ASSIGNMENT ON

Preformulation: Solubility, dissociation or ionization constant (pKa), and

partition co-efficient

Course name: Pharmaceutics II

Course code: PHRM 308

Section 2

Semester: Fall 2012

SUBMITTED TO:

Dr. Sufia Islam

Chairperson

Department of Pharmacy

SUBMITTED BY:

Samiya Khondaker Rinta (ID: 2010-3-70-048)

Kazi Mashreha Mahmud (ID: 2010-1-70-041)

Tahkib Ahsan (ID: 2010-1-70-052)

Syed Shafiqur Rahaman (ID: 2010-1-70-004)

East West University

Submission date: 20th November, 2012

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Content Page number

Introduction 1

Solubility 1

Intrinsic solubility 2

Importance of intrinsic solubility 2

How to improve solubility 3

Dissociation or ionization constant (pKa) in preformulation

3

pKa Determination 4

Henderson-Hesselbalch equations 5

Partition coefficient 5

Importance of partition coefficient 5

Drug absorption, distribution and excretion

6

Drug design 7

Biopharmaceutics 7

Conclusion 8

Reference 9

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Introduction

Before pharmaceutical drug product formulation activities are started-up,

preformulation studies are initiated. During the preformulation phase, the physical and

chemical properties of the active pharmaceutical ingredient (API) are determined. The gained

knowledge on the API is necessary for the selection of the right salt or polymorphic form and

this helps in the design and development of the dosage form.Preformulation includes many

stages (Lachman et al, 1990). Here the solubility, dissociation constant (pKa) and partition

coefficient (Log P) are discussed which are required for API characterization (Nanjwade,

2012).

Solubility

Solubility is the mass of solute that dissolves in a specific mass or volume of solvent

at a given temperature. For example 1 gm of NaCl dissolves in 2.786ml of water at 25°C. The

solubility of a molecule in various solvents is determined as a first step.  This information is

valuable in developing a formulation (Bhavin, 2011). 

Importance of solubility:

Solubility is one of the most important physicochemical properties studied during

pharmaceutical preformulation because it affects the bioavailability of the drug, the rate of

drug release into the dissolution medium, and consequently, the therapeutic efficacy of the

pharmaceutical product (Nanjwade, 2012). For liquid dosage form development, accurate

solubility data are essential to ensure the robustness of the finished product. For solid dosage

forms, solubility data are important in determining if an adequate amount of drug is available

for absorption ‘in vivo’. If a compound has a low aqueous solubility, it may be subject to

dissolution rate-limited or solubility-limited absorption within the gastrointestinal (GI)

residence time (Lobenberg et al, 2000).

Determination of solubility in preformulation:

Solubility is usually determined in a variety of commonly used solvents and some oils

if the molecule is lipophilic.The solubility of a 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 period until equilibrium is

achieved. This method is known as equilibrium solubility method. Common solvents used for

solubility determination are as follows:3

1. Water

2. Glycols for example, polyethylene glycols, propylene glycol, glycerin and sorbitol

3. Alcohols for example ethyl alcohol, methanol, benzyl alcohol and isopropyl alcohol

4. Surfactants for example tweens and polysorbates

5. Vegetable oils like castor oil, peanut oil and sesame oil

6. Buffers at various pHs (Bhavin, 2011).

Table: Showing the approximate solubilities of pharmacopeial and national formulary

substances are indicated by the descriptive terms. (Allaudin, 2012)

Intrinsic solubility (C₀)1:

Intrinsic solubility is defined as the maximum amount of solute dissolved in a given

amount solvent under standard conditions of temperature, pressure and pH. When the purity

of the drug sample can be assured, the solubility value obtained in acid for a weak acid or

alkali for a weak base can be assumed to be intrinsic solubility. In simple words the solubility

of a pure weak acid in acid or weak basic drug under alkaline conditions in the intrinsic

solubility. The intrinsic solubility of a weak acid is the lowest observed solubility when the

pH of the medium is more than two units below the pKa of the drug. The intrinsic solubility

of a weak base is the lowest observed solubility when the pH of the medium is more than two

units above the pKa of the drug (Bhavin, 2011).

Importance of intrinsic solubility:

1. Dissolution: Many drugs are formulated as solutions or added in powder or solution

form to liquids such as infusion fluid or injections in which they must dissolve and

remain in solution for a given period of time.

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2. Bioavailability: Drugs must be in a dispersed form in the solution before they can be

absorbed across the biological membranes.

3. Rate of drug release by diffusion: For solid dosage forms such as tablets, capsules or

even drugs that have precipitated from solution, the dissolution of the drug is

necessary before it can be released from the dosage form for it to be absorbed.

4. Therapeutic efficacy: these factors discussed above will ultimately determine how

effective the drug will be when administered (Lachman et al, 1990).

How to improve solubility:

Solubility may be improved with the addition of an acidic or basic excipient. For

example, solubilization of aspirin may be increased by addition of an alkaline buffer. General

methods of increasing the solubility are:

1. Addition of co-solvent, for example, water, alcohol, glycerin

2. Addition of complexingagent, for example, caffeine, EDTA

3. Chemical modification of the drug

4. pH adjustment

5. Addition of surfactant, for example, span, tween, SLS (Bhavin, 2011).

Poor solubility has issues not only with formulating the drug; it also imposes problems in

evaluating the physicochemical properties of the molecule itself. An aqueous solubility of

10mg/L or more is required over pH range of 1-7 to achieve adequate absorption of a drug

from the intestines. A solubility of less than 1mg/mL suggests the need for forming a salt of

the drug especially for tablets and capsules (PointCross Inc., 2011).

Dissociation or ionization constant (pKa) in preformulation

The dissociation or ionization constant is one of the most important characteristics of

a pharmaceutical chemical moiety which has to be estimated with accuracy. The pKa is

among the parameters that have to be estimated with accuracy, irrespective of solubility

constraints (Doria, 2011).The pKa values obtained for the selected drugs are compared with

the literature values.It is required to understand the site of absorption, distribution to various

organs and excretion of drugs.pKa is also helpful in screening salts, developing pre-clinical

and clinical formulation [4, 5, 6]. It is also useful in developing analytical methods, like

HPLC (PointCross Inc., 2011).

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pKa Determination: pKa can be determined by the following methods:

Henderson-Hesselbalch equations

UV-VIS. Spectra

Potentiometric titration

Solubility measurements

HPLC techniques

Capillary zone electrophoresis

Foaming activity (Ravichandiran et al, 2011)

Henderson-Hesselbalch equations:

The pKa is the negative logarithm of the equilibrium constant (Ka) of the acid-base

reaction of the compound of interest (Bhavin, 2011). 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-Hasselbalch equation provides an estimate of the ionized and un-ionized

drug concentration at a particular pH (Allaudin, 2012).

For acidic compounds:

pH = pKa + log ([ionized drug]/[un-ionized drug])

For basic compounds:

pH = pKa + log ([un-ionized drug]/[ionized drug])

pKa of a compound is thus a measure of drug un-ionized at a certain pH

pKa = -log Ka, where Ka is the acidity or ionization constant of a weak acid.

For a weak base, Ka = Kw/Kb, where Kw is the ionic product of water(Kw=[H3O+] x [OH-]) and

Kb is the basicity or ionization constant of the weak (Bhavin, 2011).

Other methods:

There are many experimental methods used for determining the pKa as mentioned

above. Simple fitting of the pH-solubility profile can be used if solubility measurements have

already been made at multiple pH values. Different methods are available to determine the

pKa of drugs, such as potentiometry, spectrophotometry and solubility methods [4]. The 6

potentiometric titration and spectrometric method are commonly used and widely accepted

techniques [4]. Poor solubility of the compounds hampers traditional potentiometric methods

[1]. This method can be applied only for compounds having solubility greater than 100 µM.

Spectrophotometric methods can be applied to the compounds having solubility even 1 µM.

However, this method is limited to molecules having chromophores at ionization center,

which shows spectral dissimilarity at protonated and deprotonated form (Ravichandiran et al,

2011).

The importance of pKain biologic systems needs to preserve a relatively constant

environment, including control over the pH of the organism’s fluids. One way to achieve this

is through the use of "buffers" – a buffer is a compound which due to its acid-base chemistry

reacts to changes in the environment to preserve a near constant pH that is near the pKa of the

buffering compound (Nanjwade, 2012). Thus, the acid dissociation constant (pKa) is a useful

physicochemical parameter to evaluate the extent of ionization of functional groups related to

pH, often of vital significance to understand the pharmacokinetic and pharmacodynamic

behavior of a drug substance (Avdeef and Testa, 2012).

Partition coefficient

Partition coefficient (oil/water) is a measure of a drug's lipophilicity and an indication

of its ability to cross cell membranes. It is defined as the ratio of the concentration of

unionized drug in oil phase with the concentration of the unionized drug in the aqueous

phase, at equilibrium (Lachman et al, 1990).

Po/w = (Coil/CWater) equilibrium

For series of compounds, the partition coefficient can provide an empiric handle in

screening for 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 characterizing 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 (Lachman et al, 1990).

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Importance of partition coefficient: Partition coefficient is important for the following

factors-

1) Drug absorption, distribution and excretion

2) Drug design

3) Biopharmaceutics

1) Drug absorption, distribution and excretion:

Lipophilicity affects the following aspects of the drug-

I. Solubility: If partition coefficient increases then according to the partition

coefficient ratio discussed previously, the concentration of the unionized drug in the

oil phase increases. This indicates thatwith increase in partition coefficient,aqueous

solubility decreases and lipid solubility increases and vice versa.

II. Membrane permeability: If partition coefficient increases, lipophilicity increases so

membrane permeability increases.

III. Absorption potential: With increase in partition coefficient, lipid solubility increases

so absorption across the lipoidal membrane increases and hence absorption potential

increases (Folkers et al, 2003).

IV. Plasma protein binding: If partition coefficient increases,lipophilicity increases

which in turn increases the drugs tendency to bind with plasma protein. Increase in

plasma protein binding lessens the amount of drug entering the target tissues and

cells.

V. Tissue distribution: After absorption into the systemic circulation the blood

distributes the drug to tissues. The blood carries the drug through the liver for

hepatic first pass metabolism. If the drug is highly lipophilic then while passing

through the liver it may get deposited in the adipose tissues (fat cells) in the liver,

resulting in significant decrease in tissue distribution of drug.

VI. Renal or hepatic clearance: for renal and hepatic clearance of the drug, it must have

lower partition coefficient values (Lachman et al, 1990).

The partition coefficient of a drug is dependent on its ionization state and therefore on

pH. Only unionized drug with sufficient lipid solubility is absorbed across the biological

membrane and into the systemic circulation. Polar drugs or ionized drugs are hydrophilic in

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nature and so are poorly absorbed into the systemic circulation. Drugs with low partition

coefficient value have higher aqueous solubility and are unable to cross the lipid biological

membrane. Lipid soluble drugs with high partition coefficient value have low aqueous

solubility and are able to cross the lipid barrier and reach the systemic circulation.

Drugs with partition coefficient value between 1 and3 are well absorbed. Those with

partition coefficient value less than 3 or greater than 6 have poor transport characteristics.

Highly non-polar drugs have poor transport across membranes due to their tendency to reside

in the lipophilic regions (form a depot) of membranes. Highly polar compounds show poor

bioavailability because they are unable to penetrate the lipid membrane barriers (Lachman et

al, 1990).

Compounds with a balance between permeability and transport across the membrane

show the best oral bioavailability. For optimum absorption, the drug should have sufficient

aqueous solubility to dissolve in the gastrointestinal tract at the site of absorption and lipid

solubility should be high enough to facilitate the penetration of the drug across the lipiodal

membrane and into the systemic circulation.

The higher the value of the partition coefficient the greater is the lipophilicity of the

drug and vice versa. Drugs having values of Po/w much greater than 1 are classified as

lipophilic, whereas those with partition coefficients 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 (Folkers et al, 2003).

2. Drug design:

Lipophilicity can be exploited in designing certain drugs to either increase their

duration of action, target a particular organ or to reduce their toxicity. For example, most

central nervous system (CNS) acting drugs have high partition coefficient values making

them very lipophilic and thereby the drugs are able to cross the blood brain barrier to exert

their anesthetic effect because they remain largely in unionized form (Nanjwade, 2012).

3. Biopharmaceutics:

According to Biopharmaceutics Classification System (BCS) drug substances are

classified as:9

Class I – High permeability and high solubility

Class II – High permeability and low solubility

Class III– Low permeability and high solubility

Class IV – Low permeability and low solubility (Folkers et al, 2003).

FDA Centre for Drug Evaluation and Research (CDER) defines these drug boundaries as:

I. Highly soluble: When the highest dose strength is soluble in 250ml water over a pH

range of 1 to 7.5.

II. Highly permeable: When the extent of absorption in humans is determined to be

greater than 90% of an administered dose based on mass balance or in comparison to

an intravenous reference dose.

III. Rapidly dissolving: When greater than 85% of the labeled amount of drug substance

dissolves within 30 minutes using USP apparatus I or II in a volume less than 90 ml

of buffer solutions (Folkers et al, 2003).

Conclusion

Preformulation studies on a new drug molecule provide useful information for

subsequent formulation of a physicochemically stable and biopharmaceutically suitable

dosage form. Thorough preformulation work is the foundation of developing efficatious and

economical formulations.

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Reference

Allaudin, P.M.S. (2012) A Review on Preformulation Studies on Drugs. International

Journal of on Pharmaceutical Research and Development.4(5), 64-74

Avdeef, A. and Testa, B. (2002) Physicochemical profiling in drug research: a brief survey

of the state-of-the-art of experimental techniques. Cell Mol Life Sci. 59: 1681–1689.

Bhavin, V. (2011) Preformulation Parameters pKa and Solubility [Online] Available from:

http://mypharmaguide.com/wp.../Preformulation%20Parameters.ppt [Accessed 18th

November 2012].

Doria, M. C. C. (2011) Dosage Form Design: Pharmaceutical and Formulation

Considerations [Online] Available from:

http://images.rashiela28.multiply.multiplycontent.com [Accessed 18th November 2012].

Folkers, G., Han van de, W., Hans, L., Artursson, P., Mannhold, R., and Kubinyi, H.

(2003).Drug Bioavailability : Estimation of Solubility, Permeability, Absorption and

Bioavailability (Methods and Principles in Medicinal Chemistry).Weinheim, Wiley-VCH.

Lachman, L. Lieberman, H. A., and Kanig, J. L. (1990) The Theory and Practice of Industrial

Pharmacy.3rd edn. Mumbai, Varghese Publishing House.

Lobenberg, R., Amidon, G.L. and Vieira M. (2000), Solubility as a Limiting Factor to Drug

Absorption. In: Dressman JB and Lennernas H (eds.), Oral Drug Absorption, Prediction and

Assessment. New York, Marcel Dekker, Inc.

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Nanjwade B. K. (2012), Preformulation [Online] Available from:

http://api.ning.com/files/...xA-43prW/PREFORMULATION.ppt [Accessed 18th November

2012].

PointCross Inc. (2011), Drug Development Guide, Preformulation: Solubility determination

[Online] Available from:

http://www1.pointcross.com/source/ddg/steps/preclinical/preformulation/

Solubility_determination/index.html [Accessed 18th November 2012].

Ravichandiran, V, Devarajan, V. and Masilamani, K. (2011) Determination of ionization

constant (pka) for poorly soluble drugs by using surfactants: a novel approach.Scholars

Research Library. 3(4); 183-192

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