Synthesis of MMP inhibitor radiotracer [11C]CGS 25966, a new potential pet tumor imaging agent

Molecular Imaging Probes Based on Matrix Metalloproteinase Inhibitors (MMPIs)

Matrix metalloproteinases (MMPs) are a family of zinc- and calcium-dependent endopeptidases which are secreted or anchored in the cell membrane and are capable of degrading the multiple components of the extracellular matrix (ECM). MMPs are frequently overexpressed or highly activated in numerous human diseases. Owing to the important role of MMPs in human diseases, many MMP inhibitors (MMPIs) have been developed as novel therapeutics, and some of them have entered clinical trials. However, so far, only one MMPI (doxycycline) has been approved by the FDA. Therefore, the evaluation of the activity of a specific subset of MMPs in human diseases using clinically relevant imaging techniques would be a powerful tool for the early diagnosis and assessment of the efficacy of therapy. In recent years, numerous MMPIs labeled imaging agents have emerged. This article begins by providing an overview of the MMP subfamily and its structure and function. The latest advances in the design of subtype selective MMPIs and their biological evaluation are then summarized. Subsequently, the potential use of MMPI-labeled diagnostic agents in clinical imaging techniques are discussed, including positron emission tomography (PET), single-photon emission computed tomography (SPECT) and optical imaging (OI). Finally, this article concludes with future perspectives and clinical utility.

A comprehensive review on controls in molecular imaging: lessons from MMP-2 imaging

Metalloproteinases (MMPs), including MMP-2, play critical roles in tissue remodeling and are involved in a large array of pathologies, including cancer, arthritis and atherosclerosis. Their prognostic value warranted a large investment or resources in the development of noninvasive detection methods, based on probes for many current clinical and pre-clinical imaging modalities. However, the potential of imaging techniques is only matched by the complexity of the data they generate. This complexity must be properly assessed and accounted for in the early steps of probe design and testing in order to accurately determine the efficacy and efficiency of an imaging strategy. This review proposes basic rules for the evaluation of novel probes by addressing the specific case of MMP targeted probes.

Positron emission tomography tracers for imaging angiogenesis

Position emission tomography imaging of angiogenesis may provide non-invasive insights into the corresponding molecular processes and may be applied for individualized treatment planning of antiangiogenic therapies. At the moment, most strategies are focusing on the development of radiolabelled proteins and antibody formats targeting VEGF and its receptor or the ED-B domain of a fibronectin isoform as well as radiolabelled matrix metalloproteinase inhibitors or alpha(v)beta(3) integrin antagonists. Great efforts are being made to develop suitable tracers for different target structures. All of the major strategies focusing on the development of radiolabelled compounds for use with positron emission tomography are summarized in this review. However, because the most intensive work is concentrated on the development of radiolabelled RGD peptides for imaging alpha(v)beta(3) expression, which has successfully made its way from bench to bedside, these developments are especially emphasized.

Radionuclide imaging of tumor angiogenesis

Angiogenesis is a multistep process regulated by pro- and antiangiogenic factors. In order to grow and metastasize, tumors need a constant supply of oxygen and nutrients. For growth beyond 1-2 mm in size, tumors are dependent on angiogenesis. Inhibition of angiogenesis is a new cancer treatment strategy that is now widely investigated clinically. Researchers have begun to search for objective measures that indicate pharmacologic responses to antiangiogenic drugs. Therefore, there is a great interest in techniques to visualize angiogenesis in growing tumors noninvasively. Several markers have been described that are preferentially expressed on newly formed blood vessels in tumors (alpha(v)beta(3) integrin, vascular endothelial growth factor, and its receptor, prostate-specific membrane antigen) and in the extracellular matrix surrounding newly formed blood vessels (extra domain B of fibronectin, Tenascin-C, matrix metalloproteinases, and Robo-4). Several ligands targeting these markers have been tested as a radiotracer for imaging angiogenesis in tumors. The potential of some of these tracers, such as radiolabeled cyclic RGD peptides and radiolabeled anti-PSMA antibodies, has already been tested in cancer patients, while for markers such as Robo-4, the ligand has not yet been identified. In this review, an overview on the currently used nuclear imaging probes for noninvasive visualization of tumor angiogenesis is given.

Noninvasive tracer techniques to characterize angiogenesis

Great efforts are being made to develop antiangiogenesis drugs for treatment of cancer as well as other diseases. Some of the compounds are already in clinical trials. Imaging techniques allowing noninvasive monitoring of corresponding molecular processes can provide helpful information for planning and controlling corresponding therapeutic approaches but will also be of interest for basic science. Current nuclear medicine techniques focus on the development of tracer targeting the vascular endothelial growth factor (VEGF) system, matrix metalloproteinases (MMP), the ED-B domain of a fibronectin isoform, and the integrin alphavbeta3. In this chapter, the recent tracer developments as well as the preclinical and the clinical evaluations are summarized and the potential of the different approaches to characterize angiogenesis are discussed.

Synthesis and preliminary biological evaluation of new radioiodinated MMP inhibitors for imaging MMP activity in vivo

Non-invasive measurement of matrix metalloproteinase (MMP) activity in vivo is a clinical challenge in many disease processes such as inflammation, tumor metastasis and atherosclerosis. Therefore, radioiodinated analogues of the non-peptidyl broad-spectrum MMP inhibitor (MMPI) CGS 27023A 1a were synthesized for non-invasive detection of MMP activity in vivo using single photon emission computed tomography (SPECT). The compounds Br-CGS 27023A 1b and HO-CGS 27023A 1d were synthesized from the amino acid D-valine and used as precursors for radioiodinated derivatives of CGS 27023A and their non-radioactive references I-CGS 27023A 1c and HO-I-CGS 27023A 1e. Radioiodination of the precursors with [(123)I]NaI or [(125)I]NaI produced the no-carrier-added MMP inhibitors [(123)I]I-CGS 27023A 1f, [(125)I]I-CGS 27023A 1g, HO-[(123)I]I-CGS27023A 1h, and HO-[(125)I]I-CGS 27023A 1i. In vitro studies showed that the non-radioactive analogues of the MMP inhibitors exhibited affinities against gelatinase A (MMP-2) and gelatinase B (MMP-9) in the nanomolar range, comparable to the parent compound CGS 27023A. In vivo biodistribution using HO-[(125)I]I-CGS 27023A 1i in CL57 Bl6 mice showed rapid blood and plasma clearance and low retention in normal tissues. The preliminary biological evaluation warrant further studies of these radioiodinated MMP inhibitors as potential new radiotracers for imaging MMP activity in vivo.

Synthesis of [18F]Xeloda as a novel potential PET radiotracer for imaging enzymes in cancers

Xeloda (Capecitabine), a prodrug of antitumor agent 5-fluorouracil, is the first and only oral fluoropyrimidine to be approved for use as second-line therapy in metastatic breast cancer, colorectal cancer, and other solid malignancies. Fluorine-18 labeled Xeloda may serve as a novel radiotracer for positron emission tomography (PET) to image enzymes such as thymidine phosphorylase and uridine phosphorylase in cancers. The precursor 2',3'-di-O-acetyl-5'-deoxy-5-nitro-N(4)-(pentyloxycarbonyl)cytidine (11) was synthesized from D-ribose and cytosine in 8 steps with approximately 18% overall chemical yield. The reference standard 5'-deoxy-5-fluoro-N(4)-(pentyloxycarbonyl)cytidine (Xeloda; 1) was synthesized from D-ribose and 5-fluorocytosine in eight steps with approximately 28% overall chemical yield. The target radiotracer 5'-deoxy-5-[(18)F]fluoro-N(4)-(pentyloxycarbonyl)cytidine ([(18)F]Xeloda; [(18)F]1) was prepared by nucleophilic substitution of the nitro-precursor with K(18)F/Kryptofix 2.2.2 followed by a quick deprotection reaction and purification with the HPLC method in 20-30% radiochemical yields.

Comparative studies of potential cancer biomarkers carbon-11 labeled MMP inhibitors (S)-2-(4'-[11C]methoxybiphenyl-4-sulfonylamino)-3-methylbutyric acid and N-hydroxy-(R)-2-[[(4'-[11C]methoxyphenyl)sulfonyl]benzylamino]-3-methylbutanamide

(S)-2-(4'-[11C]methoxybiphenyl-4-sulfonylamino)-3-methylbutyric acid ([11C]MSMA) and N-hydroxy-(R)-2-[[(4'-[11C]methoxyphenyl)sulfonyl]benzylamino]-3-methylbutanamide ([11C]CGS 25966), carbon-11 labeled matrix metalloproteinase (MMP) inhibitors, have been synthesized for evaluation as new potential positron emission tomography (PET) cancer biomarkers. [11C]MSMA was prepared by appropriate precursor (S)-2-(4'-hydroxybiphenyl-4-sulfonylamino)-3-methylbutyric acid tert-butyl ester, which was synthesized in eight steps from amino acid (L)-valine in 39.4% chemical yield. This precursor was labeled by [11C]methyl triflate through O-[11C]methylation method at the hydroxyl position of biphenol under basic conditions, followed by a quick acid hydrolysis and isolated by solid-phase extraction (SPE) purification to produce pure target compound [11C]MSMA in 35-55% radiochemical yield, based on 11CO2, decay corrected to end of bombardment (EOB), and 20-25 min synthesis time. [11C]CGS 25966 was prepared in our previous work starting from amino acid (D)-valine. The biodistribution of [11C]MSMA and [11C]CGS 25966 were determined at 45 min post iv injection in breast cancer animal models MCF-7's transfected with IL-1alpha implanted athymic mice and MDA-MB-435 implanted athymic mice. The results showed the uptakes of [11C]MSMA and [11C]CGS 25966 in these tumors were 0.95 and 0.42%dose/g in MCF-7's transfected with IL-1alpha implanted mice, 0.98 and 1.53%dose/g in MDA-MB-435 implanted mice, respectively; the ratios of tumor/muscle (T/M) and tumor/blood (T/B) were 1.21 and 1.09 (T/M, MCF-7's), 0.99 and 0.84 (T/B, MCF-7's), 1.38 and 1.27 (T/M, MDA-MB-435), 1.27 and 1.95 (T/B, MDA-MB-435), respectively. The micro-PET images of [11C]MSMA and [11C]CGS 25966 in both breast cancer athymic mice were acquired for 15 min from a MCF-7's transfected with IL-1alpha and/or MDA-MB-435 implanted mouse at 45 min post iv injection of 1 mCi of the tracer using a dedicated high resolution (<3 mm full-width at half-maximum) small FOV (field-of-view) PET imaging system, Indy-PET II scanner, developed in our laboratory, which showed both tumors were invisible with both tracers. The results were compared. From our results, we concluded that both [11C]MSMA and [11C]CGS 25966 might be unsuitable as PET tracers for cancer imaging.

Synthesis, biodistribution and micro-PET imaging of a potential cancer biomarker carbon-11 labeled MMP inhibitor (2R)-2-[[4-(6-fluorohex-1-ynyl)phenyl]sulfonylamino]-3-methylbutyric acid [11C]methyl ester

(2R)-2-[[4-(6-fluorohex-1-ynyl)phenyl]sulfonylamino]-3-methylbutyric acid [(11)C]methyl ester ([(11)C]FMAME), a novel carbon-11 labeled matrix metalloproteinase (MMP) inhibitor, has been synthesized for evaluation as new potential positron emission tomography (PET) cancer biomarker. [(11)C]FMAME was prepared by appropriate precursor (2R)-2-[[4-(6-fluorohex-1-ynyl)phenyl]sulfonylamino]-3-methylbutyric acid (FMA), which was synthesized in six steps from (D)-valine in 71% chemical yield. This acid precursor was labeled by [(11)C]methyl triflate through O-[(11)C]methylation method under basic conditions and isolated by solid-phase extraction (SPE) purification to produce pure target compound in 40-55% radiochemical yield, based on (11)CO(2), decay corrected to end of bombardment, and 15-20 min synthesis time. The biodistribution of [(11)C]FMAME was determined at 30 min post IV injection in breast cancer animal models MCF-7 transfected with IL-1 alpha implanted athymic mice and MDA-MB-435 implanted athymic mice. The results showed the uptakes of [(11)C]FMAME in these tumors were 1.13% dose/g in MCF-7 transfected with IL-1 alpha implanted mice and 1.37% dose/g in MDA-MB-435 implanted mice, respectively; the ratios of tumor/muscle (T/M) and tumor/blood (T/B) were 1.05 +/- 0.29 (T/M, MCF-7's), 0.77 +/- 0.20 (T/B, MCF-7's) and 0.99 +/- 0.35 (T/M, MDA-MB-435), 1.44 +/- 0.69 (T/B, MDA-MB-435), respectively. Pretreatment of MCF-7 transfected with IL-1 alpha tumor-bearing mice with MMP inhibitor FMA had no effect on [(11)C]FMAME biodistribution. Likewise, pretreatment of MDA-MB-435 tumor-bearing mice with FMA also showed no effect on [(11)C]FMAME biodistribution. The micro-PET images were acquired for 15 min from a MCF-7 transfected with IL-1 alpha tumor-bearing mouse or a MDA-MB-435 tumor-bearing mouse at 30 min post IV injection of 1 mCi of [(11)C]FMAME using a dedicated high resolution (<3 mm full-width at half-maximum) PET imaging system (Indy-PET II scanner). The initial dynamic micro-PET images of [(11)C]FMAME in a MCF-7 transfected with IL-1 alpha tumor-bearing mouse during different time periods of 0-15, 15-30, 30-45 and 45-60 min were performed by Indy-PET II. The PET images clearly showed both tumors were visible with [(11)C]FMAME. These results suggest that the localization of [(11)C]FMAME in the tumor is mediated by non-specific processes, and the visualization of [(11)C]FMAME on the tumor using the Indy-PET II scanner is related to non-specific binding.

Synthesis of radiolabeled biphenylsulfonamide matrix metalloproteinase inhibitors as new potential PET cancer imaging agents

Novel matrix metalloproteinase (MMP) inhibitor radiotracers, (S)-3-methyl-2-(2',3',4'-methoxybiphenyl-4-sulfonylamino)-butyric acid [(11)C]methyl ester (1a-c), (S)-3-methyl-2-(2',3',4'-fluorobiphenyl-4-sulfonylamino)-butyric acid [(11)C]methyl ester (1d-f), and (S)-3-methyl-2-(4'-nitrobiphenyl-4-sulfonylamino)-butyric acid [(11)C]methyl ester (1g), a series of substituted biphenylsulfonamide derivatives, have been synthesized for evaluation as new potential positron emission tomography (PET) cancer imaging agents.