Multimodal multiphoton and vibrational microscopy reveals early therapy-induced senescence in human tumors: a non-invasive tool to prevent the risk of cancer relapse

Recent studies have shown that common anticancer treatments can induce cell senescence rather than death, a critical phenotype governing tumor recurrence. This calls for the urgent development of safe, precise, and quick tools to unveil critical therapy-induced senescence (TIS). Merging different coherent Raman and multiphoton techniques, we present label-free multimodal nonlinear optical (NLO) microscopy as a powerful tool to spot early TIS. We home-built a microscope including different NLO modalities: Stimulated Raman Scattering (SRS), forward and epi-detected Coherent Anti-Stokes Raman Scattering (CARS and E-CARS), and Two-Photon Excited Fluorescence (TPEF). The infrared laser source outputs synchronized narrowband 780 nm pump pulses and 950-1050 nm tunable Stokes pulses, so to match the CH-stretching region of the Raman spectrum. Thanks to the co-registration of these diverse techniques applied on label-free TIS cells and controls, we exposed quantitative hallmarks of early TIS, confirmed by comparing different optical signals monitored over 72 hours of treatment. TPEF from metabolic coenzymes combined with E-CARS from cardiolipin and cytochrome C indicated an early shrinking of mitochondria. CARS and SRS revealed lipid vesicles overproduction and accumulation. Nuclei enlarged irregularly, visualized via subtraction of SRS signals of proteins and lipids, and CARS from deoxyribose. We consider our results will strongly influence anticancer pre-clinical studies and translated clinical applications, helping to identify quickly, non-invasively, and quantitatively TIS in human tumors.