Drug absorption
Diffusion across cell membranes – absorption occurs alongwith bulk flow of water. This
is the major mechanism across capillary endothelial cell membrane. The drug’s molecular
size should be less than 100 to 200 daltons and only unbound drug is absorbed.
Paracellular transport through intercellular gaps commonly occurs at glomerular
membranes in the kidney. At some sites there are tight junctions – in the central nervous
system there is blood brain barrier where there are no gaps between endothelial cells in
the capillaries in brain parenchyma. The epithelial cells of the choroid plexus (which
divides blood and plasma from the cerebrospinal fluid) also have tight junctions. These
tight junctions protect the brain from harmful chemicals.
Passive membrane transport
Weak electrolytes and influence of pH: A number of drugs are either weak acids or weak
bases and they tend to ionize to varying degrees depending on the pH of the fluid they are
in.
The pKa of a drug or a chemical is the pH at which half of the drug/chemical is ionized.
At steady state – an acidic drug would accumulate on the more basic side of a membrane
and a basic drug on the more acidic side – this is known as ‘ion trapping’
Carrier-mediated transport – facilitated diffusion – no input of energy, down an
electrochemical gradient
P-glycoprotein is an important efflux transporter. It can be responsible for some drug
interactions. It is found in intestinal epithelial cells, also in brain capillary endothelial
cells, renal tubular cells (apical brush border), liver canalicular cells and possibly at other
sites. This protein removes drugs from inside the cells to outside. Thus, in intestines it
limits oral absorption of transported drugs – it exports the compound back into the
intestinal tract subsequent to its absorption by passive diffusion (e.g. digoxin used in
heart failure is removed by this protein into the intestines and also into renal tubules). So
if a drug such as quinidine or verapamil inhibits the function of P-glycoprotein, it will
also inhibit the excretion of digoxin by P-glycoprotein leading to increased plasma levels
and toxicity due to digoxin.
P glycoprotein (encoded by multidrug resistance-1 gene) is also important in resistance to
cancer chemotherapy. It removes the chemotherapeutic drug out of the tumour cells and
reduces its effectiveness.
Distribution
Lipid solubility is a very important factor affecting the distribution of drugs in various
body compartments. Drugs which are highly lipid soluble have a very high Vd (volume
of distribution) because their plasma concentrations are very low. Vd = Total dose or
amount of drug / plasma concentration.
Ion trapping is not very important for distribution because the pH difference between
plasma and intracellular fluid (7.4 vs 7.0) is small.
pH gradient between intra- and extracellular fluid affects distribution of weak acids or
bases
Blood:tissue partitioning – binding to plasma proteins and tissue macromolecules is
important in the distribution of a drug between plasma and extracellular fluid. Plasma
albumin binds acidic drugs whereas α1 acid glycoprotein binds basic drugs. The
binding is saturable and non-linear. For most drugs the therapeutic range of
concentrations is limited and the extent of binding and the unbound fraction is
relatively constant.
In cases of hypoalbuminemia, e.g. secondary to liver disease or the nephrotic syndrome
(kidney function not normal – loss of protein in the urine), there is reduced binding of
drugs to plasma proteins.
In conditions such as cancer, arthritis, myocardial infarction, Crohn’s disease there is
increased α1 acid glycoprotein causing increased binding of basic drugs.
Displacement of drugs from their plasma protein binding sites leads to drug interactions.
An example: Sulfonamides can displace unconjugated bilirubin from albumin, increasing
the risk of bilirubin encephalopathy in the newborn.
It is important to remember that most assays do not distinguish free drug from bound
drug.
Steady-state unbound concentrations will only change when either drug input (dosing
rate) or clearance of unbound drug is changed.
Steady-state unbound concentrations are independent of the extent of protein binding
because the unbound fracytion of a drug is in equilibrium with various body
compartments (which have a high capacity compared to the plasma volume).
TISSUE BINDING of a drug can lead to a high concentration gradient between plasma
and tissue compartments, e.g. qinacrine, an anti-malarial drug, is concentrated in the liver
– several thousandfold higher than in blood.
Fat: In obese people the adipose tissue is as high as 50% of body wt. This can act as a
major store of lipid soluble drugs, leading to a high volume of distribution. e.g. 70% of
thiopental is found in fat 3 h after administration.
BONE can act as a store of some drugs such as tetracyclines (other di-valent metal-ion
chrelating agents) and heavy metals.
Bone can act as a reservoir for slow release of toxic lead or radium into the blood.
Redistribution is the phenomenon of rapid rise in concentration of a drug (normally given
intravenously) in highly perfused organs such as the brain and the heart (which receive a
major fraction of the cardiac output), followed by a fall in concentration in these organs
as the drug is distributed to other organs with a lower perfusion rate such as the skeletal
muscles and adipose tissue and the plasma concentration falls. Here the drug is given in a
lower concentration to take advantage of redistribution. e.g. use of intravenous thiopental
(a lipid soluble barbiturate drug) to induce anaesthesia (induction and recovery from
anaesthesia are rapid).
The blood brain barrier in the brain parenchyma does not allow drugs (especially polar
drugs) to enter the brain tissue from the plasma because the capillary endothelial cells
have tight junctions (no gaps) between them. The glial cells also form a barrier.
Advantage is taken of this barrier in designing less lipophilic antihistamines that cannot
enter the CNS and thus do not cause sedation (non-sedative).
Similarly at the choroid plexus the epithelial cells form a barrier between the blood and
the cerebrospinal fluid.
Efflux carriers – P-glycoprotein is present in brain capillary endothelial cells and
choroids plexus cells.It does not allow a drug to translocate across endothelial cells, and
exports it out, e.g. brain (and also testes) – drug concentrations are below those necessary
to achieve a desired effect even though blood levels are adequate – HIV protease
inhibitors, loperamide (a potent opioid) that lacks any CNS effects.
Placental transfer of drugs is determined by lipid solubility, extent of plasma binding and
degree of ionization of a drug. Fetal plasma is slightly more acidic (7.0 to 7.2) than that
of mother (7.4) leading to ion trapping of basic drugs.
There is also P-glycoprotein in placenta that keeps the drug out of the fetal circulation
(protective effect).
The fetus is to at least some extent exposed to essentially all drugs taken by the mother.
Excretion
P-glycoprotein and multidrug resistance-associated protein type-2 (MRP2) are present in
the apical brush border membrane of kidney tubule cells.
P-glycoprotein secretes amphipathic anions into the tubule lumen from the plasma.
MRP2 secretes conjugated metabolites such as glucuronides, sulfates and glutathione
adducts.
In the liver P-glycoprotein is present in the canalicular membrane of hepatocytes. MRP2
secretes conjugated metabolites and endogenous compounds into the bile.
Drugs and metabolites thus secreted into the intestinal lumen through bile can be
reabsorbed back (conjugated metabolites may be hydrolyzed by intestinal microflora) –
this is termed as enterohepatic recycling. This leads to an increase in the half-life of a
drug.
Drugs in breast milk: Milk is more acidic than plasma. This can lead to higher
concentrations of basic drugs in breast milk due to ion trapping.
Drug metabolism
Phase I – functionalization reactions - expose or introduce a functional group
Phase II reactions – biosynthetic (conjugation) reactions – covalent linkage between a
functional group with endogenously derived glucuronic acid, sulfate, glutathione, amino
acids or acetate (generally inactive) except: 6-glucuronide of morphine is more active.
Most metabolizing enzymes are found in the liver, gastrointestinal tract, kidneys, lungs.
In the cell they are located in the endoplasmic reticulum (ER) and in the cytosol.
When cells are homogenized and subjected to differential centrifugation we can separate
a fraction of microvescicles derived from the ER. This is known as microsomes and it
contains phase I enzymes.
The most important phase I enzymes are the Cyt. P450 monooxygenases. This is a
superfamily of heme thiolate proteins – function as a terminal oxidase – introduce a
single atom of molecular oxygen into the substrate, other atom into water.
Electrons for this reaction are supplied from NADPH via Cyt. P450 reductase
1000 currently known cyt. P450s, 50 active in human beings.
Categorized into 17 families and subfamilies designate as CYP1, CYP2, CYP3 families.
Enzymes within a family have amino acid sequences with greater than 40% identity.
CYP 3A4 & CYP3A5 – very similar isoforms – together involved in the metabolism
of about 50% of drugs
CYP3A4 – in intestinal epithelium and kidney, besides the liver – metabolism by
CYP3A4 during intestinal absorption is important in first-pass metabolism, along with
hepatic metabolism.
CYP 2C and CYP 2D6 – also important in drug metabolism
CYP1A1/2, CYP2A6, CYP2B1, CYP2E1 – not involved in drug metabolism in a major
way, but they catalyze procarcinogenic environmental chemicals to carcinogenic form,
e.g. tobacco smoking-associated lung cancer.
CYP3A – important metabolism in intestines
Antifungal – ketoconazole, itraconazole, HIV protease inhibitors – ritonavir, macrolide
antibiotics – erythromycin and clarithromycin (but not azithromycin)– all potent CYP3A
inhibitors. Also, diltiazem, nicardipine and verapamil, grapefruit juice.
Many CYP3A inhibitors also reduce P-glycoprotein causing impaired excretion of
digoxin by quinidine. Digoxin is excreted by P-glycoprotein in the kidney and the
intestines and when P-glycoprotein is inhibited by quinidine it leads to a drug interaction.
CYP2D6 – quinidine, selective serotonin reuptake inhibitors are potent inhibitors
Amiodarone, cimetidine (but not ranitidine), paroxitene, fluoxetine – reduce metabolic
activity of several CYP isoforms
Valproic acid – inhibits epoxide hydrolase another drug metabolizing enzyme.
Allopurinol inhibits xanthine oxidase which also metabolizes 6-marcaptopurine (6-MP)
(This causes life threatening toxicity in patients concurrently receiving 6-MP)
Autoinduction is when a drug hastens its own metabolism by inducing its metabolizing
enzymes –e.g. anticonvulsant carbamazepine
Rifampin – Strong inducer of metabolizing enzymes – makes oral contraceptives
ineffective
Chronic alcohol induces CYP2E1 – risk of acetaminophen hepatotoxicity is higher (Nacetyl-p-benzoquinoneimine)
Rifampin and carbamazepine – induce CYP1A2, CYP2C9 and CYP 2C19
Rifabutin, barbiturates, glucocorticoids, St. John’s Wort – induce CYP3A4 and
other isoforms