Establishment of a cell line for assessing drugs as canine P-glycoprotein substrates: proof of principle

P-glycoprotein (P-gp), encoded by the ABCB1 (MDR1) gene, dramatically impacts drug disposition. P-gp is expressed in the intestines, biliary canaliculi, renal tubules, and brain capillaries where it functions to efflux substrate drugs. In this capacity, P-gp restricts oral absorption, enhances biliary and renal excretion, and inhibits central nervous system entry of substrate drugs. Many drugs commonly used in veterinary medicine are known substrates for canine P-gp (vincristine, loperamide, ivermectin, others). Because these drugs have a narrow therapeutic index, defective P-gp function can cause serious adverse drug reactions due to enhanced brain penetration and/or decreased clearance. P-gp dysfunction in dogs can be intrinsic (dogs harboring ABCB1-1Δ) or acquired (drug interactions between a P-gp inhibitor and P-gp substrate). New human drug candidates are required to undergo assessment for P-gp interactions according to FDA and EMA regulations to avoid adverse drug reactions and drug-drug interactions. Similar information regarding canine P-gp could prevent adverse drug reactions in dogs. Because differences in P-gp substrates have been documented between species, one should not presume that human or murine P-gp substrates are necessarily canine P-gp substrates. Thus, our goal was to develop a cell line for assessing drugs as canine P-gp substrates.

The Effect of the Canine ABCB1-1Δ Mutation on Sedation after Intravenous Administration of Acepromazine

BACKGROUND

Dog breeds with the ABCB1-1Δ mutation have substantially truncated nonfunctional P-glycoprotein. Dogs homozygous for this mutation (mut/mut) are susceptible to the toxic adverse effects of ivermectin, loperamide, and vincristine. Anecdotal reports suggested ABCB1 mut/mut dogs showed increased depth and duration of acepromazine sedation.

HYPOTHESIS/OBJECTIVES

That ABCB1 mut/mut dogs have increased depth and duration of sedation after acepromazine IV compared to normal dogs (nor/nor).

ANIMALS

Twenty-nine rough-coated collies were divided into 3 groups of dogs based on their ABCB1 genotype: 10 mut/mut, 10 mut/nor, and 9 nor/nor.

METHODS

Dogs were given 0.04 mg/kg of acepromazine IV. Level of sedation, heart rate, respiratory rate, and blood pressure were recorded for 6 hours after acepromazine administration. Area under the curves (AUCs) of the normalized sedation score results were calculated and compared.

RESULTS

The median sedation scores for ABCB1 mut/mut dogs were higher than nor/nor dogs at all time points and were higher in mut/nor dogs for the first 2 hours. These differences were not found to be significant for any individual time point (P > .05). The median sedation score AUC for mut/mut dogs was significantly higher than nor/nor dogs (P = .028), but the AUC for mut/nor dogs was not (P = .45). There were no significant differences between groups for heart rate, respiratory rate, and blood pressure (P > .05).

CONCLUSIONS AND CLINICAL IMPORTANCE

In ABCB1 mut/mut dogs acepromazine dose rates should be reduced and careful monitoring performed during sedation.

P-glycoprotein mediated drug interactions in animals and humans with cancer

Drug–drug interactions can cause unanticipated patient morbidity and mortality. The consequences of drug–drug interactions can be especially severe when anticancer drugs are involved because of their narrow therapeutic index. Veterinary clinicians have traditionally been taught that drug–drug interactions result from alterations in drug metabolism, renal excretion or protein binding. More recently, drug–drug interactions resulting from inhibition of P‐glycoprotein‐mediated drug transport have been identified in both human and veterinary patients. Many drugs commonly used in veterinary patients are capable of inhibiting P‐glycoprotein function and thereby causing an interaction that results in severe chemotherapeutic drug toxicity. The intent of this review is to describe the mechanism and clinical implications of drug–drug interactions involving P‐glycoprotein and anticancer drugs. Equipped with this information, veterinarians can prevent serious drug–drug interactions by selecting alternate drugs or adjusting the dose of interacting drugs.

Comparison of chemotherapeutic drug resistance in cells transfected with canine ABCG2 or feline ABCG2

ABCG2 (ATP binding cassette subfamily G, member 2) mediates resistance to a variety of cytotoxic agents. Although human ABCG2 is well characterized, the function of canine ABCG2 has not been studied previously. Feline ABCG2 has an amino acid substitution in the adenosine triphosphate-binding domain that decreases its transport capacity relative to human ABCG2. Our goal was to compare canine ABCG2-mediated chemotherapeutic drug resistance to feline ABCG2-mediated chemotherapeutic drug resistance. HEK-293 cells stably transfected with plasmid containing canine ABCG2, feline ABCG2 or no ABCG2 were exposed to carboplatin, doxorubicin, mitoxantrone, toceranib or vincristine, and cell survival was subsequently determined. Canine ABCG2 conferred a greater degree of chemotherapy resistance than feline ABCG2 for mitoxantrone. Neither canine nor feline ABCG2 conferred resistance to doxorubicin, vincristine or toceranib. Canine, but not feline, ABCG2 conferred resistance to carboplatin, a drug that is not reported to be a substrate for ABCG2 in other species.

Polymorphisms in the canine glucocorticoid receptor alpha gene (NR3C1α)

Corticosteroids are one of the most extensively used class of therapeutic agents in dogs. In human patients, response to corticosteroid therapy has been correlated with the presence of certain polymorphisms of the glucocorticoid receptor gene (NR3C1). Depending on the polymorphism present, patients may show either increased sensitivity to glucocorticoid-induced adverse effects or resistance to their therapeutic effects. Because response to corticosteroid therapy in dogs can also be variable and unpredictable, we hypothesized that genetic variability exists in the canine NR3C1 gene. The aim of this study was to sequence the coding regions of the canine NR3C1 gene in a representative sample of dogs. Samples from 97 dogs from four previously identified genetic groupings of domestic breeds (Asian/Ancient, Herding, Hunting, and Mastiff) were sequenced and evaluated. Four exons contained polymorphisms and four exons showed no variation from the reference sequence. A total of six single nucleotide polymorphisms (SNPs) were identified including four synonymous SNPs and two nonsynonymous SNPs (c.811A>T and c.2111T>C). No dogs were homozygous for either variant allele, while 23 dogs were heterozygous for the c.811A>T allele and 2 were heterozygous for c.2111T>C allele. The amino acid changes caused by c.811A>T (serine to cysteine) and c.2111T>C (isoleucine to threonine) were both predicted by in silico analysis to be 'probably damaging' to structure and function of the resulting protein. We conclude that NR3C1 polymorphisms occur in dogs and may cause individual variation in response to corticosteroid therapy.

Identification of a nonsense mutation in feline ABCB1

The aim of this study was to sequence all exons of the ABCB1 (MDR1) gene in cats that had experienced adverse reactions to P-glycoprotein substrate drugs (phenotyped cats). Eight phenotyped cats were included in the study consisting of eight cats that experienced central nervous system toxicosis after receiving ivermectin (n = 2), a combination product containing moxidectin and imidacloprid (n = 3), a combination product containing praziquantel and emodepside (n = 1) or selamectin (n = 2), and 1 cat that received the product containing praziquantel and emodepside but did not experience toxicity (n = 1). Fifteen exons contained polymorphisms and twelve exons showed no variation from the reference sequence. The most significant finding was a nonsense mutation (ABCB11930_1931del TC) in one of the ivermectin-treated cats. This cat was homozygous for the deletion mutation. All of the other phenotyped cats were homozygous for the wild-type allele. However, 14 missense mutations were identified in one or more phenotyped cats. ABCB11930_1931del TC was also identified in four nonphenotyped cats (one homozygous and three heterozygous for the mutant allele). Cats affected by ABCB11930_1931del TC would be expected to have a similar phenotype as dogs with the previously characterized ABCB1-1Δ mutation.

Identification of DNA variants in the canine beta-1 adrenergic receptor gene

Beta-adrenergic receptor antagonists are utilized for the management of several cardiac diseases in the dog. In humans the beneficial effects of beta-adrenergic receptor antagonists are variable and are associated with a genetic variability in the beta one adrenergic receptor gene (ADRB1). To determine if DNA variants were present in the canine ADRB1 gene, DNA from five breeds of dogs was evaluated. Two deletions were identified within the region of the gene that encodes the cytoplasmic tail of ADRB1. The functions of this region are not well understood although it is important in differentiating subtypes of adrenergic receptors and may be associated with control of receptor downregulation. The functional consequences of these identified variants deserve further study.

Vincristine-induced central neurotoxicity in a collie homozygous for the ABCB1Δ mutation

A six-year-old, neutered, female collie was presented to an oncology specialty service after developing tetraparesis and self-mutilation that progressively worsened while receiving chemotherapy for lymphoma. Neurologic examination revealed ataxia, paresis and diminished conscious proprioception in all limbs with entire spinal reflexes. Magnetic resonance imaging of the brain and spinal cord was normal. Electromyography of the limbs ruled out a vincristine-induced peripheral neuropathy. Cerebrospinal fluid analysis and cerebrospinal fluid and serum testing for Neospora and Toxoplasma were normal. Results of MDR1 genotyping revealed that the dog was homozygous for the ABCB1-1Δ (MDR1) mutation. This clinical presentation strongly resembled the effects seen from inadvertent intrathecal administration of vincristine in humans. Dogs that are homozygous for the ABCB1-1Δ (MDR1) mutation should not receive standard dosages of chemotherapy drugs known to be eliminated by P-glycoprotein, the gene product of ABCB1. Testing for this mutation is strongly recommended before chemotherapy initiation for at-risk breeds.

Normal dogs treated with famotidine for 14 days have only transient increases in serum gastrin concentrations

BACKGROUND

In people, serum gastrin concentrations increase in response to administration of H(2) receptor antagonists, but the effect of famotidine administration on serum gastrin concentrations has not been evaluated in dogs.

OBJECTIVES

To determine if serum gastrin concentrations increase in response to 14 days of famotidine treatment and the time needed to return to baseline after discontinuation of famotidine; define stability of gastrin in samples held at room temperature.

ANIMALS

Eleven healthy dogs were included in part A (famotidine treatment) and 7 healthy dogs in Part B (serum gastrin stability). In part A, famotidine (0.5 mg/kg p.o. q12h) was administered for 14 days. Fasting blood samples were collected on days 0, 3, 7, 11, 14, 16, 18, 20, and 22. In part B, blood was collected after a 12-hour fast. Gastrin concentrations in serum samples held at room temperature for ≤30 minutes after sampling were compared to concentrations in samples held at room temperature for 150 minutes after sampling.

RESULTS

Serum gastrin concentrations increased by day 3 of famotidine administration and returned to baseline concentrations in all dogs by day 14 despite continued famotidine administration. Serum gastrin concentrations were lower (20% mean decrease; P = .0005) in samples held at room temperature for 150 minutes.

CONCLUSIONS AND CLINICAL IMPORTANCE

After 14 days of famotidine administration, clinically healthy dogs have normal serum gastrin concentrations. In a dog with clinical features consistent with gastrinoma, chronic famotidine administration is unlikely to contribute to increases in serum gastrin concentrations.

The effects of inhibiting cytochrome P450 3A, p-glycoprotein, and gastric acid secretion on the oral bioavailability of methadone in dogs

Methadone is an opioid, which has a high oral bioavailability (>70%) and a long elimination half-life (>20 h) in human beings. The purpose of this study was to evaluate the effects of ketoconazole [a CYP3A and p-glycoprotein (p-gp) inhibitor] and omeprazole (an H+,K(+)-ATPase proton-pump inhibitor) on oral methadone bioavailability in dogs. Six healthy dogs were used in a crossover design. Methadone was administered i.v. (1 mg/kg), orally (2 mg/kg), again orally following oral ketoconazole (10 mg/kg q12 h for two doses), and following omeprazole (1 mg/kg p.o. q12 h for five doses). Plasma concentrations of methadone were analyzed by high-pressure liquid chromatography or fluorescence polarization immunoassay. The mean +/- SD for the elimination half-life, volume of distribution, and clearance were 1.75 +/- 0.25 h, 3.46 +/- 1.09 L/kg, and 25.14 +/- 9.79 mL/min.kg, respectively following i.v. administration. Methadone was not detected in any sample following oral administration alone or following oral administration with omeprazole. Following administration with ketoconazole, detectable concentrations of methadone were present in one dog with a 29% bioavailability. MDR-1 genotyping, encoding p-gp, was normal in all dogs. In contrast to its pharmacokinetics humans, methadone has a short elimination half-life, rapid clearance, and low oral bioavailability in dogs and the extent of absorption is not affected by inhibition of CYP3A, p-gp, and gastric acid secretion.

Therapeutic implications of the MDR-1 gene

Drug transporters significantly influence drug pharmacokinetics and pharmacodynamics. P-glycoprotein (P-gp), the product of the MDR1 (ABCB1) gene, is among the most well-characterized drug transporters, particularly in veterinary medicine. A number of clinically relevant, structurally and functionally unrelated drugs are substrates for P-gp. P-gp is expressed by a variety of normal tissues including the intestines, renal tubular cells, brain capillary endothelial cells, biliary canalicular cells, and others, where it functions to actively extrude substrate drugs. In this capacity, P-gp limits oral absorption and central nervous system entry of many substrate drugs. A number of MDR1 polymorphisms have been described in human patients, some of which result in altered drug pharmacokinetics and susceptibility to diseases such as Parkinson's disease, inflammatory bowel disease, refractory seizures, and others. An MDR1 polymorphism in herding breed dogs, including collies and Australian shepherds, has been demonstrated to be the cause of ivermectin sensitivity in these breeds. Recent evidence suggests that this polymorphism, a 4-bp deletion mutation, results in increased susceptibility to the toxicity of several drugs in addition to ivermectin. Furthermore, data in rodent models suggest that P-gp may play an important role in regulating the hypothalamic-pituitary-adrenal axis.

Frequency of the mutant MDR1 allele associated with multidrug sensitivity in a sample of collies from France

A study was performed to determine the frequency of the mutant MDR1 allele associated with ivermectin sensitivity in a sample of collies living in France. Buccal swab samples were collected from approximately 83 collies for determination of MDR1 genotype. DNA was extracted and the polymerase chain reaction was performed to amplify a 148 bp (wildtype MDR1 genotype) or 144 bp (mutant MDR1 genotype) amplicon containing the MDR1 mutation. Sequence analysis was performed to determine the genotype of each dog. Adequate quantities of DNA for unequivocal genotyping were obtained from only 25 of 83 swabs. Twenty percent (5/25) of the collies studied were homozygous for the normal allele (normal), 32% (8/25) were heterozygous (carrier), and 48% (12/25) were homozygous for the mutant allele (affected). The results of this study indicate that a high percentage of collies presenting to veterinarians in France harbor the MDR1 mutation, thus impacting some therapeutic decisions.

Systemic absorption of amitriptyline and buspirone after oral and transdermal administration to healthy cats

A prospective study was performed to determine the relative availability of buspirone and amitriptyline after oral and transdermal routes of administration in 6 adult cats. For topical administration, drugs were compounded in a transdermal organogel containing pluronic and lecithin (PLO). Using a crossover design, each cat received a single dose of amitriptyline (5 mg) and buspirone (2.5 mg) by the transdermal and oral route of administration with at least a 2-week washout interval between drug treatments. Blood samples were obtained at 0, 0.5, 1, 2, 4, 6, 8, 10, and 12 hours after drug administration for determination of plasma drug concentrations. Plasma concentrations of immunoreactive amitriptyline and buspirone were determined using commercial enzyme-linked immunosorbent assay (ELISA) tests. Systemic absorption of amitriptyline and buspirone administered by the transdermal route was poor compared with the oral route of administration. Until supporting pharmacokinetic data are available, veterinarians and cat owners should not rely on the transdermal route of administration for treating cats with amitriptyline or buspirone.

Ivermectin sensitivity in collies is associated with a deletion mutation of the mdr1 gene

A subpopulation of collie dogs is extremely sensitive to neurotoxicity induced by ivermectin. The aim of this study was to determine the mechanistic basis for this phenomenon. The multi-drug-resistance gene (mdr1) encodes a large transmembrane protein, P-glycoprotein (P-gp), that is an integral part of the blood-brain barrier. P-gp functions as a drug-transport pump at the blood-brain barrier, transporting a variety of drugs from the brain back into the blood. Since ivermectin is a substrate for P-gp, we hypothesized that ivermectin-sensitive collies had altered mdr1 expression compared with unaffected collies. We report a deletion mutation of the mdr1 gene that is associated with ivermectin sensitivity. The 4-bp deletion results in a frame shift, generating several stop codons that prematurely terminate P-gp synthesis. Dogs that are homozygous for the deletion mutation display the ivermectin-sensitive phenotype, while those that are homozygous normal or heterozygous do not display increased sensitivity to ivermectin.

Functional cystic thyroid adenoma in a cat

A 9-year-old cat with hyperthyroidism was referred for radioactive iodine treatment. The cat also had a ventral cervical mass that the owners reported had been present for several years and had increased in size during the past few weeks. On physical examination, the mass was found to have caused lateral displacement of the trachea, esophagus, jugular vein, and common carotid artery. The mass was aspirated and was determined to be cystic in nature. Concentrations of thyroid hormones in the cystic fluid were similar to serum concentrations, and nuclear scintigraphy revealed thyroactive tissue lining the cyst wall. Magnetic resonance imaging suggested that the cyst originated from the right lobe of the thyroid gland. The cat was treated with sodium iodide I 131 but died 4 days later, presumably as a result of aspiration of gastric or esophageal contents. A necropsy was not performed, but histologic examination of a biopsy specimen of the mass indicated that it was a cystic thyroid adenoma.

Penicillins and beta-lactamase inhibitor combinations

beta-Lactamase production by bacteria continues to be one of the main mechanisms of bacterial resistance to beta-lactam antibiotics, and it seems likely to remain so. beta-Lactamase inhibitors provide 1 strategy to overcome this mechanism of bacterial resistance. Although 3 beta-lactamase inhibitor/antibiotic combinations are currently available, only 1 is approved for veterinary use. Because the beta-lactamase inhibitor must be present concurrently with the antibiotic for synergistic activity, it is important to consider the pharmacokinetic profile of these drugs in combination. These combinations were developed and optimized for human patients, so it is unlikely that they would achieve the ideal plasma and tissue concentrations and ratios in veterinary patients. Indeed, several differences in pharmacokinetic variables of beta-lactam antibiotic/beta-lactamase inhibitor agents have been described in dogs, compared with people. Such pharmacokinetic differences should be considered when interpreting in vitro susceptibility results in veterinary species, because these tests use ratios of drug that were established for humans. The beta-lactamase inhibitors represent a successful example of targeted drug development. However, the currently available inhibitors are active primarily against class-A beta-lactamases. Because the frequency with which class-C beta-lactamases are recognized is rapidly increasing in human isolates, and because beta-lactamase enzymes continue to evolve, new beta-lactamase inhibitors will need to be developed to target these enzymes.

Pharmacokinetics of phenylbutazone and its metabolite oxyphenbutazone in miniature donkeys

OBJECTIVE

To describe the pharmacokinetics of phenylbutazone and oxyphenbutazone after IV administration in miniature donkeys.

ANIMALS

6 clinically normal miniature donkeys.

PROCEDURE

Blood samples were collected before and 5, 10, 20, 30, 45, 60, 90, 120, 180, 240, 300, 360, and 480 minutes after IV administration of phenylbutazone (4.4 mg/kg of body weight). Serum was analyzed in triplicate by use of high-performance liquid chromatography for determination of phenylbutazone and oxyphenbutazone concentrations. The serum concentration-time curve for each donkey was analyzed separately to estimate model-independent pharmacokinetic variables.

RESULTS

Serum concentrations decreased rapidly after IV administration of phenylbutazone, and they reached undetectable concentrations within 4 hours. Values for mean residence time ranged from 0.5 to 3.0 hours (median, 1.1 hour), whereas total body clearance ranged from 4.2 to 7.5 ml/kg/min (mean, 5.8 ml/kg/min). Oxyphenbutazone appeared rapidly in the serum; time to peak concentration ranged from 13 to 41 minutes (mean, 26.4 minutes), and peak concentration in serum ranged from 2.8 to 4.0 mg/ml (mean, 3.5 microg/ml).

CONCLUSION AND CLINICAL RELEVANCE

Clearance of phenylbutazone in miniature donkeys after injection of a single dose (4.4 mg/kg, IV) is rapid. Compared with horses, miniature donkeys may require more frequent administration of phenylbutazone to achieve therapeutic efficacy.

Pharmacokinetics of imipramine in narcoleptic horses

OBJECTIVE

To validate use of high-performance liquid chromatography (HPLC) in determining imipramine concentrations in equine serum and to determine pharmacokinetics of imipramine in narcoleptic horses.

ANIMALS

5 horses with adult-onset narcolepsy.

PROCEDURE

Blood samples were collected before (time 0) and 3, 5, 10, 15, 20, 30, and 45 minutes and 1, 2, 3, 4, 6, 8, 12, and 24 hours after IV administration of imipramine hydrochloride (2 or 4 mg/kg of body weight). Serum was analyzed, using HPLC, to determine imipramine concentration. The serum concentration-versus-time curve for each horse was analyzed separately to estimate pharmacokinetic values.

RESULTS

Adverse effects (muscle fasciculations, tachycardia, hyperresponsiveness to sound, and hemolysis) were detected in most horses when serum imipramine concentrations were high, and these effects were most severe in horses receiving 4 mg of imipramine/kg. Residual adverse effects were not apparent. Value (mean +/- SD) for area under the curve was 3.9 +/- 0.7 h X microg/ml, whereas volume of distribution was 584 +/- 161.7 ml/kg, total body clearance was 522 +/- 102 ml/kg/h, and mean residence time was 1.8 +/- 0.6 hours. One horse had signs of narcolepsy 6 and 12 hours after imipramine administration; corrresponding serum imipramine concentrations were less than the therapeutic range.

CONCLUSIONS AND CLINICAL RELEVANCE

Potentially serious adverse effects may be seen in horses administered doses of imipramine that exceed a dosage of 2 mg/kg. Total body clearance of imipramine in horses is slower than that in humans; thus, the interval between subsequent doses should be longer in horses.

Pharmacokinetics of flunixin meglumine in donkeys, mules, and horses

OBJECTIVE

To compare serum disposition of flunixin meglumine after i.v. administration of a bolus to horses, donkeys, and mules.

ANIMALS

3 clinically normal horses, 5 clinically normal donkeys, and 5 clinically normal mules.

PROCEDURE

Blood samples were collected at time zero (before) and 5, 10, 15, 30, and 45 minutes, and at 1, 1.25, 1.5, 1.75, 2, 2.5, 2.75, 3, 3.5, 4, 4.5, 5, 5.5, 6, and 8 hours after i.v. administration of a bolus of flunixin meglumine (1.1 mg/kg of body weight). Serum was analyzed in duplicate by the use of high-performance liquid chromatography for determination of flunixin meglumine concentrations. The serum concentration-time curve for each horse, donkey, and mule were analyzed separately to estimate noncompartmental pharmacokinetic variables

RESULTS

Mean (+/-SD) area under the curve for donkeys (646 +/- 148 minute x microg/ml) was significantly less than for horses (976 +/- 168 minute x microg/ml) or for mules (860 +/- 343 minute x microg/ml). Mean residence time for donkeys (54.6 +/- 7 minutes) was significantly less than for horses (110 +/- 24 minutes) or for mules (93 +/- 30 minutes). Mean total body clearance for donkeys (1.78 +/- 0.5 ml/kg/h) was significantly different from that for horses (1.14 +/- 0.18 ml/kg/h) but not from that for mules (1.4 +/- 0.5 ml/kg/h). Significant differences were not found between horses and mules for any pharmacokinetic variable.

CONCLUSION AND CLINICAL RELEVANCE

Significant differences exist with regard to serum disposition of flunixin meglumine in donkeys, compared with that for horses and mules. Consequently, flunixin meglumine dosing regimens used in horses may be inappropriate for use in donkeys.

Hypercalcemia associated with granulomatous disease in a cat

A 6-year-old cat was examined because of recurrence of a draining mass involving skin and subcutaneous tissues of the caudoventral aspect of the abdomen. Previous treatment included administration of antimicrobial drugs and corticosteroids and surgical excision. Atypical mycobacteria were seen during cytologic examination of biopsy specimens of the mass; Nocardia sp was cultured. While hospitalized, the cat developed hypercalcemia and was found to have high serum calcitriol concentrations. Treatment consisted of administration of ciprofloxacin and trimethoprim-sulfadiazine because of the infection and administration of sodium chloride solution, furosemide, and calcitonin because of the hypercalcemia. The cat recovered.

Immunosuppressant inhibition of P-glycoprotein function is independent of drug-induced suppression of peptide-prolyl isomerase and calcineurin activity

PURPOSE

P-glycoprotein is a 170-kDa plasma membrane multidrug transporter that actively exports cytotoxic substances from cells. Overexpression of P-glycoprotein by tumor cells is associated with a multidrug-resistant phenotype. Immunosuppressive agents such as cyclosporins and macrolides, have been shown to attenuate P-glycoprotein activity. However, the mechanism by which some immunosuppressants inhibit P-glycoprotein function has not been determined. Since cyclosporin and macrolide immunosuppressants inhibit calcineurin (CaN) phosphatase and FKBP12 peptideprolyl isomerase (FKBP12 PPI) activity, studies were conducted to determine if these effects are directly related to the inhibitory effects these immunosuppressants have on P-glycoprotein function.

METHODS

Western blot analysis was performed to assess CaN and FKBP12 protein levels in P-glycoprotein-negative (MCF-7) and -positive (MCF-7/Adr) breast cancer cell lines. P-glycoprotein function was determined by intracellular doxorubicin accumulation and/or cytotoxicity assays before and after CaN and FKBP12 were independently inhibited by pharmacological antagonists.

RESULTS

CaN and FKBP12 levels were similar in MCF-7 and MCF-7/Adr cells. P-glycoprotein function was not affected by treatment of P-glycoprotein-expressing MCF-7/Adr cells with CaN and FKBP12 antagonists.

CONCLUSIONS

These results demonstrate that the inhibitory effects of immunosuppressive agents on P-glycoprotein function are independent of CaN or FKBP12 PPI activity.

Neuromuscular effects of doxacurium chloride in isoflurane-anesthetized dogs

OBJECTIVE

To determine the neuromuscular effects of doxacurium chloride and to construct a dose-response curve for the drug in isoflurane-anesthetized dogs.

DESIGN

Randomized, controlled trial.

ANIMALS

Six healthy, adult, mixed-breed dogs (five female, one male) weighing 24.8 +/- 2.8 kg.

METHODS

Anesthesia was induced with isoflurane in oxygen and maintained with 1.9% to 2.3% end-tidal isoflurane concentration. PaCO2 was maintained between 35 and 45 mm Hg with mechanical ventilation. Mechanomyography was used to quantitate the evoked twitch response of the paw after supramaximal train-of-four stimulation of the superficial peroneal nerve. After baseline values were recorded, the dogs received one of three doses of doxacurium (2.0, 3.5, 4.5 microg/kg of body weight) or a saline placebo intravenously in random order. All dogs received all treatments with at least 7 days between studies. After drug administration, the degree of maximal first twitch depression compared with baseline (T1%) was recorded. Dose-response relations of doxacurium were plotted in log dose-probit format and analyzed by linear regression to determine effective dose (ED50 and ED90) values for doxacurium.

RESULTS

The median log dose-probit response curve showed good data correlation (r = .999) with estimates of the ED50 (2.1 microg/kg) and ED90 (3.5 microg/kg) for doxacurium in isoflurane-anesthetized dogs. Mean +/- SD values for T1% (first twitch tension compared with baseline) at maximal depression after drug administration, onset (time from drug administration to maximal depression of T1%), duration (time from maximal depression of T1% to 25% recovery of T1%), and recovery (time from 25% to 75% recovery of T1%) times were 92% +/- 4%, 40 +/- 5 minutes, 108 +/- 31 minutes, and 42 +/- 11 minutes for dogs treated with 3.5 microg/kg of doxacurium and 94% +/- 7%, 41 +/- 8 minutes, 111 +/- 33 minutes, and 37 +/- 10 minutes for dogs treated with 4.5 microg/kg of doxacurium.

CONCLUSION AND CLINICAL RELEVANCE

We conclude that doxacurium is a long-acting neuromuscular blocking agent with a slow onset of action. Doxacurium can be used to provide muscle relaxation for long surgical procedures in isoflurane-anesthetized dogs. Interpatient variability, particularly of duration of drug action, may exist in the neuromuscular response to the administration of doxacurium in dogs.

Doxorubicin induced expression of P-glycoprotein in a canine osteosarcoma cell line

Canine and human osteosarcoma are very similar with respect to clinical presentation, radiological and histopathological features, metastatic rate and pattern and response to therapy. For these reasons, canine osteosarcoma is a useful intermediate model for the disease in humans. Overexpression of P-glycoprotein, the product of the MDR1 gene, is the most important predictor of an adverse clinical course in human patients with osteosarcoma. Exposure of canine osteosarcoma cells to doxorubicin resulted in overexpression of MDR1 mRNA and P-glycoprotein. Furthermore, these cells failed to accumulate doxorubicin intracellularly and were less sensitive to vincristine-induced cytotoxicity as compared to parental cells.

Pharmacokinetics and cardiopulmonary effects of guaifenesin in donkeys

Five donkeys and three horses were given guaifenesin, intravenously, by gravity administration, until recumbency was produced. The time and dose required to produce recumbency, recovery time to sternal and standing were recorded. Blood samples were collected for guaifenesin assay at 10, 20, 30, 40, 50, 60 min, and 2, 3, 4 and 6 h after guaifenesin administration. Serum was analysed for guaifenesin using HPLC and pharmacokinetic values were calculated using a computer software package (RSTRIP). In donkeys, heart and respiratory rates and blood pressures were recorded before and at 5-min intervals during recumbency. Arterial blood samples were collected before and at 5 and 15 min intervals during recumbency for analysis of pH, CO2, and O2. ANOVA was used to evaluate dynamic data, while t-tests were used for kinetic values. Respiratory rate was decreased significantly during recumbency, but no other significant changes from baseline occurred. The mean (+/- SD) recumbency dose of guaifenesin was 131 mg/kg (27) for donkeys and 211 mg/kg (8) for horses. Recovery time to sternal (min) was 15 (SD, 11) for donkeys and 34 (SD, 1.4) for horses. Time to standing was 32 min for donkeys and 36 min for horses. Calculation of AUC (area under the concentration-time curve) microgram/mL) (dose-dependent variable) was 231 (SD, 33) for donkeys and 688 (SD, 110) for horses. The clearance (CL) (mL/h.kg) was 546 (SD, 73) for donkeys, which was significantly different from 313 (SD, 62) for horses. Mean residence time (MRT) (h) was 1.2 (SD, 0.1) for donkeys and 2.6 (SD, 0.5) for horses. Volume of distribution Vd(area) (mL/kg) was 678 (SD, 92) for donkeys and 794 (SD, 25) for horses. At the rate of administration used in this study, donkeys required less guaifenesin than horses to produce recumbency, but cleared it more rapidly.

Comparative pharmacokinetics of caffeine and three metabolites in clinically normal horses and donkeys

OBJECTIVE

To determine whether clearance of capacity-limited drugs in horses differs from that in donkeys by comparing the serum disposition of caffeine and its metabolites, theophylline, theobromine, and paraxanthine after i.v. administration of caffeine to horses and donkeys.

ANIMALS

4 healthy horses and 5 healthy donkeys.

PROCEDURE

Blood samples were collected from each animal at time 0 (before) and 5, 10, 15, 20, 30, and 45 minutes, and 1, 2, 3, 4, 6, 8, 12, 24, 30, 36, 48, 54, 60, 72, and 96 hours after IV administration of a bolus of caffeine. Serum was analyzed in triplicate by high-performance liquid chromatography to determine caffeine, theophylline, theobromine, and paraxanthine concentrations. The serum concentration-time curves for each animal were analyzed separately to estimate model-independent pharmacokinetic variables.

RESULTS

Mean pharmacokinetic values for caffeine, theophylline, and paraxanthine did not differ significantly in horses, compared with donkeys. Mean peak serum concentration of theobromine was significantly higher in donkeys, compared with horses.

CONCLUSION

Clearance of the capacity-limited drug caffeine does not appear to differ in horses, compared with donkeys.

CLINICAL RELEVANCE

For some drugs that undergo hepatic metabolism, the dose and dose interval used for horses may be appropriate for use in donkeys.

Comparative pharmacokinetics of phenylbutazone and its metabolite oxyphenbutazone in clinically normal horses and donkeys

OBJECTIVE

To compare plasma disposition of phenylbutazone and its metabolite oxyphenbutazone after i.v. administration of phenylbutazone in horses and donkeys.

ANIMALS

4 clinically normal horses and 6 clinically normal donkeys.

PROCEDURE

Blood samples were collected from each animal at time 0 (before) and 5, 10, 20, 30, 45, 60, 90, 120, 180, 240, 300, 360, and 480 minutes after i.v. administration of a bolus dose of phenylbutazone. Serum was analyzed in triplicate by use of high-performance liquid chromatography for determination of phenylbutazone and oxyphenbutazone concentrations. The serum concentration-time curve for each horse and donkey was analyzed separately to estimate model-independent pharmacokinetic variables.

RESULTS

Significant differences were found in several pharmacokinetic variables of phenylbutazone and oxyphenbutazone in horses, compared with donkeys. Mean total body clearance of phenylbutazone in horses was fivefold less than that in donkeys (29.3 and 170.3 ml/kg/h, respectively). Mean values for area under the curve and mean residence time in horses (118.3 micrograms/h/ml and 3.6 hours, respectively) were significantly greater than values in donkeys (28.3 micrograms/h/ml and 1.7 hours, respectively). Mean values for apparent volume of distribution at steady state were not significantly different between horses and donkeys. For oxyphenbutazone, mean time to peak concentration in donkeys was significantly less than that in horses (1.6 and 6.4 hours, respectively).

CONCLUSION

Phenylbutazone clearance in donkeys was higher than that in horses, and appearance of the metabolite oxyphenbutazone in serum was more rapid in donkeys than in horses, indicating that hepatic metabolism of phenylbutazone is more rapid in donkeys than in horses.

CLINICAL RELEVANCE

Because serum concentration of phenylbutazone after single i.v. bolus administration (4.4 mg/kg of body weight) decreases more rapidly in donkeys, compared with horses, phenylbutazone may require more frequent administration in donkeys to achieve therapeutic efficacy.

Pharmacokinetics, effects on renal function, and potentiation of atracurium-induced neuromuscular blockade after administration of a high dose of gentamicin in isoflurane-anesthetized dogs

OBJECTIVE

To determine pharmacokinetics, renal effects, and effect on atracurium-induced neuromuscular blockade of a high dose of gentamicin in isoflurane-anesthetized dogs.

ANIMALS

6 healthy, adult, mixed-breed dogs, anesthetized twice and receiving gentamicin (6 mg/kg of body weight, i.v.) or saline solution.

PROCEDURE

Blood samples were collected before and at intervals after gentamicin administration. Pharmacokinetic values were evaluated by use of multivariant stepwise linear regression analysis. Gentamicin-induced renal changes were assessed by comparing pretreatment and 12- to 24-hour posttreatment values for serum urea nitrogen, serum creatinine, urine creatinine-to-gamma-glutamyl-transferase ratio, and urinalysis. Neuromuscular blockade, maintained by atracurium infusion, was assessed, using the train-of-four response. At stable 50% depression of first twitch (T1), gentamicin or saline solution was given. Before and at posttreatment intervals for 60 minutes, T1% and fourth twitch-to-T1 ratio were recorded. The infusion was discontinued and 50 to 75% T1 recovery time was recorded. At 75% T1, edrophonium (0.5 mg/kg) was administered i.v..

RESULTS

Mean values for volume of distribution and clearance were 0.263 L/kg and 2.0 ml/min/kg, respectively. Mean maximal serum concentration of gentamicin was 46.4 micrograms/ml. Pre and posttreatment values for serum urea nitrogen, serum creatinine, urine creatinine-to-gamma-glutamyltransferase ratio, and other urine analytes were not significantly different. Mean (+/- SD) values for T1% and fourth twitch-to-T1 ratio decreased significantly after gentamicin (depression was maximal at 5 minutes). Recovery time (50 to 75% T1) was not different between groups. Edrophonium restored twitch to baseline.

CONCLUSIONS

Mean values for apparent volume of distribution and total body clearance of gentamicin were similar to values in unanesthetized dogs. Mean maximal serum concentration of gentamicin was greater than that in unanesthetized dogs. Renal function was unaffected. Gentamicin potentiated atracurium-induced neuromuscular blockade, but did not affect recovery time.

Nephrotoxicosis associated with topical administration of gentamicin in a cat

A 4-year-old cat was referred for treatment of a large, open wound. The wound had been lavaged twice, approximately 12 hours apart, with approximately 10 ml of 5% gentamicin solution prior to referral, because of infection caused by Pseudomonas spp. Results of initial serum biochemical analyses were within reference ranges, and the cat was anesthetized for surgical wound debridement and closure. Two days later, the cat was azotemic (SUN concentration, 113 mg/dl; serum creatinine, concentration, 9.8 mg/dl) and had a urine specific gravity of 1.008. Granular casts were seen in the urine. The azotemia became more severe over the next 2 1/4 hours, despite treatment for acute renal failure, and the cat was euthanatized. Severe acute proximal tubular necrosis, consistent with gentamicin-induced nephrotoxicosis, was diagnosed histologically. Retrospectively, serum samples obtained for biochemical analyses were assayed for gentamicin concentration. Serum concentration of gentamicin 8 hours after topical lavage was 58.07 micrograms/ml. This was approximately 6 times greater than the desired peak concentration for gentamicin and suggested that gentamicin had been absorbed systemically following topical application.