Synthesis and evaluation of technetium-99m- and rhenium-labeled inhibitors of the prostate-specific membrane antigen (PSMA)

Abstract

The prostate-specific membrane antigen (PSMA) is increasingly recognized as a viable target for imaging and therapy of cancer. We prepared seven (99m)Tc/Re-labeled compounds by attaching known Tc/Re chelating agents to an amino-functionalized PSMA inhibitor (lys-NHCONH-glu) with or without a variable length linker moiety. K i values ranged from 0.17 to 199 nM. Ex vivo biodistribution and in vivo imaging demonstrated the degree of specific binding to engineered PSMA+ PC3 PIP tumors. PC3-PIP cells are derived from PC3 that have been transduced with the gene for PSMA. Despite demonstrating nearly the lowest PSMA inhibitory potency of this series, [(99m)Tc(CO)3( L1)] (+) ( L1 = (2-pyridylmethyl)2N(CH2) 4CH(CO2H)NHCO-(CH2) 6CO-NH-lys-NHCONH-glu) showed the highest, most selective PIP tumor uptake, at 7.9 +/- 4.0% injected dose per gram of tissue at 30 min postinjection. Radioactivity cleared from nontarget tissues to produce a PIP to flu (PSMA-PC3) ratio of 44:1 at 120 min postinjection. PSMA can accommodate the steric requirements of (99m)Tc/Re complexes within PSMA inhibitors, the best results achieved with a linker moiety between the epsilon amine of the urea lysine and the chelator.

Chart 1

Urea-Based PSMA Inhibitors^a

Chart 2

Urea-Based PSMA Ligands L1–L7

Figure 1

Binding mode of L1 to the active site of PSMA (A). The corresponding contour map is shown in (B).

Figure 2

Fluorescence microscopy of PSMA+ PC-3 PIP cells and PSMA– PC-3 flu cells using ReL2.

Figure 3

SPECT-CT imaging of tumor bearing mice with [^99mTc]L1–L4 (A–D, respectively). Dual pinhole SPECT-CT of PC-3 PIP and PC-3 flu tumor bearing mice. Mice were injected with 0.5–1 mCi (19–37 MBq) of radiopharmaceutical iv followed by a 45 min uptake period. Note essentially no uptake in the PSMA– flu tumors in each case. Abdominal radioactivity is primarily due to uptake within liver, spleen and kidneys. The horizontal lines in B are due to a reconstruction artifact at the boundaries of the field-of-view. PIP = PC-3 PIP; flu = PC-3 flu; GB = gallbladder in C; red circles highlight the location of the kidneys in D; L = left, R = right.

Figure 4

SPECT-CT imaging of tumor bearing mice with [^99mTc]L1 and [^99mTc]L3 (A and B, respectively). Dual pinhole SPECT-CT of PC-3 PIP and PC-3 flu tumor bearing mice. Mice were injected with 0.5–1 mCi (19–37 MBq) of radiopharmaceutical iv followed by a 3.5–4 h uptake period. Note lack of radiopharmaceutical outside of tumor in (A); however, the kidneys are outside of the field of view.

Figure 5

SPECT-CT imaging of LNCaP (PSMA+) tumor bearing mice with [^99mTc]L1 with (left) and without (right) blockade of PSMA using the potent, selective PSMA inhibitor, PMPA, as the blocking agent. Lack of radiopharmaceutical in both the tumor and kidneys (another PSMA+ site) upon cotreatment with PMPA provides a further check on PSMA-specific binding. Images were acquired from 30–60 min postinjection. T = tumor; K = kidney.

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