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# First Aid USMLE Step 1: Pharmacology

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In the Michaelis-Menten kinetics what do the following mean:
Km
Vmax

Draw a typical hyperbolic curve on a Michaelis-Menten Plot, then draw how the addition of a competitive and non-competitive inhibitor would change the kinetics of the reaction.
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In the Michaelis-Menten kinetics what do the following mean:

Km:
- the concentration of substrate at 1/2 Vmax
- this is INVERSELY related to the affinity of enzyme for its substrate

Vmax:
- maximal velocity at which enzymatic reaction occurs
- is directly proportional to the enzyme concentration

Competitive inhibitor: would raise Km but Vmax is the same

Non-competitive inhibitor: would lower Vmax?
Draw a Lineweaver-Burke plot. What information do the following intercept points correspond to?

X axis: 1/ [S]
Y axis: 1/V
slope: Km/Vmax
y intercept: 1/Vmax
x intercept: 1/-Km

How do a competitive and non-competitive inhibitor would change the plot?
- reversible competitors cross each other competitively
- non-competitive inhibitors DO NOT!
FOR:
reversible competitive inhibitors
irreversible competitive inhibitors
non-competitive inhibitors

Explain the following properties:
- do they resemble their substrate?
- overcome by an increase in [S]
- bind to the active site of the enzyme
- have an effect on Vmax
- have an effect on Km
- how does it alter kinematics of the original substrate?
What is the volume of distribution (Vd) of a drug? How do you calculate it?

How does the apparent Vd of a plasma-protein bound drug change in disease states?

For the following compartments, what would Vd be approximated as and what kinds of drugs are primarily found in these compartments?
- Blood
- ECF
- All tissue (including fat)
What is the volume of distribution of a drug? How do you calculate it?
- the theoretical volume occupied by a total amount of drug in the body relative to its plasma concentration

Vd = amount of drug in the body/plasma drug concentration

How does the apparent Vd of a plasma-protein bound drug change in disease states?
- apparent Vd of a plasma-protein bound drug can be altered by liver and kidney disease
- decreased protein binding leads to increased Vd

For the following compartments, what would Vd be approximated as and what kinds of drugs are primarily found in these compartments?
See image included

What is the equation for calculating maintenance dose?

In renal or liver disease however is maintenance dose and loading dose changed?
- maintenance dose is usually decreased

What factors influence the time to steady state of a drug?
- depends primarily on t1/2 and is independent of dose and dosing frequency

Note the Cp = target plasma concentration at steady state
What is an additive drug interaction?

What is an permissive drug interaction?

What is an synergistic drug interaction?

What is an tachyphylactic drug interaction?

For the following drug interactions, state what kind of interaction it is:
- aspirin and acetaminophen
- clopidogrel with ASA
- MDMA and LSD
- cortisol on catecholamine responsiveness
What is an additive drug interaction?
- effect of A and B together is equal to the sum of their individual effects

What is an permissive drug interaction?
- presence of substance A is required for full effects of substance B

What is an synergistic drug interaction?
- effect of A and B together is greater than sum of their parts

What is an tachyphylactic drug interaction?
- acute decrease in response to a drug after initial/repeated administration

For the following drug interactions, state what kind of interaction it is:
- aspirin and acetaminophen
- clopidogrel with ASA
- MDMA and LSD
- cortisol on catecholamine responsiveness
What is phase I and phase II drug metabolism?

PHASE I:
- reduction, oxidation, hydrolysis with cytochrome P-450 to yield a slightly polar, water soluble metabolite (often still active)

PHASE II:
- Conjugation (via: Methylation, Glucoronidation, Acetylation, Sulfation)
- usually yields very polar, inactive metabolites (renally excreted)

Which is usually impaired in geriatric patients?
- geriatric patients lose phase I first

"Geriatric paitnest have More GAS (phase II)"
What is the difference between efficacy and potency?

Efficacy:
- is the maximal effect a drug can produce
- unrelated to potency
- represented by the y value, with increased y = increased Vmax = increased efficacy
- note that partial agonists have less efficacy than full agonists

Potency:
- the amount of drug needed for a given effect
- represented by the X value (EC50)
- left shifting = decreased EC50 = increased potency = decrease amount of drug needed
- again, unrelated to efficacy.