Nanocrystals-Universal approach for poorly hydrophilic drug

Shivsambh Narwade*, Dipak Bagad

Sinhgad College of Pharmacy, Vadgaon (Bk.), Pune, Maharashtra, India

ABSTRACT:

The technologies such as high throughput screening, combinatorial chemistry and computer-aided drug design leads to drug candidate with poor solubility, which has erratic absorption and low oral bioavailability. Nanocrystal is universal approach for these drugs. A nanocrystal consists of drug, stabilizers as surfactants or polymers and a liquid dispersion medium which can be aqueous or non-aqueous. The nanocrystals are produced by two approaches i.e. bottom up and top down techniques. The ionic and non-ionic stabilizers avoid aggregation of drug particles by ionic or steric barriers respectively. This review article highlights on nanocrystals, their methods of preparation, special features, role of stabilizers and application of nanocrystals in drug delivery.

KEYWORDS: Nanocrystals, stabililizer, bottom-up and top-down technique, nanosuspension.

1. INTRODUCTION:

It is found that 40% or more of drug obtained from combinatorial screening programs are poorly soluble in water¹. This poor solubility creates major hurdle in formulation development i.e. it affects dissolution, absorption and bioavailability of drug which finally affects the pharmacological activity of drug. Drug nanocrystals are pure solid drug particles (mean diameter below 1000 nm)². Nanocrystals are particles made from 100% drug; typically, they are stabilized by surfactants or polymeric steric stabilizers³. Though the term nanocrystal describe a crystalline state but it can be in crystalline, partially amorphous or amorphous form based on method of production. It took about 25 years for the liposomes to appear in pharmaceutical products on the market. It was less than 10 years for the nanocrystals, having the first patent applications filed at the beginning of the 1990s, and the first product Emend on the market in 2000^4. A suitable drug candidate for nanocrystal formulation will be a drug from BCS class II and IV. Nanocrystal can be administered as oral, parenteral, pulmonary and topical drug delivery system.

2. NANOCRYSTAL AS A UNIVERSAL APPROACH

Salt formation will not be suitable for compound which is non-ionizable^5. Potential disadvantages of salt formation include, high reactivity with atmospheric carbon dioxide and water resulting in precipitation of the poorly water-soluble drug. Cyclodextrin complexation is suitable for drug having the right molecular size to fit in the cyclodextrin ring². Solid dispersions though increases dissolution rate, but molecules in the amorphous state are not thermodynamically stable; they can be convert to the crystal form during storage⁶.

Solid lipid nanoparticles (SLN) also suffer from problem like drug expulsion, gelation tendency and drug loading which depends on the solubility of drug in lipid⁷. Nanostructured lipid carrier (NLC), nanoemulsion, and polymeric nanoparticles in all these formulation drugs is distributed throughout the matrix i.e. it provides less than 100% drug loading¹⁵. Micronisation of poorly soluble drugs, increases the dissolution rate of the drug due to the increase in surface area, but does not change the saturation solubility of drug⁶. Nanocrystal preparation can tackle all the above problems exising with different formulation. This proves that nanocrystal is a universal approach for formulation of poorly soluble drugs.

3. SPECIAL FEATURES OF NANOCRYSTALS:-

3.1 Dissolution velocity:- In case of many poorly soluble drugs micronization does not create a sufficiently large surface area to adequately enhance the dissolution velocity. As a consequent next step one moved from micronization to nanonization, i.e. production of drug nanocrystals significantly enhanced the dissolution rate.

3.2 Saturation solubility:- The saturation solubility is defined as a drug-specific constant depending only on the solvent and the temperature². However below a size of 1-2 µm, the saturation solubility is also a function of particle size. The theoretical backgrounds are Ostwald-freundlich equation, kelvin equation, prandlt equation. The Kelvin equation describes the vapor pressure over a curved surface of a liquid droplet in gas. The vapor pressure increases with increasing curvature, which means decreasing particle size The equilibrium between dissolving molecules and molecules recrystallizing on particle surfaces (determining the extend of this might be too long for many nanosuspensions of saturation solubility) is shifted in favour of the of the dissolution process. As a result of this increased dissolution tendency, the saturation solubility increases⁸. The dependency of the saturation solubility is also described in the Ostwald–Freundlich equation comparing the solubilities of two particles with different diameters. The Noyes–Whitney equation describes the dissolution velocity dc/dt which depends on the surface area A and difference (cs-cx)/h (cx — equilibrium concentration in bulk phase, h — diffusional distance)⁹. According to the Prandtl equation, the diffusional distance h is reduced for small particles. The simultaneous increase in saturation solubility cs decrease in h leads to an increased concentration gradient (cs-cx)/h, thus enhancing the dissolution velocity in addition to the surface effect¹¹.

3.3 Adhesiveness to gut wall:- There is a distinct increase in adhesiveness of ultra fine powder compared to coarse powders. A nice example from daily life is differently sized sugar. Coarse sugar crystals adhere less effectively to a cake or cookie compared to iced sugar. This adhesiveness of small drug nanoparticles can be exploited for improved oral delivery of poorly soluble drugs¹¹.

4. METHODS OF PREPARATION

4.1 Bottum up method:-

4.1.1 Precipitation by antisolvent addition:- Among nanoprecipitation, antisolvent addition method has been the most commonly used as it is simplest and the most cost effective method. A drug substance is dissolved in a solvent, preferably in a water miscible solvent, in which the drug has an appropriate solubility. Thereafter, this solution is mixed with an antisolvent (water, in most of the cases), which has to be miscible with the solvent phase¹².

4.1.2 Sonoprecipitation method:- The ultrasonic energy can be introduced simply by dipping a probe sonicator in a vessel kept under stirring for mixing a solvent with an antisolvent. Sonication increases micromixing, reduces particle growth and agglomeration, and it is possible to obtain spherical amorphous particle with uniform size distribution. The particle size is dependent on sonication duration and intensity, horn length, depth of horn immersion and cavitation.

4.1.3 Evaporative precipitation technique:- Evaporative precipitation into aqueous solution (EPAS) is another modified solvent antisolvent precipitation method, In this process, the drug is dissolved in a low-boiling-point solvent and heated above its boiling point. Thereafter, the heated solution is sprayed into a heated aqueous medium with stabilizer⁹.

4.1.4 Flash precipitation-Confined liquid impinging jets:- Precipitation occurs in a region of extreme turbulence and intense mixing created by a jet of drug solution impinging a jet of anti-solvent coming through two opposing nozzles mounted in a small chamber . Since the liquid jets are directly opposing each other, precise velocity control of the two jets is critical to prevent unbalanced flow and mixing. As the two liquid jets mix, the anti-solvent will cause the drug to precipitate as fine particles¹³.

4.1.5 Controlled crystallization during freeze drying (CCDF):- This technology is based on freeze-drying a mixture of a non-toxic organic solvent (tertiary butyl alcohol) in which the drug is dissolved and an aqueous solution containing a matrix material (e.g. mannitol). Freeze-drying is performed at a relatively high temperature (above the glass transition temperature of the maximally freeze-concentrated fraction) to allow the drug and matrix to crystallize ¹⁴.

4.1.6 Precipitation in supercritical fluid:- Supercritical fluids (SCF) are mostly useful for thermolabile drugs. The commonly used supercritical solvents include carbon dioxide, ammonia, fluoroform, ethane and ethylene. Although, the toxicity and flammability issue of some of these solvents may limit their uses in pharmaceutical applications. Supercritical techniques used for precipitation of organic compounds are RESS (rapid expansion of supercritical solution), RESOLV (rapid expansion of a supercritical solution into a liquid solvent) and SAS (supercritical antisolvent).

4.1.7. Spray-freezing into liquid (SFL):- It is similar to that of CCDF. A mixture of drug, polymer, organic solvent, and water is rapidly frozen and then freeze-dried. The most important difference is that the solutions used during SFL are, at least at room temperature, thermodynamically stable, while the mixture used during CCDF is not. Until SFL is only reported to yield amorphous nanoparticles, but when process conditions, such as the concentration organic solvent or the freeze-drying temperature are changed, crystalline nanoparticles may be obtained¹⁴.

4.2 Top-down method:-

4.2.1 Media milling:- A milling chamber is filled with milling media, dispersion medium (normally water), stabilizer, and the drug. The drug particles are reduced in size by shear forces and forces of impaction generated by a movement of the milling media. Small milling pearls or larger milling balls are used as milling media¹².

4.2.2 The Microfluidizer technology:- It is based on the jet stream principle generates small particles by a frontal collision of two fluid streams in a Y-type or Z-type chamber under pressures up to 1700 bar. The jet streams lead to particle collision, shear forces and cavitations forces⁴. This technique has been acquire by Skypharma PLC.

Disadvantages:-

a. It is not production friendly.

b. The product obtained through micro fluidizer may contain high number of microparticles.

c. In many cases, 50 to 100 passes are necessary for a sufficient particle size reduction i.e. time consuming process.

4.2.3 High pressure homogenization (Dissocube technology):- A piston in a large bore cylinder creates pressure up to 2000bar. The suspension is passed through a very narrow ring gap. The gap width is typically in the range of 3-25µ. There is a high streaming velocity in the gap according to the Bernouli equation. Due to the reduction in diameter from the large bore cylinder (e.g. 3 cm) to the homogenization gap, the dynamic pressure increases and simultaneously decreases the static pressure on the liquid. The liquid starts boiling, and gas bubbles occur which subsequently implode, when the suspension leaves the gap and is again under normal pressure (cavitation). Gas bubble formation and implosion lead to shock waves which cause particle diminution¹¹. The patent describes cavitation as the reason for the achieved size diminution. Piston gap homogenizers which can be used for the production of nanosuspensions are e.g. from the companies APV Gaulin, Avestin or Niro Soavi. The technology was aquired by Skyepharma PLC at the end of the 90s and employed in its formulation development.

Advantages:-

a. Drugs that are poorly soluble in both aqueous and organic media can be easily formulated into nanosuspensions.

b. Ease of scale-up and little batch-to-batch variation.

c. Narrow size distribution of the nanoparticulate drug present in the final product.

d. Allows aseptic production of nanosuspensions for parenteral administration.

e. Flexibility in handling the drug quantity, ranging from 1 to 400 mg/mL, thus enabling formulation of very dilute as well as highly concentrated nanosuspensions¹⁶.

Disadvantages:-

a. Hydrolysis of water-sensitive drugs can occur, as well as problems during drying steps.

b. In cases of thermolabile drugs or drugs possessing a low melting point, a complete water removal requires relatively expensive techniques, such as lyophilization. Prerequisite of micronized drug particles.

c. Prerequisite of suspension formation using high-speed mixers before subjecting it to homogenization

4.2.4 Homogenisation in water free media and water mixture (nanopure):- The Nanopure technology is another approach using the piston-gap homogenizer. This technology uses a primary dispersion medium, non-aqueous liquids, e.g. oils, liquid and solid (melted) PEG, or water reduced media (e.g. glycerol–water, ethanol–water mixtures), and optionally homogenization at low temperatures. These media have low vapor pressure, cavitation takes place very limited or not at all even without cavitation, the size diminution is sufficient because of shear forces, particle collisions and turbulences⁸.

Advantages:-

Highly chemical labile drugs can be produced in such non-aqueous media (water-glycerol mixture) at very mild condition.

4.3 Combination technologies:-The combination technologies combine generally a pre-treatment step followed by a high energy process.

4.3.1 Nanoedge technology:- Baxter developed the NANOEDGE technology. In the first step, crystals are precipitated, and the obtained suspension is then subjected to a high energy process, typically high pressure homogenization. Baxter focuses mainly on the development of intravenous nanosuspensions. By now, no products are on the market³.

4.3.2 Smartcrystal technology:- This technology was first developed by PharmaSol GmbH and was later acquired by Abbott. It is a tool-box of different combination processes in which process variations can be chosen depending upon the physical characteristics of the drug (such as hardness). The process H42 involves a combination of spray-drying and HPH. Drug nanocrystals can be produced much faster in one to a few homogenization cycles. Process H69 (Precipitation and HPH) and H96 (lyophilization and HPH) yield nanocrystals of amphotericin B within a size range of about 50 nm¹⁷.

5. ADVANTAGES OF NANOCRYSTAL¹⁸

a. Increased rate of absorption,

b. Increased oral bioavailability,

c. Rapid effect,

d. Improved dose proportionality,

e. Reduction in required dose,

f. Applicability to all routes of administration in any dosage form.

g. Nanosuspensions can also be administered via the intravenous route due to very small particle size, and in this way, bioavailability can reach 100 %.

h. Reduction in fed/fasted variability,

6. NANOCRYSTAL PRODUCT AVAILABLE IN MARKET¹⁷

Table 1. NANOCRYSTAL PRODUCT AVAILABLE IN MARKET

Trade name	Drug	Company	Applied technology
Rapamune	Rapamycin	Wyth	Ball milling
Emend	Aprepitant	Merck	Ball milling
Tricor	Fenofibrate	Abbott	Ball milling
Megace EZ	Megestrol	Par pharmaceutical companies	Ball milling
Avinza	Morphine sulphate	King pharmaceutical	Ball milling
FocalinXR	Dexmethylphenidate	Novartis	Ball milling
Zanafex capsules TM	Tizanidine	Accorda	Ball milling
Triglide	Fenofibrate	Sciele pharma inc.	HPH (Microfluidizer)
Ritalin LA	Methylphenidate	Accorda	Ball milling

7. OVERVIEW OF PRODUCTS UNDER CLINICAL TRIALS¹⁰

Table 2. PRODUCTS UNDER CLINICAL TRIALS

Trade name	Therapeutic use	Applied technology	Pharma company	Administration route	Status(phase)
Fenofibrate	Lipid lowering	SkyePharma	Undisclosed	Oral	I
Insulin	Diabetes	BioSante	Self developed	Oral	I
Busulphan	Anti-cancer	SkyePharma	Supergen	Intrathecal	I
Budesonide	Asthama	Elan nanocrystal	Sheffield Pharmaceutics	Pulmonary	I
Calcium phosphate	Mucosal vaccine adjuvant for herpes	Biosante	Self developed	Oral	I/II
Thymectatin	Anticancer	Elan nanocrystal	Newbiotics/llex oncology	Intravenous	II
Megesstol cetate	AIDS related weight loss	Elan nanocrystal	Par Pharmaceuticals inc.	Oral	II
Panzem NCD(2-methoxy estradiol)	Ovarian cancer	Elan nanocrystal	EntreMed	Oral	II
Panzem NCD	Recurrant gliblatoma multiforme	Elan nanocrystal	EntreMed	Orally	II
Panzem NCD and Tamozolomide	Anti-cancer	Elan nanocrystal	EntreMed	Oral	II
Panzem NCD and avastin	Carcinoid tumor	Elan nanocrystal	EntreMed	Panzem- orally	II
Panzem NCD with and without sanitinib malate	Renal cell carcinoma	Elan nanocrystal	EntreMed	Oral	II
Panzem NCD	Prostate cancer	Elan nanocrystal	EntreMed	Oral	II
Fenofibrate	Sleep apnea syndrome	Elan nanocrystal	Solvay pharmaceuticals	Oral	II
Paclitaxel	Anticancer		American Pharmaceutical partners	Itravenous	II

8. ROLE OF STABILIZERS

Stabilizers are surfactant, co-surfactant or polymers which prevent the agglomeration of nanocrystals. The high surface energy of nanocrystals is reduced by stabilizers. The main functions of a stabilizer are to wet the drug particles thoroughly, and to prevent Ostwald’s ripening¹⁶. The ionic and non-ionic stabilizers provide stability by electrostatic repulsion and steric mechanism respectively. Stabilization from electrostatic repulsion can be described by the classic Derjaguin–Landau–Verwey–Overbeek (DLVO). The DLVO theory assumes that the forces acting on the colloidal particles in a medium include repulsive electrostatic forces and attractive vanderwals forces. The repulsive forces are originated from the overlapping of electrical double layer (EDL) surrounding the particles in the medium, and thus preventing colloidal agglomeration¹⁹. Equally “charged” particles repel each other, which is the basis of general electrostatic stabilization (Muller et al). Steric stabilization of colloidal particles is achieved by attaching (grafting or chemisorption) macromolecules to the surfaces of the particles²⁰. Polymeric stabilizers are absorbed onto the drug particles through an anchor segment that strongly interacts with the dispersed particles, while the other well solvated tail segment extends into the bulk medium. Nonionic surfactants do not produce ions in aqueous solution and are very less affected by electrolytes present in git tract. The combination of steric and electrostatic repulsion mechanism is called electrosteric stabilization. The best stabilizer is one which is bulky (non-ionic) and highly charged (ionic) enough and meet the conceptions of steric and electrostatic stabilization simultaneously. Physically stable nanocrystalline formulations are obtained when the weight ratio of drug to stabilizer is 20:1 to 2:1 (Merisko-Liversidge et al, 2003). Examples of ionic stabilizers are sodium dodecyl sulfate (SDS), sodium lauryl sulfate (SLS), lecithin and docusate sodium and that of non-ionic are Pluronic surfactants, Tween 80, polyethylene glycol (PEG), polyvinyl alcohol (PVA) polyvinylpyrrolidone (PVP) and cellulose polymers such as hydroxypropyl cellulose (HPC) and hydroxypropyl methylcellulose (HPMC).

9. VARIOUS METHOD TO CONVERT NANOSUSPENSION INTO NANOCRYSTALS

9.1 Spray drying:- In spray drying process the nanosuspension is atomized using a rotary or air-jet atomizer. In this process fast drying of the liquid feed happens due to the large surface area created by the atomization of the liquid feed into fine droplets and high heat transfer coefficients generated²¹. It is simple and inexpensive method and is therefore suitable for industrial production.

9.2 Lyophilization:- This technique is also known as freeze drying. The process invovlves the D-freezing of nanosuspension at -80°C and after which it is keep in lyophilizer, which undergoes primary drying and secondary drying process. To protect from freezing stress different cryoprotectants are added like mannitol, trehalose, glucose, PEG-6000. This technique is expensive as compared to spray drying.

9.3 Pelletization:- The most commonly used techniques are extrusion-spheronization and drug coating onto sugar spheres. It is selected on the basis of the required drug content, properties of the drug and the available equipment¹⁷.

9.4 Production in hot melted matrices:- The production can directly be performed by hot high pressure homogenization in melted material . The homogenizers are equipped with temperature control jackets placed around the sample/product containers. Working temperatures up to 100°C (heated with water) or higher (heated with silicon oil) can be selected depending on the melting temperature of the used matrix material. For batch operation, solidification has to be averted between each homogenizing cycle²².

10. APPLICATION OF NANOCRYSTALS IN DRUG DELIVERY

10.1 Oral administration:- Due to fast dissolution of nanocrystals, the drug solubility is enhanced, making it bioequivalent in fed and fasting conditions. The bioadhesive nature of nanocrystals offers additional advantage of increased stay in the gastro-intestinal tract which enhances bioavailability. Amphotericin B administered orally as a nanosuspension showed dramatically improved bioavailability as compared to its conventional oral commercial products such as Fungizone, AmBisome and micronized Amphotericin B¹⁷.

10.2 Pulmonary drug delivery:- Application can simply be performed by placing aqueous nanosuspensions in an aqueous nebulizer. The nebulizer generates an aerosol, with a droplet size suitable for pulmonary administration, e.g. 1–5 μm droplets.The nanocrystals are contained inside these droplets. Budesonide nanosuspension showed a long-term stability; no aggregates and particle growth occurred over the examined period of 1 year.

10.3 Parenteral drug delivery:- Drug nanocrystals can be given in the form nanosuspension through parenteral route. Clofazimine nanosuspension, poorly water soluble anti leprosy drug, reveals an improvement in stability and efficacy over the liposomal clofazimine.

10.4 Dermal application:- Nanocrystals exhibit the properties like increased penetration into a membrane, enhanced permeation and bioadhesiveness. This changed when the poorly soluble antioxidants rutin, apigenin and hesperidin were formulated as nanosuspension for application in skin-protective, anti-aging cosmetic products (Muller et al, 2010).

10.5 Targeted drug delivery:- Their versatility and ease of scale up and commercial production enables the development of commercially viable nanosuspensions for targeted drug delivery. Kayser developed the formulation of aphidicolin as a nanosuspension to improve the drug targeting effect against Leishmania-infected macrophages. He stated that aphidicolin was highly active at a concentration in the microgram range.

11. CONCLUSION:

The most important aspect of technology is that it is applicable to any type of drug which is poorly soluble in water. Nanocrystal can be given by various routes like oral, parenteral, pulmonary topical and can be used as a targeted drug delivery system. Within a very short period of time (20 years) 9 formulation of nanocrystal has reached to market and number of formulation are under clinical trials. Various methods are available to formulate nanocrystals. It is possible to scale up from laboratory to industrial level. It is anticipated that nanosizing will attract increased attention as an formulation option for oral administration dut to increase in number of poorly soluble drugs.

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Received on 15.07.2013

Modified on 02.08.2013

Accepted on 06.08.2013

Research Journal of Pharmaceutical Dosage Forms and Technology. 5(5): September-October, 2013, 245-251