Utilizing the Multiradionuclide Resolving Power of SPECT and Dual Radiolabeled Single Molecules to Assess Treatment Response of Tumors

Abstract

Purpose

Single photon emission computed tomography (SPECT) radionuclide pairs having distinct decay rates and different energy maxima enable simultaneous detection of dual gamma signals and real-time assessment of dynamic functional and molecular processes in vivo. Here, we report image acquisition and quantification protocols for a single molecule labeled with two different radionuclides for functional SPECT imaging.

Procedures

LS370 and LS734 were prepared using modular solid phase peptide synthesis. Each agent has a caspase-3 cleavable reporting motif, flanked by a tyrosine residue and a chelator at the opposite end of molecule. Cell uptake and efflux were assessed in human MDA-MB-231 breast cancer cells. Biodistribution studies were conducted in tumor naive and orthotopic 4T1 metastatic breast cancer tumor mice. NanoSPECT dual-imaging validation and attenuation correction parameters were developed using phantom vials containing varying radionuclide concentrations. Proof-of-principle SPECT imaging was performed in MMTV-PyMT transgenic mice.

Results

LS370 and LS734 were singly or dually radiolabeled with ¹²⁵I and ¹¹¹In or ^99mTc. Cell assays demonstrated 11-fold higher percent uptake (P < 0.001) of [¹²⁵I]LS734 (3.6 ± 0.5) compared to [¹²⁵I]LS370 (0.3 ± 0.3) at 2 h. Following chemotherapy, cellular retention of [¹²⁵I]LS734 was 3-fold higher (P < 0.05) than untreated cells. Pharmacokinetics at 1 h postinjection demonstrated longer blood retention (%ID/g) for [¹²⁵I]LS734 (3.2 ± 0.9) compared to [¹²⁵I]LS370 (1.6 ± 0.1). In mice bearing bilateral orthotopic 4T1 tumors, the uptake (%ID/g) was 2.4 ± 0.3 for [¹²⁵I]LS734 and 1.2 ± 0.03 for [¹²⁵I]LS370. The iodinated tyrosine peptide residue label was stable under in vitro conditions for up to 24 h; rapid systemic deiodination (high thyroid uptake) was observed in vivo. Phantom studies using standards demonstrated deconvolution of radionuclide signals based on different gamma ray energies. In MMTV-PyMT mice imaged with dual-labeled [¹¹¹In]–[¹²⁵I]LS734, the gamma signals were separable and quantifiable.

Conclusions

Image processing protocols were developed for quantitative signal separation resulting from a caspase-3 responsive dual-radiolabeled SPECT probe. Crosstalk unmixing was obtained for multiradionuclide NanoSPECT imaging. In vitro and in vivo data demonstrated structure–activity relationships for developing functional agents for ratiometric SPECT imaging.