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Photoacoustic imaging of chemotherapy-induced apoptosis in squamous cell carcinoma

Anti-cancer drugs typically exert their pharmacological effect on tumors by inducing apoptosis, or programmed cell death, within the cancer cells, with PCD occurring as soon as 4 hours after treatment. Detection of apoptosis in patients could decisively report a response to treatment days or even weeks before MRI, CAT, and ultrasound indicate morphological changes in the tumor. Here we developed a novel near-infrared dye based imaging probe to directly detect apoptosis with high specificity in cancer cells by utilizing a non-invasive photoacoustic imaging technique. Nude mice bearing head and neck tumors received cisplatin chemotherapy were imaged by PAI after tail vein injection of the contrast agent. In vivo PAI indicated a strong apoptotic response to chemotherapy on the peripheral margins of tumors, whereas untreated controls showed no contrast enhancement by PAI. The apoptotic status of the mouse tumor tissue was verified by immunohistochemical techniques staining for cleaved caspase-3 p11 subunit. The results demonstrated the potential of this imaging probe to guide the evaluation of chemotherapy treatment.