INTRODUCTION Preformulation study is the foundation of developing robust formulation. It can be defined as a phase of research & development process for an investigation of physical and chemical properties of new drug substance alone or in combination with other excepients in order to development of safe and effective dosage form. OBJECTIVE • The overall objective of preformulation testing is to generate information useful to the formulator: • To formulate stable and effective dosage form To increased drug stability To improve drug bioavailability Reduce drug excipient incompatibility • • • APPLICATIONS… Preformulation studies begins or shall be updated immediately after the synthesis and initial toxicity screening of a new drug. when a newly synthesized drug shows pharmacological evidence that requires further evaluation in man when formulation and dosage form changes are required when solid form changes of DS are required Protocol for preformulation studies OUTLINE OF PRINCIPAL AREAS OF PREFORMULATION RESEARCH Principal areas Physico-chemical properties Organoleptic properties Particle size and shape Purity Surface area Stability analysis Bulk characterisation Crystallinity and polimorphism Hygroscopicity Particle size characterization Bulk density Solubility analysis Ionization constant pka PH solubility profile Common ion effect ksp Thermal effects Partition co-efficient solubilization Powder flow properties Solution stability Dissolution Solid state stability Bulk stability Compatibilty PREFORMULATION PARAMETERS PREFORMULATION PARAMETERS METHOD USED Organoleptic Properties Colour and Odour Determination Crystallinity & Polymorphyism X-ray Diffraction Studies (Lachman, 1991) Fine Particle Characterization Microscopic Method (Lachman, 1991) Solubility Profile Equilibrium Solubility Method (I.P. 2007) Solubilization (Lachman, 1991) Analytical Method Development UV Spectroscopic Method, HPLC Method Ionization Constant, pKa Determination of Spectral Shifts by UV Spectroscopy (Lachman, 1991) Partition Coefficient Using octanol / water,(Lachman, 1991) Bulk Density Tapping Method (Lachman, 1991) Powder Flow Properties % Compressibility Determination, Angle of Repose (Lachman, 1991) Compatibility With Excipients DSC (Stulzer and Rodriques et al., 2008) Stability Solution and Solid State Stability (PCT/US03/35012) Stability Indicating Method Development Forced Degradation Studies (Rao et al., 2009) 7 ORGANOLEPTIC PROPRTIES Colour:-Stability problems, improve appearance by including dye in body or coating Taste:-palatability, flavours, and excepient may be added. Odour:-degradation products, eg. Aspirin stable form of drug to be used , flavours and excepients may be used. SUGGESTED TERMINOLOGY TO DESCRIBE ORGANOLEPTIC PROPERTIES OF PHARMACEUTICAL POWDERS Colour Odour Taste Off-white Cream yellow Tan Shiny Pungent sulfurous fruity Aromatic odourless acidic bitter bland intense sweet Tasteless PURITY Purity studies are essential for further studies to be carried out safely. Impurities may make a compound toxic or render it unstable. TLC,HPLC,GC and Paper chromatography used. HPLC-Impurity Index(II), Homogeneity index(HI). DTA, gravimetric analysis and melting point by hot stage microscopy are other techniques. Impurity index(II):defined as the ratio of all responses (peak areas) due to components other than the main one to the total area response. Homogeneity index(HI): defined as the ratio of response(peak area) due to main component to the total response. Eg.: main component –retention time: 4.39min -area response: 4620 Impurities – 7 minor peaks ;area response: 251 - total area response : 251+4620 Impurity index : = 251/(4620+ 251) = .0515 Homogeneity index : = 1 - .0515 = .9485 OTHER TOOLS IN ASSESMENT OF IMPURITY Differential thermal analysis(DTA) Thermogravimetric analysis(TGA) Differential scanning calorimetry(DSC) Powder X-Ray Diffraction(PXRD) PARTICLE SIZE AND SHAPE Various chemical and physical properties of drug substances are affected by their particle size distribution and shapes. The effect is not only on physical properties as well as biopharmaceutical behavior. It also influence the flow and the mixing efficacy of powders and granules. Fine materials are relatively more open to attack from atmospheric oxygen, humidity, than that of coarse material. PARTICLE SIZE DETERMINATION Microscopy. Eg. Light microscope, electron microscope. Anderson Pipette Sieving method Instruments based on light blockage(HIAC) and blockage of electrical conductivity path(coulter counter are available). COMMON TECHNIQUES FOR MEASURING FINE PARTICLES OF VARIOUS SIZES Technique Microscopic Sieve Sedimentation Elutriation Centrifugal Permeability Light scattering Particle size (mm) 1 - 100 > 50 >1 1 - 50 < 50 >1 0.5 - 50 SHAPE DETERMINATION….. Microscopy should be caarried out to determine the ratio of longest to shortest dimension. It is a shape factor. SHAPE FACTOR Commonly used shape factor converts volume of particle ‘v’to its volumetric mean diameter ‘av’ V=αv.av³ Shape factor may be defined which converts the surface area ‘s’of a particle to its surface mean diameter ‘as’ S=αs.as² Fractal Dimensions are carried out by imaging techniques. In(N)=-nIn(g)+q where:N=no.of squares. g=length of grid size. n&q are constants. SURFACE AREA DETERMINATION It is determined based on Brunaver Emitter Teller (BET)theory of adsorption. Most substances adsorb mono molecular layer of gas (Nitrogen) and temperature. Air adsorption and permeability methods. CRYSTALLINITY AND POLYMORPHISM Crystal habit and internal structure of a drug can affect bulk and physiochemical properties HABBIT: Outer appearance of crystal. Internal structure Crystalline Amorphous CHARACTERIZATION OF SOLID FORMS Solid forms Crystalline Amorphous Repeated spacing of atoms in three dimentional structure Randomly placed atoms or molecules Have less energy Need less energy to break crystalline form So solubility is less Have high energy Need less energy to break solubility is high ANALYTICAL METHODS FOR CHARACTERISATION OF SOLID FORMS Method Microscopy Fusion methods (hot stage microscopy) Infrared spectroscopy X-ray powder diffraction Scanning electron microscopy Thermogravimetric analysis Dissolution/Solubility analysis Material reqired per sample 1mg 1mg 2-20mg 500mg 2mg 10mg mg to gm MICROSCOPY All substances are transparent examined under microscope – are either isotropic or anisotropic Isotropic substances do not transmit the light – appears black – and have single refractive index. E.g. Sodium Chloride Anisotropic substances – more than one refractive index – appear bright and brilliant color – uniaxial and biaxial Color depends upon – thickness of crystal and diff. in refractive indices. THERMAL ANALYSIS Differential Scanning Colorimetry (DSC) and Differential Thermal Analysis (DTH) measures the heat loss or heat gain resulting from physical or chemical changes. Two types of processes 1) Endothermic : like fusion, boiling, sublimation, vaporization, desolvation Exothermic : like crystallization ,degradation Quantitative measurement of these process have many application in preformulation study including Purity, Polymorphism, solvation, degradation X-RAY DIFFRACTION Crystalline materials gives characteristics pattern – by peaks in certain position & varying intensities different Polymorphs – different x-ray diffraction pattern due to crystal lattice. Single crystal x-ray analysis provides precise identification & description of a crystalline substances. POLYMORPHISM - substances can exist in more than one crystalline form Polymorphic forms – diff. physical-chemical properties (incl. melting pt. & solubility) Enatiotropic Polymorphs: Monotropic Determination method: Thermodyanamically-van,t Hoff plot(solubility vs temperature) Directly – by microscopic determination HYGROSCOPICITY Many substances,particularly water soluble salt form have a tendency to absorb atmospheric pressure. Change in moisture level can influence chemical stability , flowability, and compactibility. It can be monitored by Karl Fischer titration,TGA POWDER FLOW The pharmaceutical powders are classified as --FREE FLOWING COHESIVE OR NON FREE FLOWING The powder flow are affected by the changes in – Density Particle Size Shape Free flowing drug may become cohesive and Electrostatic Charge necessitates an entirely new formulation strategy Adsorbed Moisture Several rates of flow (g/sec) determinations at each of variety of orifice sizes (1/8 to ½ inches) should be made. Greater the standard deviation between multiple flow rate measurements greater is the weight variation in products produced from the powder. It was found that the dependence of flow rate (w) on the true particle density(ρ),gravity(g), orifice diameter(D) by D= A (4w/60πρ√g)^1/n A and n are constants dependent upon material and particle size Other measures of free flowing powders is COMPRESSIBILITY. %compressibility=(ρ1-ρ0)/ρ1 Angle of repose is not much useful as lack of precision Cohesive Powders are characterized by TENSILE TESTING and evaluated in a SHEAR CELL SOLUBILITY Solubility > 1 % w/v => no dissolution-related absorption problem Highly insoluble drug administered in small doses may exhibit good absorption Unstable drug in highly acidic environment of stomach, high solubility and consequent rapid dissolution could result in a decreased bioavailability The solubility of every new drug must be determined as a function of pH over the physiological pH range of 1 - 8 DETERMINATION OF SOLUBILITY Semiquantitative determination: Solvent (fixed volume) Examine visually Vigorously shaking Adding solute in small incremental amounts Undissolved solute particles ? No Yes “LAW OF MASS ACTION” Estimated solubility Total amount added up Accurately Quantitative determination: Excess drug powder 150 mg/ml (15 %) + solvent Ampul/vial (2-5 ml) Shaking at constant temperature (25 or 37 oC) 2 - 8 oC ? The first few ml’s of the filtrates should be discarded due to possible filter adsorption Same concentration ? 48 hr Determine the drug concentration in the filtrate Membrane filter 0.45 mm Determine the drug concentration in the filtrate Membrane filter 0.45 mm 72 hr ? hr Solubility Determine the drug concentration in the filtrate Membrane filter 0.45 mm General Method of Increasing the Solubility Addition of co-solvent pH change method Reduction of particle size Temperature change method Hydotrophy Addition of Surfactant Dielectrical Constant Complexation UNIQUE PROBLEMS IN SOLUBILITY DETERMINATION OF POORLY SOLUBLE COMPOUNDS Solubility could be over estimated due to the presence of soluble impurities Saturation solubility is not reached in a reasonable length of time unless the amount of solid used is greatly in excess of that needed to saturation Many compounds in solution degrade, thus making an accurate determination of solubility difficult Difficulty is also encountered in the determination of solubility of metastable forms that transform to more stable forms when exposed to solvents pH-Solubility Profile Stir in beaker with distilled water Excess drug powder Determine the concentration of drug in the filtrate SOLUBILITY Filter Measure pH of suspension pH Continuous stirring of suspension Stirring Add acid/base Poorly-soluble weakly-acidic drugs: pH = pKa + log [(St - So)/So] (2) Poorly-soluble weakly-basic drugs: pH = pKa + log [So/(St - So)] (3) where So = solubility of unionized free acid or base St = total solubility (unionized + ionized) The process of solubilization involves the breaking of inter-ionic or intermolecular bonds in the solute, the separation of the molecules of the solvent to provide space in the solvent for the solute, interaction between the solvent and the solute molecule or ion. Step 1: Holes opens in the solvent Step2: Molecules of the solid breaks away from the bulk Step 3: The free solid molecule is intergraded into the hole in the solvent 37 SOLUBILIZATION CAN BE ENHANCED BY: Use more soluble metastable polymorph Use of complexation (eg.Ribloflavin-xanthinescomplex) Use of high-energy co-precipitates that are mixtures of solid solutions and solid dispersions (eg. Griseofulvin in PEG 4000, 6000, and 20,000) in PEG 4000 and 20,000 -> supersaturated solutions in PEG 6000 -> bioavailability in human twice > micronized drug Use of suitable surfactant Partition Coefficient It is the ratio of unionized drug distributed between organic and aqueous phase at equilibrium. P o/w = ( C oil / C water )equilibrium •It ratio of unionised drug in organic & aq.Phase •It measure lipophilicity •Major role in drug transport •Analytical separation IONIZATION CONSTANT The unionized species are more lipid-soluble and hence more readily absorbed. The gi absorption of weakly acidic or basic drugs is related to the fraction of unionized drug in solution. Factors affecting absorption: - pH at the site of absorption - Ionization constant - Lipid solubility of unionized species “pH-partition theory” Henderson-Hasselbalch equation For acids: pH = pKa + log [ionized form]/[unionized form] For bases: pH = pKa + log [unionized form]/[ionized form] Determination of Ionization Constant 1. Potentiometric pH-Titration 2. pH-Spectrophotometry Method 3. pH-Solubility Analysis Dissolution kd << ka => “dissolution rate-limited” D kd Xg Dissolution ka Absorption Absorption site (gi-tract) C, Vc Xc ke Central compartment (blood circulation) Diagram showing dissolution and absorption of solid dosage form into blood circulation PADDLE TYPE BASKET TYPE 2 types of systems to maintain uniform hydrodynamic conditions 1.STATIC DISC DISSOLUTION APPARATUS 2.ROTATING DISC APPARATUS INTRINSIC DISSOLUTION FILM THEORY The dissolution of a solid in its own solution is adequately described by Noyes-Nernst’s “Film Theory” -dW = DAK (Cs - C) (9) dt h where dW/dt = dissolution rate A = surface area of the dissolving solid D = diffusion coefficient K = partition coefficient h = aqueous diffusion layer Cs = solubility of solute C = solute concentration in the bulk medium Intrinsic dissolution rate (mg/cm2/min) is characteristics of each solid compound in a given solvent under fixed hydrodynamic conditions Intrinsic dissolution rate helps in predicting if absorption would be dissolution rate-limited > 1 mg/cm2/min --> not likely to present dissolution ratelimited absorption problems < 0.1 mg/cm2/min --> usually exhibit dissolution ratelimited absorption 0.1 - 1.0 mg/cm2/min --> more information is needed before making any prediction Amount Dissolved (mg in 500 ml) Effect of particle size of phenacetin on dissolution rate of the drug from granules 0.11 - 0.15 mm 0.15 - 0.21 mm 0.21 - 0.30 mm 0.50 - 0.71 mm SOLID STATE STABILITY for identification of stable storage condition also for identification of compatibale excipient for a formulation extent a product retains within specified limits and through its period of storage and use Stability studies conducted in the preformulation phase: Solid-state of the drug alone Solution phase with the expected excipients PHOTOLYTIC STABILITY Many drugs fade or dropped on exposure light. 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 for 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 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. SOLUTION PHASE STABILITY As compared with the dry form, the degradation is much rapid in solution form. It is important ascertain that the drug doesn’t degrade when exposed to GI fluid. The pH based stability study, using different stimulator 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 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 5mg of drug in a 50% mixture with the excipients to maximize the likelihood of obscuring an interaction . Mixtures should be examined under nitrogen to ultimate oxidation and paralytic 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 one or more peaks in themogrames of drug excipient mixtures are considered of indication of interaction. Flow diagram to identify excepient compatibility with drug No interaction Drug DSC 50%Mixture of Drug & excepient Excepient recommend ed Interaction Excepient HPLC Or TLC Alternative excepient suggested Yes Drug breakdown No FORMULATION RECOMMENDATION Upon completion of preformulation evaluation of a new drug candidate, it recommended that a comprehensive report be prepared highlighting problems associated with this molecule These Reports re extremely important in preparing regulatory documents 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. By comparing the physicochemical properties of each drug candidate with in 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.