Chapter 26: Autacoid Drugs That Mimic Endogenous Substances

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Welcome back to the Deep Dive.

I am so glad you are here with us today because we are about to open up a corner of medical science that I think is, well, it's vastly underappreciated.

You know, usually when we think about how the human body communicates, how it sends messages from point A to point B, we tend to think about the big flashy systems.

Right, the super highways.

The super highways.

We think about the brain sending electrical lightning down the nerves to move a muscle or, you know, we think about the endocrine system, the pituitary glands sitting up there like a general in a tower, shouting out hormones that travel miles through the bloodstream to tell the adrenal glands or the thyroid what to do.

It's all very command and control, very top down.

Exactly.

It's the classic central government model of biology.

A centralized decision is made and the order is broadcast to the entire nation.

It is.

A memo gets sent out to every single department, whether they need it or not.

But today, today we are looking at something completely different.

We are ignoring the federal government and we are looking at the local neighborhood I like that analogy.

We're talking about chemicals that are produced right there in the tissue, act on the cell next door, and then poof, they vanish.

We are talking about otocoids.

Otocoids, it's a fascinating term.

It comes from the Greek autos meaning self and achos meaning remedy or a medicinal agent.

So literally these are self remedies.

Self remedies.

And to really understand them, we are going to be doing a comprehensive deep dive summary of chapter 26 from Brenner and

the chapter is titled Otocoid Drugs That Mimic Endogenous Substances.

And just to set the stage for everyone listening, our mission today is to decode three major families of these local chemicals.

We aren't just skimming the surface.

We are going deep into the weeds.

We've got histamine, which I think most people know as the villain of allergy season.

We've got serotonin, which everyone thinks is just the happy chemical in the brain, but oh boy, are we going to bunk that myth today.

Oh, absolutely.

And then we have the icosinoids, specifically prostaglandins and leukotrinies, which sound like sci -fi villains, but are actually controlling everything from inflammation to blood pressure.

The really cool thing is that because these systems are so powerful, pharmacology has hijacked them.

We're going to see how manipulating these local hormones allows us to treat things as diverse as stomach ulcers, glaucoma, migraines, and even pulmonary hypertension.

It's amazing.

It's really a masterclass in how understanding the mechanism leads to the cure.

And just a heads up on the scope here, we're sticking strictly to the text of chapter 26.

We're going to walk through the biology, the drug classes, the mechanisms, and the clinical uses exactly as they're laid out.

No outside fluff, just the core science.

Just the facts from the book.

So let's start at the very beginning.

What exactly makes an odicoid an odicoid?

How is it fundamentally different from, say, a hormone like testosterone?

The key difference, the defining characteristic is the range.

It's all about locality.

A hormone like insulin or cortisol or testosterone is released into the systemic circulation.

It enters the blood and travels everywhere.

It hits every zip code in the body.

An odicoid is different.

It is produced by neural or non -neural tissues.

It acts locally to modulate the activity of smooth muscles, nerves, glands, or platelets.

And then, this is crucial, it is typically destroyed very quickly.

So it's a short range weapon.

It's a whisper, not a shout.

Precisely.

It allows tissues to self -regulate.

For example, if you scratch your skin, you need inflammation right there at the scratch.

You need increased blood flow and immune cells at that specific coordinate.

You certainly do not need inflammation in your liver or your kidneys or your brain just because you scratched your arm.

That'll be a huge overreaction.

A massive overreaction.

So your skin releases odicoids to handle it locally.

That makes perfect sense.

It's efficiency.

It is efficiency.

But, and here is the catch that sets up the rest of our discussion.

While this local nature is great for regulation,

when these things are overproduced or released systemically, they cause absolute chaos.

Chaos.

Like what?

Give me an example.

Well, think about anaphylactic shock.

That is a massive systemic release of histamine.

The body isn't designed to handle histamine everywhere at once.

Your blood pressure bottoms out.

Your airways close up.

It's a systemic catastrophe caused by a local mediator going global.

And you also mentioned carcinoid tumors.

Yes.

Those are tumors that can dump huge amounts of serotonin into the system.

Again, a local actor goes on a world tour and the body really struggles to cope with the consequences.

Okay, let's unpack the first one on our list.

Histamine.

I feel like everyone has a personal relationship with histamine, usually involving a box of tissues and some red eyes.

I think that's fair to say.

But let's look at the biology first.

Where does it actually come from?

Is it something we eat?

We sort of eat the

precursor.

Biologically, histamine is what we call a biogenic eminine.

It's chemically quite simple.

It's formed from an amino acid called histidine.

And histidine is just in our food.

It's one of the essential amino acids you get from protein in your diet.

Inside the body, there's a specific enzyme called L -histidine decarboxylase.

This enzyme acts like a pair of molecular scissors.

Okay.

It snips off a carboxyl group from the histidine and voila, you have histamine.

Okay, so we are making this stuff, but we aren't constantly inflamed and itchy, so the body must be hiding it somewhere, right?

Where's the storage locker?

Primarily, it is stored in granules.

Think of them as little pressurized storage sacks or vesicles inside specific immune cells.

Like little water balloons.

Exactly.

Little water balloons filled with histamine.

And the big players here are mast cells and basophils.

Mast cells and basophils.

And where are these cells hanging out in the body?

They are essentially guards stationed at the borders.

You find high concentrations of mast cells in the skin, the lining of the gastrointestinal tract, and the respiratory tract.

So the points of entry.

Basically, anywhere the outside world meets the inside world, you have mast cells loaded with histamine grenades waiting for an intruder.

Which explains so much.

It explains why allergies usually affect your skin hives, or your stomach cramps, or your nose sneezing.

The guards are fighting the battle at the gates.

Exactly.

Now to be complete, histamine is also found in a couple other places.

It's in the gastric fundus, that's the upper part of the stomach, within what are called gastric paracrine cells.

Now what's it doing there?

There, its job is to help signal acid production.

It's part of the machinery for digestion.

And it's also found in some neurons in the central nervous system, where it acts as a neurotransmitter to promote wakefulness.

Interesting.

But the vast majority of the body's histamine is sitting in those mast cells just waiting for a trigger.

Okay, so let's talk about that trigger.

Let's look at figure 26 .1 in the text, which breaks down this release mechanism.

Because the cell doesn't just leak histamine for fun, it has to be told to drop the bomb.

How does that happen?

Right.

There are two main categories of triggers.

The classic way, the one everyone thinks of, is immunologic.

This involves antibodies, specifically immunoglobulin E or IgE.

This is the allergy mechanism, the pollen, the cat dander, all that stuff.

Yes.

Imagine a mast cell is covered in little Y -shaped antenna.

Those are the IgE antibodies.

If you are allergic to pollen, you have IgE specific to pollen attached to your mast cells.

Okay, so they're primed.

They are primed.

When an antigen, like a grain of pollen or peanut dust, floats by and hits those antibodies, it acts like a key.

It cross -links two of those IgE antibodies,

and that physical connection triggers the cell to degranulate.

And degranulate just means?

The cell fuses those storage sacs with its outer membrane and dumps the histamine out into the tissue.

The grenades explode.

Okay, so that's the immune response.

Very clear.

But the text mentions there are chemical triggers too.

And this part really surprised me.

Certain drugs can cause histamine release directly without an allergy involved.

That's a very, very important clinical distinction.

It's a non -allergic histamine release.

What kind of things can do that?

Bacterial toxins can do it, but so can drugs like tubucurine and, crucially, morphine.

Morphine?

So if someone takes morphine for pain and they get itchy or flushed, it's not necessarily that they are allergic to opiates.

Exactly.

That's a common misconception.

And the text makes a specific point here that I want to highlight.

This morphine -induced histamine release is not mediated by opioid receptors.

How do we know that?

That seems like a very specific detail.

We know this because if you give naloxone, which is the antidote that blocks opioid receptors, the ones that cause the high -end pain relief, it doesn't stop the release.

Okay.

Morphine just chemically displaces the histamine from the granules.

It effectively kicks the histamine out of the storage locker directly.

That is wild.

So it's a direct chemical interaction, almost like shaking a vending machine until the candy falls out, rather than using the keypad.

That is a fantastic analogy, yes.

And, clinically, it matters because you don't want to label a patient as allergic to morphine if they just experienced a common chemical side effect.

That label follows them forever.

That's a great point.

Okay, so the histamine is out.

It's floating around in the tissue.

But a chemical is just a message.

It needs a receiver to hear it.

Talk to me about the receptors.

We have three main types to worry about in this context.

H1, H2, and H3.

All of them are G protein -coupled receptors, or GPCRs, which is a very common signaling architecture for this kind of system.

Let's break them down one by one because they do very different things.

Let's start with H1.

This is the one we know and loathe.

This is the thing that happens.

First,

in the skin and ucus membranes, it causes vasodilation.

The blood vessels widen.

Okay, and that causes the redness and the heat, right?

The redness and the heat you feel, correct.

Second, it increases vascular permeability.

What does that mean in plain English?

The cells lining the blood vessels pull apart slightly, allowing fluid to leak out from the blood into the tissue.

So it gets leaky.

It gets leaky.

That creates edema or swelling.

If it's in your nose, it's congestion.

If it's on your skin, it's a hive or a wheel.

And the itching, that's the worst part.

That is H1 stimulation of mucocutaneous nerve endings.

The medical term is pruritus.

It tickles the nerves directly.

And in the lungs, H1 activation causes bronchoconstriction tightening of the airways, and it also triggers the cough reflex.

So H1 is the misery receptor.

It makes you red, swollen, itchy and wheezy.

Got it.

What about H2?

H2 is the stomach receptor.

Its primary job when we exploit pharmacologically is to tell the parietal cells in the stomach lining to secrete gastric acid.

It's a major signal for acid production.

Does it do anything else?

The text mentions it has a minor role in the heart.

It can increase heart rate and contractility.

But clinically, when we talk about H2, we are almost always talking about stomach acid.

Okay, finally H3.

The text calls this one the break.

That sounds important.

H3 is fascinating.

It's located mostly on nerve terminals, specifically presynaptically.

What does presynaptic mean?

It means it sits on the nerve that is doing the releasing.

When histamine hits an H3 receptor, it tells that same nerve, okay, that's enough.

Stop releasing neurotransmitters.

It inhibits the release of histamine itself and other transmitters.

So it's a negative feedback loop.

It's a perfect negative feedback loop to prevent the signal from getting too loud.

It's the body's own off switch.

So just to summarize in my own head, H1 makes you sneeze and itch.

H2 gives you heartburn and H3 tries to calm everything down.

That is a very useful simplification.

And that geography of receptors, where they are and what they do, it dictates the entire field of pharmacology here.

Oh, so.

If you want to treat allergies, you block H1.

If you want to treat ulcers or GERD, you block H2.

It's target identification 101.

Which brings us perfectly to part two of our deep dive, the antihistamines, specifically the H1 antagonists, the drugs that block that misery receptor.

Right.

The text immediately splits these into two generations.

What is the big divide here?

Why do we have two families?

The divide is the blood brain barrier.

The first generation antihistamines are lipophilic, meaning they are fat soluble.

Because cell membranes, including the blood brain barrier, are made of fat, these drugs slip right across and get into the central nervous system.

And that's what causes the sedation, the sleepiness.

That is exactly what causes the sedation.

And the second generation.

The second generation drugs were designed specifically not to cross that barrier.

They're either more hydrophilic, they love water more than fat, or they're just too bulky to get through.

So they stay in the peripheral body.

So they work on the nose and the skin without putting the brain to sleep.

That's the goal, yes.

Let's look at the first generation roster.

These are the classics.

We've got dephanhydramine, that's benadryl, hydroxazine, promethazine, chlorphenamine,

meclazine, diamond hydranate.

They're all over the counter.

They are.

And mechanistically, they act as competitive antagonists.

What does that mean?

It means they compete with histamine for the same parking spot on the H1 receptor.

They have a chemical group, an alkylamine group, in their structure that looks just enough like to sit in the receptor and block it.

So the key doesn't turn the lock, but it prevents the real key histamine from getting in.

That's a perfect way to put it.

But because they are dirty drugs, meaning they hit other receptors too, they do more than just stop allergies.

The text mentions they are used for sedation, motion sickness, and nausea.

Let's talk about those other uses.

Yes, this is really important.

Because they enter the brain, dephanhydramine and hydroxazine are excellent sedatives.

You'll see them used for insomnia or to calm patients before surgery.

Okay, that makes sense.

But what about motion sickness?

That feels different.

It is.

There is a subclass here that is fascinating.

Yeah.

The antimetics.

Meclazine and diamond hydranate, which you might know as dramamine, are used for motion sickness and vertigo.

How do they work for that?

They work by dampening the signals in the vestibular system of the inner ear and the part of the brain that processes motion.

It's a central nervous system effect.

And promethazine.

The text says it has high antimetic activity.

Promethazine is a heavy hitter for nausea and vomiting, especially after surgery or with certain illnesses.

Doxilamine is another one, often paired with vitamin B6, specifically for morning sickness in pregnancy.

So these old drugs are kind of a Swiss army knife?

They are, but there is a cost to using them.

Right.

We mentioned sedation, which can be dangerous if you're driving or operating machinery.

But table 26 .2 in the text highlights some other nasty side effects.

Right.

What happens if you take too much of these first gen drugs?

You get anticholinergic toxicity.

What's that?

These drugs don't just block histamine receptors.

They also block muscarinic receptors, which are part of the cholinergic system.

The rest and digest system.

The rest and digest system, exactly.

So when you block those receptors, you get dry mouth, blurred vision because your eyes can't focus, urinary retention.

You can't pee and tachycardia, a racing heart.

I've heard the mnemonic for this.

Mad as a hatter, red as a beat, dry as bone, blind as a bat.

That's the one.

And the text mentions a specific antidote if someone overdoses on these and gets that full -blown toxicity.

Fisostigmine.

Right.

Fisostigmine is a cholinesterase inhibitor that can cross the blood -brain barrier to counteract that toxicity in the CNS.

It boosts the body's natural acetylcholine signals to overcome the blockade.

There is also a weird paradox mentioned for kids.

I've heard about this.

My sister gives her kid Benadryl and he bounces off the walls.

Yes, paradoxical excitement.

It's a real thing.

While adults usually get sleepy, infants and children can sometimes get hyperactive, agitated, and excited on these drugs.

What does that happen?

The mechanism isn't perfectly understood, but it's a known phenomenon and a safety watch out for parents.

Yeah.

Don't assume it will make your kid sleepy on a long flight.

It might do the opposite.

Okay, so that's the old guard.

They work, but they have baggage.

Now let's look at the second generation antihistamines.

The non -drowsy options.

Who is on this team?

This includes Ceterazine, Loratidine, Fexofanidine, Desloranidine, and Low Ceterini.

Your Claritins, Zyrtex, Allegra.

And the main selling point is simply allergy relief without the nap.

Exactly.

They treat the allergy without the nap.

They don't cross into the brain significantly.

And for that same reason, they also lack that anti -emetic activity.

Oh, that's a good point.

Don't take Claritin for seasickness.

It won't work.

It doesn't get to the part of the brain that controls nausea.

There is a bit of a history lesson in the text here regarding safety.

Some early second gen drugs got banned.

What happened there?

You're thinking of Turfenidine, which was a cell dane, and another one called Astamazole.

They were miracle drugs when they came out, non -sedating allergy relief.

Sounds great.

What was the problem?

They had a fatal flaw.

They blocked potassium channels in the heart, specifically the HERG channels.

This led to something called QT prolongation on the ECG.

And that's bad.

It's very bad.

It can lead to a specific deadly arrhythmia called torsades de pointe, basically a chaotic heart rhythm.

Yikes.

So people were dropping dead from heart attacks while treating their hay fever?

It was a major safety crisis in pharmacology in the 90s.

But the story has a happy ending, in a way.

How so?

It turns out that Fexofenidine, which is the drug in Allegra, is actually the active metabolite of Turfenidine.

Wait, explain that.

What does that mean?

So when you took Turfenidine, your liver immediately converted it into Fexofenidine again.

Fexofenidine was the thing that was actually stopping your allergies.

But the original drug, the parent drug Turfenidine, was the thing stopping your heart.

Oh, wow.

So scientists realized, hey, let's just skip the dangerous middleman.

Let's manufacture the metabolite directly.

Fexofenidine gives you the antihistamine effect without the heart risk.

That is just brilliant.

And that's why Fexofenidine is on the shelf today and Turfenidine is gone.

It's a great example of how pharmacology evolves.

We refine the molecule to keep the good and strip out the bad.

Before we leave antihistamines, we should mention the specialized formulations.

If you just have itchy eyes or runny nose, you don't always need a pill that goes everywhere.

Right.

Local problems, local solutions.

We have intranasal antihistamines like azaleastine.

It's quite potent and it actually has a dual mechanism.

It blocks the receptor and inhibits histamine release too.

What are the downsides?

Well, the text notes it has a bitter taste, which people complain about, and can cause nasal irritation because, well, you're spraying chemicals up your nose.

And for eyes?

For eyes, we have ophthalmic drops like ketufin and olipatidine.

Ketutifin is interesting.

The text says it's a mast cell stabilizer plus an H1 blocker.

It is.

It's a double agent.

It blocks the H1 receptor so histamine can't act, and it stabilizes the mast cell membrane to prevent it from degranulating in the first place.

So it stops the key from turning and it reinforces the door.

An excellent way to put it.

The text actually has a case study here called the Sneezing Stock Broker.

Let's quickly touch on that because I think it puts this all into a very practical perspective.

It's a classic scenario.

A man takes an over -the -counter allergy med.

The text implies it's probably a first generation one like divinitramine, and he feels like he's in a SOG.

He can't focus on his trades.

Which is bad if you're a stockbroker.

Very bad.

The resolution is simply good pharmacology.

He gets a diagnosis of allergic rhinitis, hay fever, and the provider switches him to a drug that respects the blood -brain barrier.

So a zelestine spray or a second -gen pill.

Exactly.

Switch to a zelestine nasal spray or a second generation oral drug like loratidine or fexofinidine.

He gets the same symptom relief without the cognitive penalty.

It highlights why understanding that blood -brain barrier difference is so critical for quality of life.

One last thing on histamine before we move on.

We talked about H3 being the break.

Is there a drug called pitolusant?

It's an H3 antagonist or more accurately an inverse agonist.

And why on earth would you want to do that?

Why would you want more histamine?

Narcolepsy.

Think about it.

We said histamine in the brain promotes wakefulness.

So if you block the break, the brain releases more histamine.

Pitolusant helps keep narcoleptic patients awake by turning up the volume on the brain's natural wake -promoting histamine system.

It's a first -in -class drug, right?

It is a very clever application of physiology.

Fascinating.

So histamine keeps you awake, which explains why antihistamines knock you out and why blocking the break on histamine wakes you up.

It all connects.

Exactly.

It's all about turning the volume knob on that specific neurotransmitter.

All right.

Let's shift gears.

We're moving from the nose to the, well, mostly the gut actually.

Let's talk about serotonin.

Serotonin or its chemical name, 5 -hydroxy tryptamine, which is why you see it abbreviated

Now, I think most people assume serotonin is all about mood, depression, and happiness, that it's a brain chemical.

But the text says the highest concentration is actually in the enterochromathin cells in the gut.

That's right.

It's one of the biggest myths in popular science.

Over 90 % of the body's serotonin is not in the brain.

It is in the gut.

Wow.

And what's it doing there?

Serotonin is a major regulator of GI motility.

It tells the intestines to contract and move food along.

It's also found in platelets, where it helps with clotting by causing platelet aggregation and, of course, in neurons in the brain.

But the gut is the main factory and storage facility.

How do we make it?

Is it like histamine?

Very similar process.

It's synthesized from an amino acid, this time as tryptophan.

You eat protein, you get tryptophan.

And enzymes in the body convert it to 5 -HT.

So that's the whole Thanksgiving turkey making you sleepy thing.

That's the folk wisdom, yeah.

High tryptophan meal.

Only when it breaks down.

It turns into a metabolite called 5 -HIAA,

which is then excreted in the urine.

That's actually a useful clinical marker.

Doctors can measure 5 -HIAA in a 24 -hour urine sample.

And if it's high.

If you have really high levels of 5 -HIAA in your urine, it might be a sign that you have a certain type of tumor, a carcinoid tumor, that's just pumping out excess serotonin.

Okay.

So if histamine had three main receptors, serotonin seems to have a lot more.

The list in the text, 5 -HT1234, it's intimidating.

It is a complex family.

There are actually even more.

But 5 -HT1 through 5 -HT4 are the main ones we discuss pharmacologically here.

But there is a key fundamental distinction to make that simplifies things a lot.

Please simplify.

The 5 -HT3 receptor is different from all the others.

It is a ligand -gated ion channel.

Meaning when serotonin binds, it opens a physical gate for ions to flow through directly.

Yes.

It's a direct, fast -acting channel.

All the others, 1, 2, and 4, are G protein -coupled receptors.

They work through a slower, multi -step second messenger system.

And why does that matter?

That structural difference makes 5 -HT3 unique in how it signals.

It's very fast.

And it makes it particularly important for the sensation of nausea and for the action of our anti -nausea drugs.

Got it.

Let's look at the drugs.

The text organizes them by what they do.

Agonists, which are activators, and antagonists, which are blockers.

Let's start with the agonists.

If we activate serotonin receptors, what can we treat?

Well, starting with the brain.

For anxiety and depression, we have buspirone, which is a partial agonist at the 5 -HT1A receptor.

But for migraines, the big players are the tryptans like sumatriptan.

Migraines are brutal.

How do tryptans actually work?

What are they activating?

They agonize or stimulate the 5 -HT1D and 1 -B receptors.

These specific receptors are found on cranial blood vessels and on nerve endings in the trigeminal system.

And what does that do?

Activating them causes vasoconstriction in the dilated, throbbing cranial blood vessels that are part of a migraine.

It also inhibits the release of inflammatory neuropeptides.

Essentially, it shuts down the vascular throbbing and the inflammation that cause the pain of a migraine.

So a very targeted attack on the migraine process.

Extremely targeted.

Then there are the GI drugs.

We mentioned serotonin drives gut motility.

So if we stimulate serotonin receptors in the gut, we should get movement, right?

That's the idea.

So we have 5 -HT4 agonists.

The history here is a bit rocky.

Cisipride was the first, but it was pulled from the market due to cardiac issues.

That's the same QT prolongation we saw with the old antihistamines.

That seems to be a recurring theme.

It is.

Then came Tigacerod, which was restricted because of increased risks of stroke and heart attack.

But more recently, a drug called Prucalipride was approved.

And what's that for?

It's for chronic idiopathic constipation, for people whose guts are just sluggish.

It stimulates those 5 -HT4 receptors and basically tells the gut to get moving.

There are a couple of newer, more niche drugs mentioned in the agonist table, too.

Lorcaserin for obesity.

Yes.

That one acts on 5 -HTTCC receptors in the hypothalamus of the brain.

It basically tricks the brain into feeling full, so it decreases food intake.

And then there is flubanserin or Adi.

Right.

That one is for hypoactive sexual desire disorder in premenopausal women.

It has a complex mechanism, acting as an agonist at some serotonin receptors and an antagonist at others to improve libido.

Okay, flip side.

Let's talk about serotonin antagonists.

When do we want to block serotonin?

The most famous and life -changing use is for nausea, specifically chemotherapy -induced nausea and vomiting, or CINV.

Before these drugs existed, chemo was a nightmare of uncontrollable vomiting for many patients.

These drugs are the citrons on dancitron, which is zofran, granistron, planicetron, and so on.

And this goes back to that unique receptor we talked about, the 5 -HT3 ion channel.

Yes.

The citrons are all 5 -HT3 receptor antagonists.

They block that specific channel.

And where are those channels?

They are located in two key places.

In the brain, in an area called the chemoreceptor trigger zone, or CTZ, which is the vomit center.

And they're also on the vagal nerve afferents in the gut.

So how does it all work during chemo?

Chemotherapy damages the cells lining the gut, causing them to release a huge amount of serotonin.

That serotonin hits the vagus nerve's 5 -HT3 receptors and sends a powerful vomit signal to the brain.

On dancitron stands in the way and blocks that signal from ever reaching the brain.

It is incredibly effective.

What about blocking serotonin for other things?

Well, some atypical antipsychotics like lisapine work in part by blocking 5 -HT2 receptors, which helps in treating schizophrenia.

And then there's a really interesting utility player called cyberheptadiene.

Cyberheptadiene seems like it does everything.

The tech says it blocks serotonin and histamine.

It does.

It's a 5 -HT2 antagonist and an H1 blocker.

This unique combination makes it specifically useful for treating a rare condition called carcinoid syndrome.

Right.

We mentioned that before, the tumors that release massive amounts of serotonin.

Exactly.

Patients get violent diarrhea and flushing from the serotonin, and sometimes wheezing from other substances released.

Like histamine.

Because cyberheptadiene blocks both serotonin and histamine, it can help dampen those terrible symptoms.

Speaking of carcinoid syndrome, the text mentions one more drug that doesn't just block the receptor, but actually stops the production of serotonin in the first place.

Yes, teletristadethyl.

This is for when the receptor blockers aren't enough to control the diarrhea.

It's a tryptophan hydroxylase inhibitor.

The very first enzyme in the production line?

It blocks the very first enzyme that converts tryptophan into the precursor for serotonin.

So if you can't block the message at the receiver, you go upstream and shut down the factory.

You cut off the supply at the source.

That is some elegant chemistry.

Okay, we have covered the aminauticoids, histamine, and serotonin.

Now we need to wade into the fats, the icosinoids.

This is a huge topic, and honestly, often the most confusing for students.

The name helps, though.

Icos means 20 in Greek.

And that refers to the fact that they're derived from 20 carbon fatty acids.

Precisely.

The grandfather of them all is arachidonic acid.

And where does that come from?

It sits in your cell membranes, literally part of the wall structure tucked away.

When a cell is injured or stimulated, an enzyme called phospholipase A2 acts like a crowbar.

It pries that arachidonic acid molecule out and releases it from the membrane into the cell.

And then according to figure 26 .2, we hit a fork in the road.

A critical metabolic fork.

Once arachidonic acid is free, it can go down one of two paths.

Path A is led by the enzyme cyclooxygenase, or COX.

The COX enzymes, like in COX inhibitors, aspirin.

The very same.

That path leads to the creation of prostaglandins and thromboxanes.

Path B is led by the enzyme 5 -lipoxygenase, or LOX.

And that path leads to the leukotrienes.

Let's pause on the source material, the fatty acid itself.

The text brings up the diet connection here, which I found fascinating.

Omega 6 versus omega 3.

Why does this matter for these pathways?

It matters immensely.

It's fundamental.

Arachidonic acid is an omega 6 fatty acid.

It's very common in western diets.

Vegetable oils, conventionally raised meats.

Right.

But if you eat a lot of cold water fish, like salmon or mackerel, you get eicospentanoic acid, or EPA, which is an omega 3 fatty acid.

It has the same number of carbons, but a slightly different structure.

And the enzymes, COX and LOX, they process these different fatty acids.

Yes.

But they create different end products with different biological activities.

This is so important.

For example, when COX acts on arachidonic acid, omega 6, it produces thromboxane A2.

TXA2 is a potent vasoconstrictor and a powerful platelet aggregator.

It causes clots.

Okay, so omega 6 leads to clots.

What about omega 3?

If you have more omega 3s in your cell membranes, the COX enzyme makes thromboxane A3 instead.

And thromboxane A3 is much, much weaker.

It barely causes any platelet aggregation.

So eating fish oil literally changes the chemical structure of your clotting signals to be less clotty.

Exactly.

You are changing the raw materials that your body uses to build these signaling molecules.

It's a huge part of the biochemical explanation for why high fish diets correlate with fewer thrombotic events, like strokes and heart attacks.

You are building your house with different bricks.

Precisely.

Bricks that are less likely to stick together and form a blockage.

That is the kind of so -what insight I love.

It connects the biochemistry to the grocery list.

Okay, back to the pathways.

Let's talk about the physiological effects of these things.

We have thromboxane A2, TXA2, and another one called prostacyclin, PGI2.

The text describes them as rivals, don't they?

They are the absolute yin and yang of blood flow and clotting.

They are in constant balance.

Tell me more.

Thromboxane A2 is released by platelets when they get activated by an injury.

It screams to other platelets, clot, constrict.

Its job is to form a plug and stop bleeding.

And prostacyclin.

Prostacyclin, PGI2, is released by the healthy endothelial cells that lie in the inside of the blood vessel.

Its job is to say,

flow, dilate, don't clot.

It actively inhibits platelet aggregation.

And health is the balance between them.

Precisely.

If thromboxane wins, you get a clot of thrombosis.

If prostacyclin wins, you bleed.

Your body is constantly tweaking this balance.

And what about the other path?

The leukotrienes from the LOX enzyme.

Those are the inflammatory heavyweights, especially in the lungs.

Leucotrienes C4 and D4 are the components of what used to be called the slow -reacting substance of anaphylaxis.

What does that mean?

It means they are a major cause of the bronchospasm you see in asthma.

In fact, they are far more potent and longer -lasting bronchoconstrictors in the lungs than histamine is.

Okay, let's get to the drugs.

This is part five.

Prostaglandin drugs.

This can get like alphabet soup, so let's take them one by one.

The text groups them by which prostaglandin they mimic.

Let's start with PGE1 derivatives.

The star here is a drug called alprostadil.

It's chemically identical to natural PGE1.

And it has two very, very different uses based on who the patient is.

Okay, use number one, babies.

Specifically,

neonates with certain congenital heart defects.

In the womb, a baby has a blood vessel called the ductus arteriosus that allows blood to bypass the not -yet -working lungs.

It's supposed to close shortly after birth.

Right.

But if a baby is born with a heart defect -like transposition of the great vessels where they need that bypass to stay open to mix blood and survive until they can have surgery, we give a continuous infusion of alprostadil.

It keeps that vessel open.

The medical term is maintaining patency of the ductus arteriosus.

That's incredible.

It's a life -saving bridge to surgery.

So what's use number two for adults?

Erectile dysfunction.

That is a big switch.

It is.

Alprostadil is a potent vasodilator.

It can be injected directly into the penis or used as a urethral suppository to increase blood flow.

It's mostly used for men who can't take or don't respond to oral drugs like Viagra.

Okay, next up is misoprostol.

This is a PGE -1 analog, so it's slightly modified.

Misoprostol is primarily known as a cytoprotective agent for the stomach.

Cyt -o means cell.

It protects the cells of the stomach lining.

How does it do that?

It increases mucus secretion and bicarbonate production, and it decreases acid secretion.

It's often prescribed to prevent gastric ulcers in people who have to take high doses of NSAIDs, like for severe arthritis.

But it has a major contraindication, a huge warning.

A huge one.

Pregnancy.

Prostaglandins stimulate uterine contractions.

Misoprostol can induce labor or cause an abortion.

It's an abortifacient.

So it's actually used for that purpose clinically.

Yes, it is used clinically, often in combination with another drug called mefapristone for medical termination of pregnancy.

It is a powerful uterine stimulant.

Moving on to PGE -2 derivatives.

The drug here is dinoprostone.

Dinoprostone is used in obstetrics for cervical ripening.

What's that?

If a woman is at term but her labor hasn't started and the cervix isn't ready, dinoprostone can be applied locally as a gel or insert to help soften and dilate the cervix to get labor going.

So to induce labor.

To induce labor, yes.

It can also be used to help evacuate the uterus in cases of fetal death or for pregnancy termination.

And now PGE -2 alpha.

The drug is carboprostremethamine.

This one is a potent oxytocic, meaning it causes uterine contraction.

It's injected intramuscularly to control postpartum bleeding that isn't responding to other methods.

So after the baby is delivered and the mother is bleeding too much.

Exactly.

It squeezes the uterus down hard to clamp off the bleeding blood vessels.

It can be life -saving.

But because it stimulates smooth muscle everywhere, the side effects are rough.

They are.

The text describes them as violent.

Upset vomiting and diarrhea are very common.

Because while it's squeezing the smooth muscle of the uterus, it's also squeezing the smooth muscle of the gut.

Now here is the twist I didn't see coming.

PGF -2 alpha drugs for the eyes.

Yes.

This was a revolutionary discovery.

The drugs are latanoprost and others like pamatoprost and travoprost.

They are game changers for treating open angle glaucoma.

How does a uterine contraction drug help the eye?

That seems completely unrelated.

It's all about receptor location.

In the eye, these drugs don't cause contraction.

Instead, they increase the outflow of the aqueous humor, the fluid inside the eye.

They open the drain?

They open a secondary drain, the uveous -cleral pathway.

By letting more fluid out, they lower the pressure inside the eye, which is the whole goal of glaucoma treatment.

And the side effect, I hear it's a very noticeable one.

It is.

It can permanently change your eye color.

It increases the melanin content in the iris.

So blue, green, or hazel eyes can slowly turn brown over time.

Permanently.

Permanently.

And it also makes your eyelashes grow longer, thicker, and darker.

I feel like people would pay extra for the eyelash part.

They do.

Bimatoprost is marketed cosmetically under the brand name Lattice for that exact purpose now.

It's a classic case of a side effect becoming a lucrative feature.

Finally, PGI -2 prostacyclin.

We said this is the natural vasodilator.

What do we treat with it?

The main target is a serious condition called pulmonary arterial hypertension,

or PAH.

What is that?

This is a disease where the blood vessels in the lungs are clamped down and constricted, causing dangerously high pressure.

This puts a huge strain on the right side of the heart and leads to heart failure.

So we need to dilate those vessels.

Urgently.

The drugs are derivatives of prostacyclin.

Eoprostenol is one.

It has a very short half -life, just a few minutes.

So it has to be given by a continuous IV infusion through a pump.

The patient has to wear this pumped 2947.

Yes.

It's a constant infusion.

Driprostenol is a bit more stable, so it can be infused under the skin with a small pump.

And Celexipag is a newer oral drug that acts as an agonist at the same IP receptor.

These drugs dilate those pulmonary vessels and can save and extend lives.

Which brings us to our final section, which is another way to treat that same disease, PAH, endothelin -wash antagonists.

Right.

So we talked about prostacyclin being the good guy, vasodilator in the lungs.

Endothelin -1, or ET -1, is the bad guy.

The villain.

The villain.

It is a potent peptide vasoconstrictor produced by the endothelial cells.

And in PAH patients, the levels of ET -1 in their blood are about 10 -fold higher than normal.

It's a key driver of the disease.

So we want to block it.

We want to block it.

End of the drugs.

Bozentan, brand name Treglare, was the first big one.

It blocks both types of endothelin receptors, ETA and ETB.

By blocking the receptor,

it stops the vasoconstriction signal.

It improves the patient's ability to walk, the exercise tolerance, and it decreases the pulmonary resistance.

But it comes with a big warning label, right?

Very big one.

A black box warning for liver toxicity.

Patients on Bozentan need to have their liver enzymes monitored every single month.

And it also causes major birth defects.

So there's a strict program to prevent pregnancy in women taking it.

Are there newer ones?

Yes.

The text mentions Ambracentan, which is more selective for just the ETA receptor, and Macetantan, which is a newer agent in the class.

But that safety profile is something that always has to be carefully managed with these drugs.

And just to close the loop on PAH, the text mentions our old friend Sildenafil Viagra is used here too.

Yes, under the brand name Revachio for this indication.

We know Sildenafil inhibits an enzyme called PDE5.

This leads to an increase in cyclic GMP, which causes smooth muscle relaxation and vasodilation.

And it works in the lungs just like it works elsewhere.

It works in the lungs just like it works in erectile tissue.

It opens the vessels and lowers the pressure.

Wow.

We have covered a lot of ground.

I mean, from the histamine in your nose causing a sneeze, to the serotonin in your gut causing diarrhea, to the prostaglandins keeping a baby's heart defect stable until surgery.

It really highlights how the body uses these local strategies to manage incredibly complex systems.

And it shows the elegance of pharmacology.

We identify the key local player.

We identify its receptor.

We build a key that fits.

And we can turn the volume up or down on these natural processes.

So what does this all mean for the listener?

Why should they care about autochoids beyond just passing an exam?

I think it empowers you to understand the why behind your meds.

If you understand that first generation antihistamines dry you out because they also block cholinergic receptors, you understand why you need to drink more water when you take them.

If you understand that NSAIDs like ibuprofen work by blocking prostaglandins and that prostaglandins are what protect your stomach lining and maintain blood flow to your kidneys, you understand why you shouldn't pop ibuprofen like candy for every little ache.

Right.

It's not free.

There's a biological cost.

There is.

This knowledge moves you from just following orders to understanding the machine.

It connects the molecular to the clinical.

Exactly.

Here is my final thought, something to chew on.

We talked about how fish oil, the omega -3s, literally changes the structure of your prostaglandins and thromboxanes.

It makes me wonder if our diet so directly determines the structural signaling molecules in our blood, how much of what we call chronic disease is just incorrect building materials.

That's a profound question.

Are we building our signaling systems out of straw instead of bricks just based on what's in the grocery cart?

The substrate dictates the signal.

It suggests that the old saying, you are what you eat is not just a metaphor.

It's a direct pharmacological fact.

On that note, we will leave you to ponder your lunch choices.

Thanks for listening from the Last Minute Lecture Team.

Bye, everyone.