Synthesis and preclinical evaluation of novel ^99mTc-labeled PSMA ligands for radioguided surgery of prostate cancer

Background: Radioguided surgery (RGS) has recently emerged as a valuable new tool in the management of recurrent prostate cancer (PCa). After preoperative injection of a ^99mTc-labeled prostate-specific membrane antigen (PSMA) inhibitor, radioguided intraoperative identification and resection of lesions is facilitated by means of suitable γ-probes. First clinical experiences show the feasibility of RGS and suggest superiority over conventional lymph node dissection in recurrent PCa. However, commonly used [^99mTc]Tc-PSMA-I&S exhibits slow whole-body clearance, thus hampering optimal tumor-to-background ratios (TBR) during surgery. We therefore aimed to develop novel ^99mTc-labeled, PSMA-targeted radioligands with optimized pharmacokinetic profile to increase TBR at the time of surgery. Methods: Three ^99mTc-labeled N4-PSMA ligands were preclinically evaluated and compared to [^99mTc]Tc-PSMA-I&S. PSMA affinity (IC₅₀) and internalization were determined on LNCaP cells. Lipophilicity was assessed by means of the distribution coefficient logD_7.4 and an ultrafiltration method was used to determine binding to human plasma proteins. Biodistribution studies and static µSPECT/CT-imaging were performed at 6 h p.i. on LNCaP tumor-bearing CB17-SCID mice. Results: The novel N4-PSMA tracers were readily labeled with [^99mTc]TcO₄⁻ with RCP > 95%. Comparable and high PSMA affinity was observed for all [^99mTc]Tc-N4-PSMA-ligands. The ligands showed variable binding to human plasma and medium to low lipophilicity (logD_7.4 − 2.6 to − 3.4), both consistently decreased compared to [^99mTc]Tc-PSMA-I&S. Biodistribution studies revealed comparable tumor uptake among all [^99mTc]Tc-N4-PSMA-ligands and [^99mTc]Tc-PSMA-I&S, while clearance from most organs was superior for the novel tracers. Accordingly, increased TBR were achieved. [^99mTc]Tc-N4-PSMA-12 showed higher TBR than [^99mTc]Tc-PSMA-I&S for blood and all evaluated tissue. In addition, a procedure suitable for routine clinical production of [^99mTc]Tc-N4-PSMA-12 was established. Labeling with 553 ± 187 MBq was achieved with RCP of 98.5 ± 0.6% (n = 10). Conclusion: High tumor accumulation and favorable clearance from blood and non-target tissue make [^99mTc]Tc-N4-PSMA-12 an attractive tracer for RGS, possibly superior to currently established [^99mTc]Tc-PSMA-I&S. Its GMP-production according to a method presented here and first clinical investigations with this novel radioligand is highly recommended.