Preclinical Evaluation and First Patient Application of 99mTc-PSMA-I&S for SPECT Imaging and Radioguided Surgery in Prostate Cancer

Initial studies in patients have demonstrated the suitability of ¹¹¹In-PSMA-I&T (¹¹¹In-DOTAGA-(3-iodo-y)-f-k-Sub(KuE)) (PSMA is prostate-specific membrane antigen and I&T is imaging and therapy) for radioguided surgery (RGS) of small metastatic prostate Cancer (PCa) soft-tissue lesions. To meet the clinical need for a more cost-effective alternative, the PSMA-I&T-based tracer concept was adapted to ^99mTc-labeling chemistry. Two PSMA-I&T-derived inhibitors with all-L-serine- (MAS₃) and all-D-serine- (mas₃) chelating moieties were evaluated in parallel, and a kit procedure for routine ^99mTc labeling was developed.

Methods: PSMA affinities (IC₅₀) and internalization kinetics of ^99mTc-MAS₃-y-nal-k(Sub-KuE) and ^99mTc-mas₃-y-nal-k(Sub-KuE) (^99mTc-PSMA-I&S for imaging and surgery) were determined using LNCaP cells and (¹²⁵I-BA)KuE as a radioligand and reference standard. In vivo metabolite analyses and biodistribution studies were performed using CD-1 nu/nu and LNCaP tumor-bearing CB-17 severe combined immunodeficiency mice. The pharmacokinetics of ^99mTc-PSMA-I&S in humans were investigated in a patient with advanced metastatic PCa via sequential planar whole-body SPECT imaging at 1, 3, 5, and 21 h after injection. Additionally, preoperative SPECT/CT (12 h after injection) and ^99mTc-PSMA-I&S-supported RGS (16 h after injection) were performed in 1 PCa patient with proven iliac and inguinal lymph node metastases.

Results: A robust and reliable kit-labeling procedure was established, allowing the preparation of ^99mTc-MAS₃-y-nal-k(Sub-KuE) and ^99mTc-PSMA-I&S in consistently high radiochemical yield and purity (≥98%, n > 50 preparations). Because of its improved internalization efficiency and superior in vivo stability, ^99mTc-PSMA-I&S was selected for further in vivo evaluation. Compared with ¹¹¹In-PSMA-I&T, ^99mTc-PSMA-I&S showed delayed clearance kinetics but identical uptake in PSMA-positive tissues in the LNCaP xenograft model (1 h after injection). In exemplary PCa patients, a relatively slow whole-body clearance of ^99mTc-PSMA-I&S was observed due to high plasma protein binding (94%) of the tracer. This, however, promoted efficient tracer uptake in PCa lesions over time and led to steadily increasing lesion-to-background ratios up to 21 h after injection. Preoperative SPECT/CT showed a high ^99mTc-PSMA-I&S uptake in all suspect lesions identified in previous ⁶⁸Ga-HBED-CC-Ahx-KuE (⁶⁸Ga-HBED-CC-PSMA) PET/CT, allowing for their successful intraoperative detection and resection during first-in-human RGS.

Conclusion: Because of a straightforward and reliable kit production, ^99mTc-PSMA-I&S represents a cost-effective, readily available alternative to ¹¹¹In-PSMA-I&T. Initial patient data indicate its comparable or even superior performance as a probe for PSMA-targeted RGS and also hint toward the unexpected potential of ^99mTc-PSMA-I&S as a SPECT imaging agent.