Prodrugs II
Prepared By
Professor Mohamed Ahmed Moustafa
Professor of Medicinal Chemistry
DRUG
TARGETING
Prodrugs for Site Specificity
Site-specific drug delivery attempts to obtain
very precise and direct effects at the ‘site of action’
without subjecting the rest of the body to
significant levels of the active agent.
The targeting of drugs for a specific site in the body by conversion to
a prodrug is plausible when the physicochemical properties of
prodrug are optimal for the target site.
It should be kept in mind, however, that when the lipophilicity of a drug is
increased, it would improve passive transport of the drug nonspecifically to all
tissues.
Prodrugs for (increased) Site Specificity
To increase the site specificity of certain drugs, the
following means of preparing prodrugs are used:
1. Increase or reduction in volume
2. Alteration of hydrophilicity or solubility
3. Introduction or removal of cationic or anionic
moieties
4. Change of pKa
5. Incorporation of hydrocarbon or other suitable
stable or labile moieties, and carriers that
transport the compound to specific organs or
tissues and make it to accumulate selectively
there, where it is bioactivated.
Prodrugs for GIT
A nice objective of using prodrugs is to restrict the
drug action to the upper part of the GIT
If we want to target drugs against an infection of
the GIT, then we want to prevent the drugs being
absorbed into the blood supply.
How??
Retardation of the drug absorption, as in case of
sulfathiazole
 How would you decrease the absorption of this
drug?
This can easily be done by using a fully ionized
molecule which is incapable of crossing cell
membranes.
 The incorporation of strongly hydrophilic
moieties to the sulfonamides prevents their
transport to the bloodstream.
They are incapable of crossing the gut wall and
are therefore directed efficiently against the GI
infection.
Prodrugs for (increased) Site
Specificity
These prodrugs act almost exclusively inside the intestinal tract.
The succinyl or phthalyl group is good enough to decrease
lipophilicity of the compound.
The hydrophilic group makes the molecule poorly absorbed,
preventing its transportation to the circulation.
used as intestinal antibacterial agent.
O
O
OH
O
S
O
N
H
N
H
N
S
Prodrugs for (increased) Site
Specificity
Bitolterol, a prodrug of the -
O
blocker colterol.
 The toluate groups in both aromatic
hydroxyls prevent the methylation of one
OH
H3C
O
H3C
O
NHC(CH3)3
Bitolterol
O
of the groups by the enzyme COMT, until
these groups are removed by hydrolysis by
the esterases present in the tissues and in
the blood.
Esterase
 Bitolterol accumulates selectively in the
COOH
lungs, where it partially and immediately
OH
releases colterol, which stimulate adrenergic receptors and then adenylatic
HO
NHC(CH3)3
cyclase, with the consequent relaxation of
2
the bronchial smooth muscles.
HO
CH 3
p-Toluic Acid
Colterol
Blood-brain barrier (BBB) is one of important
membrane often targeted for drug delivery.
It is unique lipid-like protective barrier that prevents
hydrophilic compounds from entering the brain
unless they are actively transported.
a The blood brain barrier contains active enzyme
systems to protect the CNS even further.
۩
Consequently, molecular size and lipophilicity are
often necessary, not sufficient, criteria for gaining
entry into the brain.
Prodrugs for (increased) Site Specificity
Bodor and co-workers have devised a reversible redox ✵
drug delivery system (RRDDS) for getting drugs into the
CNS and then, once in, preventing their efflux.
The approach is based on the attachment of a hydrophilic
drug to a lipophilic carrier (a dihydropyridine) thereby
making prodrug that actively transported into the brain.
O H
R
H
R
X
Prodrug residue
H
X
N
N
CH3
O H
CH3
Blood-brain
barrier
Prodrugs for (increased) Site Specificity
Once inside the brain, the lipophilic carrier
is converted enzymatically to a highly
hydrophilic species (positively charged),
which is then enzymatically hydrolyzed
back to the drug and N-methylnicotinic
acid, which is eliminated from the brain.
O H
R
H
X
N
♫ The oxidation of the dihydropyridine to
CH3
Enzymatic oxidation
the pyridinium ion (half-life generally 2050 min) prevents the drug from escaping
out of the brain because it becomes
charged.
♫ This drives the equilibrium of the
lipophilic precursor throughout all of the
tissues of the body to favor the brain.
O
R
X
N
CH3
Prodrugs for (increased) Site Specificity
O H
R
H
R
X
O H
H
X
N
N
CH3
Prodrug residue
CH3
Blood-brain
barrier
Enzymatic oxidation
O
HOOC
+
N
CH3
HX
Drug
R
Drug release
by a suitable
process
R
X
N
CH3
Prodrugs for (increased) Site Specificity
♫The functional group on the drug should be an amino,
hydroxyl, or carboxyl group.
♫When it is a carboxylic acid, the linkage is an
acyloxymethyl ester, which decomposes as in
pivampicillin.
Prodrugs for (increased) Site Specificity
♫Any oxidation occurring outside of the brain
produces a hydrophilic species that can be rapidly
eliminated from the body.
♫The released oxidized carrier is relatively nontoxic
and easily eliminated from the brain.
♫Although this is a carrier-linked prodrug, it
requires enzymatic oxidation to target the drug to
the brain.
♫The oxidation reaction is a bioprecursor reaction.
Prodrugs for (increased) Site Specificity
The brain delivery of -lactam antibiotics for the
possible treatment of bacterial meningitis.
 Since the -lactam antibiotics are hydrophilic, they
enter the brain very slowly, but they are actively
transported back into the blood.
 Therefore, they are not as effective in the
treatment of brain infections as elsewhere.
 Bodor and co-workers prepared a variety of
penicillin prodrugs attached to the dihydropyridine
carrier through various linkers and showed that lactam antibiotics could be delivered in high
concentrations into the brain.
RRDDS
NH2
H
N
O
S
N
O
O
O
O
O
N
Prodrugs for (increased) Site Specificity
Increasing the brain concentration of the inhibitory neurotransmitter aminobutyric acid (GABA) results in anticonvulsant activity.
 GABA is too polar to cross the blood-brain barrier, so it is not an effective
anticonvulsant drug.
 To increase the lipophilicity of GABA, a series of GABT and -aminobutyric Schiff
bases were synthesized.
 Progabide emerged as an effective lipophilic analog of GABA that crosses the
blood-brain barrier, releases GABA inside the brain, and shows anticonvulsant
activity.
F
OH
O
N
NH2
Cl
Prodrugs for (increased) Site Specificity
3.The synthesis of a glyceryl lipid (R=linolenoyl)
containing one GABA molecule and one vigabatrin
molecule, a mechanism-based inactivator of GABA
aminotransferase and anticonvulsant drug.
 This compound inactivates GABA
aminotransferase in vitro only if brain esterases
are added to cleave the vigabatrin from the
glyceryl lipid.
 It also is 300 times more potent than vigabatrin, in
vivo, presumably because of its increased ability
to enter the brain.
Prodrugs for (increased) Site Specificity
D.the application prodrugs to site-specific drug
delivery -Glutamyl dopa is an example of a sitespecific prodrug of levodopa (L-dopa).
 L-dopa is precursor of the neurotransmitter
dopamine, which plays an important role in the
CNS and also exerts receptor-mediated
vasodilation in the kidney.
Prodrugs for (increased) Site
Specificity
 Intraperitoneal injection of -
glutamyl dopa into mice led to the
selective generation of dopamine in
HO
O
OH
the kidney as a consequence of the
sequential actions of -glutamyl
transpeptidase and L-aromatic
HO
NH
amino acid decarboxylase, two
enzymes that are highly
concentrated in the kidney.
 The concentration of dopamine in
the kidney after -glutamyl dopa
administration was five times higher
than that after administration of an
equivalent dose of L-dopa.
O
H2
C
C
O
CH
NH 2
C
C
H2
OH
Prodrugs for (increased) Site Specificity
► This does occur in a very
O
O
selective manner, suggesting
that some N-acyl-glutamylsulfamethoxazole
O
S
O
R
N
H
N
N
H
R'HN
O
derivatives may be useful as
specific renal antibacterial
agents and -glutamyllevodopa
R=
OH
R' = Acetyl, Butyl
N- Acetyl-L--glutamyl-sulfamethoxazole
or -glutamyldopamine as
specific renal vasodilators.
Acetylase
Acetate +
H 2O
L--glutamyl-sulfamethoxazole
-glutamyl transpeptidase
Sulfamethoxazole + Glutamic acid
Prodrugs for (increased) Site Specificity
E.Design a prodrug that requires activation by an
enzyme found predominantly at the desired site of
action.
For
example,
tumor
cells
contain
a
higher
concentration of phosphatases and amidase than
do normal cells.
Consequently, a prodrug of a cytotoxic agent could
be directed to tumor cells if either of these
enzymes were important to the prodrug activation
process.
Prodrugs for (increased) Site Specificity
Diethylstilbestrol diphosphate was originally
designed for site-specific delivery of
diethylstilbestrol to prostatic carcinoma tissue.
In general, though, this tumor-selective approach
has not been very successful because the
appropriate prodrugs are too polar to reach the
enzyme site, the relative enzymatic selectivity is
insufficient, and the tumor cell perfusion rate is
too poor.
designing prodrugs that are activated by enzymes
found mainly at the target site.
This strategy has been used to design antitumour
drugs because tumors contain higher proportions of
phosphatases and peptidases than normal tissues.
For example, diethylstilboestrol diphosphate
(Fosfestrol) has been used to deliver the oestrogen
agonist diethylstilboestrol to prostatic carcinomas.
C2H5
O
HO P O
OH
C C
C2H5
C2H5
O
O P OH
OH
Diethylstilboestrol diphosphate
H2O
C
C
C2H5
Diethylstilboestrol e
OH
Prodrugs for (increased) Site Specificity
F.Certain glycosides of antiinflammatory steroids
designed to release the parent drugs in the colon.
 Since drug glycosides are bulkier and generally
more hydrophilic than the corresponding drugs,
their ability to cross the biological membranes is
reduced.
 Steroid glycosides are not cleaved by the enzymes
of the small intestine.
 In the colon, however, they are hydrolyzed by the
bacterial glycosidases, thus liberating the parent
drugs in the large intestine.
Prodrugs for (increased) Site Specificity
G.Two interesting examples are the prodrugs of N-
acetylated sulfamethoxazole and of levodopa in
which the carrier moieties are glutamic acid
derivatives.
►These prodrugs were designed on basis of the
finding of a particularly high concentration of an Nacylamino acid deacylase and -glutamyl
transpeptidase in the kidneys and that these
enzymes would hopefully release the parent drugs
in those organs.
Mutual OR
Reciprocal prodrugs
Aspirin + Paracetamol
 The best aryl ester
O
is the benorylate,
which is a mutual
prodrug of Aspirin
and Paracetamol.
 Advantages?
O
O
O
O
NH
Sultamicillin (Self Protection)
O
NH 2
O
S
S
H
N
N
N
O
O
COOH
O
COOH
Sulbactam
(Beta-lactamase inhibitor)
Ampicillin
O
NH2
S
H
N
N
O
N
O
C
O
C
O
O
O
O
S
O
Mutual Prodrug (Reciprocal Prodrugs)
OH
CH3
OPO3Na2
CH3
Sodium phosphate
and
Carbon dioxide
O
Cl
N
Cl
O
Estermustine Sodium Phosphate
Emcyt® - Pharmacia & Upjohn
HO
Cl
NH
NH+ClAziridine
Cl
Nornitrogen mustard
Cl
Actual alkylating species
• Used for metastatic carcinoma of the prostate
• Promoiety also a drug!
• Prodrug is selectively taken up into estrogen receptor positive cells then
urethane linkage is hydroylzed
• 17-alphaestradiol slow prostate cell growth
• Nornitrogen mustard is a weak alkylating agent
Famciclovir is a prodrug of penciclovir which is itself a
prodrug
Penciclovir is poorly absorbed from the gut owing to
its polarity.
Famciclovir is less polar and is absorbed more easily.
It is then metabolized, mainly in the liver, to form
penciclovir which is phosphorylated in virally infected
cells.
N
N
AcO
N
N
Liver
N
HO
N
NH2
OAc
Famciclovir
P
P
P
O
N
NH2
N
Viral thymidine
kinase
OH
Penciclovir
N
N
OH
N
N
NH2
Cell
kinase
P
O
N
N
OH
N
N
NH2
Famciclovir
O
N
N
N
N
H2N
N
N
H2N
O
N
HN
N
N
H2N
N
O
HO
O
HO
O
HO
HO
Famciclovir
O
N
HN
H 2N
P
P
Penciclovir
N
O
P
HO
N
N
Bioprecursors
Bioprecursor Prodrugs
Bioprecursor” prodrug
A compound that is metabolized into an active drug, usually
by Phase I reactions; eg acetanilide
Do NOT contain a carrier or promoiety
Contain latent functionality –
Metabolically or chemically transformed into an active –
drug
Types of activation at are predictable–
Oxidative (most common method)
Reductive
Phosphorylation (antiviral agents)
–
Non-steroidal
antiinflammatory
Use: Arthritis
Oxidation Example – Nabumetone –
Relafen® – Smith Kline Beecham
O
Series of oxidative
OH
CH3
CH3 O
decarboxylation
CH 3O
O
Active form of the drug
that inhibits Prostaglandin
biosynthesis by
cyclooxygenase
Bioprecursor Prodrugs
Reduction example - Mitomycin C - Mutamycin® - Bristol
Adenocarcinoma of the stomach and
Myers
pancreas
H 2N
H 2N
OH
CH3
O
H2N
O
H 2N
O
O
O
OHH O
O
H
OMe ReductionH2N
OMe -OCH3 H2N
NH
NH
NH
N
N
CH3
CH3
+N
OH
A quinone electron withdrawing
A hydroquinone electron donating
-H+
H+
OH
NH
Further alkylation
H2N
DNA
H2N
H2N
N
CH3
OH
OH
DNA
-CO 2
N
CH3
+
OH
Electrophile
NH
-NH3
OH
O
O
H 2N
N
CH3
OH
NH
Examples - Bioprecursor
Minoxidil is an antihypertensive that is also used as to
prevent hair loss (Rogaine/Regaine)
The active constituent is a Phase II metabolite, minoxidil
sulfate
H 2N
O
N
NH 2
H 2N
OSO 3
N
NH 2
Phase II
metabolism
N
minoxidil
N
minoxidil sulfate
Pilocarpine Bioprecurses
N
O
OR
HO
N
Invivo
N
O
OR
O
N
N
O
O
N
R = Ethyl, Butyl, Benzyl, OR others
b) Sulfoxide Reduction
The antiarthritis drug sulindac is an indene isoster of
indomethacin, which originally was designed as a
serotonin analog.
Sulindac is a prodrug for Sulindac sulfide, the
metabolitc reductive product
CH2COOH
F
CH3
H
O
R=
R
S CH3
S CH3
O
S CH3
O
N-Oxidation
N-Oxidation prodrug activation
reaction is the reversible redox drug
delivery strategy for getting drugs
into the brain.
In case of N-Methylpyridinium-2carbaldoxime chloride (2-PAM) is
used as an antidote to poisoning with
cholinesterase inhibitors.
Because of its charge and
hydrophilicity it does not penetrate
the blood-brain barrier.
C
H
N
CH3
2-PAM
NOH
.Cl
Pro-2-PAM
Pro-2-PAM
,
the
reduced
dihydropyridine form of 2-PAM,
readily
enters
she
central
nervous system.
C
H
N
CH3
NOH
.Cl
2-PAM
There it is oxidized to form 2PAM, which remains trapped in
the CNS since its charge reduces
its rate of transfer from the brain
back into the blood.
N
C
H
CH3
Pro-2-PAM
NOH
Bioprecursor Prodrugs
HN
O
HO O
O
Phosphorylation example –
I
HN
O O
OPO O
O-
Viral Thymidine
Kinase
OH
Iodoxuridine - Herplex®
Allergan - lipid soluble!
Opthalmic product for
Herpes simplex keratitis
Higher affininty for viral
kinases than mammalian
kinases but some toxicity
O
I
Not lipid soluble
OH
ATP
-O
HN
O O O O
P -O P -O P -O O
O O O
O
I
OH
TWO mechanisms of action: 1. Inhibits DNA polymerase 2. Incorporated
into DNA affording incorrect base pairing and template activity
Chemical Delivery Systems
We have already seen 2 examples of this:
– Sulfasalazine – an azo compound
– Methenamine – An urinary antibacterial agent
Requirements
– Prodrug reach the site of action in high
concentrations
– Knowledge of high metabolism at site
– Other factors
Extent of organ or site perfusion
Information on the rate of prodrug conversion to the active form
at both target and non-target sites
Rate of input/output of prodrug from the target site
Limit side effects and increase effectiveness