Dans le cancer du sein non métastatique, l'approche néoadjuvante permet d'étudier les paramètres liés à la tumeur et/ou à l'hôte influençant l'efficacité des traitements. Les protéines de la famille MDR et plus particulièrement ABCB1 sont impliqués dans les mécanismes de résistance aux anthracyclines et taxanes. Les corrélations entre efficacité (réponse histologique), polymorphisme d'ABCB1 (patientes et tumeurs) et pharmacocinétique de la doxorubicine et du docétaxel ont été étudiées dans le cadre d'un essai thérapeutique. Dans cette étude, le polymorphisme de l'exon 26 d'ABCB1 (rs1045642) est le seul qui influence la pharmacocinétique du docétaxel et ceci uniquement chez les patientes non ménopausées. Les patientes porteuses du génotype CC (40%) ont une valeur moyenne de l'AUC du docétaxel statistiquement inférieure à celles porteuses des génotypes CT (45%) et TT (15%) (p<0.0001). Par ailleurs il a été constaté chez les patientes non ménopausées une corrélation statistiquement significative entre des taux d'AUC bas du docétaxel et le diplotype 2677GG-3435CC ainsi que l'haplotype 61AA-1236CC-2677GG-3435CC. Il n'a pas été trouvé de lien entre les différents polymorphismes d'ABCB1 et la pharmacocinétique de la doxorubicine. Il existe par ailleurs une relation négative entre l'AUC du docétaxel et l'obtention d'une réponse complète histologique. Il semble exister donc une valeur minimale d'AUC du docétaxel afin d'obtenir une réponse. Par ailleurs, nous avons retrouvé un lien entre réponse tumorale et le polymorphisme d'ABCB1 (génotype C3435T, TT vs CT et CC)

Multidrug resistance phosphoglycoprotein (MDR1/P-gp) and breast cancer resistance protein (BCRP) were first isolated in chemoresistant cancer cells and have since been found in a variety of normal tissue, including the placenta. The potential function of MDR1/P-gp and BCRP in the human placenta is to protect the fetus from maternally circulating endogenous steroids and hormones, therapeutic drugs and toxins. The objective of this study was to examine the role of maternal steroids in the regulation of MDR1/P-gp and BCRP in the human placenta. Trophoblast cells were isolated from term placenta tissues and immunohistochemistry, western blot analysis and transport studies were used to determine the effect of maternal steroids on MDR1/P-gp and BCRP regulation. Maternal steroids, present at high concentrations in maternal serum, did not have an effect on BCRP in human syncytiotrophoblast. Estrogen and progesterone did not alter MDR1/P-gp levels in human syncytiotrophoblast, but cortisol significantly decreased MDR1/P-gp levels.

Multidrug resistance phosphoglycoprotein (MDR1/P-gp) and breast cancer resistance protein (BCRP) were first isolated in chemoresistant cancer cells and have since been found in a variety of normal tissue, including the placenta. The potential function of MDR1/P-gp and BCRP in the human placenta is to protect the fetus from maternally circulating endogenous steroids and hormones, therapeutic drugs and toxins. The objective of this study was to examine the role of maternal steroids in the regulation of MDR1/P-gp and BCRP in the human placenta. Trophoblast cells were isolated from term placenta tissues and immunohistochemistry, western blot analysis and transport studies were used to determine the effect of maternal steroids on MDR1/P-gp and BCRP regulation. Maternal steroids, present at high concentrations in maternal serum, did not have an effect on BCRP in human syncytiotrophoblast. Estrogen and progesterone did not alter MDR1/P-gp levels in human syncytiotrophoblast, but cortisol significantly decreased MDR1/P-gp levels.

Multidrug resistance phosphoglycoprotein (MDR1/P-gp) and breast cancer resistance protein (BCRP) were first isolated in chemoresistant cancer cells and have since been found in a variety of normal tissue, including the placenta. The potential function of MDR1/P-gp and BCRP in the human placenta is to protect the fetus from maternally circulating endogenous steroids and hormones, therapeutic drugs and toxins. The objective of this study was to examine the role of maternal steroids in the regulation of MDR1/P-gp and BCRP in the human placenta. Trophoblast cells were isolated from term placenta tissues and immunohistochemistry, western blot analysis and transport studies were used to determine the effect of maternal steroids on MDR1/P-gp and BCRP regulation. Maternal steroids, present at high concentrations in maternal serum, did not have an effect on BCRP in human syncytiotrophoblast. Estrogen and progesterone did not alter MDR1/P-gp levels in human syncytiotrophoblast, but cortisol significantly decreased MDR1/P-gp levels.

ABSTRACT MULTIDRUG RESISTANCE IN LOCALLY ADVANCED BREAST CANCER ATALAY, Mustafa Can Ph. D., Department of Biotechnology Supervisor: Prof. Dr. Ufuk G&Uuml / ND&Uuml / Z June 2004, 70 pages Breast cancer is the most frequently detected cancer among women. Early diagnosis leads to long term survival when the patients are treated with surgery, radiotherapy, chemotherapy, and hormone therapy. Unfortunately, advanced disease could still be encountered in some patients resulting in a poorer prognosis. The primary treatment modality is chemotherapy for this group of patients. Drug resistance is a serious problem resulting in the use of different drugs during chemotherapy and knowing the possibility of resistance before initiating first line chemotherapy may save time and money, and most importantly, may increase patient&rsquo / s survival. Therefore in this study, multidrug resistance is studied in locally advanced breast cancer patients. The breast tissues obtained from 25 patients both before and after chemotherapy were examined for drug resistance. Reverse transcriptase polymerase chain reaction was used for the detection of mdr1 and mrp1 gene expression. In addition, immunohistochemistry technique was used for P-glycoprotein and MRP1 detection. JSB-1 and QCRL-1 monoclonal antibodies were utilized to detect P-glycoprotein and MRP1, respectively. Five patients were unresponsive to chemotherapy. In four of these patients mdr1 gene expression was induced by chemotherapy where as the fifth patient initially had mdr1 gene expression. In addition, Pgp positivity was detected in 9 patients after chemotherapy. Both the induction of mdr1 gene expression (p&lt / 0.001) and Pgp positivity (p&lt / 0.001) during chemotherapy were significantly related with clinical response. On the other hand, mrp1 gene expression and MRP1 positivity were detected in 68% of the patients before the therapy. After chemotherapy, mrp1 expression increased to 84%. Although 80% of the clinically unresponsive patients had mrp1 gene expression, the relation between mrp1 expression and clinical drug response was not strong. Thus, it can be concluded that in locally advanced breast cancer mdr1 gene expression during chemotherapy contributed to clinical unresponsiveness. However, mrp1 gene expression did not correlate strongly with the clinical response. When RT-PCR and immunohistochemistry methods are compared in terms of detection of drug resistance, it seems that both methods gave similar and reliable results.

O estudo da expressão gênica é de grande aplicabilidade nas áreas de pesquisa científica e clínica. Por ser um método pouco invasivo e permitir coletas seriadas, a utilização de amostras sanguíneas facilita a análise da transcrição gênica. O maior desafio para a precisão nos ensaios moleculares é manter uma amostra com qualidade desde a coleta, armazenamento e transporte até o momento de sua análise. Este estudo utilizou um sistema de conservação de RNA sanguíneo que manteve amostras de qualidade após a coleta, transporte e armazenamento, permitindo extração de RNA e identificação da expressão do gene MDR1 em cavalos da raça Crioulo. Foram coletadas amostras sanguíneas de 27 cavalos a campo em tubos PAXgene® Blood RNA, que após o transporte e armazenamento a -20°C por 90 dias foram processadas em laboratório. A extração do RNA sanguíneo foi realizada com o kit Nucleo Spin® RNA II, a conversão em cDNA foi com o kit High-Capacity cDNA Reverse Transcription, utilizando a fluorimetria para as avaliações. A identificação da expressão do gene MDR1 em sangue conservado utilizou primer específico para cDNA e PCR em tempo real. Houve extração de RNA em todas as amostras coletadas, sendo que as leituras das concentrações de RNA das amostras com DNA contaminante não tiveram diferença estatística das amostras sem DNA contaminante. Ocorreu amplificação do gene MDR1 a partir do RNA das amostras, independente de contaminação de DNA. A coleta de sangue venoso dos cavalos a campo com os tubos PAXgene foi realizada sem complicações. A amplificação do mRNA com primer específico para transcrito do gene MDR1, de amostras submetidas aos métodos de coleta, armazenamento e processamento executados neste trabalho, confirma que o mRNA extraído, com a metodologia descrita, é viável, sendo sua expressão identificada em leucócitos sanguíneos de equinos da raça Crioulo. / Efficient nucleic acid extraction methods are paramount for gene expression studies and applications in the scientific and clinical research. Whole blood samples provide material for gene transcription analysis allowing serial trials with a not much invasive procedure. Still, a major challenge for molecular assays accuracy and reliability is to maintain RNA stability during sample collection and storage. A whole blood collection system for RNA preservation was used during collection, transport and storage of samples intended for RNA extraction and identification of the MDR1 gene in Crioulo horses. The blood samples were obtained from 27 horses in the field using the PAXgene® Blood RNA tubes. After 2h transport at room temperature, the samples were stored at -20oC for 90 days before processing. RNA extraction was performed with the Nucleo Spin® RNA II kit and cDNA conversion carried out with the High-Capacity cDNA Reverse Transcription kit. RNA concentration was determined by fluorometry. MDR1 gene expression was assessed using real time polymerase chain reaction (PCR) with a specific primer for cDNA. RNA was successfully extracted from all samples. Total RNA yield from DNA contaminated samples was not statistically different from those without DNA contamination. MDR1 gene amplification occurred regardless of DNA contamination. There were no complications during horse handling and blood sampling direct into PAXgene tubes in the field. Successful amplification of MDR1 gene transcript with a specific primer showed that blood samples collected, stored and processed under the described methodology provided viable mRNA for down-stream applications of molecular analyses. This study allowed the identification of the gene MDR1 in blood leucocytes of Crioulo horses.

This thesis is focused on polymorphism of MDR1 gene and related drug resistance. Resistance is caused by deletion of four nucleotids, that resulting in a frame shift and synthesis of nonfunctional transport of P-glycoprotein. The text describes a polymorphism of MDR1 (ABCB1) gene, which results in reduced resistance to drugs belonging to the group of macrocyclic lactones. It also describes inheritance of this phenomenon and it deals with the detection of mutation using PCR (polymerase chain reaction) and by fragmentation analyses. A review of literature study is a form of research solely from scientific publications. 128 dogs were included into the own analysis. The results confirmed that Collies had the highest presence of deletions (29,73 %) with a high number of carriers in the study population of dogs (54,05 %). The percentage of affected individuals in the breed of Australian Shepherd and Sheltie was significantly lower (7,32 % and 6 %), but the percentage of carriers were also high in both Australian Shepherds (34,14 %) and the breed Sheltie (48 %).

A P-glicoproteína (P-gp) é uma transportadora transmembrana de múltiplos fármacos, produto do gene MDR1 (ABCB1). A P-gp contribui para a função de barreira de vários tecidos e órgãos, funcionando como uma bomba de efluxo para muitos substratos. Diminuição na expressão desta proteína é associada à sensibilidade a fármacos. Cães da linhagem dos Collies possuem uma alta incidência de uma mutação no gene MDR1, denominada nt230(del4). Animais homozigotos para a mutação apresentam a supressão total de uma P-gp funcional e um animal heterozigoto apresenta uma maior sensibilidade para substratos da P-gp, devido a uma diminuição na expressão da mesma. Alguns fármacos opioides, como a morfina e a metadona, foram identificados como substratos da P-gp. O tramadol é um dos analgésicos opioides mais utilizados em cães. No presente trabalho, a mutação MDR1 nt230(del4) foi analisada em vinte cães Collie, utilizando reação em cadeia de polimerase (PCR). A identificação foi realizada por eletroforese em gel de poliacrilamida de alta resolução, sendo o resultado confirmado por análise de sequênciamento de DNA. Como resultado, seis cães apresentaram normalidade nos dois alelos e 14 apresentaram heterozigose para a mutação. Estes animais foram submetidos à segunda fase do experimento, quando se administrou uma dose única de 100 mg de tramadol oral de liberação prolongada (SR), objetivando investigar o tramadol como sendo substrato da P-gp. Outro objetivo foi avaliar a farmacocinética deste tipo de formulação, pois ainda não foi estabelecida para cães. A análise farmacocinética do tramadol foi realizada utilizando cromatografia líquida de alta eficiência (CLAE) com detecção por espectrometria de massas para a determinação e quantificação de tramadol no soro canino. O analito e o padrão interno foram extraídos do soro por método líquido-líquido. A separação cromatográfica foi obtida a partir de uma coluna analítica C18, mantida a 30°C, sob condições isocráticas de uma fase móvel constituída por uma mistura de acetonitrila e ácido fórmico a 0,1% (80:20). A concentração de tramadol no soro foi maior do que o limite de quantificação (LOQ), em 17 cães. Os cães foram divididos em dois grupos, cães normais (MDR1 +/+) e heterozigotos (MDR1 +/-). Os cálculos farmacocinéticos para o tramadol oral SR obtiveram valores médios de concentração máxima no soro (Cmax) de 63,12 ng/mL ± 33,35 para o grupo normal e 58,00 ng/mL ± 27,29 para o grupo heterozigoto. Tmax (tempo de concentração plasmática maxima) foi de 4 h para ambos os grupos e t ½ (meia-vida) foram 2,85 h ± 1,61 e 2,81 ± 1,46 h para os cães normais e heterozigotos, respectivamente. A área sob a curva (AUC) média para o tramadol oral SR para o grupo normal e heterozigoto foram 350,20 ± 216,61 e 312,15 ± 155,43 ng.h/mL, respectivamente. A biodisponibilidade foi de 22% e 23% para os cães normais e heterozigotos, respectivamente. Não houve diferença estatística entre os grupos em todos os parâmetros farmacocinéticos. Os resultados sugerem que o tramadol não é um substrato da Pgp. A quantidade de dados farmacocinéticos do tramadol oral na formulação de liberação prolongada (SR) em cães é escassa, sendo necessários mais estudos farmacocinéticos e farmacodinâmicos para o tramadol oral de liberação prolongada em cães para estabelecer adequada dose e frequência de administração em cães. / The P-glycoprotein (P-gp) is a transmembrane multidrug transporter, product of the MDR1 (ABCB1) gene. P-gp contributes to the barrier function of several tissues and organs, acting as an efflux pump for many substrates. Decreased expression of this protein is associated with sensitivity to drugs. Collie dogs have a high incidence of a mutation in MDR1 gene, denominated MDR1 nt230 (del4). In homozygosis, this mutation results in the total absence of a functional P-gp and a heterozygote animal presents a greater sensibility to P-gp substrates, probably due to a decrease in the expression thereof. Some opioid drugs such as morphine and methadone were identified as P-gp substrates. Tramadol is one of the most commonly opioid used in dogs. In the present work MDR1nt230 (del4) mutation was analyzed in 20 healthy Collie dogs using allele-specific polymerase chain reaction (PCR) method. Thereby, 6 homozygous intact and 14 heterozygous mutated MDR1 genotypes can be differentiated by high resolution polyacrylamide gel electrophoresis, confirmed by DNA sequence analysis. These animals underwent the second phase of the experiment, when a single oral administration of 100 mg of sustained release (SR) tramadol was administrated to investigate the tramadol as P-gp substrate. In addition, another aim was evaluate the pharmacokinetics of sustained release formulation, which has not been established for dogs. Pharmacokinetic analysis of tramadol was evaluated using high performance liquid chromatography (HPLC) with tandem mass spectrometry for determination and quantification of tramadol in canine serum. The analyte and internal standard (IS) were extracted from serum using liquid-liquid method. Chromatographic separation was achieved on a C18 analytical column, kept at 30°C, under isocratic conditions of a mobile phase consisted by a mixture of acetonitrile and water contained 0,1% formic acid (80:20). Serum tramadol concentration was greater than the limit of quantification (LOQ) in 17 dogs. The dogs were divided into two groups, normal dogs (MDR1 +/+) and heterozygous (MDR1 +/-) according to the MDR1 genotype. The median values of maximum serum concentration (Cmax) were 63.13 ng/mL ± 33.35 for the normal group and 58.01 ng/mL ± 27.29 for the heterozygous group. Tmax (time to maximum serum concentration) was 4 h for both groups and t ½ (half-life) were 2,85h ± 1,61 e 2,81h ± 1,46 for normal and heterozygous dogs, respectively. The mean area-under-the-curve (AUC) values for the sustained release tramadol compounds for the normal and heterozygous group were 350,20 ±216,61 and 312,15 ± 155,43 ng.h/mL, respectively. The bioavailability was 22% and 23% for normal and heterozygous dogs respectively. There was no statistic difference between groups in all pharmacokinetics parameters. The findings suggest that tramadol is not a P-gp substrate. The amount of pharmacokinetics data of SR formulation of tramadol in dogs is sparse. Therefore, more studies of oral SR tramadol in dogs are needed to establish appropriate dose and frequency of administration in dogs.

O estudo da expressão gênica é de grande aplicabilidade nas áreas de pesquisa científica e clínica. Por ser um método pouco invasivo e permitir coletas seriadas, a utilização de amostras sanguíneas facilita a análise da transcrição gênica. O maior desafio para a precisão nos ensaios moleculares é manter uma amostra com qualidade desde a coleta, armazenamento e transporte até o momento de sua análise. Este estudo utilizou um sistema de conservação de RNA sanguíneo que manteve amostras de qualidade após a coleta, transporte e armazenamento, permitindo extração de RNA e identificação da expressão do gene MDR1 em cavalos da raça Crioulo. Foram coletadas amostras sanguíneas de 27 cavalos a campo em tubos PAXgene® Blood RNA, que após o transporte e armazenamento a -20°C por 90 dias foram processadas em laboratório. A extração do RNA sanguíneo foi realizada com o kit Nucleo Spin® RNA II, a conversão em cDNA foi com o kit High-Capacity cDNA Reverse Transcription, utilizando a fluorimetria para as avaliações. A identificação da expressão do gene MDR1 em sangue conservado utilizou primer específico para cDNA e PCR em tempo real. Houve extração de RNA em todas as amostras coletadas, sendo que as leituras das concentrações de RNA das amostras com DNA contaminante não tiveram diferença estatística das amostras sem DNA contaminante. Ocorreu amplificação do gene MDR1 a partir do RNA das amostras, independente de contaminação de DNA. A coleta de sangue venoso dos cavalos a campo com os tubos PAXgene foi realizada sem complicações. A amplificação do mRNA com primer específico para transcrito do gene MDR1, de amostras submetidas aos métodos de coleta, armazenamento e processamento executados neste trabalho, confirma que o mRNA extraído, com a metodologia descrita, é viável, sendo sua expressão identificada em leucócitos sanguíneos de equinos da raça Crioulo. / Efficient nucleic acid extraction methods are paramount for gene expression studies and applications in the scientific and clinical research. Whole blood samples provide material for gene transcription analysis allowing serial trials with a not much invasive procedure. Still, a major challenge for molecular assays accuracy and reliability is to maintain RNA stability during sample collection and storage. A whole blood collection system for RNA preservation was used during collection, transport and storage of samples intended for RNA extraction and identification of the MDR1 gene in Crioulo horses. The blood samples were obtained from 27 horses in the field using the PAXgene® Blood RNA tubes. After 2h transport at room temperature, the samples were stored at -20oC for 90 days before processing. RNA extraction was performed with the Nucleo Spin® RNA II kit and cDNA conversion carried out with the High-Capacity cDNA Reverse Transcription kit. RNA concentration was determined by fluorometry. MDR1 gene expression was assessed using real time polymerase chain reaction (PCR) with a specific primer for cDNA. RNA was successfully extracted from all samples. Total RNA yield from DNA contaminated samples was not statistically different from those without DNA contamination. MDR1 gene amplification occurred regardless of DNA contamination. There were no complications during horse handling and blood sampling direct into PAXgene tubes in the field. Successful amplification of MDR1 gene transcript with a specific primer showed that blood samples collected, stored and processed under the described methodology provided viable mRNA for down-stream applications of molecular analyses. This study allowed the identification of the gene MDR1 in blood leucocytes of Crioulo horses.

A P-glicoproteína (P-gp) é uma transportadora transmembrana de múltiplos fármacos, produto do gene MDR1 (ABCB1). A P-gp contribui para a função de barreira de vários tecidos e órgãos, funcionando como uma bomba de efluxo para muitos substratos. Diminuição na expressão desta proteína é associada à sensibilidade a fármacos. Cães da linhagem dos Collies possuem uma alta incidência de uma mutação no gene MDR1, denominada nt230(del4). Animais homozigotos para a mutação apresentam a supressão total de uma P-gp funcional e um animal heterozigoto apresenta uma maior sensibilidade para substratos da P-gp, devido a uma diminuição na expressão da mesma. Alguns fármacos opioides, como a morfina e a metadona, foram identificados como substratos da P-gp. O tramadol é um dos analgésicos opioides mais utilizados em cães. No presente trabalho, a mutação MDR1 nt230(del4) foi analisada em vinte cães Collie, utilizando reação em cadeia de polimerase (PCR). A identificação foi realizada por eletroforese em gel de poliacrilamida de alta resolução, sendo o resultado confirmado por análise de sequênciamento de DNA. Como resultado, seis cães apresentaram normalidade nos dois alelos e 14 apresentaram heterozigose para a mutação. Estes animais foram submetidos à segunda fase do experimento, quando se administrou uma dose única de 100 mg de tramadol oral de liberação prolongada (SR), objetivando investigar o tramadol como sendo substrato da P-gp. Outro objetivo foi avaliar a farmacocinética deste tipo de formulação, pois ainda não foi estabelecida para cães. A análise farmacocinética do tramadol foi realizada utilizando cromatografia líquida de alta eficiência (CLAE) com detecção por espectrometria de massas para a determinação e quantificação de tramadol no soro canino. O analito e o padrão interno foram extraídos do soro por método líquido-líquido. A separação cromatográfica foi obtida a partir de uma coluna analítica C18, mantida a 30°C, sob condições isocráticas de uma fase móvel constituída por uma mistura de acetonitrila e ácido fórmico a 0,1% (80:20). A concentração de tramadol no soro foi maior do que o limite de quantificação (LOQ), em 17 cães. Os cães foram divididos em dois grupos, cães normais (MDR1 +/+) e heterozigotos (MDR1 +/-). Os cálculos farmacocinéticos para o tramadol oral SR obtiveram valores médios de concentração máxima no soro (Cmax) de 63,12 ng/mL ± 33,35 para o grupo normal e 58,00 ng/mL ± 27,29 para o grupo heterozigoto. Tmax (tempo de concentração plasmática maxima) foi de 4 h para ambos os grupos e t ½ (meia-vida) foram 2,85 h ± 1,61 e 2,81 ± 1,46 h para os cães normais e heterozigotos, respectivamente. A área sob a curva (AUC) média para o tramadol oral SR para o grupo normal e heterozigoto foram 350,20 ± 216,61 e 312,15 ± 155,43 ng.h/mL, respectivamente. A biodisponibilidade foi de 22% e 23% para os cães normais e heterozigotos, respectivamente. Não houve diferença estatística entre os grupos em todos os parâmetros farmacocinéticos. Os resultados sugerem que o tramadol não é um substrato da Pgp. A quantidade de dados farmacocinéticos do tramadol oral na formulação de liberação prolongada (SR) em cães é escassa, sendo necessários mais estudos farmacocinéticos e farmacodinâmicos para o tramadol oral de liberação prolongada em cães para estabelecer adequada dose e frequência de administração em cães. / The P-glycoprotein (P-gp) is a transmembrane multidrug transporter, product of the MDR1 (ABCB1) gene. P-gp contributes to the barrier function of several tissues and organs, acting as an efflux pump for many substrates. Decreased expression of this protein is associated with sensitivity to drugs. Collie dogs have a high incidence of a mutation in MDR1 gene, denominated MDR1 nt230 (del4). In homozygosis, this mutation results in the total absence of a functional P-gp and a heterozygote animal presents a greater sensibility to P-gp substrates, probably due to a decrease in the expression thereof. Some opioid drugs such as morphine and methadone were identified as P-gp substrates. Tramadol is one of the most commonly opioid used in dogs. In the present work MDR1nt230 (del4) mutation was analyzed in 20 healthy Collie dogs using allele-specific polymerase chain reaction (PCR) method. Thereby, 6 homozygous intact and 14 heterozygous mutated MDR1 genotypes can be differentiated by high resolution polyacrylamide gel electrophoresis, confirmed by DNA sequence analysis. These animals underwent the second phase of the experiment, when a single oral administration of 100 mg of sustained release (SR) tramadol was administrated to investigate the tramadol as P-gp substrate. In addition, another aim was evaluate the pharmacokinetics of sustained release formulation, which has not been established for dogs. Pharmacokinetic analysis of tramadol was evaluated using high performance liquid chromatography (HPLC) with tandem mass spectrometry for determination and quantification of tramadol in canine serum. The analyte and internal standard (IS) were extracted from serum using liquid-liquid method. Chromatographic separation was achieved on a C18 analytical column, kept at 30°C, under isocratic conditions of a mobile phase consisted by a mixture of acetonitrile and water contained 0,1% formic acid (80:20). Serum tramadol concentration was greater than the limit of quantification (LOQ) in 17 dogs. The dogs were divided into two groups, normal dogs (MDR1 +/+) and heterozygous (MDR1 +/-) according to the MDR1 genotype. The median values of maximum serum concentration (Cmax) were 63.13 ng/mL ± 33.35 for the normal group and 58.01 ng/mL ± 27.29 for the heterozygous group. Tmax (time to maximum serum concentration) was 4 h for both groups and t ½ (half-life) were 2,85h ± 1,61 e 2,81h ± 1,46 for normal and heterozygous dogs, respectively. The mean area-under-the-curve (AUC) values for the sustained release tramadol compounds for the normal and heterozygous group were 350,20 ±216,61 and 312,15 ± 155,43 ng.h/mL, respectively. The bioavailability was 22% and 23% for normal and heterozygous dogs respectively. There was no statistic difference between groups in all pharmacokinetics parameters. The findings suggest that tramadol is not a P-gp substrate. The amount of pharmacokinetics data of SR formulation of tramadol in dogs is sparse. Therefore, more studies of oral SR tramadol in dogs are needed to establish appropriate dose and frequency of administration in dogs.