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11.4 Excipient applications in formulation design
and drug delivery
Excipients are inactive components of a pharmaceutical composition that improve its
bioavailability, stability, and patient acceptance without affecting the medicine’s thera-
peutic effect. The design and development of an effective medication delivery system
relies heavily on these excipients. Binders, disintegrants, lubricants, fillers, surfactants,
and solvents are all examples of excipients with specific purposes [26]. This paper will
focus on the use of excipients in pharmaceutical formulation and administration. Exci-
pients play an important role in keeping drug formulations stable during handling, ship-
ping, and storage. The physical and chemical stability of a pharmacological formulation
can be preserved by using excipients to shield the medicine from oxygen, moisture, and
light. In order to keep medications from degrading due to oxidation, antioxidants, in-
cluding ascorbic acid, tocopherol, and butylated hydroxyanisole (BHA), are frequently
utilized. Desiccants like silica gel, magnesium oxide, or calcium oxide are used in the
formulation of moisture-sensitive medications for the same reason [27 ]. To improve
drug solubility and bioavailability is another crucial role of excipients. Low solubility
reduces the bioavailability and effectiveness of many medications. Poorly soluble medi-
cations can be made more soluble and dissolve more quickly with the help of excipients
Table 11.2: List of commonly used molecular simulation software [25].
Software Distribution Maintained by Features
Materials
studio
Commercial Biovia MD, MC,
QM, MM
DL_POLY Commercial W. Smith and T.R. Forester, Daresbury Laboratory MD
NAMD Open source Klaus Schulten, University of Illinois at Urbana Champaign MD
CHARMM Commercial Martin Karplus and others (Academic version), Biovia
(Commercial version)
MD
GROMACS Opensource University of Groningen, Netherlands MD
MacroModel Commercial Schrödinger LLC MM, MD,
MC
ACEMD Commercial Acellera Solutions MD
DESMOND Commercial D. E. Shaw Research MD
AMBER Commercial David Case, Rutgers University, and others MD
LAMMPS Open source Sandia National Laboratories MD
MC, Monte Carlo; MD, molecular dynamics; MM, molecular mechanics; QM, quantum mechanics.
11 Applications and molecular simulation strategies 253
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like solubilizers, surfactants, and cosolvents. Solubilizing compounds, like cyclodextrins,
are frequently employed to dissolve medications that are otherwise difficult to dissolve.
By enclosing the hydrophobic drug molecule in the cyclodextrin’s hydrophobic cavity,
solubility and bioavailability can be improved [28]. The release of pharmaceuticals from
the formulation is another important function performed by excipients. To achieve the
appropriate release profile, different excipients are needed for drug delivery systems
such immediate-release, sustained-release, and targeted-release. For instance, in sus-
tained-release formulations, hydrophilic polymers like hydroxypropyl methylcellulose
and sodium carboxymethylcellulose are utilized as matrix-forming agents. As the poly-
mer slowly dissolves in the stomach acid, the medication is freed from the matrix. The
flavor and texture of medicines can also be enhanced with the help of excipients. The
unpleasant taste of many medications is a major barrier to their regular use. Patients
are more likely to take a medicine if they enjoy the way it tastes, hence excipients, in-
cluding sweeteners, flavors, and masking agents, are often used. Sweeteners like aspar-
tame and sucrose, and masking agents like peppermint and apple tastes are two
examples. Excipients can enhance the drug’s look and texture in addition to its flavor
[29]. In order to expand the volume of the medication formulation and enhance its flow
qualities, fillers, including lactose, microcrystalline cellulose , and mannitol, are fre-
quently utilized. Compressing powder into tablets is a rather smooth process thanks to
the use of lubricants like magnesium stearate and stearic acid. Improved medicine tar-
geting and fewer unwanted effects are two additional benefits of using excipients. Exci-
pients used in targeted medication delivery systems must improve drug absorption and
retention at the intended location of action [30]. Drugs can be encapsulated in liposomes
or polymeric nanoparticles and then transported to the desired location. These drug
transporters can keep the medication safe from degradation and improve cellular ab-
sorption. Finally, the addition of excipients can boost the drug’s stability and safety [31].
Preservatives like benzalkonium chloride and methylparaben are frequently used in
multidose formulations to stop the growth of bacteria and other germs. Antioxidants
can stop the drug’s chemical breakdown caused by reactive oxygen species (ROS) [32].
11.5 Excipient–excipient compatibility
The term “excipient” is used to describe compounds that are included in a drug’s formu-
lation but do not contribute to the drug’s therapeutic action. Ingredients may be used as
fillers, binders, disintegrants, lubricants, preservatives, flavorings, or any combination
of these. When two excipients are said to be “excipient–excipient compatible,” it means
they can be used together without compromising the efficacy or safety of the medicine
in any way [33]. When creating a pharmaceutical formulation, it is essential that all of
the excipients work well together. The solubility, stability, bioavailability, and efficacy
of a medicinal product can be drastically altered by the selection and mix of excipients.
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A medicine’s safety and effectiveness can be jeopardized if two or more of its excipients
are incompatible [34]. This can manifest as physical instability, chemical degradation, or
changed drug release. The availability of a large variety of excipients, each with its own
individual features and characteristics, presents a significant barrier to obtaining
excipient–excipient compatibility. Several criteria determine whether two excipients
are compatible with one another; they include the molecular weight, solubility, melting
temperature, and chemical structure of the two substances [35]. Compatibility between
excipients is enhanced when they share comparable properties; incompatibil ity can
occur when they have significantly different features. Scientists in the pharmaceutical
industry employ a wide range of methods, such as physical and chemical compatibility
testing, formulation optimization, and stability testing, to guarantee that their excipients
are compatible with one another. Examining the particle size, shape, and surface area
of a mixture of excipients is an example of physical compatibility testing. The chemical
interactions between excipients, like the generation of new compounds or breakdown
products, are evaluated during chemical compatibility testing. Excipient–excipient com-
patibility can also be achieved by formulation improvement [36]. This requires picking
the right excipients and the right amounts of those excipients so that unwanted interac-
tions are kept to a minimum. Optimal formulations may also require tweaks to produc-
tion that lessen the possibility of product incompatibility [37]. Excipient–excipient
compatibility also includes stability testing. To determine the physical and chemical sta-
bility of a medicine over time, it must be subjected to a variety of environmental cir-
cumstances. Incompatibilities between excipients can be uncovered by stability testing,
which in turn aids in formulation improvement. Excipient incompatibility can be
caused by a number of different things [38]. New compounds formed between exci-
pients are a typical cause, and they can change the medicinal product’s physicochemical
properties. Excipients can interact with one another chemically, as in oxidation or hydro-
lysis, or physically, as in aggregation or precipitation, to cause this. Differences in pH is
another typical reason for incompatibility between excipients; this might have an impact
on the therapeutic product’s solubility and stability [39]. Degradation of alkaline pharma-
ceuticals by acidic excipients and acidic drugs by alkaline excipients are two examples.
The use of incompatible production procedures or equipment, as well as variations in
temperature, humidity, and light exposure, can also contribute to incompatibility be-
tween excipients. Pharmaceutical researchers face many obstacles while trying to ensure
excipient compatibility. Methods to reduce the likelihood of interactions during produc-
tion include using excipients with similar qualities, finding the optimal concentration
and ratio of excipients, and adjusting the manufacturing process [40]. When developing a
pharmaceutical formulation, it is essential to ensure that all excipients are compati-
ble with one another, as this can have a major effect on the drug’s safety and effective-
ness [41–43]. Selecting and combining excipients with care, in addition to rigorous
testing and optimizing the formulation and manufacturing processes, is necessary for
achieving excipient–excipient compatibility [44–47]. Scientists in the pharmaceutical in-
dustry can better serve the requirements of patients by creating safe and effective med-
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ications by gaining a deeper understanding of the causes of excipient–excipient incom-
patibility [48–50]. All compatibility tests are shown in Figure 11.3.
11.6 Excipient–excipient interaction
When two or more excipients are mixed to create a medicinal product, they might have
both chemical and physical interactions with one another. A drug’s final quality, safety,
and efficacy can all be negatively impacted by these interactions. Depending on their
chemical and physical qualities, different excipients might interact with one another in
different ways. Some excipients, for instance, may not work well together because they
react badly with one another or would create dangerous byproducts if mixed [51]. Parti-
cle size, surface area, and solubility are just a few physical features that might deter-
mine how well one excipient mixes with anothe r, and hence whether or not two
excipients are compatible. A common form of interaction between excipients is chemi-
cal incompatibility. This takes place whenever a combination of two or more excipients
forms a new compound. The deterioration of the drug substance, the production of
harmful by-products, and the modification of the therapeutic action are all issues that
might arise from chemical incompatibility [52]. When alkaline medications are used
with acidic excipients, for instance, the dr ug substance can be hydrolyzed and de-
graded. Another kind of excipient–excipient interaction is physical incompatibility. This
happens when there is insufficient mixing between two or more excipients because of
their inherent distinctions. Problems with flow, mixing, and distribution may arise if
the drug component and container are not physically compatible. Drug material can get
segregated and distributed unevenly in the final dose form, for instance, if two exci-
pients with differing particle sizes are mixed [53]. The environment can also play a role
in causing excipient interactions. If the excipients are subjected to extreme conditions,
such as high heat or humidity, they may undergo physical and chemical changes that
Figure 11.3: Excipient compatibility testing.
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render them incompatible. As an added complication, the medicinal ingredient can be
degraded due to photochemical processes if it is exposed to light. There are a number
of approaches taken by pharmaceutical experts to deal with excipient–excipient inter-
actions. Selecting excipients with similar qualities, optimizing the concentration and
ratio of excipients, and adjusting the manufacturing process are all ways to reduce the
likelihood of interactions [54]. To guarantee that the excipients are compatible with one
another and do not compromise the drug product’s safety, efficacy, or stability, it is im-
portant to conduct stability and compatibility testing on the final dosage form. The in-
teraction between excipients is a crucial factor in drug development. If you want a safe,
effective, and stable medicine, you need to know what kinds of interactions can happen
and how they might affect the formulation. Scientists in the pharmaceutical industry
can create safe, effective medications for patients by carefully selecting excipients that
are compatible with the drug and adjusting the formulation and production procedures
to maximize quality and stability [55].
11.7 Excipient–excipient incompatibility
The addition of excipients to an API aids in its creation by increasing its bulk, provid-
ing stability, enhancing its bioavailability, and ensuring that it reaches its intended
target. Above the safe range, the ingredients in the formulation may start to interact
with one another or become incompatible with one another, which could have nega-
tive consequences on the human body. Product viability, medicinal efficacy, and pa-
tient safety are all s eriously jeopardized b y these incompatibilities [56]. Physical
incompatibilities between excipients include, for example, changes in dissolving rate
or dose uniformity in a solid formulation, whereas chemical incompatibilities include,
for example, the generation of degradants due to impurities in excipients or func-
tional groups on excipients. Oxidation, hydrolysis, auto-oxidation, racemization, and
photolysis are all examples of degradation processes. Antioxidants, nitrogen blanket-
ing, and scavengers can help, but it is best to be cautious when choosing excipients
for formulation development to avoid drug–excipient incompatibility altogether [57].
Poor product performance is the end result of any interaction between APIs and exci-
pients, as well as between excipients themselves. Excipient incompatibilities are illus-
trated in [Table 11.3].
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11.8 Cause of incompatibilities
The production of excipients is complex since it requires raw ingredients, solvents, re-
action initiators, and other processing aids to ensure that the final product meets cGMP
standards. Methods of producing such excipients include grinding, acid hydrolysis, and
chain reactions. Thus, they may have contaminants such as residual solvents, reaction
initiators (such peroxides in polymer manufacture), or even metallic catalysts that were
not removed throughout the manufacturing process. Although the manufacturer main-
tains low levels of these pollutants, the amount may increase during storage or trans-
Table 11.3: Some examples of excipient–excipient incompatibilities compiled from literature [58].
Name of the
excipient
Incompatible excipient Remarks/outcome
Glycerin Zinc oxide and basic bismuth
nitrate
Black discoloration of glycerin
HPC Parabens and anionic
polymers
Increase in viscosity of HPC due to anionic polymers
Low substituted
HPC
Alkaline substances Increase in disintegration time of a table
Hypromellose
phthalate
MCC and CMC calcium Splitting of film coatings
Magnesium oxide Eudragit RS Retard drug release
Mannitol Xylitol, potassium chloride
and sodium chloride
Salted out by KCl and NaCl
Croscarmellose
sodium
Basic excipients and Sorbitol Dissolution slowdown of a tablet due to interaction of
CCS with basic excipients. Sorbitol reduces
disintegrant efficiency of CCS in tablet formulations
Citric acid
monohydrate
Sucrose Citric acid can cause precipitation of sucrose from the
syrups
Carboxymethyl
cellulose sodium
Gelatin and pectin Complex or coacervate formation
Benzyl alcohol Methyl cellulose and
polysorbate
Polysorbate reduces the antimicrobial activity of
benzyl alcohol
Benzoic acid Kaolin The interaction of benzoic acid with kaolin may reduce
benzoic acid’s preservative activity
Aspartame Magnesium stearate and
dibasic calcium phosphate
Magnesium stearate may elevate pH
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port, leading to interactions between the functional groups of an API and other exci-
pients and due to various degradation events [59]. As an alternative, the presence of
aldehydes and peroxides in an excipient may trigger oxidatio n, and the release of
bound water in an excipient may commence a hydrolysis process. Incompatibility in
excipients has been observed to be induced by the most prevalent chemical functional
groups, including esters, amides, hydroxyl groups, and amines [60].
11.9 Chemistry of drug–excipient interactions
Some of the most common reactions in pharmaceuticals include hydrolysis, dehydra-
tion, isomerization, elimination, cyclization, oxidation, photodegradation, and specific
interactions with formulation components (excipients and the associated contami-
nants). Temperature, pH, moisture in solids, relative humidity of the surroundings, cat-
alyst presence, light, oxygen, drug and excipient physical shape, and particle size are
among the most influential elements in these interactions. The scope of this chapter
does not allow for a deep dive into the chemistry of medication stability [61]. Excipients
may affect the stability of pharmacological compounds that are sensitive to a variety of
degradation mechanisms, therefore it is necessary to think about why that would be.
Excipients can affect drug stability in the dosage form in a number of ways, including
by changing the dosage form’s moisture content, the pH of the microenvironment, act-
ing as a general acid-base catalyst, reacting with the drug itself, or providing a source of
impurities that can either react with the drug directly or act as catalysts in its degrada-
tion. By forming eutectic or solid solutions, or by exchanging ions with the medication,
excipients can affect the drug’s physical and/or chemical form [62]. The drug’s chemical
stability could be affected by these shifts in physical or chemical state. Physical changes,
like altered organoleptic properties and slower dissolution, and chemical changes, like
drug degradation, are two ways in which drug–excipient interactions in solid dosage
forms can affect drug product stability. The chemical instability of drugs can be caused
either by interactions between the drug and the excipient directly or by interactions
between the drug and contaminants in the excipient. Reviewing the mechanisms of
chemical instability in solid dosage forms sheds light on universal themes that can be
applied across different types of degradation [63]. Some examples of these recurring
ideas involve water’s function and the microenvironmental pH. Solid-state reactions
containing excipients and/or excipient impurities provide their own unique challenges
when trying to understand and predict reaction pathways [64].
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11.10 Impact of excipients on drug release
and bioavailability
This discussion focuses on the role that excipients play in the dosage form, and how
they might be used to improve medication release and bioavailability. Drug delivery
applications of smart polymers inside the encapsulated drug are known as stimuli-
Table 11.4: Physical interactions [65].
Interaction Beneficial effect examples Detrimental effect examples
Adsorption: When a medicine is
absorbed by an excipient, it is no
longer available to be dissolved,
resulting in decreased
bioavailability. The bioavailability
of a medicine can be improved
through the use of an excipient
that facilitates drug adsorption.
A dosage form of the
nonsteroidal anti-inflammatory
medicine (NSAID), indomethacin,
utilizing kaolin as an adsorbent,
improved drug solubility and,
thus, bioavailability.
) The lubricant, magnesium
stearate, in a tablet of cetyl
pyridinium chloride adsorbs the
cations of cetyl pyridinium
chloride. The drug’s ability to kill
bacteria is drastically diminished
as a result.
) Dicumarol’s absorption is
reduced in formulations when
excipients like aluminum
hydroxide, starch, and talc are
present due to these excipients’
adsorbing capabilities.
Complexation: It forms complexes
with pharmaceuticals, usually
reversibly, but insoluble
complexes can also develop,
resulting in slower drug
breakdown and absorption. The
effect is negative when this
occurs. To improve the
bioavailability of medications that
are not very water soluble,
complexing agents can be
utilized – the positive outcome
observed in this scenario.
Cyclodextrin is commonly used to
increase the bioavailability of
medications that are not well
soluble in water. When the drug’s
mucosal permeability is raised, its
stability is also raised, which
improves its bioavailability, and
the pace and depth of its
dissolution.
Tetracycline, combined with
calcium carbonate, to produce an
insoluble compound, resulting in
slowed breakdown and
absorption. Chlorpromazine’s
membrane permeability was
lowered when it was combined
with polysorbate and sodium
lauryl sulfate.
Solid dispersion: Hydrophobic
medications benefit from these
interactions because they
increase their solubility and
bioavailability. Slow medication
dissolving may occur as a result
of solid dispersion interactions.
Drugs like piroxicam, norfloxacin,
nifedipine, and ibuprofen have
better dissolving rates when
synthesized as solid dispersions
with Polyethylene glycol of
varying molecular weights.
Drugs in a capsule containing a
solid dispersion product,
generated from the interaction of
povidone and stearic acid,
dissolved slowly.
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responsive polymers. In response to subtle shifts in the input, the physical characteris-
tics and/or structure of these polymers often shift as well. Stimuli are changes in the
environment that have an effect on the polymer, and responses are changes in the poly-
mer and the system (such as the polymer’s dissolved state in a solvent). Environmental
factors like pH, ionic strength, temperature, light, and magnetic or electric field have all
been used in biopharmaceutical applications [66]. The pharmaceutical industry relies
heavily on modified release technologies to boost the effectiveness, safety, and patient
compliance of its products. When it comes to how fast a medicine is released from a
solid oral dosage form, polymeric excipients play a crucial role. Site-specific, sustained
drug delivery is now feasible, regardless of the physicochemical qualities of drug mole-
cules or the body’s physiological state. These polymeric excipients are commonly em-
ployed to control drug release and improve oral drug absorption. When it comes to
controlled-release systems, lipid excipients have their own set of advantages and disad-
vantages. Numerous recent papers detail the variety of processing methods used to cre-
ate matrix-modified-release dosage forms using lipid excipients [67]. While excipients
themselves are not pharmacologically active, they can impact a drug product’sbioavail-
ability through interactions with the active ingredients in the dosage form and the
physiological variables present at the site of absorption [68]. Robust pharmacological
product design requires an overarching mechanistic comprehension of the foundations
of these interactions [69]. The effects of excipients on physiological processes are dis-
cussed, as are the drug–excipient interactions that occur in solid dosage forms and re-
duce drug bioavailability. Factors such as the drug’s potency and dose, the rapeutic
window, site of absorption, rate-limiting factor in drug absorption (e.g., permeability or
solubility limited), and whether drug metabolism, efflux, complexation, or degradation
at the site of absorption plays a role in determining its bioavailability would determine
the extent to which these interactions affect drug bioavailability on an individual basis.
However, formulations with enhanced drug bioavailability can be created with the aid
of a mechanistic understanding of drug–excipient interactions and their effect on drug
release and absorption. The use of excipients as biopolymers in implants for everything
from bone replacement to nerve regeneration is a prime example of the field of special-
ized excipient applications. These biopolymers are superior materials because of their
biocompatibility, biodegradability, and adaptability [70].
11.11 Importance of excipient functionality in drug
delivery applications
Excipients are inactive chemicals added to pharmaceuticals to improve their pharma-
cological qualities such as stability, solubility, and bioavailability. In drug delivery ap-
plications, excipient functionality is crucial sinc e it affects the medicine’ssafety,
effectiveness, and quality [71]. Many medications need to have their solubility im-
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proved in order to boost their bioavailability and therapeutic efficacy. Surfactants, cy-
clodextrins, and cosolvents are examples of excipients that can improve the solubility,
absorption, and distribution of a drug [72]. By preventing drugs from degrading when
exposed to air, moisture, and light, excipients improve their stability. Antioxidants,
chelating agents, and pH regulators are all examples of stabilizers that can be used to
keep drugs effective and stable. Excipients can be used to control the rate of drug re-
lease from a formulation. Controlled-release technologies, including matrix systems,
liposomes, and nanoparticles, can delay the onset of drug effects and improve patient
compliance by reducing the number of times a day the patient must take the medica-
tion [73]. Excipients, including as sweeteners, flavors, and coloring agents, are used to
improve the taste and appearance of pharmaceuticals and make them more accept-
able to patients.
11.12 Keeping faithfulness alive
Although excipients have a lengthy track record of safe use in human medicine, they
may nevertheless cause adverse responses in some patients. When excipients are cho-
sen and formulated with care, toxicity and adverse responses can be minimized [74].
Excipients’ functioning and compatibil ity with the API must be well understood be-
fore they can be selected and optimized for use in drug delivery systems. When choos-
ing an excipient, it is important to think about the API’s physicochemical qualities, the
method of administration, the desired release profile, and the intended patients [75].
The regulatory requirements for drug approval and market authorization are intrinsi-
cally tied to the performance of excipients in drug delivery applications. Excipient
safety, effectiveness, and quality data are required by the US Food and Drug Adminis-
tration (FDA) and other regulatory bodies before drugs can be marketed [76, 77]. The
drug’s manufacturer must provide evidence that the excipients used in the pr oduct
are safe and effective for human consumption. Safe, effective, and high-quality drug
products rely heavily on the performance of excipients in drug delivery applications.
Improved drug solubility, stability, and release, as well as enhanced flavor and pre-
sentation, are just some of the many benefits of using excipients. Excipients in thera-
peutic formulations must be selected and optimized with consideration given to their
compatibility with the API and their effect on patient safety and efficacy. The signifi-
cance of excipient functioning in drug delivery applications is further highlighted by
regulatory regulations for excipient use in drug products [78, 79].
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