Feasibility of Developing Radiotracers for MDM2: Synthesis and Preliminary Evaluation of an 18F-Labeled Analogue of the MDM2 Inhibitor SP-141

Abstract

Murine double minute 2 (MDM2), a negative regulator of the p53 tumor suppressor protein, is overexpressed in several human cancers. Herein we investigate the feasibility of developing 18F-labeled compounds based on the small molecule inhibitor SP-141 for imaging tumor MDM2 expression levels with positron emission tomography (PET). Three nonradioactive fluorinated SP-141 analogues, 1–3, were synthesized, and their binding to the MDM2 protein was analyzed by surface plasmon resonance (SPR). One of these, a fluoroethoxy analogue, was labeled with fluorine-18 (18F) using 18F-fluorethyl bromide to provide [18F]1 and evaluated in vitro and in vivo. SPR analysis confirmed the binding of the fluorinated analogues to MDM2 at 1.25–20 µM concentrations. Cell uptake studies revealed high uptake (67.5–71.4 %/mg protein) and specificity of [18F]1 in MCF7 and HepG2 cells. The uptake of [18F]1 in these cells could be modulated using 100 µM SP-141, potentially reflecting changes in MDM2 expression because of p53 activation by SP-141. [18F]1 exhibited stable uptake and retention in HepG2 tumor xenografts (~3 %ID/g) in vivo, but poor clearance from blood and other normal tissues, yielding low tumor-to-background ratios (< 2) at 2 h post injection. Our results suggest that [18F]1 has suboptimal characteristics for in vivo evaluation as a PET tracer for MDM2, but warrant radiolabeling and assessment of the other fluorinated analogues synthesized in this work, 2 and 3, and potentially other molecular scaffolds for developing MDM2 targeted radiotracers.

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

“…The lipophilicity of [ 18 F] 6 was determined by its partitioning between PBS (pH 7.4) and n -octanol by the shake-flask method . [ 18 F] 6 exhibited a log D 7.4 of 3.0 ± 0.1 ( n = 12).…”

“…With the goal of developing 18 F-labeled agents for imaging MDM2 with PET, we have previously evaluated an 18 F-labeled analogue of the well-studied small molecule MDM2 inhibitor SP-141 for this purpose. While the labeled SP-141 derivative had high uptake and specificity in MDM2 expressing tumor cell lines, it exhibited suboptimal biodistribution characteristics in mice, with poor clearance of 18 F-activity from normal tissues …”

“…With the emergence of molecularly specific imaging agents, the concept of utilizing PET for guiding patient selection and monitoring therapeutic response for targeted therapeutics has become feasible and attractive . With a goal of developing a PET agent for imaging MDM2, we previously evaluated a fluorine-18 ( 18 F, t 1/2 = 109.8 min) labeled MDM2 inhibitor based on the well-studied MDM2 inhibitor SP-141. , The 18 F-labeled SP-141 analogue showed high uptake and specific binding in MDM2 expressing wild-type p53 tumor cell lines but had suboptimal pharmacokinetics in mice bearing HepG2 tumor xenografts . In the present study, the potential utility of RG7388 (Idasanutlin) was evaluated for developing MDM2 targeted imaging agents based on its well-characterized nultin chemical scaffold .…”

“…13,14 The 18 F-labeled SP-141 analogue showed high uptake and specific binding in MDM2 expressing wild-type p53 tumor cell lines but had suboptimal pharmacokinetics in mice bearing HepG2 tumor xenografts. 13 In the present study, the potential utility of RG7388 (Idasanutlin) was evaluated for developing MDM2 targeted imaging agents based on its well-characterized nultin chemical scaffold. 15 RG7388 is a potent, oral MDM2 inhibitor that has been extensively investigated in both preclinical and clinical studies.…”

MDM2/X Inhibitors as Radiosensitizers for Glioblastoma Targeted Therapy

“…PET/SPECT imaging agents for the oncoprotein MDM2 and p53 are limited at the moment. MDM2 antisense oligonucleotides were radiolabeled with [ 99m Tc], MDM2 inhibitor SP-141 was radiolabeled with [ 18 F] and the peptide PM2 was radiolabeled with [ 125 I], all in a pre-clinical stage ( 137 , 240 ). Next to diagnostic information that radiolabeled MDM2/X inhibitors can reveal, they could also be useful for targeted radionuclide therapy when labelled with therapeutic radionuclides.…”