Prodrug Strategy
(Concept & Applications)
Presented By: Anvita Jadhav
M.Pharm(Pharmaceutics)
1
Prodrug concept
• The concept of “prodrug” was first introduced by Adrian
Albert in 1958 to describe compounds that undergo
biotransformation prior to eliciting their pharmacological
effect.
• A prodrug is defined as a biologically inactive derivative of
a parent drug molecule that usually requires a chemical or
enzymatic transformation within the body to release the
active drug, and possess improved delivery properties over
the parent molecule.
• The development of prodrugs is now well established as a
strategy
to
improve
the
physicochemical,
biopharmaceutical or pharmacokinetic properties of
pharmacologically potent compounds, and thereby
increase usefulness of a potential drug.
2
Schematic illustration of the prodrug concept
Pharmacokinetic or
Physicochemical
barrier
Extracellular Fluid
Site of Action
(cell or cell surface)
3
History and the Present of Prodrug Design
1899
Methenamine
First
intentional
Prodrug
1935
Protonsil
Antibiotic
1958
Adrien Albert
First
introduced
the term
“pro-drug”
1960
An explosive
increase in
the use of
prodrugs in
drug
discovery and
development.
2009
15% of the
100 best
selling drugs
were
Prodrugs
4
Rationale for prodrug design
A. Improving formulation and administration
B. Enhancing permeability and absorption
C. Changing the distribution profile
D. Protecting from rapid metabolism
E. Overcoming toxicity problems
5
Properties of ideal prodrug
1.
2.
3.
• Pharmacological Inertness
• Rapid transformation, chemically or
enzymatically, into the active form at the target
site
• Non-toxic metabolic fragments followed by
their rapid elimination
6
Classification of Prodrugs
Bipartite
prodrug
Carrier linked
prodrug
Tripartite
prodrug
Bioprecursors
Mutual
Prodrugs
Prodrugs
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A) Carrier linked prodrug
 Carrier linked prodrug consists of the attachment of a
carrier group to the active drug to alter its physicochemical
properties.
 The subsequent enzymatic or non-enzymatic mechanism
releases the active drug moiety.
Active Drug
Chemical Prodrug Formation
Covalent Bond
Inert Carrier
Chemical/Enzymatic cleavage
in vivo
8
It can be further subdivided into
1. Bipartite prodrug
• It is composed of one carrier (group) attached to
the drugs.
• Such
prodrugs
have
greatly
modified
lipophilicity due to the attached carrier. The
active drug is released by hydrolytic cleavage
either chemically or enzymatically.
• E.g. Tolmetin-glycine prodrug.
Glycine
Tolmetin
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2. Tripartite prodrugThe carrier group is attached via linker to drug.
Drug
Linking
Structure
Carrier
10
3. Mutual Prodrugs
• A mutual prodrug consists of two pharmacologically active agents
coupled together so that each acts as a promoiety for the other
agent and vice versa.
• A mutual prodrug is a bipartite or tripartite prodrug in which the
carrier is a synergistic drug with the drug to which it is linked.
• Benorylate is a mutual prodrug aspirin and paracetamol.
• Sultamicillin, which on hydrolysis by an esterase produces
ampicillin & sulbactum.
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Paracetamol
Aspirin
Ampicillin
Sulbactum
Benorylate/Benorilate
Sultamicillin
12
B) Bioprecursors
• Bio- precursor prodrugs produce their effects after in vivo chemical
modification of their inactive form.
• Bioprecursor prodrugs rely on oxidative or reductive activation reactions unlike
the hydrolytic activation of carrier-linked prodrugs.
• They metabolized into a new compound that may itself be active or further
metabolized to an active metabolite
13
Classification based on the site of conversion
Type I – Metabolized
Intracellularly
Type IA prodrugs
Metabolized at the cellular
targets of their therapeutic
actions
E.g., acyclovir,
cyclophosphamide, LDOPA, zidovudine
Type IB prodrugs
It converts into parent
drugs by metabolic tissues,
namely by the liver
E.g., carbamazepine,
captopril, heroin,
primidone
14
Type II – Metabolized
Extracellularly
Type IIA
In the milieu of
gastrointestinal
fluid
E.g., loperamide
oxide, sulfsalazine,
Type IIB
Within the
circulatory system
and/or other
extracellular fluid
compartments
E.g., aspirin,
fosphenytoin
Type IIC
Near
therapeutic
target/cells
E.g. ADEPT,
GDEPT
15
Functional Groups Amenable to Prodrug Design
1. Esters as prodrugs of carboxyl, hydroxyl and thiol functionalities
• Esters are the most common prodrugs used, and it is estimated
that approximately 49% of all marketed prodrugs are activated
by enzymatic hydrolysis.
• Ester prodrugs are most often used to enhance the lipophilicity,
and thus the passive membrane permeability, of water soluble
drugs by masking charged groups such as carboxylic acids and
phosphates.
• The synthesis of an ester prodrug is often straightforward. Once
in the body, the ester bond is readily hydrolysed by ubiquitous
esterases found in the blood, liver and other organs and tissues,
including
carboxyl
esterases,
acetylcholinesterases,
butyrylcholinesterases, paraoxonases and arylesterases.
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2. Carbonates and carbamates as prodrugs of
carboxyl, hydroxyl or amine functionalities:
• Carbonates and carbamates differ from esters by the
presence of an oxygen or nitrogen on both sides of the
carbonyl carbon.
• They are often enzymatically more stable than the
corresponding esters but are more susceptible to hydrolysis
than amides.
• Carbonates are derivatives of carboxylic acids and alcohols,
and carbamates are carboxylic acid and amine derivatives.
• The bioconversion of many carbonate and carbamate
prodrugs requires esterases for the formation of the parent
drug.
17
3. Amides as prodrugs of carboxylic acids and
amines
• Amides are derivatives of amine and carboxyl functionalities of a
molecule. In prodrug design, amides have been used only to a
limited extent owing to their relatively high enzymatic stability
in vivo.
• An amide bond is usually hydrolyzed by ubiquitous
carboxylesterases, peptidases or proteases. Amides are often
designed for enhanced oral absorption.
• Lipophilicity of various compounds like acid chlorides and acids
can be altered in many groups of compounds by amide
formation.
• This approach is successful to improve the stability of drug in
vivo in many of the pharmaceutical agents and gives targeted
drug delivery due to presence of enzyme amydase.
18
4. Oximes as derivatives of ketones, amidines and
guanidines
• Oximes (for example, ketoximes, amidoximes and
guanidoximes) are derivatives of ketones, amidines and
guanidines, thus providing an opportunity to modify
molecules that lack hydroxyl, amine or carboxyl
functionalities.
• Oximes are hydrolyzed by the versatile
cytochrome P450 (CYP450) enzymes.
microsomal
• Oximes, especially strongly basic amidines and guanidoximes,
can be used to enhance the membrane permeability and
absorption of a parent drug.
19
Applications of prodrugs
Pharmaceutical Applications
Masking Taste & Odor
Minimizing Pain at Site of Injection
Alteration of Drug Solubility
Enhancement of Chemical Stability
Reduction of G.I. irritation
Change of physical form of the drug
Pharmacokinetic Applications
Enhancement of bioavailability
(Lipophilicity)
Prevention of Pre-systemic Metabolism
Prolongation of duration of action
Reduction of toxicity
Site specific drug delivery
20
Pharmaceutical Applications
Masking Taste & Odor
Taste Masking:
• The undesirable taste arises due to adequate solubility and interaction
of drug with taste receptors, which can be solved by lowering the
solubility of drug or prodrug in saliva.
• Chloramphenicol, an extremely bitter drug has been derivatized to
chloramphenicol palmitate, a sparingly soluble ester.
• It possesses low aqueous solubility which makes it tasteless and later
undergoes in vivo hydrolysis to active chloramphenicol by the action of
pancreatic lipase.
Odor Masking:
• The ethyl mercaptan (tuberculostatic agent)has a boiling point of 25ºC
and a strong disagreeable odour.
• Diethyl dithio isophthalate, a prodrug of ethyl mercaptan has a higher
boiling point and is relatively odourless.
21
Minimizing Pain at Site of Injection
• Pain caused by intramuscular injection is mainly due to the
weakly acidic nature or poor aqueous solubility of drugs.
• Example, intramuscular injection of antibiotic like
clindamycin and anticonvulsant drug like phenytoin was
found painful due to poor aqueous solubility and could be
overcome by making phosphate ester prodrugs
respectively and maintaining the formulations at pH 12.
22
Alteration of Drug Solubility
• The prodrug approach can be used to increase or decrease
the solubility of a drug, depending on its ultimate use.
Example• The solubility of betamethasone in water is 58 μg/ml at
25⁰C. The solubility of its disodium phosphate ester (a
charged ester promoeity) is more than 100 mg/ml, an
increase in water solubility greater than 1500-fold.
• Acetylated sulfonamide moiety enhanced the aqueous
solubility of the poorly water-soluble sodium salt of the
COX-2 inhibitor Parecoxib ~300-fold.
23
Enhancement of Chemical Stability
• Although chemical unstability can be solved to a greater extent by
appropriate formulations, its failure necessitates the use of prodrug
approach. The prodrug approach is based on
1.
modification of the functional group responsible for the instability or
2.
by changing the physical properties of the drug resulting in the
reduction of contact between the drug and the media in which it is
unstable.
• E. g. Antineoplastic drug- Azacytidine.
• The aqueous solution of azacytidine is
readily hydrolyzed but the bisulfite prodrug
shows stability to such degradation at acidic
pH and is also more water soluble than the
parent drug.
• The prodrug gets converted to active drug
at the physiological pH
24
Reduction of G.I. irritation
• Several drugs cause irritation and damage to the gastric
mucosa through direct contact, increased stimulation of
acid secretion or through interference with protective
mucosal layer.
Drug
Prodrug
Salicylic acid
Salsalate, Aspirin
Diethyl stilbestrol
Fosfestrol
Kanamycin
Kanamycin pamoate
Phenylbutazone
N-methyl piperazine salt
Nicotinic acid
Nicotinic acid hydrazide
25
Change of physical form of the drug
• Some drugs which are in liquid form are unsuitable for formulation
as a tablet especially if their dose is high.
• The method of converting such a liquid drug into solid prodrug
involves formation of symmetrical molecules having a higher
tendency to crystallize e.g. ester of Ethyl mercaptan and trichloro
ethanol.
26
Pharmacokinetic Applications
Enhancement of bioavailability (Lipophilicity)
• Passive diffusion is the commonest pathway for transportation of
drug from site of administration to systemic circulation through a
lipoidal membrane.
• Thus improvement in the lipophilic character serves as a tool for
betterment of bioavailability. Two reasons can be attributed to the
enhanced oral bioavailability of lipophilic compound a) The lipophilic form of a drug has enhanced membrane /water
partition coefficient as compared to the hydrophilic form thus
favoring passive diffusion e.g. Bacampicillin prodrugs of Ampicillin
is more lipophilic, better absorbed and rapidly hydrolyzed to the
parent drug in blood.
b) The lipophilic prodrugs have poor solubility in gastric fluids and
thus greater stability and absorption e.g. ester of Erythromycin.
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Prevention of Pre-systemic Metabolism
• The first pass metabolism of a drug can be prevented if the
functional group susceptible to metabolism is protected
temporarily by derivatization.
• Alternatively manipulation of the drug to alter its
physicochemical properties may also alter the drug –
enzyme complex formation.
Drug
Prodrug
Propranolol
Propranolol hemisuccinate
Dopamine
L-DOPA
Morphine
Heroin
28
Prolongation of duration of action
• Drugs with short half life require frequent dosing with conventional
dosage forms to maintain adequate plasma concentration of the
particular drug.
• In plasma level time profile and consequently patient compliance is
often poor.
• Prolongation of duration of action of a drug can be accomplished by the
prodrug . Prodrug can be formed by two approaches1.
To control the release rate of prodrug.
Drug
Ester Prodrug
Testosterone
Testosterone propionate
Estradiol
Estradiol propionate
Fluphenazine
Fluphenazine deaconate
29
2.
To control the rate of conversion of prodrug into active
drug in the blood.
• This second approach of controlled conversion of prodrug to
active drug was difficult, it was successfully utilized to deliver
Pilocarpine to eyes in the treatment of glaucoma.
• The diesters of drug when applied as ophthalmic solution showed
better intra-ocular penetration due to improved lipophilicity and
slow conversion of the ester prodrug to active Pilocarpine resulted
into prolonged the therapeutic effect
30
Reduction of toxicity
• An important objective of drug design is to develop a moiety with
high activity and low toxicity
• NSAIDs local side effects like gastric distress with various, which can
be overcome by prodrug design.
• Another example is the bioprecursor Sulindac, as it is a sulphoxide,
it doesn’t cause any gastric irritation and also better absorbed.
• The prodrug Ibuterol is diisobutyrate ester of Terbutaline (a
selective β-agonist useful) in glaucoma. This prodrug, is 100 times
more potent, has longer duration of action and is free from both
local and systemic toxicity.
31
Site specific drug delivery
• After its absorption into the systemic circulation, the drug
is distributed to the various parts of the body including the
target site as well as the non-target tissue.
• These problems can be overcome by targeting the drug
specifically to its site of action by prodrug design
• The prodrug is converted into its active form only in the
target organ/tissue by utilizing either specific enzymes or
a pH value different from the normal pH for activation e.g.
5-amino salicylic acid.
32
Site specific drug delivery for cancer
• As oncostatic drugs are endowed with poor selectivity. The lack of selectivity of
anticancer drugs, and associated toxicity, hampers their effectiveness and long term
use. Hence, not surprisingly, there is an urgent need to improve their selectivity.
• prodrug technology can be used to site specific delivery of anticancer drugs.
• Anticancer prodrugs can be designed to target specific molecules (enzymes, peptide
transporters, antigens) that are overexpressed in tumor cells in comparison to normal
cells. The new promising chemotherapeutic prodrugs include:
1. Enzyme-activated
prodrugs
2. Targeting-ligand
conjugated prodrugs
• ADEPT
• GDEPT
• Antibody-drug conjugates
• Peptide-drug conjugates
33
1. Enzyme-activated prodrugs
• One approach toward improving the specificity of
chemotherapy is enzyme-activated prodrug therapy in
which a non-toxic drug is converted into a cytotoxic agents,
i.e. antimetabolites and alkylating agents.
• E.g. ADEPT, GDEPT
34
Antibody-directed enzyme prodrug therapy (ADEPT)
• The principle of ADEPT is to use an antibody directed at a tumor-associated
antigen which localizes the enzyme in the vicinity of the tumor.
• A non-toxic prodrug, a substrate for the enzyme, is then given intravenously
and converted to a cytotoxic drug only at the tumor site where the enzyme is
localized, resulting in tumor cell death.
Antibody
Prodrug
Drug
Tumor target
L6
Mitomycin C
phosphate
Mitomycin C
Lung
adenocarcinoma
BW413
Etoposide
phosphate
Etoposide
Colon carcinoma
L6
Doxorubicin
phosphate
Doxorubicin
Lung
adenocarcinoma
35
Schematic presentation of antibody-directed enzyme prodrug therapy (ADEPT).
mAb-enzyme conjugate is given first, which binds to antigens expressed on tumor
surfaces. Prodrug is given next, which is converted to active drug by the pre-targeted
enzyme.
36
Gene-directed enzyme prodrug therapy - GDEPT
• GDEPT, is a two-step process. In the first step, the gene for a foreign
enzyme is delivered to tumor cells. In the second step, a non-toxic agent is
administered systematically and converted by the enzyme to its cytotoxic
metabolite.
Enzyme
Prodrug
Drug
Cytochrome
p450
Cyclophosphamide,
ifosfamide
Phosphamide
mustard, acrolein
Cytosine
deaminase
5-Fluorocytosine
5-Fluorouridine
5-Fluorouracyl
Nitroreductase
5-(Aziridin-1-yl)-2,4- 5-(Aziridin-1-yl)-4dinitrobenzamide
hydroxylamino-2nitrobenzamide
37
Schematic presentation of gene-directed enzyme prodrug therapy (GDEPT).
Gene for foreign enzyme is transfected to tumor cells, which express the enzyme to
activate the systemically administered prodrug
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2. Targeting-ligand conjugated prodrugs
• Antibody-drug conjugates:
• Tumor-specific monoclonal antibodies (or fragments of antibodies)
are conjugated to oncostatic drugs such as antifolates, anthracyclines,
taxanes and vinca alkaloids.
• The antibody delivers the therapeutic agent to tumor cells. After
reaching its target, the conjugate is internalized through a receptormediated pinocytosis, and the pharmacologically active compound is
released in the cell.
• Peptide-drug conjugates:
• Peptide-conjugated prodrugs for cancer therapy utilize peptide
ligands designed to bind with a tumor specific antigen or a peptide
transporter which is overexpressed in neoplastic cells.
• These ligands are conjugated to a chemotherapic agent either directly
or by a linker.
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Marketed Prodrugs
Fenofibrate
Fosphenytoin
Rabeprazole
40
Limitations of Prodrug Design
• Formation of unexpected metabolite from the total
prodrug that may be toxic.
• The inert carrier generated following cleavage of prodrug
may also transform into a toxic metabolite.
• During its activation stage, the prodrug might consume a
vital cell constituent leading to its depletion.
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CONCLUSION
Prodrug design is a part of the general drug discovery
process, in which a unique combination of therapeutically
active substances is observed to have desirable
pharmacological effects.
In human therapy prodrug designing has given successful
results in overcoming undesirable properties like
absorption, nonspecificity, and poor bioavailability and GI
toxicity.
Thus, prodrug approach offers a wide range of options in
drug design and delivery for improving the clinical and
therapeutic effectiveness of drug.
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