Prodrug All Lectures (2) PDF

The Organic Chemistry of Drug Design and Drug Action Prodrugs and Drug Delivery Systems Dr.yahya saad yaseen  Prodrugs and Drug Delivery Systems Drug Discovery: 1. Drug design and development 2. Drug Design: optimizing target interaction 3. Drug Design: Optimizing access to t...

The Organic Chemistry of Drug Design and Drug Action Prodrugs and Drug Delivery Systems Dr.yahya saad yaseen  Prodrugs and Drug Delivery Systems Drug Discovery: 1. Drug design and development 2. Drug Design: optimizing target interaction 3. Drug Design: Optimizing access to the target  Prodrugs and Drug Delivery Systems Prodrug : a pharmacologically inactive compound that is converted to an active drug by a metabolic biotransformation Ideally, conversion occurs as soon as the desired goal for designing the prodrug is achieved.  Utility of Prodrugs 1. Aqueous Solubility : to increase water solubility so it can be injected in a small volume 2. Absorption and Distribution: to increase lipid solubility to penetrate membranes for better absorption 3. Site Specificity :the extent to which a drug produces only the desired therapeutic effect without causing any other physiological changes. A drug with high specificity exhibits a strong drug–receptor interaction, ensuring targeted action and minimal side effects  Utility of Prodrugs (cont’d) 4. Instability :to prevent rapid metabolism; avoid first- pass effect 5. Prolonged Release: to attain a slow, steady release of the drug 6. Toxicity : to make less toxic until it reaches the site of action 7. Poor Patient Acceptability :to remove an unpleasant taste or odor; gastric irritation 8. Formulation Problems : to convert a drug that is a gas or volatile liquid into a solid Types of Prodrugs rational prodrug design  Types of Prodrugs rational prodrug design I. Carrier-linked prodrug: A compound that contains an active drug linked to a carrier group that is removed enzymatically A. bipartate : comprised of one carrier attached to drug B. tripartate : carrier connected to a linker that is connected to drug #tripartite agreement involves three parties while bipartite agreement involves two parties II. Bioprecursor prodrug A compound metabolized by molecular modification into a new compound, which is a drug or is metabolized further to a drug - not just simple cleavage of a group from the prodrug—e.g., amine getting oxidized to CO2H, to afford the active.  Mechanisms of Prodrug Activation Carrier-Linked Prodrugs Most common activation reaction is hydrolysis. Rate of hydrolysis can be modified by locating alkyl groups in area of the carbonyl group to Increase steric hindrance, and retard hydrolysis rate.  Ideal Drug Carriers 1. Protect the drug until it reaches the site of action 2. Localize the drug at the site of action 3. Allow for release of drug 4. Minimize host toxicity 5. Are biodegradable, inert, and nonimmunogenic 6. Are easily prepared and inexpensive 7. Are stable in the dosage form  Carrier Linkages for Various Functional Groups Alcohols, Carboxylic Acids, and Related Groups Most common prodrug form is an ester esterases are ubiquitous can prepare esters with any degree of hydrophilicity or lipophilicity ester stability can be controlled by appropriate electronic and steric manipulations Prodrugs for Alcohol- Drug—OH Drug—OX Containing Drugs alcohols X Effect on Water Solubility O (R = aliphatic or aromatic) C—R decreases (increases lipophilicity) O + increases (pKa ~ 8) C—CH 2NHMe2 O increases (pKa ~ 5) C—CH 2CH2COO- O increases (pKa ~ 4) C Ester analogs as NH + prodrugs can affect PO3= (phosphate ester) increases (pKa ~ 2 and ~ 6) lipophilicity or O hydrophilicity CCH 2SO3- increases (pKa ~ 1) To accelerate hydrolysis rate: attach an electron-withdrawing group if a base hydrolysis mechanism is important attach an electron-donating group if an acid hydolysis mechanism is important To slow down hydrolysis rate: make sterically-hindered esters make long-chain fatty acid esters  Another Approach to Accelerate Hydrolysis Intramolecular hydrolysis of succinate esters O H OH O -O COO- Drug—O - O Drug—OH + -OOC Drug—O O O Also, acetals or ketals can be made for rapid hydrolysis in the acidic medium of the GI tract. Enolic hydroxyl groups can be esterified as well. HOOC S OH O F O S F O N O Cl Cl N O O H2N H2N oxindole 8.1 8.2 antirheumatic agent Carboxylic Acid-Containing Groups Esterify as with alcohols  Maintaining Water Solubility of Carboxylate Prodrugs O + Drug—C—O—CH2CH2—NRR' 2 8.3 Can vary pKa by appropriate choice of R and R  Prodrugs for Phosphate- or Phosphonate-Containing Drugs O O O P O 2 8.4  Amine Prodrugs Drug—NH 2 Drug—NHX X O O O O + CR CCHNH3 C OPh —CH 2NHCAr CHAr NAr R #Amides are commonly not used because of stability #Activated amides (low basicity amines or amino acids) are effective # pKa of amines can be lowered by 3 units by conversion to N- Mannich bases (X = CH2NHCOAr) N-Mannich base (R = CH2NHCOPh) has a log D7.4 two units greater than the parent compound. OH CH3 NHR phenylp ropanolamine hydrochlo ride (R = H. HCl) 8.5 Another approach to lower pKa of amines and make more lipophilic. Imine (Schiff base) prodrug OH O N F NH 2 hydrolyze imine and amide to GABA anticonvulsant Cl inside brain proga bide 8.6  A Reductive Carrier-Linked Prodrug Approach OH OH N NO2 :NH Y Y Y bioreductio n XH Drug + X Z X Z Z Drug Drug 8.7 8.8  Prodrugs of Sulfonamides A water soluble prodrug of the anti-inflammatory drug valdecoxib (8.9) has been made (8.10). Ph N Ph N O O Na+ H2N CH3 N CH3 S S O O O O O valdecoxib parecoxib sodiu m 8.9 8.10  Prodrug Analogs of Carbonyl Compounds Drug Drug X C=O C R R Y X C Y OR' O S C=NR' C=NOH C C C OR' N N H H imines oximes ketals Examples of Carrier-Linked Bipartate Prodrugs  1. Prodrug for O Increased Water R' = CCH2CH2CO2 Na Solubility OH OR' Me Prodrug forms HO O Me R' = PO3Na 2 for aqueous injection or opthalmic use O R predniso lone (R = R' = H ) poor water methylpredniso lone (R = CH3 , R' = H) solubility 8.11 corticosteroid Choice of water solubilizing group: The ester must be stable enough in water for a shelf life of > 2 years (13 year half-life), but must be hydrolyzed in vivo with a half-life < 10 minutes. Therefore, in vivo/in vitro lability ratio about 106. To avoid formulation of etoposide with detergent, PEG, and EtOH (used to increase water solubility), it has been converted to the phosphate prodrug. O O CH 3 O HO O HO O O O O CH 3O OCH 3 OR etoposide (R = H) etoposide phosph ate (R = PO3 H2 ) 8.12 2. Prodrug for Improved Absorption Through Skin OR O CH3 CH3 HO O CH3 CH3 O F O F fluocinolone acetonide (R = H) fluocinon ide (R = COCH3 ) 8.14 corticosteroids - inflammation, allergic, pruritic skin conditions Better absorption into cornea for the treatment of glaucoma RO OH RO NHCH3 dipivefrin (R = Me3 CCO) epinephr ine (R = H ) 8.15 The cornea has significant esterase activity 3.Prodrug for Site Specificity RO Bowel sterilant OR O N H oxyphenisatin (R = H) (administer rectally) 8.16 prodrug R = Ac (administer orally) hydrolyzed in intestines  3. Prodrug for Site Specificity The blood-brain barrier prevents hydrophilic molecules from entering the brain, unless actively transported. The anticonvulsant drug vigabatrin.crosses poorly. A glyceryl lipid (8.17, R = linolenoyl) containing one GABA ester and one vigabatrin ester was 300 times more potent in vivo than vigabatrin. OCOR O NH2 O O NH2 O 8.17  OR Site Specificity Using Enzymes at the Site of Action RO diethylstilbestrol diphosphate (R = PO3= ) diethylstilbestrol (R = H) 8.18 Phosphatase should release the drug selectively in tumor cells. This approach has not been successful because the prodrugs are too polar, enzyme selectivity is not sufficient, or tumor cell perfusion rate is poor.  Enzyme-Prodrug Therapies For selective activation of prodrugs in tumor cells Two steps: 1. incorporate a prodrug-activating enzyme into a target tumor cell 2. administer a nontoxic prodrug which is a substrate for the exogenous enzyme incorporated  Criteria for Success with Enzyme-Prodrug Therapies 1.The prodrug-activating enzyme is either nonhuman or a human protein expressed poorly 2.The prodrug-activating enzyme must have high catalytic activity 3.The prodrug must be a good substrate for the incorporated enzyme and not for other endogenous enzymes 4.The prodrug must be able to cross tumor cell membranes 5. The prodrug should have low cytotoxicity and the drug high cytotoxicity 6. The activated drug should be highly diffusible to kill neighboring non expressing cells (bystander killing effect) 7. The half-life of the active drug is long enough for bystander killing effect but short enough to avoid leaking out of tumor cells  Antibody-Directed Enzyme Prodrug Therapy (ADEPT) An approach for site-specific delivery of cancer drugs. Phase One: An antibody-enzyme conjugate is administered which binds to the surface of the tumor cells. The antibody used has been targeted for the particular tumor cell. The enzyme chosen for the conjugate is one that will be used to cleave the carrier group off of the prodrug administered in the next phase. Phase Two: After the antibody-enzyme has accumulated on the tumor cell and the excess conjugate is cleared from the blood and normal tissues, the prodrug is administered. The enzyme conjugated with the antibody at the tumor cell surface catalyzes the conversion of the prodrug to the drug when it reaches the tumor cell. Advantages: ADEPT 1. Increased selectivity for targeted cell 2. Each enzyme molecule converts many prodrug molecules 3. The released drug is at the site of action 4. Demonstrated to be effective at the clinical level 5. Concentrates the drug at the site of action Disadvantages: 1. Immunogenicity and rejection of antibody-enzyme conjugate 2. Complexity of the two-phase system and i.v. administration 3. Potential for leakback of the active drug An example is carboxypeptidase G2 or alkaline phosphatase linked to an antibody to activate a nitrogen mustard prodrug. I I I I N N carboxypeptidase G2 + L-Glu + CO2 H O N CO2H OH O CO2H electron-do nating; 8.19 activates nitrogen mustard electron-withdrawing; deactivates nitrogen mustard Humanization of antibodies minimizes immunogenicity. Note the prodrug-activating enzyme is a bacterial enzyme. Antibody-Directed Abzyme Prodrug Therapy (ADAPT) Instead of using a prodrug-activating enzyme, humanized prodrug-activating catalytic antibody (abzyme) can be used. Ideally, the abzyme catalyzes a reaction not known to occur in humans, so the only site where the prodrug could be activated is at the tumor cell where the abzyme is bound. Antibody 38C2 catalyzes sequential retro-aldol and retro-Michael reactions not catalyzed by any known human enzyme. Found to be long-lived in vivo, to activate prodrugs selectively, and to kill colon and prostate cancer cells.  Abzyme 38C2 Activation of a Doxorubicin Prodrug O OH O O OH O O OH O OH OH OH 38C2 38C2 OH OH OH CH3O O OH H O CH3O O OH H O B- O CH3O O OH H O O CH3 O retro-aldol O retro-Michael O CH3 H CH3 NH O NH O NH O HO HO HO O H O O O O O- 8.20 -CO2 +H+ B- O OH O OH OH CH3O O OH H O CH3 O NH2 HO doxorubicin  Gene-Directed Enzyme Prodrug Therapy (GDEPT) Also known as suicide gene therapy A gene encoding the prodrug-activating enzyme is expressed in target cancer cells under the control of tumor-selective promoters or by viral transfection. These cells activate the prodrug as in ADEPT. Cl Cl OMe OMe N nitroreductase H N OMe OMe N N HO N: H O2N O H O OMe OMe NMe NMe N O NH N O NH -CO2 O O +H+ Cl OMe N OMe N O H OMe NH2 amino-seco-CBI-TMI 8.21 Nitroreductase gene-directed enzyme prodrug therapy suicide gene therapy suicide gene therapy  4.Prodrug for Stability protection from first-pass effect Oral administration has lower bioavailability than i.v. injection. O NHCH(CH3 )2 OR' R propanolol (R = R' = H) 8.22 antihypertension O prodrug R' = CCH2 CH2COOH plasma levels 8 times that with propanolol 5.Prodrugs for Slow and Prolonged Release 1. To reduce the number and frequency of doses 2. To eliminate night time administration 3. To minimize patient noncompliance 4. To eliminate peaks and valleys of fast release (relieve strain on cells) 5. To reduce toxic levels 6. To reduce GI side effects #Long-chain fatty acid esters hydrolyze slowly #Intramuscular injection is used also  O OR F N Cl haloperid ol (R = H) haloperid ol decan oate (R = CO(CH2) 8CH3 ) 8.24 Sedative/tranquilizer/antipsychotic O prodrug R' = C(CH2) 8CH3 slow release inject i.m. Antipsychotic activity for about 1 month  6.Prodrugs to Minimize Toxicity Many of the prodrugs just discussed also have lowered toxicity. For example, epinephrine (for glaucoma) has ocular and systemic side effects not found in dipivaloylepinephrine. 7.Prodrug to Increase Patient Acceptance CH3 The antibacterial drug N H H N Cl clindamycin (8.28) is bitter and not well H O HO OH O tolerated by children. SCH3 OR Clindamycin palmitate clindomycin (R = H) is not bitter. clindomycin pho sphate (R = PO3H2) clindomycin palmitate (R = O(CH 2) 1 4CH3 ) 8.28 Either not soluble in saliva or does not bind to the bitter taste receptor or both.  8.Prodrug to Eliminate Formulation Problems Formaldehyde is a gas with a pungent odor that is used as a disinfectant. Too toxic for direct use. N H+ CH2O + NH3 N N H3O+ N methenamine 8.30 It is a stable solid that decomposes in aqueous acid. The pH of urine in the bladder is about 4.8, so methenamine is used as a urinary tract antiseptic. Has to be enteric coated to prevent hydrolysis in the stomach.  Areas of Improvement for Prodrugs site specificity protection of drug from biodegradation minimization of side effects  Macromolecular Drug Delivery To address these shortcomings, macromolecular drug delivery systems have been developed. A bipartate carrier-linked prodrug in which the drug is attached to a macromolecule, such as a synthetic polymer, protein, lectin, antibody, cell, etc. Absorption/distribution depends on the physicochemical properties of macromolecular carrier, not of the drug. Therefore, attain better targeting. Minimize interactions with other tissues or enzymes. Fewer metabolic problems; increased therapeutic index.  Disadvantages of Macromolecular Delivery Systems Macromolecules may not be well absorbed Alternative means of administration may be needed (injection) Immunogenicity problems  Macromolecular Drug Carriers Synthetic polymers CH2 CH CH2 CH x y OH O O O 8.32 O Aspirin linked to poly(vinyl alcohol) has about the same potency as aspirin, but less toxic.  Steric Hindrance by Polymer Carrier poly(methacrylate) CH3 CH3 CH2 C CH2 C x y C=O C=O O O to enhance water S=O solubility O testosterone 8.34 #No androgenic effect #Polymer backbone may be sterically hindering the release of the testosterone. A spacer arm was added, and it was as effective as testosterone. CH3 CH3 CH2 C CH2 C x y C=O C=O O O to enhance - water + Cl spacer NMe2 S=O solubility arm O H3 C O O 8.35  Poly(-Amino Acid) Carriers O poly(L-glutamine) NH CH C x spacer O O N O O H C CH O 8.37 norethindrone - contraceptive #Slow release over nine months in rats General Site-Specific Macromolecular Drug Delivery System Polymer chain Solubilizer spacer Homing device either hydrophilic or hydrophobic Drug for site specificity 8.38 Site-Specific Delivery of a Nitrogen Mustard O O O NHCH C NH CH C y NHCH C z poly(L-Glu) x spacer O arm O O NH O- NH water-solubilizing Ig antibody from rabbit antiserum N against mouse Cl Cl lymphoma cells 8.39 All 5 mice tested were alive and tumor free after 60 days (all controls died). Also, therapeutic index greatly enhanced (40 fold).  Tumor Cell Selectivity Drug attached to albumin (R = albumin) NH2 Tumor cells take up proteins rapidly. Proteins N broken down inside cells, O N releasing the drug. RO O HO Shown to inhibit growth of Ectomelia virus in OH mouse liver, whereas free cytosine arabinoside (R = H) inhibitor did not. 8.41 antitumor  Antibody-Targeted Chemotherapy calicheamicin, except as disulfide HO instead of trisulfide O NH O CH3O H polysaccharide HO O O CH3 S H O N S NH CH3O N H polysaccharide H N RS - O O antibody Lys O S O gemtuz umab ozo gamicin 8.42 8.43 humanized spacer Does not release calicheamicin nonenzymatically. Exhibits no immune response.  Tripartate Drugs (Self-immolative Prodrugs) A bipartate prodrug may be ineffective because the linkage is too labile or too stable. In a tripartate prodrug, the carrier is not attached to the drug; rather, to the linker. Therefore, more flexibility in the types of functional groups and linkages that can be used, and it moves the cleavage site away from the carrier. The linker-drug bond must cleave spontaneously (self-immolative) after the carrier-linker bond is broken.  Tripartate Prodrugs enzyme Carrier Linker Drug Carrier + Linker Drug spontaneous Linker + Drug  Typical Approach O Drug—X—CH2 —O—CR esterase Drug—X—CH2 —O- + RCOOH fast Drug—X - + CH2O  Tripartate Prodrugs of Ampicillin O Poor oral absorption (40%) NH S NH2 Excess antibiotic may destroy N important intestinal bacteria O used in digestion and for COO- biosynthesis of cofactors. ampicillin 8.44 Also, more rapid onset of antibacterial resistance. Various esters made were too stable in humans (although they were hydrolyzed in rodents) - thought the thiazolidine ring sterically hindered the esterase.  Tripartate Prodrugs of Ampicillin O O Ph Ph NH NH S S NH2 esterase NH2 + R'COOH N N O O.. O O R' O OH when O O R' = OEt R O R 8.46 bacampicillin (R = CH 3 , R' = OEt) EtOH pivampicillin (R = H, R' = t-Bu) O + CO2 8.45 R H 98-99% absorbed 8.44 Ampicillin is released in < 15 minutes  hydrolysis activated hydrolysis deactivated O O O Drug—X crosses Drug—X enzyme Drug—X blood-brain oxidation N barrier N N+ CH3 CH3 CH3 8.48 8.47 hydrophilic HOOC enzyme hydrolysis drug + Drug — XH electron- N+ electron-donating, withdrawing; CH3 lipophilic carrier 8.49 hydrophilic Passive diffusion of 8.47 into the brain; active transport of 8.49 out of the brain XH of the drug is NH2, OH, or COOH If oxidation occurs before it gets into the brain, it cannot cross the blood-brain barrier.  When the drug is a carboxylic acid, a self-immolative reaction also can be used. O O O O Drug C OCH2 O C carrier esterase Drug— C— O—CH2 —O- + HOC carrier 8.50 fast —CH2O Drug—COO-  Example of Redox Drug Delivery Antibody generation in the brain is not significant. -Lactams are too hydrophilic to cross the blood-brain barrier effectively. H R N S H R N S O O N enzymatic O O O N O O oxidation O O O O N O N Me 8.51 Me esterase H H R N S N O R S -CH2O O N O N O O O O- O O N O O Me 8.49 High concentrations of -lactams delivered into brain.  Tripartate Prodrug for Delivery of Antibacterials Permeases are bacterial transport proteins for uptake of peptides.  Mutual Prodrugs A bipartite or tripartite prodrug in which the carrier is a synergistic drug with the drug to which it is linked. Antibacterial -lactamase inactivator ampicillin penicillanic acid sulfone O Ph N O O H S NH2 S N O O O N O O O sultamicillin 8.59 Hydrolysis gives 1:1:1 ampicillin : penicillanic acid sulfone : formaldehyde  Ideal Mutual Prodrugs Well absorbed Both components are released together and quantitatively after absorption Maximal effect of the combination of the two drugs occurs at 1:1 ratio Distribution/elimination of components are similar  II.Bioprecursor Prodrugs Carrier-linked prodrugs largely use hydrolytic activation Bioprecursor drugs mostly use oxidative or reductive activation #The metabolically-activated alkylating agents are actually examples of bioprecursor prodrugs.  Protonation Activation Discovery of Omeprazole Cimetidine and ranitidine reduce gastric acid secretion by antagonizing the H2 histamine receptor. Another way to lower gastric acid secretion is by inhibition of the enzyme H+,K+-ATPase (also called the proton pump), which exchanges protons for potassium ions in parietal stomach cells, thereby increasing stomach acidity.  Lead Discovery Lead compound found in a random screen. S N NH2 8.60 Liver toxicity observed thought to be because of the thioamide group.  Lead Modification Related analogs made, and 8.61 had good antisecretory activity. N S N N 8.61 H Modification gave 8.62 with high activity. N S N N 8.62 H The sulfoxide (8.63) was more potent, but it blocked iodine uptake into the thyroid. O N S N N H timoprazole 8.63  Lead Modification (cont’d) CH3 Modification of 8.63 gave O N CO2CH3 8.64 having no iodine N S blockage activity. N H CH3 picopraz ole 8.64 Picoprazole shown to be an inhibitor of H+,K+-ATPase. SAR of analogs indicated OCH3 CH3 electron-donating groups on H3 C O N OCH3 the pyridine ring were S N favorable. Increased inhibition N H of H+,K+-ATPase. omepraz ole Best analog was omeprazole 8.65 (8.65). The pKa of the pyridine ring of omeprazole is about 4, so it is not protonated and able to cross the secretory canaliculus of the parietal cell. The pH inside the cell is below 1, so this initiates the protonation reaction below. OCH3 OCH3 OCH3 OCH3 H3 C CH3 CH3 CH3 CH3 H3 C H3 C H3 C N -H2 O.. N N N S S.. S S O HN S HN HN O OH N N N N N H H+ OCH3 OCH3 OCH3 OCH3 8.66 omepraz ole 8.65 +H+ OCH3 H3 C CH3 Formation of 8.66 leads to covalent attachment. N Omeprazole also inhibits isozymes of carbonic N S S NH anhydrase, another mechanism for lowering gastric acid secretion. OCH3  Hydrolytic Activation Hydrolysis can be a mechanism for bioprecursor prodrug activation, if the product requires additional activation. R Me S O S O- Me S O- O S+ N N S+ N S H N Me K2CO3/RX HO O O Me OH Me HO O Me HO O leinamycin 8.70 8.71 antitumor agent prodrugs of leinamycin HO- O Hydrolysis of these analogs gives an intermediate Me O O that reacts further to the activated form. S- O Me S S Me O- O O- Me S O O- S S+ N N S+ N S+ N S H N N Me Me HO O HO O O Me OH DNA cleavage Me O Me Me O O O O O O this was O O synthesized and 8.72 8.73 gave 8.74 as well increased stability as DNA cleavage Me over leinamycin HO2 C O S N N Me H HO S HO Me O O O 8.74  Elimination Activation CF 3 CF 3 B: O O H N N C H N H N O CH3 HO CH3 H B 8.76 potent inhibitor of leflunom ide 8.75 dihydroorotate rheumatoid arthritis drug dehydrogenase Inhibition of dihydroorotate dehydrogenase blocks pyrimidine biosynthesis in human T lymphocytes.  Oxidative Activation N-Dealkylation Sedative CH3 CH3 CH3 N N N N N N N N N CH3 P450 CH3 P450 NH.. 2 O N N O Cl O CH3 Cl H Cl X X X CH3 N CH3 N N N N -H2 O N Cl N.. Cl NH HO X X alpraz oalam (X = H) triaz olam (X = Cl) 8.77  O-Dealkylation Analgesic activity of phenacetin is a result of O-dealkylation to acetaminophen. O HN CH3 OR phenacetin (R = C H2 CH3 ) acetaminophen (R = H) 8.78  Oxidative Deamination Neoplastic (cancer) cells have a high concentration of phosphoramidases, so hundreds of phosphamide analogs of nitrogen mustards were made for selective activation in these cells. HO: H H O N P-450 N H NH O O 2 P Cl P Cl H P O O N O N O Cl N B: Cl Cl Cl cyclophosphamide 8.80 8.81 8.79 H 2N O - P O O Cl Cl spontaneous N H HPO4= + NH3 + HN or + phosphoramidase Cl Cl 8.83 Cyclophosphamide was very 8.84 8.82 effective, but it required liver DNA DNA homogenates (contains P450) O H N O H N + OH OH for activation. Therefore HN N HN N N N oxidation is required, not H 2N N DNA H 2N N hydrolysis. 8.86 8.85 isolated  N-Oxidation identified O O O P450 -H2 O CH3 NHNHCH2 CNHCHMe 2 CH3 NHNHCH2 CNHCHMe 2 CH3N=N—CH CNHCHMe 2 + HO B—H H procarbaz ine 8.88 8.87 B: advanced Hodgkin’s disease O O H2O CH3N-NH2 + OHC CNHCHMe 2 CH3N-N=CH CNHCHMe 2 H H 8.89 CH3N=NH O CH3 O CH3 + +N CH3-N N N DNA HN HN CH3 + N2 H2N N N H2N N N DNA Scheme 8.22 8.90 identified  N-Oxidation Pralidoxime chloride is an antidote for nerve poisons. It reacts with acetylcholinesterase that has been inactivated by organophosphorus toxins. N OH - +N Cl CH3 pralidoxime chloride 8.91  To increase the permeability of pralidoxime into the CNS, the pyridinium ring was reduced (8.92). N OH N CH3 8.92 oxidation into brain N OH - +N Cl CH3 pralidoxime chloride 8.91 Similar to the reversible redox drug delivery strategy for getting drugs into the brain by attaching them to a dihydronicotinic acid, hydrophobic 8.92 crosses the blood-brain barrier; oxidation to 8.91 prevents efflux from brain.  Prodrugs for (increased) Site Specificity ✵Bodor and co-workers have devised a reversible redox drug delivery system (RRDDS) for getting drugs into the CNS and then, once in, preventing their efflux. The approach is based on the attachment of a hydrophilic drug to a lipophilic carrier (a dihydropyridine) thereby making prodrug that actively transported into the brain. O H H O H H R R X X N N CH3 CH3 Blood-brain Prodrug residue barrier  Prodrugs for (increased) Site Specificity Once inside the brain, the lipophilic carrier is converted enzymatically to a highly hydrophilic species (positively charged), which O H is then enzymatically hydrolyzed back to the H R drug and N-methylnicotinic acid, which is X eliminated from the brain. N CH3 ♫The oxidation of the dihydropyridine to the Enzymatic oxidation pyridinium ion (half-life generally 20-50 min) O prevents the drug from escaping out of the R brain because it becomes charged. X ♫This drives the equilibrium of the lipophilic N precursor throughout all of the tissues of the CH3 body to favor the brain. Prodrugs for (increased) Site Specificity O H H O H H R R X X N N CH3 CH3 Blood-brain Enzymatic oxidation Prodrug residue barrier O HOOC R Drug release HX R X + Drug by a suitable N process N CH3 CH3 Mechanism of Acetylcholinesterase O Me 3N—CH2CH2—O Me 3N—CH2CH2—OH Trp H CH3 Trp O Phe Trp Trp + B+ H :B Phe B: HB O CH3 O H2O + Me 3NCH2CH2OH + CH3 COOH  Inactivation of Acetylcholinesterase by Diisopropyl Phosphorofluoridate Scheme 8.24 affinity labeling agent O O O O F P Trp P H O Trp O Phe Trp B+ :B O H Phe Trp B HB O 8.93 irreversible inhibition Inactivation prevents degradation of the excitatory neurotransmitter acetylcholine. Accumulation of acetylcholine causes muscle cells in airways to contract and secrete mucous, then muscles become paralyzed.  Reactivation of Inactivated Acetylcholinesterase by Pralidoxime Scheme 8.25 O O H O N N O P O: Trp pralidoxime P O Trp Me O O Phe Trp B HB Phe Trp O B HB O O N O N O P O N P O Trp Me N O Me O Phe Trp BH OH :B  Temporary inhibition of acetylcholinesterase enhances cholinergic action on skeletal muscle. Scheme 8.26 O Me 3N O Me 3N OH Trp H NMe2 Trp O + Phe Trp B+ :B Trp B: HB H Phe O NMe2 O neostigmine covalent, but 8.94 reversible slow H2O Used for the neuromuscular inhibition disease myasthenia gravis + CO2 + Me2NH Me 3N OH  Reversible (noncovalent) inhibitors of acetylcholinesterase, such as donepezil and tacrine are used for Alzheimer’s disease. Enhances neurotransmission involved in memory. O H3CO NH2 H3CO N N.. HCl HCl donepez il hydrochloride tacrine hydrochloride 8.95 8.96  S-Oxidation Poor oral bioavailability of brefeldin A. Converted to Michael addition sulfide prodrug (8.98). S-Oxidation and elimination gives brefeldin A. Scheme 8.27 :B H HO H HO H HO H H O O HO RSH [O] O O CH3 HO RS H HO RS O CH3 H O CH3 H H O H brefeldin A 8.97 8.98 8.99 -RSOH antitumor, antiviral agent  Aromatic Hydroxylation Cyclohexenones as prodrugs for catechols Scheme 8.28 N -H2 O N N P450 N HO O O OH O P450 8.100 aromatic hydroxylation oxidation next to sp2 carbonyl N HO OH 8.101  Alkene Epoxidation O N N O NH2 O NH2 carbamaz epine 8.102 8.103 active anticonvulsant agent  Transamination Stimulation of pyruvate dehydrogenase results in a change of myocardial metabolism from fatty acid to glucose utilization. Glucose metabolism requires less O2 consumption. Therefore, utilization of glucose metabolism would be beneficial to patients with ischemic heart disease (arterial blood flow blocked; less O2 available). Arylglyoxylic acids (8.104) stimulate pyruvate dehydrogenase, but have a short duration of action. O COOH R 8.104 Oxfenicine (8.105, R = OH) is actively transported and is transaminated (a PLP aminotransferase) in the heart to 8.104 (R = OH). NH 2 COOH R oxfenicine (R = OH) 8.105  Reductive Activation Azo Reduction Scheme 8.29 COOH NHSO2 N=N OH Anaerobic cleavage N by bacteria in lower sulfasalazine 8.106 bowel ulcerative colitis COOH H2N OH + NHSO2 NH2 N 8.107 sulfapyridine 8.108 For inflammatory Causes side effects bowel disease To prevent side effect by sulfapyridine a macromolecular delivery system was developed. n poly(vinylamine) NH SO2 Not absorbed or metabolized in small spacer CO2Na intestine. N N OH 8.109 CO2Na NH2 OH Released by reduction at the disease site. Sulfapyridine is not released (still attached to polymer). More potent than sulfasalazine.  Azido Reduction R N N N N HO O OH HO vidarabine (R = NH2) 8.110 antiviral drug Vidarabine is rapidly deaminated by adenosine deaminase. 8.110, R = N3 is not a substrate for adenosine deaminase and also can cross the blood-brain barrier for brain infections.  Sulfoxide Reduction CO2H F CH3 CH3 S O sulindac 8.111 anti-arthritis Sulindac is inactive in vitro; the sulfide is active in vitro and in vivo. Sulindac is an indane isostere of indomethacin, which was designed as a serotonin analog. The 5-F replaced the 5-OMe group to increase analgesic properties. The p-SOCH3(sulfine) group replaced p-Cl to increase water solubility. CO2H CO2H F MeO CH3 CH3 N NH2 O HO CH3 S N Cl H O indometh acin serotonin sulindac 8.112 8.111  Disulfide Reduction To increase the lipophilicity of thiamin for absorption into the CNS. OH OH Scheme 8.30.. N GSH N N S S O N S- O H O H CH3 N NH2 CH3 N NH2 +B H 8.113 nonenzymatic OH + N: N N OH N S S CH3 N NH2 CH3 N NH2 OH thiamin 8.114 poorly absorbed into CNS CH3O To diminish toxicity of primaquine and target it for cells with the malaria parasite, a N HN macromolecular drug delivery system was NH2 designed. primaquine 8.115 antimalarial, toxic Intracellular thiol much higher than in blood; selective reduction inside the cell CH3O lactose N O HN serum N S S H albumin NH 3+ primaquine 8.116 for lactose improved uptake in liver Therapeutic index of 8.116 is 12 times higher than 8.115 in mice.  Nitro Reduction Scheme 8.32 O O O F F Cl F HN HN HN O O O Cl Cl N P O O N bioreductio n O.. N P O N N P O O N H3 C O O H3 C O O O H3 C O- HO HO HO O O 8.120 O2N.. 8.118 HO NH 8.119 O O O F F F HN HN HN O O O -O O N H2O P O O N N P O O N P O O O O H3 C O- O O- HO HO HO 8.121 Mechanism-based inactivator of thymidylate synthase  Nucleotide Activation Scheme 8.33 =O SH 3PO O O O SH N N O—P—O—P—O- N N N N HO OH O - O - = O3PO O N N hypoxanthine-guanine H phosphorib osyltransferase 6-mercaptopurine HO OH 8.122 Anti-leukemia drug 8.123 Inhibits several enzymes in the purine nucleotide biosynthesis pathway.  Phosphorylation Activation O O N HN HN N H 2N N N H2N N N HO RO O O HO acyclovir (R = H) 8.124 8.125 antiviral 2-deoxyguanosine Resembles structure of 2-deoxyguanosine viral thymidine kinase Uninfected cells do not R = PO3= phosphorylate acyclovir (selective toxicity) viral guanylate kinase R = P2O6-3 viral phosphoglycerate kinase R = P3O9-4 Acyclovir triphosphate is a substrate for viral -DNA polymerase but not for normal -DNA polymerase Incorporation into viral DNA leads to a dead-end complex (not active). Disrupts viral replication cycle and destroys the virus. Even if the triphosphate of acyclovir were released, it is too polar to be taken up by normal cells. High selective toxicity Only 15-20% of acyclovir is absorbed Therefore, prodrugs have been designed to increase oral absorption. Hydrolyzes NH2 here Prodrug for N N a prodrug H2N N N HO O 8.126 adenosine deaminase acyclovir Hydroxylates here N N H 2N N N HO O 8.127 xanthine oxidase acyclovir A bipartate carrier-linked prodrug of acyclovir, the L-valyl ester of acyclovir, has 3-5 fold higher oral bioavailability with the same safety profile. O HN N H 2N H 2N N N O O O valaciclovir 8.128 An analog of acyclovir whose structure is even closer to that of 2-deoxyguanosine is ganciclovir. O HN N H 2N N N HO O HO ganciclovir 8.129 More potent than acyclovir against human cytomegalovirus. Two carbon isosteres of ganciclovir are available. O N N N HN H 2N N H 2N N N N HO AcO C in place AcO C in place HO of O of O penciclovir famciclovir 8.130 8.131 Better oral absorption than penciclovir Converted to penciclovir rapidly  Sulfation Activation Activity of minoxidil requires sulfotransferase-catalyzed sulfation to minoxidil sulfate. OR H2N N NH2 N N minoxidil (R = ) minoxidil sulfate (R = SO3- ) 8.132 hair growth Inhibitors of the sulfotransferase inhibit the activity of minoxidil, but not minoxidil sulfate.  Decarboxylation Activation An imbalance in the inhibitory neurotransmitter dopamine and the excitatory neurotransmitter acetylcholine produces movement disorders, e.g. Parkinson’s disease. In Parkinson’s there is a loss of dopaminergic neurons and a low dopamine concentration. Dopamine treatment does not work because it cannot cross blood-brain barrier, but there is an active transport system for L-dopa (levodopa, 8.133, R = COOH).  HO HO NH2 H R levodop a (R = COOH) 8.133 After crossing blood- L-aromatic amino acid brain barrier decarboxylase (PLP) dopamine (R = H) Does not reverse the disease, only slows progression.  Combination Therapy Monoamine oxidase B (MAO B) degrades dopamine in the brain. Therefore, a MAO B-selective inactivator is used to protect the dopamine - selegiline (L-deprenyl). Peripheral L-aromatic amino acid decarboxylase destroys >95% of the L-dopa in the first pass. Maybe only 1% actually gets into brain. To protect L-dopa from peripheral (but not CNS) degradation, inhibit peripheral L-aromatic amino acid decarboxylase with a charged molecule that does not cross the blood-brain barrier. HO HO OH HO O NH2 NHNH3+ HO H3 C N N OH COO- H H carbidopa benseraz ide 8.134 8.135 (used in U.S.) (used in Europe and Canada) Selectively inhibits peripheral L-amino acid decarboxylase. The dose of L-dopa can be reduced by 75%.  Selective Inactivation of Brain Monoamine Oxidase A Earlier we found that inactivation of brain MAO A has an antidepressant effect, but a cardiovascular side effect occurs from inactivation of peripheral MAO A. 8.136 (R = COOH) is actively transported into the brain, where L-aromatic amino acid decarboxylase converts it to 8.136 (R = H), a selective MAO A mechanism-based inactivator. Carbidopa protects 8.136 (R = COOH) from decarboxylation outside of the brain. F NH 2 R OH 8.136  Selective Delivery of Dopamine to Kidneys Dopamine increases renal blood flow. Prodrugs were designed for selective renal vasodilation. Scheme 8.34 OH OH COO- L--glutamyl H transpeptidase + N OH + OH NH3 NH3 O COO- COO- Glu 8.137 L-dopa L--glutamyl-L-dopa L-aromatic amino acid decarboxylase + Dopamine accumulates in the kidneys because of NH3 OH high concentrations of L--glutamyltranspeptidase and L-aromatic amino acid decarboxylase there. OH