FORMULATION FORUM – Nanosuspensions – An Enabling Formulation for Improving Solubility & Bioavailability of Drugs


INTRODUCTION

In the recent past, the industry is adapting innovative technologies as more new chemical entities (NCEs) coming out of discovery are poorly soluble, and hence, enhancing their performances, solubility, permeation, and bioavailability remain at the forefront of development.1 By utilizing new approaches like amorphous solid dispersions, such as spray drying, hot melt extrusion, co-precipitation, among others, a considerable number of the NCEs have been marketed in the past decades.

Nonetheless, utilization of the most classical methods like nano-milling, complexation, and salt formation has also yielded a number of drugs to market as innovation and more NCEs continue to fill the pipelines.

Nanosuspensions are colloidal dispersions composed of nano-sized drug particulates stabilized by suitable stabilizers.2 Nanosuspension formulation technology has overcome many of the challenges posed by insoluble molecules stemming from high melting and/or higher logP. In spite of all of these enabling technologies, the nanosuspension is ideally relevant for poorly soluble drugs with high crystal energy in lipids or aqueous solutions. The nanosizing by homogenization or milling results in drugs with reduced particle size, which allows higher per volume loading, thereby making them ideal for depot delivery via the subcutaneous route for long-acting injectable or oral administration of insoluble molecules.3 Examples of drugs marketed as nanosuspensions include griseofulvin, fenofibrate (Tricor®), megestrol acetate (Megace® ES), sirolimus (Rapamune®), among others that utilize one or more excipients, polymers, and/or surfactants to stabilize these nanoparticles by minimizing the electrostatic interactions, thereby preventing flocculation and enhancing long-term stability in the crystalline state.4 Unforeseeable challenges could stem from maintaining the supersaturation of drugs, especially from those in amorphous states. However, nanosuspensions are proven to be ideal when administered, eg, IV, IM, or SubQ as the macrophages act as the reservoirs for slow release of the molecules, resulting in prolonged pharmacokinetics profiles while maintaining the therapeutic’s concentration and minimizing the drug’s systemic toxicity, and enhancing the efficacy at higher dosing levels. Likewise, the oral administration of nanosuspensions resolves many challenges stemming from dose dumping and variabilities, faster onset, and minimizing the food effects in fed and fasted states. Nanosuspensions can also be sterilized by filtration, dry heat, steam, and radiation.3

The following focuses on the role of excipients and methods for preparation and application of nanosuspensions in injectable, ocular, topical, and oral drug delivery.

ROLE OF EXCIPIENTS IN NANOSUSPENSIONS

Stabilizers
Povidone, Polysorbate 80, synthetic and natural lipids, cellulosic ingredients (MC, HPMC, EC, HEC, HPC, CMC), Soluplus®, Poloxamer 188, Poloxamer 407, sodium lauryl sulfate, polyvinyl alcohol, vitamin E TPGS, polyethylene glycols, among others are commonly used stabilizers for nanosuspensions. They play an important role in nanosuspensions for stabilization of high energy nano-sized crystals prepared via milling or homogenization. One stabilizer is preferred over others depending upon the nature of administration routes, such as oral versus injectable. In certain cases, a mixture of two or three stabilizers are used. The mechanism by which these particles are stabilized is wetting of the particle  size, thereby preventing agglomeration by steric repulsion barrier around and minimizing Ostwald’s ripenings. The chemical nature and composition of the stabilizers have pronounced effects on the physico-chemical stability and in vivo performance of nanosuspensions.5

Organic solvents and lipid-based surfactants/co-surfactants are also predominantly used when using microemulsions for formulation of nanosuspensions. These include bile salts, transcutol, and solvents like glycofurol, ethanol, and isopropanol.

PREPARATION OF NANOSUSPENSIONS

For preparation of nanosuspensions, top-down and bottom-up approaches are commonly used, as shown in Figure 1.6-8 In addition, there are other ways to prepare nanosuspensions, such as emulsion and microemulsion processes.

TOP-DOWN PROCESS FOR NANOSUSPENSIONS

Wet/Media Milling
Nanosytems originally developed wet/media milling technology, which involves the use of high-shear milling of drug, stabilizers, and water.5 The high-shear grinding results in breaking down large particles into nano-sized crystals under controlled temperature until a desired unimodal particle size distribution of 200 nm is obtained. This process is simple and results in a desired particle size of drugs. For instance, after high-shear wet milling, the particle size of naproxen is reduced to a mean particle size of 0.147 microns (D90 0.205 microns) from a mean particle size of 24 microns (D90 47 microns) within 30 minutes.9

The advantages of high-shear wet milling involve include easy scalability, less variations from batch to batch, drugs compatible to 1organic or aqueous milling, and milling flexibility with handling of drug concentrations as high as 400 mg/ml in nanosuspensions.

High-Pressure Homogenization
This method requires high-pressure, high-shear homogenization using microfluidization, Disso Cubes technology, or Avestin’s piston gap homogenization. This homogenization process requires the fracture of drug particles by cavitation, high-shear force collision of particles against each other. The collision of the particles at high speed helps break the particles into smaller particles to achieve nano-sizing. In some cases, increasing the viscosity of media helps enhance the nano-sizing efficiency by increasing the powder density within the dispersion media. Usually, multiple homogenization cycles (5-10 cycles) are used to achieve the desired unimodal particle size distribution with a narrow polydispersity index. Similar to wet-milling technology, the advantages of high-shear homogenization include easy scalability, compatible to drugs in organic and aqueous solutions, flexibility for higher drug loading (>400 mg/ml), and suitability for aseptic production of parenteral nanosuspensions.

BOTTOM-UP PROCESS FOR NANOSUSPENSIONS

The bottom-up method involves a number of processes, including precipitation, microemulsions, among others as described further. This method requires the dispersion of drug dissolved in organic solvent to aqueous solution containing a suitable solubilizer/co-surfactant followed by agitation to yield precipitates/emulsions. Following evaporation of solvent, the drug immediately precipitates into aqueous phase, resulting in surfactant-stabilized nanosuspensions. Organic solvents, such as methylene chloride, chloroform, ethyl acetate, ethyl format, ethanol, among others, are used in preparation of nanosuspensions via precipitates/emulsions. These solvents can also control the emulsion droplets, which in turn control the particle size of nanosuspensions. In some cases, a hybrid process combining bottom-up and top-down is utilized to ensure better control of size distribution.

Microemulsions are thermodynamically stable, clear dispersions of two immiscible liquids (oil and water), which are stabilized by interfacial films of a surfactant and co-surfactant. They offer advantages of easy scale up and manufacturing, high levels of solubilization and drug loading, and long shelf-life. Griseofulvin, an anti-fungal drug, for example, has been investigated as a microemulsion in nanosuspensions showing a 3-fold faster dissolution compared to a marketed drug, suggesting the co-surfactant caused faster dispersion of drug from nanosuspensions.10

CHARACTERIZATION OF NANOSUSPENSIONS

Particle Size Distribution (PSD)
The mean particle size of nanosuspensions is a key governing factor for saturation solubility, wettability, and dispersion ability and physical stability. In general, large particle size dispersibility is much slower compared to smaller particle nanosuspensions, which dictates the performance of drugs. There are several methods to measure the PSD, including photon correlation spectroscopy (PCS) or dynamic light scattering (DLS), which are used to measure rapid and accurate PSD and polydispersity index (PDI) of nanosuspensions. PDI is an important parameter that dictates the unimodal (PDI 0.1-0.25) or multimodal distribution (PDI > 0.5) of the particulates and governs the physical stability of nanosuspensions. With its limited ability to measure the particle size in the range of 3 nm to 3 microns, PCS may not be applied to large particulates of the suspensions for contaminants averaging particle size > 3 microns. In those cases, the laser diffraction (LD) technique is used. It has the ability to measure particle size in the range of 0.05-80 microns or much higher (2000 microns). The samples are diluted to measure the PSD by LD or PCS, and both use different principles to measure the particle size, eg, LD measurement is volume based, whereas PCS is light-intensity weighted size. Therefore, measurement from each differs considerably, and LD often yields higher values than PCS.11 In other techniques, for example, Coulter counter gives more precise measurement for PSD per volume unit compared to LD and/or PCS, which is important to identify the contamination with lager particles of nanosuspensions intended for IV administration.

Zeta potential of nanosuspensions is an important attribute for physical stability and performance of drugs. A zeta potential of ±30 mV is required to electro-statistically stabilize nanosuspensions. In some cases, a zeta potential of ±20 mV is desirable for better stability and preventing aggregation of nanosuspensions.12 In addition, morphological and surface properties, rheological properties, and solid state characterization methods, such as DSC, XRPD, and FTIR and Raman spectroscopy, are used to characterize nanosuspensions.

Solidification & Stability of Nanosuspensions
Nano-sized drug suspensions are in a kinetic stable state. In spite of a range of polymers and solubilizers used to stabilize these particulates in nanosuspensions, it is often impossible to inhibit the nucleation of nano-sized particles. To alleviate these challenges, in addition to stabilizers, other methods, such as drying or lyophilization of the formulation, is necessary for long-term stability-enhanced shelf-life and avoiding aggregation and degradation by hydrolysis. Matrix former water-soluble cryoprotectants, such as mannitol, sucrose, glucose, dextran, and trehalose, are commonly used to preserve the physio-chemical properties of nanosuspensions in lyophilized or freeze-dried forms.13 Spray drying can also be used to preserve the physio-chemical stability of drugs in nanosuspensions.14

APPLICATION OF NANOSUSPENSIONS IN DRUG DELIVERY

Oral Drug Delivery
Nanosuspensions have been used in formulation of several marketed drugs with the aim to improve solubility and bioavailability of poorly soluble molecules. Nano-sizing helps increase the surface area, enhances wettability and hence the dispersibility in aqueous medium, and significantly increases kinetic solubility, permeability, and absorption of drug molecules. A few examples of orally marketed drugs includes the following:

  • Atovaquone, an antibiotic for treatment of viral infection in HIV patients, with bioavail­ability 10%-15%, improved the oral bioavail­ability by 2.5-fold by nanosuspensions com­pared to the marketed drug Wellvone®.
  • Danazol, a poorly soluble and bioavailable drug, showed a significant increase in oral bioavailability of 82% in nanosuspensions with respect to 5.2% of the marketed drug Danocrine. In addition, it also minimized the dose variability and food effect in fed/fasted states when dosed at 200 mg.15
  • In naproxen’s case, when this drug was administered as a nanosuspension, it enhanced faster absorption, reduced the tmax to half, increased 2.5- to 4.5-fold AUCs during the first hour of intake compared to suspension (Naprosyn) and tablet (Anaprox) formulations. In addition, it also minimized food effect in fed/fasted states and reduced the dose variability.9

Like several marketed drugs, a number of drugs have also been investigated in oral nanosuspensions.16 Nanosuspensions are ideally dosed as liquid capsules in hard gels and/or soft gels because many of the poorly soluble drugs require technologies for immediate delivery of molecules, and formulation of these molecules for oral sustained or controlled release may be challenging due to risk of dose dumping and poor in vivo performances. However, finding the appropriate excipients that could incorporate nanosuspensions as tablets or fast melt caplets might be advantageous for prolonged delivery, reduced food effects, and inter-subject dose variability. Ketoprofen, for example, has been successfully incorporated into pellets for sustained release of drug over 24 hours.17

Parenteral Drug Delivery
A range of molecules in several marketed drugs have been investigated as nanosuspensions in injectable formulations.16 Parenteral routes of administration (especially, IV, IM, or SubQ for nanosuspension) offer a limited choice of excipients not only in selection of key ingredients, but also the amounts used in such formulations often dictated by the FDA’ s inactive ingredient database. This database also creates a bottleneck of finding the appropriate excipients for such use, primarily driven by lack of safety data and bioburden for a particular ingredient. Furthermore, stringent requirements for aseptic processing, concerns of cross-contamination, safety and foreign particulates in manufacturing and, on the other hand, the lack of patient compliance and concerns of allergic reactions and any adverse effects, all impede the development of injectable formulations. In spite of all the hurdles, the injectable route, though is invasive, remains widely used for potent and high-valued drugs for quick onset for regular and emergency use and reduces dosing frequencies for long-acting injectables and for targeted delivery. Nanosuspensions offer solutions to all the challenges due, in part, to low-volume administration especially via IM and SubQ, reducing dose frequencies and maintaining the drugs over extended periods by the lymphatic system.18 It is therefore ideal for drug delivery of small and large molecules. Sequestered by mononuclear phagocytic systems (MPS) upon IV administration, nanosuspensions offer higher efficacy of anti-biotics and anti-infective drugs for which they can be targeted by macrophages leading to prolonged and sustained release.19 Unlike IM and SubQ administration, IV administration of nanosuspensions may cause incontrollable dissolution rates, leading to accumulation of drugs in the reticuloendothelial system or macrophage systems (MPS) in liver and spleen, with increased risk of organ damage and unpredictability in safety and efficacy of drugs.20 These challenges have resulted in the launch of several oral and IM drugs to the market, and fewer by IV route.21

Pulmonary Administration of Nanosuspensions
Pinar et al 2003, describe a number of drugs in pulmonary applications. Increasing the surface area to improve bioavailability by reducing particle size of nanocrystals with prolonged residence time in lungs provides a better strategy for achieving the desired efficacy though inhalation. It can lead to rapid onset, thereby avoiding systemic toxicity in the mouth and/or pharynx by unwanted drug accumulation. This is achieved by using nebulizers (jet or mesh) or aerosol meter dose or powder inhalers. In such cases, the particle size of nanocrystals and their controls with aerodynamic diameter of 1-5 microns are critical for optimal pulmonary delivery.

Ocular Drug Delivery
Ocular drug delivery via nanosuspensions remains at the forefront of treatment of eye diseases for both hydrophobic and hydrophilic drugs across the ocular mucosa.22 These formulations are designed to permeate the corneal membrane with controlled-release excipients for sustained and prolonged release with their abilities to enhance the absorption and bioavailability of drug molecules and with intent to reduce toxicity, side effects, and drug dosing. Treatments for eye diseases, such as diabetic retinopathy, glaucoma, macular degeneration, conjunctivitis, proliferative vitreoretinopathy, among others, are within reach due in part to formulation of drugs in nanosuspensions composed of solubilizers, viscosity enhancers, charge modifiers, stabilizers, and polymeric excipients to achieve optimal retention, permeation, and tolerability at the ocular site.23

SUMMARY & FUTURE PERSPECTIVES

As more insoluble NCEs are discovered, nanosuspensions have become a popular choice to enhance the solubility and bioavailability of BCS II and IV drugs, driven by their low solubility and permeability. Nanosuspensions containing nano-sized crystals and stabilized with polymeric solubilizers and permeation enhancers leverage added benefits for screening of a broad range of NCEs across all modalities via oral, injectable (IV, IM, and SubQ), ocular, pulmonary, and topical routes of administration. Evidently, this has resulted in the launch of many drugs over the years, and as the new molecules continue to be discovered, the industry is open to adapt all the classical and non-classical technologies to bring NCEs to the pipeline, clinical development, and commercialization. The advent of antibody drug conjugates (ADCs) for delivery of high-concentration monoclonal antibodies (mAbs) will open doors for SubQ administration of nanosuspensions.24

Ascendia remains at the innovation front of utilizing its enabling platform technologies to screen, formulate, and manufacture NCEs for its clients. Nanosuspensions applicable to molecules across all dosage forms can easily be adaptable via seamless transition from the early research phase to clinical manufacturing of new drug candidates. With Ascendia’s expertise in top-down and bottom-up (solvent inversion) approaches coupled with its cGMP sterile and non-sterile capabilities and state of-the-art facility and equipment, our team is ready to handle small molecules and biologics across all modalities for oral, injectable, ocular, and inhalation delivery in nanosuspensions.

REFERENCES 

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Jim Huang, PhD
Founder & CEO
Ascendia Pharmaceuticals
j.huang@ascendiapharma.com
www.ascendiapharma.com

Dr. Jim Huang is the Founder and CEO of Ascendia Pharmaceuticals, Inc. he earned his PhD in Pharmaceutics from the University of the Sciences in Philadelphia (formerly Philadelphia College of Pharmacy and Sciences) under Joseph B. Schwartz. He has more than 20 years of pharmaceutical experience in preclinical and clinical formulation development, manufacturing, and commercialization of oral and parenteral dosage forms. His research interests are centered on solubility/bioavailability improvement and controlled delivery of poorly water-soluble drugs through nano-based technologies.

Shauket Ali, PhD
Sr. Director, Scientific Affairs & Technical Marketing

Ascendia Pharmaceuticals
shaukat.ali@ascendiapharma.com
www.ascendiapharma.com

Dr. Shaukat Ali joins Ascendia Pharmaceuticals Inc. as Senior Director of Scientific Affairs and Technical Marketing after having worked in the pharma industry for many years. His areas of expertise include lipid chemistry, liposomes, lipid nanoparticles, surfactant-based drug delivery systems, SEDDS/SMEDDS, oral and parenteral, topical and transdermal drug delivery, immediate- and controlled-release formulations. He earned his PhD in Organic Chemistry from the City University of New York and carried out his post-doctoral research in Physical Biochemistry at the University of Minnesota and Cornell University. He has published extensively in scientific journals and is inventor/co-inventor of several US and European patents.