Factors Influencing Drug action

Question 1

List the factors affecting drug action

Answer 1

1. Species/bloodlines
2. Age
3. Health status/condition
4. Dosage
5. Route of application
6. Feeding
7. Gender/sex
8. Tolerance
9. Dependence/abuse
10. Allergy

Question 2

Factors affecting species: Receptorial examples

Answer 2

• Cattle → Xylazine
• Cat → Morphine

Question 3

Factors affecting species: Absorption

Answer 3

• Horse → Ampicillin
• Horse & Rabbit → Atropine

Question 4

Factors affecting species: Distribution

Answer 4

Dog → Ivermectin

Question 5

Factors affecting species: Metabolism

Answer 5

• Cat → Salycilates, phenols
• Dog → Sulphonamides
• Pig → Procaine
• Rabbit & goat → Atropine

Question 6

Factors affecting species: Gut flora

Answer 6

Rabbit & hamster → Penicillin

Question 7

Compare mydriatic effect of an antagonist on birds and mammals

Answer 7

• Mammals: Effect on muscarinic cholinergic neuromuscular junction E.g Atropine
• Birds: Effect on nicotinic cholinergic neuromuscular junction E.g d-tubocurarine

Question 8

Factor influencing drug action: Health status/condition

Answer 8

• Fever - Reduced emptying of stomach, negative effect on absorption
• Diarrhoea:
  
  • Increased gut motility, decreased the bioavailability
  • Exsiccation → Altered distribution, decreased urinary excretion

Question 9

Factor influencing drug action: Dosage

Answer 9

• Atropine: Antimuscarinic < pre-anasthetic < antidote
• Xylazine: Weak sedation < milk analgesia < deep sedation, strong analgesic

Question 10

Factor influencing drug action: Route of application

Answer 10

MgSO₄, Insulin

Question 11

Factor influencing drug action: Feeding

Answer 11

Type and composition of feed

• Decreased bioavailability (Ca²⁺ → Antibiotics)
• Increased activity of metabolising enzymes (plant-steroids)

Question 12

Factor influencing drug action: Sex

Answer 12

Males: Faster metabolism

• LD₅₀ values may be higher (not always higher)

Question 13

Factors affecting drug disposition and responses in older patients: Absorption

Answer 13

Question 14

Factors affecting drug disposition and responses in older patients: Body composition & protein binding

Answer 14

Question 15

Factors affecting drug disposition and responses in older patients: Metabolism & excretion

Answer 15

Question 16

Factor influencing drug action: Tolerance

Answer 16

• Subject’s reaction to a drug decreases
• The organism builds up a resistance to the effects
  
  • Tachyphylaxis (rapid development of drug tolerance)
  • Drug immunity

Question 17

Factor influencing drug action: Dependence, abuse

Answer 17

• Habituation - Force of habit, mental, not really psychological
• Addiction - Strong psychological and frequently physiological

Question 18

Factor influencing drug action: Allergic reaction

Answer 18

• Uncommon
• Caused by an oversensitive immune system
• Usually occurs the second time of taking a drug
  
  • Contamination of skin
  • Inhalation
  • Depot preparations
  • Chronic diseases, atopy

Question 19

Types of drug allergy

Answer 19

Type I → IV

Question 20

Type I drug allergy

Answer 20

Immediate/anaphylactic reaction

• IgE on mass cells/basophils receptors
  
  • Histamine, prostaglandins
  • Vasodilation, oedema, inflammation
  • Anaphylactic shock
• Elimination of clinical signs is rapid

Question 21

Type II drug allergy

Answer 21

Cytolytic

• IgG & IgM maintained, complement activation-dependent
• Haemolytic anaemia, thrombocytopenia
• Signs last over months

Question 22

Type III drug allergy

Answer 22

Immune-complex associated

• Immune-complex formation, mainly IgG + antigens
• Serum sickness, urticaria, drug fever, cutaneous eruptions

Question 23

Type IV drug allergy

Answer 23

Delayed or cellular

• T-lymphocytes & macrophages, maintained hypersensitivity
• Contact dermatitis, morbilliform eruptions