Fluorine-18 Labeling of the MDM2 Inhibitor RG7388 for PET Imaging: Chemistry and Preliminary Evaluation

Abstract

RG7388 (Idasanutlin) is a potent inhibitor of oncoprotein murine double minute 2 (MDM2). Herein we investigated the feasibility of developing 18 F-labeled RG7388 as a radiotracer for imaging MDM2 expression in tumors with positron emission tomography (PET). Two fluorinated analogues of RG7388, 6 and 7, were synthesized by attaching a fluoronicotinyl moiety to RG7388 via a polyethylene glycol (PEG 3 ) or a propyl linker. The inhibitory potency (IC 50 ) of 6 and 7 against MDM2 was determined by a fluorescence polarization (FP)-based assay. Next, compound 6 was labeled with 18 F using a trimethylammonium triflate precursor to obtain [ 18 F]FN-PEG 3 -RG7388 ([ 18 F]6), and its properties were evaluated in MDM2 expressing wild-type p53 tumor cell lines (SJSA-1 and HepG2) in vitro and in tumor xenografts in vivo. The FP assays revealed an IC 50 against MDM2 of 119 nM and 160 nM for 6 and 7, respectively. 18 F-labeling of 6 was achieved in 50.3 ± 7.5% radiochemical yield. [ 18 F]6 exhibited a high uptake (∼70% of input dose) and specificity in SJSA-1 and HepG2 cell lines. Saturation binding assays revealed a binding affinity (K d ) of 128 nM for [ 18 F]6 on SJSA-1 cells. In mice, [ 18 F]6 showed fast clearance from blood with a maximum tumor uptake of 3.80 ± 0.85% injected dose per gram (ID/g) in HepG2 xenografts at 30 min postinjection (p.i.) and 1.32 ± 0.32% ID/g in SJSA-1 xenografts at 1 h p.i. Specificity of [ 18 F]6 uptake in tumors was demonstrated by pretreatment of mice with SJSA-xenografts with a blocking dose of RG7388 (35 mg/kg body weight, i.p.). In vivo stability studies in mice using HPLC showed ∼60% and ∼30% intact [ 18 F]6 remaining in plasma at 30 min and 1 h p.i., respectively, with the remaining activity attributed to polar peaks. Our results suggest that RG7388 is a promising molecular scaffold for 18 F-labeled probe development for MDM2. Additional labeling strategies and functionalizing locations on RG7388 are under development to improve binding affinity and in vivo stability of the 18 F-labeled compound to make it more amenable for PET imaging of MDM2 in vivo.

Affinity probes based on small-molecule inhibitors for tumor imaging

“…( 40 ) attempted to change the glutamine structure in the linker to a benzene structure to obtain 16 based on the structure of 15 , the probe showed slower clearance and lower affinity than 15 because of the introduction of the benzene ring structure on the linker. The same is true for the principle of designing radionuclide probes and modifications in the linker area can significantly improve the tumor uptake rate and in vivo pharmacokinetics ( 37 , 62 ).…”

“…However, this loss is acceptable because it does not considerably affect the imaging effect of the probe (Figures 6A, B). Another method to add F to the noncritical area of the inhibitor, such as the PEG chain (28), which can also avoid the damage of steric hindrance to the affinity, can improve the metabolic kinetics of the probe and is conducive to the imaging effect (62,85). However, it is necessary to verify the affinity of probes by molecular docking and affinity experiments.…”

Chemical Probes and Activity-Based Protein Profiling for Cancer Research

“…Moreover, dual imaging of Bcl-2/MDM2 facilitated the detection of apoptosis marker proteins ITPR1, GSR, RER1, PDIA3, Apoa1, and Tnfrsf17 by LC–MS/MS in various cancer cells [ 68 ]. Another class of MDM2-targeted probes, RG7388 (idasanutlin), was developed using an 18 F-labeled radiotracer to determine MDM2 expression in tumors [ 69 ].…”