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  #1  
Old 03-06-2012
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Drug Enzyme induction and drug clearance!

Verapamil and phenytoin are both eliminated from the body by metabolism in the liver.Verapamil has a clearance rate of 1.5 L/min,approximately equal to liver blood flow,whereas phenytoin has a clearance of 0.1 L/min. When these compounds are administered along with rifampin, a drug that increases hepatic drug metabolizing enzymes, which of the following is most likely?

(A) The clearance of both verapamil and phenytoin will be increased.
(B) The clearance of both verapamil and phenytoin will be decreased.
(C) The clearance of verapamil will be unchanged, whereas the clearance of phenytoin will be increased.
(D) The clearance of phenytoin will be unchanged, whereas the clearance of verapamil will be increased.

Answers with explanation must:clearance of conception
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Old 03-06-2012
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Wow a good one!

phenytoin and rifampin do both enhance CyP450!

But if Verapamil is already having liver blood flow, so somethinbg tells me its being cleared at the maximal rate maybe?

I would go with C
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Old 03-06-2012
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i'm embarassed to say A...
because thats clearly too easy a question then lol
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Old 03-06-2012
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it should be A
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Old 03-06-2012
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Quote:
Originally Posted by Dr.NickRiviera View Post
i'm embarassed to say A...
because thats clearly too easy a question then lol
This is how I often make mistakes... I always think that there is something inbetween the lines, but the answer is usually simple...
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Old 03-06-2012
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Its D .... becoz phenytoin is eliminated by zero order kinectics so ..... the amt eliminated is always fixed no matter what the conc ...... so its clearance stays the same but verapamil clearance increases ......
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Old 03-06-2012
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Quote:
Originally Posted by Hitman View Post
Its D .... becoz phenytoin is eliminated by zero order kinectics so ..... the amt eliminated is always fixed no matter what the conc ...... so its clearance stays the same but verapamil clearance increases ......
ahh i was thinking along those lines but got confused and gave up and went with A "-_- lol
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Old 03-06-2012
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the ANSWER is D
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Old 03-06-2012
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Guys make a note in ur FA these drugs follow Zero-order kinetics and all else is first order
the drugs in the question can be changed in ur exam lol

Peas & WHEATS

Phenytoin, Phenylbutazone
Warfarin
Heparin
Ethanol
Aspirin
Theophylline, Tolbutamide
Salicylates
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Old 03-07-2012
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Default Correct answer is here

Apparently Verapamil is metabolized so rapidly that only the rate of delivery to the liver regulates the disappearance, i.e. it is blood flow limited.Further increase in liver enzymes could not increase it's elimination.However,the rate of elimination of phenytoin is apparently limited by its rate of metabolism since clearance is much less than hepatic blood flow.Therefore,the clearance of phenytoin can rise if some agent causes an increase in liver enzymes

. So the right answer here is (C)

Think all of you got it :-)
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Old 03-07-2012
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hey where is this Qs from ????
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  #12  
Old 03-07-2012
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Wooot wooot!

A morning confidence booster!
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Old 03-07-2012
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wait, so is it C or D?
and why?
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Old 03-07-2012
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Please someone tell the correct answer...
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Old 03-07-2012
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Clearance x urine concentration=blood flow x plasmaconcentration (if no secretion). So verapamil has reached the highest limit, ie what is in the plasma will be in the urine at the same rate.
Phenytoin has not reached its limit, more enzymes induced will clear more phenytoin ( same amount in the same amunt of time).
So using rifampin will not increase verapamil clr, only increase phenytoin clr. Trick Q.

Ans is C
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  #16  
Old 03-08-2012
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Question is from Katzung and trevor's pharmacology(Exam and board review).......you can easily find it at the end of chapte
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