Preparation of NanoSuspensions

By: Pharma Tips | Views: 5193 | Date: 02-Jun-2011

Nanosuspensions can be produced by bottom-up or top-down techniques. In thecase of bottom-up technique, the strategy is to build particles from their constituentunits, i.e., molecules, atoms. Bottom-up technique is a classical precipitation processin which the drug is dissolved in a solvent, which is subsequently added to anonsolvent to precipitate the drug crystals.

Nanosuspensions can be produced by bottom-up or top-down techniques. In the
case of bottom-up technique, the strategy is to build particles from their constituent
units, i.e., molecules, atoms. Bottom-up technique is a classical precipitation process
in which the drug is dissolved in a solvent, which is subsequently added to a
nonsolvent to precipitate the drug crystals.

Use of solvents, the difficulty to avoid the formation of microcrystals, and the poor solubility
 of an increasing number of drugs in all media limit this approach. In the case of top-down technique,
the coarse material is subsequently broken down until nanoscopic dimensions are reached.

There are two basic techniques widely used: (1) Pearl/ball milling (Nanocrystal
technology®, élan) and (2) High-pressure homogenization (Dissocubes®,
Skyepharma). In pearl milling, an aqueous suspension of the drug is fed into the
mill containing milling pearls made of glass, zirconium oxide, ceramic sintered
aluminium oxide, or hard polystyrene with high abrasion resistance. As the pearls
are rotated at a very high shear rate, the drug particles are ground into nano-sized
particles between the moving milling pearls. Depending upon the hardness, the
drug needs to be milled from hours to several days.

In the high-pressure homogenization technique, the drug suspension is forced
under pressure through a valve that has a nanoaperture. Typical pressures applied
are between 100 and 1,500 bars. As the drug suspension passes through the nanoaperture
with high velocity, the static pressure is decreased, leading to the formation
of small gas bubbles, which implode as they exit the valve.

The cavitation forces created breakdown the drug microparticles
into nano-sized particles.

It is not possible to obtain the desired particle size for many drugs in a single homogenization
cycle. Multiple homogenization cycles are required depending on the hardness
of the drug, desired mean particle size, and required homogeneity of the product.
To avoid the removal of water after high-pressure homogenization in aqueous
media, nanosuspensions were produced using high-pressure homogenization in
nonaqueous media or in water with water-miscible liquids (Nanopure®, Pharmasol).

A combinative technology (NANOEDGE TM) was introduced by Baxter Healthcare
in which precipitation step is followed by high-pressure homogenization to prevent
the precipitate from crystal growth. Very recently, Moschwitzer and Muller (2006)
reported a new combination method for the production of ultrafine submicron

This method involves an evaporation step to provide a solventfree
modified starting material followed by high-pressure homogenization.
Hydrocortisone acetate nanosuspensions produced using this method with reduced
homogenization cycles and the nanosuspensions demonstrated excellent long-term
storage stability. Application of supercritical fluid process for production of nanosuspensions
has increased in recent years.

The most widely used methods are rapid expansion of supercritical solution process, gas antisolvent process,
and supercritical antisolvent process. Cyclosporine, budesonide, and griseofulvin nanosuspensions
have been prepared using these methods (Young et al. 2000; Steckel et al.
1997; Chattopadhyay and Gupta 2001).
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