BIOTRANSFORMATION Chemical modification of drugs or foreign
compounds (xenobiotics) in the body. The apparent function of drug
or xenobiotics metabolism is their transformation into water-
soluble derivative which can be easily eliminated via renal
route.
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Implications For Drug Metabolism 1.Termination of drug action
2.Activation of prodrug 3.Bioactivation and toxication
4.Carcinogenesis Objective of studying drug metabolism - medicinal
chemists aware of the chemical processes involved in the
biotransformation of drugs - so design of new more safe drugs.
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BIOTRANSFORMATION CAN LEAD TO THE FOLLOWING Inactivation Eg:
drugs like paracetomol, ibubrufen, chloromphenical etc and their
metabolites are rendered inactive or less active. Active metabolite
from active drug Activation of inactive drug Certain drugs are
inactive and need activation in the body. Prodrug gives more
bioavailability and less toxic effects. ACTIVE DRUGACTIVE
METABOLITE allopurinolalloxanthine DigitoxinDigoxin
MorphineMorphine 6 glucoronide Cholrol hydratetrichloroethanol
ProdrugActive metabolite LevodopaDopamine SulindacSulfide
metabolite PredinosonePrednisolone
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Effect of drug metabolism Active drugInactive Drug Active drug
Inactive prodrug Inactive metabolite Metabolic activation Lipid
Solubility R-H R-OH Chemical hook
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Two categories of xenobiotics are not or are hardly subject to
metabolic transformations : 1- Hydrophilic compounds (e.g.
saccharine, strong acids or bases) 2- Highly lipophilic
polyhalogenated xenobiotics such as some insecticides e.g.
DDT.
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Lethal synthesis: Lethal synthesis: a classic example of lethal
synthesis is provided by the metabolic conversion of the nontoxic
insecticide parathion into its oxygenated isostere paraoxon, a
potent acetylcholinesterase inhibitor
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Major Sites of Metabolism Major Sites of Metabolism Drug
metabolism can occur in every tissue (e.g. gut, lung, kidney).. The
LIVER is the chief organ for drug metabolism because: The blood
flow through the liver is high Hepatocytes contain numerous
metabolic enzymes High liver Mediumlung, kidney, intestine Lowskin,
testes, placenta, adrenals Very lownervous system
PHASE I Oxidation Involves addition of oxygen or removal of
hydrogen. Reactions mostly occur in liver by a group of
mono-oxygenases, cyt P450 reductase oxygen. Various oxidation
reactions are Hydroxylation Oxygenation at C,N or S atoms N or O
dealkylation Oxidative deamination Eg: barbiturates,
phenothiazines, steroids, paracetomol. 60% of drugs are metabolized
primarily by CYPs.
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The great majority of these oxidations are carried out by the
haemoprotein cytochrome P- 450 which is embedded within the
phospholipid environment of the microsomes derived from the
endoplasmic reticulum of living cells. 1. Oxidation reactions
Cytochrome P 450 (CYPs) Cytochrome P 450 is a family of enzymes
located in the endoplasmic reticulum of liver. When liver is
homogenized and biochemically fractionated these enzymes are found
in the microsomal fraction (small closed ER membrane fragments).
Thus these enzymes are called microsomal enzymes.
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Cytochrome P 450 enzymes perform many of the most important
biotransformation reactions. These enzymes oxidize a wide variety
of compounds foreign to the body. There are at least 18 different
forms of cytochrome P 450 identified in humans, produced by
different genes. Carbon monoxide binds to the reduced Fe(II) heme
and absorbs at 450 nm (origin of enzyme family name). 60% of drugs
are metabolized primarily by CYPs.
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Important cytochrome P 450 enzymes: 1. CYP3A4. 2. CYP1A2. 3.
CYP2C. 3. CYP2D6. Individual enzymes differ in their substrate
specificity and regulatory properties.
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The reaction sequence illustrated by CyP Rrdox Cycle :
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2. Reduction reactions Reduction Involves removal of oxygen or
addition of hydrogen. Reactions mostly occur in liver by a group of
cyt P450 reductase Eg: Alcohols, Warfarin, Chloramphenicol etc. It
is a major route of metabolism for aromatic nitro and azo compound
as well as for a wide variety of aliphatic and aromatic N-oxides
which are reduced to tertiary amines.
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Hydrolysis Cleavage of drug by addition of water. Ester + water
acid + alcohol in presence of enz esterase. Occurs in liver,
intestine, plasma. Similarly, amides and polypeptides are
hydrolysed by amidases and peptidase enz. Eg: Aspirin, procaine,
oxytocin etc.
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Cyclization Involves formation of ring structure from straight
compound. Eg: Proguanil De- Cyclization Involves formation of open
structure from ring compound. Eg: barbiturates, phenytoin etc. Is a
minor pathway.
Phase II or Conjugation Reactions Conjugation reactions link an
endogenous moiety (an endocon) to the original drug (if polar
functions are already present) or to the Phase I metabolite. They
are catalyzed by enzymes known as transeferases. They involve a
cofactor which binds to the enzyme in the close proximity of the
substrate and carries the endogenous molecule or moiety to be
transferred. Forms highly ionized organic acid which is excreted in
urine or bile. Generally conjugation reactions have high energy
requirements.
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Glucoronide conjugation Most important reaction carried out by
UDP glucoronsyl transferases (UGT). Drugs with hydroxyl and
carboxylic acid are easily conjugated with glucoronic acid.
Glucoronidation increases the mol. Wt of the drug and thus favors
excretion in bile. Drug excreted in gut by bile is hydrolyzed
bacteria and is reabsorped back and undergo same process, so this
enterohepatic circulation prolongs the action of drugs. Eg:
Aspirin, paracetamol, morphine etc.
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Beside xenobiotics, a number of endogenous substrates, notably
bilirubin and steroids are eliminated as glucuronide conjugates. In
neonates and children, glucuronidation processes often are not
fully developed. In such subjects, drugs and endogenous compounds
(e.g. bilirubin) that are normally metabolized by glucuronidation
may accumulate and cause serious toxicity. For example, neonatal
hyperbilirubinemia and gray baby syndrome, result from accumulation
of toxic levels of the free chloramphenicol.
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Acetylation Drugs having amino or hydrazine are conjugated by
acetyl co-enzyme. Multiple genes control the N-Acetyl transferases
and rate of acetylation shows genetic polymorphism Eg:
Sulphonamides, hydralazine, clonazepam etc. Methylation Methionine
and cysteine are methyl donors. Amines and phenols are methylated.
Eg: Adrenaline, histamine, nicotinic acid etc. Suphate conjugation
Phenolic compounds and steroids are sulphated by sulfotransferases.
Eg: Cholramphenicol, Adrenal, Methyldopa etc.
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Glycine Conjugation Salicylates and other drugs with carboxylic
acid are conjugated with glycine. Glutathione Conjugation is a
minor pathway. It also serves to inactivate highly reactive quinone
or epoxide intermediate. Presence of large amount of adducts
results in tissue damage. E.g. Paracetomol Ribonucleoside /
nucleotide synthesis is a important pathway for activation of drugs
in chemotherapy. Mainly purine and pyramidine drugs are used.
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Conjugation with glutathione (GSH) Glutathione (GSH,
-glutamylcysteinylglycine) is a thiol-containing tripeptide. GSH
conjugation is an important pathway by which chemical reactive
electrophilic compounds are detoxified. Many serious drug
toxicities may be explainable also in terms of covalent interaction
of metabolically generated electrophilic intermediates with
cellular nucleophiles. GSH protects vital cellular constituents
against chemically reactive species by virtue of its nucleophilic
sulfhydryl (SH) group.
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Unlike other conjugative phase II reactions, GSH conjugation
does not require the initial formation of an activated enzyme or
substrate. Two general mechanisms are known for the reaction of
compounds with GSH: 1. Nucleophilic displacement at an
electron-deficient carbon or heteroatom. 2- Nucleophilic addition
to an electron-double bond.
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Excretion Drugs and its metabolites are excreted in Urine
Faeces E.g. erythromycin, ampicillin, rifampin etc. Exhaled air
e.g. anesthetics, alcohol etc. Saliva & sweat e.g. lithium,
rifampin etc Milk Renal excretion Renal excretion is responsible
for excreting almost all drugs. Net renal excretion (glomerular
filtration + tubular secretion) (or) the amount = - tubular
reabsorption of drug in urine
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Glomerular filtration Glomerular capillaries have pores larger
and thus non- protein bound drug is filtered in the glomerulus,
thus gfr filtration drug depends on plasma binding protein and
renal blood flow (irrespective of lipid soluble and lipid insoluble
drugs). Glomerular filtration rate (g f r) ~ 120ml/min and declines
progressively after the age of 50 and is low in case of renal
failure. Tubular reabsorption Occurs by passive diffusion Depends
on lipid solubility and ionization of drug at urinary pH. Non -
lipid drugs are completely excreted out The rate of drug =
glomerular filtration rate Change in pH of urine is also used in
eliminating drug during drug poisoning
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Tubular secretion This is active transfer of organic acids and
bases by two separate class of transporters OCT &OAT. In
addition, efflux transporter P-GP &MRP2 are located in luminal
membrane of proximal tubules. Active transport across tubules
reduces concentration of its free form in the tubular vessels and
promotes dissociation of protein bound drug which again is
secreted. Renal excretion Renal excretion is responsible for
excreting almost all drugs. Net renal excretion (glomerular
filtration + tubular secretion) (or) the amount = tubular
reabsorption of drug in urine
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Kinetics of Elimination Bioavailability (F), Vol. of
distribution (V) and clearance (CL). Clearance = Rate of
elimination / conc. of drug in plasma. Drug is eliminated by zero
and first order reaction. Half - life The half -life of a drug is
the time taken for its plasma concentration to be reduced to half
its original value. T1/2 = 0.693 * vd/ CL Where vd vol. of
distribution CL clearance
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Factors affecting half life As patients age the skeletal muscle
mass decreases which could decrease the vol. of distribution. In
obese person, due to increased adipose tissue higher dose of drug
to be given to reach the target organ. Some of drug get into the
pathologic fluid space like ascites, pleural membrane causing long
term toxicity. Factors affecting half lifeMost common effect on
half-life Effects on volume of distribution Aging decreased
ObesityIncreased Pathologic fluidincreased Effects on clearance Cyt
P 450 induction (increased metabolism) decreased Cyt P 450
inhbition( decreased metabolism) Increased Cardiac failure
(decreased clearance)Increased Hepatic failure (decreased
clearance)Increased Renal failure (decreased
clearance)Increased
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ENZYME INDUCTION Certain drug substrates of CYP, upon repeated
administration, induce or elevate the amounts of specific forms of
the enzyme. Induction results in accelerated metabolism of the drug
inducer and any other drugs metabolized by the induced enzyme.
Classic Enzyme Inducers Chronic ethanol Phenobarbital Tobacco
(benzo[a]pyrene) PCBs (Poly chlorinated biphenyls), dioxin
(environmental) Glucocorticoids
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ENZYME INHIBITION Some drugs are capable of inhibiting
metabolizing enzymes. This is usually only important if a patient
on a stable drug regimen starts taking a second drug metabolized by
the same enzyme. Examples: cimetidine, ethanol
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Drugs that Inhibit Drug Metabolism by Forming Complexes with
CYPs
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Non-nitrogenous Substances that Effect Drug Metabolism by
Forming Complexes with CYPs Grapefruit juice - CYP 3A4 inhibitor;
highly variable effects; unknown constituents Isosafrole, safrole -
CYP1A1, CYP1A2 inhibitor; found in root pear, perfume Piperonyl
butoxide & alcohol -CYP1A1, CYP1A2 inducer; insecticide
constituent
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Effect of Grapefruit Juice on Felodipine Plasma Concentration
5mg tablet with juice without
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STATE OF HEALTH Liver disease Kidney disease Endocrine diseases
Reduced blood flow
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Optimising Pharmacokinetic Properties
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Designing Prodrugs and Bioprecursors Prodrug is any compound
that undergoes biotransformation prior to exhibiting its
pharmacological effects. Prodrugs can be divided into two classes:
The Carrier-prodrugs : Result from a temporary linkage of the
active molecule with a transport moiety that is frequently of
lipophilic nature.
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Example: Aspirin for salicylic acid Prodrugs to mask toxicity
and side effects Mask groups responsible for toxicity/side
effectsMask groups responsible for toxicity/side effects Used when
groups are important for activityUsed when groups are important for
activity Salicylic acid Analgesic, but causes stomach ulcers due to
phenol group Aspirin Phenol masked by ester Hydrolysed in body
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Drug Barrier to delivery of drug Drug Barrier to delivery of
drug Pro-moiety Drug Biotransformation Efficacy Drug + + Non-toxic
& rapidly excreted
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Practical applications of carrier-prodrug design To 1.Improve
of the lipophilicity 2.Enhancement of water solubility 3. mask
toxicity and side effects 4.Increase the duration of
Pharmacological effects 5.Mask polar groups 6. target drugs
7.Improvement of patient acceptance 8.Increased
site-specificity
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Prodrugs to increase water solubility Often used for i.v.
drugsOften used for i.v. drugs Allows higher concentration and
smaller dose volumeAllows higher concentration and smaller dose
volume May decrease pain at site of injectionMay decrease pain at
site of injection Example: Succinate ester of chloramphenicol
(antibiotic) Succinate ester Esterase Chloramphenicol
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Prodrugs to increase water solubility Example: Phosphate ester
of clindamycin (antibacterial) Less painful on injection
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Prodrugs to increase water solubility Example: Lysine ester of
oestrone Lysine ester of oestrone is better absorbed orally than
oestroneLysine ester of oestrone is better absorbed orally than
oestrone Increased water solubility prevents formation of fat
globules in gutIncreased water solubility prevents formation of fat
globules in gut Better interaction with the gut wallBetter
interaction with the gut wall Hydrolysis in blood releases oestrone
and a non toxic amino acidHydrolysis in blood releases oestrone and
a non toxic amino acid
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Increase the duration of Pharmacological effects Add
hydrophobic groups
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Example: Hydrophobic esters of fluphenazine (antipsychotic)
Hydrophobic esters of fluphenazine (antipsychotic) Given by
intramuscular injectionGiven by intramuscular injection
Concentrated in fatty tissueConcentrated in fatty tissue Slowly
released into the blood supplySlowly released into the blood supply
Rapidly hydrolysed in the blood supplyRapidly hydrolysed in the
blood supply
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Example: Azathioprine for 6-mercaptopurine 6-Mercaptopurine
(suppresses immune response) Short lifetime - eliminated too
quickly Azathioprine Slow conversion to 6-mercaptopurine Longer
lifetime Mask polar groups Mask polar groups Reduces rate of
excretionReduces rate of excretion
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Improvement of patient acceptance Used to reduce solubility of
foul tasting orally active drugs Less soluble on tongue Less
revolting taste Example: Palmitate ester of chloramphenicol
(antibiotic) Palmitate ester Esterase Chloramphenicol
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Prodrugs used to target drugs Example:Hexamine Stable and
inactive at pH>5Stable and inactive at pH>5 Stable at blood
pHStable at blood pH Used for urinary infections where pH