The transamination-chemistry-based process for sitagliptin is a through-process, which challenges the crystallization of the active pharmaceutical ingredient (API) in a batch stream composed of multiple components. Risk-assessment-based design of experiment (DoE) studies of particle size distribution (PSD) and crystallization showed that the final API PSD strongly depends on the seeding-point temperature, which in turn relies on the solution composition.

To determine the solution composition, near-infrared (NIR) methods had been developed with partial least squares (PLS) regression on spectra of simulated process samples whose compositions were made by spiking each pure component, either sitagliptin free base (FB), water, isopropyl alcohol (IPA), dimethyl sulfoxide (DMSO), or isopropyl acetate (IPAc), into the process stream according to a DoE. An additional update to the PLS models was made by incorporating the matrix difference between simulated samples in lab and factory batches.

Overall, at temperatures of 20–35 °C, the NIR models provided a standard error of prediction (SEP) of less than 0.23 wt % for FB in 10.56–32.91 wt %, 0.22 wt % for DMSO in 3.77–19.18 wt %, 0.32 wt % for IPAc in 0.00–5.70 wt %, and 0.23 wt % for water in 11.20–28.58 wt %. After passing the performance qualification, these on-line NIR methods were successfully established and applied for the on-line analysis of production batches for compositions prior to the seeding point of sitagliptin crystallization.