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Question

Posted on: July 25 2013

A dermatologist asks us to make Dalacin lotion.

This is the following recipe: Salicylic acid 5g; Clindamycin 600mg (under the form of phosphate 712mg), propylene glycol 6g, aqua purificata 20g, isopropyl alcohol 120g In This solution a precipitation arises. We suspect by the presence of an acid and a base, which are not soluble at the same pH (?). We therefore suspect that we have to prepare a buffer since after adding ' a little base ' we get a transparent (colourless) solution (instead of a cloudy).

Answer

I immediately make the comparison with Clindamycne 1.5% solution of the TMF. This preparation contains: Clindamycin phosphate 540 mg, propylene glycol 3 g, water 10 g and isopropyl alcohol up to 30 g  

Dalacin Lotion contains 1% clindamycin in a mixture of propylene glycol, isopropyl alcohol, HCl, NaOH and water.

There is therefore a difference between the TMF formula and the speciality where the presence of HCl and NAOH indicate the setting of a certain pH.

This is confirmed by following information  

Clindamycin phosphate is a water soluble ESTER   of the semi-synthetic antibiotic produced by a 7 (S)-chloro-Substitution of the 7 (R)-hydroxyl group of the parent antibiotic lincomycin.

The solution contains isopropyl alcohol 50% v/V, propylene glycol, sodium hydroxide (to adjust The pH to between 4.0 – 7.0) and purified water. The structural formula is represented below:

 

 

By acidifying under the influence of the large amount of salicylic acid, the dissociation of the coupled phosphoric acid is reduced and the clindamycin phosphate becomes less soluble as the cloudy designates. Adding a small amount of base again brings the pH between pH   4 and 7. As base I would preferably use NaHCO3. Add small amounts (per 50 mg) to clear and note the added quantity for the following prescriptions.