Prodrug

1. Prodrugs
Basic concept, Prodrugs of functional group, Prodrugs to
improve patient acceptability, Drug solubility, Drug
absorption and distribution, site specific drug delivery and
sustained drug action. Rationale of prodrug design and
practical consideration of prodrug design.

2. BARRIERS TO THE THERAPEUTIC UTILITY OF A DRUG
Drug
Drug in sol. at
admin site
Dosage form
Manufacture
Release
Pharmaceutical phase
Pharmacokinetic phase
Elimination
Drug in target
organ
Drug in blood
Drug in various
body
compartments
Absorption
Elimination
Distribution
Elimination
Transport
Pharmacodynamic phase

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
3

4. VARIOUS APPROACHES TO ENHANCE THE EFFICACY OF A DRUG:
The therapeutic efficacy can be improved by minimizing or eliminating the
undesirable properties while retaining the desirable ones.
 Physical approach
 Chemical approach

5. CHEMICAL MEANS OF OPTIMIZING THE DRUG THERAPEUTICS
1. Design and development of new drugs.
2. Design of hard and soft drugs.
3. Design of prodrugs.

6. HARD DRUGS
 Resistant to biotransformation
 Has a long half life
 Design of hard drug involves metabolic stabilization
ADVANTAGES:
 Enhanced duration of action
 Avoids generation of potentially active harmful metabolites
HOWEVER, less readily eliminated due to lack of metabolism
Ex: Conversion of tolbutamide to chlorpropamide (Hypoglycemic urea's)

7. SOFT DRUGS
A soft drug is a biologically active compound that is biotransformed in vivo in a
rapid and predictable manner in to non toxic metabolites.
Design of synthetic soft drug involves introduction of a group or a bond
susceptible to rapid metabolic action.
Ex: Natural endogenous compounds such as adrenalin and insulin.

8. SOFT DRUGS
• Short duration of action prevents possibility of toxicity and increases
therapeutic index
N
NO
O
N
NHO
O
N+
O
O
O
O
N+
Cl-
Cl-
COOH
COOH
Etomidate (hypnotic) metabolite
Succinylcholine (neuromuscular blocking agent)
HO +N(CH3)3
+

9. Prodrugs
Prodrugs are pharmacologically inactive derivatives of active drugs that are
designed to maximize the amount of active drug that reaches its site of action,
through manipulation of physicochemical, biopharmaceutical and
pharmacokinetic properties of drug.
They are converted into active drug within the body through enzymatic or non-
enzymatic reactions. Also called drug latentiation.

10. “Drug Latentiation”
Process of purposely designing and synthesizing a
molecule that specifically requires “bioactivation”
to a pharmacologically active substance

11. Classification of Prodrugs
Prodrugs
Carrier linked
prodrug
Bipartite
prodrug
Tripartite
prodrug
Mutual
Prodrugs
Bioprecursors
11

12. CLASSIFICATION OF PRODRUGS:
1.Carrier linked prodrugs:
Active drug is covalently linked to an inert carrier or transporter
moiety. They have enhanced lipophilicity due to attached carrier. The active
drug is released by hydrolytic cleavage, either chemically or enzymatically.
2.Bioprecursors:
These are inert molecules obtained by chemical modification of
the drug but do not contain a carrier. Such a molecule has the same
lipophilicity as the parent drug and is bioactivated generally by redox
biotransformation, only enzymatically. Ex: NSAID – nabumetone (relafen) -
arthritis.
CH3
O
CH3
O
OH
CH3O
O
Series of oxidative
decarboxylation
Active form of the drug
that inhibits Prostaglandin
biosynthesis by
cyclooxygenase

13. 3. Mutual prodrugs:
The prodrug comprises of two pharmacologically active agents coupled
together to form a single molecule such that each acts as the carrier for the
other.
Ex: Benorylate is mutual prodrug of NSAIDs aspirin and
paracetamol.
Ex: Emcyt is a mutual prodrug containing estramustine and
nornitrogen mustard linked to each other.
Actual alkylating species
CH3
OH
OH
NH
Cl
Cl
NH+Cl-
Cl
Nornitrogen mustard
Aziridine
Sodium phosphate
and
Carbon dioxide
CH3
OPO3
Na2
ON
O
Cl
Cl Estermustine Sodium Phosphate
Emcyt® - Pharmacia & Upjohn

14. 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 (NSAID).
It can be further subdivided into
TolmetinGlycine
14

15. 2. Tripartite prodrug-
Drug Linking
Structure Carrier
The carrier group is attached via linker to drug.
15

16. 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.
16

17. AspirinParacetamol
Sulbactum
Ampicillin
Benorylate/Benorilate Sultamicillin
17

18. 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
18

19. Strategies for the design of prodrugs:
1. Carriers:
 Carrier is an inert molecule or the promoiety attached to the active drug
moiety through a metabolically labile linkage.
 The carrier imparts some desirable property to the drug such as
increased lipid or water solubility.
 Carriers that help in directing the active moiety to the target site is called
as specifier.
Specifiers:
Targeting unit part of the prodrug which directs the active moiety to the target
site.
 Antibody Directed Enzyme Prodrug Therapy
 Gene Directed Enzyme Prodrug Therapy
 Polymer Directed Enzyme Prodrug Therapy/ Macromolecule Directed
Prodrug Therapy

20. linkers:
A releasable linker or spacer is incorporated between the specifier/carrier and the
parent drug.
Reasons for the application of linkers:
1. Incorporation of appropriate linkage between the promoiety and the active drug.
2. Facilitation of enzymatic action on carrier linked prodrugs.
Classification of linkers:
• Electronic cascade linkers (Cleavage occurs by mesomeric effect)
• Cyclization linkers (cleavage occurs by cyclisation)
Doxorubicin (anticancer) attached with β-glucuronide was inert towards clevage
by β-glucuronidase

22. Multiple drug release:
Simple Drug-Linker-specifier:

23. Functional groups amenable to prodrug design

24. Prodrug linkage and enzymes involved in the hydrolysis of linkage:
Prodrug linkage Hydrolyzing enzymes
Ester Esterase
Short and medium chain Cholinesterases
Aliphatic Ester hydrolase , Lipase, Cholesterol
Esterase, Acetylcholinesterase,
Aldehyde oxidase, Carboxypeptidase
Long chain aliphatic carbonate Pancreatic lipase,Pancreatin,Lipase,
Carboxypeptidase, Cholinesterase
Phosphate, Organic Acid phosphatase, Alkaline
Phosphatase
Sulfate, organic Steroid sulfatase

25. Prodrug Linkage Hydrolyzing Enzyme
Amide Amidase
Amino acid Proteolytic enzymes, Trypsin,
Carboxypeptidase A and B,
Azo Azo reductase
Carbamate Carbamidase
Phosphamide Phosphoramidases
β-Glucuronide β-Glucuronidase
N-Acetylglucosaminide α- N-Acetylglucosaminidase
β-Glucoside β-Glucosidase

26. APPLICATIONS OF PRODRUGS:
1.Prodrugs to improve patient acceptability
2.Enhancing Drug solubility
3. Enhancing Drug absorption and distribution
4.Site specific drug delivery
5. Sustained drug action

27. 1. Pharmaceutical applications/Prodrugs to improve patient acceptability :
a).Improvement of taste:
 Bitter taste of the drug
 Unsuitable for preparation of suspension.
 Reduce the solubility of the drug in the saliva.
PARENT DRUG PRODRUG WITH IMPROVED TASTE
chloramphenicol Palmitate ester
clindamycin Palmitate ester
sulfisoxazole Diacetate ester
Lipase
O2N CH
CH3
CH
NH
O
CHCl2
CH2 OHO2N CH
CH3
CH
NH
O
CH3
CH2OC
O
(CH2)14CH3
Chloramphenicol palmitate chloramphenicol

28. b) Improvement of odour:
•The odor of the compound depends upon its vapor pressure.
Ex: ethyl mercaptan is a foul smelling liquid of b.p. 35 C. It is converted into
its phthalate ester, diethyldithio-isophthalate ester which is odorless and
has higher boiling point.(Used in the treatment ofTuberculosis)
thioesterase
Diethyldithio-isophthalate ethyl mercaptan
CO SC2H5
C
O
SC2H5
CH3CH2SH

29. c). Reduction of gastric irritation:
• Increased stimulation of acid secretion.
• Interference with the protective mucosal layer.
parent drug prodrugs
Salicylic acid Aspirin
Diethyl stilbestrol Fosfestrol
Phenyl butazone N-methyl piperazine salt
Oleandrin Oleandrine acetate
d). Reduction of pain on injection
• Drug precipitates or penetrates into the surrounding tissues.
• The solution is strongly acidic, alkaline or alcoholic.
Clindamycin-2’-phosphate ester Phosphatase Clindamycin

30. 2.Enhancement of solubility and dissolution rate:
 Dissolution is the rate limiting step in the absorption of drug.
 Parenteral and ophthalmic formulations of such agents required.
Parent drug Prodrugs with enhanced
hydrophilicity
Chloramphenicol Sodium succinate ester
Tocopherols Sodium succinate ester
Testosterone Phosphate ester
Diazepam L-lysine ester

31. 1. Prodrugs for Increased Water Solubility
O
HO OH
O
O
OHO
O
methylprednisolone
succinate ester
esterases
O
HO OH
O
OH
H H
H
R
R = H prednisolone (corticosteroid)
R = CH3 methyl prednisolone
a.
b.
N
H
O
OR
H3N+
O
amidases
H2N
O
O
amino acid amide of benzocaine Benzocaine (local anesthetic)

32. • 3. Prodrugs for improved Absorption and Distribution
• Ampicillin when administered orally only about 40% of
dose is absorbed. Therefore, ampicillin when presented
in the form of its esters has increased absorption.
• Eg: Pivampicilline and Becampicillin
S
N
O
N
H
O
NH2
COOH
N
S
O
O
H
HN
O
NH2
O O
O
O
becampicillin

33. Prodrugs for improved Absorption and Distribution
a.
HO
HO
OH
H
N
NH
OH
O
O
O
O
Epinephrine (anti-glaucoma)Dipivefrin (with better corneal penetration
esterase
4. Prodrugs for Site Specificity
2-O3PO
OPO3
2-
HO
OH
phosphatase
a.
diethlystilbestrol diphosphate
Diethlystilbestrol
(breast cancer treatment)

34. N
ODRUG
O
CH3
N
ODRUG
O
CH3
N+
ODRUG
O
CH3
N+
HO
O
CH3
+DRUG
b.
N
S
O
O
HH
N
R
O
O
N
O
O
CH3
-lactam antibiotic delivery to brain
In the treatment of meningitis
BBB
oxidation
hydrolysis

35. O NH
OH
O NH
O O
O
OH
O
O
-
O
O
DRUG
O
O
O
4. Prodrugs for Stability (first-pass metabolism)
propranolol (antihypertensive)
glucuronidation
propranolol O-glucuronide
propranolol hemisuccinate
Drug OH
+
a. O NH
O
HOOC
OH
OH
OH
O

36. O
N
O
F
Cl
O
OH
N
O
F
Cl
S
N
N
N
O
F
F
F
(CH2)5CH3
O
S
N
N
N
OH
F
F
F
5. Prodrugs for Slow and Prolonged Release
haloperidol decanoate (activity ~ 1 month i.m.)
haloperidol (tranquilizer)
(peak plasma ~ 2-6 hr oral)
fluphenazine enanthate
(duration of activity ~ 1 month)
fluphenazine (antipsychotic)
(duration of activity 6-8 hrs)
a.
b.

37. O O
OHO
O
O
O
O
NR1R2
6. Prodrugs to minimize toxicity
aspirin (anti-inflammatory)ester derivative of aspirin (without gastric irritation)
a.
clindamycin (R = H)
clindamycin phosphate (R = PO3H2)
clindamycin palmitate (R = CO(CH2)14CH3)

38. 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
38

39. TRIPARTATE PRODRUGS
• The carrier is not linked directly to the drug but instead through a linker
• Allows for decreased steric hindrance during enzymatic cleavage that may
occur with bipartate prodrugs
• Carrier is enzymatically cleaved from Linker
• Linker spontaneously cleaves from Drug
CARRIER DRUG CARRIER
Inactive Prodrug inactiveNontoxic
+
Enzyme
Linker DRUGLinker
DRUGLinker + spontaneous

40. DRUG
O
O O CARRIER
O
DRUG
O
O O-
CARRIER
O
HO
+esterases
DRUG
O
OH
+
H2CO
TRI PARTATE PRODRUGS - Doubl e Pr odr ugs

41. N
S
O
O
H
HN
O
NH2
O
O
O
N
S
O
O
H
HN
O
NH2
O O
O
O
N
S
O
OH
H
HN
O
NH2
O
ampicillin (antibiotic)
bacampicillin pivampicillin
a.
Examples of Carrier-linked Tripartate Prodrugs

42. MUTUAL PRODRUGS
• Useful when 2 synergistic drugs need to be administered at the same site
at the same time
• Mutual prodrug is bipartate or tripartate where a synergistic drug acts as
the carrier
O
O
N
OH
Cl
Cl
NH
Cl
Cl
HO
OH
estramustine (prostatic cancer treatment)
normustard
estradiol

43. • Used for metastatic carcinoma of the prostate
• Promoiety also a drug!
• 17-alphaestradiol slow prostate cell growth
• Nornitrogen mustard is a weak alkylating agent

44. BIOPRECURSOR PRODRUGS
Bioprecursor prodrugs rely on oxidative or reductive activation reaction
unlike the hydrolytic activation of carrier-linked prodrugs
Metabolic Activation of Bioprecursor Prodrugs:
1. Oxidative Activation
• N- and O-Dealkylation
• Oxidative Deamination
• N-Oxidation
• Epoxidation
2. Reductive Activation
• Azo Reduction
• Sulfoxide Reduction
• Disulfide Reduction
• Bioreductive Alkylation
• Nitro Reduction
3. Nucleotide Activation
4. Phosphorylation Activation
5. Decarboxylation Activation

45. 1. Oxidative Activation
a. N- Dealkylation
O
N
Cl
N
N
X
N
CH3
CH3
O
N
Cl
N
N
X
NH2
N
N
X
NN
X
b. O- Dealkylation
OHN
O
C2H5
HN
O
OH
phenacetin
acetaminophen
benzodiazepine (anxiolytic & sedative)
P450
P450
alprazolam (x = H)
triazolam (x = Cl)

46. N
P O
N
H
O
Cl
Cl
HO
N
P O
H2N
O
Cl
Cl
O
H
H
O
H
N
P O
H2N
OH
Cl
Cl
HN
Cl
Cl
H3PO4+NH3
N
P O
N
H
O
Cl
Cl
P450
c. Oxidative deamination
+
cyclophosphamide
acrolein
(toxic)
nitrogen mustard

47. N
N
OH
H
H
N+
N
OH
Cl-
5, 6 dihydropyridine
bioprecursor
d. N-Oxidation
pralidoxime
(acetylcholine esterase activator)
e. Epoxidation
N
O NH2
N
O NH2
O
carbamazepine (anticonvulsant) carbamazepine 10, 11-oxide

48. 2. Reductive Activation
a. Azo reduction
N S NH2
O
O
N
NH2
H2N H2N S NH2
O
O
prontosil sulfanilamide
OH
O
F
S
OH
O
F
S
O
b. Sulfoxide reduction
Sulindac (NSAID) sulfide derivative

49. N
N
N
S
NH2
O
S
O
OH
c. Disulfide reduction
thiamine tetrahydrofurfuryl disulfide
Thiamine B1
3. Nucleotide Activation
SH
N
N
N
N
H
SH
N
N
N
N
O
HO OH
2-O3PO
6-mercaptopurine
inhibits purine synthesis
HO
H2N
N+
S
N
N
Cl-

50. NH2HO
HO
COOH
HN
N N
N
H2N
O
O
OH
HO
HN
N N
N
H2N
O
O
4-
O9P3O
OH
4. Phosphorylation Activation
ganciclovir
substrate for DNA polymerase
5. Decarboxylation Activation
+ CO2
levodopa dopamine
NH2HO
HO