Studies toward the Development of New Silicon-Containing Building Blocks for the Direct ¹⁸F‑Labeling of Peptides

Silicon-containing prosthetic groups have been conjugated to peptides to allow for a single-step labeling with ¹⁸F radioisotope. The fairly lipophilic di-<i>tert</i>-butylphenylsilane building block contributes unfavorably to the pharmacokinetic profile of bombesin conjugates. In this article, theoretical and experimental studies toward the development of more hydrophilic silicon-based building blocks are presented. Density functional theory calculations were used to predict the hydrolytic stability of di-<i>tert</i>-butylfluorosilanes <b>2</b>–<b>23</b> with the aim to improve the in vivo properties of ¹⁸F-labeled silicon-containing biomolecules. As a further step toward improving the pharmacokinetic profile, hydrophilic linkers were introduced between the lipophilic di-<i>tert</i>-butylphenylsilane building block and the bombesin congeners. Increased tumor uptake was shown with two of these peptides in xenograft-bearing mice using positron emission tomography and biodistribution studies. The introduction of a hydrophilic linker is thus a viable approach to improve the tumor uptake of ¹⁸F-labeled silicon–bombesin conjugates

Design, Synthesis, and Initial Evaluation of a High Affinity Positron Emission Tomography Probe for Imaging Matrix Metalloproteinases <b>2</b> and <b>9</b>

The activity of matrix metalloproteinases (MMPs) is elevated locally under many pathological conditions. Gelatinases MMP2 and MMP9 are of particular interest because of their implication in angiogenesis, cancer cell proliferation and metastasis, and atherosclerotic plaque rupture. The aim of this study was to identify and develop a selective gelatinase inhibitor for imaging active MMP2/MMP9 in vivo. We synthesized a series of <i>N</i>-sulfonylamino acid derivatives with low to high nanomolar inhibitory potencies. (<i>R</i>)-2-(4-(4-Fluorobenzamido)­phenylsulfonamido)-3-(1<i>H</i>-indol-3-yl)­propanoic acid (<b>7</b>) exhibited the best in vitro binding properties: MMP2 IC₅₀ = 1.8 nM, MMP9 IC₅₀ = 7.2 nM. Radiolabeling of <b>7</b> with no carrier added ¹⁸F-radioisotope was accomplished starting from iodonium salts as precursors. The radiochemical yield strongly depended on the iodonium counteranion (ClO₄^– > Br^– > TFA^– > tosylate). ¹⁸F-<b>7</b> was obtained in up to 20% radiochemical yield (decay corrected), high radiochemical purity, and >90 GBq/μmol specific radioactivity. The radiolabeled compound showed excellent stability in vitro and in mice in vivo

Pilot Preclinical and Clinical Evaluation of (4S)-4-(3-[18F]Fluoropropyl)-L-Glutamate (18F-FSPG) for PET/CT Imaging of Intracranial Malignancies

<div><p>Purpose</p><p>(S)-4-(3-[¹⁸F]Fluoropropyl)-<i>L</i>-glutamic acid (18F-FSPG) is a novel radiopharmaceutical for Positron Emission Tomography (PET) imaging. It is a glutamate analogue that can be used to measure x_C^- transporter activity. This study was performed to assess the feasibility of 18F-FSPG for imaging orthotopic brain tumors in small animals and the translation of this approach in human subjects with intracranial malignancies.</p><p>Experimental Design</p><p>For the small animal study, GS9L glioblastoma cells were implanted into brains of Fischer rats and studied with 18F-FSPG, the 18F-labeled glucose derivative 18F-FDG and with the 18F-labeled amino acid derivative 18F-FET. For the human study, five subjects with either primary or metastatic brain cancer were recruited (mean age 50.4 years). After injection of 300 MBq of 18F-FSPG, 3 whole-body PET/Computed Tomography (CT) scans were obtained and safety parameters were measured. The three subjects with brain metastases also had an 18F-FDG PET/CT scan. Quantitative and qualitative comparison of the scans was performed to assess kinetics, biodistribution, and relative efficacy of the tracers.</p><p>Results</p><p>In the small animals, the orthotopic brain tumors were visualized well with 18F-FSPG. The high tumor uptake of 18F-FSPG in the GS9L model and the absence of background signal led to good tumor visualization with high contrast (tumor/brain ratio: 32.7). 18F-FDG and 18F-FET showed T/B ratios of 1.7 and 2.8, respectively. In the human pilot study, 18F-FSPG was well tolerated and there was similar distribution in all patients. All malignant lesions were positive with 18F-FSPG except for one low-grade primary brain tumor. In the 18F-FSPG-PET-positive tumors a similar T/B ratio was observed as in the animal model.</p><p>Conclusions</p><p>18F-FSPG is a novel PET radiopharmaceutical that demonstrates good uptake in both small animal and human studies of intracranial malignancies. Future studies on larger numbers of subjects and a wider array of brain tumors are planned.</p><p>Trial Registration</p><p>ClinicalTrials.gov <a href="https://clinicaltrials.gov/ct2/results?term=NCT01186601" target="_blank">NCT01186601</a></p></div

Demographic information for the 5 human subjects imaged with 18F-FSPG.

<p>Demographic information for the 5 human subjects imaged with 18F-FSPG.</p

Comparison of 18F-FSPG, 18F-FET and 18F-FDG uptake in rat GS9L brain tumors, healthy brain and blood at approximately 1 hr post injection.

<p>The tracer uptake was measured ex vivo and is shown as % ID/g values with standard deviation and ranges. Tumor-to-normal brain (T/brain) and tumor-to-blood ratios (T/blood) were derived from the respective mean values.</p

Overview of amino acid transporters being studied clinically using various PET tracers.

<p>Overview of amino acid transporters being studied clinically using various PET tracers.</p

Time-activity curves of 18F-FSPG, 18F-FDG and 18F-FET tumor-uptake ratio in orthotopic GS9L rat brain tumor lesions.

<p>A volume of interest (VOI) analysis over time was performed from the tumor lesions and the ratio calculated using the VOI from the healthy brain for each tracer. 18F-FSPG showed increasing VOI ratio activity over time. In contrast, 18F-FDG and 18F-FET VOI ratios remained rather constant and at a lower level.</p

18F-FSPG PET images, in comparison with MRI and 18F-FDG PET images of the three patients with lung cancer metastases to the brain.

<p>For each subject, the whole-body Maximum Intensity Projection (MIP) image of the 18F-FSPG PET scan is shown on the left. In the right column, the axial images through the level of the brain metastasis include the post-contrast T1 MRI (top), 18F-FSPG PET (middle), and 18F-FDG PET (bottom). Physiologic distribution to normal organs are highlighted on the MIP image for subject A, including the liver (l), pancreas (p), kidneys (k), and bladder (b). For the first two subjects (A, B), the small lesions (below 1.5 cm) are clearly visible on MRI and with 18F-FSPG (SUV-A: 11.0, SUV-B: 4.7). With 18F-FDG, however, there is no discernible activity in these lesions. The larger lesion for subject C, who had previously been treated in this region, is again clearly discernable with MRI although the etiology of the enhancement was unclear whether representing residual/recurrent disease versus post-therapy changes. Both the 18F-FDG (SUV 10.1) and 18F-FSPG PET (SUV 21.8) are positive for this subject, but the accumulation of the latter is stronger, and further enhanced by the lack of uptake in the surrounding normal brain tissue (FDG SUV 4.8, FSPG SUV 0.1).</p

Comparison of the SUV mean values at 60 minutes post-injection for 18F-FSPG and 18F-FDG across all patients (N = 5).

<p>Comparison of the SUV mean values at 60 minutes post-injection for 18F-FSPG and 18F-FDG across all patients (N = 5).</p

Comparison of axial post-contrast T1 MRI (left column) and 18F-FSPG PET (right column) in the two subjects with primary brain tumor.

<p>Subject A has recurrent high-grade glioblastoma, while subject B has an enlarging, partially resected low-grade oligodendroglioma. In each case, the arrows point to the primary lesions. For subject A, there is intense accumulation of 18F-FSPG in the enhancing lesion (SUV 5.2), while in subject B, there is no accumulation of the radiotracer (SUV 0.7) in the non-enhancing lesion. Comparatively, the background brain SUV is 0.1 for both subjects. There is incidental note of prominent physiologic uptake of 18F-FSPG in the scalp.</p