Ali Nokhodchi, PharmD, PhD, is an associate professor of pharmaceutics and visiting professor at King's College, Department of Pharmacy, School of Health and Life Sciences, Franklin-Wilkins Building, 150 Stamford St., London, Englad SE1 8WA.

Ali Nokhodchi, PharmD, PhD

Water interacts with pharmaceutical solids at virtually all stages of manufacture, from synthesis of raw materials to the storage of the final dosage form. The interactions of water with powders is, therefore, a major factor in the formulation, processing and product performance of solid pharmaceutical dosage forms.  

The amount of water associated with a solid at a particular relative humidity (RH) and temperature depends on its chemical affinity for the solid and the number of available sites of interaction, the surface area and the nature of material.

  The physicochemical properties of pharmaceutical solids (such as flow, compaction, hardness and dissolution rate) are dependent on the presence of moisture, and several reports have been made in the literature to this effect.

Because of the importance of polymers in tablet and sustained-release matrices, the influence of storage conditions on the mechanical properties of tablets will be discussed separately. Compaction of powders into coherent specimens requires the formation of interparticulate attractions between the particles during the compression phase.  

However, after ejection of the tablet from the die, the tablet strength can change.

  This means that interparticulate attractions are also formed or disturbed during the postcompaction storage phase. Three main mechanistic explanations for the formation of interparticulate attractions have been proposed: 					

Rearrangement of solid material at the particle surfaces within the tablet.

Continuing deformation of particles in the compact after compaction.

Crystallization of dissolved material between neighbouring particles because of the movement of water within the tablet.

The first of these mechanisms is one of the main reasons for the postcompaction changes in the tensile strength of tablets. The rearrangement in the second mechanism can lead to either the formation of solid bridges between particles or to an increase in the bonding surface area of intermolecular attraction forces.  

It seems that this restructuring of solid materials is mediated or facilitated by the presence of adsorbed water and that water can show this function — although the point has not been reached where the state and thickness of the adsorbed water vapour allows a true dissolution of the solid material. 

For example, the first mechanism was proposed as an explanation for the postcompaction increase in tensile strength of tablets compacted from fine particulates (20–40 μm) of two water-soluble materials, saccharose and sodium chloride.

There are several factors that can change the effect of storage conditions including RH on the tensile strength of tablets. These factors will be discussed in this article. 

For tablets of saccharose and sodium chloride, the tablet strength increased generally during the storage.

  Although saccharose and sodium chloride are both water-soluble, larger differences in tensile strength of saccharose tablets were observed in comparison with tablets of sodium chloride after storage for 1 week. 

 For water-insoluble excipients, such as tablets of calcium hydrogen phosphate dihydrate, the tablet strength did not vary with time or storage conditions.

				  Changes in tensile strengths of sodium chloride and saccharose could be correlated to the changes in surface area of tablets.

It has been shown that there is transport or movement of solid material within the tablet during the storage results in a rearrangement of the material such that new interparticulate attractions are formed. This seems to be facilitated by the uptake of water from the environment. It seems reasonable that the RH of the environment during storage of tablets affects both the rate and the amount of water uptake. Thus, both the rate and the total degree of movement of solid material and the subsequent formation of interparticulate attractions are related to the amount of sorbed water.  			

This was also verified by a study on the effect of precompression processing and storage conditions on compaction properties of sodium chloride, sucrose, calcium hydrogen phosphate dihydrate and acetylsalicylic acid, which showed that the tablet strength increased with the postcompaction storage time for both sodium chloride and sucrose (water-soluble materials) at intermediate and high RH. However, for water insoluble materials of calcium hydrogen phosphate and acetylsalicylic acid no change was observed (Table 1).

 Table 1 The effect of storage condition on the tablet strength of some pharmaceutical powders.*			

Furthermore, for sodium chloride tablets it has been shown that the increase in compact strength occurred at 0, 33 and 57% RH, mainly within the first 4 h of storage.

				  However, only a minor change, if any, in the postcompaction tablet strength was noticed when the tablets were stored at 0% RH (Table 1). Although there are various studies on the effect of RH on the mechanical properties of tablet during the storage,

  it is very difficult to predict the effect of RH on the hardness of tablets as it has a complex effect on the physical ageing of compressed tablets and the variation in hardness.

 investigated the effect of temperature and humidity on the hardness of sustained release diphenhydramine HCl matrices.

				  The obtained results are tabulated in Table 2, which shows that the plateau hardness is achieved much more quickly for the tablets stored at 40 °C/75% RH than for the tablets stored at 25 °C/60% RH, which could be attributed to a faster acquisition of equilibrium moisture content at high temperature and RH (e.g., 40 °C/75% RH). 

Table 2 Hardness values of diphenhydramine  HCl-kollidon tablets.*			

Most of the changes in the mechanical properties of tablets could be related to the deliquescent point of the salts. As the humidity increases (but remains below the critical deliquescent point), the hardness of the salts increases. At a particular humidity, hardness would probably change until the compact attains its equilibrium moisture content. Thereafter the hardness would remain constant.  

Above the critical deliquescent point, the hardness of the salts decreases considerably.

  In the case of polymer tablets, the effect of moisture is not as straightforward as salts. For example, Reir and Shangraw reported adverse effects on the strength of Avicel tablets upon storage under elevated humidity. They attributed this to the absorption of water into the Avicel tablets causing bond disruption and loss in tablet strength.

Initial moisture content of pharmaceutical powders is an important factor affecting physicomechanical properties of tablets.

 Some investigations suggest only a limited effect for the powder storage conditions before preparation of compacts on the postcompaction strength changes even for materials prone to crystallize readily when exposed to humid atmospheres.

  A possible explanation is that the particle surfaces might be activated during compression because of interparticulate friction. However, a major effect has been observed in some cases. For example, Nyqvist and Nicklasson compared the effects of inducing moisture into Avicel powder prior to compression with the storage of 'dry' Avicel tablets at elevated RHs.

  The former produced an increase in tablet strength while the latter treatment produced a marked decrease.  

Partial moisture loss is the main factor increasing the hardness of tablets because of recrystallization of water-soluble excipients or a water-soluble drug in the void spaces on compression. 

The extent of the increase in hardness depends on the specific combination of drug and excipient and their physical properties (such as aqueous solubility, crystallinity and hygroscopicity), as well as the tabletting method.

  Therefore, recrystallization could be avoided by careful selection of excipients. Tablets prepared by direct compression are more susceptible to changes caused by humidity than tablets prepared by wet granulation. The changes in tablet behaviour upon ageing can be correlated with the moisture content of granules in tablets made by wet granulation. 

For tablets made by direct compression, it has been shown that those with high initial moisture content increase in hardness upon storage, with the magnitude of such increases being dependent upon the physical properties of the base and the absolute moisture content.

				  Among the tablets studied, lactose-based tablets with different initial moisture contents were found to be the most resistant to changes upon storage.

Effect of storage conditions on mechanical properties 

The effects of RH on the mechanical properties of hydroxypropyl methyl cellulose (HPMC) polymers have been extensively studied.

  It has been shown that the tensile strength of tablets obtained from HPMC powders equilibrated at 0% RH and then stored at different RHs before testing showed an initial plateau at about 5% wt moisture content and then a decrease that should be related to weakening of the interparticle bonds because of the softening of the HPMC polymers.  

But when HPMC powders compressed at the same compression force after equilibration at different RHs were stored at the same RH, the tensile strength of the tablets showed the plateau at 10% wt moisture content and then decreased. 

The effect of moisture content on a polymer's compaction properties has been reviewed briefly.

				  The presence of moisture in varying quantities could both increase or reduce the mechanical strengths depending on the formulation content.

explained the increase/decrease in tensile strength with moisture according to the type of moisture associated with polymers (Table 3).

Table 3 The effect of the type of moisture on the tensile strength and elastic recovery of HPMC K4M tablets stored at various relative humidities.*							

An increase in RH from 23% to 75% caused 7.5-,4.8-,and 2.3-fold increases in internally absorbed water, externally adsorbed water, and monolayer-adsorbed water, respectively. Similarly, the tensile strengths of HPMC K4M tablets concomitantly increased.  

The distribution of moisture in a material; the range and magnitude of the van der Waals' forces between the particles; and the development of additional bonds by plastic deformation and/or melting of powder particles should control the tensile strength of the tablets. Water molecules initially adsorbed on the surfaces may form a monomolecular layer and increase the van der Waals forces, thereby smoothing out the surface microirregularities and reducing interparticle separation.

  This monolayer-bound water can be regarded as part of the particles' surface molecular structure.

				  These effects would increase the tensile strength of HPMC K4M tablets with increasing moisture content. As more water molecules adhere to the surface, moisture may transfer into the material.

  This effect may soften the particles' surfaces; under high pressure, the area of contact between the particles will increase with plastic deformation and more solid bonds may form.

  These effects could account for the tablets' increased tensile strength. 

It has been suggested that water adsorption reduced tablet tensile strength because of condensation and multilayer adsorption.

				  The extent of multilayer adsorption in Table 3 can be estimated by subtracting the monolayer adsorption from the externally adsorbed moisture values. Although the amount of multilayer adsorption increased, tensile strength did not decrease. Therefore, the effect of moisture on tablet tensile strength is the result of the balance between the amount of monolayer-adsorbed moisture, internally absorbed moisture and externally adsorbed moisture.  

Maltodextrines have been extensively studied as excipients.

  They display a completely different type of profile compared with emcompress or Fast-Flo lactose, with significant decreases in crushing strength occurring from the time immediately after ejection until 1 day after ejection. Emcompress and Fast-Flo lactose are primarily brittle materials so the tablet porosity would not be expected to change much over time, whereas the plastically-deforming viscoelastic expands after ejection from the die. 

The maltodextrins had an initial rapid increase in tablet porosity, from the time immediately after ejection until 1 day after ejection, after which only small changes in porosity were seen. This initial change was probably a result of viscoelastic expansion of the material and can explain the large loss in crushing force that was seen. 

 The porosity changes observed after 1day were slight and not sufficient to explain how the consistent maltodextrin tablet-crushing force decreases with time. Bonds weakened within the tablet without concurrent changes in tablet size or other gross changes, such as increased tablet porosity.  

The influence of storage conditions on the disintegration time and dissolution rate

Following the administration of a drug through a solid dosage form, a sequence of steps is required before the drug reaches the systemic circulation. An orally administered solid dosage form undergoes disintegration and deaggregation, followed by the dissolution of the drug. The dissolved drug molecules must penetrate the gastrointestinal membrane and be picked up by the blood. Each of the steps involved may limit how fast the drug molecules reach the general circulation. Therefore, it is important to review the effect of storage conditions on the disintegration time of tablets and dissolution rate of drugs. 

Several studies have been reported on the changes in behaviour of disintegration of tablets upon storage.

Such changes were found to be responsible for some bioavailability problems.

  The water sorption and its effect on disintegration of tablets made of various excipients have shown that the physical properties of the tablets were influenced by water uptake or loss during storage. At low RHs, the disintegration times increased and at high RHs decreases were observed.

It has been shown that tablets prepared by direct compression were more susceptible to changes caused by humidity than tablets prepared by wet-granulation.

  There are many studies that examine the effect of ageing on tablet disintegration. Large differences are apparent among different formulations and tabletting methods.

 The changes in tablet behaviour upon ageing were correlated with the moisture content of granules in tablets made by wet granulation. Tablets made from granules with high initial moisture content(4%), on storage, showed increases in hardness induced by partial loss of moisture.

It has been shown that tablets made from granules with low-moisture content (2%) gained hardness during storage as a consequence of the partial loss of moisture. Such increases in hardness, which were not related to initial tablet hardness, showed minimum effect on tablet disintegration.

 investigated the ageing of tablets prepared by direct compression of bases of different moisture contents.

  The storage-induced changes in disintegration were evaluated for tablets made by direct compression of three different bases with different initial moisture contents. 

The increases in hardness may increase the disintegration time. The tablets with low initial moisture content were minimally affected by storage.

  The gain of moisture by some of these tablets led to reduction of the disintegration times. Among the tablets studied, lactose-based tablets with different initial moisture content were found to be the most resistant to changes upon storage.  

In the development of tablets, it is generally recognized that ageing may cause chemical and physical changes that may modify the dissolution of drugs from compressed tablets. These changes in dissolution may alter the bioavailability of the active drug substance. 

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