Answer to Practice Problem #1 Metabolite Kinetics Tutorial:

 

Drug B. Recall that the problem states that the metabolites are active. Thus, the duration will be dependent upon the rate of elimination of the metabolites. Since the metabolite of Drug A exhibits formation rate-limited disposition, the “apparent” half-life of this metabolite is equal the half-life of Drug A. Since the metabolite of Drug B exhibits elimination rate-limited elimination, the half-life of the metabolite is longer than Drug B. Recall that the problem states:

 

t_1/2 of Drug A =  t_1/2 of Drug B

 

Moreover, we know that the

t_1/2 of Drug A = apparent t_1/2 of the metabolite of Drug A

            and,

t_1/2 of the metabolite of Drug B >  t_1/2 of Drug B

 

Thus,

t_1/2 of Drug A = apparent t_1/2 of the metabolite of Drug A =  t_1/2 of Drug B < t_1/2 of the metabolite of Drug B

 

            Therefore, the half-life of the metabolite of Drug B is longer than any other species, which means the duration of action after administration of Drug B will be the longest.

 

Last revised 07/19/04

 

ã 2004 -  Craig K. Svensson, Pharm.D., Ph.D.

 

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