general principles in pharmacology
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General Principles in Pharmacology. Ma. Victoria M. Villarica M.D. Basic Principles:. Pharmacology – study of substances that interact with living systems to produce an effect Pharmacotherapeutics – drugs used in the diagnosis, treatment and prevention of diseases - PowerPoint PPT PresentationTRANSCRIPT
General Principlesin
Pharmacology
Ma. Victoria M. Villarica M.D.
Basic Principles:
• Pharmacology – study of substances that interact with living systems to produce an effect
• Pharmacotherapeutics – drugs used in the diagnosis, treatment and prevention of
diseases• Toxicology – toxic effects of drugs• Pharmacognosy – drugs in their unaltered
state
• Pharmacogenetics
• Pharmacoeconomics
• Drug – substance that brings about change through its’ chemical action
• Physical properties of a drug:
a. physical nature of a drug
b. drug size
c. chemical forces – covalent, electrostatic, hydrophobic
Basic Pharmacologic concepts:
1. Pharmacokinetics – body → drug
- “ drug-concentration” relationship
4 processes:
A. absorption – rate → circulating fluids
factors: drug solubility, drug concentration, local conditions, blood flow, surface area
Routes of drug administration:
a. enteral – oral , rectal b. parenteral – IV, IM, SC, intraperitoneal,
intrathecal, intraarterial, inhalational, otic, optic
c. topical
B. distribution – site of administration →site of action
factors: size of the organ, blood flow, solubility, binding
Permeation – how a drug transverses the plasma membrane
• Passive diffusion, active transport, facilitated diffusion, pinocytosis
C. metabolism – biotransformation; liver
2 phases:
1. phase I – introduce or expose a functional group
e.g. dealkylation, oxidation, reduction,
hydrolysis, deamination, cytochrome p450
2. Phase II – formation of covalent linkage between the functional group on the
parent compound; cytosol
e.g. glucoronidation, sulfation, acetylation
Factors
Inducer
Inhibitor
D. excretion – elimination; kidneys
Factors
2 Basic Parameters of Pharmacokinetics
1. Volume of distribution (Vd) – amount of apparent space in the body able to contain a drug
Vd = amt of drug in body / concentration (C)
2. Clearance (Cl) – ability of the body to eliminate a drug
Cl = rate of elimination / C
Clearance:a. capacity-limited elimination – varies,
depending upon concentration of the drug that is achieved; saturable; dose/concentration dependent
e.g. phenytoin, ethanol, aspirin rate of elimination = Vmax x C
Km x C Vmax – maximum elimination capacity Km – drug conc. at w/c rate of elimination is
50% of Vmax “pseudo-zero order kinetics” – elimination is
independent of concentration
b. Flow dependent elimination – dependent on the rate of delivery of the drug to the organ
“high extraction” drugs
“first order kinetics” – a constant fraction of drug is eliminated/unit of time; not saturated
“zero-order kinetics” – a constant amount of drug is eliminated/unit of time; saturable
Other parameters:
• Half-life (t ½) – time required to change the amount of drug by ½
t ½ = 0.7 x Vd
Cl• Drug accumulation – drug interval is shorter
than 4 t ½ , accumulation is detectable
Accumulation factor = 1/ 1 – fraction
remaining before next dose
•Bioavailability – fraction of unchanged drug reaching the circulation; extent of absorption varies first pass elimination ER = C liver Q (hepatic blood flow)• Steady state – achieved when rate of
elimination = rate of administration rate in = rate out• Area under the curve (AUC) – 1st order
elimination; time concentration profile after a dose; C is constant
•Minimum effective concentration• Loading dose = Vd x desired plasma conc. bioavailability - initial dose that is given• Maintenance dose = Cl x desired plasma conc. bioavailability• Therapeutic index (TI) – dose to produce
desired effect• Intermittent dose: peak – high pts. of
fluctuations (toxic effects) troughs – low pts. of fluctuations (lack drug
of effects)
2. Pharmacodynamics – drug →body
• Receptors
inert binding site – binds with a drug w/out initiating events leading to any of the drug’s effects; buffers concentration gradient that drives diffusion
active site – recognition site
Principles:
a. Concentration effect curve – response to low dose increases in direct proportion to dose; however, as dose increases, the response increment diminishes that finally, doses may be reached at w/c no further increase in response can be achieved
b. Receptor-effector coupling – transduction process that occurs between occupancy of the receptors and drug response
• Spare receptor• Receptor antagonists – prevent agonist from
binding and activating receptors• 2 classes:
a. competitive antagonist
b. irreversible antagonist – unavailable
chemical antagonist – protamine and warfarin or heparin
Physiologic antagonist – steroids and insulin• Receptor agonist – full agonist and partial
agonist
Signaling mechanism and drug action
1. Intracellular receptors for lipid soluble agents – NO, hormones, corticosteroids, sex hormones, vit D, thyroid hormone
2. Ligand regulated transmembrane enzymes – insulin, growth factor
3. Cytokine receptor – JAK enzyme, STAT; growth hormone, erythropoietin, interferon
4. Ligand-gated channels – ACTH, GABA5. G-proteins and 2nd messengers – cAMP, Ca,
cGMP
Relation between drug dose and clinical response
A. Graded dose response pharmacologic potency – EC50 and ED50 maximal efficacy – extent or degree of an effect that can be achieved by the patientB. Quantal dose effect responsemargin of safety;
indicates variability of responsiveness; ED50, LD50, TD50, TI = TD50
ED50
Variations in drug responsiveness: - mechanisms involve alteration in
concentration of a drug and changes in the receptor
• Hyporeactive
• Hyperreactive
• Tolerance - ↓ responsiveness due to continued drug administration
• Tachyphylaxis – rapid, diminishing responsiveness
Basic and Clinical Evaluation of a New Drug• 1st step – discovery of a potential molecule
(chemical modification, random screening of natural products, rational drug design, biotechnology and cloning)
• 2nd step – drug screening → LEAD compound• 3rd step – preclinical and toxicity testing; limitations (acute and chronic toxicity, teratogenicity,
carcinogenicity, mutagenicity, investigative toxicology)
• 4th step – evaluation in humans ; factors
Phases of clinical trial:
• Phase 1 – 25-50 healthy volunteers
• Phase 2 – 10-200 patients with target disease
• Phase 3 – larger population; difficult phase; NDA is submitted and approval takes place 3 yrs or more
• Phase 4 post-marketing surveillance;
apply for a patent (20 yrs.)
Maraming Salamat