Mdr1a/b (multidrug resistance protein 1a/b), rat
Aliases: Mdr1a: Abcb4, Evi32, Mdr3, P-gp, Pgp, Pgy-3, Pgy3, mdr-3; Mdr1b: Abcb1, Mdr1, Mdr1b, Pgy-1, Pgy1, mdr
Gene names: ATP-binding family, sub-family B (MDR/TAP), member 1A/B (Abcb1a/b)
While humans have only one MDR1 gene to encode a single P-gp drug transporter, there are two genes in rodents, Mdr1a and Mdr1b, that encode P-gps with largely overlapping functions and extensive sequence identity to the single human gene . The 775-kb-long mMdr gene cluster is located on mouse chromosome 5 , while the rMdr cluster is found in the rat chromosomal region 4q11-12 .
All rodent Mdr1 transporters consist of 1272-1277 amino acids, have a molecular mass of 170 kDa , , ,  and are arranged in two homologous halves with six transmembrane domains each . Their protein structure is highly similar to human MDR1: mouse Mdr1a and Mdr1b share 87% and 80% amino acid homology, respectively, with human MDR1, while rat Mdr1 transporter amino acid sequences are 80% homologous to human , . The amino acid-level homology between mouse and rat Mdr1 transporters is as high as 93% .
Murine Mdr1 transporters show a similar tissue distribution to that seen in humans. mMdr1a is predominantly expressed in the intestine, lung, liver, as well as the blood-brain and the blood-testes barriers, while mMdr1b is highly expressed in the adrenal, placenta, ovary, and the pregnant uterus . Both genes are expressed at similar levels in the mouse kidney. These distribution data suggest that the two murine Mdr1 genes jointly accomplish the same function as the single MDR1 in humans.
Rat Mdr1 proteins display a slightly different tissue distribution compared to human or mouse. Expression of rMdr1a was primarily detected in the gastrointestinal tract and ovary, with lower levels in the brain, testis, kidney, lung and liver, whereas rMdr1b was found to reside mainly in the distal gastrointestinal tract, ovary, lung, and heart, with lower levels in the ileum, spleen, thymus, liver, kidney, and the embryo . In the brain, rMdr1a was found to be expressed on the luminal (apical) membrane of capillary endothelial cells, whereas rMdr1b was detected in the parenchyma .
There is a significant overlap in the substrate-interacting residues of the MDR1/Mdr1 ligand binding site across human, rat and mouse . Consistently, murine Mdr1 transporters have wide substrate specificities overlapping with human MDR1: they mainly accept amphiphilic and hydrophobic substrates such as anthracyclines, vinca alcaloids, epipodophyllotoxins and other chemoterapy drugs, various antibiotics, as well as endogenous compounds (e.g. steroids, cytokines, bilirubin) and natural products [15-21]. There are some exceptions, however, such as the antiepileptic drugs phenytoin and levetiracetam that were directionally transported by mMdr1a but not by hMDR1 . Bisphenol A is a substrate for rMdr1b but not for hMDR1 or rMdr1a . An in vitro directional transport study revealed that human, mouse and rat Mdr1 transporter liabilities for some compounds may be species-specific . Also, some studies have reported that mouse Mdr1a and Mdr1b have different propensities for the transport of progesterone , ramosetron , diltiazem, cyclosporin A, or dexamethasone . Overexpression of human MDR1 versus mouse Mdr1a or Mdr1b in CHO cells conferred different levels of drug resistance due to the distinct capacities of these transporters . Cutler et al. have shown that in order to obtain the same inhibition as in rats and mice, higher blood concentration of the P-gp inhibitor elacridar was needed in guinea pigs . It was also demonstrated that the brain penetration of [18F]altanserin and [11C]GR205171 was 4.5 and 8.6 times lower in rodents than in humans . These results indicate higher activity and/or expression of Mdr1 isoforms in the murine and rat BBB compared to humans. Differences between human, rodent and other species were also found with regard to the inhibitory effects of quinidine and verapamil on Mdr1-mediated daunorubicin, digoxin and cyclosporin A transport in Mdr1-transfected cell lines . These differences may result from divergent amino acid residues at the drug binding sites or distinct tissue-specific expression patterns between species.
For a long time, little was known of Mdr1 function in non-human species. The generation of Mdr1a-knockout mice marked a breakthrough in investigating the in vivo role of Mdr1 and its importance to drug transport . These mice were viable and fertile; however, Mdr1 substrates tended to accumulate in their brains and they were thus more susceptible than wild-type to central neurotoxicity.
Mdr1a(-/-) / Mdr1b(-/-) mice generated by Schinkel et al.  were also healthy as long as left unchallenged, but turned out to be 50-100 times more sensitive to the neurotoxic effects of the pesticide ivermectin, and the accumulation of this drug in their brains was 80-100-fold increased over wild-types. In the double knock-outs vinblastine was 3-fold more likely to cause sensitivity, and acute arsenic toxicity was more pronounced compared to wild-type mice . Furthermore, Mdr1 double-deficient mice showed increased oral drug absorption and impaired rate of drug elimination . Some drugs penetrating into the CNS, such as cyclosporine A, were lethal to the double knock-outs , . Hence, Mdr1a/1b(-/-) mice were utilized in a large number of studies as a model to explore the role of Mdr1 transporters in the in vivo disposition and toxicity of various organic chemicals including trabectedin  (which caused severe liver toxicity in Mdr1-deficient mice), digitoxin, vincristine, taxol, tributylmethyl ammonium or asimadoline . The intestinal and hepatobiliary clearance of cationic amphiphilic drugs was reduced in double knock-outs , . These animals showed increased bioavailability of paclitaxel , and elevated tissue levels of digoxin , dexamethasone , rifampicin , and morphine  in comparison with wild-type mice.
When Mdr1 transporters were down-regulated in normal mice, significantly increased triptolide exposure in the liver and plasma was strongly correlated with enhanced hepatotoxicity . Increases in xenobiotic penetration have also been observed when mouse Mdr1 transporters were inhibited with cyclosporine A , .
A novel MDR1-humanized mouse model was generated on the Mdr1a/b double knockout background by using a mouse artificial chromosome (MAC) vector. hMDR1-MAC mice are the first to express functional human MDR1 at the blood-brain barrier, and can be used estimate the human brain distribution of P-gp-substrate drugs in a nonhuman in vivo system .
Multiple rat Mdr1 knock-out strains developed by different laboratories are becoming employed in pharmacokinetic studies more often than knock-out mice, due to the practical advantages of using rats instead of mice, as well as the superior relevance of rats to drug development studies. Quantitatively consistent with murine models, distribution of some Mdr1 substrates to the brain was increased in both SAGE-type and Wistar-type Mdr1-deficient rats , . Despite some changes observed in the gene expression of SAGE knock-out rats, they offer a useful tool for the study of transporter-mediated pharmacokinetics . Recently, Mdr1a/b double-knockout rats were generated by the CRISPR/Cas9 system without any off-target effect, and significantly different digoxin absorption was observed in the KO animals .
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