Practical Aspects of Solubility Determination and Considerations for Enabling Formulation Technologies

Outline

Aqueous solubility is an important aspect of pharmaceutical drug development. Very often structure-activity relationships of small molecule pharmaceuticals are tuned to maximize aqueous solubility to facilitate in vivo delivery of the drug substance. Solubility is also important in the synthesis, crystallization and purification of the drug substance. Aqueous solubility consideration is paramount to understanding results from in vitro biological testing of efficacy and planning for formulation approaches to conduct pharmacokinetic and toxicological screening. Lastly, but often not highlighted, is the fact that aqueous solubility is vital to understanding the long-term impact of pharmaceuticals in our environment. The content of this article was presented as a pre-conference workshop at the IQPC 7th Annual Improving Solubility Summit.

Simply stated, aqueous solubility is the equilibrium partitioning of a compound between its pure phase and water. At equilibrium, there is a dynamic, balanced process of dissolution of the solid into solution and crystallization of the drug from solution. Aqueous solubility (Sw) can be empirically correlated to activity and crystal lattice energy via the general solubility equation (GSE) proposed by Yalkowsky [1,2] where, Log S_w = -0.01 (MP-25)-log K_ow+0.5, with MP= melting point and K_ow designated as the octanol-water partition coefficient. There are also multiple computational tools for prediction of aqueous solubility including ACD Labs, COSMO-RS, QikPro and others [3]. While the accuracy of these models is often limited by the accuracy in the prediction of the lattice energy of small molecule pharmaceuticals [4], it is now often possible to “train” some systems with compounds of a particular structural series to enhance the accuracy of the models. With more accuracy, the models will have more of an impact in guiding structural modifications to optimize aqueous solubility.

During the stages of drug development, from high-throughput screening through lead development to candidate selection [5], solubility determination methodologies are mainly differentiated by the physical state of the starting material and equilibration time. Early automated high-throughput solubility determination methods are generally used to guide medicinal chemistry efforts during lead optimization and provide a rank order for compounds in libraries [6].

These early methods are typically kinetic methods, where solubility is evaluated using concentrated stock solutions of the drug substances diluted into standard aqueous vehicles, filtered and read via platebased ultraviolet detection, nephelomtery (light scattering) methods [7] or other methods [8]. As development progresses and the number of viable compounds decreases, more definitive, lower throughput methods of aqueous solubility are employed [9, 10]. These assays often begin with a more well-defined solid form of the drug substance, employ longer equilibration times, have more accurate analytical processes and fully characterize the resulting solid form. At this stage, characterization of the form (initial and final) during solubility determination should be a focus [11].

The impact of the solid form on aqueous solubility is paramount. It is known that the relative difference in aqueous solubility between crystalline and amorphous forms of the same drug substance is often 10-fold or higher [12, 13]. Differences between two distinct crystalline polymorphs of the same drug substance typically range only circa two-fold [14]. The expected difference in aqueous solubility between hydrated and non-hydrated forms of small molecule drug substances range from two to ten- fold; the hydrated forms being less soluble [15]. These designations all serve to emphasize the importance of rigorous solid form characterization before and after aqueous solubility determinations.

It is important to fully understand the solubility data that is being used to drive development of small molecules. This includes understanding the design details of each screen/solubility evaluation and a critical evaluation of the performance of the drug substance in that screen, i.e. does the data make sense based on what is known or predicted about the molecule? Consideration of the buffer systems used for solubility determination is important as ‘salting out’ or the formation of atypical soluble or insoluble complexes is always possible [16]. At later stages of drug substance solubility evaluation, the purity of the material under evaluation should be charted and monitored with an eye towards understanding that impurities may delay or reduce precipitation (and enhance supersaturation) and increase apparent solubility by reducing drug substance crystallinity. Similarly, de-solvation in aqueous media may yield a high-energy lattice (a “desolvated solvate”) which yields a high apparent solubility. At this later screening stage, the characterization of impurities and an understanding of chemical degradation during solubility determination should be in focus.

It is recommended that some effort be made to understand or characterize the species in solution during aqueous solubility determination, to more fully understand the system at equilibrium and the factors that affect it. For example, self-association behavior of surfactant-like molecules can lead to unusually high observed solubility, resulting from facile super (over) saturation of the aqueous test systems [17]. Unexpected ion-pair formation can lead to anomalous pHsolubility behavior, with small organic acids such as methanesulfonic, acetic and lactic acids being common ion-pair formers.

When structural modifications are not adequate to achieve necessary aqueous solubility of a drug substance, formulation approaches are often employed. There are many documented formulation decision trees published that can guide formulation efforts [18, 19]. These guidances cover several standard approaches to increase aqueous solubility in dosing formulations, such as pH manipulation for ionizable drug candidates, addition of co-solvents, cyclodextrin complexation, emulsion solublization, employment of solid dispersion technologies, formation of prodrugs and application of nanotechnology. Several of these approaches are highlighted for discussion below.

Manipulation of the pH, so that an ionizable drug substance is more soluble in aqueous media, and co-solvency are classical approaches to increase solubility. It is generally recognized that optimal pHassisted formulation of an ionizable drug substance at a pH that is ≥2 pH units away from the compound’s pKa (≥2 pH units above for acids and ≥2 pH units below for bases) is the optimal range for maximal solubility increase. Different couterions may result in different solubilization of the drug substance due to differences in the solubilities of the salts formed, so that some screening may be required to identify the optimal counterion. Further, some counterions are designed to achieve high solubility with limited precipitation of the salt (i.e. mesylate); a tendency which may be leveraged for weakly basic (or acidic) compounds [20] . Addition of co-solvents to aqueous systems is known to boost solubility and the solublizing effects can be predicted to some extent based on the work of Yalkowsky [21]. These water-miscible co-solvents include polyethylene glycols (PEGs), propylene glycol and strong organic cosolvents such as dimethysulfoxide and pyrrolidone.

Cyclodextrin solubilization via complexation is also a common approach for enabling insoluble drug candidates in drug development [22, 23]. The most common pharmaceutical cyclodextrins utilized are the substituted beta cyclodextrins (sulfabutyl ether and hydroxypropyl); however, gamma and substituted gamma cyclodextrins (sulfabutyl ether and hydroxypropyl substitution) are becoming more widely used throughout the pharmaceutical industry [24] as the size and complexity of smallmolecule pharmaceuticals continue to increase. While it is possible to estimate the extent of cyclodextrin complexation for a given small molecule drug candidate using predictive models based on calculated molecular parameters, it may be beneficial to screen the substituted cyclodextrin systems (beta and/or gamma) to determine which systems provide maximal solubility enhancement, as small factors such as charge, molecule shape and volume, and molecular flexibility can favor the use of certain substituted cyclodextrins over others [25].

One of the optimal and most definitive ways to increase aqueous solubility is to produce a prodrug of an insoluble small molecule. In a recent review of the physciochemical properties of marketed prodrugs [26], it was shown that preparation of a prodrug of an insoluble drug increases its aqueous solubility (on average) by two-three orders of magnitude. The most common type of prodrug produced to increase solubility involves the introduction of a phosphate group on the molecular scaffold, such as in the cases of Prednisolone phosphate (Orapred® ODT), hydrocortisone phosphate (Efcortesol®), clinadamycin phosphate (Dalacin®) and phosphazide (Nikavir®). Phosphate prodrugs are released to the active drug through the action of ubiquitous phosphates present in large number and distribution within the body. This approach does require the availability of a derivatizable functional moiety on the molecule, preferably a free hydroxyl group, which can be introduced into the molecular scaffold during the design stage of drug development.

The majority of the remaining solubility enabled formulation approaches are adequately reviewed elsewhere [27, 28]. Currently, the use of attrition milling to achieve nano-sized crystalline particles has not proved to provide any significant increases in aqueous solubility [29] but does provide significant dissolution enhancement, which has proven beneficial to enhance systemic exposure for high melting, highly crystalline drug substances. Various additional nanotechnologies are being developed to enable solubility enhancement of poorly soluble drug substances [30] and significant progress is being made in the delivery of nanotechnology-based marketed pharmaceutical products [31].

One of the new challenges in solubility screening involves small peptides, as these molecules appear to be making significant inroads in the drug discovery and development process [32] . Their promise of pinpoint selectivity and reduced off-target toxicity make them attractive new molecular targets. However, their more complex structure and unique molecular and physical properties will present unique challenges for solubility characterization and enhancement in the near future.

In summary, the importance of solubility in the drug development process cannot be overstated. During the Discovery stage, a general knowledge of solubility behavior is needed to understand the performance of drug candidates in in vitro assays and to guide synthetic efforts towards ‘drug-able’ compounds. Later, a more robust understanding becomes critical to guide formulation efforts, and interpret and optimize preclinical and clinical bio-performance. Solubility methodologies suited to these paradigms range from high throughput kinetic assays to low throughput equilibrium assays. Critical to both, however, is the need to thoroughly understand the methods, the nature of the materials going into the assays, and the behavior of the compounds during the assays in order to understand the data that is generated.

Author Biographies

Marc Tesconi, Ph.D., is an Associate Research Fellow and Group Leader in Pharmaceutical Sciences at Pfizer, Marc has thirteen years of experience in the pharmaceutical industry and expertise in solid form selection, discovery support, early clinical formulation development and project management. Research interests have focused on enabling technologies for oral and sterile injectable dosage forms.

Margaret Landis, Ph.D., is currently a Senior Principal Scientist at Pfizer Inc (Groton, CT). She joined Pfizer in 1998, after receiving her Ph.D. in organic photochemistry. She performs pre-formulation analysis and formulation development of Discovery, Toxicology and early Clinical dosage forms. Her interests lie in the areas of pharmaceutics of small molecules and small peptides and chemical reactivity.

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