In the development phase of all drugs, the active pharmaceutical ingredient (API) often dictates the type of dosage form that the final marketable product will be. It all comes down to how the API will remain stable in storage and of course, once it enters the body before it reaches its target release site. In order to ensure the stability of the final dosage form, formulation studies must be efficiently done.
Achieving a successful formulation study requires a systematic planning and design of the manufacturing process. To obtain such success, the following basic components must be considered:
- Physicochemical attributes of the API
- Compatibility of excipients
- Manufacturing procedure
The major concern of formulation studies is really on the interaction of the API and its excipients as these are hard to predetermine. The output of such studies (physical/chemical/therapeutic interactions) will then help dictate the final recipe for a certain drug dosage form.
Moreover, it must also be noted that pharmaceutical formulation studies must be free from any untoward microbial or chemical contamination as these events may change both the physicochemical and therapeutic properties of the formulation. To ensure this, preservatives are added both to prolong shelf life and to maintain sterility for a specific period of time.
The formulation process may be divided into:
- Process Development
Prior to the process development part, formulation studies involve understanding the biological API on the level of is amino acid sequence, biochemical and biophysical characteristics (pH), ionic strength, possible excipients, and even stability-indicating assays to check possible degradation pathways for the biologic under study.
Nowadays, the field of biologicals and biopharmaceuticals is growing rapidly so it is important to determine all potential issues in every stage of its development. Along with the rise of such drugs comes also the increase in complexity of formulating longer-acting formulations with larger injection volumes and longer injection durations. One of the major challenges faced by the biopharmaceutical industry is the stabilisation of the drug during processing. Main reason is that these substances are inherently unstable and prone to degradation, hence, a variety of excipients is required to stabilise aseptic/sterile biologicals during manufacturing processes.
On this note, formulation studies are critical as this is where the final excipients for drug stability, and of course efficiency, is determined. Finalizing the drug recipe at an early stage will also lessen operational costs as it can prevent drug product recalls and any unwanted adverse drug reactions once the drug is released in the market. Therefore, having the most suitable excipients combined with the selected API will dictate the success of developing a possible novel drug.
Excipients are a very important factor in formulation studies as they contribute in the development of both small and large molecule drugs. These are pharmacologically inert substances on their own, but offers tons of advatnages once combined with the API.
Historically speaking, such excipients were first used as a vehicle for the API or even a flavorant. But now, these substances have their own functional roles, and for large molecule manufacturing (vaccines and proteins), the offer the following advatnages:
- Enhance solubility of the API
- Enhance stability of the final product (during manufacturing and storage processes)
- Control pH and tonicity
- Stabilize conformation of the large molecules (vaccines and proteins) including exposure of the functional epitope
- Prevent degradation of the API
- Act as bulking agent
- Provide antioxidant properties
- Act as preservatives to the formulation
- Enhance therapeutic effect of the biological (adjuvant)
Understanding the mechanism of action on how the excipients stabilise biological products is key to a successful formulation study. The following factors can impact conformational stability:
- Electrostatic interactions
- Van der Waals interactions
As an example, a biological or large molecule drug in its native conformation (biologically active state) can be stabilised with the use of high concentrations of saccharides (sucrose, trehalose, lactose etch.) or a high concentration of polyhydric alcohols (sorbitol, mannitol etc.), all owing to their preferential exclusion from the surface of said biological.
Moreover, another class of excipients known as surfactants or surface active agents, can be added to prevent protein denaturation at interfaces of both air-water and solid-water, thus, enhancing stability via differential binding. In the prevention of both soluble and insoluble protein aggregation, on the other-hand, Polysorbate 20/80, or even Pluronic F68 may be used.
Additionally, for liquid formulations of multi-dose biological products, it is important to use antimicrobial preservatives to increase final product stability. Possible cases wherein the biological in an aqueous solution is still unstable, may force the inclusion of lyophilisation or freeze drying in the final manufacturing process; and this will dissuade the use for any preservative excipients even for multi-dose drugs. Such formulation will then be reconstituted via bacteriostatic water for injection (BWFI).
More often than not, adjuvants are used in the formulation of vaccines in order to enhance its therapeutic properties by increasing the stimulation of the body’s immune response.
Some vaccines are made from weakened microbes and these naturally contain adjuvants to promote a stronger immune response from the body. However, nowadays most of the developed vaccines constitute only a small component of the microbe/virus/bacteria rather than the whole thing, so adjuvants must be added to help generate a stronger immune response and overall, protect the person form contracting the disease any longer.
It must be noted however, that adjuvants may cause unwanted adverse reactions after vaccination such as:
- Pain at the injection site
- Fevers and chills
- Body aches
In the US, there are several types of adjuvants approved for use such as:
Several different adjuvants are used in U.S. vaccines.
Table 1. List of Adjuvants in US vaccines, CDC (link)
||One or more of the following: amorphous aluminum hydroxyphosphate sulfate (AAHS), aluminum hydroxide, aluminum phosphate,
potassium aluminum sulfate (Alum)
|Anthrax, DT, DTaP (Daptacel), DTaP (Infanrix), DTaP-IPV (Kinrix), DTaP-IPV (Quadracel), DTaP-HepB-IPV (Pediarix), DTaP –IPV/Hib (Pentacel), Hep A (Havrix), Hep A (Vaqta), Hep B (Engerix-B), Hep B (Recombivax), HepA/Hep B (Twinrix), HIB (PedvaxHIB), HPV (Gardasil 9), Japanese encephalitis (Ixiaro), MenB (Bexsero, Trumenba), Pneumococcal (Prevnar 13), Td (Tenivac), Td (Mass Biologics), Tdap (Adacel), Tdap (Boostrix)
||Monophosphoryl lipid A (MPL) + aluminum salt
||Oil in water emulsion composed of squalene
||Monophosphoryl lipid A (MPL) and QS-21, a natural compound extracted from the Chilean soapbark tree, combined in a liposomal formulation
||Cytosine phosphoguanine (CpG), a synthetic form of DNA that mimics bacterial and viral genetic material
||ActHIB, chickenpox, live zoster (Zostavax), measles, mumps & rubella (MMR), meningococcal (Menactra, Menveo), rotavirus, seasonal influenza (except Fluad), single antigen polio (IPOL), yellow fever
Vaccine in a final drug product, is composed of several ingredients each serving a specific purpose such as to provide immunity, to keep the vaccine safe and long lasting, and to produce a safe and efficient vaccine.
The Centers for Disease Control and Prevention (CDC) listed out the usual excipients included in the composition of a stable vaccine.
Table 2. Vaccine composition as per CDC (https://www.cdc.gov/vaccines/vac-gen/additives.htm)
||Thimerosal (only in multi-dose vials of flu vaccine)*
||To prevent contamination
||To help boost the body’s response to the vaccine
||To keep the vaccine effective after manufactured.
|Residual cell culture materials
||To grow enough of the virus or bacteria to make the vaccine
|Residual inactivating ingredients
||To kill viruses or inactivate toxins during the manufacturing process
||To prevent contamination by bacteria during the vaccine manufacturing process
- Centers for Disease Control and Prevention. (2019). Vaccines & Immunizations. Retrieved from: https://www.cdc.gov/vaccines/vac-gen/additives.htm
- Centers for Disease Control and Prevention. (2019). Vaccines for Your Children: Making the Vaccine Decision. Retrieved from: https://www.cdc.gov/vaccines/parents/why-vaccinate/vaccine-decision.html
- Centers for Disease Control and Prevention.(2020). Vaccine Safety: Adjuvants. Retrieved from: https://www.cdc.gov/vaccinesafety/concerns/adjuvants.html#:~:text=An%20adjuvant%20is%20an%20ingredient,adjuvants%20help%20vaccines%20work%20better.
- Crommelin, D., Hawe, A., & Jiskoot, W. (2019). Formulation of Biologics Including Biopharmaceutical Considerations. Pharmaceutical Biotechnology, 83-103. doi: https://doi.org/10.1007/978-3-030-00710-2_5
- Medi, B., Chintala, R., & Bhambhani, A. (2014). Excipient selection in biologics and vaccines formulation development. Eurpoean Pharmaceutical Review. Retrieved from: https://www.europeanpharmaceuticalreview.com/article/24136/excipient-selection-biologics-vaccines-formulation-development/
- Pasquale, A., Preiss, S., Silva, F., & Garcon, N. (2015). Vaccine Adjuvants: from 1920 to 2015 and Beyond. Vaccines (Basel), 3(2), 320-343. doi: 10.3390/vaccines3020320
- ScienceDirect. (n.d.). Pharmaceutical Formulation. Retrieved from: https://www.sciencedirect.com/topics/medicine-and-dentistry/pharmaceutical-formulation
- Thomas, F. (2019). Formulation and Development Considerations for Biologics. PharmTech. Retrieved from: https://www.pharmtech.com/view/formulation-and-development-considerations-biologics
- U.S. Food and Drug Administration. (2020). Inactive Ingredient Search for Approved Drug Products. Retrieved from: https://www.accessdata.fda.gov/scripts/cder/iig/index.cfm