The Organic Chemistry of Drug Design and Drug Action

Chapter 8

Prodrugs and Drug Delivery Systems

Prodrugs and Drug Delivery SystemsProdrug - 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 excretionA pro-soft drug would require metabolism to convert it to a soft drug

Utility of Prodrugs1. 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 - 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---often tried to limit the toxicity of anticancer drugs.

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 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—e.g., amine getting oxidized to CO2H, to afford the active.

Protecting Group Analogy for the Concept of Prodrugs

A. RCO2H RCO2 Etreaction R'CO2Et R'CO2H

R'CO2H1. O3

R'CH=CH2B. RCH=CH2

EtOHHCl

Δon R

reactionon R 2. H2O2

H3O+

Δ

Scheme 8.1analogous tocarrier-linkedThe ester group can be usedTo modify properties of The drug

bioprecursor

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

analogous to

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 Carriers1. 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-Containing Drugs

Table 8.1

Ester analogs as prodrugs can affect lipophilicity or hydrophilicity

Drug—OXDrug—OH

alcoholsX Effect on Water Solubility

C—R

O(R = aliphatic or aromatic)decreases (increases lipophilicity)

C—CH2NHMe2

O+ increases (pKa ~ 8)

C—CH2CH2COO-

Oincreases (pKa ~ 5)

C

O

NH+

increases (pKa ~ 4)

PO3= (phosphate ester) increases (pKa ~ 2 and ~ 6)

CCH2SO3-

Oincreases (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

Drug—OH +Drug—O-

Drug—O

-O

O

O

O

O

O

-OOCCOO-

H OH

Scheme 8.2

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.

N

OHS

O

O

H2N

F

Cl

oxindole8.1

NO

O

H2N

F

Cl

8.2

S

OO

HOOC

antirheumatic agent

Carboxylic Acid-Containing Groups

Esterify as with alcohols

Maintaining Water Solubility of Carboxylate Prodrugs

Drug—C—O—CH2CH2—NRR'2+

8.3

O

Can vary pKa by appropriate choice of R and R′

Prodrugs for Phosphate- or Phosphonate-Containing Drugs

P

O

O

OO

2

8.4

Amine ProdrugsTable 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)

Drug—NH2 Drug—NHX

X

—CH2NHCAr

O

CHAr NAr+

CR

O

CCHNH3

R

O

C OPh

O

N-Mannich base (R = CH2NHCOPh) has a log D7.4two units greater than the parent compound.

phenylpropanolamine hydrochloride (R = H HCl)8.5

NHR

OH

CH3

.

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

Imine (Schiff base) prodrug

progabide8.6

O

Cl

NNH2F

OH

anticonvulsant

hydrolyze imine and amide to GABA inside brain

A Reductive Carrier-Linked ProdrugApproach

DrugX Z

YNO2

bioreduction

DrugX Z

YNH

OH

:

DrugXH

Z

YN

OH

8.7 8.8

+

Scheme 8.3

Prodrugs of Sulfonamides

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

S

ONPh

CH3H2N

O O

valdecoxib8.9

S

ONPh

CH3N

O O

parecoxib sodium8.10

O

Na+

Prodrug Analogs of Carbonyl Compounds

C=NOHOR'

C

OR'

XC

Y

DrugC

Drug

RC=O

R

X

Y

CNH

OC

NH

SC=NR'

imines oximes ketals

Table 8.3

Examples of Carrier-Linked Bipartate Prodrugs

Prodrug for Increased Water Solubility

prednisolone (R = R' = H)methylprednisolone (R = CH3, R' = H)

8.11

O

Me

HO

R

Me OH

O

OR'

poor water solubility

corticosteroid

R' = PO3Na2

R' = CCH2CH2CO2 Na

O

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.

for aqueous injection or opthalmic use

Prodrug forms

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

etoposide (R = H)etoposide phosphate (R = PO3H2)

8.12

OO

OO

ORCH3O OCH3

OOO

OCH3HO

HO

Prodrug for Improved Absorption Through Skin

corticosteroids - inflammation, allergic, pruritic skin conditions

fluocinolone acetonide (R = H)fluocinonide (R = COCH3)

8.14

O

OCH3

HOCH3

OOR

F

F

OCH3

CH3

Better absorption into cornea for the treatment of glaucoma

The cornea has significant esterase activity

dipivefrin (R = Me3CCO)epinephrine (R = H)

8.15

OH

RO

RO

NHCH3

Prodrug for Site Specificity

oxyphenisatin (R = H)8.16

NH

RO

OR

O

Bowel sterilant

(administer rectally)

prodrug R = Ac (administer orally)hydrolyzed in intestines

Prodrug for Site SpecificityThe 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.

8.17

O

O

OCOR

NH2

ONH2

O

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.

diethylstilbestrol diphosphate (R = PO3=)

diethylstilbestrol (R = H)8.18

OR

RO

Enzyme-Prodrug Therapies

For selective activation of prodrugs in tumor cellsTwo steps:1. incorporate a prodrug-activating enzyme into a target tumor cell2. administer a nontoxic prodrug which is a substrate for the exogenous enzyme incorporated

1. The prodrug-activating enzyme is either nonhuman or a human protein expressed poorly2. 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

Criteria for Success with Enzyme-Prodrug Therapies

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.

ADEPTAdvantages:1. Increased selectivity for targeted cell2. Each enzyme molecule converts many prodrugmolecules3. The released drug is at the site of action4. Demonstrated to be effective at the clinical level5. Concentrates the drug at the site of action

Disadvantages:1. Immunogenicity and rejection of antibody-enzyme conjugate2. Complexity of the two-phase system and i.v. administration3. Potential for leakback of the active drug

An example is carboxypeptidase G2 or alkaline phosphataselinked to an antibody to activate a nitrogen mustard prodrug.

N

O

O

IIN

OH

II

carboxypeptidase G2

electron-withdrawing;deactivates nitrogen mustard

electron-donating;activates nitrogen mustard

+

HN CO2H

CO2H

L-Glu + CO2

8.19

Scheme 8.4

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, 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 prodrugcould 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 prodrugsselectively, and to kill colon and prostate cancer cells.

Abzyme 38C2 Activation of a Doxorubicin Prodrug

Scheme 8.5

8.20

O

O

OH

OHCH3O H O

OH

O

NHHO

CH3

OH

O

O

O

O OH

38C2

O

O

OH

OHCH3O H O

OH

O

NHHO

CH3

OH

O

O

O

O

O

H

38C2

O

O

O

OH

OHCH3O H O

OH

O

NHHO

CH3

OH

O

O

O-

O

O

OH

OHCH3O H O

OH

O

NH2HO

CH3

OH

O

-CO2 +H+

retro-aldol retro-Michael

doxorubicin

B-

B-

NNH

OMe

OMe

OMe

Cl

NH

O

O

NMe

N

O2N O

amino-seco-CBI-TMI8.21

nitroreductase NNH

OMe

OMe

OMe

Cl

NH

O

O

NMe

N

N O

H

HO :

NNH

OMe

OMe

OMe

Cl

NH2

O

-CO2+H+

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

Gene-Directed Enzyme Prodrug Therapy (GDEPT)

Also known as suicide gene therapy

Prodrug for Stabilityprotection from first-pass effect

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

propanolol (R = R' = H)8.22

OR'

O NHCH(CH3 )2

R

prodrug R' =OCCH2 CH2COOH

plasma levels 8 times that with propanolol

antihypertension

Prodrugs for Slow and Prolonged Release1. 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 slowlyIntramuscular injection is used also

haloperidol (R = H)haloperidol decanoate (R = CO(CH2)8CH3)

8.24

FO

NOR

Cl

Sedative/tranquilizer/antipsychotic

prodrug R' =

OC(CH2)8CH3 slow release

inject i.m.Antipsychotic activity for about 1 month

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.

Prodrug to Increase Patient Acceptance

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

Clindamycin palmitateis not bitter.

clindomycin (R = H)clindomycin phosphate (R = PO3H2)

clindomycin palmitate (R = O(CH2)14CH3)8.28

N HCH3

H O

HN Cl

OHO

ORSCH3

OH

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

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 methenamineis used as a urinary tract antiseptic.

Has to be enteric coated to prevent hydrolysis in the stomach.

methenamine8.30

NN

N

N

CH2O + NH3H+

H3O+

Areas of Improvement for Prodrugs

• site specificity

• protection of drug from biodegradation

• minimization of side effects

Macromolecular Drug DeliveryTo 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

yx

8.32

OO

O

O

CH2 CH

OH

CH2 CH

Synthetic polymers

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

Steric Hindrance by Polymer Carrier

yx

8.34

O

O

C=O

CCH2

CH3

C

O

S=O

CH2

CH3

C=O

poly(methacrylate)

to enhance water solubility

testosterone

No androgenic effectPolymer backbone may be sterically hindering the release of the testosterone.

A spacer arm was added, and it was as effective as testosterone.

x y

spacer arm

Cl-+

8.35O

O

C

C=O

CH2

O

S=OH3 CO

NMe2

O

C=O

C

CH3

CH2

CH3

to enhance water solubility

Poly(α-Amino Acid) Carriers

poly(L-glutamine)

spacer

norethindrone - contraceptive

Slow release over nine months in rats

8.37

x

O

OO

O

NH C

O

NH CH

CH

C

O

General Site-Specific Macromolecular Drug Delivery System

either hydrophilic or hydrophobic for site specificity

Polymer chain

Solubilizer spacer Homing device

Drug

8.38

Site-Specific Delivery of a Nitrogen Mustard

antibody from rabbit antiserum against mouse lymphoma cells

spacer arm

All 5 mice tested were alive and tumor free after 60 days (all controls died).Also, therapeutic index greatly enhanced (40 fold).

water-solubilizing

poly(L-Glu)zyx

NH

N

CHCH C NH

ClCl

O

C NH

O

O- NH

NHCH

O O

C

O

Ig

O

8.39

Tumor Cell Selectivity

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.

Drug attached to albumin (R = albumin)

antitumor

O

OH

NRO

N

NH2

O

HO

cytosine arabinoside (R = H)8.41

Antibody-Targeted Chemotherapy

O

NH

O

CH3O

HO

S

O

H polysaccharide

S

O

HN

N

CH3

O

O

HN

Lysantibody

gemtuzumab ozogamicin8.42

RS-

O

NH

O

CH3O

HO

S

O

H polysaccharide

8.43

calicheamicin, except as disulfide instead of trisulfide

spacerhumanized

Does not release calicheamicin nonenzymatically.Exhibits no immune response.

Scheme 8.7

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

Tripartate Prodrugs

Carrier Linker Drugenzyme

DrugLinkerCarrier +

spontaneous

Linker Drug+

Scheme 8.8

Typical Approach

fast

Drug—X- + CH2O

Drug—X—CH2 —O- + RCOOHesteraseDrug—X—CH2 —O—CR

O

Scheme 8.9

Tripartate Prodrugs of Ampicillin

ampicillin8.44

O

NNH2

NH

O

S

COO-

antibacterial

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.

Various esters made were too stable in humans (although they were hydrolyzed in rodents) - thought the thiazolidinering sterically hindered the esterase.

Tripartate Prodrugs of Ampicillin

esterase

bacampicillin (R = CH3, R' = OEt)pivampicillin (R = H, R' = t-Bu)

8.45

8.46

..

+ R'COOH

whenR' = OEt

EtOH+ CO2

8.44

NH2

N

SNH

O

Ph

O

OO O

R

R'

O

NH2

N

SNH

O

Ph

O

OO

R

R H

O

OH

98-99% absorbedAmpicillin is released in < 15 minutes

Scheme 8.10

8.49

+ Drug — XH

enzymehydrolysis

8.48

enzymecrosses

8.47

CH3

N

CH3

N

Drug—X

O

Drug—X

O

CH3

N+

Drug—X

O

N+

HOOC

CH3

blood-brainbarrier

oxidation

hydrolysis deactivatedhydrolysis activated

hydrophilic drug

electron-donating, lipophilic carrier

electron-withdrawing; hydrophilic

Reversible Redox Drug Delivery System to the CNSScheme 8.11

Passive diffusion of 8.47 into the brain; active transport of 8.49out of the brain

If oxidation occurs before it gets into the brain, it cannot cross the blood-brain barrier.

XH of the drug is NH2, OH, or COOH

When the drug is a carboxylic acid, a self-immolative reaction also can be used.

Scheme 8.12

Drug—COO-

fast —CH2O

Drug— —O—CH2 —O-esterase

8.50

C

OHOC

O

carrier+Drug C OCH2 O C carrier

O O

8.51

O

HN

ON

SR

OO

O

N

Me

O

enzymatic

oxidation

N

Me

O

HN

ON

SR

OO

O

O

esterase

N

Me

O

OO

HN

ON

SR

OO O

-CH2OO

HN

ON

SR

OO-

8.49High concentrations of β-lactams delivered into brain.

Scheme 8.13

Example of Redox Drug DeliveryAntibody generation in the brain is not significant.

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

Tripartate Prodrug for Delivery of Antibacterials

Permeases are bacterial transport proteins for uptake of peptides.

+NH4 +

++

+..

+1. permease

+

8.53

CH3

OCH3

HN

NH

N

COO-

N

H3N

HN F

O

O COO-H3N

H2N

O

F

O

HN

COO-

NH

O

F

O

O

COO-H2N

COO-

H

2. peptidase

H

Scheme 8.14

Only L,L-dipeptides are active

Mutual ProdrugsA bipartate or tripartate prodrug in which the carrier is a synergistic drug with the drug to which it is linked.

sultamicillin8.59

NH2

Ph

O O

NH S

NON

O

O

S

O

OO

O

Antibacterial ampicillin

β-lactamase inactivatorpenicillanic acid sulfone

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

Bioprecursor Prodrugs

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 bioprecursor prodrugs.

Protonation ActivationDiscovery of Omeprazole

Cimetidine and ranitidine (Chapter 3) reduce gastric acid secretion by antagonizing the H2histamine 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.

8.60

N

S

NH2

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

Lead ModificationRelated analogs made, and 8.61 had good antisecretoryactivity.

Modification gave 8.62 with high activity.

8.62

NS

NH

N

timoprazole8.63

NS

NH

NO

8.61

NS

NH

N

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

Lead Modification (cont’d)

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).

picoprazole8.64

NS

NH

NO

CH3

CH3

CO2CH3

omeprazole8.65

NS

NH

NO

CH3

OCH3

OCH3

H3 C

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.

omeprazole8.65

N

SHN

N

O

CH3

OCH3

OCH3

H3 C

..

H+

N

SHN

NO

CH3

OCH3

OCH3

H3 C

H

N

SHN

NOH

CH3

OCH3

OCH3

H3 C

..N

SN N

CH3

OCH3

H3 C

OCH3

8.66

S

N

SN NH

CH3

OCH3

H3 C

OCH3

S

-H2 O

+H+

Formation of 8.66 leads to covalent attachment.Omeprazole also inhibits isozymes of carbonic anhydrase, another mechanism for lowering gastric acid secretion.

Scheme 8.16

Hydrolytic Activation

leinamycin8.70

K2CO3/RX

8.71

SS+

O

NH N

S

O

O-

O OHMe

Me

Me

HO

N N

S

OMe

Me

HO

S+

O

O-O

S

R

MeHO

Scheme 8.17

antitumor agent prodrugs of leinamycin

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

8.72

N N

S

OMe

Me

O

S+

O

O-O

S

MeHO

OO

O

Me

HO-

N N

S

OMe

Me

S+

O

O-O

S-

MeHO

SS+

O

NH

N

S

O

O-

O OHMe

Me

Me

O

O

O

8.73

O

O

DNAcleavage

O

NH

N

S

O

Me

8.74

O

O

S

MeHO

HO2 C

MeHO

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

increased stability over leinamycin

this was synthesized

and gave 8.74 as well

as DNA cleavage

Scheme 8.18

Elimination Activation

NO CH3

O

NH

CF3

H

leflunomide8.75

B:

CN

HO CH3

O

NH

CF3

8.76BH

rheumatoid arthritis drug

potent inhibitor of dihydroorotatedehydrogenase

Scheme 8.19

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

Oxidative ActivationN-Dealkylation

Sedative 8.20

..

..P450P450

Cl

N N

N

N

CH3

CH3

O

X

CH3

Cl

N N

N

N

CH3

HO

X

CH3

Cl

N N

N

NH2

CH3

O

X

NH

NN

NCH3

ClHO X

N

NN

NCH3

Cl

X

alprazoalam (X = H)triazolam (X = Cl)

8.77

-H2 O

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

phenacetin (R = CH2CH3)acetaminophen (R = H)

8.78

CH3HN

OR

O

Oxidative DeaminationNeoplastic (cancer) cells have a high concentration of phosphoramidases, so hundreds of phosphamide analogs of nitrogen mustards were made for selective activation in these cells.

+

8.868.85

DNA DNA

8.84

HPO4= + NH3 + spontaneous

orphosphoramidase

8.82 8.83

+

B:

8.81

P-450

8.80cyclophosphamide8.79

HN

OP

O

N

HN

O

Cl

PClO

N

Cl

Cl

HO:

H

OP

NH2O

N

Cl

Cl

O

H

-O PH2N

O

H

O

N

Cl

ClHN

Cl

Cl

HN

N N

N

H2NDNA

O

HN

N

HN

N

OH N

H2N

O HN

OH

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

Scheme 8.21

N-Oxidationidentified

identified

advanced Hodgkin’s disease

Scheme 8.22

+

H

procarbazine8.87

P450

8.88B:

B—H+

8.89

H2O

CH3 NHNHCH2 CNHCHMe2

O

CH3N=N—CH CNHCHMe2

O

H

CH3N-NH2 + OHC CNHCHMe2

O

CH3N-N=CH CNHCHMe2

O

HN

N N

N

H2N

CH3

DNA

O

N

H

CH3N=NH

CH3-N

CH3 + N2

+

DNA

8.90

HN

N N

N

H2N

CH3O

CH3 NHNHCH2 CNHCHMe2

O

HO

-H2 O

N-Oxidation

+

pralidoxime chloride8.91

Cl- N

N

CH3

OH

Pralidoxime chloride is an antidote for nerve poisons.

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

To increase the permeability of pralidoxime into the CNS, the pyridinium ring was reduced (8.92).

+

pralidoxime chloride8.91

Cl- N

N

CH3

OH

oxidation into brain

8.92

NN

CH3

OH

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 8.91prevents efflux from brain.

Mechanism of Acetylcholinesterase

+ :B

Me3NCH2CH2OH + CH3 COOH+

H2O

+

OCH3

OHB

Me3N—CH2CH2—OO

CH3

OB

H

H

Me3N—CH2CH2—OH

B:

Trp

Phe TrpTrp

PheTrp

Scheme 8.23

Inactivation of Acetylcholinesterase by Diisopropyl Phosphorofluoridate

8.93

+ :BO

B

H

H

Trp

Phe Trp

OP

OF

O

OB

Trp

Phe Trp

OP

O

O

HB

affinity labeling agent

irreversible inhibition

Scheme 8.24

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

pralidoxime

ON

NO

Me

P

O

OB

Trp

Phe Trp

OP

O

O

HBO

B

Trp

Phe Trp

OP

O

O

HB

NMe

NO:

H

OHBH

Trp

Phe Trp:B

NON

Me

P

OO

OO

Scheme 8.25

Temporary inhibition of acetylcholinesteraseenhances cholinergic action on skeletal muscle.

+ :B

H2O

+

ONMe2

O HB

OBH

H B:

TrpPhe Trp

Trp

PheTrp

O

O

NMe2

Me3N OHMe3N

slow

OHMe3N+ CO2 + Me2NH

neostigmine8.94

Scheme 8.26

Used for the neuromuscular disease myasthenia gravis

covalent, but reversible inhibition

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

donepezil hydrochloride8.95

H3CO

H3CO

O

N

. HClN

NH2

tacrine hydrochloride8.96

. HCl

S-OxidationPoor 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

O

O

CH3HO

HO

H

H

O

O

CH3HO

HO

H

H

RS HO

O

CH3HO

HO

H

H

RSHO

HH

-RSOH

:B

[O]

brefeldin A8.97 8.98

8.99

RSH

Aromatic HydroxylationCyclohexenones as prodrugs for catechols

aromatic hydroxylationoxidation next to sp2 carbonyl

Scheme 8.28

O

N

8.100

P450

O

N

8.101

-H2 O

O

N

OH

N

P450

OH

N

HO

HO

Alkene Epoxidation

carbamazepine8.102

N

O NH2

8.103

N

O NH2

O

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 O2available).

8.104

COOH

O

R

Arylglyoxylic acids (8.104) stimulate pyruvatedehydrogenase, 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).

oxfenicine (R = OH)8.105

COOH

NH2

R

+

sulfapyridine8.108

8.107

sulfasalazine8.106

NNHSO2 N=N OH

COOH

H2N OH

COOH

NNHSO2 NH2

ulcerative colitis

For inflammatory bowel disease

Causes side effects

Reductive ActivationAzo Reduction

Scheme 8.29

Anaerobic cleavage by bacteria in lower bowel

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

8.109

n

N

SO2

NH

N

CO2Na

OH

poly(vinylamine)

spacer

Not absorbed or metabolized in small intestine.

NH2

CO2Na

OH

Sulfapyridine is not released (still attached to polymer).More potent than sulfasalazine.

Released by reduction at the disease site.

Azido Reduction

vidarabine (R = NH2)8.110

N

N N

N

O

R

HO

HO

OH

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

sulindac8.111

CH3

CO2H

S

F

CH3

O

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 group replaced p-Cl to increase water solubility.

sulindac8.111

CH3

CO2H

S

F

CH3

Oindomethacin

8.112

NCH3

CO2H

Cl

MeO

O

NH

HO

NH2

serotonin

Disulfide ReductionTo increase the lipophilicity of thiamin for absorption into the CNS.

+

+

GSH..

8.113H

thiamin8.114

N

N

NH2

N

HCH3

SO

OH

O

N

N

NH2

N

HCH3

S-

O

OH

N

N

NH2

N

SCH3

N

N

NH2

N:S

CH3

OH

OH

S

B

OH

Scheme 8.30

nonenzymatic

poorly absorbed into CNS

primaquine8.115

N

CH3O

HNNH2

serumalbumin

lactose

lactose8.116

N

CH3O

HN

O

NH3+

SSNH

antimalarial, toxic

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

primaquine

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

for improved uptake in liver

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

Nitro Reduction

HN

N

O

O

OO

HO

F

P

O

O

NCl

H3 C

O

O2N

bioreduction

HN

N

O

O

OO

HO

F

P

O

O

NCl

H3 C

O

NHHO..

HN

N

O

O

O

O

HO

F

P

O

O-

N

H3 C

Cl

..

HN

N

O

O

OO

HO

F

P

O

O-

N

H3 C

HN

N

O

O

OO

HO

F

P

O

O

H2O

HN

N

O

O

OO

HO

F

P

O

O-

-O

8.118 8.119

8.120

8.121

Mechanism-based inactivator of thymidylate synthase

Scheme 8.32

Nucleotide Activation

8.123

hypoxanthine-guaninephosphoribosyltransferase

6-mercaptopurine8.122

N

N NH

N

SHO

HO

O—P—O—P—O-

OH

=O3PO O O

O- O-

N

N N

N

SH

O

HO OH

=O3PO

Anti-leukemia drugInhibits several enzymes in the purine nucleotide biosynthesis pathway.

Scheme 8.33

Phosphorylation Activation

acyclovir (R = H)8.124

HN

N N

N

O

H2N

ORO

8.125

HOO

NH2N

O

NHN

N

HO

antiviral 2′-deoxyguanosine

Resembles structure of 2′-deoxyguanosine

R = PO3=

viral guanylate kinase

viral thymidine kinase

R = P2O6-3

viral phosphoglycerate kinaseR = P3O9

-4

Uninfected cells do not phosphorylate acyclovir (selective toxicity)

Acyclovir triphosphate is a substrate for viral α-DNA polymerase but not for normal α-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

Modification of thymidine kinase

Change in substrate specificity for thymidinekinase

Altered viral α-DNA polymerase

Only 15-20% of acyclovir is absorbed

Therefore, prodrugs have been designed to increase oral absorption.

Hydrolyzes here Prodrug for

a prodrug

adenosine deaminase

acyclovir

8.126

HOO

NH2N

NH2

NN

N

Hydroxylateshere

acyclovir

xanthine oxidase

This prodrug is 18 times more water soluble than acyclovir.

8.127

N

N N

N

H2N

OHO

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.

valaciclovir8.128

HN

N N

N

O

H2N

OO

O

H2N

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

More potent than acyclovir against human cytomegalovirus.

ganciclovir8.129

HOO

NH2N

O

NHN

N

HO

Two carbon isosteres of ganciclovir are available.

penciclovir8.130

HONH2N

O

NHN

N

HO

famciclovir8.131

AcONH2N

NN

N

AcOC in place of O

C in place of O

Better oral absorption than penciclovirConverted to penciclovirrapidly

Sulfation ActivationActivity of minoxidil requires sulfotransferase-catalyzed sulfation to minoxidil sulfate.

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

hair growth

N

N

OR

NH2H2N

N

minoxidil (R = )minoxidil sulfate (R = SO 3

- )8.132

Decarboxylation ActivationAn 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 dopaminergicneurons 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).

levodopa (R = COOH)8.133

HO

HONH2

RH

After crossing blood-brain barrier

L-aromatic amino acid 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 decarboxylasedestroys >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.

(used in U.S.)Selectively inhibits peripheral L-amino acid decarboxylase.

The dose of L-dopa can be reduced by 75%.

carbidopa8.134

H3 C COO-

NHNH3+

HO

HO

benserazide8.135

HH

HO

HON N

OHO NH2

OH

(used in Europe and Canada)

Selective Inactivation of Brain Monoamine Oxidase A

8.136

OH

NH2

R

F

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.

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.

+

L-aromatic amino aciddecarboxylase

+

Glu

L-γ-glutamyltranspeptidase

8.137

+

COO-

O

NH3

OH

COO-

HN OH

COO-

OH

OHNH3

OH

OH

NH3

Scheme 8.34

L-γ-glutamyl-L-dopa

Dopamine accumulates in the kidneys because of high concentrations of L-γ-glutamyltranspeptidaseand L-aromatic amino acid decarboxylase there.

L-dopa

Selective Delivery of Dopamine to KidneysDopamine increases renal blood flow.Prodrugs were designed for selective renal vasodilation.

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