Protein-structure-sensitive mid-infrared optoacoustic microscopy enables label-free assessment of drug therapy in myeloma cells

Conventional live-cell optical microscopy lacks sensitivity and specificity for label-free detection of intracellular protein-structure dynamics, such as conformational transition from α-helix to β-sheet. Detecting intermolecular β-sheet formation, for instance, is important because it is a hallmark of misfolded proteins and aggresome formation—which are intrinsic indicators of cell apoptosis in myeloma therapy. Going beyond conventional optical microscopy, we introduce a single-cell imaging technology with label-free sensitivity to intracellular intermolecular β-sheet formation in living cells. This unique ability was attained by exploiting the spectral specificity of the mid-infrared amide I region (1700 – 1600 cm ^-1 ) to protein structure and the positive-contrast nature of optoacoustic microscopy. By means of this technology, we were able to monitor the efficiency of proteasome inhibition in a myeloma cell line and—as a first demonstration towards clinical translation—in biopsied myeloma cells from patients. Achieving label-free monitoring of treatment at a single-cell level allows longitudinal assessment of response heterogeneity, which could provide crucial therapeutic information, such as patient-specific sensitivity to treatment, thus facilitating personalized medicine in myeloma therapy.