Chapter 36: Immunosuppressants

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Imagine a highly trained,

just incredibly aggressive army living inside your blood.

Right, an army whose only mission, I mean every second of every day, is to hunt down and destroy anything it doesn't immediately recognize as you.

Yeah, and you know, most of the time we are incredibly grateful for that army, but what happens when we actually want to smuggle a foreign object past those defenses, like say a life -saving kidney?

Or you know, what happens when that army gets confused and just starts attacking your own healthy tissue?

Exactly.

Welcome to a very special Last Minute Lecture Deep Dive.

If you are joining us today, chances are you are staring down a pharmacology exam,

and well, you need to get a handle on how we manipulate one of the body's most complex organ systems.

Consider us your study buddies.

Our mission today is to completely master Chapter 36, which covers immunosuppressants.

And the immune system really is an astonishing machine.

I mean, the challenge in pharmacology has always been figuring out how to actively sabotage it without killing the patient.

Oh, absolutely.

Historically, our tools for doing this were incredibly blunt.

The earliest immunosuppressants were basically like taking a sledgehammer to the body's entire defense network.

Right.

They were completely non -selective.

They would wipe out both the antibody -mediated and cell -mediated arms simultaneously.

Yeah, which was tragic because patients who received a successful organ transplant would then frequently succumb to simple, everyday infections.

I mean, they had zero defenses left to fight off a common cold.

But looking at the modern landscape of immunosuppression, we really don't use sledgehammers anymore.

Like, as shown in the chapter's opening summary chart, Figure 36 .1, we use a highly targeted multi -drug approach.

Right.

A patient might be on a regimen of two to four different agents at once, and each one acts like a very precise scalpel.

And the current strategy is fundamentally broken down into two distinct phases, right?

First, you have induction therapy.

Yes.

Induction is this powerful short -term immune suppression.

It's delivered directly at the time of the transplant surgery.

You hit the immune system hard and fast with a heavy strike.

Because the goal there is to clear the battlefield, right?

So the new organ can settle in without immediately triggering acute rejection.

Exactly.

And then you transition the patient to maintenance therapy.

These are less potent drugs that provide long -term baseline immunological protection.

And the brilliance of the modern approach is mixing several different drugs at lower doses.

By combining them, you get this synergistic protective effect.

Yeah.

While minimizing the toxic side effects that would happen if you just pushed one single drug to its maximum dose.

Right.

So to understand how these modern drugs actually work, we really have to understand the target.

We need to visualize T cell activation.

And it's super helpful to picture T cell activation like a nuclear launch sequence.

Right.

You know, you can't just bump a button and start a war.

Right.

Like, figure 36 .2 lays out the cell requires a very specific sequential verification process.

Three signals, to be exact.

Grasping this three -signal model is the absolute key to mastering this material.

I mean, if you understand the sequence, you will naturally understand where almost every single drug we discussed today exerts its effect.

Okay.

So let's break it down.

Signal one is the initial trigger.

Right.

It happens when an antigen presenting cell or an APC presents a foreign substance directly to the T cell's CD3 receptor complex.

Wait, I need to pause here.

Turning one key isn't enough to launch the missile, right?

If an antigen presenting cell flags down a T cell and triggers CD3, the T cell doesn't just immediately mobilize.

No, it does not.

Signal one alone will leave the T cell totally inert.

You absolutely need signal two, which we call costimulation.

So imagine the APC and the T cell have to complete a secondary secret handshake to verify the threat.

That's exactly it.

The APC has proteins on its surface called CD80 and CD86.

These specific proteins have to reach out and engage with a receptor called CD28 on the T cell.

Okay.

So the antigen hits CD3 and the APC shakes hands with CD28.

Now we have signal one and signal two successfully engaged.

Right.

And that triggers a massive internal cascade within the T cell.

Specifically, it activates the calcium -calcinurin pathway.

Okay.

So calcinurin is a phosphatase enzyme.

And the textbook notes its job is to dephosphorylate a molecule called NFAT.

That's a nuclear factor of activated T cells.

Let me track this.

Calcinurin strips a phosphate off of NFAT.

Why does that matter?

Because that phosphate group is essentially acting as an anchor.

When calcinurin strips it away, the NFA molecule is freed.

Oh, I see.

So it moves directly from the cytoplasm into the nucleus of the T cell.

Exactly.

Once inside the nucleus, NFA binds into the DNA and activates the genes responsible for producing cytokines.

And the most important of these cytokines is interleukin 2 or IL2.

Which brings us to signal three.

The T cell has now manufactured all this IL2 and basically releases it into its surrounding environment.

Right.

And that IL2 then circles back and binds to the IL2 receptor, which is also known as CD25, on the surface of itself and other T cells.

And that binding event is signal three.

Yes.

When IL2 hits the CD25 receptor, it activates an intracellular protein called MTOR.

That's the mammalian target of rapamycin.

So activating MTOR is like the final green light.

It kicks the cell cycle into gear, causing those T cells to rapidly multiply and proliferate.

Exactly.

The immune RE is officially mobilized and cloning itself.

Okay.

For everyone listening, pause and burn that sequence into your brain.

Signal one is the CD3 trigger.

Signal two is the CD8086 to CD28 handshake.

That internal cascade frees NFA to make IL2.

And signal three is IL2 binding to CD25, activating MTOR and causing cell division.

Memorizing that logic flow is the ultimate cheat code for this entire topic.

Every drug we talk about from here on out is just a clever way to hijack that exact sequence.

So let's start hijacking it by looking at the heavy hitters, the induction and acute rejection medications shown in figure 36 .3.

The stars of this initial surgical phase are the antibodies.

Now the science behind making these antibodies sounds a bit like science fiction.

I mean, are we literally designing these complex proteins from scratch in a test tube or are we hijacking a biological process?

We are entirely hijacking biology using something called hybridoma technology.

Scientists basically immunize mice with the target antigen, extract their antibody -producing B cells, and then physically fuse those B cells with tumor cells.

Wait, tumor cells?

Why are we injecting cancer biology into a manufacturing process?

Well because tumor cells are effectively immortal.

Normal cells die off after a certain number of divisions, but tumor cells just keep dividing endlessly.

Oh wow.

So by fusing a specific antibody -producing cell with a tumor cell, you create this hybridoma factory that turns out massive limitless quantities of one specific antibody.

You got it.

And then using recombinant DNA technology, scientists swap out the mouse genetic sequences with human genetics.

They humanize the antibodies.

Right, because if we injected pure mouse antibodies into a human, the human's immune system would immediately recognize them as foreign and destroy the drug itself.

Exactly.

And there's actually a secret to decoding the names of these drugs on your exam.

If you see a drug ending in MAB,

it stands for monoclonal antibody.

And if it has D in the middle of the name, it's chimerized, right?

A hybrid mix of mouse and human.

Right.

And if it has ZOO, it is fully humanized.

But let's look at the first induction agent,

antithemocyte globulins.

Now these don't end in MAB.

Yeah, because they are polyclonal, meaning they aren't those hyper -specific lab -made clones.

They are made by taking human lymphoid cells and immunizing horses or rabbits.

And rabbit preparations are highly preferred in the clinic because they are far more potent and have a better safety profile than the horse versions.

And their mechanism of action is pretty brutal, right?

Very.

They bind to a wide variety of receptors on T cells and mark them for destruction,

causing profound depletion of circulating T cells.

They essentially sweep the board clear.

There is a major clinical point here regarding how we administer them.

When you give these rabbit antibodies, you have to infuse them very slowly, and you absolutely must pre -medicate the patient.

Right, with corticosteroids, acetaminophen, and antihistamines.

Destroying that many T cells all at once causes those dying cells to release a massive wave of cytokines into the bloodstream.

Which is known as cytotine release syndrome.

Exactly.

If you don't pre -medicate to blunt that inflammatory response, the patient will experience severe infusion -related reactions, spiking fevers, violent chills, plunging blood pressure.

You have to brace the body for the impact of all those bursting cells.

That makes total physiological sense.

Okay, next up is basaliximab.

This is a chimerized monoclonal antibody.

And right away, I see our cheat code coming into play.

The mechanism of action is that it binds specifically to CD25.

Which is the IL -2 receptor.

So by sitting on that receptor, basaliximab completely blocks signal 3.

Wow.

So the T cells might get activated by signal 1 and 2.

They might even manufacture IL -2, but that IL -2 has nowhere to bind.

Right.

The cell cycle cannot advance.

It's like putting a physical padlock over the ignition.

The key is there, but you can't insert it.

And clinically, it's fascinating because it is non -depleting.

It doesn't destroy the T cells.

It just paralyzes their ability to multiply.

And because of that gentle approach, it's generally very well tolerated.

Patients just get two IV doses, one right before surgery, one four days later, and that's it.

That's wild.

Contrast that with alemtuzumab.

This is a humanized monoclonal antibody that binds to CD52.

Right.

And CD52 is a marker found on both T cells, A and D, B cells.

So alemtuzumab depletes both lines of defense simultaneously.

That sounds like a much bigger hammer than basaliximab.

It is a massive hammer.

It causes such profound and prolonged immunosuppression that the patient's defenses are virtually offline.

Which is why you actually have to put the patient on prophylactic medications, right?

Yes, for pneumocystis pneumonia and severe herpesvirus insufflations for months after the dose.

Okay, moving down the induction list, we have rituximab.

Now we are specifically targeting the B cells.

Rituximab binds to CD20, an antigen found on pre -B cells, mature B cells, and memory B cells.

It binds to them and induces them to rupture.

Correct.

But wait, if B cells eventually mature into plasma cells, why doesn't this drug kill plasma cells too?

I mean, they are the factories that actually pump out the antibodies.

It's a great question.

As a B cell matures into a fully formed plasma cell, it physically sheds the CD20 marker from its surface.

Oh.

So rituximab wipes out all the immature and mature B cells in circulation, but the plasma cells that have already fully developed are completely invisible to the drug.

Exactly.

And rituximab carries a really severe boxed warning that you need to watch out for.

It can cause reactivation of the J .C.

virus, which leads to a devastating, often fatal brain infection called progressive multifocal leukoencephalopathy, or PML.

Wow.

And it can also reactivate hepatitis B, right?

So monitoring those viral serologies before giving the drug is critical.

Right.

Now, following up on your earlier point, if rituximab misses the plasma cells and those plasma cells are churning out antibodies that are actively attacking the transplanted organ.

Which is antibiotic -mediated rejection.

Exactly.

How do we stop them?

Right.

If the antibody factories are already built and running, what drug can shut them down?

We use bortezomib.

This is a proteasome inhibitor.

It targets those normal plasma cells.

And the proteasome is essentially the cell's internal garbage disposal, right?

It clears out damaged or misfolded proteins.

Right.

And bortezomib blocks that disposal system.

As a result, toxic junk proteins rapidly build up inside the plasma cell until the cell is forced into epoptosis, or programmed cell death.

The factory basically gets choked by its own waste and shuts down.

That is so clever.

Now, I've always wondered about our final induction agent, IVU, or intravenous immunoglobulin.

You are giving a patient a massive infusion of pooled antibodies from thousands of human donors.

Using other people's antibodies to stop an immune response just feels completely backward.

It does seem counterintuitive.

But when you flood the system with that high dose of pooled human plasma, you're essentially overwhelming the patient's immune receptors.

The body senses this massive glut of antibodies and sends a negative feedback signal to its own B cells, inducing apoptosis and simultaneously down -regulating the complement system, which is a cascade of proteins that destroys tissues.

Oh, that makes sense.

And a practical tip for the exam.

Patients often get headaches, fevers, or blood pressure changes during the IV -UGIN fusion.

The easiest way to manage that is simply slowing down the IV drip rate.

So we survived the wartime surgical strike.

We used our heavy induction antibodies to stop acute rejection.

Now, we transition the patient to a long -term peacekeeping mission, the daily maintenance regimen, outlined in Figure 36 .4.

Right.

And remember, the strategy here is combining different classes of drugs to get the immune suppression we need without destroying the patient's organs.

And the absolute cornerstone of almost every maintenance regimen is a class called calcineurin inhibitors.

The two big names you must know are cyclosporine and tacrolimus.

Here is where our cheat code pays off again.

We know what calcineurin is.

It's the enzyme from the internal cascade downstream of signal 1.

Right.

Cyclosporine enters the cell and binds to a protein called cyclophilin.

Tacrolimus enters and binds to a different protein called FK binding protein.

But both of these unique drug protein complexes do the exact same thing.

They inhibit calcineurin.

Because calcineurin is blocked, it can't strip that anchor phosphate off of NFAT.

So NFAT remains trapped in the cytoplasm, it never makes it to the nucleus, and the cell never produces IL -2.

Signal 3 is completely sabotaged from the inside before it even begins.

Exactly.

Now, between the two drugs, tacrolimus is the preferred mainstay for all solid organ transplants because clinical data shows it simply results in less rejection.

And both require very strict monitoring, right?

You have to draw the patient's blood every 12 hours to check for trough levels, the lowest concentration of the drug in the blood right before the next dose is due.

Right.

And this is because they're heavily metabolized by CYP3A4 enzymes in the liver and the gut.

We hear metabolized by CYP3A4 a lot in pharmacology.

Think of CYP3A4 as the body's main chemical incinerator.

If a patient's incinerator is working too fast, the drug disappears before it can protect the organ.

If it's working too slowly, the drug builds up to incredibly toxic levels.

And that toxicity is severe.

The major limitation of calcineurin inhibitors is severe nephrotoxicity.

They're incredibly hard on the kidneys, but they also have unique side effects.

Yeah.

Let's look at a clinical vignette from the chapter study questions.

Imagine an 18 -year -old female patient walks into your clinic six months post kidney transplant.

She is highly distressed because she is rapidly growing thick, dark facial and body hair.

What's happening here?

Well, that condition is called hirsutism, and it is a classic, well -documented adverse effect of cyclosporine.

It can be devastating for a young patient's quality of life.

So the clinical move here is simple but life -changing.

You switch her from cyclosporine to tacrolimus.

Tacrolimus is just as effective at preventing rejection, but it does not cause that excessive hair growth.

Exactly.

Moving on to our second class of maintenance drugs,

the co -stimulation blocker, belatacept.

The name tells you exactly what it does.

It intercepts signal two.

Remember that secret handshake between the antigen presenting cell and the T cell?

Belatacept is a recombinant protein engineered to mimic CD28.

It binds directly to the CD80 and CD86 proteins on the antigen presenting cell.

Right.

Essentially covering them up so they can't complete the handshake with the actual T cell.

Signal two is blocked.

T cell activation fails.

This is a milestone drug because it's the first IV maintenance therapy, which is fantastic for patients who struggle with the burden of remembering to take daily pills.

You can dose it once a month.

However, there's a massive safety warning.

Belatacept is strictly contraindicated in patients who are seronegative for the Epstein -Barr virus, or EBV.

Why does prior exposure to EBV matter so much for this specific drug?

If a patient has never been exposed to EBV, their body doesn't have any underlying memory cell immunity to it.

If you completely disable their T cell response with belatacept and they catch EBV for the first time post -transplant, the virus can run completely unchecked.

Wow.

Yeah, this puts them at an unacceptably high risk of developing a deadly condition called post -transplant lymphoperliferative disease, or PTLD, specifically manifesting as tumors in the central nervous system.

Checking their serological titers before starting this drug is mandatory.

Okay, our third class, shown in figure 7 -6 .5, is the MTOR inhibitors, serolimus and everolimus.

Remember how ticrolimus binds to FK -binding protein to stop calcineurin?

Well, serolimus also binds to FK -binding protein, but instead of targeting calcineurin, this new complex directly inhibits MTOR.

Which is the exact target of signal 3.

By blocking MTOR, the T cell gets arrested right in the G1 phase of the cell cycle.

It's like taking the key out of the ignition just so the engine is trying to turn over.

The fuel is there, the IL -2 has bound to the receptor, but the cell physically cannot split and divide.

And the beauty of MTOR inhibitors is that they spare the kidneys.

They don't have the niferotoxicity of calcineurin inhibitors, so they're great to combine in a regimen to keep those toxic doses lower.

Fun fact, serolimus is also heavily used in cardiology to coat cardiac stents.

Its anti -proliferative effect stops the blood vessel's endothelial cells from multiplying and closing the stent back up over time.

That's a great crossover point.

But let's look at another clinical mystery.

You have a patient whose recent lab work shows massive spikes in cholesterol and triglycerides, and they are struggling with a surgical incision that refuses to close.

They are on an immunosuppressant regimen.

Which drug is the culprit?

That points squarely to serolimus.

It is notorious for causing impaired wound healing and severe hyperlipidemia.

If you have a patient with an already abnormal lipid profile, or who just underwent major surgery,

serolimus is likely the wrong choice for their immediate maintenance therapy.

Right.

So our fourth class is the anti -proliferatives, as ethioprine and mycophenolate.

Figure 36 .6 lays out the chemical blockade.

Mycophenolate works by inhibiting a specific enzyme called

IMPDhydrogenase.

Right.

And by stopping that enzyme, it halts the creation of guanosine monophosphate, or GMP, which is an essential building block for DNA.

Exactly.

But why doesn't this drug just kill every dividing cell in the patient's body?

What makes it specific to the immune system?

It comes down to cellular supply lines.

Most cells in the human body have two distinct ways to make nucleotides for DNA.

A de novo pathway, meaning they can build them from scratch, and a salvage pathway, meaning they can recycle old genetic materials.

But lymphocytes, our T and B cells, completely lack that salvage pathway.

They rely entirely on the from scratch de novo method.

That is an incredibly elegant bottleneck.

When mycophenolate blocks that specific de novo enzyme, the rest of the body's cells just switch over to recycling.

But it cuts off the lymphocytes' only supply line.

Exactly.

It completely starves the T and B cells of the precursors they need for nucleic acid synthesis,

absolutely stopping them from cloning themselves.

And clinically, mycophenolate has mostly replaced the older drug, azathioprine.

Azathioprine causes severe widespread bone marrow suppression.

Furthermore, there's a dangerous drug interaction you must remember for the exam.

Oh, right.

If a patient is taking the common gout medication allopurinol, it inhibits the bodily enzymes responsible for breaking down azathioprine.

This leads to massive, potentially fatal toxicity.

You have to severely reduce the azathioprine dose if they are on allopurinol.

It's also worth noting that mycophenolate can be notoriously rough on the stomach, causing severe nausea and diarrhea.

To get around this, pharmacologists created an active drug formulation called mycophenolic acid, which is an enteric -coated tablet.

So it physically bypasses the stomach and dissolves in the intestines to drastically reduce that GI upset.

Finally, rounding out our maintenance list, we have the absolute OGs of immunosuppression,

corticosteroids, drugs like prednisone and methylprednisolone.

How do these old -school drugs fit into this highly targeted modern model?

Well, they don't target surface receptors like the modern biologics.

They are much more

Steroids are lipophilic, meaning they smash right through the cell membrane, bind to an internal glucocorticoid receptor, dive straight into the cell's nucleus, and rewrite the DNA instructions.

They actively suppress the transcription of genes involved in inflammation, causing rapid lymphocyte lysis or redistribution out of the bloodstream.

I mean, they are undeniably effective.

We use them across the board, not just for transplants but for raging autoimmune conditions like severe rheumatoid arthritis and asthma exacerbations.

But the catch is the long -term toxicity.

Chronic steroid use exacts a heavy toll on the body.

We're talking profound diabetogenic effects meaning they can induce clinical diabetes, they cause osteoporosis, cataracts, severe weight gain, and hypertension.

Which is why the ultimate goal of modern transplant pharmacology is to design maintenance regimens that minimize or completely eliminate the need for long -term steroids altogether.

Exactly.

Okay, let's step back and recap the logic flow we've covered.

When a patient gets a transplant, we hit them with a heavy surgical strike of induction therapy.

Maybe a lymphocyte -depleting antithemocyte globulin to clear the board, or a targeted IL -2 padlock like Bacilliximab to survive the immediate shock of surgery without acute rejection.

Following that, we transition them to a delicate, carefully balanced maintenance talktail.

We might use Tachrolimus to block the downstream effects of signal 1, combine it with mycophenolate to starve the DNA supply lines, and maybe a rapidly tapering low -dose steroid.

So we are simultaneously blocking signals 1, 2, and 3 from multiple angles, keeping the immune army pacified without destroying the patient's kidneys or leaving them totally defenseless against the outside world.

It is a phenomenal high -stakes balancing act.

But before we wrap up, I want to leave you with a provocative thought to mull over as you prepare for your exam.

Let's hear it.

We just spent this entire deep dive talking about how to suppress the immune system with these powerful, complex, and sometimes highly toxic drugs.

But the holy grail of transplantation isn't inventing a slightly better immunosuppressant.

The absolute holy grail is immune tolerance.

Imagine if we didn't have to suppress the immune system at all.

What happens to this entire field of pharmacology when we figure out how to teach a patient's immune system to look at a newly transplanted kidney and permanently recognize it not as a foreign invader, but as self?

Man, if we can crack that biological code, we completely eliminate the need for every single medication we just discussed today.

No more nephrotoxicity, no more opportunistic infections, no more increased cancer risks, just the body wholly accepting the gift of life.

Exactly.

Something to think about as you review your notes tonight.

Well, we want to thank you so much for joining us on this deep dive.

From all of us at the Last Minute Lecture Team, study hard, trust your knowledge, and good luck on that pharmacology exam.