Preformulation Studies

Intended Learning Objectives

At the end of the lecture, students will be able to

• Explain preformulation and its role played in formulation development
• Enlist some of the preformulation studies carried out on drug molecule
• Discuss the importance of properties like partition coefficient, dissociation constant, and chirality
• Describe factors affecting the stability of drug molecules and their importance in preformulation with examples
• Explain the importance of solubility determination in Preformulation studies
• Discuss the importance of crystallinity, density, particle size, and polymorphism in formulation development
• Describe the importance of determining drug excipient compatibility during the preformulation stages and the methods for the same

Preformulation Studies

Preformulation – Concept

• Almost all drugs are marketed as tablets, capsules, or both
• Prior to the development of these major dosage forms, it is essential that certain fundamental, physical, and chemical properties of the drug molecule and other properties of the drug powder are determined
• This information decides many of the subsequent events and approaches in formulation development
• This first learning phase is known as preformulation

Preformulation – Definition

• It can be defined as an investigation of physical and chemical properties of a drug substance – alone and or when combined with excipients

Drug                                                                                           Drug + excipients

Preformulation – Objectives 

• To lay down the foundation for transforming a new drug entity into a pharmaceutical formulation in such a way that it can be administered in the right way, in the right amount, and on the right target
• To provide longer stability to the formulation by properly designing and protecting drug components from environmental conditions and to evaluate the performance of the developed formulation

Peformulation studies on drug molecule

• Partition coefficient (Log P)
• Dissociation constant (pKa or pKb)
• Stability of molecule under a variety of conditions

Partition coefficient

• The partition coefficient (Log P) value is defined as the ratio of the unionized drugs distributed between the aqueous and organic phase
• Oil-water partition coefficient gives the idea about the drug’s ability to cross the lipidic membrane
• Lipophilic/hydrophilic balance is one of the most important contributing factors for optimum drug absorption and delivery

Partition coefficient determination

In vitro methods

• Shake flask method
• Chromatographic method (HPLC)
• Computation based on software
• Countercurrent/filter probe method

Shake flask method

• Highly used method is shake flask method that utilizes octanol-water system to determine drug’s partitioning behaviors

Why Octanol???

• Octanol is believed to mimic the lipoidal character of biological membrane as it contains polar head and nonpolar tail
• Octanol is organic compound that is immiscible with water; however, some of the water is expected to be present in polar head portion
• Solubility parameter for most of the drugs resembles with that of octanol

Dissociation constant

• Dissociation constant (pKa) is the property that determines the solubility in pH-dependent environment and extent of ionization
• Extent of ionization determines absorption
• Only unionized form can be absorbed

pKa value determination gives idea about site of absorption

• Weakly acidic drugs having pKa value around 4 are best absorbed from stomach as they are predominantly present in unionized form
• Basic drugs having pKa value of around 8 are best absorbed from intestine as they are predominantly present in unionized form

Partition coefficient and absorption

Chirality

• Many molecular entities exist in racemic form, but only one form gives the desirable pharmacological activity
• Other present isomer may be devoid of pharmacological activity or may exhibit deleterious side effects
• Thalidomide exists as racemic form
• It was introduced as a sedative agent
• The S-enantiomer of thalidomide was a teratogenic agent (causes birth defects), while R-enantiomer was effective as a sedative agent

Stability of Molecule

• The main objective of determining stability of molecule is to identify the conditions in which molecule is susceptible to deteriorate and to determine degradation pathway
• The mechanism of degradation and condition provides the idea about proper designing of formulation, suitable molecular modification, appropriate storage condition, and selection of proper packaging material

The major mechanisms by which a molecule undergoes degradation are

• Hydrolysis
• Oxidation
• Photolysis
• Racemization.

Hydrolysis 

• Hydrolysis involves reaction of a molecule with water resulting in cleavage of a chemical bond within the molecule
• Presence of hydrolyzable functional groupsàfaster hydrolysis of the molecule

Functional groups like esters and amide are prone to hydrolysis and especially the ester derivatives

• Beta-lactam antibiotics are susceptible to hydrolysis
• Hence they are supplied as dry powder injection where they are reconstituted before intravenous administration

Oxidation 

• Many molecules can undergo oxidative degradation, which involves exposure of molecule to atmospheric oxygen or autoxidation by free radicals
• However, in some cases, oxidation can be initiated in presence of light or elevated temperature
• The extent of oxidation for a given substance can be studied by passing oxygen through the solution of substance, or it can be achieved by addition of hydrogen peroxide to the solution of substance
• So degree of oxidation can be controlled by avoiding exposure to lights and storage at controlled temperatures
• The extent of oxidation can be controlled by addition of antioxidants

Photolysis

• Photolysis refers to decomposition of a molecule by absorption of energy when exposed to light
• Exposure to light not only brings photodegradation but may trigger oxidation
• Prior knowledge of photochemical behavior can provide guidance regarding storage condition, packaging, and handling condition
• Example, riboflavin and vitamin B12 are susceptible to photodegradation directly and oxidation induced by light

So to avoid the decomposition, the formulation containing vitamin B12 and riboflavin is stored in amber color vials

Amber color bottles do not allow the ultraviolet radiation to pass through, which is the main factor for photodegradation

Racemization

• It is an event where optically active molecule becomes inactive without any change in molecular composition
• Racemization leads to either loss of pharmacological action or toxic effect may be enhanced by several fold
• Racemization is mostly affected by the conditions like pH, type of solvents, presence of light, and temperature
• Main goal in this study is to design optimum condition in which molecule can remain stable

Physical Characterization of Molecules

• Most of the newly discovered drugs are solids
• Properties under study during preformulation phase are bulk property characterization and micromeritic property characterization

Solubility & Permeability 

Solubility 

• The solubility of a drug is the amount of the drug that dissolves in a given solvent to produce a saturated solution at constant temperature and pressure

Solubility is not an independent parameter but it relies on several properties like crystal characteristics, temperature, pH, complexation, and molecular structure

• Most widely studied techniques during preformulation analysis is solubility profile of drug candidate
• Solubility and permeability forms the scientific basis of Biopharmaceutics Classification System (BCS)
• BCS provides framework for designing type of drug delivery system

Solubility, permeability and type of formulation

Solubility Improvement Techniques

• Chemical modification of drug
• Addition of cosolvent or surfactant
• Particle size reduction
• Hydrotropy
• Complexation

Crystalline & Amorphous form

Techniques to study Crystallinity

1. Scanning electron microscopy
2. X-ray
3. Differential scanning microscopy differential thermal analysis
4. Hot stage microscopy,

Polymorphism

• Polymorphism is the ability of a compound to crystallize as more than one distinct chemically identical crystalline species with different internal lattices or crystal packing arrangement

Polymorphic forms – Significance

• Different types of polymorphs exhibit different types of solubility, stability, and therapeutic activity
• Chloramphenicol palmitate exists in three different polymorphic forms, namely, A, B, and C. Form B has higher solubility and better dissolution profile, while form A is more stable one but low serum concentration was observed
• During formulating suspension of an anthelmintic drug oxyclozanide, transformation of unstable polymorph to more stable leads to different crystal size and causes caking

Deliquescency & Hygroscopicity

• Hygroscopicitycan be defined as the capacity of a compound to absorb atmospheric moisture
• Deliquescent substancesabsorb atmospheric moisture absorbing to a EXTREME extent and liquefy itself

Why study hygrocopicity?

• Changes in the moisture level can influence

      Chemical stability

      Flowability

      Compressibility to a greater extent

• Moisture level uptake can be monitored by techniques like thermogravimetric analysis (TGA), Karl Fischer titration, and gas chromatography

Classes of Hygrocopicity

Hygroscopicity is described by four different classes after being stored at 25°C at relative humidity of 80% for 24 hours

• Slightly hygroscopic:After abovementioned storage condition, if overall increase in weight is greater or equal to 0.2% but less than 2% w/w
• Hygroscopic: After abovementioned storage condition, if overall increase in weight is greater or equal to 0.2% but less than 15% w/w.
• Very hygroscopic:After abovementioned storage condition, if overall increase in weight is greater than 15% w/w.

Particle Size

• Particle size greatly affects

      dissolution rate

      Solubility

      Bioavailability

      content uniformity

      lack of grittiness

Application of particle size in preformulation

• When solubility is major issue, one may significantly improve the solubility by reducing the particle size (increased surface area)
• In case of suspension, particle size is the most important parameter, which determines the stability and quality of formulation. Too much reduction in the particle size leads to generation of charged particle and hence unstable system. On other hand, larger particle size leads to caking.
• Due to non-uniform particle size distribution, there is significant risk associated with content uniformity in case of potent formulations.

Particle size determination

• Microscopy
• Sedimentation rate
• Coulter counter method
• Surface area determination by nitrogen adsorption method

Density – Issues

• Capsules:With drugs having low density, the bulk becomes more and hence capsule formulation is quite difficult to formulate as capsule can incorporate limited volume
• Tablets:In development of tablet formulation, low-density drug creates difficulties as they are having low compressibility and hardness in tablet is difficult to achieve
• Homogeinity:If the difference of density is more between drug substances and excipient is more, homogeneity in the formulation is difficult to achieve

Flow properties

• Efficient flow of drug substance powder is needed for effective tablet formulation

Drug excipient compatibility

Effects of drug excipient incompatibility

• Change in organoleptic properties of formulation
• Changes in in vivo performance of formulation, that is, dissolution
• Decreased potency of active ingredient
• Generation of toxic degradation product
• Change in physical appearance of formulation, that is, color, phase conversion

Physical incompatibility

• Such interaction results in changes like change in color, odor, flow properties, and sedimentation rate
• Example of physical incompatibility is between tetracycline and calcium carbonate
• It results in formation of insoluble complex with calcium carbonate, leading to slower dissolution and decreased absorption in the gastrointestinal tract

Chemical incompatibility

• Chemical reaction may take place as hydrolysis, oxidation racemization, and Maillard reactions
• Examples of chemical incompatibility is exhibited by reaction of lactose with amino group of active pharmaceutical ingredient referred to as “Maillard reaction” and results into darkening of formulation with characteristic odor.
• Bronchodilator aminophylline + lactoseàbrown discoloration appeared in samples after storing at 60°C for 3 weeks

Therapeutic incompatibility

• Interaction will take place once the formulation is administered into the body
• Example: enteric coated polymers, when administered along with antacids dissolve prematurely and release the drug that is liable to acid degradation or may cause adverse effect in GI, that is, gastric bleeding

Determination of drug excipient incompatibility

• Thermal Analysis

Properties of materials are studied as they change with temperature

    DSC – Differential Scanning Calorimetry

    TGA – Thermo Gravimetric Analysis

    DTA – Differential Thermal Analysis

• High performance liquid chromatography (HPLC)
• FTIR

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