An 85 year old male is brought
to the physician by his wife
for reappearance of his Parkinson’s
symptoms for the last few months.
He has been treated with various
drugs over the last 20 years.
The patient’s wife says that his symptoms are worse
as he nears the time for the next dose of medication.
The patient’s movements have been slower and
it’s difficult to initiate voluntary movements.
He was diagnosed with hypertension 10 years ago
and has been compliant with his medications.
His current medications are carbidopa/levodopa,
rasalagine, aspirin and captopril.
Vitals are a pulse 70, respirations 15, blood pressure
130/76 and temperature of 36.7 degrees celsius.
Examination reveals the expected, ‘pill-rolling’
resting tremor which is alleviated by movement.
Increased tone of arm muscles and resistance
to passive movements at joints is noted.
When asked to walk across the room, he
has difficulty taking the first step
and has a stooped posture and
takes short, rapid steps.
Laboratory examination reveal: serum glucose fasting of
97 (mg/dL), sodium 141 (mEq/L), potassium 4.0 (mEq/L),
Chloride 100 (mEq/L), Cholesterol, total 190 (mg/dL), HDL
(mg/dL), LDL 70 (mg/dL), and Triglycerides 184 (mg/dL).
He is started on a drug that increases the efficacy
of his current anti-Parkinson’s medications.
Which of the following is most likely
added to his current medication regimen?
Answer choice (A) - Benztropine
Answer choice (B) - Selegiline
Answer choice (C) - Atorvastatin
Answer choice (D) - Entacapone
Answer choice (E) - Bromocriptine
Now take a moment to come to an answer
yourself before we go through it together.
Now let's jump right in.
Now let's first tackle the question characteristics.
Now this is a question that concerns
both pathology and pharmacology.
We have to be aware that this is Parkinson's
disease but we must be able to understand
the underlying pharmacology of the
medications listed in the answer choices.
Now this is a 2-step question.
We have to first determine what
each of these medications does
but then be able to as a second step
determine which of the following medications
will actually help increase the efficacy of
his current Parkinsons medication regimen.
And the stem is absolutely required in this case
because we need to know the patient's history,
his clinical progression and what medications he's
on and we need to look at the laboratory values.
So let's walk through this question together.
Step 1 - we have to determine the likely diagnosis.
Well it's clear this patient has Parkinsons
disease but the key here in the diagnosis,
is that the patient is showing "end-of-dose"
deterioration or it's also called 'wearing-off'
where Parkinsonian symptoms seem to worsen a few hours
before the next dose of carbidopa/levodopa is required.
Now, end-of dose deterioration is seen after long term use
of carbidopa/levodopa in advanced parkinsons disease.
and that's very important to know to
appreciate the diagnosis in a clinical setting.
Now looking at the patient's vitals and
labs, his blood pressure is unremarkable
and his blood labs are grossly unremarkable except
that he has a slightly raised triglyeride level.
Now step 2 - let's determine which drug would increase the
efficacy of his current carbidopa/levodopa medication.
Now inhibitors of the
catechol-O-methyltransferase or COMT inhibitors
increase the efficacy of carbidopa/levodopa by inhibiting
the breakdown of catecholamine neurotransmitters.
now let's refer to our image to
better understand this pathway.
Now we can see on the image that we have a split
between the peripheral and central nervous system.
Now let's look at the left side
first, we have the peripheral system.
Now there in the middle we see L-Dopa.
Now L-Dopa can be broken down in two different ways
to give you either dopamine or another substance.
Now what we want is peripherally
for L-Dopa to remain unchanged
so that it can cross the blood-brain barrier
and go in to the central nervous system.
And then we convert it to
dopamine where it can affect
the basal ganglia and help us with our
movement disorder in parkinsons disease.
we dont want to lose our L-Dopa peripherally thus we want to give medications to prevent the L-dopa breakdown peripherally.
Now levodopa is the medication we're
looking at there in the middle.
Now if you've noticed, Sinemet or
carbidopa/levodopa always go together.
And that's because carbidopa prevents the
peripheral breakdown of L-dopa into dopamine.
now the medication in this case we're recommending as
a possibility are what are called COMT inhibitors.
now if you see there, COMT also helps break,
inhibit the breakdown of the COMT inhibitor
so that you don't break down your L-Dopa.
So Tolcapone , Entacapone will prevent the
COMT inhibitor from breaking it down.
So thus with both carbidopa on board, and a -capone,
we can prevent peripheral breakdown of L-Dopa
and have them cross the blood brain barier into a
higher concentration dose into the central system.
Then centrally, we can see that
again, the COMT will help us.
It will help prevent even central breakdown of
dopamine and increase the central dose of the -capone.
So there's Tolcapone again helping us out.
And here is another point, you can just see that if you
use low dose Selegeline or if you use Rasalagine,
you can also then prevent,
these are both MAO inhibitors,
you can also inhibit the central breakdown of
dopamine and increase dopamine levels centrally again
you have to give more at the basal
ganglia to help the Parkinsons.
So thus we can see how the -capones or the COMT
inhibitors and also even here the MNLB inhibitors
can actually help us in increasing dopamine
peripherally and centrally respectively.
Now, so what we've shown here is that inhibitors of
catechol-O-methyltransferase will actually increase the efficacy
of carbidopa/levodopa because we'll have more levodopa in
the total form peripherally which can then go centrally,
and even centrally will stay more as
dopamine as opposed to being broken down.
Now of the answer choices, Entacapone is a
selective and reversible COMT inhibitor.
Now if we look in the patient's medication
list, the patient's already on Rasalagine
which is a MAO-B inhibitor which as we can see on the image
also helps keep dopamine levels elevated centrally.
Thus, the answer to this question
is answer choice (D)- Entacapone
as it will keep levels of dopamine higher by
preventing peripheral breakdown of L-dopa.
Now let's go through some high-yield
facts of Parkinsons disease.
Now levodopa remains the most effective
symptomatic treatment for Parkinsons disease.
levodopa is always given with a peripheral
decarboxylase inhibitor carbidopa most consistently
or benserazide to reduce
its conversion to dopamine
allowing lower doses of levodopa to
be given to reduce side effects.
With advancing disease, there is progression of loss of neurons
with a duration of effect, each levodopa dose will shorten
and the return of symptoms some
hours before the next dose
and this is called 'end-of-dose
deterioration' or 'wearing off'.
Now, end of dose deterioration can
be improved at least initially
by increasing the frequency of levodopa doses or
by adding a COMT inhibitor such as Entacapone
which reduces the breakdown of catecholamine
neurotransmitters both peripherally and centrally.