Yuchuan Gong

A microfluidic platform for pharmaceutical salt screening

We describe a microfluidic platform comprised of 48 wells to screen for pharmaceutical salts. Solutions of pharmaceutical parent compounds (PCs) and salt formers (SFs) are mixed on-chip in a combinatorial fashion in arrays of 87.5-nanolitre wells, which constitutes a drastic reduction of the volume of PC solution needed per condition screened compared to typical high throughput pharmaceutical screening approaches. Nucleation and growth of salt crystals is induced by diffusive and/or convective mixing of solutions containing, respectively, PCs and SFs in a variety of solvents. To enable long term experiments, solvent loss was minimized by reducing the thickness of the absorptive polymeric material, polydimethylsiloxane (PDMS), and by using solvent impermeable top and bottom layers. Additionally, well isolation was enhanced via the incorporation of pneumatic valves that are closed at rest. Brightfield and polarized light microscopy and Raman spectroscopy were used for on-chip analysis and crystal identification. Using a gold-coated glass substrate and minimizing the thickness of the PDMS control layer drastically improved the signal-to-noise ratio for Raman spectra. Two drugs, naproxen (acid) and ephedrine (base), were used for validation of the platforms ability to screen for salts. Each PC was mixed combinatorially with potential SFs in a variety of solvents. Crystals were visualized using brightfield polarized light microscopy. Subsequent on-chip analyses of the crystals with Raman spectroscopy identified four different naproxen salts and five different ephedrine salts.

Low-concentration polymers inhibit and accelerate crystal growth in organic glasses in correlation with segmental mobility.

Crystal growth in organic glasses has been studied in the presence of low-concentration polymers. Doping the organic glass nifedipine (NIF) with 1 wt % polymer has no measurable effect on the glass transition temperature Tg of host molecules, but substantially alters the rate of crystal growth, from a 10-fold reduction to a 30% increase at 12 °C below the host Tg. Among the polymers tested, all but polyethylene oxide (PEO) inhibit growth. The inhibitory effects greatly diminish in the liquid state (at Tg + 38 °C), but PEO persists to speed crystal growth. The crystal growth rate varies exponentially with polymer concentration, in analogy with the polymer effect on solvent mobility, though the effect on crystal growth can be much stronger. The ability to inhibit crystal growth is not well ordered by the strength of host-polymer hydrogen bonds, but correlates remarkably well with the neat polymers Tg, suggesting that the mobility of polymer chains is an important factor in inhibiting crystal growth in organic glasses. The polymer dopants also affect crystal growth at the free surface of NIF glasses, but the effect is attenuated according to the power law us ∝ ub(0.35), where us and ub are the surface and bulk growth rates.

Microfluidic approach to polymorph screening through antisolvent crystallization

Here we present a microfluidic platform comprised of 48 wells to screen for polymorphs of active pharmaceutical ingredients (API) through antisolvent crystallization. API solutions and anti-solvents are precisely metered in various volumetric ratios (range from 50 : 10 to 10 : 50), and mixed via diffusive mixing on-chip. Optical microscopy and Raman spectroscopy were used to analyze the resultant solids. The small volumes (37 nL) and the ability to screen a wide range of supersaturations through diffusive mixing make this platform especially useful for solid form development at discovery and early development stages in pharmaceutical industry. To validate this microfluidic approach, we conducted on-chip antisolvent crystallization using indomethacin. Solvent choice, supersaturation level, and antisolvent-to-solution ratio were found to affect the resulting crystal form of the solids prepared on chip. We modelled the representative time-dependent concentration profiles during the mixing of the antisolvent and API solutions. Combining this analysis with solubility data yielded spatiotemporal supersaturation profiles, which we correlated with solid formation as observed experimentally.

A Microfluidic Platform for Evaporation-based Salt Screening of Pharmaceutical Parent compounds

We describe a microfluidic platform to screen for salt forms of pharmaceutical compounds (PCs) via controlled evaporation. The platform enables on-chip combinatorial mixing of PC and salt former solutions in a 24-well array (~200 nL/well), which is a drastic reduction in the amount of PC needed per condition screened compared to traditional screening approaches that require ~100 μL/well. The reduced sample needs enable salt screening at a much earlier stage in the drug development process, when only limited quantities of PCs are available. Compatibility with (i) solvents commonly used in the pharmaceutical industry, and (ii) Raman spectroscopy for solid form identification was ensured by using a hybrid microfluidic platform. A thin layer of elastomeric PDMS was utilized to retain pneumatic valving capabilities. This layer is sandwiched between layers of cyclic-olefin copolymer, a material with low air and solvent permeability and low Raman background to yield a physically rigid and Raman compatible chip. A solvent-impermeable thiolene layer patterned with evaporation channels permits control over the rate of solvent evaporation. Control over the rate of solvent evaporation (2-15 nL h(-1)) results in consistent, known rates of increase in the supersaturation levels attained on-chip, and increases the probability for crystalline solids to form. The modular nature of the platform enables on-chip Raman and birefringence analysis of the solid forms. Model compounds, tamoxifen and ephedrine, were used to validate the platforms ability to screen for salts. On-chip Raman analysis helped to identify six different salts each of tamoxifen and ephedrine.

Solvent compatible microfluidic platforms for pharmaceutical solid form screening

We describe a microfluidic platform with enhanced solvent compatibility to screen solid forms of pharmaceutical parent compounds including salts, cocrystals, and their crystal forms via controlled solvent evaporation and antisolvent addition techniques. The platform enables on-chip combinatorial mixing of parent compound, auxiliary materials, or non-solvents in a 24- to 72-well array (∼100–200 nL per well). This approach enables screening with very small quantities of material per condition compared to traditional screening approaches that require larger volumes, ∼100 μL per well. Compatibility with (i) polar as well as non-polar organic solvents commonly employed in crystallization of pharmaceuticals, such as ethanol, methanol, tetrahydrofuran, acetonitrile, chloroform, hexane, and toluene, (ii) Raman spectroscopy used for on-line identification of the resulting solids was achieved by using a perfluoropolyether-based microfluidic platform. Integration of a hybrid thin layer assembly of elastomeric PDMS–SIFEL–SIFEL ensures that pneumatic valving capabilities are retained. This assembly was sandwiched between layers of cyclic-olefin copolymer (COC) at the top and Teflon FEP or COC (depending on the solvent) at the bottom to yield a physically rigid, Raman compatible crystallization chip. In addition, a solvent-impermeable thiolene layer patterned with evaporation channels was employed to permit control over the rate of solvent evaporation for solvent evaporation experiments. The resulting hybrid microfluidic platforms enabled enhanced compatibility with a variety of polar and non-polar organic solvents such as methanol, ethanol, isopropyl alcohol, acetonitrile, tetrahydrofuran, hexane, heptane, and toluene, which is especially critical for antisolvent crystallization experiments. In solvent evaporation experiments with these platforms the rate of solvent evaporation can be controlled consistently (5–20 nL h−1), thereby facilitating nucleation and crystal growth. Model compounds, theophylline and carbamazepine, were used to validate the platforms ability to screen for cocrystals via solvent evaporation and for polymorphs via antisolvent addition. On-chip Raman analysis was used to identify different cocrystals and polymorphs.

Modeling Physical Stability of Amorphous Solids Based on Temperature and Moisture Stresses

Isothermal microcalorimetry was utilized to monitor the crystallization process of amorphous ritonavir (RTV) and its hydroxypropylmethylcellulose acetate succinate-based amorphous solid dispersion under various stressed conditions. An empirical model was developed: ln(τ)=ln(A)+EaRT-b⋅wc, where τ is the crystallization induction period, A is a pre-exponential factor, Ea is the apparent activation energy, b is the moisture sensitivity parameter, and wc is water content. To minimize the propagation of errors associated with the estimates, a nonlinear approach was used to calculate mean estimates and confidence intervals. The physical stability of neat amorphous RTV and RTV in hydroxypropylmethylcellulose acetate succinate solid dispersions was found to be mainly governed by the nucleation kinetic process. The impact of polymers and moisture on the crystallization process can be quantitatively described by Ea and b in this Arrhenius-type model. The good agreement between the measured values under some less stressful test conditions and those predicted, reflected by the slope and R(2) of the correlation plot of these 2 sets of data on a natural logarithm scale, indicates its predictability of long-term physical stability of amorphous RTV in solid dispersions. To further improve the model, more understanding of the impact of temperature and moisture on the amorphous physical stability and fundamentals regarding nucleation and crystallization is needed.

Crystallization and characterization of cocrystals of piroxicam and 2,5-dihydroxybenzoic acid

A cocrystal of piroxicam (PRX) and 2,5-dihydroxybenzoic acid (HBA), PRX–HBA, and an acetone (ACT) solvate of the cocrystal, PRX–HBA–ACT, were crystallized on a microfluidic platform via solvent evaporation. The inclusion of ACT was found serendipitously, with the ACT probably being introduced into the crystallization system as an impurity in HBA. The crystal structures of PRX–HBA and PRX–HBA–ACT obtained by X-ray diffraction both exhibit the P21/c space group. Bulk quantities of the cocrystals were synthesized using a slurry method and their physical properties were characterized with thermogravimetric analysis and differential scanning calorimetry. Additionally, the phase behavior of PRX–HBA–ACT during desolvation was monitored with powder X-ray diffraction. The powder diffraction of PRX–HBA–ACT after complete desolvation has the same peak positions as the PRX–HBA, suggesting that PRX–HBA–ACT re-arranges to the structure of PRX–HBA.