plasma etoposide concentration–time data. Pharmacokinetic parameters were estimated by use of a computer software program,q and etoposide concentration-versus-time data were analyzed by use of an open noncompartmental model for both the oral and IV datasets. When adequate datum points yielded measurable etoposide concentrations, ke was determined with a linear regression of the terminal 3 to 5 datum points of the logarithmic plasma etoposide concentration–time plot by use of a weighting paradigm of 1/Y, where Y is the observed concentration value. The t1/2 was estimated as 0.693/ke. The AUC0–t was estimated by use of the linearlog trapezoidal rule.

Etoposide bioavailability was estimated on the basis of AUC by use of the equation (AUC0–t PO/AUC0–t IV) × 100, where AUC0–t PO is the AUC0–t for orally administered etoposide and AUC0–t IV is the AUC0–t for etoposide administered IV. Bioavailability was also estimated on the basis of Cmax by use of the equation (Cmax PO/Cmax IV) × 100, where Cmax PO is the Cmax for orally administered etoposide and Cmax IV is the Cmax for etoposide administered IV.

Statistical analysis—Median and range values were computed for plasma etoposide concentrations and for pharmacokinetic parameters. Values for Tmax, Cmax, t1/2, and AUC were compared for each route of administration by use of Wilcoxon signed rank tests. The relationship between t1/2 for the oral and IV routes was examined by use of linear regression analysis. Analyses were performed with a computer software program,r and 2-sided values of P b 0.05 were considered significant.

Results

Animals—The 8 client-owned dogs with tumors enrolled in the study ranged from 3 to 9 years of age (median, 7 years) and weighed 26 to 43 kg (median, 30 kg). Three dogs were mixed-breed dogs, and 5 were purebred dogs (1 Golden Retriever, 1 Chesapeake Bay Retriever, 1 Bernese Mountain Dog, 1 Doberman Pinscher, and 1 Shetland Sheepdog). Five dogs had lymphoma, 1 dog had acute myeloid leukemia, 1 dog had metastatic osteosarcoma, and 1 dog had disseminated histiocytic sarcoma. All dogs had adequate values for hematopoietic and serum biochemical variables at the time of etoposide administration.

Oral administration of etoposide—The median dose of etoposide administered orally to the 8 dogs was 48.5 mg (range, 42.5 to 61.5 mg). All dogs readily swallowed the orally administered dose of etoposide. No adverse effects were detected for any of the dogs at the time the drug was given or during the 24-hour postadministration observation period.

IV administration of etoposide—The median dose of etoposide administered IV to the 8 dogs was 48 mg (range, 44.0 to 61.6 mg). During IV etoposide administration, 4 dogs had signs consistent with hypersensitivity reactions. Two dogs had intense, widespread pruritis with severe erythema (grade 3 hypersensitivity), and 2 dogs had mild pruritis with head shaking (grade 1 hypersensitivity). Duration of the etoposide infusion was extended to 2 hours in the dogs with grade 3 hypersensitivity reactions. None of the dogs had the administration of etoposide discontinued because of an infusionrelated hypersensitivity reaction.

Pharmacokinetics—Plasma etoposide concentrations were less than the limit of detection (0.025 μg/mL) of the HPLC assay at all time points for samples obtained from 2 dogs (a mixed-breed dog and the Bernese Mountain Dog) after oral administration of etoposide. Plasma concentrations over time after oral and IV administration of etoposide were plotted (Figure 1). Standard pharmacokinetic variables after oral and IV administration of etoposide were tabulated (Table 1). There was no significant (P = 0.16) difference in Tmax after oral and IV administration of etoposide, which was not unexpected because the IV dose was infused over a period of at least 1 hour. The etoposide Cmax was > 1 μg/mL in 7 of 8 dogs after IV administration and < 1 μg/mL in all dogs after oral administration. Oral administration of etoposide resulted in significantly lower values for Cmax (P = 0.031) and AUC0–t (P = 0.031), compared with values after IV administration. Median bioavailability after oral administration of etoposide, as determined on the basis of AUC, was 13.4% (range, 5.7% to 57.3%). Oral bioavailability was > 30% in only 2 dogs (the Golden Retriever and the Shetland Sheepdog). Because the etoposide concentrations were less than the limit of detection of the assay after oral administration in 2 dogs, bioavailability was not estimated in those dogs. There was no significant (P = 0.25) difference in etoposide t1/2 after oral or IV administration. Predictably, there was a strong relationship (R2 = 0.9995; P = 0.014; n = 3) between the t1/2 for oral and IV administration of etoposide in dogs.

Figure 1—
Figure 1—

Plasma etoposide concentration-versus-time data for 8 tumor-bearing dogs administered a single dose of etoposide (50 mg/m2) IV during a 60-minute period (A) and administered a single dose of the parenteral formulation of etoposide (50 mg/m2) orally (B). The initial route of administration was randomly assigned, and the second dose was administered via the alternate route 3 to 7 days after the first dose. Each symbol represents results for 1 dog. Plasma etoposide concentrations after oral administration are reported for only 6 dogs because they were less than the limit of detection in 2 dogs. Time 0 was the onset of the infusion and time of administration of the etoposide for IV and oral administration, respectively.

Citation: American Journal of Veterinary Research 69, 10; 10.2460/ajvr.69.10.1316

Table 1—

Median (range) values for pharmacokinetic parameters determined after oral and IV administration of a single dose (50 mg/m2) of the parenteral formulation of etoposide to 8 tumorbearing dogs.

Pharmacokinetic parameterOral(n = 6)*IV (n = 8)
Tmax(min)60(30–60)65(60–90)
Cmax(μ/mL)0.22(0.14–0.71)1.53(0.72–9.68)
t1/2(min)91 (72–197)74 (46–284)
AUC0–1t([min* μig]/mL)24.0 (5.6–89.8)105.9(77.3–374.4)
Bioavailability
 AUC method13.4(5.7–57.3)NA
Cmax method16.3(2.9–34.5)NA

Pharmacokinetic parameters are for the 6 dogs in which plasma etoposide concentrations were measurable; for 2 dogs, the plasma concentrations of etoposide at all time points after oral administration were less than the limit of quantification of the assay (0.025 μg/mL).

Adequate data points were available for only 3 dogs to estimate t1/2 after oral administration.

The equation for the AUC method is (ALJC0-tp0/AUC0-tIV)X 100, where AUC0-tp0 is the AUC0-t for orally administered etoposide and AUC0-tIv is the AUC0-tfor etoposide administered IV. The equation for the Cmax method is (CmaxPQ/CmaxIV) X100, where Cmaxpo is the Cmax for orally administered etoposide and Cmaxiv is the Cmax for etoposide administered IV.

NA= Not applicable.

Discussion

The results of the study reported here on the pharmacokinetics of etoposide after oral and IV administration indicated that the bioavailability of etoposide in dogs was low (median bioavailability was 13.4%) and that interdog variability was high (range, 5.7% to 57.3%). In addition, 2 dogs had undetectable plasma etoposide concentrations after oral administration, so bioavailability could not be quantitated in these dogs. Neither technical problems nor emesis were evident when the formulation was administered orally or during collection or analysis of the blood samples for determining pharmacokinetics in these dogs, but oral administration of etoposide was not repeated in these dogs to eliminate these possibilities. Numerous studies18–20 have been conducted to evaluate the bioavailability of oral administration of etoposide capsules in adult humans. Bioavailability in patients is considered to be approximately 50%,21 but mean values ranging between 17%22 and 76%18 have been reported. Oral absorption of etoposide capsules in humans is a dose-dependent event, with a greater percentage of drug being absorbed at lower absolute doses.23

To our knowledge, information about the pharmacokinetic profile after IV administration of etoposide in dogs has not been published. However, Cmax, t1/2, and AUC values after IV administration of etoposide (50 mg/m2) obtained in this study are in agreement with the mean ± SD values of 5.46 ± 1.96 μg/mL, 1.43 ± 0.54 hours, and 2.28 ± 0.54 (h · μg)/mL, respectively, reported24 after IV administration of etoposide phosphate (57 mg/m2) to dogs. Etoposide phosphate is a water-soluble prodrug of etoposide and is rapidly and extensively converted to etoposide.24 Hypersensitivity reactions were common (4/8 dogs) during IV infusion of etoposide to the dogs in the study reported here, despite use of premedications to lessen adverse reactions. Hypersensitivity reactions during IV infusion of etoposide are the result of the solubilizing agent (ie, polysorbate) in which the parenteral formulation of etoposide is prepared.11 This finding and the findings in other reports9,10 confirm that IV administration of etoposide to dogs is neither practical nor safe.

All the dogs in our study readily accepted and tolerated the orally administered parenteral formulation of etoposide, which was delivered in a pleasant-tasting oral preparation, and no adverse effects were detected after administration. Pharmacokinetic data of etoposide after oral administration to dogs is limited. After oral administration of 2 mg of etoposide/kg, etoposide blood concentrations were < 0.3 μg/mL in 1 healthy dog; however, the etoposide formulation was not reported.25 In another study26 of 3 dogs with naturally developing hemangiosarcoma, serum etoposide concentrations of < 150 ng/mL were detected after oral administration of a single dose of a parenteral etoposide formulation at a dose of 50 mg/m2, which is similar to the dose used in the study reported here.

Several preclinical findings suggest that the duration of exposure of neoplastic cells to etoposide is important for maximal antitumor activity.27,28 Etoposide's target (ie, DNA topoisomerase II) is substantially expressed only in dividing cells during selected mitotic phases of the cell cycle.29 Therefore, chronic dosing may be advantageous because it maximizes the likelihood of exposing malignant cells to etoposide during sensitive periods of the cell cycle. Cytotoxic effects of topoisomerase II–targeting drugs relate to the magnitude of formation of drug-induced, enzyme-mediated DNA strand breaks and to the intracellular persistence of these lesions.30 Therefore, antineoplastic agents or protracted dosing schemes that prolong the presence of DNA strand breaks in cells would be expected to result in superior efficacy.

Although the importance of the dosing schedule for drug administration on the antineoplastic activity of etoposide was suggested in early preclinical and clinical trials, the most informative study5 on the importance of schedule dependence included patients with small-cell lung cancer who received 500 mg of etoposide/m2 as a 24-hour IV infusion or as a daily 2-hour infusion for 5 days. Although both groups received the same total dose of drug, differences in response rates were dramatic. For the 24-hour IV infusion, 10% of patients responded to treatment, compared with a response rate of 89% for the 5-day treatment group. Pharmacokinetic data from that study5 revealed prolonged maintenance of serum etoposide concentrations (r 1 μg/mL) was associated with superior efficacy in the 5-day treatment group. Clinical studies on the efficacy of etoposide in dogs with cancer are limited. Investigators in 1 study9 reported on the use of etoposide in dogs with lymphoma. Dogs received 100 mg of etoposide/m2 as a 30-minute IV infusion or as a daily IV bolus at a fractionated dosage of 25 mg/m2 each day for 4 days. Interestingly, 1 of the 3 dogs given the fractionated dose of etoposide had complete remission after treatment, whereas none of the 10 dogs given the drug as a 1-day treatment responded.9 These data suggest that a protracted duration of exposure to relatively low etoposide concentrations, instead of higher doses given over shorter periods, may also be important in dogs, similar to results in humans.

Currently, the FDA-approved soft-gelatin capsule formulation of etoposide provides a convenient means for protracted oral administration. However, in pediatric oncology, commercially available capsule sizes (50-, 100-, and 500-mg capsules) compromise the accurate delivery of smaller doses required for children. Thus, the parenteral formulation of etoposide is routinely given orally to circumvent this limitation.7,8 The bioavailability of etoposide when the parenteral formulation is given orally to people has not been reported, but pharmacokinetic data reveal that with the doses administered, most achieve plasma concentrations > 1 μg/mL and antitumor responses are common.8 Dose-dependent absorption of etoposide is not apparent when the parenteral formulation of etoposide is administered orally.8

Oral bioavailability of etoposide is primarily limited by Pgp expression on the luminal surface of intestinal epithelial cells.31 The Pgp is a product of the MDR1 gene, and it confers chemotherapy resistance when overexpressed on cancer cells.14 The functional orientation of Pgp throughout the intestinal tract results in the secretion of etoposide back into the intestinal lumen.32–34 Administration of cyclosporin, a modulator of Pgp, increases the plasma concentration of orally administered etoposide by at least 10-fold in rats.33 Coadministration of etoposide and cyclosporine in humans also results in dramatic improvement in oral uptake.35,36 Cyclosporine substantially improves the disposition of orally administered docetaxel (another Pgp substrate) in dogs,13 and it may be worthwhile to investigate such an approach in dogs treated by oral administration of etoposide.

A commercially available flavoring agentb was mixed with the parenteral formulation of etoposide. We selected this product because it is commonly used in our clinic as a vehicle for oral administration of drugs and as a flavoring agent. Etoposide is chemically unstable in an acidic pH (pH < 2)37; simple syrups typically are acidic and can alter drug pH.38 The flavoring agent is acidic (pH, 3.01),s but bioavailability may still have been improved in the study reported here if etoposide had not been mixed with any vehicle. When children are orally administered injectable etoposide, it is typically suspended in juice (eg, apple juice, orange juice, or lemonade) or a flavored syrup.7 Finally, it is possible that the flavoring agent altered intestinal Pgp-mediated etoposide transport, as has been reported39 with coadministration of grapefruit juice. However, because the commercially available flavoring agent consisted of only sucrose, glycerin, sorbitol, and water, this alteration in Pgp-mediated transport was unlikely.

The duration of serum etoposide concentrations > 1 μg/mL is associated with superior efficacy in humans.7,8,40 Once absorbed, there is no pharmacologic difference between oral and IV administration of etoposide with respect to mechanism of action.2 The study reported here was not an efficacy trial, and serum concentrations needed in dogs with cancer are not known. Nonetheless, concentrations > 1 μg/mL were not achieved in any dog orally administered etoposide in other studies25,26 or in our study. The dosage of etoposide (50 mg/m2) used in the study reported here was based on reports9,10 that dogs tolerate IV administration of etoposide at dosages of 25 to 100 mg/m2 and on the clinical experience of one of the authors (KMR) that dogs tolerate etoposide administered as capsules at a dosage of 50 mg/m2/d for 21 days. Controlled studies to determine the maximum dosage of etoposide that is tolerated in dogs are not available. Increasing the dosage of orally administered etoposide in dogs may allow better systemic exposure; however, the wide interpatient variability in Cmax and AUC detected in our study may contribute to overexposure in some dogs.41

The parenteral formulation of etoposide administered orally to dogs was tolerated well and provided easy and accurate dosing. However, pharmacokinetic results of the study reported here revealed that the bioavailability was low and interpatient variability was wide. Methods to improve bioavailability and systemic exposure of this formulation of etoposide in dogs are warranted before routine use can be advocated. This could include oral administration of etoposide with an agent that inhibits intestinal Pgp or use of higher doses of etoposide.

Abbreviations

AUC

Area under the plasma etoposide concentration-versus-time curve

AUC0–t

Area under the plasma etoposide concentration-versus-time curve from time 0 to the last datum point

Cmax

Maximum plasma concentration

HPLC

High-performance liquid chromatography

ke

Terminal elimination rate constant

Pgp

P-glycoprotein

t1/2

Terminal elimination half-life

Tmax

Time at which maximum plasma concentration was achieved

a.

Vepesid, Ben Venue Labs, Bedford, Ohio.

b.

Ora Sweet, Paddock Labs, Minneapolis, Minn.

c.

Cortef, Pharmacia & Upjohn, Kalamazoo, Mich.

d.

Benadryl capsules, Major Pharmaceuticals, Livionia, Mich.

e.

Pepcid tablets, Ivax, Miami, Fla.

f.

Solu-cortef, Pharmacia & Upjohn, Kalamazoo, Mich.

g.

Benedryl injection, Baxter Healthcare Corp, Deerfield, Ill.

h.

Pepcid injection, Ivax, Miami, Fla.

i.

Podophyllotoxin, Sigma-Aldrich Co, St Louis, Mo.

j.

Fisher micro-centrifuge, model 235B, Fisher Scientific, Waltham, Ma.

k.

Dual-model 210 pumps, Varian, Palo Alto, Calif.

l.

Model 410 autosampler, Varian, Palo Alto, Calif.

m.

Pro Star model 345 UV/Vis detector, Varian, Palo Alto, Calif.

n.

Varian LC workstation software, version 6.3, Varian, Palo Alto, Calif.

o.

Ultrasphere ODS column, Beckman Coulter Inc, Fullerton, Calif.

p.

NewGuard RP-18 column, Applied Biosystems, Foster City, Calif.

q.

WinNonlin Pro pharmacokinetic program, version 4.01, Pharsight Corp, Mountain View, Calif.

r.

SPSS, version 10, SPSS Inc, Chicago, Ill.

s.

Lynch M. Cornell University Hospital for Animals, College of Veterinary Medicine, Cornell University, Ithaca, NY: Personal communication, 2007.

References

  • 1.

    Montecucco A, Biamonti G. Cellular response to etoposide treatment. Cancer Lett 2007;252:918.

  • 2.

    Hande KR. Etoposide: four decades of development of a topoisomerase II inhibitor. Eur J Cancer 1998;34:15141521.

  • 3.

    Ross W, Rowe T, Glisson B, et al. Role of topoisomerase II in mediating epipodophyllotoxin-induced DNA cleavage. Cancer Res 1984;44:58575860.

    • Search Google Scholar
    • Export Citation
  • 4.

    Long BH, Musial ST, Brattain MG. Single- and double-strand DNA breakage and repair in human lung adenocarcinoma cells exposed to etoposide and teniposide. Cancer Res 1985;45:31063112.

    • Search Google Scholar
    • Export Citation
  • 5.

    Slevin ML, Clark PI, Joel SP, et al. A randomized trial to evaluate the effect of schedule on the activity of etoposide in small-cell lung cancer. J Clin Oncol 1989;7:13331340.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6.

    McLeod HL, Relling MV. Stability of etoposide solution for oral use. Am J Hosp Pharm 1992;49:27842785.

  • 7.

    Mathew P, Ribeiro RC, Sonnichsen D, et al. Phase I study of oral etoposide in children with refractory solid tumors. J Clin Oncol 1994;12:14521457.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8.

    Edick MJ, Gajjar A, Mahmoud HH, et al. Pharmacokinetics and pharmacodynamics of oral etoposide in children with relapsed or refractory acute lymphoblastic leukemia. J Clin Oncol 2003;21:13401346.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9.

    Hohenhaus AE, Matus RE. Etoposide (VP-16). Retrospective analysis of treatment in 13 dogs with lymphoma. J Vet Intern Med 1990;4:239241.

    • Search Google Scholar
    • Export Citation
  • 10.

    Ogilvie GK, Cockburn CA, Tranquilli WJ, et al. Hypotension and cutaneous reactions associated with intravenous administration of etoposide in the dog (Erratum published in Am J Vet Res 1988;49:1765). Am J Vet Res 1988;49:13671370.

    • Search Google Scholar
    • Export Citation
  • 11.

    Masini E, Planchenault J, Pezziardi F, et al. Histamine-releasing properties of polysorbate 80 in vitro and in vivo: correlation with its hypotensive action in the dog. Agents Actions 1985;16:470477.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12.

    Gough WB, Zeiler RH, Barreca P, et al. Hypotensive action of commercial intravenous amiodarone and polysorbate 80 in dogs. J Cardiovasc Pharmacol 1982;4:375380.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13.

    McEntee M, Silverman JA, Rassnick K, et al. Enhanced bioavailability of oral docetaxel by co-administration of cyclosporin A in dogs and rats. Vet Comp Oncol 2003;1:105112.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14.

    Pommier Y, Goldwater F. Topoisomerase inhibitors: the epipodophyllotoxins, acridines, and ellipticines. In: Chabner BA, Longo DL, eds. Cancer chemotherapy and biotherapy: principles and practice. 4th ed. Philadelphia: Lippincott Williams & Wilkins, 2005;451475.

    • Search Google Scholar
    • Export Citation
  • 15.

    Veterinary co-operative oncology group—common terminology criteria for adverse events (VCOG-CTCAE) following chemotherapy or biological antineoplastic therapy in dogs and cats v1.0. Vet Comp Oncol 2004;2:195213.

    • Search Google Scholar
    • Export Citation
  • 16.

    Robieux I, Aita P, Sorio R, et al. Determination of unbound etoposide concentration in ultrafiltered plasma by high-performance liquid chromatography with fluorimetric detection. J Chromatogr B Biomed Appl 1996;686:3541.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 17.

    Kato Y, Mawatari H, Nishimura S, et al. Determination of etoposide serum concentrations in small pediatric samples by an improved method of reversed-phase high-performance liquid chromatography. Acta Med Okayama 2003;57:2124.

    • Search Google Scholar
    • Export Citation
  • 18.

    Stewart DJ, Nundy D, Maroun JA, et al. Bioavailability, pharmacokinetics, and clinical effects of an oral preparation of etoposide. Cancer Treat Rep 1985;69:269273.

    • Search Google Scholar
    • Export Citation
  • 19.

    Cunningham D, McTaggart L, Soukop M, et al. Etoposide: a pharmacokinetic profile including an assessment of bioavailability. Med Oncol Tumor Pharmacother 1986;3:9599.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20.

    Slevin ML, Joel SP, Whomsley R, et al. The effect of dose on the bioavailability of oral etoposide: confirmation of a clinically relevant observation. Cancer Chemother Pharmacol 1989;24:329331.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 21.

    Hainsworth JD, Johnson DH, Frazier SR, et al. Chronic daily administration of oral etoposide—a phase I trial. J Clin Oncol 1989;7:396401.

  • 22.

    Hainsworth JD, Williams SD, Einhorn LH, et al. Successful treatment of resistant germinal neoplasms with VP-16 and cisplatin: results of a Southeastern Cancer Study Group trial. J Clin Oncol 1985;3:666671.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 23.

    Hande KR, Krozely MG, Greco FA, et al. Bioavailability of low-dose oral etoposide. J Clin Oncol 1993;11:374377.

  • 24.

    Igwemezie LN, Kaul S, Barbhaiya RH. Assessment of toxicokinetics and toxicodynamics following intravenous administration of etoposide phosphate in beagle dogs. Pharm Res 1995;12:117123.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 25.

    Eisenberg EJ, Eickhoff WM. Determination of etoposide in blood by liquid chromatography with electrochemical detection. J Chromatogr 1993;621:110114.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 26.

    Lana S, U'ren L, Plaza S, et al. Continuous low-dose oral chemotherapy for adjuvant therapy of splenic hemangiosarcoma in dogs. J Vet Intern Med 2007;21:764769.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 27.

    Wolff SN, Grosh WW, Prater K, et al. In vitro pharmacodynamic evaluation of VP-16–213 and implications for chemotherapy. Cancer Chemother Pharmacol 1987;19:246249.

    • Search Google Scholar
    • Export Citation
  • 28.

    Drewinko B, Barlogie B. Survival and cycle-progression delay of human lymphoma cells in vitro exposed to VP-16–213. Cancer Treat Rep 1976;60:12951306.

    • Search Google Scholar
    • Export Citation
  • 29.

    Heck MM, Hittelman WN, Earnshaw WC. Differential expression of DNA topoisomerases I and II during the eukaryotic cell cycle. Proc Natl Acad Sci U S A 1988;85:10861090.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 30.

    Fox ME, Smith PJ. Long-term inhibition of DNA synthesis and the persistence of trapped topoisomerase II complexes in determining the toxicity of the antitumor DNA intercalators mAMSA and mitoxantrone. Cancer Res 1990;50:58135818.

    • Search Google Scholar
    • Export Citation
  • 31.

    Allen JD, Brinkhuis RF, Wijnholds J, et al. The mouse Bcrp1/Mxr/Abcp gene: amplification and overexpression in cell lines selected for resistance to topotecan, mitoxantrone, or doxorubicin. Cancer Res 1999;59:42374241.

    • Search Google Scholar
    • Export Citation
  • 32.

    Leu BL, Huang JD. Inhibition of intestinal P-glycoprotein and effects on etoposide absorption. Cancer Chemother Pharmacol 1995;35:432436.

  • 33.

    Keller RP, Altermatt HJ, Donatsch P, et al. Pharmacologic interactions between the resistance-modifying cyclosporine SDZ PSC 833 and etoposide (VP 16–213) enhance in vivo cytostatic activity and toxicity. Int J Cancer 1992;51:433438.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 34.

    Allen JD, Van Dort SC, Buitelaar M, et al. Mouse breast cancer resistance protein (Bcrp1/Abcg2) mediates etoposide resistance and transport, but etoposide oral availability is limited primarily by P-glycoprotein. Cancer Res 2003;63:13391344.

    • Search Google Scholar
    • Export Citation
  • 35.

    Yahanda AM, Alder KM, Fisher GA, et al. Phase I trial of etoposide with cyclosporine as a modulator of multidrug resistance. J Clin Oncol 1992;10:16241634.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 36.

    Lum BL, Kaubisch S, Yahanda AM, et al. Alteration of etoposide pharmacokinetics and pharmacodynamics by cyclosporine in a phase I trial to modulate multidrug resistance. J Clin Oncol 1992;10:16351642.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 37.

    Shah JC, Chen JR, Chow D. Preformulation study of etoposide: identification of physicochemical characteristics responsible for the low and erratic oral bioavailability of etoposide. Pharm Res 1989;6:408412.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 38.

    Boothe DM. Veterinary compounding in small animals: a clinical pharmacologist's perspective. Vet Clin North Am Small Anim Pract 2006;36:11291173.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 39.

    Reif S, Nicolson MC, Bisset D, et al. Effect of grapefruit juice intake on etoposide bioavailability. Eur J Clin Pharmacol 2002;58:491494.

  • 40.

    Clark PI, Slevin ML, Joel SP, et al. A randomized trial of two etoposide schedules in small-cell lung cancer: the influence of pharmacokinetics on efficacy and toxicity. J Clin Oncol 1994;12:14271435.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 41.

    Toutain PL, Bousquet-Melou A. Bioavailability and its assessment. J Vet Pharmacol Ther 2004;27:455466.

Contributor Notes

Dr. Flory's present address is Bobst Hospital of the Animal Medical Center, 510 E 62nd St, New York, NY 10021.

Dr. Autio's present address is Department of Equine and Small Animal Medicine, College of Veterinary Medicine, Helsinki University, Helsinki, 00014, Finland.

Presented in part at the 26th Annual Conference of the Veterinary Cancer Society, Pine Mountain, Ga, October 2006.

The authors thank Mary Lynch for technical assistance.

Address correspondence to Dr. Rassnick.