Maintenance of supersaturation I: indomethacin crystal growth kinetic modeling using an online second-derivative ultraviolet spectroscopic method

Effect of Structurally Related Compounds on Desupersaturation Kinetics of Indomethacin

PURPOSE

The pharmaceutical literature contains examples wherein desupersaturation from high concentrations does not proceed to equilibrium concentration of the thermodynamically most stable form but remains above equilibrium. The purpose of the current research was to investigate the effect of structurally related compounds on desupersaturation kinetics as a possible explanation for a higher than equilibrium solubility after crystal growth of γ-indomethacin (γ-IMC).

METHODS

Three structurally related compounds (SRC) - cis-sulindac (c-SUL), trans-sulindac (t-SUL) and indomethacin-related compound-A (IMC-A) -were investigated. Desupersaturation kinetics to the most stable γ-IMC, in the presence of c-SUL, t-SUL or IMC-A, was measured at pH 2.0.

RESULTS

The SRCs c-SUL and t-SUL were effective crystallization inhibitors of IMC, while IMC-A was not a potent crystallization inhibitor of IMC. Among the sulindac isomers, t-SUL was a stronger crystallization inhibitor. The apparent solubility of γ-IMC crystals grown from supersaturated solutions in the presence of SRCs matched the equilibrium solubility of γ-IMC. During crystallization of IMC in the presence of IMC-A, the concentration of IMC-A declined initially but rebounded as supersaturation and crystallization rate of IMC declined, suggesting that IMC-A itself became incorporated in the IMC crystal lattice at higher degrees of IMC supersaturation.

CONCLUSIONS

The results suggest that high apparent solubility after crystallization of IMC reported by several authors is not related to the presence of IMC-A impurity. The greater IMC crystal growth rate inhibition by t-SUL than by c-SUL was consistent with the proposed orientation of SUL molecules adsorbed on the IMC crystal, providing a mechanistic understanding of the inhibition.

Mechanisms for the Slowing of Desupersaturation of a Weak Acid at Elevated pH

Supersaturating drug delivery systems are used to achieve higher oral bioavailability for poorly soluble drugs. However, supersaturated solutions often decline to lower concentrations by precipitation and crystallization. The purpose of the current research is to provide a mechanistic understanding of drug crystallization as a function of pH, using indomethacin (IMC, pK_a 4.18) as a model compound. Desupersaturation kinetics to the γ-form of IMC was measured at pH 2.0, 3.0, 4.0, and 4.5 from an initial degree of supersaturation of 2.5-6. At equivalent levels of supersaturation, crystal growth rates decreased with an increase in solution pH. Two mechanisms for this phenomenon, reactive diffusion (resulting in a higher surface pH as compared to bulk pH) and inhibition of crystallization by structurally similar ionized IMC at higher pH, were explored. Non-steady-state models for reactive diffusion showed that the surface pH was only 0.01 units above that of the bulk solution pH. Mass transport models for reactive diffusion during crystallization could not explain the decrease in desupersaturation kinetics at higher pH. However, zeta potentials as high as -70 mV suggested that IMC^- is adsorbed on the surface of the particles. A mathematical model for inhibition of crystal growth by IMC^- accounted for the pH effect suggesting that ionized IMC acts as an effective crystallization inhibitor of IMC.

Classification of the crystallization tendency of active pharmaceutical ingredients (APIs) and nutraceuticals based on their nucleation and crystal growth behaviour in solution state

Supersaturated drug delivery systems are commonly used to address the problems of poor aqueous solubility posed by most of the active pharmaceutical ingredients (APIs). However, the supersaturated systems are highly unstable due to their high free energy levels and demonstrate a tendency to precipitate. Understanding the crystallization tendency based on the mechanisms of crystallization, that is nucleation and crystal growth, is imperative to design formulation strategies and select appropriate precipitation inhibitors. This study aims to provide a classification system, based on both the nucleation and crystal growth tendency in the solution state of 60 APIs and nutraceuticals (in absence of polymer) from their desupersaturation profiles monitored by UV-Visible spectroscopy. The APIs and nutraceuticals are divided into four classes based on their induction time (t_ind) and crystal growth rate as fast nucleators-fast crystal growth (class I), fast nucleators-slow crystal growth (class II), slow nucleators-fast crystal growth (class III) and slow nucleators-slow crystal growth (class IV). Most of the molecules fall in the class I and class IV. An easy-to-use protocol for nucleation and crystal growth studies has been optimized. This protocol will find application to assess the crystallization tendency of the molecules in the preliminary screening stages, enabling appropriate formulation strategies to inhibit crystallization.

The Opposed Effects of Polyvinylpyrrolidone K30 on Dissolution and Precipitation for Indomethacin Supersaturating Drug Delivery Systems

Amorphous solid dispersions (ASD) are one of the most important supersaturating drug delivery systems (SDDS) for poorly water-soluble drugs to improve their bioavailability. As a result of thermodynamic instability, drug molecules tend to precipitate during storage and dissolution in gastrointestinal tract. Various precipitation inhibitors (PI) have been widely used to improve the stability in the past decade. However, most studies have investigated the inhibiting capability of PI on drug precipitation, rarely considering their potential hindering effect on the drug dissolution. The present study designed an ASD of Indomethacin (IND) and Eudragit® EPO by hot melt extrusion to investigate the influence of the added PI (PVP-K30) into ASD both on dissolution and precipitation. The precipitation study by solvent shift method indicated PVP-K30 could inhibit the precipitation of IND significantly. The dissolution study in different concentrations of PVP-K30 showed when the concentration increased above 50 μg/mL, PVP-K30 displayed an acceptable precipitation inhibition without drug concentration decline but an unexpected dissolution impediment with the reduction of maximum concentration platform. The dissolution tests of physical mixtures (PMs) of ASD and PVP-K30 also showed the precipitation inhibition and dissolution impediment when more than 2% PVP-K30 in PMs. This opposed effect of PVP-K30 was strengthen in ternary systems prepared by hot melt extruding the mixtures of IND, Eudragit® EPO and PVP-K30. All of these results proved the PI may be a double-edged sword for the opposed effects of precipitation inhibition and dissolution impediment, which should be carefully considered in the design and development of SDDS.

Cocrystal Solubility Advantage Diagrams as a Means to Control Dissolution, Supersaturation, and Precipitation

Cocrystals are often more soluble than needed and pose unnecessary risks for precipitation of less soluble forms of the drug during processing and dissolution. Such conversions lead to erratic cocrystal behavior and nullify the cocrystal solubility advantage over parent drug (SA = Scocrystal/Sdrug). This work demonstrates a quantitative method for additive selection to control cocrystal disproportionation based on cocrystal solubility advantage (SA) diagrams. The tunability of cocrystal SA is dependent on the selective drug-solubilizing power of surfactants (SPdrug = (ST/Saq)drug). This cocrystal property is used to generate SA-SP diagrams that facilitate surfactant selection and provide a framework for evaluating how SA influences drug concentration-time profiles associated with cocrystal dissolution, drug supersaturation, and precipitation (DSP). Experimental results with indomethacin-saccharin cocrystal and surfactants (sodium lauryl sulfate, Brij, and Myrj) demonstrate the log-linear relationship characteristic of SA-SP diagrams and the dependence of σmax and dissolution area under the curve (AUC) on SA with characteristic maxima at a threshold supersaturation where drug nucleation occurs. This approach is expected to streamline cocrystal formulation as it facilitates additive selection by considering the interplay between thermodynamic (SA) and kinetic (DSP) processes.

Impacts of Polymeric Additives on Nucleation and Crystal Growth of Indomethacin from Supersaturated Solutions

Three polymers, polyvinylpyrrolidone (PVP K30), hydroxypropyl methyl cellulose (HPMC E5), and Kollidone VA64 (PVP-VA64), have been assessed for their impact on the nucleation and crystal growth of indomethacin (IND) from supersaturation solutions. PVP was the most effective inhibitor on IND nucleation among three polymers, but the effect of three polymers on inhibiting nucleation is quite limited when the degree of supersaturation S is higher than about 9. Analysis of the nucleation data by classical nucleation theory model generally afforded good data fitting with the model and showed that addition of polymers may affect the crystal/solution interfacial free energy γ and also the pre-exponential kinetic factor. PVP-VA showed better inhibitory effects on crystal growth of IND when the polymer concentration is high (0.1%, w/w) as reflected by the crystal growth inhibition factor R, and PVP exhibited relatively stronger effects on inhibiting crystal growth at low polymer concentrations (0.005%, w/w). The crystal growth inhibitory effect of polymers should be attributable to the retardation of the surface integration of the drug, and such effect should also be polymer and drug dependent. The enhancement of supersaturation level of IND should be attributable to both nucleation and crystal growth inhibition by polymers. The nucleation and crystal growth rate of α-polymorph IND is higher than that of γ-polymorph, and α-polymorph is the predominant form appeared in supersaturated solutions. A rational selection of the appropriate polymer for specific drug is critical for developing supersaturated drug delivery formulations.

Cellulose based polymers in development of amorphous solid dispersions

Cellulose derivatives have gained immense popularity as stabilizers for amorphous solid dispersion owing to their diverse physicochemical properties. More than 20 amorphous solid dispersion-based products that have been approved for marketing consist of cellulose derivatives as stabilizers, thus highlighting their importance in generation of amorphous solid dispersions. These polymers offer numerous advantages like drug solubilization, crystallization inhibition and improvement in release patterns of drugs. Exploring their potential and exploiting their chemistry and pH responsive behaviour have led to the synthesis of new derivatives that has broadened the scope of the use of cellulose derivatives in amorphous formulation development. The present review aims to provide an overview of different mechanisms by which these cellulose derivatives inhibit the crystallization of drugs in the solid state and from supersaturated solution. A summary of different categories of cellulose derivatives along with the newly explored polymers has been provided. A special segment on strengths, weaknesses, opportunities, and threats (SWOT) analysis and critical quality attributes (CQAs) which affect the performance of the cellulose based amorphous solid dispersion will aid the researchers in identifying the major challenges in the development of cellulose based solid dispersion and serve as a guide for further formulation development.

Approaches to increase mechanistic understanding and aid in the selection of precipitation inhibitors for supersaturating formulations – a PEARRL review

Objectives

Supersaturating formulations hold great promise for delivery of poorly soluble active pharmaceutical ingredients (APIs). To profit from supersaturating formulations, precipitation is hindered with precipitation inhibitors (PIs), maintaining drug concentrations for as long as possible. This review provides a brief overview of supersaturation and precipitation, focusing on precipitation inhibition. Trial-and-error PI selection will be examined alongside established PI screening techniques. Primarily, however, this review will focus on recent advances that utilise advanced analytical techniques to increase mechanistic understanding of PI action and systematic PI selection.

Key findings

Advances in mechanistic understanding have been made possible by the use of analytical tools such as spectroscopy, microscopy and mathematical and molecular modelling, which have been reviewed herein. Using these techniques, PI selection can be guided by molecular rationale. However, more work is required to see widespread application of such an approach for PI selection.

Summary

Precipitation inhibitors are becoming increasingly important in enabling formulations. Trial-and-error approaches have seen success thus far. However, it is essential to learn more about the mode of action of PIs if the most optimal formulations are to be realised. Robust analytical tools, and the knowledge of where and how they can be applied, will be essential in this endeavour.

Supersaturable solid self-microemulsifying drug delivery system: precipitation inhibition and bioavailability enhancement

Solid self-emulsifying drug delivery system (SSEDDS), which incorporates liquid SEDDS into a solid dosage form, has been recently introduced to improve the oral bioavail-ability of poorly water-soluble drugs. However, supersaturated drug generated by SSEDDS is thermodynamically unstable and tends to precipitate rapidly prior to absorption, resulting in compromised bioavailability. The aim of this study was to construct a novel supersaturated SSEDDS (super-SSEDDS) by combining SSEDDS with appropriate precipitation inhibitor. Fenofibrate (FNB), a sparingly soluble drug, was selected as a model drug in this study. An optimized SSEDDS was prepared by solvent evaporation by using mesoporous silica Santa Barbara Amorphous-15 as the inert carrier. Supersaturation assay was conducted to evaluate the precipitation inhibition capacity of different polymers, and the results showed that Soluplus^® could retard the FNB precipitation more effectively and sustain a higher apparent concentration for ~120 min. This effect was also clearly observed in the dissolution profiles of FNB from SSEDDS under supersaturated condition. The study of the mechanism suggested that the inhibition effect might be achieved both thermodynamically and kinetically. The area under the concentration–time curve of the super-SSEDDS was 1.4-fold greater than that of SSEDDS in the absence of Soluplus, based on an in vivo pharmacokinetic study conducted in beagle dogs. This study has demonstrated that the approach of combining SSEDDS with Soluplus as a supersaturation stabilizer constitutes a potential tool to improve the absorption of poorly water-soluble drugs.

Maintenance of supersaturation II: indomethacin crystal growth kinetics versus degree of supersaturation

This study compares the kinetics of crystal growth of indomethacin from supersaturated suspensions at varying degrees of supersaturation (2 ≤ S ≥ 9) in the presence of seed crystals of the γ-form of indomethacin, the lowest energy polymorph. At high S (6 ≤ S ≥ 9), the crystal growth was first order with rate coefficients (kG ) that were nearly constant and consistent with the value predicted for bulk-diffusion control. At lower S (<6), kG values were significantly smaller, decreasing approximately linearly with a decrease in S. The decline in kG at low S was attributed to a prolonged period during the initial stages of crystal growth in which surface integration was rate limiting. The apparent solubility of indomethacin after crystal growth for 3 days increased by ∼1.6-fold at both low (S = 2) and high (S = 6) degrees of supersaturation suggesting that a higher energy surface layer was deposited on the γ-form seed crystals during crystal growth. When growth experiments were repeated at low S in the presence of indomethacin seed crystals isolated from a previous crystal growth experiment (i.e., seed crystals having higher energy surface), kG matched the higher values observed for bulk diffusion-controlled crystal growth. Crystal growth experiments were also conducted at S < 1.6 using a constant infusion of an indomethacin solution in the presence of γ-form seed crystals to obtain kG under conditions where deposition of a higher energy surface could not occur. At these conditions, the smaller value of kG indicative of surface integration control was again observed and the apparent solubility of indomethacin after crystal growth matched that of the γ-form. A quantitative mechanistic understanding of the crystal growth kinetics of indomethacin derived from experiments at high and low S may be useful in future studies aimed at understanding the inhibitory effects of pharmaceutical excipients on the crystal growth of poorly soluble compounds and their utility in maintaining drug supersaturation during oral absorption.

Molecular dynamics simulation of amorphous indomethacin

Molecular dynamics (MD) simulations have been conducted using an assembly consisting of 105 indomethacin (IMC) molecules and 12 water molecules to investigate the underlying dynamic (e.g., rotational and translational diffusivities and conformation relaxation rates) and structural properties (e.g., conformation, hydrogen-bonding distributions, and interactions of water with IMC) of amorphous IMC. These properties may be important in predicting physical stability of this metastable material. The IMC model was constructed using X-ray diffraction data with the force-field parameters mostly assigned by analogy with similar groups in Amber-ff03 and atomic charges calculated with the B3LYP/ccpVTZ30, IEFPCM, and RESP models. The assemblies were initially equilibrated in their molten state and cooled through the glass transition temperature to form amorphous solids. Constant temperature dynamic runs were then carried out above and below the T(g) (i.e., at 600 K (10 ns), 400 K (350 ns), and 298 K (240 ns)). The density (1.312 ± 0.003 g/cm(3)) of the simulated amorphous solid at 298 K was close to the experimental value (1.32 g/cm(3)) while the estimated T(g) (384 K) was ~64 degrees higher than the experimental value (320 K) due to the faster cooling rate. Due to the hindered rotation of its amide bond, IMC can exist in different diastereomeric states. Different IMC conformations were sufficiently sampled in the IMC melt or vapor, but transitions occurred rarely in the glass. The hydrogen-bonding patterns in amorphous IMC are more complex in the amorphous state than in the crystalline polymorphs. Carboxylic dimers that are dominant in α- and γ-crystals were found to occur at a much lower probability in the simulated IMC glasses while hydrogen-bonded IMC chains were more easily identified patterns in the simulated amorphous solids. To determine molecular diffusivity, a novel analytical method is proposed to deal with the non-Einsteinian behavior, in which the temporal evolution of the apparent diffusivity D(t) is described by a relaxation model such as the KWW function and extrapolated to infinite time. The diffusion coefficient found for water diffusing in amorphous indomethacin at 298 K (2.7 × 10(-9) cm(2)/s) compares favorably to results obtained in experimental IMC glasses (0.9-2.0 × 10(-9) cm(2)/s) and is mechanistically associated with β-relaxation processes that are dominant in sub-T(g) glasses.

Characterization of supersaturatable formulations for improved absorption of poorly soluble drugs

With the increasing number of poorly water-soluble compounds in contemporary drug discovery pipelines, the concept of supersaturation as an effective formulation approach for enhancing bioavailability is gaining momentum. This is intended to design the formulation to yield significantly high intraluminal concentrations of the drug than the thermodynamic equilibrium solubility through achieving supersaturation and thus to enhance the intestinal absorption. The major challenges faced by scientists developing supersaturatable formulations include controlling the rate and degree of supersaturation with the application of polymeric precipitation inhibitor and maintenance of post-administration supersaturation. This review is intended to cover publications on this topic since April 2009. Scientific publications associated with characterization of supersaturatable systems and related preclinical and clinical pharmacokinetics (PK) studies are reviewed. Specifically, this review will address issues related to assessing the performance of supersaturatable systems including: (1) Diversified approaches for developing supersaturatable formulations, (2) meaningful in vitro test methods to evaluate supersaturatable formulations, and (3) in vivo PK study cases which have demonstrated direct relevance between the supersaturation state and the exposure observed in animal models and human subjects.