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Chapter 8 Prodrugs and Drug Delivery Systems. The Organic Chemistry of Drug Design and Drug Action. Prodrugs and Drug Delivery Systems. Prodrug - a pharmacologically inactive compound that is converted to an active drug by a metabolic biotransformation

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Chapter 8 prodrugs and drug delivery systems

Chapter 8

Prodrugs and Drug Delivery Systems

The Organic Chemistry of Drug Design and Drug Action


Prodrugs and drug delivery systems
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.

  • Prodrugs and soft drugs are opposite:

  • a prodrug is inactive - requires metabolism to give active form

  • a soft drug is active - uses metabolism to promote excretion

  • A pro-soft drug would require metabolism to convert it to a soft drug


Utility of prodrugs
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 and ability to reach site of action

3. Site Specificity - to target a particular organ or tissue if a high concentration of certain enzymes is at a particular site or append something that directs the drug to a particular site


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 drug latentiation rational prodrug design
Types of ProdrugsDrug Latentiation - 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

C. mutual - two, usually synergistic, drugs attached to each other

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


Protecting group analogy for the concept of prodrugs
Protecting Group Analogy for the Concept of Prodrugs

Scheme 8.1

analogous to

carrier-linked

analogous to

bioprecursor

Keep in mind that a prodrug whose design is based on rat metabolism may not be effective in humans.


Mechanisms of prodrug activation carrier linked prodrugs
Mechanisms of Prodrug ActivationCarrier-Linked Prodrugs

Most common activation reaction is hydrolysis.


Ideal drug carriers
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
Carrier Linkages for Various Functional GroupsAlcohols, 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 containing drugs
Prodrugs for Alcohol-Containing Drugs

Table 8.1

Ester analogs as prodrugs can affect lipophilicity or hydrophilicity


  • To accelerate the hydrolysis rate:

  • attach an electron-withdrawing group if a base hydrolysis mechanism is important

  • attach an electron-donating group if an acid hydrolysis mechanism is important

  • To slow down the hydrolysis rate:

  • make sterically-hindered esters

  • make long-chain fatty acid esters


Another approach to accelerate hydrolysis intramolecular hydrolysis of succinate esters
Another Approach to Accelerate HydrolysisIntramolecular hydrolysis of succinate esters

Scheme 8.2



Carboxylic acid containing groups esterify as with alcohols
Carboxylic Acid-Containing GroupsEsterify as with alcohols


Maintaining water solubility of carboxylate prodrugs
Maintaining Water Solubility of Carboxylate Prodrugs

Can vary pKa by appropriate choice of R and R



Amine prodrugs
Amine Prodrugs

Table 8.2

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.


Another approach to lower pKa of amines and make more lipophilic.

Imine (Schiff base) prodrug

hydrolyze imine and amide to GABA inside brain

anticonvulsant



Prodrugs of sulfonamides
Prodrugs of Sulfonamides

A water soluble prodrug of the anti-inflammatory drug valdecoxib (8.9) has been made (8.10).


Prodrug analogs of carbonyl compounds
Prodrug Analogs of Carbonyl Compounds

Table 8.3

imines

oximes

ketals

Acetals or ketals can be made for rapid hydrolysis in the acidic medium of the GI tract.



Prodrug Stability Properties

Choice of hydrolytic prodrug group: The group 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.


Prodrug for increased water solubility
Prodrug for Increased Water Solubility

Prodrug forms

for aqueous injection or opthalmic use

poor water solubility

corticosteroid


To avoid formulation of etoposide with detergent, PEG, and EtOH (used to increase water solubility), it has been converted to the phosphate prodrug.


Prodrug for improved absorption through skin
Prodrug for Improved Absorption Through Skin

corticosteroids - inflammation, allergic, pruritic skin conditions


Better absorption into cornea for the treatment of glaucoma

The cornea has significant esterase activity


Prodrug for site specificity
Prodrug for Site Specificity

Bowel sterilant

(administer rectally)

Prodrug R = Ac (administered orally)

Hydrolyzed in the intestines


Prodrug for site specificity1
Prodrug for Site Specificity

The blood-brain barrier prevents hydrophilic molecules from entering the brain, unless actively transported. The anticonvulsant drug vigabatrin (see Chapter 5) 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.


Site specificity using enzymes at the site of action
Site Specificity Using Enzymes at the Site of Action

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
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

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 diffusable to kill neighboring nonexpressing 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
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.


Adept
ADEPT

Advantages:

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. Concentrates the drug at the site of action

5. Demonstrated to be effective at the clinical level

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.

Scheme 8.4

Humanization of antibodies minimizes immunogenicity. Note the prodrug-activating enzyme is a bacterial enzyme.


Antibody directed abzyme prodrug therapy adapt
Antibody-Directed Abzyme Prodrug Therapy (ADAPT)

Instead of using a prodrug-activating enzyme, a 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.



Gene directed enzyme prodrug therapy gdept also known as suicide gene therapy
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.

Scheme 8.6


Prodrug for stability protection from first pass effect
Prodrug for Stabilityprotection from first-pass effect

Oral administration has lower bioavailability than i.v. injection.

antihypertension

plasma levels 8 times that with propranolol


Prodrugs for slow and prolonged release
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


Sedative/tranquilizer/antipsychotic

slow release

inject i.m.

Antipsychotic activity for about 1 month


Prodrugs to minimize toxicity
Prodrugs to Minimize Toxicity

Many of the prodrugs just discussed also have lower toxicity.

For example, epinephrine (for glaucoma) has ocular and systemic side effects not found in dipivaloylepinephrine.


Prodrug to increase patient acceptance
Prodrug to Increase Patient Acceptance

The antibacterial drug clindamycin (8.28) is bitter and not well tolerated by children.

Clindamycin palmitate is not bitter.

Either not soluble in saliva or does not bind to the bitter taste receptor or both.


Prodrug to eliminate formulation problems
Prodrug to Eliminate Formulation Problems

Formaldehyde is a gas with a pungent odor that is used as a disinfectant. Too toxic for direct use.

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
Areas of Improvement for Prodrugs

  • site specificity

  • protection of drug from biodegradation

  • minimization of side effects


Macromolecular drug delivery
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
Disadvantages of Macromolecular Delivery Systems

  • Macromolecules may not be well absorbed

  • Alternative means of administration may be needed (injection)

  • Immunogenicity problems


Macromolecular drug carriers
Macromolecular Drug Carriers

Synthetic polymers

Aspirin linked to poly(vinyl alcohol) has about the same potency as aspirin, but less toxic.


Steric hindrance by polymer carrier
Steric Hindrance by Polymer Carrier

poly(methacrylate)

to enhance water solubility

testosterone

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.

to enhance water solubility


Poly amino acid carriers
Poly( testosterone.-Amino Acid) Carriers

poly(L-glutamine)

spacer

norethindrone - contraceptive

Slow release over nine months in rats


General site specific macromolecular drug delivery system
General Site-Specific Macromolecular Drug Delivery System testosterone.

either hydrophilic or hydrophobic

for site specificity


Site specific delivery of a nitrogen mustard
Site-Specific Delivery of a Nitrogen Mustard testosterone.

poly(L-Glu)

spacer arm

water-solubilizing

antibody from rabbit antiserum against mouse lymphoma cells

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
Tumor Cell Selectivity testosterone.

Drug attached to albumin (R = albumin)

Tumor cells take up proteins rapidly. Proteins broken down inside cells, releasing the drug.

Shown to inhibit growth of Ectomelia virus in mouse liver, whereas free inhibitor did not.

antitumor


Antibody targeted chemotherapy
Antibody-Targeted Chemotherapy testosterone.

Scheme 8.7

calicheamicin, except as disulfide instead of trisulfide

humanized

spacer

Does not release calicheamicinnonenzymatically.

Exhibits no immune response.


Tripartate drugs self immolative prodrugs
Tripartate testosterone. Drugs(Self-immolativeProdrugs)

A bipartateprodrug may be ineffective because the linkage is too labile or too stable.

In a tripartateprodrug, 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 (i.e., be self-immolative) after the carrier-linker bond is broken.


Tripartate prodrugs
Tripartate Prodrugs testosterone.

Scheme 8.8


Typical approach
Typical Approach testosterone.

Scheme 8.9


Tripartate prodrugs of ampicillin
Tripartate Prodrugs of Ampicillin testosterone.

Poor oral absorption (40%)

Excess antibiotic may destroy important intestinal bacteria used in digestion and for biosynthesis of cofactors.

Also, more rapid onset of resistance.

antibacterial

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 ampicillin1
Tripartate Prodrugs of Ampicillin testosterone.

Scheme 8.10

98-99% absorbed

Ampicillin is released in < 15 minutes


Reversible redox drug delivery system to the cns
Reversible Redox Drug Delivery System to the CNS testosterone.

Scheme 8.11

hydrolysis activated

hydrolysis deactivated

hydrophilic drug

electron-withdrawing; hydrophilic

electron-donating,

lipophilic carrier

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.

Scheme 8.12


Example of tripartate redox drug delivery
Example of Tripartate Redox Drug Delivery reaction also can be used.

-Lactams are too hydrophilic to cross the blood-brain barrier effectively.

Scheme 8.13

High concentrations of -lactams delivered into brain.


Another tripartate prodrug for delivery of antibacterials
Another Tripartate Prodrug for Delivery of Antibacterials reaction also can be used.

Permeases are bacterial transport proteins for uptake of peptides.

Scheme 8.14

Only L,L-dipeptides are active


Mutual prodrugs
Mutual Prodrugs reaction also can be used.

A bipartate or tripartateprodrug in which the carrier is a synergistic drug with the drug to which it is linked.

Antibacterial ampicillin

-lactamase inactivator

penicillanic acid sulfone

Hydrolysis gives 1:1:1 ampicillin : penicillanic acid sulfone : formaldehyde


Ideal mutual prodrugs
Ideal Mutual Prodrugs reaction also can be used.

  • 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


Bioprecursor prodrugs
Bioprecursor Prodrugs reaction also can be used.

Carrier-linked prodrugs largely use hydrolytic activation

Bioprecursor drugs mostly use oxidative or reductive activation

The metabolically-activated alkylating agents discussed in Chapter 6 are actually examples of bioprecursorprodrugs.


Protonation activation discovery of omeprazole
Protonation Activation reaction also can be used.Discovery of Omeprazole

Cimetidine and ranitidine (Chapter 3) 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 Discovery reaction also can be used.

Lead compound found in a random screen.

Liver toxicity observed thought to be because of the thioamide group.


Lead modification
Lead Modification reaction also can be used.

Related analogs made, and 8.61 had good antisecretory activity.

Modification gave 8.62 with high activity.

The sulfoxide (8.63) was more potent, but it blocked iodine uptake into the thyroid.


Lead modification cont d
Lead Modification (cont’d) reaction also can be used.

Modification of 8.63 gave 8.64 having no iodine blockage activity.

Picoprazole shown to be an inhibitor of H+,K+-ATPase. SAR of analogs indicated electron-donating groups on the pyridine ring were favorable. Increased inhibition of H+,K+-ATPase.

Best analog was omeprazole (8.65).


The reaction also can be used.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.

Scheme 8.16

Formation of 8.66 leads to covalent attachment.

Omeprazole also inhibits isozymes of carbonic anhydrase, another mechanism for lowering gastric acid secretion.


Hydrolytic activation
Hydrolytic Activation reaction also can be used.

Hydrolysis can be a mechanism for bioprecursor prodrug activation, if the product requires additional activation.

Scheme 8.17

antitumor agent

prodrugs of leinamycin


Hydrolysis of these analogs gives an intermediate that reacts further to the activated form.

this was synthesized and gave 8.74 as well as DNA cleavage

increased stability over leinamycin

Scheme 8.18


Elimination activation
Elimination Activation reacts further to the activated form.

Scheme 8.19

potent inhibitor of dihydroorotate dehydrogenase

rheumatoid arthritis drug

Inhibition of dihydroorotate dehydrogenase blocks pyrimidine biosynthesis in human T lymphocytes.


Oxidative activation n dealkylation
Oxidative Activation reacts further to the activated form.N-Dealkylation

Scheme 8.20


O dealkylation
O reacts further to the activated form.-Dealkylation

Analgesic activity of phenacetin is a result of O-dealkylation to acetaminophen.


Oxidative deamination
Oxidative Deamination reacts further to the activated form.

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.

Scheme 8.21

Cyclophosphamide was very effective, but it required liver homogenates (contains P450) for activation. Therefore oxidation is required, not hydrolysis.

isolated


N oxidation
N reacts further to the activated form.-Oxidation

identified

advanced Hodgkin’s disease

Scheme 8.22

identified


N oxidation1
N reacts further to the activated form.-Oxidation

Pralidoxime chloride is an antidote for nerve poisons.

It reacts with acetylcholinesterase that has been inactivated by organophosphorus toxins.


To increase the permeability of reacts further to the activated form.pralidoxime into the CNS, the pyridinium ring was reduced (8.92).

oxidation in brain

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.


Mechanism of acetylcholinesterase
Mechanism of Acetylcholinesterase reacts further to the activated form.

Scheme 8.23


Inactivation of acetylcholinesterase by diisopropyl phosphorofluoridate
Inactivation of Acetylcholinesterase by Diisopropyl Phosphorofluoridate

Scheme 8.24

affinity labeling agent

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.



Temporary inhibition of Pralidoximeacetylcholinesterase enhances cholinergic action on skeletal muscle.

Scheme 8.26

covalent, but reversible inhibition

Used for the neuromuscular disease myasthenia gravis


Reversible ( Pralidoximenoncovalent) inhibitors of acetylcholinesterase, such as donepezil and tacrine are used for Alzheimer’s disease. Enhances neurotransmission involved in memory.


S oxidation
S Pralidoxime-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

antitumor, antiviral agent


Aromatic hydroxylation cyclohexenones as prodrugs for catechols
Aromatic Hydroxylation PralidoximeCyclohexenones as prodrugs for catechols

Scheme 8.28

aromatic hydroxylation

oxidation next to sp2 carbonyl


Alkene epoxidation
Alkene Epoxidation Pralidoxime

active anticonvulsant agent


Transamination
Transamination Pralidoxime

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 Pralidoxime acids (8.104) stimulate pyruvate dehydrogenase, but have a short duration of action.

Oxfenicine (8.105, R = OH) is actively transported and is transaminated (a PLP aminotransferase) in the heart to 8.104 (R = OH).


Reductive activation azo reduction
Reductive Activation PralidoximeAzo Reduction

Scheme 8.29

Anaerobic cleavage by bacteria in lower bowel

ulcerative colitis

For inflammatory bowel disease

Causes side effects


To prevent side effect by Pralidoximesulfapyridine a macromolecular delivery system was developed.

poly(vinylamine)

Not absorbed or metabolized in small intestine.

spacer

Released by reduction at the disease site.

Sulfapyridine is not released (still attached to polymer).

More potent than sulfasalazine.


Azido reduction
Azido Reduction Pralidoxime

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
Sulfoxide Reduction Pralidoxime

anti-arthritis

Sulindac is inactive in vitro; the sulfide is active in vitro and in vivo.


Sulindac Pralidoxime is an indaneisostere of indomethacin, which was designed as a serotonin analog.

The 5-F replaced the 5-OMe group to increase analgesic properties.

The p-SOCH3 group replaced p-Cl to increase water solubility.


Disulfide reduction
Disulfide Reduction Pralidoxime

To increase the lipophilicity of thiamin for absorption into the CNS.

Scheme 8.30

nonenzymatic

poorly absorbed into CNS


To diminish toxicity of Pralidoximeprimaquine and target it for cells with the malaria parasite, a macromolecular drug delivery system was designed.

Intracellular thiol much higher than in blood; selective reduction inside the cell

antimalarial, toxic

primaquine

for improved

uptake in liver

Therapeutic index of 8.116 is 12 times higher than 8.115 in mice.


Nitro reduction
Nitro Reduction Pralidoxime

Scheme 8.32

Mechanism-based inactivator of thymidylate synthase


Nucleotide activation
Nucleotide Activation Pralidoxime

Scheme 8.33

Anti-leukemia drug

Inhibits several enzymes in the purine nucleotide biosynthesis pathway.


Phosphorylation activation
Phosphorylation Activation Pralidoxime

2-deoxyguanosine

antiviral

Resembles structure of 2-deoxyguanosine

viral thymidine kinase

Uninfected cells do not phosphorylate acyclovir (selective toxicity)

R = PO3=

viral guanylate kinase

R = P2O63-

viral phosphoglycerate kinase

R = P3O94-


Acyclovir triphosphate is a substrate for viral Pralidoxime-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


Resistance to acyclovir
Resistance to Acyclovir Pralidoxime

Modification of thymidine kinase

Change in substrate specificity for thymidine kinase

Altered viral -DNA polymerase


Only 15-20% of acyclovir is absorbed Pralidoxime

Therefore, prodrugs have been designed to increase oral absorption.

Hydrolyzes here

Prodrug for a prodrug

adenosine deaminase

acyclovir


Hydroxylates Pralidoxime here

xanthine oxidase

acyclovir

This prodrug is 18 times more water soluble than acyclovir.


A Pralidoximebipartate carrier-linked prodrug of acyclovir, the L-valyl ester of acyclovir, has 3-5 fold higher oral bioavailability with the same safety profile.


An analog of acyclovir whose structure is even closer to that of 2-deoxyguanosine is ganciclovir.

More potent than acyclovir against human cytomegalovirus.


Two carbon that of 2isosteres of ganciclovir are available.

C in place of O

C in place of O

Better oral absorption than penciclovir

Converted to penciclovir rapidly


Sulfation activation
Sulfation that of 2Activation

Activity of minoxidil requires sulfotransferase-catalyzed sulfation to minoxidil sulfate.

hair growth

Inhibitors of the sulfotransferase inhibit the activity of minoxidil, but not minoxidil sulfate.


Decarboxylation activation
Decarboxylation Activation that of 2

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).


After crossing blood-brain barrier that of 2

L-aromatic amino acid decarboxylase (PLP)

dopamine (R = H)

Does not reverse the disease, only slows progression.


Combination therapy
Combination Therapy that of 2

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 that of 2L-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.

(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
Selective Inactivation of Brain Monoamine Oxidase A that of 2

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.


Selective delivery of dopamine to kidneys
Selective Delivery of Dopamine to Kidneys that of 2

Dopamine increases renal blood flow.

Prodrugs were designed for selective renal vasodilation.

Scheme 8.34

L-dopa

L--glutamyl-L-dopa

Dopamine accumulates in the kidneys because of high concentrations of L--glutamyltranspeptidase and L-aromatic amino acid decarboxylase there.

Example of a carrier-linked prodrug of a bioprecursorprodrug for dopamine.


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