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Karan - MBBS Year 1: Respiratory Week 2 > Histamines and Anti-Histamines > Flashcards

Histamines and Anti-Histamines Flashcards

1
Q

What cells are histamines located in and how are they released during inflammatory reactions?

A
  • Granules in mast cells and basophils
  • Released by exocytosis (i.e degranulation)
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2
Q

What are the effects of histamines?

A
  • Vasodilation
  • Increased GI and mucous secretion
  • Bronchoconstriction
  • Increased vascular permeability
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3
Q

What are histamines released in response to?

A
  • Cold
  • Bacterial toxins
  • Trauma
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4
Q

How can anaphylaxis trigger histamine release?

A
  • C3a and C5a interaction with membrane receptors
  • Antigen interaction with IgE
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5
Q

What are the different types of histamine receptors?

A
  • H1 (Gq linked) - vasodilation/increased vascular permeability/contraction of most smooth muscle except blood vessels
  • Targeted by H1 antagonists - for treatment of inflammation
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6
Q

What do the histamine receptors have in common?

A

GPCRs

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7
Q

What type of hypersensitivity reactions are histamines involved in?

A

TYPE 1

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8
Q

Does histamine have therapeutic applications?

A

NO

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9
Q

Describe the first generation antihistamines.

A
  • Sedating - cross the blood brain barrier
  • Cyclizine, chlorphenamine maleate, promethazine hydrochloride
  • Useful for children with motion sickness - can calm the patient down.
  • Not used for adults - can cause drowsiness when driving
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10
Q

Describe the second generation anti-histamines.

A
  • Non-sedating
  • No CNS activity
  • Loratidine and cetirizine hydrochloride
  • Used for adults
  • Some cardiac toxicity e.g terfenadine
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11
Q

Describe the clinical uses of antihistamines. PART 1

A
  • All agents effective against ALLERGIC REACTIONS. Cetirizine and loratadine used for rhinitis,puritis and urticaria
  • Injectable formulations as adjunct therapy used in emergency anaphylactic treatment - injected in thigh
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12
Q

Describe the clinical uses of antihistamines. PART 2

A
  • Antiemetics - prevent vomiting and motion sickness e.g cyclising
  • Sedation - in patients with sleeping difficulties e.g promethazine - sedative. Used IV during anaphylactic emergency.
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13
Q

Describe mechanism of action of anti-histamines.

A
  • COMPETITIVE ANTAGONISM OF EXCITATORY Gq receptors
  • Reversed by high concentrations of histamines (agonist)
  • Reduced release of IP3 which would usually cause bronchoconstriction
  • Reduced hypersecretions, pruritus and sneezing
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14
Q

What are the main difference between first and second generation of antihistamines?

A
  • FIRST GEN - CNS effect so causes drowsiness
  • OPPOSITE FOR SECOND GEN
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15
Q

Describe the pharmacokinetics of promethazine hydrochloride.

A
  • Well absorbed from GI sites
  • Can be administered orally, rectally, deep IM or IV (others when oral not available)
  • IM preferred over IV because of localised activity at injection site
  • Crosses placenta
  • Metabolised in liver to inactive metabolites
  • Excreted via urine
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15
Q

Describe the pharmacokinetics of chlorphenamine maleate.

A
  • Administered orally, topically
  • Hepatic metabolism via CYP450 enzymes to active and inactive metabolites
  • First pass metabolised - low bioavailability
  • Excreted via urine
16
Q

Describe the pharmacokinetics of cetirizine hydrochloride.

A
  • Administered orally
  • Rapid absorption
  • Relatively short half-life
  • Limited hepatic metabolism
  • Excreted as urine,faeces (50% IN URINE as unchanged drug so ELIMINATION)
  • Water soluble - doesn’t need to be metabolised to be excreted
17
Q

Describe the pharmacokinetics of loratadine. PART 1

A
  • Administered orally
  • Rapid absorption
  • High Vd > TBW - passes through muscle and water compartments
  • Relatively short half life but increases when active metabolite forms
18
Q

Describe the pharmacokinetics of loratadine. PART 2

A
  • Parent drug heavily bound to plasma protein. Reduces when active metabolite forms
  • Extensive hepatic metabolism via CYP2D6 and 3A4 (so no grapefruit juice) to active metabolite.
  • Elderly Cmax doubled upon hepatic function impairment so decrease the dose by half. POSSIBLE ALTERNATIVE - cetirizine (non-sedative)

Karan - MBBS Year 1: Respiratory Week 2 (10 decks)

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