Bilayer (100 A° thick).
Made up of phospholipid & cholesterol.
Polar groups are (phospholipid heads) oriented
at the two surfaces and with nonpolar
(hydrocarbon tails) embedded in the matrix to
form a continuous sheet.
This makes it electrically resistant and relatively
Their specific lipid and protein composition
differs according to cell/organelle type.
Some intrinsic proteins extend through full
thickness of membrane, surround fine aqueous
pores while others are adsorbed on the surface
have enzymatic or signal transduction
Paracellular spaces or channels also exist.
Biological membranes are highly dynamic
1. Passive Diffusion
2. Carrier mediated Transport
i) Facilitated Diffusion
ii) Active Transport
b) Secondary Symport (Cotransport)
• Transfer process from higher concentration
gradient to lower concentration gradient.
• Its called passive because it doesn’t require
• Non-ionised drugs can diffuse passively
through the biological barriers at a rate
proportional to their lipid:water partition
• For weak electrolytes (partially ionised) drugs,
diffusion would depend on-
1. Degree of ionisation
2. pH of the surrounding environment
3. Lipid:water partition coefficient of their
Influence of pH on weak electrolytes.
Weakly ACIDIC drugs
-- Form Salt with bases
-- Phenytoin, phenobarbitone,
-- Ionize more at alkaline pH
-- Absorbed readily from acidic
environment - Stomach
Weakly BASIC drugs
--Form Salt with acids
-- Atropine, morphine,
-- Ionize more at acidic pH
-- Absorbed readily from alkaline
environment - Intestine
Implications of this equation :
1. Stronger acids/ weak bases have a lower pKa value.
2. Weakly acidic drugs – stomach
Weakly basic drugs – intestine
Net absorption from intestine usually exceeds
that from stomach (short transit time and
limited surface area ).
3. At pH = pKa, the drug is 50% ionized and 50%
4. For each unit change in pH, there is 10 fold
change in ratio of log
[ionized form/ unionized form] of a drug.
5. Strongly acidic/ basic drugs and quaternary
ammonium compounds remain predominantly
ionized and are poorly absorbed.
6. Basic drugs attain higher concentration
intracellularly. (pH 7.0)
7. Ion Trapping – When a weak electrolyte crosses a
biological membrane to encounter a pH in which it
turns into ionic form and cant diffuse out, e.g when an
unionized form of acidic drugs which crosses gastric
mucosa (pH=2), reverts to the ionized form within
mucosal cells and then only slowly passes to the ECF.
#Difference in pH across membranes can result in
differential drug distribution.
• Transmembrane proteins embedded in the cell
membrane serve as carriers/transporters.
• Transporters form an intermediate complex
with the substrate.
• Much slower than flux through channels.
• Specific for the substrate/ type of substrate.
• Follows saturation kinetics.
• Competitively inhibited by analogues
which utilize the same transporter.
• What is the difference between Channels
•Types depending on energy use:
Down hill transport
Carrier moves drug along its concentration
No energy required, follows Saturation Kinetics
Drugs →Anti cancer drugs, antiviral drugs,
amino acids in brain, certain vitamins:
riboflavin, thiamine, vit B12
• Plot of rate of absorption against drug concentration.
-- linear relationship at a lower range
-- rate of ascent of the curve decreases and eventually
the curve becomes flat.
0 2 4 6 8
• Up hill transport
• Against electrochemical gradient
• Energy dependent - Generated by membrane
• Follows Saturation Kinetics
• Drugs : 5-fluorouracil, Choline, Digitalis.
• Blocked by inhibiting cell metabolism or by reducing
Sodium cyanide or Sodium fluoride or
• Depending on the source of driving force, it can be
Primary or Secondary
• Direct energy is required
• Carried by ATP Binding Cassette group of
• Unidirectional flow from cytoplasm to ECF
• E.g anticancer agents, digoxin, anti-retroviral
• One ion /solute (x) supplies driving force for
transport of other solute (y)
• Symporters : Na+glucose symporters
• Antiporters : Na+/H+ exchanger
• Cellular uptake of exogenous complexes inside
• At the expense of cellular energy.
• Rare process for engulfing relatively larger
• E.g. bacteria/ foreign bodies by macrophages
botulinum toxin, allergens
o Fluid uptake – engulfs a fluid/ drug in solution.
• Trapping of macromolecular
solutes into invaginations.
• Fusion of membrane to form
• Pinching off of the vesicle and
passing of solute/drug inside
o E.g. Insulin, Immunoglobulin in
neonates’ gut, lipid droplets.
o Physical process where passage of drugs occurs through
pores or paracellular spaces.
o Depends on
• 1. Molecular weight / size
• 2. Pressure gradient
• 3. Protein binding – free form
o E.g Glucose, urea, Alcohol
Control Influx of essential nutrients & ions.
Efflux of cellular waste, environmental toxins,
drugs and other xenobiotic.
Regulates bioavailability & distribution of
Transport of compounds out of brain across
blood brain barrier
• Involved in primary active transporters
• Energy - from ATP hydrolysis
• 7 families (ABCA TO ABCG) consisting of 49
• Unidirectional flow from cytoplasm to ECF
Best recognized transporter :
• P glycoprotein (encoded by ABCB1 also
• Cystic fibrosis transmembrane
regulators (CFTR) (encoded by ABCC7).
• Transporter Name : P-gp MDR1 (ABCB1)
• Tissue Distribution : Liver, Kidney, Intestine,
• Physiological Function : Natural
detoxification system against xenobiotic.
• Inhibitors: Quinidine, Verapamil,
Spironolactone, Clarithromycin & Ritonavir
Tariquidor and Laniquidar are under trial, they
block the efflux of drugs by inhibiting P-gp.
Ivacaftor, a first-in-class drug, was recently
approved for treatment of Cystic Fibrosis, it acts
on CFTR (ABCC7).
It is termed as a potentiator, increases the
probability that the mutated Cl- channel remains
in the opened state.
Facilitate transporters & ion coupled secondary
52 SLC families with 395 transporters present in
Many serve as drug targets or in drug
Mostly are Facilitative transporters or
sometimes Secondary Active transporters.
E.g. Serotonin transporter (SERT→SLC6A4)
Dopamine transporter (DAT →SLC6A3)
It is involved in both influx and efflux of
Polymorphism with –SLC30A8 causes type 1 DM
Polymorphism with –SLC22A4 & SLC22A5 causes IBD
• Asymmetrical transport across monolayer of polarized
• Important in transfer of solute across the epithelial &
• ABC transporters → unidirectional efflux.
• SLC transporters → either drug uptake or efflux.
• Intestine → absorption of nutrients & bile acids.
• Liver → hepatobiliary transport.
• Kidney → tubular secretion.
• Brain → barrier functions.
• Different examples illustrate the importance of
vectorial transport in determining the drug
exposure in circulating blood & liver.
1. HMG COA reductase inhibitors
2. ACE inhibitors
Membrane transporters in
Basolateral membrane of hepatocyte.
Uptake of organic anions (drugs, billirubin),
cations & bile salts.
OATPs → anions.
OCTs (organic cation transport protein) &
NTCP → cation & bile salt
Present in bile canalicular membrane of
MRP2, MDR1, BCRP, BSEP & MDR2.
Mediate efflux of drugs & their metabolites, bile
salts & phospholipids→ Against concentration
gradient from liver to bile.
HMG COA reductase inhibitors
• Inhibit cholesterol biosynthesis.
• Statins :- Parvastatin
• OATP1B1 →uptake
• MRP2 → efflux
• Gemfibrosil inhibits OATP1B1
and increases its concentration
in systemic circulation leading
• Genitic polymorphism also
Active metabolite SN 38
excreted in bile by MRP2
+ Probenecid → inhibit
- Anticancer drug
Bosentan 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.
Inhibition of its hepatic uptake by
cyclosporine, rifampicin, and sildenafil can
affect its pharmacokinetics.
• Renal transporters play an important role in drug
elimination ,toxicity and response.
• For the transepithelial flux of a compound, the
compound must traverse two membranes
sequentially, the basolateral membrane facing
the blood side and the apical membrane facing
the tubular lumen.
• Of the two steps involved in secretory transport,
transport across the luminal membrane appears
to be rate-limiting.
Cations secreted in proximal tubules
Cations →endogenous compounds e.g. choline &
Primary function for secretion →eliminate body
xenobiotic, positively charged drugs & their
Primary function → removal of body xenobiotics
including weakly acidic drug e.g. parvastatin,
captopril, & penicillins.
Two primary transporters on basolateral membrane
→ OAT1 & OAT3 →Flux organic anions from
intestinal fluid to tubular cells.
OAT4 luminal membrane transporter
oDrugs showing their action via:
• Thiazide diuretics act at
on Na+Cl- symporter.
• Loop diuretics act at
SLC12A1 transporter, on
• K+ sparing diuretics act at
• Recent studies have suggested that genetic
variations of OCT1 and OCT2 are associated
with alteration in renal elimination and
response to Metformin.
• Polymorphisms in ABCG2 have been
associated with reduced response to
Allopurinol and Oxypurinol.
• Involved in neuronal reuptake of neurotransmitters
→ SLC1 & SLC6 transporters
• SLC6 responsible for reuptake of :-
1. Norepinephrine transporters (NET/SLC6A2)
2. Dopamine transporters (DAT/SLC6A3)
3. Serotonin transporters (SERT/SLC6A4)
4. GABA transporters (GAT)
Act as pharmacological targets for
SLC6 regulate concentration of
neurotransmitter in synaptic cells
o Function in reversible direction
o Depend on Na+ gradient to transport
GAT1 → GABA transporter present on
GAT3 → on glial cells
GAT2 → Absent on presynaptic neuron
present on choroid plexus
primary role to maintain homeostasis
of GABA in CSF
On CNS, PNS &
dwell time of
limits its action
memory & mood
On CNS, PNS &
serotonin in brain
Drugs acting on CNS have to cross BBB.
Functionally, the BBB is
o (1) partly physical,
o (2) partly a consequence of selective permeability
o (3) partly a consequence of the enzymatic destruction of
certain permeates by enzymes in the barrier.
In this efflux transporters play role.
P-glycoprotein extrudes its substrate drugs on luminal
membrane of brain capillary endothelial cells into
Limiting brain penetration
The U.S. FDA has issued a draft clinical pharmacology
guidance on performing drug-drug interaction studies
during clinical drug development (FDA, 2012). The
guidance presents information on how to use in vitro
data for transporter studies to make decisions about
whether to conduct a clinical drug-drug interaction
Although only a handful of transporters (OATP1B1,
OATP1B3, P-gp, BCRP, OCT2, MATE1, OAT1, and
OAT3) are included in the FDA guidance, more might
be included in the list which cause drug-drug
Goodman L. S, Goodman & Gilman’s
Pharmacological Basis of Therapeutics, 13th
Edition, New York; New Delhi, TataMcGraw-Hill
Katzung B. G, Basic and Clinical Pharmacology,
14th Edition, New York; New Delhi, TataMcGraw-
Hill Education, 2017.
Tripathi K.D, Essentials of Medical Pharmacology,
7th Edition, New Delhi, Jaypee Brothers
Sharma H. L, Principles of Pharmacology, 3rd
Edition, New Delhi, Paras Medical Publisher, 2018