Influence of Solvent Selection on the Crystallizability and Polymorphic Selectivity Associated with the Formation of the "Disappeared" Form I Polymorph of Ritonavir

The comparative crystallizability and polymorphic selectivity of ritonavir, a novel protease inhibitor for the treatment of acquired immune-deficiency syndrome, as a function of solvent selection are examined through an integrated and self-consistent experimental and computational molecular modeling study. Recrystallization at high supersaturation by rapid cooling at 283.15 K is found to produce the metastable "disappeared" polymorphic form I from acetone, ethyl acetate, acetonitrile, and toluene solutions in contrast to ethanol which produces the stable form II. Concomitant crystallization of the other known solid forms is not found under these conditions. Isothermal crystallization studies using turbidometric detection based upon classical nucleation theory reveal that, for an equal induction time, the required driving force needed to initiate solution nucleation decreases with solubility in the order of ethanol, acetone, acetonitrile, ethyl acetate, and toluene consistent with the expected desolvation behavior predicted from the calculated solute solvation free energies. Molecular dynamics simulations of the molecular and intermolecular chemistry reveal the presence of conformational interplay between intramolecular and intermolecular interactions within the solution phase. These encompass the solvent-dependent formation of intramolecular O-H...O hydrogen bonding between the hydroxyl and carbamate groups coupled with differing conformations of the hydroxyl's shielding phenyl groups. These conformational preferences and their relative interaction propensities, as a function of solvent selection, may play a rate-limiting role in the crystallization behavior by not only inhibiting to different degrees the nucleation process but also restricting the assembly of the optimal intermolecular hydrogen bonding network needed for the formation of the stable form II polymorph.

Microfluidic Platform with Serpentine Geometry Providing Chaotic Mixing in Induction Time Experiments

We present a droplet microfluidic platform mixing the contents of the droplet chaotically in microfluidic induction time measurements, a promising method for quantifying nucleation kinetics with minute amounts of solute. The nucleation kinetics of aqueous potassium chloride droplets dispersed in mineral oil without surfactants is quantified in the presence and absence of chaotic mixing. We demonstrate the ability of the proposed platform to dictate droplet size, to provide a homogeneous temperature distribution, and to chaotically mix the droplet contents. Chaotic mixing in induction time measurements is facilitated by the motion of droplets through serpentine micromixer bends, while the extent of mixing is controlled by how much droplets move. Different nucleation kinetics are observed in experiments where the droplets are static, mixed, and in motion. We hypothesize that the droplet motion induces formation of a thin-liquid Bretherton film surrounding the droplets. The thin film shields droplets from solid boundaries that are more efficient heteronucleant surfaces compared to liquid-liquid interfaces. We observed that repeated microfluidic induction time measurements, particularly with moving droplets, produce significantly distinct cumulative nucleation probability curves, indicating that the measured nucleation kinetics depend strongly on the details of the experimental procedure, which we discuss in detail. Finally, we compare the microfluidic experiments to well-mixed, milliliter volume, turbidity-based measurements in the context of classic nucleation theory.

Comparison of the Nucleation Kinetics Obtained from the Cumulative Distributions of the Metastable Zone Width and Induction Time Data

A linearized integral model based on classical nucleation theory is applied in this work to determine the interfacial energy and pre-exponential factor using a linear plot from the cumulative distributions of the metastable zone width (MSZW) data for some systems reported in the literature, including isonicotinamide, butyl paraben, dicyandiamide, and salicylic acid. Based on the same criterion for the nucleation point, the interfacial energy and pre-exponential factor are determined using the conventional linear regression method from the cumulative distributions of the induction time data for the same systems. The results indicate that the interfacial energy and pre-exponential factor calculated from the MSZW data are consistent with those calculated from the induction time for the studied systems.

Characterization and Crystal Nucleation Kinetics of a New Metastable Polymorph of Piracetam in Alcoholic Solvents

A new polymorph of the drug active pharmaceutical ingredient piracetam (Form VI) has been discovered and characterized by X-ray powder diffraction (PXRD), solid-state Raman, attenuated total reflectance infrared spectroscopy, and differential scanning calorimetry. The PXRD diffractogram of Form VI shows a distinct peak at 24.2° (2θ) that distinguishes it from the previously known polymorphs and solvates. Form VI is metastable with respect to the previously known polymorphs Form II and Form III; in ethanol solution at 288 K, Form VI transforms into Form II within 15 min, while in isopropanol solution Form VI is kinetically stable for at least 6 h. A total of 1200 crystal nucleation induction time experiments of piracetam in ethanol and isopropanol solutions have been conducted, in sets of 40-80 repeat experiments carried out at different temperatures and solute concentrations. Each solution nucleated as a single polymorph, and each set of repeat experiments resulted in different proportions of Form II, Form III, and Form VI, with Form VI dominating at low nucleation temperatures and Form II at higher nucleation temperatures. The induction time data for Form VI at 288 K have been evaluated within the framework of the classical nucleation theory. At equal driving force, nucleation of Form VI is less obstructed in ethanol than in isopropanol, as captured by a lower interfacial energy and higher pre-exponential factor in ethanol. The proportion of Form VI obtained at a comparable driving force increases in the order ethanol < isopropanol.

Cubosomal Betamethasone-Salicylic Acid Nano Drug Delivery System for Enhanced Management of Scalp Psoriasis

Introduction

Betamethasone dipropionate (BD), a potent corticosteroid, and salicylic acid (SA), a keratolytic agent, have been used in combination to treat scalp psoriasis; however, undesirable side effects associated with their prolonged topical use are inevitable. In this study, BD and SA were loaded into cubosomes, a nanoparticulate system with outstanding biocompatibility, bio-adhesivity and penetration power.

Methods

Design of experiments (DOE) was utilized to prepare thirteen different cubosomal dispersions by emulsification technique using glycerol monoolein (GMO) as a lipid phase and Poloxamer 407 (P407) as a surfactant, sodium carboxymethyl cellulose (SCMC) was added to enhance the dispersions’ rheological properties. The thirteen dispersions were in-vitro characterized for their particle size, polydispersity index (PDI), zeta potential, BD and SA content and rheological behaviour. The desirability of an optimized formula (OF) was set to the smallest particle size, lowest zeta-potential and highest viscosity. The OF was in-vitro characterized for the same parameters in addition to transmission electron microscope imaging and in-vitro drug release. The OF’s anti-psoriatic activity was evaluated in-vivo using an imiquimod-induced psoriasis model.

Results

The OF achieved a particle size of 197.4 ± 9.47 nm, a PDI of 0.443 ± 0.025, a zeta potential of −44.4 ± 0.141mv, BD content of 105.85 ± 2.290%, SA content of 88.855 ± 2.920% with shear-thinning rheological behaviour and completed in-vitro drug release within 2–3 hours. The in-vivo studies confirmed the cubosomes’ higher anti-psoriatic efficacy over the commercial product with lower changes in ear thickness, spleen to body weight ratio, psoriasis area severity index score and improved histopathological findings.

Conclusion

The developed BD SA-loaded cubosomes exhibit promising anti-psoriatic activity attributed to its nano-size and unique lipid content, with enhanced skin penetration and modified rheological properties; increasing the formulation’s in-contact duration with the scalp resulting in lower application frequency and thus reduced BD and SA associated side effects.

The Role of the Pre-Exponential Factor in Determining the Kinetic Selection of Polymorphs During Solution Crystallization of Organic Compounds

Generally, pairs of polymorphs can be characterized by their ratios of equilibrium solubilities (C*me/C*st) and interfacial energies (γst/γme) for a given temperature and solvent. We refer to this point as...

Nucleation of the Theophylline:Salicylic Acid 1:1 Cocrystal

The nucleation behavior of the theophylline-salicylic acid 1:1 (THP:SA) cocrystal in chloroform has been investigated and compared with the corresponding behavior of the pure compounds. Induction times have been determined at different supersaturations at 10 °C under each condition in approximately 40-80 repetition experiments in 20 mL vials. Nucleation times, extracted from the median induction times by accounting for a nucleus growth time, have been used to determine the interfacial energy and the pre-exponential factor within the classical nucleation theory. Results show that the cocrystal at equal driving force has a longer nucleation time, or to reach equal nucleation time, the cocrystal requires a higher driving force. Pure theophylline is easier to nucleate than pure salicylic acid, despite the latter having a smaller molecular size, higher solubility, and is expected to form dimers already in the solution. The cocrystal is found to have an interfacial energy in between the respective values for the pure compounds. However, the higher molecular volume of the cocrystal, taken as the volume of the 1:1 theophylline-salicylic acid assembly, leads to the highest nucleation work, which, together with a low pre-exponential factor, explains why the cocrystal is the most difficult to nucleate. The experimentally extracted pre-exponential factor of the cocrystal is very similar to that of THP, and similar trends are observed from theoretical expressions of volume-diffusion- and surface-integration-controlled nucleation, respectively.

Influence of solution chemistry on the solubility, crystallisability and nucleation behaviour of eicosane in toluene : acetone mixed-solvents

Compositionally dependent solution structure is found to influence the solubility, crystallisability and nucleation mechanism of eicosane when crystallising from toluene : acetone mixed-solvent solutions.

Crystallization of paracetamol from aqueous solutions in a planar oscillatory flow crystallizer: effect of the oscillation conditions on the nucleation kinetics

Nucleation kinetic data is reported for a planar oscillatory flow crystallizer.

Crystal nucleation of salicylamide and a comparison with salicylic acid

Nucleation behaviour of salicylamide in different solvents was determined and compared with salicylic acid, attempting to progress the rationalization of the influence of the solvent and solute on crystal nucleation of organic compounds in solution.

Investigations into the Influence of Solvents on the Nucleation Kinetics for Isonicotinamide, Lovastatin, and Phenacetin

A new method of data interpretation based on classical nucleation theory is proposed in this work to elucidate the influence of solvents on the pre-exponential nucleation factor and interfacial energy using the induction time data for three crystallization systems, including isonicotinamide, lovastatin, and phenacetin. In this method, the pre-exponential nucleation factor is replaced by the intrinsic nucleation factor multiplied by temperature and divided by solution viscosity. The proposed method is applied to study the nucleation kinetics of isonicotinamide, lovastatin, and phenacetin among various solvents using the induction time data measured in this work. The results indicate that the intrinsic nucleation factor increases linearly with increasing square root of interfacial energy in various solvents for each system.

Thermodynamic vs. Kinetic Basis for Polymorph Selection

Ratios of equilibrium solubilities rarely exceed two-fold for polymorph pairs. A model has been developed based on two intrinsic properties of polymorph pairs, namely the ratio of equilibrium solubilities of the individual pairs (C*me/C*st) and the ratio of interfacial energies (γst/γme) and one applied experimental condition, namely the supersaturation identifies which one of a pair of polymorphs nucleates first. A domain diagram has been developed, which identifies the point where the critical free energy of nucleation for the polymorph pair are identical. Essentially, for a system supersaturated with respect to both polymorphs, the model identifies that low supersaturation with respect to the stable polymorph (Sst) leads to an extremely small supersaturation with respect to the metastable polymorph (Sme), radically driving up the critical free energy with respect to the metastable polymorph. Generally, high supersaturations sometimes much higher than the upper limit of the metastable zone, are required to kinetically favour the metastable polymorph.

Revealing the role of a surfactant in the nucleation and crystal growth of thiamine nitrate: experiments and simulation studies

This work combines experiments and simulations to investigate the inhibition effects of a surfactant on the nucleation and crystal growth rate of thiamine nitrate.

Rationalising crystal nucleation of organic molecules in solution using artificial neural networks

Artificial neural network modelling is used to analyse and predict primary nucleation based on various physicochemical solute and solvent parameters.

Predicting Nucleation of Isonicotinamide from the Solvent-Solute Interactions of Isonicotinamide in Common Organic Solvents

The interactions of isonicotinamide (INA) with seven common solvents (acetic acid, acetonitrile, acetone, chloroform, ethyl acetate, and methanol) have been studied to examine solute-solvent effects on the nucleation of INA from these solvents. In a simple model of 1:1 solute-solvent interactions, the strongest INA-solvent interaction is with acetic acid (binding energy, Δ E_bind = -64.05 kJ mol^-1) and the weakest is with chloroform (Δ E_bind = -24.85 kJ mol^-1). This arises since acetic acid and INA form a hydrogen-bonding motif containing two moderate strength N-H···O hydrogen bonds, while chloroform and INA have a single weak C-H···O hydrogen bond. Taking acetic acid, chloroform, and methanol, the solvents with the strongest, the weakest, and an intermediate strength INA-solvent binding energy, the solvation of INA was studied to compare it with the 1:1 model. Acetic acid has the strongest binding energy (-872.24 kJ mol^-1) and solvation energy (-341.20 kJ mol^-1) with chloroform binding energy (-517.72 kJ mol^-1) and solvation energy (-199.05 kJ mol^-1). Methanol has intermediate binding energy (-814.19 kJ mol^-1) and solvation energies (-320.81 kJ mol^-1). These results further confirm the recent the findings which indicate that the key trends in solvent-solute interactions can be determined from a simple and efficient 1:1 dimer model and can be used to predict ease of nucleation with stronger binding energies correlating to slower, more difficult nucleation. A limit of this model is revealed by considering alcohol and acid solvents with longer alkyl chains.

High-Mobility, Ultrathin Organic Semiconducting Films Realized by Surface-Mediated Crystallization

The functionality of common organic semiconductor materials is determined by their chemical structure and crystal modification. While the former can be fine-tuned via synthesis, a priori control over the crystal structure has remained elusive. We show that the surface tension is the main driver for the plate-like crystallization of a novel small organic molecule n-type semiconductor at the liquid-air interface. This interface provides an ideal environment for the growth of millimeter-sized semiconductor platelets that are only few nanometers thick and thus highly attractive for application in transistors. On the basis of the novel high-performance perylene diimide, we show in as-grown, only 3 nm thin crystals electron mobilities of above 4 cm²/(V s) and excellent bias stress stability. We suggest that the established systematics on solvent parameters can provide the basis of a general framework for a more deterministic crystallization of other small molecules.

A basis for the kinetic selection of polymorphs during solution crystallization of organic compounds

Domain diagram for supersaturation needed for a given polymorph pair to select kinetically the metastable or stable forms.

Relating induction time and metastable zone width

A relation between MSZW and induction time reveals nucleation happens when nucleation potential accumulates up to a constant value in a system. The MSZW and induction time can be estimated from each other with this relation.

Influence of solvent on crystal nucleation of risperidone

Over 2100 induction time experiments were carried out for the medium-sized, antipsychotic drug molecule, risperidone in seven different organic solvents. To reach the same induction time the required driving force increases in the order: cumene, toluene, acetone, ethyl acetate, methanol, propanol, and butanol, which reasonably well correlates to the interfacial energies as determined within classical nucleation theory. FTIR spectroscopy has been used to investigate any shifts in the spectra and to estimate the interaction of solute and solvent at the corresponding site. The solution condition has also been investigated by Density Functional Theory (DFT) calculations over (1 : 1) solvent–solute binding interactions at 8 different sites on the risperidone molecule. The DFT computational results agree with the spectroscopic data suggesting that these methods do capture the binding strength of solvent molecules to the risperidone molecule. The difficulty of nucleation correlates reasonably to the DFT computations and the spectroscopic measurements. The results of the different measurements suggest that the stronger the solvent binds to the risperidone molecule in solution, the slower the nucleation becomes.