Introduction to Preformulation Studies
Drugs are marketed as tablets, capsules, injections, solutions, suspensions, etc. Prior to the development of these dosage forms, it is essential that certain fundamental physical and chemical properties of the drug molecule and other derived properties of the drug powder (for solid dosage forms) are determined. This information determines many of the subsequent events and approaches in formulation development. This first learning phase is known as. preformulation.
Preformulation involves the application of biopharmaceutical principles to the
characterization of physicochemical parameters of drug substance with the goal of designing optimum drug delivery system.
Before beginning the formal preformulation programs the preformulation scientist must
consider the following factors :-
Factors to consider before preformulation program
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The amount of drug available.
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The physicochemical properties of the drug already known.
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Therapeutic category and anticipated dose of compound.
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The nature of information, a formulation should have or would like to have.
UV Spectroscopy
The first requirement of any Preformulation study is the development of a simple
analytical method for quantitative estimation in subsequent steps. Most drugs have aromatic rings and/or double bonds as part of their structure and absorb light in UV range. UV spectroscopy being a fairly accurate and simple method is a performed estimation technique at early
preformulation stages.
Characterization of drug molecules is a very important step at the preformulation phase of
product development. The following studies are conducted as basic preformulation studies, although special studies are conducted depending on the type of dosage form and the type of
drug molecules.
Preformulation Studies
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Solubility determination
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pKa determination
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Partition co-efficient
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Crystal properties and polymorphism
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Practical size, shape and surface area.
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Chemical stability profile.
1. Solubility determination
The solubility of a drug is an important physicochemical property because it affects the rate of drug release into dissolution medium, the bioavailability of the drug and consequently, the therapeutic efficacy of the pharmaceutical product.
The solubility of the drug molecules in various solvents is determined as a first step. This
information is valuable in developing a formulation. Solubility is usually determined in
variety of commonly used solvents and some oils if the molecule is lipophillic.
The solubility of material is usually determined by the equilibrium solubility method, which employs a saturated solution of the material, obtained by stirring an excess of material in the solvent for a prolonged time until equilibrium is achieved.
Common solvents used for solubility determination are:-
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Water
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polyethylene glycols
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propylene glycol
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glycerine
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sorbitol
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ethyl alcohol
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methanol
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benzyl alcohol,
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isopropyl alcohol,
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polysorbates,
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castor oil,
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peanut oil,
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sesame oil,
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buffer at various pHs, etc.
Aqueous solubility
The availability of a drug is always limited and the preformulation scientist may only have 50 mg. Solubility dictates the ease with which formulation for oral gavages and intravenous injection studies in animals are obtained. The pKa allows the formulator to know the pH to maintain solubility and to choose salts required to achieve good bioavailability from the solid state and improve stability and powder properties.
Intrinsic solubility (Co)
An increase in solubility in acid compared to aqueous solubility suggests a weak base and an increase in solubility in alkali, a weak acid. An increase in acidic and alkaline solubility
suggest either amphoteric or zwitterion behaviour. In this case there will be two pKas, one acidic and one basic. When the polarity of the drug sample can be assured the solubility
obtained in acid for a weak acid or alkali for a weak base can be assured to be the intrinsic
solubility (Co.) i.e. the fundamental solubility when completely unionized. The solubility
should ideally be measured at two temperatures.
Temperature to measure solubility
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4 degree Celsius to ensure physical stability on short term storage. The minimum density of water occurs at 4 oC. This leads to minimum aqueous solubility.
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37 degree Celsius to support biopharmaceutical evaluation.
pKa determination
Determination of the dissociation constant for a drug capable of ionization within a pH range of 1 to 10 is important since solubility and consequently absorption, can be altered by orders of magnitude with changing pH. The Henderson-Hasselbach equation provides an estimate of the ionized and un ionized drug concentration at a particular pH.
Partition coefficient
Partition coefficient (oil/water) is a measure of a drug’s lipophilicity and is an indication of
its ability to cross cell membranes. It is defined as the ratio of unionized drug distributed between the organic and aqueous phases at equilibrium.
Po/w = (Coil /Cwater) equilibrium.
For series of compounds, the partition coefficient can provide an empirical information for screening of some biologic properties. For drug delivery, the lipophilic/hydrophilic balance has been shown to be a contributing factor for the rate and extent of drug absorption.
Although partition coefficient data alone does not provide understanding of in vivo
absorption, it does provide a means of characterising the lipophilic/hydrophilic nature of the drug.
Since biological membranes are lipoidal in nature, the rate of drug transfer for passively
absorbed drugs is directly related to the lipophilicity of the molecule. The partition coefficient is commonly determined using an oil phase of octanol or chloroform and water.
Drugs having values of P much greater than 1 are classified as lipophilic, whereas those with partition coefficient much less than 1 are indicative of a hydrophilic drug.
Although it appears that the partition coefficient may be the best predictor of absorption rate, the effect of dissolution rate, pKa and solubility on absorption must not be neglected.
Dissolution
The dissolution rate of the a drug is only important where it is the rate limiting step in the absorption process. It is suggested that provided the solubility of a drug exceeded mg/ml at pH 7 no bioavailability or dissolution related problems were to be expected. Below mg/ml such problems were quite possible and salt formation could improve absorption and solubility by controlling the pH of the microenvironment, independently of the drug and dosage forms position within the GI tract.
Intrinsic dissolution rate
When dissolution is controlled solely by diffusion the rate of diffusion is directly proportional
to the saturated concentration of the drug in solution.
Common ion effect
A common ion significantly reduces the solubility of a slightly soluble electrolyte. The
‘salting out’ results from the removal of water molecules as solvent owing to the competing
hydration of other ions. The reverse process ‘salting in’ occurs with large anions e.g.
benzoate, salicylate which open the water structure. These hydrotrops increase the solubility
of poorly water soluble compounds such as diazepam.
Melting Point
The melting point of a drug can be measured using three techniques:
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Capillary melting
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Hot stage microscopy
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Differential scanning calorimetry or thermal analysis.
Capillary melting
Capillary melting gives information about the melting range but it is difficult to assign an
accurate melting point.
Hot stage microscopy
This is the observation of melting under a microscope equipped with a heated and lagged
sample stage. The heating rate is controllable and up to three transitions can be registered.
Differential scanning calorimetry and thermal analysis
Differential thermal analysis (DTA) measures the temperature difference between the sample
and a reference as a function of temperature or time when heating at a constant rate.
Differential scanning calorimetry (DSC) is similar to DTA except that the instrument
measures the amount of energy required to keep the sample at the same temperature as the reference i.e. it measures the enthalpy of transition.
Crystal properties and polymorphism
Many drug substance can exit in more than one crystalline form with different space lattice
arrangements. This property is known as polymorphism. Polymorphs generally have different melting points, x-ray diffraction patterns and solubility even though they are chemically identical.
Differences in the dissolution rates and solubilities of different polymorphic forms of a given drug are very commonly observed. When the absorption of a drug is dissolution rate limited, a more soluble and faster-dissolving form may be utilized to improve the rate and extent of bioavailability.
For drugs prone to degradation in the solid state, physical form of the drug influences
degradation. Selection of a polymorph that is chemically more stable in a solution in many
cases. Different polymorphs also lead to different morphology, tensile strength and density of power bed which all contribute to compression characteristics of materials. Some investigation of polymorphism and crystal habit of a drug substance as it relates to pharmaceutical processing is desirable during its preformulation evaluation especially when the active ingredient is expected to constitute the bulk of the tablet mass. Although a drug substance may exist in two or more polymorphic forms, only one form is thermodynamically stable at a given temperature and pressure. The other forms would convert to the stable form
with time. In general, the stable polymorph exhibits the highest melting point, the lowest
solubility, and the maximum chemical stability. Various techniques are available for the investigation of the solid state. These include microscopy (including hot stage microscopy), infrared spectrophotometry, single-crystal x-ray and x-ray power diffraction, thermal analysis, and dilatometry.
Particle size, shape and surface area
Involve bulk flow, formulation homogeneity and surface-area controlled processes such as dissolution and surface morphology of the drug particles. In general, each new drug candidate should be tested during preformulation with the smallest particle size as is practical to facilitate preparation of homogeneous samples and maximize the drug’s surface area for interactions.
Various chemical and physical properties of drug substances are affected by their particle size distribution and shapes. The effect is not only on the physical properties of solid drugs but also, in some instances, on their biopharmaceutical behaviour. It is generally recognized that poorly soluble drugs showing a dissolution-rate limiting step in the absorption process will be more readily bio available when administered in a finely subdivided state rather than as a coarse material.
In case of tablets, size and shape influence the flow and the mixing efficiency of powders and granules. Size can also be a factor in stability: fine materials are relatively more open to attack from atmospheric oxygen, the humidity, and interacting excipients than are coarse materials.
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Determination of particle size- carried out by different methods.
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Determination of surface area – carried out by different methods.
Particle size determination
Though microscopy is the simplest technique of estimating size ranges and shapes, it is too
slow for quantitative determination. The material is best observed as a suspension in non dissolving fluid. Sieving is less useful technique at preformulation stage due to lack of reproducibility. Andreason pipette is based on the rate difference of sedimentation of
different particles, but techniques like this are seldom used due to their tedious nature.
Instruments based on light scattering, light blockage and blockage of electrical conductivity path (coulter counter) are available.
Surface area determination
Surface area is most commonly determined based on Brunauer-Emette-Teller (BET) theory of adsorption. Most substances adsorb a monomolecular layer of gas under certain conditions of partial pressure of gas and temperature. Knowing the monolayer capacity of adsorbent and
the area of molecule of the adsorbate, the surface area can be calculated. The adsorption process is carried out with nitrogen. The adsorption takes place by virtue of van der Waal’s forces.
Power flow properties
When limited amounts of drugs are available, powder flow properties can be evaluated by
measurements of bulk density and angle of repose. Changes in particles size and shape are generally very important. An increase in crystal size or a more uniform shape will lead to a small angle of repose and a smaller Carr’s index Bulk density Knowledge of absolute and bulk density of the drug substance is very useful in having some
idea as to the size of final dosage form. The density of solids also affects their flow
properties.
Carr’s compressibility index
Carr’s compressibility index can be used to predict the flow properties based on density
measurement. A similar index has been defined by Hausner ratio.
Angle of repose
The maximum angle which is formed between the surface of a pile of powder and horizontal surface is called the angle of repose.
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Chemical stability profile
Preformulation stability studies are usually the first quantitative assessment of chemical
stability of a new drug. These studies include both solution and solid state experiments under condition typical for the handing, formulation, storage and administration of a drug candidate as well as stability in the presence of other recipients.
Factors affecting chemical stability critical in rational dosage form design include:
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temperature
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pH and
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dosage form diluents.
The method of sterilization of potential product will be largely dependent on the temperature stability of the drug. Drugs having decreased stability at elevated temperatures cannot be sterilized by autoclaving but must be sterilized by
another means, e.g., filtration. The effect of pH on drug stability is important in the
development of both oral and parenteral dosage forms. Solutions for oral administration must be protected from the highly acidic environment of the stomach. Buffer selection for potential dosage forms will be largely based on the stability characteristic of the drug.
- Solid state stability
- Solution phase stability
- Compatibility studies: stability in the presence of excipients
- Typical stability protocol for a new chemical entity
Solid state stability
Chemical instability normally results from either of the following reactions: hydrolysis,
oxidation, photolysis or pyrolysis. Chemical structure of the drug determines the response of drugs to either of these attacks. Esters and lactates and to a lesser extent, amides are prone to solvolysis. Unsaturation or electron rich centre in the structure makes the molecule vulnerable to free radical mediated or photo-catalysed oxidation. Physical properties of drug are also affected. Amorphous materials are less stable than their crystalline forms. Denser
materials are more stable to ambient stress.
Elevated temperature studies
The elevated temperatures commonly used are 40, 50 and 60 oC, with ambient humidity. The samples stored at highest temperature are observed weekly for physical and chemical changes and compared to an appropriate control. If a substantial change is seen, samples stored at lower temperatures are examined. If no changes is seen after 30 days at 60 oC, the stability prognosis is excellent.
Stability under high humidity conditions
Solid drug samples can be exposed to different relative humidity conditions by keeping them in laboratory desiccators containing saturated solutions of various salts. The closed desiccators in turn are kept in oven to provide constant temperature. The preformulation data of this nature are useful in determining if the material should be protected and stored in controlled low humidity environment or if non-aqueous solvent be used during formulation.
Photolytic stability
Many drugs fade or darken on exposure to light. Though the extent of degradations is small and limited to the exposed surface area, it presents an aesthetic problem. Exposure of drug 400 and 900 foot-candles of illumination for 4 and 2 week periods respectively is adequate to provide some idea of photosensitivity. Resulting data may be useful in determining if an amber colored container is required or if colour masking dye should be used in the formulation.
Stability to oxidation
Drug’s sensitivity to oxidation can be examined by exposing it to atmosphere of high oxygen tension. Usually a 40% oxygen atmosphere allows for rapid evaluation. A shallow layer of drug exposed to a sufficient headspace volume ensures that the system is not oxygen limited.
Samples are kept in desiccators equipped with three-way stop cocks, which are alternatively evacuated and flooded with desired atmosphere. The process is repeated 3 or 4 times to ensure 100% desired atmosphere. Results may be useful in predicting if an antioxidant is
required in the formulation or if the final product should be packaged under inert atmospheric conditions.
Compatibility studies
The knowledge of drug-excipients interaction is useful for the formulation to select
appropriate excipients. The described preformulation screening of drug-excipients interaction requires only 5 mg of drug in a 50% mixture with the excipients to maximize the likelihood of obscuring an interaction. Mixtures should be examined under nitrogen to minimize oxidation and pyrolytic effect at a standard heating rate on DSC, over a temperature range, which will encompass any thermal changes due to both the drug and appearance or disappearance of one or more peaks in themograms of drug-excipient mixtures are considered as indicative of interaction.
Solution phase stability
As compared with the dry form, the degradation is much rapid in solution form. It is important to ascertain that the drug doesn’t degrade when exposed to GI fluid. The pH based stability study, using different stimulator of GI condition can be designed. A poor solution stability of drug may urge the formulator to choose a less soluble salt form, provided the bioavailability is not compromised
Absorption behaviour
It is essential to test the in vivo behaviour of the new drug for successful formulation of a
dosage form with good bioavailability. Partial in vivo and in vitro test are designed to study
pharmacokinetic profile of the drug.
Conclusion
Preformulation studies have a significant part to play in anticipating formulation problems
and identifying logical path in both liquid and solid dosage form technology. The need for
adequate drug solubility cannot be overemphasized. The most appropriate salt for development and stability studies in solution will indicate the feasibility of parental or other liquid dosage form and can identify methods of stabilization. In parallel, solid-state stability by DSC, TLC and HPLC in the presence of tablet and capsule excipient will indicate the most acceptable vehicles for solid dosage form.
By comparing the physicochemical properties of each drug candidate within a therapeutic
group, the preformulation scientist can assist the synthetic chemist to identify the optimum
molecule, provide the biologist with suitable vehicles to elicit pharmacological response and advise the bulk chemist about the selection and production of the best salt with appropriate particle size and morphology for subsequent processing.