Drug solubility and bioavailability improvement. Possible methods with emphasis on liquisolid systems formulation

by Dr. Jan Gajdziok (Author) Barbora Vraníková (Author) Klára Kostelanská (Author) David Vetchý (Author) Jan Muselík (Author) Roman Goněc (Author)

Textbook 2018 169 Pages




1.1 Bioavailability
1.2 Physical-chemical properties of active substances
1.2.1 Solubility
1.2.2 Acidobasic properties
1.2.3 Partition coefficient
1.2.4 Size and shape of the molecule
1.2.5 Binding to plasma proteins
1.3 Factors within the organism
1.3.1 Gastric emptying and intestinal motility
1.3.2 pH in stomach and intestine
1.3.3 Impact of enzymes
1.3.4 Drug metabolism and first-pass effect
1.3.5 Biorhythms
1.3.6 Intra- and interindividual variability
1.3.7 Dosage form
1.3.8 Food

2.1 Chemical modification of active substance
2.1.1 Salts
2.1.2 Hydrates
2.1.3 Cocrystals
2.1.4 Prodrugs
2.1.5 Chelation
2.2 Physical methods
2.2.1 Amorphism and crystal polymorphism
2.2.2 Controlled crystallization Sono-crystallization Crystallization from supracritical fluids
2.2.3 Freeze drying
2.2.4 Spray drying
2.2.5 Particle size reduction of active substances Dry milling Freeze milling Wet milling
2.3 Technological means
2.3.1 Mediated dissolution Increasing of wettability Micellar solubilization Cosolvents Hydrotropes
2.3.2 Cyclodextrin complexes
2.3.3 pH adjustment
2.3.4 Solid dispersions
2.3.5 Interactive powder mixtures
2.3.6 Microgranulation
2.3.7 Self-emulsifying systems
2.3.8 Liquisolid systems

3.1 History
3.2 Advantages
3.3 Disadvantages
3.4 Technology
3.5 Preformulation testing
3.5.1 Flowable liquid retention potential
3.5.2 Liquisolid compressibility test
3.5.3 Optimal load factor
3.6 Excipients
3.6.1 Solvents Propylene glycol Liquid polyethylene glycols Polysorbates Glycerol Poloxamers Polyoxyethylated castor oil Polyoxyethylated hydrogenated castor oil Propylene glycol caprylate Macrogol-15-hydroxystearate Polyvinyl acetate
3.6.2 Conventional carriers Modified starch Lactose Microcrystalline cellulose Anhydrous dibasic calcium phosphate Mesoporous silicates Magnesium aluminometasilicates Chemical structure Properties Improvement of flow properties of powders Impact on the hardness of tablets Liquisolid systems Stabilisation of active substance Medicinal and other uses
3.6.3 Carriers for controlled release liquisolid systems Hydroxypropyl methylcellulose Polymethylacrylates
3.6.4 Other carriers for liquisolid systems F-Melt Clays and clay minerals Kaolinite Imogolite Halloysite Bentonite Montmorillonite Talc Sepiolite Laponite Zeolites
3.6.5 Coating material Fumed silica Silica gel Calcium silicate
3.7 Evaluation of liquisolid systems
3.7.1 Evaluation of liquid phase Viscosity Solubility
3.7.2 Evaluation of solid raw materials Size and shape of particles Optical microscopy Estimation of particle size by analytical sieving Laser diffraction Scanning electron microscopy Density Pycnometric density of solids Bulk and tapped density Porosity Flow properties Hausner ratio and compressibility index Flow through an orifice Angle of repose Angle of slide Specific surface area BET isotherm Transmission electron microscopy Dye adsorption Thermal analysis X-ray diffraction Fourier transformation of infra-red spectrum Nuclear magnetic resonance
3.7.3 Evaluation of final LSS dosage forms Uniformity of dosage units Hardness Friability Disintegration Dissolution Contact angle Water absorption ratio and wetting time
3.8 Use of liquisolid systems
3.8.1 Improvement of bioavailability
3.8.2 Controlled drug release
3.8.3 Oral dispersible tablets
3.8.4 Buccoadhesive tablets
3.8.5 Protection from light


The aim of this book is to provide a brief but comprehensive overview on the issue of drug bioavailability improvement by preparation of a perspective dosage form – liquisolid systems. The introduction chapter about drug solubility and bioavailability is followed by a description of the general methods which could be used to improve drug bioavailability using approaches of chemistry, physical modification, and primarily pharmaceutical technology. Benefits and practical use of each method are documented by examples. The main part of the book is aimed at characterization and description of liquisolid systems (LSS) – perspective dosage form for bioavailability improvement. Elementary principles of LSS formulation are described in detail, e.g. how to perform a preformulation study; how to choose the correct type and amount of excipients; how to evaluate the dosage forms, etc. All the above mentioned principles are documented with practical examples.

The book could be used as a textbook for students of natural, medical and pharmaceutical sciences as well as by researchers in this field or industrial area.


Contemporary pharmacotherapy is characterised by the increasing amount of active substances that are only poorly soluble in water. This may lead to the limitation of their systemic absorption on oral administration which is closely related to the bioavailability. This category is estimated to include more than forty percent of active substances that are in general use. So far, this poor aqueous solubility has been improved by physical or chemical modification of the active substance. In general, such changes are very expensive and troublesome, often leading to problems in stability, marketing authorisation process, or administration comfort of the particular drug. This is one of the reasons why modern pharmaceutical technology has focused on those dosage forms that can increase the bioavailability of some active substances while maintaining suitable stability and administration comfort. Several processes that improve solubility, respectively bioavailability have been described and published. These include micronization, nanocrystals, and formulation of solid dispersions. Only recently, a novel trend has appeared – to take advantage of good solubility of active substances in chosen solvents, that is, to use the active substances in a liquid phase. Subsequently, there is the possibility to bind the active substance in liquid phase onto highly porous carrier. Powder mixture prepared in this way can be compressed in tablets or filled in hard capsules. A “liquisolid system” is formed. Because the most limiting step of the absorption into the systemic circulation is represented by the dissolution of active substance in the gastro intestinal tract, the advantage of these systems lies in the fact that they contain the active substance already in dissolved form, therefore, this step is discounted, and the absorption, respectively the bioavailability of administered drug are improved.


liquisolid systems, pharmaceutical technology, bioavailability, solubility, carrier, preformulation study, evaluation techniques


1.1 Bioavailability

Bioavailability used to describe the ability of tablets, coated tablets, and capsules to release contained active substances so as they could be available for the absorption in the gastrointestinal tract. Nowadays, this propensity is called pharmaceutical availability. Bioavailability denotes currently the rate and extent of the active substance, whether unchanged or activated by its metabolism, reaches the site of its effect. Because the measurement of the concentration of the active substance at this site (e.g. near receptors) can be difficult to impossible, the bioavailability is defined more often as the rate and extent of the active substance reaching systemic circulation (Flynn, 2007).

The bioavailability (F) after oral administration can be measured if the active substance is administered both orally (PO) and intravenously (IV) and the plasmatic concentration timeline curves are compared. Bioavailability can be calculated according to the Equation 1, where areas under curve after oral administration AUCPO and after intravenous administration AUCIV are used (Flynn, 2007).

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Following oral administration, which is nowadays the most frequently used and the most popular route of administration of the majority of systemic drugs, the extent in which the drug reaches systemic circulation is influenced by many factors and processes (Figure 1). Although bioavailability cannot be predicted absolutely, the understanding of factors that have impact on it have risen in recent years. The factors include solubility, partition coefficient, absorption rate or penetration rate, metabolism, and elimination of the drug, etc. (Kaushal, 2016). Pharmaceutical technology can modify bioavailability on the level of pharmaceutical availability, i.e. the disintegration rate of the dosage form, the rate of liberation and dissolution of the active substance in gastrointestinal tract and on the level of absorption, to some extent, too.

Several factors that have impact on the bioavailability after oral administration are described in following chapters.

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Figure 1: The fate of the drug after oral administration

1.2 Physical-chemical properties of active substances

Physical-chemical properties of active substances that have impact on their bioavailability include aqueous and lipidic solubility, acidobasic properties (pH and pKa values), partition coefficient, molecular weight, shape of the molecule, and plasmatic protein binding (Kawakami, 2006; Jangher, 2011). The importance of physical-chemical properties of active substances is highlighted by Lipinski's rule of five (Lipinski, 1997). This rule states that an orally active drug has no more than one violation of the following criteria: not more than 5 hydrogen bond donors, not more than 10 hydrogen bond acceptors, molecular mass less than 500 daltons, and octanol-water partition coefficient not higher than 5.

1.2.1 Solubility

Aqueous solubility of a drug belongs to the most significant parameters that have impact on reaching required systemic concentration. From physical-chemical point of view, pharmacopoeias divide substances with respect to their solubility into several classes (Table 1) (Ph. Eur. 9, 1.4). From pharmacological point of view, this classification is usually insufficient as it does not take in account the dose of the drug. In some active substances, therapeutic response is achieved with a very small amount, on the other hand, in other substances, larger quantity of drug is necessary. Therefore, even a small dose of poorly soluble drug can dissolve completely in the gastrointestinal tract, while a large dose of well soluble drug does not have to dissolve (Kumar, 2014).

Table 1: Pharmacopoeial classification of solubility (Ph. Eur. 9, 1.4)

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This is the reason behind the design of Biopharmaceutics Classification System (BCS). BCS divides active substances into four classes according to their aqueous solubility and gastrointestinal permeability (Table 2). Within BCS, highly soluble drugs are those whose highest dose dissolves in 250 ml of water in pH range 1-8; highly permeable substance are defined as those absorbed from GIT to the extent of at least 90 % of administered dose (Gowardhane, 2013).

Table 2: Classes of biopharmaceutics classification system

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BCS was designed in 1990s (Amidon, 1995) and the authors intended to facilitate regulatory processes for new drugs. They hoped that i n vivo bioequivalence studies can be replaced with specific in vitro tests that would enable the prediction of the behaviour of the drug in vivo in case of immediate release dosage form (more than 85 % of dissolved active substance at 30 minutes) purposed for systemic absorption (Dvořáčková, 2009).

On the other hand, the solubility in lipids is related to the ability of the substance to permeate lipophilic membranes. The extent of absorption by passive transport, respectively mere diffusion, is proportional to the solubility of the substance in lipids (Zhu, 1995; Okáčová, 2011).

1.2.2 Acidobasic properties

The majority of drugs are weak acids or weak bases that can exist either in dissociated or in non-dissociated form. The non-dissociated form is absorbed more easily. The structure of the molecule determines if and in which environment stays the substance not polar (soluble in lipids) enough to pass through lipophilic cellular membranes. The dissociation constant (pKa) characterizes not only acidobasic, but also lipophilic and hydrophilic properties of a substance because pKa denotes pH value, at which 50 % of the drug is presented in its ionized (hydrophilic) form (Sinha, 2010; Jangher, 2011).

According to Brønsted theory of acids and bases, the bases are those substances (B) that can accept a proton, while acids are those substances (AH) that can donate a proton. The relation between pH, dissociation constant and the ratio between dissociated and non-dissociated substance is given by Henderson-Hasselbalch equation (Sinha, 2010):

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1.2.3 Partition coefficient

Partition coefficient (log P) between polar (aqueous) and nonpolar (most often octanol) phase determines the ratio of the active substance present in biomembrane lipid bilayer and neighbouring aqueous phase. Because the membranes the drug has to penetrate have hydrophilic-lipophilic nature, the absorption is the highest if the ratio between the concentration in lipophilic and hydrophilic phase is approximately one (Sinha, 2010; Jangher, 2011). In case of too lipophilic drug, i.e. partition coefficient higher than five, there is a risk of the substance remaining lodged within the cellular membrane (Baek, 2012).

1.2.4 Size and shape of the molecule

The diffusion rate of a drug is influenced by the size of its molecule. Larger molecules diffuse significantly slower than small molecules. The majority of active substances have molecular weight of 100-1000 daltons. Higher molecular weight is typical for peptides (up to 10 000 daltons) and proteins, e.g. monoclonal antibodies (about 150 000 daltons) (Zhu, 1995; Zatloukal, 2004; Sinha, 2010).

1.2.5 Binding to plasma proteins

The extent of binding to plasma proteins depends on the concentration of the active substance and its affinity to proteins. Lipophilic substances have usually higher tendency to bind to plasma proteins (chiefly albumin) than hydrophilic substances (Allahham, 2007). The bond is formed quickly and it is reversible. A change in the concentration of free unbound substance will result in corresponding change of the concentration of bound substance. Binding to plasma proteins represents a depot system that decreases the intensity of the effect, however, on the other hand, it helps to prolong the duration of the effect (Watano, 2003; Zhu, 2003).

1.3 Factors within the organism

1.3.1 Gastric emptying and intestinal motility

Intestinal motility and the velocity of gastric emptying have significant impact on the rate and extent of the absorption of the drug (Planinšek, 2011). Whether slower gastric emptying increases or decreases the absorption rate, respectively the bioavailability of drugs depends on their character. Prolonged gastric emptying was proved to decrease therapeutic response of levodopa (Mearrick, 1974). In contrary, slower gastric emptying increases the bioavailability of sodium salicylate in rabbits (Alhamami, 2007). Indeed, the difference lies in which part of the GIT is the drug absorbed. If the drug is absorbed in the stomach, the bioavailability is improved if the emptying is slower. If the predominant absorption occurs in the intestine, prolonged gastric emptying will have negative impact on bioavailability (Planinšek, 2011; Krupa, 2014).

Gastric emptying is closely related to food intake and it depends on several factors (Jangher, 2011; Planinšek, 2011; Yang, 2014). These include:

- volume of food – higher volume will at first increase, then, decrease the velocity of gastric emptying
- composition of food – fats and proteins slow down gastric emptying
- viscosity of food – higher viscosity will prolong gastric emptying
- temperature of food – warm food improves emptying
- fasting – gastric emptying is faster after fasting than after shorter interval between meals
- drugs – for example, analgesics and alcohol slow down, sodium bicarbonate and ranitidine accelerate gastric emptying

Beside food the velocity of gastric emptying is also influenced by the shape of the stomach, respectively the tonus of its wall. Horn-shaped hypertonic stomach is emptied faster than hook-shaped hypotonic stomach. Furthermore, the position of the body or exercise can have impact on the emptying. Light exercise (e.g. short walk) supports gastric emptying; however, hard exercise slows it down. The lying position on right side may accelerate gastric emptying in some people. Fear or depression lead to decrease gastric motility and decrease secretion, on the other hand, aggression accelerates motility. Acute or chronic pain in any part of the body suppresses reflexively both gastric motility and emptying (Hasuo, 2017).

Intestine motility improves the disintegration of solid dosage forms, as well as the dissolution and diffusion of the active substance towards the intestine wall. Intestinal motility can be influenced by the viscosity of food (the higher the viscosity, the lower the motility) or by drugs (cholinolytics decrease and prokinetics increase the motility) (Jangher, 2011; Krupa, 2014).

1.3.2 pH in stomach and intestine

As mentioned above, the solubility and the extent of absorption of many drugs is pH-dependent. The value of pH varies throughout the gastrointestinal tract. In the stomach, pH lies between 1.5 and 2.9 (fasting), but may increase on food intake up to 6.7 (Chen, 2009; Shaikh, 2009). The value of pH is different in different parts of the intestine. In the duodenum, pH is 6.0-6.5 (Shaikh, 2009; Kulkarni, 2010), in the jejunum, pH is about 6.8 (Kulkarni, 2010), and in the ileum, pH reaches as much as 7.4 (Kavitha, 2011). In the colon, pH varies greatly between 5.5 and 8.0, with lower values more frequent at the beginning; this is caused by the formation of acidic metabolites due to microbial metabolism (Nokhodchi, 2011).

Weak acids dissolve better in the alkaline environment in the intestine; however, in the stomach, they are present in non-ionised form and can be absorbed. On the other hand, weak bases dissolve better in the acidic gastric environment, but their non-ionised forms are absorbed better from the alkaline content of the intestine (Jangher, 2011).

1.3.3 Impact of enzymes

In particular parts of the gastrointestinal tract, there is different volume of fluids and different kinds of enzymes. Daily, the stomach produces 1.5-2.0 l of gastric fluid that contains hydrochloric acid, pepsin, mucin, and gastric lipase. The daily output of pancreas amounts to 1.0-1.5 l of fluid that contains a whole range of enzymes; these include amylase, trypsin, chymotrypsin, carboxypeptidase, lipase, esterase, and ribonuclease. In the intestine, this is joined by daily portion of 0.5-1.0 l of bile that contains bile salts (Kulkarni, 2010).

The main role of enzymes is to start the degradation of the most important constituent molecules of food, i.e. proteins, sugars, fats, and nucleotides. This is the reason why protein-based or nucleic acid-based drugs cannot be administered orally. Nevertheless, there are special pharmaceutical technologies leading to dosage forms that prevent the degradation of this kind of drugs (Baek, 2012).

1.3.4 Drug metabolism and first-pass effect

As shown in Figure 1, the drug has to undergo many processes before reaching systemic circulation or the desired site of its effect. A part of orally administered drug is not absorbed at all because of chemical or microbial degradation, or inactivation by bonding or complex formation. A further part of the substance may be metabolised when passing the gastrointestinal wall. That part of active substance that reaches portal vein can be metabolised in the liver or returned to the intestine by biliary secretion. This means, that the amount of the active substance that reaches systemic circulation depends also on the rate and extent of its gastrointestinal metabolism as well as on the first-pass effect in the liver (Spireas, 2002).

1.3.5 Biorhythms

Almost all physiologic functions, including drug absorption are influenced by time, with the most significant changes related to day time (circadian rhythms). The existence of these rhythms was proven in heart frequency, body temperature, blood pressure, blood flow in organs, plasma levels of hormones and other signal molecules (e.g. cortisol, melatonin, insulin, prolactin, etc), the amount of circulating blood particles, lung function, (tidal volume, forced expiratory volumes), liver function (metabolism), renal function (glomerular filtration rate, urine pH and volume, elimination of ions), gastrointestinal motility, gastric pH, gastric emptying, etc. Furthermore, the onset of some diseases and their symptoms depends on day time, e.g. asthma deteriorates in the small hours and heart attacks occur in the morning (Spireas, 1998B; Javadzadeh, 2007A; Jangher, 2011). These factors have to be considered when predicting the bioavailability of administered drug.

1.3.6 Intra- and interindividual variability

There are intra- and interindividual differences in the effect of drugs. The metabolism of drugs is in some cases genetically varied; e.g. N-acetyltransferase, hydrolase, and CYP2D6 can have extremely low activity in some people while significantly higher than average in others. The bioavailability is influence also by age. In children, the biotransformation is slower than in adults. In older people, blood flow through liver is reduced, and the activity of enzymes is decreased, therefore drugs are metabolised slower. Furthermore, the renal functions and thus the renal elimination are reduced with age (Zhu, 1995; Gavali, 2011). The reasons behind interindividual differences include also illnesses, organ disorders (e.g. GIT or renal diseases), eating habits, smoking, and drug interactions (Gavali, 2011).

Many studies of the impact of intra- and interindividual variability on bioavailability and effect were performed (Tayel, 2008; Gubbi, 2010; Hentzschel, 2012A), however, its extent cannot be predicted fully. If variability is likely to occur, individualized therapy may help to overcome this obstacle.

1.3.7 Dosage form

In the past, the bioavailability of the drug was perceived as tightly connected to the dosage form. At present, the bioavailability is perceived as complex property that can be influenced on many levels, however the choice of suitable dosage form remains the essential part of ensuring therapeutic effect (Spireas, 2002). An active substance administered in the form of aqueous solution is absorbed faster than when administered in oil solution or solid dosage form (Javadzadeh, 2008). There are dosage forms that are unaffected by aggressive acidic gastric environment, and/or control the release of the drug by delaying or accelerating its dissolution in particular parts of the gastrointestinal tract (Baek, 2012).

1.3.8 Food

Often, the bioavailability is influenced by food intake. This is significant chiefly in low absorption rate drugs. Food can have impact not only on the disintegration and dissolution (by changing pH of stomach or intestines, by changing the amount of secreted bile), the absorption (by increasing blood flow in the gastric or intestinal wall), or the passage of drug (by emptying or not emptying the stomach) but also on its metabolism in the intestines or liver (Tang, 2008). Some components of food may form non-absorbable or insoluble complexes with the drug (e.g. calcium with tetracyclines) and thus decrease their bioavailability (Karmarkar, 2009B; Jangher, 2011).

The impact of food on bioavailability is very complex and cannot be identified without running studies with the particular drug (Tang, 2008). For example, the bioavailability of propanthelin is significantly decreased by food (Moses, 1983). In contrary, the bioavailability of triclabendazol can be doubled if administered after food (El-Hammadi, 2012). The character of food can also play an important role. The bioavailability of erlotinib is slightly changed after standard food, but increases significantly when administered with fat-rich food (Ling, 2008).

FDA issued a guideline on testing the impact of food intake on bioavailability and conduction of non-fasting bioequivalence studies. The measurement of plasma concentration should be performed both under fasting (at least 0.5 before and 2 h after meal) and fed (after meal) conditions. In these studies, high energy food is recommended; total energy should amount to 800-1000 kcal (consisting of 500-600 kcal from fats, 150 kcal from proteins, and 250 kcal from sugars) (FDA, 2002).


There are numerous methods that improve the dissolution of drugs, respectively their bioavailability from oral solid dosage forms. Based on their nature, these processes are usually divided to chemical and physical modifications of the active substance and technological procedures (Figure 2). Chemical modifications of the active substance include prodrugs in the forms of esters, glycosides, etc, and the use of more soluble salts. This is the task of medicinal chemistry. The boundary between physical modification and technological procedure is unclear as many physical methods (e.g. micronization or freeze drying) are used to formulate the dosage form.

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Figure 2: The most important methods improving the bioavailability of poorly soluble APIs

2.1 Chemical modification of active substance

2.1.1 Salts

Salts are ionic compounds and therefore they are more soluble in polar solvents (chiefly water) than non-ionic substances. The use of salts is a popular method that changes physical-chemical properties of the active substance mainly because it is simple. A presence of ionizable group in the molecule is necessary, of course. The salt has to be chemically stable, it should not be hygroscopic, and it should dissolve quickly from the dosage form (Kratochvíl, 2010).

About one half of active substances are marketed as salts. About three quarters of salts contain the active substance as cation (the most numerous salts at all are hydrochlorides); about one quarter contain the active substance as anion (the cation is usually alkali metal or alkaline earth metal) (Kratochvíl, 2010; Okáčová, 2010; Kataria 2013). There may be significant pharmacokinetic differences between similar salts of the same drug, for instance, diclofenac potassium has higher bioavailability than diclofenac sodium in both healthy and febrile rabbits (Ahmad, 2010).

Salts with organic acids/bases are usually more soluble in water than inorganic salts. For example, avitriptan acetate is more soluble in water than avitriptan hydrochloride (16.5 mg/ml versus 3.4 mg/ml) (Good, 2009).

2.1.2 Hydrates

A hydrate is a compound that contains bound molecules of water. This is common not only in salts, but also in other substances. Each molecule of substance is connected to one or more molecule of water. Water is thus a direct component of crystalline structure and has impact on physical and chemical properties of the substances, e.g. crystal shape, colour, solubility, or melting point. Some hydrates are very stable (Good, 2009).

When a hydrate is present in anhydrous environment (low relative humidity, drying process, etc.) it releases water until there is a dynamic balance between water in the hydrate and vapour around it. The hydrate turns either into another type of hydrate containing less water molecules or into anhydrous form. In contrary, if the environment contains larger amount of vapour, a hydrate may accept water from the environment and turn into another type of hydrate containing more water molecules or the anhydrous form can turn into hydrate. These kinds of hydrates are used as desiccants of gases, liquids, as well as solids. A hydrate can have different colour or different solubility than corresponding anhydrous form. The main disadvantage of hydrates lies in their propensity to react with air vapour, thus changing their chemical and physical properties. If active substance is to be used as hydrate its use is limited by the chance to dehydrate. However, if the substance crystallizes as a hydrate under ambient conditions, the hydrate is often used for the formulation of the dosage form; an example can be seen in azithromycin dihydrate (Good, 2009).

2.1.3 Cocrystals

The notion cocrystal was used firstly by Hoogsteen in 1963 to denote the crystallization product of 1-methylthymine and 9-methyladenine (Rowe, 2006). In the course of cocrystallization, pure components do not form crystals by themselves. Pharmaceutical cocrystal is defined usually as stoichiometric multi-component crystal formed from at least two molecular or ionic compounds that are in pure state and solid at ambient temperature. This definition is still discussed because salts are also multi-component compounds, although their properties are different (Aitipamula, 2012).

If a substance does not form salts easily, for example because possessing poorly ionizable groups, pharmaceutical co-crystals present an alternative. Chemical and physical properties of the resulting co-crystal are different than those of its pure constituent, but no new covalent bonds are formed. The active substance in the form of a co-crystal retains its chemical identity, but its properties in solid phase – namely solubility and dissolution rate – can be changed. Molecular complex is another name for a co-crystal. Pharmaceutical co-crystals are host:guest compounds, where the active substance plays the role of the host and hosts a co-crystalline partner. These partners include mostly inactive substances, for example acids (adipic, citric, fumaric, succinic acid, etc), amides (nicotinamide, urea, etc), and sugar alcohols (mannitol, sorbitol, etc) (Okáčová, 2010).

Co-crystals have been tested in various active substances. For example, co-crystals of carbamazepine with 7 different substances were prepared and their solubility was found to be 2-152 times higher than that of carbamazepine alone (Good, 2009).

While carbamazepine itself has several polymorphs and is likely to form unstable solvates or hydrates, a co-crystal of carbamazepine and saccharine in the ratio 1:1 is stable, furthermore, the bioavailability is increased (Vishweshwar, 2006; Hetal, 2010).

2.1.4 Prodrugs

In 1958, Albert defined prodrugs as active substances that are biotransformed prior to be able to exert pharmacological effect. Nowadays, prodrugs are defined as biologically active substances that are without effect when administered and which are transformed into one or more active metabolites in the organism (Allen, 1978; Serajuddin, 2007). Usually, a prodrug consists of the active substance and one or more auxiliary groups, which may be rather lipophilic to facilitate transport or rather hydrophilic to improve solubility. This group has to be inactive, non-toxic, and easy to detach from the active substance (by chemical or enzymatic hydrolysis in the intestinal wall, liver, or systemic circulation) (Allen, 1978).

The use of prodrugs in dosage forms may increase solubility, improve stability, change unpleasant taste, improve absorption, decrease pre-systemic metabolism, and decrease toxicity (Allen, 1978).

Prodrugs with increased aqueous solubility are synthesised to contain either non-ionizable groups (hydroxy and ether groups of polyols, polyethylenglycol, and sugars) or ionizable groups (esters with inorganic or organic acids, esters/amids with aminoacids). The inclusion of non-ionisable groups in the molecule can increase the solubility two to three times, while that of ionisable group by several orders (Serajuddin, 2007).

An example of an ester with inorganic acid is clindamycin 2-dihydrogen phosphate (Okáčová, 2010). The use of esters is frequent because the method is relatively easy and it can change physical-chemical, biopharmaceutical, and therapeutic properties of drugs without changing their chemical structure and effect (Kataria, 2013).

In 1899, methenamin (hexamethylenetetramine) was probably the first deliberately synthesized prodrug. Experimental prodrugs include polyethylene glycol-paclitaxel (Balaraman, 2010) and N-glycyl-carbamazepine (Hoogsteen, 1963). Examples of marketed prodrugs include valaciclovir which is broken to aciclovir and valine (MacDougall, 2004), cefuroxime axetil which is metabolised to cefuroxime, acetaldehyde, and acetic acid (Sommers, 1984), or dabigatran etexilate, a double-prodrug, in which ethanol and hexyl carbonate are removed by metabolism (Blommel, 2011).

Another possibility to improve the solubility of poorly soluble drug lies in synthesizing or using a glycosylate. While the resulting substance may gain higher bioavailability, the sugar component can be uncoupled by biological processes in the body. Glycosylates permeate from the gastrointestinal tract easier that their unglycolysed constituents (Okáčová, 2010).

Silymarin is a mixture of natural flavolignans from the thistle Silybum marianum. These flavolignans are poorly soluble in water and have low bioavailability. However, if the molecules are glycosylated the bioavailability is improved (Kosina, 2002).

2.1.5 Chelation

Some active substances are able to form chelates with metallic ions. This changes their physico-chemical properties, e.g. increasing their solubility, stability, and melting point, pharmacokinetics, and pharmacodynamics. These more convenient properties allow for the use of demanding technological operations like milling. Sometimes, chelates can arise in the course of the manufacturing process of the dosage form (Okáčová, 2011).

Aluminium-magnesium gel is administered as antacid in the treatment of gastric or duodenal ulcers. The gel protects the gastric mucosa and maintains pH in the stomach at 4.5-5.5. Some antibiotics, e.g. azithromycin, are used to eradicate Helicobacter pylori or Campylobacter ieiuni because these bacteria contribute to the development and relapse of ulcers. When combining azithromycin with aluminium or magnesium antacid gels, the antibiotic and ions join to form complexes with high antibiotic effect. The chelate of azithromycin and magnesium and aluminium in ratios from 1:1 to 1:4 was proven to form gel that remains on the gastric mucosa of rats for 24 hours; therapeutic concentration of azithromycin is increased 1.5-60 times and exceeds minimal inhibition concentrations for bacteria mentioned above. Furthermore, the chelate does not change the toxicity of the active substance (Djokič, 1990).

2.2 Physical methods

2.2.1 Amorphism and crystal polymorphism

The solubility depends on the state of the substance in the solid state – whether it is crystalline or amorphous. Amorphous substances lack regular arrangement of molecules. They can be prepared by many ways, for example, by sudden cooling of the solution, by the addition of a liquid the substance is insoluble in, or by freeze-drying. In general, amorphous substances are more soluble than crystalline ones; however, they are less stable.

The solubility or the rate of the dissolution of an active substance can be changed by the choice of appropriate physical form. Some substances can form solid phases with different internal (crystalline) arrangement when influenced by external factors – temperature, pressure and crystallization conditions. This phenomenon is called polymorphism and particular forms the substance in solid phase is able to build are called polymorphic modifications or polymorphs of the particular substance (Okáčová, 2010). The polymorphism of active substances has been studied for more than 40 years. To simplify, some polymorphs can turn into other polymorphs if the temperature and/or pressure are changed. Beside solubility and the rate of the dissolution, the polymorphs can differ significantly in other physical properties, e.g., melting point, density, compactability, flow properties, and stability. This is the reason why the crystalline structure of each active substance has to be identified. The dosage form should be manufactured from that polymorph that has the most suitable properties with respect to the stability, bioavailability, and feasibility of technological processing. Polymorphism is studied and particular forms identified by using X-ray powder diffraction, differential scan calorimetry, thermogravimetric analysis, etc.

The importance of various crystalline modifications of active substances have been discussed very much recently (Okáčová, 2010; Kaur, 2012). A thermodynamically less stable polymorph with higher tension of vapour is more soluble in given solvent than a more stable polymorph. This is highly significant in poorly soluble substances. Monotrope metastable polymorphs are synthesized deliberately, because such modifications are more soluble and their dissolution from solid dosage forms is improved. The difference in solubility of particular polymorph is usually about 1:2, however, in case of premafloxacin, a difference of 1:23 was reported (Schinzer, 1997).

Desired polymorphs are manufactured usually by crystallization from suitable solvent under chosen conditions. This choice is complicated by the fact that the majority of organic solvents are toxic. While the use of some of them is prohibited altogether, the residual content of the permitted ones in the substances is limited strictly by regulations (Okáčová, 2010; Saindane, 2011).

The solubility of two crystalline and one amorphous form of indometacin was evaluated. The amorphous form was more soluble than both crystalline forms; however, the difference in solubility was not so high as predicted by thermodynamical considerations (Hancock, 2000). Atorvastatin calcium is commercially available in both crystalline and amorphous form. The wettability and intrinsic dissolution rate are higher in amorphous than in crystalline forms (Shete, 2010).

Simple milling of crystalline substance may turn it into amorphous substance or a more soluble polymorph. However, the solubility of such a polymorph is usually limited even in case large amount of surfactant or other solubilizer is added. Furthermore, amorphous substances can be influenced by humidity of the environment which can initiate the formation of agglomerates, thus decreasing the efficacy of the milling, solubility of the product and its dissolution profile. If the active substance is milled together with magnesium aluminometasilicate (Neusilin), more soluble modifications of the drug with sufficient stability can be obtained. Neusilin can help to maintain stable amorphous state of a substance by absorbing it on its amorphous surface and by building hydrogen bonds between the drug and its silanol groups (Bogner, 2006; Okáčová, 2010; Fuji Chemical Industries, 2014K).

Full transition of indometacin from crystalline to amorphous form was achieved by milling it together with Neusilin US2 in ratio 1:5 at ambient conditions for 5 days. The presence of crystalline form was checked by FT-IR. However, without Neusilin no amorphous form was observed even after 14 days and no matter if zero or increased (75 %) humidity was applied. The higher the amount of Neusilin, the faster the change; if the ratio between indometacin and Neusilin was 1:1, 8 days were necessary for total conversion to amorphous form. Higher amount of Neusilin offers larger surface for the drug to adhere to and protects its amorphous structure (Bogner, 2006).

If aceclofenac is milled together with Neusilin US2 in the ratio 1:5 at ambient temperature for 20 hours, full conversion to amorphous form occurs. The dissolution rate of amorphous aceclofenac is 103 % at 3 hours while crystalline aceclofenac requires 8 hours and reaches only 92 % (Vadher, 2009).

Neusilin does not play only the role of carrier and stabiliser of the amorphous drug, but it can increase the mechanical resistance of manufactured tablets. For instance, prednisone was milled together with Neusilin US2 in the ratio 1:7 for 90 minutes. The amount of amorphous form reached 75 %. Figure 3 shows that the particles of Neusilin US2 retained their size and spherical shape after milling which was convenient for further processing. The milled product was compressed into mini tablets (suitable for administration to children). Although compaction pressure of 4 MPa was applied, the dissolution rate at 30 minutes reached 87 % as compared to 60 % in the case of crystalline form (Lou, 2013).

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Figure 3: a) Neusilin US2, b) crystalline prednisone, c) Neusilin and prednisone milled together (Lou, 2013)

2.2.2 Controlled crystallization

The choice of crystallization method and its conditions can have impact on physical-chemical parameters of the product, chiefly its solubility. Controlled crystallization can be used to obtain crystal with optimal size, shape, and distribution. The process depends on used solvents or their mixtures, temperature, pressure, hypersaturation of the solution, the shape of the vessel, etc. Controlled crystallization is started usually by changing pH or adding another solvent. Solvents are chosen according to their polarity, dipole moment, surface tension, viscosity, boiling point, density, etc. The shape of crystals can be influenced by using various polymeric stabilisers that adsorb to some crystal surfaces and thus slow down their growth because the stabiliser binds to the surface with respect to how the molecules on the surface are oriented (Tao, 2009; Okáčová, 2010). Stabilisers include hypromellose (Yildiz, 2007; Kim, 2008B; Zhiyi, 2009), sodium carmellose, hydroxyethylcellulose, methylcellulose, hydroxyethylstarch, agar, gelatine, sodium alginate, pectin, polyvinylalcohol, povidone, polyethylene glycol, etc (Jung, 2001; Tao, 2009).

Controlled crystallization was used to reduce particle size of ibuprofen, itraconazole, and ketoconazole. Homogeneous dispersions with particle size of about 2 µm were prepared using various stabilisers, the smallest crystals were obtained when using hypromellose 4000 (Jung, 2001). Sono-crystallization

Beside above mentioned physical-chemical factors, sono-crystallization is based also on ultrasound with frequency 20-100 kHz or even up to 5 MHz. The use of ultrasound can have significant impact on the properties and shape of manufactured crystals (Sander, 2014). When prepared by melt sono-crystallization, ibuprofen crystals consisted of irregular agglomerates of needle-, desk-, and tube-like porous crystals with about three times larger surface area and improved solubility. The longer the ultrasound with the frequency of 1.25 MHz was applied, the smaller crystals were produced. Dissolution in phosphate buffer reached 90-10 %; the samples not treated with ultrasound reached only 55 % (Manish, 2005). Crystallization from supracritical fluids

Controlled crystallization can be performed using solvents in supercritical state, that is, solvents, whose temperature and pressure exceeded their critical points. In such state, the solvent has properties of both a gas and a liquid, retaining its ability to dissolve other substances but acquiring flow properties of gases. Carbon dioxide is the most frequently used supercritical solvent because its critical temperature and pressure are relatively low, it is cheap, non-toxic, and non-flammable. Other examples of supercritical solvents include the mixture of carbon dioxide and ethanol, nitrous oxide, fluoroform, ether, etc (Charoenchaitrakool, 2000; Young, 2000; Türk, 2002; Won, 2005; Perrut, 2005; Boonnoun, 2013).

If the substance is dissolved in supercritical fluid and sprayed in the environment through a narrow nozzle, micronization by rapid expansion of supercritical solution occurs. High degree of saturation together with sudden drop in pressure leads to even formation of nuclei; thus, well dispersed particles are formed (Zhang, 2009). However, if the substance is poorly soluble in supercritical solvent, supercritical anti-solvent micronization is more suitable. In that case, the solution of active substance in organic solvent is sprayed into a chamber filled with supercritical medium which acts as anti-solvent. The size and shape of manufactured crystals can be influenced by the type and diameter of the nozzle, the properties of organic solvent and manufacturing conditions (temperature, pressure, jet flow) (Steckel, 2004). Technologies using supracritical fluids can lead to very small particles (with diameter less than 500 nm). The method can be also used in thermosensitive substances as only low temperatures are used in the course of the process (Steckel, 2004; Zhang, 2009).

The effects of supracritical antisolvent technology on the recrystallization and particle size of sulfathiazole were studied. Acetone at 35 °C and 12 MPa was found out to be the most suitable antisolvent. During the process, not only the polymorph form of sulfathiazole changed from III to II, but also mean particle size decreased from 43 µm to 2 µm. Sulfathiazole processed in this way dissolved faster than the original active substance (Chen, 2010).

Micronization by rapid expansion of supercritical fluid was used to process substances while achieving low particle size, for example, griseofulvin (Zhang, 2009), β-sitosterole (Zhang, 2009), ibuprofen (Yi, 2008), and cyclosporine A (Dollo, 2003). Micronization by supercritical anti-solvent was used to manufacture solid dispersion microparticles of felodipine in HPMC (Won, 2005), or nanoparticles of atorvastatin (Kim, 2008A).

2.2.3 Freeze drying

Freeze drying is a process in the course of which substance, solution, or suspension is dried very carefully in frozen state. The solvent is removed by sublimation at first and subsequently by desorption. The resulting solid acquires special porous structure with channels. When freeze-dried structure dissolves, the process is very rapid or at least very accelerated (Okáčová, 2010; Kasper, 2013; Wanning, 2015).

The conventional course of freeze drying consists of three steps: freezing, primary drying, and secondary drying. During the first step the solution or suspension is frozen (usually at minus 40-50 °C). This causes ice crystals to form and their subsequent growth. There are two ways of freezing that may occur. Either the liquid phase becomes solid all of a sudden (eutectic freezing) at a temperature that depends on the nature of dissolved compounds or it becomes more and more viscous and dense until amorphous solid is formed (vitrification). Quick freezing leads to numerous small ice crystals while slow freezing results usually in a few large ice crystals (Wang, 2012). Unluckily, there is no universal instruction how to freeze a solution and the freezing process has to be tested experimentally for each type of product. Primary drying is based on the sublimation of ice at decreased pressure. Phase transition demands energy, of course, and it can be roughly stated that 1 watt sublimates 1 g of ice in 1 hour. However, there are freeze-driers with lower energy input. After primary drying, the product contains still about 15-20 % of water. Secondary drying is based on water desorption. The temperature is maintained higher than during primary drying and the pressure is maintained as low as possible. The main disadvantages of freeze-drying are high energy input, costs, and time consumption (Challa, 2005; Carrier, 2007).

Spray freeze drying is less energy and time consuming than conventional freeze drying. In this process, the technologies of spray- and freeze drying are combined, resulting in highly porous spherical particles of desired size. The difference lies in the first step, when the solution is sprayed to form diminutive droplets, which are subsequently frozen. Next steps are identical with conventional freeze drying (Ghorab, 2003).

In pharmaceutical industry, freeze drying is used mostly to produce injectables containing active substances that are not stable enough in aqueous solution (Carrier, 2007). Freeze drying is also the basis for some technological processes in the production of oral dispersible tablets (patented systems Zydis, Lyoc, and Quisksolv). Freeze dried films can be used for buccal administration or for the treatment of wounds (Barone, 1998).

Freeze drying can be used to improve the solubility and bioavailability of poorly soluble drugs. For example, ketoprofen and nitrendipin were processed with hydroxypropyl-β-cyclodextrin complex, with tertiary butanol used as solvent. The solubility of ketoprofen complex in simulated gastric fluid was 26 times higher when using freeze drying and the solubility of nitrendipin complex in simulated both gastric and intestinal fluid was 4 to 5 times higher (Wang, 2007) when compared with standard processing. Freeze drying was also used to produce silicate-lipid hybrid carrier with celecoxib. In this process, nanoporous structure consisting of triglyceride emulsion and silica (Aerosil 380) nanoparticles was formed. The solubility of celecoxib from this product in simulated intestinal fluid was 1.5 times higher than of the active substance alone (Yasmin, 2014).

2.2.4 Spray drying

While cabinet driers have high energy demand and use high temperature air that is not suitable for some products, spray drying is a subtle technology that dries up liquid products without affecting the quality of dissolved or suspended substances. This advantage of spray drying is underlined by the ability of the process to result in various products – powders, granules, agglomerates – that have specific properties defined by their structure, particle size distribution, density, and residual humidity. At the beginning, the solution or liquid dispersion in suitable solvent or dispersant is passing through a jet, resulting in fine spray. This is dried rapidly in flowing gas, usually at increased temperature. If the substance is prone to oxidation or flammable solvent is used, nitrogen has to be used instead of air. When required amount of liquid is evaporated from the drop, a solid particle is formed. This is removed from the gas flow by a filter or cyclone. By changing the conditions, particles of desired shape, size, and density can be obtained (Rabišková, 2007A; Okáčová, 2010).

Particles manufactured by this process are usually spherical, porous, and have good flow properties. They are easy to wet and quick to dissolve. Spray drying can increase the solubility as well as the rate of dissolution of a substance. The process can lead to suitable shape and size of particles that can be as small as 5 µm in diameter (Okáčová, 2010; Walters, 2014).

The rate and mainly the velocity of dissolution from the dosage form grows with increasing surface, respectively with decreasing size of individual particles. This is described in Noyes-Whitney equation (Equation 3).

illustration not visible in this excerpt

dn amount of substance dissolved in time interval (dt)

D diffusion coefficient of dissolved substance in the solvent

S total area of phase boundary between the solid substance and the solution

δ thickness of diffusion layer

cs concentration of the saturated solution at the phase boundary

c concentration of the dissolved substance in the total volume at given time

Spray drying consists of three steps: atomization, droplet drying/formation of particles, and collection of particles. In the course of atomization, a spray of droplets with high surface to mass ratio is formed. The second step is based on the conversion of atomized liquid droplets into solid particles, resulting in dispersed particle aerosol. Subsequently, the solid and gaseous phases have to be separated. This is done by either cyclone separation or baghouse filtration. Cyclone separation is possible thanks to the difference in density of particles and gas; baghouse filtration collects the dried particles by means of filters (Walters, 2014).

Spray drying is commonly used to process proteins and to manufacture powder for inhalation, vaccines, living organisms, and herbal extracts. Spray drying is run under atmospheric pressure, which is an advantage. Although spray drying is a time consuming process and can last for several hours, it is still significantly shorter than freeze drying (Sollohub, 2010; Vandana, 2014; Walters, 2014).

Noyes-Whitney equation states that solubility is higher in particles with lower particle size. As mentioned above, spray drying can lead to particles smaller than 5 µm. For example, artemisinine spray dried together with maltodextrine acquired much higher solubility, dissolution, and bioavailability (Sahoo, 2009).

Dissolution of felodipine is faster when using spray dried product than product manufactured by hot-melt extrusion. This was tested in combination with two different polymers – polyvinylpyrrolidone and hydroxypropyl methylcellulose acetylsuccinate. The difference in dissolution rate is caused by the fact that spray dried product consists of both smaller and more porous particles than extruded product (Mahmah, 2013).

2.2.5 Particle size reduction of active substances

The smaller the particles are the faster they dissolve. This is clear from Noyes Whitney equation and this is the reason why particle size reduction is one of the oldest and the most used technologies to increase the bioavailability of poorly soluble drugs. The most common kind of particle size reduction is milling (Jinno, 2006). The aim of the reduction is to obtain particles with diameter of 10 µm or lower because such particles can be absorbed by active transport mechanisms. Nanoparticles can be absorbed directly through the wall of intestinal cells (Okáčová, 2010).

With respect to the degree of their size reduction, particles are called microparticles or nanoparticles (Steckel, 2003; Tao, 2009). Dry milling

Dry milling of active substances is the most frequently used method to reduce particle size. The use of oscillation, hammer, or ball mills results in particles with dimensions of tens of micrometers. Jet mills are more efficient and particles with the dimensions of 0.1-1 µm can be obtained. Dry milling is widely used; however, it is not ideal, as final properties of the active substance (shape, size, morphology, surface, etc) can be influenced only partially. Furthermore, in the process of dry milling, micronized particles gain electrostatic charge, which leads to the formation of agglomerates. Agglomerates prevent liquids to penetrate the particle and thus slow down the dissolution. Mere dry milling of active substance without further processing is usually unable to increase bioavailability (Okáčová, 2010; Gowardhane, 2013).

Three ways to reduce particle size of cilostazol were tested (hammer mill, jet mill, and spray drying). In all three cases the dissolution profile was improved when compared to untreated active substance. In suspensions manufactured in hammer and jet mill, the impact of food intake on the dissolution rate was observed, however, the dissolution of product manufactured by spray drying was unaffected by food (Jinno, 2006). Freeze milling

Freeze (cryogenic) milling is convenient if the material has disadvantageous properties like low melting point, high elasticity, or high hardness and if it can be cooled in liquefied gases, for instance, carbon dioxide, hydrogen, or nitrogen. Frozen material is very brittle and easy to disintegrate into small particles in the course of milling. Mills used in this type of milling work by similar principles as dry mills, however, their construction is modified to be able to withstand liquefied gases. Freeze milling is used to reduce particle size of animal and plant tissues, and unstable or volatile substances (Chambin, 2004).

Freeze milling was proved to improve the solubility of nifedipine and indometacin in povidone polyacetate copolymer (Rasenack, 2002). Wet milling

Wet milling is becoming popular in cases where dry milling cannot be used or is not efficient enough. Conventional ball or colloidal mills can be used for wet milling; however, pearl mills are more efficient. In a pearl mill, the suspension of active substance is moved through a body of rotating beads that have diameter 1 mm or lower. In general, water or other hydrophilic nonvolatile liquid is used as dispersant with an addition of tenside that helps to suspend poorly wettable substances and also prevents agglomeration of micronized particles. The dosage form is then manufactured using dried product or directly the micronized suspension for wet granulation. The main disadvantage lies in the risk of agglomeration (Okáčová, 2010; Gowardhane, 2013).

Wet milling was used to improve the dissolution profile of noretindrone and noretisterone. By using a jet mill, mean particle size was reduced from 53 µm to 1 µm. While density, coarseness, and hardness of particles were the same in micronized and non-micronized substances, micronization increased the dissolved amount from 65 to 85 % at 30 minutes, and from 69 to 90 % at 45 minutes (Bansal, 2011).

2.3 Technological means

2.3.1 Mediated dissolution

Mediated dissolution (solubilization) is based on increasing the solubility of a substance by adding suitable excipient, i.e. a solubilizer. There are several ways how to do this; the most frequently used ones include increasing the wettability, micellar solubilization, co-solvents, and hydrotropic effect, (Sonali, 2013). Increasing of wettability

Wettability can be increased by using tenside, surfactant, or any other surface active substance. These also help to increase the dissolution profile of poorly soluble substances. Tensides decrease surface tension between solid and liquid phase and improve the dissolution of lipophilic drugs in aqueous media. They adsorb onto the particles, creating a film that prevents them from agglomeration (floculation). The dissolution is increased due to higher wettability as well as improved penetration of dissolution medium into the particles. Using tensides to improve wettability is advantageous because the processing is easy and the stability of the drug is increased, too. However, their use is limited by their toxicity (Narang, 2007; Bajaj 2011; Okáčová, 2011; Gowardhane, 2013).

Surfactants with hydrophilic-lipophilic balance (HLB) value 7-9 are usually called wetting agents, while tensides with HLB value higher than 10 are called micellar solubilizers (Table 3) (Sonali, 2013). Wetting agents used in pharmaceutical industry include sodium laurylsulphate, benzalkonium chloride, macrogol stearate, fatty acids sorbitan esters, polysorbates, polyethoxylated castor oil, etc (Janakiraman, 1984).

Table 3: The use of tensides with respect to their HLB (Sonali, 2013)

illustration not visible in this excerpt Micellar solubilization

In this case, the active substance is dissolved thanks to reversible interactions with the micelles of surfactant (tenside), resulting in a thermodynamically stable isotropic solution. The process of micellization in water is the outcome of subtle balance of intermolecular forces that include hydrophobic, electrostatic, van der Waals, and hydrogen bonds/interactions. Given amount of tenside allows for the solubilization of only certain amount of insoluble substance, which can be denoted as maximal additive concentration. Micelles are formed if the amount of dissolved tenside exceeds critical micellar concentration (CMC) (Vraníková, 2015A). The CMC of most surface active substances lies between 0.05 and 0.10 % (w/v) and depends on chemical properties of the substance (Kratochvíl, 2010; Sonali, 2013). In general, the lower the CMC is, the more stable the micelles are. This is very important in case of intravenous administration, where sudden dilution with the volume of circulating blood occurs. If the CMC is high, micelles disintegrate to their constituent molecules and their poorly soluble content may precipitate. Because of low CMC values, non-ionic tensides are better micellar solubilizers than ionic tensides. The incorporation in micelles improves bioavailability by two parallel means – by improving the solubility and by facilitating the transport of the drug through intestinal wall (Dvořáčková, 2011).

Micelles can have spherical, cylindrical, or planar shape and their size ranges from 5 to 500 nm. The shape and size of micelles can be influenced by changing the chemical structure of tenside, or by changing the conditions of their preparation (temperature, concentration, ratio of particular tensides, ionic strength, pH) (Vraníková, 2015A).

In conventional micelles, the amount of water decreases from the surface towards the core, which is entirely hydrophobic. As shown in Figure 4, an active substance can be incorporated in the micelle by several means depending on its nature. Hydrophilic active substance is adsorbed on the surface (1), amphiphilic substance can be lodged among the hydrophilic parts of tenside molecules on the surface (2) or wedged along the boundary between hydrophobic and hydrophilic molecules right beneath the surface (3), and lipophilic substances are incorporated in the lipophilic core (4) (Vraníková, 2015A). Reverse micelles can be used to solubilize hydrophilic substance in lipophilic solvents, that is, in oils (Rangel-Yagui, 2005A).

illustration not visible in this excerpt

Figure 4: Incorporation of substances in micelles

Surface active substances used to improve the solubility by micellar solubilisation include sodium laurylsulphate (Lindahl, 1997), poloxamer (Pluronic P-85) (Zhou, 2009), polyethylene glycol (Carbowax or Carbowax Sentry) (Fallingborg, 1999) , macrogol-12 laurylether (Brij 30) (Fallingborg, 1999), etc. The size of micelles can be measured using dynamic light scattering or transmission electron microscopy.

The possibilities of micellar solubilisation were tested in ibuprofen. Sodium laurylsulphate, dodecyltrimethylamonium bromide, and octaethylene glycol dodecyl ether at different concentrations were tried. The solubility of ibuprofen was found to grow linearly with increasing concentration of particular tensides. Highest solubilisation capacity was found in dodecyltrimethylammonium bromide (Lindahl, 1997).

Polymeric micelles are of special interest. These are formed from polymers containing both hydrophilic and hydrophobic units, for example, poloxamers that consist of polyethylene oxide and polypropylene oxide blocs. The core of the micelle contains hydrophobic blocs and the surface contains hydrophilic blocs. PEG surface prevents recognition by macrophagi, which means that micelles remain in systemic circulation for longer time, allowing for more effective transport of drugs to desired tissues. Moreover, polymeric micelles can be sterilised easily by filtration.

Poloxamer micelles loaded with nevirapin, without or with an addition of cosolvents (glycerol, propylene glycol, and polyethylene glycol 400) were tested and found to increase the solubility of nevirapin in water (Moretton, 2014). Cosolvents

The addition of cosolvents is a simple and highly efficient method to prepare aqueous solutions of otherwise poorly soluble substances. Cosolvent can be defined as liquid miscible with water that has the ability to disrupt some physical properties of water and so to make the drug more soluble. Surface tension of cosolvents is usually lower than surface tension of water and co-solvents decrease surface tension within the system. Unlike other methods that improve the solubility, the use of co-solvents allows for the dissolution of larger amount of active substance. However, bioavailability does not have to be increased because of precipitation after the administration. To increase the solubility even more, cosolvents are combined with other solubilization methods, for example, pH adjustment. The combination of tensides and cosolvents is also possible (for instance, sodium laurylsulphate and ethanol). The most frequently used cosolvents include glycerol, ethanol, propylene glycol, pyrrolidone, and liquid polyethylene glycols. In preclinical (toxicological and pharmacological) studies in animals, dimethyl sulfoxide and dimethyl acetamide are also used as cosolvents because of their high solubilization capacity and relatively low toxicity (Janakiraman, 1984; Zhu, 1995; Kawakami, 2006; Vemula, 2010; Jangher, 2011; Okáčová, 2011).

The main advantages of this method are simplicity, fast production, and high efficiency. Uncontrolled precipitation on dilution is the main disadvantage. Amorphous or crystalline precipitates pose a threat in case of intravenous administration. The use of some substances as cosolvents is also prohibited or limited due to their toxicity. As in other solubilized dosage forms, there is an increased risk of chemical instability of the undissolved portion of the substance (Vemula, 2010).

The formulation of oral (solid or liquid) as well as parenteral dosage form of valdecoxib is a challenge because the aqueous solubility of valdecoxib is only 10 µg/ml. Combinations of hydrophilic carriers (mannitol, PEG 4000, PEG 6000, PEG 8000, or urea), co-solvents (ethanol, methanol, or glycerol), and tensides (Tween 20, Tween 80, or sodium laurylsulphate) were tested to increase the solubility. The most efficient combination, that is, the highest solubilisation potential was identified in PEG 4000, sodium laurylsulphate, and ethanol (Desai, 2004).

In some drugs, glycerol increases the solubility only very slightly, which can be attributed to the fact that it is polar. The less polar the co-solvent is, the more effectively hydrogen bonds between water molecules are disrupted. This also reduces the possibility of nonpolar drug being precipitated from the newly formed solvent (the mixture of water and co-solvent). For example, fluasterone is the most effectively solubilized by ethanol, followed by PEG 400, propylene glycol, and glycerol. With increasing concentration of the co-solvent, the solubility of fluasterone was found out to grow exponentially (Zhao, 1999). Hydrotropes

Hydrotropic effect was first described a century ago as an increase in solubility caused by the addition of fairly high concentration of alkali metal salts of various organic acids (Neuberg, 1916). Various substances that increase the solubility of nonpolar substances in water by disrupting its associated structure can be used as hydrotropes. The solubility is increased by 100-200 times or even more and occurs at sufficient hydrotrope concentration (Atanacković, 2009). Because hydrotrope acts on the solvent, this kind of solubilisation is neither specific nor stoichiometric (Kumar, 2014).

Commonly used hydrotropic substances include urea, nicotinamide, lysine, tryptophan, citric acid, sodium benzoate, sodium salicylate, aromatic sulfonic acids and their salts, etc (Hagan, 1996; Rangel-Yagui, 2005A; Rangel-Yagui, 2005B). With several exceptions, a molecule of hydrotropic substance consists of a hydrophilic and hydrophobic part, the latter being too small to enable spontaneous aggregation, which would occur in tensides (Patil, 2013; Kumar, 2014).

The use of hydrotropes is simple and cost-effective. The active substance and the hydrotrope are just mixed together and dissolved. In contrast to other solubilisation methods as micellar solubilisation or co-solvents, the advantage of hydrotopes lies in their pH independence. No organic solvents are required, so the method is nature friendly and no limits on residual solvents have to be observed. However, high concentration of hydrotropes are required (up to 6 mol/l), there is a risk of interaction between the active substance and the hydrotrope, and some hydrotropes have pharmacological effect of their own (Rangel-Yangui, 2005A, Sajid, 2012; Patil, 2013).

Because paclitaxel is poorly soluble in almost any solvent and parenteral administration requires the use of dissolved substance, more than 60 potential hydrotropes were tested. Out of these, N,N-diethyl nicotinamide was identified as the most efficient, as it was able to increase the aqueous solubility of the active substance from 0.0003 mg/ml to 512 mg/ml (Lee, 2003).

Salicylic acid was a model drug to study the addition of hydrotropes in concentration up to 3 mol/l and temperature 30-60 °C. The solubility of salicylic acid increased with the increasing concentration of the hydrotrope as well as with increasing temperature. Hydrotropic ability was different for different hydrotropes and increased in following way: urea < citric acid < sodium salicylate < sodium acetate (Theneshkumar, 2009).

2.3.2 Cyclodextrin complexes

Inclusion complexes of active substances with cyclodextrins are used methods to increase solubility as well as permeability in gastrointestinal tract and several products based on this advantage have been marketed (Freedman, 1993; Miyake, 1999). Cyclodextrins were first isolated in 1891. These cyclic sugars are synthesised by bacterial glucosyltransferases from starch. Their ability to form complexes with other substances was discovered in 1948. Beside natural cyclodextrins (α, β, and γ) (Table 4), there are also hydrophilic, hydrophobic, or ionic derivatives with various physical-chemical properties, improved stability, lower toxicity after parenteral administration, and higher inclusion capacity.



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liquisolid systems pharmaceutical technology bioavailability solubility carrier preformulation study evaluation techniques


  • Dr. Jan Gajdziok (Author)

    2 titles published

  • Barbora Vraníková (Author)

  • Klára Kostelanská (Author)

  • David Vetchý (Author)

  • Jan Muselík (Author)

  • Roman Goněc (Author)



Title: Drug solubility and bioavailability improvement. Possible methods with emphasis on liquisolid systems formulation