This document discusses the roles of membrane transporters in pharmacokinetics. It begins with an introduction to transporters, noting that they are membrane proteins that control the uptake and efflux of nutrients, ions, drugs and waste. Approximately 2000 human genes code for transporters. The document then covers principal sites of transporters, types of transporters including ABC transporters and SLC transporters, hepatic and renal transporters, and concludes that transporters play significant roles in drug bioavailability, efficacy and pharmacokinetics.

1. Transporters in Pharmacokinetics
Dr Baheti Ashishkumar
JR1, MD Pharmacology
MGIMS,Sevagram
9/12/2019

2. 1. INTRODUCTION
2. PRINCIPAL SITES
3. MECHANISM
4. TYPES OF TRANSPORTERS
5. HEPATIC TRANSPORTERS
6. RENAL TRANSPORTERS
7. CONCLUSION

3. 3
INTRODUCTION
These are membrane proteins that are present in all organisms.
Approximately 2000 genes in the human genome (6%) code for transporters or
transporter related proteins.
These proteins control the uptake of essential nutrients and ions and the efflux of cellular
waste, environmental toxins, drugs and other xenobiotics.
Also play significant roles in determining the bioavailability, therapeutic efficacy, and
pharmacokinetics of a variety of drugs.

4. Roles of membrane transporters in pharmacokinetic pathways

5. 5
Membrane transporters principal sites

6. 6
P-glycoprotein
 (permeability glycoprotein, abbreviated as P-gp or Pgp) also known as
 multidrug resistance protein 1 (MDR1) or
 ATP-binding cassette sub-family B member 1 (ABCB1) or
 cluster of differentiation 243 (CD243)
is an important protein of the cell membrane that pumps many xenobiotics out
of cells.
MDR1/P-gp is the most important ABC transporter yet identified, and
digoxin is one of the most widely studied of its substrates.
More formally, it is an ATP-dependent efflux pump with
broad substrate specificity.

7. Some examples
• Loperamide (substrate for P-gp in brain)
+
Quinidine (P-gp inhibitor in brain)
Respiratory Depression

8. Some examples-contd.
• Digoxin (substrate for P-gp)
+
Cyclosporin A / Quinidine (P-gp inhibitor)
Toxicity

10. Membrane transport mechanisms
1. Passive diffusion : Diffusion of any solute
(including drugs) occurs down an
electrochemical potential gradient of the
solute
2. Facilitated diffusion : Facilitated
diffusion is a form of transporter-mediated
membrane transport that does not require
energy input.

11. 3. Active transport:
(i) Primary active Membrane
transport that directly couples with ATP
hydrolysis is called primary active transport.
e.g. ABC transporters, NaK ATPase pump
(ii) Secondary active
symport
antiport
The transport across a biological membrane
of one solute S1 against its concentration
gradient is energetically driven by the
transport of another solute S2 in accordance
with its concentration gradient.e.g NaCaX

12. 12
Vectorial Transport
Defn: Asymmetrical transport across a monolayer of polarized cells, such
as epithelial and endothelial cells of brain capillaries
- Important for absorption of nutrients and bile acids in the intestine.
- For lipophilic agents, ABC transporters alone are able to achieve
vectorial transport.

13. Transepithelial or transendothelial flux:-Requires distinct transporters at the two surfaces of the epithelial or endothelial
barriers, for transport across the small intestine (absorption), the kidney and liver (elimination), and the brain capillaries that
comprise the blood-brain barrier.

14. 14
Transporter superfamilies in the human genome
Only two major gene superfamilies
ATP Binding Cassette (ABC)
49 genes
Solute Linked Carrier
(SLC)
300 genes
7 subclasses:
Primary active transporters
ABC-A to ABC-G
e.g. P-gp/MDR1/ABC-B1
CFTR ABCC7
43 family/classes:
Secondary active Transporters
(facilitated/ion coupled)
e.g. SERT, SLC6A4
DAT SLC6A3

15. ATP Binding Cassette (ABC)
49 genes- 7 families
In 1976, Juliano and Ling reported that overexpression of a membrane protein in
colchicine-resistant Chinese hamster ovary cells also resulted in acquired
resistance to many structurally unrelated drugs (i.e., multidrug resistance)
Since the cDNA cloning of this first mammalian ABC protein (P-
glycoprotein/MDR1/ABCB1), knowledge of the ABC superfamily has grown to
include 49 genes, each containing one or two conserved ABC regions
Physiological Roles of ABC Transporters is illustrated by studies involving
knockout animals or patients with genetic defects in these transporters.

19. Tissue Distribution of Drug-Related ABC Transporters

21. ABC Transporters in Drug Absorption and Elimination
 Alteration of MDR1 activity by inhibitors (drug-drug interactions) affects oral absorption and
renal clearance.
 Drugs with narrow therapeutic windows should be used cautiously if MDR1-based drug-
drug interactions are likely.
 In the intestine, MRP3 can mediate the intestinal absorption in conjunction with uptake
transporters.
 Dysfunction of MRP3 results in a shortening of the elimination half-life.
 MRP4 substrates transported by the OAT family transporters on the basolateral membrane
of the epithelial cells in the kidney.
 Dysfunction of MRP4 enhances the renal concentration.

22. Solute Linked Carrier (SLC) Transporters
43 familes – represent approx. 300 genes in human genome
SLC1 – SLC 43
The first SLC family transporter was cloned in 1987 by expression cloning
in Xenopus laevis oocytes (Hediger et al., 1987).
Since then, many transporters in the SLC superfamily have been cloned
and characterized functionally.

25. 28
1. Hepatic Transporters
• Hepatic uptake of organic anions ( drugs, leukotrienes, bilirubin), cations, and
bile salts is mediated by SLC – type transporters in the sinusoidal membrane of
hepatocytes: OATs, OCTs, NTCP.
Transporters in the hepatocyte that function in the uptake and efflux of drugs across the sinusoidal membrane and efflux of drugs into
the bile across the canalicular membrane

26. 29
ABC transporters such as MRP2, MDR1, BCRP, BSEP, MDR3 in the bile canalicular
membrane of hepatocytes mediate the efflux of drugs and their metabolites, bile
salts, phospholipids against a steep concentration gradient from liver to bile.
Some ABC transporters also present in the basolateral membrane of hepatocytes
and may play a role in efflux of drugs back into the blood, although their
physiological role remains to be elucidated.

27. 1. HMG CoA Reductase inhibitors:
Statins reversibly inhibit HMG CoA Reductase
Most of the statins in the acid form are substrates of uptake transporters, so they
are taken up efficiently by the liver and undergo enterohepatic circulation.
OATP1B1 – hepatic uptake transporters
MRP2- efflux transporters act cooperatively to produce vectorial transport of
bisubstrates in the liver.
2. Temocapril: ACE inhibitor – Prodrug – Temocaprilat. (excreted both in bile and
urine by liver and kidney respectively, whereas other ACE inhibitors are excreted
mainly in urine). Temocaprilat is a bisubstrate of the OATP family and MRP2,
whereas other ACE inhibitors are not good substrates for MRP2.

28. 3. Irinotecan: Irinotecan hydrochloride (CPT-11) is a potent anticancer
drug but late onset gastrointestinal side effects make it difficult to use
safely.
After iv administration – SN-38 – conjugated with glucuronic acid –
excreted into bile by MRP2.
Some studies have shown that the inhibition of MRP2 mediated
biliary excretion of SN38 by coadministration of probenecid decreases
drug induced diarrhea, at least in rats.

29. 4. Angiotensin II Receptor Antagonists.
Angiotensin II receptor antagonists are used for the treatment of
hypertension, acting on AT 1 receptors expressed in vascular smooth
muscle, proximal tubule, and adrenal medullary cells, and elsewhere.
For most of these drugs, hepatic uptake and biliary excretion are important
factors for their pharmacokinetics as well as pharmacological effects.
Telmisartan is taken up into human hepatocytes in a saturable manner,
predominantly via OATP1B3.
On the other hand, both OATPs 1B1 and 1B3 are responsible for the
hepatic uptake of valsartan and olmesartan, although the relative
contributions of these transporters are unclear.

30. 5. Repaglinide and Nateglinide
Repaglinide is a meglitinide analog anti-diabetic drug.
Although it is eliminated almost completely by the metabolism mediated by
CYPs 2C8 and 3A4, transporter-mediated hepatic uptake is one of the
determinants of its elimination rate.
In subjects with SLCO1B1 (gene code for OATP1B1) 521CC genotype, a
significant change in the pharmacokinetics of repaglinide was observed.
Genetic polymorphism in SLCO1B1 521T>C results in altered
pharmacokinetics of nateglinide, suggesting OATP1B1 is a determinant of
its elimination, although it is subsequently metabolized by CYPs 2C9,
3A4, and 2D6.

31. 6. Fexofenadine
Fexofenadine, a histamine H1 receptor antagonist, is taken up in the liver by
OATP1B1 and OATP1B3 and excreted in the bile via transporters
including MRP2 and BSEP.
Patients with genetic polymorphism in SLCO1B1 521T>C, show altered
pharmacokinetics.
7. Bosentan
Bosentan is an endothelin antagonist used to treat pulmonary arterial
hypertension.
It is taken up in the liver by OATP1B1 and OATP1B3, and subsequently
metabolized by CYP2C9 and CYP3A4.
Transporter-mediated hepatic uptake can be a determinant of elimination of
bosentan, and the inhibition of hepatic uptake by cyclosporin A, rifampicin,
and sildenafil can affect its pharmacokinetics.

32. 36
2. Renal transporters
Specificity of secretory pathways in nephron were well characterized
using variety of physiological techniques
Isolated perfused nephrons and kidneys
Micropuncture techniques
Cell culture methods
Isolated renal plasma membrane vesicles
During the past decade, molecular studies have identified and
characterized the renal transporters that play a role in drug
elimination, toxicity and response.

33. 1. Organic cation transporter:
Many secreted organic cations are Endogenous compounds – choline,
dopamine, N methylnicotinamide.
primary function is to rid the body of xenobiotics like +vely charged drugs and
their metabolites like nicotine, ranitidine, metformin, procainamide etc.
Model of organic cation secretory
transporters In the proximal tubule
• Organic cations that are secreted by the
kidney may be either hydrophobic or
hydrophilic.
Hydrophilic organic drug cations generally
have molecular weights < 400 daltons.

34. For the transepithelial flux of a compound (e.g., secretion), the compound
must traverse two membranes sequentially, the basolateral membrane
facing the blood side and the apical membrane facing the tubular lumen.
Distinct transporters on each membrane mediate each step of transport.
Organic cations appear to cross the basolateral membrane in human
proximal tubule by two distinct transporters in the SLC family 22 (SCL22):
OCT2 (SLC22A2) and OCT3 (SLC22A3).
Organic cations are transported across this membrane down an
electrochemical gradient.

35. The recent discovery of a new transporter family, SLC47A, multidrug and
toxin extrusion family (MATEs), has provided the molecular identities of the
previously characterized electroneutral proton–organic cation antiport
mechanism.
Transporters in the MATE family, assigned to the apical membrane of the
proximal tubule, appear to play a key role in moving hydrophilic organic
cations from tubule cell to lumen. In addition, novel organic cation
transporters (OCTNs), located on the apical membrane, appear to
contribute to organic cation flux across the proximal tubule.

36. 2. Organic anion transporter:
removal of weak acidic drugs like pravastatin, captopril, PAH, penicillines,
ochratoxin etc.
Model of organic anion secretory
transporters In the proximal tubule
Rectangle – transporters in SLC 22 family
Rectangle – transporters in ABC superfamily
OA – organic anion
aKG – alpha ketoglutarate

37. OAT1 (SLC22A6)
OAT1 was cloned from rat kidney.
This transporter is > 30% identical to OCTs in the SLC22 family.
Mouse, human, pig, and rabbit orthologs have been cloned and are ∼80% identical to
human OAT1.
The gene for the human OAT1 is mapped to chromosome 11 and is found in an SLC22
cluster that includes OAT3 and OAT4.
In humans, rat, and mouse OAT1 is expressed primarily in the kidney with some
expression in brain and skeletal muscle.
Immunohistochemical studies suggest that OAT1 is expressed on the basolateral
membrane of the proximal tubule in human and rat, with highest expression in the middle
segment, S2.
Based on quantitative PCR, OAT1 is expressed at a third of the level of OAT3.

38. • OAT2 (SLC22A7)-present in both kidney and liver. In the kidney, the transporter is
localized to the basolateral membrane of the proximal tubule, and function as a transporter
for nucleotides, particularly guanine nucleotides such as cyclic GMP .
• OAT3 (SLC22A8)-Human OAT3 consists of two variants, one of which transports a wide
variety of organic anions, including model compounds, PAH and estrone sulfate, as well as
many drug products (e.g., pravastatin, cimetidine, 6-mercaptopurine, and methotrexate)
• OAT4 (SLC22A9)-OAT4 is expressed in human kidney and placenta. The specificity of
OAT4 includes the model compounds estrone sulfate and PAH, as well as zidovudine,
tetracycline, and methotrexate

39. Other Anion Transporters.
URAT1 (SLC22A12), first cloned from human kidney, is a kidney-specific transporter
confined to the apical membrane of the proximal tubule.
NPT1 (SLC17A1), cloned originally as a phosphate transporter in humans, is expressed
in abundance on the luminal membrane of the proximal tubule as well as in the brain.
NPT1 transports PAH, probenecid, and penicillin G.
MRP2 (ABCC2), an ABC transporter, initially called the GS-X pump, has been
considered to be the primary transporter involved in efflux of many drug conjugates such
as glutathione conjugates across the canalicular membrane of the hepatocyte.
MRP4 (ABCC4) is found on the apical membrane of the proximal tubule and transports a
wide array of conjugated anions, including glucuronide and glutathione conjugates.

41. 46
Thank You

42. breast cancer resistance protein (BCRP), and
multidrug resistance-associated proteins (MRPs),
multidrug resistance protein 4 (MRP4)
bile salt export pump (BSEP, ABCB11)
Na+-taurocholate cotransporting polypeptide (NTCP)
nucleotide binding domains (NBDs)
the major facilitator superfamily (MFS) and
two neurotransmitter transporter families:-
 the neurotransmitter; sodium symporter (NSS, or SLC6) and
the dicarboxylate/amino acid:cation (Na+ or H+) symporter (DAACS, or SLC1) family.
URAT1 (urate transporter 1)
Sodium-dependent phosphate transport protein 1 NPT
Single nucleotide polymorphisms, frequently called SNPs (pronounced “snips”), are the most
common type of genetic variation among people.
NTCP (sodium/taurocholate cotransporting polypeptide)
Cystic fibrosis transmembrane conductance regulator (CFTR)
PEPT1 (peptide transporter 1)

43. 48
conformation
change
conformation
change
Carrier-mediated solute transport
Basic membrane Transport Mechanisms
Channels vs Transporters

46. Vesicular monoamine/amine transporter - SLC 18
VMAT-2 inhibitors
RESERPINE – Not used now
TETRABENAZINE – Huntingtons chorea
DEUTETRABENAZINE - Huntingtons chorea
VALBENAZINE – Tardive dyskinesia