Question
Gepost op: 29 März 2026How can a 100 ml preparation of 4% erythromycin in a 1% hydroxypropyl methylcellulose (HPMC) solution be formulated to avoid precipitation, given that erythromycin is poorly soluble in water and a precipitate forms when the 1% HPMC solution is added to a 1:5 erythromycin-ethanol solution?
Answer
Gepost op: 29 März 2026Discussion of the preparation
For the preparation, reference can be made to the 4% erythromycin preparations described in the Therapeutic Magistral Formulary (TMF). These indicate that approximately 50% ethanol must be present in the preparation. The remaining 50% consists of the HAC 1% solution.
A second possibility is to ensure that the concentration of the active ingredients in the delivered preparation matches the concentration in 100 ml of HAC 1%. To achieve this, 150 mg of cetrimide and 15 mg of chlorhexidine digluconate (or 75 mg of 20% chlorhexidine digluconate solution) are added to the water used to dilute the ethanolic erythromycin solution.
Because the latter quantity cannot be accurately weighed, 20 ml of a 1% concentration is prepared first (diluting 1 g of solution to 20 ml). From this dilution, 1.5 g is then used.
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