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Total drug (ampicillin) present: If, in the constituted product, each 5
mL contains 125 mg of ampicillin, the total amount of ampicillin in the
100-mL product is:
3. Using the product in the previous example, if a physician desires an
ampicillin concentration of 100 mg/5 mL (rather than 125 mg/5 mL),
how many milliliters of water should be added to the dry powder?
Because it was determined that 2500 mg of ampicillin is in the dry
product, the volume of product that can be made with a concentration
of 100 mg/5 mL may be calculated by:
Then, because it had been determined that the dry powder occupies 22
mL of volume, it is possible to determine the amount of water to add:
4. The label of a dry powder for oral suspension states that when 111 mL
of water is added to the powder, 150 mL of a suspension containing
250 mg of ampicillin per 5 mL is prepared. How many milliliters of
purified water should be used to prepare, in each 5 mL of product, the
correct dose of ampicillin for a 60-lb child based on the dose of 8
mg/kg of body weight?
The dose of ampicillin may be determined by:
Then, the amount of ampicillin in the container is determined by:
Thus, the amount of product that can be made from 7500 mg of drug
such that each 5 mL contains 218 mg of drug may be found by:
Finally, because the volume of powder occupies 39 mL (150 mL – 111
mL), the amount of water to add is determined by:
5. The label for ERYPED-200 states that when 53 mL of purified water is
added to the powder, 100 mL of a pediatric suspension containing 200
mg of erythromycin ethylsuccinate per teaspoonful results. If a
pharmacist mistakenly added 43 mL of water to the suspension,
calculate the amount of erythromycin succinate per teaspoonful
resulting from the mistake. The amount of erythromycin ethylsuccinate
in the package is determined by:
The amount of volume displaced by the powder can be calculated as:
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The volume of the mistakenly reconstituted suspension would be
calculated as:
The concentration of erythromycin ethylsuccinate in the mistakenly
reconstituted suspension would then be:
CASE IN POINT 17.1
B
A pediatrician telephones a pharmacist asking that the concentration of
an antibiotic suspension be changed. The pediatrician wants the child–
patient to take 200 mg of amoxicillin per teaspoonful dose. The label for
amoxicillin powder for oral suspension indicates that the addition of 68
mL of purified water will result in a final volume of 100 mL with a
concentration of 250 mg amoxicillin per 5 mL of suspension.
How many milliliters of water should the pharmacist add to the
amoxicillin powder to produce a concentration of 200 mg/teaspoonful?
b
Problem courtesy of Flynn Warren, Bishop, GA.
Constitution of dry powders for parenteral
solution
Some medications intended for injection are provided as dry powder in vials
to be constituted with sterile water for injection or other designated solvent
or diluent immediately before use. Generally, these medications are smallvolume products intended for use by injection or as additives to largevolume parenterals.
In contrast to the dry powders intended for oral use after constitution,
injectable products may contain only limited amounts of specified added
ingredients to increase the stability and effectiveness of the drug (obviously,
no colorants, flavorants, or sweeteners are added). So, in effect, the bulk
volume of the dry contents of a vial is largely or entirely the medication.
If the quantity of the dry drug powder is small and does not contribute
significantly to the final volume of the constituted solution, the volume of
solvent used will approximate the final volume of solution. For example, if
1000 units of a certain antibiotic in dry form is to be dissolved, and if the
powder does not account for any significant portion of the final volume, the
addition of 5 mL of solvent will produce a solution containing 200
units/mL. However, if the dry powder, because of its bulk, contributes to the
final volume of the constituted solution, the increase in volume produced by
the drug must be considered, and this factor must then be used in calculating
the amount of solvent needed to prepare a solution of a desired
concentration. For example, the package directions for making injectable
solutions of piperacillin sodium specify that 4 mL of sterile solvent should
be added to 2 g of the dry powder to produce 5 mL of a solution that is to
contain 400 mg/mL. The drug, in this case, accounts for 1 mL of the final
volume.
Example calculations for the constitution of dry
powders for parenteral use
1. When a vial containing 3.5 mg of a sterile powder of the monoclonal
antibody bortezomib (VELCADE) is reconstituted with 3.5 mL of 0.9%
sodium chloride injection, a drug concentration of 1 mg/mL results.
Calculate the volume of injection occupied by the bortezomib powder.
1 mg/mL is equivalent to 3.5 mg/3.5 mL; thus, the volume occupied by
bortezomib may be considered negligible.
2. When a vial is reconstituted to a volume of 1.2 mL with sterile water for
injection, the resulting solution contains 20 mg/mL of drug. Calculate
the drug content of the vial.
3. Label instructions for the reconstitution of a 500-mg vial of ceftazidime
for intramuscular injection (FORTAZ) call for the addition of 1.5 mL of
diluent to prepare 1.8 mL of injection. Calculate (a) the volume
occupied by the dry drug; (b) the concentration of ceftazidime in the
injection, in mg/mL; and (c) the volume of injection to provide a dose
of 250 mg of ceftazidime.
a. 1.8 mL – 1.5 mL = 0.3 mL, volume of ceftazidime
b. 500 mg/1.8 mL = 277.8 mg/mL
c. 250 mg × 1.8 mL/500 mg = 0.9 mL
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4. The package information enclosed with a vial containing 5,000,000
units of penicillin G potassium (buffered) specifies that when 23 mL of
a sterile solvent is added to the dry powder, the resulting concentration
is 200,000 units/mL. On the basis of this information, how many
milliliters of sterile water for injection should be used in preparing the
following solution?
The package information states that the constituted solution prepared by
dissolving 5,000,000 units of the dry powder in 23 mL of sterile
solvent has a final concentration of 200,000 units/mL. The resulting
volume can be calculated as follows:
The dry powder, then, accounts for 25 mL – 23 mL = 2 mL of this
volume.
STEP 1. The final volume of the prescription is determined as follows:
STEP 2. 10 mL − 2 mL (displaced by dry powder) = 8 mL.
5. Piperacillin sodium is available in 2-gram vials, and the dry powder
accounts for 1 mL of the volume of the constituted solution. Using a 2gram vial of piperacillin sodium and sodium chloride injection as the
solvent, explain how you could fill the following medication order:
STEP 1. Dissolve the 2 g of dry powder in 9 mL of sodium chloride
injection to prepare 10 mL of solution. Each milliliter will contain 200
mg of piperacillin sodium.
STEP 2. Use 1.25 mL of the constituted solution and 13.75 mL of
sodium chloride injection.
A hospital pharmacist received the following order for a pediatric
patient weighing 32 kg:
Medication Order: Oxacillin sodium, 150 mg/kg/day IV in divided
doses every 6 hours
The following product and procedures were followed:
Product: 10-g vial oxacillin sodium
Label directions: Reconstitute vial with 93 mL of sterile water for
injection to yield oxacillin sodium, 100 mg/mL.
Pharmacy operations: For the infusion, add the calculated dose to 100
mL of sodium chloride injection. Administer over 30 minutes by
IV infusion.
a. Calculate the final volume of solution in the vial when
reconstituted according to the label directions and the volume
displaced by the powder.
b. Calculate the single dose for this patient.
c. How many milliliters of solution from the reconstituted solution
(vial) should be added to the sodium chloride injection for the
infusion?
d. If the pharmacist mistakenly reconstituted the vial with 63 mL of
water, what would be the resulting concentration in mg/mL?
Use of Prefabricated Dosage Forms in
Compounding
Pharmacists frequently find that bulk supplies of certain proprietary drug
substances are not available for extemporaneous compounding and that
prefabricated tablets, capsules, injections, and other dosage forms provide
the only available source of the medicinal agents needed.
When using commercially prepared dosage forms as the source of a
medicinal agent, the pharmacist selects products that are of the most simple,
economic, and convenient form. For example, uncoated tablets or capsules
are preferred over coated tablets or sustained-release dosage forms. For both
convenience and economy, use of the fewest dosage units is preferred; for
example, one 100-mg tablet rather than five 20-mg tablets. An injection
often provides a convenient source of medicinal agent when the volume of
injection required is small and it is compatible with the physical
characteristics of the dosage form being prepared (e.g., an oral liquid).
Occasionally, when of the prescribed strength, small whole tablets or
broken scored (grooved) tablets may be placed within capsule shells when
capsules are prescribed. In most instances, however, tablets are crushed in a
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mortar and reduced to a powder. When capsules are used as the drug source,
the capsule shells are opened and their powdered contents are expelled. The
correct quantity of powder is then used to fill the prescription or medication
order.
It is important to understand that in addition to the medicinal agent,
most solid dosage forms contain additional materials, such as fillers,
binders, and disintegrants. These ingredients may have to be considered in
the required calculations. For example, a tablet labeled to contain 10 mg of
a drug may actually weigh 200 mg or more because of the added
ingredients. Calculations involved in the use of injections generally are
simplified because injections are labeled according to quantity of drug per
unit volume, for example, milligrams per milliliter (mg/mL).
A factor to consider when using manufacturers’ dosage forms in
compounding is the uncertainty of the precise content of active therapeutic
agent. This is because there are legally allowable variances that, in some
cases, may be 90% to 110% of labeled drug content. Thus, whenever
possible, use of the bulk chemical in compounding procedure provides
better assurance of drug content.
Example calculations for the use of prefabricated
dosage forms in compounding
1. Only capsules, each containing 25 mg of indomethacin, are available.
How many capsules should be used to obtain the amount of
indomethacin needed in preparing the following prescription?
Because 2 mg/mL of indomethacin is prescribed, 300 mg is needed in
preparing the prescription. Given that each capsule contains 25 mg of
indomethacin, then 300 (mg) ÷ 25 (mg) = 12 capsules are needed.
2. The drug metoprolol tartrate (LOPRESSOR) is available as 50-mg
tablets. Before preparing the following prescription, a pharmacist
determined that each tablet weighed 120 mg. Explain how to obtain the
proper quantity of LOPRESSOR.
15 (mg) × 24 = 360 mg of LOPRESSOR needed
Crush 8 tablets, which contain:
400 mg (8 × 50 mg) of LOPRESSOR
960 mg (8 × 120 mg) of total powder
3. How many milliliters of an injection containing 40 mg of triamcinolone
(TMC) per milliliter should be used in preparing the following
prescription?
4. A pharmacist receives the following prescriptionc:
In filling the prescription, the pharmacist chooses to use 10-mg unscored
diazepam tablets.
a. How many tablets must be used?
b. If the tablets in answer (a) are powdered and weigh a total of 345 mg,
how many milligrams of the powder would provide the diazepam
required?
c. If the desired total weight for the contents of each chart (divided
powder) is 250 mg, how much lactose should be used as diluent?
Calculations:
a. 0.75 mg × 30 = 22.5 mg diazepam required.
22.5 mg/10 mg per tablet = 2.25 tablets, so 3 tablets must be used.
b.
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c. 250 mg × 30 = 7500 mg (total weight) – 258.75 mg (tablet powder
weight) = 7241.25 mg lactose
5. SJ is a 7-year-old female patient weighing 53 pounds. Her caregiver
brings the following prescription to the pharmacy:
In filling the prescription, the pharmacist chooses to use 0.2-mg clonidine
tablets.
a. How many clonidine tablets are required to compound the
prescription?
b. What volume of clonidine liquid will be required per dose?
c. If the medication is taken as directed, how many days will the
medication last?
Calculations:
a. 0.05 mg/mL × 60 mL = 3 mg of clonidine required
3 mg/0.2 mg per tablet = 15 tablets
b. 53 1b × 1 kg/2.2 lb = 24.09 kg
10 mcg/kg/day × 24.09 kg × 1 day/2 doses × 1 mg/1000 mcg = 0.12
mg clonidine required
0.12 mg × 1 mL/0.05 mg = 2.41 mL
c. 60 mL × 1 dose/2.41 mL × 1 day/2 doses = 12.45 days ≈ 12 days
c
Problem courtesy of Deborah Elder, Pharmaceutical and Biomedical Sciences, College of
Pharmacy, The University of Georgia, Athens, GA.
Following the directions for the compounding of an oral suspension
from oseltamivir phosphate (TAMIFLU) capsules,5 a pharmacist
determined that:
The suspension should have a drug concentration of 6 mg/mL
The volume to compound is based on the patient's weight, that is,
≤15 kg = 75 mL
16 to 23 kg = 100 mL
24 to 40 kg = 125 mL
≥41 kg = 150 mL
75-mg TAMIFLU capsules are to be used
Cherry Syrup or Ora-Sweet SF may be used as the vehicle
Specific compounding procedures as outlined in the reference should
be employed
5
How many 75-mg TAMIFLU capsules should be used in compounding
a suspension for a 30-kg patient?
If the prophylactic dose for a 30-kg patient is listed as 60 mg once
daily, how many milliliters of the compounded oral suspension
would constitute a dose?
Special Calculations: Capsule Filling and
Suppository Molding
Capsule filling
6
The extemporaneous filling of capsules enables the pharmacist to prepare
patient-specific doses of drugs in a conveniently administered form. Empty
capsule shells, made of gelatin, are readily available in a variety of sizes, as
shown in Figure 17.3, with size 000 being the largest and size 5 the
smallest.
FIGURE 17.3 Hard gelatin capsule sizes, from left to right: 000,
00, 0, 1, 2, 3, 4, and 5.
Filled capsules should be neither underfilled nor overfilled but should hold
the ingredients snugly. Different drug powders have different densities, and
thus different weights can be packed into a given size capsule (see Table
17.1). In filling a prescription or medication order, a pharmacist should
select a capsule size that accommodates the fill and will be easy for the
patient to swallow.
TABLE 17.1 CAPSULE SIZES AND APPROXIMATE FILL
CAPACITIES
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