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INTRODUCTION
DEFINITION
• Freeze-drying (also known as lyophilisation or cryodesiccation) is a
dehydration process typically used to preserve a perishable material or
make the material more convenient for transport.
• Freeze-drying works by freezing the material and then reducing the
surrounding pressure to allow the frozen water in the material to sublime
directly from the solid phase to the gas phase.
• Freeze drying or Lyophilization is defined as a stabilizing process in
which the substance is first frozen and then the quantity of the solvent is
reduced first by sublimation (primary drying) and then by desorption
(secondary drying) to values that will no longer support biological
growth or chemical reactions.
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•It is a process used to dry extremely heat sensitive materials. It can
allow the drying , without excessive damage , of proteins, blood
products and even microorganism which retain a small but singnificant
viability.
• In this process, the initial liquid solution or suspensions is frozen, the
pressure above the frozen state is reduced and the water is removed by
sublimation. Thus an over all liquid-to-vapour transition take place, as
with all the previous dryers ,but all three state of matter are involved:
liquid to solid, then solid to vapour.
• Lyophilization is a multistage operation in which, quite obviously, each
step is critical. The main factors of this scenario are all well known and
should be under strict control to achieve a successful operation. There
are many new parenteral products, including anti-infective,
biotechnology derived products, and in-vitro diagnostics which are
manufactured as lyophilized products.
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Fig: The phase diagram for water with freeze drying process
superimposed
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The line on phase diagram represents the interphase equilibrium lines
which shows:
•The boiling point of the water as it is lowered by reduction of the
external pressure above the water (BO).
•The variation of the melting point of ice on reduction of the external
pressure above it . there is a very slight rise in melting point (AO).
•The reduction of the vapour pressure exerted by ice as the temperature
is reduced (CO).
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Application of the phase diagram of water to freeze drying :
The process of freeze drying is superimposed on the phase diagram for
water. In its basic form freeze drying comprises three steps:
•Freezing the solution.
•Reducing the atmospheric pressure above the ice to below that of the
triple point of the product.
•Adding heat to the system to raise the temperature to the sublimation
curve (CO) to provide the latent heat of sublimation.
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THE ORIGINS OF FREEZE DRYING
•Freeze-drying was first actively developed during world war 11. Serum
being sent to Europe for medical treatment of the wounded required
refrigeration.
•Due to the lack of available refrigeration, many serum supplies were
spoiling before reaching the intended recipients.
•The freeze-drying process was developed as a commercial technique
that enabled serum to be rendered chemically stable and viable without
having to be refrigerated.
•The freeze dry process was applied to penicillin and bone, and
lyophilization became recognized as an important technique for
preservation of biological.
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PRETREATMENT
Pretreatment includes any method of treating the product prior to
freezing.
This may include concentrating the product, formulation revision (i.e.,
addition of components to increase stability and/or improve
processing), decreasing a high vapor pressure solvent or increasing the
surface area.
Methods of pretreatment include: Freeze concentration, Solution
phase concentration, Formulation to Preserve Product Appearance,
Formulation to Stabilize Reactive Products, Formulation to Increase
the Surface Area, and Decreasing High Vapor Pressure Solvents.
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FREEZING
In a lab, this is often done by placing the material in a freeze-drying
flask and rotating the flask in a bath, called a shell freezer, which is
cooled by mechanical refrigeration, dry ice and methanol, or liquid
nitrogen.
On a larger scale, freezing is usually done using a freeze-drying
machine, in which it is important to cool the material below its triple
point, the lowest temperature at which the solid and liquid phases of the
material can coexist.
To produce larger crystals, the product should be frozen slowly or
can be cycled up and down in temperature. This cycling process is
called annealing.
The freezing temperatures are between −50 °C and −80 °C.
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PRIMARY DRYING
During the primary drying phase, the pressure is lowered (to the
range of a few millibars), and enough heat is supplied to the material
for the water to sublime.
In this initial drying phase, about 95% of the water in the material is
sublimated.
This phase may be slow because, if too much heat is added, the
material’s structure could be altered and pressure is controlled
through the application of partial vacuum.
The vacuum speeds sublimation is used to deliberate drying process.
Furthermore, a cold condenser chamber and/or condenser plates
provide a surface(s) for the water vapour to re-solidify it.
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This condenser plays no role in keeping the material frozen; rather,
it prevents water vapor from reaching the vacuum pump, which could
degrade the pump's performance.
Condenser temperatures are typically below −50 °C (−60 °F).
It is important to note that, in this range of pressure, the heat is
brought mainly by conduction or radiation; the convection effect is
negligible, due to the low air density.
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SECONDARY DRYING
Secondary drying involves removal of water which did not
freeze Drying kinetics were determined gravimetrically using a
vacuum microbalance and by Karl Fischer assay of vials sealed at
selected times.
The secondary drying phase aims to remove unfrozen water
molecules. In this phase, the temperature is raised higher than in
the primary drying phase, and can even be above 0 °C, to break
any physico-chemical interactions that have formed between the
water molecules and the frozen material.
The pressure is also lowered in this stage to encourage
desorption (typically in the range of microbars, or fractions of a
Pascal)After the freeze-drying process is complete, the vacuum is
usually broken with an inert gas, such as nitrogen, before the
material is sealed.
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LYOPHILIZER STERILIZATION/DESIGN
The sterilization of the lyophilizer is one of the more frequently
encountered problems noted during inspections.
These lyophilizers inside surfaces wiped with a chemical agent
that may be a sterilant but usually has been found to be a sanitizing
agent
For such surface "sterilization" or treatment inert gas (usually
nitrogen) and sterile air for backfill or vacuum break is often
inaccessible .
Another method of sterilization that has been practiced is the use of
gaseous ethylene oxide. As with any ethylene oxide treatment,
humidification is necessary.
Employing Water For Injection as a final wash or rinse of the
lyophilizer.
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A generally recognized acceptable method of sterilizing the
lyophilizer is through the use of moist steam under pressure.
Sterilization procedures should parallel that of an autoclave, and a
typical system should include two independent temperature sensing
system.
The filters are used to sterilize inert gas and/or air, there should be
some assurance of their integrity.
Generally, lyophilizers should be sterilized after each cycle because
of the potential for contamination of the shelf support rods.
Some of the newer lyophilizers have double doors - one for loading
and the other for unloading. The typical single door lyophilizer opens
in the clean area only, and contamination between loads would be
minimal
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FINISHED PRODUCT TESTING:
(a) Dose Uniformity
These contains two types:
content uniformity and weight variation.
(b) Stability Testing
(c) Sterility Testing
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EXCIPIENTS IN FREEZE DRIED PRODUCTS
BUFFERS
•Buffer system in freeze dried parenterals is sodium phosphate, scince it
is present physiologically, has a pH of normal plasma.
•Other buffer system used in approved product includes acetate, citrate,
arginine, histidine, succinate and Tri(tri-hydroxymethyl aminomethane).
BULKING AGENTS
•Bulking agents are needed when the drug quantity is insufficient to form
a pharmaceutically acceptable freeze dried solid, and the drug is
dispersed in an inert matrix that has appropriate freeze drying
charecterestics.
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•Bulking agents are falls into two general categories- those that tends
to crystallize from frozen system and those that remain amorphous.
The most common crystallizing excipient are mannitol and glycine.
•Polyethylene glycols, which are less common, also tends crystallize
from freezing solutions.
•Mannitol is known to, in some cases, promotes vials breakage
during freeze-drying
•Amorphous excipients include disaccharide such as sucrose,
trehalose, lactose, or mannitol. These excipients play a double role in
a formulation as a bulking agent and, for proteins and other
biological products, as stabilizer
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ADDED SALTS:
•Salts such as sodium chloride are often included in the freeze dried
formulations to provide an isotonic reconstituted solution.
• Salts when used in combination with amorphous excipients, added salts
tends to decrease the collapse temperature, making the process less
efficient and, in some cases, increasing the risk of not being able to make
a pharmaceutically acceptable product.
•Added salt may also inhibits crystallization of component of the
formulation for which crystallization is needed.
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FREEZE-DRYING EQUIPMENT
•Rotary Evaporator Freeze-drier used for liquid products, such as
pharmaceutical solutions and tissue extracts.
•Manifold freeze-driers are usually used when drying a large amount
of small containers and the product will be used in a short period of
time. A manifold drier will dry the product to less than 5% moisture
content.
•Tray freeze-driers are more sophisticated and are used to dry a variety
of materials. A tray freeze-drier is used to produce the driest product for
long-term storage. Tray freeze-driers can dry products in bulk or in
vials. When drying in vials, the freeze-drier is supplied with a
stoppering mechanism that allows a stopper to be pressed into place,
sealing the vial before it is exposed to the atmosphere. This is used for
long-term storage, such as vaccines.
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NEW APPROACHES :
•The ASFD (Atmospheric Spray Freeze Drying) process has been
developed as a new method for manufacturing powder by scientists
in Edmonton, Canada.
•The goal was to reduce drying time and operate at atmospheric
conditions with reasonable collection efficiency; this work has also
been patented.
•Spray-freeze drying technologies, this new technology combines
spray freezing, deposition / collection, and convective flow drying
into one step employing co-current gas flow to spray-freeze .
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ADVANTAGES
The advantages of lyophilization include:
1 Ease of processing a liquid, which simplifies aseptic handling.
2 Enhanced stability of a dry powder.
3 Removal of water without excessive heating of the product.
4 Enhanced product stability in a dry state.
5 Rapid and easy dissolution of reconstituted product.
DISADVANTAGES
The disadvantages of lyophilization include:
1 Increased handling and processing time
2 Need for sterile diluent upon reconstitution
3 Cost and complexity of equipment
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References:
1.Thomas A Jennings ‘ Lyophilization , Introduction And Basic
Principle’ by informa healthcare USA 2008, page no 4
2. Louis Rey , Joan C. May ‘ Freeze drying ‘ , Liophilization of
pharmaceutical and biological products, Third edition , by informa
healthcare, New York , London page no
3. Michael E . Aulton ‘ Pharmaceutics ‘ The design and manufacture of
medicine, Third edition, Churchill livingstone, New York, page no 195,
435-8, 442, 620.
4. E. a Rawlins , Bentley’s ‘ Text Book Of pharmaceutics’ , Eigth edition
, Sunders Elsevier. Page no. 192-194.
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5.. Sandeep Nema and John D. Ludwing ‘Pharmaceutical Dosage Forms:
Parenteral Medications’ Third edition , volume2 : Facility Design,
Sterilization And Processing, Informa healthcare , Newyork. Page no.
353-380.
6. Kenneth E. Avis, Herbert A. Lieberman and Leon Lachman,
‘Pharmaceutical Dosage forms: parenteral medications, volume 2;
second edition, revised and expanded; Marcel Dekker, New York. Basel;
page no.163-231.
7. Adel Nowefaa Lyophilization the science of freeze drying
"lyophilization."
8. Dr. Pikal, M. and Reiter, C. (2008) "Basic Theory of Freeze Drying"
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9. Shalaev EY, Wang W, Gatlin LA, Rational choice of excipients
for use in lyophilized formulations . Drugs Pharma sci2008;
175;197-217.
10. Brian Donaldson. "Wedding Bouquet Preservation…saver
your special memories of your wedding day through the
preservation of your bridal bouquet!" (MHT). Retrieved 2010-06-
23.
11. US Department of health and human services, FDA US Food
And Drug administration ‘Inspections, Compliance, Enforcement,
and Criminal Investigations’.
