Fluorine-substituted ligands for the peroxisome proliferator-activated receptor gamma (PPARgamma): potential imaging agents for metastatic tumors

Nuclear Receptor Imaging In Vivo-Clinical and Research Advances

Nuclear receptors are transcription factors that function in normal physiology and play important roles in diseases such as cancer, inflammation, and diabetes. Noninvasive imaging of nuclear receptors can be achieved using radiolabeled ligands and positron emission tomography (PET). This quantitative imaging approach can be viewed as an in vivo equivalent of the classic radioligand binding assay. A main clinical application of nuclear receptor imaging in oncology is to identify metastatic sites expressing nuclear receptors that are targets for approved drug therapies and are capable of binding ligands to improve treatment decision-making. Research applications of nuclear receptor imaging include novel synthetic ligand and drug development by quantifying target drug engagement with the receptor for optimal therapeutic drug dosing and for fundamental research into nuclear receptor function in cells and animal models. This mini-review provides an overview of PET imaging of nuclear receptors with a focus on radioligands for estrogen receptor, progesterone receptor, and androgen receptor and their use in breast and prostate cancer.

A bile acid derivative with PPARγ-mediated anti-inflammatory activity

During our search for bioactive secondary metabolites in the jellyfish-derived fungus Penicillium chrysogenum J08NF-4, several bile acid derivatives (2-6) were isolated along with a new steroidal artifact (1). An in vitro anti-inflammatory assay showed that pretreatment with 1 suppressed NO production and the gene expressions of the pro-inflammatory mediators iNOS and TNF-α in LPS-induced RAW 264.7 macrophages. Docking analysis of 1 revealed that it might bind to the ligand binding domain (LBD) of PPARγ in a manner similar to that of the synthetic steroid mifepristone (7), which is used clinically to treat hypercortisolism and was recently reported to be a PPARγ agonist. Compound 1 activated PPARγ in murine Ac2F liver cells and suppressed the LPS-induced phosphorylation of the NF-κB p65 subunit leading to downregulation of pro-inflammatory mediators. Our findings suggest that 1 acts as a steroidal PPARγ activator that downregulates the expressions of pro-inflammatory mediators by suppressing the NF-κB signaling pathway.

Synthesis and evaluation of 18F-labeled PPARγ antagonists

INTRODUCTION

Peroxisome proliferator-activated receptor gamma (PPARγ) transcriptionally modulates fat metabolism and also plays a role in pathological conditions such as cancer, neurodegenerative disease and inflammation. PPARγ imaging agents are potential tools for investigating these diseases.

METHODS

Four analogs of GW9662, a PPARγ antagonist, with different fluorine-containing substituents at the para-position of the aniline ring were synthesized and evaluated using two different receptor binding assays for measuring PPARγ affinity. Micro-positron emission tomography (PET) imaging studies were performed in a transgenic mouse model having a heart-specific overexpression of PPARγ.

RESULTS

All four analogs were found to have binding affinities that were comparable to or better than the reference antagonist, GW9662, using a scintillation proximity assay (SPA). However, only the chloro-based analogs (compounds 3 and 4) had activity in a whole-cell assay measuring activation of the PPARγ/retinoid X receptor complex. The microPET imaging studies in an MHC-PPARγ transgenic mouse model showed high uptake and PPARγ-specific binding for the irreversible antagonist [(18)F]3, whereas the corresponding reversible methoxy analog ([(18)F]5) displayed only nonspecific uptake in heart.

CONCLUSIONS

The results of this preliminary study show that the irreversible antagonist [(18)F]3 may represent a novel strategy for imaging PPARγ in vivo with PET.

Fluorine-18 labeling and biodistribution studies on peroxisome proliferator-activated receptor-gamma ligands: potential positron emission tomography imaging agents

INTRODUCTION

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is an important regulator of lipid metabolism; it controls the differentiation of preadipocytes and is also found at high levels in small metastatic tumors. In this report, we describe the radiochemical synthesis and evaluation of two (18)F-labeled analogs of the potent and selective PPARgamma agonist farglitazar.

MATERIALS AND METHODS

The isomeric aromatic fluorine-substituted target compounds [(2S)-(2-benzoylphenylamino)-3-(4-(2-[2-(4-[(18)F]fluorophenyl)-5-methyloxazol-4-yl]ethoxy)-phenyl)propionic acid ([(18)F]-1) and (2S)-[2-(4-fluorobenzoyl)phenylamino]-3-(4-[2-(5-methyl-2-phenyloxazol-4-yl)ethoxy]-phenyl)propionic acid ([(18)F]-2)] were prepared in fluorine-18-labeled form, respectively, by radiofluorination of an iodonium salt precursor or by Ullmann-type condensation with 2-iodo-4'-[(18)F]fluorobenzophenone after nucleophilic aromatic substitution with [(18)F]fluoride ion. Each compound was obtained in high specific activity and good radiochemical yield.

RESULTS AND DISCUSSION

(18)F-1 and (18)F-2 have high and selective PPARgamma binding affinities comparable to that of the parent molecule farglitazar, and they were found to have good metabolic stability. Tissue biodistribution studies of (18)F-1 and (18)F-2 were conducted, but PPARgamma-mediated uptake of both agents was minimal.

CONCLUSION

This study completes our first look at an important class of PPARgamma ligands as potential positron emission tomography (PET) imaging agents for breast cancer and vascular disease. Although (18)F-1 and (18)F-2 have high affinities for PPARgamma and good metabolic stability, their poor target-tissue distribution properties, which likely reflect their high lipophilicity combined with the low titer of PPARgamma in target tissues, indicate that they have limited potential as PPARgamma PET imaging agents.

Synthesis and evaluation of a bromine-76-labeled PPARγ antagonist 2-bromo-5-nitro-N-phenylbenzamide

Peroxisome proliferator activated-receptor gamma (PPARgamma) binds to peroxisome receptor response elements with its heterodimeric partner, retinoid X receptor, and regulates downstream gene expression. PPARgamma transcriptionally modulates fat metabolism, and receptor agonists have been developed to treat type II diabetes. PPARgamma is also overexpressed in some tumor cell lines and primary tumors, including breast and prostate tumors. Two PPARgamma antagonists, 2-chloro-5-nitro-N-phenylbenzamide (GW9662) and 2-chloro-5-nitro-N-pyridin-4-yl-benzamide (T0070907), represent good lead compounds for radiotracer development. In the current study, four additional halogen substituted analogs were synthesized and evaluated in a whole cell screening assay for PPARgamma binding activity. Two bromine-containing analogs having EC50 values <5 nM were chosen for bromine-76 radiolabeling. Bromine-76-labeled 2-bromo-5-nitro-N-phenyl-benzamide was selected for subsequent in vitro and in vivo studies due to its superior radiolabeling yield (approximately 70%) and the well-characterized pharmacological properties of its analog GW9662. An in vitro stability study showed that 40% of the compound remained intact in plasma and about 25% in whole blood after 30 min. Biodistribution studies in MDA-MB-435 human breast tumor-bearing nude mice were carried out at 5 min, 30 min, 2 h and 24 h post injection of the radiotracer. Although in vivo metabolite studies demonstrated rapid compound degradation, at least 10% of the parent compound was delivered to the tumor. We are currently exploring second generation analogs of these lead compounds for the development of radiolabeled antagonists of the PPARgamma receptor.