Pharmaceutical Compounding Calculations, Part One–Non-Salts, Salts, and Policy-Exception Salts

Drug calculations for the formulation of dosage forms and associated preparations are key activities in pharmaceutical compounding. Calculations determine the amount of drug identified in the request for compounding. Correct drug calculations for compounded preparations are essential; if drug calculations are incorrect, resulting preparations will be incorrect—potentially with significant therapeutic and technical consequences.

This paper is the first in a series of discussions that address drug calculations and related topics for small-molecule compounding. Series discussions are applicable to essentially all compounding in 503A pharmacies, 503B outsourcing facilities, pharma industry R&D including clinical drugs, non-product laboratory preparations, and other applications. Series discussions will reflect academic teaching and compounding practice experiences. Textbooks traditionally provide the foundation for calculations (1–3); content in this paper addresses workplace applications and potential hazards (i.e., what might go wrong?) (4). Concepts that are often omitted, confused, or otherwise misunderstood that affect the accuracy of calculations are described. Topics address small-molecule compounding; large-molecule biologic drugs are not compounded (5,6).

Part one in the series addresses terminology, regulations, and labeling respective drug types. Subsequent topics will include numeric calculations, moisture, material losses, and related topics that build on prior publications (7,8). Drug calculations begin with the medication order—individual prescription, hospital order, 503B outsourcing batch, pharma clinical supply request, research formulations, or other request for a compounded preparation. The medication order identifies the desired active moiety by proprietary or generic name along with the duration of treatment. Types of drugs used in compounding are identified. Compounding personnel must select an appropriate drug for compounding—there may be multiple choices—and calculate its amount. Drug selection must integrate patient considerations, physicochemical properties, material handling, and other applicable considerations. After drug selection, calculations are executed; each drug requires different calculations. Post calculations, labeling must clearly define the compounded preparation. Figure 1 shows an overview of calculations-related activities from medication order initiation through final dosage form dispensing and/or shipping; specific part one discussion topics are highlighted in red.

Terminology

Drugs. Drugs are known by a variety of terms depending on regulatory agency, country, site vernacular, and other considerations. Some of the more familiar terms include API, active substance, drug substance, bulk drug, bulk drug substance, active, bulk, and so on. “API” terms are more common in pharma industry (e.g., International Council for Harmonisation (ICH) Q7A Good Manufacturing Practice Guidance for Active Pharmaceutical Ingredients [9]); “bulks” language is common in compounding discussions (e.g., Bulk Drug Substances Used in Compounding Under Section 503A of the US Food, Drug and Cosmetic Act (FD&C Act) [10]; Bulk Drug Substances Used in Compounding Under Section 503B of the FD&C Act [11]).

Active moiety. The active moiety is the part of the drug responsible for pharmacologic activity. In theory, the active moiety excludes molecule portions such as esters, counterions, complexes, chelates, clathrates, and waters of hydration that do not contribute to drug activity. The active moiety is contained within the drug molecule; it may be part or all of the molecule (e.g., diazepam is generally considered to be the active moiety and active drug in Valium). The distinction between active moiety and active drug is critical for understanding calculations and labeling in salt drugs.

Non-salt drugs. This discussion identifies several drug types relevant to compounding calculations; these are categorized per chemical structure as non-salt drugs and salt drugs. Non-salt drugs are also termed free acids or free bases; non-salt drugs have covalent and other non-ionic bonds. For compounding purposes, the active moiety of a non-salt drug comprises the entire drug molecule (i.e., active moiety is same as active drug).

Salt drugs. Salt drugs are molecules with ionic bonds. Salt drugs are easily recognized by name (e.g., sodium drug, potassium drug, drug hydrochloride, drug acetate, and others that include counterions). Some non-salt drugs may be confused as salts (e.g., clindamycin phosphate is an ester; clindamycin palmitate hydrochloride is a salt of an ester). Reviewing the molecule structure is essential to determine the drug type. Some drugs may be available as either non-salt (covalent) molecules or as salts (e.g., phenobarbital and sodium phenobarbital; codeine and codeine phosphate), and either may be used in compounding with appropriate evaluation; their respective properties (i.e., pharmacologic and physicochemical) must be considered.

Policy-exception salts. Another type of salt drug, policy-exception salts, has also been identified. These molecules are specified in the United States Pharmacopeia (USP)/FDA salt policy and are dosed differently than salt molecules. Policy-exception salts require different calculations and labeling than salt drugs that comply with policy. Recognizing salt molecule structures is also important in electrolyte calculations.

Peptides. Peptides may be small-molecule drugs or biologics depending on their amino acid chain length (<40 amino acids are small molecules; >40 amino acids are biologic proteins). Peptides have covalent bonds between amino acids as well as ionizable groups and salts. FDA is currently reviewing data to definitively classify peptides within compounding bulk lists (12,13).

Drug forms. This discussion also identifies several drug forms relevant to compounding calculations. Forms describe the concentration of drug moiety in the drug. Forms may be bulk solids assumed to be essentially 100% potent or pure; pharma industry and 503B outsourcing facilities commonly use bulk drugs for compounding. Forms may also be drugs at some concentration in solid or liquid mixtures; 503A hospital and community pharmacies are more likely to use drug mixtures (e.g., commercial sterile IV additives and commercial tablets) as drug sources for individual unit compounding. Commercial products used in compounding also contain functional formulation ingredients (filler, binder, coating, flavor, color, etc.) (14). These ingredients must be evaluated before use; they may cause undesirable effects on patients (e.g., allergic reactions, lactose intolerance), be restricted by religious beliefs (15), or have incompatible physicochemical properties. Part two in this series will address numeric bulk drug calculations; part three will address numeric drug mixture calculations.

Atomic mass. The atomic mass (atomic weight) is the mass of an individual element expressed in atomic mass units (amu) or daltons (Da) (e.g., sodium = 22.99 amu). Atomic masses represent protons and neutrons in the atom structure; the atomic number represents protons; atomic mass decimals represent atom isotopes. The molecular mass (molecular weight) (MW) of a compound is the combined masses of individual elements, hydrates, solvates, and other molecule components (e.g., sodium chloride MW = Na + Cl = 58.44 amu). The active moiety molecular mass may be calculated based on the atomic weights of the individual elements in the active moiety. Also relevant is the number of active moiety components in the drug molecule (e.g., divalproex sodium contains two valproic acid active moieties). The molecular masses of the active moiety and drug are correlated to their respective gravimetric weights by calculating moles (mols) and millimoles (mmols). A mole is the weight in grams of the drug molecular mass (e.g., 58.44 grams = 1 mole of NaCl; 58.44 mg = 1 mmol of NaCl). Understanding moles is important to determine equivalent dosages of different drug salts.

Non-proprietary name. The most relevant drug name in compounding is the non-proprietary name. The non-proprietary name is the generic name of the drug; diazepam is a non-proprietary name. Non-proprietary names are approved by country regulatory agencies and the World Health Organization (WHO) and usually include a drug-name stem; (e.g., -caine [local anesthetics], -olone [steroids], and many others) (16). Proprietary drug names are commercial marketing names; Valium is a proprietary name.

Pharmaceutical alternatives. Pharmaceutical alternatives are drugs that have the same active moiety but are a different molecule (i.e., different salts or esters). Metronidazole is an active moiety; metronidazole hydrochloride and metronidazole benzoate are pharmaceutical alternatives. The same moles of pharmaceutical alternatives have the same pharmacologic effect (e.g., same moles of codeine, codeine sulfate, and codeine phosphate have different gravimetric weights but same pharmacologic effect). Pharmaceutical alternatives used for compounding must be approved drugs—not unapproved synthetic salts. Different drug salts may have different properties (solubility, pKa, lipophilicity, hygroscopicity, others [17]); substituting pharmaceutical alternatives requires evaluation of respective physicochemical properties. Pharmaceutical alternative dosages are determined using molarity calculations. Prodrugs are related but are different chemical compounds; prodrugs per se are not pharmaceutical alternatives; prodrugs are compounds that convert to an active moiety drug in the body.

Several other similar terms may be confused or misused when discussing pharmaceutical alternatives. Pharmaceutical equivalents are generic drugs—same drug, same dose, same route of administration, but different appearance. Therapeutic equivalents are the same as pharmaceutical equivalents (generic drugs) that have been tested for bioequivalence. Therapeutic alternatives, therapeutic interchange, and therapeutic substitution are related terms associated with drug substitution with same clinical effects as authorized through a hospital formulary (18).

Regulations

There are numerous regulatory documents addressing product naming, dosage, and labeling for commercial products that are relevant to compounding (19,20). Respective 503A and 503B bulk drug lists identify approved drugs. Specific compounded dosage form labeling requirements are exempt from FD&C regulations identified in Section 503 (21). 503B compounded preparations must state “This is a compounded preparation” or similar wording (22). Selection, naming, and labeling of compounded dosage forms containing non-salt (covalent) active moieties/drugs is straightforward (23–25); non-salt active moieties are generally identical to their drug. Selection, naming, and labeling of compounded dosage forms containing active moieties that are salt molecules is more complex.

Salt (ionic) drugs. The 2013 USP Salt Policy (26) is the basis for FDA guidance (27,28) on calculating and naming salt drugs. The FDA guidance states:

“1. When a drug in a drug product is a salt, the drug product monograph title will contain the name of the active moiety (or neutral form) and not the name of the salt (e.g., ‘newdrug tablets’—not ‘newdrug hydrochloride tablets’).

2. The strength also will be expressed in terms of the active moiety (e.g., ‘100 mg newdrug’) rather than the salt strength equivalent (e.g., ‘123.7 mg newdrug hydrochloride’).

3. If the name and strength of a drug product are expressed in terms of the active moiety, the full name and full strength (or proportion, if CDER [Center for Drug Evaluation and Research] has determined proportion is more appropriate) of the drug (e.g., salt) will appear elsewhere on the drug product label and labeling.

4. The USP Salt Policy provides for exceptions to the ‘active moiety’ naming approach when the name of the salt conveys vital information from a clinical perspective. In these cases, the drug product monograph title will include the name of the salt, and the strength of the drug product also is expressed in terms of the salt form (drug).

5. USP does not anticipate changing existing monograph titles unless necessary for safety. USP and FDA have agreed to coordinate regarding any necessary retrospective name changes.”

Policy-exception salts. FDA expects that new chemical entities that are salts will be named using the active moiety in accordance with the USP Salt Policy described above. However, the FDA guidance also identifies exceptions (#4 above) to this approach. Therapeutic and historic reasons are cited as justification for exceptions. Many historic drugs were dosed according to their drug weights including counterions (e.g., atropine sulfate, codeine phosphate, morphine sulfate). The following are salt drug policy-exceptions per FDA guidance (28):

• Drug is a simple salt and administration is therapeutically important (e.g., lithium carbonate, iron sulfate and other intravenous iron, calcium gluconate and other calcium salts, potassium chloride, magnesium sulfate, and other common intravenous additives).
• Scientific evidence demonstrates the salt form affects the absorption, distribution, metabolism, and excretion (ADME) that influences product selection.
• Clinically significant amounts of cations (sodium, potassium, magnesium, calcium) accompany the active moiety of a drug product related to the daily electrolyte intake in special populations (e.g., sodium in cardiac treatment and potassium in renal treatment).
• There is significant evidence-based safety concern that the counter-ion part of the salt could cause acid-base disturbances; hepatic, renal, or other organ damage; or hypersensitivity reactions.
• The name of the salt is necessary to maintain consistency with other dosage forms of the same drug (salt). For example, drug products approved before 2013 with the salt in its established name and dosage based on the salt form would not change naming for a new dosage form with the same salt drug.
• The USP Salt Policy should not be applied if there are relevant, documented safety reasons in a closely related product.

Labeling

Compounded dosage forms are exempt from commercial product labeling requirements. 503A compounded dosage form labeling is also governed by state pharmacy regulations. Accurate compounded product labeling requires attention to detail (i.e., labeling “codeine” is not the same as labeling “codeine phosphate”).

Non-salt drug labeling. The labeling of covalent compounded preparations is straightforward. Covalent active moieties are often the drugs in compounded dosage forms. The stated amount of drug must clearly identify the name and drug amount (e.g., Diazepam 10 mg tablets). Approved commercial product labeling containing the compounded drug may be checked to verify equivalence of active moiety and drug in labeling.

Salt drugs labeling. FDA guidance documents illustrate labeling of dosage forms containing salt drugs. The following examples illustrate the compounding dilemma involving salt drugs—are salt drugs labeled according to their active moiety or bulk drug? Figure 2 illustrates a salt drug label with active moiety as drug name. This label represents product in compliance with the drug salt policy (i.e., active moiety is the focus of compounding and associated calculations; ionic non-active counterions are in secondary labeling). Labeling of this type would have been approved after USP salt policy was implemented. Note active moiety on primary label and the drug (larger number) on side panel of label. Naming and labeling of salt drugs is applicable to compounded dosage forms.

Policy-exception salt labeling. Figure 3 illustrates a policy-exception salt drug label with drug notation on the primary label and active moiety calculation in the side panel (i.e., opposite of Figure 2). This label demonstrates labeling for a pre-2013 approved salt drug product—the entire salt drug amount (Olddrug palmitate) is dosed as the active moiety per USP salt-guidance exceptions. This approach is applied to compounded dosage forms involving drugs identified as exceptions to the salt policy.

Calculations hazards

Several considerations are potential hazards to drugs selection, calculations, and labeling in compounded dosage forms. Understanding drug chemistry is fundamental. Misunderstanding of the USP salt policy causing dosage errors and labeling errors is an obvious problem. Labeling on approved commercial products may not be consistent with examples described previously.

Drug chemistry. Compounding personnel involved with the selection of drugs for compounding must have a good understanding of basic drug chemistry; molecular masses, moles, structures, and associated topics are relevant in compounding calculations. Selection of a drug is critical; molecule physicochemical properties may be relevant to compounding (e.g., valproic acid is an insoluble oil and divalproex sodium is a water-soluble solid). The choice of drug and evaluation thereof must be based on technical judgment. Regulations addressing drugs in compounding are also noteworthy; FDA has cautioned regarding use of semaglutide (Ozempic), semaglutide sodium, and semaglutide acetate; the aforementioned salt forms are not FDA-approved (29,30). FDA has also reminded compounding organizations to utilize sterile drugs when compounding sterile dosage forms (31).

Drug evaluation. Drug information and labeling may be confusing, inconsistent, or have unusual formats; approved labeling may have been formatted before the USP salt policy became official. Practitioners may incorrectly assume differences between active moieties and drugs are inconsequential; drug dosage errors in salt drugs will depend on the respective molecule masses of active moiety and counterion. Worst-case molecule structure affecting dosing would be a small molecular weight active moiety and a large inactive counterion (sulfate, phosphate, mesylate, etc.). Depending on drug properties, even small-molecule counterions (hydrogen, sodium, potassium) may be significant; the phenytoin package insert cautions that phenytoin and sodium phenytoin have an 8% difference in dosage effects (32). Even when clinical effects are calculated to be minimal; calculations will be incorrect if the counterion molecular weight is overlooked.

Drug overdose. Drug overdoses occur when a drug exception is erroneously calculated to provide the active moiety amount as directed by the USP salt policy (i.e., salt policy is followed when it is not appropriate). For example, if codeine 30 mg was ordered and codeine phosphate drug amount was calculated as a new drug, the codeine phosphate calculated value (40 mg) would be 33% high. Codeine analgesic dosage historically has utilized 30 mg codeine phosphate or codeine sulfate—not codeine base (33,34).

Drug underdose. Drug underdoses occur when the drug is utilized as the active moiety without calculating the equivalent active moiety amount. For example, if clopidogrel 75 mg was ordered for compounding, but clopidogrel bisulfate was used, the clopidogrel active moiety dose (58 mg) would be 23% low.

Individual compounded dosage forms must be correctly labeled reflecting the active moiety and drug content. Labeling “codeine” when dosage and content is “codeine phosphate” is not accurate. Label space limitations may encourage labeling shortcuts that are inaccurate and incorrect. The Institute for Safe Medication Practices (ISMP) has identified labeling problems in compounded dosage forms and has recommended increased FDA guidance on compounded dosage form labeling (35). Drug shortages causing purchase of uncommon concentrations (e.g., heparin) have contributed to errors including fatalities (36) when personnel do not recognize concentration changes and do not recalculate drug amounts.

Labeling variation. Compounding personnel must be wary of inconsistent, confusing, or non-standard formats in labeling (Table I) that do not follow the FDA format described above. For example, metoprolol tartrate (Lopressor) is an immediate-release beta blocker that is dosed twice daily; metoprolol succinate (Toprol XL) is an extended-release metoprolol preparation formulated for once-daily dosage. Secondary labeling on metoprolol succinate products (37–39) references metoprolol tartrate to aid in transitioning patient dosage. Potassium phosphates injection (policy-exception drug) is labeled as phosphorus mmol/mL and potassium mEq/mL on the primary label and mg/mL amounts of potassium phosphate monobasic and potassium phosphate dibasic equivalent amounts of potassium and phosphate on the side panel (40,41). Succinylcholine labeling may be inconsistent or different format (e.g., same concentration products may be labeled as total drug mg/vial (42–44). These labels were likely approved before the salt policy was implemented and have not been updated.

Summary

Part one of this series addresses pharmaceutical principles including terminology, regulations, and dosage form labeling in compounded preparations. Types of drugs are identified. Non-salt (covalent) drugs and salt (ionic) drugs are primary groups; certain salt drugs are further grouped as policy-exception salts. Policy-exception salts are dosed as non-salt drugs including their molecule counterions; labeling must identify the equivalent active drug amounts. Forms of drugs are also identified. Drug formsmay be solids or liquids; they may be bulk drugs or drugs in mixtures; mixtures may include solutions, mixtures of solids, and commercial products (e.g., tablets). Table II summarizes the drug types, dosage rationale, forms, active moiety identification, primary labeling, and secondary labeling. The USP salt policy, historical precedent, and identified exceptions are the bases for these requirements; note significant labeling differences between salt drugs and policy-exception salts. Problems associated with the above are identified. Effects resulting from misunderstanding of drug salt principles may include overdosing or underdosing of dosage form active moiety. Mislabeling of dosage form content is also possible; attention to detail is critical. Labeling may have variations for a variety of reasons. Careful reading of commercial product labeling with good understanding of principles is mandatory for compounded product applications.

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Acknowledgments

Comments from Alan M. Mancini, RPh, pharmaceutical educator; Richard Poska, RPh, Flexo CMC Consulting; Jan M. Keresztes, RPh, PharmD, pharmaceutical educator; and William R. Porter, PhD, pharmaceutical educator and principal scientist, Peak Process Performance Partners, are greatly appreciated.

About the authors

Paul L. Pluta, RPh, PhD is a pharmaceutical scientist with pharmaceutical industry, academic teaching, journal editorship, community pharmacy, and hospital pharmacy experiences.

Nishant B. Thakar, RPh, PharmD is associate professor of Clinical Sciences, Roosevelt University College of Science, Health, and Pharmacy, Schaumburg, IL, USA.

Varanya Chaiyaperm, RPh, PharmD is clinical assistant professor, University of Illinois at Chicago Retzky College of Pharmacy, Chicago, IL, USA.