CHAPTER 1: Preformulation Studies

Discovering and developing new medicines is a long, complex and expensive process and the failure rate is high during the process. To minimise attrition it is essential, therefore, to understand the physicochemical characteristics of compounds or biological entities that are candidates for development into final products.

At various stages during the development of a new medical product the candidate drug must be formulated into a dosage form that is appropriate for the intended study e.g. in vitro screening using chemical, physicochemical or biological assays, pre-clinical in vitro laboratory safety tests, in vivo efficacy and safety studies in relevant animal species, first-in-human studies to determine the optimum drug to progress into clinical development, initial volunteer/patient studies and full-scale clinical trials (Figures 1.1 and 1.2).

The nature and composition of the formulations will be different for each stage of development but the formulation chosen for full-scale clinical trials must, as far as possible, be the same as the product that is intended for marketing. Otherwise extensive clinical comparative trials may be required to demonstrate the similarity between the formulation used in the clinical trials and that proposed for subsequent marketing.

To ensure that the various formulations are optimised for their intended use, pre-formulation studies should be conducted not only to evaluate the characteristics of candidate drugs but also potential formulation excipients, and their interactions with drug substances, in order to select appropriate formulation ingredients. In addition, preformulation studies should assess the effect of possible conditions of preparation, manufacture and storage on stability, so as to give confidence that a reliable assessment of the candidate drug has been performed during development and in regular, post-marketing, use.

Data acquired from preformulation studies also forms an important basis for understanding the potential pharmacokinetics of a drug in humans and animals.

In addition, as the chosen product is scaled up in manufacture and/or further process development is carried out e.g. to use alternative equipment or technologies; preformulation data can be a useful source of information to understand the opportunities for and limitations to process change.

Furthermore, a number of the characteristics measured in preformulation studies can be used to predict the stability of the formulation during manufacture, transport and storage so as to determine the shelf life of the marketed product.

Preformulation studies can therefore be defined as; Laboratory studies to determine the characteristics of active substance and excipients that may influence formulation and process design and performance.

It has been described as “Learning before doing”.

The aqueous and lipid solubility characteristics of a drug substance are of fundamental importance in determining whether it is capable of reaching sites of absorption, its interaction with putative therapeutic targets and its ultimate metabolism and excretion.

An assessment of solubility characteristics is, therefore, usually a starting point for preformulation studies.

Using standard aqueous buffers the drug or excipient is vigorously stirred at a constant temperature, e.g. 37 °C, to achieve equilibrium, maximum (saturated) absolute solubility. For compounds with ionisable groups this equilibrium solubility of the unionised form is known as the intrinsic solubility.

Preformulation studies will start by measuring intrinsic solubility in a neutral, an acid and an alkaline environment; typically 0.1 M HCl, water and 0.1 M NaOH at 4 °C, 25 °C, 37 °C and an elevated temperature e.g. 50 °C.

These data can be recorded as the absolute (intrinsic) aqueous solubility at each pH and compared with data on known and related compounds.

The values obtained can provide insight into the state of the drug substance as it is subjected to a variety of different pH environment e.g. as it passes through the gastro-intestinal tract, circulates through various cellular, organ components, arterial and venous circulation and excretory fluids such as bile and urine.

In addition the solubility profile at different pH's can inform the type of the aqueous solvents that might potentially be used in formulations (e.g. parenteral injections, nasal or ophthalmic drops, oral solutions).

Furthermore, the information is useful to assess the possible effect that aqueous media used in dosage form manufacture, e.g. tablet wet granulation and film coating, may have on the compound.

The aqueous solubility of a compound is dependent, inter alia, on its state of ionization, including the ratio of ionised to unionised moiety.

The degree of ionisation can be estimated using the Henderson–Hasselbach equation which for weak acidic compounds (HA) is

or in its rearranged form

where Ka is the ionisation constant of the dissociation constant.

And for weakly basic compounds (BH)

Or

pK_a is obtained by measuring the pH changes of the substance in solution during potentiometric titration using either a weak base or a weak acid. When pH = pK_a the compound is 50% ionised.

The pK_a can be calculated from intrinsic solubility data; also measured using a variety of techniques e.g. conductivity, potentiometry and spectroscopy.

The pK_a value provides a useful indication as to the region of the gastrointestinal tract in which the drug will be in either the ionised or unionised state and, hence, some indication of its possible absorption characteristics.

Importantly, however, the chemical nature and concentration of the counter ion conferring solubility e.g. chloride or hydrochloride can have a significant influence on solubility and this should be examined during preformulation studies; so as to choose an optimum compound e.g. base or cation, for further development.

In addition to determining the solubility characteristics in an aqueous environment it is also useful to obtain preliminary data on the solubility of the drug/excipient in non-aqueous solvents that might be used in formulations, e.g. topical ointments/liniments or oily injections, and to provide data that can be used to select solvents for manufacture of the active ingredient, e.g. extraction or crystallisation, and for the final formulation, e.g. tablet granulation.

Since there are many organic solvents that might be employed, preliminary preformulation studies should focus on a selection of solvents such as:

For Formulation

• Ethyl alcohol

• Glycerin

• Propylene glycol

• Arachis oil

• Ethyl oleate

• Liquid paraffin

For Manufacture

• Industrial methylated spirits

• Isopropyl alcohol

• Benzyl alcohol

• Polyethylene glycol

Whilst a knowledge of intrinsic solubility is essential, the rate at which a drug or excipient dissolves in any particular medium is also important.

Solubility rate will depend on many factors, such as particle size; particle size distribution and particle porosity—and, hence, the surface area available, which is changing as dissolution occurs—the wettability of the particle surfaces, the nature of the dissolution fluid, its polarity, rheological properties and the degree of stirring or agitation during dissolution.

Therefore, initial preformulation studies should focus on a model dissolution system, e.g. using pharmacopoeial paddle dissolution methodology. Studies should be performed at constant temperature and pH's using similar particle size fractions (sieve cut of powders) when comparing with reference materials.

More discriminating studies e.g. to examine surface area, pH or particle size can be performed as further development progresses.

Once in solution in an organ or cell in a biological fluid, e.g. synovial fluid, vitreous humour, mucous etc., a drug will need to diffuse to the site of transfer or action.

The rate at which the drug can diffuse is dependent on a variety of physiochemical properties such as the viscosity of the fluid through which it is diffusing, the temperature of the fluid, the concentration gradient across the fluid—and hence the amount of drug in solution and the surface area with which it is in contact.

In a fluid with pure Newtonian rheological properties the rate of diffusion of a chemical entity can be calculated using the Noyes–Whitney Equation.

where

dM/dt is the rate of dissolution (i.e. the amount M diffusing in time t)

D is the diffusion coefficient from the saturated liquid layer adjacent to the crystal surface.

S is the surface area exposed.

Cs is the concentration in a saturated liquid layer directly adjacent to the crystalline solid surface.

Cb is the concentration in the bulk solution further out from the crystal, (Cs − Cb) is the concentration gradient.

H is the thickness of the liquid saturated layer.

Preformulation diffusion studies can be conducted using a Franz cell.¹ 

In addition to determining the rate and quantity of drug that has permeated, the diffusion coefficient provides another means of comparing related compounds and those with known in vivo characteristics.

Even when a drug substance is readily soluble at physiological pH's, its ability to transfer across membranes can be highly dependent on its capacity to partition into and cross lipophilic substrates, e.g. components of cell walls.

This lipophilicity can be quantified for comparative purposes by determining its partition coefficient P

which is a measure of the unionised drug distribution between an aqueous and an organic phase at equilibrium.

The technique used is to dissolve a known concentration of the compound in an aqueous solution and shake this together in a flask with an equal volume of the lipid. After the phases separate, the amount of drug remaining in the aqueous solution is determined, from which the amount that has partitioned into the lipid can be calculated.

Drug substances make contact with a variety of lipid substances in various compartments of the body so the choice of a lipid to determine the partition coefficient can be critical.

Over many years, n-octanol has been chosen as a model lipid in preformation studies since it has properties not too dissimilar to many biological short chain hydrocarbon lipids.

It is possible, therefore to build up a library of values for known drugs against which the new drug can be compared.

As the candidate drugs are being optimised to chose a lead compound for development, further studies can be established to examine their partition in solvents of increasing lipophilicity²  (Figure 1.3).

Since many factors determine the activity, absorption and permeation of drugs across membranes and into tissues and cells; the partition coefficient of itself is only a starting point for understanding the biopharmaceutical properties of a substance. Nevertheless it can provide valuable comparative data when examining a series of lead molecules to optimise efficacy and bioavailability.

Once in solution in physiological fluids e.g. gastric juices or plasma, a drug must permeate cells and tissues to reach its target site of action. This will involve passive and/or active transport mechanisms. For passive diffusion the drug will need to partition with the lipid components of cells and/or diffuse through aqueous pores in tissues.

An index of its permeability can be obtained in vitro by measuring the permeability across a model membrane at a constant temperature, Typically the drug in solution is placed in one side of a two-compartment cell separated from the second compartment by a polymeric membrane, the second compartment containing a physiological representation fluid, e.g. normal saline.

The amount of drug permeating through the membrane can be measured at various time intervals. A variety of membranes may be chosen each differing in their lipid composition.

The data obtained permits the calculation of the diffusion rates and a comparison of permeability with that of drugs whose properties are known or comparison with related drug candidates.

Permeability is not therefore a single characterises but depends primarily on solubility, partition (aqueous : lipids), diffusion coefficient and the nature of the membrane (chemical and biological composition and thickness).

The rate of permeability will also depend upon other physicochemical properties of solutions (e.g. fluid temperature, viscosity, density).

Combining knowledge of solubility with knowledge of permeability allows an initial estimate of bioavailability.

Amidon et al.³  suggested a Biopharmaceutical Classification Scheme (Figure 1.4, Table 1.1) which has been used as a preliminary indication of bioavailability.

This categorisation can be used to establish whether candidate compounds possess physicochemical properties that are likely to be inferior in terms of bioavailability and hence suggest that further medical chemistry should be conducted to achieve potentially better bioavailability whilst retaining potency. It also forms the basis of FDA Regulatory Guidance on the need for bioavailability and bioequivalence studies.⁴ 

It is important to recognise that this classification is only an estimate of the likely bioavailability. Following oral administration of a drug, many other processes govern its pharmacokinetic properties. These include in vivo stability and metabolism in various body fluids and compartments, receptor avidity and glomerular filtration rate.

Also, the biological fluids to which the drug is exposed may contain a variety of solutes that can affect solubility—e.g. surface active agents—so it is sometimes useful to measure solubility in “biorelevant” aqueous media in addition to standard pH buffer solutions.⁵ 

Furthermore, absorption and excretion—and hence bioavailability—can be affected by the biological nature of absorptive and efflux transporters.^6,7 

Chemical and biological materials have different capacities to adsorb and desorb water (called “hygroscopicity”) depending on their chemical and physical state.

Drug substances and excipients will be stored in warehouses prior to manufacture, and exposed to various humidly environments during manufacture.

It is important, therefore, to determine their moisture sorption characteristics to establish those conditions that are acceptable and those that should be avoided.

Hygroscopicity information can be used to select packaging for the final dosage form that can protect the product from exposure to the many different humid environments to which it may exposed be during transport and storage. This is necessary to provide a maximum shelf life against chemical/microbiological degradation or, for example, in the case of tablets, physical degradation through disintegration or discoloration.

Laboratory evaluation consists of exposing thin layers of the drug or excipient on dishes at a variety of relative humidities (RH) and at different temperatures; measuring the weight gain or loss over a few days or weeks of exposure and hence the amount of water taken up under each specific temperature and humidity condition. The moisture content at equilibrium at a specific relative humidity is called the equilibrium moisture content (emc).

The data can then be presented as a moisture sorption graph (Figure 1.5).

Various attempts have ben made to standardise the terminology used in classifying hygroscopicity. The most widely used terms are:

• Deliquescent

• Very hygroscopic

• Hygroscopic

• Non-hygroscopic

However, there is no generally recognised classification.⁸  Although, Callahan et al.⁹  have proposed a useful definition (Table 1.2).

Some materials, e.g. maize, potato and corn starches, have the capacity to retain different amounts of water at the same relative humidity depending upon their moisture exposure history.¹⁰ 

For example if a starch powder is dried completely and then exposed to a humid environment it will adsorb and absorb moisture isothermally to a maximum emc at 100% RH. When the moisture saturated powder is placed in a low-humidly environment, desorption takes place more slowly due to the amylase chemical bonding that has occurred during sorption, which resists the rapid desorption of water molecules.

The moisture sorption graph thus displays a hysteresis (Figure 1.6). At any particular relative humidity, starch powder may have a different moisture content depending upon its exposure history.

This can be important in the context of the use of materials with such moisture hysteresis properties. For example, starches are used as disintegrants in tablet formulations. Their capacity to initiate disintegration is dependent upon their swelling capacity which, in turn, is dependent upon their moisture content, which, in turn, is dependent on their sorption history. Thus a pre-dried starch is likely to be a more efficient disintegrant than a starch included in the final formulation having been pre-exposed to humid environments.

Drugs and excipients can exist in various crystalline or amorphous states depending on their chemical composition and method of isolation or crystallisation.

During crystallisation, molecules may arrange themselves in different geometric configurations such that the structure of the crystals formed has different packing arrangements or orientations. These different states are refers to as polymorphs.

Each polymorphic form may possess very different physicochemical characteristics (e.g. solubility, melting point), which can significantly affect the bioavailability of a drug as well as its stability (Figure 1.7).¹¹  In addition, polymorphism can affect the compression properties of drugs (e.g. paracetamol can exist in monoclinic or orthorhombic forms, the latter possessing preferable compaction properties).

In the amorphous state, crystal structures are generally disordered, such that the substance does not posses a sharp melting point but change its physical state slowly as temperature rises. The point at which this commences is called the glass transition temperature. This is another useful preformulation characteristic to consider when selecting processes for manufacture (including wet granulation for tableting and heat sterilisation for injectables) which might change the morphic structure and hence the physicochemical and biological properties of the product.

It is important, therefore, to establish whether a candidate for development has the propensity to exist in different polymorphic states, the properties of each polymorph (melting point, density, hardness, optical properties, hygroscopicity, solubility, stability etc.) and the conditions under which each may be formed; so as to provide guidance to establish a manufacturing process that ensures that the preferred polymorph is created and maintained.

Polymorphisms have been classified as:

1. Enantiotropic: one form changing into another form by varying temperature or pressure.

2. Monotropic: in which the polymorphic form is unstable at all temperatures and pressures.

Clearly it is highly desirable to chose a polymorph (where they exist) that is sufficiently stable at room temperature and to define the temperature conditions (during manufacture and storage) under which polymorphic change or instability could deleteriously affect the compound.

A useful staring point is to prepare samples of the drug or excipient using very different conditions e.g. a variety of solvents for crystallisation, different rates and temperature changes during crystallisation and drying. Samples can then be subjected to several analytical procedures to examine their possible polymorphism. Examples of these procedures are described in the following paragraphs.

This technique measures the heat loss or gain that results from changes (whether physical or chemical or both) as a sample is subject to a programmed temperature change.

Changes in transitions—such as melting, desolvation and degradation—can be identified for different polymorphs to determine the preferred form for future use.

This technique measures changes in sample weight at either a constant temperature over time or when subject to a programmed temperature rise.

It provides additional data to DSC and is particularly of value in examination of solvation.

This technique is very useful in establishing whether a compound exists as an amorphous state and for comparing the reproducibility of different batches of the chosen crystalline polymorph.

Crystalline polymorphic materials can exist in a number of shapes or forms (sometimes called “habits”) depending on the method and solvent used for final crystallisation.

This can range from highly angular crystals with an elongated shape, needle-like crystals and flat plate-like forms to more spherical habits.

Their shape can affect the “flowability” of the bulk powder e.g. during discharge from containers/hoppers etc. due to particle–particle mechanical and physical interactions or cohesion.

Each type of crystal may exist as well-formed, solid, structures or posses different degrees of internal stricture; sometimes leading to highly porous structures.

These differences can have profound effects on the rates of dissolution due to differences in surface area exposed to the solvent.

It is useful therefore in preformulation studies to crystallise the compound using different conditions of temperature, solvent, speed of crystallisation etc. to determine how critical the crystalline form may be and suggest preferred crystallisation conditions for further optimisation.

Clearly, the stability of a drug (and of formulation excipients) is critically important to ensure that the patient receives the correct dose of the active ingredient. Furthermore, for those drugs that can degrade to produce toxic materials, it is essential to determine the conditions under which this might occur so as to find methods of prevention or stabilisation and/or to determine limitations in terms of shelf life and storage conditions.

In addition, the stability of excipients and their stability in combination with other excipients and drug substances can be a critical factor in achieving a stable marketable product, e.g. the stability of antimicrobial or antioxidant preservatives in liquid formulations.

Degradation can occur through a number of chemical/physiochemical and biological pathways.

e.g.

• hydrolysis

• isomerisation

• oxidation

• polymerisation

• solid-state phase transformation

• dehydration or desolvation

• cyclization

• photolytic degradation

• microbial attack

Importantly, the kinetics of instability can vary according to the route and hence rate of degradation.

In addition, compounds, especially unsaturated fatty acids and oils, can degrade through different orders of reaction e.g. first, second, third etc. order kinetics. The chemical route of degradation depending on temperature.

Early preformulation studies should be designed to subject the drug or excipient to several “stress” conditions to identify key degradation pathways and the extent of degradation. From these studies it is possible to estimate the probable stability of the chemical or biological substance under the environmental conditions that it could be subject to during synthesis or extraction, manufacture, transport and storage.

The data might be used to provide feedback to the research team for modification of the labile groups to improve stability.

Alternatively, where such modification compromises the efficacy of the compound the data can be used to guide formulation stabilization strategies, to restrict the conditions to which it should be exposed during manufacture, transport and storage and to provide an early estimate of the potential shelf life of the final formulation.

Stability studies can be conducted on the materials in their solid state and in solution.

In addition to the value of such studies during preformulation, regulatory authorities require such data in submissions for product approval. This requirement is so that they can independently assess whether the product is likely to be adequately stable under the proposed conditions of manufacture, transport and storage until the shelf life or expiry date claimed on the label.

ICH (International Conference on Harmonisation of Regulatory Requirements) Drug stability test guideline Q1A (R2) requires that the drug substance be tested under different stress conditions.

It is suggested that stress testing include the effect of

• pH

• Temperature

• Humidity

• Light

• Oxidizing agents

The chemical stability of a drug substance in the solid state can be evaluated under various temperature and humidity stress conditions.

Pre-weighed samples are stored in stability cabinets in open vials or thin layers for periods of up 8 weeks under conditions such as:

• 40 °C

• 60 °C

• 80 °C

• 25 °C 85% RH

• 40 °C 75% RH

At pre-determined time intervals, e.g. 2 weeks, 4 weeks, 8 weeks, samples are removed, dissolved in an appropriate solvent, and analysed using a robust, stability-indicating assay; typically a reverse-phase high-performance liquid chromatography (HPLC) assay that allows direct injection of stability samples.¹² 

The exact temperature and humidity conditions and time intervals of storage that are chosen should take into account the chemical/biological nature of the substance and regulatory agency requirements.¹³ 

Ideally, the assay should allow detection of degradation peaks equivalent to 0.1% of the parent peak, but this is not always practical at the discovery stage.

Some techniques that perform and analyse multiple degradation experiments on drug substances under various stress conditions are amenable to high-throughput measurements in a 96-well format.

Hydrolysis can occur in many molecular species but particularly for carboxylic acid derivatives or substances containing a functional group based on carboxylic acid, e.g. ester, amide, lactone, lactam, imide and carbamate.

To identify and quantify potential degradation by this route, samples of the compound should be subject to stress testing in acidic and alkaline conditions, e.g. refluxing the drug in 0.1 N HCl and 0.1 N NaOH for 8–12 hours.

As with solubility evaluations at the preformulation stage, the stability of candidate drugs (and formulation excipients) in non-aqueous solvents that typically might be used in subsequent formulations or manufacturing processes should be examined.

Similar molecules may interact to produce complex structures—including dimers and polymers of various lengths and orientations. The potential for such interactions should be evaluated by examining the polymeric state of samples during stress testing for heat, light and solution stability.

For those substances that may degrade when exposed to light, a number of opportunities exist to prevent or minimise instability through the choice of specialised coatings or packaging.

Nevertheless, the propensity for compounds to degrade in this way should be evaluated at an early opportunity.

Solid-state photostability can be evaluated by exposing thin layers of samples to high-intensity light (HIL)/UV conditions initially at 25 °C (but subsequently at more elevated temperatures) in a photostability chamber.

The ICH guidelines recommend exposure at 1.2 million lux hours to visible light and 200 W hours m⁻² to UV to represent the frequencies of light radiation in various geographical locations.

Since the drug may be required to be formulated as a solution (e.g. oral, parenteral or topical), photostability should also be evaluated in aqueous and, where appropriate, non-aqueous solution.

For both solid and solution photostability studies, samples protected from light are stored under the same conditions and used as controls.

Stability tests should also be performed under several physiological and formulation pH conditions in order to understand the characteristics of the drug candidate under physiological conditions and to provide key information for the formulation of solution dosage forms.

Typically this would involve measuring stability at 37 °C in a range of buffer solutions e.g. pH 1, pH 4, pH 7 and pH 9 at intervals from 1 day up to 1 month.

The studies should be designed using reasonable concentration of drug or excipient to detect even minor decomposition products in the range of detection.

Drugs and excipients may be degraded by oxidation reactions of which there are two distinct types.

1. Oxidation through direct reduction reactions via atmospheric oxygen.

2. Oxidation by chain reaction involving the formation of peroxy free radicals.

This, latter, route of degradation is most likely to occur in compounds with double carbon bonds; especially long-chain unsaturated fatty acids and oils.

The oxidation process involves several steps viz.: initiation, propagation, and termination, and can be catalysed by heat, light, metals or free radicals.

Typically the reaction is as follows

If the chemical structure of the drug or excipient indicates that this route of oxidation is possible, then preformulation studies should subject samples of the substance to various elevated temperature stress conditions under open atmospheric conditions, i.e. in the presence of oxygen.

The route by which such oxidation occurs may be temperature-dependant e.g. the energetics and hence the site of peroxidation at low temperatures may be significantly different to those at high temperatures, giving rise to different orders of reaction at different exposure temperatures.

Thus, elevated temperature challenges may not reflect what happens at ambient temperatures.

In the solid state, oxidation can occur where molecular oxygen diffuses through the crystal lattice to the labile sites. These are called “oxygen” electron–transfer reactions.

Although early preclinical studies—and some animal studies on a lead candidate drug—may use simple solutions derived from preformulation studies on solubility and stability, as the candidate progresses to clinical trials, especially confirmatory large-scale trails, it will be required to be formulated with excipients.

Thus drug–excipient compatibility studies are required to determine the flexibility of choice available for various types of oral, parenteral, topical etc. formulation.

Based on a knowledge of the stability characterises of the drug substance, stability tests can be conducted on the drug in the presence of various excipients.

Clearly the range of excipients that might be eventually be chosen for the final, marketed, product can be extensive and, hence, a considerable number of possible combinations for evaluation can be identified. This is not usually justified at the early stages of development. Such initial studies should therefore be restricted to a few major potential excipients, e.g. lactose, sucrose, dextrose, magnesium stearate etc., as a prelude to more extensive evaluation later in formulation design and development.

Most drug substances that are chosen for final product development will be in powder form. The technical properties of these solid-state materials will be important in formulation and manufacture, e.g. their compression characteristics for tablet formation, their flow properties in capsule and tablet production.

Such studies are required at later stages of development and, although still “pre” formulation studies, should be performed only when it is clear that a development candidate drug has ben identified and when the formulation of a final dose form is definitely required.

The physico–technical properties can be described as “fundamental” or “derived”.^14–16 

Viz.: Fundamental. The inherent physicochemical properties of the compound (e.g. melting point, solubility, stability, taste, absolute density, hardness etc.).

Derived. Those characteristics which are dependant upon the physical state of the solid, which can vary according to how the substance is manufactured and processed, e.g. particle size, size distribution, surface area, specific surface, particle shape, bulk and tapped density, cohesiveness, dispersibility, flowability, compactability, including material tensile strength, stress relaxation and stress density; strength : pressure and force displacement profiles.

E. F. Fiese and T. A. Hagen, in Chapter 8 Preformulation in The Theory and Practice of Industrial Pharmacy, ed. L. Lachman, H. A. Lieberman and J. L. Kanig, Lea & Febiger Philadelphia, 1986.

S. Motola and S. N. Agharkar, in Chapter 4 in Pharmaceutical Dosage Forms: Parenteral Medications, ed. K. E. Avis, H. A. Lieberman and L. Lachman, Marcel Dekker, NY, 2nd edn, 1992, vol. 1.

J. T. Carstensen, in Preformulation Chapter 7 in Modern Pharmaceutics, ed. G. s. Banker and C. T. Rhodes, Marcel Dekker, 4th edn, 2002.

M. Gibson, Pharmaceutical Preformulation and Formulation, InterPharm/CRC, 2003.

A. T. Florence and D. Attwood, Physicochemical Principals of Pharmacy, Pharmaceutical Press, 5th edn, 2011.

S. Gatisford, in Part 5 in Aulton's Pharmaceutics: The Design and Manufacture of Medicines, ed. M. E. Aulton, K. Taylor, Churchill Livingstone Elsevier, 4th edn, 2013.