Pharmaceutical Freeze
Drying:
The Lyophilization Process
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
1
Outline
I. What is freeze drying?
II. Reasons for freeze drying
III. Steps in freeze drying
A. Freezing
B. Primary Drying
C. Secondary Drying
IV. Case Studies
V. Pros and Cons of Freeze Drying
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
2
I. What is Freeze Drying?
• Definition of Freeze Drying
“To dry (as food) in a frozen state under high vacuum esp.
for preservation” (Webster Dictionary)
“ ... a means of drying, achieved by freezing the wet substance
and causing ice to sublime directly to vapor by exposing
it to a low partial pressure of water vapor” (Sterile Pharmaceutical
Manufacturing - Applications for the 1990’s)
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
3
II. Reasons for Freeze Drying?
•
•
•
•
Material chemically unstable in solution
Low temperature drying process
Compatible with protein pharmaceuticals
The amorphous form of the drug is desirable
(i.e., solubility)
• Low particulate contamination
• Compatible with aseptic/sterile processing
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
4
Pharmaceutical Freeze Drying Involves:
1. Dissolving the drug and excipients in a suitable
solvent, generally water.
2. Sterilizing the bulk solution by passing it through
a bacteria-retentive filter.
3. Filling into individual sterile containers.
4. Freezing the solution by placing the open
containers on cooled shelves in a freeze drying
chamber or pre-freezing in another chamber.
5. Applying Vacuum to the chamber and heating the
shelves in order to sublime ice.
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
5
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
6
Desired Freeze Dried Characteristics
• Intact cake
• Sufficient strength
• Uniform color
• Sufficiently dry
• Sufficiently porous
• Sterile Free of Pyrogens
• Free of particulate
• Chemically stable
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
7
III. Steps in Freeze Drying
A. Freezing
– Freezing of water into ice to produce a rigid frozen
solute structure
– Solutes concentrate between ice crystals
B. Primary Drying
– Removal of ice via sublimation
– Product temperature less than Collapse temperature
C. Secondary Drying
– Remove adsorbed water
– Achieve moisture content needed for stability
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
8
Freeze Drying Equipment
Chamber
Condenser
Condensing Coils
Vacuum
Pump
Product Shelf
Compressor
Shelf Fluid
Pump
Heat
Exchanger
Heater
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
9
Steps in Freeze Drying
Shelf Temperature
40
140
Chamber Pressure
Temperature (oC)
20
120
0
100
Pressure (millitorr)
Mean Product Temperature
80
-20
60
-40
A
0
B
10
C
20
30
40
50
60
Time (Hours)
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
10
Freeze Drying
Temperature
Time
Pressure
Solution
Nathaniel Milton, Ph.D.
Powder
Product Development, Eli Lilly and Co.
11
A. Freezing Process
Cooling
Supercooling
Ice Nucleation
Crystal Growth
A. Concentration of Solutes
Ionic strength
Reaction rates
Precipitation of Buffers - pH shifts
B.
Crystallization
of solute (eutectic)
Nathaniel Milton, Ph.D.
Metastable Amorphous
Solute
annealling
D.
Crystalline /
Amorphous mixture
C.
Amorphous
solute
vitrification
Product Development, Eli Lilly and Co.
Lyotropic
liquid
crystals
12
Crystalline Solutes
Some solutes crystallize
with ice during freezing
Crystalline solutes
After Freezing
(Freeze Concentrate)
Eutectic Mixture
The temperature where solute and ice both exist in a rigid crystalline
state is the “eutectic temperature”.
For example, NaCl forms a eutectic mixture containing 23.3%NaCl
and melts at -21.13oC.
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
13
Amorphous Solutes
Most solutes don’t crystallize
and form a random (amorphous)
viscous glassy phase
Amorphous solute
After Freezing
(Freeze Concentrate)
Glassy Mixture
In these systems the viscosity of solute phase increases until the
solute is completely immobile and behaves like a glass.
The temperature where the solute behavior changes from solution
to a rigid glass is the “glass transition” temperature.
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
14
Physical State of the Solute and Temperature:
Significant Impact on Freeze-Drying Behavior
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
15
Types of Freeze-Drying Behavior:
Crystallization of Nafcillin During Annealing
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
16
B. Primary Drying
• The sublimation of ice from the frozen solution to
create a dried layer of solute
• Solute must form a rigid structure to support its
weight after the removal of ice.
• Maintaining product below the collapse temperature
is critical to produce acceptable material
• Consequences of improper temperature control
¾ Collapse product
¾ Shrunken freeze dried plug
¾ Melt-back
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
17
Product Collapse - during freeze drying product temperature
exceeds the collapse temperature and the material “collapse” as ice
is sublimed.
Fill volume
Solute
Ice
¾ After ice sublimed a dried residue of solute is produced.
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
18
Types of Freeze-Drying Behavior:
Collapse
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
19
Example Of Collapse –
Annealed vs. Unannealed Sucrose/Glycine
Formulations
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
20
Significance of Temperature
„
„
Collapse temperatures are formulation
dependent
During Freeze Drying
„
„
Primary drying (I.e., ice sublimation),
Glass transition temperature, Tg’
T<<Tg
"Rigid" Solid
Tg
Semi-solid
T>>Tg
"Fluid" Liquid
Increasing molecular mobility & "reactivity"
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
21
How is Product Temperature Controlled during Primary Drying?
Product temperature is controlled indirectly:
a. Chamber pressure
- Heat Transfer
- Mass Transfer (Product Resistance)
b. Shelf temperature
- Heat Transfer
Condensing
coils
T
Freeze Dryer Shelf
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
22
Properly dried material produces a well formed cake with no
apparent shrinkage.
Important Points about Primary Drying
• Product temperature is critical during primary drying
• Changes in product temperature during drying may influence
appearance of final product
• Damage which occurs during primary drying can not be repaired.
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
23
Freeze drying is a process where heat and mass
transfer are coupled!
•
m
Pc
Rp
Po
Kv
Ti
Ice
Tb
Q
Shelf Temp - Ts
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
24
Influence of Vapor Flow Resistance on Product
Temperature
¾ Water vapor must have enough energy to pass through the dried layer
and to the condenser
¾ As resistance increases more energy (heat) is needed for water vapor
to escape
¾ Product temperature increases with increasing resistance
•
Nitrogen
m
Mass
Vacuum
Interface
Rp
Dried Layer
Ice/Produce
Interface
Ice
Ice
Water vapor
Heat
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
Heat
25
Why Does Product Collapse => Product Resistance
Solution
Frozen
Product
Heated
Ice
Sublimes
(Heat Removed)
Vapor
Pressure
Increases
No
No
Dry
?
Is
T > Tc
Temperature
Increases
?
Yes
Yes
Dry Cake
Collapse
Nathaniel Milton, Ph.D.
Product
Resistance
Increases
Product Development, Eli Lilly and Co.
26
Heat and Mass Transfer Equations Describing Freeze Drying
⋅
m =
A p ( Po − Pc )
R p + Rs
=
A p ( Po − Pc )
∧
Rp
where
Eq. 1
Pc = chamber pressure/
(above dried solute)
Rp = product resistance
Rs =stopper resistance
.
m = rate of sublimation
Po = vapor pressure of ice
• Relationship between chamber pressure and vapor pressure
of ice (I.e., ice temperature)
⋅
Av Kv (TS − ΔT − TI )
m=
=
ΔΗ s
ΔΗ s
⋅
Q
Eq. 2
Av = surface of vial
Kv = vial heat transfer coefficient
Ts = shelf temperature
ΔHs = enthalpy of sublimation
ΔT = temperature difference
across ice slab
TI = temperature at the ice
interface
• Relationship between shelf temperature and ice temperature
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
27
•The relationship between the vapor pressure of ice and ice
temperature is
− 6144.96
− 27315
.
TI =
ln Po − 24.01849
Eq. 3
Combining Eq. 1, 2, and 3 yields Eq. 4
ΔΗ s
∧
Rp =
Ap
Av
( Po − Pc )
⎛
⎡
⎤⎞
− 6144.96
Kv ⎜ Ts − ΔT − ⎢
− 27315
. ⎥⎟
⎝
⎣ ln( Po − 24.01849)
⎦⎠
Eq. 4
• Eq. 4 describes the relationship between product resistance,
vapor pressure of ice (product temperature), the shelf temperature,
and chamber pressure).
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
28
1000
B
Vapor Pressure of Ice (torr 10-3)
A
800
600
400
C
200
0
2
4
6
8
10
Product Resistance (torr
12
cm2 hr
14
16
18
gm-1)
Regression analysis of vapor pressure of ice and product resistance data
collected at a shelf temperature of 20°C and 100 millitorr (A), shelf
temperature 0°C and 100 millitorr (B), and shelf temperature of -20°C and
chamber pressure 80 millitorr with Eq. 4 assuming a 2 degree
temperature gradient across the ice slab.
¾ Increase Rp related to increase Po (i.e., Temperature)
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
29
C. Secondary Drying
¾Removal of adsorbed water from the dried solute (no ice present)
5% water
0.1% water
• Controls moisture level in product to maintain proper
chemical and physical stability.
• Reversible process (can de-humidify and humidify product to
change moisture content)
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
30
Critical Points for Consideration
¾ Degradation
™Concentration effects during freezing
¾ Reconstitution
™Disperse material in freeze dried cake
¾Collapse
™Glycine and mannitol bulking agents raise Tc
¾ Damage during freezing and drying
™Cryoprotectants and lyoprotectants
¾ Stabilizers (amorphous)
™Sugars (sucrose, lactose), glycine
¾ Adherence to glass
™Surfactants, silicone
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
31
Critical Points for Consideration
• Physical state in frozen solution
– Excipient and active pharmaceutical ingredient
• Physical state in freeze dried powder
– Impact on physical and chemical stability
• Influence of processing conditions
– Changes in thermal history can changed the
physical state of material(s) and effect process
compatibility and chemical stability
• Understanding facilitates formulation
development, process design and control
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
32
Case Study
Nafcillin Sodium
N. Milton and S. L. Nail. The physical state of nafcillin sodium in
frozen aqueous solutions and freeze-dried powders.
Pharmaceutical Development and Tech, 1 (3), 269-277, 1996.
Buffers and pH control
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
33
Isothermal Crystallization
Photomicrographs of 25% nafcillin sodium frozen solution using
crossed polars and first order red compensator: A) frozen solution
at -10°C, B) frozen solution at -4°C, C) frozen solution after 5
minutes at -4°C, and D) frozen solution after 15 minutes at -4°C.
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
34
Solid state decomposition at 50°C of nafcillin sodium unannealed
(open symbols) and annealed (closed symbols) stored at 11%
(squares) and 23% (triangles) relative humidity.
¾Unannealed less stable than Annealed
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
35
Case Study - Buffer Selection
• Preliminary data suggested the optimal
solution pH between 4 - 5
• Formulations prepared with acetate,
citrate and tartrate buffers
• All buffers were prepare in equal molar
concentrations and adjusted with NaOH
• Acetate buffer least stable (Why?)
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
36
Effect of various Buffers on Stability (100 mM)
pH 4.0
22
20
18
16
14
12
10
8
6
4
2
0
pH 4.5
22
20
Acetic acid
Citric acid
18
Citric acid
Tartaric acid
16
Tartaric acid
% TRS
% TRS
Acetic acid
14
12
10
8
6
4
2
0
0
1
2
3
4
5
0
1
Time, (Weeks)
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
2
3
4
5
Time, (Weeks)
37
Review of Data
• pH of reconstituted acetate buffer formulation
increased 1.58 - 1.78 pH units
• Acetic acid component of buffer system
• Acetic acid is volatile and evaporates
• Loss of acetic acid leads to increase in
formulation pH and poor stability
• Avoid use of volatile buffer species or other
materials (I.e., ammonium salts)
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
38
Conclusions
Advantages of Freeze Drying
1. Low particulate contamination
2. Solid more stable than solution
3. Low temperature process => less in-process degradation
4. Compatible with aseptic processing
5. Can be easily reconstituted
Disadvantages of Freeze Drying
1. Cost => capital expenditures, process long and expensive
2. Difficult to produce crystalline material
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
39
Conclusions
‰ Freeze drying provides a method of drying temperature labile
materials.
‰ The freeze drying process is divided into 3 steps:
- Freezing
- Primary Drying
- Secondary Drying
‰ Changing the freezing, primary drying, or secondary drying
conditions can influence the physical and chemical stability
of the final product
‰ Freeze drying is often the last choice in methods for drying
materials, because the cost and time required.
Nathaniel Milton, Ph.D.
Product Development, Eli Lilly and Co.
40