Week 1 Video 1 Pharmacokinetics

Question 1

ADME

Pharmaconkenetics

Answer 1

Absorption- How the drug gets into the body
Distribution- Where the drug goes in the body
Metabolism- how the body chemically modifies the drub
Excretion- How the body gets rid of the drug

Question 2

Graph ADME

Pharmaconkenetics

Answer 2

So, I’ll begin by drawing a graph. And here on the Y-axis, we have the concentration of
the drug within the body and on the X-axis, we have time. So, if we give a person a drug
at time zero, then we will see the concentration of the drug go up and then it will fall
slowly as the drug is removed from the body. And that’s what we see on this graph. So,
looking at this graph, we can see that in this section, the drug enters the body faster
than it is being removed from it. Therefore, the concentration of the drug in the body
increases. Then after the peak of this graph, the drug is being removed from the body
faster than it is entering. So the concentration of the drug in the body starts decreasing.
This point at the top of the graph is important because it is a highest concentration of
drug in the body. Therefore, we call this the maximum concentration or Cmax.
This is important because knowing the maximum concentration can help predict the
therapeutic benefit and also the likelihood of side effects. The time at which the
maximum concentration occurs is called Tmax. Now a slightly more confusing concept is
the half-life of a drug. The half-life is by definition, the time it takes to remove half of
the current concentration of drug from the body. So, let’s consider the half-life on this
graph. If we begin at Tmax, we want to see how much time it takes for concentration of
the drug to decrease from Cmax to half of Cmax. So, here is the half Cmax line here, and
therefore the half-life, which is often denoted T1/2 is the time between Tmax and
where I’ve drawn T1/2 here.
So, just to summarize the time it is taken for the drug to drop from its maximum level to
half of its maximum level is the half-life. On this graph, this is the time between Tmax
and where I’ve drawn T1/2. There’s a lot more to talk about half-lives, including rate
Page 2 of 2
laws and orders of reactions, but that is beyond the scope of this video. So, we’ll talk
about that in a later tutorial. The last thing to look at is the area under the curve and
this area will be illustrated by shading in the space, under the graph. This represents the
total exposure to a drug that the body receives. This is a function of how high the
concentration of the drug gets as well as how quickly it’s excreted. And that’s an
introduction to the basics of pharmacokinetics.
I

Question 3

Absorption

Answer 3

Area under the curve is the same for all three, oral, IV and inhalation. The drug follows under the first order of kenetics.

Now I have three more points I want to make. Firstly, note that the area under the curve
is the same for all these routes of administration. This is because the amount of
exposure to the drug, that is the dose, is the same. Just some routes of administration
spread the dose out over a greater length of time. Secondly, you might be wondering
why the same drug is excreted at different rates or more simply why curve number
three falls quicker than curve one. Without going into too much detail, I have chosen to
show a drug that conforms to first order kinetics. If you don’t know what that means, it’s
okay. We’ll cover all this in a later tutorial, but it basically means that the rate of
excretion is proportional to the current concentration of the drug. If the concentration
of the drug is high, then it will be excreted faster than if the concentration is low. Also,
it’s important to note that the half-life of the drug is the same regardless of the route of
administration. This is because the half-life is a property of the drug, not the route. It
will take same amount of time for the drug to reach half of the C max orally as it will to
reach half of the C max intravenously. This is because the C max is different for each
route of administration. And the rate of excretion is proportional to the current
concentration of the drug.