DrVirgo
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Drug X at a dose of 20 produces a 50% increase in Vascular Resistance.
Drug Y at a dose of 40 produces a 75% increase in Vascular Resistance.
Which drug is more potent?
Which drug is more efficacious?
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| robin082006
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Which drug is more potent? Cannot answer because both drugs are given in different dose
Which drug is more efficacious? cannot answer because we do not know the maximum effect of both drugs.
If Drug X at a dose of 20 produces a 50% increase in Vascular Resistance.
And Drug Y at a dose of 20 produces a 75% increase in Vascular Resistance---> drug Y is more potent.
If Drug X reaches its maximum effect, which increase 50% total peripheral resistance, at a dose of 20. And Drug Y reaches its maximum effect, which increases 75% TPR, at a dose of 40---> drug Y is more efficacous.
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| asherif
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I don't think we can tell from the available data about the drugs.
However we can assume X is more potent (if we assume Y will cause 37.5% increase at the dose of 20)
and we can assume that Y is more effective (if X can't reach the maximum of 75%)
I think we need a dose response curve for both to be able to tell for sure.
waiting for your feedback. 
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| DrVirgo
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The answer is:
X is more potent (notice that it only takes a dose of 20 to produce a 50% increase! Drug Y at the same dose (20) would only produce a 37.5% increase!
POTENCY IS DEPENDENT UPON DOSE so X IS MORE POTENT!
Y produces a 75% increase in TPR whereas X only produces a 50% increase. Drug Y does a better job, period! SO Y IS MORE EFFICACIOUS!
EFFICACY IS NOT DEPENDENT ON DOSE!
I think this concept is important because they ask it all the time....
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| robin082006
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Asherif, think about this e.g.
Drug X at a dose of 20 produces a 50% increase in Vascular Resistance.
Drug Y at a dose of 40 produces a 75% increase in Vascular Resistance. But at dose 20 produces 60% increase in vascular resistance---> which drug is more potent? OF COURSE DRUD Y
It is a mistake if you think that Drug Y at a dose of 40 produces a 75% increase in Vascular Resistance and therefore produces 37.5% increase at dose 20.
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| robin082006
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when you see carefully and understand the figure effect-dose. you will realize that it is a mistake "Drug Y at the same dose (20) would only produce a 37.5% increase"
You can see some questions about these issues in Rapid Review CD.
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| robin082006
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Y produces a 75% increase in TPR whereas X only produces a 50% increase --> Drug Y does a better job, period! SO Y IS MORE EFFICACIOUS! --> again, it is a misunderstanding.
Assume that
Y produces a 75% increase in TPR at dose 40, but 80% at dose 100 or over.
X produces a 50% increase at dose 20, 75% at dose 60, and 90% at dose 100 or over (saturated).
Drug X is more efficicacous.
EFFICICACY MEANS MAXIMUM EFFECT OF DRUG THAT IT CAN REACH. SEE PG 197 IN FA.
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| DrVirgo
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Ok so we are in agreement that Y is more efficacious and X is less efficacious.
(Efficacy is regardless of dose)
About Potency:
If you need LESS of one drug to produce the same effect, that drug is more potent.
I'll try to find more examples... I don't have rapid review.
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| DrVirgo
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Robin,
This was a MCQ in Q-bank I think and the answer out of all the multiple choices was: "X is less efficacious" so i assume that Y is more efficacious...
They went on to say and make a point that EFFICACY IS REGARDLESS OF DOSE, and POTENCY DOES DEPEND ON DOSE!
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| robin082006
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Virgo
The thing here is that you assume drug Y would produce 37.5% at dose 20 because it produces 75% at dose 40.
See the effect-dose curve, you will know your assumption is unreasonable.
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| robin082006
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If both drugs are given at the same dose, the more effect the drug, the more potent.
If both drugs reach the maximum effect regardless of dose, the higher maximum effect the drug, the more efficicacous.
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| asherif
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robin082006 wrote:
Asherif, think about this e.g.
Drug X at a dose of 20 produces a 50% increase in Vascular Resistance.
Drug Y at a dose of 40 produces a 75% increase in Vascular Resistance. But at dose 20 produces 60% increase in vascular resistance---> which drug is more potent? OF COURSE DRUD Y
It is a mistake if you think that Drug Y at a dose of 40 produces a 75% increase in Vascular Resistance and therefore produces 37.5% increase at dose 20.
And drug X might produce 90% increase at dose 40
like I said, the data is not enough to tell, a dose response curve comparing both drugs is needed to tell for sure.
My answer was built on assumption because I know these responses can happen.
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| DrVirgo
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Ok,
here is that question:
A cardiovascular pharmacologist is researching the effects of new compounds on arteriolar resistance. Drug X
maximally increases vascular resistance by 50% at a dose of 20 mg/mL. Drug Y maximally increases vascular
resistance by 75% at a dose of 40 mg/mL. Which of the following conclusions can the researcher draw from this
experiment?
A. Drug X has a smaller volume of distribution than Drug Y
B. Drug X has a shorter half-life than Drug Y
C. Drug X is less efficacious than Drug Y
D. Drug X is less potent than Drug Y
E. Drug X has a lower LD50 than Drug Y
Explanation:
The correct answer is C. The only conclusion that can be drawn from this data is that Drug X is less efficacious
than Drug Y. Efficacy is defined as the maximum effect that can be produced by a drug, regardless of dose.
Drug X can only produce a 50% change in resistance, whereas Drug Y can produce a 75% change in
resistance. Therefore, Drug X is less efficacious than Drug Y.
A volume of distribution (choice A) is the ratio of the amount of drug in the body to its plasma concentration. In
this experiment, we do not know the total amount of drug used or the plasma concentration. For that matter, we
do not even know if this is an in vivo experiment. Thus, no conclusions can be drawn about volume of
distribution.
The half-life (choice B) is the time it takes for the concentration of a drug to fall 50% from its previous
measurement. There is no information given to determine half-life.
The potency (choice D) is the dose or concentration required to produce 50% of the drug's maximal effect. We
cannot determine the potency of Drug Y from this question.
The LD50 (choice E) is the dose that causes death in 50% of a population of subject. The experiment
described above does not describe a population study, nor does it give any indication about the toxicity of the
drug.
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| asherif
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robin082006 wrote:
Virgo
The thing here is that you assume drug Y would produce 37.5% at dose 20 because it produces 75% at dose 40.
See the effect-dose curve, you will know your assumption is unreasonable.
Yeah, in real life may be, but examwise this should be how you assume it, for an exam question it is very reasonable. 
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| DrVirgo
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Yes Robin you are right, we can't determine Potency! 
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| robin082006
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DrVirgo wrote:
Robin,
This was a MCQ in Q-bank I think and the answer out of all the multiple choices was: "X is less efficacious" so i assume that Y is more efficacious...
They went on to say and make a point that EFFICACY IS REGARDLESS OF DOSE, and POTENCY DOES DEPEND ON DOSE!
Virgo,
I finish 96% of qbank but did not see a question like this and its explanation. I will post it when I see it when doing the left. But actually I did some questions about this in Rapid review CDs and sure about my answer..
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| DrVirgo
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Robin,
You are right about potency, but as the explanation says: X is less efficacious.
I posted the question, answer and explanation in post #13.
Thanks again for your help.
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| robin082006
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It is exact like what I said.
You see: "maximally increases vascular resistance by 50% at a dose of 20 mg/mL. Drug Y maximally increases vascular resistance by 75% at a dose of 40 mg/mL."--> Y is more efficicacous.
DrVirgo wrote:
Ok,
here is that question:
A cardiovascular pharmacologist is researching the effects of new compounds on arteriolar resistance. Drug X
maximally increases vascular resistance by 50% at a dose of 20 mg/mL. Drug Y maximally increases vascular
resistance by 75% at a dose of 40 mg/mL. Which of the following conclusions can the researcher draw from this
experiment?
A. Drug X has a smaller volume of distribution than Drug Y
B. Drug X has a shorter half-life than Drug Y
C. Drug X is less efficacious than Drug Y
D. Drug X is less potent than Drug Y
E. Drug X has a lower LD50 than Drug Y
Explanation:
The correct answer is C. The only conclusion that can be drawn from this data is that Drug X is less efficacious
than Drug Y. Efficacy is defined as the maximum effect that can be produced by a drug, regardless of dose.
Drug X can only produce a 50% change in resistance, whereas Drug Y can produce a 75% change in
resistance. Therefore, Drug X is less efficacious than Drug Y.
A volume of distribution (choice A) is the ratio of the amount of drug in the body to its plasma concentration. In
this experiment, we do not know the total amount of drug used or the plasma concentration. For that matter, we
do not even know if this is an in vivo experiment. Thus, no conclusions can be drawn about volume of
distribution.
The half-life (choice B) is the time it takes for the concentration of a drug to fall 50% from its previous
measurement. There is no information given to determine half-life.
The potency (choice D) is the dose or concentration required to produce 50% of the drug's maximal effect. We
cannot determine the potency of Drug Y from this question.
The LD50 (choice E) is the dose that causes death in 50% of a population of subject. The experiment
described above does not describe a population study, nor does it give any indication about the toxicity of the
drug.
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| robin082006
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In this case you should mention: MAXIMALLY EFFECT, THIS MEANS EFFICACY.
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| robin082006
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QUESTION 1. Drug X at a dose of 20 produces a 50% increase in Vascular Resistance.
Drug Y at a dose of 40 produces a 75% increase in Vascular Resistance
QUESTION 2. Drug X maximally increases vascular resistance by 50% at a dose of 20 mg/mL.
Drug Y maximally increases vascular resistance by 75% at a dose of 40 mg/mL.
VIRGO, DO YOU REALIZE THE DIFFERENCE BETWEEN TWO QUESTIONS?
for question 1. We cannot say about potency and efficacy (your question).
but for question 2. Y is more efficicacous or X is less efficicacous (qbank) because maximal effect of X is 50% and that of Y is 75%.
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| DrVirgo
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Yes, I see! I forgot to mention maximal effect!
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| nadiabarati
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Yes robin. you are right. nice explanation.
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