Summary: Principles Of Pharmacology

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• 1 Principles of Pharmacology

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• What is meant by ‘formulation’? Which type of substances, apart from the pharmacological agent, may be used in the preparation of drugs? 

  
  
  
  
  Formulation = the preparation of a drug according to a formula specifying:
  

  • ○  Pharmacological substances.
    
  • ○  Solvent (water, alcohol, air, oil/fat, jelly, etc.).
    
  • ○  Binders, stabilizers, preservatives.
    
  • ○  Colours, flavors, odorants.
    
  • ○  Coating. 
    

• Explain why the following factors are relevant for the formulation of a drug: barriers; setting; urgency; stability. 

  
  
  
  
  Formulation depends on the following factors:
  

  • ○  The barriers that the drug is capable of passing(first pass effect) (intravenous, oral, etc.).
    
  • ○  The setting in which the drug will be used (surgery i.v., home tablet, etc.).
    
  • ○  The urgency of the medical situation / delay before onset of action (e.g.
    intravenous administration in emergency situations).
    
  • ○  Stability of the drug (drugs denatured by acid are not taken orally). 
    

• What is the ‘first pass effect’ and how is it explained? 

  Blood from the gastrointestinal tract passes through the liver before entering any other organs. During this first pass through the liver, a fraction of the drug can be metabolised to an inactive or less active derivative.

• What are the advantages and disadvantages of intravenous, intramuscular and subcutaneous injection of a drug? 

  
  
  
  
  Parenteral (via injection / “around the GI tract”): (+) more rapid and predictable absorption, more accurate dose selection (­) need for strict asepsis (= the state of being free from disease­causing contaminants), risk of infection, pain, local irritation.
  

  • ○  Intravenous (i.v.): # rapid onset.
    
  • ○  Intramuscular (i.m.): # semi rapid onset.
    
  • ○  Subcutaneous (SubQ, s.c.): # slow onset (not as well perfused as muscle). 
    

• State 3 non­invasive modes of drug administration suitable for circumventing the first pass effect.

  
  
  
  
  Other routes to bypass the liver:
  

  • ○  Sublingual: absorption through capillary bed under the tongue, easily self
    administered (e.g. nitroglycerin).
    
  • ○  Rectal (PR): for unconscious or vomiting patients, or small children,
    absorption unreliable (e.g. analgesics, anticonvulsants).
    
  • ○  Inhalation: rapid absorption (e.g. nitrous oxide, ether, halothane).
    
  • ○  Transdermal: “patch” applied to the skin and then passes into the capillary
    bed, good for self­administration (e.g. patch >> nicotine, nitroglycerin, fentanyl). 
    

• Define distribution half life.

  
  
  
  
  Distribution half life (t1/2α) = the time for the rapid decline in plasma drug concentration as a dose of drug is distributed throughout the body in fat tissue storage.

• Define elimination half life

  
  
  
  
  Elimination half life (t1/2β) = the time for disappearance from the plasma due to the metabolism and excretion of the drug (bigger than distribution half life). 

•  In case of continuous infusion of a drug, how long does it take the plasma concentration to reach a steady state? What would happen if the administered dose were doubled? 

  
  
  
  
  Continuous infusion IV: steady state C reached after continuous infusion for 4­5 half lives. increasing the rate of infusion will not decrease the time needed to reach steady state. It will, however, increase the plasma drug concentration at steady state. 

• What is the characteristic course of the plasma concentration of a drug administered in an intermittent dose? What is the rationale of giving a ‘loading dose’? 

  
  
  
  
  ● Intermittent dose: peaks = high points of fluctuation, toxic effects most likely to be observed during peak. troughs = low points of fluctuation, lack of drug effect (e.g. more pain before the 2nd dose than midway between 1st and 2nd dose).
  ● Loading dose = initial dose higher than subsequent doses for the purpose of achieving therapeutic drug concentrations in the serum rapidly. 

• In which way is the plasma half­life of a drug influenced by its distribution; metabolism; elimination? 

  Distribution: An increased distribution causes a longer life
  
  
  Metabolism: An increased metabolism causes a shorter life
  
  Elimination: An increased elimination causes a shorter life