Enantiomerically pure active pharmaceutical ingredient (API) formulations are often essential in delivering an effective and bioavailable drug. APIs can exhibit polymorphism, meaning that the small molecule can crystallize into multiple forms. Both, thermodynamics and kinetics play a crucial role in determining which form of the API is generated at a higher yield. Each polymorphic form and its corresponding enantiomers, can have very different properties and result in vastly differing efficacies of the final drug formulation. The desired form of the API is not necessarily the most stable or kinetically preferred, which is why process development and formulations play a key role in the manufacturing process. As such, the Food and Drug Administration (FDA) requires rigorous characterization and analysis, both during in-line process development and final formulation of the API, to ensure delivery of a safe, effective, and bio-available drug. The use of Second Harmonic Generation (SHG) coupled with Raman spectroscopy offers the ability to rapidly characterize samples, distinguish between polymorphs, and provide quantitative information with limit of detections (LOD) down to 100 parts-per-million (ppm).*
Chowdhury et al.* have recently demonstrated the ability to utilize SHG to very sensitively discriminate between two different polymorphic forms of clopidogrel bisulfate. White spherical particles of the API were characterized with SHG imaging utilizing a commercially available SONICC instrument from Formulatrix with the ability to achieve an LOD of 0.1% within 1 minute and 0.01% in less than 10 minutes. The versatility of this analysis is increased with the addition of Raman spectroscopy integrated into the instrument. SHG can be utilized to choose which particles to analyze (high or low intensity) and Raman can be used to ascertain chemical information. In general, Raman spectroscopy is a very slow technique that takes over 15 hours to scan enough particles in order to achieve 0.1% LODs. However, SHG can be used to quickly image the sample area and then direct the Raman probe to areas of interest, significantly reducing analysis time.
Tools for rapidly identifying polymorphic composition are critical in process development and stability of the final drug. Manufacturing yields and purity of one form or another can be significantly increased by detecting low amounts of an undesired polymorph sooner in the process. Reactions and chemistries can be altered based on the feedback from composition analysis to drive the end product to one form or another. SHG coupled with Raman serves as a rapid means to identify the presence of undesirable polymorphs and provides feedback into process methods. Shelf life studies can also be expedited with lower limits of detection and yield greater information on the kinetics behind shifts from one polymorph to another over time. Even the presence of a very small amount (ppm) of a thermodynamically more stable form can disrupt bioavailability and shelf life making SHG-guided Raman a remarkable tool in API characterization.
*Chowdhury AU, Ye DH, Song Z, Zhang S, Hedderich HG, Mallick B, Thirunahari S, Ramakrishnan S, Sengupta A, Gualtieri EJ, Bouman CA, Simpson GJ, Anal Chem. 2017; 89 (11): 5958-65.