PHASE 3: Transporterbased DDIs 101

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PHASE 3 Transporter-based DDIs 101

• Jessica Oesterheld, MD

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What you need to know to figure out
transporter-based DDIs

• Different knowledge set than for CYPs
• Answer 3 questions
• on which organ does the transporter based DDI
  occur BBB, intestine,liver, renal proximal cell
• Is the transporter an ABC or SLC- know if
  effluxer or influxer
• on what membrane does the transporter reside
• intestinal pgp / CYP3A4 tag team- DDI multiplier

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Transporters

• Until 30 years ago thought only lipid soluble
  drugs diffuse across intestine
• Actually non-lipid soluble drugs can be
  transported (both influxed and effuxed) across
  both the apical and the basolateral borders by
  membrane-embedded transporters-way drugs move
  across cells.

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Focus on ABCB1(p-gp)

• ABCB1(p-pg) exist in membranes of tumor cells
  BBB, testes, placenta,they block entry of
  compounds into these sanctuaries
• In kidney, liver and small intestine,
  ABCB1(p-gp) efflux compounds into the collecting
  tubule, biliary system, gut lumen - ABCB1 always
  on apical side (brush border, luminal)
• Transports wide variety of substrates especially
  neutral and hydrophobic cations or amphipathic
  (mostly non-polar with one end polar

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Transporters- functions

• play a part in resistance to cancer cells
• genetic polymorphisms are responsible for
  diseases
• In intestine, hepatocytes, renal proximal tubule
  cells, blood transfer drugs in and out of organs
  into blood and target cells to breast milk, and
  protect sanctuaries like the brain, testes,
  placenta
• Can cause DDIs
• Considered Phase 3 reactions

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Transporters-2 superfamilies

• ATP-binding cassette (ABC) transporters
• http//nutrigene.4t.com/humanabc.htm
• Solute-Linked Carrier (SLC) transporters
• www.bioparadigms.org/slc/menu.asp

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ABC and SLCs

• The ABC transporters are in a single family with
  a shared structural element and ontologic
  history-effluxing or exporting of endogenous and
  exogenous products
• The SLC transporter families are a diverse group
  whose families share no common structural
  elements, and they are all transporters of a wide
  array of exogenous and endogenous products- some
  bidirectonal- but most influxing, importing

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Nomenclature- ABCs

• Usual family, subfamily and isoform convention
  -an integer represents the family (e.g., 1, 2), a
  letter, the subfamily (e.g., A, B, C) and an
  integer, the isoform (e.g., 1,2). e.g., CYP2D6,
  UGT1A1
• Members of the ABC family all have the same
  systemic ABC designation since they are all in a
  single family and the first integer is
  skipped---- eg ABCB1(p-gp) --commonly followed by
  alias or common name

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Nomenclature SLCs

• SLC superfamily are not in a single family, their
  designation, SLC, is followed by an integer
  (family), letter (subfamily) and integer
  (isoform). There is a single exception in the SLC
  nomenclature. The SLC subfamily 21 is designated
  as SLCO instead of SLC-e.g., SLCO1B1 (OATP1B1)

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ABCs/SLCs

• organ specific and present on a particular
  membrane either luninal or basolateral border
  (Bblood)
• have overlapping substrates, inhibitors and
  inducers
• can have genetic variations
• cause transporter-based DDIs in intestine, BBB,
  liver and kidney

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History of transporters

• 30 years ago, Juliano and Ling noted after
  initial efficacy, many anti-CA drugs stopped
  being effective at the same time Multiple Drug
  Resistance (MDR)
• The gene MDR1 on chromosome 7 encodes a
  glycoprotein named P (permeability) glycoprotein
  that actively pumps out compounds from cell
• Multiple drugs are effected at same time since
  ALL are P-gp substrates
• Approximately 50 of human cancers express
  P-glycoprotein at levels sufficient to confer MDR
  
• Intensive search for p-gp inhibitors (lately
  fluoxetine used with CA drug for colon cancer)

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ABCs

• There are ABC 49 transporters divided into 7
  subfamilies
• ABCA (12), ABCB (11), ABCC (13), ABCD (4),
  ABCE (1), ABCF (3), ABCG (6)
• flip compounds across a cellular gradient
  flippases or bouncersor hydrophobic vacuum
  cleaners
• Different transporters involved with exogenous
  and endogenous compounds---- bile, glucuronides,
  organic anions, lipids.
• Use ATP hydrolysis as driving force

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Human diseases associated with an ABC Transporter

• Disease Transporter
• Cystic fibrosis ABCC7 (CFTR)
• Stargardt disease AMD ABCA4 (ABCR)
• Tangier Disease and Familial HDL deficiency
  ABCA1 (ABC1)
• Progressive familial intrahepatic cholestasis
  ABCB11 (SPGP), ABCB4 (MDR2)
• Dubin-Johnson syndrome ABCC2 (MRP2)
• Pseudoxanthoma elasticum ABCC6 (MRP6)
  
• Persistent hypoglycemia of infancy ABCC8
  (SUR1), ABCC9 (SUR2)
• Sideroblastic anemia and ataxia
  ABCB7 (ABC7)
• Adrenoleukodystrophy ABCD1 (ALD)
• Sitosterolemia ABCG5, ABCG8
  
• Immune deficiency ABCB2 (Tap1), ABCB3
  (Tap2)

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Sub families B,C,G

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(No Transcript)
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Decrease bioavailability

Villus tip of enterocyte
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BILIARY EXCRETION INTO GUT
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Tubular secretion

reabsorbtion
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Other BBB ABC transportersABCC4(MRP4),
ABCC5(MRP5), ABCG2(BCRP)
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BBB-Some drugs are ABCB1(p-gp)substrates

• Drugs can be ABCB1(p-gp) substrates or not
• Older antihistamines are not ABCB1(p-gp)
  substrates- therefore they enter the BBB, but
  non-sedating antihistamines are ABCB1(p-gp)
  substrates and are excluded from the CNS and have
  less CNS side effects

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BBB-Some drugs are ABCB1(p-gp) inhibitors/inducers

• New kind of drug-drug interaction
• Usually anti-diarrheal drug loperamide (Imodium)
  is excluded by BBB from CNS because it is a
  ABCB1(p-gp) substrate. If add quinidine which is
  a ABCB1(p-gp) inhibitor at sufficient dosage,
  patient will get CNS side effect or respiratory
  depression (Sadeque and Wandel 2000)

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Intestine/kidney-Other ABCB1(p-gp) DDIs

• Long known that when quinidine given with
  digoxin, it increased digoxin concentrations
• But only a small portion of digoxin is handled by
  P450 enzymes
• digoxin is a ABCB1(p-gp) substrate
• Quinidine inhibits ABCB1 in intestine and kidney
  leading to more digoxin retained

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dig
quinidine
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TUBULAR SECRETION
dig

ACTIVE REABSORBTION
quinidine
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CYP3A and ABCB1(p-gp) in gut-Intestinal Tag Team

• Many substrates of ABCB1(p-gp) are also CYP3A4
  substrates
• ABCB1(p-gps) efflux compound to lumen, then
  compound is reabsorbed this shuttle leads to
  increased exposure of compounds to CYP3A4 and
  maximizes their activity
• If inhibit ABCB1(p-gps) then decrease CYP3A4
  efficiency (inhibit CYP3A4)
• If inhibit both CYP3A4 and ABCB1 (p-gps) mulitply
  the combined inhibition of the CYP

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Intestinal Cells
drug
pgp
3A4
B
L
drug
o
LUMEN
pgp
3A4
0
D
drug
pgp
3A4
P-gps in front- with repeated shuttles of
absorption/efflux Since substrates can diffuse
back in
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Double Inhibitors/Double Inducers

• Not only are many drugs both ABCB1 (p-gp) and
  CYP3A4 substrates, but many drugs are both ABCB1
  and CYP3A4 inhibitors (e.g., gfj, erythromycin,
  cyclosporine)
• Many drugs are both ABCB1 and CYP3A4 inducers
  (e.g., St Johns wort, rifampin)
• Therefore the effects of DDIs on double
  substrates by double inhibitors or double
  inducers are substantially increased
• Coordination of their activity by nuclear
  receptor coordination -.Pregnane X Receptor (PXR)
  Constitutive Androstane Receptor (CAR) and
  perhaps others

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Very difficult to predict and tease out
ABCB1(p-gp)-based DDIs

• At least 2 binding sites and 2 ATP-binding sites
  on 2 symmetric non-identical halves of P-gp
• Sites work cooperatively
• Competitive inhibition, non-competitive
  inhibition and collective stimulation
• Must also tease out CYP3A4 effects and other
  transporters

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Intestinal/hepatic ABCB1(p-gp) Activity (Lin and
Yamazaki 2003)

• ABCB1(p-gp) functional activity is saturable,
  therefore only important when drug is active at
  low dose or poor solubility
• Only certain drugs are affected, especially those
  with high permeability and poor solubility

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ABCB1(p-gp) Variability

• Controversial whether men have 2.4 fold higher
  levels of ABCB1(P-gp) than women
• May not be fully developed until 8 years of age
• Increased in pregnancy (Hebert et al 2008)
• SNP at exon 26 3435TT associated with lower
  ABCB1(p-gp) activity in duodenum and 2-fold
  higher digoxin concentration (Hoffmeyer et al
  2000)- these findings not consistently replicated
  (Dragonas 2008), part of haplotype
• Haplotypehaploid genotype group of alleles of
  linked genes
• However association of this SNP with postural
  hypotension from NT (Roberts 2002)
• Varies from 16 to 27 in subjects of African
  descent to 48 to 57 in whites and 41 to 66 in
  Asians

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SLCs

• Diverse group without a common structure
• 364 members 46 families
• Members of same families must have 20 amino
  acids in common
• Dont use ATP, but couple to another solute
  transporter or use favorable gradients
• Generally influxers
• Split into those that handle anions or cations
• Transporters that handle anions (weak
  acids)OATs, OATPs, also MRPs
• Transporters that handle cations (weak bases)
  OCTs, OCTNs, MATEs, also ABCB1

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SLC6A4SERT
Also SLC46 and 47
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SLC families involved in drug transport

• (1) Proton Oligopeptide Transporters (SLC15) 4
  members
• (2) Organic Anion Transporting Polpeptides
  (SLC21 known as SLCO) 20 members
• (3) Organic Cation, Anion, and Zwitterions
  Transporters (SLC22) 18 members
• (4) Multidrug and Toxin Extrusion Transporters
  (SLC47) 3 members

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SLCO1A2 (OATP1A2)

• Fexofenadine (Allegra) is a substrate
• Grapefruit juice (naringin) inhibits this
  transporter and prevents entry to the intestinal
  cell and enters lumen instead-gt lower systemic
  levels of fexofenadine
• Other juices- apple and orange do the same thing
  (Bailey et al 2007)

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fexofexadine
gfj
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SLC22A6 (OAT1)- organic anion transporters

• Cidofovir (Vistide) used for CMD retinitis- in
  order to prevent renal toxicity if it accumulates
  in kidney, given with probenecid and blocks
  entrance to kidney
• Probenecid originally developed to increase
  systemic levels of penicillin during WW2 when
  penicillin was scarce

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Cidofovir
oct2
Probenecid
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Liver

• SLC transporters on the basolateral or portal
  side of liver- influxes the substrate into the
  liver
• If transporter is inhibited, leads to increased
  levels of the substrate
• ABC transporters embedded in the opposite
  membrane effluxes the substrate into the biliary
  caniculi

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SLCO1B1 (OAT1B1)

• Substrates which can be influxed into liver
  include irinotecan, penicillin G, most statins,
  methotrexate, repaglinide, rifampin
• Inhibitors of SLCO1B1 include cyclosporine,
  gemfibrozil, clarithromycin, indinavir,ritonavir,
  saquinavir and surprisingly rifampin

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pravastatin

• Influxed into liver via SLCO1B1 (OATP1B1) and
  effluxed into billiary caniculi via ABCC2
  (MRP2)----- with other minor transporter pathways
• Phase 1 and 2 enzymes in liver, but their effects
  minimal.
• Gemfibrozil inhibits SLCO1B1 and increases AUC
  more than 200 (Pravachol website-combination
  therapy not recommended)

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Pravastatin and Gemfibrozil DDI

• This DDI is a risk factor for the development of
  rhabdomyolitis
• Independent of risk factor from additive risk
  factor from both drugs individually
• In this case there is not separate effect via
  CYPs

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Transporter CYP DDIs

• Gemfibrozil is not only a SLCO1B1 inhibitor, but
  it is also a CYP2C8 inhibitor
• Plasma concentration of replaginide (Prandin) is
  increased 20 fold because it is both a SLCO1B1
  substrate and CYP2C8 substrate (Niemi et al.
  2003)

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replaginide

Genetic variations of SLCO1B1- decreased
transporter activity also leads to increased AUC
of substrate
gemfibrozil
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Genetics of SLCO1B1 haplotypes

• SLCO1B115 is present in 15 of
  European-Americans associated with higher
  pravastatin levels and several other SLCO1B1
  (OATP1B1) substrates
• Also associated with increased risk of
  simvastatin-induced myopathy
• SLCO1B11B associated with reduced plasma
  concentrations of some SLCO1B1 substrates
• Haplotypehaploid genotype group of alleles of
  linked genes

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Status of transporter-based DDIs

• Nomenclature of SLCs standardized with new SLCs
  added in last 2 years
• Few selective substrate or inhibitors
• In vitro DDIs may not translate into in vivo ones
• In vivo, knock-out mice may not translate to
  humans-recent development of humanized
  transgenetic mice or humanized by transplanting
  hepatocytes
• In vivo human studies need to tease out effect
  from CYPs, UGTs and other transporters