Case - Ointment from Injection

3. Percent, Ratio, mg/mL, PPM, PPB 3.5) Cases 3.5.1) Cases Case - Ointment from Injection Hard 1

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120 g of an ointment needs to be compounded containing 4.4% of a hydrophobic drug. The pharmacist does not have the pure form of the drug, but has an injection which he is considering to use as the source of the drug. The injection uses sesame oil as the vehicle, and his ointment will be made using cold cream base. Therefore, he concluded that he could compound the product by incorporating some amount of liquid injection into some amount of ointment base to give it the desired final weight of 120 g. The label of the injection states that 'Each 1 mL contains 115 mg of drug'. The literature search revealed that the injection has a specific gravity of 0.63.

1. How many grams of the hydrophobic drug are needed to fill the prescription?
2. How many milliliters of the liquid injection would provide the correct amount of the drug?
3. In order to compound the product, how much cold cream base does the pharmacist need?

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lb equals `5.28g; 45.91 mL and 91.07 g` kg

    The ointment is a semisolid preparation and both of the weights are expressed in g unit. Therefore, the strength should be in percent `w/w`.
    The sign `4.4\quad%` means `4.4\quad g` of drug is present per `100\quadg` of the ointment.
    Thus, `(4.4\quad g)/(100\quad g) = x/(120\quad g)`, or, `x = 5.28\quad g`. (Ans 1)

    Now, this `5.28` g amount of the drug needs to be collected from liquid injection, where each `1\quad mL` contains `115\quad mg` of the drug.
    Thus, `(1\quad mL)/(115\quad mg)×(1000\quad mg)/(1\quad g)× 5.28\quad g = 45.91\quad mL` of the liquid injection will be needed. (Ans 2)

    In order to determine the weight of the cold cream base to be used, we first need the weight of the liquid injection, which will be deducted from `120\quad g` later.
    Using the concept of specific gravity, we know:
    `SG = (weight\quad i n\quad g)/(volume\quad i n\quad mL)`
    Thus, `Weight\quad i n\quad g = SG × (volume\quad i n\quad mL)`
    `= 0.63 × 45.91 = 28.93\quad g`.
    Subtract this weight from the desired weight of the finished product (which is `120\quad g`).
    Weight of ointment base = `120 - 28.93 = 91.07\quad g` (Ans 3)

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