Introduction to Elimination / What Is The Elimination / Defination Of Elimination / KCL pharmacy notes / B.pharma Notes / solution pharmacy

 Introduction to Elimination

DRUG METABOLISM

3.1.1. Introduction 

Elimination is the major process for the removal of a drug from the body and the termination of its action. It is defined as the irreversible loss of drug from the body. Elimination occurs by t wo processes, i.e., metabolism and excretion. The body metabolises some drugs chemically. Metabolism may either result in the formation of inactive substances (metabolites) or substances that may resemble to the original drug in terms of therapeutic activi ty or toxicity or substances that may differ from the original drug. 

 Liver is the main site of metabolism for most drugs, and hence, almost every drug passes through the liver. For the conversion of prodrug to active metabolites or for the conversion of active drugs to inactive forms, the drugs should enter the liver to be acted upon by many enzymes.

 A specific group of cytochrome P-450 enzymes involves in liver’s primary mechanism for metabolising drugs. The metabolism rate of many drugs is controlled by the level of these cytochrome P - 450 enzymes. Since the enzymes possess a limited capacity of metabolism, they are burdened, when the levels of drug present in the blood are high. It is difficult for an infant to metabolise certain drugs, because at the t ime of birth, the metabolic enzyme systems are not developed completely. Also, the activity of the enzymes reduces with an increase in the age of an individual. This is the reason why aged individuals and infants are unable to metabolise drugs as efficiently as the younger adults and children. As a result, often smaller doses per pound of body weight are dispensed for infants and aged individuals as compared to young individuals or middle-aged adults. 

The biochemical modification of a drug in the body is t ermed drug metabolism or biotransformation . Once a drug enters the body it undergoes absorption, distribution, metabolism, and finally the metabolites (resultant products formed after metabolism) undergo excretion.

3.1.2. Organs Involved in Drug 

Metabolism Though each biological tissue possesses a certain capability to metabolise drugs, but the principal site of drug metabolism is the smooth endoplasmic reticulum of the liver cell. Due to the given factors, liver functions as the major organ for metabolism: 

 1) Size of the liver is quite large. 

2) Chemicals absorbed by the gut perfuse the liver first. 

3) Most of the drug -metabolising enzymes are present in very high concentrations in the liver as compared to other organs. After being swallowed, a drug is absorbed by the digestive system and the portal vein carries it to the hepatic portal system, where it undergoes maximum metabolism. Thus, the drug is said to exhibit first -pass effect. Epithelial cells of the GIT, lungs, kidneys, and skin are the other possible sites where the drug can undergo metabolism. However, localised side effects are often seen at these sites.

 3.1.3. Enzymes Involved in Drug Metabolism

 Metabolism of drugs involves many important enzymes and pathways. Based on the type of reaction catalysed by them, they can be categorised as:

 1) Enzymes involved in Phase I Metabolism include: 

i) In Oxidation: Cytochrome P -450 monooxygenase system, flavincontaining monooxygenase system, alcohol dehydrogenase, aldehyde dehydrogenase, monoamine oxidase, and co-oxidation by peroxidase. 

ii) In Reduction: NADPH-cytochrome P -450 reductase and reduced (ferrous) cytochrome P -450. It is important to note that a chemical can enter substrate cycle during reduction reactions. In this cycle, a free - radical electron is gained by the chemical which is also quickly lost towards oxygen to form a superoxide anion. 

iii) In Hydrolysis: Esterase, amidase, and epoxide hydrolase. 

2) Enzymes involved in Phase II Metabolism include:

 i) In Methylation: Methyltransferase. 

ii) In Sulphation: Glutathione S-transferases and sulfotransferases. 

iii) In Acetylation: N-acetyltransferases and amino acid N-acyl transferases. 

iv) In Glucuronidation: UDP-glucuronosyltransferases. 

3.1.4. Factors Affecting Drug Metabolism

 The following factors affect the biotransformation of a drug: 

1) Inhibitors: Certain drugs, e.g., cimetidine, omeprazole, and ciprofloxacin, can inhibit enzymes that metabolise a drug. Since the metabolising enzymes are inhibited, metabolism of the administered drug decreases, which in turn leads to an increase in the duration of its action. 

2) Stimulators: Certain drugs like phenobarbitone and rifampic in can increase the activity of enzymes that metabolise a drug. Hence, it proves advantageous when drugs like phenytoin and warfarin are administered, as it increases their metabolism.

3) Age: Young children show poor drug metabolism as metabolic enzyme systems are not developed completely. For example, grey baby syndrome is seen in infants on administration of chloramphenicol as they lack glucuronyl transferase required for the inactivation of chloramphenicol. 

4) Sex: In comparison to males, the females possess lesser ability for drug metabolism. 

5) Species: Some enzymes may be species -specific. For example , rabbits possess atropinase enzyme and hence are able to metabolise atropine (therefore, atropine is non -toxic for rabbits); however, atropinase enzyme is absent in humans and hence, atropine proves toxic for humans.

 6) Genetics: Drug metabolising enzymes show hereditary patterns; deficiencies of either of the enzymes belonging to the enzyme system can be inherited from one generation to the other. For example, an individual in whom Glucose-6-Phosphate Dehydrogenase (G-6-PD) enzyme is genetically deficient shows haemolysis when primaquine is administered to them.

 7) Body Temperature: Temperature of the body is directly proportional to drug metabolism. Drug metabolism h as been found to increase with an increase in body temperature and vice versa

 

BASIC UNDERSTANDING OF METABOLIC PATHWAYS

3.2.1. Introduction 

Drug biotransformation involves enzymatic reactions that are divided into Phase I and Phase II reactions ( table 3.1 ). Phase I reactions include hydrolysis, reduction, and oxidation. These reactions slightly increase the hydrophilicity. Phase II reactions include glucuronidation, sulfonation (or sulfation), acetylation, methylation, conjugation with glutathione, and conjugat ion with amino acids. These reactions increase the hydrophilicity by a greater extent. Phase I reactions may or may not go before the Phase II reactions. For example, heroin forms morphine -3-glucuronide by undergoing hydrolysis (Phase I) and then conjuga tion with glucuronic acid (Phase II). However, morphine forms morphine-3-glucuronide by direct conjugation with glucuronic acid (Phase II). 

3.2.2. Phase I Metabolic Pathways A molecule of drug initially enters phase I metabolism, where it undergoes a sequence of reactions, and at the end of this phase, the molecule shows the following changes: 

1) Forms a reactive site or a functional group, like OH, SH, or NH2. During phase II, they successively conjugate with molecules, such as glucuronic acid, acetyl CoA, etc.

 2) Converts itself into forms that show reduced solubility in lipids as well as in water so that its excretion is facilitated.

3.2.2.1. Hydrolysis Reaction

 Drugs containing carboxylic acid (esterprocaine), amide (procainamide), thioesters (spironolactone), phosphoric acid ester (paraoxon), and aci d anhydride (diisopropylfluoro-phosphate) functional groups undergo hydrolysis. Hydrolysis of carboxylic acid esters, amides, and thioesters is catalysed by carboxylesterases located in various tissues and serum. Hydrolysis of phosphoric acid esters is catalysed by paraoxonase (or organophosphatase), which is a serum enzyme. Hydrolysis of phosphoric acid anhydrides is catalysed by diisopropylflurophosphatase. Carboxylesterases catalyse the trans -esterification of drugs in the presence of alcohol, e.g., conversion of cocaine to ethylcocaine. 

 1) Carboxylic Acid Esters

N-hydroxylation of amides generates reactive intermediates that covalently bind to macromolecules, e.g., paracetamol, and cause toxicity. Prolonge d usage or overdosage of paracetamol causes liver damage. Hepatotoxicity is caused by N - acetyl-p-benzoquinone metabolite, which is inactivated by glutathione conjugation. Prolonged use or overdose of paracetamol causes depletion of glutathione and the toxic metabolite of paracetamol causes liver damage. 

During azo reduction, N=N undergoes sequential reduction and gets cleaved into two primary amines. Four reducing equivalents are required for this reaction; whereas, six reducing equivalents are required for nitro reduction.

 2) Carbonyl Reduction: This reaction is catalysed by carbonyl reductases present in blood and cytosolic fraction of the liver, kidney, brain, and other tissues. Reduction of aldehydes i nto primary alcohols and reduction of ketones into secondary alcohols are examples of carbonyl reduction.  

3.2.2.3. Oxidation Reaction

 Liver cells (or hepatocytes) are the most common site for oxidation of a drug molecule and microsomes form the site of oxidation within the hepatocytes. Mixed Function Oxidase (MFO) is the enzyme system that causes oxidation of drug. 

The following components make up the enzyme system: 

1) Cytochrome Oxidase Enzyme: It is commonly known as CytochromeP-450 (CYP-450) and is chemically a haemoprotein. It is the terminal enzyme of the enzyme system that plays a role in drug oxidation. Oxidation of drug is caused by reduction of CYP -450 (as it exhibits maximum absorption at 450nm), hence, extra electrons are transferred to molecular oxygen, and so oxygen is reduced (reduced form of oxygen is known as activated oxygen). Once the electrons have been transferred, CYP -450 is free to again accept another electron, i.e., it is recovered. 

2) NADPH: A co -enzyme that forms one more significant member of this system of oxidation.

 3) Oxygen. 

4) NADPH Cytochrome Reductase: It is chemically a flavoprotein, and another enzyme involved in the process. 

Steps of Oxidation 

 1) The drug along with oxidised cytochrome P -450 (Fe +++) enzyme forms a binary complex. 

2) The oxidised CYP-450 now receives one electron from the reduced flavoprotein, and becomes reduced P-450 (Fe++) but still remains attached to the drug. 

3) Subsequently, activation of an oxygen molecule is seen. Oxygen attaches to the binary complex [obtained in step (1)], and forms a drug-reduced P-450 (Fe++)- activated oxygen complex, generally, abbreviated as drug-Fe++ -oxygen complex. 

4) Lastly, the oxidised drug splits and CYP -450 again converts into oxidised (Fe+++) form; thus the enzyme is recovered.