General Mechanisms of Toxin Induced Cell General Mechanisms of Toxin Induced Cell DamageDamage

Toxic metabolites can form covalent bonds with Toxic metabolites can form covalent bonds with target molecules or alter the target molecule by target molecules or alter the target molecule by non-covalent interactions.non-covalent interactions.

Covalent interactions:Covalent interactions:Targets for the covalent interactions include DNA, Targets for the covalent interactions include DNA, Protein, lipids and carbohydratesProtein, lipids and carbohydrates

Non-Covalent interactions includesNon-Covalent interactions includes-Lipid peroxidation-Lipid peroxidation-Generation of toxic oxygen radicals -Generation of toxic oxygen radicals ( superoxide anion, O( superoxide anion, O--

2, 2, singlet oxygen 1Osinglet oxygen 1O22) ) followed by enzymetic conversion to Hfollowed by enzymetic conversion to H22OO2 2 or or

hydroperoxy, HOOhydroperoxy, HOO-- and OH and OH- - radicals. radicals.-Reactions causing depletion of -Reactions causing depletion of Glutathione( one of the four enzymes having Glutathione( one of the four enzymes having strong tendency to neutralize free radicals) strong tendency to neutralize free radicals) result in oxidative stress.result in oxidative stress.-Modification of sulphydryl (-SH) group- -Modification of sulphydryl (-SH) group- causes the inactivation of the key enzymes.causes the inactivation of the key enzymes.

are examples of heterocyclic compoundsare examples of heterocyclic compounds

three-membered rings that contain oxygenthree-membered rings that contain oxygen

ethylene oxideethylene oxide propylene oxidepropylene oxide

HH22CC CHCH22

OO

HH22CC CHCHCHCH33

OO

EpoxidesEpoxides

peroxy acidperoxy acid

CC CC ++

OO

RCORCOOOHH

CCCC

OO

++

OO

RCOHRCOH

Epoxidation of AlkenesEpoxidation of Alkenes

+ CH+ CH33COCOOOHH

OO

(52%)(52%)

+ CH+ CH33COHCOH

OO

ExampleExample OO

CC CC ++

OO

RCORCOOOHH

CCCC

OO

++

OO

RCOHRCOH

syn additionsyn addition

Stereochemistry of EpoxidationStereochemistry of Epoxidation

ethyleneethylene HH22C=CHC=CH22 11

propenepropene CHCH33CH=CHCH=CH22 2222

2-methylpropene 2-methylpropene (CH(CH33))22C=CHC=CH22 484484

2-methyl-2-butene 2-methyl-2-butene (CH(CH33))22C=CHCHC=CHCH33 65266526

More highly substituted double bonds react faster.More highly substituted double bonds react faster.Alkyl groups on the double bond make itAlkyl groups on the double bond make itmore “electron rich.”more “electron rich.”

Relative Rates of EpoxidationRelative Rates of Epoxidation

Formation of highly reactive metabolites(from Formation of highly reactive metabolites(from relatively inert chemical compounds) which relatively inert chemical compounds) which interact with the tissues to precipitate one or interact with the tissues to precipitate one or more of the several form of toxicities such as more of the several form of toxicities such as carcinogenesis and teratogenesis is called as carcinogenesis and teratogenesis is called as bioactivation or toxicological activation.” e.g. bioactivation or toxicological activation.” e.g. Benzopyrene : aromatic epoxidation : lung Benzopyrene : aromatic epoxidation : lung cancer cancer Aflatoxin B1 : olefin epoxidation : Hepatic Aflatoxin B1 : olefin epoxidation : Hepatic cancer. cancer. Thalidomide : hydrolytic cleavage of lactam : Thalidomide : hydrolytic cleavage of lactam : Teratogenesis Teratogenesis

1974 – Montgomery – Catalysis of 1974 – Montgomery – Catalysis of Peroxymonosulfate Reactions by Ketones – Peroxymonosulfate Reactions by Ketones – proposed formation of a highly reactive proposed formation of a highly reactive intermediate – dioxirane. (intermediate – dioxirane. (J. Am. Chem. Soc.J. Am. Chem. Soc., , 19741974, 7820)., 7820).1980 – Curci – Achieved epoxidation of 1980 – Curci – Achieved epoxidation of unreactive alkenes using KHSOunreactive alkenes using KHSO5 5 and acetone, and acetone,

via a dioxirane intermediate. (via a dioxirane intermediate. (J. Org. Chem.J. Org. Chem., , 19801980, 4758)., 4758).Reactive intermediates include epoxides and Reactive intermediates include epoxides and free radical species (unpaired electrons) that free radical species (unpaired electrons) that are short-lived and hence highly reactiveare short-lived and hence highly reactive

Phase I reactionsPhase I reactions

OxidationOxidationHydroxylation (addition of -OH Hydroxylation (addition of -OH group)group)N- and O- Dealkylation (removal of N- and O- Dealkylation (removal of -CH side chains)-CH side chains)Deamination (removal of -NH side Deamination (removal of -NH side chains)chains)Epoxidation (formation of epoxides)Epoxidation (formation of epoxides)

Oxygen addition (sulfoxidation, N-Oxygen addition (sulfoxidation, N-oxidation)oxidation)Hydrogen removalHydrogen removal

These are the types of reactions These are the types of reactions performed by the Cytochrome P450 performed by the Cytochrome P450

systemsystem• Aromatic hydroxylationAromatic hydroxylation Phenobarbital, Phenobarbital,

amphetamineamphetamine• Aliphatic hydroxylationAliphatic hydroxylation Ibuprofen, cyclosponineIbuprofen, cyclosponine• EpoxidationEpoxidation Benzo [a] pyreneBenzo [a] pyrene• N-DealkylationN-Dealkylation DiazepamDiazepam• O- DealkylationO- Dealkylation CodeineCodeine• S- DealkylationS- Dealkylation 6-Methylthiopurine6-Methylthiopurine• Alcohol oxidationAlcohol oxidation EthanolEthanol

Oxidative DeaminationOxidative Deamination Diazepam, amphetamineDiazepam, amphetamine

N-OxidationN-Oxidation ChlorpheniramineChlorpheniramine

S-OxidationS-Oxidation Chlorpromazine,cimetidineChlorpromazine,cimetidine

Phosphothionate oxidationPhosphothionate oxidation ParathionParathion

DehalogenationDehalogenation HalothaneHalothane

Metabolism of Metabolism of benzo(a)pyrene to 9,10 benzo(a)pyrene to 9,10 epoxide:epoxide:Potent mutagen that binds Potent mutagen that binds DNADNA

N-OxidationN-OxidationCircumstantial evidence strongly suggests that most Circumstantial evidence strongly suggests that most idiosyncratic drug reactions are due to reactive idiosyncratic drug reactions are due to reactive metabolites of drugs rather than due to the drugs metabolites of drugs rather than due to the drugs themselves. themselves. Many of the drugs that are associated Many of the drugs that are associated with idiosyncratic drug reactions contain nitrogen. with idiosyncratic drug reactions contain nitrogen. There are many possible reasons for this There are many possible reasons for this association. One is simply that many drugs, association. One is simply that many drugs, especially CNS active drugs, contain nitrogen. In especially CNS active drugs, contain nitrogen. In addition, addition, nitrogen is relatively easy to oxidize nitrogen is relatively easy to oxidize because of its lone pair of electrons and many because of its lone pair of electrons and many nitrogen-containing compounds readily undergo nitrogen-containing compounds readily undergo redox cycling, which can generate reactive oxygen redox cycling, which can generate reactive oxygen species. species.

There are several nitrogen-containing function groups There are several nitrogen-containing function groups that are especially associated with adverse reactions. that are especially associated with adverse reactions. These include aromatic amines, nitro compounds (nitro These include aromatic amines, nitro compounds (nitro compounds are reduced to the same reactive compounds are reduced to the same reactive intermediates as are formed by oxidation of the intermediates as are formed by oxidation of the corresponding aromatic amine), hydrazines and corresponding aromatic amine), hydrazines and compounds that can be oxidized to iminoquinone and compounds that can be oxidized to iminoquinone and related compounds. A greater attention to the issue of related compounds. A greater attention to the issue of reactive metabolites during drug development would reactive metabolites during drug development would likely lead to safer drugs; however, not all drugs that likely lead to safer drugs; however, not all drugs that form reactive metabolites are associated with a high form reactive metabolites are associated with a high incidence of idiosyncratic drug reactions. incidence of idiosyncratic drug reactions.

In addition to the presence of such a group, In addition to the presence of such a group, other factors, such as dose and electron density other factors, such as dose and electron density of the compound, appear to play a role in of the compound, appear to play a role in whether the drug containing such functional whether the drug containing such functional groups will be associated with a relatively high groups will be associated with a relatively high incidence of idiosyncratic drug reactions.incidence of idiosyncratic drug reactions.

reab

sorp

tion

reabso

rptio

n

Ready for eliminationReady for elimination