Development and characterization of a triple-modality anti-PSMA targeting agent for immuno-SPECT/CT, near-infrared fluorescence imaging and targeted photodynamic therapy of PSMA-expressing tumors

Objectives Despite improvements in diagnosis and therapy, prostate cancer remains a significant health problem in the Western world. Targeted photodynamic therapy (tPDT) potentially is a highly selective cancer treatment based on targeting molecules conjugated to photosensitizers, aiming at inducing cell death upon exposure to near-infrared (NIR) light following specific targeting of a fluorophore to the tumor cells. Here, the development and in vivo characterization of the triple-modality anti-PSMA targeting agent 111In-DTPA-D2B-IRDye700DX is described for both pre- and intra-operative tumor localization, image-guided surgery as well as for eradication of residual tumor tissue by tPDT.

Methods The anti-PSMA monoclonal antibody, D2B, was conjugated with IRDye700DX and DTPA and subsequently radiolabeled with 111In. To determine the optimal dose and time point of NIR light irradiation, BALB/c nude mice with s.c. LS174T-PSMA xenografts received 3, 10, or 30 µg of the conjugate intravenously (8 MBq/mouse, n=5 per group), followed by µSPECT/CT and NIR fluorescence imaging at 24, 48, 72, and 168 h p.i. to assess the localization of the IRDye700DX conjugate in the tumor. Tumor growth and overall survival of 3 mice injected with 30 µg of the conjugate followed by 30 min of NIR light irradiation at 72 h p.i. was compared to mice that did not receive any treatment.

Results Localization of the D2B-700DX conjugate in the PSMA-expressing tumors was clearly visualized both with SPECT/CT and NIR fluorescence imaging. Highest tumor accumulation of the conjugate was observed in mice that received 30 µg at 72 h p.i., which was confirmed by quantitative analysis of the SPECT data (73 ± 10% ID/g) and biodistribution studies at 168 h p.i. (52 ± 16% ID/g). tPDT with 30 µg of the tracer and irradiation at 72 h p.i. caused significant tumor growth inhibition compared to the tumor growth in control mice. Median survival in the treatment group was significantly improved from 12.6 to 23.3 days (p = 0.0339). Tumor lesions could be resected with image-guidance using intraoperative NIR fluorescence imaging.

Conclusions This study provided proof-of-principle that 111In-DTPA-D2B-IRDye700DX enables pre- and intra-operative visualization of PSMA+ tumors with radionuclide and fluorescence imaging, image-guided surgery and PSMA-targeted PDT. Optimal tumor destruction was achieved at a conjugate dose of 30 µg and a NIR-light irradiation at 72 h after injection of the conjugate.