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Prodrugs

Prodrugs

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PRODRUGS
BASIC CONCEPTS AND APPLICATION OF
PRODRUGS DESIGN
1.1. CONCEPT OF PRODRUGS
The prodrug concept was first proposed by Albert in 1958. Albert and his co-workers
described prodrugs as pharmacologically inactive chemical derivatives that could be used
to alter the physicochemical properties of drugs, in a temporary manner, to increase their
usefulness and/ or to decrease associated toxicity.
Prodrug is a chemically modified inert drug precursor which upon biotransformation
liberates the pharmacologically active parent compound. A prodrug by definition is
inactive and must be converted into an active species within the biological system. A
prodrug is also called as latentiated drug, bioreversible derivative, or congener.
Most chemically designed prodrugs are composed of two parts in which the active drug is
linked to a pharmacologically inert molecule.
The chemical bond between the two parts of the prodrug must be sufficiently stable to
withstand the pharmaceutical formulation of the prodrug while permitting chemical or
enzymatic cleavage at the appropriate time or site.
After administration or absorption of the prodrug, the active drug is usually released
either by catalyzed hydrolysis by the liver or intestinal enzymes or simply by hydrolysis
although reductive processes have also been utilized.
Prodrugs
Objectives of prodrugs
 Pharmaceutical: To improve solubility, chemical stability, and organoleptic properties
 Pharmacokinetic: To improve absorption, to decrease presystemic metabolism, to
improve pharmacokinetic profile.
 Pharmacodynamic: To decrease toxicity and improve therapeutic index, to design
single chemical entities combining two drugs.
Ideal requirements of prodrugs
 It shouldn’t have intrinsic pharmacological activity.
 The carrier molecule released in vivo must be non-toxic.
 The metabolic fragments apart from active drug should be non-toxic.
 It should rapidly transform chemically or enzymatically into the active form where
desired.

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Prodrugs

  • 1. PRODRUGS BASIC CONCEPTS AND APPLICATION OF PRODRUGS DESIGN
  • 2. 1.1. CONCEPT OF PRODRUGS The prodrug concept was first proposed by Albert in 1958. Albert and his co-workers described prodrugs as pharmacologically inactive chemical derivatives that could be used to alter the physicochemical properties of drugs, in a temporary manner, to increase their usefulness and/ or to decrease associated toxicity. Prodrug is a chemically modified inert drug precursor which upon biotransformation liberates the pharmacologically active parent compound. A prodrug by definition is inactive and must be converted into an active species within the biological system. A prodrug is also called as latentiated drug, bioreversible derivative, or congener.
  • 3. Most chemically designed prodrugs are composed of two parts in which the active drug is linked to a pharmacologically inert molecule. The chemical bond between the two parts of the prodrug must be sufficiently stable to withstand the pharmaceutical formulation of the prodrug while permitting chemical or enzymatic cleavage at the appropriate time or site. After administration or absorption of the prodrug, the active drug is usually released either by catalyzed hydrolysis by the liver or intestinal enzymes or simply by hydrolysis although reductive processes have also been utilized.
  • 5. Objectives of prodrugs  Pharmaceutical: To improve solubility, chemical stability, and organoleptic properties  Pharmacokinetic: To improve absorption, to decrease presystemic metabolism, to improve pharmacokinetic profile.  Pharmacodynamic: To decrease toxicity and improve therapeutic index, to design single chemical entities combining two drugs.
  • 6. Ideal requirements of prodrugs  It shouldn’t have intrinsic pharmacological activity.  The carrier molecule released in vivo must be non-toxic.  The metabolic fragments apart from active drug should be non-toxic.  It should rapidly transform chemically or enzymatically into the active form where desired.
  • 7. Types of prodrugs  Carrier-linked prodrugs  Bio-precursors  Macromolecular prodrugs  Mutual prodrugs  Bio-conjugates
  • 8. Carrier-linked prodrugs: These are prodrugs where active drug is covalently bonded to an inert carrier or transport moiety which is usually lipophilic in nature. They are generally ester or amides. The active drug is released by hydrolytic cleavage, either enzymatically or chemically, e.g., Aspirin. Bio-precursors: These are also called as metabolic precursors and are inert molecules obtained by chemical modification of the active drugs but do not contain a carrier. Such prodrugs have the almost same lipophilicity and are generally bioactivated by redox reactions, only enzymatically. For example: Fenbufen.
  • 9. Macromolecular prodrugs: These prodrugs have macromolecules like polyethylene glycol, cyclodextrin, chondroitin sulphate as carriers. Macromolecular prodrugs/ polymeric prodrugs consist of drug covalently attached to polymeric backbone. For example: Naproxen polymeric prodrug. Mutual prodrugs: Here two active agents are combined to give prodrug which separates in the body. The mutual prodrug concept is one step ahead as it minimizes side effects along with increase/ addition in activity. For example: Naproxen-paracetamol mutual prodrug.
  • 10. Bio-conjugates: These are prodrugs where the carrier is an antibody raised against tumor cells. Antibody conjugated delivery systems, e.g., monoclonal antibodies (Mab), which are directed against tumor antigens may be used to target anticancer drugs to target cells for selective killing of cancer cells; mainly these antibodies are “magic bullet” and lead to the formation of immuno-conjugates.
  • 11. Prodrugs can also be classified into two major types, based on how the body converts the prodrug into the final active drug form: Type I prodrugs: These are bioactivated inside the cells (intracellularly). Examples of these are anti-viral nucleoside analogues that must be phosphorlyated and the lipid- lowering statins. Type II prodrugs: These are bioactivated outside sells (extracellularly), especially in digestive fluids or in body’s circulatory system (particularly in the blood). Examples of these are salicin, aspirin and certain antibody, gene- or virus-directed enzyme prodrugs used in chemotherapy of immunotherapy.
  • 12. Prodrug type Site of conversion Sub- type Tissue/ location of conversion Examples Type I Intra- cellular A Therapeutic target tissues/ cells Acyclovir 5-Fluorouracil B Metabolic tissues (Liver, GIT, mucosal cells, lungs, etc.) Carbamazepine Sulindac Type II Extra- cellular A GI fluids Loperamide oxide Sulfasalazine B Systemic circulation Aspirin, Becampicillin C Therapeutic target ADEPs
  • 13. Common pro-moieties A wide variety of pro-moieties have been used to overcome liabilities associated with drugs. The selection of pro-moeity depends on:  Purpose of the prodrug  Type of functional groups available on the parent drug  Chemical and enzymatic conversion mechanisms of prodrug to parent drug  Safety of the pro-moeity, and ease of manufacturing
  • 14. Several chemical classes of prodrug can be obtained based on functional groups available in drug and pro-moeity used: Functional Groups Prodrug Chemical Class Carboxylic (-COOH) Simple esters, acyloxyalkyl esters, amides, phosphoxyalkyl esters Hydroxyl (-OH) Simple esters, acyloxyalkyl esters, carbamates, phosphate esters carbonates, phosphoxyalkyl esters. Sulfhydril (-SH) Thioesters, acyloxyalkyl thioesters, disulphides. Amine (-NH) Amides, carbamates, imines, enamines Carbonyl (>C=O) Oximes, imines, enamines, acetals, hydrazones
  • 15. Carboxylic acids and alcohols: Prodrugs of agents that contain carboxylic acid or alcohol functional group can often be prepared by conversion to an ester. Hydrolysis is normally accompanied by the esterase enzymes present in the plasma and other tissues that are capable of hydrolysing a wide variety of ester linkages, e.g., ester hydrolase, lipase, acetylcholine esterase, cholesterol esterase, etc.
  • 16. When it is desired to decrease water solubility, a non-polar alcohol or carboxylic acid is used as the pro-drug moiety; leading to increased absorption and a longer duration of action, e.g., Dipivefrin HCl (prodrug of epinephrine). Phosphate esters of alcohols offer a method of increasing solubility of an agent, e.g., Clindamycin phosphate (less pain on injection). Succinic esters increase water solubility of the drugs.
  • 17. Amino acids: The peptide linkages can be employed to increase the cellular uptake by use of an amino acid transporter. The amino acids are then cleaved by specific peptidase enzymes. A more common approach is the use of Mannich bases as prodrug form of the amines, e.g., Hetacillin.
  • 18. Amines: Several compounds like propyl-amine, diethylamine, cyclohexyl amine, 2–amino ethyl amine, 2-hydroxyl ethyl amine, ethylenediamine, benzathine and cysteamine are used as pro-moieties in synthesis of amide prodrugs. These form amide bonds with carboxylic groups of drug moiety.
  • 19. Polysaccharides: NSAIDs conjugated with polysaccharide specifically for colon targeting have been studied with cyclodextrin, dextran, pectin, chitosan and chondroitin.
  • 20. Phytophenols: Phytophenols are used traditionally for their medicinal as well as flavoring properties, with well documented safety profiles. Phytophenols are used as carriers for prodrugs in an attempt to combine anti-inflammatory and antioxidant properties. Naturally occurring phenolic antioxidants are thymol, guiacol and eugenol whereas menthol is alcoholic compound. Menthol, thymol, eugenol, guiacol, vanillin, umbelliferone are the pro- moieties used in prodrug synthesis.
  • 21. Polymers: The conjugation of a biologically active compound with a polymer is one of the many methods for altering and controlling the pharmacokinetics, bio-distribution and often toxicity of various drugs. Polyethylene glycols (PEGs) appear to be particularly convenient as oligomeric matrices, since they are easily available in wide range of well definite molecular weights.
  • 22. Mutual prodrugs: Mutual prodrugs involve combining two different pharmacophores with similar pharmacological activities to give synergistic action or different pharmacological activities whose action is needed together.
  • 23. Reversal of prodrugs Reversal of prodrugs is the bio-activation of the prodrug of an agent to its active form. It may be: Enzymatic Chemical
  • 24. Enzymatic: Esterases such as ester hydrolase, lipase, cholesterol esterase, choline esterases, acetylcholine esterases, etc., hydrolyse the ester type prodrugs like carboxylic acid esters, succinic esters, phosphate esters, e.g., Chloramphenicol palmitate, clindamycin phosphate. Azo reductase hydrolyses azo-linkage, e.g., Prontosil and sulfasalazine. Peptidases hydrolyse the peptide linkage prodrugs.
  • 25. Chemical: A serious drawback of the prodrugs requiring chemical activation is the inherent ability of these compounds (triggered by change in pH), therefore raising some stability problems, e.g. Mannich bases.
  • 26. 1.2. EXAMPLES OF PRODRUGS The earliest example of a prodrug is arsphenamine (7.1) used by Ehrlich for the treatment of syphilis. Later, Voegtlin demonstrated that the activity of this compound against the syphilis organism was attributable to the metabolite oxophenarsine (7.2) which was less toxic at the dose required for effective therapy.
  • 27. Domagk (1935) reported that the azo dye prontosil (7.3) had antibacterial activity. Prontosil was subsequently shown to be a precursor which was metabolized to the active agent, p-amino-benzene sulfonamide (7.4), in vivo. This led to the subsequent development of a wide range of therapeutically superior sulfonamides through modification of amino-benzene sulfonamide molecule.
  • 28. Pamaquin (7.5) is dealkylated and oxidized to the quinone (7.6), which is 16 times more active in vivo than the parent compound whereas paludrine (7.7) cyclizes to give the active dihydrotriazine (7.8), which has structural similarities to the antimalarial drug pyrimethamine (7.9).
  • 30. The development of depressants based on trichloroethanol (7.11) was shown to be the active-metabolite of the once used hypnotic chloral hydrate (7.10). This led to the use of trichloroethanol acid phosphate (7.12) for patients where choral hydrate was found to be either unpalatable or caused gastric irritation.
  • 31. Antiepileptic activities of methylphenobarbitone (7.13), primidone (7.14), and methsuximide (7.15) were related to the plasma levels of active metabolites (obtained by demethylation, oxidation and demethylation, respectively).
  • 32. The nonsteroidal anti-inflammatory drug sulindac (Clinoril) (7.16) is also a prodrug, which is reduced to the active metabolite (7.17), although some of the inactive sulfone (7.18) is formed on oxidation.
  • 33. The in vivo hydrolysis of aspirin (7.19) to salicylic acid (7.20) by esterases allows the administration of aspirin in preference to salicylic acid, which is more corrosive to the gastrointestinal mucosa.
  • 34. Hexamine (Hiprex, Mandelamine) (7.21) is administered as a prodrug of formaldehyde (7.22) for the treatment of urinary tract infections although it was initially used to dissolve renal stones. An enteric coat is used to protect the prodrug from stomach acid; however, on reaching the acidic environment of the urine, the formaldehyde is released and exerts its antiseptic action.
  • 35. Phenylbutazone (7.23) is converted by the body into hydroxylated forms, oxyphenbutazone (7.24) and (7.25). The drug is used in therapy under hospital supervision, mainly as an anti-inflammatory agent, and this activity resides in form (7.24). However, another use of the drug is as a uricosuric agent, in the treatment of gout, and this action is attributable to the form (7.25). The observation that substitution in the side chain of phenylbutazone results in enhanced uricosuric action has led to the discovery of several other agents which have this action, inparticular sulfinpyrazone (7.26).
  • 37. In addition to those drugs detailed above several drugs which were metabolized to active compounds were initially considered to be prodrugs but later shown to possess activity themselves. For example, phenacetin (7.27), an analgesic and antipyretic agent, is mainly metabolized in the body to an active metabolite, N-acetyl-p-aminophenol (paracetamol) (7.28), as well as to an inactive metabolite, the glucuronide of 2-hydroxyl phenacetin (7.29), in small amounts.
  • 38. 1.3. APPLICATION TO PHARMACEUTICAL PROBLEMS The pharmaceutical problems that have been addressed using the concept of prodrug design include problems related to patient acceptability (unpalatability, gastric irritation and pain on injection), insolubility, and drug instability. 1.3.1. Patient acceptability 1.2.2. Gastric irritability 1.2.3. Drug solubility 1.2.4. Drug stability
  • 39. 1.3.1. Patient acceptability Unpleasant tastes and odors may often affect patient compliance. Very young children generally require liquid medication since they are usually not amenable to swallowing capsules or coated tablets.
  • 40. Despite the life-threatening toxicity the antibiotic chloramphenicol (7.30) it is still administered orally for the treatment of typhoid fever and salmonella infections. However, the drug has an extremely bitter taste and is entirely unsuitable for administration as a suspension to such patients. To overcome this problem orally administered chloramphenicol is usually formulated as the inactive tasteless palmitate (7.31) or cinnamate (7.32) esters.
  • 41. The bitter taste of the antibiotics clindamycin and erythromycin has been similarly masked using the palmitate ester and hemisuccinate ester prodrugs, respectively. The antimicrobial metronidazole (7.33) is another example of a drug with an unacceptably bitter taste. To overcome thisproblem, it is administered as a suspension of benzoyl- metronidazole (Flagel S) (7.34).
  • 42. Ethyl mercaptan (useful in the treatment of leprosy) is foul-smelling liquid, it is converted to its phthalate ester which has lower vapour pressure and is odourless (when rubbed on skin it is metabolized to parent drug by thioesterase). Parent drug Prodrug with improved taste Tetracycline 3,4,5-Trimethoxy benzoate salts Lincomycin Phosphate or alkyl ester Triamcinolone Diacetate ester Sulfisoxazole Acetyl ester
  • 43. 1.3.2. Gastric irritability Several drugs cause irritation and damage to the gastric mucosa through direct contact, increased stimulation of gastric secretion or through interference with the protective mucosal layer. The NSAIDs, especially salicylates, have such a tendency. They lower the gastric pH and induce ulceration.
  • 44. Parent drug Prodrug Salicylic acid Aspirin (acetylsalicylic acid) Diethyl stilbestrol Fosfestrol Kanamycin Kanamycin pamoate Phenylbutazone N-Methyl piperazine salt Nicotinic acid Nicotinic acid hydrazide
  • 45. 1.3.3. Drug solubility The formulation of insoluble compounds for parenteral delivery represents a major problem as the insoluble drug will have a tendency to precipitate on injection in an organic solvent. The solubility of such compounds can be improved by the use of phosphate or hemisuccinate (ester type) prodrugs.
  • 46. The insoluble glucocorticoids (betamethasone, prednisolone, methylprednisolone, hydrocortisone, and dexamethasone) are available for injection as the water-soluble prodrugs (disodium phosphate (RO-PO3 2- ) or sodium hemisuccinate (RO-CO- CH2CH2COO- Na+ ) salts). The phosphate esters are rapidly hydrolyzed to the active steroid by phosphatases, whereas the hemisuccinate salts are less efficiently hydrolyzed by esterases, possibly due to the presence of an anionic center (COO- ) near the hydrolyzable ester bond.
  • 47. The poorly water-soluble anti-inflammatory steroidal alcohol dexamethasone has been shown to rapidly (t1/2= 10 min) liberate the active steroid in vivo when injected as the water-soluble phosphate (7.35). The water-soluble phosphate ester of agent oxyphenbutazone (7.36) is rapidly hydrolyzed in vivo and gives higher blood levels of oxyphenbutazone on oral or intramuscular administration than attained on administration of the same doses of the parent drug.
  • 48. Difficulties in the formulation of anticonvulsant drug phenytoin (7.37) as a soluble injection have led to the development of water-soluble prodrugs which have been shown to have a superior in vivo performance in rats. The prodrug is prepared by reacting phenytoin with an excess of formaldehyde to give the 3-hydroxymethyl intermediate (7.38), which is unstable in absence of excess reagent. Conversion of the intermediate (7.38) to disodium phosphate ester prodrug (7.39) gives a water-soluble derivative. This is metabolized in vivo by phosphatases to 3-hydroxymethyl intermediate (7.38), which rapidly breaks down (t1/2= 2 s) at 37 °C (pH 7.4) to give the active drug, phenytoin.
  • 50. 1.3.4. Drug stability Many drugs are unstable and may either breakdown on prolonged storage or are degraded rapidly on administration. This is a particular problem on oral administration as drugs are often unstable in gastric acid. Although enteric coatings may be used, it is also possible to utilize prodrug design to overcome this problem.
  • 51. The antibiotic erythromycin is destroyed by gastric acid and, as an alternative to enteric- coated tablets; it is administered orally as a more stable ester. The inactive erythromycin stolate (lauryl sulfate salt of the propionyl ester), when administered as a suspension, is rapidly absorbed and the propionyl ester is converted by body esterases to the active erythromycin.
  • 52. 5-Aminosalicylic acid (mesalazine) useful in the treatment of ulcerative colitis cannot be administered orally since firstly, it is unstable in gastric acid and secondly, it would not reach its site of action in the ileum/colon since it would be absorbed in the small intestine. Sulfasalazine (7.40), where mesalazine is covalently linked with sulfapyridine, is broken down in the colon by bacteria to the two components and in this way 5-aminosalicylic acid is delivered to the required site of action.
  • 53. However, sulfapyridine is responsible for major side effects attributable to this combination. An alternative prodrug, osalazine (7.41), consisting of two molecules of 5- aminosalicylic acid was developed to overcome this problem. Reduction of azo bond by the colonic microflora liberates two molecules of mesalazine. Mesalazine has also been administered orally as tablets coated with a pH-dependent acrylic-based resin, which disintegrates in the terminal ileum/colon as the environment pH rises above pH 7.
  • 54. Microbial metabolism of prodrugs has also been utilized in the delivery of corticosteroids to the colon. Such compounds are generally readily absorbed from the upper gastrointestinal tract and therefore, delivered ineffectively to the colon. Administration of corticosteroids, such as dexamethasone (7.42), as glycoside prodrugs overcomes these problems by reducing systemic uptake in the small intestine. Prodrugs, such as dexamethasone-β-D-glucoside (7.43), are hydrolyzed by the specific glycosidases produced by the colonic bacteria and the parent corticosteroid absorbed from the lumen of the large intestine, resulting in much higher concentrations in the colonic tissues.
  • 56. 5-Aminosalicyclic acid has been linked to poly(sulfonamidoethylene) to give another mesalazine prodrug known as polyasa (7.44), which has been shown to have less side effects than sulfasalazine (7.40).
  • 57. 1.4. APPLICATION TO PHARMACOLOGICAL PROBLEMS There are a number of pharmacological problems which may be addressed by prodrug design. These problems may be either related to pharmacokinetic, pharmacodynamic, or toxic properties of the drug. Inappropriate pharmacokinetics may result in an undesirable rate of onset or duration of action of a drug.
  • 58. Poor pharmacodynamics may be a consequence of inefficient or unpredictable drug adsorption from GIT, inappropriate distribution, and variable bioavailability as a consequence of pre-systemic metabolism or the inability to reach the site of action from the systemic circulation, e.g., penetration of the blood-brain barrier. Toxic side effects (adverse reactions) may be due to non-specific drug delivery to the site of action of drug.
  • 59. 1.4.1. Drug absorption 1.4.2. Drug distribution 1.4.3. Drug metabolism 1.4.4. Site-specific drug delivery 1.4.5. Sustaining drug action
  • 60. 1.4.1. Drug absorption Many drugs are either poorly or unpredictably adsorbed from the gastrointestinal tract resulting in variation in efficacy between patients. Prodrug design has been utilized in a number of cases to optimize the adsorption of such drugs thereby improving their bioavailability.
  • 61. Many penicillins are not absorbed efficiently when administered orally and their lipophilic esters have been used to improve absorption. Ampicillin (7.45), a wide-spectrum antibiotic, is readily absorbed orally as the inactive prodrugs, pivampicillin (7.46), bacampicillin (7.47), and talampicillin (7.48) which are then converted by enzymatic hydrolysis to ampicillin. The preferred prodrug is pivampicillin since minimal hydrolysis occurs in the intestine before absorption into the systemic circulation. Pivampicillin, the pivaloyloxy methyl ester, contains anacyloxy-methyl function which is rapidly hydrolysed by enzymes to the hydroxymethyl ester.
  • 63. Acyclovir (7.49) has been widely used for the treatment of herpes simplex and herpes- zoster infections. This prodrug is activated through phosphorylation by the viral thymidine kinase to acyclovir monophosphate, which is then converted to the triphosphate, which inhibits DNA polymerase, by host cellular enzymes. However, the use of this drug has been limited to some extent by low oral absorption; only 20% of a 200-mg dose is absorbed and little improvement is seen with doses above 800 mg.
  • 64. This has led to the development of a range of acyclovir prodrugs including “6- deoxyacyclovir” (7.50) which has been used for prophylaxis of herpes-virus infections in patients with haematological malignancies. It is well absorbed orally and produces plasma concentrations of the drug which are much higher than those obtained by oral administration of acyclovir. The drug (7.50) is converted to acyclovir in vitro by xanthine oxidase.
  • 66. An alternative orally active prodrug is valacyclovir (7.51), the L-valyl ester of acyclovir, which is rapidly hydrolysed by first pass intestinal and hepatic metabolism. The mechanism of this biotransformation has yet to be fully elucidated but is thought to be enzymatic in nature.
  • 67. Recently famciclovir (7.52) has been licensed in UK for the treatment of herpes-zoster infections. Famciclovir is an orally absorbed 6-deoxy, diacetyl ester prodrug of penciclovir (7.53). This prodrug is rapidly deacetylated and oxidized in the intestinal wall and liver to give a systemic availability of penciclovir from famciclovir of 77% on oral administration. Penciclovir is selectively phosphorylated by viral thymidine kinase in the same way as acyclovir.
  • 69. Several 2’,3’-dideoxynucleoside analogs such as zidovudine (azidothymidine, AZT) (7.54) and 2’,3’-didehydro-3’-deoxythymidine (D4T) (7.55) have potent antiviral activity against human immunodeficiency virus (HIV). These compounds are phosphorylated intracellularly to the 5’-triphosphate derivatives which inhibits the viral reverse transcriptase. Such prodrugs increase the circulating half- life while limiting the elevation of the plasmaconcentration of the parent nucleoside. Some of the ester prodrugs were also shown to have higher absolute oral bio-availabilities than the parent nucleoside drug.
  • 71. The use of these nucleoside analogs as antiviral and antineoplastic agents is also limited by their absolute requirement for kinase-mediated intracellular phosphorylation. Nucleotide phosphates are unable to readily penetrate membranes and therefore have little therapeutic utility. This has led to the development of masked-phosphate prodrugs of anti- HIV nucleoside analogs, such as (7.56), which facilitate intracellular delivery of the bioactive free phosphate. These compounds have been shown to be 25 times more potent and 100 times more selective than the parent nucleosides. Unlike the parent drugs they also retain good activity against kinase-deficient cells.
  • 73. In another example, the antihypertensive effects after oral administration of the angiotensin converting enzyme inhibitor enalaprilat (7.57) have been improved by conversion to the more efficiently absorbed ethyl ester, enalapril (7.58). In the active form, less than 12% is adsorbed whereas the inactive derivative has an improved adsorption of between 50% and 75%. The prodrug enalapril is converted in vivo to the active enalaprilat by hydrolysis in the liver following absorption from the gastrointestinal tract.
  • 74. Animal studies have shown that the oral absorption of certain basic drugs may be increased by the preparation of “soft” quaternary salts. The “soft” quaternary salt is formed by reaction between α-chloromethyl ester (7.59) and the amino group of the drug. The quaternary salt formed is termed as “soft” quaternary salt since, unlike normal quaternary salts; it can release the active basic drug on hydrolysis.
  • 75. “Soft” quaternary salts have useful physical properties compared with the basic drug or its salts. Water solubility may be increased compared with other salts, such as the hydrochloride, but more importantly there may be an increased absorption of the drug from the intestine. Increased absorption is probably due to the fact that the “soft” quaternary salts have surfactant properties and are capable of forming micelles and unionized ion pairs with bile acids, etc., which are able to penetrate the intestinal epithelium more effectively. The prodrug, after absorption, is rapidly hydrolyzed with release of the active parent drug as illustrated below.
  • 77. Such an approach has also been utilized to achieve improved bioavailability of pilocarpine on ocular administration. Pilocarpine is rapidly drained from the eye resulting in a short duration of action. The “soft” quaternary salt (7.60) has a lipophilic side-chain which has been shown to improve absorption in rabbits and gives a more prolonged effect at one tenth of the concentration of pilocarpine. The action of this compound has been shown to be due to the release of pilocarpine on hydrolytic cleavage of the ester followed by release of formaldehyde.
  • 78. Topical administration is also used in the treatment of glaucoma with adrenaline (7.61), which lowers the intraocular pressure. Enhanced therapeutic efficacy may be achieved using a more lipophilic prodrug dipivefrin (7.62) which is 100 times more active than adrenaline as a consequence of more efficient corneal transport, followed by de-esterification by the corneal tissue and release of adrenaline in the aqueous humor. Consequently, lower doses of dipivefrin than adrenaline can be administered to achieve the same therapeutic effect with reducing the side effects associated with the use of adrenaline including cardiac effects due to systemic absorption and the accumulation of melanin deposits in the eye.
  • 80. 1.4.2. Drug distribution The modification of a drug to a prodrug may lead to enhanced efficacy for the drug by differential distribution of the prodrug in body tissues before the release of the active form. For example, more extensive distribution of ampicillin occurs in the body tissues when the methoxymethyl ester of hetacillin is administered, than is obtained with ampicillin itself.
  • 81. Conversely, decreased tissue distribution of a drug may occur, as was observed when adriamycin as its DNA-complex was administered as a prodrug. Decreased tissue distribution restricts the action of a drug to a specific target site in the body and may therefore decrease its toxic side effects, resulting from its reaction at other sites. Anticancer drugs can suppress growth in normal as well as neoplastic tissue. Improved selective localization has been achieved using nontoxic prodrugs which release the active drug within the cancer cell as a result of either the enhanced enzyme activity in the cell or enhancement of reductase activity in the absence of molecular oxygen in the hypoxic cells.
  • 82. The prodrug cyclophosphamide (7.63) is used for the treatment of certain forms of cancer and as an immunosuppressant after organ transplant. It does not possess alkylating properties and thus is not a tissue vesicant since electron withdrawing properties of the adjacent phosphono-function decrease the nucleophilic properties of β-chloroethylamino-nitrogen atom and prevent formation of reactive alkylating ethyleniminium ion.
  • 83. The prodrug requires hepatic oxidase-mediated metabolic activation to generate 4- hydroxy-cyclophosphamide (7.64). This compound exists in equilibrium with its open ring tautomer aldophosphamide (7.65), which undergoes β-elimination to produce alkylating cytotoxic phosphoramide mustard (7.66) in target cells. Cyclophosphamide is also metabolized by aldehyde dehydrogenase to the inactive carboxyphosphoramide (7.67).
  • 85. Recently, thiophosphate prodrug amifostine (7.68) has been introduced as a cytoprotective agent to reduce toxic effects of cyclophosphamide on bone marrow. As tumor cells are often hypoxic, poorly vascularized, and have a low pH environment they also have reduced alkaline phosphatase activity. Amifostine exploits these differences in uptake and enzyme activity to ensure that the prodrug is only dephosphorylated to active drug in healthy tissues. Active drug therefore selectively deactivates reactive cytotoxic species produced by cyclophosphamide in non-tumor tissue without compromising the efficacy of chemotherapy.
  • 87. Acrolein produced during ring opening of 4-hydroxycyclophosphamide (7.64) was initially found to cause bladder trouble. This problem has been overcome by use of a modified cyclophosphamide (7.68), which does not form acrolein after ring opening. The anticancer effect of the prodrug procarbazine (7.69) has also been attributed to the formation of a cytotoxic species in the target cells. In this case, procarbazine is metabolized by the mixed function oxidase to azoprocarbazine (7.70) which undergoes further cytochrome P450-mediated oxidation to azoxy procarbazine isomers (7.71, 7.72) which liberate the diazomethane alkylating agent in the target cells.
  • 89. 1.4.3. Drug metabolism Many drugs are efficiently absorbed from gastrointestinal tract but undergo pre-systemic metabolism or inactivation before reaching the systemic circulation. A major class of drugs that undergo marked pre-systemic metabolism are those containing phenolic moieties.
  • 90. The phenolic group can be derivatised to either an ester or an ether group. Phenacetin (used as an analgesic) is the prodrug of paracetamol (acetaminophen). Propranolol (an antihypertensive drug) has high first-pass hepatic extraction but its hemisuccinate prodrug is resistant to esterases of liver. Corticosteroids undergo extensive hepatic first-pass (pre-systemic) metabolism which can be prevented by use of their ester or ether prodrugs (e.g., Triamcinolone acetamide).
  • 91. 1.4.4. Site-specific drug delivery Prodrugs have more recently been used to achieve site-specific drug delivery (drug targeting) to various tissues. Such prodrugs are designed to ensure that the release of the active drug only occurs at its site of action thereby reducing toxic side effects due to high plasma concentrations of the drug or nonspecific uptake by other body tissues. The prodrug is converted to its active form only in the target organ/ tissue by utilizing either specific enzymes or pH value. This has led to the development of systems for site- specific delivery to the brain and to cancer cells.
  • 92. The blood-brain barrier is impenetrable to lipid insoluble and highly polar drugs. Although lipophilic prodrugs may be used to overcome this physiological barrier, the increased lipid solubility may enhance uptake in other tissues with a consequential increase in toxicity. Furthermore, therapeutic levels of such lipophilic prodrugs can only be maintained if there is a constant plasma concentration. These problems may be overcome by utilizing a dihydropyridine-pyridinium salt type redox system. This approach was first used to enhance the penetration of the nerve gas antagonist pralidoxine into CNS using (7.73) as a nonpolar prodrug, which can cross the barrier and is then rapidly oxidized to active form and trapped in the CNS.
  • 94. More recently this approach has been developed as a general rationale for the site-specific and sustained delivery of drugs, which either do not cross the blood-brain barrier readily or are rapidly metabolized.
  • 95. The delivery system is prepared by condensing phenylethylamine with nicotinic acid to give (7.74) which is then quaternized to give (7.75). The quaternary ammonium salt (7.75) is then reduced to the 1,4-dihydro-derivative (7.76). The prodrug (7.76) is delivered directly to the brain, where it is oxidized and trapped as the prodrug (7.75). Quaternary ammonium salt (7.75) is slowly cleaved by enzymatic action with sustained release of the active phenylethylamine and the facile elimination of the carrier molecule. Elimination of drug from general circulation is by comparison accelerated, either as (7.75) or (7.76) or as cleavage products.
  • 97. This rationale removes excess drug and metabolic products during or after onset of the required action. This is in contrast to normal penetration of the brain by a drug from plasma, where plasma levels must be maintained to produce the required effect and can thus lead to the systemic side effects.
  • 98. In animal experiments the anti-inflammatory effect of topically applied hydrocortisone has been increased, and its systemic effects after absorption decreased, by use of the prodrug spirothiazolidine derivative (7.77). These beneficial effects are due to restriction of the action of hydrocortisone within the skin. After absorption, (7.77) is hydrolyzed in a stepwise manner with eventual release of hydrocortisone within the skin from the accumulated prodrug, resulting in a more intense anti-inflammatory effect and a decrease in its rate of leaching into the blood stream to produce systemic effects. The sustained release of hydrocortisone is due to retardation of the intermediate hydrolytic product (7.78) by disulfide formation (7.79) between its thiol group and a thiol group of the skin, followed by a slow breakdown of (7.79) to give hydrocortisone.
  • 100. The 2-nitro-imidazole compound misonidazole (7.80) is selectively cytotoxic to cultured hypoxic cells. Reduction of the nitro group to the hydroxylamine (R-NH2OH) probably occurs, with further fragmentation occurring to give the DNA-alkylating species including glyoxal ((CHO)2).
  • 101. Nitracine (7.81) is selective alkylating agent for hypoxic mammalian cells in culture after reduction, although the identity of the active species is unknown. Although nitracine is 105 times more potent than misonidazole (parent drug) in this system, it lacks activity in murine or human xenografted tumors.
  • 102. The p-nitro substituent in the nitrogen mustard (7.82), by exerting an electron withdrawing effect, reduces the electron density on the nitrogen thereby inhibiting the formation of the alkylating carbonium ion. Reduction of the nitro group in the nitrogen mustard (7.82) in a hypoxic environment removes its electron withdrawing effect and restores the ability of the compound to form the alkylating species via an SN1 reaction pathway. Whether reduction gives the hydroxylamine (7.83) or the amine (7.84) is uncertain, but both species have greater activity than the nitro compound.
  • 103. The aziridine (7.85) may be activated in a similar manner and has been shown to be selectively toxic to hypoxic cells. It should be noted that the presence of additional groups in the aryl ring may affect the actual electron density on the nitrogen atom, and hence the reactivity of the alkylating species generated, despite the activation process occurring on reduction.
  • 104. Soluble macromolecular prodrug delivery systems have also been developed to improve the pharmacokinetic profile of pharmaceutical agents by the controlled release of the active agent. Recently the potential of N-(2-hydroxypropyl)-methacrylamide (HPMA) copolymers as carriers for the antitumor agent doxorubicin has been investigated. Doxorubicin was linked to the polymeric carrier by peptidyl spacers designed to be cleaved by lysosomal thiol-dependent proteases, which are known to have increased activity in metastatic tumors. Such conjugates have been shown to have a broad range of antitumor activities against leukemic, solid tumor and metastatic models. Fluorescein- labelled HPMA copolymers have been shown to accumulate in vascularized stromal regions, particularly in new growth sites in the tumor periphery.
  • 105. Recent research has been directed toward alternative approaches to obtain site-specific activation of prodrugs for cancer chemotherapy using antibody-directed enzyme prodrug therapy (ADEPT). The ADEPT approach employs an enzyme, not normally present in the extracellular fluid or on cell membranes, conjugated to an antitumor antibody which localizes in the tumor via an antibody-antigen interaction on administration. Once any unbound antibody conjugate has been cleared from the systemic circulation, a prodrug, which is specifically activated by the enzyme conjugate, is administered.
  • 106. The bound enzyme-antibody conjugate ensures that the prodrug is converted to cytotoxic parent compound only at the tumor site thereby reducing systemic toxicity. It has been shown that in systems utilizing cytosine deaminase to generate 5-fluorouracil from the 5- fluorocytosine prodrug at tumor sites, 17 times more drug can be delivered within a tumor than on administration of 5-fluorouracil alone.
  • 107. 1.4.5. Sustaining drug action For drugs rapidly cleared from the body frequent dosing is required. Alterations in the form of physiochemical properties of a drug in the form of a prodrug can be used to prolong drug action (duration of action). Prodrug design has also been successfully used to modify the duration of action of the parent drug by either reducing the clearance of drug or by providing a depot of the parent drug.
  • 108. Intramuscular depot injections of lipophilic ester prodrugs of steroids (testosterone propionate and estradiol propionate) and antipsychotics (fluphenazine decanoate) resulted in controlled release rate. The prodrug bitolterol (7.86), which is the di-p-toluate ester of N-t-butyl noradrenaline (7.87), has been shown to provide a longer duration of bronchodilator activity than the parent drug (in dog model).
  • 109. The phenothiazine group of drugs, acting as tranquillizers, have been converted to long acting prodrugs which are administered by intramuscular injection. Not only is the frequency of administration reduced, but also the problem associated with patient compliance is also eliminated. Flupenthixol (7.88) when administered as the decanoate ester (7.89) in an oily vehicle for the treatment of schizophrenia is released intact from the depot and subsequently hydrolyzed to the parent drug, possibly after penetration of the blood-brain barrier. Maximum blood levels are observed within 11-17 days after injection and the plateau serum levels averaged 2-3 weeks in duration.
  • 111. Similarly, perphenazine (7.90) has been used as the enanthrate ester (7.91) and pipothiazine (7.92) as the undecanoate (7.93) ester and palmitate (7.94) ester (with prolonged duration of action of prodrug).
  • 112. 1.5. DRAWBACKS OF PRODRUG APPROACH Although the prodrug concept may be utilized to improve the undesirable properties of the drug, it may become a potential source of toxicities if, a) The prodrug generates toxic metabolites which are not generated by the parent drug; b) Increased consumption of glutathione during the conversion of prodrug to active metabolite may leave vital cell constituents unprotected; c) Inert carrier moiety could not remain inert and leads to formation of toxic metabolites; d) The prodrug or/and carrier moiety generate such metabolites which alter the pharmacokinetic features of the parent drug by either inducing metabolic enzymes or by competing the active drug for binding with plasma-proteins.