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Chapter • Inhibitors of Hydrolyzing Metalloenzymes
25
unpleasant dry cough, and occasionally life-threatening
angioedema (acute swelling of the mucous membranes)
can occur. This is thought to be related to the inhibition
of the degradation of the described peptides, especially
bradykinin. The catalytic activity of the C-domain appears to be responsible for blood pressure regulation under invivo conditions, where angiotensinI is efciently
cleaved. Bradykinin, on the other hand, is cleaved equally
well by both domains. By using compounds that are selective for the C-domain, it may be possible to lower
blood pressure, while leaving aresidual degradation of
bradykinin intact. Excessive levels of this peptide could
then be avoided. The structure determination of ACE,
thus, opens up anew perspective for the development
of selective inhibitors that allow efcient regulation of
blood pressure according to an established principle.
Hopefully, they will have fewer side effects.
25.6 Inhibitors of Matrix
Metalloproteases: An Approach to
Treat Cancer and Rheumatoid
Arthritis?
Matrix metalloproteases (MMPs) are afamily of neutral zinc endopeptidases. They play an important role in
the formation and degradation of connective tissue, for
example, after injury or during angiogenesis (the proliferation of blood vessels). In ahealthy state, these proteases
are kept in balance by tightly controlled mechanisms.
In this way, active proteases are released from inactive
precursors only when needed, or our body has sufcient
endogenous inhibitors to mediate the balance between
matrix synthesis and matrix degradation. In adisease
state, this complex equilibrium is disrupted and various
. Fig. 25.12 Crystal structure of lisinopril 25.19 (. Fig.25.11) with
t-ACE. The central carboxylate group of the inhibitor coordinates to
the zinc ion. The NH group on the lysine residue of lisinopril forms
an H-bond to the C═O group of Ala 354 in the
carbonyl group also forms hydrogen bonds with His 353 and His 513.
The terminal ammonium group of the lysine residue forms an H-bond
to Glu 162. The acid group of the proline residue forms an H-bond
pocket, and the
contact with Lys 511 and Tyr 520. The phenethyl side chain is placed
in the S
pocket. (7 https://sn.pub/R8rq3h)
1
. • Inhibitors of Matrix Metalloproteases: An Approach to Treat Cancer and Rheumatoid Arthritis?
. Table 25.5 Domain-specic inhibition of angiotensin-converting enzyme by structurally deviating compounds
Compound N-domain inhibition (nM) C-domain inhibition (nM)
RXP A380 25.31 10,000 3.0
Captopril 25.13
Enalapril 25.18
RXP407 25.30 2.0 2500
Lisinopril 25.19
Keto-ACE 25.29 15,000 40.0
a
. Fig.25.9, b. Fig.25.11
a
b
b
8.9 14.0
26.0 6.3
44.0 2.4
MMPs are produced in excess. This leads to pathological
situations associated with the construction and degradation of extracellular tissues.
The etiology of rheumatoid arthritis is based on such
chronic destructive processes that lead to the loss of bone
and cartilage. Cartilage tissue consists of aglycoprotein
matrix that is cross-linked and reinforced by collagen.
MMPs cleave these scaffold proteins. In rheumatoid arthritis, the balance between matrix synthesis and degradation appears to be lost. Excessive activity of matrix
metalloproteases leads to excessive degradation of cartilage. Inhibition of these proteases may, therefore, be
apromising approach to the treatment of rheumatoid
arthritis. Degradation of the extracellular matrix is also
critical for malignant tumor growth, tumor cell invasion,
metastasis, and angiogenesis. Therefore, the inhibition of
MMPs could also lead to cancer therapy.
Nearly 30MMPs have been identied, including collagenases (MMP-1, -8, -13), gelatinases (MMP-2, -9),
stromelysins (MMP-3, -10, -11), matrilysin (MMP-7),
macrophage metalloelastases (MMP-12, -19), and enamelysin (MMP-20). The collagenases, gelatinases, and
stromelysin recognize collagen as asubstrate. Collagen
is composed of three intertwined, left-handed α-helical
chains. Each individual chain is more than 1000 amino
acids long and contains the repeating sequence –(Gly–
X–Y)n–, where the Xposition is usually occupied by
aproline or an alanine and the Yposition by ahydroxyproline or an alanine. Collagenases cleave collagen in its
native triple helical structure, gelatinases cleave collagen
in adenatured form, and stromelysins are thought to
cleave proteoglycans.
Anumber of different collagens are cleaved by collagenases between the glycine and leucine or isoleucine
residues. Asubstrate comparison between human, bovine, mouse, and chicken showed that three amino acids
to the right and left of the cleavage site are conserved.
Therefore, the N- or C-terminal protected hexapeptide
Ac–Pro–Leu/Gln–Gly–Leu/Ile–Leu/Ala–Gly–OEt, for
example, 25.32 (. Fig.25.13), is recognized as aminimal substrate.
This established the starting point for the design of
collagenase inhibitors. The peptide bond to be cleaved
in the minimal substrate 25.32 is replaced with anoncleavable isostere. The replacement of the amide bond
between Gly and Leu with a ketomethylene group –
COCH2–, ahydroxymethylene group –CH(OH)CH2–,
or ahydroxylamine derivative led to inactive compounds
in all cases. These groups are apparently unable to form
afavorable interaction with the zinc ion. Finally, the use
of aphosphinate group yielded apotent collagenase inhibitor 25.33. However, if only the N-terminal proline is
removed from this hexapeptide, the inhibitory activity
will be largely lost. The search for collagenase inhibitors
based on the N-terminal tripeptide fragment led to modestly active compounds such as 25.34. The synthesis of
potential inhibitors containing the C-terminal tripeptide
sequence Leu–Leu–Gly–O-alkyl was much more successful. Coupling these structural elements with the potent
hydroxamic acid head group to bind the zinc ion yielded
collagenase inhibitors with nanomolar afnity such as
Ro 31-4724, 25.35, and Ro 31-9790, 25.36. The X-ray
structure of 25.35 complexed with human broblast collagenase was solved. As expected, the compound binds
25
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Chapter • Inhibitors of Hydrolyzing Metalloenzymes
. Fig. 25.13 Collagenase inhibitors prepared from
substrate analogues. Compound 25.32 covers the substrate sequence from P3 to
bond by a–PO2– group25.33 leads to apotent inhibitor. Compound 25.34 contains only the three amino
acids prior to the cleavage site as well as the C-terminal
hydroxamic acid as azinc-binding group. Compounds
25.35 and 25.36 contain the three or two amino acid side
chains following the cleavage site in their structures; this
time they are augmented with an N-terminal hydroxamic
acid group. The two inhibitors marimastat 25.37 and
batimastat 25.38 were in clinical trials for several years as
compounds for treatment of cancer
. Replacement of the amide
to the zinc ion as abidentate ligand. The leucine side
chain in the
methyl group binds in the
chain at position
position lls the
pocket and the alanine
pocket. The leucine side
, which should formally occupy the
pocket, is oriented away from the enzyme. The binding
mode is shown in . Fig.25.14.
Interestingly, replacing the iso-butyl side chain at po-
sition
with atert-butyl group in 25.36 resulted in an
increase in afnity, even though the group is not in direct
contact with the enzyme. This result has been attributed
to conformational stabilization. The bulky tert-butyl
group limits the mobility of the inhibitor so that the
conformation adopted in the enzyme is still energetically
favorable. Compound 25.36 showed some activity after
oral administration in an animal model and was selected
for clinical trials as adrug to treat arthritis. The structurally similar inhibitors marimastat 25.37 and batimastat
25.38 from British Biotech have been in development for
many years as broad-spectrum MMP inhibitors for the
treatment of cancer. Finally, in the eld of matrix metalloproteases, many lead structures have been discovered
and developed into potent inhibitors. . Fig. 25.15 lists
some of these substances (25.39–25.48), almost all of
which are derived from hydroxamic acids. Hardly any
peptidic character can be inferred from them. The only
problem is that none of these compounds has made it
through clinical trials to the market. The results of the
clinical trials studies were rather sobering. Bayer’s development product tanomastat 25.47 for the prevention of
angiogenesis, tumor growth, and metastasis performed
worse than aplacebo sample. Novartis’ CGS 27023A
25.48 did not fare much better.
What is the reason for the lack of success in these
drug development projects so far? One of the reasons
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. • Inhibitors of Matrix Metalloproteases: An Approach to Treat Cancer and Rheumatoid Arthritis?
. Fig. 25.14 Crystal structure of collagenase with Ro 31-4724 (25.35,
IC50 = 9 nM) shows the adopted binding mode. The hydroxamic acid
binds in abidentate-like manner to the zinc ion. Both amide groups
form hydrogen bonds to the enzyme. The leucine side chain of the
inhibitor in the
the protein’s interior. The alanine methyl group binds in the
whereas the leucine side chain in position
position lls the
pocket, which is oriented toward
pocket,
protrudes into the solvent
may be alack of selectivity of the designed compounds.
At the time these compounds were designed, only afew
of the relevant MMPs had been identied. The different
members of the MMP family are very similar to one another. Overlapping substrate proles were observed. In
some cases, another member of the family can take over
the task of aprotease that has been deactivated by inhibition. When comparing the proteases, it is striking that
practically only the
pockets S3, S2, S1,
pocket is deeply buried. All other
,
are relatively shallow and easily
accessible from the outside. In addition, proteins in the
pocket have been shown to be highly adaptable to bound
substrates and inhibitors. While this may provide an opportunity for the development of selective inhibitors, it
does not generally facilitate drug development for such
pockets. For the collagenase MMP-1, aconformational
change at Arg 214 was shown to open amuch larger
pocket (. Fig.25.16). In the conformation determined
with the rst crystal structures, the available space was
because the
(7 https://sn.pub/1LoZeU)
pocket is practically nonexistent.
sufcient to accommodate asec-butyl group as in 25.49.
However, after the rearrangement of the arginine, much
longer biaryl ether residues (see 25.50) can be accommodated in the
pocket!
Afurther complication is that there is another family
of zinc proteases, the ADAM family (adisintegrin and
metalloprotease, or adamlysines), whose members share
little sequence homology with MMPs but have catalytic
centers that are very similar to MMPs. This family was
discovered after the rst MMP inhibitors were in clinical
trials. TNF-α converting enzyme (TACE) is amember of
this family. Blocking this enzyme affects the function of
TNF-α, the proinammatory cytokine that plays acentral role in immune response (Sect.29.8). The enzyme
itself is being investigated as atarget for drug therapy of
autoimmune diseases. Cross-reactivity with MMP inhibitors is not desired. Unfortunately, it has also been shown
that MMP inhibitors have no effect against advanced and
late stage cancer. However, in the early days of MMP
25
−
3
Chapter • Inhibitors of Hydrolyzing Metalloenzymes
. Fig. 25.15 Development
candidates 25.39–25.48 from
various companies as potent MMP
isoenzyme inhibitors. Hydroxymates, inverse hydroxymates, and
carboxylates were used as anchor
groups for the zinc ion. Tanomastat
25.47 from Bayer and CGS27023A
25.48 from Novartis were clinically
developed for several years
research, models from an early stage of tumorigenesis
were used.
Despite the availability of many potent inhibitors,
drug development using members of the MMP family
has not yet resulted in asuccessful marketed product.
The problem of selectivity in this family seems to be too
high an obstacle to make precise regulation of these enzymes by drugs apromising therapy.
of these enzymes are known, which are called the α-,
β-, γ-, and δ-CAs. Sixteen isoforms of the α-CAs occur
in mammals. Some are cytosolic and others are membrane-anchored. They are involved in many physiologically important processes such as respiration, CO2/
transport between metabolizing tissues and the lungs, pH
homeostasis, electrolyte secretion, biochemical reactions
requiring C1 building blocks, bone resorption and calcication, and tumor growth.
The zinc ion is located at the end of afunnel-shaped
25.7 Carbonic Anhydrases: Catalysts of
aSimple but Essential Reaction
catalytic site in the α-carbonic anhydrases. It is held in
place by three histidine residues. The fourth coordina-
tion site is occupied by awater molecule. It is strongly
Another group of zinc-dependent enzymes that share
avery similar catalytic mechanism with the zinc proteases are the carbonic anhydrases (CAs). They catalyze
avery important reaction in our body, the xation of
carbon dioxide from bicarbonate, or the reverse reaction
for the release of CO2. In total, four different families
polarized by coordination to Zn2+. Most likely, this water
molecule is present as an OH− ion. There is also ahy-
drogen bond acceptor group found in the OH group of
Thr 199 (. Fig.25.17). The proton of this OH group
at Thr 199 forms ahydrogen bond with the carboxylate
group of Glu 106. The water (or OH− ion), which has
−
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0
1
. • Carbonic Anhydrases: Catalysts of aSimple but Essential Reaction
. Fig. 25.16 Crystal structure of the collagenase MMP-1 with two
different inhibitors 25.49 and 25.50. Because of aconformational rearrangement of Arg 214, the
can be accommodated. This adaptive ability of the specicity pockets
in MMPs makes the development of selective inhibitors extremely difcult. (7 https://sn.pub/kLL5Ky)
greatly enhanced nucleophilicity, attacks aCO2 molecule
located in ahydrophobic niche near Val 121, Val 143, and
Leu 198 at the bottom of the binding pocket. One of the
oxygen atoms of the CO2 nds ahydrogen bonding partner in the NH function of Thr 199. The newly formed
bicarbonate is displaced from the temporarily pentacoordinated zinc ion, and anew water molecule takes its
position at the zinc ion. Anew catalytic cycle can start.
CAII is one of the fastest enzymes known. The acquisition or removal of aproton is the rate-limiting step in the
reaction cycle. Carbonic anhydrases have aseries of multiple histidine residues that deliver the protons from the
edge of the funnel-shaped binding pocket. This arrange-
pockets with their voluminous groups
ment also makes the funnel amphiphilic, meaning one
side is hydrophobic and the other is hydrophilic. The very
narrow area around the catalytic zinc ion provides only
enough space for CO2 and
. Putative inhibitors must
be able to form equivalent interactions as the bicarbonate
ion, while at the same time occupying the funnel opening.
In addition to ions such as cyanide, thiocyanate, or isocyanate, especially sulfonamides, sulfamates, and sulfamides
have the appropriate head group for coordination in the
catalytic site. The amino group attached to these sulfur
derivatives is acidic enough to easily release aproton and
coordinate to the zinc ion in acharged state, analogously
to the OH− ion. The remaining proton interacts with the
Chapter • Inhibitors of Hydrolyzing Metalloenzymes
25
threonine OH group. An oxygen atom of the SO2 function
satises the NH function of the latter amino acid. The
second S═O group expands the tetrahedral coordination
state on zinc to pentavalency. An aromatic carbon that is
part of aheterocyclic ring system is usually found at the
fourth bond of the central sulfur atom of most known
inhibitors. In some examples, there is another oxygen or
nitrogen atom acting as alinker to this heterocycle.
In the case of carbonic anhydrases, the coordination
of the ligands to the zinc ion in the catalytic site is essen-
tial for good binding. In this way, small ligands such as
phenylsulfonamide 25.51 or its isostere thiophene-2-sulfonamide 25.52 achieve submicromolar inhibition of car-
bonic anhydraseII (. Fig.25.18). More than 50years
ago, the replacement of these aromatic rings by other
heterocycles led to the rst marketed products, which
were introduced into therapy as sulfonamides under the
names acetazolamide 25.53 and methazolamide 25.54. In
1954, acetazolamide represented the rst mercury-free
diuretic (Sect.30.12). It has also been used as asystemic
treatment for glaucoma. Glaucoma is an eye disease that
causes visual eld loss and, in severe cases, to blindness.
It is caused by insufcient drainage of the aqueous humor
from the eye. As aresult, pressure builds up inside the eye,
damaging the optical nerve if left untreated. Carbonic
anhydraseII inhibitors reduce the production of aqueous
humor and can reduce the pressure inside the eye.
Acetazolamide 25.53 and methazolamide 25.54 were
used for many years to treat glaucoma. They must be administered systemically. Direct application in the form of
eye drops does not work because the compounds cannot
penetrate the eye from the outside. Systemic administration and low selectivity with respect to the different
isoforms of carbonic anhydrase means that these enzymes are also inhibited outside of the eye. Unwanted
side effects are the consequence. As aresult, both of these
compounds have largely disappeared from therapy today.
For along time, it was assumed that carbonic anhydrase inhibitors could not be used as eye drops due to of
their unfavorable physicochemical properties. In 1983, to
general surprise, the topically active carbonic anhydrase
inhibitor 25.55 was reported for the rst time. The single
exchange of amethyl for atriuoromethyl group caused
this transformation! As aconsequence of this discovery,
the lipophilic range of alarge number of carbonic anhydrase inhibitors was characterized, within which topical
application is possible.
This led to the development of dorzolamide 25.57. In
fact, its design is the rst example of adrug optimized
by structure-based design with the support of ab initio
calculations along with crystal structure determinations.
After the X-ray structure of carbonic anhydraseII became available, structure-based design of carbonic anhydrase inhibitors began at Merck Sharp & Dohme in the
. Fig. 25.17 The catalytic site in α-carbonic anhydrases is found at
the end of afunnel-shaped binding pocket. There an OH− ion which
is coordinated to the Zn2+ ion nucleophilically attacks aCO2 mole-
cule. Abicarbonate ion is formed, which is held in place by Thr 199
(left). Asulfonamide, deprotonated at nitrogen, ts at the site of the
carbonate in the very narrow binding pocket (right). Because of the
tetravalency of the sulfur, this site can be tted with another substit-
uent, as is shown in the present case with ap-uorophenyl group.
(7 https://sn.pub/mEDTzP)
. • Carbonic Anhydrases: Catalysts of aSimple but Essential Reaction
. Fig. 25.18 The small aromatic sulfonamides 25.51 and 25.52 bind
to carbonic anhydraseII with submicromolar afnity. By exchanging aheterocycle, acetazolamide 25.53 and methazolamide 25.54 are
obtained. Both drugs were used for along time as systemic carbonic
anhydrase inhibitors for diuresis and for the treatment of glaucoma.
Compound 25.55 was the rst topically active CA inhibitor that is useable as eye drops. The structure-based design of new inhibitors led
mid-1980s. One of the rst compounds to emerge from
this effort was thienothiopyranosulfonamide 25.56 (MK
927). It binds to carbonic anhydrase with asubnanomolar
inhibition constant (Ki = 0.7 nM). The crystal structure
with the enzyme shows the expected coordination of the
sulfonamide group to the zinc ion in the active site. In
addition to hydrogen bonding, the inhibitor forms hydrophobic interactions with the protein. The observation
that the iso-propylamino group occupies an energetically
unfavorable position with an axial orientation on the ring
was asurprise. Apparently, the compound ts better into
the binding pocket in this unfavorable conformation. To
to the marketed product dorzolamide 25.57 by way of 25.56. Compounds 25.58–25.61 are further drugs that inhibit carbonic anhydrases and are used for the treatment of glaucoma or as a diuretic. Even
celecoxib 25.62, topiramate 25.63, and the articial sweetener saccharin 25.64 inhibit carbonic anhydrases and this explains some of their
observed side effects
enhance the afnity to the enzyme, amodication of the
molecule was planned to decrease the energetic penalty
between equatorial and axial orientation of the side chain.
This was achieved by stereospecically adding another
methyl group to the six-membered ring. To compensate for
the increased lipophilicity, the iso-propylamino group was
reduced to an ethyl group. The result of this modeling was
dorzolamide 25.57. It binds to carbonic anhydraseII with
Ki = 0.37 nM. Dorzolamide has successfully completed
all clinical trials. It has been marketed under the name
Trusopt® since 1995 and was the rst topically active
carbonic anhydrase inhibitor marketed for the treatment
Chapter • Inhibitors of Hydrolyzing Metalloenzymes
25
of glaucoma. Some other important drugs (25.58–25.61)
that inhibit carbonic anhydrase are shown in . Fig.25.18.
They serve as diuretics, glaucoma inhibitors, antiepileptics, and as treatments for altitude sickness, for peptic
ulcer disease, or for ankylosing spondylitis (also known
as Bechterew’s disease, achronic autoimmune inammatory disease that leads to spinal fusion). Since tumors require an acidic environment, carbonic anhydrases such as
CAIX and CA XII may be responsible for maintaining
these conditions. Therefore, they are potential targets for
cancer treatment because inhibition of CA would disrupt
acid homeostasis. The 16human α-carbonic anhydrase
isoenzymes characterized to date are highly homologous.
Small differences, such as the exchange of athreonine for
ahistidine at position 200, distinguish the CAI and CAII
isoforms. Drugs must exploit these differences to achieve
the desired selectivity between these isoforms (Sect.18.14).
In the meantime, some very surprising adverse effects
of known drugs can be attributed to carbonic anhydrases.
To improve solubility, terminal sulfonamide groups have often been incorporated into drug candidates as functional
groups. The analgesic celecoxib 25.62 is a cyclooxygenaseII inhibitor (Sect.27.9). It can also bind to carbonic an-
hydrase with nanomolar afnity through its sulfonamide
group. In patients with familial adenomatous polyposis
(FAP), adisease that leads to the development of polyps
in the colon, areduction in the number of tumors has
been clinically observed in patients treated with celecoxib.
This result may be consistent with carbonic anhydrase inhibition. One of the side effects of the antiepileptic drug
topiramate 25.63 is loss of appetite. As asulfamate, this
compound is apotent mitochondrial CAV inhibitor. This
isoenzyme is involved in de novo lipogenesis. This observa-
tion led to athorough investigation of CAV as apossible
therapeutic principle for obesity therapy. Even the very
old and widely used articial sweetener saccharin 25.64,
which contains acyclic sulfonamide unit, can inhibit some
carbonic anhydrases very strongly. Other clinically used
carbonic anhydrase inhibitors, like saccharin, are known
to have an unpleasant metallic aftertaste. This property
is thought to be due to the inhibition of CAVI, which is
produced in the oral cavity. Its inhibition affects the pH
and may cause the bitter metallic taste sensation. Presumably, other drugs with terminal sulfonamide groups also
have effects on carbonic anhydrases. Only time will tell
whether the major problem of achieving sufcient selectivity within this class of enzymes can be solved.
25.8 A Case for Two: Zinc and Magnesium
in the Catalytic Centers of
Phosphodiesterases
Phosphodiesterases (PDEs) are aclass of metalloenzymes
with at least 12gene families that hydrolyze the intracellular second messengers cAMP 25.65 and cGMP 25.67
(cyclic AMP and GMP) to their open-chain analogues
(. Fig.25.19). They are widely distributed in various
tissues and organs and control important processes in
the regulation of calcium channels, sense of smell, platelet aggregation, aldosterone release, cell proliferation,
myocardial contractility, insulin release, inammation
modulation, smooth muscle contraction, mood, penile
erectile function, or muscle metabolism. Among family
members, the sequences are highly conserved.
First, the crystal structures of PDE4 and PDE5 were
solved. To date, eight PDEs have been crystallographically characterized. While inhibition of PDE4 may lead
to the treatment of asthma, chronic obstructive pulmonary disease, or autoimmune diseases, PDE 5inhibitors
have been developed for the treatment of erectile dysfunction. The PDE 5 enzyme is expressed in various
tissues and is specic for the hydrolysis of cGMP. In
addition to the zinc ion, which is essential for hydrolytic
cleavage, there is an additional magnesium ion found in
the active site. The zinc ion is coordinated by two histidine and two aspartic acid residues. At the fth position,
there is awater molecule that, together with one of the
two aspartic acid residues, forms abridge to the magnesium ion (. Fig.25.20). The other coordination site of
the octahedrally surrounded Mg2+ is occupied by awater
molecule. Zn2+ also prefers an octahedral geometry in
phosphodiesterases. The sixth coordination position is
occupied by awater molecule. This water molecule presumably takes over the role of the nucleophilic OH− for
the hydrolytic cleavage of the cyclic phosphodiester.
Three PDE 5inhibitors were brought to the market as
drugs to treat erectile dysfunction. Aside from sildenal
25.69 (Viagra®), the rst to be introduced by Pzer, var-
denal 25.70 (Levitra®) and tadalal 25.71 (Cialis®) have
passed clinical trials (. Fig.25.21). Interestingly, these
inhibitors bind to the catalytic site of PDE5, but do not
make direct contact with the zinc ion (. Fig.25.20). In
fact, the binding of the basic nitrogen to the metal ion is
mediated by two water molecules. The pyrazolopyrimidinone moiety in sildenal replaces the analogous group
in the natural substrate cGMP. The relationship with
cGMP is even more obvious when it is considered that
the 2-phenyl-substituted purines such as 25.72 served as
lead structures (. Fig.25.21). The pyrazolopyrimidine
25.73 or imidazotriazenone 25.74 that are contained in
sildenal and vardenal, respectively, were developed
from them. The chemically closely related vardenal
adopts avery similar binding mode as sildenal. The
structurally different tadalal, on the other hand, adopts
adistinctly different orientation.
The discovery of the effects of sildenal was achieved
once again by serendipity. The compound was in clinical trials at Pzer for the treatment of angina pectoris.
However, it proved to be no better than the classic nitro
compounds (i.e., nitroglycerin or isosorbide dinitrate).
These nitro derivatives release NO under reductive con-
. • A Case for Two: Zinc and Magnesium in the Catalytic Centers of Phosphodiesterases
. Fig. 25.19 cAMP 25.65 and cGMP 25.67
are hydrolyzed into their open-chain analogues
AMP 25.66 and GMP 25.68, respectively, by
phosphodiesterases
. Fig. 25.20 Crystal structure of sildenal 25.69 (. Fig. 25.21) in
PDE5. The pyrazolopyrimidinone moiety of the inhibitor is recognized by Gln 817 through two parallel hydrogen bonds and binds to
the catalytic zinc ion (blue-gray) via awater molecule. It is found in the
vicinity of amagnesium ion (light green), which is coordinated by ve
ditions, which stimulates guanylate cyclase. cGMP is
then formed, which in turn inuences vasoconstriction.
Aphosphodiesterase inhibitor also increases cGMP levels
by blocking the degradation of this second messenger. In
clinical trials, however, one side effect in male probands
was noteworthy: it stimulated penile erections. NO is re-
water molecules and Asp 654. Abridging water molecule is shared by
Mg2+ and Zn2+. (7 https://sn.pub/mLzaFK)
leased into the cavernous body of the penis and increased
cGMP is produced by activation of guanylyl cyclase. This
causes increased blood ow to the cavernous body and
stimulates penile erection. Sildenal enhances the effect
by inhibiting the degradation of cGMP. Sildenal was approved for the treatment of erectile dysfunction in 1998.