Moisture Effects on Solid Dosage Forms—Formulation


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PHARMACEUTICAL PROCESSES
Moisture Effects on Solid Dosage Forms—Formulation, Processing, and Stability
Armin H. Gerhardt
“Pharmaceutical Processes” discusses scientific and technical principles associated with pharmaceutical unit operations useful to practitioners in compliance and validation. We intend this column to be a useful resource for daily work applications.
Reader comments, questions, and suggestions are needed to help us fulfill our objectives. Suggestions for future discussion topics or questions to be addressed are requested. Case studies submitted by readers are also most welcome. We need your help to make “Pharmaceutical Processes” a useful resource. Please send your comments and suggestions to column coordinator Armin Gerhardt at [email protected] or to journal coordinating editor Susan Haigney at [email protected]
KEY POINTS DISCUSSED The following key points are discussed:
• Moisture may have a significant impact on a wide range of chemical, physical, and microbial properties of the finished pharmaceutical product
• Moisture in dosage forms comes from many sources including bulk drug, inactive excipients, manufacturing processes, and environmental conditions, and is a result of a variety of causes
• Water may interact in distinct ways including surface adsorption, as a crystal hydrate, by deliquescence, and by capillary condensation. Examples of these interactions are discussed.
• Water may have significant effects on product stability, tablet compaction, wet granulation, powder flow properties, and microbial growth
• Compliance personnel must be knowledgeable of areas with potential for moisture problems. Change control of these areas is especially important to maintain compliant manufacturing. Be prepared for potential problems.
• The effects of moisture should be considered in troubleshooting investigations and root cause analyses.
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INTRODUCTION The moisture content of drugs, excipients combined with the drugs to manufacture a final dosage form (i.e., compressed tablets), and/or processing manipulations involving moisture may have a significant impact on a wide range of chemical and physical properties of the finished product. Properties such as powder compressibility, flow rate, compactability, drug degradation, and microbial growth may be affected. Various processing steps require water (or other solvent) to accomplish their intended result. These include wet granulation, particle or film coating, spray drying, lyophilization, and crystallization. Control of these operations is best accomplished when the underlying mechanisms of water’s interactions with solid particles are recognized. Quality and compliance practitioners must be aware of the potential deleterious effects of moisture on tablet products and manufacturing processes.
Sources of Moisture Moisture in tablet products comes from many sources. Moisture may come from the bulk drug or inactive excipients in the formulation. In pure chemicals, moisture may be present as water of crystallization and/or as adsorbed water. The amount of moisture in these ingredients should be controlled by specification, but may vary within acceptable limits. Additional sources of water come from the various manufacturing processes. For example, significant amounts of water may be added in the wet granulation process. This water is then essentially removed during the drying process. Water may also be added and removed in the tablet coating process. Water may come from the environmental conditions of manufacturing (i.e., the relative humidity of the manufacturing and packaging areas). Certain formulation ingredients function to attract water and swell upon contact with moisture—and can do so when exposed to high atmospheric humidity. Water also may contact the product by transfer through packaging materials. Tablet products may be packaged in bottles with desiccants to prevent moisture uptake by the product and subsequent potential negative

effects. Moisture vapor transmission through films used in blister packaging has been quantitated. All these sources of moisture may impact the stability of susceptible products.
RELEVANT CHARACTERISTICS OF WATER At low concentration, water possesses a number of traits that combine to make it very effective at increasing molecular mobility (i.e., a plasticizer). It has a low molecular weight (18 daltons), small size, low density, high dielectric constant, and high ability to form hydrogen bonds (1). It may move via the vapor phase to localized molecular regions with charged and polar groups and concentrate there, thus reducing hydrogen bonding between adjoining molecules, increasing the free volume by dilution, and reducing the viscosity and glass transition temperature. Defined as the point at which a solid material converts from a rigid structure to a more flexible rubbery material, the glass transition temperature is important because of its correlation with chemical reactivity. In terms of physical behavior, these effects include material softening; its viscosity is reduced; and there is a reduction in mechanical strength as measured by either the tensile strength or Young’s Modulus.
Water is commonly present at low concentration on or within solid phase pharmaceutical materials, such as less than 1.5% w/w. Though water acts as a solvent when it is the predominant component as in a dilute solution, water is present in a very limited proportion in typical pharmaceutical solids systems. Under these conditions, water may be viewed as dissolving within void spaces or regions of individual particles or sorbing to their surface; it is in these situations that water exhibits its potential to be a highly effective plasticizer. At the molecular level, water increases torsion of side groups or end segments, and macromolecular chain segments or chains have increased mobility.
MECHANISMS OF WATER INTERACTIONS For crystalline solids, water may interact in the following four distinct ways (2):
• It may adsorb to the surface • It may be present within the crystalline lat-

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Figure:
A plate-shaped crystalline particle with partial surface adsorption of water and preferential concentration at a corner with crack defects and on a hydrophilic edge.

Non-speci c surface adsorption of water as individual molecules or small clusters.

Crystal corner with fractures from milling showing preferential water concentration on exposed higher energy surfaces and amorphous regions.

Preferential water concentration on crystal edge with most polar groups.

tice in a stoichiometric quantity (i.e., crystal hydrate) • The crystalline solid may deliquesce (i.e., liquefaction of the solid to form a solution by continued water vapor adsorption) • Capillary condensation may occur when microporous regions are present in the solid.
Adsorbed water on the solid surface is generally hydrogen bonded to the surface. There may be an additional two to three layers possible when high relative humidity is present, all of which is readily reversible by small temperature increases or small decreases in relative humidity (3). While the water of a crystal hydrate is found at predictable locations within the solid and is held in place by hydrogen bonds, it is not generally available to reactions.
With amorphous solids, long-range ordered structure is not present. This produces localized empty voids which allow for the possibility of water to dissolve in the solid (see Figure). Though the vast majority of material in an individual solid particle may be crystalline, imperfections in the regularly repeating arrangement of atoms and molecules may also be present, and it is in these zones of disorder that significantly higher rates or reactivity may be found. Processes within the pharmaceutical indus-

try such as mechanical milling, lyophilization, spray drying, or rapid removal of solvent may produce significant quantities of amorphous material. These zones are potentially more reactive, and may produce significant increases of molecular mobility when water concentrates in them. Whereas adsorption to a crystalline solid is limited to the available surface area, water uptake of an amorphous material is limited to the quantity of amorphous solid. Polymeric materials, by virtue of their irregular chain length and myriad configurations possible in a solution, are difficult to crystallize. During a drying step, solvent removal takes place rapidly and does not permit sufficient time for individual molecules to align in a crystalline pattern; they are predominantly amorphous in structure when solvent removal is completed. Materials such as polyvinylpyrrolidone, hydroxypropyl cellulose, hydroxypropylmethyl cellulose and starch are free-flowing powders that are predominantly amorphous.
Model calculations have been made to link this qualitative picture with relevant examples (Tables I and II). Table I depicts the situation for spherical sucrose particles in the size range from 1 to 100 µm with their specific surface area. Assuming 0.1% w/w water is at the particle surface, the calculated number of water layers ranges from 1.1 to 110.

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Practical experience with pharmaceutical materials of 100 µm size (roughly 140 US mesh size) with 0.1% water would not suggest the presence of 110 layers of surface water. This indicates that a significant portion of the water is dissolved within the disordered regions in addition to water present on the surface.
Further to this example, Table II shows the calculated amount of water found in varying quantities of the amorphous regions of sucrose with either 0.1% or 0.5% moisture content. With the assumption that all water is preferentially found in the amorphous regions, the range of values is from 2% (for 5% amorphous material with 0.1% water) to 50% (for 0.5% amorphous material and 0.5% water). Connecting this to the results for the glass transition temperature, it was calculated to be in the range of -73 to 49ºC [the glass transition temperature for pure water is -108ºC (4)]. For sucrose with 2.5% amorphous material holding 0.5% water, the resultant glass transition temperature of 9ºC is below room temperature conditions. Thus molecular mobility may be enhanced and permit reactions such as chemical degradation or re-crystallization.
EFFECTS OF WATER ON TABLET FORMULATIONS Water may significantly affect the chemical, physical, and microbial properties of tablets. Examples of these interactions include the following:
• Product stability • Tablet compaction • Moisture-activated dry granulation • Wet granulation • Effervescent tablets • Solids flow properties • Microbial growth.
Product Stability Product stability considerations comprise two areas: Drug potency and drug dissolution. When formulations are considered, the water present in excipients or employed in processing may preferentially concentrate in amorphous regions of the drug. This may occur via the vapor phase, or water concentrated regions may be brought in direct contact with the drug. Depending on the drug properties, this moisture might then increase the rate of drug

Table I: Sucrose sphere specific surface area and the theoretical number of water layers on the surface when 0.1% moisture is adsorbed (Adapted from Reference 2).

Particle Size (µm)

Specific Surface Area (cm2/g)

Number of water layers

1

38,000

1.1

10

3,800

11

38

1,000

42

100

380

110

Table II: Moisture content in amorphous region and glass transition temperature when either 0.1% or 0.5% moisture is sorbed (Adapted from Reference 2).

Moisture Amount (%)

Amount of Amorphous Material (%)

Moisture content in amorphous zone (mg water/ 100mg solid)

Glass Transition Temperature (ºC)

0.5

20

9

1

10

27

0.1

2.5

4

45

5

2

49

0.5

100

-73

1

50

-36

0.5

2.5

20

9

5

10

27

degradation. Hydrolysis is a well-known mechanism for drug degradation reactions. High moisture levels might also cause drug dissolution to be adversely affected, potentially resulting in reduced drug bioavailability. In this case, the drug would be fully potent, but would not dissolve as needed for absorption and product therapeutic efficacy.
Tablet Compaction Moisture is an important factor in compaction of blended powders or dried granulation to form tablets. Tensile strength is generally low at low

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moisture content (approximately 0.1 to 0.2% w/w). As the moisture level increases, the compact tensile strength also increases to a maximum level; higher moisture contents then lead to decreased compact tensile strength (5). Due to stability requirements, moisture levels above 1.5% w/w are seldom found in compressed tablets. Possible mechanisms for increasing the tensile strength are adsorbed water may alter surface structure such that there are more solid bridges, or immobile water at a particle surface may enhance interactions between particles. With the presence of relatively more surface moisture, it is possible for permeation into the particle, which may plasticize or soften the material. During compression, there are increased bonding surfaces as particles yield and flow under the pressure applied by the tablet press. Actual results are dependent on the material(s) being compacted. A wide range of materials have been studied and occasional anomalous results have been found. Each product requires a solid foundation of data from which to select the appropriate balance range for moisture that delivers robust compaction and stability throughout the product expiry period.
Moisture-Activated Dry Granulation The technique of moisture-activated dry granulation provides the advantages of wet granulation while the drying step is eliminated. This process is composed of two distinct stages including agglomeration followed by moisture adsorption (6). Agglomeration is accomplished by the addition of a relatively small amount of water, roughly 1-4%, to the combination of drug substance, binder, and filler. This is then followed by the moisture adsorption step of addition and mixing microcrystalline cellulose, potato starch, or a combination of these materials.
Wet Granulation Wet granulation is an important process in tablet manufacturing. It has been shown that the compressibility of both microcrystalline cellulose and silicified microcrystalline cellulose are progressively reduced with increasing quantities of water when

wet granulated (7). Using a laboratory scale highshear granulator and varying the amount of water from zero to 100% w/w of the powder, both materials produced compacts of approximately 3.8 MPa tensile strength at a compression force of 50 MPa. However, when 100% water was used the maximum compact tensile strength was 1.0 MPa at the much higher compression pressure of 180MPa. At the more typical wet granulation condition where 20% water was used, there was also a significant decline in compact tensile strength. Reduction of 3.8 MPa to 1.5 MPa at compression pressure of 50 MPa was observed, a 39% loss of compact tensile strength.
For microcrystalline cellulose, the presence of low quantities of moisture has been shown to produce an antiplasticization effect (8). Specifications for this material set the moisture limit to less than 5%; a typical lot may have water content in the range of 3 to 4%. Based on the water sorption isotherm, the equilibrium moisture content for this material at commonly found manufacturing environments of 20% and 40% relative humidity is approximately 3.2% and 4.3%, respectively. Study results indicate maximal compact tensile strength at a moisture content of approximately 4.2%; however, the dried MCC compacts had less than half this tensile strength (Table III). The authors of the study attributed the reduced compact tensile strength at low moisture content to an antiplasticization effect, in essence the presence of a small quantity of water makes MCC more ductile and improves bonding. Beyond roughly 4% water content, there is reduction in bond strength as the plasticization effect weakens or limits bonding.
Effervescent Tablets One specialty dosage form requiring exquisite control of moisture in processing is the effervescent tablet (9). This dosage form offers distinct advantages in terms of the following:
• Capacity to accommodate large dose drug within a typical 2,000 mg tablet
• No requirement to swallow the dosage form • Buffering the acid environment of the stom-
ach with carbonation and induction of rapid

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emptying that aids drugs susceptible to acidic degradation • The foil packaging required for stability offers protection for drugs sensitive to light, oxygen, or moisture.
Selection of formulation components requires the presence of an organic acid and metal carbonate that release carbon dioxide when exposed to water. Because this reaction is autocatalytic (i.e., it also produces water that further speeds the reaction) there is a requirement for strict relative humidity control and processing steps during all phases of production and packaging. Environmental processing conditions are typically 10% relative humidity. Minimal quantities of water are employed during granulation and spray nozzles deliver this over a large powder bed surface to minimize local concentrations.
Solids Flow Properties Excessive moisture in blended solids may be the result of incomplete drying, the presence of large blocks of granulated material that retain moisture in their core, or permeation through poor barrier materials during storage. These occurrences may cause significant effect on the flow properties of solids in processing.
Within a powder bed, the forces of cohesion, adhesion, and friction may impact the rate of flow. Cohesion and adhesion are a mutual attraction and resistance to separation between the same material and different materials, respectively; friction originates from points of direct contact between particles and is a resistance of one particle’s movement caused by other particle(s). Moisture may impact these interactions by adsorbing to the surface of particles and altering their surface energy or surface electrostatic charge. Moisture may condense from the atmosphere to porous or amorphous regions at the area of contact to soften a material and allow greater areas of contact. With the majority of materials, a relative increase in the moisture content will diminish flow rates due to an increase in the forces of cohesion and adhesion. There can be a significant range of moisture content in which

Table III: Microcrystalline cellulose.

Water content (% w/w)
0

Compact Tensile strength (MPa)
1.5

1.6

2.6

4.2

3.3

5.2

2.8

10.8

2.6

12.0

2.4

Crystallinity index 2.3 2.7 3.2 3.3 3.4 3.3

there appears to be little impact on flow properties, particularly at the lowest moisture values (less than 0.5% w/w). Each formulation requires sufficient data to justify its optimal moisture content range.
Materials that have been micronized or produced by other techniques such that their size is less than approximately 5 µm are particularly sensitive to the effects of electrostatic charge accumulation. This is frequently observed when fine powders cling to plastic storage materials. There may be varying quantities of material retained in the container during transfer steps in processing, which may be operator or seasonally dependent. For those items present in small quantities (e.g., potent drugs), it may be necessary to include a second step in which the empty plastic bag has a second type of material added to it with agitation or shaking to free the charged particles and permit a quantitative transfer to the product. This technique essentially becomes a rinse step for the container that uses a second component of the formulation. Process vessels made of metal serve to effectively ground a material and dissipate the electrostatic charges. Also, increasing the moisture content of a material generally results in reduced static charge accumulation. During material movement operations with large volumes of air (e.g., fluid bed drying and vacuum loading), static charges may build to significant levels, and they may produce sparks that have the potential to cause explosions. Many of the powdered organic solids found in pharmaceutical prod-

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ucts have the capability to support an explosion should it be initiated. Although this is a rare event, it is important to ensure equipment is adequately grounded and enforce all safety precautions. This is particularly important where organic solvents are used near moving powder materials (e.g., transfer of powder from a plastic bag into a vessel with organic or mixed organic-aqueous solvents). The organic vapors may spread along equipment surfaces and ignite with the smallest electrical discharge.
Microbial Growth The presence of moisture is also important to microbiological stability. The food industry has made extensive use of testing water activity to control microbial growth and monitor chemical stability (10). Water activity measurements differ from water content in that water activity reflects the energy status of water in a system. Water activity is defined as the vapor pressure of water above the sample divided by the vapor pressure of pure water at the same temperature. Values range from 0 for totally dry samples to 1.0 for pure water. Use of water activity testing has begun being adapted within the pharmaceutical industry, in part because of the introduction of US Pharmacopeia <1112> “Application of Water Activity Determination to Nonsterile Pharmaceutical Products.” Water quantitative determination is based on either a Karl Fischer titration or loss on drying in most pharmacopeial monographs. The Karl Fischer method requires meticulous analytical technique to yield accurate results; side reactions are possible and stringent drying of methanol is necessary. Most importantly, results from either technique may not be predictive of product stability outcomes. Among the advantages of water activity testing are the simplicity of this non-destructive assay, reduced likelihood of handling problems, and comparable or improved correlation to physical and chemical changes. Typical water activity values range from 0.3 to 0.5 for oral solid dosage forms. Most microbial growth is inhibited at values less than approximately 0.91, and yeasts and molds generally cease growing below 0.87 and 0.80, respectively.

IMPLICATIONS FOR COMPLIANCE, TROUBLESHOOTING, AND ROOT CAUSE ANALYSIS The significant effects of moisture on solids processing and tablet products as described above indicate the need for good control of moisture in development and routine manufacturing and packaging. The following are potential areas of concern:
• Active drug and excipient ingredients moisture content
• Product characteristics • Manufacturing processes • Environmental controls • Packaging materials.
Change in moisture levels of all associated with these areas of concern is critically important. Changes may be unintended, such as seasonal variation of environmental conditions. Changes may also be planned, such as with a new source of raw material with different moisture content. Change control of planned changes is critically important. Changes in moisture levels, both increased and decreased levels, should be considered as possible causes of manufacturing or stability problems in troubleshooting and root cause analysis.
Active Drug and Excipient Ingredients Moisture Content The moisture content of all ingredients in the formulation should be known, including the active drug and all excipients. Also important are the specification limits for incoming materials. For example, one supplier of an excipient may reliably provide material containing 0.5% moisture. Less expensive material from a new vendor may be obtained. The new material may contain higher moisture—still within acceptable limits—but with potential for affecting product stability, processing, compaction, or other properties. Any changes to incoming materials with changes in moisture content must be carefully monitored for susceptible products.
Product Characteristics Product characteristics impacted by the effects of moisture should be known. Products with an active drug that is chemically sensitive to moisture

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or products with sensitive dissolution performance must be carefully monitored regarding exposure to moisture or to changes in moisture content. An effervescent tablet formulation containing acid and base components will be stable in a low-moisture-content environment, but is highly likely to undergo the effervescent acid-base reaction prematurely in the presence of even relatively low levels of environmental moisture.
Manufacturing Processes Any manufacturing process that may impact the product moisture content of susceptible products should be carefully monitored. For example, there may be seasonal changes in the efficiency of drying processes caused by high humidity conditions in summer and dry air conditions in winter; air conditioning units need to have sufficient capacity to control processing within specifications. Tablet compressing problems may occur when the granulation moisture content is too high or too low. Manually controlled processes such as granulation and drying should be carefully monitored. Compliance personnel must be mindful of changes to equipment associated with manufacturing processes associated with moisture content.
Environmental Controls Environmental conditions for moisture-sensitive materials and products must be carefully controlled. Seasonal humidity variation may impact product quality during manufacturing or packaging processes. A manufacturing area may easily run below facility limits during low humidity seasons, but may struggle to be compliant during the rainy season. Compliance personnel must be watchful of changes to equipment associated with environmental controls.
Packaging Materials and Processes Packaging materials used for moisture sensitive products must be carefully monitored. Blister films may have widely differing moisture permeability. Data on moisture vapor permeation through flat sheets of polymeric materials (e.g., PVC, HDPE) are

readily available from the supplier for comparison. In addition to the flat sheet permeation data, it is important to realize the heating and cavity formation process may stretch and diminish the thickness in the region of bends by as much as 75%, thereby significantly reducing its moisture barrier protection in the relatively thin film regions. Metal foil materials are significantly better moisture barriers than the organic polymers, though they are more susceptible to punctures or tears that destroy their barrier function. Equipment changes which may affect the sealing of blister packages or foil pouches may cause problems with package integrity and product stability.
Change Control Any changes affecting materials, products, manufacturing processes, environment controls, packaging materials, and packaging processes for susceptible products must be carefully evaluated. A thorough and comprehensive change control program will facilitate monitoring of change.
Troubleshooting and Root Cause Analysis When it is necessary to troubleshoot a manufacturing challenge or out-of-specification analytical result, a practical understanding of the many ways water may impact the product can be of significant benefit in establishing a root cause with optimal remedial actions. The insight gained by asking the question ”what impact did water have in causing this situation?” can be very helpful. Trace the data reported on the moisture content of materials at key points and ensure sufficient samples have been obtained to establish confidence in their accuracy and reliability. For example, tablet hardness, thickness, weight variation, compression force, ejection force, frequency of sticking to punches, disintegration, dissolution and chemical stability may be significantly different with moisture content variation of as little as 0.3% w/w. Low granulation moisture levels are well known to cause problems in tablet compressing. Water is found in nearly all pharmaceutical powders, it plays a vital role in granulation, compaction, and coating operations, yet it may have

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a deleterious effect during the product’s shelf life. It is essential that the effects of moisture be understood and controlled.
CONCLUSIONS Water is present in nearly every pharmaceutical powder. It functions as a highly effective plasticizer to increase molecular mobility when present in low concentrations and may localize in surprisingly high quantities at regions of amorphous structure. Water is utilized to great benefit in granulation, compaction, and coating processes, yet may have a deleterious impact during a product’s shelf life via an increase in chemical degradation or support of microbial growth. Controlling the impact of water begins with specifications and testing of all components, controlling the manufacturing and packaging processes along with the environment, selection and testing of packaging materials, and vigilance during the product’s life cycle. Changes in component or product moisture content, either unintended such as those in seasonal variation or planned moisture changes such as with new sources of excipients, must be carefully monitored by compliance practitioners. In addition, understanding the myriad capabilities of water often offers valuable insight to determining root cause and corrective actions when troubleshooting.
REFERENCES
1. Tolstogazov, V.B., The Importance of Glassy Biopolymer Components in Foods, Nahrung 44 76, 2000.
2. Ahlneck, C. and Zografi, G., “The Molecular Basis of Moisture Effects on the Physical and Chemical Stability of Drugs in the Solid State,” Int. J. Pharm. 62, 87-95, 1990.

3. Thiel, P.A. and Madey, T.E., “The Interaction of Water Solid Surfaces: Fundamental Aspects,” Surface Sci. Rep., 7 211385, 1987.
4. Velikov, V., Borick, S., Angell, C.A., The Glass Transition of Water, Based on Hyperquenching Experiments,” Science 294, 2335-2338, 2001.
5. Nokhodchi, A., “An Overview Of The Effect Of Moisture On Compaction And Compression,” Tablets & Capsules 29(1), 46-66, 2005.
6. Christensen, L.H., Johansen, H.E., Schaefer, T., “MoistureActivated Dry Granulation in a High Shear Mixer,” Drug Dev. Ind. Pharm. 20(14), 2195-2213, 1994.
7. Habib, Y.S., Abramowitz, R., Jerzewski, R.L., Jain, N.B., Agharkar, S.N., “Is Silicified Wet-Granulated Microcrystalline Cellulose Better Than Original wet-Granulated Microcrystalline Cellulose?” Pharm. Dev. and Tech. 4(3) 431-437, 1999.
8. Charmathy, S.P. and Pinal, R., “Moisture-Induced Antiplasticization In Microcrystalline Cellulose Compacts,” Tablets & Capsules 5(5), 22-33, 2007.
9. Lee, R.E., “Effervescent Tablets,” Tablets & Capsules 2(4), 2004, 40-45.
10. Snider, B., Liang, P., Pearson, N., “Implementation Of Water-Activity Testing To Replace Karl Fischer Testing For Solid Oral-Dosage Forms,” Pharm. Tech. 31(2), 56-71, 2007. GXP
ABOUT THE AUTHOR
Armin H. Gerhardt, Ph.D., is an industry consultant who spent more than 16 years at Abbott split between formulation services in R&D and project management for new drug product development teams. Armin retired from Abbott in 2007. He has taught various courses in pharmaceutical processing for many years. Armin has also authored book chapters on pharmaceutical unit operations. He can be reached at [email protected]

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Moisture Effects on Solid Dosage Forms—Formulation