Unit 1 General Pharmacology (As per PCI syllabus)

UNIT :1
General Pharmacology
Presented by: Prof.Mirza Anwar Baig
Anjuman-I-Islam's Kalsekar Technical Campus
School of Pharmacy,New Pavel,Navi Mumbai,Maharashtra
1
1
Presented by: Prof.Mirza Anwar
Baig

Contents:
A. Introduction to Pharmacology-
 Definition, historical landmarks and scope of pharmacology,
nature and source of drugs, essential drugs concept and routes
of drug administration.
 Agonists, antagonists( competitive and non competitive),
spare receptors, addiction, tolerance, dependence,
tachyphylaxis, idiosyncrasy, allergy.
B. Pharmacokinetics-
 Membrane transport, absorption, distribution, metabolism and
excretion of drugs.
 Enzyme induction, enzyme inhibition, kinetics of elimination
2
Baig

At the end of topic you should be able to....
1. Compare advantage and disadvantages of routes of drug
administration.
2. Explain the essential characteristics of drug for proper
absorption & excretion.
3. Summarize the role of distribution and metabolism
in drug actions.
3
Baig

1.Pharmacology:
• Science of drugs (Greek:Pharmacon--drug; logos-discourse in).
• Deals with interaction of exogenously administered chemical
molecules (drugs) with living systems.
The two main divisions of pharmacology are:
• Pharmacodynamics: knowledge about drugs, but most importantly -
What the drug does to the body.
• Pharmacokinetics (Greek: Kinesis-movement):- What the body
does to the drug.
4
Baig

2. Drug (French: Drogue -a dry herb)
"Drug is any substance or product that is used or is intended to be
used to modify or explore physiological systems or pathological
states for the benefit of the recipient."
3. Pharmacotherapeutics:
• It is the application of pharmacological information together with
knowledge of the disease for its prevention, mitigation or cure.
• Selection of the most appropriate drug, dosage and duration of
treatment taking into account the specific features of a patient are
a part of pharmacotherapeutics.
5
Baig

4.Clinical pharmacology:
 Scientific study of drugs in man.
 Pharmacodynamic and pharmacokinetic investigation in
healthy volunteers and in patients.
 Evaluation of efficacy and safety of drugs
 Comparative trials with other forms of treatment;
 Surveillance of patterns of drug use, adverse effects etc.
5. Chemotherapy:
 Treatment of systemic infection/malignancy
 Specific drugs that have selective toxicity for the infecting
 organism malignant cell with no/minimal effects on the host cells
6
Baig

6.Toxicology:
It is the study of poisonous effect of drugs and other
chemicals (household, environmental pollutant,
industrial, agricultural, homicidal) with emphasis on
detection, prevention and treatment of poisonings.
It also includes the study of adverse effects of drugs,
since the same substance can be a drug or a poison,
depending on the dose.
7
Baig

DRUG NOMENCLATURE
Three categories of names:
(a) Chemical name:
It describes the substance chemically, e.g.1-(lsopropylamino)-
3-(1-naphthyloxy) propan-2-ol for propranolol.
This is cumbersome and not suitable for use in prescribing.
A code name, e.g. RO 15-1788 (later named flumazenil) may
be assigned by he manufacturer for convenience and
simplicity before an approved name is coined.
b) Brand name –
Original drug which is defended by patent and may be produced
during patent term only by this pharmaceutical firm
c) Generic name–
When term of patent is discontinued the drug may be produced by
different pharmaceutical companies under new product (trade) names
but at the basis of original active substance (similar quantity, route of
administration etc.) 8
Baig

ESSENTIAL DRUGS (MEDICINES) CONCEPT
The WHO has defined Essential Drugs (medicines) as "those that
satisfy the priority healthcare needs of the population.
They are selected with due regard to public health relevance,
evidence on efficacy and safety, and comparative cost effective.
Essential medicines are intended to be available within the context
of functioning health systems at all times and in adequate amounts,
in appropriate dosage forms, with assured quality and adequate
information, and at a affordable price.
For optimum utilization of resources, governments (especially in
developing countries) should concentrate on well
tested,cheaper,safe and efficacious drugs by identifying them as
Essential medicines.
9
Baig

WHO criteria for the selection of an essential
medicine.
a) Adequate data on its efficacy and safety should be available.
b) Available in a form in which quality, including bioavailability, and stability on
storage can be assured.
c) Its choice should depend upon pattern of prevalent diseases; availability of
facilities and trained personnel; financial resources; genetic, demographic and
enviromental factors.
d) In case of two or more similar medicines, choice should be made on the basis of
their relative efficacy, safety, quality, price and availability, by comparative
pharmacokinetic properties and local facilities for manufacture and storage.
f) Most essential medicines should be single compounds.
Fixed ratio combination products should be included only when dosage of each
ingradient meets the requirements of a defined population group, and when the
combination has a proven advantage in therapeutic effect, safety, adherence or in
decreasing the emergence of drug resistance.
10
Baig

(g) Selection of essential medicines should be a continuous process
which should take into account the changing priorities for public
health action, epidemiological conditions as well as availability of
better medicines/ formulations and progress in pharmacological
knowledge.
(h) Recently, it has been emphasized to select essential medicines
based on rationally developed treatment guidelines.
 First Model List of Essential Drugs along with their dosage forms
and strengths in 1977 by WHO which could be adopted after
suitable modifications according to local needs.
 India produced its National Essential Drugs List in 1996 and has
revised it in 2003 with the title "National List of Essential Medicines".
This includes 354 medicines which are considered to be adequate
to meet the priority healthcare needs of the general population of the
country.
 Adoption of the essential medicines list for procurement and supply
of medicines, especially in the public sector healthcare system, has
resulted in improved availability of medicines, cost saving and more
rational use of drugs.
11
Baig

Orphan Drugs
• These are drugs or biological products for diagnosis/treatment/
prevention of a rare disease or condition, or a more common
disease (endemic only in resource poor countries) for which there is
no reasonable expectation that the cost of developing and marketing
it will be recovered from the sales of that drug.
• The list includes:
Sodium nitrite, fomepizole, liposomal amphotericin 8, ancrod,
rifabutin, succimer, somatropin, digoxin immune Fab (digoxin
antibody), liothyronine (T3) and many more.
• Though these drugs may be life saving for some patients, they are
commercially difficult to obtain.
• Governments in developed countries offer tax benefits and other
incentives to pharmaceutical companies for developing and
marketing orphan drugs (e.g. Orphan Drug Act in USA).
12
Baig

Route of drug adminsteration
Definition:
A route of administration is the path by which a
drug, fluid, poison or other substance is brought
into contact with the body.
13
Baig

Classification
Routes of administration can broadly be divided into:
1. Topical:
Drugs are applied topically to the skin or mucous membranes, mainly for
local action.
2. Oral:
used for systemic (non-local) effect, substance is given via the digestive
tract.
3. Parenteral:
A drug administered parenterally is one injected via a hollow needle into
the body at various sites and to varying depth.
4. Rectal: Drugs given through the rectum by suppositories or enema.
5. Inhalation: The lungs provide an excellent surface for absorption when
the drug is delivered in gaseous, aerosol or ultrafine solid particle form
14
Baig

ROUTES OF DRUG ADMINISTRATION
Mostly common considerations are:
1. Physical and chemical properties of the drug
- Solid/liquid/ gas
– Solubility and stability
– PH and irritancy
2. Site of desired action - localized and aprochable
3. Rate and extent of absorption of the drug from different
routes.
4. Effect of digestive juices and first pass metabolism of the
drug.
5. Rapidity with which the response is desired (eg.routine
treatment or emergency).
6. Accuracy of dosage required (i.v. and inhalation).
7. Condition of the patient (unconscious, vomiting) etc.
15
Baig

Local route
• These routes can only be used for localized lesions at
accessible sites and for drugs whose systemic absorption
from these sites is minimal or absent.
• Thus, high concentrations are attained at the desired site
without exposing the rest of the body.
• Systemic side effects or toxicity are consequently absent or
minimal.
• The same can serve as systemic route of administration, e.g.
glyceryl trinitrate (GTN) applied on the skin as ointment or
transdermal patch.
16
Baig

A.Topical route
This refers to external application of the drug to the surface
for localized action.
It is often more convenient as well as encouraging to the
patient.
Drugs can be efficiently delivered to the localized lesions on
skin, oropharyngeal/nasal mucosa, eyes, ear canal, anal
canal or vagina.
The dosage forms are lotion, ointment, cream,powder, paints,
drops, spray, lozengens, suppositories or pesseries.
Nonabsorbable drugs given orally (sucralfate,
vancomycin), inhalation of drugs for action on bronchi
(salbutamol, cromolyn sodium) and irrigating solutions/jellys
(povidone iodine,lidocaine) applied to urethra
17
Baig

1- Topical route:
I Skin
A-Dermal – cream, ointment (local action)
B- Transdermal- absorption of drug through skin (i.e systemic action)
I. stable blood levels(controlled drug delivery system)
II. No first pass metabolism
III. Drug must be potent or patch becomes too large
II Mucosal membranes
•eye drops (onto the conjunctiva)
• ear drops
• intranasal route (into the nose)
18
Baig

2- Oral route:
- By swallowing.
- It is intended for systemic effects resulting
from drug absorption through the various
epithelia and mucosa of the
gastrointestinal tract.
19
Baig

Advantages:
1- Convenient - portable, no pain, easy to take.
2- Cheap - no need to sterilize, compact, multi-dose bottles,
automated machines produce tablets in large quantities.
3- Variety - tablets, capsules, suspensions, mixtures .
Disadvantages:
1- Sometimes inefficient - low solubility drugs may suffer poor
availability e.g. Griseofulvin
2- First-pass effect - drugs absorbed orally are transported to the
general circulation via the liver. Thus drugs which are extensively
metabolized will be metabolized in the liver during absorption. e.g.
propranolol
20
Baig

First pass effect:
First pass effect
21
Baig

1. The first pass effect is the term used for the hepatic metabolism of a
pharmacological agent when it is absorbed from the gut and
delivered to the liver via the portal circulation.
2. The greater the first pass effect, the lower the bioavailability of the
drug(the rate and extent of the drug reaching systemic circulation).
3. Food and G-I motility can affect drug absorption.Often patient
instructions include a direction to take with food or take on an empty
stomach.
4. Absorption is slower with food(milk and milk products) for
tetracyclines and penicillins, etc. However, for propranolol
bioavailability is higher after food, and for griseofulvin absorption is
higher after a fatty meal.
First pass effect (Cont.):
22
Baig

5. Sometimes may have adverse reactions – e.g.
Antibiotics may kill normal gut flora and allow
overgrowth of fungal varieties. Thus, antifungal
agent may be included with an antibiotic.
6. Not suitable for unconscious patient - Patient
must be able to swallow solid dosage forms.
Liquids may be given by tube.
7.May cause irritation to gastric mucosa, nausea
and vomiting.
8.Effect too slow for emergencies.
23
Baig

3- Buccal/Sublingual route:
• Some drugs are taken as smaller tablets which are
held in the mouth (buccal tablet) or under the
tongue (sublingual tablet).
• Buccal tablets are often harder tablets [4 hour
disintegration time], designed to dissolve slowly.
• E.g Nitroglycerin, as a softer sublingual tablet [2
min disintegration time], may be used for the rapid
relief of angina.
24
Baig

Advantages
1- Avoid hepatic first pass - The liver is by-passed thus
there is no loss of drug by first pass effect for buccal
administration. Bioavailability is higher.
2- Rapid absorption - Because of the good blood supply
to the area, absorption is usually quite rapid.
3- Drug stability - pH in mouth relatively neutral (Except.
stomach - acidic). Thus a drug may be more stable.
3- Buccal/Sublingual route (Cont.)
25
Baig

Disadvantages
1- Holding the dose in the mouth is
inconvenient.
2- Small doses only can be accommodated
easily.
3- Buccal/Sublingual route (Cont.)
26
Baig

4- Parenteral route:
27
Baig

A- Intravascular (IV, IA):
- placing a drug directly into blood stream.
-May be - Intravenous (into a vein) or - intraarterial (into an artery).
Advantages
1- precise, accurate and immediate onset of action, 100%
bioavailability.
Disadvantages
1- risk of embolism.
2- high concentrations attained rapidly leading to greater risk of
adverse effects.
4- Parenteral route (Cont.)
28
Baig

4- Parenteral route (Cont)
B-Intramuscular :(into the skeletal muscle).
Advantages
1- suitable for injection of drug in aqueous solution (rapid
action) and drug in suspension or emulsion (sustained
release).
Disadvantages
1- Pain at injection sites for certain drugs.
29
Baig

C- Subcutaneous (under the skin), e.g. insulin.
D- Intradermal, (into the skin itself) is used for skin testing some
allergens.
E- Intrathecal (into the spinal canal) is most commonly used for
spinal anesthesia .
F- Intraperitoneal, (infusion or injection into the peritoneum)
e.g. peritoneal dialysis in case of renal insuffeciency.
4- Parenteral route (Cont)
30
Baig

5-Rectal route:
Most commonly by suppository or enema.
Advantages
1- By-pass liver - Some of the veins draining the rectum lead
directly to the general circulation, thus by-passing the liver.
Reduced first-pass effect.
2- Useful - This route may be most useful for patients unable
to take drugs orally (unconscious patients) or with younger
children.
- if patient is nauseous or vomiting
31
Baig

Disadvantages :
1- Erratic absorption - Absorption is often incomplete
and erratic.
2- Not well accepted.
5- Rectal route (Cont.)
32
Baig

6- Inhalation route:
- Used for gaseous and volatile agents and aerosols.
- Solids and liquids are excluded if larger than 20 micron.
- Smaller than 0.5 micron , they aren't retained.
Advantages
A- Large surface area
B- thin membranes separate alveoli from circulation
C- high blood flow
- As result of that a rapid onset of action due to rapid access
to circulation.
33
Baig

Disadvantages
1- Most addictive route of administration because it hits the brain
so quickly.
2- Difficulties in regulating the exact amount of dosage.
3- Sometimes patient having difficulties in giving themselves a
drug by inhaler.
6- Inhalation route (Cont.)
34
Baig

Comparisons:
35
Baig

Comparisons:
36
Baig

Pharmacokinetics
Pharmacokinetics refers to what the
body does to a drug & phar-
macodynamics describes what the
drug does to the body.
• Four pharmacokinetic properties determine
the onset, intensity,and the duration of drug
action
• Absorption:
• Distribution:
• Metabolism:
• Elimination:
Using knowledge of pharmacokinetic
parameters, clinicians can design
optimal drug regimens, including the
route of administration, the dose,the
frequency, and the duration of
treatment.
37
Baig

Drug absorption:
38
Baig

B.Factors influencing absorption:
1.Effect of pH on drug absorption:
2. Blood flow to the absorption site:
3. Total surface area available for absorption:
4. Contact time at the absorption surface:
5. Expression of P-glycoprotein:
39
Baig

1.Effect of pH on drug absorption:
5. Expression of P-glycoprotein:
40
Baig

Bioavailability
• Bioavailability is the rate and extent to
which an administered drug reaches
the systemic circulation.
• For example, if 100 mg of a drug is
administered orally and 70 mg is
absorbed unchanged, the bio-
availability is 0.7 or 70%.
• Determining bioavailability is
important for calculating drug dosages
for nonintravenous routes of
administration.
Determination of bioavailability:
41
Baig

2.Factors that influence bioavailability:
a.First-pass hepatic
metabolism:
b.Solubility of the drug:
c.Chemical instability:
d.Nature of the drug
formulation:
42
Baig

a. First-pass hepatic metabolism :
What is FPHM:
When a drug is absorbed from the GI tract, it enters the portal circulation
before entering the systemic circulation.
If the drug is rapidly metabolized in the liver or gut wall during this initial
passage, the amount of unchanged drug entering the systemic
circulation is decreased. This is referred to as first-pass hepatic
metabolism.
First-pass metabolism by the intestine or liver limits the efficacy of many
oral medications.
For example:
More than 90% of nitroglycerin is cleared during
first-pass metabolism. Hence, it is primarily administered
via the sublingual or transdermal route.
Drugs with high first-pass metabolism should be given in doses
sufficient to ensure that enough active drug reaches the desired site of
action.
43
Baig

Examples:
a. Nitroglycerin pacth b. First pass effect
44
Baig

b.Solubility of the drug:
• Very hydrophilic drugs are poorly absorbed because of their inability
to cross lipid-rich cell mem-branes.
• Paradoxically, drugs that are extremely lipophilic are
also poorly absorbed, because they are totally insoluble in
aqueous body fluids and, therefore, cannot gain access to the
surface of cells.
• For a drug to be readily absorbed, it must be largely lipophilic, yet
have some solubility in aqueous solutions.
• This is one reason why many drugs are either weak acids or
weak bases.
c.Chemical instability:
Some drugs, such as penicillin G, are unstable in the pH of the
gastric contents. Others, such as insulin, are destroyed in the
GI tract by degradative enzymes.
45
Baig

d.Nature of the drug formulation:
Drug absorption may be altered by factors unrelated to the
chemistry of the drug.
For example,
Particle size
Salt form
Crystal polymorphism
Enteric coatings,
Presence of excipients (such as binders and dispersing agents)
can influence the ease of dissolution and,therefore, alter the rate of
absorption.
46
Baig

D.Bioequivalence
Two drug formulations are bioequivalent if they show
comparable bioavailability and similar times to achieve peak blood
concentrations.
Clinical effectiveness often depends on both the maximum
serum drug con-centration and the time required (after administration) to
reach peak concentration
E.Therapeutic equivalence
Two drug formulations are therapeutically equivalent if they are
pharmaceutically equivalent (that is, they have the same dosage
form, contain the same active ingredient, and use the same route
of administration) with similar clinical and safety profiles.
Therefore, two drugs that are bioequivalent may not be therapeutically
equivalent.
47
Baig

IV. DRUG DISTRIBUTION
• Drug distribution is the process by which a drug reversibly leaves
the bloodstream and enters the interstitium (extracellular fluid)
and the tissues.
• For drugs administered IV, absorption is not a factor, and the
initial phase (from immediately after administration through the
rapid fall in concentration) represents the distribution phase,
during which the drug rapidly leaves the circulation and enters the
tissues.
• The distribution of a drug from the plasma to the interstitium
depends on cardiac output and local blood flow, capillary
permeability, the tissue volume,the degree of binding of the drug
to plasma and tissue proteins, and the relative lipophilicity of the
drug.
48
Baig

49
Baig

Factors affecting distribution:
A. Blood flow
B. Capillary permeability
C. Binding of drugs to plasma proteins and tissues
D. Lipophilicity
E. Volume of distribution
50
Baig

A. Blood flow
1. The rate of blood flow to the tissue capillaries varies widely.
2. For instance, blood flow to the “vessel-rich organs” (brain, liver,
and kidney) is greater than that to the skeletal muscles. Adipose
tissue, skin, and viscera have still lower rates of blood flow.
3. Propofol has short duration of hypnosis produced by an IV
bolus.
4. High blood flow, together with high lipophilicity of propofol,
permits rapid distribution into the CNS and produces anesthesia.
5. A subsequent slower distribution to skel-etal muscle and adipose
tissue lowers the plasma concentration so that the drug diffuses
out of the CNS, down the concentration gradient, and
consciousness is regained.
51
Baig

B.Capillary permeability
• Capillary permeability is determined by capillary structure and by the chemical
nature of the drug.
1. LIVER AND SPLEEN:
• A significant portion of the basement membrane is exposed due to
large,discontinuous capillaries through which large plasma proteins can pass.
2. BRAIN:
The capillary structure is continuous, and there are no slit junctions.
• These closely jux-taposed cells form tight junctions that constitute the blood–
brain barrier.
• For example, a specific transporter carries levodopa into the brain. By contrast,
lipid-soluble drugs readily penetrate the CNS because they dissolve in the
endothelial cell membrane.
• Ionized or polar drugs generally fail to enter the CNS because they cannot pass
through the endothelial cells that have no slit junctions.
52
Baig

C.Lipophilicity
1. The chemical nature of a drug strongly influences its ability to cross
cell membranes.
2. Lipophilic drugs readily move across most biologic membranes.
3. These drugs dissolve in the lipid membranes and penetrate the
entire cell surface.
4. The major factor influencing the distribution of lipophilic drugs is
blood flow to the area.
5. In contrast, hydrophilic drugs do not readily penetrate cell
membranes and must pass through slit junctions.
53
Baig

D.Volume of distribution
The apparent volume of distribution, Vd , is defined as the
fluid volume that is required to contain the entire drug in the
body at the same concentration measured in the plasma.
Vd = Amount of drug in to the body
(C0)
Plasma concentration at time zero (C0).
Although Vd has no physiologic or physical basis, it can be
useful to compare the distribution of a drug with the volumes
of the water compartments in the body.
54
Baig

1. Distribution into the water compartments in
the body:
• Once a drug enters the body, it has the potential to distribute into
any one of the three functionally distinct compartments of body
water or to become sequestered in a cellular site.
a. Plasma compartment:
If a drug has a high molecular weight or is extensively protein
bound, it is too large to pass through the slit junctions of the
capillaries and, thus, is effectively trapped within the plasma
(vascular) compartment.
As a result, it has a low Vd that approximates the plasma volume
or about 4 L in a 70-kg individual. Heparin shows this type of
distribution.
55
Baig

b. Extracellular fluid:
• Drug with low molecular weight but is hydrophilic, it can pass
through the endothelial slit junctions of the capillaries into the
interstitial fluid.
• Hydrophilic drugs cannot move across the lipid membranes of cells
to enter the intracellular fluid.
• Therefore, these drugs distribute into a volume that is the sum of the
plasma volume and the interstitial fluid, which together constitute
the extracellular fluid (about 20% of body weight or 14 L in a 70-kg
individual).
• Example:Aminoglycoside antibiotics
c. Total body water:
Drug with LMW and is lipophilic, it can move into the interstitium
through the slit junctions and also pass through the cell membranes
into the intracellular fluid.
These drugs distribute into a volume of about 60% of body weight
or about 42 L in a 70-kg individual.
56
Baig

2.Apparent volume of distribution:
1. A drug rarely associates exclusively with only one of the water
compartments of the body.
2. Instead, the vast majority of drugs distribute into several
compartments, often avidly binding cellular components, such as
lipids (abundant in adipocytes and cell membranes), proteins
(abundant in plasma and cells), and nucleic acids (abundant in cell
nuclei).
3. Therefore, the volume into which drugs distribute is called the
apparent volume of distribution (Vd ).
4. Vd is a useful pharmacokinetic parameter for calculating the
loading dose of a drug.
57
Baig

3.Determination of Vd :
• The fact that drug clearance is usually
a first-order process allows calculation
of Vd . First order means that a
constant fraction of the drug is
eliminated per unit of time.
• This process can be most easily
analyzed by plotting the log of the
plasma drug concentration (Cp ) versus
time.
• The concentration of drug in the
plasma can be extrapolated back to
time zero (the time of IV bolus) on the
Y axis to determine C0 , which is the
concentration of drug that would have
been achieved if the distribution phase
had occurred instantly.
This allows calculation of Vd as
Vd= Dose/Co
58
Baig

V. DRUG CLEARANCE THROUGH
METABOLISM
Once a drug enters the body, the process of elimination begins.
The three major routes of elimination are hepatic metabolism,
biliary elimination,and urinary elimination.
Together, these elimination processes decrease the plasma
concentration exponentially.
That is, a constant fraction of the drug present is eliminated in a
given unit of time.
Most drugs are eliminated according to first-order kinetics,
although some,such as aspirin in high doses, are eliminated
according to zero-order or nonlinear kinetics.
Metabolism leads to production of products with
increased polarity, which allows the drug to be eliminated.
59
Baig

• Clearance(CL) estimates the amount of
drug cleared from the body per unit of
time.
• Total CL is a composite estimate
reflecting all mechanisms of drug
elimination and is calculated as follows:
CL = 0 . 693 × Vd / t 1/2
60
Baig

Kinetics of metabolism:
1.First-order kinetics:
The metabolic transformation of drugs is catalyzed by enzymes,
and most of the reactions obey Michaelis-Menten kinetics.
V max [ C ]
v = Rate of drug metabolism = ------------------------
K m + [ C ]
In most clinical situations, the concentration of the drug, [C], is
much less than the Michaelis constant, K m , and the Michaelis-
Menten equation reduces to
v = Rate of drug metabolism = V max [ C ]
K m
This means that a constant fraction of drug is metabolized per
unit of time (that is, with each half-life,the concentration
decreases by 50%). First-order kinetics is also referred to as linear
kinetics.
61
Baig

2.Zero-order kinetics:
• With a few drugs, such as aspirin, ethanol,and phenytoin, the doses
are very large. Therefore, [C] is much
greater than Km , and the velocity equation becomes
V max [ C ]
v = Rate of drug metabolism = [ C ] = V max
• The enzyme is saturated by a high free drug concentration,
and the rate of metabolism remains constant over time. This is
called zero-order kinetics (also called nonlinear kinetics).
• A constant amount of drug is metabolized per unit of time. The rate
of elimination is constant and does not depend on the drug
concentration.
62
Baig

B.Reactions of drug metabolism
The kidney cannot efficiently eliminate lipophilic drugs that
readily cross cell membranes and are reabsorbed in the distal
convoluted tubules.
Therefore, lipid-soluble agents are first metabolized into more
polar (hydrophilic) substances in the liver via two general sets
of reactions, called phase I and phase II.
63
Baig

1.Phase I:
• Phase I reactions convert lipophilic drugs into more polar
molecules by introducing or unmasking a polar functional group,such
as –OH or –NH 2 .
• Phase I reactions usually involve reduction, oxidation, or hydrolysis.
• Phase I metabolism may increase,decrease, or have no effect on
pharmacologic activity.
• a.Phase I reactions utilizing the P450 system:
• Most frequently involved reactions in drug metabolism are catalyzed by
the cytochrome P450
• The P450 system is important for the metabolism of many endogenous
compounds (such as ste-roids, lipids) and for the biotransformation of
exogenous substances (xenobiotics).
• Cytochrome P450, designated as CYP,is a superfamily of heme-
containing isozymes that are located in most cells, but primarily in the
liver and GI tract.
64
Baig

[1] Nomenclature: The family name is indicated by the Arabic
number that follows CYP, and the capital letter designates
the subfamily, for example, CYP3A. A second
number indicates the specific isozyme, as in CYP3A4.
[2] Specificity:
Different P450 isoforms present.
Have the capacity to modify a large number of structurally
diverse substrates.
An individual drug may be a substrate for more than one
isozyme.
Four isozymes are CYP3A4/5,CYP2D6, CYP2C8/9, and CYP1A2.
Most of the CYP3A4 are found in intestinal mucosa, accounting for
first-pass metabolism of drugs such as chlorpromazine and
clonazepam.
65
Baig

[3] Genetic variability:
P450 enzymes exhibit considerable genetic variability among
individuals and racial groups.
Variations in P450 activity may alter drug efficacy and the
risk of adverse events.
CYP2D6, in particular, has been shown to exhibit genetic
polymorphism.
CYP2D6 mutations result in very low capacities to metabolize
substrates.
No benefit from the opioid analgesic codeine due to lack the
CYP2D6 enzyme that activates the drug.
Although CYP3A4 exhibits a greater than 10 fold variability
between individuals, no polymorphisms have been identified so far
for this P450 isozyme.
66
Baig

[4] Inducers:
I. The CYP450-dependent enzymes are an important target for
pharmacokinetic drug interactions through inducing the CYP
isozymes.
II. Certain drugs (phenobarbital, rifampin, and carbamazepine)
increasing the synthesis of one or more CYP isozymes. results
in loss of pharmacological effect of drugs which are
metabolized by these CYP isozymes.
III. Rifampin significantly decreases the plasma concentrations of
HIV pro- tease inhibitors, thereby diminishing their ability to
suppress HIV replication.
IV. St. John’s wort is a widely used herbal product and is a potent
CYP3A4 inducer.
V. Many drug interactions have been reported with concomitant
use of St. John’s wort.
67
Baig

[5] Inhibitors:
An important source of drug interactions that lead to serious adverse
events.
Inhibition of drugs metabolism is through competition for the same
isozyme.
Omeprazole is a potent inhibitor of three of the CYP isozymes
responsible for warfarin metabolism.
If the two drugs are taken together, plasma concentrations of warfarin
increase, which leads to greater anticoagulant effect and increased risk of
bleeding.
More important CYP inhibitors are erythromycin,ketoconazole, and
ritonavir, because they each inhibit several CYP isozymes.
Grapefruit juice inhibits CYP3A4 and leads to higher levels and/or greater
potential for toxic effects with drugs, such as nifedipine, clarithromycin,
and simvastatin, that are metabolized by this system.
68
Baig

b.Phase I reactions not involving the P450
system:
These include
Amine oxidation (catecholamines or histamine)
Alcohol dehydrogenation (ethanol oxidation)
Esterases (metabolism of aspirin in the liver)
Hydrolysis (procaine).
69
Baig

2.Phase II:(conjugation reactions)
• Polar metabolite of drugs can be excreted by the kidneys.
• Lipophillic metabolite (after phase I metabolism) ,subsequent
conjugation reaction with an endogenous substrate, such as
glucuronic acid, sulfuric acid, acetic acid, or an amino acid, results in
polar, usually more water-soluble compounds that are often
therapeutically inactive.
• A notable exception is morphine-6-glucuronide, which is more potent
than morphine.
• Glucuronidation is the most common and the most important
conjugation reaction.
• Drugs already possessing an –OH, –NH 2 , or –COOH group may
enter phase II directly and become conjugated
• The highly polar drug conjugates are then excreted by the kidney or
in bile.
70
Baig

VI. DRUG CLEARANCE BY THE KIDNEY
Polar drugs are eliminated easily from the body.
Number of routes are available for elimination, the most
important being elimination through the kidney into the urine.
Patients with renal dysfunction may be unable to excrete drugs
and are at risk for drug accumulation and adverse effects.
Elimination of drugs via the kidneys into urine involves the
processes
– Glomerular filtration,
– Active tubular secretion
– Passive tubular reabsorption.
71
Baig

Summery Drug elimation:
72
Baig

1.Glomerular filtration:
Free drug (not bound to albumin) flows through the capillary
slits into the Bowman space as part of the glomerular filtrate.
The glomerular filtration rate (GFR) is normally about 125
mL/min but may diminish significantly in renal disease.
Lipid solubility and pH do not influence the passage of drugs
into the glomerular filtrate.
However, variations in GFR and protein binding of drugs do
affect this process.
73
Baig

2. Proximal tubular secretion:
Drugs that were not transferred into the glomerular filtrate leave the
glomeruli through efferent arterioles,(network)
Two energy-requiring active transport systems take part in secretion of
drugs:
a. For anions (deprotonated forms of weak acids)
b. For cations (protonated forms of weak bases).
Each of these transport systems shows low specificity and can transport
many compounds.
Thus, competition between drugs for these carriers can occur within each
transport system.
74
Baig

3.Distal tubular reabsorption:
As a drug moves toward the distal convoluted tubule, its concentration
increases and exceeds that of the perivascular space.
The uncharged drug may diffuse out of the nephric lumen, back into the
systemic circulation.
Manipulating the urine pH to increase the fraction of ionized drug in the
lumen may be done to minimize the amount of back diffusion and increase
the clearance of an undesirable drug.
As a general rule, weak acids can be eliminated by alkalinization of the
urine,whereas elimination of weak bases may be increased by acidification of
the urine. This process is called “ion trapping.”
For example,a patient presenting with phenobarbital (weak acid) overdose
can be given bicarbonate, which alkalinizes the urine and keeps the drug
ionized, thereby decreasing its reabsorption. 75
Baig

4.Role of drug metabolism:
Most drugs are lipid soluble and, without chemical
modification, would diffuse from the tubular lumen to
perivascular space (why) due to concentration gradient
between the drug concentration in the filtrate and perivascular
space.
To minimize this reabsorption, drugs are modified primarily in
the liver into more polar substances via phase I and
phase II reactions.
The polar or ionized conjugates are unable to back diffuse out
of the kidney lumen.
76
Baig

Summery Drug elimation:
77
Baig

VII. CLEARANCE BY OTHER ROUTES
Drug clearance may also occur via the intestines, bile, lungs, and breast,milk etc.
Drugs that are not absorbed after oral administration or drugs that are secreted
directly into the intestines or into bile are eliminated in the feces.
The lungs are primarily involved in the elimination of anesthetic gases (for
example, isoflurane).
Elimination of drugs in breast milk may expose the breast-feeding infant to
medications and/or metabolites being taken by the mother and is a potential
source of undesirable side effects to the infant.
Excretion of most drugs into sweat, saliva, tears,hair, and skin occurs only to a
small extent.
Total body clearance and drug half-life are important measures of drug clearance
that are used to optimize drug therapy and minimize toxicity.
78
Baig

A.Total body clearance:
The total body (systemic) clearance, CL total , is the sum of all
clearances from the drug-metabolizing and drug-eliminating
organs.
The kidney is often the major organ of elimination. The liver
also contributes to drug clearance through metabolism and/or
excretion into the bile.
Total clearance is calculated using the following equation:
CL total = CL hepatic + CL renal + CL pulmonary + CL other
where CL hepatic + CL renal are typically the most important.
79
Baig

B.Clinical situations resulting in changes in drug half-life
• Adjustment in dosage is required when a patient has an abnormality.
Increase in drug half-life include in case of
1) diminished renal or hepatic blood flow
2) decreased ability to eliminate drug from plasma
3) decreased metabolism,
These patients may require a decrease in dosage or less frequent dosing intervals.
In contrast, the half-life of a drug may be
1) decreased by increased hepatic blood flow
2)decreased protein binding, or increased metabolism.
This may necessitate higher doses or more frequent dosing intervals.
80
Baig

FACTORS MODIFYING
DRUG ACTION
81
Baig

Factors:
1. Body weight
2. Age
3. Sex
4. Species and race
5. Genetics
6. Route of administration
7. Environmental factors and time of administration
8. Psychological factor
9. Pathological states
82
Baig

Introduction:
 Variation in response to the same dose of a drug between different
patients and even in the same patient on different occasions was
observed.
 Categories of differences among individuals are responsible for
the variations in drug response:
(1) Individuals differ in pharmacokinetic handling of drugs:
(2) Variations in number or state of receptors, coupling proteins or other
components of response effectuation.
(3) Variations in neurogenic/hormonal tone or concentrations of specific
constituents, e.g. atropine tachycardia depends on vagal tone,
propranolol bradycardia depends on sympathetic tone
83
Baig

The factors modify drug action either:
a) Quantitatively:
The plasma concentration of the drug is increased or decreased. Most
of the factors introduce this type of change and can be dealt with by
adjustment of drug dosage.
b) Qualitatively:
The type of response is altered, e.g. drug allergy or idiosyncrasy. This
is less common but often precludes further use of that drug in the
affected patient.
84
Baig

Factors affecting dose response:
Fall in two categories
1. Genetic 2. Non genetic
Provide guidance for the selection of appropriate drug and
dose for an individual patient.
However, final adjustments have to be made by observing the
response in a given patient on a given occasion.
85
Baig

Factors modifying drug action:
1. Body size:
It influences the concentration of the drug attained at the site of action.
The average adult dose refers to individuals of medium built.
For exceptionally obese or lean individuals and for children dose may be calculated
on body weight (BW) basis:
It has been argued that body surface area (BSA) provides a more accurate basis for
dose calculation, because total body water, extracellular fluid volume and
metabolic activity are better paralleled by BSA.
86
Baig

2. Age:
Infants and children have important physiological differences from adults.
 The newborn has low g.f.r. and tubular transport is immature.
 The t1/2 of drugs excreted by glomerular filtration (gentamicin) and tubular
secretion (penicillin) is prolonged by 3 to 5 times.
 Glomerular filtration reaches adult rates by 5 month of age and tubular
secretion takes about 7 months to mature.
 Hepatic drug metabolizing system is inadequate in newborns -
chloramphenicol can produce gray baby syndrorne.
 Blood-brain barrier is more permeable-drugs attain higher concentration in
the CNS (accumulation of unconjugated bilirubin causes kernicterus).
 Drug absorptjon may also be altered in infants because of lower gastric
acidity and slower intestinal transit.
 Transdermal absorption, is faster because their skin is thin and more
permeable.
87
Baig

 After the first year of life, drug metabolism is often faster than
in adults, e.g. theophylline, phenytoin, carbamazepine t1/2 is
shorter in children.
 Solid dosage forms and aerosol inhalations are difficult to
administer to young children.
 Children are growing and are susceptible to special adverse
effects of drugs eg: suppression of growth can occur with
corticosteroiods.
 In the elderly, renal function progressively declines (intact
nephron loss) so that is - 75% at 50 years and - 50% at 75 year'
compared to young adults. Drug doses has to be reduced
88
Baig

3. Sex
a. Females have smaller body size and require doses that are on the lower side
of the range.
b. Subjective effects of drugs may differ in females because of their mental
makeup .
c. Maintenance treatment of heart failure with digoxin is reported to be
associated with higher mortality among women than among men.
d. A number of antihypertensives ( clonidine, methyldopa, Beta-blockers,
diuretics) interfere with sexual function in males but not in females .
e. Gynaecomastia is a side effect (of ketoconazole, metoclopramide,
chlorpromazine, digitalis) that can occur only in men.
f. Ketoconazole causes loss of libido in men but not in women.
g. Androgens are unacceptable to women and estrogens to men. In women
consideration must also be given to menstruation, pregnancy and lactation.
89
Baig

Drugs given during pregnancy can affect the foetus .
There are marked and progressive physiological changes during pregnancy,
especially in the third trimester, which can alter drug disposition.
(i) Gastrointestinal motility is reduced -> delayed absorption of orally
administered drug.
(ii) Plasma and extracellular fluid volume expands-volume of drug distribution may
increase.
(iii) While plasma albumin level falls, -the unbound fraction of acidic drugs
increases but that of basic drugs decreases.
(iv) Renal blood flow increases markedly polar drugs are eliminated faster.
(v) Hepatic microsomal enzymes undergo induction-many drugs are metabolized
faster.
Thus, the overall effect on drug disposition is complex and often difficult to predict.
90
Baig

4. Species and race
• Rabbits are resistant to atropine.
• Rats and mice are resistant to digitalis and rat is more sensitive to curare than
cat.
• These differences are important while extrapolating results from experimental
animals to man.
• Blacks require higher and Mongols require lower concentrations of atropine
and ephedrine to dilate their pupil.
• Beta-blockers are less effective as antihypertensive in AfroCaribbeans.
• Indians tolerate thiacetazone better than whites.
91
Baig

5. Genetics
The dose of a drug to produce the same effect may vary by 4--6 fold among
different individuals.
All key determinants of drug response, viz. transporters, metabolizing
enzymes, ion channels, receptors with their couplers and effectors are
controlled genetically.
The study of genetic basis for variability in drug response is called
'Pharmacogenetics'.
As the genomic technology has advanced, gene libraries and huge data bases
(like 'pharmacogenetics and pharmacogenomics knowledge base', 'Human
genome variation database', etc.) have been created aiming at improving
precision in drug therapy.
92
Baig

6. Route of administration
• It governs the speed and intensity of duration of response.
• Parenteral administration is often resorted to for more rapid, more
pronounce and more predictable drug action.
• A drug may have entirely different uses through different routes,
e.g. magnesium sulfate given orally cause: purgation, applied on
sprained joints-decreases swelling, intravenously it produces
depression and hypotension.
93
Baig

7. Environmental factors and time of administration
Exposure to insecticides, carcinogens,tobacco smoke and consumption of
charcoil,broiled meat are well known to induce drug metabolism.
8. Psychological factor:
Efficacy of a drug can be affected by patient's beliefs, attitudes and
expectations.
This is particularly applicable to centrally acting drugs, e.g. a nervous and
anxious patient requires more general anaesthetic; Alcohol generally
impairs performance.
Punishment (which induces anxiety) is introduced, it may actually improve
performance.
94
Baig

References:
1. Rang H. P., Dale M. M., Ritter J. M., Flower R. J., Rang and Dale‘s
Pharmacology,.Churchil Livingstone Elsevier
2. Katzung B. G., Masters S. B., Trevor A. J., Basic and clinical pharmacology, Tata
Mc Graw-Hill
3. Goodman and Gilman‘s, The Pharmacological Basis of Therapeutics
4. Marry Anne K. K., Lloyd Yee Y., Brian K. A., Robbin L.C., Joseph G. B., Wayne A.
K.,Bradley R.W., Applied Therapeutics, The Clinical use of Drugs, The Point
Lippincott Williams & Wilkins
5. Mycek M.J, Gelnet S.B and Perper M.M. Lippincott‘s Illustrated Reviews-
Pharmacology
6. K.D.Tripathi. Essentials of Medical Pharmacology, JAYPEE Brothers Medical
Publishers (P) Ltd, New Delhi.
Baig
95

Thank You
96
Baig

Unit 1 General Pharmacology (As per PCI syllabus)

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Ähnlich wie Unit 1 General Pharmacology (As per PCI syllabus)

Ähnlich wie Unit 1 General Pharmacology (As per PCI syllabus) (20)

Mehr von Mirza Anwar Baig

Mehr von Mirza Anwar Baig (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

Unit 1 General Pharmacology (As per PCI syllabus)