02 Drug interactions in pharmacology.ppt

DRUG INTERACTIONS
Ali Mohammed
Assistant Professor

Outcomes of drug interactions
1) Loss of therapeutic effect
2) Toxicity
3) Unexpected increase in pharmacological activity
4) Beneficial effects e.g additive & potentiation (intended)
or antagonism (unintended).
Definition;
It is the modification of the effect of one drug (the object drug ) by
the prior concomitant administration of another (precipitant
drug).

Every Drug Interaction is Harmful ????
NO
• Several drug interactions are deliberately employed in therapeutics,
e.g.
– ACE inhibitors + diuretics to treat hypertension or
– Sulfamethoxazole + Trimethoprim to treat bacterial infection or
– Furosemide + amiloride to prevent hypokalaemia.

Drugs more likely to be involved in drug interactions
 With Narrow therapeutic index (Low Safety Margin)
 Aminoglycosides
 Digitalis
 Lithium
 Affecting vital physiology of the body
 Antihypertensive drugs
 Anti-diabetic drugs
 Anticoagulants
 With high plasma protein binding capacity
 NSAIDs
 Warfarin
 Sulfonylureas
 Metabolized by Zero Order Kinetics or Saturation Kinetics
 Phenytoin
 Theophyllin

MECHANISM OF DRUG INTERACTIONS
• Drug interactions can be broadly divided into
– Pharmaceutical Interaction
• During dosage form preparation or at time of administrations.
• Dissolving the drug in solvent,
• Mixing drugs in powder, solution or injection forms.
– Pharmacokinetic (ADME)
• Absorption (Complex or Chelate formation, Altered stomach pH, Ionization,
GIT motility, First Pass Metabolism)
• Distribution (Protein binding)
• Metabolism ( Enzyme induction/inhibition)
• Excretion (Altered pH, Ionization, Entero-hepatic recirculation)
– Pharmacodynamic ( At receptor or tissue level)
• e.g synergism.antagonism, altered cellular transport, effect on the
receptor site.

Pharmacokinetic interactions
1) Altered GIT absorption.
•Altered pH, Altered bacterial flora, formation of drug chelates or complexes,
drug induced mucosal damage and altered GIT motility.
Ex1., antiacids
pH
Decrease the tablet dissolution
of Ketoconazole (acidic)
H2 antagonists
pH
Therefore, these drugs must be separated by at least 2h in the time of administration of
both .
 Insoluble and poorly absorbed complexes in the gut
 Example:-
 Tetracyclines and calcium/iron salts, antacids or sucralfate
 Phenytoin absorption is decreased by sucralfate
 Minimized by administering the two drugs with a gap of 2-3 hours.

b) Altered intestinal bacterial flora ;
EX., In 10% 0f patients receive digoxin…..40% or more
of the administered dose is metabolized by the intestinal flora
Antibiotics kill a large number of the normal flora of the intestine
Increase digoxin conc.
and increase its toxicity
a) Altered pH;
The non-ionized form of a drug is more lipid
soluble and more readily absorbed from GIT than the
ionized form does.
Alteration in Entero-hepatic recirculation
•Antibiotics like Tetracyclines (Broad Spectrum) markedly reduce gut flora that
normally deconjugates oral contraceptive steroids secreted in the bile as glucuronides
and permits their Entero-hepatic recirculation. Contraceptive failure when
concurrent use of antibiotics due to lowering of the contraceptive blood levels.

c) Complexation or chelation;
EX1., Tetracycline interacts with iron preparations
or
Milk (Ca2+ ) Unabsorpable complex
Ex2., Antacid (aluminum or magnesium) hydroxide
Decrease absorption of
ciprofloxacin by 85%
due to chelation

d) Drug-induced mucosal damage.
Antineoplastic agents e.g., cyclophosphamide
vincristine
procarbazine
Inhibit absorption
of several drugs
eg., digoxin
e) Altered motility
Metoclopramide (antiemitic)
Increase absorption of cyclosporine due
to the increase of stomach empting time
Increase the toxicity
of cyclosporine

2) Displaced protein binding
It depends on the affinity of the drug to plasma protein.
The most likely bound drugs is capable to displace others.
The free drug is increased by displacement by another drug
with higher affinity.
Phenytoin is a highly bound to plasma protein (90%),
Tolbutamide (96%), and warfarin (99%)
Drugs that displace these agents are Aspirin
Sulfonamides
phenylbutazone

3) Altered metabolism
The effect of one drug on the metabolism of the
other is well documented. The liver is the major site of drug
metabolism but other organs can also do e.g., WBC,skin,lung,
and GIT.
CYP450 family is the major metabolizing enzyme
in phase I (oxidation process).
Therefore, the effect of drugs on the rate of metabolism
of others can involve the following examples.

EX1., Enzyme induction
A drug may induce the enzyme that is responsible
for the metabolism of another drug or even itself e.g.,
Carbamazepine (antiepileptic drug ) increases its own
metabolism
Phenytoin increases hepatic metabolism of theophylline
Leading to decrease its level Reduces its action
and
Vice versa
N.B enzyme induction involves protein synthesis .Therefore,
it needs time up to 3 weeks to reach a maximal effect

EX2., Enzyme inhibition;
It is the decrease of the rate of metabolism of a drug by
another one.This will lead to the increase of the concentration
of the target drug and leading to the increase of its toxicity .
Inhibition of the enzyme may be due to the competition
on its binding sites , so the onset of action is short
may be within 24h.
N.B; When an enzyme inducer (e.g.carbamazepine) is
administered with an inhibitor (verapamil)
The effect of the
inhibitor will be
predominant

Ex.,Erythromycin inhibit metabolism of astemazole and terfenadine
Increase the serum conc.
of the antihistaminic leading to
increasing the life threatening
cardiotoxicity
EX., Omeprazole
Inhibits oxidative
metabolism
of diazepam

First-pass metabolism:
Oral administration increases the chance for liver
and GIT metabolism of drugs leading to the loss of a
part of the drug dose decreasing its action. This is
more clear when such drug is an enzyme inducer
or inhibitor.
EX., Rifampin lowers serum con. of verapamil level by
increase its first pass . Also, Rifampin induces the
hepatic metabolism of verapamil

4) Renal excretion:
•Active tubular secretion;
It occurs in the proximal tubules (a portion of renal tubules).
The drug combines with a specific protein to pass through
the proximal tubules.
When a drug has a competitive reactivity to the protein that is
responsible for active transport of another drug .This will reduce
such a drug excretion increasing its con. and hence its toxicity.
EX., Probenecid ….. Decreases tubular secretion of
methotrexate.

* Passive tubular reabsorption;
Excretion and reabsorption of drugs occur in the tubules
By passive diffusion which is regulated by concentration
and lipid solubility.
N.B., Ionized drugs are reabsorbed lower than non-ionized ones
Ex1., Sod.bicarb. Increases lithium clearance
and decreases its action
Ex2., Antacids
Increases salicylates
clearance and decreases its
action

Pharmacodynamic interactions;
It means alteration of the dug action without change in its
serum concentration by pharmacokinetic factors.
EX., Propranolol + verapamil
Synergistic or additive
effect
Synergism means =1+1=3
Additive means= 1+1=2
Potentiation means= 1+0=2
Antagonism means 1+1=0 or 0.5
Effect at the receptor site
•Antiadrenegic
•anticholinergic
On the other hand

* Risk factors:
1) High risk drugs; these are the drugs that show a narrow
therapeutic index e.g., corticosteroids, rifampin,
oral contraceptives, quindine, lidoquine
2) High risk patients; these are the groups of patients
that should be treated with caution due to a specific
heath condition e.g., pregnant women, malignant cases,
diabetic patients, patients with liver or kidney disorders
asthmatic patients and cardiac disorders.

•Onset of drug interaction
It may be seconds up to weeks for example in
case of enzyme induction, it needs weeks for protein synthesis
, while enzyme inhibition occurs rapidly.
The onset of action of a drug may be affected by the half
lives
of the drugs e.g., cimitidine inhibits metabolism of
theophylline.
Cimitidine has a long half life, while, theophylline has a short
one.
When cimitidine is administered to a patient regimen for
Theophylline, interaction takes place in one day.

* Prevention of drug interaction
1) Monitoring therapy and making adjustments
2) Monitoring blood level of some drugs with narrow
therapeutic index e.g., digoxin, anticancer agents…etc
3) Monitoring some parameters that may help to
characterize the the early events of interaction
or toxicity e.g., with warffarin administration, it
is recommended to monitor the prothrombin time
to detect any change in the drug activity.
4) Increase the interest of case report studies to
report different possibilities of drug interaction

02 Drug interactions in pharmacology.ppt

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