Prodrug Strategy

(Concept & Applications)

Presented By: Anvita Jadhav

M.Pharm(Pharmaceutics)

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 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.
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Schematic illustration of the prodrug concept
Pharmacokinetic or
Physicochemical
barrier
Site of Action
Extracellular Fluid (cell or cell surface)

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History and the Present of Prodrug Design

2009
1960 15% of the
An explosive 100 best
1958 increase in selling drugs
the use of were
Adrien Albert Prodrugs
First prodrugs in
1935 introduced drug
Protonsil the term discovery and
“pro-drug” development.
Antibiotic
1899
Methenamine
First
intentional
Prodrug
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 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

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Properties of ideal prodrug

• Pharmacological Inertness
1.
• Rapid transformation, chemically or
enzymatically, into the active form at the target
2. site

• Non-toxic metabolic fragments followed by

their rapid elimination
3.

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 Classification of Prodrugs

Bipartite
prodrug

Carrier linked Tripartite

prodrug prodrug
Prodrugs
Mutual
Bioprecursors
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
Chemical/Enzymatic cleavage
Inert Carrier in vivo

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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 prodrug-
The carrier group is attached via linker to drug.

Linking
Drug Structure Carrier

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 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

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 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

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Classification based on the site of conversion
Type I – Metabolized
Intracellularly
Type IA prodrugs Type IB prodrugs
Metabolized at the cellular It converts into parent
targets of their therapeutic drugs by metabolic tissues,
actions namely by the liver
E.g., acyclovir, E.g., carbamazepine,
cyclophosphamide, L- captopril, heroin,
DOPA, zidovudine primidone

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 Type II – Metabolized
Extracellularly
Type IIB
Type IIA Type IIC
Within the
In the milieu of circulatory system Near
gastrointestinal and/or other therapeutic
fluid extracellular fluid target/cells
E.g., loperamide compartments
E.g. ADEPT,
oxide, sulfsalazine, E.g., aspirin, GDEPT
fosphenytoin

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 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.

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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.

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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 microsomal

cytochrome P450 (CYP450) enzymes.

• Oximes, especially strongly basic amidines and guanidoximes,

can be used to enhance the membrane permeability and
absorption of a parent drug.

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 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
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 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.
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 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.

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 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.

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 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

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 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.

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 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

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 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 approaches-
1. To control the release rate of prodrug.

Drug Ester Prodrug

Testosterone Testosterone propionate
Estradiol Estradiol propionate
Fluphenazine Fluphenazine deaconate

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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

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 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.

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 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.

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 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 2. Targeting-ligand
prodrugs conjugated prodrugs
• ADEPT • Antibody-drug conjugates
• GDEPT • Peptide-drug conjugates

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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

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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 Mitomycin C Lung

phosphate adenocarcinoma
BW413 Etoposide Etoposide Colon carcinoma
phosphate
L6 Doxorubicin Doxorubicin Lung
phosphate adenocarcinoma

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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 Cyclophosphamide, Phosphamide
p450 ifosfamide mustard, acrolein
Cytosine 5-Fluorocytosine 5-Fluorouracyl
deaminase 5-Fluorouridine
Nitroreductase 5-(Aziridin-1-yl)-2,4- 5-(Aziridin-1-yl)-4-
dinitrobenzamide hydroxylamino-2-
nitrobenzamide

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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 38
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 receptor-
mediated 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

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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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