Chapter 1: Pharmacokinetics

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Have you ever wondered why taking an allergy pill can take a full hour to kick in?

But an IV at the hospital starts working in like a matter of seconds.

Right.

It's night and day.

Yeah, exactly.

Or why taking a medicine with a glass of grapefruit juice might accidentally send you to the emergency room.

Which is a very real thing, by the way.

It is.

Well, welcome to a custom -tailored deep dive.

Today we are exploring the invisible microscopic battle that your body fights every single time you swallow a pill.

It really is a battle.

Our mission today is to master the absolute foundation of pharmacology.

And we're doing this by going straight to the gold standard, chapter one of Lippincott Illustrated Reviews, pharmacology, the seventh edition.

Such a great text.

It is.

And if you are, you know, gearing up to conquer the subject for the very first time, you are in the perfect place.

We are going to translate all of this dense drug information into something actually human.

I love that because it really is the ultimate starting point.

You know, people typically think of pharmacology as the study of what a drug does to the body.

Right.

Like you take a pill, your blood pressure drops.

That mechanism, the actual effect, is called pharmacodynamics.

But to really understand how to use these tools clinically, we have to look at the exact opposite process.

Pharmacokinetics.

Yes.

Pharmacokinetics is the story of what your body does to the drug.

From the very second a foreign substance enters your system, your body is actively moving it.

It's physically altering its chemical structure and desperately trying to throw it out.

Because your body essentially treats the medicine like an invader.

Basically, yeah.

And to track how it handles that invasion, the chapter introduces a really simple conceptual roadmap in Figure 1 .1.

It's the ADME flowchart.

ADME.

Right.

Yeah.

So can you sort of describe that visual for everyone listening?

Sure.

So Figure 1 .1 lays it out like a journey.

First, the drug has to cross over into the bloodstream that's absorption.

The A.

Right.

Then it leaves the blood to travel out into the tissues and the cells that's distribution.

Okay.

Got it.

Next, the body, which is usually the liver, chemically alters the drug into different shapes called metabolites.

That's metabolism.

Which is the M.

Exactly.

And finally, the drug and those broken down metabolites are permanently flushed out of the body, which is elimination.

The E.

So ADME.

Absorption, distribution, metabolism, and elimination.

You got it.

And understanding those four stages takes all the guesswork out of clinical decisions.

Yeah.

I mean, it explains exactly why a doctor might choose like a skin patch for one patient, but a deep muscle injection for another.

But before we can even begin tracking that ADME journey, we have to actually get the drug inside the body, right?

We need an entry point.

The gateway.

The gateway.

And the options here are surprisingly vast.

Figure 1 .2 in the text is this great diagram of a human body with arrows pointing to all these different entry points.

You've got oral, sublingual under the tongue, IV, topical.

Yeah, there are a lot.

But the most common divide you need to know is between enteral and parenteral routes.

Enteral versus parenteral.

Let's break that down.

So enteral basically just means we're using the gastrointestinal tract, giving the drug by mouth.

Which is incredibly convenient and cheap.

Super cheap.

But from the drugs perspective,

it's a brutal gauntlet.

I mean, to get to the bloodstream, it first has to survive the stomach.

Right.

And the stomach is essentially a vat of hydrochloric acid.

Exactly.

A lot of active ingredients would be completely destroyed if we just dropped them in their bear.

So pharmacologists get around this by developing specialized physical preparations.

Like what?

Well, drugs like omeprazole or daily aspirin are often given an enteric coating.

It's this tough chemical envelope designed to withstand high acidity.

It simply refuses to dissolve in the stomach.

Oh, wow.

So it just waits.

Yeah.

It waits until it reaches the much calmer, less acidic environment of the intestine before it finally releases the drug.

That's brilliant.

And then you have those labels you always see on over -the -counter bottles like ER, XR, SR,

the extended release preparations.

Right.

Those use different proprietary coatings to control the speed of release.

It slows the absorption down to a trickle, which totally changes how a drug acts over time.

The text mentions oral morphine as an example of this, right?

Yes.

Oral morphine is a perfect example.

Morphine naturally has a very short half -life in the body, around two to four hours.

Okay.

So if a patient is in severe chronic pain and they take a standard fast -acting pill, they'd have to wake up and take another pill, like six times a day, just to keep the pain at bay.

Which is awful for quality of life.

Terrible.

But by packaging it in an extended release tablet, the drug drips into the system so slowly that the patient only needs two doses a day to maintain a continuous steady curtain of relief.

That's amazing.

So that's the enteral route through the stomach.

Now let's contrast that with the parenteral route, which usually means a needle.

The way I visualize this is with airline travel.

Swallowing a pill orally is like booking a connecting flight with long layovers, unpredictable weather delays, and like strict security checkpoints.

It takes hours and you might lose some luggage along the way.

I like that analogy.

Thanks.

And a parenteral route, specifically an intravenous injection or IV, is a direct, nonstop private jet landing right at your destination.

The drug goes directly into the systemic blood circulation.

A very apt comparison.

You completely bypass the security checkpoints.

So thinking about this logically, if an IV is a direct flight, it works instantly and gives a doctor 100 % control over how much drug enters the blood.

Why bother with pills at all?

Why don't we just use IVs for every treatment?

It fundamentally comes down to safety and reversibility.

Reversibility?

Once you push a medication directly into a vein,

you cannot take it back.

If a patient accidentally swallows too many pills, they can be taken to the hospital and doctors can pump their stomach or give them activated charcoal to stop the absorption.

But with an IV, that drug is instantly circulating through the heart and brain.

There's no retrieving it.

Furthermore, the sheer act of breaking the skin barrier with a needle introduces massive risks.

Like infection.

Exactly.

Local tissue necrosis, pain, and especially infection.

You're basically creating a direct highway for bacteria to enter the blood.

Which perfectly explains why we use different depths for injections, depending on the goal.

We don't always go straight for the vein.

No we don't.

And figure 1 .3 actually maps out those depths beautifully.

Yeah, if you imagine a cross section of the skin,

the IV needle punches straight down past all the skin layers and directly into the vein.

But if a doctor uses an intramuscular or IM injection, that needle goes much deeper.

It intentionally bypasses the veins and deposits the drug deep into the muscle tissue.

We rely on the muscle for what we call depot preparation.

Depot!

Like a storage depot.

Exactly.

The drug is suspended in a special vehicle like polyethylene glycol.

When it hits the muscle, it precipitates out and forms a little depot that very slowly dissolves into the surrounding capillaries.

Oh, so it just sits there and leaks out.

Yes.

It provides a sustained dose over days or sometimes even weeks from a single shot.

Wow.

On the other hand, if we want a slow constant diffusion, like with daily insulin, we use a subcutaneous or SC injection.

That needle stops short, intentionally depositing the drug in the fatty layer just beneath the skin for slower diffusion.

So we have IV, IM, and SC.

We also have fascinating ways to completely bypass the needle while still avoiding the stomach.

Like transdermal.

Yes.

Figure 1 .4 breaks down the transdermal patch.

It's basically a patch with a backing, a drug reservoir, and a release membrane that delivers the drug right across the skin and into a blood vessel.

That's very clever.

And then there's the rectal route, which feels a bit unglamorous, but the text says it's clinically incredibly useful.

It is invaluable in an emergency.

If a patient is vomiting violently or is unconscious, they just cannot take a pill.

A rectal suppository avoids the stomach completely and crucially, it bypasses about 50 % of the portal circulation.

Meaning the blood flow that drags the drug to the liver.

Exactly.

It allows more of the medication to actually reach the rest of the body intact.

Okay.

So we've chosen our entry point.

The drug is waiting at the border.

This kicks off the very first stage of ADME, absorption.

Border crossing.

Right.

How does the drug physically leave the gut or the muscle and get inside the bloodstream?

Well, the fundamental barrier is the cell membrane.

To get into the blood, the drug has to cross it.

And figure 1 .6 details the four ways this happens.

Okay, let's go through them.

The vast majority of drugs manage this through passive diffusion.

They follow the path of least resistance, moving from a high concentration, like inside the intestines, toward an area of low concentration, like the bloodstream.

So they just drift over.

If the drug is highly water soluble, it slips through the microscopic aqueous pores.

If it's lipid soluble, meaning it mixes well with fats, it just dissolves right through the fatty lipid bilayer of the cell membrane itself.

No energy required.

But some molecules need a little help, though.

Figure 1 .6 also shows facilitated diffusion, which uses specialized carrier proteins.

They act like revolving doors for larger molecules, spinning them inside without easing up any energy.

Right.

Then there's active transport, where the cell actually burns ATP energy to force a drug inside against the natural gradient.

It's actively pumping it.

Yeah.

And for truly massive molecules, like vitamin B12, the cell performs endocytosis.

The membrane reaches out, engulfs the giant molecule, and pinches off a little bubble to drag it inside.

It's really cool to visualize.

But, you know, the mechanism of drug use is entirely dictated by its unique chemistry.

And this is where things get really fascinating.

Ooh, the pH factor.

Figures 1 .7 and 1 .8 go heavy into this.

They do.

Because the vast majority of medications are either weak acids or weak bases.

Depending on the ambient pH of the fluid the drug is floating in, whether it's the highly acidic stomach acid or the more neutral blood the drug will either take on an electrical charge or it'll remain uncharged.

Okay, wait.

Let's pause here, because this sounds like deep chemistry, and I want to make sure it's clear for you listening.

Why does the human body care if a microscopic molecule has a positive or negative charge?

What does that actually change?

It changes everything, mainly because of what cell membranes are made of.

Lipids, fats.

Right.

Think about trying to mix oil and water.

A drug molecule that carries an electrical charge is highly polar.

It acts like water.

It strongly repels fat.

Okay.

So if a charged or ionized drug hits a cell membrane, it literally bounces off.

It gets stuck.

Oh, wow.

But an uncharged, non -ionized drug molecule is lipid soluble.

It slips through the fatty cell membrane like a ghost walking through a solid wall.

Ah.

So the acidity of the environment completely controls whether the drug is wearing its charged armor or not.

Exactly.

If it is charged, it is trapped and cannot be absorbed.

If it's uncharged, it glides right through.

That perfectly explains why certain drugs are only absorbed in specific parts of the digestive tract.

Yep.

And even if a drug gets its charge just right and successfully crosses the membrane, it might immediately run into a cellular bouncer.

The bouncer.

I love this concept.

Figure 1 .9 shows this transmembrane pump called P -glycoprotein.

Yes.

P -glycoprotein is wild.

It uses ATP energy to grab foreign molecules that have just crossed into the cell and violently hurls them right back out.

It's just throwing them out the club.

Literally.

It pumps drugs out of the intestinal lining back into the gut or out of the brain capillaries back into the blood.

The body is actively working against the treatment, and the text notes that overexpression of these bouncers is actually a major reason why some patients develop multidrug resistance during treatment.

Their cells simply pump the medicine out faster than it can get in.

That battle at the border dictates a concept called bioavailability,

which is the critical measurement of how much the drug we swallowed actually made it into the systemic bloodstream alive and intact.

And figure 1 .10 shows this perfectly with an area under the curve, or AUC, graph.

Right.

If you plot the concentration of a drug in the plasma over time, an IV injection would start at 100 % instantly.

The line shoots straight up.

Massive spike.

But if you give that exact same dose in pill form, the curve on the graph is much lower and much wider.

A huge chunk of the drug is just missing.

Where did it go?

It got caught at the liver's tollbooth.

This is what we call first pass metabolism, which is mapped out in figure 1 .11.

The first pass.

Yeah, when a drug is absorbed through the stomach or the intestines, that blood does not go straight to the heart.

It gets funneled into the portal circulation, which leads directly to the liver.

And the liver is the body's ultimate security filter.

Exactly.

It aggressively inspects all the blood coming from the gut and will metabolize and destroy massive quantities of foreign chemicals before they ever get the chance to reach the rest of the body.

Like nitroglycerin.

Right.

The tech says it's famously vulnerable to this.

Oh, absolutely.

The liver destroys more than 90 % of an oral nitroglycerin dose on the very first pass.

Which is why patients place nitroglycerin tablets under the tongue instead.

The tissue there drains directly into the general circulation, completely bypassing that liver tollbooth.

Exactly.

Okay.

So let's say our drugs survived the stomach acid, found the right pH, snuck past the P -glycoprotein bouncer and navigated around the liver's first pass metabolism.

It is finally in the systemic bloodstream.

Time for step two.

Step two of ADME distribution.

The hide -and -seek phase.

It's in the blood, but where does it travel next?

Well, that depends largely on whether it prefers fat or water.

Lepophilic, or fat -soluble drugs,

easily dissolve through lipid cell membranes.

They rapidly leave the bloodstream and distribute widely into all the tissues, muscles, and organs.

And the water -soluble ones?

Hydrophilic, or water -soluble drugs, have a much harder time.

They cannot cross those cell membranes easily, so they are mostly confined to the water and the blood plasma.

To get anywhere, they have to slowly squeeze through tiny slit junctions between the capillaries.

Okay, hold on.

I was looking through the math on how we track this in figure 1 .14, and something seems physically impossible.

Oh, the volume of distribution.

Yes.

Pharmacologists use this calculation called the volume of distribution, or VDIDR.

But the calculations often suggest a drug can distribute into hundreds of liters of fluid.

A normal human body only contains about, what, 42 liters of water in total?

Roughly, yeah.

So how is that physically possible?

Where is the rest of this volume coming from?

It sounds like magic, I know.

But volume of distribution is actually a theoretical mathematical concept, not a literal physical bucket of water.

We calculate it simply by taking the total dose of the drug we injected into the patient,

and dividing it by the concentration of the drug we met her floating in the blood plasma.

So dose divided by plasma concentration.

Now let's imagine a highly fat -soluble drug.

You inject a large dose, and because it loves fat, it immediately escapes the bloodstream and hides out deep inside the body's fat tissues,

or binds incredibly tightly to proteins in the muscle.

This is called tissue sequestration.

When you draw a vial of blood a few hours later to measure the plasma concentration,

you barely find any drug in the sample at all, because almost all of it is hiding in the fat.

Oh.

So the denominator in our equation, the plasma concentration, plummets to almost zero.

Exactly.

And mathematically, when you divide a normal dose by a microscopic plasma concentration,

it artificially inflates the volume of distribution to a massive number.

That makes so much sense.

Yeah, so when a clinician runs the numbers and sees an impossible volume of distribution of, like, 500 liters, they instantly know the drug is not swimming in the blood, it's hiding deep in the tissues.

It's a mathematical red flag that the drug is playing hide and seek.

And if the drug is sequestered in the body fat, the kidneys cannot possibly filter it out.

Which means the half -life of the drug extends dramatically.

Which leads us directly into step three, metabolism.

The makeover.

DM and ADME.

Right.

The drug is circulated, it's done its therapeutic job, and the body wants to sweep it out.

But as you noted, the kidneys have a real problem with fat -soluble drugs.

Because they just reabsorb.

Right.

Exactly.

The kidney tries to filter the drug into the urine, but because the drug is lipophilic, it just naturally diffuses right back across the kidney cells and reenters the blood.

The kidneys alone cannot clear it.

So the body has to give the drug a chemical makeover.

It has to physically change the lipophilic drug into a hydrophilic, highly polar molecule so it gets permanently trapped in the watery urine and flushed away.

And that transformation happens on the liver's factory floor.

Let's talk about the speed of that factory.

Figure 1 .15 introduces Michaelis Menten Kinetics.

Right, which maps out how fast the liver processes these drugs.

Most of the time, drugs are metabolized using first -order kinetics.

I picture first -order kinetics like a well -staffed shipping warehouse.

Okay, I like this.

If the warehouse processes exactly 10 % of whatever inventory comes in the door every hour, the workers just adapt to the volume.

If 100 boxes arrive, they process 10.

If a massive truck drops off 1 ,000 boxes, the workers speed up and process 100.

They consistently clear a constant fraction of the drug per unit of time.

That's a perfect way to look at it.

The rate of elimination stays directly proportional to the concentration of the drug.

But that warehouse has a limit.

Right.

With massive doses of certain drugs like aspirin, or high amounts of ethanol from alcohol,

the system switches to zero -order kinetics.

Meaning, the factory is completely overwhelmed.

The delivery trucks have dumped so many boxes that every single worker is busy, every machine is maxed out, the system is totally saturated.

Yes.

At this point, it doesn't matter if you drop off 1 ,000 boxes or 10 ,000 boxes.

The factory can only process 50 boxes an hour.

Period.

In zero -order kinetics, the liver enzymes are completely saturated.

The body can only clear a constant amount of the drug over time, rather than a constant fraction.

It becomes a bottleneck, which is why alcohol takes so long to clear your system.

So what are the workers in this factory actually doing to the drug molecules?

Figure 1 .16 outlines this chemical makeover in two main phases, phase one and phase two.

Yes.

Phase I is predominantly handled by the cytochrome P450 system in the liver.

You can think of these CYP enzymes as tiny chemical scalpels.

Scalpels.

Yeah, they slice into the drug to introduce or unmask a polar functional group, like adding an oxygen -hydrogen pair to the structure.

The goal is to make it slightly more water -soluble.

And sometimes that one slices enough to send it to the kidneys.

But if the drug is stubborn and remains too fat -soluble, it moves down the assembly line to phase two.

Right.

Phase two relies on conjugation.

The liver finds a massive, highly polar, incredibly water -soluble molecule, most commonly glucuronic acid, and physically attaches it, or conjugates it, to the drug.

It's like strapping a giant heavy water balloon to the drug molecule.

That's exactly what it is.

The drug is almost always rendered completely inactive, and it's now so polar and heavy that it is permanently trapped in water.

Which brings us to the final letter in ADME, elimination.

The final exit.

Our drug is now a highly polar, water -soluble conjugate.

It travels through the blood and arrives at the kidney's filtration system.

Figure 1 .20 shows this whole journey through the nephron.

Right.

The blood is filtered, and the modified drug drops into the start of the nephron, which are the microscopic tubes in the kidney.

It flows down the proximal tubule, through the loop of Henle, and into the distal tubule to eventually become urine.

And here is where we see a truly brilliant clinical application of everything we've discussed so far.

Even in the distal tubule, there's a risk of passive reabsorption.

Yes, there is.

If a drug molecule manages to lose its electrical charge, it can slip back into the blood.

But emergency room doctors can actually use the pH rules we learned earlier to trap the drug on purpose.

We call this ion trapping.

Let's say a patient is rushed into the emergency room having overdosed on phenobarbital, which is a weak acid.

The doctor needs to force the kidneys to excrete as much of it as possible, rapidly.

So they'll administer an IV of bicarbonate to the patient.

To change the pH.

Exactly.

This drastically alters the patient's urine, making it alkaline or basic.

When the weak acid phenobarbital flows into this newly basic urine, a chemical reaction occurs.

The phenobarbital loses a proton and becomes highly charged.

And remember our golden rule.

Charged molecules cannot cross lipid membranes.

The phenobarbital is now electrically trapped inside the nephron tube.

Yes.

By deliberately manipulating the pH of the urine, the doctor actively blocks the drug from reabsorbing into the blood, forcing the body to urinate it out.

That is just elegant chemistry saving a life in real time.

It really is amazing.

And you know, while the kidneys handle the bulk of elimination, drugs can also exit through other routes.

Right.

Unabsorbed drugs leave via feces.

Volatile drugs like anesthetic gases used in surgery are literally exhaled through the lungs.

And crucially, drugs can be excreted in breast milk.

That's a huge point for nursing mothers.

Yes.

They require intense clinical monitoring because the infant is inadvertently receiving a dose of whatever medication the mother consumes.

Wow.

Okay.

So we have traced the complete journey from entry to exit.

ADME.

But to wrap this up, how does a clinician use all this kinetic data to actually write a prescription?

Well, clinical dosing is about finding the perfect schedule to keep a patient safe but cured.

The ultimate goal of a dosing schedule is to reach a steady state.

Figure 1 .23 illustrates this perfectly.

It shows the oscillating waves of drug concentration in the plasma.

Exactly.

Smaller, more frequent doses mean smaller peaks and valleys.

The concentration rises when they take the pill and drops as the liver and kidneys clear it.

The steady state is the perfect equilibrium where the amount of drug entering the body exactly matches the amount being cleared out.

So you're perfectly balanced within the therapeutic window.

Right.

But there is a golden rule in pharmacokinetics.

Regardless of the dose size, it always takes about four to five half -lives of repeated dosing for a drug to accumulate and hit that steady state plateau.

But wait, what if we can't wait four to five half -lives?

What if a patient comes into the ICU with a severe life -threatening infection right now?

Waiting days to reach steady state could be fatal.

It could be.

And that's why doctors use a loading dose.

A loading dose.

Yeah.

Instead of starting with a normal daily pill, they give a massive initial dose up front.

The goal is to instantly fill up that mathematical volume of distribution we talked about earlier.

You intentionally saturate the body's tissues and instantly spike the plasma concentration directly into the therapeutic window.

So you force the body up to that steady state level on day one?

Yes.

And then you immediately switch to a much smaller maintenance dose.

To maintain it.

Exactly.

The maintenance dose only needs to replace the exact amount of drug that the kidneys are clearing every few hours.

You use the heavy loading dose to get there instantly and the later maintenance dose to stay there safely.

It's exactly like gunning a car's engine to quickly merge onto the highway and then immediately switching on cruise control to maintain your speed.

That's a great way to put it.

Well, as we wrap up this deep dive, I want to leave you listening with a thought that struck me while reviewing this material.

There is a deeply fascinating, almost poetic irony in pharmacokinetics.

Think about all the physiological hurdles we just broke down.

The p -glycoprotein bouncers forcefully pumping chemicals out of our cells.

The CYP450 liver enzymes acting as aggressive toll booths.

The kidneys' complex filtration traps.

Over millions of years, human evolution built these incredibly sophisticated defenses for one specific reason.

To protect us.

Right.

They are perfectly designed to save your life if you accidentally eat a toxic berry or ingest a deadly poison in the wild.

But the body cannot distinguish between a toxic plant and a life -saving synthetic molecule.

It treats all of them as foreign invaders that must be dismantled.

Exactly.

In modern medicine, these ancient life -saving evolutionary defenses are the exact hurdles that pharmacologists have to outsmart every single day.

We really do.

I mean, we have to design brilliant enteric coatings to trip the stomach acid, precisely calculate pK values to bypass the cellular membranes, and deliberately overload liver enzymes just to get helpful medicine to the brain or the heart.

We spend billions of dollars constantly fighting our own body's desperate attempts to keep us safe.

It is a profound tug of war at the molecular level.

It truly is.

So tomorrow morning, when you take your daily allergy pill or even just sip your morning cup of coffee, take a second to appreciate the invisible microscopic battle happening inside your bloodstream.

Your body is actively fighting and cooperating with every single molecule.

Well said.

Thank you so much for joining us on this Deep Dive.

On behalf of the Last Minute Lecture team, thank you for listening.