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CHAPTER 6 (EFFICACY (Dependant on the number of D-R complexes
formed and…
CHAPTER 6
EFFICACY
- Dependant on the number of D-R complexes
formed and the efficiency of the coupling of
receptor activation to cellular responses
- A drug with greater efficacy is more therapeutically
beneficial than one that is more potent
- Emax= assumes that all receptors are occupied
by the drug and no increase in response will be
observed if more drugs are added
- Efficacy: the ability of a drug to elicit a
response when it interacts with a receptor
- Often used to describe the maximal
level of response a drug can produce
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D-R COMPLEX
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Specific for a particular ligand and produces a unique response. E.g, Hearts contain muscarinic acetylcholine receptors specific for acetylcholine.
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POTENCY
- Potency: A measure of the amount of drug necessary to produce an effect of a given magnitude
- Efficacy concentration at 50% is used to determine potency and is commonly designated as EC50
- Effective concentration (EC50) is the concentration of an agonist needed to elicit half of the maximum biological response of the agonist
- The D-R curve become sigmoidal in shape, which simplifies the interpretation of the dose response
DRUG-DRUG
INTERACTIONS
- With pharmacodynamic drug interactions, two drugs have interactive effects.
- In terms of efficacy, there can be several types of interactions between medications: cumulative, additive, synergistic.
- Cumulative effects
The condition in which repeated administration of a drug may produce effects that are more pronounced than those produced by the first dose.
- Additive effects
i. The effect of two chemicals is equal to the sum of the effect of the two chemicals taken separately, eg., aspirin and motrin.
ii. The effect of the two drugs is in the same direction and simply adds up: effect of drugs
A + B = effect of drug A + effect of drug B
- Synergestic efects
i. The effect of two chemicals taken together is greater than the sum of their separate effect at the same doses,
Ex: Alcohol & hypnotic drugs
DRUGS
vs
RECEPTORS
Not all drugs exert their effects by interacting with a receptor.
- Antacids
Act chemically to neutralize the HCl produced by gastric parietal cells and thereby raise the pH of gastric fluid (reduce heartburn symptom)
- Metal chelating agents
Dimercaprol. Combine with toxic metals to form a complex that can be more readily excreted.
- Osmotic diuretics
Mannitol. Inhibit water absorption from kidney tubules
SIGNAL
TRANSDUCTION
Drugs = Signals,
Receptors = Signal Detectors
Ligand = Small molecule that binds to a site on a receptor protein. Can be a naturally occurring molecule (endogenous) or a drugs.
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Lead to initiating a series of reactions that ultimately result in a specific intracellular response
RECEPTORS
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Most receptors are named to indicate the type of drug/chemical that interact best with it. E.g. The receptor for histamine is called a histamine receptor.
Divided into 4 mechanism:
- Ligand-gated ion channels
- G protein-coupled protein
- Enzyme-linked receptors
- Intracellular receptors
THERAPEUTIC INDEX
Drug safety measurement, because a larger value indicates a wide margin between doses that are effective and doses that are toxic.
the ratio of the dose that produces toxicity to the dose that produces a clinically desired or effective response in a population of individuals.
TI = LD50/ED50, an indication of safety of a drug
(higher is better)
Effectiveness, Toxicity,
Lethality
- ED50 - Median Effective Dose 50; the dose at which 50 percent of the population or sample manifests a given effect
- TD50 - Median Toxic Dose 50 - dose at which 50 percent of the population manifests a given toxic effect
- LD50 - Median Toxic Dose 50 - dose which kills 50 percent of the subjects
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