Download The Organic Chemistry of Drug Design and Drug Action

The Organic Chemistry of
Drug Design and Drug Action
Prodrugs and Drug Delivery Systems
Nohad A Atrushi
• 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.
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
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 - 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 firstpass 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
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
Scheme 8.1
A.
B.
RCO2H
RCH=CH2
EtOH
HCl

reaction
on R
RCO2 Et
reaction
on R
R'CH=CH2
R'CO2Et
1 . O3
2 . H2O2
H3O+

R'CO2H
R'CO2H
analogous to
carrier-linked
The ester group can be used
To modify properties of
The drug
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
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
Drug—OH
Prodrugs for AlcoholContaining Drugs
Table 8.1
Ester analogs as
prodrugs can affect
lipophilicity or
hydrophilicity
Drug—OX
alcoho ls
X
O
Effect on Water Solubility
C—R
(R = aliphatic or aromatic)
decreases (increases lipophilicity)
O
+
C—CH 2NHMe2
increases (pKa ~ 8)
O
C—CH 2CH2COOO
C
increases (pKa ~ 5)
increases (pKa ~ 4)
NH
+
PO3= (phosphate ester)
O
CCH 2SO3-
increases (pKa ~ 2 and ~ 6)
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
Scheme 8.2
O
-O
H
Drug—O-
OH
O
Drug—OH +
O
-
OOC
Drug—O
O
O
Also, acetals or ketals can be made for rapid
hydrolysis in the acidic medium of the GI tract.
COO-
Enolic hydroxyl groups can be esterified as well.
HOOC
S
F
O
Cl
O
OH
N
O
H 2N
oxind ole
8.1
antirheumatic agent
O
S
F
O
Cl
N
O
H 2N
8.2
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
P
O
O
O
2
8.4
Amine Prodrugs
Table 8.2
Drug—NH 2
Drug—NHX
X
O
CR
O
+
CCHNH3
R
O
O
C OPh —CH 2NHCAr
CHAr
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 NMannich bases (X = CH2NHCOAr)
NAr
N-Mannich base (R = CH2NHCOPh) has a log D7.4
two units greater than the parent compound.
OH
CH3
NHR
ph enylpr opanolam ine h yd ro chlor ide (R =. H HCl)
8.5
Another approach to lower pKa of amines and
make more lipophilic.
Imine (Schiff base) prodrug
OH
N
F
Cl
pr ogabid e
8.6
anticonvulsant
O
NH2
hydrolyze imine and
amide to GABA
inside brain
A Reductive Carrier-Linked Prodrug
Approach
Scheme 8.3
OH
OH
NO2
Y
Drug
X
Z
8.7
Y
bio reduction
Drug
X
N
:NH
Z
Drug
XH
Y
+
Z
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
O
H 2N
O
CH 3
S
O
valde coxib
8. 9
N
O
Na+
N
O
CH3
S
O
O
par eco xib sod ium
8.10
Prodrug Analogs of
Carbonyl Compounds
Table 8.3
Drug
Drug
X
C
R Y
C=O
R
C
X
Y
C=NR'
C=NOH
OR'
C
OR'
imines oximes ketals
O
C
N
H
S
C
N
H
Examples of Carrier-Linked
Bipartate Prodrugs
1. Prodrug for
Increased Water
Solubility
OH
HO
Me
O
O
R' = CCH2CH2CO2 Na
OR'
Me
O
Prodrug forms
R' = PO3Na2
for aqueous injection or
opthalmic use
R
pr ed niso lone (R = R' = H)
m e thylpr ednisolo ne (R = CH
3 , R' = H)
8.11
poor water
solubility
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
CH 3 O
HO
O
O
O
HO
O
O
O
CH 3O
OR
OCH 3
etop osid e (R = H)
etop osid e ph osph ate (R = PO
3 H2 )
8. 12
2. Prodrug for Improved Absorption
Through Skin
HO
CH3
O
CH3
OR
CH3
O
O
CH3
F
O
F
flu ocinolo ne ace tonid e (R = H)
flu ocinonide (R = COCH3 )
8.14
corticosteroids - inflammation, allergic,
pruritic skin conditions
Better absorption into cornea for the treatment of
glaucoma
RO
OH
RO
NHCH3
dipivefr in (R = Me
3 CCO)
ep inep hr ine (R = H)
8.15
The cornea has significant esterase activity
3.Prodrug for Site Specificity
RO
Bowel sterilant
OR
N
H
O
oxyph enis atin (R = H) (administer
8.16
prodrug R = Ac (administer orally)
hydrolyzed in intestines
rectally)
3. 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.
OCOR
O
NH2
O
O
NH2
O
8.17
Site Specificity
Using Enzymes at
the Site of Action
OR
RO
=
diethylstilbestr ol dip hosp hate (R = PO
3 )
diethylstilbestr ol (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 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)
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
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. 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
N
N
carbo xy pep tidase G2
H
N
O
O
CO2H
CO2H
8.19
I
+
L-Glu
+
CO2
OH
electron -do natin g;
activates n itrogen mustard
electron -with drawing;
deactiv ates nitro gen 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, 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.
Abzyme 38C2 Activation of a
Doxorubicin Prodrug
O
OH
O
O
OH
O
O
OH
OH
CH3O
O
OH H O
O
CH3
NH O
HO
CH3O
O
retro-aldo l
H O
O
8.20
OH
3 8C2
O
OH
O
OH H O
O
CH3
NH O
HO
O
H
O
OH
3 8C2
OH
CH3O
O
retro-M ichael
B-
OH
O
OH H O
O
CH3
NH O
HO
O
-CO2
B-
O
+H+
O-
OH
O
OH
OH
CH3O
OH H O
O
CH3
O
NH2
HO
do xor ub icin
Lectin-directed enzyme-activated prodrug therapy
The LEAPT strategy. (A) Concept. LEAPT is a bipartite delivery system.
(Step 1) Site-selective delivery of a glycosylated rhamnosidase (Rhacleaving) enzyme by sugar-mediated RME. (Step 2) Delivery of a Rhacapped prodrug that can be cleaved only by the delivered glycosylated
rhamnosidase. When these two steps are combined, activation of the
prodrug results in site-selective release of the parent drug (Step 3).
(B) Glycosylated enzyme construction. Pure wild-type α-L-rhamnosidase
N-WT with native “Y-linked” glycosylation was (i) chemically
glycosylated with sugar-IME reagents to yield a N-WT-Xxx (Xxx = Gal,
Man, or dGal) series with mixed synthetic (“X-linked”) and native (Ylinked) glycosylation or (ii) enzymatically DG with endo-H, yielding NDG. N-DG was then chemically reglycosylated with sugar-IME reagents
to yield only synthetic (X-linked) N-DG-Xxx (15). An IME reagent
bearing two terminal Gal units (dGal) was also synthesized from Dgalactose.
Lectin-directed enzyme-activated prodrug 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.
Cl
Cl
N
O2N
N
O
NMe
O
OMe
OMe
NH
O
N
H
OMe
OMe
n it roreduct ase
N
H
HO N:
O
NMe
N
O
NH
N
H
OMe
OMe
-CO 2
+H+
O
Cl
OMe
N
O
Scheme 8.6
N
H
OMe
OMe
NH2
am ino -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
pr op ano lol (R = R' = H)
8.22
antihypertension
O
pr od ru g 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
F
N
OR
Cl
halop er idol (R = H)
halop er idol decan oate (R = CO(CH
2) 8CH3 )
8.24
Sedative/tranquilizer/antipsychotic
O
prod ru g 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
The antibacterial drug
clindamycin (8.28) is
bitter and not well
tolerated by children.
CH3
N H H
N
H
O HO
Cl
O
OH
SCH3
OR
Clindamycin palmitate
clind om ycin (R = H)
clind om ycin pho sphate (R = PO
is not bitter.
3H 2)
clind om ycin palm itate (R = O(CH
2) 14 CH3 )
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
CH2O + NH3
H+
H3O+
N
N
N
me thenamin e
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
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
x
OH
CH
y
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
CH2
C
x
C=O
O
CH3
CH2 C
y
C=O
O
S=O
O
testosterone
to enhance
water
solubility
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
x
CH2 C y
C=O
C=O
O
sp acer
arm
O
+ Cl
NMe2
-
O
O
O
8.35
S=O
H3 C
to enhance
water
solubility
Poly(-Amino Acid) Carriers
poly(L-glutamine)
O
NH CH C
x
spacer
O
N
H
O
O
O
C CH
O
8.37
norethindrone - contraceptive
Slow release over nine months in rats
General Site-Specific Macromolecular
Drug Delivery System
Po ly mer ch ain
Solubilizer
either hydrophilic
or hydrophobic
sp acer
Drug
8.38
Ho ming
dev ice
for site specificity
Site-Specific Delivery of a Nitrogen Mustard
O
NHCH C
O
x
O
NHCH C
NH
poly(L-Glu)
O
NH
water-solubilizing
Ig
Cl
z
spacer
arm
O
O-
y
O
NHCH C
N
Cl
antibody from
rabbit antiserum
against mouse
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)
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.
NH2
N
RO
O N
O
HO
OH
cytosine arabinoside (R = H)
8.41
antitumor
Antibody-Targeted Chemotherapy
Scheme 8.7
calicheamicin, except as disulfide
instead of trisulfide
HO
O
O
NH
CH3O
H
HO
polysaccharide
O
CH3
antibo dy
Lys
S
O
O
O
humanized
H
N
N
H
N
O
S
RS -
O
NH
CH3O
H
O
S
ge m tuz um ab oz ogam icin
8.42
spacer
Does not release calicheamicin nonenzymatically.
Exhibits no immune response.
8.43
polysaccharide
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
Scheme 8.8
Carrier
Link er
Drug
enzy me
Carrier
+
Link er
Drug
sp on tan eous
Link er
+
Drug
Typical Approach
Scheme 8.9
O
Drug—X—CH2 —O— CR
esterase
Drug—X—CH2 —O-
+
fast
Drug—X-
+
CH2O
RCOOH
Tripartate Prodrugs of Ampicillin
O
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.
NH
S
NH2
O
N
COO-
am picillin
8.44
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 Ampicillin
Scheme 8.10
O
Ph
O
Ph
NH
S
NH2
O
esterase
N
O
O
O
R
R'
NH
S
NH2
O
+
N
..
OH
O
O
O
R'COOH
wh en
R' = OEt
R
8.46
bacam picillin (R = CH3 , R' = OEt)
pivam picillin (R = H, R' =t-Bu )
8.45
98-99% absorbed
EtOH
+ CO2
O
R
8.44
Ampicillin is released in < 15 minutes
H
Reversible Redox Drug Delivery System to the CNS
(Nitrogen Mustard?????HW)
Scheme 8.11
hydrolysis
activated
hydrolysis deactivated
O
O
Drug—X
N
CH3
crosses
blo od-brain
barrier
O
Drug—X
N
CH3
enzy me
o xidatio n
8.47
hydrophilic
drug
HOOC
electron-donating,
lipophilic carrier
+
N+
CH3
8.49
Drug — XH
Drug—X
8.48
N+
CH3
enzy me
h ydrolysis
electronwithdrawing;
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 selfimmolative reaction also can be used.
Scheme 8.12
O
O
Drug C OCH2
8.50
O C carrier
esterase
O
O
Drug—C— O—CH2 —O- + HOC
fast
—CH2O
Drug—COO-
carrier
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
N
R
O
O
S
O
N
O
enzy matic
O
o xidatio n
O
O
H
N
R
S
O
O
O
O
N
Me
8.51
O
N
N
Me
esterase
H
N
R
O
Scheme 8.13
O
S
H
N
R
-CH 2O
N
O-
O
O
O
S
N
O
High concentrations of -lactams delivered into brain.
O
O
O
O
N
Me
8.49
Tripartate Prodrug for Delivery of
Antibacterials
Permeases are bacterial transport proteins for uptake
of peptides.
O
Scheme 8.14
HN
+
H3N
O
O
CH3
O
F
N
N
H
+
H3N
1 . p ermease
COO
-
2 . p ep tidase
COO-
O
+
CH3
..
H2N
O
8.53
+
H2N
+
NH4
+
O
H
COO
H
COO-
Only L,L-dipeptides are active
F
HN
-
+
F
HN
O
N
H
N
COO-
Mutual Prodrugs
A bipartate or tripartate prodrug in which the carrier is a
synergistic drug with the drug to which it is linked.
Antibacterial
ampicillin
-lactamase inactivator
penicillanic acid sulfone
O
Ph
NH2
N
H
O
O
S
N
O
S
O
O
O
N
O
O
su ltam icillin
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
NH2
N
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
8. 61
N
H
Modification gave 8.62 with high activity.
S
N
8.62
N
N
H
The sulfoxide (8.63) was more potent, but it blocked
iodine uptake into the thyroid.
O
S
N
N
N
H
tim opr az ole
8.63
Lead Modification (cont’d)
Modification of 8.63 gave
8.64 having no iodine
blockage activity.
CH3
N
Picoprazole shown to be an
inhibitor of H+,K+-ATPase.
SAR of analogs indicated
electron-donating groups on H3 C
the pyridine ring were
favorable. Increased inhibition
of H+,K+-ATPase.
Best analog was omeprazole
(8.65).
O
S
CO2CH3
N
N
H
picop razo le
8.64
OCH3
CH3
N
O
S
CH3
N
N
H
om epr az ole
8.65
OCH3
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.
Scheme 8.16
OCH3
CH3
H3 C
OCH3
CH3
H3 C
N
..
OCH3
H3 C
N
S
HN
N
O
S
H+
OCH3
OCH3
O
..
HN
S
N
H
OCH3
CH3
H3 C
-H2 O
N
HN
N
CH3
N
OH
S
S
N
N
OCH3
OCH3
8.66
om epr az ole
8.65
Formation of 8.66 leads to covalent attachment.
Omeprazole also inhibits isozymes of carbonic
anhydrase, another mechanism for lowering gastric
acid secretion.
+H+
OCH3
CH3
H3 C
N
N
S
NH
OCH3
S
Hydrolytic Activation
Hydrolysis can be a mechanism for bioprecursor
prodrug activation, if the product requires additional
activation.
Scheme 8.17
S
OS+
Me
O
N
H
R
S
S
O
Me
O-
S
S+ N
N
K2CO3/RX
N
Me
HO
O
O Me OH
Me HO
leinam ycin
8.70
O
antitumor agent
Me
HO
8.71
O
prodrugs of leinamycin
HO-
Hydrolysis of these analogs gives an intermediate
that reacts further to the activated form.
O
O
O
Me
S
O
O
Me
-
S+ N
S
O
N
Me
HO
S-
S+ N
O
O
O
S
N
Me
HO
O
Me
Me
O-
O
Me
S
OS+
Me
O
S
N
H
N
O Me OH
DNA
cleavage
Me
O
O
O
O
this was
synthesized and
gave 8.74 as well
as DNA cleavage
O
O
8.73
8.72
increased stability
over leinamycin
HO2 C
Me
HO
Scheme 8.18
Me
O
N
H
S
N
S
HO Me
O
O
8.74
O
Elimination Activation
Scheme 8.19
B:
CF 3
O
H
N
N
H
N
O
B
CH3
H
le fluno m ide
8.75
rheumatoid arthritis drug
CF 3
O
C
HO
N
H
CH3
8.76
potent inhibitor of
dihydroorotate
dehydrogenase
Inhibition of dihydroorotate dehydrogenase blocks
pyrimidine biosynthesis in human T lymphocytes.
Oxidative Activation
N-Dealkylation
Sedative 8.20
CH3
N
N
N
Cl
N
O
CH3
CH3
CH3
N
N
CH3 P4 50
N
O
Cl
X
CH3
P4 50
Cl
X
CH3
N
Cl
X
alpr az oalam (X = H)
tr iazolam (X = Cl)
8.77
N
N
N
-H2 O
N
NH
.. 2
O
H
N
N
N
N
X
CH3
N
N
..
NH
Cl
HO
X
O-Dealkylation
Analgesic activity of phenacetin is a result
of O-dealkylation to acetaminophen.
O
HN
CH 3
OR
phe nacetin (R = CH2 CH3 )
acetam ino phen (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
N
P
P-4 50
O
Scheme 8.21
P
Cl
O N
+
NH3
Cyclophosphamide was very
effective, but it required liver
homogenates (contains P450)
for activation. Therefore
oxidation is required, not
hydrolysis.
H NH
2
O
Cl
O N
Cl
cycloph osph am ide
8.79
HPO4=
O
H
N
H
O
8.80
Cl
O
Cl
DNA
H
+
Cl
8.83
8.82
8.84
DNA
O
+
N
HN
H 2N
Cl
8.81
H 2N
O
-O P
spo ntan eo us
N
or
p ho sph oramidase
Cl
Cl
B:
Cl
+ HN
P O
N
N
N
DNA
H
N
H
N
O
OH
N
HN
H 2N
N
N
8.86
8.85
isolated
OH
N-Oxidation
identified
O
CH3 NHNHCH2
CNHCHMe 2
O
O
P4 50
CH3 NHNHCH2
CNHCHMe 2
-H2 O
HO
pr ocar bazine
8.87
CH3N=N—CH
+
B— H
H
CNHCHMe 2
8.88
B:
advanced Hodgkin’s
disease
O
CH3N-NH2 + OHC
H
O
H 2O
CNHCHMe 2
CH3N-N=CH
H
CNHCHMe 2
8.89
CH3N=NH
O
+
CH3-N
CH3
Scheme 8.22
N
+ N2
DNA
HN
H 2N
N
CH3
+N
N
DNA
O
CH3
N
N
N
HN
H2N
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
Cl
CH3
-
N OH
pr alido xim e chlor id e
8.91
To increase the permeability of pralidoxime into
the CNS, the pyridinium ring was reduced (8.92).
N
CH 3
8.92
N OH
oxidation into brain
Cl -
+N
N OH
CH3
pr alido xim e chlor id e
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
R
H
R
X
Prodrug residue
H
X
N
N
CH3
O H
CH3
Blood-brain
barrier
Prodrugs for (increased) Site Specificity
Once inside the brain, the lipophilic carrier is
converted enzymatically to a highly
hydrophilic species (positively charged), which
is then enzymatically hydrolyzed back to the
drug and N-methylnicotinic acid, which is
eliminated from the brain.
O H
R
H
X
N
♫The oxidation of the dihydropyridine to the
CH3
Enzymatic oxidation
pyridinium ion (half-life generally 20-50 min)
prevents the drug from escaping out of the
brain because it becomes charged.
♫This drives the equilibrium of the lipophilic
precursor throughout all of the tissues of the
body to favor the brain.
O
R
X
N
CH3
Prodrugs for (increased) Site Specificity
O H
R
H
R
X
O H
H
X
N
N
CH3
Prodrug residue
CH3
Blood-brain
barrier
Enzymatic oxidation
O
HOOC
+
N
CH3
HX
Drug
R
Drug release
by a suitable
process
R
X
N
CH3
Mechanism of Acetylcholinesterase
Scheme 8.23
O
Me 3N—CH2CH2—O
Trp
H
CH3
Trp
Phe
B+
:B
H
O
Me 3N—CH2CH2—OH
Trp
Trp
Phe
O
B:
O
CH3
+
HB
H 2O
+
M e3NCH2CH2OH
+
CH3 COOH
Inactivation of Acetylcholinesterase by
Diisopropyl Phosphorofluoridate
Scheme 8.24
affinity labeling agent
O
Trp
Phe Trp
O
O
F P
O
H
B+
O
H
8.93
P
Trp
:B
Phe Trp
O
B
O
O
HB
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
P
Trp
Phe Trp
O
B
O
p ralidox ime
O
HB
O
N
Me
N
O
P
O
O
N
Trp Me
Phe Trp
N
Trp Me
Phe Trp
N
H O
O:
B
O
P
O
O
HB
O
N
P O
O
O
BH
OH
:B
Temporary inhibition of acetylcholinesterase
enhances cholinergic action on skeletal muscle.
Scheme 8.26
O
Me 3N
Trp
Phe Trp
O
H
B+
O
NMe2
:B
H
ne ostig m ine
8.94
Used for the neuromuscular
disease myasthenia gravis
Me 3N
Trp
Trp
Phe
OH
B:
NMe2
O
covalent, but
reversible slo w
inhibition
H2O
+
Me 3N
OH
+
HB
O
CO2 + Me2NH
Reversible (noncovalent) inhibitors of
acetylcholinesterase, such as donepezil and tacrine
are used for Alzheimer’s disease. Enhances
neurotransmission involved in memory.
H 3CO
H 3CO
O
NH 2
N
.
HCl
do nepe zil hydr och lor ide
8.95
N
.
HCl
tacr ine hydr och lor ide
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
H HO
O
HO
O
H
:B
H HO
CH3
RSH
HO
RS H
H
brefeldin A
8.97
O
O
[O]
CH3
HO
H
8.98
-RSOH
antitumor,
antiviral agent
H HO
RS
O
H
H
O
H
8.99
O
CH3
Aromatic Hydroxylation
Cyclohexenones as prodrugs for catechols
Scheme 8.28
N
N
P4 50
N
N
-H2 O
HO
O
O
8.100
oxidation next to
sp2 carbonyl
OH
O
aromatic hydroxylation
P4 50
N
HO
OH
8.101
Alkene Epoxidation
O
N
N
O
NH2
car bam az epine
8.102
O
NH2
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).
NH2
COOH
R
oxfenicin e (R = OH)
8.105
Reductive Activation
Azo Reduction
Scheme 8.29
COOH
N
NHSO 2
N=N
Anaerobic cleavage
by bacteria in lower
bowel
OH
su lfas alazi ne
8.106
ulcerative colitis
COOH
H 2N
OH
+
8.107
For inflammatory
bowel disease
N
NHSO2
NH2
su lfap yr idine
8.108
Causes side effects
To prevent side effect by sulfapyridine a
macromolecular delivery system was developed.
poly(vinylamine)
n
Not absorbed or
metabolized in small
intestine.
NH
SO2
spacer
CO2Na
N
N
OH
8.109
CO 2Na
NH 2
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
HO
N
N
O
OH
HO
vidarabin e (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
CO 2H
F
CH 3
CH 3
S
O
sul indac
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 group replaced p-Cl to increase water solubility.
CO 2H
F
CO2H
MeO
CH 3
CH3
N
O
CH 3
S
O
sul indac
8. 111
Cl
in dome thacin
8.112
NH2
HO
N
H
se ro tonin
Disulfide Reduction
To increase the lipophilicity of thiamin for absorption into
the CNS.
OH
Scheme 8.30
CH 3
..
GSH
N
N
S
S
NH2
N
N
O
H
O
N
OH
N
CH 3
O
H
NH 2
+B
8.113
S-
H
nonenzymatic
OH
+
N
N
CH 3
N
NH 2
S
thiam in
8.114
poorly absorbed into CNS
N:
N
OH
CH 3
N
S
NH 2 OH
CH3O
N
HN
NH2
To diminish toxicity of primaquine and
target it for cells with the malaria parasite, a
macromolecular drug delivery system was
designed.
pr im aqu ine
8.115
Intracellular thiol much higher than
in blood; selective reduction inside
the cell
antimalarial, toxic
CH3O
lactose
N
HN
primaquine
O
N
H
S S
NH3+
8.116
serum
albumin
for
improved
uptake in liver
lactose
Therapeutic index of 8.116 is 12 times higher than 8.115 in mice.
Nitro Reduction
Scheme 8.32
O
HN
O
Cl
N
H3 C
P
O
N
Cl
bio reduction
N
H3 C
HO
P
O
O
O
-O
P
O-
O
O
O
O
N
O
O-
O
H 2O
P O
O
HO
8.121
Mechanism-based inactivator of
thymidylate synthase
HO
N
H3 C
P
F
HN
O
N
O
O
O
F
HN
O
N
O
8.119
F
O
P
F
HN
O
HO
8.120
O
HN
H3 C
HO
..
HO NH
O
Cl
..
N
N
O
O
8.118
F
HN
O
O
O
O
O 2N
O
O
F
O
O
N
O
OHO
Nucleotide Activation
Scheme 8.33
=
O3PO
O
SH
N
N
N
N
H
6-mer captop ur ine
8.122
Anti-leukemia drug
O
SH
O
O—P—O—P—OHO OH
O
-
O
-
N
N
=
O3PO
N
N
O
h yp ox an thine-guan ine
p ho sph oribo syltran sferase
HO OH
8.123
Inhibits several enzymes in
the purine nucleotide
biosynthesis pathway.
Phosphorylation Activation
O
O
N
HN
H2N
RO
N
N
N
HN
H 2N
HO
N
N
O
O
acyclo vir (R = H)
8.124
antiviral
HO
8.125
2-deoxyguanosine
Resembles structure of 2-deoxyguanosine
Uninfected cells do not
phosphorylate acyclovir
(selective toxicity)
viral thymidine kinase
R = PO3=
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
Resistance to Acyclovir
Modification of thymidine kinase
Change in substrate specificity for thymidine
kinase
Altered viral -DNA polymerase
Only 15-20% of acyclovir is absorbed
Therefore, prodrugs have been designed to
increase oral absorption.
Hydrolyzes
here
NH2
N
N
H 2N
N
N
Prodrug for
a prodrug
HO
O
8.126
adenosine deaminase
acyclovir
Hydroxylates
here
N
N
H 2N
HO
N
N
O
8.127
xanthine oxidase
acyclovir
This prodrug is 18 times more water soluble than
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
N
HN
H2N
H2N
O
N
N
O
O
valaciclo vir
8.128
An analog of acyclovir whose structure is even
closer to that of 2-deoxyguanosine is ganciclovir.
O
N
HN
H 2N
HO
N
N
O
HO
ganciclovir
8.129
More potent than acyclovir against human
cytomegalovirus.
Two carbon isosteres of ganciclovir are available.
O
N
HN
H 2N
HO
N
H 2N
AcO
N
HO
pen ciclovir
8. 130
N
N
C in place
of O
N
N
AcO
C in place
of O
fam ciclo vir
8.131
Better oral absorption
than penciclovir
Converted to penciclovir
rapidly
Sulfation Activation
Activity of minoxidil requires sulfotransferase-catalyzed
sulfation to minoxidil sulfate.
OR
H 2N
N
NH2
N
N
minoxidil (R = )
minoxidil sulfate (R = SO
8.132
3
-
)
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
NH 2
H
R
levodo pa (R = COOH)
8.133
After crossing bloodbrain 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 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
HO
NHNH 3
H3 C
COO -
car bid opa
8.134
+
OH
O
HO
N N
H H
NH2
OH
be nser azid e
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
COO
OH
-
H
N
+
NH3
O
OH
COO-
L--glutamy l
transp eptidase
OH
+
NH3
Glu
8.137
OH
COO-
L-dopa
L-aromatic amino acid
decarbox ylase
L--glutamyl-L-dopa
Dopamine accumulates in the kidneys because of
high concentrations of L--glutamyltranspeptidase
and L-aromatic amino acid decarboxylase there.
+
NH3
OH
OH
Example of a carrier-linked prodrug of a bioprecursor
prodrug for dopamine.