Name: 什麼是藥代動力學？ADME -第一講 (What is Pharmacokinetics? ADME -Lect 1)
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So let's jump right in to pharmacokinetics.

So the first thing we want to do is be able to answer the question, what is pharmacokinetics and cover some basic definitions.

And the easiest way to understand pharmacokinetics is to differentiate it from pharmacodynamics.

So when you look at this word, pharmacokinetics, what is the first thing that you think of?

Well, you should say pharmaco that means drug and kinetics, you should be thinking motion.

And this is to differentiate pharmacokinetics from pharmacodynamics.

Now, pharmaco again means drug. Dynamics comes from this word dynamus which really means power.

So when I think of pharmacokinetics, I'm thinking of a drug and I'm thinking of that drug moving through our body.

So what happens as a drug moves through our body?

Well the concentration changes and that's really what pharmacokinetics is looking at.

PK, pharmacokinetics, is really answering the question of how does the drug concentration change as it moves through the different compartments of your body.

This is different than pharmacodynamics. We're dealing with power.

When you think of somebody who is powerful, what can they do?

They can exert their power and change other people right?

So the same idea goes with pharmacodynamics.

It's how does the drug exert its effects on your body, how powerful is this drug, how potent is it?

Potency is a component of pharmacodynamics but by enlarge, the big thing we're looking at with pharmacodynamics is drug receptor interactions.

Now we also look at efficacy of the effects of the drug. Potency refers to the you know how much of the drug is required to get a certain effect

Today we're really focused on pharmacokinetics.

Now before we move any further, I want to explain how many books define this and how it can be confusing.

So a lot of books define pharmacokinetics as what your body does to the drug

and conversely, they define pharmacodynamics  as what the drug does to your body.

I always mix these things two up and I have to think about it and I always return back to my bread and butter right here.

Pharmacokinetics, oh that has to do with motion, that has to do with the drug concentration changing as it's moving through my body.

Pharmacodynamics, I think power. How does the drug exert its effects, drug receptor interactions, potency, efficacy - all that jack.

So when I think of kinetics, where is it moving?

Well drugs need to enter our body and so, half of pharmacokinetics is dealing with drugs entering our body

and the other half of it is dealing with drugs leaving our body.

So, what are the components of pharmacokinetics?

So instead of writing enter and leave, I wrote in vs. out.

So there are 2 components of drugs coming into our body.

Those two components are represented with absorption (A) and distribution (D).

I'm just going to write an A here and a D there.

Well, here's a book definition. Absorption is the process of a substance entering the blood circulation.

It's okay. How can we make this definition a little better?

Well, instead of saying our blood circulation, let's say it's the process of a substance entering the systemic circulation.

and the reason I like to say systemic is because absorption is everything that happens before this drug enters our systemic circulation.

So maybe I'll even add a little before right here.

And if you think about, you take a drug orally, it goes through your intestines.

It needs to go through your portal circulation before it gets to your liver and then it finally gets into your IVC then it's in our systemic circulation.

So everything before that is what we call absorption.

Now, to differentiate that from distribution, after it's in our systemic circulation, that drug can now be distributed to the rest of the body.

Well, here's a definition that is nothing special.

The dispersion of well, maybe a substance throughout fluids and tissues of the body.

It makes sense. You know you don't need a definition for distribution. It's going from one place to another.

So those are the 2 ins. What are the 2 outs?

Well one of them is metabolism and the other is elimination (E).

And so, I'm going to write those right here. A, D, M, E.

This is a little acronym that is commonly used to define the 4 components of pharmacokinetics.

Absorption, Distribution, Metabolism, Elimination.

Metabolism is the irreversible transformation of parent compounds into daughter compounds.

Well, let's say I start with Drug A. I transform Drug A - it's the parent - into a very structurally similar daughter compound that is Drug B.

Now, this doesn't mean that I am actually breaking down the drug. In pharmacology, it's actually the opposite.

Many times we don't actually break down the drug but we actually add something to it

and we add 1 or 2 molecules and the reason we do that is to make it more polar.

And so, the reason we make it more polar is to help us eliminate it.

Now before we move on to the next step, what are we doing here?

We're taking a biologically active molecule and we're transforming it.

So, a common name for metabolism is biotransformation.

And so, what is doing this biotransformation?

And the reason we do it again is to make it more polar to get it ready for elimination.

Now this isn't the only reason that this happens but it's the big one

and right now, we're focused on the big picture.

Elimination is the removal of substances from the body.

Now, the definition is not bad but the word itself is something I don't like. I don't like calling this elimination. Here's why.

Look at what we did to Drug A right here. We started with Drug A, we biotransformed it and now we have Drug B.

So in essence, what have we done with Drug A?

And we no longer have Drug A. It goes bye-bye and now, we have Drug B.

and instead of calling this elimination, let's call this excretion.

Now a lot of books mess this up and confuse these two.

You might see this in your book written as elimination. Don't let yourself get confused but make sure you differentiate elimination from excretion.

So it's important to know these definitions but it's even more important to know where these things are occurring in the body. So, let's do that.

So where are you excreting or how do you excrete most of you know foreign substances in your body, most drugs?

And most of the time, this is happening in your renal circulation. Let's write renal, let's write this is happening in your kidney.

So when you see a kidney, a renal in some sort of exam question dealing with pharmacology, you can be thinking about excretion.

Whereas metabolism, where is most biotransformation occurring?

Let me ask you a question. Do you guys remember unconjugated bilirubin which came from red blood cells?

Well, what was the problem with unconjugated bilirubin right?

It needed to be conjugated and become direct bilirubin so that we can excrete it.

Well that occurred in the liver. The same place where metabolism is occurring here.

When you think liver, you should be thinking metabolism.

Now, I'm going to note. We don't always do this to make it more polar.

In some cases, we actually metabolize drugs to activate them and we've seen this in the body too.

Where else have you seen the liver activate certain endogenous substances?

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什麼是藥代動力學？ADME -第一講 (What is Pharmacokinetics? ADME -Lect 1)

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