Abstract
The prostate-specific membrane antigen (PSMA)–targeted radiotracers 68Ga/177Lu-PSMA-I&T and 99mTc-PSMA-I&S (for imaging and surgery) are currently successfully used for clinical PET imaging, radionuclide therapy, and radioguided surgery of metastatic prostate cancer. To additionally exploit the high sensitivity and spatial resolution of fluorescence imaging for improved surgical guidance, a PSMA-I&T–based hybrid tracer, PSMA-I&F (DOTAGA-k(Sulfo-Cy5)-y-nal-k-Sub-KuE), has been developed and evaluated. Methods: The in vitro PSMA-targeting efficiency of PSMA-I&F, the reference PSMA-I&T, and their corresponding natGa-/68Ga- and natLu/177Lu counterparts was determined in LNCaP cells via competitive binding assays (IC50) and dual-tracer radioligand and fluorescence internalization studies. Biodistribution and small-animal PET imaging studies were performed in CB17 SCID and LNCaP xenograft–bearing SHO mice, respectively, and complemented by intraoperative far-red fluorescence imaging using a clinical laparoscope. Additionally, fully automated serial cryosectioning and fluorescence imaging of 1 tumor-bearing animal as well as PSMA immunohistochemistry and fluorescence microscopy of organ cryosections (tumor, kidney, spleen) were also performed. Results: Compared with the parent PSMA-I&T analogs, the PSMA affinities of PSMA-I&F and its natGa-/natLu-complexes remained high and unaffected by dye conjugation (7.9 < IC50 < 10.5 nM for all ligands). The same was observed for the internalization of 68Ga- and 177Lu-PSMA-I&F. In vivo, blood clearance of 68Ga- and 177Lu-PSMA-I&F was only slightly delayed by high plasma protein binding (94%–95%), and very low accumulation in nontarget organs was observed already at 1 h after injection. Dynamic PET imaging confirmed PSMA-specific (as demonstrated by coinjection of 2-PMPA) uptake into the LNCaP xenograft (4.5% ± 1.8 percentage injected dose per gram) and the kidneys (106% ± 23 percentage injected dose per gram). Tumor-to-background ratios of 2.1, 5.2, 9.6, and 9.6 for blood, liver, intestines, and muscle, respectively, at 1 h after injection led to excellent imaging contrast in 68Ga-PSMA-I&F PET and in intraoperative fluorescence imaging. Furthermore, fluorescence imaging of tissue cryosections allowed high-resolution visualization of intraorgan PSMA-I&F distribution in vivo and its correlation with PSMA expression as determined by immunohistochemistry. Conclusion: Thus, with its high PSMA-targeting efficiency and favorable pharmacokinetic profile, 68Ga/177Lu-PSMA-I&F serves as an excellent proof-of-concept compound for the general feasibility of PSMA-I&T–based hybrid imaging. The PSMA-I&T scaffold represents a versatile PSMA-targeted lead structure, allowing relatively straightforward adaptation to the different structural requirements of dedicated nuclear or hybrid imaging agents.
Highlights
In 68Ga-prostate-specific membrane antigen (PSMA)-I&F PET and in intraoperative fluorescence imaging
Synthesis and Radiolabeling PSMA-I&F was synthesized via a mixed solid-phase/solutionphase synthetic strategy, with coupling of Sulfo-Cy5-COOH to the fully deprotected, DOTAGA-functionalized backbone as the last step
The specific activities of 68Ga-PSMAI&F and 177Lu-PSMA-I&F were 61 and 55 GBq/mmol, respectively, and radiochemical purity as determined by radio–thin-layer chromatography was greater than 98% for both compounds
Summary
In 68Ga-PSMA-I&F PET and in intraoperative fluorescence imaging. fluorescence imaging of tissue cryosections allowed highresolution visualization of intraorgan PSMA-I&F distribution in vivo and its correlation with PSMA expression as determined by immunohistochemistry. In turn, has been facilitated by the relative tolerance of the most commonly used central PSMA binding motif, EuX (glutamate-urea-X, with X 5 lysine, glutamate, or cysteine), toward diverse and even bulky chemical modifications [1,4,10] This allows relatively straightforward adjustment of the tracer structure to the requirements for the respective labeling strategy (99mTc, 18F, diagnostic and therapeutic M31 radiometals) without compromising PSMA-targeting efficiency and has promoted the rapid expansion of PSMA-targeted theranostics toward a-therapy using 213Bi [11,12] or 225Ac [13]. Fluorescence imaging supports real-time visual image guidance, it suffers from severe attenuation in tissue and is not suitable to detect metastases in deeper-lying lymph nodes [25] In this context, the use of multimodal, or rather, hybrid tracers that combine g-emission and fluorescence in one (targeted) molecule merges the best of both modalities. The various prostate cancer–related studies performed with this tracer have demonstrated how a hybrid tracer design can help improve radioguidance procedures [27], thereby triggering the recent development of various dedicated hybrid nuclear/fluorescence probes [28,29,30,31,32,33]
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