General Mechanisms of Toxin Induced Cell Damage Toxic metabolites can form covalent bonds with target molecules or alter the target molecule by non-covalent interactions. Covalent interactions: Targets for the covalent interactions include DNA, Protein, lipids and carbohydrates

Non-Covalent interactions includes -Lipid peroxidation -Generation of toxic oxygen radicals ( superoxide anion, O - 2, singlet oxygen 1O 2 ) followed by enzymetic conversion to H 2 O 2 or hydroperoxy, HOO - and OH - radicals. -Reactions causing depletion of Glutathione( one of the four enzymes having strong tendency to neutralize free radicals) result in oxidative stress. -Modification of sulphydryl (-SH) group- causes the inactivation of the key enzymes.

are examples of heterocyclic compounds three-membered rings that contain oxygen ethylene oxidepropylene oxide H2CH2CH2CH2C CH 2 O H2CH2CH2CH2C CHCH 3 O Epoxides

peroxy acid C C + O RCOOH C C O + O RCOH Epoxidation of Alkenes

+ CH 3 COOH O(52%) + CH 3 COH OExample O

C C + O RCOOH C C O + O RCOH syn addition Stereochemistry of Epoxidation

ethyleneH 2 C=CH 2 1 propeneCH 3 CH=CH methylpropene (CH 3 ) 2 C=CH methyl-2-butene (CH 3 ) 2 C=CHCH More highly substituted double bonds react faster. Alkyl groups on the double bond make it more “electron rich.” Relative Rates of Epoxidation

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

1974 – Montgomery – Catalysis of Peroxymonosulfate Reactions by Ketones – proposed formation of a highly reactive intermediate – dioxirane. (J. Am. Chem. Soc., 1974, 7820) – Curci – Achieved epoxidation of unreactive alkenes using KHSO 5 and acetone, via a dioxirane intermediate. (J. Org. Chem., 1980, 4758). Reactive intermediates include epoxides and free radical species (unpaired electrons) that are short-lived and hence highly reactive

Phase I reactions  Oxidation Hydroxylation (addition of -OH group) Hydroxylation (addition of -OH group) N- and O- Dealkylation (removal of - CH side chains) N- and O- Dealkylation (removal of - CH side chains) Deamination (removal of -NH side chains) Deamination (removal of -NH side chains) Epoxidation (formation of epoxides) Epoxidation (formation of epoxides) Oxygen addition (sulfoxidation, N- oxidation) Oxygen addition (sulfoxidation, N- oxidation) Hydrogen removal Hydrogen removal

These are the types of reactions performed by the Cytochrome P450 system Aromatic hydroxylationPhenobarbital, amphetamineAromatic hydroxylationPhenobarbital, amphetamine Aliphatic hydroxylationIbuprofen, cyclosponineAliphatic hydroxylationIbuprofen, cyclosponine EpoxidationBenzo [a] pyreneEpoxidationBenzo [a] pyrene N-DealkylationDiazepamN-DealkylationDiazepam O- DealkylationCodeineO- DealkylationCodeine S- Dealkylation6-MethylthiopurineS- Dealkylation6-Methylthiopurine Alcohol oxidationEthanolAlcohol oxidationEthanol

Oxidative DeaminationDiazepam, amphetamine N-OxidationChlorpheniramine S-OxidationChlorpromazine,cimetidine Phosphothionate oxidationParathion DehalogenationHalothane

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

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

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

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

reabsorption Ready for elimination