Correlation Between Molecular Mobility and Crystal Growth of Amorphous Phenobarbital and Phenobarbital with Polyvinylpyrrolidone and L‐proline

Amorphous solid dispersions: Stability mechanism, design strategy and key production technique of hot melt extrusion

Solid dispersion (SD) system has been used as an effective formulation strategy to increase in vitro and in vivo performances of poorly water-soluble drugs, such as solubility/dissolution, stability and bioavailability. This review provides a comprehensive SD classification and identifies the most popular amorphous solid dispersions (ASDs). Meanwhile, this review further puts forward the systematic design strategy of satisfactory ASDs in terms of drug properties, carrier selection, preparation methods and stabilization mechanisms. In addition, hot melt extrusion (HME) as the continuous manufacturing technique is described including the principle and structure of HME instrument, key process parameters and production application, in order to guide the scale-up of ASDs and develop more ASD products to the market in pharmaceutical industry.

Amorphous Solid Dispersion as Drug Delivery Vehicles in Cancer

Cancer treatment has improved over the past decades, but a major challenge lies in drug formulation, specifically for oral administration. Most anticancer drugs have poor water solubility which can affect their bioavailability. This causes suboptimal pharmacokinetic performance, resulting in limited efficacy and safety when administered orally. As a result, it is essential to develop a strategy to modify the solubility of anticancer drugs in oral formulations to improve their efficacy and safety. A promising approach that can be implemented is amorphous solid dispersion (ASD) which can enhance the aqueous solubility and bioavailability of poorly water-soluble drugs. The addition of a polymer can cause stability in the formulations and maintain a high supersaturation in bulk medium. Therefore, this study aimed to summarize and elucidate the mechanisms and impact of an amorphous solid dispersion system on cancer therapy. To gather relevant information, a comprehensive search was conducted using keywords such as "anticancer drug" and "amorphous solid dispersion" in the PubMed, Scopus, and Google Scholar databases. The review provides an overview and discussion of the issues related to the ASD system used to improve the bioavailability of anticancer drugs based on molecular pharmaceutics. A thorough understanding of anticancer drugs in this system at a molecular level is imperative for the rational design of the products.

Crystallization Propensity of Amorphous Pharmaceuticals: Kinetics and Thermodynamics

Four model compounds, nifedipine, indomethacin, felodipine, and ketoconazole, all with nearly identical glass transition temperatures, were chosen to study the effects of thermodynamics and molecular mobility on their crystallization propensities. The time and temperature dependence of the crystallization induction time of each compound was determined by differential scanning calorimetry (DSC) and enabled the generation of their time-temperature-transformation (TTT) diagrams. The relaxation times (τ_α) were measured by dielectric spectroscopy, and the Gibbs free energy (ΔG) and entropy (ΔS) difference between the crystalline and amorphous states were obtained by DSC. The temperature dependence of the crystallization induction time (τ₀(T)) is a function of the thermodynamic activation barrier and the frequency of "attempted jumps" (1/τ_α(T)) to overcome the barrier. Even though the four model compounds exhibited very similar molecular mobility (relaxation time) over a wide range of temperatures, their crystallization propensities were very different. The observed difference in crystallization propensity was explained in terms of the difference in the thermodynamic barrier, and it is correlated to the empirical relation (TΔS³)/ΔG².

Amorphous solid dispersions: An update for preparation, characterization, mechanism on bioavailability, stability, regulatory considerations and marketed products

Amorphous solid dispersions (ASDs) are being employed frequently to improve bioavailability of poorly soluble molecules by enhancing the rate and extant of dissolution in drug product development process. These systems comprise of an amorphous active pharmaceutical ingredient stabilized by a polymer matrix to provide enhanced stability. This review discussed the methodologies of preparation and characterization of ASDs with an emphasis on understanding and predicting stability. Rational selection of polymers, preparation techniques with its advantages and disadvantages and characterization of polymeric amorphous solid dispersions have discussed. Stability aspects have been described as per ICH guidelines which intend to depend on selection of polymers and preparation methods of ASD. The mechanism involved on improvement of bioavailability also considered. Regulatory importance of ASD and current evolving details of QBD approach were reviewed. Amorphous products and particularly ASDs are currently most emerging area in the pharmaceutical field. This strategic approach presents huge impact and advantageous features concerning the overall improvement of drug product performance in clinical settings which ultimately lead to drug product approval by leading regulatory agencies into the market.

Influence of the Internal Structure and Intermolecular Interactions on the Correlation between Structural (α) and Secondary (β-JG) Relaxation below the Glass Transition Temperature in Neat Probucol and Its Binary Mixtures with Modified Saccharides

Broadband dielectric spectroscopy (BDS) has been used to study the molecular dynamics and aging process in neat probucol (PRO) as well as its binary mixtures with selected acetylated saccharides. In particular, we applied the Casalini and Roland approach to determine structural relaxation times in the glassy state of the examined systems (so-called isostructural times, τ_iso). Next, using the calculated τ_iso, primitive relaxation times of the coupling model were obtained and compared to the experimental secondary β (Johari-Goldstein (JG) type) relaxation times. Interestingly, it turned out that there is a correlation between the β-JG and the structural (α)-relaxation processes below the glass transition temperature (T < T_g) in each investigated sample. This is a new observation compared to previous studies demonstrating that such a relationship exists only in the supercooled liquid state of neat PRO. Moreover, it was revealed that the stretching parameters obtained from the aging procedure are very close to the ones determined by fitting the dielectric data above the T_g with the use of the Kohlrausch-Williams-Watts function, indicating that the aging process is governed by the α-relaxation. Complementary Fourier transform infrared and X-ray diffraction measurements allowed us to find a possible reason for these findings. It was demonstrated that although there are very weak intermolecular interactions between PRO and modified saccharides, the intra- and intermolecular structure of PRO is practically unaffected by the presence of modified saccharides.

Investigation of physical properties and stability of indomethacin–cimetidine and naproxen–cimetidine co-amorphous systems prepared by quench cooling, coprecipitation and ball milling

Objectives

The objective was to characterize the structural behaviour of indomethacin–cimetidine and naproxen–cimetidine co-amorphous systems (1 : 1 molar ratio) prepared by quench cooling, co-evaporation and ball milling.

Methods

Powder X-ray diffraction (PXRD) and DSC were used to characterise the samples. Structural relaxation (i.e. molecular mobility) behaviour was obtained from the Kohlrausch–Williams–Watts (KWW) relationship.

Key findings

A glass transition temperature (Tg), on average 20 °C higher than the predicted Tg (calculated from the Fox equation), was observed in all samples. The structural relaxation was dependent on the preparative methods. At a storage temperature of 40 °C, a comparatively higher molecular mobility was observed in indomethacin–cimetidine samples prepared by ball milling (ln τβ = 0.8), while similar molecular mobility was found for the same sample prepared by quench cooling (ln τβ = 2.4) and co-evaporation (ln τβ = 2.5). In contrast, molecular mobility of the naproxen–cimetidine samples followed the order co-evaporation (ln τβ = 0.8), quench cooling (ln τβ = 1.6) and ball milling (ln τβ = 1.8).

Conclusion

The estimated relaxation times by the DSC-KWW method suggest that different preparative methods resulted in a variation of structural characteristics. Despite the differences in molecular mobility, all sample remained co-amorphous for up to 7 months.

Mechanically induced amorphization of drugs: a study of the thermal behavior of cryomilled compounds

The purpose of this work was to determine what aspect of the milled compound influences its thermal profile. For this, six different compounds with different properties were chosen and cryomilled for different times to get an amorphous solid. Differential scanning calorimetry (DSC) and X-ray powder diffraction were used to characterize the material and look at the thermal behavior. Melt-quenched samples were also prepared, and the thermal profile upon milling was determined and correlated with the thermal behavior of the cryomilled samples. Growth rates were determined by hot-stage microscopy. Ketoconazole, when cryomilled, showed only one crystallization exotherm in the DSC profile. Ursodiol, and to some extent indomethacin, initially showed a double exotherm which eventually become a single exotherm on further milling. Griseofulvin, carbamazepine, and piroxicam exhibited a double exotherm in the DSC profile upon cryomilling to the amorphous state. Surface crystal growth rates around T (g) were found to be highest for compounds showing the double exotherm in the DSC. Thus, it was seen that compounds which have high surface crystallization tendency will exhibit the double exotherm during heating.

Evaluation of amorphous solid dispersion properties using thermal analysis techniques

Amorphous solid dispersions are an increasingly important formulation approach to improve the dissolution rate and apparent solubility of poorly water soluble compounds. Due to their complex physicochemical properties, there is a need for multi-faceted analytical methods to enable comprehensive characterization, and thermal techniques are widely employed for this purpose. Key parameters of interest that can influence product performance include the glass transition temperature (T(g)), molecular mobility of the drug, miscibility between the drug and excipients, and the rate and extent of drug crystallization. It is important to evaluate the type of information pertaining to the aforementioned properties that can be extracted from thermal analytical measurements, in addition to considering any inherent assumptions or limitations of the various analytical approaches. Although differential scanning calorimetry (DSC) is the most widely used thermal analytical technique applied to the characterization of amorphous solid dispersions, there are many established and emerging techniques which have been shown to provide useful information. Comprehensive characterization of fundamental material descriptors will ultimately lead to the formulation of more robust solid dispersion products.

Molecular dynamics and crystallization phenomenon of supercooled and glassy DNA and RNA nucleosides: β-adenosine, β-thymidine, and β-uridine

Nucleosides are chemical compounds that have an extremely important biological role; they can be found in all types of living organisms. They are crucial components from which DNA and RNA acids are built. In addition, nucleosides are key regulators of many physiological processes. In this paper, the molecular dynamics in the liquid and glassy state of three selected nucleosides, β-adenosine, β-thymidine, and β-uridine, was investigated by means of dielectric spectroscopy. Our results revealed multiple relaxation processes associated with different types of molecular motions. Besides the primary α relaxation, two secondary modes in the glassy states of examined compounds were identified. Crystallization progress monitored by dielectric spectroscopy and x-ray diffraction technique at isostructural relaxation conditions revealed that the examined nucleosides possess completely different tendencies to recrystallize from the liquid as well as the glassy state. We have also made an attempt to predict the time scale of molecular motion below the glass transition temperatures of the respective nucleosides to discuss their potential stability at room temperature over prolonged storage time. Finally, combination of molecular mobility studies with evaluation of thermodynamic parameters from calorimetric measurements allowed us to discuss the fundamental roles of both kinetic and thermodynamic factors in governing the physical stability of the glassy state.

Review: physical chemistry of solid dispersions

Objectives

With poorly soluble drug candidates emerging in the drug discovery pipeline, the importance of the solid dispersion formulation approach is increasing. This strategy includes complete removal of drug crystallinity, and molecular dispersion of the poorly soluble compound in a hydrophilic polymeric carrier. The potential of this technique to increase oral absorption and hence bioavailability is enormous. Nevertheless, some issues have to be considered regarding thermodynamic instability, as well in supersaturated solutions that are formed upon dissolution as in the solid state.

Key findings

After a brief discussion on the historical background of solid dispersions and their current role in formulation, an overview will be given on the physical chemistry and stability of glass solutions as they form supersaturated solutions, and during their shelf life.

Conclusions

Thorough understanding of these aspects will elicit conscious evaluation of carrier properties and eventually facilitate rational excipient selection. Thus, full exploitation of the solid dispersion strategy may provide an appropriate answer to drug attrition due to low aqueous solubility in later stages of development.

Prediction of Onset of Crystallization from Experimental Relaxation Times. II. Comparison between Predicted and Experimental Onset Times

Given a good correlation between onsets of crystallization and mobility above T(g), one might be able to predict crystallization onsets at a temperature of interest far below T(g) from this correlation and measurement of mobility at a temperature below T(g). Such predictions require that: (a) correlation between crystallization onset and mobility is the same above and below T(g), and (b) techniques used to measure mobility above and below T(g) measure the same kind of mobility [(b) demonstrated previously using dielectric and calorimetric techniques]. The objective of present work is to determine whether crystallization onset times couple with relaxation times determined above T(g), and if so to verify predictions made below T(g) (from data above T(g)) with experimental data. Model compounds were indomethacin, ketoconazole, flopropione, nifedipine, and felodipine. Onsets of crystallization measured above T(g) were coupled with dielectric mobility for indomethacin, felodipine, and flopropione. Prediction of crystallization onset times for temperatures below T(g) matched well with experimental data for indomethacin (25 degrees C, 35 degrees C: Predicted 473, 95 h;

EXPERIMENTAL

624 +/- 158, 139 +/- 49 h) and flopropione (35 degrees C, 40 degrees C; Predicted 115, 58 h;

EXPERIMENTAL

96 +/- 30, 59 +/- 10 h). The data suggests that coupling between crystallization onsets and molecular mobility at temperatures above T(g) may be exploited to develop stability testing protocol for crystallization from amorphous state.