L20 The role of transporter proteins in xenobiotic disposition

how are drug transporters useful in absorption

• role in the intestine in facilitating entry of some drugs into enterocytes

• also preventing access by some xenobiotics

how are drug transporters useful in distribution

role in allowing entry into target organs for activity but also liver for metabolism

how are drug transporters useful in excretion

roles in both renal and biliary excretion

ABC proteins (ATP dependent) types

• MRP

• MDR

• BSEP

• BCRP

solute carrier (SLC) S2 different gene families exist

• code for membrane proteins but these proteins have functions including roles in normal physiology

what do facilitative transporters allow

substrates to flow downhill with their electrochemical gradients

what do secondary active transporters allow

substrates to flow uphill against their electrochemical gradient by coupling of transport to that of a co-substrate

facilitative and secondary active transporters are not

ATP-dependent

SLC proteins have 12 membrane spanning domains and are often present on the membrane of

polarised cells with specific location (apical or basolateral side)

SLC familly genes

• OAT anion transporter

• OATP anion transporter

• OCT cation transporter

• MATE cation transporter

• PEPT mostly peptides but also penicillins

OATP stands for

organic anion-transporting polypeptide

what are OATP proteins encoded by

the SLCO1 to SLCO6 genes

families 1 and 2 of OATP transporters are the most important in relation to

drug disposition

• SLCO1 and SLCO2 (OATP1 and OATP2)

OATP proteins are important in drug transport across sinusoidal membranes in the liver

OATP1B1, OATP1B3, OATP2B1 are most important in the liver

kidney also has OATPs present but they have a more limited role in

renal excretion

drug substrates are normally anionic with high molecular weight (>450)

generally protein bound in plasma

examples of OATP1B1 substrates are

• statins

• rifampicin

• benzylpenicillin

• ACE inhibitors

• some anti cancer drugs

OAT proteins are members of the

SLC22A subfamily

• key role in renal excretion but also expressed elsewhere e.g. liver and small intestine

OAT1, OAT2, OAT3 found on basolateral membrane of

proximal tubule cells, facing blood vessels

OAT4 is on the apical membrane of the PCT facing…

urine

• can sometimes transport in both directions

secretion of anions/drugs

OA is a drug substrate

A is OAT1, 2, 3

B is a transporter for dicarboxylic acid

C is Na/K transporter

E is OAT4

OAT1-3 are coupled to

dicarboxylic acid (energy source)

• e.g. alpha-ketoglutarate - sodium dependent

OAT4 transport reabsorbs drugs from

urine and may be bidirectional

OAT 1 to 4 substrates are likely to be

sulfate or glucuronide conjugates

drugs the dont undergo metabolism may also be substrates for

OAT1-4

• e.g. penicillins

give an example of an OAT1 substrate

tetracycline

give an example of an OAT2 substrate

• AZT (HIV)

• diclofenac

• diclofenac glucuronide

give an example of an OAT3 substrate

• oestrone sulfate

• benzylpenicillin

• rosuvastatin

give an example of an OAT4 substrate

• oestrone sulfate

• diclofenac glucuronide

OCT

organic cation transporters

• members of SLC22A

OCT1

• important transporter of sinusoidal face of liver

• substrates include metformin and cisplatin

OCT2

• high levels on basolateral membrane of kidney

• blood side of PCT

• substrates metformin, cisplatin and cimetidine

cationic drug and chemical transport

PEPT transporters PEPT1 and 2 of the POT family are

SLC15A

• secondary active transporters

where is PEPT1 expressed

intestine and kidney

where is PEPT2 expressed

kidney

PEPT1 and PEPT2 express similar substrate specificities with…

penicillins, ACE inhibitors and valcyclovir

• proton dependent transporters -in kidney contribute to drug reabsorption from urine to PCT

MATE transporters

multidrug and toxic extrusion family

• identified in bacteria -highly conserved

• export pumps contributing to renal and biliary excretion of cationic drugs

where is MATE1 expressed

liver and kidney

where is MATE2-K expressed

kidney only

MCT1

monocarboxylate transporter

ASBT

NA-dependent bile salt transporter

transporter proteins role in liver

xenobiotics need to be transported into hepatocyte across sinusoidal membrane for both metabolism and to reach targets

• e.g. HMGCoA reductase for statins

compounds need to enter enterocytes through the brush border membrane and then cross the…

basolateral membrane into the hepatic portal vein

following metabolism, compounds usually need to be transported out either across canalicular membrane (to bile) or …

sinusoidal membrane (back to blood for renal excretion)

describe the basolateral membrane of the kidney tubule

• compounds excreted by tubular secretion need to enter

• those being reabsorbed need to return to circulation

describe the brush border membrane of the kidney tubule

• compounds need to cross this membrane to enter the lumen for final renal excretion

• reabsorption may occur

urate transporter transports urate from the…

lumen, back into the proximal tubule cell

what does diclofenac undergo metabolism by

CYP2C9 and UGT2B7

UGT2B7 produces …

acylglucuronide (DF-AG)

• only a proportion of diclofenac is metabolised/conjugated

OAT2 and OAT4 is important in renal excretion of…

diclofenac acylglucuronide

diclofenac acylglucuronide can also be excreted via

bile

give an example of inhibition of transport through Organic Anion Transporters

penicillin and probenecid

• valuable as penicillin is cleared more slowly

give an example of inhibition of transport through ABCB1

digoxin excretion can be affected by a range of drugs e.g. erythromycin, statins

• problematic as high levels are toxic

• digoxin is not extensively metabolised -only cleared by kidney

describe induction via PXR

• ABCB1, ABCC2, SLCO1B1

• potential for drug-drug interactions when rifampicin is prescribed due to induction

• results in a rapid clearance of other drugs e.g. cyclosporin, digoxin

• st johns wort has a similar effect

OATP1B1 genetic polymorphism is associated with…

higher plasma levels of some statins due to impaired ability to enter hepatocyte

• higher plasma level leads to toxicity in muscle cells → potentially fatal rhabdomyolysis where muscle protein dissolves

OCT2 is linked to nephrotoxicity with

cisplatin

OCT2 may also involve…

MATE2

OCT2 drug accumulates in…

tubule cells

cisplatin is very nephrotoxic because…

it is efficiently taken up into the PCT

• but rate of secretion is lower

• cimetidine is co-administered to inhibit uptake of PCT and prevent cimetidine accumulation