Augmentation of [¹⁸F]-C-SNAT4 PET imaging of apoptosis after radiotherapy using a cold mixing strategy

Positron Emission Tomography (PET) imaging can monitor cancer treatment response by non-invasively detecting apoptosis in vivo. Signal-to-noise (SNR) remains one of the critical barriers to approval for clinical use. We have previously developed a PET tracer [18 F]-C-SNAT4 for imaging capase-3 activity in apoptotic tumors induced by chemo- and immunotherapy. [18 F]-C-SNAT4 is designed to undergo caspase-3 activated intramolecular cyclization. The product then self-assembles in situ into nanoparticles to generate preferential retention of F18 radioactivity in apoptotic cells. This unique mechanism prompted us to investigate if a cold mixture could enhance the probe retention and further augment the sensitivity for imaging radiotherapy.

[18 F]-C-SNAT4 and hot/cold mixture [18 F]/[19 F]-C-SNAT4 were used to detect human NSCLC (NCI-H460) apoptosis induced by radiation. Both hot [18 F]-C-SNAT4 and hot/cold mixture [18 F]/[19 F]-C-SNAT4 had significantly increased uptake in radiation treated vs. untreated NCI-H460 cells in vitro. A 1: 80 hot/cold mixture increased signal by 1.6x compared to [18 F]-C-SNAT4 alone. In vivo studies were performed in murine xenograft models in high-dose radiation and low-dose radiation treatment groups. The hot/cold mixture showed an increase in the signal by 2.5x in high-dose radiation treated murine NCI-H460 xenograft models. Low-dose radiation induced apoptosis was only detected with the hot/cold mixture with 2.4x signal compared to hot [18 F]-C-SNAT4. Toxicity and dosimetry safety were evaluated at 250x and 10x respective dosages, then normalized to human dose equivalent.

A hot/cold mixture of [18 F]/[19 F]-C-SNAT4 generates significantly more signal compared to hot [18 F]-C-SNAT4, leading to higher sensitivity in detecting treatment response. This may present a solution to low sensitivity in the translation of apoptosis-specific radionuclides to clinical application.