Pilot study of a novel (18)F-labeled FSHR probe for tumor imaging

Follicle-stimulating hormone receptor (FSHR) is overexpressed in primary and metastatic tumor. Molecular imaging of FSHR is beneficial for prognosis and therapy of cancer. FSHβ(33-53) (YTRDLVYKDPARPKIQKTCTF), denoted as FSH1, is a FSHR antagonist. In the present study, maleimide-NOTA conjugate of FSH1 (NOTA-MAL-FSH1) was designed and labeled with [(18)F] aluminum fluoride. The resulting tracer, (18)F-Al-NOTA-MAL-FSH1, was preliminarily evaluated in PET imaging of FSHR-positive tumor.

NOTA-MAL-FSH1 was synthesized and radiolabeled with Al(18)F complex. The tumor-targeting potential and pharmacokinetic profile of the (18)F-labeled compound were evaluated in vitro and in vivo using a PC3 human prostate tumor model.

(18)F-Al-NOTA-MAL-FSH1 can be efficiently produced within 30 min with a non-decay-corrected yield of 48.6 ± 2.1 % and a radiochemical purity of more than 95 %. The specific activity was at least 30 GBq/μmol. The radiotracer was stable in phosphate-buffered saline and human serum for at least 2 h. The IC50 values of displacement (18)F-Al-NOTA-MAL-FSH1 with FSH1 were 252 ± 1.12 nM. The PC3 human prostate tumor xenografts were clearly visible with high contrast after injection of (18)F-Al-NOTA-MAL-FSH1 via microPET. At 30, 60 and 120 min postinjection, the tumor uptakes were 2.98 ± 0.29 % injected dose (ID)/g, 2.53 ± 0.20 %ID/g and 1.36 ± 0.12 %ID/g, respectively. Dynamic PET scanning showed that tumor uptake reached a plateau by about 6 min. Heart peaked earlier and then cleared quickly. Biodistribution studies confirmed that the normal organs except kidney uptakes were all below 1 %ID/g at 1 h p.i. The tumor-to-blood and tumor-to-muscle ratio at 10 min, 0.5, 1, and 2 h after injection were 1.64 ± 0.36, 2.97 ± 0.40, 9.31 ± 1.06, and 13.59 ± 2.33 and 7.05 ± 1.10, 10.10 ± 1.48, 16.17 ± 3.29, and 30.88 ± 4.67, respectively. The tracer was excreted mainly through the renal system, as evidenced by high levels of radioactivity in the kidneys. FSHR-binding specificity was also demonstrated by reduced tumor uptake of (18)F-Al-NOTA-MAL-FSH1 after coinjection with an excess of unlabeled FSH1 peptide.

NOTA-MAL-FSH1 could be labeled rapidly and efficiently with (18)F using one step method. Favorable preclinical data suggest that (18)F-Al-NOTA-MAL-FSH1 may be a suitable radiotracer for the non-invasive visualization of FSHR positive tumor in vivo.