An Introduction To Action And Uses Of Medicines Biology Essay

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AGONISTS: An agonist is a drug that binds to a receptor of a cell and triggers a response by the cell. An agonist often mimics the action of a naturally occurring substance

An agonist produces an action. An antagonist blocks an action of an agonist.

E.g. Sertraline increased serotonergic effects when 5HT1 agonists given with duloxetine,

ANTAGONIST: If a drug is an antagonist it blocks the action of a natural body chemical. It inhibits a response, antagonists have the ability to bind to the receptor but do not initiate a change in cellular function, and Antagonists are also referred to as blockers.

E.g. H2- antagonists reduce absorption of itraconazole and ketaconazole.

P1 (b) Write a brief account of the routes available for drug delivery. Give drug examples for each route.

Oral e.g.: tablets (paracetamol), capsules (tramadol), liquid (paracetamol syrup.)

Sublingual e.g. Glycerine trinitrate tablets

Buccal e.g. buccastem (prochlorperazine)

Rectal e.g. glycerol suppositories

Intravenous e.g. injection (ampicillin, tazocin)

Intramuscular e.g. injection (cefalexin,cefotaxime)

Subcutaneous e.g. insulin,actrapid

Topical e.g. creams diprobase

Eye e.g. celluvisc eye drop

Ear e.g. otex,waxsol

Inhalation e.g. sevoflurane, desflurane

Nasal e.g. rhinolast( azelastine hydrochloride)

Vaginal e.g. canesten, gyno-daktarin

Source: British National Formulary (BNF) (2008) (1)

P1(c) Write an account on how the different routes of administration.You have mentioned above, can affect drug action.

Oral Route is most convenient and safest. When a drug is taken orally, absorption starts from mouth to stomach. Most drugs are absorbed in intestine via the portal vein and travel to the liver and beak down before reach to the bloodstream. When a drug is taken orally it may affect how much and how fast the drug is absorbed and some drugs are taken with foods (e.g. diclofanic sodium) and some should be taken with empty stomach and others should not be taken with other drugs due to interaction (e.g. aspirin and clopidogril)

Subcutaneous route: the drug is injected just beneath the skin, then drug moves into the blood vessels and reaches the blood stream through lymphatic vessels. E.g.: insulin (actrapid). This route has slow affect because drug move slowly from the blood vessels.

Intravenous: Needle is inserted directly into a vein and drug immediately into the bloodstream and takes effect more quickly. It is painful and not easy to find vein some time. E.g. penicillin injection,

Intramuscular: needle is inserted directly in to the muscle of upper arm, thigh, and buttock. Drug absorbed in to the blood stream through tissues and vessels. It is painful and some time site of injection has swelling. E.g. diazepam injection.

Sublingual: drug are placed under the tongue and absorbed into the small blood vessels under the tongue. E.g. Glycerine trinitrate.

Buccal: Drug are placed under the upper or lower lip and absorbed into the small blood vessels. E.g. buccastem (prochlorperazine)

Rectal: Drug is inserted in to the rectum because rectum wall is thin and its blood supply rich. Drug absorbed through blood vessels and locally effect for pain. E.g. glycerol suppositories, paracetamol suppositories.

Vaginal route: Drug is absorbed through the vaginal walls through muscle tissues and blood vessels. E.g. canesten, gyno-daktarin

Topical: drug is apply on the skin and absorbed through skin slowly. It has slow effect and used mostly for skin disease. E.g. creams, oils, ( diprobase)

Eye: drug is absorbed through eye wall and blood vessels and enter the blood stream. This is mostly use for locally effect for eye infection or dry eyes. E.g. celluvics for dry eyes.

Ear : drug is absorbed though ear blood vessels and enter in to bloodstream and some time have unwanted effects mostly this route is use for ear infection e.g. otitis media ( gentamicin ear drop)

Nasal: drug is absorbed through nasal thin mucous membrane. This route is used for nasal infection and sinusitis, nasal allergies, drugs work quickly for local effect and some time irritates the nasal passage. E.g.rhinolast( azelastine hydrochloride)

Inhalation: Drugs inhale through the mouth and nasal passage in to small particles and pass through trachea and then absorbed into lungs and blood stream. E.g. sevoflurane, desoflurane.

Ref: medical library. BNF (2)

P1 (d) Explain with examples the following terms drug/food interaction, and drug/ drug interaction

Drug/ Food interaction: An interaction is occurring when the effects of one drug are changed by the presence of food, and drink.

Examples: Alcohol: If you are taking any sort of medication, it's recommended that you avoid alcohol, which can increase or decrease the effect of many drugs. E.g. alcohol with antihypertensive drugs, sleeping tablets, antihistamines (citrizine), antipsychotics, aspirin, metronidazole etc

All food delays the absorption of drug if drug been taken with food. Food affects the bioavailability, metabolism and excretion. Some drugs have their absorption increased by food (enzyme inducer) which may or may not be important:

Barbiturates

rifampicin

griseofulvin

Drug/Drug interaction: An interaction is occurring when the effects of one drug are changed by the presence of another drug.

When one drug inhibits the metabolism of another drug. Higher plasma concentrations are produced, resulting in an increased effect with risk of toxicity. E.g. warfare and phenytoin

Affect of a drug and food on the process of absorption, distribution, metabolism and excretion of another drug.

Absorption:

Change in GI pH

Delayed absorption and other mechanisms.

Change in gastric motility

Induction or inhibition of drug transport proteins.

Distribution

Protein binding: due to changes in protein binding, one drug can displace another thereby increasing its proportion free to diffuse from plasma to its site of action.

Induction or inhibition of drug transport proteins. E.g.verapamil inhibits, increasing digoxin.

Metabolism:

Changes in first pass metabolism. E.g. grapefruit juice and calcium channel blockers.

Drug can gradually increase the rate of metabolism of another and reduced effect.

Enzyme induction: e.g. phenytoin, rifampicin, smoking, chronic alcohol.

Enzyme inhibition : e.g. erythromycin, omeprazole, acute alcohol

Elimination :

Change in urinary pH e.g.; methotrexate and sodium bicarbonate.

Changes in active renal tubular transport e.g. methotrexate and NSAIDsa

Changes in renal blood flow e.g.: NSAIDs and lithium.

Biliary excretion and entero-hepatic circulation e.g. penicillin's and oral contraceptives.

Ref: class work and notes, BNF (3)

P1 (e) List the main types of adverse drug reactions.

Type A

Because the action of drug.

Dose related

Related to pharmacology action

These can occur because the patient is receiving too high dose or because of other factors which patient been given a reduced dose. Such as low body weight, age, reduced liver or kidney function. Ant cholinergic effect may occur with administration of antihistamines (cetirizine hydrochloride). Ant cholinergic means anti acetylcholine. Acetylcholine is the transmitter in the parasympathetic system so the effects will mimic those of the other system- the sympathetic one. Old people are more susceptible adverse reaction e.g. gastrointestinal effects, confusion, constipation, and postural hypotension.

Type B

Those which are unusual, these are unpredictable reactions. These include allergic reactions ranging from mild rashes to anaphylaxis which can be fatal.

Non dose related

Idiosyncratic

Bizarre

Unrelated to pharmacology action

Drugs which are most likely to cause adverse reaction are:

Those with a narrow therapeutic index

Those effect on liver

Those which at high levels cause

Type C

Dose and time related

Related to cumulative drug use.

Type D

Delayed effect

Apparent after some times of drug use

Type E

Withdrawal

Discontinuation that is too abrupt

Ref: clinical book, BNF (4)

M1 (a) Explain in detail with examples agonist and antagonist.

Agonists: An agonist is a drug that binds to a receptor of a cell and triggers a response by the cell. An agonist often mimics the action of a naturally occurring substance

An agonist produces an action. An antagonist blocks an action of an agonist.

Its open the cells door open. A drug that binds to a receptor of a cell and triggers a response by the cell. An agonist often mimics the action of a naturally occurring substance. An agonist produces an action.

E.g. Sertraline increased serotonergic effects when 5HT1 agonists given with duloxetine

Antagonist: If a drug is an antagonist it blocks the action of a natural body chemical. It inhibits a response, antagonists have the ability to bind to the receptor but do not initiate a change in cellular function, and Antagonists are also referred to as blockers.

it blocks the cell door. If a drug is an antagonist it blocks the action of a natural body chemical. It inhibits a response. Antagonists have the ability to bind to the receptor but do not initiate a change in cellular function. Antagonists are also referred to as blockers.

E.g. H2- antagonists reduce absorption of itraconazole and ketaconazole.

E.g. H2 antagonist, histamine causes acid production in the stomach. They are two types1. Competitive (reversible and irreversible) 2. Non-competitive.

Ref: class work and BNF(5)

1 (b) with examples, explain how drugs act on ion channels and drugs that act on enzymes. Design a diagram to illustrate your answer.

Drugs act on ion channels: it is divided in to two

Blockers: blocking drugs block the channels and prevent the ion specific to that channel from moving in or out of the cell. E.g. lignocaine blocks the sodium channels in pain neurons.

Modulators: do not block the ion channel but bind to sites on the channel or inhibit. E.g. verapamil a drug used to slow the heart, binding to calcium channels in artioventricular nodes inhibiting the carrier's opening.

Figure 08 - Example of a drug that work on ion channels - lignocaine (Ref. Birmingham City University material)

Drugs acting on enzymes: enzymes catalyse reactions. Enzymes are specific for one chemical reaction. Drugs can inhibit or induce enzyme activity. Many different enzymes active in the body, intracellular and extracellular, responsible for catalysis of the biochemical reaction .drug that act on enzymes is the active site where the enzymes interacts with its substrate. Drug that has similar shape to the substrate can bind to the active site and inhibit the action.

The action of drugs on enzymes (Ref. Birmingham City University material)

Enzyme inhibition: ACE (angiotensin converting enzyme) inhibits the enzymes and powerful vasoconstrictor. E.g. Lisinopril is ACE inhibitor.

Drugs that act on enzymes: non steroidal anti-inflammatory (NSAIDs) e.g. aspirin, ibuprofen.

NSAIDs inhibit the formation of prostaglandins which occur naturally in the body and are responsible for causing inflammation.

The prostaglandins do have other effects.

They reduce acid production and increase the protective mucus lining in the stomach

They increase blood flow to kidneys.

So the side effects of NSAIDs when the prostaglandin production is reduced

Peptic ulceration

Salt and water retention

 M1(c) Write an account of the mechanism of drug /drug and food/drug interactions giving examples of drugs affected.

An interaction is occurring when the effects of one drug are changed by the presence of another drug, herbal medicines, food, and drink.

Affect of a drug and food on the process of absorption, distribution, metabolism and excretion of another drug.

Absorption:

Change in GI pH

Delayed absorption and other mechanisms.

Change in gastric motility

Induction or inhibition of drug transport proteins.

Distribution

Protein binding: due to changes in protein binding, one drug can displace another thereby increasing its proportion free to diffuse from plasma to its site of action.

Induction or inhibition of drug transport proteins. E.g. verapamil inhibits, increasing digoxin.

Metabolism:

Changes in first pass metabolism. E.g. grapefruit juice and calcium channel blockers.

Drug can graqdually increase the rate of metabolism of another and reduced effect.

Enzyme induction e.g; phenytoin, rifampicin, smoking, chronic alcohol.

Enzyme inhibition : eg: erythromycin, omeprazole, acute alcohol

Elimination :

Change in urinary pH e.g.; methotrexate and sodium bicarbonate.

Changes in active renal tubular transport e.g. methotrexate and NSAIDsa

Changes in renal blood flow e.g: NSAIDs and lithium.

Biliary excretion and entero-hepatic circulation e.g. penicillins and oral contraceptives.

All food delays the absorption of drug if drug been taken with food. Food affects the bioavailability, metabolism and excretion. Some drugs have their absorption increased by food (enzyme inducer) which may or may not be important:

Barbiturates

rifampicin

griseofulvin

When one drug inhibits the metabolism of another drug. Higher plasma concentrations are produced, resulting in an increased effect with risk of toxicity. E.g. warfare and phenytoin.

E.g. alcohol with antihypertensive drugs, sleeping tablets, antihistamines (citrizine), antipsychotics, aspirin, metronidazole etc

Ref: class work and notes, BNF (3)

D1 (a) Design a table to illustrate the advantage and disadvantages of the different routes of administration of medicines.

Route

Advantages

Disadvantages

Oral

Convenient

Large surface area for absorption

Drug metabolism

Incomplete absorption

First pass effect

Gastrointestinal upset

Intravenous ( I/V)

Direct

No first pass effect

Slow infusion or rapid onset of action.

Easier to titrate dose.

Requires I/V access

Hard to remove

Vascular injury

Extra vacation

Intra arterial

To specific organ eg: brain, heart

Intra muscular (I/M)

Good for depot storage( if oil based)

Rapid onset of action

Pain at site of injection

Subcutaneous(SC)

Non irritating small volumes even slow absorption

Adrenaline in local anesthestics

Pain at site of injection

Topical

Convenient localized limited systemic absorption

Effects are limited to area of application.

D1 (b) write an explanation of Zero-order pharmacokinetics and first-order pharmacokinetics and half life.

First order and Zero order Kinetics:

First Order Kinetics:

A constant fraction of the drug in the body is eliminated per unit time. The rate of elimination is proportional to the amount of drug in the body. The majority of drugs are eliminated in this way.

  

such as digoxin, have a very high volume of distribution (500 litres). Drugs which are lipid insoluble, such as neuromuscular blockers, remain in the blood, and have a low Volume distribution..

The fraction of the drug in the body eliminated per unit time is determined by the elimination constant (kel). This is represented by the slope of the line of the log plasma concentration versus time.

Clearance = elimination constant x volume of distribution

Rate of elimination = clearance x concentration in the blood.

Elimination half life (t1/2): the time taken for plasma concentration to reduce by 50%. After 4 half lives, elimination is 94% complete.

Zero Order Elimination.

The metabolic pathways responsible for alcohol metabolism are rapidly saturated and that clearance is determined by how fast these pathways can work. The metabolic pathways work to their limit. This is known as zero order kinetics

e.g. phenytoin, salicylates, theophylline, and thiopentone (at very large doses). Because high dose this is very slow to clear,

Ref:4um.com, owned and operated by Pat Neligan

Half Life:

The period of time required for the concentration or amount of drug in the body to be reduced to exactly one-half of a given concentration or amount. Steady state is achieved when

rate in = rate out

Examples of half-life: Amiodarone 25 days, digoxin 24 to 36 days, fluoxetine 1 to 6 days, methadone 15 to 60 hours or 190 hours, salbutamol 7 hours.

D1(c) Write an explanation of how pharmacokinetics can influence the dose of a drug using examples to illustrate your ideas.

Pharmacokinetics is the study of what the body does to a drug. Four Pharmacokinetics process Absorption, Distribution, Metabolism and Excretion (ADME).

Absorption:

Absorption is defined as the passage of a drug from its site of administration into the plasma.

Drug must enter plasma before reaching its action.

Factors affecting absorption:

Route of administration

Aqueous solubility of the drug( dissolution)

Lipid solubility of drug

Formulation e.g. coating, etc.

Extent of drug metabolism (in gut, in GI mucosa and liver) before reaching the systemic circulation-first passes effect.

First pass effect.

Aqueous solubility of the drugs.

Interaction of the drug.

Distribution:

The transfer of the drug from general circulation of the body to different part of the body or organs such as muscles, kidneys, liver, etc.

Protein binding: many drug bind to plasma proteins (e.g. albumin, globulins.).Protein bound drugs usually too big to diffuse across biological membrane. Most drugs must be unbound (free) to be biologically active. Changes of protein binding only clinically important for few drugs that are clinically important for a few drugs that are highly protein bound e.g. phenytoin, warfarin.

Factor affecting protein binding:

Renal impairment end stage.

Hypoalbuminaemia

Pregnancy last trimester

Displacement by other drugs.

Saturability of plasma protein binding within therapeutic range. E.g. phenytoin.

Distribution in elderly: low body mass, low body water, high body fat, serum albumin.

Distribution in neonates and children: adipose tissue varies suatantially in neonates, amount of albumin related to length of gestation, protein binding not occurs in neonates until 7yrs.

Half life time required for the amount of drug in the body to be reduced one-half of a given amount. E.g.: digoxin 24 to 36 hours.

Metabolism:

The process by which drugs are modified to allow their degradation and removal from the body. The liver is the main organ involved in drug metabolism. Others also metabolise like kidney, lung, gut. Some drugs are extensively metabolised as they pass through the liver. This is FIRST PASS EFFECT. Glyceryl trinitrate as a high degree of first pass effect. Two ways to bypass first pass metabolism involving giving the drug by sublingual and buccal routes.

Stages of metabolism: liver contains all the enzymes. The main enzymes is P450 group. Metabolism is divided into two phase of biochemical reaction: phase1 and phase2.

Phase1: reduction or hydrolysis of the drug. But most common is oxidation and catalysed by cytochrome P450 enzymes. Drug is now oxidised. Pro-drug intentionally activated by phase1 metabolism.

Phase2: involves conjugation that is the attachement of ionised group to the drug.these are glutathione,methyl or acetyl groups. The attachement of an ionised group makes the metabolite more water soluble this facilitates excretion.

Factors affecting liver metabolism: age, disease, shock, genetic, other drugs, food etc.

Cytochrome P450 : oral medications are absorbed through the small intestine into the portal circulation. Initial metabolism by cytochrome P450 isoenzymes occurs in bowel wall and in the liver before entering the systemic circulation. This is known as the first pass effect. Enzymes induction or inhibition.

Inhibitor: is a compound that slows down the metabolism of a substrate by a given enzyme.rate of substrate metabolism is decreased, resuling in increased drug concentrations. E.g: amiodarone, cimetidine, ciprofloxacin, erythromycin,grapefruit.

Inducer: is a compound that speeds up the metabolism of a substrate by a given enzymes.eg. carbamazepine speeds up metabolism of clozapine and if carbamazepine discontinued ,clozapine levels will rise and occurs unanticipated seizure. E.g. barbiturates, carbamazepine, phenytoin, rifampicin.

Elimination: drug elimination is removal of drugs from the body. All drugs are eliminated from the body in a metabolized form by excretion

e.g.: saliva, sweats, breast milk, and exhaled air. Few drug not metabolised at all.

Renal excretion: water-soluble drugs and their metabolites are eliminated by the kidneys in urine. Renal excretion decrease with increasing age.

Steady state: As repeated dose of a drug are administered its plasma concentration builds up and reaches what is known as a steady state.

Loading dose: sometimes a loading dose may be administered so that a steady state is reached more quickly then smaller maintenance dose are given to ensure that the drug levels stay within the steady state.

For all drugs there is a minimum effective concentration and a maximum safe concentration between these two values is called the therapeutic range. Therapeutic drug monitoring for drugs with narrow therapeutic range e.g. digoxin, etc.

Ref: class work notes and handout./ Birmingham city uni. Material/ BNF.

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