Formulation in Pharmacy Practice  - eMixt

Acetazolamide
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Acetazolamide is a weak acid with a pKa of 7.2.  Acetazolamide is very slightly soluble in water, but dissolves in solutions of alkali hydroxides.  Acetazolamide sodium, the injectable form, is freely soluble in water.  The injection has a pH of 9 - 10 and is chemically stable at 2 - 8°C for 7 days after reconstitution.
The optimum pH for stability is about 4 - 5.1,2  Alkaline solutions such as the injection are less stable.
Acetazolamide sodium can be given orally and this is a convenient method when small doses are required.

A New Zealand standardised formula is available Formula and Batch Sheet
This formula uses Ora-Blend/SF or Ora-Plus and Ora-Sweet/SF in equal parts, and has been approved by the Extemporaneous Compounding Pharmacists' Group.

Alternatives
Formula A uses commercially available suspending bases.  Formula B is a suggested alternative if these products are not available, and is based on published studies and stability information.1,2,3,4

Formula A3

Acetazolamide 25mg per mL
Acetazolamide tablets  250 mg 12 
*Vehicle to   120  mL
*Either:
1:1 mixture of Ora-Sweet and Ora-Plus,
1:1 mixture of Ora-Sweet SF and Ora-Plus, or
or Cherry Syrup.
Expiry:  30 days.
Stability:   60 days.  Potential for microbial growth not assessed.
Storage:  Refrigerate.
Shake well before use

 
Formula B

Acetazolamide 25mg per mL
 
Acetazolamide tablets  250 mg 10 
Methylcellulose 1% 50  mL
Glycerol or Syrup 20  mL
Parabens 0.1  %
*Citric acid qs 
Water to   100  mL
 
*If possible check the final pH.  If necessary, add sufficient citric acid to give a final pH of about 5.
Expiry:  30 days.
Storage:  Refrigerate.  Protect from light.
Shake well before use
 

Other Formulations
Alexander et al4 studied acetazolamide suspension formulated in a base of water, sorbitol, glycerol and syrup with carboxymethylcellulose and aluminium magnesium silicate as suspending agents.  The final preparation was adjusted to about pH 5.  This preparation was reported to be chemically stable for at least 79 days at 5 - 22°C.

REFERENCES
  1. Parasrampuria J., Das Gupta V.  Preformulation Studies of acetazolamide.  Effect of pH, two buffer species, ionic strength and temperature of its stability.  J Pharm Sci 1989; 78: 855-7.
  2. Parasrampuria J., Das Gupta V.  Development of oral liquid dosage forms of acetazolamide.  J Pharm Sci 1990; 79 (9): 835-6.
  3. Allen LV, Erickson MA.  Stability of acetazolamide, allopurinol, azathioprine, clonazepam, and flucytosine in extemporaneously compounded oral liquids.  Am J Health-Syst Pharm 1996; 53: 1944-9.
  4. Alexander K.S., Haribhakti R.P., Parker G.A.  Stability of acetazolamide in suspension compounded from tablets.  Am J Hosp Pharm1991; 48: 1241-4.

Formulation in Pharmacy Practice
Page Updated August 2011