Lab Training notes 22 7 13 2003 format
PreFormulation
The word preformulation is composed of two words pre and formulation. Studies done on a drug molecule before formulation development are called as preformulation studies.
The various physical, chemical and physicotechnical properties of the drug molecule are found out in pre formulation studies. The information obtained in preformulation studies will be used in formulation development. Preformulation can be considered as the learning phase about the molecule. Example: If the drug molecule is found to be unstable in GIT, it has to be formulated as an injection.
Example: If the drug molecule is found to be unstable in presence of light, the product should be manufactured in a dark area and the final product is to be stored in light resistant containers.
Example: If the drug molecule is found to be unstable in presence of oxygen, filling of ampoules will be done under nitrogen blanket atmosphere.
Example: If the drug molecule is found to have poor solubility, it is converted into salt form. Diclofenac is converted into diclofenac sodium salt for this reason.
Example: If the drug molecule is unstable in gastric fluids, formulate enteric coated tablets.
Example: If the drug molecule is having very poor solubility and poor tissue permeability, avoid this molecule. Fail early – fail cheap.
Example: If the drug molecule is found to be incompatible with lactose, avoid lactose in the formulation.
The various properties of a drug that are determined in preformulation studies are fundamental properties, derived properties and drug excipients in compatibilities.
Fundamental properties: These properties depend on the chemical structure of the drug molecule. They include
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2. Dissolution kinetics 3. pKa
4. Partition or distribution coefficient (log P or log D) 5. Permeability of a drug across biological membranes 6. Solution state stability
7. Solid state stability 8. Solid state properties
Derived preformulation properties: The properties that a drug substance should have to develop a particular dosage form like solid, oral, liquid oral or parenteral are called derived properties.
Derived preformulation properties for solid oral dosage form like tablet, include
1. Particle size 2. Bulk density 3. Flow properties
4. Compaction behavior
Derived preformulation properties are specific to a particular dosage form to be developed. To develop capsules, compaction behavior is not required.
Drug Excipients interactions: The last activity in preformulation studies is compatibility studies; the physical and chemical stability of the drug molecule is studied in presence of excipients. The choice of excipients selected for the study depends on the type of dosage form to be developed.
1. Solubility:
Concept: The maximum extent to which a drug dissolves in a solvent at a particular temperature is called solubility.
Example: Solubility of paracetamol is 1 g in 70 ml water, solubility of NaCl is 35 g in 100 ml water.
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Importance: Solubility is an important preformulation property that affects the performance of a drug molecule. Solubility plays an important role during formulation development.
Bioavailability of tablets and capsules depend on solubility and dissolution rate of the drug. Orally administered drug has to first dissolve in gastrointestinal fluid before it can be absorbed. Dissolved drug then passes through the intestinal membrane and enters into blood.
Solubility studies in nonaqueous solvents are important for development of analytical methods, crystallization method and formulation development of soluble oral or Parenteral formulations.
About 40% of drug molecules fail to come into the market because of poor aqueous solubility.
Determination: Solubility of a drug is determined using shake flask method. Preformulation solubility studies are carried out in distilled water, 0.9 % NaCl (isotonic solvent), 0.01 M HCl, 0.1 M HCl, 0.1 M NaOH (all at room temperature and at 37 degree centigrade).
Solubility studies are also carried out in fasted state simulating intestinal fluid/ the fed state simulated intestinal fluid.
The effect of temperature and pH on solubility of drug is also studied.
If a drug is having poor solubility and dissolution rate, limited solubilization experiments are carried out using co solvency/ micellar solubilization /complexation/solid dispersions/ salt formation principles. This information will be useful for the formulation scientist to formulate his dosage form.
Example: Diclofenac is converted into diclofenac sodium to increase its aqueous solubility and dissolution rate.
Example: Nimesulide is complexed with beta cyclo dextrin to increase its aqueous solubility, dissolution rate and bioavailability.
2. Dissolution Rate:
Concept: The process by which a drug goes into a solvent to form a solution is called dissolution. The speed of dissolution is called dissolution rate. For
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example salt dissolves fast in water when compared with sugar.
Importance: Orally administered drug has to first dissolve in gastrointestinal fluid before it can be absorbed. Dissolved drug then passes through the intestinal membrane and enters into blood circulation.
Dissolution rate (speed of dissolving) of drug particles depend on several physico chemical properties like chemical form, crystal habit, particle size, solubility, surface area and wetting properties. Dissolution experiments help to identify the solvate / polymorph / complex / solid dispersions/ solvent deposited systems/ salt forms having high dissolution rates. The fast dissolving forms will be used by the formulation scientist to develop tablets.
Example: Diclofenac is converted into diclofenac sodium to increase its aqueous solubility and dissolution rate.
Example: Nimesulide is complexed with beta cyclo dextrin to increase its aqueous solubility, dissolution rate and bioavailability.
Example: Generally solid dispersions of drugs in hydrophilic carriers have good dissolution rate and oral bioavailability.
Determination: Dissolution studies are carried out using official dissolution testing apparatus.
3. Dissociation Coefficient (pKa ):
Concept: Most of the drugs are weak acids or weak bases. The dissociation coefficient is a measure of the drugs ability to ionize in water. The extent of ionization of a drug depends on its dissociation coefficient value and pH of the solvent.
The Henderson – Hasselbalch equation can be used to calculate the extent of ionization of a drug at a particular pH value.
For weak acids, pH = pKa + Log [ionized form/unionized form]
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Importance: Ionized drug molecules have more water solubility but less tissue permeability. Unionized drug molecules have less water solubility but more tissue permeability. This is because unionized molecules are more lipophillic and can easily cross biological membranes.
Gastrointestinal tract exposes the drug molecule to a wide range of pH values. Information on pKa value allows understanding the behavior of ionizable drug molecule, under different pH conditions.
For example a weakly acidic drug with a pKa value greater than three remains unionized in stomach (pH is 1) and is ionized in intestine. Hence we can predict that this drug is going to be rapidly absorbed in the stomach. Due to large surface area of the intestine this drug will be absorbed from the intestine also. Determination: pKa values of a drug can be determined by potentiometric method or by spectral shifts in UV/Visible spectroscopy. Buffers, temperature, ionic strength and co solvents affect the pKa value and should be controlled during pKa experiments.
4. Partition coefficient:
Concept: It indicates the lipophillic nature of a drug molecule and its ability to cross biological membranes. It is given by the equation: P = Ko/w = Co / Cw
P = Partition coefficient of drug between n octanol and water Co = Concentration of drug in octanol
Cw = Concentration of drug in pH 7.4 buffer
Importance: The lipophillic nature of a drug molecule is expressed in terms of P and Log P values. Higher log P values indicate that the drug is more lipophillic in nature. For example, Log P value of diclofenac is 4.0 and Log P value of ibuprofen is 3.65.
If a drug is highly lipophillic, it will easily cross biological membranes in the body.
A small log P value indicates that the drug is hydrophilic in nature, it will have good water solubility but it will not easily cross biological membranes. A higher log P value indicates that the drug is strongly lipophillic. It has poor aqueous solubility, but once it goes into solution in G.I fluids, it enters into the biological membrane and stays there only. It will not go into the blood. This drug gets accumulated in the membrane.
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Hence a drug should have optimum hydrophilic and lipophillic nature. It should dissolve easily in G.I fluids and pass through biological membrane easily. A log P value of two is considered optimal for good bioavailability. Determination: It is determined by shaking the drug between octanol and pH 7.4 buffer until equilibrium is achieved. The two layers are separated and assayed for drug content.
5. Permeability:
Concept: The ability of a drug molecule to cross biological membranes is called permeability. Drugs after dissolution have to cross the membrane for reaching into blood.
Permeability along with solubility forms the scientific basis of Biopharmaceutics Classification System (BCS). According to the BCS, drug substances, are classified as follows:
Class 1: High Solubility – High Permeability Class 2: Low Solubility – High Permeability Class 3: High Solubility – Low Permeability Class 4: Low solubility – Low permeability
A drug substance is considered highly soluble when the highest dose strength is soluble in 250 ml or less of aqueous media over the pH range of 17.5.
A drug substance is considered to be highly permeable when the extent of absorption in humans is determined to be 90% or more of an administered dose.
In terms of apparent permeability coefficient (Papp) across Caco2 cell line, a value greater than 2×105 cm/s indicates good permeability.
Importance: A successful drug molecule should possess a balance between hydrophilic and lipophillic nature, to achieve optimal solubility and permeability. A log P value of about 2 is considered optimal for good permeability.
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system. Drugs belonging to BCS class 3 and 4 have poor permeability and are hence administered using Parenteral route.
Permeability of a drug can be increased by use of permeability enhancers. Some of the commonly used excipients like polysorbate 80, poloxamer and cremophor RH 40, are used for enhancing permeability.
Determination: Permeability can be determined using in vitro tools like PAMPA, Caco2 cell lines, everted rat intestine; in situ experiments like single pass intestinal perfusion in rats and in vivo data like human absorption data.
6. Solution state stability:
Concept: A drug is said to be stable, if it does not undergo any physical and chemical degradation in solution state during storage. Most common pathways for degradation are hydrolysis, oxidation and light.
Importance: From solution state stability studies, we can identify the conditions to prepare a stable solution and for developing liquid formulations. Example: Ampicillin undergoes degradation very fast in solution state. Hence it is formulated as dry syrup. It is to be reconstituted before use and this syrup should be used in 4 days.
Example: Insulin is stable at pH.
Example: If a drug undergoes oxidative degradation, the ampoules are filled under nitrogen blanket atmosphere.
Storage conditions for the dosage form can be recommended basing on this information.
Determination:
1. Solution state stability is first studied in extremes of pH and temperature. Drug is dissolved in 0.1N HCL / water/ 0.1N NaOH, the solutions are filled into ampoules and are sealed. These ampoules are stored at 90 0C. At different time intervals these solutions are assayed by HPLC for drug content /degraded products.
2. Drug is dissolved in solvents having different pH and the solutions are filled into ampoules and are sealed. These ampoules are stored at
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elevated temperatures (40 0C, 50 0C and 60 0C). At different time intervals these solutions are assayed by HPLC for drug content /degraded products. From this information pH of optimum stability can be found out. Order of decomposition and shelf life of the solutions can be found out by constructing Arrhenius plots.
3. To check for oxidative degradation of drugs, drug solutions are filled in vials and are capped with Teflon coated rubber stoppers. After penetrating the stoppers with needles, the head space is flooded with oxygen, needles are removed and the needle holes are sealed with wax to prevent degassing. At different time intervals samples are analyzed for
drug content.
4. If the drug is found to undergo oxidative degradation, drug solutions having antioxidants / nitrogen atmosphere are treated as in the above step.
5. Photostability testing of solutions is carried out by exposing the drug solution in sealed ampoules to artificial light of defined intensity. Controls covered with aluminum foil are also placed along with the test sample. Analysis of drug solution in test and control is carried out to predict photodegradation of drugs.
7. Solid State Stability Studies:
Concept: A drug is said to be stable if it does not undergo any physical and chemical degradation in solid state during storage. Solid state reactions are slower than solution state reactions. Solid state stability depends on the nature of the chemical substance and its type of crystalline form.
Importance: Stability studies provide useful information for formulation development, processing and packaging development.
Addition of stabilizing excipients, environment control during manufacturing and protective packaging systems, can be identified from these stability studies.
Determination:
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directly to a variety of temperatures, humidity conditions and light intensities up to 12 weeks. Samples are analyzed by HPLC/DSC/FT IR techniques. As the quantity of drug decomposed is very less to be detected by HPLC, it is better to estimate degraded products. The various conditions of stability testing are as follows.
S.NO Test Condition Storage condition
1 5 0C Refrigerator
2 22 0C Room temperature
3 37 0C Ambient humidity
4 37 0C and 75 % RH Accelerated testing 5 50 0C ambient humidity Accelerated testing 6 50 0C, O
2 head space Accelerated testing 7 50 0C ambient humidity, N
2 head space Accelerated testing 8 70 0C ambient humidity Accelerated testing 9 90 0C ambient humidity, Accelerated testing
2. Polymorphic changes are detected by DSC or XRD techniques.
3. Surface discoloration due to oxidation is studied using diffuse reflectance equipment.
4. To check for oxidative degradation of drugs, samples are filled in 25 ml vials and are capped with Teflon coated rubber stoppers. After penetrating the stoppers with needles, the head space is flooded with oxygen, needles are removed and the needle holes are sealed with wax to prevent degassing. Controls are filled with nitrogen. At different time intervals samples are analyzed for drug content, polymorphic changes
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5. The decay process should be analyzed for zero order / first order kinetics and shelf life determined.
6. To check for oxidative degradation of drugs, drug samples are spread in a watch glass and exposed to artificial light of defined intensity. Controls covered with aluminum foil are also placed along with the test sample. Analysis of drug samples in test and control is carried out to predict
photodegradation of drugs.
7. The hygroscopic nature of a drug can be determined by gravimetry or Karl Fischer titration. Drug is spread as a thin layer on a watch glass and is exposed to 75 % RH. The increase in weight is monitored to measure moisture uptake. Moisture uptake is monitored for 24 hours. 8.Solid State Properties:
Concept: A number of drugs are used in the solid form and their solid state properties influence their stability and performance. The various solid state properties are
1. Crystalline form determined by XRD
2. Solvated state determined by XRD
3. Crystal habit by microscopy
4. Particle size determined by microscopy/sieving/light scattering
5. Particle shape determined by shape factors
6. Particle surface area determined by adsorption/air permeability method
7. Bulk density determined by three tap method
8. Flow properties determined by angle of repose/Hausner ratio/ flowability/ compressibility index method.
9. Compaction behavior determined by using instrumented tabletting machines.
Importance: Solid state properties of a drug influence solubility, dissolution rate and stability. Most stable solid form has the lowest solubility and reactivity. It is better to develop a drug product using most stable form because metastable form may convert to stable form, during shelf life.
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Particle size and shape of the drug influence flow of powders. It is difficult to compress powders having low bulk density. Granulation is done to improve particle size, increase density and flow of powders.
If the drug powders do not have satisfactory compaction profiles, binders have to be included in tablet formulations.
9. Compatibility studies for selection of excipients:
Concept: Compatibility studies are carried out to study the interactions of drug substance with excipients. Selection of excipients is important for development of a quality drug product. Choice of excipients depends on the type of dosage form. For example, for a tablet dosage form, diluents, binders, disintegrants, lubricants, glidants are selected for compatibility studies. If necessary, solubilizer, stabilizing agent, buffer and rate controlling polymers can also be included.
Mechanisms for incompatibility: Compatibility studies are done to identify physical and chemical incompatibility between drug substance and excipients. Excipients may be responsible for incompatibility by
(i) altering the moisture content
(ii) altering the microenvironment pH
(iii) acting as a catalyst for degradation
(iv) acting as an impurity that causes degradation
Compatibility study procedures: Isothermal stress testing, differential scanning calorimetry and isothermal micro calorimetry are used for conducting compatibility studies.
Stress conditions like thermal, humidity, oxidation and mechanical, are used in compatibility studies. Studies can be performed between two or multicomponent systems. Miniformulation studies are also used to understand compatibility in multicomponent systems.
Drug and excipients in powder or compacted form are taken in vials and are stored at temperatures of 50, 60 oC or even higher. Additional stress like moisture can also be included. The samples are monitored on weekly basis (for a period of 4 weeks). If there is color, odor, deliquescence and flow behavior changes, incompatibility can be suspected. Samples are also analyzed using
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UV spectroscopy or HPLC. Samples are examined by X ray diffraction, FTIR and solid state NMR for change in crystalline form /incompatibility.
Importance of compatibility studies in formulation development: Compatibility studies allow systematic selection of excipients, for formulation development.
Preformulation Studies for Generic Products
A generic product is defined as “a drug product that is comparable to branded innovator product in dosage form, strength, route of administration, quality and performance characteristics”. As per the description given in USFDA, a generic product is pharmaceutical equivalent and bioequivalent to the innovator product. A generic product is a duplicate of the innovator product. If the innovator and generic product are being developed using same solid form, fundamental preformulation properties like aqueous solubility, pKa, log P, pHsolubility profile, intestinal permeability, stability and solid state properties, are expected to be similar. However, derived properties like bulk density, flow and compactability may differ. However, some of these studies may have to be repeated in case the solid form used by the innovator and generic products is different.
The most important preformulation study for a generic product is compatibility studies. From a formulation development point, a generic product may or may not have same excipients as the innovator. Let us consider that the generic product is qualitatively similar to innovator product. Qualitative similarity means that both innovator and generic product have similar excipients. Theoretically, there should not be a need for compatibility studies. However, a need for compatibility studies could arise, because of differences in (i) solid state properties of API, (ii) impurity profile of the API and (iii) source / vendor of excipients. The second situation is when excipients in innovator and generic formulation differ qualitatively. This would call for a more rigorous compatibility studies as some of the excipients have not been used earlier with that API.
Conclusion: Many preformulation studies should be performed, though generic product can be considered a duplicate of the innovator product. Differences in the material properties of the API and excipients, used by innovator and generic manufacturer, can affect performance of the dosage form. Thus it is important to perform preformulation studies for generic products also.
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Preformulation report is like the resume of the molecule and provides scientific basis for drug discovery and development.
By
Dr. M. Eswar Gupta
Professor and Head, Dept. of Pharmaceutics, Sir C.R.R College of Pharmaceutical Sciences, Eluru, A.P., India. Email: meguptas@yahoo.com
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elevated temperatures (40 0C, 50 0C and 60 0C). At different time intervals these solutions are assayed by HPLC for drug content /degraded products. From this information pH of optimum stability can be found out. Order of decomposition and shelf life of the solutions can be found
out by constructing Arrhenius plots.
3. To check for oxidative degradation of drugs, drug solutions are filled in vials and are capped with Teflon coated rubber stoppers. After penetrating the stoppers with needles, the head space is flooded with oxygen, needles are removed and the needle holes are sealed with wax to prevent degassing. At different time intervals samples are analyzed for
drug content.
4. If the drug is found to undergo oxidative degradation, drug solutions having antioxidants / nitrogen atmosphere are treated as in the above step.
5. Photostability testing of solutions is carried out by exposing the drug solution in sealed ampoules to artificial light of defined intensity. Controls covered with aluminum foil are also placed along with the test sample. Analysis of drug solution in test and control is carried out to predict photodegradation of drugs.
7. Solid State Stability Studies:
Concept: A drug is said to be stable if it does not undergo any physical and chemical degradation in solid state during storage. Solid state reactions are slower than solution state reactions. Solid state stability depends on the nature of the chemical substance and its type of crystalline form.
Importance: Stability studies provide useful information for formulation development, processing and packaging development.
Addition of stabilizing excipients, environment control during manufacturing and protective packaging systems, can be identified from these stability studies.
Determination:
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directly to a variety of temperatures, humidity conditions and light intensities up to 12 weeks. Samples are analyzed by HPLC/DSC/FT IR techniques. As the quantity of drug decomposed is very less to be detected by HPLC, it is better to estimate degraded products. The various conditions of stability testing are as follows.
S.NO Test Condition Storage condition
1 5 0C Refrigerator
2 22 0C Room temperature
3 37 0C Ambient humidity
4 37 0C and 75 % RH Accelerated testing
5 50 0C ambient humidity Accelerated testing 6 50 0C, O2 head space Accelerated testing 7 50 0C ambient humidity, N2 head space Accelerated testing 8 70 0C ambient humidity Accelerated testing 9 90 0C ambient humidity, Accelerated testing
2. Polymorphic changes are detected by DSC or XRD techniques.
3. Surface discoloration due to oxidation is studied using diffuse reflectance equipment.
4. To check for oxidative degradation of drugs, samples are filled in 25 ml vials and are capped with Teflon coated rubber stoppers. After penetrating the stoppers with needles, the head space is flooded with oxygen, needles are removed and the needle holes are sealed with wax to prevent degassing. Controls are filled with nitrogen. At different time intervals samples are analyzed for drug content, polymorphic changes
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5. The decay process should be analyzed for zero order / first order kinetics and shelf life determined.
6. To check for oxidative degradation of drugs, drug samples are spread in a watch glass and exposed to artificial light of defined intensity. Controls covered with aluminum foil are also placed along with the test sample. Analysis of drug samples in test and control is carried out to predict
photodegradation of drugs.
7. The hygroscopic nature of a drug can be determined by gravimetry or Karl Fischer titration. Drug is spread as a thin layer on a watch glass and is exposed to 75 % RH. The increase in weight is monitored to measure moisture uptake. Moisture uptake is monitored for 24 hours.
8.Solid State Properties:
Concept: A number of drugs are used in the solid form and their solid state properties influence their stability and performance. The various solid state properties are
1. Crystalline form determined by XRD 2. Solvated state determined by XRD 3. Crystal habit by microscopy
4. Particle size determined by microscopy/sieving/light scattering 5. Particle shape determined by shape factors
6. Particle surface area determined by adsorption/air permeability method 7. Bulk density determined by three tap method
8. Flow properties determined by angle of repose/Hausner ratio/ flowability/ compressibility index method.
9. Compaction behavior determined by using instrumented tabletting machines.
Importance: Solid state properties of a drug influence solubility, dissolution rate and stability. Most stable solid form has the lowest solubility and reactivity. It is better to develop a drug product using most stable form because metastable form may convert to stable form, during shelf life.
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Particle size and shape of the drug influence flow of powders. It is difficult to compress powders having low bulk density. Granulation is done to improve particle size, increase density and flow of powders.
If the drug powders do not have satisfactory compaction profiles, binders have to be included in tablet formulations.
9. Compatibility studies for selection of excipients:
Concept: Compatibility studies are carried out to study the interactions of drug substance with excipients. Selection of excipients is important for development of a quality drug product. Choice of excipients depends on the type of dosage form. For example, for a tablet dosage form, diluents, binders, disintegrants, lubricants, glidants are selected for compatibility studies. If necessary, solubilizer, stabilizing agent, buffer and rate controlling polymers can also be included.
Mechanisms for incompatibility: Compatibility studies are done to identify physical and chemical incompatibility between drug substance and excipients. Excipients may be responsible for incompatibility by
(i) altering the moisture content (ii) altering the microenvironment pH (iii) acting as a catalyst for degradation
(iv) acting as an impurity that causes degradation
Compatibility study procedures: Isothermal stress testing, differential scanning calorimetry and isothermal micro calorimetry are used for conducting compatibility studies.
Stress conditions like thermal, humidity, oxidation and mechanical, are used in compatibility studies. Studies can be performed between two or multicomponent systems. Miniformulation studies are also used to understand compatibility in multicomponent systems.
Drug and excipients in powder or compacted form are taken in vials and are stored at temperatures of 50, 60 oC or even higher. Additional stress like moisture can also be included. The samples are monitored on weekly basis (for a period of 4 weeks). If there is color, odor, deliquescence and flow behavior changes, incompatibility can be suspected. Samples are also analyzed using
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UV spectroscopy or HPLC. Samples are examined by X ray diffraction, FTIR and solid state NMR for change in crystalline form /incompatibility.
Importance of compatibility studies in formulation development:
Compatibility studies allow systematic selection of excipients, for formulation development.
Preformulation Studies for Generic Products
A generic product is defined as “a drug product that is comparable to branded innovator product in dosage form, strength, route of administration, quality and performance characteristics”. As per the description given in USFDA, a generic product is pharmaceutical equivalent and bioequivalent to the innovator product. A generic product is a duplicate of the innovator product. If the innovator and generic product are being developed using same solid form, fundamental preformulation properties like aqueous solubility, pKa, log P, pHsolubility profile, intestinal permeability, stability and solid state properties, are expected to be similar. However, derived properties like bulk density, flow and compactability may differ. However, some of these studies may have to be repeated in case the solid form used by the innovator and generic products is different.
The most important preformulation study for a generic product is compatibility studies. From a formulation development point, a generic product may or may not have same excipients as the innovator. Let us consider that the generic product is qualitatively similar to innovator product. Qualitative similarity means that both innovator and generic product have similar excipients. Theoretically, there should not be a need for compatibility studies. However, a need for compatibility studies could arise, because of differences in (i) solid state properties of API, (ii) impurity profile of the API and (iii) source / vendor of excipients. The second situation is when excipients in innovator and generic formulation differ qualitatively. This would call for a more rigorous compatibility studies as some of the excipients have not been used earlier with that API.
Conclusion: Many preformulation studies should be performed, though generic product can be considered a duplicate of the innovator product. Differences in the material properties of the API and excipients, used by innovator and generic manufacturer, can affect performance of the dosage form. Thus it is important to perform preformulation studies for generic products also.
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Preformulation report is like the resume of the molecule and provides scientific basis for drug discovery and development.
By
Dr. M. Eswar Gupta
Professor and Head, Dept. of Pharmaceutics, Sir C.R.R College of Pharmaceutical Sciences, Eluru, A.P., India. Email: meguptas@yahoo.com