concepts in drug metabolism

Things to remember when applying to drugs

2008-06-13T16:39:00.001-07:00

Phase I

*Cytochrome P450 looks for sites most electron rich and least sterically hindered.

So aromatic rings with electron withdrawing groups (Halogens, Nitro, carboxylic acid etc..) are not oxidized.
*The same drug can be oxidized at different sites.

so when looking at a drug we look for

1-Aromatic (benzene ring) → para hydroxylation.

(R is the rest of the molecule)

2-Olifin (double bond) → diol 3- Methyl group attached to benzene (benzylic carbon) → alcohol
4-Methyl group attached to C=C (allylic carbon) → alcohol.

5-Carbon next to carbonyl and imin (benzodiazepine) → secondary alcohol
6-long chain → oxidation of ω (terminal) carbon or ω -1 (carbon before the terminal)
...........................→ alcohol

7-Alicyclic (saturated ring) → alcohol on para position .

8-Tertiary or secondary aliphatic amine → removal of a methyl group to give amine
.......(make sure to remove the smaller group)
.......(called oxidative dealkylation)
.......(make sure the alkyl group removed has H on it )
.......(if the other group is small will be removed in a second step)

9-Tertiary nitrogen (nitrogen with no hydrogen on it) → N-oxide.

10-Primary amine (R-CH2-NH2) → NH3 (amonia) + R-CHO (carbonyl)
.........(oxidative deamination)
.........(make sure the carbon next to N has H on it)

........R- NH2 → R-NO2 (nitro).

11-An OCH3 group R-OCH3 → R-OH (alcohol)+ HCHO

12-An (R-S-CH3) → removal of CH3 → R-SH + HCHO
..........the carbon removed should have H

..............→ Oxidation of the sulfur → Sulfoxide or sulfone.

13-A Sulfur attached to a carbon with double bond C=S → C=O (oxidative desufuration).

14- R-CH2OH (alcohol) → R-CHO (aldehyde) → R-COOH (acid).

15- R-COOCH2R' (ester) → RCOOH (acid)+ R'-CH2OH (alcohol).

16-R-HNCO-R' (amide) → R-NH2 (amine) + R-'COOH (acid)

all the functional groups produced above (aromatic OH ; NH2 ; C=O ; COOH ; SO2 ; NO2 ; SH etc...) will be conjugated in phase II.
*If a drug has a functional group before metbolism e.g. OH or COOH or aromatic NH2 etc.. It can go directly to phase II and conjugates this functional group to a conjugating agent (mostly glucoronic acid)
*Aromatic amines are famous for conjugation to Acetyl group.

Please apply all these rules to the drugs given in the lectures.

Good Luck

Points to remember

2008-06-13T14:46:00.000-07:00

1-Metabolism is a Detoxification process .........(with exceptions)
2-Phase I Changes the structure by adding a functional group in order to
a-end the pharmacological activity............(with exceptions)
b-Increase polarity (convert it to water soluble).
c-Introduce Functional group to react in phase II

d-Liver is the main site of metabolism.

e-Kidney is the main site of excretion

Phase I metabolism

Microsomal Oxidation

(Introduction of one atom of Oxygen into the molecule (Cytochrome P450)).

*Introduction of a fumctional group

1-Carbon-Carb0n system

*Aromatic Oxidation (major ) → arene oxide → Para hydroxylation of only one electron rich (no electron withdraing groups) aromatic ring.
*Olifins → epoxide → Diols
*Benzylic cabon →→ alcohol
* allylic carbon →→ alcohol
* alicyclic →→ alcohol (para position)
*carbon next to carbonyl imin →→ alcohol
*long chain hydrocarbon → ω or ω -1 oxidation → alcohol
........The alcohol produced may be conjugated by phase II directly or
........if primary alcohols produced →→→ aldehyde then acid.
........if secondary alcohol is produced →→→ keton (revercible)

2-Oxidation of carbon - Heteroatom

Carbon-Nitrogen
......a-Oxidation of α carbon → Carbinolamine → Carbonyl (aldehyde or keton) + Amine(Oxidative N-dealkylation)

..................Small alkyl groups removed faster
...................α-carbon must have a hydrogen
...................first alkyl group removed faster.

b-Oxidation of the nitrogen
................Tertiary amine →→→ N-oxide (revesible)
................Secondary amine → hydroxylamine → nitrone
................Primary amine → hydroxylamine → nitroso → nitro

*Oxidation of aromatic amines and of amides will follow the same rules.
*The same rules are also true for Carbon - Oxygen system →→ carbonyl + alcohol.
*Carbon Sulfur System→→→
..........oxidation of α-carbon → SH and carbonyl
..........oxidation of S →→ S oxide →→ sulfone
...........Oxidative desulfuration (replace C =S with C=O).

Non- microsomal oxidation

Oxidation of alcohol to aldehydes and ketons by alcohol dehydrogenase (non microsomal),
Oxidation of aldehydes to acids by aldehyde dehydrogenase (non microsomal).

Reduction reactions

Ketones reduced (by alcohol dehydrogenase) →→ secondary alcohols (symetric center)

Reduction of nitro and azo compounds →→ amines (the opposit of oxidation reaction).

Hydrolysis reactions

*Hydrolysis of esters →→→ alcohol and acid (Very fast) (major)

*Hydrolysis of amides →→→ amine and acid (Slower)

Phase II (Conjugation)

*if the drug has a funcional group may go to phase tow directly.

*Requires activation (except glutatione conjugtion).

*Activation of the conjugating agent in

...........Glucuronic acid conjugaiotn (Major for all drugs) →active form UDPGA.

...........Sulfate (for alcohols)(minor) → active form PAPS

...........Methyl (catichols and phenols)(for indogenous compounds) → active form SAM

...........Acetyl (aromatic amines) → active form Acetyl COA

*activation of the the drug (aromatic acids)(Minor) → Active form of the drug Acyl COA

phase II (Conjugation produces non toxic metabolites (Except sulfate conjugation)

phase II increased solubility (except methylation and acetylation)

Glutatione conjugation is most important for protection againest electrophilic agents

Things to remember when applying to drugs

2008-04-25T11:52:00.001-07:00

Phase I

*Cytochrome P450 looks for sites most electron rich and least sterically hindered
So aromatic rings with electron withdrawing groups (Halogens, Nitro, carboxylic acid etc..) are not oxidized.

*The same drug can be oxidized at different sites.

so when looking at a drug we look for
(R is the rest of the molecule)

1-Aromatic (benzene ring) → para hydroxylation
2-Olifin (double bond) → diol

3- Methyl group attached to benzene (benzylic carbon) → alcohol
4-Methyl group attached to C=C (allylic carbon) → alcohol
5-Carbon next to carbonyl and imin (benzodiazepine) → secondary alcohol
6-long chain → ω (terminal) carbon or ω -1 (carbon before the terminal) → alcohol
7-Alicyclic (saturated ring) → alcohol on para position.

8-Tertiary or secondary aliphatic amine → removal of a methyl group to give amine

.....(make sure to remove the smaller group)

.....(called oxidative dealkylation)

.....(make sure the alkyl group removed has H on it )

.....(if the other group is small will be removed too

9-Tertiary nitrogen (nitrogen with no hydrogen on it) → N-oxide.

10-Primary amine (R-CH2-NH2) → NH3 (amonia) + R-CHO (carbonyl)

.......(oxidative deamination)

........(make sure the carbon next to N has H on it)

........ NH2 → NO2 (nitro). 11-An OCH3 group → OH +OCHO

12-An (R-S-CH3) → removal of CH3 → R-SH + HCHO

..........the carbon removed should have H

..............→ Oxidation of the sulfur → Sulfoxide or sulfone.

13-A Sulfur attached to a carbon with double bond C=S → C=O (oxidative desufuration)

14- R-CH2OH (alcohol) → R-CHO (aldehyde) → R-CH2CH2CH2COOH (acid)

15- R-COOCH2R' (ester) → RCOOH (acid)+ R'-CH2OH (alcohol)

16-R-HNCO-R' (amide) → R-NH2 (amine) + R-'COOH (acid)

*all the functional groups produced above (aromatic OH ; NH2 ; C=O ; COOH ; SO2 ; NO2 ; SH etc...) will be conjugated in phase II.

*If a drug has a functional group before metbolism e.g. OH or COOH or aromatic NH2 etc.. It can go directly to phase II and conjugates this functional group to a conjugating agent (mostly glucoronic acid)

*Aromatic amines are famous for conjugation to aAcetyl group.

Please apply all these rules to the drugs given in the lectures.

Good Luck

Summary of Lecture I

2008-04-10T12:37:00.001-07:00

DRUG METABOLISM

Structural changes that occur to a foreign molecule (xenobiotic) in the body
Drugs should have certain lipid solubility (lipophilic) to be absorbed.
In the body will mostly be passively transferred into the tissues where they act on the receptors (active).
If highly lipid soluble will not be eliminated form the body.
To be excreted in the urine (major excretion site) a drug has to be polar enough to dissolve in water.
Other excretion sites are sweat, saliva, bile etc…
The introduction of small polar groups into the molecule (metabolism):
A-ends pharmacological activity (alters fitting in the receptor) (Key & lock)
B-ends toxicity (detoxification)
C-increases polarity and water solubility (elimination).

Which fulfill the purpose of drug metabolism

In general metabolism means detoxification
Exceptions:

A-Inactive drug (mostly a pro-drug) by metabolism will produce active drug (bio-activation) e.g. prontosil.

B- metabolism of the activedrug (imipramine) gives the active metabolite (desipramine).
C-Some drugs give toxic metabolites e.g. chloarmphenicol.

Sites of drug metabolism

divided into hepatic and extra hepatic:

Hepatic metabolism in the liver (major site of metabolism) for the following reasons:
i- Rich in almost all drug-metabolizing enzymes
ii-A well-perfused organ i.e easy access to the metabolizing enzymes.
iii-Orally administered drugs will all pass through the liver before reaching systemic
circulation thus extensively metabolised (first pass effect) the extent of metabolism may affect the bio-availability (Lidocain and nitroglycerine)
Extra hepatic metabolism: overall metabolism in tissues other than the liver.
i-Intestine:
a- estrases and lipases are abundant in intestinal wall leading to the hydrolysis of many ester pro-drugs.
b-Intestinal b-glucuronidase enzyme hydrolyzes glucuronide conjugates the product will be reabsorbed into the blood stream (enterohepatic circulation or recycling) thus the drug stays longer in the body (affect duration of action).
c- Bacterial flora present in the intestine and colon also contribute to the
overall drug metabolism.
ii-Kidney, lungs, adrenal glands, placenta, brain etc….

Figure 1 showing the fate of the drug in the body after administration

General pathways of drug metabolism

Phase I (Functionalization Reactions):

Introduces a polar functional group e.g. OH, COOH, NH2, SH, into drug molecule
a- direct introduction of the functional group (e.g. aromatic & aliphatic hydroxylation)
b- modifying existing functionalities (e.g. reduction of ketones & aldehydes to alcohols, oxidation of alcohols to acids, hydrolysis of esters & amides to yield COOH, OH , NH groups, reduction of azo & nitro compounds to give NH2).
Generally provides a functional group or handle in the molecule in preparation for phase II reactions.
Product may be polar enough to be excreted.

Phase II (Conjugation Reactions):

Attaches a small, polar& ionizable endogenous compounds such as glucuronic acid, sulfate, glycine and other amino acids to the functional group"handle" of phase I metabolites to produce water soluble conjugates.
Conjugated metabolites are readily excreted in the urine and generally devoid of pharmacological activity and toxicity.
Compounds with existing functional groups may go directly to phase II.
Drugs which are already hydrophilic are largely excreted unchanged.

PHASE I (FUNCTIONALIZATION REACTIONS):

1-OXIDATION REACTIONS

Requires molecular oxygen + reducing agent NADPH (reduced form of nicotinamide
adenosine dinucleotide phosphate).

according to the following equation:

RH +NADPH+ O2+ H+ = ROH+ NADP+ +H2O

The enzyme systems involved is called mixed function oxidases or monooxygenases. Composed of :
a-Cytochrome P-450 (heme-protein). Transfers an oxygen atom to the substrate
(RH) through the activation of molecular oxygen, introducing one oxygen atom into drug molecule & reduction of other to water
b-NADPH- dependent cytochrome P-450 reductase.
c- NADH- linked cytochrome b5.
a + b + cofactors NADPH& NADH provide reducing equivalents (electrons).

Cytochrome P450 is present in high concentration in liver. Also present in other tissues e.g kdney
The name cytochrome P-450 is due to the reduced (Fe2+) form enzyme – substrate complex binding with carbon monoxide giving a complex with UV-absorption maximum at 450 nm.

Types of oxidation reactions catalyzed by Cytochrome P450

1-Oxidation of aromatic moieties (aromatic hydroxylation):

Oxidation of aromatic compounds (arenes) by mixed function oxidase enzymes to phenolic metabolites (arenols) through epoxide intermediate (arene oxide).
Arene oxides intermediates are extremely toxic, are highly electron deficient thus react with any nucleophils in the body.
Body rids itself by detoxification pathways.
Aromatic oxidation is a major pathway in human.

Scheme 1: Possible reaction pathways for the detoxification of arene oxid.

Rules for hydroxylation aromatic rings

Similar to electrophilic substitution rules:

Para hydroxylation usually predominates (preferred) (less sterically hindered) e.g. phenobarbital.

e.g.phenobarbital

Sometimes ortho oxidation may take place.

If two identical aromatic rings are present, oxidation of only one ring e.g.. phenybutazone

phenylbutazone

Chlorpromazine

Substituents on aromatic ring influence the ease of hydroxylation. activated (electron rich) rings e.g. Diazepam arfe more susceptible.

Diazepam

Clonidine hydrochloride

4-2,3,7,6-tetrachlorodibenzo-p-dioxins (environmental pollutants) highly toxic: resistant to aromatic oxidation
Resistance to metabolism + lipid solubility will lead to residance in the body.

2-Oxidation of olefins:

Olefinic carbon–carbonwill produce epoxides (more stable than arene oxide)
The epoxide is susceptible to enzymatic hydration by epoxide hydras giving the1, 2- dihydrodiols.

example is the oxidation of chlorpromazine (above) into diole metabolite through the stable active epoxide intermediate which contributes to the overall activity of the drug.

also the oxidation of alcofenac side chain to the diol metabolite (above).
simillarly secobarbital is metabolized to diol metabolite through the formation of the epoxide.

Epoxide intermediate is very toxic and is highly electron deficient will attack nucleophils in surrounding enzymeleading to covalent bonding to the enzyme and its destruction (suicidal complex). Thus causing inhibition of the enzyme and prolong duration.

3- Oxidation at benzyllic carbon atoms:

Carbon atoms attached to aromatic rings (benzyllic) oxidation produces alcohols (carbinol) ) metabolite.
Primary alcohols metabolites are further oxidized to aldehydes and carboxylic acids (CH2OH→CHO→COOH). Eg. tolbutamide.

4- Oxidation at allylic carbon atoms:

Commonly observed in drug metabolism

Tolbutamide........................................ THC

more preferable, less

sterically hindered

metabolically active site.

5-Oxidation at carbon atoms alpha to carbonyl and imines:
benzodiazepines an important class of drugs undergo oxidation to 3-hydroxyl metabolite

6-Oxidation at aliphatic and alicyclic carbon atoms:

Oxidation at the terminal carbon (ω – oxidation)
ω -1 also called penultimate carbon atom (carbon next to the last carbon)
The alcohol metabolites undergoes further oxidation giving aldehydes and ketones or carboxylic acid. e.g. anticonvulsant valproic acid

valproic acid

secobarbital

Phenylbutazone

Summary of Lecture II

2008-04-10T12:36:00.002-07:00

7-Oxidation involving carbon-heteroatom systems:

Nitrogen, oxygen & sulfer.
Two types of biotransformation reaction
1-Hydroxylation of the α-carbon atom attached directly to the heteroatom (N, O, S) giving an unstable intermediate which decomposes leading to the break of the carbon-heteroatom bond thus causing Oxidative N-, O-, and S-dealkylation and oxidative deamination.

The presence of a- hydrogen is a must

2-Hydroxylation or oxidation of the heteroatom (N, S only) → N-hydroxylation, N-oxide formation, sulfoxidation and sulphone formation.

a-Oxidation involving carbon–nitrogen systems
1-Oxidation of tertiary aliphatic and alicyclic amines:

Two types of oxidation reactions.
Catalyzed by Cytochrome P450

i-Oxidation of the carbon atom α to the nitrogen:
oxidation reaction leading to the elimination of alkyl groups (especially methyl groups). oxidative N-dealkylation.

carbinolamine

α-carbon hydroxylation gives unstable carbinolamine intermediate which undergoes spontaneous heterolytic cleavage of the C-N bond giving secondary amine + carbonyl moiety.

I-Tertiary alphatic amines:

rules which govern the N-dealkylation reactions:

1-small alkyl groups e.g. methyl, ethyl and isopropyl are removed rapidly

2- α-carbon involved has to be attached to a hydrogen atom.
Exception N-dealkylation of the t-butyl group through carbinolamine intermediate not possible since α-carbon hydroxylation cannot take place (no α H).

Can take place indirectly e.g. t-butylnorchlorocyclizine is metabolized by the removal of the t-butyl gp giving norchlorcyclizine.

3-The removal of first alkyl gp from tertiary amines is faster than second alkyl gp.

4- In tertiary amines with different substituents smaller alkyl group is more rapidly removed.e.g. benzphetamine.

5- Bisdealkylation of tertiary amines will give primary aliphatic amine which will lead to further oxidation as shown later.
6-Alicyclic tertiary amines undergo oxidative N-dealkylation. e.g. meperidine .

7-carbon α to an alicyclic nitrogen will undergo hydroxylation giving lactam metabolite. e.g.nicotine (below).
8-Dealkylation of tertiary amines is faster than secondary.

b- Oxidation of the nitrogen gives N-oxide which may undergo reduction by reductase to give tertiary amine again.

N-oxide metabolite may possess pharmacological activity. e.g. imipramine oxidized to active N-oxide.

N- oxidation is a minor metabolic pathway.
Dealkylation more predominant.

II- Secondary aliphatic and alicyclic amines:

The secondary amine may be parent drug or metabolite which will undergo metabolism by
a- oxidation of the carbon α to the nitrogen giving.

i-N-dealkylation through carbinolamine intermediate giving primary amine metabolite.

propranolol..................Crbinolamine..........primary.........ketone
β-adrinergic blocker..............................aliphatic amine ..metabolite

ii-Oxidative deamination:

similar to N-dealkylation.
larger bulk of the molecule will be eliminated as a carbonyl metabolit.
smaller bulk is removed as a small amine moiety.
The mechanism is again through α-carbon hydroxylation giving carbinolamine intermediate leading to carbon-nitrogen cleavage and the formation of carbonyl + amine.

as shown below:
In case of a drug like Norketamine (below)

α-carbon has no H thus no α-carbon hydroxylation is possible thus deamination is not possible.

Both reactions possible dealkylation first then deamination is more favourable.
Secondary amines will undergo N-dealkylation before deamination.
direct deamination of secondary amines takes place.
propranolol has two α- carbon atoms which contain H (both subject to hydroxylation) leading to two possibilities :

A- N-Dealkylation followed by elimination of acetone moiety giving primary amine metabolite then deamination giving carbonyl metabolite.

OR
B-Direct dealkylation → elimination of isopropylamine → aldehyde metabolit.

Secondary alicyclic amines undergo α hydroxylation giving lactam metabolite e.g. phenmetrazine (shown below).
ii-Oxidation of the nitrogen atom:

Results inseveral oxygenated products.

N-Hydroxylation will give the corresponding N-hydroxylamine metabolites which will undergo further oxidation (spontaneously or enzymatically) giving nitrone derivatives.

Example is N-benzylampheatmine sahown below).

III-Primary aliphatic amines:

Parent drug or metabolite (Dealkylation of primary amine or bis dealkylation of secondary amine) will undergo
a- Oxidation of the carbon α to the nitrogen leading to oxidative deamination (catalized by mixed function oxidases).

Endogenous primary amines e.g. dopamine, nor epinephrine → oxidative deamination
(monoamine oxidases (MAO)) inactivation.
b- Oxidation of the nitrogen atomleading to formation of N-hydroxyl amine (chemically unstable) which undergoes spontaneous or enzymatic oxidation giving nitroso and nitro derivatives.

Structural features of the α- substituent will determine which oxidation will take place that of the carbon or the nitrogen.
E.g. in phenteramine the Oxidation of Nitrogen inevitableas shown below:

normally if α-carbon contains a H then either oxidation of N to give N oxide or Oxidation of C to give deamination as shown

Dealkylation is a more common pathway than N-oxidation in human
IIII-Oxidation of aromatic amines and heterocyclic nitrogen compounds

1-For tertiary aromatic amines: N-dealkylation and N-oxidation e.g. N,N-dimethylaniline.

2-Secondary aromatic amines:

N-dealkylation →primary amine or N-hydroxylation to give nitrone e.g. N-methyl-4-aminoazobenzene.

3- Primary aromatic amines:

more common in drug
Can be formed from metabolism of other compounds.
Will be N-hydroxylated giving N-hydroxylamine which is further oxidized to the nitroso then nitro derivatives.
Aromatic hydroxylamine.
Nitrogen atoms in aromatic heterocyclic moieties undergo N-oxidation (minor extent)

Summary of Lecture III

2008-04-10T12:36:00.001-07:00

IV-Oxidation of amides

Oxidative carbon- nitrogen cleavage.

Oxidative dealkylation of N- substituted amide.

diazepam..............................................desmethyldiazepam
parent drug..................................................active metabolite

Chlorpropamide...........................carbinplamid

Hyopglycemic..................................Intermediate

2. Cyclic amides or lactams will undergo hydroxylation of carbon α to the nitrogen leading to ring opening.

3. Aromatic amides will undergo N- Hydroxylation (minor extent) giving chemically reactive
metabolites (Toxic).

Acetaminophen...........N-acety- acetaminophen.......N-acetyl-imidoquinone

N-acetylimidoquinone is electrophilic thus may react with either

Liver macromplecules causing liver necrosis.
GSH as a protection mechanism.

b-Oxidation involving Carbon-Oxygen System

Attack only on the α-carbon atom causing dealkylation (mainly ethers) .
Mixed function oxidases will undergo α-carbon hydroxylation giving hemiacetal or hemiketal intermediate the equivalant of carbinolamine in amines.

Hemiacetal

If several non-equivalent methoxy groups are present only one will undergo O-dealkylated selectively.

c-Oxidation of carbon-sulfur system

Three types of oxidative metabolic reactions.

1......Cleavage of carbon-sulfur bonds (S-Dealkylation).

similar to O- and N- dealkylation reactions (α-carbon hydroxylation} + (α-hydrogen atom).

2.......Desulfuration:
oxidative conversion of carbon-Sulfur double bonds (C=S) to carbon-oxygen double bond (C=O).

Similar desulfuration takes place in P=S in organophosphorous insecticides.
Mechanism is still unknown (microsomal oxidation of the C=S or C=P).

3.......Oxidation of the sulfer or S-Oxidation.

Common metabolic pathway.
Major metabolic pathways for several phenothiazines.

Thioridazine (Mellaril) undergoes S-oxidation (2-methylthio group) giving the active sulfoxide metabolite mesoridazine twice as active an antipsychotic thus was synthesized and introduced in the market as sentril. Further S-oxidation will produce sulfones (-SO2-).

Oxidation of alcohols and aldehydes

non-microsomal

Alcoholic metabolites produced by microsomal oxidation will undergo phase Π conjugation or will be oxidized. Primary alcohols will give aldehydes and secondary alcohols will give ketones.
Catalyzed by a non-microsomal enzyme alcohol dehydrogenase in the liver and
other tissues and requires NAD+ or NADP+ as coenzyme.
Reversible i.e. if not immediately oxidized will be reduced to the alcohol again by
alcohol dehydrogenase.
Aldehyde produced from primary alcohol drug or from the deamination aliphatic amines will be further oxidized to the corresponding carboxylic acids catalyzed by aldehyde dehydrogenase enzymes e.g. aldehyde oxidase and xanthine oxidase.
As mentioned before cyclic amines give lactam metabolites.

First oxidized by microsomal oxidation of ring carbon α- to the nitrogen giving carbinolamine intermediate then carbinolamine will be oxidized tocarbonyl moiety by non microsomal alcohol dehydrogenase.

2-Reductive reactions

i-Reduction of aldehydes and ketones:

The aldehyde or keton may be parent drugs or metabolits (oxidative deamination of primary and secondary amines) will undergo reductive metabolic reactions.
Reduction is not a major metabolic pathway for aldehydes since the undergo rapid oxidation to carboxylic acids.
Ketones are not easily oxidized so they undergo reduction to secondary alcohols then are conjugated to glucuronic acid and are excreted in urine.
The reduction is catalyzed by enzymes called aldo-keto reductases (nammely alcohol dehydrogenase) undergo bioreduction of aldehydes and ketones.
In liver and other tissues and require NADPH as cofactor.
The same enzyme undergoes both the oxidation and the reduction reactions e.g. alcohol dehydrogenase.
Bioreduction of aldehydes to alcohols is not common, a known example.

Reduction of ketones will produce asymmetric center i.e. two possible isomers ( formation of one isomer more favorable).
The preferable production of one stereoisomer over the other is called product stereoselectivity.

ii-Reduction of azo and nitro compounds:

Leads to formation of primary amines.
Aromatic nitro compounds are reduced initially to the nitroso and hydroxylamine
intermediates.

Another example is Clonazepam reduction of the aromatic nitro group to amino.

Performed by NADPH-dependant microsomal and nitro reductase enzymes in the liver and bacterial reducrases in intestine.
Clonazepam and nitrazepam are metabolized extensively by reduction of the nitro group to give the 7-amino metabolite in human.

Most famous example is metabolic redcution of azo drugs e.g. sulphamidochyrosidine (prontosil) to give sulfanilamide (liver). This lead to discovery of sulfonamides as antibacterial. (structures before).
Bacterial reductase in the intestine also undergo reduction of nitro compounds ehich are excreted in the bile.
Bacterial reductases also contributeto the reduction of azo drugs (poorly absorbed) e.g. sulfasalazine (in ulcerative colitis) (poorly absorbed) undergoes reductive cleavage of the azo linkage in the colon producing Sulfapyridine and 5-aminosalicylic acid in thus becomes active.

iii-Miscellaneous reductions :
1-N-oxides will givete rtiary amines

Important reaction.
Several tertiary amines are polar, excretable N-oxide.
Extensive reduction reaction → delay elimination of the tertiary amine parent drug

2- Reduction of sulfoxides (limited extent)

More common is the oxidation of sulfoxides to sulfone (i.e. the opposit reaction).
Sulindac (anti-inflammatory) will undergo reduction to give sulfide metabolite (active and is responsible for the overall anti-inflammatory activity of the parent drug.
Sulfone metabolite have little anti-inflammatory activity.

3-Hydrolytic reactions

1-Hydrolysis of esters:

Catalyzed by esterasesin the liver, kidney and intestine.
Hydrolysis is major metabolic pathway for esters (ease of hydrolysis of the ester
linkage).
Hydrolysis of esters will produce alcohol and acid functional groups which will undergo conjugation.
Hydrolysis → pharmacologically active metabolites.

P-chlorophenoxyisobutyric acid is a major metabolite and is responsible for hypolipidimic effect of the drug.
The ease of hydrolysis of esters and the presence of estrases in many tissues and plasma leds to the use of ester derivatives as prodrugs to overcome side effects (bitter taste, poor absorption, and poor solubility irritation at site of injection).
Once inside is biotransformed to the active drugs.
A famous example is the antibiotic chloramphenicol ( bitter taste).
Palmitate ester will mask the taste in pediatric preparations.
Upon oral administration intestinal estrases and lipases will undergo hydrolyis liberating active chloramphenicol.

2-Hydrolysis of amides:

Microsomal amidases and deacylases will produce carboxylic acid + amine metabolites.
Hydrolyis of amides is slowler than ester hydrolysis.

Hydrolysis of procainamide is much slower than procain (ester) (cannot be administered orally).

Another example is the hydrolysis of the amide linkage in.

Another example is indomethacine.

summary of Lecture IV

2008-04-10T12:22:00.000-07:00

PHASE II (CONJUGATION REACTIONS)

Or Synthetic reaction

Capable of converting parent xenobiotics or phase I metabolites into polar and water soluble products.
Conjugated products are relatively excretable, biologically inactive and non toxic.
Some phase II reactions such as methylation and acetylation only terminate the
pharmacological activity.but does not not increase solubility.
Phase II reactions is the truly detoxifying pathway in drug metabolism (few exceptions).

Drug + Conjugating Agent → Conjugating Product

The conjugated agent (e.g. glucuronic acid, sulfate, methyl & acetyl) is activated
to give coenzyme with the appropriate transferase enzyme leads to the attachment to the accepting substrate.
In case of glycine and glutamine → substrate is initially activated.
Glutathione does not require initial formation of an activated coenzyme or substrate.

I- Glucuronic acid conjugation

Most common conjugative pathway in drug metabolism

Readily available supply of D- glucuronic acid in the body (from D- glucose).
A large number of functional groups can combine enzymatically with glucuronic acid e.g. hydroxyl groups (phenols, alcohols, enols, N-hydroxylamines, N- hydroxylamides) carboxyl groups (aryl acids, arylalkyl acids). nitrogen groups (arylamines, alkylamines, amides, sulfonamides, tertiary amines) sulfhydryl groups.
The conjugated products are water soluble and are eliminated very fast.

Involves two steps
a-Synthesis of the activated coenzyme, uridine-5'-diphospho –α-D- glucuronic
acid (UDPGA).
b-Transfer of glucuronyl group from UDPGA to an appropriate substrate.
catalyzed by microsomal enzymes called UDP-glucuronyltransferases.

α-D-Glucose will be transformed to α-D-Glucose-1-phosphate which is then converted to Uridine-5'-diphospho-α-D-glucose (UDPG) then by UDP-Glucoronyl-transferase will produce-B-Glucoronide.

II- Sulfate conjugation

The amount of sulfate available is limited.
Utilized to conjugate endogenous compounds e.g. steroids, catecholamines, and thyroxin.
Phenols and alcohols, aromatic amines, and N- hydroxyl compounds can combine
enzymatically with sulfate.
The sulfate conjugation process involves:
a-Activation of inorganic sulfate into the coenzyme 3-phosphoadenosine -5'-
phosphosulfate (PAPS).
b-Sulfate group by sulfotransferases (in liver, kidney and intestine) is transferred to the substrate.

III-Conjugation with glycine, glutamine and other amino acids

Availability of amino limited acids in the body.
Glycine and glutamine are conjugated to carboxylic acids (aromatic acids and arylalkyl acids).
Glycine and glutamine are not activated to coenzymes. the carboxylic acid substrate is activated.
The conjugation process involves:
a-The carboxylic substrate in presence of ATP and coenzyme A is converted to acyl coenzyme A complex
b- Acyl coenzyme A complex react with glycine or glutamine in presence of N- acyl transferase enzymes is transferred onto glycine or glutamine causing acylation(conjugation).

IV- Glutathione or mercapturic acid conjugation.

Covalent interaction of electrophilic metabolites with cellular nucleophiles causes toxicity.
Glutathione conjugation (nucleophilic sulfhydryl group) is a pathway to detoxifiy chemically reactive electrophilic compounds.
No initial formation of an activated coenzyme or substrate is required.
The inherent reactivity of the nucleophilic GSH towards an electrophilic substrate will sufficient driving force to cause conjugation.

The conjugation process involves:

The substrate conjugates with glutathione (γ- glutamylcysteinylglycine) by enzymes known as glutathione S- transferases gives (glutathione adduct).
Glutathione adduct undergoes sequential enzymatic cleavage of two amino acids (glutamic acid and glycine).
N- Acetylation of the S- substituted cysteine residue will produce mercapturic acid.

V-Acetylation

Important metabolic route for drugs containing primary amino groups (e.g. primary
aromatic amines sulfonamides, hydrazines, hydrazides, and primary aliphatic
amines).
The produc is inactive and non toxic but water solubility is not enhanced.
The conjugation process involves:

Formation of Acetylcoenzyme A conjugating agent (active form).
Transfer of the acetyl group from accetyl coenzyme A to the accepting amino substrate by N- acetyltransferases enzyme.

VI-Methylation

Methylation does notproduce polar or water soluble metabolites except when it produces ammonium derivative.
Methylated products are mostly pharmacologically inactive.
Methylation plays an important role in the biosynthesis of many endogenous
compounds (e.g. epinephrine) in addition to the inactivation of physiologically active biogenic amines (e.g. norepinpherine, serotonin and histamine).
Groups which under go methylation are: catechols, phenols, amines, N-heterocyclic and thiol compounds.
The conjugation process involves:

The methyl group is converted into its active form (S- adenosylmethionine (SAM)).
The activated methyl group is transferred onto accepting substrate by methyltransferases
enzyme.

FACTORS AFFECTING DRUG METABOLISM

A drug is not metabolized by a single metabolic pathway.
A single drug may be metabolized by different phase I and phase II metabolic pathways giving several metabolites from the same parent drug.
Relative amount of metabolites depends on the concentration and the activity of the enzymes metabolizing them.

The following factors may affect the metabolic rate of the drug:

1.Age-related differences:

Newborn suffer from underdevelopment or deficiency of oxidative and conjugative enzymes leading to reduction of metabolic capacity. e.g.oxidative (cytochrome P-450) metabolism of tolbutamide is reduced in newborn leading to increased t ½ (40 hours Vs 8 hours in adults).
Glucuronyltransferase activity reductuin leading to the reduction inchloramphenicol conjugation with glucuronic causing the accumulation of toxic levels of the drug causing gray baby syndrome.
Elderly patients show evidence of inefficientdrug metabolism due to compromised liver functions.

2-Genetic or hereditary Factors:

There are differences in the rate of metabolism observed of some drugs in hum.
Acetylation shows rate differences in human e.g. procainamide undergoes conjugation of amino groups with acetyl group.
Marked difference among individuals in the rate of acetylation of these drugs leads to it's being either slowly or rapidly metabolized (bimodal distribution).
Rapid acetylators (have more acetyl transferase in their liver).
The rate of acetylation affects the therapeutic response and toxicity of these drugs.
Rapid acetylators undergo fast elimination leading to inadequate therapeutic response to the drug.
Slow acetylators are subject to toxicity due to accumulation of the drug.

3-Species differences:

A particular drug may be metabolized differently by different species.
Strain differences in metabolism may also take place e.g. oxidative deamination or aromatic hydroxylationare two metabolic pathways of amphetamine.
In human, guinea pig and rabbit oxidative deamination predominates.
In rats aromatic hydroxylation is the predominates.
There is also some difference due to presence or absence of the particular transferase enzyme e.g. cats lack glucuronyl transferase while pigs lack sulfotransferase enzyme and thus unable to conjugate phenols .

4-Sex differences:

Sex differences is species dependent.
Rabbits and mice do not show sex differences in human there arevery few reports of
sex differences in metabolism e.g. the metabolism of nicotine and aspirin seem to differ between men and women.

5-Enzyme induction:

Exposure to certain drugs leads to markedly increased activity of hepatic microsomal enzymes e.g. cytochrome P450 (enzyme induction), E.g pesticides and polycyclic aromatic hydrocarbons and other environmental pollutants.
Due to the increase in the synthesis of more enzymes in the liver.
Enzyme induction leads to increase in the rate of metabolism of drugs including the the those drugs that induced the enzyme activity in the first place.
Drug induction leads to serious drug interaction if a known enzyme-inducing drug is coadministered with one or two drugs which are known to be largely metabolized by liver enzymes.
This will lead to marked decrease in the bioavailability of the drugs leading to decrease in activity.
Coadministration of warfarine (anticoagulant) ((largely eliminated through liver metabolism)) and the hypnotic phenobarbitone (efficient enzyme enducer) leads to marked decrease in anticoagulant activity of warfarine.
Dosage readjustment of warfarin is very important when it is coadministered with phenobarbital and also when the patient suddenly stops taking the barbiturate.
Poycyclic hydrocarbons e.g. benzo[α] pyrine and environmental pollutants e.g. polychlorinated biphenyls induce certain oxidative pathways and consequently the metabolism of drugs.
Enzyme induction increases toxicity of some drugs by increasing the production of a certain chemically reactive metabolite. E.g oxidation of acetophenone → reactive imidoquinones is catalyzed by phenobarbital-inducible form of cytochrome P450 enzymes in rats.
Phenobarbital pretreatment leads to in vivo hepatotoxicity and covalent binding of phenacetine.

6-Enzyme inhibition:

Certain drugs are capable of inhibiting drug metabolizing enzymes.
Through substrate competition, inhibition of protein synthesis, inactivation of drug-metabolizing enzyme and hepatotoxicity leads to impairment of drug metabolism causing accumulation of the drug in the body.
Phenylbutazone stereoselectively inhibits the metabolism of the more potent (S)(-)- enantiomer of warfarine.This explains the increased hypoprothrombinemia and hemorrhaging in patients taking both warfarine and phenylbutazone.

7-Miscellaneous factors affecting drug metabolism:

Dietary factors e.g. protein/carbohydrate ratio, indoles present in some vegetables e.g. cabbage, vitamins and minerals, starvation and malnutrition will all influence drug metabolism.
Physiologic state of the liver (Hepatic cancer, cirrhosis, hepatitis).
Pregnancy, hormonal disturbances (thyroxin, steroids).
Circadian rhythm may all influence the metabolism of drugs.