Lokesh Kumar

A number of drugs are unstable in an aqueous environment, even when exposed for a short duration, thus requiring packaging, storage, and shipping in a powder or lyophilized state to keep the product stable during its shelf life. Used for parenteral administration, these drugs are commonly known as powder for injection (PI), powder for reconstitution, dry powder injection, or powder for constitution. 

 24 describes more than 100 drugs available as dry powder for injection (1).  

Typically, PI drugs are supplied in glass vials with rubber plugs and are mixed or reconstituted with a diluent (usually 5% dextrose solution, normal saline, bacteriostatic water, or sterile water for injection) before administration. An incompletely dissolved product can be hazardous to the patient, thereby making reconstitution a critical performance parameter for these products (1). 

 defines the completeness of the reconstitution procedure as the state in which the solid dissolves completely; leaving no visible residue as undissolved matter or the constituted solution is not significantly less clear than an equal volume of the diluent or purified water present in a similar vessel and examined similarly (1). The International Conference on Harmonization (

) Q6A guideline on drug specifications recommends that "for dry products for injection, acceptance criteria for redispersibility or reconstitution time should be provided and the procedure for resuspension or reconstitution (mechanical or manual) should be indicated. Time required to achieve resuspension or reconstitution by the indicated procedure should be clearly defined" (2). 

Variability in the reconstitution time of a product from the same or different manufacturers can seriously affect patient safety. The mention of a specific reconstitution time in the product literature or label could ensure a reproducible performance of the product in terms of complete reconstitution. Few 

 monographs of PIs specify the reconstitution time. For example, the monograph for amifostine sulfate, an anticancer drug, specifies the reconstitution time in 0.9% w/v sodium chloride solution to be not more than 45 s (3). Few PI products available in the market mention the time required for their reconstitution either on the label or in the literature supplied with the product. For instance, haemosolvate factor VIII, a glycoprotein necessary for blood clotting and hemostasis, specifies the reconstitution time to be not greater than 30 min. The long reconstitution time required for this product is because reconstitution must be carried out by gentle swirling to avoid foaming and gel formation in the product (4). However, for some drugs such as decitabine hydrochloride, a prolonged reconstitution time might be detrimental to drug stability (5). Therefore, special inputs from formulation scientists are required to optimize the reconstitution time during the development of these dosage forms. 

Keeping in mind the importance of reconstitution, the knowledge of parameters affecting reconstitution time is critical for product development, quality control, and overall product performance. This article analyzes these parameters by classifying them as extrinsic or intrinsic and examines how they contribute to the variability of reconstitution time of PI drugs (see Figure 1). 

Intrinsic parameters affecting reconstitution time

Intrinsic parameters are inherent to an active pharmaceutical ingredient (API) or a formulation and include the molecular-, particle-, and bulk-level solid-state properties of the API. Any added excipients also influence the inherent reconstitution properties of the product. 

 Powder dissolution is a kinetic phenomenon that depends primarily on the surface area exposed to the solvent, which in turn is a function of particle size. Particle size, therefore, influences reconstitution time. Coarser particles lower the dissolution rate because of their lower specific surface area in accordance with the Noyes Whitney equation (6). Conversely, very fine particles agglomerate because they are more cohesive (7) and therefore show longer wetting time, resulting in prolonged reconstitution (8). Therefore, particle size must be optimized and controlled to obtain short and reproducible reconstitution times. Particle shape and size affect the closeness of the powder packing, which in turn affects the penetration of water into interstitial spaces of the powder bed (9). Water penetration promotes wetting of each particle that is required before dissolution. Small particle size and symmetrical shape enhance close packing of particles, thus preventing the required penetration of water. Larger particles are usually more irregular in shape and therefore provide more space in the interstices for wetting. 

 The porosity of dry powder may have a significant effect on its reconstitution. Haeger et al. showed that an increase in porosity of the lyophilized cake by decreasing the concentration of the fill solution before lyophilization yielded a fluffy, low-bulk density lyophilized parenteral formulation with the desired characteristic of rapid reconstitution time (10). Several methods for determining the porosity of powder by means of mercury intrusion porosimetry (11–14), nitrogen gas adsorption (14, 15), and X-ray tomography (16) have been reported. 

 A drug can exist in crystalline, solvate, or amorphous form. Polymorphism, the existence of several crystalline forms of a single compound, affects dissolution properties of the drug (17). Different pseudopolymorphic forms also have different dissolution rates. For example, acyclovir exists as a hydrate and as an anhydrate, with each  form having significantly different dissolution rates (18).  

The amorphous form of a drug has higher kinetic solubility compared with the crystalline form as a result of its inherent high-energy state. Amorphization is, therefore, used to enhance a drug's dissolution properties in PI formulations. Bornstein et al. reported that the reconstitution time of the freeze-dried amorphous form of cefazolin sodium was decreased by 50% when compared with its crystalline form (19). 

 The degree of crystallinity, which is the percentage of crystalline form of any compound in the amorphous matrix, can markedly affect dissolution properties. Analytical techniques used to determine the degree of crystallinity include powder X ray diffractometry (20), near infrared spectroscopy (21), Raman spectrometry (22), solid-state nuclear magnetic resonance (23), dynamic vapor sorption (24), and thermoanalytical techniques such as isothermal microcalorimetry (24–26), differential scanning calorimetry (27), modulated temperature differential scanning calorimetry (28), and solution calorimetry (29). Reliability of the results may improve if several techniques are used in parallel (30). 

 Wetting is the primary step in the reconstitution process that is followed by the submergence, dispersion, and dissolution of the particle (31). Wettability, is a measure of the ability of a bulk powder to imbibe the liquid under the influence of capillary forces, and it depends on variables such as particle size, density, porosity, surface charge, surface area, and surface activity (32). Although, several techniques are available to determine the wettability of a powder, it is difficult to accurately assess powder wettability because of the complexity of this phenomenon in powder systems.  

Wettability is largely reflected by the contact angle between the powder surface and the penetrating diluent (32). The commonly used contact angle measurement on solid surfaces is influenced not only by the physicochemical properties of the powder, but also by factors such as surface roughness, chemical heterogeneity, sorption layers, molecular orientation, swelling, and partial dissolution of the solid in the liquid (33). Consequently, there is no universal test to measure the wettability of powders and each powder–liquid pair must be examined on a case-by-case basis to select the most appropriate method (33). 

Methods to determine wettability of powders have been classified according to the size of the sample and the mechanism of wetting as shown in Figure 2 (33). Details of each method can be found in the cited literature (9, 34–45). 

 The inherently low aqueous solubility of an API in formulation may contribute to incomplete reconstitution. Several formulation interventions are used in such instances, including the use of cosolvents, cyclodextrin complexation, lipidic systems, and amorphization by freeze drying. A rapid dissolution of  anidulafungin, an antifungal compound, was observed with 5–30% w/v ethanolic solution used as a diluent (46). The reconstitution behavior of melphalan, an anticancer drug, was improved by using cyclodextrins ((SBE)7m-β-CD or HP-β-CD) solution as the reconstitution diluent (47). 

Freeze drying is an important technique to formulate PI because it enhances the solubility and stability of the final product (48). High moisture content in the end product adversely affects not only the stability but also the reconstitution properties. Excipients used during freeze drying also have an impact on reconstitution time. Use of (SBE)7m-β-CD as a freeze-drying excipient in melphalan formulation improved the formulation's reconstitution behavior (47). The chemically modified cyclodextrins such as 2-hydroxypropyl-β-cyclodextrin (HPβCD), 2-hydroxypropyl-γ-cyclodextrin (HPγCD) and γ-cyclodextrin (γCD) enhanced the rate of dissolution of prototype lyophilized formulations of doxorubicin. It has been reported that HPγCD, present in five-fold excess relative to doxorubicin, decreased the reconstitution time from 26 min to less than 9 min (49). Using organic-aqueous mixtures instead of only aqueous solution during freeze-drying generates amorphous cakes with a large surface area, resulting in enhanced reconstitutability. Freeze drying of sucrose and lactose solutions from a 

-butanol and water mixture demonstrated rapid reconstitution, which was attributed to higher cake porosity (48, 50). 

 The presence of excess moisture in the formulation can sometimes accelerate chemical and physical degradation, which results in the formation of less-soluble degradation products that slow the reconstitution process. A lyophilized bovine serum albumin formulation when stored at elevated temperature (37 °C) and high relative humidity exhibited a loss of solubility that was attributed to protein aggregation occurring in the wet solid. Similar observations were made for recombinant human albumin, insulin, and tetanus toxoid (51). In such cases, special attention should be given to the choice of rubber plugs because it is the most potential source of ingress of moisture to the product (50). 

 Foaming during reconstitution is a serious problem for biopharmaceutical drugs because it may lead to protein denaturation and a consequent loss in their activity. This situation  necessitates determination of the protein activity that is lost as a result of shaking, which may further assist in setting specifications for reconstitution time. Kanavage et al. reported that the usual reconstitution time for lyophilized antivenom preparations by gentle swirling is approximately 45 min. Minimal changes observed in activity of the antivenom formulation, even with severe intentional foaming, obviated the need for any special reconstitution specifications (52). Apart from product stability, foaming also causes problems during administration, requiring it to be dissipated before use. In case of lyophilized palivizumab (Synagis, MedImmune), a rapid spurt of water into the vial may cause immediate and prolonged foaming that will require up to 2 h to dissipate (53).  

 Formation of a gelatinous mass (lump) during reconstitution is a major reason for a long reconstitution time. The gel layer that forms immediately after drug–diluent contact is highly viscous and sticky and adversely influences dissolution kinetics, thereby causing extreme difficulties in measuring reconstitution time (54). Farina et al. reported that two anthracyclines (antitumour drugs), doxorubicin (Adriamycin) and epirubicin (Pharmorubicin), marketed as lyophilized formulations showed a formation of a gel mass during reconstitution. A new formulation (i.e., rapid dissolution formula, RDF) containing parabens (hydroxybenzoate esters) as anti-aggregants manufactured by freeze drying was suggested to overcome the problem (55). 

Extrinsic parameters affecting reconstitution time

Extrinsic parameters include variables related to the container of the product and the conditions used on the product in storage and during the reconstitution process.  

Reconstitution of ziprasidone (Geodon), an antipsychotic drug, was found to be quick and more effective when carried out by hand shaking rather than using a commercially available agitation machine (56). Stogniew made a comparison between shaking and hand swirling for anidulafungin, an antifungal drug, and found that swirling generally resulted in longer reconstitution times, although with reduced foaming. With shaking, reconstitution times were shortened, but foaming was invariably observed (46).  

Within a particular method, reconstitution time is further dependent on several variables, including shaking frequency, height of stroke, inclination of the shaking axis, and the dimensions of the mockups. Thiermann et al. studied the reconstitution of HI 6 dichloride, a broad-spectrum cholinesterase reactivator, and observed that horizontal shaking reduced the frequency required for reconstitution (57). Shaking intensities may vary significantly among patients, healthy subjects, doctors, and nurses resulting in differences in reconstitution time (57). Freeze-dried or lyophilized protein drugs need unique reconstitution procedures to avoid excessive stress that might lead to foaming. Foaming causes physical instability of proteins such as denaturation, surface adsorption, aggregation, and precipitation, which might evoke undesired immune response in a patient. It is, therefore, recommended to reconstitute these drugs by either rolling them between palms or swirling thrm despite higher reconstitution times observed with these methods (58). 

 A change in reconstitution time may be observed during storage of a PI. Common reasons include cohesion, crystal growth, and/or moisture sorption, which promote lump formation. Cohesional forces increase with reduced distance between the particles and become prominent in the powder cake, which is usually a result of vibrations experienced by the product during transit or storage. Stresses during storage might induce crystal-form transformations and ultimately alter the reconstitution time (59). 

 Glass vials are sometimes coated internally with a silicone fluid to produce a hydrophobic surface. This operation is usually performed to avoid sticking or adherence of drug to the glass vial, thereby ensuring better reconstitutability (60). However, use of organic cosolvents as diluent must be avoided with siliconized glass vials or stoppers because organic cosolvents

can solubilize or extract silicone oil from the package component. The extracted silicone oil can impede the wetting of the affected portions of the cake, resulting in incomplete reconstitution (48). 

 PIs are considered small-volume parenterals and therefore packaged mostly in glass vials of volumes not more than 100 mL. The amount of headspace available determines the volume available for agitation of the product. A decrease in reconstitution time of dry-fill cyclophosphamide was observed by increasing the headspace of glass vials, which obviated the need of an expensive freeze drying process for improving the reconstitution time (61). 

 The temperature of the diluent influences the reconstitution time. A significant reduction in reconstitution time was observed using diluent prewarmed to 41 °C for dantrolene (62). Although a warmed diluent would speed up the reconstitution of almost any powdered substance, it might adversely affect the drug stability, which may necessitate the evaluation of the effect of temperature by conducting appropriate stability studies (62). Reconstitution of lyophilized amifostine powder (500 mg) with 2.9 mL sodium chloride solution or sterile water for injection at 20–25 °C resulted in complete dissolution within minutes. Reconstitution with smaller volumes (2.0 or 2.5 mL), although feasible, required more time and/or a warmer temperature (63). Dissolution of amifostine for subcutaneous injection was thus proved to be volume and temperature dependent (63). Mitchell et al. showed that dantrolene solubility increases linearly with increasing temperature between 20 and 40 °C (64). However, a temperature higher than 40 °C is not recommended because it could cause red blood cell lysis and local tissue burns (64, 65).  

Reconstitution, a pharmaceutical prerequisite for powder for injection, has direct implications on patient safety, making it a critical parameter to evaluate powder for injection formulations. Numerous intrinsic and extrinsic parameters affect the time and reproducibility of reconstitution of powders. Identification and control of these parameters during product development and quality control can reduce batch-to-batch variability in reconstitution time and provide confidence among the end users, thereby making the powder for injection formulation an attractive and safe alternative for moisture-sensitive drugs. A short and reproducible reconstitution time will also save the precious time and labor of caregivers while dealing in emergency situations, ensuring a more effective critical care during compromised clinical situations. 

Pradip Hiwale, Aeshna Amin, and Lokesh Kumar

 is an associate professor in the Department of Pharmaceutical Technology (Formulations) at the National Institute of Pharmaceutical Education and Research, Sector 67, Phase X, SAS Nagar, Punjab 160 062, India, tel. 91 172 2214682-87, fax 91 172 2214692, 

*To whom all correspondence should be addressed. 

Submitted: June 27, 2007. Accepted: Aug. 2, 2007. 

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 24 (United States Pharmacopeial Convention, Rockville, MD, 2006), pp. 2455–2458. 

 2. International Conference on Harmonization, 

Q6A: Specifications: Test Procedures and Acceptance Criteria For New Drug Substances and New Drug Products:Chemical Substances 

24 (United States Pharmacopeial Convention, Rockville, MD, 2006), p. 126. 

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www.home.intekom.com/pharm/nbi/viii1000.html

 5. S. Redkar and P. Phiasivongsa, "Salts of 5-Azacytidine," US Patent 2006/0063735 A1 (2006). 

 6. A.A. Noyes and W.R. Whitney, "The Rate of Solution of Solid Substances in Their Own Solutions," 

 7. M. Melgardt, "Influence of Agglomeration of Cohesive Particles on the Dissolution Behaviour of Furosemide Powder," 

 8. P.K. Hla and S. Hogekamp, "Wetting Behaviour of Instantized Cocoa Beverage Powders," 

 9. W. Wu and G.H. Nancollas, "Determination of Interfacial Tension from Crystallization and Dissolution Data: A Comparison with Other Methods," 

 10. H.E. Bruce, "Composition of Matter Comprising a Lyophilized Preparation of a Penicillin Derivative," US Patent 4477452 (1984). 

11.  L. Moscou and S. Lub, "Practical Use of Mercury Porosimetry in the Study of Porous Solids," 

12. S. Westermarck et al., "Mercury Porosimetry of Pharmaceutical Powders and Granules," 

13. P.J. Dees and J. Polderman, "Mercury Porosimetry in Pharmaceutical Technology,"

14. S. Westermarck et al., "Pore Structure and Surface Area of Mannitol Powder, Granules and Tablets Determined with Mercury Porosimetry and Nitrogen Adsorption," 

Articles

The authors describe the factors affecting reconstitution time of dry powder for injection and classifies them as intrinsic and extrinsic parameters.

Variables Affecting Reconstitution Time of Dry Powder for Injection

An estimated 50% of all drug molecules used in medicinal therapy are administered as salts. This fact indicates that the salification, or salt formation, of a drug substance is a critical step in drug development (1, 2). A drug substance often has certain suboptimal physicochemical or biopharmaceutical properties that can be overcome by pairing a basic or acidic drug molecule with a counterion to create a salt version of the drug (3). The process is a simple way to modify the properties of a drug with ionizable functional groups to overcome undesirable features of the parent drug (4). This fact underlines the importance of salt formation for drugs that are designed, developed, and marketed after a rigorous research and development program (1). 

Salts are formed when a compound that is ionized in solution forms a strong ionic interaction with an oppositely charged counterion, leading to crystallization of the salt form (5). In the aqueous or organic phase, the drug and counterion are ionized according to the dielectric constant of the liquid medium. The charged groups in the drug's structure and the counterion are attracted by an intermolecular coulombic force. During favorable conditions, this force crystallizes the salt form (see Figure 1). All acidic and basic compounds can participate in salt formation (4). However, the success and stability of salt formation depends upon the relative strength of the acid or base or the acidity or basicity constants of the species involved (6). 

Figure 1: Diagrammatic representation of salt formation.  

The salt form is separated into individual entities (i.e., the ionized drug and the counterion) in liquid medium, and its solubility depends upon the solvation energy in the solvent. The solvent must overcome the crystal lattice energy of the solid salt and create space for the solute. Thus, the solubility of a salt depends on its polarity, lipophilicity, ionization potential, and size. A salt's solubility also depends on the properties of solvent and solid such as the crystal packing and presence of solvates (7). 

Table Ia: Advantages of salt formation for drug properties. 

Salt forms of drugs have a large effect on the drugs' quality, safety, and performance. The properties of salt-forming species (i.e., counterions) significantly affect the pharmaceutical properties of a drug (see Tables Ia and Ib) and can greatly benefit chemists and formulators in various facets of drug discovery and development (6). 

Table Ib: Disadvantages of salt formation for drug properties. 

The stage of salt selection in drug development

 Pharmaceutical companies previously selected salts at various stages in drug development. However, companies now tend to move the salt-selection process to the research phase to make the process more foolproof (25). Ideally, the salt form should be chosen before long-term toxicology studies are performed (i.e., at the start of Phase I clinical trials) (24). This timing is an important factor in the early stages of new-drug development because changing the salt form at a later stage may force a repetition of toxicological, formulation, and stability studies, thus increasing development time and cost (26). A new salt form introduced at a late stage must also be evaluated for potential impurity changes, and its bioequivalence (bio-bridge), pharmacokinetic equivalence (PK-bridge), and toxicity equivalence (tox-bridge) to the previous salt form must be proven.  

 Innumerable salt forms are available to pharmaceutical scientists. The selection process must therefore be rational and streamlined. A lack of proper planning may lead to the synthesis of several salt forms of the drug candidate for preformulation testing. Moreover, this hit-or-miss approach results in many failures and may cause the loss of test substance and time. These considerations underscore the need for a well-formatted decision tree to help scientists choose a suitable salt form in an efficient and timely manner, depending upon the intended use, with a minimum number of failures and expended resources. 

The main objective of a salt-selection study is to identify the salt form most suitable for development. The following four parameters are often considered primary or essential criteria for the selection of a particular form:  

Aqueous solubility measured at various pH values, depending upon the intended pharmaceutical profile

Low hygroscopicity (i.e., water absorption versus relative humidity), which gives consistent performance

Optimal chemical and solid-state stability under accelerated conditions (i.e., minimal chemical degradation or solid-state changes when stored at 40 °C and 75% relative humidity).

A serious deficiency in any of these characteristics should exclude that form from further development. In addition to these essential criteria, the following desirable criteria also influence salt-form selection:  

Limited number of polymorphs or absence of variability because of polymorphism

Ease of synthesis, handling, or formulation development (27).

A single salt form generally cannot satisfy all the requirements for developing drug-substance dosage forms. However, introducing a second or third salt form consumes additional developmental resources and increases the cost of manufacturing, handling, storing, and characterizing the additional salt forms. Therefore, the dosage form is developed with a single salt form whenever possible (9). The major drug-development issues are addressed by choosing the appropriate salt form. Minor issues can be addressed using other development tools. Decreasing development timelines intensify the pressure to select the right salt form the first time. Salt selection is a critical step in the preformulation stage of drug development. Gould says that "the balance required in assessing the correct salt from to progress into drug development makes it a difficult semiempirical exercise" (8). This statement emphasizes the need to prioritize the salt-selection process so that various development issues are addressed as early as possible. 

 The decision about whether salt or free acid or base should be developed depends on these forms' relative pharmaceutical and commercial merits. If the active compound is a liquid, a solid dosage form is usually preferred because oil is difficult to purify, characterize, and maintain in its effective form. Oil is also difficult to transport, sensitive to oxygen, and susceptible to batch-to-batch variations. If the free acid or base is a water-soluble solid with a high melting point, preparing a salt form is generally unnecessary. Alternatively, several useful properties of salt forms may be explored (6). 

In spite of the numerous advantages associated with salt forms, developing them is not always feasible. The preparation of a stable salt may not be possible for some drugs. The salt may have certain undesirable properties compared with the free acid or base, and it would thus be appropriate to develop the free acid or base (28). In a salt-screening study of RPR111423, a pyridine base, hydrochloride and mesylate salts were formed. The hydrochloride salt showed a loss of hydrogen chloride at high temperatures (110–120 °C) and precipitation at an acidic pH because of the common-ion effect. The mesylate salt also showed precipitation at acidic pH. The two salts were polymorphic and hygroscopic in comparison with free base, which was nonpolymorphic and nonhygroscopic. These results proved the free base to be a better candidate than the salt forms (29). 

Pharmacological indications also help determine whether the salt form or the free acid or base should be pursued. For example, when a slow onset or a constant plasma level is required, a highly ionized salt form may be inappropriate if the free acid or base provides a sufficient plasma level. Tolbutamide sodium, an antihyperglycemic agent, causes a rapid fall in blood glucose levels because it is highly ionized. This characteristic causes hypoglycemia in patients with normal insulinomas. Therefore, tolbutamide sodium's corresponding free acid was preferred for oral administration. The salt form's only application is the diagnosis of pancreatic adenomas (30). 

 The choice of salt is governed largely by the acidity or basicity of the ionizable group, the safety of the counterion, the drug indications, the route of administration, and the intended dosage form. The expectations of the salt form must be outlined as a desirable pharmaceutical profile that guides the synthesis of the salt forms (see Table II). 

Table II: The selection of potential salt forms based on intended pharmaceutical profile. 

 The degree of ionization is a critical parameter for the physiological performance of the drug and for its formulation development (25). The p

 of the drug and counterion is important for successful salt formation as well. For the preparation of salt forms of basic drugs, the p

 of the counterion should be at least 2 pH lower than the p

 of the drug (34). Similarly, for the preparation of salt forms of acidic drugs, the p

 of the counterion should be at least 2 pH higher than the p

 of the drug. These specifications are required because the counterion must bring the solution's pH to a level lower than the pH

 (see Figure 2) to reach the salt plateau, at which the solubility of salt prevails over the solubility of free acid or base (5). The generalized rule of difference in p

 units of the ionizable group in the active pharmaceutical ingredient (API) and in the acid or base is meant to make the proton transfer energetically favorable. When the p

 of the API and counterion are not significantly different, a solid complex may form but may rapidly disproportionate (i.e., break down into the individual entities of drug and counterion) in an aqueous environment (35). 

 Salts can be prepared on a small scale using various methods. Forming salts from free acid or base is the most common method. The free acid or base of the drug substance is combined with the counterion base or acid in specific molar ratios in a suitable solvent system. The salt form is then isolated, and the solid precipitate is recrystallized. A less common method is to form salts through salt exchange. In this method, a counterion salt is treated with a free acid or base in a specific molar concentration in a suitable solvent. The solid is then isolated and recrystallized. The sulfate salt of methyl pyridinium-2-aldoxime was prepared using silver sulfate as a counterion. The unwanted silver ions were removed as insoluble iodide salt, and the desired sulfate salt was precipitated by adding antisolvent (36). A wide range of salts are generally prepared for each new substance. Their properties are compared during a preformulation program that improves the chances of selecting the optimal salt form (29). However, a balanced approach should be adopted because limited resources are available at this early stage of drug development. Commonly used salts such as hydrochlorides and sodium have advantages over other salt-forming moieties. For example, they have low molecular weight and low toxicity. However, other salt forms such as mesylate may sometimes offer advantages such as higher solubility and bioavailability (37).  

High-throughput synthesis has gained greater importance in the salt-selection process. This technique allows many counterions and crystallization solvents to be evaluated using as little as 50 mg of drug substance. After the optimum drug-substance form is selected at the microlevel, the synthesis of the compound can be scaled up to several hundred grams to test for other stages of preformulation (27). 

 salt screening also offers a viable alternative to traditional salt screening (34). This method has a special relevance for poorly soluble compounds because it can rapidly rank compounds based on their solubility, effectively screening out insoluble compounds immediately (13). During 

 salt screening, a known concentration of drug is added to a concentrated counterion solution sufficient to obtain the pH

 (see Figure 2) for successful salt formation that may be subjected to solubility screening. Tong applied 

 salt screening to GW1818, an alpha 1A adrenergic receptor antagonist, and short-listed four salts—the phosphate, succinate, mesylate, and hydrochloride—for further development. These salts were found to be crystalline with adequate solubility (comparable with the authentic salts later prepared by the traditional method) and were selected for further evaluation (34). 

Figure 2: The pH-solubility profile for a compound with a single, basic pKa value of 5. The four regions of pH-dependent solubility are: salt plateau, pHmax, ionized compound, and unionized compound. 

In spite of the abundance of available counterions, few are used frequently. The preference for pharmaceutical counter-ions is explained by studying the distribution of different counterions of medicinal compounds in 

2006 (38) (see Table III). The table shows that salt forms of drugs (56.15%) are preferred over free forms (43.85%). Hydrochloride and sodium remain the favorite counterions for the salt formation of medicinal compounds. However, the availability of many pharmaceutically acceptable counterions makes the salt-selection process difficult and cumbersome. 

 The generated salt forms are compared for the desired physico-chemical and biopharmaceutical properties, which guide the final selection of an optimal salt form. 

Techniques for the characterization of salts.

 After synthesis, the formation of the salt forms must be confirmed. Next, their pharmaceutical properties must be assessed. Several characterization techniques provide valuable information for salt screening (see Table IV). 

Table IV: Techniques for the characterization of salt forms. 

 Morris et al. adopted a multitiered approach to screen salts for their optimal physical forms (39). In this approach, physicochemical tests are conducted in several tiers, and a go–no-go decision is made after each tier. Only appropriate salts, free acids, or bases are tested further, thus avoiding the generation of extensive data about each salt form generated. The studies can be planned so that the least time-consuming experiments that could still prompt a go–no-go decision are conducted in the first tier. Experiments that are more time consuming and labor intensive can be conducted at later tiers. In this way, many salt forms can be screened with a minimum of experimental effort. If the tiered approach eliminates all the candidates, additional salts must be considered before reevaluating any salt rejected in an earlier tier.  

For a rational approach to salt screening, the tiered approach should be combined with a goal-oriented approach in which the main problems associated with the free acid or base are handled first, followed by secondary problems. For example, ranitidine hydrochloride is hygroscopic with a critical relative humidity of approximately 67% (40). However, the hydrochloride salt of ranitidine has better absorption properties compared with the free base and is one of the most successful drugs ever marketed. In a multitiered approach, the hydrochloride salt would have been rejected after hygroscopicity testing, in spite of its better absorption profile (41). High hygroscopicity could be mitigated by developing proper packaging. Similarly, the hydrochloride form of sertaline (i.e., Pfizer's Zoloft) might have been rejected because of its reported 28 polymorphic forms (42). This fact underlines the importance of a goal-oriented approach that addresses the most critical problems first. Less critical problems could be overcome by a proper development strategy. The final salt form selected should have a fine balance of the optimal physicochemical and biopharmaceutical properties. Each stage of salt selection (see Figure 3) is relevant and contributes to the selection of the optimal salt form. However, salt selection can be a difficult task because each salt imparts unique properties to the parent compound. 

Figure 3: Flow diagram for selecting the optimal salt form of a drug. 

 Salt screening starts with the characterization of free acid or base, followed by the identification of possible counterions. The acid or base characterization provides information for potential counterion selection and for planning relevant crystallization experiments. This stage is followed by a screening of crystallization conditions for the desired salts, salt formation and its confirmation, and finally the preformulation characterization of generated salts (20).  

During salt-form selection, the determination of p

 and corresponding ionizable groups gives an idea of the feasibility of salt formation. This information is the basis for selecting suitable counterions and a preliminary synthesis of salt forms, preferably at the microlevel, coupled with characterization for salt formation. After the confirmation of salt formation, the prepared salts are screened for various biopharmaceutical properties with a view to selecting the optimal salt form. 

Assessment of crystallinity is the first stage of salt selection. The salt form should preferably be crystalline so that its properties remain constant during pharmaceutical handling, transportation, and use. However, the amorphous form may have advantages (e.g., solubility) that can be harnessed by proper formulation development. On the other hand, stabilizing the amorphous form for devitrification to crystalline form may lead to the loss of these advantages. Atorvastatin calcium was originally developed in an amorphous form. During Phase III clinical trials, it reverted to crystalline form, and the final product was developed using a crystalline form (43). 

After crystallinity assessment, the salt form's hygroscopicity profile is assessed to find a salt form that retains its properties in the varying humidity conditions of pharmaceutical operations. This assessment can be performed using methods such as traditional saturated salt solutions in a desiccator or more advanced dynamic vapor sorption methods. The salt forms with acceptable hygroscopicity profiles are then evaluated for their solubility. The salts with adequate solubility are tested for their physicochemical stability, including polymorphic stability and excipient compatibility. These tests are especially relevant in combination formulations such as aspirin–propoxyphene. Aspirin–propoxyphene hydrochloride is unstable, but aspirin–propoxyphene napsylate is stable (8). 

Salt forms having adequate stability are assessed for variability in their properties resulting from polymorphism. Compounds with a limited number of polymorphs are preferred because their performance during pharmaceutical operations and performance is predictable. The salt forms that qualify the stage of polymorphism are tested for process control, economic feasibility, and processability (including parameters such as corrosiveness, taste, wettability, and flowability). These criteria are generally evaluated at a small scale by a medicinal chemist, who narrows the choice to a particular salt form. However, after a particular salt form is selected, these parameters are evaluated at a larger scale so that the selected salt form has properties that are easily controlled batchwise and over time. 

The selected salt form is subjected to pharmacological testing for drug release as per the requirements of onset and the duration of activity. Pharmacological safety studies are also performed. For example, epinephrine borate causes occasional mild stinging in the eye, compared with hydrochloride and bitartrate salt, which cause moderate to severe stinging (44). The selected salt form may then be subjected to extensive long-term toxicology studies in Phase I clinical trials of drug development. 

Salt-selection studies provide a viable extension of a drug's patent because salts with superior properties can be patent-protected. New salt forms often have novel physical properties related to processability (e.g., crystallization, morphology, and filtration) and formulation (e.g., stability) (45). They may also result in the detection of new polymorphs (1). 

A new salt form may have a profile that makes it suitable for a new route of administration. For example, diclofenac sodium salt (Ciba-Geigy) was marketed as Voltaren. Before the Voltaren patent expired, other salts (e.g., diclofenac diethylamine) with substantially better skin-penetration properties were discovered and patented. These salts, in corresponding formulations, were particularly suitable for topical applications. Patenting new salts, therefore secures an exclusive position in the market (20). 

Selecting an appropriate salt form of an API may also play a role in blocking the development of generic drug products. Dr. Reddy's Laboratories tried to obtain marketing authorization for amlodipine maleate, a different salt version of amlodipine besylate. However, the US Court of Appeals for the Federal Circuit concluded that the basic patent for amlodipine covers other salt forms of the drug, including its maleate salt. The verdict against Dr. Reddy's Laboratories effectively prevented the generic version from entering the market (19). 

Salt-selection studies should consider the regulatory aspects of introducing a new salt form. A new salt form of an approved drug substance is considered a new chemical entity, thus requiring a full dossier to be submitted for marketing approval (1). For regulatory purposes, a new salt form is designated a "pharmaceutical alternative" to the original form (46, 47). However, the approval process for a new salt may use some of the details already known about the active entity of a related, previously accepted salt (1). Therefore, when scientists change the active moiety of a salt that is already marketed, they may usually submit an abbreviated application, popularly known as the 505 b(2) filings (or the hybrid NDA), if they can prove that the new salt form's active moiety has the same pharmacokinetics, pharmacodynamic, and toxicity characteristics as the original (19). A generic version of a drug based on an alternative salt form may also be approved in a similar way. However, the benefits expected from the introduction of a new salt form must be weighed against the cost and time involved in the studies required for regulatory approval. 

The selection of an appropriate salt form for a potential drug candidate is an opportunity to modulate its characteristics to improve bioavailability, stability, manufacturability, and patient compliance.