Second order nonlinear optical imaging of chiral crystals (SONICC) was applied to selectively detect crystal formation at early stages and characterize the kinetics of nucleation and growth. SONICC relies on second harmonic generation (SHG), a nonlinear optical effect that only arises from noncentosymmetric ordered domain structures, which include crystals of chiral molecules. The model systems studied include pharmaceutically relevant compounds: griseofulvin and chlorpropamide. SONICC demonstrates low detection limits producing an 8 order of magnitude improvement relative to macroscopic average techniques and 5 order of magnitude improvement relative to optical microscopy. SONICC was also applied to examine the kinetics of crystallization in amorphous griseofulvin. The results show that SONICC enables simultaneous monitoring of individual crystal growth, nucleation rate, and macroscopic crystallization kinetics.

The SAMPL-1 hydration free energy blind prediction challenge data set includes 63 compounds that are more chemically diverse, polyfunctional, drug-like, and with examples of transfer free energies and molecular weights larger than ever before seen in previously tabulated data sets of neutral compounds. For the prospective SAMPL-1 study, we employed a continuum model including a boundary element solution of the Poisson equation to describe electrostatic solvation, a molecular surface area-based cost of cavity formation in water, and a continuum Lennard-Jones potential to account for dispersion-repulsion solute-solvent effects. For the latter contribution, continuum van der Waals atom-type coefficients were calibrated and validated on previously available hydration data sets. In the prospective study, this continuum hydration model yielded SAMPL-1 predictions highly correlated with experimental data, albeit with a slope of slightly above 0.5, suggesting a valid model but with a systematic error. Analysis of the major outliers, all overestimating the experimental hydration data, highlights a common structural theme as a possible cause of the prediction errors: densely polar and hydrogen-bond-capable structures, featuring primarily substituted (sulfon)amide groups, often in conjugated systems. By examining analog pairs within the SAMPL-1 data set, it was also noted that certain solvation trends are captured neither by chemical sense nor by our hydration model, which seem too additive. A retrospective analysis of model transferability between hydration data sets as a function of its parameters and complexity indicates that the electrostatic component of the model is fairly transferrable across data sets, but the nonelectrostatic terms are less so. For the chemical space covered in SAMPL-1, absolute prediction errors indicate that the simpler transferrable electrostatics-only model outperforms the more complex model including cavity and continuum dispersion terms. Possible directions to further improve this continuum hydration model are proposed.

Microscopic characterization of crystallinity in powders can reveal information lost in ensemble-averaged measurements. Nonlinear optical imaging based on second harmonic generation (SHG) provides rapid and highly selective detection of individual chiral microcrystals, enabling insights into the fundamental mechanism of action for the observed crystallinity loss of an organic powder induced by mechanical grinding. Using griseofulvin as the model compound, the results from second order nonlinear optical imaging of chiral crystals (SONICC) compared favorably with those of powder X-ray diffraction (PXRD) over the linear dynamic range of the PXRD measurements. However, the SHG measurements demonstrated three decade improvements in linear dynamic range. The detection limit of SHG was estimated to be 4 ppm crystallinity in the powder. The rate of crystallinity loss induced by milling followed a first order process with a half-life of 15 ± 1 min. Recrystallization of cryomilled powder is ~40 times faster than that prepared by melt-quenched powder, suggesting that the disordered state obtained by exhaustive cryomilling appears to contain ordered domains that are larger than the critical nucleation size, but below the detection limit of SONICC. The presence of such domains provides a barrier-less nucleation source resulting in rapid crystallization, the kinetics of which depends only on crystal growth.

The primary aim of this study was to evaluate the utility of second-order nonlinear imaging of chiral crystals (SONICC) to quantify crystallinity in drug–polymer blends, including solid dispersions. Second harmonic generation (SHG) can potentially exhibit scaling with crystallinity between linear and quadratic depending on the nature of the source, and thus, it is important to determine the response of pharmaceutical powders. Physical mixtures containing different proportions of crystalline naproxen and hydroxyl propyl methyl cellulose acetate succinate (HPMCAS) were prepared by blending and a dispersion was produced by solvent evaporation. A custom-built SONICC instrument was used to characterize the SHG intensity as a function of the crystalline drug fraction in the various samples. Powder X-ray diffraction (PXRD) and Raman spectroscopy were used as complementary methods known to exhibit linear scaling. SONICC was able to detect crystalline drug even in the presence of 99.9 wt % HPMCAS in the binary mixtures. The calibration curve revealed a linear dynamic range with a R2 value of 0.99 spanning the range from 0.1 to 100 wt % naproxen with a root mean square error of prediction of 2.7%. Using the calibration curve, the errors in the validation samples were in the range of 5%–10%. Analysis of a 75 wt % HPMCAS–naproxen solid dispersion with SONICC revealed the presence of crystallites at an earlier time point than could be detected with PXRD and Raman spectroscopy. In addition, results from the crystallization kinetics experiment using SONICC were in good agreement with Raman spectroscopy and PXRD. In conclusion, SONICC has been found to be a sensitive technique for detecting low levels (0.1% or lower) of crystallinity, even in the presence of large quantities of a polymer.