Photoimmunotherapy (PIT) is an upcoming potential cancer treatment modality, the effect of which is improved in combination with chemotherapy. PIT causes a super-enhanced permeability and retention (SUPR) effect. Here, we quantitatively evaluated the SUPR effect using radiolabeled drugs of varying molecular weights (18F-5FU, 111In-DTPA, 99mTc-HSA-D, and 111In-IgG) to determine the appropriate drug size. PIT was conducted with an indocyanine green-labeled anti-HER2 antibody and an 808 nm laser irradiation. Mice were subcutaneously inoculated with HER2-positive cells in both hindlimbs. The tumor on one side was treated with PIT, and the contralateral side was not treated. The differences between tumor accumulations were evaluated using positron emission tomography or single-photon emission computed tomography. Imaging studies found increased tumor accumulation of agents after PIT. PIT-treated tumors showed significantly increased uptake of 18F-5FU (p < 0.001) and 99mTc-HSA-D (p < 0.001). A tendency toward increased accumulation of 111In-DTPA and 111In-IgG was observed. These findings suggest that some low- and medium-molecular-weight agents are promising candidates for combined PIT, as are macromolecules; hence, administration after PIT could enhance their efficacy. Our findings encourage further preclinical and clinical studies to develop a combination therapy of PIT with conventional anticancer drugs.
Keywords: indocyanine green; photoimmunotherapy (PIT); positron emission tomography (PET); radionuclide imaging; single-photon emission computed tomography (SPECT); super-enhanced permeability and retention (SUPR).