Spatially-offset Raman spectroscopy for the non-invasive, real-time characterization of in situ skin implant formation and drug release kinetics

In situ forming implants (ISFIs) present a promising approach for sustained parenteral drug delivery, offering reduced first pass metabolism, fewer systemic side effects, and improved patient compliance. This study investigates defocused spatially-offset Raman spectroscopy (SORS) as a real-time, non-invasive, and label-free method for monitoring implant formation and drug release from ISFIs loaded with the model drugs 4-cyanophenol (4-CP) or all-trans retinoic acid (RA). A custom-designed flow-through diffusion cell, incorporating full-thickness porcine skin, was developed to enable ex vivo release studies, allowing simultaneous defocused SORS measurements of subcutaneously-implanted formulations and HPLC quantification of drug released into the receptor medium. Confocal Raman microscopy cross-section imaging validated defocused SORS findings, while static ex vivo release studies using Franz diffusion cells served as a reference for flow-through experiments. Both Franz cell and flow-through release studies revealed a pronounced burst release for the hydrophilic 4-CP after injection (90.7 % and 94.8 % released after 2.5 d under static and flow-through conditions, respectively) and a delayed, attenuated release for the hydrophobic RA (3.3 % and 2.1 % after 2.5 d). Defocused SORS further confirmed a correlation between implant formation and drug release, highlighting solvent exchange as a key driver of burst release. These findings demonstrate the unique capability of defocused SORS for the comprehensive, non-invasive, and real-time characterization of ISFI formulations. By combining the relevance of biological tissue with experimental accessibility, the ex vivo flow-through configuration (enhanced by defocused SORS) provides a novel and powerful platform for formulation development and individualized therapeutic drug monitoring, with strong potential for clinical translation.