Kinetic Modeling of Accelerated Stability Testing Enabled by Second Harmonic Generation Microscopy.

The low limits of detection afforded by second harmonic generation (SHG) microscopy coupled with image analysis algorithms enabled quantitative modeling of the temperature-dependent crystallization of active pharmaceutical ingredients (APIs) within amorphous solid dispersions (ASDs). ASDs, in which an API is maintained in an amorphous state within a polymer matrix, are finding increasing use to address solubility limitations of small-molecule APIs. Extensive stability testing is typically performed for ASD characterization, the time frame for which is often dictated by the earliest detectable onset of crystal formation. Here a study of accelerated stability testing on ritonavir, a human immunodeficiency virus (HIV) protease inhibitor, has been conducted. Under the condition for accelerated stability testing at 50 °C/75%RH and 40 °C/75%RH, ritonavir crystallization kinetics from amorphous solid dispersions were monitored by SHG microscopy. SHG microscopy coupled by image analysis yielded limits of detection for ritonavir crystals as low as 10 ppm, which is about 2 orders of magnitude lower than other methods currently available for crystallinity detection in ASDs. The four decade dynamic range of SHG microscopy enabled quantitative modeling with an established (JMAK) kinetic model. From the SHG images, nucleation and crystal growth rates were independently determined.

Second Harmonic Generation Guided Raman Spectroscopy for Sensitive Detection of Polymorph Transitions.

Second harmonic generation (SHG) was integrated with Raman spectroscopy for the analysis of pharmaceutical materials. Particulate formulations of clopidogrel bisulfate were prepared in two crystal forms (Form I and Form II). Image analysis approaches enable automated identification of particles by bright field imaging, followed by classification by SHG. Quantitative SHG microscopy enabled discrimination of crystal form on a per particle basis with 99.95% confidence in a total measurement time of ∼10 ms per particle. Complementary measurements by Raman and synchrotron XRD are in excellent agreement with the classifications made by SHG, with measurement times of ∼1 min and several seconds per particle, respectively. Coupling these capabilities with at-line monitoring may enable real-time feedback for reaction monitoring during pharmaceutical production to favor the more bioavailable but metastable Form I with limits of detection in the ppm regime.

The detection and subsequent volume optimization of biological nanocrystals.

Identifying and then optimizing initial crystallization conditions is a prerequisite for macromolecular structure determination by crystallography. Improved technologies enable data collection on crystals that are difficult if not impossible to detect using visible imaging. The application of second-order nonlinear imaging of chiral crystals and ultraviolet two-photon excited fluorescence detection is shown to be applicable in a high-throughput manner to rapidly verify the presence of nanocrystals in crystallization screening conditions. It is noted that the nanocrystals are rarely seen without also producing microcrystals from other chemical conditions. A crystal volume optimization method is described and associated with a phase diagram for crystallization.

Nonlinear optical imaging of integral membrane protein crystals in lipidic mesophases.

Second-order nonlinear optical imaging of chiral crystals (SONICC) is explored for selective detection of integral membrane protein crystals grown in opaque and turbid environments. High turbidity is a hallmark of membrane protein crystallization due to the extensive use of detergent and/or lipids that often form various mesophases. Detection of crystals in such media by conventional optical methods (e.g., intrinsic UV fluorescence, birefringence, bright-field image analysis, etc.) is often complicated by optical scattering and by the small sizes of the crystals that routinely form. SONICC is shown to be well-suited for this application, by nature of its compatibility with imaging in scattering media and its high selectivity for protein crystals. Bright second harmonic generation (SHG) (up to 18 million counts/s) was observed from even relatively small crystals (5 mum) with a minimal background due to the surrounding lipid mesophase ( approximately 1 thousand counts/s). The low background nature of the resulting protein crystal images permitted the use of a relatively simple, particle counting analysis for preliminary scoring. Comparisons between a particle counting analysis of SONICC images and protocols based on the human expert analysis of conventional bright-field and birefringence images were performed.

Selective detection of protein crystals by second harmonic microscopy.

The unique symmetry properties of second harmonic generation (SHG) microscopy enabled sensitive and selective imaging of protein microcrystals with negligible contributions from solvated proteins or amorphous protein aggregates. In studies of microcrystallites of green fluorescent protein (GFP) prepared in 500 pL droplets, the SHG intensities rivaled those of fluorescence, but with superb selectivity for crystalline regions. GFP in amorphous aggregates and in solution produced substantial background fluorescence, but no detectable SHG. The ratio of the forward-to-backward detected SHG provides a measure of the particle size, suggesting detection limits down to crystallites 100 nm in diameter under low magnification (10x). In addition to being sensitive and highly selective, second-order nonlinear optical imaging of chiral crystals (SONICC) is directly compatibility with virtually all common protein crystallization platforms.