Second generation Al18F-labeled D-amino acid peptide for CXCR4 targeted molecular imaging

BACKGROUND

The C-X-C chemokine receptor type 4 (CXCR4) is overexpressed in many cancers, e.g. multiple myeloma and acute leukemia, yet solely [68Ga]PentixaFor is used for clinical PET imaging. The aim of this study was to develop and assess a second generation Al18F-labeled D-amino acid peptide based on the viral macrophage inflammatory protein II for CXCR4 targeted molecular imaging.

METHODS

We designed a library of monomer and multimer constructs and evaluated their binding affinity for human and mouse CXCR4. Based on these results, we selected the best vector molecule for development of an Al18F-labeled ligand, [18F]AlF-NOTA-2xDV1(c11sc12s), which was further evaluated in a cell-based binding assay to assess its binding properties and specificity for CXCR4. Next, pharmacokinetics and tumor uptake of [18F]AlF-NOTA-2xDV1(c11sc12s) were evaluated in naïve mice and mice with xenografts derived from U87.CXCR4 cells. Finally, we performed an imaging study in a non-human primate to assess the in vivo distribution of this novel radioligand in a species closely related to humans.

RESULTS

The lead ligand AlF-NOTA-2xDV1(c11sc12s) showed six-fold higher affinity for human CXCR4 compared to Ga-Pentixafor. The corresponding radiotracer was obtained in a good radiochemical yield of 40.1 ± 13.5 % (n = 4) and apparent molar activity of 20.4 ± 3.3 MBq/nmol (n = 4) after optimization. In U87.CD4.CXCR4 cell binding assays, the total bound fraction of [18F]AlF-NOTA-(2×)DV1(c11sc12s) was 32.4 ± 1.8 %. This fraction could be reduced by 82.5 % in the presence of 75 μM AMD3100. In naïve mice, [18F]AlF-NOTA-2xDV1(c11sc12s) accumulated in organs expressing mouse CXCR4, e.g. the liver (SUVmean (mean standardized uptake value) 75 min p.i. 11.7 ± 0.6), which was blockable by co-injecting AMD3100 (5 mg/kg). In U87.CXCR4 xenografted tumor mice, the tumor uptake of [18F]AlF-NOTA-2xDV1(c11sc12s) remained low (SUVmean 0.5 ± 0.1), but was reduced by co-administration of AMD3100. Surprisingly, [18F]AlF-NOTA-2xDV1(c11sc12s) exhibited a similar biodistribution in a non-human primate as in mice indicating off-target binding of [18F]AlF-NOTA-2xDV1(c11sc12s) in liver tissue. We confirmed that [18F]AlF-NOTA-2xDV1(c11sc12s) is taken up by hepatocytes using in vitro studies and that the uptake can be blocked with AMD3100 and rifampicin, a potent organic anion-transporting-polypeptide (OATP)1B1 and OATP1B3 inhibitor.

CONCLUSION

The second generation D-peptide AlF-NOTA-2xDV1(c11sc12s) showed high affinity for human CXCR4 and the corresponding radiotracer was produced in good radiochemical yields. However, [18F]AlF-NOTA-2xDV1(c11sc12s) is not specific for CXCR4 and is also a substrate for OATP1B1 and/or OATP1B3, known to mediate hepatic uptake. Therefore, D-amino acid peptides, based on the viral macrophage inflammatory protein II, are not the prefered vector molecule for the development of CXCR4 targeting molecular imaging tools.

Preparation and Evaluation of [18F]AlF-NOTA-PBB for PET Imaging of Cyclin-dependent Kinase 4/6 in Tumors

Cyclin-dependent kinases (CDKs), especially cyclin-dependent kinase 4/6 (CDK4/6), have been targets for the development of specific tumor imaging agents. Palbociclib is a highly selective CDK4/6 inhibitor. In this study, to develop a novel 18F-labeled palbociclib derivative for specific tumor imaging, we designed and synthesized a ligand (NOTA-PBB) consisting of palbociclib as the targeted pharmacophore and NOTA as the macrocyclic bifunctional chelator. The corresponding [18F]AlF-NOTA-PBB complex was prepared with high radiochemical purity (98.4 ± 0.15%) and yield (58.7 ± 4.5%) within 35 min without requiring HPLC purification through a simple one-step 18F-labeling strategy of NOTA-AlF chelation chemistry. The radiotracer was lipophilic (log P = 0.095 ± 0.003) and had good stability in vitro and in vivo. The cellular uptake studies performed on the MCF-7 breast cancer cell line (ER-positive and HER2-negative) showed that radioactive uptake was blocked by preincubating with a molar dose of palbociclib and it had a nanomolar binding affinity to CDK4/6 (IC50 = 16.23 ± 1.84 nM), demonstrating a CDK4/6-mediated uptake mechanism. Its ex vivo biodistribution in nude mice-bearing MCF-7 tumors showed obvious tumor uptake and a high tumor/muscle ratio of [18F]AlF-NOTA-PBB, and tumor uptake was inhibited with 100 μg of palbociclib, demonstrating specific binding to CDK4/6. Radioactivity accumulation in MCF-7 tumors was observed in PET imaging with [18F]AlF-NOTA-PBB. Based on the results of this work, [18F]AlF-NOTA-PBB has the promising capability as a CDK4/6-targeted tumor imaging agent.

3p-C-NETA: A versatile and effective chelator for development of Al18F-labeled and therapeutic radiopharmaceuticals

Background: Radiolabeled somatostatin analogues (e.g. [⁶⁸Ga]Ga-DOTATATE and [¹⁷⁷Lu]Lu-DOTATATE) have been used to diagnose, monitor, and treat neuroendocrine tumour (NET) patients with great success. [¹⁸F]AlF-NOTA-octreotide, a promising ¹⁸F-labeled somatostatin analogue and potential alternative for ⁶⁸Ga-DOTA-peptides, is under clinical evaluation. However, ideally, the same precursor (combination of chelator-linker-vector) can be used for production of both diagnostic and therapeutic radiopharmaceuticals with very similar (e.g. Al¹⁸F-method in combination with therapeutic radiometals ²¹³Bi/¹⁷⁷Lu) or identical (e.g. complementary Tb-radionuclides) pharmacokinetic properties, allowing for accurate personalised dosimetry estimation and radionuclide therapy of NET patients. In this study we evaluated 3p-C-NETA, as potential theranostic Al¹⁸F-chelator and present first results of radiosynthesis and preclinical evaluation of [¹⁸F]AlF-3p-C-NETA-TATE. Methods: 3p-C-NETA was synthesized and radiolabeled with diagnostic (⁶⁸Ga, Al¹⁸F) or therapeutic (¹⁷⁷Lu, ¹⁶¹Tb, ²¹³Bi, ²²⁵Ac and ⁶⁷Cu) radionuclides at different temperatures (25-95 °C). The in vitro stability of the corresponding radiocomplexes was determined in phosphate-buffered saline (PBS) and human serum. 3p-C-NETA-TATE was synthesized using standard solid/liquid-phase peptide synthesis. [¹⁸F]AlF-3p-C-NETA-TATE was synthesized in an automated AllinOne® synthesis module and the in vitro stability of [¹⁸F]AlF-3p-C-NETA-TATE was evaluated in formulation buffer, PBS and human serum. [¹⁸F]AlF-3p-C-NETA-TATE pharmacokinetics were evaluated using µPET/MRI in healthy rats, with [¹⁸F]AlF-NOTA-Octreotide as benchmark. Results: 3p-C-NETA quantitatively sequestered ¹⁷⁷Lu, ²¹³Bi and ⁶⁷Cu at 25 °C while heating was required to bind Al¹⁸F, ⁶⁸Ga, ¹⁶¹Tb and ²²⁵Ac efficiently. The [¹⁸F]AlF-, [¹⁷⁷Lu]Lu- and [¹⁶¹Tb]Tb-3p-C-NETA-complex showed excellent in vitro stability in both PBS and human serum over the study period. In contrast, [⁶⁷Cu]Cu- and [²²⁵Ac]Ac-, [⁶⁸Ga]Ga-3p-C-NETA were stable in PBS, but not in human serum. [¹⁸F]AlF-3p-C-NETA-TATE was obtained in good radiochemical yield and radiochemical purity. [¹⁸F]AlF-3p-C-NETA-TATE displayed good in vitro stability for 4 h in all tested conditions. Finally, [¹⁸F]AlF-3p-C-NETA-TATE showed excellent pharmacokinetic properties comparable with the results obtained for [¹⁸F]AlF-NOTA-Octreotide. Conclusions: 3p-C-NETA is a versatile chelator that can be used for both diagnostic applications (Al¹⁸F) and targeted radionuclide therapy (²¹³Bi, ¹⁷⁷Lu, ¹⁶¹Tb). It has the potential to be the new theranostic chelator of choice for clinical applications in nuclear medicine.

Synthesis, preclinical evaluation, and first-in-human study of Al18F-PSMA-Q for prostate cancer imaging

PURPOSE

To investigate the potential of a novel Al¹⁸F-labeled PSMA-targeted radiotracer for PCa diagnosis through both preclinical and pilot clinical studies.

METHODS

Al¹⁸F-PSMA-Q was prepared automatically. The binding affinity to PSMA was evaluated in vitro using the 22Rv1 (PSMA +) and PC-3 (PSMA -) cell lines. Pharmacokinetics evaluation, biodistribution study, Micro-PET imaging of Al¹⁸F-PSMA-Q in normal mice and tumor-bearing mice, and a comparison with ¹⁸F-DCFPyL were performed. PET/CT imaging was performed on 8 healthy volunteers and 20 newly diagnosed PCa patients at 1 h post-injection (p.i.). The biodistribution in human and preliminary diagnostic efficacy of Al¹⁸F-PSMA-Q were evaluated, and the radiation dosimetry was estimated using OLINDA/EXM 2.0 software.

RESULT

Qualified Al¹⁸F-PSMA-Q was efficiently prepared with a non-decay-corrected radiochemical yield (RCY) of 22.0-28.3%, a specific activity (SA) of > 50 GBq/μmol. The hydrophilicity was comparably high with a log P value of - 3.69 ± 0.39. Al¹⁸F-PSMA-Q was found to bind to PSMA specifically with a Ki value of 17.05 ± 1.14 nM. The distribution and elimination half-lives of Al¹⁸F-PSMA-Q were 3.93 min and 14.22 min, respectively, which were shorter than those of ¹⁸F-DCFPyL. Micro-PET imaging of Al¹⁸F-PSMA-Q can clearly differentiate 22Rv1 tumors from PC-3 tumors and background with a high SUVmax of 2.17 ± 0.42 and a tumor-to-muscle ratio of 84.37 ± 31.62, which were higher than those of ¹⁸F-DCFPyL (1.79 ± 0.39 and 13.25 ± 1.65). The uptake of Al¹⁸F-PSMA-Q in 22Rv1 cells and tumors can be substantially blocked by 2-PMPA. High level accumulation of Al¹⁸F-PSMA-Q was observed in organs physiologically expressing PSMA. Twenty-six tumor lesions were detected in 20 PCa patients, and the mean SUVmax values of primary tumors, lymph node metastasis, bone metastases, and tumor-muscle ratios were 19.71 ± 16.52, 5.11, 31.30 ± 29.85, and 44.77 ± 22.29, respectively. The mean SUVmax of tumors in patients with PSA > 10 ng/mL was significantly higher than that in patients with PSA ≤ 10 ng/mL (25.97 ± 18.64 vs. 10.33 ± 3.74). Meanwhile, the mean SUVmax of tumors in patients with a Gleason score ≥ 8 was significantly higher than that in patients with a Gleason score < 8 (31.85 ± 22.09 vs. 13.18 ± 11.58). The kidneys received the highest estimated dose of 0.098 ± 0.006 mGy/MBq, and the effective dose was calculated as 0.0128 ± 0.007 mGy/MBq.

CONCLUSION

The novel qualified PSMA-targeted radiotracer Al¹⁸F-PSMA-Q was conveniently prepared with favorable yield and SA. The results of preclinical and pilot clinical studies exhibited a high specific uptake in PCa lesions and an excellent tumor-to-background ratio with a reasonable radiation exposure, which indicated the great potential of Al¹⁸F-PSMA-Q for PCa imaging.

TRIAL REGISTRATION

Chinese Clinical trial registry ChiCTR2100053507, Registered 23 November 2021, retrospectively registered.

Clinical translational evaluation of Al18F-NOTA-FAPI for fibroblast activation protein-targeted tumour imaging

PURPOSE

In this study, a novel aluminium-[¹⁸F]fluoride (Al¹⁸F)-labelled 1,4,7‑triazacyclononane-N,N',N″-triacetic acid (NOTA)-conjugated fibroblast activation protein inhibitor (FAPI) probe, named Al¹⁸F-NOTA-FAPI, was developed for fibroblast activation protein (FAP)-targeted tumour imaging; it could deliver hundreds of millicuries of radioactivity using automated synthesis. The tumour detection efficacy of Al¹⁸F-NOTA-FAPI was further validated in both preclinical and clinical translational studies.

METHODS

The radiolabelling procedure of Al¹⁸F-NOTA-FAPI was optimized. Cell uptake and competitive binding assays were completed with the U87MG and A549 cell lines to evaluate the affinity and specificity of the Al¹⁸F-NOTA-FAPI probe. The biodistribution, pharmacokinetics, radiation dosimetry and tumour imaging efficacy of the Al¹⁸F-NOTA-FAPI probe were researched in healthy Kunming (KM) and/or U87MG model mice. After the approval of the ethical committee, the Al¹⁸F-NOTA-FAPI probe was translated into the clinic for PET/CT imaging of the first 10 cancer patients.

RESULTS

The radiolabelling yield of Al¹⁸F-NOTA-FAPI was 33.8 ± 3.2% using manual synthesis (n = 10), with a radiochemical purity over 99% and the specific activity of 9.3-55.5 MBq/nmol. The whole body effective dose of Al¹⁸F-NOTA-FAPI was estimated to be 1.24E - 02 mSv/MBq, which was lower than several other FAPI probes (⁶⁸Ga-FAPI-04, ⁶⁸Ga-FAPI-46 and ⁶⁸Ga-FAPI-74). In U87MG tumour-bearing mice, Al¹⁸F-NOTA-FAPI showed good tumour detection efficacy based on the results of micro PET/CT imaging and biodistribution studies. In an organ biodistribution study of patients, Al¹⁸F-NOTA-FAPI showed a lower SUVmean than 2-[¹⁸F]-fluoro-2-deoxy-D-glucose (2-[¹⁸F]FDG) in most organs, especially in the liver (1.1 ± 0.2 vs. 2.0 ± 0.9), brain (0.1 ± 0.0 vs. 5.9 ± 1.3), and bone marrow (0.9 ± 0.1 vs. 1.7 ± 0.4). Meanwhile, Al¹⁸F-NOTA-FAPI did not show extensive bone uptake, and was able to detect more lesions than 2-[¹⁸F]FDG in the PET/CT imaging of several patients.

CONCLUSION

The Al¹⁸F-NOTA-FAPI probe was successfully fabricated and applied in fibroblast activation protein-targeted tumour PET/CT imaging, which showed excellent imaging quality and tumour detection efficacy in U87MG tumour-bearing mice as well as in cancer patients.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR2000038080. Registered 09 September 2020. http://www.chictr.org.cn/showproj.aspx?proj=61192.

Automatic radiosynthesis and preclinical evaluation of 18F-AlF-PSMA-NF as a potential PET probe for prostate cancer imaging

Facile automatic production is important for the application of prostate-specific membrane antigen (PSMA) tracers in clinical practice. We developed a new ¹⁸F-AlF-labelled PSMA probe-¹⁸F-AlF-PSMA-NF-and explore its automated production method and potential value in clinical settings. ¹⁸F-AlF-PSMA-NF was prepared using an automated method with dimethylformamide (DMF) as the solvent in a positron emission tomography (PET)-MF-2 V-IT-I synthesizer. Tracer characteristics were examined both in vitro and in vivo. Micro-PET/computed tomography (CT) was performed to investigate the utility of ¹⁸F-AlF-PSMA-NF for imaging PSMA-positive tumours in vivo. ¹⁸F-AlF-PSMA-NF was prepared automatically within 35 min with a non-attenuation correction yield of 37.9 ± 11.2%. The tracer was hydrophilic, had a high affinity for PSMA (Kd = 2.58 ± 0.81 nM), and showed stability in both in vitro and in vivo conditions. In the cellular experiments, ¹⁸F-AlF-PSMA-NF uptake in PSMA-positive LNCaP cells was significantly higher than that in PSMA-negative PC-3 cells (P < 0.001), and could be blocked by excess ZJ-43-a PSMA inhibitor (P < 0.001). LNCaP tumours were clearly visualized by ¹⁸F-AlF-PSMA-NF on micro-PET/CT, with a high level of uptake (13.72 ± 2.01 percent injected dose per gram of tissue [%ID/g]) and high tumour/muscle ratio (close to 50:1). The PSMA-positive LNCaP tumours had a significantly higher uptake than PSMA-negative PC-3 tumours (13.72 ± 2.01%ID/g vs. 1.07 ± 0.48%ID/g, t = 10.382, P < 0.001), and could be blocked by ZJ-43 (13.72 ± 2.01%ID/g vs. 2.77 ± 1.44%ID/g, t = 8.14, P < 0.001). A new ¹⁸F-AlF-labelled PSMA probe-¹⁸F-AlF-PSMA-NF-was successfully developed and can be prepared automatically. It has the biological characteristics resembling that of a PSMA-based probe and can potentially be used in clinical settings.

Development of an Albumin-Based PSMA Probe With Prolonged Half-Life

Prostate-specific membrane antigen (PSMA) is an attractive target for the diagnosis and therapy of prostate cancer as it is specifically overexpressed in prostate cancer cells. Improving the circulation of radioligands in the blood is considered as an effective strategy that can improve tumor burden, which benefits detection of small lesions and improves the effect of PSMA radioligand therapy (PRLT). In this study, we introduced maleimidopropionic acid (MPA) to a PSMA-targeted tracer and developed Al¹⁸F-PSMA-CM, which targets human serum albumin (HSA) binding and PSMA. Al¹⁸F-PSMA-CM is evaluated in vitro and in vivo for stability, PSMA specificity, and biodistribution in 22Rv1 tumor-bearing mice. Al¹⁸F-PSMA-CM was prepared with a radiochemical purity of >99% and specific activity of 11.22–18.70 MBq/nmol. Al¹⁸F-PSMA-CM was stable in vitro and in vivo and prolonged circulation in blood with a binding ratio of 47 ± 3.2% and Kd value of 3.08 ± 0.45 nM to HSA. The uptake of Al¹⁸F-PSMA-CM in PSMA(+) 22Rv1 cells was increased in 2 h, and the uptake was blocked by a PSMA inhibitor, ZJ-43. The Kd value of Al¹⁸F-PSMA-CM to PSMA was 8.46 ± 0.24 nM. Al¹⁸F-PSMA-CM was accumulated in kidneys and 22Rv1 tumors [74.76 ± 15.42 and 6.16 ± 0.74 ID%/g at 2 h post injection (p.i.)], which were decreased by −80.0 and −84.3% when co-injected with ZJ-43. Al¹⁸F-PSMA-CM showed high PSMA specificity and accumulated in 22Rv1 tumors with increasing uptake in 4 h. MPA moiety showed the ability to prolong the half-life of tracers, and the MPA-conjugated tracer showed the potential to improve tumor uptake. MPA may be a choice to develop radiopharmaceuticals for PRLT of prostate cancer.

Positron emission tomography imaging of a novel Anxa1-targeted peptide 18 F-Al-NODA-Bn-p-SCN-GGGRDN-IF7 in A431 cancer mouse models

Annexin 1 (Anxa1) is a highly specific surface marker of tumor vasculature in the lung and prostate solid tumors. The IF7 peptide was modified with a hydrophilic linker, GGGRDN, and coupled with a new bifunctional chelating agent NODA-Bn-p-SCN. The resulting peptides (NODA-Bn-p-SCN-GGGRDN-IF7) were successfully labeled with Al¹⁸ F. The targeting characteristics of the radiolabeled peptides were evaluated in the Anxa1 positive A431 tumor model. Micro-positron emission tomography (micro-PET) imaging revealed that the A431 tumors were clearly visualized (5.74 ± 1.13%ID/g, 3.92 ± 0.78%ID/g and 1.30 ± 0.43%ID/g at 0.5, 1, and 2 h post-injection, respectively). Anxa1 binding specificity was also demonstrated by reduced tumor uptake after co-injection with excessive unlabeled GGGRDN-IF7 peptide at 30, 60, and 120 min post-injection. ¹⁸ F-Al-NODA-Bn-p-SCN-GGGRDN-IF7 might be a potential PET imaging agent for detecting Anxa1 levels in cancers due to the favorable characteristics such as convenient synthesis, specific Anxa1 targeting, and good tumor uptakes.

Automated GMP compliant production of [18F]AlF-NOTA-octreotide

BACKGROUND

Gallium-68 labeled synthetic somatostatin analogs for PET/CT imaging are the current gold standard for somatostatin receptor imaging in neuroendocrine tumor patients. Despite good imaging properties, their use in clinical practice is hampered by the low production levels of 68Ga eluted from a 68Ge/68Ga generator. In contrast, 18F-tracers can be produced in large quantities allowing centralized production and distribution to distant PET centers. [18F]AlF-NOTA-octreotide is a promising tracer that combines a straightforward Al18F-based production procedure with excellent in vivo pharmacokinetics and specific tumor uptake, demonstrated in SSTR2 positive tumor mice. However, advancing towards clinical studies with [18F]AlF-NOTA-octreotide requires the development of an efficient automated GMP production process and additional preclinical studies are necessary to further evaluate the in vivo properties of [18F]AlF-NOTA-octreotide. In this study, we present the automated GMP production of [18F]AlF-NOTA-octreotide on the Trasis AllinOne® radio-synthesizer platform and quality control of the drug product in accordance with GMP. Further, radiometabolite studies were performed and the pharmacokinetics and biodistribution of [18F]AlF-NOTA-octreotide were assessed in healthy rats using μPET/MR.

RESULTS

The production process of [18F]AlF-NOTA-octreotide has been validated by three validation production runs and the tracer was obtained with a final batch activity of 10.8 ± 1.3 GBq at end of synthesis with a radiochemical yield of 26.1 ± 3.6% (dc), high radiochemical purity and stability (96.3 ± 0.2% up to 6 h post synthesis) and an apparent molar activity of 160.5 ± 75.3 GBq/μmol. The total synthesis time was 40 ± 3 min. Further, the quality control was successfully implemented using validated analytical procedures. Finally, [18F]AlF-NOTA-octreotide showed high in vivo stability and favorable pharmacokinetics with high and specific accumulation in SSTR2-expressing organs in rats.

CONCLUSION

This robust and automated production process provides high batch activity of [18F]AlF-NOTA-octreotide allowing centralized production and shipment of the compound to remote PET centers. Further, the production process and quality control developed for [18F]AlF-NOTA-octreotide is easily implementable in a clinical setting and the tracer is a potential clinical alternative for somatostatin directed 68Ga labeled peptides obviating the need for a 68Ge/68Ga-generator. Finally, the favorable in vivo properties of [18F]AlF-NOTA-octreotide in rats, with high and specific accumulation in SSTR2 expressing organs, supports clinical translation.

Effect of Chelate Type and Radioisotope on the Imaging Efficacy of 4 Fibrin-Specific PET Probes

Thrombus formation plays a major role in cardiovascular diseases, but noninvasive thrombus imaging is still challenging. Fibrin is a major component of both arterial and venous thrombi and represents an ideal candidate for imaging of thrombosis. Recently, we showed that (64)Cu-DOTA-labeled PET probes based on fibrin-specific peptides are suitable for thrombus imaging in vivo; however, the metabolic stability of these probes was limited. Here, we describe 4 new probes using either (64)Cu or aluminum fluoride (Al(18)F) chelated to 2 NOTA derivatives.

METHODS

Probes were synthesized using a known fibrin-specific peptide conjugated to either NODAGA (FBP8, FBP10) or NOTA-monoamide (FBP9, FBP11) as chelators, followed by labeling with (64)Cu (FBP8 and FBP9) or Al(18)F (FBP10 and FBP11). PET imaging efficacy, pharmacokinetics, biodistribution, and metabolic stability were assessed in a rat model of arterial thrombosis.

RESULTS

All probes had similar nanomolar affinity (435-760 nM) for the soluble fibrin fragment DD(E). PET imaging allowed clear visualization of thrombus by all probes, with a 5-fold or higher thrombus-to-background ratio. Compared with the previous DOTA derivative, the new (64)Cu probes FBP8 and FBP9 showed substantially improved metabolic stability (>85% intact in blood at 4 h after injection), resulting in high uptake at the target site (0.5-0.8 percentage injected dose per gram) that persisted over 5 h, producing increasingly greater target-to-background ratios. The thrombus uptake was 5- to 20-fold higher than the uptake in the contralateral artery, blood, muscle, lungs, bone, spleen, large intestine, and heart at 2 h after injection and 10- to 40-fold higher at 5 h. The Al(18)F derivatives FBP10 and FBP11 were less stable, in particular the NODAGA conjugate (FBP10, <30% intact in blood at 4 h after injection), which showed high bone uptake and low thrombus-to-background ratios that decreased over time. The high thrombus-to-contralateral ratios for all probes were confirmed by ex vivo biodistribution and autoradiography. The uptake in the liver (<0.5 percentage injected dose per gram), kidneys, and blood were similar for all tracers, and they all showed predominant renal clearance.

CONCLUSION

FBP8, FBP9, and FBP11 showed excellent metabolic stability and high thrombus-to-background ratios and represent promising candidates for imaging of thrombosis in vivo.

Assessment of an elastin binding molecule for PET imaging of atherosclerotic plaques

Elastin is considered as a key player in human vascular diseases and it might contribute to the development of atherosclerosis. The elastin binding radiotracer, [(18)F]AlF-NOTA-EBM ([(18)F]2), was evaluated in a wild type mouse to determine its in vivo distribution and on human carotid atherosclerotic plaque tissues to assess its utility as a PET imaging agent for visualizing human atherosclerotic plaque lesions. The free ligand NOTA-EBM, which served as the precursor, was obtained in 25% chemical yield. The radiosynthesis of [(18)F]2 was accomplished by coordination of Al(18)F to NOTA-EBM in 8-13% decay corrected radiochemical yield (n = 7) and specific radioactivity of 59 ± 12 GBq/μmol. A dynamic in vivo PET scan in a healthy wild type mouse (C57BL/6) showed high accumulation of radioactivity in heart and lungs, organs reported to have high elastin content. Excretion of [(18)F]2 proceeded via the renal pathway and through the hepatobiliary system as indicated by a high uptake of radioactivity in the liver, intestines and gall bladder. In vitro autoradiography on human atherosclerotic plaque sections showed a heterogeneous distribution of [(18)F]2 with an elevated accumulation in stable and vulnerable atherosclerotic plaques compared to control samples of normal arteries. However, there was no statistical significance between the different plaque phenotypes and control samples. Competition experiments with 10.000-fold excess of free ligand NOTA-EBM resulted in a marked decrease of radioactivity accumulation, consistent with a target-specific ligand.