3 - Pharmacokinetics and Pharmacodynamics Flashcards
What is pharmacokinetics and pharmacodynamics?
- Pharmacokinetics: what the body does to the drug
- Pharmacodynamics: what the drug does to the body
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What are some of the pharmacokinetic facts we need to know about a drug before it can be approved for use?
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What are some factors that can affect the pharmacokinetics/Vd of a drug?
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What is bioavailabilty and what is it affected by?
Amount of drug that gets to the systemic circulation unchanged!
Affected by: formulation, age, food (chelation), vomiting, malabsorption (Crohn’s), first pass metabolism in liver and gut
100% in IV
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Why is metformin available in different preparations?
- Modified slow release only needs to be taken once a day, less likely to have GI side effects
- More expensive to have extended release but patients more likely to adhere if less side effects
- Most of the time different preparations are just branding, e.g nurofen period pain
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What are some factors that affect the distribution of a drug?
- Blood flow
- Lipophilicity
- Hydrophilicty
- Protein binding (difficult to distribute when bound)
- Vd
- Pregnancy
- Hypoalbuminaemia
- Renal failure
- Other drugs displacing the original drug from protein binding
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What is the multicompartment model of drug distribution?
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When prescribing a patient a second drug, when should you be really careful with the first drug interactions?
- Second drug may displace first so more first drug that can elicit a response
- Be careful if first drug is highly protein bound, has a narrow therapeutic index, low Vd
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How do you work out the volume of distribution?
How does weight affect Vd?
- Larger Vd than body water means it is absorbed by the adipose tissue
- Larger weight, more adipose so larger Vd so need higher dose
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What are the two phases of drug metabolism?
Phase I reactions of drug metabolism involve oxidation, reduction, or hydrolysis of the parent drug, resulting in its conversion to a more polar molecule. (Assisted by CYP)
Phase II reactions involve conjugation by coupling the drug or its metabolites to another molecule.
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What CYP’s are affected by smoking and alcohol, and what else can they be affected by?
- Age
- Hepatic disease
- Blood flow to liver
- Alcohol and Cigarette smoking (CYP450 inhibited in acute, induced in chronic)
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What changes can CYP450 enzymes make to a drug?
- Can inactivate active drugs
- Can activate inactive drugs, e.g Levodopa
- Can covert active drug into another one e.g codeine to morphine
What is the most common CYP that metabolises drugs and what CYP is an example of genetic variations?
- CYP3A4
- CYP2D6 absent in some caucasians and over expressed in 30% of Africans. Has substrates like beta blockers and SSRIs
When dosing two different patients for a drug, what should you think about in terms of the metabolism of the drug?
- Race
- Sex (women slower ethanol metaboliser)
- Self induced metabolism with drugs e.g Carbamazepine
- If they are taking St John’s Wort (inducer of CYP3A4)
- Weight
- Smoking and drinking status
- Age
What routes can drugs be eliminated from the body?
- Mainly through the kidney
- Faeces
- Hair
- Sweat
- Tears
- Saliva
- Breast milk
- Genital secretions
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What type of drug molecules are excreted renally?
- Low weight polar molecules
- Clearance mainly affected by GFR
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What type of drug molecules are excreted hepatically and therefore into the faeces?
Usually high molecular weight molecules, conjugated with glucaronic acid
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