We have reported a new molecular-targeted cancer phototherapy, photoimmunotherapy (PIT), which
killed implanted tumors in mice without side-effects. To understand the mechanism of cell killing
with PIT, three-dimentional dynamic low-coherence quantitative phase microscopy (3D LC-QPM),
a device developed by Hamamatsu Photonics K.K, was used to detect morphologic changes in
cancer cells during PIT. 3T3/HER2 cells were incubated with anti-HER2 trastuzumab-IR700 (10
μg/mL, 0.1 μM as IR700) for 24 hours, then, three-dimensionally imaged with the LC-QPM during
the exposure of two different optically filtered lights for excitation of IR700 (500-780 nm) and
imaging (780-950 nm). For comparison with traditional PDT, the same experiments were performed
with Photofrin (10 and 1 μM).
Serial changes in the cell membrane were readily visualized on 3D LC-QPM. 3T3/HER2 cells began
to swell rapidly after exposure to 500-780 nm light excitation. The cell volume reached a maximum
within 1 min after continuous exposure, and then the cells appeared to burst. This finding suggests
that PIT damages the cell membrane by photo-reaction inducing an influx of water into the cell
causing swelling and bursting of the cells. Interestingly, even after only 5 seconds of light exposure,
the cells demonstrated swelling and bursting albeit more slowly, implying that sufficient cumulative
damage occurs on the cell membrane to induce lethal damage to cells even at minimal light exposure.
Similar but non-selective membrane damage was shown in PDT-treated cells Photofrin.
Thus, PIT induces sufficient damage to the cell membrane within 5 seconds to induce rapid necrotic
cell death which can be observed directly with 3D LC-QPM. Further investigation is needed to
evaluate the biochemical mechanisms underlying PIT-induced cellular membrane damage.