An Albumin-Binding PSMA Ligand with Higher Tumor Accumulation for PET Imaging of Prostate Cancer

Abstract

Prostate-specific membrane antigen (PSMA) is an ideal target for the diagnosis and treatment of prostate cancer. Due to the short half-life in blood, small molecules/peptides are rapidly cleared by the circulatory system. Prolonging the half-life of PSMA probes has been considered as an effective strategy to improve the tumor detection. Herein, we reported a ⁶⁴Cu-labeled PSMA tracer conjugating with maleimidopropionic acid (MPA), ⁶⁴Cu-PSMA-CM, which showed an excellent ability to detect PSMA-overexpressing tumors in delayed time. Cell experiments in PSMA-positive 22Rv1 cells, human serum albumin binding affinity, and micro-PET imaging studies in 22Rv1 model were performed to investigate the albumin binding capacity and PSMA specificity. Comparisons with ⁶⁴Cu-PSMA-BCH were performed to explore the influence of MPA on the biological properties. ⁶⁴Cu-PSMA-CM could be quickly prepared within 30 min. The uptake of ⁶⁴Cu-PSMA-CM in 22Rv1 cells increased over time and it could bind to HSA with a high protein binding ratio (67.8 ± 1.5%). When compared to ⁶⁴Cu-PSMA-BCH, ⁶⁴Cu-PSMA-CM demonstrated higher and prolonged accumulation in 22Rv1 tumors, contributing to high tumor-to-organ ratios. These results showed that ⁶⁴Cu-PSMA-CM was PSMA specific with a higher tumor uptake, which demonstrated that MPA is an optional strategy for improving the radioactivity concentration in PSMA-expressing tumors and for developing the ligands for PSMA radioligand therapy.

Keywords: PET imaging; PSMA; copper radioisotopes; maleimidopropionic acid; prostate cancer.

Figure 1

The radio-labeling process of ⁶⁴Cu-PSMA-CM.

Figure 2

In vitro stability of ⁶⁴Cu-PSMA-CM. The radio-HPLC patterns of (a) ⁶⁴Cu-PSMA-CM after incubation in saline and (b) 5% HSA for 12 h. (c) The radio-chemical purity of ⁶⁴Cu-PSMA-CM in saline (blue) and 5% HSA (red) at 1 h, 4 h, 12 h, 24 h, and 48 h.

Figure 3

(a,b) were relative PSMA expression in 22Rv1 and PC3 cell lines. The expression of PSMA in 22Rv1 cells was significantly higher than in PC3 cells. The cell uptake of ⁶⁴Cu-PSMA-BCH and ⁶⁴Cu-PSMA-CM in (c) 22Rv1 cells or (d) PC-3 cells. *: p < 0.05, ***: p < 0.001, NS = not statistically significant. Block = 1 μg of ZJ-43.

Figure 4

The binding affinity of ⁶⁴Cu-PSMA-CM and ⁶⁴Cu-PSMA-BCH to PSMA in 22Rv1 cells. The dissociation constant values of ⁶⁴Cu-PSMA-BCH and ⁶⁴Cu-PSMA-CM were 0.59 nM and 4.58 nM, respectively.

Figure 5

Blood activity–time profile of ⁶⁴Cu-PSMA-CM in normal Kunming mice. The half-life of the distribution phase and elimination phase were 0.16 h and 3.93 h, respectively.

Figure 6

(a) Bio-distribution of ⁶⁴Cu-PSMA-CM in 22Rv1 mice at 1 h, 6 h, 24 h, and 48 h. (b) Tumor-to-organ ratios according to the distribution. LI—large intestine; SI—small intestine; T—tumor.

Figure 7

Micro-PET images of ⁶⁴Cu-PSMA-CM and comparison with ⁶⁴Cu-PSMA-BCH in 22Rv1 xenograft nude mice. Maximum intensity projection (MIP) images of ⁶⁴Cu-PSMA-CM in mice bearing 22Rv1 tumors (a) without or (b) with the co-injection of blocker ZJ-43 at 4 h, 8 h, 12 h, 24 h, and 48 h p.i., and (c) images of ⁶⁴Cu-PSMA-BCH at 4 h p.i. 12 h and 24 h p.i. in mice bearing 22Rv1 tumors. White arrows indicated 22Rv1 tumors. Immunohistochemical staining of (d) 22Rv1 xenografts tumor showed PSMA-positive (20 × amplifcation).