Intestinal Transporters in drug

absorption

Omer mustapha Lab of physical and industrial pharmacyMS leading to phd Hanyang university erica korea

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Contents

1. Organic Anion Transporters (OAT)

2. Organic Cation Transporters (OCT)

3. Nucleoside Transporters

4. Monocarboxylate Transporters

5. Impact of intestinal transporters on bioavailability

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1. Organic Anion Transporters (OAT, SLCO, OATP)

• Ionic agents generally exhibit low passive membrane permeability, resulting in their poor bioavailability.

• An organic anion-transporter is a membrane transport protein or 'transporter' that transports organic anions across the cell membrane.

• These are present in the lipid bilayer of the cell membrane. OATs belong to the Solute Carrier Family (SLC), more specifically the Solute Carrier Organic Anion (SLCO) gene subfamily.

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• Organic anion transporters are classified into

1. Organic anion transporters (OATs)

2. Organic anion transporting polypeptides (OATPs)

3. Multiple drug resistance-associated proteins(MRPs)

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Organic anion transporters (OAT)

• Five structurally related isoforms i.e. OAT 1-OAT 5 have been identified.

• These are expressed in the kidney and have important functions in renal clearance.

• OAT2 is expressed at higher levels in liver as compared to kidney.

• OATs are also expressed to a lesser extent in brain, muscle, eye and placenta.

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• Topology characteristics include twelve 12 α-helix two transmembrane domains (TMD), one large hydrophilic extracellular loop between TMD 1 and TMD 2 carrying glycosylation sites and a large intracellular loop containing multiple phosphorylation sitesbetween TMD 6 and TMD 7.

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• OATs are polyspecific transporters that interact with various clinically significant organic anion drugs such as

• Non steroidal anti-inflammatory drugs (NSAIDs), β-lactam antibiotics, antiviral drugs, diuretics, antitumor drugs and angiotensin-converting enzyme inhibitors .

• The role of OAT family in the intestinal absorption of drugs seems negligible because they have not been identified in human intestine.

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Organic anion transporting polypeptides (OATPs)

• OATPs are the rapidly expanding family of mammalian transporters and transport a wide range of amphipathic endogenous and exogenous organic compounds.

• The topology include twelve TMD containing a large extracellular domain with multiple glycosylation sites.

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• OATP mediate transmembrane transport of wide range of organic compounds

1. organic anions, such as bromosulfophthalein (BSP), bile salts, bilirubin, prostaglandins, and estrogen-conjugates;

2. neural steroids and steroid conjugates; 3. lipophilic organic cations, e.g., rocuronium; and 4. organic dyes, thyroid hormones, and anionic

oligopeptides. • Various pharmaceutically relevant compounds such as

digoxin, pravastatin, methotrexate, temocaprilat, benzylpenicillin, fexofenadine,(D-Pen2, D-Pen5)-enkephalin (DPDPE), as well as some NSAIDs.

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• Mechanism driving OATP mediated transport are

1. Na+-independent manner

2. Anion exchange mechanism

3. Glutathione substrate transport

4. Proton-coupled transport mechanism

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• Expression of OATPs (OATP-C and OATP-8 occur at the basolateral membrane of hepatocytes

• While other OATPs like OATP-B, OATP-D and OATP-E are fairly expressed in tissues including blood-brain barrier, lung, heart, kidney, placenta and intestine.

• The regulation of OATPs expression and functional kinetics can occur at both transcriptional and post-transcriptional levels.

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2. Organic Cation Transporters (OCT, OCTN)

• An organic Cation transport protein mediates the transport of organic cations, zwitterionsand anions across the cell membrane. These proteins are members of the solute carrier family, subfamily 22.

• Topology characteristics are similar as OATs.

• They have two distinct subfamilies i.e. OCT and OCTN both of them having multiple species.

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• OCT1-3, are polyspecific transporters capable of transporting wide range of organic cations liketetraethylammonium (TEA) and N-methylquinine, 1-methyl-4-phenylpyridium (MPP+), antiviral, metformin monoamine neurotransmitters acetylcholine, dopamine, serotonin, histamine, choline etc.

• Physiological compounds such as creatinine, guanidine, and thiamine.

• Pharmaceuticals like desipramine, an antidepressantcimetidine, acyclovir, ganicyclovir and α Blocker Prazosin.

• Members of the OCTN subfamily have differential abilities to interact with a variety of organic cation drugs, as well as carnitine.

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• OCTs translocate organic cations and compounds in an electrogenic manner (i.e.producing a change in the electrical potential of a cell).

• Na+ ions and H+ independent.

• Driving force is provided by substrate concentration gradient and the membrane potential.

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• OCTs isoforms are mainly expressed in liver or kidney.

• To a lesser extent in heart, skeletal muscles, placenta, prostrate and small intestine.

• OSTNs were also detected in intetestinal enterocytes.

• The regulation of OCTs is associated with phosphorylation / dephosphorylation of the transporters.

• A gender dependent difference was also observed in a study in rats.

• Another study showed OCTs level gradually increase from infants to adults.

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Nucleoside Transporters (CNT, ENT)

• Nucleosides are the ribosylated precursors of purine and pyrimidine nucleotides, these not only help in cellular energy and signal transduction in the form of their phosphorylated analogs (e.g., ATP and cAMP, respectively), but also have other physiological functions.

• E.g. cardiac and vascular effects, adenosine act as a neuromodulator, inhibit lipolysis in fat cells and act as an anti-inflammatory.

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• The cellular transport of nucleosides is mediated by two distinct families the high affinity, concentrative nucleoside transporters(CNT; SLC28) and the low affinity, equilibrativenucleoside transporters (ENT; SLC29).

• These transporter have distinguished structural features and transport mechanisms.

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Concentrative Nucleoside Transporter (CNT)

• CNT consists of three subtypes CNT1-CNT3.

• These are comprised of 13 putative TMD and exhibits several protein kinase C phosphorylation sites.

• They have wide range of substrate specificities e.g. pyrimidine nucleotides, purine adenosine, uridine and purine nucleotides.

• Pharmaceutical compounds include nucleoside analogs e.g zidovudine, lamivudine, cytidinecladrabine etc.

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• CNT expression has been shown in small intestine, kideny, liver, heart, brain, placenta, pancreas, skeletal muscle, colon, rectum mammary gland, bone marrow, trachea prostrate and testis.

• CNT family is sodium dependent and works through an active symport mechanism.

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Equilibrative Nucleoside Transporters (ENT; SLC29)

• ENT consist of four subtypes ENT1- ENT4.

• They are characterized by 11 putative TMD with an intracellular amine terminus and extracellular carboxy terminus and exhibit glycosylation sites in extracellular loops.

• ENT transport is mediated via a bidirectional facilitated diffusion mechanism which is dependent upon substrate concentration gradient.

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• The substrate specificities are for endogenous nucleosides and therapeutic nucleoside analogs.

• Phamaceuticalcompounds like cladribine, gemicitabine, fludarabine, cytarabine, purine and pyrimidine analogs and transport nucleobase adenine.

• These transporters are widely expressed on the mRNA level. Distributed between the apical and basolateral membranes to mediate flux of nucloesides.

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Monocarboxylate Transporters (MCT)

• MCTs, constitute a family of proton-linked plasma membrane transporters that carry molecules having one carboxylate group (monocarboxylates).

• 14 members of this family are identified each having unique tissue distribution.

• MCT isoforms exhibit 12 putative TMD, with both the amine and carboxy termini located intracellularly.

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• MCTs have a high affinity for many endogenous and exogenous short chain anionic compounds e.g. lactic acid, pyruvate, acetoacetate, β-hydroxybutyrate, acetate, propionate and butyrate.

• MCT isoforms mainly expressed in gastrointestinal tract. Localized apically or basolaterally.

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4. Impact of Intestinal Transporters on Bioavailability

• The interplay of various transporter protein mechanisms along with transport by parallel pathways can significantly impact on the overall bioavailability of a compound.

• Drug transporters play an important role in intestinal drug absorption and secretion, and can be major determinants of oral bioavailability.

• Transporters exhibit affinity for an extraordinary range of compounds and provide great insight for advancing the field of rational drug design.

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• By understanding the substrate specificity, transport mechanism, and expression profile of transporters, efficient intestinal absorption may be made feasible by strategies including appropriately modifying either the structural recognition elements of NCEs or through rational formulation design to tailor optimized drug delivery.

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