The presentation provides the brief description about bioiprecursor prodrugs and site specific drug delivery approach.

1. PRODRUGSWith Special Reference to
(Bioprecursors and Site Specific
Drug Delivery)
Presented by:-
Sobhi Gaba

2. PRODRUGS
• The reversibly modified compound usually inactive in
itself and releases the active compound as a
metabolite is called a PRODRUG
OR
• Prodrug is defined as biologically inactive derivative
of a parent drug molecules that usually requires a
chemical or enzymatic transformation within the
body to release the active drug and possess improved
delivery properties over parent molecule

3. OR
• A prodrug is a medication or compound that after
administration is metabolised (in body) into
pharmacologically active drug.
PRODRUG ACTIVE DRUG MOEITY
For Example:-
Sulfasalazine Amino Salicylic Acid+
Sulfapyridine
CHEMICAL OR
ENZYMATIC ATTACK

4. CLASSIFICATION OF PRODRUGS
ON THE BASIS OF STRUCTURAL ASSOCIATION
PRODRUG
CARRIER LINKED PRODRUGS
BIPARTITE
PRODRUGS
TRIPARTITE
PRODRUGS
POLYMERIC
PRODRUGS
MUTUAL PRODRUGS
BIOPRECURSORS

5. ON THE BASIS OF SITES OF CONVERSION INTO ACTIVE DRUG FORM
PRODRUG
TYPE 1
TYPE 1A TYPE 1B
TYPE 2
TYPE 2A TYPE 2B

6. BIOPRECURSORS
• A Bioprecursor prodrug is a prodrug that does not
imply the linkage to a carrier group, but results from
a molecular modification of the active principle itself.
• This modification generates new compound, able to
be transformed metabolically or chemically, the
resulting compound being the active principle.
• These types of prodrugs not only need hydrolysis to
get converted to its active compound bound but also
requires oxidation or reduction reactions(PHASE 1
Metabolism Reaction)

7. ORIGIN OF BIOPRECURSORS
• The birth of bioprecursor prodrugs occurred when it was
demonstrated that the antibacterial agent “PRONTOSIL” was active
only in vivo because it was metabolised to the actual drug
”SULFANILAMIDE”
PRONTOSIL(Azo Prodrug) SULFANILAMIDE(Amine
sulfa drug)

8. ACTIVATION OF BIOPRECURSOR
PRODRUGS
• The bioprecursor prodrug show their action after undergoing
metabolism reaction in the body (in vivo). The activation of these
prodrugs can be by following pathways:-
I. PROTON ACTIVATION
II. ELIMINATION ACTIVATION
III. OXIDATIVE ACTIVATION
IV. REDUCTIVE ACTIVATION
V. PHOSPHORYLATION ACTIVATION
VI. DECARBOXYLATION ACTIVATION

9. I. PROTON ACTIVATION
• Some bioprecursors activates in the body through
proton activation mechanism.
• For example: OMEPERAZOLE (Proton Pump Inhibitor)
• It is used in the treatment of gastric ulceration.

10. PROTON ACTIVATION OF OMEPERAZOLE
CYSTEINE RESIDUE
BLOCKS PROTON
PUMP

11. OMEPERAZOLE(WEAK BASE WITH pka=4)
DONOT GET ACTIVATED IN GASTRIC PH
SO IS LIPID MEMBRANE PERMEABLE AND ABLE TO DIFFUSE INTO SECRETORY CANALICUS OF PARIETAL
CELLS (pH= 1)
MECHANISM OF ACTION OF OMEPERAZOLE

12. PROTON PUMP IS BLOCKED AND THUS GASTRIC ACID SECRETION
IS INHIBITED

13. II. ELIMINATION ACTIVATION
• LEFLUNAMIDE METABOLITE
(Used in treatment
of rheumatoid arthritis)
• Leflunamide is not itself active
• It undergoes elimination and gets converted to its
metabolite and show antiarthritic activity.
• The metabolite inhibit the synthesis of pyrimidine.

14. MECHANISM OF ACTION OF
LEFLUNAMIDE
LEFLUNAMIDE

15. (INACTIVE)
(ACTIVE) SHOWS
ANTI ARTHRITIC
ACTIVITY
MECHANISM OF ELIMINATION
REACTION OF LEFLUNAMIDE

16. III. OXIDATIVE ACTIVATION
• The most common pathway for the action of several
bioprecusors is oxidation. The various types of
oxidation occurring in the compounds are as follows:-
1. N and O Dealkylation
2. Oxidative Deamination
3. N-Oxidation
4. S-Oxidation

17. 1. N AND O DEALKYLATION
• N DEALKYLATION:-
Open ring analogues of benzodiazepines such as anxiolytic drug
“ALPRAZOLAM” and sedative “TRIAZOLAM”, undergoes metabolic N-
dealkylation and spontaneous cyclization.

18. MECHANISM OF ACTION OF ALPRAZOLAM

19. •O-DEALKYLATION:-
•The example of bioprecursor prodrug that is activated
by O-dealkylation is an analgesic drug i.e.
“PHENACITIN”
•PHENACITIN ACETAMINOPHEN
(Has more affinity
to bind with COX2)
O DEALKYLATION

20. 2. OXIDATIVE DEAMINATION
• Various antitumour agents gets activated by this
mechanism
• These anti tumour compounds donot act only by hydrolysis
but they need activation by liver enzymes. For example:-
CYCLOPHOPHAMIDE (Nitrogen Mustard).
• The liver enzymes CYP450 cause deamination of
cyclophosphamide and then phosphoramidase enzyme in
tumour cells helps the intermediate formed to bind with
N7 guanine molecule of DNA of tumour cells and thus
causing death of tumour cells.

21. MECHANISM OF OXIDATIVE DEAMINATION
IN CYCLOPHOPHAMIDE

22. 3. N OXIDATION
• For example:- PRALIDOXIME (antidote for poisoning by
organophosphorous compounds)
BIOPRECURSOR PRODRUG ACTIVE FORM

23. MECHANISM OF ACTION OF
PRALIDOXIME
• ACETYLCHOLINESTERASE CATALYZED HYDROLYSIS OF ACETYL CHOLINE

24. PHOSPHORYLATION OF ACETYLCHOLINESTERASE BY
DIISOPROPYL
PHOSPHOROFLUORIDATE(ORGANOPHOSPHOROUS
COMPOUND)
Acetylcholinesterase gets blocked by organophosphorous
compound thus increasing the concentration of acetyl
choline and high cholinergic activity is observed

25. REACTIVATION OF PHOPHORYLATED ACETYL
CHOLINESTERASE BY PRALIDOXIME CHLORIDE
PRALIDOXIME is not able to cross BBB but PLARIDOXIME CHLORIDE can.
Pralidoxime chloride after crossing the BBB gets converted to Pralidoxime through
N Oxidation which further acts with phosphorous to treat organophophorous
poisoning

26. 4. S OXIDATION
• BREFELDIN A is an antitumour and antiviral antibiotic, has poor
bioavailability and is rapidly cleared.
• A series of sulphide prodrugs were prepared having greater aqueous
solubilities than BREFELDIN A.
• The prodrugs were converted back to BREFELDIN A by S Oxidation to
the sulfoxide.

27. IV. REDUCTIVE ACTIVATION
• Bioprecursors can be activated through reduction too.
• The functional groups which undergoes reduction for
activation are as follows:-
1. AZO REDUCTION
2. SULFOXIDE REDUCTION
3. DISULFIDE REDUCTION

28. 1. AZO REDUCTION
• The most common example for azo reduction is PRONTOSIL from
where the discovery of bioprecursor prodrug occurred.
• Prontosil is not active but its metabolite “SULFANILAMIDE” has
antibacterial activity
• SULFANILAMIDE
• (ANTIBACTERIAL ACTION)

29. • Another example of azo reduction is the reduction of SULFASALAZINE
to active drug SULGAPYRIDINE and 7-amino salicylic acid.
• SULFASALAZINE is itself inactive as it has azo group and not free
amino group which binds with the enzyme DIHYDROFOLATE
REDUCTASE and inhibits FOLIC ACID synthesis in bacterial cell but on
the other hand the metabolite of sulfasalazine has free amino group.

30. 2. SULFOXIDE REDUCTION
• The most common example of sulfoxide reduction is SULINDAC
• SULINDAC is a Non Steroidal Antinflammatory Drug (NSAID) and is
used in treatment of arthritis.
• The sulfoxide group present in SULINDAC doesnot allow the drug to
bind with COX2.
• The drug on reduction gets converted to its corresponding sulphide
which is active.

31. 3. DISULFIDE REDUCTION
• To diminish the toxicity of the antimalarial drug PRIMAQUINE and
target it for cells that contain the malarial parasite, a macromolecular
drug delivery system was designed.
• The LACTOSE LINKED ALBUMIN was used for improved intake in the
liver via ASILAOGLYCOPROTEIN RECEPTOR SYSTEM

32. V. PHOPHORYLATION ACTIVATION
• The drug undergoing phosphorylation activation is ACYCLOVIR.
• It is highly effective against genital HERPES SIMPLEX VIRUS and
VARICELLA ZOSTER VIRUS infection.

33. Acyclovir itself is inactive but it is selectively phosphorylated by a viral
thymidine kinase to corresponding monophosphate (uninfected cells do not
phosphorylate acyclovir and this accounts for the selective toxicity towards
viral cells.
The conversion of monophosphate to diphosphate catalysed by guanylate kinase.
The conversion of diphosphate to triphosphate ( catalysed by phosphoglycerate
kinase)
Acyclovir triphosphate is a substrate for the viral alpha DNA polymerase but
not for human cellular alpha DNA polymerase
Incorporation of acyclovir triphosphate into the viral DNA leads to the
formation of dead and complex and result in viral cell death.

34. VI.DECARBOXYLATION ACTIVATION
• The removal of carboxylic group group from the drug lead to an active
metabolite.
• For example:- LEVODOPA AND DOPAMINE
• Levodopa is a bioprecursor produg for dopamine
• In Parkinson disease, the concentration of dopamine is decreased,
which can be treated by increasing level of DOPAMINE
• DOPAMINE is impermeable to BBB and will have no effect.
• So LEVODOPA (bioprecursor prodrug) is used which has high
permeability into brain through BBB.
• After entering into brain it gets converted to DOPAMINE and thus
PARKINSON DISEASE can be treated.

35. MECHANISM OF ACTION OF LEVODOPA

36. SITE SPECIFIC DRUG DELIVERY
MECHANISMS
• Several strategies, under the rubric of ENZYME PRODRUG THERAPIES, have
been developed to achieve selective activation of prodrugs at a desired
site, typically in TUMUOR CELLS.
• COMMON PROCEDURE FOR THIS APPROACH:-
STEP 1- A prodrug activating enzyme is incorporated into the target tumour
cells
STEP 2- A non toxic prodrug, which is a substrate of the exogenous enzyme
that was incorporated into the tumour cells, is administerd systemically.
STEP 3- The prodrug is actively converted into the the active anticancer drug
in high local concentration inside the tumour cells

37. DIFFERENT APPROACHES
•The different approaches used for site specific drug
delivery into tumour cells are as follows:-
1. ADEPT (ANTIBODY DIRECTED ENZYME PRODRUG
THERAPY)
2. GDEPT (GENE DIRECTED ENZYME PRODRUG
THERAPY)
3. VDEPT (VIRUS DIRECTED ENZYME PRODRUG
THERAPY)

38. CERTAIN CRITERIAS FOR THESE APPROACHES
TO BE EFFECTIVE
I. The prodrug-activating enzyme should be either of non
human origin or a human protein that is absent or
expressed only at lower concentrations in normal tissues.
II. The prodrug-activating enzyme must achieve adequate
expression in the targeted tumour cells and have high
catalytic activity
III. The prodrug should be a good substrate for the enzyme
incorporated in the tumours but nit be activated by
endogeneous enzymes outside the tumours

39. IV. The prodrug must be able to cross the tumour cell
membrane for intracellular activation.
V. The cytotoxicity difference between the prodrug and its
corresponding active drug should be high.
VI. The activated drug should be highly diffusible or be actively
taken up by adjacent non expressing cancer cells (BYSTANDER
KILLER EFFECT i.e. the ability of the drug to kill neighbouring
non expressing cells).
VII. The half life of the active drug should be long enough to
induce a BYSTANDER KILLING EFFECT but short enough to
avoid the drug leaking out of tumour cells and causing
damage elsewhere.

40. ADEPT (ANTIBODY DIRECTED ENZYME
PRODRUG THERAPY)
STEP 1:- AN ANTIBODY RAISED AGAINST A PARTICULAR TUMOUR CELL
LINE IS CONJUGATED WITH THE ENZYME(NEEDED FOR ACTIVATION OF
PRODRUG)
ADMINISTERED THIS CONJUGATE IN VIVO AND ALLOWED IT TO ACCUMULATE ON TUMOUR CELL
EXCESS CONJUGATE NOT BOUND IS GIVEN TIME TO CLEAR FROM THE BLOOD AND NORMAL
TISSUES

41. PRODRUG IS ADMINISTERED
THE ENZYME CONJUGATED WITH THE ANTIBODY AT THE TUMOUR
CELL SURFACE CATALYSES THE CONVERSION OF PRODRUG TO ACTIVE
DRUF WHEN IT REACHES THE TUMOUR CELLS
TUMOUR CELLS ARE KILLED SELECTIVELY

42. MECHANISM OF ACTION OF
ADEPT

43. EXAMPLE OF ADEPT
NITROGEN MUSTARD ACTIVATION BY CARBOXYPEPTIDASE G2
(BACTERIAL ENZYME) FOR USE CONJUGATED WITH
HUMANIZED MONOCLONAL ANTIBODIES

44. ADVANTAGES AND DISADVANTAGES OF ADEPT
• ADVANTAGE
1. INCREASED SELECTIVITY OF FOR THE RELEASE OF HIGH
CONCENTRATION OF DRUG AT TARGETED CELLS.
(Advantage is only evident if enough time is allowed for the clearance
of antibdody enzyme conjugate that is not bound to tumor cells.
• DISADVANTAGE:-
1. REJECTION OF ANTIBODY ENZYME CONJUGATE
2. THE POTENTIAL FOR LEAKBACK OF THE ACTIVE DRUG
3. REQUIREMENT OF I.V ADMINISTRATION

45. GDEPT (GENE DIRECTED ENZYME
PRODRUG THERAPY)
• Also called SUICIDE GENE THERAPY
A GENE ENCODING THE PRODRUG ACTIVATING ENZYME IS INTEGRATED INTO THWEE GENOME
OF THE TUMOR CELLS UNDER THE CONTROL OF TUMOR SELECTIVE PROMOTORS OF VIRAL
TRANSFECTION
THESE CELLS, THEN EXPRESS THE ENZYME THAT ACTIVATES THE PRODRUG
PRODRUG CONVERTS TO ACTIVE FORM AND KILL TUMOR CELLS SELECTIVELY

46. MECHANISM OF ACTION OF
VDEPT