C4. The chemical basis of drug action Flashcards
Describe normalities
-Normality describes the number of equivalents per litre. It is now rarely used as a unit.
-For hydrochloric acid (considering its dissociation to give H+)
1M HCl = 1N HCl (because it produces one equivalent (of protons) per L)
-For sulphuric acid (considering its dissociation to give H+)
1M H2SO4 = 2N H2SO4 (because it produces two equivalents (of protons) per L)
Total mEq equation?
one note
Ideal characteristics of an antacid?
-Acts locally – therefore insoluble in water and not absorbed into the circulation
-Maintains pH in the range 3 – 6
-Exerts its effect swiftly and with a good duration of action
-Does not produce large volumes of gas when reacting with HCl
-Does not cause laxative effects or constipation
Compare a potent antacid and a less potent antacid
ONE NOTE
Effect of combining a number of antacids in one medicine?
-Can optimise the rate of onset of action, duration of action, range of pH reached
-Perhaps moderate constipating and laxative effects
-Calcium and aluminium salts have constipating effect
-Magnesium has laxative effect
Active ingredients in Gaviscon (Double Action)- Raft forming agent?
Sodium alginate
Sodium bicarbonate
Calcium carbonate
Most raft forming agents have what two main ingredients?
i) an alginate
ii) a gas-forming agent
Describe alginic acids?
-Extracted from brown seaweeds
-Polymer composed of sugar monomer units- i.e. is a polysaccharide
-Can be 80 residues or more in length, molecular weight over 150,000 Daltons
-Alginic acid and most metal alginates insoluble in water
-Alkali metal salts of alginates are water soluble
ONE NOTE
Structure of raft forming agents?
ONE NOTE
Describe the alginic acid raft?
-If pH drops below 3.5 (the
approximate pKa) – alginic acid precipitates as a gel
-The precipitate/gel swells and
absorbs water
-The raft is formed
Describe alginic acid and carbon dioxide raft forming agents
-Many alginate based medicines also contain bicarbonate or carbonate
In acid:
i) The bicarbonate or carbonate produces carbon dioxide
ii) The soluble alginate reacts to form an insoluble alginic acid
precipitate/gel
iii)The precipitate/gel traps the carbon dioxide gas, to produce a low density raft (viscous foam) that can float on
the stomach contents
Effect of calcium cations on raft forming agents?
-Calcium cations interact with the alginate/alginic acid and aid cross-
linking in the polymer
-This increases the viscosity of the gel/raft e.g. BP Raft-forming ability method
-Evidence that the presence of calcium cations increases the raft strength (in vitro)
ONE NOTE
Describe anti-foaming agent, simeticone
-This is dimethicone (polydimethylsiloxane) and silica, and is also known as activated dimethicone
-Silicone polymer
-Believed to alter (lower) the surface tension of small gas bubbles so they co-join, to form larger bubbles
-Allows gas in stomach to be more easily lost
-Evidence that this relieves trapped wind and bloating in dyspepsia
