Nanoparticle Technology for the Delivery of Poorly Water-Soluble Drugs

The emergence of high-throughput screening (i.e., a method for screening thousands of potential drug candidates) has led to an increase in the number of poorly water-soluble drugs (1, 2). The delivery of such drugs into the body in a sufficiently bioavailable form has been challenging for formulation researchers, especially if a drug also is insoluble in an organic medium. Although several approaches such as solubilization (3, 4), cosolvency (5), complexation with beta-cyclodextrin (6, 7), and solid dispersion (8–10) can enhance a drug's dissolution, these methods are limited and suffer from disadvantages such as environmental concerns (e.g., because of the need for organic solvents), low drug loading, and large doses.

In the recent years, nanoparticle technology has emerged as a strategy to tackle such formulation problems associated with poorly water-soluble and poorly water- and lipid-soluble drugs (2, 11–13). Nanoparticles are solid colloidal particles ranging in size from 1 to 1000 nm that are used as drug delivery agents (14). The reduction of drug particles to the nano-scale increases dissolution velocity and saturation solubility, which leads to improved in vivo drug performance (15, 16). The various advantages of formulating water-insoluble drugs as nanoparticles are provided in the sidebar, "Benefits of pharmaceutical nanoparticles" (17–21).

Nanoparticle production processes

Nanoparticles can be produced by either dispersion-based processes (which involves breaking larger micrometer-sized particles into nanoparticles) or precipitation-based processes (which involves nucleation of particles from the molecular state). Nanoparticle production processes should:

• be simple, continuous, and efficient;

• be viable for large-scale production;

• effectively screen out most microsized particles;

• be acceptable to regulatory authorities;

• require flexible amounts of drug.

Various processes such as wet milling, high-pressure homogenization, emulsification, precipitation, rapid expansion, and spray freezing can be used to produce drug nanoparticles.

Wet milling. Wet milling is an attrition-based process in which the drug is dispersed first in an aqueous-based surfactant solution. The resulting suspension is subjected to wet milling using a pearl mill in the presence of milling media (13, 22). The impaction of a drug particle with milling media generates enough energy to convert the drug crystals into nanoparticles. The grinding media generally is composed of glass, zirconium oxide stabilized with zirconium silicate, or a highly crosslinked polystyrene resin in a spherical form (0.4–3.0-mm diameter). The processing temperature is less than 40 °C and processing pressures are as high as ~20 psi. Milling times range from hours to days depending upon the drug's hardness.

In a study conducted by Liversidge and Cundy, the bioavailability of danazol, a poorly bioavailable gonadotropin inhibitor, improved when administered to subjects as a nanocrystal suspension prepared by wet milling compared with a danazol macrosuspension. By formulating danazol as a nanocrystal suspension, the absolute bioavailabilty increased to 82.3%, compared with 5.2% of a commercial danazol suspension (23). Nanocrystals of naproxen were prepared using wet milling; their bioavailabilities were compared with Naprosyn (naproxen suspension, Roche, Geneva Switzerland) and Anaprox (naproxen tablet, Roche). The study revealed that the time required to achieve the maximum concentration was reduced by ~50% for the nanocrystal dispersion. In addition the area under curve increased by 2.5–4.5-fold during the first hour of study (24, 25).

Nanocrystal suspensions also increase drug loading, which can be significant for injectable products. In one study, researchers found that the maximum tolerated dose of the nanoparticle paclitaxel, an anticancer drug, was greater than the marketed product Taxol (Bristol-Myers Squibb, New York, NY) which uses cremophorEl–ethanol mixtures (26). This effect leads to increased efficacy of poorly water-soluble drugs.

One limitation of wet milling is that products can become contaminated because of abrasion occurring on grinding beads, which is intolerable for parenterally administered drugs. Other limitations are the possibility of only batchwise production and variations in particle-size distribution (12). In contrast, only limited feed is subjected to size reduction during wet milling.

High-pressure homogenization. High-pressure homogenization is based on the principle of cavitation (i.e., the formation, growth, and implosive collapse of vapor bubbles in a liquid [11, 12, 27–29]). In this process, a drug presuspension (containing drug in the micrometer range) is prepared by subjecting the drug to air jet milling in the presence of an aqueous surfactant solution. The presuspension is subjected to high-pressure homogenization in which it passes a very small homogenizer gap of ~25 mm. Cavitation forces are created, which are sufficiently high to disintegrate drug microparticles to nanoparticles as the suspension leaves the gap and normal air pressure is reached again. The homogenization pressure and number of homogenization cycles are key parameters in optimizing the process. The homogenization pressures generally range from 100 to 1500 bar and the number of homogenization cycles could be 3, 5, or 10 depending upon the drug's hardness, the desired mean particle size, and the product's required homogeneity (30–32).

The main advantage of high-pressure homogenization is that it is suitable for both large- and laboratory-scale production because high-pressure homogenizers are available in various sizes. In addition, homogenization creates negligible nanoparticle contamination, which is one of the most important objectives of a nanoparticle production process.

In addition, the dissolution and bioavailability of poorly soluble drugs is improved. Scholer et al. found that the bioavailability of atovaquone, a drug used to treat leishmaniasis, was increased by formulating it as nanosuspension (33). In comparison with micronized drugs, nanosuspensions of atovaquone at equivalent doses reduced infectivity from 40 to 15% at a reduced concentration of 7.5 mg/kg. Drug load decreased from 22.5 (micronized drug) to 7.5 mg/kg (nanosuspension), but the activity was increased by 2.5-fold at the same time (33).

Kayser et al. found that the bioavailability of amphotericin B, a highly effective antimycotic and leishmanicidal drug, was enhanced by formulating it as nanosuspension. In a comparison with commercial crude amphotericin B (micronized drug particles) and intravenous administered liposomes entrapping amphotericin B, drug absorption markedly improved. This effect confirms the ability of nanosuspensions to increase bioavailability (34). In a recent study, the dissolution characteristics of nifedipine were significantly increased with regard to the commercial product by preparating nanoparticles using high-pressure homogenization. After 60 min, 95% of drug nanoparticles were dissolved ~5% of the unmilled drug was dissolved (35).

Nanosuspensions also eliminate the possibility of Ostwald ripening (i.e., the growth of larger particles at the dispense of smaller one), thereby facilitating physical long-term stability as an aqueous suspension. Ostwald ripening is caused by the various saturation solubilities of differently sized particles that are near each other and the concentration gradient between them. Molecules from the more highly concentrated solution around very small particles diffuse to the vicinity of larger particles where a lower concentration is present. This effect leads to supersaturation, drug crystallization, and, thus, the growth of the larger particles. Ostwald ripening does not occur in nanosuspensions because the homogenization process makes only uniform particle sizes (12).

A limitation of this process is that the pressure used is so high that in some cases, the crystal structure changed. This effect leads to increased amorphous fraction (11). The variation in crystallinity can result in instability and also poses quality control problems.

Emulsification technology. Emulsification also can be used to prepare nanoparticle suspensions. In this method, the drug solution in an organic solvent is dispersed in the aqueous phase containing surfactant. This step is followed by the evaporation of organic solvent under reduced pressure, which results in the precipitation of drug particles to form a nanoparticle suspension which is stabilized by the added surfactant.

Alternatively, the nanoparticle suspension can be obtained by diluting an emulsion prepared by conventional methods, which results in the complete diffusion of the internal phase into the external phase leading to nanoparticle suspension. By forming drug nanosuspensions using emulsification technology, the mitotane anticancer drug dissolution rate was increased by five-fold compared with commercial products (36).

The use of microemulsion as templates for producing drug nanosuspensions also has been reported in literature. The dissolution rate of the antifungal griseofulvin drug was enhanced three-fold compared with the commercial products by formatting a nanosuspension using microemulsion (37). The emulsification technology cannot be used for drugs that are poorly soluble drugs in both aqueous and organic media. Moreover, the use of organic solvents also poses environmental concerns.

Precipitation with a compressed fluid antisolvent (PCA). In the PCA process (patented by RTP Pharmaceuticals and licensed to SkyePharma Plc [London, UK]), supercritical carbon dioxide is mixed with organic solvents containing drug compounds. The solvent expands into supercritical carbon dioxide, thus increasing the concentration of the solute in the solution, making it supersaturated, and causing the solute to precipitate or crystallize out of solution. Microparticles and nanoparticles are formed after drug precipitation by mass transfer because of organic solvent extraction into carbon dioxide and the diffusion of carbon dioxide into the droplets (38, 39). High mass-transfer rate is important to minimize particle agglomeration and reduce drying time (40).

Hanna and York attempted to increase the mass transfer rate by using a coaxial nozzle design with a mixing chamber. In this process, the drug in organic solvent interacts with the compressed fluid carbon dioxide antisolvent in the mixing chamber before dispersion and then flows through a restricted orifice into a particle-formation vessel. The high frictional surface forces that are generated cause the solution to disintegrate into droplets (41). This process was further modified by Subramaniam et al. who used an ultrasonic nozzle-based process for producing discrete nanoparticles in a narrow size range. A key step in the formation of nanoparticles is to enhance the mass transfer rate between the droplets and the antisolvent before the droplets coalesce to form bigger droplets. With an ultrasonic nozzle, the sound waves assist the antisolvent's disintegration into small droplets, thereby increasing the interfacial area. In addition, the turbulence created by the focused sound waves also enhances the mass transfer rate between the droplets and the carbon dioxide. Sound waves, rather than inertial or frictional forces, are exploited for droplet formation. Hence, the diameter of the line into which the spray solution is introduced can be larger than that of either capillary or micro-orifice nozzles (42). This larger diameter allows for higher solution throughput and reduces the probability of nozzle plugging, thereby dramatically increasing manufacturing efficiency.

Rapid expansion from a liquefied-gas solution (RESS). In an RESS process, a solution or dispersion of phospholipids or other suitable surfactant in the supercritical fluid is formed. Then, rapid nucleation of drug is induced in the supercritical fluid containing surfactant. This process allows rapid, intimate contact of the drug dissolved in supercritical fluid and the surfactant which inhibits the growth of the newly formed particles (43, 44). This process was used by Young et al. to prepare nanoparticles of cyclosporine in the size range of 500–700 nm (45). Tween-80 solution was used as a surfactant to prevent flocculation and agglomeration of nanoparticles. Researchers reported that the cyclosporine particles formed by this process could be stabilized for drug concentrations as high as 6.2 and 37.5 mg/mL in 1.0 and 5% (w/w) Tween-80 solutions. RESS process was combined with high-pressure homogenization by Pace et al. to prepare a physically stable nanosuspension. In this process, the poorly soluble drugs and surface modifier were first dissolved in a liquefied, compressed gas solvent, which was subsequently expanded into an aqueous solution containing surfactant. The suspension so formed was further subjected a to high-pressure homogenization process to produce a stable nanosuspension (46).

Spray freezing into liquid (SFL). In this process, developed at the University of Texas at Austin (Austin, TX) and commercialized by Dow Chemical Company (Midland, MI), an aqueous, organic, or aqueous–organic cosolvent solution; aqueous–organic emulsion; or drug suspension is atomized into a cryogenic liquid such as liquid nitrogen to produce frozen nanoparticles which are subsequently lyophilized to obtain free-flowing powder (47–51). The rapid freezing rate caused by the low temperature of liquid nitrogen and the high degree of atomization resulting from the impingement occurring between drug solution and cryogenic liquid leads to the formation of amorphous nanoparticles. Apart from liquid nitrogen, the drug solution also can be atomized into compressed fluid carbon dioxide, helium, propane, or other cryogenic liquids such as argon or hydrofluoroethers. Highly potent danazol nanoparticles contained in larger structured aggregates were produced by the SFL process (52). The SFL powders exhibited significantly enhanced dissolution rates. The micronized bulk danazol exhibited a slow dissolution rate; only 30% of the danazol was dissolved in 2 min. Nonetheless, 95% of the danazol was dissolved in only 2 min for the SFL highly potent powders. In a recent study, SFL danazol/PVP K-15 powders with high surface areas and high glass transition temperatures remained amorphous and exhibited rapid dissolution rates after 6 months in storage (53).

Evaporative precipitation into aqueous solution (EPAS). The EPAS process also was developed by the University of Texas at Austin and commercialized by Dow Chemical Company. In this process, the drug solution in a low boiling liquid organic solvent is heated under pressure to a temperature above the solvent's normal boiling point and then atomized into a heated aqueous solution containing stabilizing surfactant (54). The surfactant also can be added to organic solvent along with an aqueous solution to inhibit crystallization and growth of nucleating drug particles. The EPAS process was used to produce a nanoparticle suspensions of cyclosporine A and danazol, which showed high dissolution rates. Nanoparticle suspensions produced by the EPAS process can be incorporated into a parenteral dosage form or can be dried to produce solid oral dosage forms (55, 56). In a recent study, danazol particles formed by both EPAS and SFL processes produced amorphous powders with high glass transition temperature and low contact angle values. The dissolution rates were faster for the SFL particles, although both techniques enhanced dissolution rates of the active ingredient (57).

Commercialized nanotechnology

Various nanotechnologies already have been commercialized to help deliver poorly water-soluble drugs into the body. A review of some of these technologies will follow.

Dissocubes. Dissocubes technology (a patented technology currently owned by SkyePharma Plc) is based on piston–gap high-pressure homogenization (APV Micron LAB 40, APV Deutschland GmbH, Lubeck, Germany) and has already been described. The main advantages of this technology are ease of scale-up, little batch-to-batch variation, and aseptic production for parenteral administration.

Nanocrystal technology. Nanocrystal technology (Elan Corporation, Dublin, Ireland) can be used to formulate and improve compound activity and final product characteristics of poorly water-soluble compounds. The nanocrystal technology can be incorporated into all parenteral and oral dosage forms, including solid, liquid, fast-melt, pulsed-release, and controlled-release dosage forms.

Nanocrystal particles are produced by milling the drug substance using a proprietary wet-milling technique (13, 22). The Nanocrystal drug particles are stabilized against agglomeration by surface adsorption of selected generally regarded as safe (GRAS) stabilizers (see Figure 1). The result is an aqueous dispersion of the drug substance that behaves like a solution—a nanocrystal colloidal dispersion that can be processed into finished dosage forms for all routes of administration. For example, the Rapamune (sirolimus, Wyeth, Madison, NJ) immunosuppressant tablet developed with nanocrystal technology is designed to give patients more convenient administration and storage than the oral solution. Nanocrystal technology is an enabling technology for evaluating new chemical entities that exhibit poor water solubility. In addition, it is a valuable tool for optimizing the performance of established drugs.

Figure 1: Nanocrystal particles adsorbed with surface stabilizers (photo courtesy of Elan Corporation).

Nanomorph technology. Nanomorph technology (Soliqs Abbott GmbH & Co. Kg, Ludwigshafen, Germany) converts drug substances with low water solubility from a coarse crystalline state into amorphous nanoparticles (see Figure 2). Nanomorph technology is based on a dissolution–precipitation concept that operates using water-miscible solvents for dissolution, followed by precipitation by aqueous polymer solutions. In this technology, the drug suspension in solvent is fed into a chamber, where it is rapidly mixed with another solvent. Immediately, the drug substance suspension is converted into a true molecular solution. The admixture of an aqueous polymer solution induces precipitation of the drug substance. The polymer keeps the drug substance particles in their nanoparticulate state and prevents them from aggregation or growth. Water redispersable dry powders can be obtained from the nanosized dispersion by conventional methods (e.g., spray drying). Nanomorph formulations can be incorporated into the whole range of standard galenic application forms.

Figure 2: Development of colloidal amorphous particles using nanomorph technology (photo courtesy of Soliqs Abbott GmbhH & Co. Kg, Germany).

Nanoedge technology. Nanoedge technology (Baxter Healthcare Corporation, Deerfield, IL) is a formulation toolbox for poorly water-soluble drugs. It is a useful technology for active ingredients that have high melting points and high octanol-water partition coefficients, log P. It is based on direct homogenization, microprecipitation, and lipid emulsions.

In microprecipitation, the drug first is dissolved in a water-miscible solvent to form a solution. Then, the solution is mixed with a second solvent to form a presuspension and energy is added to the presuspension to form particles having an average effective particle size of 400 nm to 2 μm (58). The energy-addition step involves adding energy through sonication, homogenization, countercurrent flow homogenization, microfluidization, or other methods of providing impact, shear, or cavitation forces. A drug suspension resulting from these processes may be administered directly as an injectable solution, provided water-for-injection is used in the formulation and an appropriate means for solution sterilization is applied. Nanoedge technology facilitates small particle sizes (<1000 nm [volume weighted mean]), high drug loading (10–200 mg/mL), long-term stability (up to 2 years at room temperature or temperatures as low as 5 °C), the elimination of cosolvents, reduced levels of surfactants, and the use of safe, well-tolerated surfactants.

Nanopure technology. In Nanopure technology (PharmaSol GmbH, Germany), poorly water-soluble drugs are transferred to drug nanocrystals via a high-pressure homogenization process (2). The drug powder is dispersed in a surfactant solution and the forces in the high-pressure homogenizer are strong enough to disintegrate the coarse drug powder into drug nanoparticles with a mean diameter, typically between 200–600 nm.

The drug powder is dispersed in a nonaqueous medium (e.g., PEG 600, Miglyol 812) or a water-reduced mixture (e.g., water-ethanol) and the obtained presuspension is homogenized in a piston-gap homogenizer. A suitable machine for the laboratory scale is the Micron Lab 40 (APV Deutschland GmbH). Nonaqueous dispersion media such as PEG or oils yield suspensions that are suitable for the direct filling of capsules and thus an intermediate step required when using pure aqueous nanosuspensions is avoided. With Nanopure technology, homogenization can be performed in a nonaqueous phase or phases with reduced water content. And, in contrast to more pronounced cavitation at higher temperatures, homogenization was similar or more efficient at lower temperatures, even below the freezing point of water.

Crititech technology. Crititech Technology (CritiTech, Inc., Lawrence, KS) is based on PCA. Crititech uses ultrasonic energy produced by a converging–diverging nozzle or an electromechanical oscillator to shatter droplets into even droplets. This technique alone would not cause submicron particles to form because the droplets tend to coalesce immediately into larger drops. In the crititech procedure, the drug-laden solvent is sprayed into a flowing stream of supercritical carbon dioxide, which allows for a rapid mass transfer of solvent into the stream of supercritical carbon dioxide. This rapid mass transfer forces precipitation or crystallization to occur before the coalescence of droplets. The ultrasonic nozzle-based process is capable of producing discrete nanoparticles in a narrow size range (42). Moreover, crititech's proprietary particle-harvesting device allows continuous processing of compounds in closed systems with complete recovery of solvents and carbon dioxide for reuse or safe disposal.

Nanocochleate technology. Nanocochleate delivery vehicles (also known as bioral technology) are a broad-based enabling technology for the delivery many therapeutic products. These molecules are stable phospholipid-cation precipitates composed of simple, naturally occurring materials such as phosphatidylserine and calcium. They consist of alternating layers of phospholipid and multivalent cations existing as stacked sheets, or continuous, solid, lipid bilayer sheets rolled up in a spiral configuration, with little or no internal aqueous space (see Figure 3). Unique properties of nanocochleates have been used to mediate and enhance the oral bioavailability of a broad spectrum of important but difficult-to-formulate biopharmaceuticals, including compounds with poor water solubility, protein and peptide drugs, and large hydrophilic molecules. Nanocochleate formulations are widely suitable to a broad range of therapeutic applications which include the oral delivery of amphotericin B (bioral amphotericin B); large DNA constructs and plasmids (bioral DNA vaccines and bioral gene therapy); peptide formulations; anti-inflammatory formulations (bioral aspirin); and peptide-based vaccines.

Figure 3: Composition of nanocochleate delivery vehicles (photo courtesy of BioDeliverySciences International).

Controlled-flow cavitation (CFC) technology. CFC (Five Star Technologies, Cleveland, OH) can be used to develop advanced materials for emerging applications and to design processes that enhance existing products and processes. CFC technology is based on hydrodynamic cavitation, which involves the formation, growth, and implosive collapse of vapor bubbles in a liquid created by fluctuations in fluid pressure (see Figure 4). In this process, the formation, size, density, speed of collapse, intensity of implosion and other energetics of cavitation bubble creation, and collapse are controlled to produce the necessary energy dissipation levels and desired effects on the process medium (59).

Figure 4: Controlled-flow cavitation reactor (photo courtesy of Five Star Technologies, Inc.).

Conclusion

Nanoparticle technology has shown potential for enhancing bioavailabilty of poorly water-soluble drugs. Many drug delivery and pharmaceutical companies are exploiting this technology to reexamine active ingredients that were abandoned from formulation programs because of their poor solubility. With the advent of new technologies and commercial success, the nanoparticle technology will continue to appeal to both pharmaceutical researchers and the pharmaceutical industry.

Vivek Kharb and Meenakshi Bhatia are postgraduate students, Harish Dureja and Deepak Kaushik* are lecturers of pharmaceutical sciences at the M.D. University, Rohtak-124001, Haryana, India, tel. +01262 272535, deepkaushik@rediffmail.com

*To whom all correspondence should be addressed.

Submitted: May 23, 2005. Accepted: Oct. 5, 2005.

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