Introduction to Pharmacology

Introduction to Pharmacology

Prepared by:Abraham Daniel C. Cruz, MD, MS Pharmacology (cand.)

Objectives

• By the end of this lecture, the student should be able to:• Define pharmacology, medical pharmacology and toxicology• Describe the history of pharmacology• Explain the basic principles of pharmacogenetics and its clinical

applications• Describe the different steps in drug development• Acquire an overview of basic pharmacodynamic and

pharmacokinetic principles

What is Pharmacology?

HISTORY OF PHARMACOLOGYPREHISTORY- Egypt, China, India recognize

beneficial and toxic effects from plant and animals

- most were worthless and harmful

END OF 17TH CENTURY - Observations & experiments- Development of materia medica- lack of methods of purifying

active agents and testing hypothesis on MOA

LATE 18th & 19th CENTURY- Francois Magendie and Claude

Bernard develop methods for experimental pharmacology and physiology

1940s & 50s- Introduction of rational

therapeutics and controlled clinical trial

LAST 30 - 50 YEARS- receptor pharmacology- molecular MOA- orphan receptors- pharmacogenomics

Basic Principle 1

Basic Principle 2 Good Evidence Good Medicine

Pharmacology and Genetics

Definitions

• Pharmacogenetics• Genetic differences in metabolic pathways which can affect

individual responses to drugs (therapeutic AND adverse effects)• Pharmacogenomics

• the technology that analyses how genetic makeup affects an individual's response to drugs

• deals with the influence of genetic variation on drug response in patients by correlating gene expression or single-nucleotide polymorphisms with a drug's efficacy or toxicity

HeredityHardy Weinberg LawHarnessing the Power of PharmacogeneticsAdapted from:

Irma R. Makalinao MD FPPS FPSCOT

Professor of Pharmacology and Toxicology

UP College of Medicine

Terminology of Mendelian Genetics

• Parental,F1, and F2

• Dominant and recessive• Phenotype and genotype• Monohybrid cross

Mendel’s Results

•Principle of Dominance: One form of a hereditary trait dominates or prevents the expression of the recessive trait• Dominant: trait that is expressed in the F1

generation• Recessive: Trait that is not expressed in the

F1 generation

Mendel’s Hypothesis

• For every trait there must be a pair of factors- one maternal, one paternal These factors are called genes

• Dominant represented by a capital

• Recessive represented by lower case

• Gene Segregation: genes segregate when gametes are formed

• Genetic polymorphism• Hereditary variations in which sharply distinct qualities

coexist along side each other in a population • May refer to genetic loci in which the variants occur with a

frequency of 1-2%

• Twin studies• Family studies

• Enables the investigator to discriminate: 1. various modes of genetic transmission2. Dominance – recessivity relationships that characterize expression of the trait

Genetic Profile of Human Drug Response

Estimating Heritability from Twin Studies

• Technique of using twin studies was originally devised by Francis Galton during the 19th century

• Index of Heritability (Holzinger index)• If H>1 phenotypic variation due to heredity• If H =0 attributable to environment

Twin Studies in Pharmacogenetics

• Acetylation polymorphism among the first one studied• Studied first in the German population• Japanese study showed good correlation (r=0.95) among

identical twin, among fraternal twins (r=0.25)• Concept of concordance vs. discordance

• Relative importance of heredity and environment to more complicated pharmacological phenomena can also be better appreciated from a twin study

• Value of data from identical and fraternal twins

Urinary elimination of INH Identical Twins Fraternal Twin

Sex INH Eliminated

Sex INH Eliminated

MM

8.88.3

FF

12.113.7

FF

26.025.2

FF

10.94.6

MM

11.812.4

MM

11.08.5

FF

12.211.5

FF

3.915.2

FF

4.14.4

MM

10.515.6

Basic Patterns of Inheritance

• Autosomal Dominant• Autosomal Recessive• X-linked

• G6PD deficieny• Pyridoxine senstitive anemia• Vasopressin resistance

• Mitochondrial inheritance • Aminoglycoside induced deafness• Predominantly maternally inherited

Hardy-Weinberg Law• Developed in 1908• algebraic formula to estimate the frequency of a dominant or recessive

gene in a population based on the frequency with which the trait or condition is found in that population

p2 + 2pq + q2 = 1• p2 = frequency of homozygous dominant population• 2pq = frequency of heterozygous population• q2 = frequency of homozygous recessive population• p = frequency of the dominant allele in a population • q = frequency of the recessive allele in the population, and • p + q = 1

Population studies

Population studies

Rx + =

Rx + = Rx + =

Imagine being able to

walk into your doctor’s office and present

a “smart card” encoded either with the

sequence of your genome itself or with an access

code granting permission to log on to a secure database

containing your genomic information.

Pharmacogenomics significantly impacts this development model by identifying people

whose genetic profiles or "bar codes" predict that they are inappropriate for a given

medication, whether due to poor efficacy and/or adverse side effects.

Pharmacogenomics“Drugs by Design?”

“In the very near future, primary care physicians will routinely perform genetic tests before writing a prescription because (they will) want to identify the poor responders.”F. Collins (AAFP Annual Meeting, 1998)

Environmental Health Perspectives • VOLUME 111 | NUMBER 11 | August 2003

The Benefits of Personalized MedicineExperts believe that minimizing adverse drug reactions are likely to be the first area in which Pharmacogenomics will benefit patients

Anticipated Benefits of Pharmacogenomics

• More Powerful Medicines • drugs based on the proteins, enzymes, and RNA molecules associated with genes

and diseases. • facilitate drug discovery and allow drug makers to produce a therapy more

targeted to specific diseases. • maximize therapeutic effects and decrease damage to nearby healthy cells

Anticipated Benefits of Pharmacogenomics

• More Accurate Methods of Determining Appropriate Drug Dosage• Current methods

• of based on weight and age replaced with dosages based on a person's genetics • maximize value of therapy's value and decrease the likelihood of overdose

Anticipated Benefits of Pharmacogenomics

• Better Vaccines • Vaccines made of genetic material, either DNA or RNA, promise all

the benefits of existing vaccines without all the risks. • activate the immune system without causing infections. • inexpensive, stable, easy to store, and capable of being

engineered to carry • several strains of a pathogen at once

Anticipated Benefits of Pharmacogenomics

• Better and safer medications the first time• Instead of the standard trial-and-error method prescribe the best available drug

therapy from the beginning• take guesswork out of finding the right drug• speed recovery time• increase safety as the likelihood of adverse reactions is eliminated

Anticipated benefits of Pharmacogenomics

• Decrease in the Overall Cost of Health Care • Decreases in:

• ADRs• failed drug trials• time to get a drug approved• time patients are on medication• medications patients must take to find an effective therapy• effects of a disease on the body (through early detection)

• An increase in the range of possible drug targets will promote a net decrease in the cost of health care

GENETIC POLYMORPHISMS

Pharmacokinetic Pharmacodynamic

•Transporters•Plasma protein binding•Metabolism

•Receptors•Ion channels•Enzymes•Immune molecules

Genetic Polymorphisms of Drug Metabolizing Enzymes are significant if:

• The enzyme changes the way the drug is metabolized in the human body (quantitative disposition)

• Active metabolites are formed

Genetic Polymorphisms of Drug Metabolizing Enzymes are significant if:

• Therapeutic index is narrow and the risk for toxicity is high with small changes in the concentration

(Ex. Phenytoin)

• There are significant interactions with other drugs, food or disease

From: Evans WE, Relling MV. Pharmacogenomics: Translating functional genomics into rational therapeutics. Science 286:487-491, 1999.

Genetic polymorphisms in human drug metabolizing enzymes

Succinylcholine Induced Paralysis

N-acetyltransferase Polymorphism • Acetylation polymorphism of Isoniaizd was first

discovered in 1950s• Trait and mode of inheritance correlated with adverse

reactions

Inherited Variations in

Pharmacodynamics

Warfarin Resistance• Discontinuous variation in response to warfarin

• Resistance to anticoagulation • Present theory:

• Decreased sensitivity of liver enzyme or receptor sites to anticoagulants

• Increased sensitivity to Vitamin K1

INDEX CASE: HM, 73 year old male with MIWarfarin dose: 145 mg/day as maintenance doseVitamin K1: 1/2 mg to increase prothrombin time to 25-43% activity

G6PD Deficiency Drug-induced Hemolysis

• X-linked trait affecting nearly 400 M people• Predisposes an individual to hemolytic anemia induced by a specific drug• G6PD catalyzes the first step in HMP oxidation pathway of carbohydrate

metabolism leading to the oxidation of NADP to NADPH• NADPH needed by to maintain reduced GSH

INDEX CASE:

Among Blacks who developed hemolytic anemia after treatment with PRIMAQUINE an antimalarial

Drugs and other agents causing clinically significant hemolysis in G6PD deficiency

Acetanilid Phenylbutazone Sulfanilamide Sulfacetamide Sulfapyridine Sulfamethoxazole Thiazolesulfone Diaminodiphenylsulfone Trinitrotoluene

Nitrofurazone Nitrofurantoin Furazolidone Furaltoldone Pamaquine Primaquine Pentaquine Naphthalene Fava beans

Inherited Variations in Pharmacodynamics

Condition Abnormal Enzyme

Inheritance Frequency

Drugs

Inability to taste phenylthiourea

Unknown Autosomal recessive

Approx. 30% of Caucasians

Drugs containing N-C-S group Phenylthiourmethyl Propylthiouracil

Glaucoma due to abnormal intraocular pressure to steroids

Unknown Autosomal recessive

Approx. 5% of USA population

Corticosteroids

Inherited Variations in Pharmacodynamics

Condition AbnormalEnzyme

InheritanceFrequency

Drugs

MalignantHyperthermiawith muscularrigidity

Unknown Autosomaldominant

Approx. 1 in 20000anesthetized patients

Halothane Other

generalanesthetics

MethemoglobinReductasedeficiency

Methemo-globinReductase

Autosomal recessive Heterozygous

(HZG) carriersaffected

Approx. 1 in 100HZG carriers

Manydifferentdrugs

Specific Applications of

Pharmacogenetics

Personalizing cancer chemotherapy

• Thioupurine S-methyltransferase (TPMT) • essential for the metabolism of thiopurine medications used to

treat acute lymphoblastic leukemia (ALL) most common form of childhood cancer

Application of pharmacogenetics to cancer therapy• There is now a commercially available diagnostic test measuring a

patient’s ability to produce the metabolic enzyme thiopurine S-methyltransferase (TPMT)

TPMT deficiency screening

• Genetic testing gives clinicians the ability to classify ALL patients according their TPMT genotype, which allows for optimized dosing.

Why do you need to screen?

• Doses in patients with alleles rendering them deficient in TPMT (who are thus less tolerant of thiopurine medications) are reduced by as much as 95%.

Cytochrome p450 enzymes

Ecogenetics and cancer

• Genetic differences in the metabolic activation or detoxification of carcinogenic chemicals as determinants or risk

• Increased risk of cancer associated with the following polymorphism• CYP (2A1, 1A2, 2E1)• Gluthathione transferases (GSMT1, GSTT1)• Epoxide hydrolase• NAT2

Neurotoxicology, 21(1-2) 2000

Issues related to ecogenetic research

• Functional significance of the polymorphism• Interaction/combination of susceptibility genotypes

• Increased risk of Lung cancer (Hayashi et al)• CYP 1A1 Val/Val genotype = 2.0• GSTM (-) = 1.44• CYP + GSTM = 5.58

• Ethical, legal, social issues surrounding studies of susceptible individuals

Environmental Genome Project

• The mission of the EGP is to improve understanding of human genetic susceptibility to environmental exposures

The power of pharmacogenetics

• These new tools must be used for the purposes of identifying and controlling harmful exposures rather than to exclude the genetically predisposed

(Eaton et al 1998)

DRUG DEVELOPMENT

Drug Development and Evaluation

• Possible therapeutic value drug development

• DOH-BFAD FDA• Ensure safety and reliability• Must undergo pre-clinical trials and clinical

trials (phase I, II, III)

Drug Development and EvaluationPre -clinical

Pre-Clinical Trials

In vitro and animal studiesPurpose

Evaluate toxicity Determine presumed effects

Reasons for dropping Lack therapeutic activity Toxic to living animals Teratogenic (adverse effect on fetus) Small or narrow margin of safety

Phase I

20-50 HEALTHY volunteers (young men)Clinical investigators evaluate:

Safety (adverse effects) Pharmacokinetics Therapeutic effects (?)

Reasons for dropping Lack therapeutic effect Unacceptable adverse effects Highly teratogenic Too toxic

Phase II

20 – 300 subjects; patients WITH DISEASEEvaluate

Efficacy – does it work? Safety

Reasons for dropping Less effective than anticipated Too toxic, unacceptable adverse effect Low benefit-t0-risk ratio No more effective than other available drugs

Phase III

• Randomized, controlled multicenter trials• Double blind• Large patient groups (300 – 3000)• Patients WITH DISEASE• Usually compared with “gold standard” and placebo• Most expensive, time consuming, difficult trials to design and run

Phase IV

• Post marketing surveillance• Pharmacovigilance• Detect any rare or long term adverse effects• Report any untoward or unexpected adverse effect not seen during

pre-clinical and phases I – III• After prolonged use and wide distribution

• Risk of malignancy• Thrombotic events• Idiosyncratic side effects in special populations

Drug Development and Evaluation

Drugs Withdrawn from the Market

• Troglitazone (Rezulin) – liver failure• Rofecoxib (Vioxx) – thrombotic events• Dexfenfluramine (Redux)– cardiotoxicity

Further Drug Classification

• Pregnancy Category• A, B, C, D, X

• Controlled drugs• Closely monitored by the Dangerous Drugs Board of the DOH• Need S-2 license• Prescription in triplicate

FDA Pregnancy Categories

• A- Adequate studies in pregnant , no risk• B- Animal studies no fetal risk, but human not adequate

OR Animal toxicity but human studies no risk• C- Animal studies show toxicity, human studies inadequate

but benefit of use may exceed risk• D- Evidence of human risk, but benefits outweigh risks• X- Fetal abnormalities in humans, risk greater than benefit

GENERAL PRINCIPLES IN PHARMACOLOGY

Nature of Drugs

• Drug - any substance that brings about a change in biologic function through its chemical action

• interacts as a (pharmacologic) agonist (activator) or antagonist (inhibitor) with a specific molecule in the biologic system that plays a regulatory role (receptor)

• Chemical antagonists - interact directly with other drugs• Osmotic agents - interact almost exclusively with water

molecules; concept of receptor pharmacology does not apply.

Sources of DrugsSource Example Generic/Trade Name Classification

Plants Cinchona barkPurple FoxglovePoppy Plant

QuinidineDigitalisMorphineCodeine

Anti-arrhythmicInotropeAnalgesicAnalgesic, Antitussive

Minerals MagnesiumZincGold

Milk of MagnesiaZinc Oxide OintmentAurafonin

Antacid, LaxativeSuncreen, Skin ProtectantAnti-inflammatory; used in RA

Animals Pancreas (Cow, Pig)Stomach (Cow, Pig)Thyroid Gland

Insulin

Pepsin

Thyroid, USP

Antidiabetes

Digestion

Hormone

Synthetic MeperidineDiphenoxylateCo-trimoxazole

DemerolLomotilBactrim, Septa

AnalgesicAntidiarrhealAnti-infective

Nature of Drugs

• Poisons - drugs that have almost exclusively harmful effects• Paracelsus (1493–1541) - "the dose makes the poison" any

substance can be harmful if taken in the wrong dosage• Toxins - poisons of biologic origin (from plants or animals);

different from inorganic poisons (heavy metals)• Requirements for drug – receptor interaction

• Appropriate size, electrical charge, shape, and atomic composition• Permeation - Can be transported from its site of administration to its site of

action absorption and distribution• Appropriate duration of action inactivation or excretion

Physical Nature of Drugs

• State at room temperature - determines best route of administration

• solid (aspirin, atropine); liquid (nicotine, ethanol); gas (nitrous oxide)

Organic drugs• carbohydrates, proteins, lipids, and their constituents• weak acids or bases implications on kinetics and

compartmentalization (ion trapping)

Inorganic drugs• lithium, iron, and heavy metals

Drug Size

• 100 – 1000 MW range of molecular weight of most drugs

• 100 – lower limit; minimum molecular weight of drug to achieve selective binding

• upper limit – determined by the need of the of the drug to traverse membranes

• drugs larger than 1000 MW do not diffuse readily diffuse between compartments of the body

• implication – very large drugs (proteins) must be administered directly in their site of drug action

Drug Reactivity and Drug-Receptor Bonds

BOND TYPE MECHANISM BOND STRENGTH

van der Waals Shifting electron density in areas of a molecule, or in a molecule as a whole, results in the generation of transient positive or negative charges. These areas interact with transient areas of opposite charge on another molecule.

+

Hydrogen Hydrogen atoms bound to nitrogen or oxygen become more positively polarized, allowing them to bond to more negatively polarized atoms such as oxygen, nitrogen, or sulfur.

++

Ionic Atoms with an excess of electrons (imparting an overall negative charge on the atom) are attracted to atoms with a deficiency of electrons (imparting an overall positive charge on the atom).

+++

Covalent Two bonding atoms share electrons. ++++

• DRUG SHAPE• Chirality – molecule has a non-superposable mirror image

• chiral center (asymmetric carbon) S)(-) isomer - “left-oriented” or (R)(+) isomer – “right-oriented” per chiral center

• Implications: potency, toxicity, metabolism • most drugs are administered as racemic mixtures (50% or more is less

active, inactive, or actively toxic)

• RATIONAL DRUG DESIGN• based on SARs and info about receptors• computer models + Human Genome Project

• RECEPTOR NOMENCLATURE • IUPHAR Committee on Receptor Nomenclature and Drug

Classification

DRUG-BODY INTERACTIONSPharmacodynamics and Pharmacokinetics

Pharmacodynamics

1. Drug (D) + receptor-effector (R) D-R-effector complex effect

2. D + R D-R complex effector molecule effect3. D + R D-R complex activation of coupling molecule

effector molecule effect4. Inhibition of metabolism of endogenous activator

increased activator increased effect*effector may be part of the receptor molecule or may be a separate molecule

Types of Drug-Receptor Interactions

Drugs That Inhibit Their Binding Molecules

• “indirect agonist” - mimic agonists by inhibiting molecules responsible for terminating the action of an endogenous agonist

• amplify effects of physiologically released agonists

• effects are more selective and less toxic than those of exogenous agonists

Agonists, Partial Agonists, Inverse Agonists

• Receptor status• Ri - inactive, nonfunctional • Ra - activated

• constitutive activity (-) agonist; some of the receptors are activated; produce same physiologic effect as agonist-induced activity

• Agonists - ↑ affinity to Ra = ↑ effect• full agonists vs. partial agonists

• Antagonists – Ri = Ra blocks access of agonists to receptor prevent usual agonist effect no change

• Inverse agonists - ↑ affinity to Ri = produce opposite effects when compared with agonists

Receptors and Inert Binding Sites

Receptor• Selective in “choosing” ligands

to bind avoid constant activation of the receptor by many different ligands

• Function changes upon ligand binding alters biologic system pharmacologic effect

Inert Binding Site• ex. Albumin

• bind drugs but (-) regulatory function no detectable change

• Significant in pharmacokinetics• Distribution • Bioavailability

Pharmacokinetics

• drug should reach its site of action; scenarios:• Drug is active, lipid soluble, stable given as such• Prodrug absorbed and distributed converted to the active drug by

metabolic processes• Apply drug directly to target tissue• (most common) administer drug in one compartment move to site of action

in another compartment; requires:• Permeation – perfusion-rate limited versus permeability-rate limited

• absorption • distribution

• Elimination• metabolic inactivation• excretion

Permeation

• Aqueous diffusion• Through aqueous pores• Not present in some tissues• Driven by concentration gradient• Does not occur if drug is protein-bound

• Lipid diffusion• Most important factor for drug

permeation• lipid: aqueous partition coefficient• for weak acids and bases

• Charged molecules attract water• Dissociation depends on pH of medium and

pKa of drug• Henderson-Hasselbalch equation –

determines ratio of lipid-soluble form to water-soluble form

• Special carriers• peptides, amino acids, and glucose• via active transport or facilitated

diffusion• selective, saturable, and inhibitable

• Endocytosis • Vit B12• Iron

• Exocytosis• Neurotransmitters• Thyroid hormones

Permeation, Blood Flow, and Protein BindingPerfusion-Rate Limitation• Membrane offers no resistance• Drug in the blood leaving the

tissue is in equilibrium with that of the tissue blood and tissue viewed as one equilibrium achieved instantaneously

• Alteration in protein content is NOT expected to affect rate of transport at a given concentration

Permeability-Rate Limitation• Membrane resistance to drug

movement is high• Movement is slow and

insensitive to changes in perfusion equilibrium is not achieved by the time the blood leaves tissue view blood and tissue as separate

• Altered protein-binding influences rate of transport by affecting unbound concentration

Fick’s Law of Diffusion

• Where:• C1 = higher concentration• C2 = lower concentration• Area = cross-sectional area of the diffusion path• Permeability coefficient = measure of the mobility of the drug molecules in the

medium of the diffusion path• thickness = length of the diffusion path

• lipid diffusion• lipid: aqueous partition coefficient - major determinant of drug mobility

Ionization of Weak Acids and Bases

• Many drugs are weak acids or bases

• ionized molecules attract water dipoles polar, relatively water – soluble, lipid – insoluble complex

• lipid diffusion depends on relatively high lipid solubility drug ionization may markedly reduce the ability to permeate membranes

Henderson-Hasselbach Equation

Weak Acid Weak Base

• REMEMBER: • neutral uncharged/unionized/non-polar more lipid soluble• law of mass action reactions move to the:

• left in an acid environment (low pH, excess protons available) • right in an alkaline environment• the lower the pH relative to the pKa , the greater will be the fraction of drug in the protonated form

unprotonatedprotonated

Ion Trapping

Weak Bases

APPLICATIONS

Case

• A 12 year old child has bacterial pharyngitis and is to receive an oral antibiotic. Ampicillin is a weak organic acid with a pKa of 2.5. What percentage of a given dose will be in the lipid soluble form in the duodenum at a pH of 4.5?

• A. about 1%• B. about 10%• C. about 50%• D. about 90%• E. about 99%

Case

• A 12 year old child has bacterial pharyngitis and is to receive an oral antibiotic. Ampicillin is a weak organic acid with a pKa of 2.5. What percentage of a given dose will be in the lipid soluble form in the duodenum at a pH of 4.5?

• A. about 1%• B. about 10%• C. about 50%• D. about 90%• E. about 99%

Explanation

• Ampicillin is an acid, so it is more ionized in an alkaline pH and less ionized in an acidic pH. The Henderson-Hasselbach equation predicts that the ratio changes from 50/50 at the pH equal to the pKa, to 1/10 (protonated/unprotonated) at 1 pH unit more alkaline than the pKa, and 1/100 at 2 pH units more alkaline. For acids, the protonated form is the non-ionized, more lipid-soluble form

Computation

• log (protonated/unprotonated) = pKa - pH• substituting the values, we get log (protonated/unprotonated) = 2.5 - 4.5• log (protonated/unprotonated) = -2• to get the actual value of (protonated/unprotonated), you need a

scientific calculator and get the antilog of -2• if u remember a little bit of calculus, the antilog of -2 is also equal to 10

raised to the exponent of -2• 10 raised to the exponent of -2 is equal to .01• .01 = 1/100 = 1%

Drug Groups

• one or more prototype drugs can be identified that typify the most important characteristics of the group

• Study in detail• permits classification of other drugs as variants

• study differences from prototype

Sources of Information

• Pharmacology: Examination and Board Review, by Trevor, Katzung, and Masters (McGraw-Hill, 2010)

• USMLE Road Map: Pharmacology, by Katzung and Trevor (McGraw-Hill, 2006)

• references at the end of each chapter of Katzung

• Periodicals/journals• The New England Journal of Medicine• The Medical Letter on Drugs and Therapeutics• Drugs

• Physicians’ Desk Reference

• Package inserts

• Micromedex

• Drug Interactions: Analysis and Management

• US and Philippine FDA

THANK YOU!!!