3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.3. Pharmacology
                  3.1.3.1. Pharmacology principles
                      3.1.3.1.3. Pharmacogenetics
 3.1.3.1.3.1. Porphyria 

porphyria

[TFC2:p148-p149; RDM6:p1098]

@Unfinished

Porphyrin metabolism

  • Porphyrins are cyclic structures formed by linkage of four pyrrole rings through methene bridges
  • Haem is the most important porphyrins in human physiology

Biosynthesis of haem

  • Start with formation of delta-aminolaevulinic acid (ALA)
    * By condensation and subsequent decarboxylation of succinyl CoA and glycine
    * Catalysed by ALA synthetase
    * Take place in the mitochondria
    * ALA synthetase is the rate-limiting factor in porphyrin biosynthesis

ALA synthetase

  • Control of haem production is primarily through ALA synthetase
  • Haem provides a negative feedback on the formation of ALA synthetase
  • ALA synthetase is readily inducible
  • In porphyria, there is a partial block in the synthetic pathway
  • Therefore in porphyria, increase in ALA synthetase
    --> Increase in the intermediate products before the block

Classifications of porphyria

  • 5 major syndromes in adults
    --> All autosomal dominant
  • Hepatic Porphyria
    * Acute intermittent porphyria (AIP)
    * Variegate porphyria (VP)
    * Porphyria cutanea tarda
    * Hereditary coproporphyria (very rare)
  • Erythropoietic Porphyria
    * Erythropoietic protoporphyria

Hepatic porphyria

Acute intermittent porphyria (AIP)

Defect

  • Partial deficiency of porphobilinogen deaminase

Clinical features

Clinical signs and symptoms are similar to lead poisoning

  • Abdominal pain, vomiting, constipation
  • Peripheral neuropathy (lower motor neuron lesion) --> Can progress to bulbar paralysis
  • Psychiatric disorders
  • Skin is NEVER affected
  • Triggers include
    * Certain drugs can induce the enzyme ALA synthetase
    * Infection, fasting, menstruation

Drugs which may trigger attacks of AIP

Sensitising drugs include:

  • Barbiturates
  • Diazepam
  • Phenytoin
  • Pentazocine
  • Birth control pills
  • Ethyl alcohol
  • Sulfonamides
  • Ergotamine preparation
  • Others: (not found in textbook but in Prof Kam's lecture notes include)
    * Cephalosporin
    * Sulphonylurea
    * Steroids

Drugs which are SAFE in AIP

[RDM6:p1098]

Safe drugs include:

  • Neostigmine, atropine
  • Suxamethonium, pancuronium
  • N2O
  • Procaine
  • Propofol, etomidate
  • Morphine, pethidine, fentanyl
  • Droperidol, promethazine, chlorpromazine, promazine

NB:

  • Etomidate is NOT safe according to [PHW2:p92]

Investigation

  • Acute attacks
    * Increased urinary porphobilinogen and delta-aminolevulinic acid (ALA) --> dark urine
    * Faecal porphyrins normal
  • Remission
    * Same findings

Variegate porphyria (VP)

Defect

  • Protoporphyrinogen oxidase deficiency

Clinical features

  • Acute attacks = similar to AIP, except also affects skin
  • Skin is photosensitised --> fragility, pigmentation, and hypertrichosis in light-exposed areas

Investigations

  • Acute attacks
    * Increased urinary porphobilinogen and delta-aminolevulinic acid (ALA) --> same as AIP
    * Faeces contain excess coproporphyrins and protoporphyrins
  • Remission
    * Urine may be normal

Porphyria cutanea tarda

Defect

  • Uroporphyrinogen decarboxylase deficiency

Clinical features

  • No acute attacks and are not usually drug-related
  • Skin = bullae and hyperpigmentation on exposed area (like VP)
  • Underlying alcoholic liver disease or hepatitis C common
  • May have a history of exposure to toxins e.g. hexachlorobenzene

Investigations

  • Normal porphobilinogen (PBG) and ALA in urine
  • Normal porphyrins in stools
  • Greatly increased uroporphyrin in urine
    --> Red urine

Hereditary coproporphyria

  • Very rare
  • Clinically like variegate porphyria
  • With raised faecal and sometimes urinary coproporphyrin

Erythropoietic porphyria

Erythropoietic protoporphyria

  • Defect = Ferrochelatase deficiency
  • Clinical features = skin photosensitivity
  • Investigations = Free protoporphyrin in RBC, normal urine