[^99mTc]Tc-PSMA-HSG for PSMA-targeted Hybrid Surgical Guidance: a new addition to the PSMA-I&S/I&T family

To develop a dual-labeled PSMA-targeted tracer for radio- and fluorescence-guided surgery (RGS/FGS) with enhanced clinical utility due to optimized pharmacokinetics and tumor targeting.

Four novel hybrid PSMA ligands with varying cyanine-based fluorophores were comprehensively characterized preclinically. On the basis of its excellent in vitro (logD, plasma protein binding, PSMA-affinity, internalization) and in vivo (stability, clearance kinetics, tumor uptake in LNCaP and PC3-PIp tumor-bearing mice) profile, [^99mTc]Tc-PSMA-HSG was selected as the clinical lead compound. Five patients with primary and recurrent prostate cancer underwent [^99mTc]Tc-PSMA-HSG SPECT/CT and RGS. Tracer dosimetry was calculated using a MIRDcalc v1.22 protocol.

The PSMA affinity (IC₅₀=38.4 ± 5.3 nM), hydrophilicity (logD =-2.94), and human plasma protein binding of [^99mTc]Tc-PSMA-HSG were all nearly identical to those of the non-fluorescent parent compound [^99mTc]Tc-PSMA-I&S. Tumor uptake in mice was 11.8 ± 1.5%ID/g at 6 h p.i. (vs. 6.4 ± 1.0%ID/g for [^99mTc]Tc-PSMA-I&S). In and ex vivo fluorescence imaging in mice confirmed tumor localization with high signal-to-background ratios. In patients, [^99mTc]Tc-PSMA-HSG showed faster clearance and less background uptake than [^99mTc]Tc-PSMA-I&S, with notably reduced salivary gland and intestinal accumulation, but a slightly higher whole body effective dose (0.011 ± 0.003 vs. 0.0052 mSv/MBq). Intraoperative gamma detection revealed lymph node metastases in 6/6 tracer-avid lesions, which were confirmed by PSMA-HSG fluorescence microscopy and histopathology. The specificity, selectivity, NPV and PPV of [^99mTc]Tc-PSMA-HSG in RGS were 100%, respectively.

The hybrid tracer [^99mTc]Tc-PSMA-HSG demonstrates high specificity and favorable pharmacokinetics. Its successful first-in-human application highlights its translational potential for precise intraoperative detection of PSMA-positive lymph node metastases.