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 AdrianAlbert in 1958 to describe compounds that undergobiotransformation prior to eliciting their pharmacologicaleffect.

• A prodrug is defined as a biologically inactive derivative ofa parent drug molecule that usually requires a chemical orenzymatic transformation within the body to release theactive drug, and possess improved delivery properties overthe parent molecule.

• The development of prodrugs is now well established as astrategy to improve the physicochemical,biopharmaceutical or pharmacokinetic properties ofpharmacologically potent compounds, and therebyincrease usefulness of a potential drug.

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Schematic illustration of the prodrug concept

Extracellular Fluid

Site of Action(cell or cell surface)

Pharmacokinetic or Physicochemical barrier

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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

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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

1.• Pharmacological Inertness

2.

• Rapid transformation, chemically or enzymatically, into the active form at the target site

3.• Non-toxic metabolic fragments followed by

their rapid elimination

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Classification of Prodrugs

Prodrugs

Carrier linked prodrug

Bipartite prodrug

Tripartite prodrug

Mutual Prodrugs

Bioprecursors

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Active Drug

Inert Carrier

A) Carrier linked prodrug

Chemical Prodrug Formation

Chemical/Enzymatic cleavage in vivo

Covalent Bond

Carrier linked prodrug consists of the attachment of acarrier group to the active drug to alter its physicochemicalproperties.

The subsequent enzymatic or non-enzymatic mechanismreleases the active drug moiety.

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1. Bipartite prodrug

• It is composed of one carrier (group) attached tothe drugs.

• Such prodrugs have greatly modifiedlipophilicity due to the attached carrier. Theactive drug is released by hydrolytic cleavageeither chemically or enzymatically.

• E.g. Tolmetin-glycine prodrug.

It can be further subdivided into

TolmetinGlycine

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2. Tripartite prodrug-

Drug Linking Structure Carrier

The carrier group is attached via linker to drug.

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3. Mutual Prodrugs

• A mutual prodrug consists of two pharmacologically active agentscoupled together so that each acts as a promoiety for the otheragent and vice versa.

• A mutual prodrug is a bipartite or tripartite prodrug in which thecarrier 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 producesampicillin & sulbactum.

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AspirinParacetamol

Sulbactum

Ampicillin

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 unlikethe hydrolytic activation of carrier-linked prodrugs.

• They metabolized into a new compound that may itself be active or furthermetabolized to an active metabolite

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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, L-DOPA, zidovudine

Type IB prodrugs

It converts into parent drugs by metabolic tissues, namely by the liver

E.g., carbamazepine, captopril, heroin, primidone

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Type II – Metabolized Extracellularly

Type IIA

In the milieu of gastrointestinal fluid

E.g., loperamideoxide, 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

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1. Esters as prodrugs of carboxyl, hydroxyl and thiol functionalities

• Esters are the most common prodrugs used, and it is estimatedthat approximately 49% of all marketed prodrugs are activatedby enzymatic hydrolysis.

• Ester prodrugs are most often used to enhance the lipophilicity,and thus the passive membrane permeability, of water solubledrugs by masking charged groups such as carboxylic acids andphosphates.

• The synthesis of an ester prodrug is often straightforward. Oncein the body, the ester bond is readily hydrolysed by ubiquitousesterases found in the blood, liver and other organs and tissues,including carboxyl esterases, acetylcholinesterases,butyrylcholinesterases, paraoxonases and arylesterases.

Functional Groups Amenable to Prodrug Design

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2. Carbonates and carbamates as prodrugs ofcarboxyl, hydroxyl or amine functionalities:

• Carbonates and carbamates differ from esters by thepresence of an oxygen or nitrogen on both sides of thecarbonyl carbon.

• They are often enzymatically more stable than thecorresponding esters but are more susceptible to hydrolysisthan amides.

• Carbonates are derivatives of carboxylic acids and alcohols,and carbamates are carboxylic acid and amine derivatives.

• The bioconversion of many carbonate and carbamateprodrugs requires esterases for the formation of the parentdrug.

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3. Amides as prodrugs of carboxylic acids and amines

• Amides are derivatives of amine and carboxyl functionalities of amolecule. In prodrug design, amides have been used only to alimited extent owing to their relatively high enzymatic stabilityin vivo.

• An amide bond is usually hydrolyzed by ubiquitouscarboxylesterases, peptidases or proteases. Amides are oftendesigned for enhanced oral absorption.

• Lipophilicity of various compounds like acid chlorides and acidscan be altered in many groups of compounds by amideformation.

• This approach is successful to improve the stability of drug invivo in many of the pharmaceutical agents and gives targeteddrug 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 andguanidoximes) are derivatives of ketones, amidines andguanidines, thus providing an opportunity to modifymolecules that lack hydroxyl, amine or carboxylfunctionalities.

• Oximes are hydrolyzed by the versatile microsomalcytochrome P450 (CYP450) enzymes.

• Oximes, especially strongly basic amidines and guanidoximes,can be used to enhance the membrane permeability andabsorption of a parent drug.

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Applications of prodrugsPharmaceutical ApplicationsMasking 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 ApplicationsEnhancement of bioavailability (Lipophilicity)

Prevention of Pre-systemic Metabolism

Prolongation of duration of action

Reduction of toxicity

Site specific drug delivery

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Masking Taste & Odor

Taste Masking:

• The undesirable taste arises due to adequate solubility and interactionof drug with taste receptors, which can be solved by lowering thesolubility of drug or prodrug in saliva.

• Chloramphenicol, an extremely bitter drug has been derivatized tochloramphenicol palmitate, a sparingly soluble ester.

• It possesses low aqueous solubility which makes it tasteless and laterundergoes in vivo hydrolysis to active chloramphenicol by the action ofpancreatic lipase.

Odor Masking:

• The ethyl mercaptan (tuberculostatic agent)has a boiling point of 25ºCand a strong disagreeable odour.

• Diethyl dithio isophthalate, a prodrug of ethyl mercaptan has a higherboiling point and is relatively odourless.

Pharmaceutical Applications

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Minimizing Pain at Site of Injection

• Pain caused by intramuscular injection is mainly due to theweakly acidic nature or poor aqueous solubility of drugs.

• Example, intramuscular injection of antibiotic likeclindamycin and anticonvulsant drug like phenytoin wasfound painful due to poor aqueous solubility and could beovercome by making phosphate ester prodrugsrespectively and maintaining the formulations at pH 12.

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Alteration of Drug Solubility

• The prodrug approach can be used to increase or decreasethe solubility of a drug, depending on its ultimate use.

Example-

• The solubility of betamethasone in water is 58 μg/ml at25⁰C. The solubility of its disodium phosphate ester (acharged ester promoeity) is more than 100 mg/ml, anincrease in water solubility greater than 1500-fold.

• Acetylated sulfonamide moiety enhanced the aqueoussolubility of the poorly water-soluble sodium salt of theCOX-2 inhibitor Parecoxib ~300-fold.

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Enhancement of Chemical Stability

• Although chemical unstability can be solved to a greater extent byappropriate formulations, its failure necessitates the use of prodrugapproach. 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 thereduction of contact between the drug and the media in which it isunstable.

• E. g. Antineoplastic drug- Azacytidine.

• The aqueous solution of azacytidine isreadily hydrolyzed but the bisulfite prodrugshows stability to such degradation at acidicpH and is also more water soluble than theparent drug.

• The prodrug gets converted to active drugat the physiological pH 24

Reduction of G.I. irritation

• Several drugs cause irritation and damage to the gastricmucosa through direct contact, increased stimulation ofacid secretion or through interference with protectivemucosal 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 formulationas a tablet especially if their dose is high.

• The method of converting such a liquid drug into solid prodruginvolves formation of symmetrical molecules having a highertendency to crystallize e.g. ester of Ethyl mercaptan and trichloroethanol.

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Enhancement of bioavailability (Lipophilicity)

• Passive diffusion is the commonest pathway for transportation ofdrug from site of administration to systemic circulation through alipoidal membrane.

• Thus improvement in the lipophilic character serves as a tool forbetterment of bioavailability. Two reasons can be attributed to theenhanced oral bioavailability of lipophilic compound -

a) The lipophilic form of a drug has enhanced membrane /waterpartition coefficient as compared to the hydrophilic form thusfavoring passive diffusion e.g. Bacampicillin prodrugs of Ampicillinis more lipophilic, better absorbed and rapidly hydrolyzed to theparent drug in blood.

b) The lipophilic prodrugs have poor solubility in gastric fluids andthus greater stability and absorption e.g. ester of Erythromycin.

Pharmacokinetic Applications

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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 conventionaldosage forms to maintain adequate plasma concentration of theparticular drug.

• In plasma level time profile and consequently patient compliance isoften poor.

• Prolongation of duration of action of a drug can be accomplished by theprodrug . Prodrug can be formed by two approaches-

Drug Ester Prodrug

Testosterone Testosterone propionate

Estradiol Estradiol propionate

Fluphenazine Fluphenazine deaconate

1. To control the release rate of prodrug.

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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 toactive drug was difficult, it was successfully utilized to deliverPilocarpine to eyes in the treatment of glaucoma.

• The diesters of drug when applied as ophthalmic solution showedbetter intra-ocular penetration due to improved lipophilicity andslow conversion of the ester prodrug to active Pilocarpine resultedinto 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 canbe 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 (aselective β-agonist useful) in glaucoma. This prodrug, is 100 timesmore potent, has longer duration of action and is free from bothlocal and systemic toxicity.

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Site specific drug delivery

• After its absorption into the systemic circulation, the drugis distributed to the various parts of the body including thetarget site as well as the non-target tissue.

• These problems can be overcome by targeting the drugspecifically to its site of action by prodrug design

• The prodrug is converted into its active form only in thetarget organ/tissue by utilizing either specific enzymes ora 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 ofanticancer drugs, and associated toxicity, hampers their effectiveness and long termuse. 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, peptidetransporters, antigens) that are overexpressed in tumor cells in comparison to normalcells. The new promising chemotherapeutic prodrugs include:

1. Enzyme-activated prodrugs

• ADEPT• GDEPT

2. Targeting-ligand conjugated prodrugs

• Antibody-drug conjugates • Peptide-drug conjugates

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1. Enzyme-activated prodrugs

• One approach toward improving the specificity ofchemotherapy is enzyme-activated prodrug therapy inwhich 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

phosphate

Mitomycin C Lung

adenocarcinoma

BW413 Etoposide

phosphate

Etoposide Colon carcinoma

L6 Doxorubicin

phosphate

Doxorubicin Lung

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 foreignenzyme is delivered to tumor cells. In the second step, a non-toxic agent isadministered systematically and converted by the enzyme to its cytotoxicmetabolite.

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-

dinitrobenzamide

5-(Aziridin-1-yl)-4-

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. Afterreaching its target, the conjugate is internalized through a receptor-mediated pinocytosis, and the pharmacologically active compound isreleased in the cell.

• Peptide-drug conjugates:

• Peptide-conjugated prodrugs for cancer therapy utilize peptideligands designed to bind with a tumor specific antigen or a peptidetransporter which is overexpressed in neoplastic cells.

• These ligands are conjugated to a chemotherapic agent either directlyor by a linker.

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Marketed Prodrugs

Fosphenytoin

Fenofibrate

Rabeprazole

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Limitations of Prodrug Design

• Formation of unexpected metabolite from the totalprodrug that may be toxic.

• The inert carrier generated following cleavage of prodrugmay also transform into a toxic metabolite.

• During its activation stage, the prodrug might consume avital cell constituent leading to its depletion.

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CONCLUSION

Prodrug design is a part of the general drug discoveryprocess, in which a unique combination of therapeuticallyactive substances is observed to have desirablepharmacological effects.

In human therapy prodrug designing has given successfulresults in overcoming undesirable properties likeabsorption, nonspecificity, and poor bioavailability and GItoxicity.

Thus, prodrug approach offers a wide range of options indrug design and delivery for improving the clinical andtherapeutic effectiveness of drug.

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• Jarkko Rautio, Hanna Kumpulainen, Tycho Heimbach, Reza Oliyai§, DoomanOh|, Tomi Järvinen and Jouko Savolainen, Prodrugs: design and clinicalapplications, Nature Reviews: Drug Discovery, Vol.7, Pg. No.255-270, March2008.

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