(730c) Continuous Nanoparticle Production Using Focused Acoustics

Continuous nanoparticle
production using focused acoustics

Many
drugs currently in development are either water insoluble or lipophilic, thus creating
formulation challenges to administer these drugs at a high dose.  In some situations, nanoparticles are an appropriate
formulation strategy as a result of their intrinsic properties such as small
shape and size (e.g., radius) and large surface area to volume ratios.  Nanoparticles can be generated by grounds up crystallization,
which is typically processed in batch mode.  One basic approach of crystallization is to
vary the solubility of the solute of interest with different methods, such as vary
the ratio to non-solvent vehicle (anti-solvent).  

Low
frequency, unfocused ultrasound has previously been demonstrated for improving crystallization
kinetics.  However, unfocused ultrasound
has efficiency limitations due the inability to focus the energy, creating
issues in thermal management such as non-uniform mixing, and uncontrolled
thermal fields.  Additionally, these
unfocused solutions are not able to provide sufficiently tight, uniform
nucleation control at the chemical solvent boundary interface that is required
for nanoparticle production.  The Covaris
Adaptive Focused Acoustics? (AFA) technology is a highly
controlled mechanical based process that provides a practical tool to precisely
control both the nucleation formation and the rate of crystal growth.  The high energy efficiency of AFA avoids
excessive heat generation, reducing potential thermal damages to the sample and
allowing greater control over thermally dependent chemical reactions.  Moreover, the precisely controlled energy delivery
improves the reproducibility of the sample processing for higher quality control.

As
a model of nanoparticle formation, a Solvent:Anti-solvent crystallization method with an active
AFA energy field was tested in different batch volumes as well as in a continuous
flow format.  Cinnarizine was used as a
model compound.  Nanoparticles were
measured using a Malvern Mastersizer 3000, and
particle sizes were reported for 10th percentile, 50th
percentile, and 90th percentile populations (d10, d50, and d90
populations respectively). 

Initial
studies were performed in triplicate on batch scale processing of 2 mL
samples.  Parameters evaluated include:

Solvent                                                 Dimethylacetamide (DMA)

Drug solvent concentrations      
50 mM, 125 mM, 250 mM

Stabilizer                                              0.2%
PVP30 and 0.25 mM SLS

Stabilizer percentage                    
0.2% 1.5%, 3.0%

Solvent:Anti-solvent
ratio            1:29, 1:14, 1:6.6

For comparison, a control was run at each setting.  The control was mixing 2 mL samples via a magnetic
stir bar in a 20 mL scintillation vial for the same time duration of 5 minutes.

In
all cases, the particle sizes of the AFA-processed samples were lower and more
reproducible than the control.  A typical
comparison is shown in the following tables:

S220x        Covaris Focused-ultrasonicator

300 W       Peak Incident Power (PIP)

50%           Duty Factor (DF)

1,000        Cycles Per Burst (CPB)

12x24        2 mL process vessel

300            processing time (seconds)

6°C            water bath temperature

Table
1. Results from the stir bar method (Control).

125mM-Control

Ratio

d10-Control

d50-Control

d90-Control

Span

1:6.6

9.0

29.9

1730.0

57.5

1:6.6

8.0

26.8

66.5

2.1

1:6.6

8.0

26.5

64.2

2.1

Average

8.3

27.7

620.2

20.6

SD

0.45

1.54

784.70

26.1

All values are in microns

Table
2. Results from the AFA method.

125mM-AFA

Ratio

d10-AFA

d50-AFA

d90-AFA

Span

1:6.6

0.0406

0.122

0.325

2.335

1:6.6

0.0406

0.122

0.333

2.387

1:6.6

0.0407

0.123

0.334

2.395

Average

0.041

0.122

0.331

2.372

SD

0.000

0.000

0.004

0.027

All values are in microns

d50 particle size populations for the
control sample averaged 27.7 microns while the same method processed with AFA
yielded average d50 particle size populations of 122 nanometers.

A
continuous manufacturing process was evaluated with the following settings:

Drug solvent concentration         125 mM  (46 mg/mL)

Solvent flow rate                             800 µL/min

Anti-solvent flow rate                    10 mL/min

Stabilizer                                              0.2% PVP30 and 0.25 mM SLS

Stabilizer percentage                     0.2%

Solvent:Antisolvent ratios 
          1:12.5  (10 mL/min anti-solvent, 800 µL/min solvent)

With 125 mM at a 800
µL/min solvent flow rate, the production rate was 36.5 mg/min.

Higher
total acoustic power input (Peak Incident Power) yielded both smaller particle
sizes and repeatable results.  At PIP
values of 500 Watt, particle size distributions are listed in the following
table.

Table
3. Particle size distributions at PIP = 500 W.

Sample 9

Sample 10

Sample 11

Sample 12

d10

0.085

0.091

0.098

0.090

d50

0.180

0.191

0.202

0.187

d90

0.415

0.409

0.423

0.399

Span

1.834

1.663

1.611

1.654

All values are in microns

Control of particle size was
demonstrated by varying PIP.  Particles
sizes at 300 PIP are listed in the following table.

Table
4. Particle size distributions at PIP = 300 W.

Sample 5

Sample 6

Sample 7

Sample 8

d10

2.15

1.79

1.89

1.78

d50

3.48

3.23

3.03

2.86

d90

5.55

5.01

4.95

4.73

Span

0.98

1.32

1.01

1.03

All values are in microns

The
current study demonstrates the technical feasibility of using Adaptive Focused
Acoustics (AFA) to control nucleation and crystal growth for continuous flow Solvent:Anti-solvent
crystallization.  Nanoparticles were
successfully and reliably produced in a continuous flow process with d50 particle
size populations below 200 nm.  By using
the AFA system to control the crystallization process, a choice of nano- or micron-sized crystals can be easily generated with
a simple change of PIP process parameter.  In addition, the isothermal, reproducible AFA process
enables control of fluid shear effects on secondary nucleation to minimize
crystal size, accelerate the overall rate of crystal formation, and improve
yield.  The effect of AFA on the
morphology of the crystal is currently under investigation.