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Pharmacology > Pharmacokinetics > Flashcards

Pharmacokinetics Flashcards

1
Q

Is the drug getting into the patient?

A

PHARMACEUTICAL PROCESS

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2
Q

Is the drug getting to its site of action?

A

PHARMACOKINETIC PROCESS

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3
Q

Is the drug producing the required pharmacological effect?

A

PHARMACODYNAMIC PROCESS

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4
Q

Is the pharmacological effect being translated into a therapeutic (or toxic) effect?

A

THERAPEUTIC PROCESS

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5
Q

What your body does to the drug is called

A

Pharmacokinetics

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6
Q

The quantitative analysis of the time course of drug:

A

Absorption
Distribution
Metabolism
Excretion

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7
Q

Routes of Administration

A

Oral
Sublingual
Rectal
Parenteral : Intravenous, Intramuscular, and Subcutaneous
Inhaled
Topical : Intranasal, Eye, and Cutaneous

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8
Q

In oral administration, mechanism of absorption can be of 2 types
enteral:

A

1. Mechanism of absorption
* Passive non-ionic diffusion
-Majority of drugs

2. Specialised transporters
* Large neutral amino acid transporter
-L-dopa, Methyldopa, Baclofen

  • Oligopeptide transporter (PEPT-1)
    -Amino beta lactams, ACE inhibitors
  • Monocarboxylic acid transporter
    -Salicylic acid, pravastatin

Enteral: absorption from mouth, stomach and small intestine
Mouth and stomach: minority
Small Intestine: majority
Passive > Active

  • Rate ~ 75% in 1-3 hours. Depends on:
    Motility e.g. diarrhoea decreases absorption
    Blood flow
    Food: Enhances or impairs
    Particle size and formulation
    Physico-chemical factors: Unionised/Lipid soluble
  • Rate of gastric emptying is a rate limiting step
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9
Q

Effects of Food on Oral Drug Absorption

A

1. Poor acid stability
Prolonged gastric exposure  Degradation
E.g. isoniazid
**2. Require acid environment **
E.g. ketoconazole
3. Fat or bile acids enhance absorption
E.g. tacrolimus,
4.Bind to fibre, reducing absorption
E.g. digoxin
5. Bind to calcium (chelate), reducing absorption
E.g. tetracyclines,

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10
Q

Effect of Formulation on Absorption

A

** Rate of disintegration of tablet**
-Tablet compression
-Bulk excipients
Rate of dissolution of drug particles in intestinal fluid
-Particle size: smaller dissolve quicker
Modified Release
-Reduce frequency of oral administration
E.g. morphine

-Deliver contents to site of action
E.g. mesalazine: pH sensitive coating – 5-ASA (5-aminosalicylic acid) released in distal small bowel and colon; used for ulcerative colitis

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11
Q

Sublingual Administration

A

From blood vessels at base of tongue

Lipid soluble drugs only
-E.g. Nitroglycerin [used for angina (chest pain)]

Small surface area
-Potent drugs only

Avoids first pass metabolism

Rapid absorption: minutes

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12
Q

Rectal Formulations

A

Avoid first pass metabolism

Erratic absorption because of rectal contents

Acceptable to patients?

Useful if unable to take oral medications. E.g. paracetamol

Useful if unable to get IV access. E.g. diazepam in status epilepticus

Direct effect on large bowel. E.g. Corticosteroids in Inflammatory Bowel Disease

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13
Q

Parenteral: Subcutaneous

A

Very small injection volume
Drug absorbed very slowly into blood through capillaries
Larger drug molecules such as protein drugs reach blood through lymphatic system
E.g., insulin

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14
Q

Parenteral: Intramuscular

A

Slow absorption, but faster compared to subcutaneous due to better supply of blood vessels than skin
Larger injection volume compared to subcutaneous
Less invasive compared to IV
Painful

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15
Q

Parenteral: Intravenous

A

Direct delivery to plasma : No absorption required

Rapid effect

Avoids first pass metabolism

Risks: Infection, embolism (blockage due to clot formation)

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16
Q

Inhaled Medications

A
  • Formulations
    Powders, Aerosol solutions, Nebulised solutions
  • Delivery to bronchioles
    ~10%
    Depends on type of inhaler and how it is used
  • Local effects
    E.g. infections
  • Some systemic absorption
    Salbutamol: Used for Asthma. Can cause tremor (Tachycardia)
    Corticosteroids: Used for Asthma. Can cause osteoporosis
    Ipratropium bromide: Used for bronchitis. Can cause anticholinergic ‘dry mouth’
17
Q

Topical: Intranasal Formulations

A
  • Direct therapeutic effect
    Sodium chromoglycate for rhinitis
  • Systemic effect
    Sumatriptan in migraine
  • Local toxicity
    Cocaine: Necrosis of nasal septum
18
Q

Topical: Eye Drops

A

Absorption through conjunctival sac epithelium
Local effects in eyes with minimal systemic effects
Some systemic absorption
E.g. Timolol for glaucoma may precipitate bronchospasm in asthma

19
Q

Topical: Cutaneous Administration

A
  • Local effect on skin
    Steroids
  • Slow systemic absorption (patch)
    Lipid soluble drugs only
    -Oestrogen
    -Opioids – Fentanyl
20
Q

First Pass Metabolism of Oral Drugs

A

Gut
Liver

21
Q

First Pass Metabolism in Gut Lumen

A
  • Gastric acid
    E.g. Inactivation of benzylpenicillin
  • Proteolytic enzymes
    E.g. Inactivation of insulin
  • Microbial enzymes
22
Q

First Pass Metabolism in Gut Wall

A
  1. Monoamine oxidase: Metabolises monoamines
    * Irreversible MAO inhibitors + amine-containing foods
    -Tyramine not metabolised by MAO in gut wall
    Enters systemic circulation
    Releases NAD from stores in nerve endings causing hypertensive crisis

** 2. CYP 3A4**
Many drugs act as inducers, inhibitors or substrates
Blocked by grapefruit juice

3. P-glycoprotein (enterocytes of gut lumen)
Interactions b/w inhibitors (e.g. verapamil, macrolides) and substrates (e.g. digoxin)

23
Q

Hepatic First Pass Metabolism

A
  • Reduced amount of parent drug
  • Metabolites
    -More water soluble: Facilitates excretion
    -Activity
    Decreased
    Increased: Pro-drugs
    - Inactive precursors, metabolised to active metabolites
    -E.g. Cyclophosphamide, simvastatin, ramipril, perindopril
    -Reduced first pass metabolism ->Reduced bioavailability of pro-drugs
24
Q

Bioavailability

A

The percentage of an ingested dose of a drug that enters systemic circulation

Implications for oral and parenteral dosing
-High bioavailability -> Dose same for IV and p.o. routes
E.g. Metronidazole
-Low bioavailability -> Higher dose for PO than parenteral routes
E.g. Morphine: 10 mg s/c or IM  30 mg PO

25
Q

Bioavailability of Thyroxine (T4)

A

Certain agents reduce bioavailability of oral thyroxine

may need to ↑ dose T4

-Drugs that decrease absorption of oral T4
Cholestyramine
Soy bean formulations
Sucralfate
Ferrous sulfate

-Drugs that increase hepatic metabolism of T4
Phenobarbitone
Carbamazepine
Rifampicin

26
Q

Bioequivalence

A
  • Pharmaceutically equivalent and equal systemic bioavailability
  • Generics
    -Must be bioequivalent to innovator (80–125%)
  • Phenytoin toxicity outbreak (Australia 1968)
    -‘Inert’ excipient changed: CaSO4 to lactose
    -Increased solubility and systemic availability
27
Q

drug binding

Distribution

A

It describes the reversible transfer of drug from one location to another within the body

It is important that drugs present in plasma in free form can be transported to the tissues

Some drugs can bind certain plasma proteins

Drug binding
It is rarely specific, usuallylabileand reversible
Involves:
Ionic bonds
Hydrogen bonds
van der Waals forces and,
Covalent bonds (less often)

28
Q

Protein Binding

A
  • Drugs bind to different plasma proteins
    Albumin (acidic drugs, e.g. warfarin, NSAIDs)
    Alpha-1 acid glycoprotein (basic drugs, e.g. Quinine)
    Lipoproteins (basic drugs)
    Globulins (hormones)
  • Only free drug can bind to receptors
  • Therefore, protein-bound drug can be considered to be in a temporary storage
29
Q

Clinical implications of changes in protein binding

A

Changes in protein binding may be caused by
Disease
Nutrition
Protein binding displacement interactions

E.g. valproate displaces phenytoin

Clinically relevant effects if
> 90% of drug is protein bound
E.g. phenytoin, warfarin
Small volume of distribution

30
Q
A

Pharmacology (8 decks)

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