Potency Calculation of API in Manufacturing Formula

The purpose of this topic is to provide information to assist pharmacists and support personnel in performing the necessary calculations for compounding and dispensing materials for production.

The pharmacist must be able to calculate the amount or concentration of API in each unit or dosage portion of a compounded preparation at the time it is prepared and again at the time it is dispensed. 

Pharmacists must perform calculations and measurements to obtain, theoretically, 100% of the amount of each ingredient in compounded formulations. Calculations must account for the active pharmaceutical ingredient or active moiety, and water content of drug substances, which includes that in the chemical formulas of hydrates.

Potency calculation of Active pharmaceutical ingredients is the most common activity which is performed on the daily basis by pharmacists in the pharmaceutical industry. It is a common observation that the majority of pharmaceutical professionals don't know how to calculate the potency or assay of an active pharmaceutical ingredient.

What is API (Active Pharmaceutical Ingredients)
  • API is an Active pharmaceutical ingredient that gives pharmacological or therapeutic activity.
  • Only API is the element in a complete dosage form that gives effect to cure, treat or prevent disease. All other ingredients are called excipients which have no pharmacological effect.
  • API is a pharmacological agent so its quantity should always be measured in a precise manner.
  • API is delivered according to the label claim.
  • To calculate the potency of API following factors are considered.
  1. Moisture Content of API
  2. Salt removal or factor adjustment.

Moisture Content
  • The amount of moisture content in the API is checked in the certificate of Analysis of that specific API and its adjustment is made to maintain its efficacy.

Salt removal or factor adjustment.
  • Some active pharmaceutical ingredients are used in salt form like Metformin HCl and some are used without salt like Sitagliptin Phosphate in which the phosphate factor is removed from sitagliptin.

Question: How will we know when to use API with salt and when to use API without salt?

Answer: we have to check or read the label claim. If the label claims that each tablet contains Sitagliptin phosphate then we do not need to remove the salt form and if the label claim states that each tablet contains Sitagliptin "as phosphate" then we have to remove the salt form from API.

  • Many active pharmaceutical ingredients give pharmacological effects in salt form and some APIs give effects without salt. The attached salt is used just as a carrier to improve solubility or dissolution etc.
  • There are such cases in which the required amount of a dose is specified in terms of a cation (e.g., Li+), an anion (e.g., F−), or a molecule (e.g., theophylline in aminophylline).
  • In these instances, the drug substance weighed is a salt or complex, a portion of which represents the pharmacologically active moiety. Thus, the exact amount of such substances weighed must be calculated on the basis of the required quantity of the pharmacological moiety.
  • The following formula may be used to calculate the theoretical weight of an ingredient in a compounded preparation:
W = actual weighed amount
A = prescribed or pharmacist-determined weight of the active moiety of the drug
B = molecular weight (MW) of the ingredient, including waters of hydration for hydrous ingredients
C = MW of the active drug that is provided in the MW of the weighed ingredient
D = the fraction of dry weight when the % by weight of adsorbed moisture content is known from the loss on drying procedure or from the CoA. The CoA should be lot specific.

Potency Calculation of API in Manufacturing Formula

If the % Assay of API is less than 100%, adjust the Quantity as per the below Equation,

Extra Quantity of API dispensed should be compensated with an equivalent deduction in the Quantity of Excipient.

NOTE: Water content of API already considered while arriving at results of % Assay of API on a dried basis.

Calculation of Drugs dosed as salt form and hydrate

1. Drugs dosed as salt form and hydrate
  • Triturate morphine sulfate and lactose to obtain 10g in which there are 30 mg of morphine sulfate for each 200mg of the morphine–lactose mixture. 
[NOTE: Morphine is dosed as morphine sulfate, which is the pentahydrate.]

W = weight of morphine sulfate (g)
A = weight of morphine sulfate pentahydrate in the prescription, 1.5 g
B = MW of morphine sulfate pentahydrate, 759 g/mol
C = MW of morphine sulfate pentahydrate, 759 g/mol
D = 1.0
To solve the equation:
W = (1.5 g × 759 g/mol) / (759 g/mol × 1) = 1.5 g of morphine sulfate pentahydrate

2. Active drug moiety and correction for moisture
Accurately weigh an amount of aminophylline to obtain 250mg of anhydrous theophylline. 
[NOTE: The powdered aminophylline dihydrate weighed contains 0.4% w/w absorbed moisture as stated in the CoA received.]


W = weight of aminophylline dihydrate (mg)
A = weight of anhydrous theophylline, 250 mg
B = MW of aminophylline dihydrate, 456 g/mol
C = MW of anhydrous theophylline, 360 g/mol
D = 0.996
[NOTE: One mol of aminophylline contains 2 mol of theophylline. Theophylline has an MW of 180.]

To solve the equation:
W = (250 mg × 456 g/mol) / (360 g/mol × 0.996) = 318 mg of aminophylline dihydrate

Calculation of Hydrates, salts, and esters

1. Hydrates
  • If production for 100g of lidocaine hydrochloride 2% gel is to be made, 2 g of anhydrous lidocaine hydrochloride could be used.
  • The equivalent amount of lidocaine hydrochloride monohydrate could be calculated as follows:
W = weight of lidocaine hydrochloride monohydrate (g)
A = weight of anhydrous lidocaine hydrochloride in the prescription, 2 g
B = MW of lidocaine hydrochloride monohydrate, 288.81 g/mol
C = MW of anhydrous lidocaine hydrochloride, 270.80 g/mol
D = 1.0

To solve the equation:
W = (2 g × 288.81 g/mol) / (270.80 g/mol × 1) = 2.133 g of lidocaine hydrochloride monohydrate

2. Salts
  • A production calls for 10 mL of a fentanyl topical gel at a concentration 50 μg fentanyl/0.1 mL prepared from fentanyl citrate. 
  • The amount of fentanyl citrate required for the preparation could be calculated as follows:
Amount of fentanyl needed for the preparation: (50 μg fentanyl/0.1 mL) × 10 mL = 5000 μg of fentanyl

W = weight of fentanyl citrate in the prescription (μg)
A = weight of fentanyl in the prescription, 5000 μg
B = MW of fentanyl citrate, 528.59 g/mol
C = MW of fentanyl, 336.47 g/mol
D = 1.0

To solve the equation:
W = (5000 μg × 528.59 g/mol)/(336.47 g/mol × 1) = 7855 μg of fentanyl citrate

3. Esters
The amount of cefuroxime axetil contained in a single 250mg cefuroxime tablet can be calculated as follows:

W = weight of cefuroxime axetil in tablet (mg)
A = weight of cefuroxime in the prescription, 250 mg
B = MW of cefuroxime axetil, 510.47 mg/mmol
C = MW of cefuroxime, 424.39 mg/mmol
D = 1.0

To solve the equation:
W = (250 mg × 510.47 g/mol)/(424.39 g/mol × 1) = 300 mg of cefuroxime axetil

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