Drug-excipient compatibility studies represent an important phase in drug development. Before a drug substance is formulated into the desired dosage form, there is need for the formulation scientist to fully consider the chemical structure of the drug substance, the type of delivery system required and the proposed manufacturing process. Drug substances are usually combined with excipients which serve different and specialized purpose. Although excipients are pharmacologically inert, they can undergo chemical reactions and physical interactions with drug substances under favourable environmental conditions. These interactions can lead to instability resulting in the formation of new entities with different physicochemical properties and pharmacological effects.
Drug-excipient compatibility studies have been used as an approach for accepting/rejecting excipients for use in pharmaceutical formulations, thus allowing the rapid optimization of a dosage form with respect to patentability, processing, drug release, elegance, and physicochemical stability. In order to obtain rapid stability assessment of drug and excipients, drug stability is investigated under the stress condition according to standard protocol and/or existing knowledge on potential degradation pathway or incompatibilities.
- 1 Importance of Drug-Excipient compatibility studies
- 2 Goals of Drug-Excipient compatibility studies
- 3 Mechanism of Drug-Excipient(s) interactions
- 4 Analytical Methods for Drug – Excipient Incompatibility
- 4.1 1. Thermal Techniques
- 4.1.1 a. Differential Scanning Calorimetry (DSC)
- 4.1.2 b. Isothermal microcalorimetry
- 4.1.3 c. Differential Thermal Analysis
- 4.2 2. Spectroscopic Techniques
- 4.3 3. Chromatographic Techniques
- 4.3.1 a. Thin Layer Chromatography (TLC)
- 4.3.2 b. High Performance Liquid Chromatography/ High Pressure Liquid Chromatography (HPLC)
- 4.1 1. Thermal Techniques
- 5 Conclusion
Importance of Drug-Excipient compatibility studies
- It maximizes the stability of a dosage form.
- It bridges drug discovery and development.
- It is essential investigational new drug submission (IND).
- It helps to avoid surprise problems during formulation processes.
Goals of Drug-Excipient compatibility studies
- To find out how compatible an excipient is with Active Pharmaceutical Ingredient (API) or candidate drug molecules.
- To find out the excipient that stabilizes an unstable API.
- To assign a relative risk level to each excipient.
- To design and develop selective and stability indicating analytical methods to determine their impurities.
Mechanism of Drug-Excipient(s) interactions
The mechanisms of drug-excipient(s) interactions are not fully understood despite the best efforts of several eminent investigators in the field. However, some of the common ways by which excipients may alter drug stability in a dosage form include:
a. Physical drug-excipient interactions
These types of interactions are quite common but are very difficult to detect in dosage forms. Drug substances and excipients interact without undergoing changes involving breaking or formation of new bonds. The components of the drug product retain their chemical structure but undergo changes which alter their physical properties. Physical interactions may result in changes in dosage uniformity, colour, odour, flow properties, solubility, sedimentation rate, dissolution rate etc. Incompatibilities are assessed by physically observing the test samples. Physical interactions can be either beneficial or detrimental to the product performance depending on its application.
Benefits of physical drug-excipient interactions
- Improves bioavailability of sparingly water-soluble drugs: The bioavailability of sparingly water-soluble drugs can be enhanced using complexing agents e.g., complexation of cyclodextrin with ursodeoxycholic acid increases the rate and extent of drug dissolution which in turn increases the bioavailability of the drug substance.
- Increases surface area of drugs available for dissolution: Adsorption of drugs on excipient surface can increase the surface area of the drug available for dissolution. Thus, an increase in bioavailability of drug substance. E.g., formulation of indomethacin using kaolin as adsorbent increases its bioavailability as a result of increased dissolution rate.
- Improves dissolution rate and bioavailability of hydrophobic drugs: Physical interactions of drugs with excipient improve the dissolution rate and bioavailability of hydrophobic drugs. E.g., improved dissolution rates of drugs like piroxicam, norfloxacin, nifedipine and ibuprofen were achieved when these drugs were formulated into solid dispersions using polyethylene glycol of different molecular weights.
Detrimental effects of physical drug-excipient interactions
- Decreases dissolution and absorption rates of drug substances due to the formation of insoluble complexes e.g., tetracycline forms an insoluble complex with calcium carbonate leading to slower dissolution and decreased absorption in the gastrointestinal tract. Formulation of chlorpromazine with polysorbate 80 and sodium lauryl sulphate decreased membrane permeability of the drug.
- Reduces bioavailability of drugs available for dissolution: Adsorption of drugs on excipient surface can also lead to reduced bioavailability as the drug is not available for dissolution. E.g., the marked reduction in the antibacterial activity of cetyl pyridinium chloride cations in tablets containing cetyl pyridinium chloride is due to the adsorption of cetyl pyridinium chloride on the surface of magnesium stearate which acts as a lubricant.
- Slow dissolution of drugs: Ion interactions can result in slow dissolution of drugs. E.g., solid dispersion product formed due to interactions between povidone and stearic acid in a capsule showed slow dissolution of the drugs.
b. Chemical drug-excipient interactions
This involves the interaction of drug substance and excipient through chemical degradation pathway. The formulation undergoes a chemical reaction in which the constituent atoms are rearranged via bond breakage and bond formation to produce an unstable chemical entity. Generally, chemical interactions have a deleterious effect on the formulation hence; such kind of interactions must be avoided. Chemical interactions can be in the form of hydrolysis, oxidation, racemization, polymerization, Maillard reactions, photolysis etc., and changes in the study samples are analyzed by a chromatographic-based assessment of potency and formation of degradants or by any other analytical method depending on the nature of the candidate drug molecule, available literature and the goals of the study. Some examples of chemical drug-excipient interactions include
- Inhibition of diclofenac sodium release from matrix tablet by polymer chitosan at low pH. This occurs possibly via formation of ionic complex between diclofenac sodium and ionized cationic polymer.
- Oxidation of diethylstilbestrol to the peroxide and conjugated quinone degradation products by Silicon dioxide which acts as a catalyst.
c. Physiological/Biopharmaceutical drug-excipient interactions
By this, we mean interactions that occur after the drug product has been administered to the patient. These interactions are similar to physical interactions but differ in the sense that
- The interaction is between the medicine (drug substance and excipients) and the body fluids.
- The interactions have the tendency to influence the rate of absorption of the drug.
All excipients interact in a physiological sense when they are administered as part of a dosage form. They are included in a formulation specifically because they interact with the physiological fluids and function in certain ways e.g., disintegrants in immediate release tablets and capsule formulations. On the other hand, physiological interactions can be detrimental to the patient. Examples of such interactions include
- Premature breakdown of enteric coat – Enteric coating polymers e.g., cellulose acetate phthalate and hydroxyl propyl cellulose acetate phthalate, dissolve prematurely in the stomach in the presence of antacids or drugs that cause increase in the pH of the stomach. This results in premature release of active pharmaceutical ingredient in stomach itself, which case results in degradation of drug in stomach e.g., pro-drugs or side effects like gastric bleeding as in the case of NSAIDs.
- Interactions due to adjunct therapy– A classic biopharmaceutical incompatibility is the interaction between tetracycline antibiotics and antacids containing aluminium, calcium, magnesium, bismuth and zinc ions. The tetracycline antibiotics chelates with these metallic ions to form complexes which only are not poorly absorbed, but also have reduced antibacterial effects.
- Increase in gastrointestinal motility – Certain excipients such as sorbitol and xylitol have the tendency to increase gastrointestinal motility, thus reducing the available time for absorption of drugs like metoprolol. The effect is very much dependent on the amount of the excipient administered at one time. Polyethylene glycol 400 has also been reported to influence the absorption of ranitidine.
Analytical Methods for Drug – Excipient Incompatibility
The key to the early assessment of instability in formulations is the availability of analytical methods to detect low levels of degradation products, generally less than 2%. Below are some of the analytical methods which are used in drug-excipient compatibility studies.
1. Thermal Techniques
Thermal methods of analysis comprise a group of techniques in which the physicochemical properties of drug substances are measured as a function of temperature. In this method, the test samples are subjected to a controlled temperature over a given period of time. This method of analyses plays a vital role in drug-excipient compatibility studies and has been frequently used for quick identification of physicochemical interaction between drugs and excipients.
a. Differential Scanning Calorimetry (DSC)
DSC represents a leading thermal screening technique that has been increasingly used for excipient compatibility studies for over five decades. In this technique, the DSC curves of pure samples are compared to that obtained from 50% mixture of the drug and excipient (usually 5mg of the drug in a ratio of 1:1 with the excipient). It is assumed that the thermal properties (melting point, change in enthalpy, etc.) of blends are the sum of the individual components if the components are compatible with each other. An absence, a significant shift in the melting of the components or appearance of a new exo/endothermic peak and/or variation in the corresponding enthalpies of reaction in the physical mixture indicates incompatibility. However, slight changes in peak shape height and width are expected due to possible differences in the mixture geometry.
Advantages of Differential Scanning Calorimetry
- Requires of short time of analysis
- Low sample consumption
- Provides useful indications of any potential incompatibility
Limitations of Differential Scanning Calorimetry
- Conclusions based on DSC results alone may be misleading and have to be interpreted carefully.
- DSC cannot be used if thermal changes are very small. Therefore, it should always be supported by some non-thermal methods like TLC or FT-IR or XRPD.
- DSC cannot detect the incompatibilities which might occur after long-term storage.
b. Isothermal microcalorimetry
This is an extremely sensitive and invaluable tool used to determine drug-excipient incompatibilities. It measures minute amounts of heat emitted or absorbed by a sample in a variety of processes. This method of analysis is used to characterize pharmaceutical solid to obtain heats of solution, heats of crystallization, heats of reaction, heats of dilution and heats of adsorption – since nearly all physicochemical processes are accompanied by a heat exchange within their surroundings. In a typical drug-excipient compatibility study, a solution, suspension, or solid mixture of drug substance and excipient is placed in the calorimeter and the thermal activity (heat gained or evolved) at a constant temperature is monitored. The thermal activity observed is assumed to be proportional to the rate of chemical and/or physical processes taking place in the sample. The thermal activity of the test sample is compared to the “non-interaction” curve constructed from the control (i.e., thermal activity of drug substance and excipient that were measured individually). If an experimentally significant difference is observed, the excipient is considered to be potentially incompatible with the drug substance.
Advantages of Isothermal microcalorimetry
- Samples are not heated, and so the changes are observed as it might typically occur at ambient conditions.
- It is sensitive to small changes in heat gained or evolved, thus small samples, or slow processes, may be investigated.
- It gives meaningful results without requirement of multiple sample preparations
- Does not require long storage times, thus saving valuable time and effort during the formulation process.
Limitations of Isothermal microcalorimetry
- Isothermal microcalorimetry is not discriminatory. The exact nature of the transition must be known in order to interpret the data.
c. Differential Thermal Analysis
Differential Thermal Analysis (DTA) is an analytical technique in which the changes in temperature between a test sample and an inert reference under controlled and identical conditions is used to identify and quantitatively analyze the chemical composition of a substance. When the test sample and inert reference are heated to a sufficient temperature, the thermal changes in the test sample which lead to the absorption or emission of heat can be detected relative to the inert reference (control). The differences in temperature are then plotted against time, or against temperature. Drug-excipient interactions can be identified by comparing DTA curves obtained from the test sample with those of inert reference. Incompatibilities are indicated by the appearance of one or more new DTA peaks or the disappearance of one or more DTA peaks corresponding to those of the components of the test sample. In the absence of any interaction, the DTA peak of the test sample show patterns corresponding to those of the individual components.
Advantages of Differential Thermal Analysis
- DTA technique yield data that are considerably more fundamental in nature.
- Enthalpy change (under a DTA peak) is not affected by the heat capacity of the sample.
Limitations Differential Thermal Analysis
- Differential Thermal Analysis is usually performed on powders and for this reason, the resulting data may not be representative of bulk samples, where transformations may be controlled by the buildup of strain energy.
- The rate of heat evolution may be high enough to saturate the response capability of the measuring system. This limitation may be overcome by diluting the test sample with inert material.
- Problems are encountered in transferring heat uniformly away from the specimen at temperature range of 200 to 500◦C. This problem may be solved by using flat disc-like thermocouples to ensure optimum thermal contact with the now flat bottomed sample container.
2. Spectroscopic Techniques
Spectroscopic analytical methods include all techniques which probe certain features of a given sample by measuring the amount of radiation emitted or absorbed by molecular or atomic species of interest. This method of analysis uses electromagnetic radiation to interact with matter and thus investigate certain features of a sample as a function of wavelength (λ). Because these methods of analysis use a common set of optical devices for collimating and focusing the radiation, they often are identified as optical spectroscopies. Some of the most frequently used spectroscopic methods of analysis include vibrational spectroscopy, diffuse reflectance spectroscopy, fluorescence spectroscopy, FT-IR spectroscopy etc. and each operates over different, limited frequency ranges within this broad spectrum, depending on the processes and degree of the energy changes.
a. Vibrational spectroscopy
Using this method, information on the molecular structure and environment of organic compounds are generated by measuring the vibrations of chemical bonds that result from exposure to electromagnetic energy at various frequencies. These vibrations are commonly studied by infrared and Raman spectroscopies. While infrared spectroscopy uses the infrared region of the electromagnetic spectrum (from about 400 cm-1 to 4000 cm-1) to measures the change in dipole moment, Raman spectroscopy uses inelastic scattering process to measures the change in polarization of the sample. The spectra obtained are indicative of the nature of chemical bonds present in the test sample, and when pieced together can be used to identify the chemical structure or composition of a given sample. Vibrational spectroscopy are not only used to investigate solid state properties of drug substances and their formulations, but are also used as compatibility study tool as the vibrational changes serve as probe of potential intermolecular interactions among the components. Thus, drug-excipient interactions that occur during processing can easily be detected with the aid of these spectroscopic techniques.
Advantages of vibrational spectroscopy
- Sensitive and can be used for process monitoring.
- Requires short time of analysis
- Nondestructive method of analysis with the exception of some UV-Vis applications
- Requires minimal or no sampling preparation (Raman spectroscopy)
- Provides complex fingerprint which is unique to the compound under investigation (IR spectroscopy).
Limitations of vibrational spectroscopy
- Presence of overlapping peaks in the spectra may hinder the analysis.
- Solvent may interfere if samples are run in solution (Raman spectroscopy)
- Rarely used as a quantitative technique because of relative difficulty in sample preparation and complexity of spectra (IR spectroscopy).
b. Flourescence Spectroscopy/ Fluorometry/ Spectrofluorometry
This is a type of spectroscopic techniques which analyzes fluorescence properties of samples in order to provide information regarding their concentration and molecular environments. It involves using a beam of light, usually UV/visible radiation, to excite the electrons in molecules of certain compounds particularly those with chromophore and rigid structure, causing them to emit the radiation at a longer wavelength. The radiation emitted (emission spectrum) and/or the radiation absorbed by the sample (excitation spectrum) can then be measured and compared with the control. Apart from determining the stability of peptide drugs in solution, fluorescence spectroscopy has also been used in
- Carrying out limit test where the impurities are fluorescent or can simply be rendered fluorescent.
- Determination of fluorescent drugs in low dose formulations containing non-fluorescent excipients.
- Studying the binding of drugs to components in complex formulations and measuring small amount of drugs and for studying drug-protein binding in bioanalysis.
Advantages Fluorescence Spectroscopy
- It is highly sensitive, specific and easy to carry out.
- Samples are analyzed at low cost as compared to other analytical techniques.
- It is a selective detection method, thus, it can be used to quantify a strongly fluorescent compound in the presence of a larger amount of non-fluorescent materials.
- Can be used to monitor changes in complex molecules e.g., proteins which are increasingly used as drugs.
Limitations of Fluorescence Spectroscopy
- The technique only applies to a limited number of molecules as there are relatively small numbers of compounds that have characteristic fluorescence.
- The technique is subject to interferences by UV absorbing species and heavy ions in solution.
- Fluorescence is affected by temperature.
3. Chromatographic Techniques
Chromatography is an analytical technique frequently used in pharmaceutical research for separating sample mixture into its individual components. This technique is based on selective adsorption of the components on a stationary phase (usually a solid or liquid with high surface area). As the solute mixture passes over the stationary phase, the components are adsorbed and released at the surface at varying rates depending on differential affinities of individual components towards stationary and mobile phase. Compared to other available analytical techniques used in drug-excipient compatibility studies, chromatography is known for its characteristics of high resolution and detection power, making it suitable for detecting multiple components in a complex mixture with high accuracy, precision, specificity, and sensitivity. Various chromatographic methods of analysis have been used in drug-excipient compatibility studies, all following the same basic principles of operation.
a. Thin Layer Chromatography (TLC)
TLC is a chromatographic method of analysis carried out on glass, plastic or metal plates coated on one side with a thin layer of adsorbent. The thin layer of adsorbent serves as the stationary phase and is usually made of silica, alumina, polyamide, cellulose or ion exchange resin. In TLC, solutions of the test samples (that is, a mixture of the drug and the excipient) and the controls (individual drug and excipients) are prepared and spotted on the same baseline at the end of the plate (the origin). The plate is then placed upright in a closed chamber containing mixtures of organic solvents which serve as the mobile phase. The analyte moves up the plate, under the influence of the mobile phase which moves through the stationary phase by capillary action. The distance moved by the analyte is dependent on its relative affinity for the stationary or the mobile phase. Incompatibilities are indicated by the formation of a spot with Rf value (retardation factor) different from that of the controls after the plate has been developed with solvent. An excipient on the other hand is considered to be potentially compatible with the drug substance if the spots produced have identical Rf value with those of the controls. Because some samples undergo negligible thermal changes which might be difficult to detect by thermal methods of analysis, TLC is widely used in drug-excipient compatibility study as a confirmative test of compatibility after performing DSC.
Advantages of Thin Layer Chromatography
- The technique is robust and cheap
- The compound formed as a result of incompatibilities between the drug and the excipient can be detected if a suitable detection reagent is used.
- Unlike gas chromatography and high-performance liquid chromatography in which some components of a mixture may elute from the chromatographic system, there is no risk of losing any component of the mixture in TLC since all component of a mixture can be seen in the chromatographic system.
- Batch chromatography can be used to analyze many samples at a time, thus increasing the speed of analysis.
Limitations of Thin Layer Chromatography
- This technique is not suitable for volatile substances.
- Sensitivity in often limited.
- Requires more operators skill for optimal use than high-performance liquid chromatography
b. High Performance Liquid Chromatography/ High Pressure Liquid Chromatography (HPLC)
HPLC is a chromatographic technique widely used in drug-excipient compatibility studies by quantitative estimation of test samples that have been subjected to isothermal stress testing (IST). This method of analysis is based on mechanisms of adsorption, partition and ion exchange, depending on the nature of the stationary phase used. In HPLC, a liquid mobile phase is pumped under high pressure through the stationary phase (a stainless-steel column packed with tiny particles with a diameter of 3 to 10 micron). A small volume of the test sample is loaded onto the head stainless-steel column via a loop valve. Separation of a sample mixture occurs according to the relative lengths of time spent by its components in the stationary phase. Column effluent can be monitored with a variety of flow-through device/detector that measures the amount of the separated components. HPLC results that show a percentage loss similar to the control (drug considered individually) indicate no interaction between drug and the excipients and vice versa.
Advantages of High-performance liquid chromatography
- Suitable for separating nonvolatile or thermally sensitive molecules such as amino acids, steroids etc.
- Has broad applicability, that is, it can be used for both organic and inorganic samples.
- Can be very sensitive and accurate.
- Provides better precision relative to the changes being investigated.
- Can be readily automated.
- Less risk of sample degradation since heating is not required in the process.
Limitations of High-performance liquid chromatography
- Takes considerable time and resources
- Solvents used cannot be recycled.
- There is still need for reliable and inexpensive detectors which can monitor compounds that lack chromophores.
Drug-excipient compatibility study is a necessary prerequisite to the development of drug products that are safe and stable for use. Proper selection and assessment of possible incompatibilities between the drug and excipients during preformulation studies is of paramount importance to accomplish the target product profile and critical quality attributes. In order to avoid stability problems encountered during drug development and post-commercialization, there is need for proper assessment of possible incompatibilities between the drug and excipients using appropriate analytical techniques. These analytical techniques are needed not only to generate useful information with regards to which excipient is compatible with a drug substance, but also for troubleshooting unexpected problems which might arise during formulation processes. Drug-excipient interactions may take a long time to be manifested in conventional stability testing programs, and are not always predicted by stress and pre-formulation studies. It is hoped that this write-up provides valuable information concerning the drug–excipient interactions that aid in the selection of appropriate excipients for safe, stable and bioavailable dosage form.
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