- The onset of pharmacological response depends upon 2 pharmacokinetic processes: Absorption & Distribution.
- The duration & intensity of action depends upon the rate of drug removal from the body or rate of elimination & tissue redistribution of the drug.
- Biotransformation (metabolism) of drug is defined as the conversion from one chemical form to another.
- The chemical changes are usually affected enzymatically in the body & thus this definition excludes chemical instability. for example:
Xenobiotics: All chemical substances that are not nutrients for the body & enter the body through ingestion, inhalation or absorption are called xenobiotics.
- Conversion of penicillin to penicilloic acid (Bacterial/mammalian enzymes) is metabolism.
- Conversion of penicillin to penicilleinic acid by stomach acid is chemical instability.
Biotransformation often results in:
a) Pharmacological inactivation of the drug:
|Inactivation of drug with biotransformation|
b) No change in pharmacological activity:
|No change in activity of the drugs after metabolism|
c) Toxicological activation:
|Toxic metabolites of metabolism|
d) Pharmacological activation:
|Activation of drugs after metabolism|
i) Iproniazid (antidepressant) -------> Isoniazid (Anti-Tb)
ii) Diazepam (Tranquilizer) --------> Oxazepam (Anticonvulsant).
Drug metabolizing organs: Liver is the major site for the drug metabolism. Other sites for the drug metabolism in the decreasing order includes: Lungs > Kidney > Intestine > Placenta > Skin.
Drug metabolizing enzymes:
a) Microsomal enzymes: Catalyzes a majority of drug biotransformation reactions. They are located in the microsomes of rough endoplasmic reticulum (RER). Some important characteristics of these enzymes include:
- Intact nature of lipoidal membrane bound enzymes of the microsomes is essential for the activity.
- No. of lipid soluble substances can interact wih these enzymes.
- Lipid soluble substance is biotransformed to water soluble substances.
RT Williams divided the pathways of drug metabolism reactions into two general categories:
a) Phase I reactions: (Functionalization/ Asynthetic reactions):
- It includes oxidative, reductive & hydrolytic reactions.
- A polar function group is either introduced or unmasked if already present on the otherwise lipid soluble substrate (OH, COOH, NH2 & SH).
- The resulting product of phase I reaction is susceptible to phase II reactions.
Also known as conjugation or true detoxification reactions.
Various reactions occurring in metabolism:
a) Phase I reactions:
1) Oxidative reactions:
- Oxidation of aromatic carbon atom.
- Oxidation of olefins (C=C).
- Oxidation of benzylic, allylic carbon atoms & carbon atoms alpha to carbonyl & imines.
- Oxidation of aliphatic carbon atoms.
- Oxidation of alicyclic carbon atoms.
- Oxidation of carbon-heteroatom
- Carbon Nitrogen system
- Oxidative deamination
- N-Oxide formation
- Carbon-sulphur system
- Oxidation of OH, C=O & COOH groups.
- Miscellaneous oxidative reactions
- Reduction of carbonyl functions (Aldehydes/ketones).
- Reduction of alcohols & C=C bonds.
- Reduction of N compounds (nitro, azo & N-oxide)
- Miscellaneous reductive reactions.
- Hydrolysis of esters & amides.
- Hydrolysis of amides.
- Hydrolytic cleavage of nonaromatic heterocyclics.
- Hydrolytic dehalogenation.
- Miscellaneous hydrolytic reactions.
- Conjugation with glucouronic acid.
- Conjugation with sulfate moieties.
- Conjugation with alpha-amino acids.
- Conjugation with glutathione & mercaptopuric acid formation.
- Accetylation reaction
- Methylation reaction
- Miscellaneous reaction
- Oxidative reactions are the most important and most common metabolic reactions.
- Almost all drugs in Phase I biotransformation undergo oxidation at some stage or the other.
- Oxidative reactions increase hydrophilicity of xenobiotiocs by introducing polar functinal groups (OH).
- Oxidation of xenobiotics is non sppecifically catalyzed by a number of enzymes located in the microsomes. Such enzymes require both molecular oxygen (O2) & the reducing agent NADPH to effect reactions. They are therefore referred as mixed function oxidases. The overall stoichometry of the reaction is:
- Since only one oxygen atom from the molecular oxygen is incorporated in the product formed hence mixed function oxidases are also known as mono-oxygenases.
- The multienzyme function oxidase system located in the endoplasmic reticulum of hepatic cells is composed of an electron transfer system chain consisting of three components:
Mg2+ ions are required for maximum action
- A heme protein known as Cyt. P450, which is actually a family of enzymes.
- A second enzyme, the flavoprotein known as Cyt. P450 reductase which is NADPH dependent and function as an electron carrier, catalyzing the reduction of cyt. P450 to the ferrous form.
- Heat stable lipid component known as phosphatidyl choline (Facilitate electron transfer from NADPH).
Different types of oxidation reaction reactions occurring in Phase I biotransformation include:
- Oxidation of aromatic carbon atoms:
Oxidation of aromatic carbon atoms
- Oxidation of olefins:
Oxidation of olefins
- Oxidation of benzylic carbon atoms:
|Oxidation of benzylic carbon atoms:|
- Oxidation of Diazepam:
|Oxidation Oo Diazepam|
- Oxidation of Aliphatic carbon atoms
|Oxidation of Valproic Acid|
- Oxidation of Alicyclic Carbon Atoms
|Oxidation of Minoxidil|
- Oxidation of Carbon-Heteroatom system:
|Metabolism of Imipramine|
|Metabolism of Trimethoprim|
- Reductive reactions also play an important role in generating polar groups in the drugs which can further undergo Phase II reactions and further elimination of the drug from the body.
- Reduction of Carbonyl and Nitrogen-containing drugs undergo bioreduction reaction in Phase I of the drug metabolism.
- Following are the examples of bioreduction reactions of some important drugs:
|BIOREDUCTION IN METABOLISM OF DRUGS|
- Hydrolytic reactions involve the cleavage of larger molecules into smaller ones.
- These hydrolytic biotransformation phase I reactions are not confined to the liver only instead, it occurs in all major organs such as the intestine, lungs, kidney, etc.
- All the endogenous substances are also metabolized by these hydrolytic enzymes.
- Examples of hydrolytic phase I metabolism reactions are:
|HYDROLYTIC PHASE I METABOLISM REACTIONS|
- Phase II reactions of the drug metabolism are considered as a true detoxification pathway of the biotransformation as large molecular moieties combine with the drug or its metabolite to make it more polar & non-toxic.
- High molecular weight conjugates ( More than 350 Da) are excreted through bile whereas low molecular weight conjugates excrete through urine ( less than 250 da).
- Glutathione conjugates always get excreted through bile.
- Phase II metabolism reactions are always capacity limited.
- The table below summarises the Phase II metabolism reactions:
|Name of Conjugation reaction||Conjugating agent||Enzyme||Intermediate used|
|Glucouronidation||Glucouronic acid||UDP-Glucouronyl transferase||UDPGA|
|Amino acid Conjugation||Glycine||Avyl transferase||Acyl CoA|
|Glutathione||Glutathione||Glutathione S transferase||----|
|Methylation||L-Methionine||Methyl Transferase||S-adenosyl methionine|