ABCB1-1Δ Polymorphism Can Predict Hematologic Toxicity in Dogs Treated with Vincristine

Assessment of verdinexor as a canine P-glycoprotein substrate

The P-glycoprotein (P-gp) substrate status of antineoplastic drugs intended for veterinary patients is an important characteristic to define for two reasons. First, neoplastic cells expressing P-gp can actively efflux drugs that are P-gp substrates curtailing their efficacy. Second, antineoplastic drugs tend to have a narrow therapeutic index. Antineoplastic drugs that are P-gp substrates can cause severe adverse reactions in animals with P-gp dysfunction such as dogs with ABCB1-1Δ and cats with ABCB11930_1931del TC. Animals with P-gp dysfunction experience greater overall exposure to P-gp substrate drugs due to mechanisms such as increased intestinal absorption, decreased biliary clearance and greater central nervous system penetration compared with animals with normal P-gp function. Accordingly, knowing the P-gp substrate status of antineoplastic drugs is an important safety consideration prior to use in canine or feline cancer patients. This study used a cell line overexpressing canine P-gp to assess the P-gp substrate status of verdinexor. Based on both a cytotoxicity assay and a competitive flow cytometry assay verdinexor is not a substrate for canine P-gp.

Canine and feline P-glycoprotein deficiency: What we know and where we need to go

In 2001 the molecular genetic basis of so-called "ivermectin sensitivity" in herding breed dogs was determined to be a P-glycoprotein deficiency caused by a genetic variant of the MDR1 (ABCB1) gene often called "the MDR1 mutation." We have learned a great deal about P-glycoprotein's role in drug disposition since that discovery, namely that P-glycoprotein transports many more drugs than just macrocyclic lactones that P-glycoprotein mediated drug transport is present in more places than just the blood brain barrier, that some cats have a genetic variant of MDR1 that results in P-glycoprotein deficiency, that P-glycoprotein dysfunction can occur as a result of drug-drug interactions in any dog or cat, and that the concept of P-glycoprotein "inhibitors" versus P-glycoprotein substrates is somewhat arbitrary and artificial. This paper will review these discoveries and discuss how they impact drug selection and dosing in dogs and cats with genetically mediated P-glycoprotein deficiency or P-glycoprotein dysfunction resulting from drug-drug interactions.

The prevalence of the ABCB1-1Δ variant in a clinical veterinary setting: The risk of not genotyping

Multidrug sensitivity is an autosomal recessive disorder in dogs caused by a 4-bp deletion in the ABCB1 gene, often referred to as the ABCB1-1Δ variant. This disease has a high prevalence in some breeds and causes adverse reactions to certain drugs when given in normal doses. Though most dogs known to be at risk are of the collie lineage or were traced back to it, the variant has also been described in several seemingly unrelated breeds. It is generally advised to genotype dogs at risk before treating them. However, there seems to be a discrepancy between the advice and current veterinary practices, as a recent study in Belgium and the Netherlands showed that most veterinarians never order a DNA test. To assess the possible risk of not testing for multidrug sensitivity in a clinical setting, the ABCB1-1Δ variant allele frequency was established in a sample of 286 dogs from a veterinary clinic. This frequency was compared to the allelic frequency in 599 samples specifically sent for genetic testing. While the allelic frequency in the sample for genetic testing was high (21.6%) and in line with the general reports, the allelic frequency in the clinical setting was low (0.2%), demonstrating an enormous difference between laboratory and clinical frequencies. Because of the low frequency of the disease-causing variant in the general clinical population, the risk of encountering a dog displaying multidrug sensitivity despite not genotyping seems to be low. As the variant was only found in an at-risk breed, the current recommendation of routinely genotyping at-risk breeds before treatment seems justified.

Tolerance and Pharmacokinetics of Galliprant™ Administered Orally to Collies Homozygous for MDR1-1Δ

The objectives of the study were to evaluate the pharmacokinetics and tolerance of grapiprant, a substrate of the human P-gp transporter, in collies homozygous for MDR1-1Δ when administered at the labeled dosage of 2 mg/kg once daily for 28 days. Twelve collie dogs with homozygous for MDR1-1Δ genotype from a commercial colony were used in the study, eight in the treated group and four as placebo-treated controls. The only treatment-related clinical sign was self-limiting vomiting (in 2/8 treated animals) and the only treatment-related clinical pathological changes seen were a slight decrease in serum albumin in one dog (2.6 g/dL; reference 2.7 to 3.9 g/dL) and total protein (5.1 g/dL; reference 5.5 to 7.7 g/dL). Absorption of grapiprant was rapid with a median T_max of 1 h, C_max of 5.2 μg/mL, AUC_0-24 of 17.3 ± 7.1 h^* μg/mL and median terminal t½ of 4.3 h after the first dose. To determine whether MDR1-1Δ animals handle grapiprant differently from normal dogs, a population pharmacokinetic analysis was performed utilizing data from the collies and historical beagle data. Volume of the peripheral compartment of collies was estimated to be 45% that of beagles, and clearance from the central compartment was 71% less in collies than in beagles. Self-liming vomiting events occurred at a numerically higher rate (2/8; 25%) in this group of P-gp-deficient dogs than seen in a clinical study (17%) composed of various dog breeds but limited numbers in this PK study make comparisons difficult. Grapiprant was otherwise well tolerated in collies homozygous for MDR1-1Δ despite increased drug exposure compared to dogs without this mutation.

Molecular pathology in the cancer clinic - where are we now and where are we headed?

Molecular pathology is a developing sub-microscopic discipline of pathology that studies the effects of molecular variations and mutations on disease processes. The ultimate goal of molecular pathology in cancer is to predict risk, facilitate diagnosis and improve prognostication based on a complete understanding of the biological impact of specific molecular variations, mutations and dysregulations. This knowledge will provide the basis for customised cancer treatment, so-called precision medicine. Rapid developments in genomics have placed this field at the forefront of clinical molecular pathology and there are already a number of well-established genetic tests available for clinical use including PCR of antigen receptor rearrangement and KIT mutational analysis. Moving beyond tests assessing a single gene, there are significant research efforts utilising genomics to predict cancer risk, forecast aggressive behaviour and identify druggable mutations and therapeutic biomarkers. Researchers are also investigating the use of circulating cells and nucleic acid for clinically useful low morbidity genomic assessments. If we are to realise the full potential of molecular pathology and precision medicine there are a number of challenges to overcome. These include developing our understanding of the underlying biology (in particular intra-tumoural heterogeneity), methodological standardisation of assays, provision of adequate infrastructure and production of novel therapeutics backed by high-quality clinical data supporting the precision medicine approach. The era of molecular pathology holds the potential to revolutionise veterinary cancer care, but its impact on clinical practice will depend upon the extent to which the inherent challenges can be overcome.

Neutropenia in dogs receiving vincristine for treatment of presumptive immune-mediated thrombocytopenia

BACKGROUND

Neutropenia is an adverse effect of vincristine when used in multidrug chemotherapy protocols.

OBJECTIVE

To determine the incidence of neutropenia, identify potential risk factors for neutropenia, and determine the effect of neutropenia on outcome, in dogs receiving vincristine for treatment of immune-mediated thrombocytopenia (ITP).

ANIMALS

One hundred twenty-seven client-owned dogs presumptively diagnosed with ITP.

METHODS

In this retrospective cohort study, medical records were reviewed to identify dogs presumptively diagnosed with ITP, and treated with vincristine, over a 15-year period. Logistic regression was used to identify risk factors for the development of neutropenia in dogs receiving vincristine. Time to platelet count ≥40 000 platelets/μL, survival, and duration of hospitalization were compared between neutropenic and non-neutropenic dogs.

RESULTS

Vincristine was administered to 127 dogs with presumptive ITP; 19 became neutropenic. Administration of cyclosporine was significantly (P < .001) associated with the development of neutropenia (odds ratio: 12.97, 95% confidence interval: 4.17, 40.35). There was no difference in median time to ≥40 000 platelets/μL between neutropenic dogs (4 days; range, 1-14 days) and non-neutropenic dogs (3 days; range, 0-48 days). Percentage survival to discharge was 95% in both groups, but median duration of hospitalization was significantly longer in neutropenic dogs (6 days; range, 3-22 days) compared to non-neutropenic dogs (4 days; range, 2-15 days).

CONCLUSIONS AND CLINICAL IMPORTANCE

Cyclosporine administration was associated with the development of neutropenia in dogs receiving vincristine, which might be related to effects on metabolism of vincristine. Neutrophil counts should be monitored in dogs receiving vincristine treatment for ITP, particularly if administered in conjunction with cyclosporine.

Biodynamic signatures from ex vivo bone marrow aspirates are associated with chemotherapy-induced neutropenia in cancer-bearing dogs

Background

Neutropenia is the most common dose‐limiting side effect of cytotoxic chemotherapy in cancer‐bearing dogs. Biodynamic imaging (BDI) is a functional imaging technology that measures dynamic light scattering from living, three‐dimensional tissues to characterize intracellular motion within those tissues. Previous studies have associated BDI biomarkers with tumour sensitivity to chemotherapy agents in dogs with naturally occurring cancer. We hypothesized that BDI, performed ex vivo on bone marrow aspirate samples, would identify dynamic biomarkers associated with the occurrence of specific degrees of neutropenia in tumour‐bearing dogs receiving doxorubicin chemotherapy.

Materials and Methods

Bone marrow aspirates were collected from 10 dogs with naturally occurring cancers prior to initiation of doxorubicin treatment. BDI was performed on bone marrow samples treated ex vivo with doxorubicin at 0.1, 1, 10 and 100 μM along with 0.1% DMSO as a control. Dogs then were treated with doxorubicin (30 mg/m², intravenously). Peripheral blood neutrophil counts were obtained on the day of treatment and again 7 days later. Receiver operating characteristic curves identified provisional breakpoints for BDI biomarkers that correlated with specific changes in neutrophil counts between the two time points.

Results

Provisional breakpoints for several BDI biomarkers were identified, specifying dogs with the largest proportionate change in neutrophils and with neutropenia that was grade 2 or higher following doxorubicin treatment.

Conclusions

Biodynamic imaging of bone marrow aspirates may identify those dogs at greater risk for neutropenia following doxorubicin chemotherapy. This approach may be useful for pre‐emptively modifying chemotherapy dosing in dogs to avoid unacceptable side effects.

Vincristine and ivermectin combination chemotherapy in dogs with natural transmissible venereal tumor of different cyto-morphological patterns: A prospective outcome evaluation

Vincristine is the first-line drug for the chemotherapy of canine transmissible venereal tumor (CTVT). Drug resistance is related to tumor cyto-morphological patterns of CTVT. There are anti-cancer properties of ivermectin, thus, a combination of ivermectin and vincristine could be an effective chemo-therapeutic treatment regimen for CTVT. Study aims, therefore, were to (1) assess the frequency of CTVT cyto-morphologies, and (2) evaluate treatment efficacy and possible adverse reactions to vincristine compared with a combination vincristine and ivermectin. Dogs (n = 41) with CTVT were characterized by tumor cyto-morphology and disease severity and of those, 20 were randomly allocated into two groups. There was a control group (G-Vin; n = 10) in which there was treatment with vincristine; and an experimental group (G-Iv/Vin; n = 10) in which there was treatment with the ivermectin/vincristine combination. Although dogs in the G-Iv/Vin group had more severe disease at the beginning of the study (P = 0.0031), the number of weeks and chemotherapy sessions until tumor remission were similar among dogs of the two groups, indicating both treatments were effective. There was a decrease in the leukocyte counts (P = 0.0020), related to neutropenia (P = 0.0371) in the G-Vin but not the G-Iv/Vin treatment group. There was no tumor resistance that developed during the study regardless of the treatment regimen used or tumor cytomorphology. In summary, the use of the vincristine/ivermectin combination was well tolerated and efficacious for CTVT treatment.

Drug dose and drug choice: Optimizing medical therapy for veterinary cancer

Although novel agents hold great promise for the treatment of animal neoplasia, there may be room for significant improvement in the use of currently available agents. These improvements include altered dosing schemes, novel combinations, and patient-specific dosing or selection of agents. Previous studies have identified surrogates for "individualized dose intensity,", for example, patient size, development of adverse effects, and pharmacokinetic parameters, as potential indicators of treatment efficacy in canine lymphoma, and strategies for patient-specific dose escalation are discussed. Strategies for treatment selection in individual patients include conventional histopathology, protein-based target assessment (eg, flow cytometry, immunohistochemistry, and mass spectrometry), and gene-based target assessment (gene expression profiling and targeted or global sequencing strategies). Currently available data in animal cancer evaluating these strategies are reviewed, as well as ongoing studies and suggestions for future directions.

Personalized medicine: going to the dogs?

Interindividual variation in drug response occurs in canine patients just as it does in human patients. Although canine pharmacogenetics still lags behind human pharmacogenetics, significant life-saving discoveries in the field have been made over the last 20 years, but much remains to be done. This article summarizes the available published data about the presence and impact of genetic polymorphisms on canine drug transporters, drug-metabolizing enzymes, drug receptors/targets, and plasma protein binding while comparing them to their human counterparts when applicable. In addition, precision medicine in cancer treatment as an application of canine pharmacogenetics and pertinent considerations for canine pharmacogenetics testing is reviewed. The field is poised to transition from single pharmacogene-based studies, pharmacogenetics, to pharmacogenomic-based studies to enhance our understanding of interindividual variation of drug response in dogs. Advances made in the field of canine pharmacogenetics will not only improve the health and well-being of dogs and dog breeds, but may provide insight into individual drug efficacy and toxicity in human patients as well.

Translating Pharmacogenetics and Pharmacogenomics to the Clinic: Progress in Human and Veterinary Medicine

As targeted personalized therapy becomes more widely used in human medicine, clients will expect the veterinary clinician to be able to implement an evidence-based strategy regarding both the prescribing of medicines and also recognition of the potential for adverse drug reactions (ADR) for their pet, at breed and individual level. This review aims to provide an overview of current developments and challenges in pharmacogenetics in medicine for a veterinary audience and to map these to developments in veterinary pharmacogenetics. Pharmacogenetics has been in development over the past 100 years but has been revolutionized following the publication of the human, and then veterinary species genomes. Genetic biomarkers called pharmacogenes have been identified as specific genetic loci on chromosomes which are associated with either positive or adverse drug responses. Pharmacogene variation may be classified according to the associated drug response, such as a change in (1) the pharmacokinetics; (2) the pharmacodynamics; (3) genes in the downstream pathway of the drug or (4) the effect of “off-target” genes resulting in a response that is unrelated to the intended target. There are many barriers to translation of pharmacogenetic information to the clinic, however, in human medicine, international initiatives are promising real change in the delivery of personalized medicine by 2025. We argue that for effective translation into the veterinary clinic, clinicians, international experts, and stakeholders must collaborate to ensure quality assurance and genetic test validation so that animals may also benefit from this genomics revolution.

Development of an oligonucleotide microarray for simultaneous detection of two canine MDR1 genotypes and association between genotypes and chemotherapy side effects

Canine MDR1 gene mutations produce translated P-glycoprotein, an active drug efflux transporter, resulting in dysfunction or over-expression. The 4-base deletion at exon 4 of MDR1 at nucleotide position 230 (nt230[del4]) in exon 4 makes P-glycoprotein lose function, leading to drug accumulation and toxicity. The G allele of the c.-6-180T>G variation in intron 1 of MDR1 (single nucleotide polymorphism [SNP] 180) causes P-glycoprotein over-expression, making epileptic dogs resistant to phenobarbital treatment. Both of these mutations are reported to be common in collies. This study develops a more efficient method to detect these two mutations simultaneously, and clarifies the genotype association with the side effects of chemotherapy. Genotype distribution in Taiwan was also investigated. An oligonucleotide microarray was successfully developed for the detection of both genotypes and was applied to clinical samples. No 4-base deletion mutant allele was detected in dogs in Taiwan. However, the G allele variation of SNP 180 was spread across all dog breeds, not only in collies. The chemotherapy adverse effect percentages of the SNP 180 T/T, T/G, and G/G genotypes were 16.7%, 6.3%, and 0%, respectively. This study describes an efficient way for MDR1 gene mutation detection, clarifying genotype distribution, and the association with chemotherapy.

Population variability in animal health: Influence on dose-exposure-response relationships: Part I: Drug metabolism and transporter systems

There is an increasing effort to understand the many sources of population variability that can influence drug absorption, metabolism, disposition, and clearance in veterinary species. This growing interest reflects the recognition that this diversity can influence dose-exposure-response relationships and can affect the drug residues present in the edible tissues of food-producing animals. To appreciate the pharmacokinetic diversity that may exist across a population of potential drug product recipients, both endogenous and exogenous variables need to be considered. The American Academy of Veterinary Pharmacology and Therapeutics hosted a 1-day session during the 2017 Biennial meeting to explore the sources of population variability recognized to impact veterinary medicine. The following review highlights the information shared during that session. In Part I of this workshop report, we consider sources of population variability associated with drug metabolism and membrane transport. Part II of this report highlights the use of modeling and simulation to support an appreciation of the variability in dose-exposure-response relationships.

Antibiotic prophylaxis in veterinary cancer chemotherapy: A review and recommendations

Bacterial infection following cancer chemotherapy-induced neutropenia is a serious cause of morbidity and mortality in human and veterinary patients. Antimicrobial prophylaxis is controversial in the human oncology field, as any decreased incidence in bacterial infections is countered by patient adverse effects and increased antimicrobial resistance. Comprehensive guidelines exist to aid human oncologists in prescribing antimicrobial prophylaxis but similar recommendations are not available in veterinary literature. As the veterinarian's role in antimicrobial stewardship is increasingly emphasized, it is vital that veterinary oncologists implement appropriate antimicrobial use. By considering the available human and veterinary literature we present an overview of current clinical practices and are able to suggest recommendations for prophylactic antimicrobial use in veterinary cancer chemotherapy patients.

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.

Species differences in tumour responses to cancer chemotherapy

Despite advances in chemotherapy, radiotherapy and targeted drug development, cancer remains a disease of high morbidity and mortality. The treatment of human cancer patients with chemotherapy has become commonplace and accepted over the past 100 years. In recent years, and with a similar incidence of cancer to people, the use of cancer chemotherapy drugs in veterinary patients such as the dog has also become accepted clinical practice. The poor predictability of tumour responses to cancer chemotherapy drugs in rodent models means that the standard drug development pathway is costly, both in terms of money and time, leading to many drugs failing in Phase I and II clinical trials. This has led to the suggestion that naturally occurring cancers in pet dogs may offer an alternative model system to inform rational drug development in human oncology. In this review, we will explore the species variation in tumour responses to conventional chemotherapy and highlight our understanding of the differences in pharmacodynamics, pharmacokinetics and pharmacogenomics between humans and dogs. Finally, we explore the potential hurdles that need to be overcome to gain the greatest value from comparative oncology studies.

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.

Determination of MDR1 gene expression for prediction of chemotherapy tolerance and treatment outcome in dogs with lymphoma

The multidrug resistance gene 1(MDR1) expression levels were analysed in 27 dogs with different types of malignant lymphomas receiving a standard chemotherapy protocol. Blood samples were used for MDR1 real-time PCR expression analysis. Treatment tolerance and outcome were evaluated on a regular basis by clinical examination and client questioning. Dogs developing severe adverse effects under treatment showed significantly lower basal MDR1 gene expression levels when compared with those who tolerated the drugs well. In the longitudinal MDR1 gene expression analysis during treatment, four dogs showed a greater than two-fold MDR1 up-regulation, compared to baseline expression. All four of these dogs, but none of the others, showed disease progression. In conclusion, basal and follow-up MDR1 gene expression levels could be of predictive value for the occurrence of severe adverse drug reactions and/or the development of MDR during chemotherapy for lymphoma in dogs.

Treatment of MDR1 mutant dogs with macrocyclic lactones

P-glycoprotein, encoded by the multidrug resistance gene MDR1, is an ATP-driven drug efflux pump which is highly expressed at the blood-brain barrier of vertebrates. Drug efflux of macrocyclic lactones by P-glycoprotein is highly relevant for the therapeutic safety of macrocyclic lactones, as thereby GABA-gated chloride channels, which are confined to the central nervous system in vertebrates, are protected from high drug concentrations that otherwise would induce neurological toxicity. A 4-bp deletion mutation exists in the MDR1 gene of many dog breeds such as the Collie and the Australian Shepherd, which results in the expression of a non-functional P-glycoprotein and is associated with multiple drug sensitivity. Accordingly, dogs with homozygous MDR1 mutation are in general prone to neurotoxicity by macrocyclic lactones due to their increased brain penetration. Nevertheless, treatment of these dogs with macrocyclic lactones does not inevitably result in neurological symptoms, since, the safety of treatment highly depends on the treatment indication, dosage, route of application, and the individual compound used as outlined in this review. Whereas all available macrocyclic lactones can safely be administered to MDR1 mutant dogs at doses usually used for heartworm prevention, these dogs will experience neurological toxicity following a high dose regimen which is common for mange treatment in dogs. Here, we review and discuss the neurotoxicological potential of different macrocyclic lactones as well as their treatment options in MDR1 mutant dogs.

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.

Pharmacokinetic modeling of doxorubicin pharmacokinetics in dogs deficient in ABCB1 drug transporters

BACKGROUND

The identification of dogs defective in ATP-binding cassette transporter B1 (ABCB1, MDR1) activity has prompted questions regarding pharmacokinetics (PK), efficacy and toxicity of ABCB1 substrates in these dogs.

HYPOTHESIS/OBJECTIVES

Dogs defective in ABCB1 activity (ABCB1(null)) have doxorubicin (DOX) PK different from that of normal dogs (ABCB1(wt)). Utilization of a physiologically based pharmacokinetic (PBPK) model allows computer simulation to study this polymorphism's impact on DOX PK.

ANIMALS

None.

METHODS

A virtual ABCB1(wt) dog population was generated and DOX distribution, elimination, and metabolism simulated by PBPK modeling. An in silico population of virtual dogs was generated by Monte Carlo simulation, with variability in physiologic and biochemical parameters consistent with the dog population. This population was used in the PBPK model. The ABCB1 components of the model were inactivated to generate an ABCB1(null) population and simulations repeated at multiple doses. Resulting DOX levels were used to generate PK parameters.

RESULTS

DOX exposures in the ABCB1(null) population were increased in all simulated tissues including serum (24%) and gut (174%). Estimated dosages in the ABCB1(null) population to approximate exposure in the ABCB1(wt) population at a dose of 30 mg/m(2) were 24.8 +/- 3.5 mg/m(2) for serum and 10.7 +/- 5.9 mg/m(2) for gut.

CONCLUSIONS AND CLINICAL IMPORTANCE

These results suggest that serum DOX concentrations are not indicative of tissue exposure, especially those with appreciable ABCB1 activity, and that gastrointestinal (GI) toxicosis would be dose limiting in ABCB1(null) populations. Dosage reductions necessary to prevent GI toxicosis likely result in subtherapeutic concentrations, thereby reducing DOXs efficacy in ABCB1(null) dogs.

Supporting the veterinary cancer patient on chemotherapy: neutropenia and gastrointestinal toxicity

The application of effective chemotherapy protocols in companion animals with sensitive tumors may result in return to a good quality of life and extended long-term survival. However, the potential for adding adverse events that impact overall quality of life in veterinary cancer patients is an important consideration. To ensure the experience is a positive one for our clients and patients alike, the practicing veterinary oncologist must ensure clients are appropriately educated as to the type and likelihood of adverse events that may occur in their companions and should plan for appropriate preventative and therapeutic protocols to manage them. Such attention to detail will help guarantee their clients' experience with medical treatment is positive and worthwhile, and will result in improvement in the well-being of their companion.

Detection of the nt230[del4] MDR1 mutation in dogs by a fluorogenic 5' nuclease TaqMan allelic discrimination method

For detection of the nt230[del4] MDR1 mutation, a 4-bp deletion in the canine MDR1 (ABCB1) gene, a TaqMan allelic discrimination assay was designed that allows for MDR1 genotyping without post-PCR processing. Directly after completion of the PCR amplification, the MDR1 genotype can be assigned based on selective fluorescence measurement. For primer selection the locus of a potential 265A>G single nucleotide polymorphism was omitted; this locus is covered by the oligonucleotide PCR primers from most of the hitherto established MDR1 genotyping methods. Dogs homozygous for the nt230[del4] MDR1 mutation show highly increased susceptibility to many drugs commonly used in veterinary medicine including ivermectin. As more than 10 dog breeds are predisposed to this mutation, reliable genotyping methods are necessary to identify affected dogs before drug treatment. This study provides a new allelic discrimination method that detects the MDR1 mutation with high specificity and reliability and is useful for routine diagnostics.