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Monday, September 19, 2011

Acid-Peptic Diseases: Treatment of acid peptic disease with antacid

Antacids

Antacids are weak basic drugs that react with gastric hydrochloric acid to form a salt and water thus reducing the intragastric acidity. They are used to treat peptic ulcer pain associated with hyperchlorhydria.


How do antacids act? Mechanism of action

1.      By chemically neutralizing hydrochloric acid
2.      By inhibiting pepsin at ph>3.5

After a meal about 45mEq/h of hydrochloric acid is secreted. A single dose of 156 mEq of antacid given after 1 hour of meal neutralizes gastric acidity for 2 hours. However the acid neutralizing ability of antacids depend upon different formulations.


Antacids don’t have any effect on the underlying cause of ulceration.

Parts of an antacid:
Weak basic part
In the form of OH, oxides, HCO3, trisilicate
Responsible for neutralization of hydrochloric acid
Metallic cation
Al, Mg, Na, Ca, K
Responsible for pharmadynamic property and adverse effects
Classification of antacids:
Property
Non-systemic antacids
Systemic antacids
Definition
Drugs that neutralizes the local pH of the stomach
Drugs that enters into the systemic circulation and modifies the pH of the body
Example
Al(OH)3 gel
NaHCO3

MgCO3
NaCO3

Mg(OH)2
CaCO3

Mg trisilicate

Solubility
Insoluble
Soluble
Absorbed into system
No
Yes
Onset of action
Slow
Quick
Adverse effect
Diarrhea, Constipation
Acid base abnormality such as metabolic alkalosis

Sodium bicarbonate (NaHCO3) (eg, baking soda, Alka Seltzer) reacts with hydrochloric acid to form sodium chloride and carbon dioxide.

The CO2 produced cause gastric distension and belching.

Sodium chloride is absorbed and this may worsen condition in a patient with heart failure, hypertension and renal failure by excessive fluid retention.

Any unreacted alkali is rapidly absorbed and may cause metabolic alkalosis when given in high doses or in patients with renal insufficiency.

Calcium Carbonate(CaCO3)(eg Tums, Os-Cal) is less soluble and reacts more slowly to from calcium chloride and carbon dioxide. So just like sodium bicarbonate, calcium carbonate may produce belching or metabolic acidosis.

Excessive dose of either NaHCO3 or CaCO3 with calcium containing dairy products can lead to the following conditions:
1.      Hypocalcaemia
2.      Renal insufficiency
3.      Metabolic acidosis (milk alkali syndrome)

Aluminium hydroxide(Al(OH)3) and Magnesium hydroxide(Mg(OH)2) both react slowly with HCL to form aluminium chloride or magnesium chloride and water.

Because no CO2 gas is not produces, belching does not occur.

Metabolic acidosis is also uncommon because of the efficiency of the neutralization reaction.

But there are adverse effects. Unabsorbed magnesium salt cause osmotic diarrhea and aluminium salts cause constipation. To prevent this they both are mixed together in a single tablet to minimize the impact on bowel function.

Both magnesium and aluminium are excreted through the kidney. So patients with renal insufficiency should not take these agents long term.

Antacids should not be given within 2 hours of doses of other drugs/ Drug interaction:

Antacids may affect the absorption of other drugs. There are two mechanisms:

1.      By binding with the drug(reducing its absorption)
2.      By increasing the intragastric pH(affecting the drugs solubility especially weak basic or acidic drugs)

So antacids should not be given with 2 hours of doses of tetracyclines, fluroquinolones, itraconazole and iron.


Interaction
Possible cause
Antacid + Iron, Digoxin= Reduced bioavailability of iron and digoxin
Antacid prevents the absorption of iron and digoxin
Antacid + Tetracyclin = Reduced bioavailability of tetracycline
Reduced gastric absorption of tetracycline
Antacid + Warfarin = Reduced bioavailability of warfarin
Blockage of absorption of warfarin by antacid.

Properties of an ideal antacid:

1.      Should be palatable, non irritant, not toxic
2.      Should have good patient compliant, ie, should have:
a.       Quick onset of action
b.      Prolong duration of action
c.       Minimum or no adverse effects like metabolic acidity, diarrhea, constipation, rebound acidity.
3.      Should be cheap and easily available
4.      Should not interfere with the absoroption of other materials like food and drugs.

So the ideal antacid is the mixture of aluminium and magnesium hydroxide.

What is rebound acidity?




Pharmacological action of antacid:

1.      Acid neutralization (by chemical antagonism)
2.      Pepsin inactivation: Antacid neutralizaes the gastric acid and increase the intragastric pH to 3.5. At pH of 3.5 pepsin becomes inactivated
3.      Aluminium containing antacids coagulates the mucoprotein at the base of ulcer
4.      Prevents acid reflux to the lower part of the oesophagus
5.      Effective in prompt healing of duodenal ulcer


Indications/Clinical use:

1.      Hyperacidity
2.      Peptic ulcer disease(gastric and duodenal)
3.      Gastritis
4.      Reflux oesophagitis
5.      Non ulcer dyspepsia
6.      Ulcer due to stress

Adverse effects:
Systemic antacids
Non systemic antacids
1.      Metabolic alkalosis due to absorption of basic part.
1.      Constipation: Aluminium relaxed the smooth muscle of the gut thus reducing the gut motility
2.      Na containing antacids are dangeraous in patients with renal insuffiency and cardiac problems.
2.    Diarrhoe and acid rebound with magnesium antacids

3.      Constipation, hypercalcemia, milk alkali syndrome, cardiac dysarrythmia, encephalopathy, acid rebound with calcium antacids

Antacid Plus

Antacid plus contains aluminum hydroxide gel, magnesium hydroxide along with simethicone.
Al(OH)3 gel + Mg(OH)2 + Simethicone.

Simethicone is an antiflatulent agent.

Taken in an empty stomach the effect lasts for 20 to 40 minutes but taken one hour after meal the effect lasts for 2 to 3 hours.

Antacid Plus may be considered an ideal antacid as it has all the properties of and ideal antacid but it is contraindicated in pregnancy.

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