Download URIC ACID METABOLISM AND MEDICATIONS I`m here to talk

URIC ACID METABOLISM AND MEDICATIONS
I’m here to talk about uric acid and so it’s in the context of gout and what we
are going to be considering is what is uric acid? How does it lead to gouty
arthritis and, for that matter, how does uric acid also lead to kidney stones
otherwise known as renal calculi? Where does uric acid come from? And
we’ll see, actually, that it’s the tail end of biochemical pathway linked to
purines; purine nucleotides, nucleocides and even just the straight purine bases.
So it’s right at the tail end of breakdown of DNA / RNA, for example. And
we’ll see that can be clinically very important, particularly in a situation where
you have a patient with a malignancy that is treated with an anti-cancer drug
and the malignancy, the malignant cells, respond in a dramatic way.
If you think just for a moment about all the DNA and all those dividing cells,
what happens when those cells die? Suddenly you’ve got a big load of
biochemical substrate and we’ll see that, at least we’ll consider, the
consequences of all of the DNA and RNA in those cells breaking down to
purines and the load that that then creates in terms of uric acid production.
We’ll also consider a couple of pathways for purine nucleocide synthesis. One
is how do you make a purine nucleocide from scratch and what’s its relevance
to uric acid production? And we’ll also consider a so-called salvage pathway
mechanism which is a rather neat way of retrieving some purine bases before
they get converted to uric acid and, in that way of course, you potentially can
reduce the amount of uric acid production but, of course, there are also some
economies involved in this in terms of retrieving those purine bases and
returning them back into the nucleotide… nucleocide, nucleotide pool,
remembering that a nucleocide is essentially a purine or a pyrimadine attached
to a ribose, to sugar; a nucleotide is a purine or pyrimadine base attached to a
sugar attached to phosphates and, of course, its in that phosphorylated form, that
phosphated acid form, that DNA and RNA can be made.
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All right: so let’s get going.
I’m not going to say a lot about the aetiology of gout, partly because I’m not a
pathologist, partly because I’m not a rheumatologist and partly because not all
that much is known about it either, but the basic point seems to be that the
crystallisation of uric acid in the joints acts as the trigger for an acute
inflammatory reaction and that acute inflammation involves the recruitment of
neutrophils and the attempt by the neutrophils to engulf uric acid crystal.
If you like, it treats the uric acid crystal as it would a bacteria. It tries to
phagocytose and it tries to kill it. The only problem is it’s not a living
organism. It’s a crystal and one of the consequences of that is that the
neutrophils are not particularly effective because the crystals actually survive
the action of the neutrophils, so that means more neutrophils tend to get
recruited and, as a consequence of that, you get this vicious cycle whilst you get
the release locally of the enzymes and the oxygen metabolites that the
neutrophil is trying to use to kill the crystals and, as a consequence of that, you
get local joint damage.
I think I have said all those things there.
Notice that I’ve also used the term sodium urate and they can, to some extent,
be used interchangeably. Uric acid; urate. What’s uric acid? How are its levels
for influence? And we’ll see some of the main influences on that as this lecture
unfolds.
Here we have the structures of uric acid in at least one of its forms on the left
and urate on the right so you can see that, essentially, there’s an equilibrium
between uric acid form and the urate form. It’s a ph-dependent equilibrium, as
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you would expect for an acid. The ph at which you have equal concentrations
of the acid form and the anion form is, as you can see there, around 5.8, so it’s
actually below the normal serum ph. So that means that, under normal
conditions with, essentially, anions accumulating at pHs above 5.8 and acids
accumulating, at least the proteinated forms, accumulating at pHs below that
pKa value 5.8, but normal neutral ph around 7.4. The predominant form is
urate, right? It’s predominantly up to that right hand side. Many of you who
are familiar with this ph equilibrium will see that immediately.
So, under normal circumstances then, in a joint, its sodium urate, its crystals of
sodium urate, typically monosodium urate since sodium is the primary cationic
species of blood plasma that crystallises and causes the problem. Of course, if
the ph equilibrium is disturbed in the joint, then things can change.
In the kidney where you get calculi formed from uric acid/urate crystals, then
there probably is, in fact, more uric acid because the ph in the renal tubules, as
many of you will be aware, progressively acidifies as you head down from the
glomerulus where the blood is first filtered down to the collecting tubules where
it will ultimately enter the pelvis of the kidney to transfer to the bladder.
So we have to deal with these two different terms – uric acid and urate. I think
you can see, for all intents and purposes, the names are interchangeable when
we’re talking about this among colleagues and, indeed, among patients that
there are some specific theoretical points that relate to what is the major species
in a certain location under certain conditions?
I think I’ve said this already. Uric acid is a purine base. So if we have
nucleocides being either purines or pyrimadines, then we’re talking about the
purine half of the nucleotide metabolism here. And here are three of the main
purines, adenine and guanine, we’ve talked a lot about in different contexts.
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They are, of course, the “A’s” and “G’s” in DNA and RNA. Hypoxanthine is a
close relative. I think you can see in structural terms and it’s an important
member in the pathway.
This slide seeks to make the global point that it doesn’t matter whether we’re
talking nucleotides with the sugars and phosphates, or nucleocides just with the
sugars, or just the purine bases themselves. Just those two rings fused together,
the six-membered ring, the five-membered ring with the nitrogens.
When we come to consider the breakdown of these species, there is a final
common pathway.
Its essentially nucleotide, remove the phosphates,
nucleocide – now remove nucleocides, now remove the sugars, purine bases on
to uric acid.
Serum uric acid or, I should say urate concentrations in men and women, are
roughly the same. As you can see, they tend to be slightly higher in males than
in females. The curious thing about this is that the solubility of urate in
solution is extremely close to its actual concentration in plasma, so much so that
its probably true that there are not particularly well-defined substances in the
plasma which act to limit the crystallisation of the uric acid so you can imagine
a situation where a biochemical inhibitor of crystallization, if its levels can be
adjusted either up or down, can have a potential impact on whether or not
crystals form.
Why are uric acid levels so high in humans? It’s curious. Humans, amongst
primates and other mammals, are unusual in not converting uric acid to another
much more water soluble product known as allantoin – a-l-l-a-n-t-o-i-n,
allantoin. Now just imagine that six-membered ring of the purines opening up
from uric acid or from the purine and essentially you’ve got the idea; there is an
enzyme in all those other species which converts uric acid or urate, converts it
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to allantoin. Allantoin is much more water-soluble and readily excreted,
doesn’t tend to crystallize, and doesn’t tend to cause gout or cause kidney stones
– renal calculi.
So what are the concentrations so high in humans? There’s one line of
argument - there are a couple of different lines of argument – the one there
about promoting salt retention under low salt conditions, I’m not sure just how
robust that idea is. The one above that relating to a possible role of uric acid as
an anti-oxidant I think does need to be taken seriously because it can be shown
in vitro in the test tube that, in fact, it does have anti-oxidant effects and those
anti-oxidant effects are quite impressive. It seems to be in many cases more
powerful as an anti-oxidant than Vitamin C, even in school days. So there is a
serious view, and this needs to be taken seriously, that uric acid has a role as a
biological anti-oxidant in humans and that this may actually have some
protective effect.
It makes it very difficult when you look at a patient who may have a number of
metabolic problems, including an increase in uric acid concentration, to try to
dissect out what is cause and effect but one view is that the uric acid is actually
… its levels are increasing as a response to a pro-inflammatory situation with a
view to suppressing the damage that arises from oxidation.
Here is some data of the type we are just talking about. So we’re looking at the
percent remaining of different species. So here we have uric acid as you can see
in the presence of… In this case it’s a radical oxygen known as singlet oxygen.
It’s rather an unusual form of oxygen in which the unpaired electrons end up in
an excited state and it turns out to be quite a reactive form of oxygen, or
superoxide or hydroxyl radicals and so on.
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You can see that in the presence of, in this case, singlet oxygen that uric acid
rapidly gets degraded. This is a sign that it’s actually interacting with and
detoxifying, if you like, the singlet oxygen. Ascorbate also works, all right, in
his assay. Interestingly, so does deoxyguanine, but none of the other purines or
pyrimadines that were tried here actually had any effect or were able to interact
with these radicals.
This table is from a different study basically showing the same kind of thing.
You can see here… What’s happening here essentially is that either ascorbate
or uric acid is acting to interact or inhibit the formation of radical cations so, in
fact, that’s what we’re seeing here is that inhibitory effect. Now you can see
that, at a concentration of around a plasma-like concentration of uric acid,
there’s actually quite a powerful effect here. The concentration of ascorbate on
supplements is around about 100 to 200 mmol/L. You can see that, in fact, the
effect of uric acid is much greater in this assay system than the effect of
ascorbate than concentrations that would be achieved under normal
circumstances in plasma.
There’s also a refinement here: human serum albumen also interacts with the
system and does have an effect as well.
So, those of you who would like to pursue this and there are some more recent
papers asking the question whether, under some circumstances, uric acid may
be anti-oxidant in its effects and, in other circumstances, may be pro-oxidant in
its effects. For those of you who would like to pursue these issues, here are
some of the papers that are relevant to the anti-oxidant case and I would be very
happy to send you any references related to possible pro-oxidant effects as well,
if you’re interested.
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How do we eliminate uric acid? Essentially via the kidneys and so there’s
filtration across the glomeruli, it’s a small molecule, certainly small enough to
cross the filtration barrier of the glomeruli. It then gets secreted and reabsorbed.
Now the reabsorption, as you can see, is not entirely designed to eliminate uric
acid and has the effect of putting a brake on elimination. And so you can boost
the elimination of uric acid by blocking that reabsorption mechanism and we’ll
look at a couple of drugs a little later that actually do that.
Does diet have an effect on serum uric acid levels? The answer to that does
seem to be yes, and so although uric acid sits at the bottom end of the metabolic
pathway that is essentially breaking down purines and so therefore the rate of
endogenous breakdown of purines is, perhaps, the primary determinant of what
uric acid levels will, ultimately, be. It is a partially open-ended system and we
are consuming foods that are rich or not rich in purines and they can have an
effect so foods that are high in purines include seafoods, meat, alcoholic
beverages, especially beer. Foods that are low in purines are fruits, grains,
vegetables and a few other things that are listed there.
So, the ultimate level of uric acid in serum is dependent then on quite a number
of different factors: Purine nucleotide synthesis, simply because if you don’t
have purines you can’t break them down and make uric acid, so how fast are
purine nucleocides being made? That will have an impact, ultimately, on how
much uric acid is ultimately produced. What is the rate of puric breakdown?
What is the rate of puric intake and how fast is the uric acid being excreted by
the kidneys?
Now, we are going to have a look at the mechanism of purine nucleotide
synthesis – the so-called salvage pathways that I mentioned right at the very
beginning of the lecture and how they interact with one another to determine
uric acid production rates. Don’t be scared by these pathways.
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Down here we’ve got urate. Here are the nucleotides’, nucleocides’ purine
bases. We have just been talking about that. This little branch here is
essentially talking about the production of purine nucleotides starting with a
sugar phosphate, ribose phosphate. Remember that we, in the last lecture,
considered the mechanism of synthesis of ribose, ribose 5-phosphate via the
pentose phosphate pathway so we have glycolysis, glucose entering the cell, its
converted to glucose 6-phosphate, is trapped, and then potentially can be
shunted out through the pentose phosphate pathway to make ribose, right?
Okay. So here’s ribose. Here’s the ribose 5-phosphate. This intermediate gets
generated. We’ll have a look at its structure in just a minute. It sounds terrible
– it’s not so terrible - and then amino acids are also required but, essentially, it
can then make the nucleotides. They can then be used to make nucleic acids or
they can be a source of energy for biochemical pathways.
Notice that the diet largely provides nucleocides or purine bases. Here are the
so-called salvage pathways. So free purine bases can either be converted to
urates, in which case they need to be excreted, or via the salvage pathways they
can be converted back to nucleotides, in which case they can return to the pore,
they can be used in the synthesis of DNA and RNA and so on.
So, in terms of the fluxes, then, through this overall mechanism, we have got a
dietary input, we’ve got an input from the synthesis of purine nucleotides and
then we’ve got this salvage pathway mechanism. So let’s just have a slightly
closer look at these different inputs.
I think that just having looked at… seen that diagram, I think you’ll probably
agree with me that the de novo pathway promotes uric acid production. It’s
seen to be providing substrates. The salvage pathways inhibit uric acid
production.
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We’re at the top end of the de novo purine nucleotide synthesis pathway here.
With ribose phosphate being converted to this metabolite, phosphoribosyl
pyrophosphate. Now here’s ribosyl phosphate over here, here’s our sugar,
here’s our phosphate and here’s the dotted line, and the dotted line ultimately,
as the purine nucleotide gets synthesized, will take on the familiar shape, the
double ring shape. But it has to be built up in sequences, in specific
biochemical reactions, and it starts with two phosphates joined together sitting
here “just like an activated intermediate” would be the way a biochemist would
refer to this. And so you can see then that ribosyl phosphate gets converted to
PRPP, so we have our two phosphates attached at this point and then
pyrophosphate is often also used as a source of energy and it’s useful in this
context as well for a source of energy for driving the reaction forward.
The next thing that happens essentially is that the pyrophosphate, those two
phosphates, are knocked off and replaced by an amino group. And then it gets
built up, first of all to IMP which is a hypoxanthine-base attached to ribose
attached to a phosphate. And then, from there, down to ATP and GTP and off
you go with DNA and RNA synthesis and so on.
There’s another point that the slide seeks to make, and that is that not only is
PRPP, so this nucleus with these two additional phosphates added to it are
substrate, it also has an influence on the pathway and so his red arrow here with
the plus sign beside it is intended to get across the point that the level of
phosphoribosyl pyrophosphate (PRPP) also has an impact on the rate at which
the pathway proceeds. If you like it can activate the enzyme, not just be a
substrate for the enzyme, it can actually switch the enzyme on. That suggests
there is a second site on the enzyme, one that’s involved in the catalytic
pathway, another one which is actually influencing the rate at which the enzyme
works.
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Now, when we consider the salvage pathways and what happens when you
block those, we’ll see that in fact PRPP can accumulate in a way that is not to
the advantage of the system because it can, inappropriately, drive… purine
nucleotide synthesis can switch it on really powerfully. So it’s worthwhile just
noting that – the text there and the plus sign in red.
What about salvage pathways? Basically controlled by two enzymes and here is
the basic point I think I’ve now tried to make a couple of times is to take a
purine base, right? Ribose is not there, the phosphates are not there and convert
it back to a nucleotide with the ribose with the phosphate. And so that’s what
we’re seeing here in the case of adenine and what we’re seeing in the case of
guanine and hypoxanthine.
Notice that PRPP, that we’ve just met, also participates in these salvage
pathways. If they work, they’re a neat way of returning the purine bases back to
the nucleotide pool for biochemical uses. If they don’t work, then the adenine,
the guanine, the hypoxanthine are going to be broken down and converted to
uric acid. So the existence of the salvage pathways then provides a brake
potentially on uric acid production.
So, the roles of salvage pathway enzymes to minimise uric acid production
recover purine bases for use as purine nucleotides. It’s interesting. In certain
tissues the levels of the salvage enzymes are extremely high - much higher than
in other tissues. They brain is one site where they are particularly high. The
levels are something like 10 to 20 times higher so that’s a sign that in the brain
the pathways are orientated in such a way as to be extremely efficient in the
purine nucleotide economy. Purine nucleotides are required as a source of
energy. They are also required in certain biosynthetic pathways. You can see
here; I’ve made the point that GTP is required for the synthesis of
tetrahydrobiopterin. We’ve actually metabolic tetrahydrobiopterin before. Can
anybody remember where?
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Is anybody going to help me? I’ve just had a mental blank. Well, if I had just
had a mental blank, nobody’s helping me! I remember as a first year intern I
am assisting at an operation and I’ve got the Registrar beside me, I’ve got the
surgeon on the other side; I’m holding my (inaudible ) – I think it’s a (inaudible
) – which is supposed to be holding the liver back and I feel as though I’m
doing a really good job and, after a little while of this, the Registrar turns to me
and says: “Help him! Help him! He needs help!”. Then I was taking much
more interest in the operation, trying to put instruments in the right spot and not
get in the way and so on. I guess you guys are not ready to be interns, right?
(wry laugh)
Tetrahydrobiopterin – we met it when we considered the pathway to convert
phenylalanine to tyrosine in (inaudible 0:34:45.9). It’s a co-enzyme for that
pathway. It turns out that BH4 is also a co-enzyme for the next step in the
pathway which converts tyrosine to dopa which is just in front of the synthesis
of the important neurotransmitter, dopamine. So, GTP is required for the
synthesis of tetrahydrobiopterin and thus dopamine, a neurotransmitter in the
brain. So that’s one reason why the brain actually goes to the trouble of making
substantial amount of these salvage enzymes to hold on to these purine
nucleotides.
What we’re considering here is an X-linked condition, so it’s mainly boys, in
which there’s a deficiency of the hypoxanthine/guanine phosphoribosyl
transferase enzyme – one of these salvage enzymes we’ve just met.
It’s a condition known as the Lesch Nyhan Syndrome. Here’s the
reaction that should be catalysed so hypoxanthine or guanine in the
presence of PRPP is converted either, in the case of hypoxanthine, to IMP
or guanine and in the case of guanine to GMP. If there’s a block in this
enzyme, if the enzyme doesn’t work or its seriously deficient as in Lesch
Nyhan Syndrome, then you don’t make IMP or GMP; the salvage
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pathway doesn’t work and, instead, hypoxanthine and guanine are
converted to xanthine and, in turn, uric acid. So, you would expect that in
these people they would have gout, which is to say high levels of uric
acid in the blood, and if this pathway is really important for the
metabolism, for the synthesis of neurotransmitters in the brain, there may
be a problem in CNS function. Now, in fact, that turns out to be true.
They do get serious CNS effects and here are some of the individuals
affected; they’re mainly boys there. Mental retardation, self-mutilation,
growth retardation, choreoathetosis, jerky movements of the body along
with sinuous snake-like movements so the “choreo” is the jerking fast
kind of movement; “athetosis” is the snake-like movement, so
combinations of those – spacicity – and the effects do indeed arise from
disturbed neurotransmitter synthesis in the brain.
So, unfortunately, in neurons they don’t have a really robust mechanism for
making purine nucleotides by the de novo pathway so the salvage pathway
becomes important. And different cell types then have different levels of de
novo purine synthesis enzymes. The liver has particularly high levels of those
enzymes. Other cells, including neurons in the brain, have particularly high
levels of salvage pathway enzymes.
Since a lot of metabolic processing occurs through the liver, the fact that the
salvage enzyme pathways are relatively low in the liver means that if there is a
substantial drive to purine nucleotide synthesis, then there can be also a
substantial drive to uric acid production.
Let me just go back for a moment. Notice that, if we block this pathway, in
addition to hypoxanthine/guanine now being substrate uric acid production,
there’s also an increase in PRPP. So that, in certain tissues, where the purine
nucleotide synthesis, the de novo synthesis pathway is available, you’ll get
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enhanced drive for that pathway and, again, potentially enhanced drive of uric
acid production.
I think most of you are aware that, at least it’s a conversational point, that
there’s an interaction between ethanol. Traditionally, it was the idea of
champagne, I think, or port or sherry, and uric acid production but there does
appear to be a real link between ethanol and uric acid production and we’ll just
have a look at one way in which that works in just a moment.
Just before we do that, just to remind you that in the pathway of ethanol
oxidation, acetic acid or acetate, because the proton comes off here, is the
primary product of the breakdown pathway and that’s irrespective of whether it
goes via ADH, alcohol dehydrogenase, or via the cytochrome P450 system; you
end up with acetate.
Now when we last talked about this, we made the point that acetate can be
converted to Acetyl CoA and that has some implications in terms of the caloric
value of ethanol.
Here’s the actual mechanism by which this occurs. Here we have ethanol being
converted to acetate again, acetate being converted to Acetyl CoA. Notice that
that enzyme requires ATP and that, in the process of Acetyl CoA production,
AMP gets generated. RO phosphate comes off so the two terminal phosphates
of ATP come off – it’s a source of energy for the reaction but AMP then gets
generated.
Now that AMP can be, as you can see, a source of purines in the production of
uric acid so in acute experiments where this has been looked at carefully, you
need to take the serum ethanol concentration up to around about 20 mV/L and
you may recall that we made the point that at 0.05g percent, so the cut-off for
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what the police would consider the cut-off for safe drinking and driving, the
concentration is 10 mM - .05%, so we’re double that here and above so, around
about .1% and above, at those levels you actually can see an impact on serum
uric acid levels and they can go up by as much as 10%, 20% even 30% and
since the uric acid concentration, even under normal circumstances, sits right on
that knife-edge between crystallization and not crystallizing. The 10% or 20%
increase, although it doesn’t sound like much, can be enough to predispose the
individual to hyperuricemia and gout.
So the clinical effects then of hyperuricemia: gouty arthritis, as you know or as
we have been talking about, tophi - basically subcutaneous deposits of uric acid.
Imagine a big lump on the top of my ear stuffed full of monosodium urate
crystals – that’s a tophis, you know? That’s one of the common sites around
joints. There’s another common site – renal calculi, kidney stones.
So, clinical context in which it’s desirable to lower uric acid levels? If you
would like to prevent acute attacks, you’ve got somebody who is having
recurrent, acute attacks of gouty arthritis, one satisfactory approach is to lower
the serum uric acid level.
You might want to eliminate tophi. That can be painful. They’re certainly
disfiguring and the cosmetic effect, as you can imagine, can be quite unpleasant.
Suppress uric acid levels in the context of Tumor Lysis Syndrome. I mentioned
this right back at the start. If you have a patient with a malignancy that happens
to be a malignancy that is particularly sensitive to chemotherapy, as you know
many leukaemias and lymphomas fall into that bracket. People go into hospital,
have the regime of chemotherapy, the tumour melts away. As it does so, uric
acid gets generated in large quantities along with a number of other biochemical
by-products.
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Since you’re producing such a large load of uric acid in this Tumor Lysis
Syndrome, the chances of attacks of acute gout, of uric acid crystallizing in
subcutaneous sites and, indeed, renal calculi are expected to become a serious
problem and so its now very standard in anti-cancer regimens in this scenario to
actually not only manage but to seek to prevent the consequences of the Tumor
Lysis Syndrome in terms of marked increases in serum uric acid level and it
would be very appropriate, and it is very appropriate, to monitor the uric acid
level on a daily basis in these patients to make sure that you are actually
controlling the situation, that the uric acid level does remain at around .4mM/L
and doesn’t go up to .5, .6 and so on.
Here are some examples of gouty tophi around the joints of the hands. That one
has clearly been around for a while.
Okay. Approaches to treating acute gout generally relate to trying to suppress
the acute inflammation and so non-steroidal anti-inflammatory drugs,
colchicines, which inhibits phagocytosis, and so therefore inhibits the neutrophil
response, Cox-2 inhibitors, so the primary target in the case of acute gouty
arthritis is, in fact, to control the inflammation, not to control the uric acid levels
and, in a strange way that is not well understood, if you try to control acute gout
or somebody who is having quite frequent attacks of acute gout, the agents that
lower the serum uric acid level will often make the attack worse, prolong it and
even trigger it so, in the acute attack, controlling the inflammation is essentially
what you’re trying to do.
Subsequently, once the acute attack is over, then it’s appropriate to seek to
reduce the serum uric acid level with a view to preventing subsequent attacks
and that strategy seems to work quite well.
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So prevent attacks or treat tophi might be two of the aims of the management of
gout.
In terms of how do you go about lowering the serum uric acid levels, one is
essentially the idea is trying to block uric acid synthesis and we’ll have a look at
the drug that many of you are probably familiar with already, Allopurinol, how
it does that, in just a moment, or stimulate its conversion to allantoin which is a
relatively new approach, or promote urinary excretion of uric acid.
Now, I think you can see that there is one potential problem of promoting
urinary excretion of uric acid and that is that you can potentially induce the
production of calculi. You can actually make your patient develop kidney
stones and so, as a consequence of that, most of the strategies of lowering uric
acid do not involve promoting urinary excretion.
Nevertheless there is a drug, Probenecid, and we’ll look at it in just a moment,
that is occasionally used for this purpose.
Here’s the tail end of the pathway now. So the nucleotides have gone. The
nucleocides have gone. We are down into the base territory. Here’s
hypoxanthine, here’s xanthine, here’s uric acid. The red star there, or asterisk,
basically is the site of action of the enzyme xanthine oxidase, and that’s the
enzyme that the drug Allipurinol targets. Okay, we’ll spell Allipurinol in a
moment. I think you’ve got it in your notes.
Notice it doesn’t really matter whether the purines are coming from the adenine
pathway, AMP adenosine, or from IMP or GMP; they all ultimately converge
onto it with the production of uric acid. So xanthine oxidase then is a useful
therapeutic target and this then is the at least part of the mechanism by which
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Allipurinol works. Notice that in, in fact, Allipurinol is itself a purine. It’s got
the six-membered ring, it’s got the five-membered ring, and it’s got the
nitrogens. It’s an analog of the purines and, indeed, it actually gets acted upon
via xanthine oxidase and, in the process, this compound, alloxanthine, is
generated. Now it turns out that alloxanthine binds really tightly, in fact I think
there’s even a covalent bond between alloxanthine and the binding site, the
substrate binding site in xanthine oxidase. This is a form of so-called suicide
inhibition and, as a consequence of that, Allipurinol is quite specific in its
effects and it’s very powerful in its effects and it’s a great drug.
This is some information on xanthine oxidase. Maybe the main point to make
from this slide, which otherwise is kind of useful background information while
we’re thinking about how Allipurinol works, is that its mainly in hepatacytes
where we’re seeking to block it clinically and in the hepatacytes in the
cytomplasmic enzyme.
Here’s a new xanthine oxidase inhibitor. It’s not on the market in this country.
I don’t know… I think there’ll be some resistance to this drug coming onto the
market simply because Allipurinol is so successful and relatively cheap and
there are generic forms of it and so on.
Here’s Febuxostat; this new one that we’re just talking about, and here’s
Allipurinol. Allipurinol is a purine and you can see that Febuxostat is not a
purine. Nevertheless, its binds very tightly in the substrate binding site for
xanthine oxidase and blocks it.
Alternative approaches to lowering uric acid levels: the so-called uricosurics
are the agents which promote excretion of uric acid in the kidney. They do so
by blocking re-absorption in the proximal tubule of the kidney but they do have
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this side-effect of promoting uric acid delivery into the fluid of the renal tubules
and therefore promoting renal calculus formation.
This drug, Benzbromerone, is a new version – here it is again here – of these
uricosurics; Probenecid is the one that’s on the market here at present. You
will, occasionally see it being used but I think, in general, I would think it
would be a good idea to avoid this drug, at least in the first instance, and start
with Allopurinol in terms of trying to lower serum uric acid levels.
There’s a potential advantage of Benzbromerone that it is effective in the
context of renal failure so if the patient has renal failure, there is a need to try to
promote or boost uric acid excretion. Probenecid is not to work in patients with
renal failure. Benzbromerone, on the other hand, does seem to be effective in
that context.
We said right back at the start of the lecture it seems odd that humans, unlike
other primates, other mammals, lack the enzyme uricase, the enzyme that
converts uric acid to allantoin, the water-soluble metabolite.
So, on the other hand, if you deliver uricase to a patient, for example in the form
of Rasburicase, which is on the market here for the specific case of trying to
cope with the Tumor Lysis Syndrome, then you can convert uric acid to the
water-soluble form, allantoin, and promote its elimination. So it’s highly
effective in mobilising uric acid from tophi and in the context of the Tumor
Lysis Syndrome. So there are situations in which this approach can be very
powerful. It wouldn’t normally be used just for somebody with recurrent
attacks of gouty arthritis.
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It’s for the special case where you want to try to mobilize uric acid from
subcutaneous deposits associated with tophi. You need to lower the serum uric
acid levels substantially to be able to start to mobilize uric acid under those
circumstances from those subcutaneous deposits; also in the situation in which
the purine load is really massive as in the acute Tumor Lysis Syndrome.
And this table then seeks to just contrast the potential benefits and potential
downsides of different types of therapy.
In terms of drugs that are currently on the Australian market, Allanpurinol is
there sort of blocking uric acid synthesis via blocking xanthine oxidase,
Probenecid is there to promote uric acid excretion by blocking re-absorption in
the kidney and uricase is there for the dissolution of tophi or, perhaps more
commonly, in the Tumor Lysis Syndrome.
Okay, so we’ve reached the end. Gout is a crystal arthopathy so it’s an acute
form of arthritis. It tends to be one joint at a time and its due to the local
formation of crystals and the local inflammatory response to those crystals in
the joints mediated by neutrophils.
Uric acid is derived from the breakdown of purines and we’ve seen both the
impact of the de novo synthesis pathway to promote uric acid production and
the role of the salvage pathways particularly in certain tissues to recover purine
bases back into the purine nucleotype core and, as a consequence of that,
suppress uric acid synthesis.
Uric acid levels will rise if the de novo pathway synthesis is switched on, or of
the salvage pathway is switched off and the most dramatic example of salvage
pathways being switched off is in the Lesch Nyhan Syndrome where, in fact,
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the enzyme that’s responsible for catalyzing that particular salvage pathway,
which is hypozanthine or guanine, was not available.
There’s also a dietary impact on uric acid levels and we considered the way in
which ethanol can have an impact on uric acid via that process whereby acetate
gets converted to Acetyl CoA, acetate being at the bottom of the ethanol
oxidation pathway. Ethanol, acid aldehye, acetate, in the mechanism in the
process of acetate being converted to Acetyl CoA, we saw that ATP was needed
and that ATP being converted to AMP can then serve as a source of additional
purines for the production of uric acid.
It doesn’t look like a particularly impressive pathway; how can it happen? I
think one thing we need to remember is that the ethanol concentration in those
that use it are actually quite high. Remember we made the point in the ethanol
lecture that ethanol is a weak drug. The levels actually have to be quite high to
achieve any effect and, as a consequence of that, the levels get quite high inside
the cells and therefore there’s a substantial impact on purine nucleotide
metabolism inside the cells as well. So it’s about load. What concentrations of
purines are actually being generated inside the cell to fuel uric acid production?
We’ve seen the treatment of acute gout focuses on suppressing inflammation
but that its also useful to treat hyperuricemia, either with a view to preventing
subsequent attacks of acute gout or to mobilize uric acid in tophi or to control
uric acid production in the Tumor Lysis Syndrome.
Are there any question? Yes? Okay, Ben?
(INAUDIBLE QUESTION FROM BEN IN AUDITORIUM)
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So the question is why in treating acute gout with an agent that lowers the
serum uric acid level, in some way worsen the acute attack? It’s not
understood, Ben. Whether it means that there are sudden shifts in uric acid
levels that can lead to crystallization or whether there is some other biochemical
factor is not known. So its hand waving at the moment.
We’ve got a question here but there’s one over here first.
So the question is if you lose weight quickly, can that have an impact on the
uric acid level? And I guess the implication is that it could rise. So, just as in
the case of the Tumor Lysis Syndrome providing a large amount of
biochemistry substrate for uric acid production, I would think that in sudden
loss of weight it could have the same effect.
(INAUDIBLE QUESTION FROM AUDITORIUM)
Yes, potentially exercise… It depends… I guess the key question is whether
there is a fairly rapid mobilization of nucleotides out of DNA or RNA because
there’s a substantial load potentially of purine nucleotides in those biochemical
substances. Yes?
(INAUDIBLE QUESTION FROM AUDITORIUM)
So the question is why is it in particular the hallux joint of the great toe, you
know, the great toe is often one? I can’t explain to you about why it should be
that particular joint other than to say I suppose that one of the key questions
here is what is actually leading to the formation of the crystals in the first place
and if there’s, say, local damage in a particular joint? Of course the hallux, you
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know, the great toe is exposed to repetitive minor injury on a daily basis. It’s
possible that that environment makes it more likely for crystals to actually form.
Any other questions? Right, if not, thank you.
[APPLAUSE]
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