Chapter 16: Antiparkinsonian Drugs – Treatments & Mechanisms

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Welcome to the Depth Dive.

Today, we're cutting through some real complexity in chronic disease management.

We're plunging right into how we treat Parkinson's disease, specifically the anti -Parkinsonian drugs, how they actually work at a cellular level.

Okay, let's unpack this then.

We're basically looking at chapter 16 from Lilly's.

It focuses on managing Parkinson's disease or PD, you know, that chronic progressive neurodegenerative disorder.

Our mission here is to walk you through the pathophysiology, then the major drug classes, and finally those absolutely critical nursing process steps.

Exactly.

And you have to start with the core problem, those classic PD symptoms, TRIRA, tremor, rigidity, akinesia or bradykinesia, and postural instability.

They often don't even show up until maybe 80 % of the dopamine neurons in the substantia negra are already gone, depleted.

And knowing that, well, it shapes the entire therapeutic strategy we have to use.

Right, 80 % loss before you even see it.

That's huge.

So before we get to the drugs, let's nail down some key terms you'll hear.

Bradykinesia.

That's the slowness of movement.

It's really characteristic.

And related is akinesia, sort of the absence or poverty of movement.

It can lead to that mask -like face, the expressionless face.

And then there are these phenomena that happen during treatment, the on -off phenomenon.

Yeah, that's a tough one.

It's where you get these erratic swings in how well the drugs are working based on plasma levels.

Good control one minute, then suddenly symptoms return.

It's unpredictable.

And the wearing off phenomenon?

That's a bit rapidly before the next dose is scheduled, the patient feels it wearing off.

Okay, so all this stems from this underlying neurotransmitter imbalance, right, in the basal ganglia.

Precisely.

It's all about the balance, or lack thereof, between two key players.

Dopamine, which acts as an inhibitor, and acetylcholine, ACIC, which is excitatory.

In PD, the nerve terminals fail to produce enough dopamine, so you lose that inhibitory signal, what's left.

You get excessive, unopposed excitatory activity from the acetylcholine.

Think of it like figure 16 .2 in the text, probably shows a balance scale.

Normally, dopamine and AC are balanced.

In PD, the dopamine side is drastically lower, so the AC side dominates.

Drug therapy, then, is really all about trying to restore that balance, either by boosting dopamine levels or function, or by blocking that excess AC activity.

Makes sense.

We're fighting the imbalance.

What about the cause?

Does the chapter touch on why this happens?

It mentions a few theories.

Sometimes it's just idiopathic, we don't know.

But possibilities include things like head injury, maybe excess iron causing free radical damage, or environmental toxins like pesticides, certain metals, even just premature aging of those specific cells.

Okay, so we know we need to boost dopamine.

The traditional cornerstone, especially later on, is levodopa carbidopa.

Cinnamate is the common brand name.

But here's where it gets interesting, like you said.

Why can't we just give the patient dopamine directly?

Why the levodopa middleman?

The blood -brain barrier, that's the key.

Exogenous dopamine, dopamine you give as a drug, it simply cannot cross that protective barrier to get into the central nervous system where it's needed.

Levodopa, though, is dopamine's biological precursor.

It can cross the blood -brain barrier.

Think of it as sneaking the building blocks inside.

Once it's in the CNS, it gets converted into dopamine.

Clever.

But levodopa itself is vulnerable on the journey, isn't it?

Very vulnerable.

Outside the CNS and the periphery, there's an enzyme called dopedicarboxylase that just loves to break down levodopa before it even gets close to the brain.

And that's where carbidopa comes in.

It's the bodyguard.

Carbidopa is a peripheral decarboxylase inhibitor.

It blocks that enzyme outside the brain.

So it protects the levodopa.

Exactly.

It shields the levodopa, allowing more of it to actually reach the brain.

This means we can use smaller doses of levodopa, which is crucial.

Right.

Because smaller doses mean fewer of those nasty peripheral side effects you'd get with levodopa alone, like cardiac dysrhythmias or really bad nausea and vomiting.

Precisely.

But even with carbidopa, long -term levodopa therapy isn't without its own problems.

Those motor complications we mentioned earlier.

The on -off effect, yeah.

And you also mentioned dyskinesia.

Right.

Dyskinesia, difficulty controlling movements.

It often shows up as correa, those involuntary jerky movements, maybe in the limbs or face.

Or dystonia, which involves abnormal muscle tone, often causing painful postures of the head, neck, or tongue.

Okay.

And are there situations where levodopa -carbidopa is just off the table, contraindicated?

Yes, definitely.

Primary angle closure glaucoma is one absolute contraindication, and another big one.

You don't give it to patients with undiagnosed skin conditions.

There's a risk it could activate malignant melanoma.

Very important to remember.

Good flag.

Okay, moving on from dopamine replacement.

What about mimicking dopamine?

That brings us to the direct -acting dopamine receptor agonists.

Drugs like ropinrol, pramipexol.

Bromocryptine as well.

Right.

These often get used first line, don't they?

Especially in younger patients.

The strategy is to delay using levodopa.

That's exactly the idea.

Delay levodopa as long as possible, because its effectiveness can wane over time.

These agonists stimulate dopamine receptors directly.

We can split them into two main subclasses.

First, there's the ergot derivative, that's bromocryptine.

It primarily activates presynaptic D2 receptors, stimulating dopamine production.

Interestingly, it's also used for

hyperprolactinemia conditions where you have too much prolactin.

But a key caution, because it's an ergot derivative, it can cause peripheral vasoconstriction.

So it's a no -go in patients with severe ischemic disease like peripheral vascular disease.

Okay.

And the other subclass, the non -ergot drugs, like ropinrol...

Yes, ropinrol and pramipexol.

These are generally preferred now.

They tend to be more specific for the dopamine D2 receptor subfamily involved in Parkinson's.

This often translates to a better side effect profile, particularly fewer dyskinesias compared to bromocryptine, and they do a good job of delaying the need for levodopa.

But there's a really significant potential side effect with non -ergots we need to highlight, isn't there?

Impulse control.

Oh, absolutely.

This is a huge one.

Impulse control disorders.

We're talking about things like compulsive gambling, hypersexuality, binge eating, excessive shopping.

It can be devastating for patients and families.

Yeah, definitely something that needs careful monitoring and patient education.

Beyond that, you might also see things like edema, fatigue, dizziness with these agents.

Right.

Okay.

So we've covered replacing dopamine and mimicking it.

What about preserving it?

Making the dopamine that is there last longer.

This is where the indirect acting dopaminergic drugs come in.

There are three main classes here.

First up, the MAOB inhibitors.

Celageline and raceline are the key examples.

They're a mechanism.

They inhibit the MAOB enzyme.

This is an enzyme in the brain that specifically breaks down dopamine as well as norepinephrine and epidefrine to some extent.

So by blocking MAOB, you keep more dopamine around for longer.

They're often used as adjuncts added on when the response to levodopa starts to fluctuate.

And this is where we get that famous cheese effect warning, right?

Exactly.

With Celageline specifically.

At the usual doses, it's selective for MAOB.

But if the dose goes above 10 milligrams per day, it loses that selectivity.

It becomes a non -selective MAO inhibitor, blocking MAOA as well.

And MAOA is the enzyme that breaks down tiramine.

Precisely.

Tiramine is found in aged cheeses, red wine, beer, smoked meats, things like that.

If a patient on high -go Celageline eats these foods, the tiramine builds up, potentially causing a severe hypertensive crisis.

It's dangerous.

So strict dietary restrictions if the dose goes high.

And another critical point,

never use these with meparidine hydrochloride, the opioid.

That's a major contraindication.

Absolutely.

A potentially fatal interaction there.

OK, second indirect acting class,

the dopamine modulator, amantadine.

The antiviral drug that turned out to help Parkinson's.

That's the one.

It's a bit of an oddball.

Its mechanism seems to involve causing dopamine release from storage sites in the neurons and also blocking dopamine reuptake.

So it increases dopamine availability in the synapse.

How effective is it?

It can be quite effective, especially for dyskinesia.

But often the benefit only lasts for about six to 12 months.

Its effectiveness tends to wane over time.

OK, and the third class in this preservation squad.

The key OMT inhibitors.

Enticone is the main one here.

COMT stands for catecholomethyltransferase.

It's another enzyme that breaks down catecholamines, including dopamine's precursor, levodopa.

Crucially, enticone works peripherally.

It blocks COMT outside the brain.

Why is that important?

Because it protects the levodopa from being broken down before it reaches the brain.

Exactly.

By inhibiting peripheral COMT, it prolongs the duration of action of levodopa.

It makes the levodopa dose last longer, directly combating that wearing off phenomenon we talked about.

That's a neat mechanism.

Any memorable side effects with enticone?

Well, there's one that's harmless, but good to warn patients about.

It can cause a brownish -orange discoloration of the urine.

Totally benign, but alarming if you're not expecting it.

Good to know.

Clinically more significant.

Clinically, we need to watch for an increased risk of orthostatic hypotension that drop in blood pressure on standing and potentially syncope or fainting.

OK, so we've boosted, mimicked, and preserved dopamine.

What about the other side of that imbalanced seesaw, the excess acetylcholine?

Right, that brings us to our final major class,

anticholinergic therapy.

Drugs like benztropine.

The goal here is simple.

Block the effects of acetylcholine.

This helps reduce the symptoms caused by that unopposed cholinergic activity, specifically tremor and muscle rigidity.

So they target tremor and rigidity.

What about

bradykinesia?

They really don't do much for bradykinesia, the slowness.

Their main benefit is for tremor and rigidity.

OK, and thinking about side effects.

Cholinergic effects are sometimes remembered by the acronym SLUDGE, right?

Salivation, lacrimation, urination, diarrhea, GI motility, emesis.

That's a helpful one.

So, anticholinergic is doing the opposite will cause.

Dry mouth, blurred vision, urinary retention, constipation, the drying effects.

Exactly.

And this leads to a really important nursing caution, especially in older adults.

These drugs need to be used very cautiously in the elderly because they're much more susceptible to adverse effects like confusion, disorientation, urinary retention, constipation, and even hyperthermia overheating.

And the text specifically flags benztropine as contraindicated in hot weather or during exercise.

Yes, because blocking sweat gland activity impairs the body's ability to cool down, increasing the risk of heatstroke.

It's a serious concern.

Wow.

OK, so we've covered a lot of complex pharmacology.

Let's pivot now and tie this back to patient care.

How does this translate into the nursing process?

Assessment.

Assessment is absolutely fundamental.

You're constantly monitoring for those TRAP symptoms.

How severe are they?

Are they changing?

You need baseline measurements.

You also have to be vigilant about orthostatic hypotension.

That means checking blood pressure line down and then standing up looking for that significant drop.

And you're watching for signs of drug toxicity.

Things like excessive twitching, drooling, perhaps eye spasms.

These can indicate the dose might be too high.

What about lab monitoring?

Crucial.

Liver function tests like alkaline phosphatase, serum transaminases, and kidney function tests.

These drugs are metabolized and excreted, so we need to make sure those organs are handling them OK.

Catching toxicity or dysfunction early is key.

OK, assessment covered.

Now, implementation.

This seems like where the really tricky details lie for making treatment work.

Let's talk about that levodopa and protein interaction again.

Yes, this trips people up.

Amino acids from dietary protein compete with levodopa for absorption from the gut and transport across the blood -brain barrier.

So the practical advice isn't necessarily to cut out protein overall, but to manage the timing.

Patients should try to take their levodopa dose about 30 minutes before eating a protein -containing meal.

And limit large portions of protein at one time.

The text suggests a deck of card -sized portions of meat.

That's a good visual guide.

The key is to separate the drug dose from the big influx of competing amino acids.

It's about timing around the meals, rather than just total daily intake.

Timing seems critical everywhere here, giving doses hours before bed to avoid insomnia.

Yes, dopaminergic drugs can be stimulating.

And adherence, this is non -negotiable.

We use the ACT on time principle.

Doses must not be stopped abruptly, ever.

And missing a dose, even by just 30 minutes, can sometimes trigger a prolonged, really disabling off period where symptoms return full force.

Consistency is paramount.

What about the adjunct drugs during implementation?

COMT inhibitors, like Entacapone.

Entacapone starts working pretty quickly, usually within days.

It could be taken with or without food, which is convenient.

But you do need that liver function monitoring we mentioned.

And the MAOB inhibitors, managing orthostatic hypotension.

Absolutely.

Teach patients to change positions slowly.

Sit on the edge of the bed for a minute before standing.

Rise slowly from a chair.

Purposeful movements to allow the blood pressure to adjust.

Okay.

So after assessment and implementation, we evaluate.

What does success look like?

Therapeutic success isn't just about reducing symptoms, though that's key.

It's also about an improved overall sense of well -being.

Increased appetite, better mental status, more energy.

And yes, objectively, we look for decreased symptom intensity.

Less tremor, reduced rigidity, a smoother gait with less shuffling.

Measurable improvements in those TROP signs.

Right.

Let's do a really quick recap, then, before we finish.

Parkinson's is this profound dopamine deficit in the brain.

Our main strategies are replace the dopamine using Lovodopa, protected by Carbidopa, mimic dopamine using agonists like Ruppenerol, or preserve the dopamine that's still there using MAOB or COMT inhibitors.

And sometimes we block excess acetylcholine with anticholinergic.

That sums it up well.

And the complexity comes from the timing, the side effects, especially motor complications and things like impulse control, which means ongoing patient education is just absolutely essential.

It really is.

And the critical implication here, thinking longer term, is that Lovodopa's effectiveness window inevitably shrinks over time.

That's why developing and using these adjunctive drugs as COMT inhibitors, the non -or -god agonists, is so vital.

They help extend that window of good function for as long as possible.

Managing advanced PD becomes this constant dynamic challenge for the patient and the whole healthcare team.

It requires real collaboration.

A very challenging condition to manage.

Well, that was quite the deep dive.

Thank you for joining us today as we explored this really critical area of pharmacology and patient care.

And thank you, our listener, for being a part of our little last -minute lecture family.

Absolutely.

And maybe just one final thought for you to ponder.

Considering that progressive nature of PD, and the fact that symptoms usually only appear after that massive 80 % neuronal loss,

what kind of future drug strategies might we see?

Could we perhaps focus on protecting or somehow boosting the function of that remaining 20 % of neurons before symptoms even start?

Or slowing down that initial decline, rather than just managing the deficit after it's already profound?

Something to think about.