PET imaging of α<Subscript>v</Subscript>β<Subscript>3</Subscript> integrin expression in tumours with <Superscript>68</Superscript>Ga-labelled mono-, di- and tetrameric RGD peptides

Purpose  

Due to the restricted expression of α_vβ₃ in tumours, α_vβ₃ is considered a suitable receptor for tumour targeting. In this study the α_vβ₃-binding characteristics of ⁶⁸Ga-labelled monomeric, dimeric and tetrameric RGD peptides were determined and compared with their ¹¹¹In-labelled counterparts.     

Improved targeting of the α<Subscript>v</Subscript>β<Subscript>3</Subscript> integrin by multimerisation of RGD peptides

Purpose The integrin α_vβ₃ is expressed on sprouting endothelial cells and on various tumour cell types. Due to the restricted expression of α_vβ₃ in tumours, α_vβ₃ is considered a suitable receptor for tumour targeting. In this study the α_vβ₃ binding characteristics of an ¹¹¹In-labelled monomeric, dimeric and tetrameric RGD analogue were compared. Methods A monomeric (E-c(RGDfK)), dimeric (E-[c(RGDfK)]₂), and tetrameric (E{E[c(RGDfK)]₂}₂) RGD peptide were synthesised, conjugated with DOTA and radiolabelled with ¹¹¹In. In vitro α_vβ₃ binding characteristics were determined in a competitive binding assay. In vivo α_vβ₃ targeting characteristics of the compounds were assessed in mice with SK-RC-52 xenografts. Results The IC₅₀ values for DOTA-E-c(RGDfK), DOTA-E-[c(RGDfK)]₂, and DOTA-E{E[c(RGDfK)]₂}₂were 120 nM, 69.9 nM and 19.6 nM, respectively. At all time points, the tumour uptake of the dimer was significantly higher as compared to that of the monomer. At 8 h p.i., tumour uptake of the tetramer (7.40±1.12%ID/g) was significantly higher than that of the monomer (2.30±0.34%ID/g), p<0.001, and the dimer (5.17±1.22%ID/g), p<0.05. At 24 h p.i., the tumour uptake was significantly higher for the tetramer (6.82±1.41%ID/g) than for the dimer (4.22±0.96%ID/g), p<0.01, and the monomer (1.90±0.29%ID/g), p<0.001. Conclusion Multimerisation of c(RGDfK) resulted in enhanced affinity for α_vβ₃ as determined in vitro. Tumour uptake of a tetrameric RGD peptide was significantly higher than that of the monomeric and dimeric analogues, presumably owing to the enhanced statistical likelihood for rebinding to α_vβ₃.     

<Superscript>68</Superscript>Ga- and <Superscript>111</Superscript>In-labelled DOTA-RGD peptides for imaging of αvβ3 integrin expression

PURPOSE: alphavbeta3 integrins are important cell adhesion receptors involved in angiogenic processes. Recently, we demonstrated using [(18)F]Galacto-RGD that monitoring of alphavbeta3 expression is feasible. Here, we introduce (68)Ga- and (111)In-labelled derivatives and compare them with [(18)F]Galacto-RGD. METHODS: For radiolabelling, cyclo(RGDfK(DOTA)) was synthesised using SPPS. For in vitro characterisation determination of partition coefficients, protein binding, metabolic stability, alphavbeta3 affinity and cell uptake and for in vivo characterization, biodistribution studies and micro positron emission tomography (PET) imaging were carried out. For in vivo and in vitro studies, human melanoma M21 (alphavbeta3 positive) and M21-L (alphavbeta3 negative) cells were used. RESULTS: Both tracers can be synthesised straightforward. The compounds showed hydrophilic properties and high metabolic stability. Up to 23% protein-bound activity for [(68)Ga]DOTA-RGD and only up to 1.4% for [(111)In]DOTA-RGD was found. Cell uptake studies indicate receptor-specific accumulation. This is confirmed by the biodistribution data. One hour p.i. accumulation in alphavbeta3-positive tumours was 2.9 +/- 0.3%ID/g and in alphavbeta3-negative tumours 0.8 +/- 0.1%ID/g for [(68)Ga]DOTA-RGD ([(111)In]DOTA-RGD: 1.9 +/- 0.3%ID/g and 0.5 +/- 0.2%ID/g; [(18)F]Galacto-RGD: 1.6 +/- 0.2%ID/g and 0.4 +/- 0.1%ID/g). Thus, tumour uptake ratios were comparable. Due to approx. 3-fold higher blood pool activities for [(68)Ga]DOTA-RGD, tumour/blood ratios were higher for [(111)In]DOTA-RGD and [(18)F]Galacto-RGD. However, microPET studies demonstrated that visualisation of alphavbeta3-positive tumours using [(68)Ga]DOTA-RGD is possible. CONCLUSIONS: Our data indicate that [(68)Ga]DOTA-RGD allows monitoring of alphavbeta3 expression. Especially, the much easier radiosynthesis compared to [(18)F]Galacto-RGD would make it an attractive alternative. However, due to higher blood pool activity, [(18)F]Galacto-RGD remains superior for imaging alphavbeta3 expression. Introduction of alternative chelator systems may overcome the disadvantages.     

<Superscript>18</Superscript>F-labeled mini-PEG spacered RGD dimer (<Superscript>18</Superscript>F-FPRGD2): synthesis and microPET imaging of α<Subscript>v</Subscript>β<Subscript>3</Subscript> integrin expression

Purpose We have previously reported that ¹⁸F-FB-E[c(RGDyK)]₂ (¹⁸F-FRGD2) allows quantitative PET imaging of integrin α_vβ₃ expression. However, the potential clinical translation was hampered by the relatively low radiochemical yield. The goal of this study was to improve the radiolabeling yield, without compromising the tumor targeting efficiency and in vivo kinetics, by incorporating a hydrophilic bifunctional mini-PEG spacer. Methods ¹⁸F-FB-mini-PEG-E[c(RGDyK)]₂ (¹⁸F-FPRGD2) was synthesized by coupling N-succinimidyl-4-¹⁸F-fluorobenzoate (¹⁸F-SFB) with NH₂-mini-PEG-E[c(RGDyK)]₂ (denoted as PRGD2). In vitro receptor binding affinity, metabolic stability, and integrin α_vβ₃ specificity of the new tracer ¹⁸F-FPRGD2 were assessed. The diagnostic value of ¹⁸F-FPRGD2 was evaluated in subcutaneous U87MG glioblastoma xenografted mice and in c-neu transgenic mice by quantitative microPET imaging studies. Results The decay-corrected radiochemical yield based on ¹⁸F-SFB was more than 60% with radiochemical purity of >99%. ¹⁸F-FPRGD2 had high receptor binding affinity, metabolic stability, and integrin α_vβ₃-specific tumor uptake in the U87MG glioma xenograft model comparable to those of ¹⁸F-FRGD2. The kidney uptake was appreciably lower for ¹⁸F-FPRGD2 compared with ¹⁸F-FRGD2 [2.0 ± 0.2%ID/g for ¹⁸F-FPRGD2 vs 3.0 ± 0.2%ID/g for ¹⁸F-FRGD2 at 1 h post injection (p.i.)]. The uptake in all the other organs except the urinary bladder was at background level. ¹⁸F-FPRGD2 also exhibited excellent tumor uptake in c-neu oncomice (3.6 ± 0.1%ID/g at 30 min p.i.). Conclusion Incorporation of a mini-PEG spacer significantly improved the overall radiolabeling yield of ¹⁸F-FPRGD2. ¹⁸F-FPRGD2 also had reduced renal uptake and similar tumor targeting efficacy as compared with ¹⁸F-FRGD2. Further testing and clinical translation of ¹⁸F-FPRGD2 are warranted. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Zhanhong Wu and Zi-Bo Li contributed equally to this work.     

<Superscript>68</Superscript>Ga-labeled cyclic RGD dimers with Gly<Subscript>3</Subscript> and PEG<Subscript>4</Subscript> linkers: promising agents for tumor integrin α<Subscript>v</Subscript>β<Subscript>3</Subscript> PET imaging

Purpose  Radiolabeled cyclic RGD (Arg-Gly-Asp) peptides have great potential for the early tumor detection and noninvasive monitoring of tumor metastasis and therapeutic response. ¹⁸F-labeled RGD analogs ([¹⁸F]-AH111585 and [¹⁸F]Galacto-RGD) have been investigated in clinical trials for positron emission tomography (PET) imaging of integrin expression in cancer patients. To develop new RGD radiotracers with higher tumor accumulation, improved in vivo kinetics, easy availability and low cost, we developed two new RGD peptides and labeled them with generator-eluted ⁶⁸Ga (t_1/2 = 68 min) for PET imaging of integrin α_vβ₃ expression in tumor xenograft models. Materials and methods  The two new cyclic RGD dimers, E[PEG₄-c(RGDfK)]₂ (P₄-RGD2, PEG₄ = 15-amino-4,7,10,13-tetraoxapentadecanoic acid) and E[Gly₃-c(RGDfK)]₂ (G₃-RGD2, G₃ = Gly-Gly-Gly) were designed, synthesized and conjugated with 1,4,7-triazacyclononanetriacetic acid (NOTA) for ⁶⁸Ga labeling. The microPET imaging and biodistribution of the ⁶⁸Ga labeled RGD tracers were investigated in integrin α_vβ₃-positive tumor xenografts. Results  The new RGD dimers with the Gly₃ and PEG₄ linkers showed higher integrin α_vβ₃ binding affinity than no-linker RGD dimer (RGD2). NOTA-G₃-RGD2 and NOTA-P₄-RGD2 could be labeled with ⁶⁸Ga within 30 min with higher purity (>98%) and specific activity (8.88–11.84 MBq/nmol). Both ⁶⁸Ga-NOTA-P₄-RGD2 and ⁶⁸Ga-NOTA-G₃-RGD2 exhibited significantly higher tumor uptake and tumor-to-normal tissue ratios than ⁶⁸Ga-NOTA-RGD2. Conclusion  Because of their high affinity, high specificity and excellent pharmacokinetic properties, further investigation of the two novel RGD dimers for clinical PET imaging of integrin α_vβ₃ expression in cancer patients is warranted. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Design,synthesis and validation of integrin α<Subscript>2</Subscript>β<Subscript>1</Subscript>-targeted probe for microPET imaging of prostate cancer

Purpose  

The ability of PET to aid in the diagnosis and management of recurrent and/or disseminated metastatic prostate cancer may be enhanced by the development of novel prognostic imaging probes. Accumulating experimental evidence indicates that overexpression of integrin α₂β₁ may correlate with progression in human prostate cancer. In this study, ⁶⁴Cu-labeled integrin α₂β₁-targeted PET probes were designed and evaluated for the imaging of prostate cancer.     

In vivo imaging of tumour angiogenesis in mice with the α<Subscript>v</Subscript>β<Subscript>3</Subscript> integrin-targeted tracer <Superscript>99m</Superscript>Tc-RAFT-RGD

Purpose The molecular imaging of tumour neoangiogenesis currently represents a major field of research for the diagnostic and treatment strategy of solid tumours. Endothelial cells from tumour neovessels overexpress the α_vβ₃ integrin, which selectively binds to Arg-Gly-Asp (RGD)-containing peptides. We evaluated the potential of the novel radiotracer ^99mTc-RAFT-RGD for the non-invasive molecular imaging of α_vβ₃ integrin expression in mice models of tumour development. Methods ^99mTc-RAFT-RGD, ^99mTc-cRGD (specific control) and ^99mTc-RAFT-RAD (non-specific control) were injected intravenously to mice bearing B16F0 or TS/A-pc tumours. In vivo whole-body tomographic imaging and post-mortem biodistribution studies were performed 60 min following tracer injection. Adjacent tumour slices were used to compare the localisation of neovessels from immunostaining and the pattern of ^99mTc-RAFT-RGD uptake from autoradiographic ex vivo imaging. Results Biodistribution studies indicated that ^99mTc-RAFT-RGD tumour uptake was significantly higher than that of ^99mTc-RAFT-RAD in B16F0 (2.4±0.5 vs 1.0±0.1%ID/g, respectively) and in TS/A-pc tumours (2.7±0.8 vs 0.7±0.1%ID/g, respectively). Immunohistochemical and autoradiographic studies indicated that ^99mTc-RAFT-RGD intratumoural uptake preferentially occurred in angiogenic areas. Tomographic imaging allowed tumour visualisation following injection of ^99mTc-RAFT-RGD and ^99mTc-cRGD with similar tumour-to-contralateral muscle (T/CM) ratios in B16F0 and in TS/A-pc tumours whereas ^99mTc-RAFT-RAD T/CM ratios did not allow tumour imaging. In accordance with the higher level of α_vβ₃ integrin expression on TS/A-pc tumours than on B16F0 tumours as determined from western blot and immunoprecipitation analyses, the ^99mTc-RAFT-RGD T/CM ratio was significantly higher in TS/A-pc than in B16F0 tumours. Conclusion ^99mTc-RAFT-RGD allowed the in vivo imaging of α_vβ₃ integrin tumour expression.     

[<Superscript>18</Superscript>F]FPRGD<Subscript>2</Subscript> PET/CT imaging of integrin α<Subscript>v</Subscript>β<Subscript>3</Subscript> levels in patients with locally advanced rectal carcinoma

Purpose

Our primary objective was to determine if [¹⁸F]FPRGD₂ PET/CT performed at baseline and/or after chemoradiotherapy (CRT) could predict tumour regression grade (TRG) in locally advanced rectal cancer (LARC). Secondary objectives were to compare baseline [¹⁸F]FPRGD₂ and [¹⁸F]FDG uptake, to evaluate the correlation between posttreatment [¹⁸F]FPRGD₂ uptake and tumour microvessel density (MVD) and to determine if [¹⁸F]FPRGD₂ and FDG PET/CT could predict disease-free survival.

Methods

Baseline [¹⁸F]FPRGD₂ and FDG PET/CT were performed in 32 consecutive patients (23 men, 9 women; mean age 63?±?8 years) with LARC before starting any therapy. A posttreatment [¹⁸F]FPRGD₂ PET/CT scan was performed in 24 patients after the end of CRT (median interval 7 weeks, range 3 – 15 weeks) and before surgery (median interval 4 days, range 1 – 15 days).

Results

All LARC showed uptake of both [¹⁸F]FPRGD₂ (SUV_max 5.4?±?1.5, range 2.7 – 9) and FDG (SUV_max 16.5?±?8, range 7.1 – 36.5). There was a moderate positive correlation between [¹⁸F]FPRGD₂ and FDG SUV_max (Pearson’s r?=?0.49, p?=?0.0026). There was a moderate negative correlation between baseline [¹⁸F]FPRGD₂ SUV_max and the TRG (Spearman’s r?=??0.37, p?=?0.037), and a [¹⁸F]FPRGD₂ SUV_max of >5.6 identified all patients with a complete response (TRG 0; AUC 0.84, 95 % CI 0.68 - 1, p?=?0.029). In the 24 patients who underwent a posttreatment [¹⁸F]FPRGD₂ PET/CT scan the response index, calculated as [(SUV_max1 ? SUV_max2)/SUV_max1] × 100 %, was not associated with TRG. Post-treatment [¹⁸F]FPRGD₂ uptake was not correlated with tumour MVD. Neither [¹⁸F]FPRGD₂ nor FDG uptake predicted disease-free survival.

Conclusion

Baseline [¹⁸F]FPRGD₂ uptake was correlated with the pathological response in patients with LARC treated with CRT. However, the specificity was too low to consider its clinical routine use.

     

Imaging α<Subscript>v</Subscript>β<Subscript>3</Subscript> integrin expression in skeletal metastases with <Superscript>99m</Superscript>Tc-maraciclatide single-photon emission computed tomography: detection and therapy response assessment

Purpose

Osteoclast activity is an important factor in the pathogenesis of skeletal metastases and is a potential therapeutic target. This study aimed to determine if selective uptake of ^99mTc-maraciclatide, a radiopharmaceutical targeting α_vβ₃ integrin, occurs in prostate cancer (PCa) bone metastases and to observe the changes following systemic therapy.

Methods

The study group comprised 17 men with bone-predominant metastatic PCa who underwent whole-body planar and single-photon emission computed tomography/computed tomography (SPECT/CT) imaging with ^99mTc-maraciclatide before (n?=?17) and 12 weeks after (n?=?11) starting treatment with abiraterone. Tumour to normal bone (T:N) ratios, tumour to muscle (T:M) ratios and CT Hounsfield units (HU) were measured in up to five target metastases in each subject. An oncologist blinded to study scans assessed clinical responses up to 24 weeks using conventional criteria.

Results

Before treatment, metastases showed specific ^99mTc-maraciclatide accumulation (mean planar T:N and T:M ratios 1.43 and 3.06; SPECT T:N?and T:M ratios 3.1 and 5.19, respectively). Baseline sclerotic lesions (389–740 HU) showed lower T:M ratios (4.22 vs. 7.04, p?=?0.02) than less sclerotic/lytic lesions (46–381 HU). Patients with progressive disease (PD; n?=?5) showed increased planar T:N and T:M ratios (0.29 and 12.1%, respectively) and SPECT T:N and T:M ratios (11.9 and 20.2%, respectively). Patients without progression showed decreased planar T:N and T:M ratios (0.27 and ?8.0%, p?=?1.0 and 0.044, respectively) and SPECT T:N and T:M ratios (?21.9, and ?27.2%, p?=?0.3 and 0.036, respectively). The percentage change in CT HU was inversely correlated with the percentage change in SPECT T:M ratios (r?=??0.59, p?=?0.006).

Conclusions

^99mTc-maraciclatide accumulates in PCa bone metastases in keeping with increased α_vβ₃ integrin expression. Greater activity in metastases with lower CT density suggests that uptake is related to osteoclast activity. Changes in planar and SPECT T:M ratios after 12 weeks of treatment differed between patients with and without PD and ^99mTc-maraciclatide imaging may be a potential method for assessing early response.

     

Inter-heterogeneity and intra-heterogeneity of α<Subscript>v</Subscript>β<Subscript>3</Subscript> in non-small cell lung cancer and small cell lung cancer patients as revealed by <Superscript>68</Superscript>Ga-RGD<Subscript>2</Subscript> PET imaging

Purpose

Integrin α_vβ₃ is the therapeutic target of the anti-angiogenic drug cilengitide. The objective of this study was to compare α_vβ₃ levels in non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC) patients, by using the positron emission tomography (PET) tracer ⁶⁸Ga-labeled dimerized-RGD (⁶⁸Ga-RGD₂).

Methods

Thirty-one patients with pathologically confirmed lung cancer were enrolled (21 were NSCLC and 10 were SCLC). PET/CT images were acquired using ⁶⁸Ga-RGD₂.¹⁸F-FDG PET/CT images were also acquired on the consecutive day as reference. The standard uptake values (SUV) and the tumor/nontarget (T/NT) values were quantitatively compared. Expression of the angiogenesis marker α_vβ₃ in NSCLC and SCLC lesions was analyzed by immunohistochemistry.

Results

The ¹⁸F-FDG SUVmax and the SUVmean were not significantly different between NSCLC and SCLC patients. The ⁶⁸Ga-RGD₂ uptake of SCLC patients was at background levels in both SUV and T/NT measurements and was significantly lower than that of NSCLC patients. The range value of ⁶⁸Ga-RGD₂ SUVmean was 4.5 in the NSCLC group and 2.2 in the SCLC group, while the variation coefficient was 36.2% and 39.3% in NSCLC and SCLC primary lesions, respectively. Heterogeneity between primary lesions and putative distant metastases was also observed in some NSCLC cases. Immunostaining showed that α_vβ₃ integrin was expressed in the cells and neovasculature of NSCLC lesions, while SCLC samples had negative expression.

Conclusions

The uptake of ⁶⁸Ga-RGD₂ in SCLC patients is significantly lower than that in NSCLC patients, indicating a lower α_vβ₃ target level for cilengitide in SCLC. Apparent intra-tumor heterogeneities of α_vβ₃ also exist in both NSCLC and SCLC. Such inter- and intra-heterogeneity of α_vβ₃ may potentially improve current applications of α_vβ₃-targeted therapy and diagnostic imaging in lung cancer.

     

In vivo imaging of integrin ανβ<Subscript>3</Subscript> expression using fluorescence-mediated tomography

Purpose Optical imaging would be desirable for cancer diagnostics since it can potentially resolve relevant oncological target structures in vivo. We therefore synthesised an αvβ₃ targeted fluorochrome and imaged tumour xenografts with different αvβ₃ expression levels using both planar and tomographic optical imaging methods. Methods An αvβ₃-targeted RGD peptide was labelled with a cyanine dye (Cy 5.5). Binding of the optical tracer was tested on M21 melanoma (n = 5), HT-1080 fibrosarcoma (n = 6) and MCF-7 adenocarcinoma (n = 5) cells and their tumour xenografts. All optical imaging studies were performed using two-dimensional planar fluorescence reflectance imaging (FRI) technology and three-dimensional fluorescence-mediated tomography (FMT). Results In vitro, the peptide-dye conjugate showed a clear binding affinity to αvβ₃-positive M21 and HT-1080 cells while αvβ₃-negative MCF-7 cells and pre-dosing with the free RGD peptide revealed little to no fluorescence. In vivo, tumour xenografts were clearly visualised by FRI and FMT up to 24 h post injection. FMT allowed quantification of the fluorochrome distribution in deeper tissue sections showing an average fluorochrome concentration of 417.61 ± 105.82 nM Cy 5.5 (M21), 353.68 ± 54.02 nM Cy 5.5 (HT-1080) and 262.83 ± 155.36 nM Cy 5.5 (MCF-7) in the target tissue 60 min after tracer administration. Competition with the free RGD peptide resulted in a reduction in the fluorochrome concentration in M21 tumour tissue (294.35 ± 84.27 nM). Conclusion RGD-Cy 5.5 combined with novel tomographic optical imaging methods allows non-invasive imaging of tumour-associated αvβ₃ expression and may thus be a promising strategy for sensitive evaluation of tumour target expression.     

<Superscript>68</Superscript>Ga-DOTATATE PET–CT imaging in carotid body paragangliomas

Objective

The aim of this study was to present our experience in the baseline evaluation of carotid body paragangliomas (CBP) with ⁶⁸Ga-DOTATATE PET–CT.

Methods

Five patients (4F, 1M; age 24–73 years) with CBPs who underwent ⁶⁸Ga-DOTATATE PET–CT scan before the treatment were evaluated retrospectively. PET–CT images were analyzed visually as well as semiquantitatively, with measurement of maximum standardized uptake value (SUV_max).

Results

All patients had unilateral CBP lesion, showed intense ⁶⁸Ga-DOTATATE uptake in PET–CT. Additionally, ⁶⁸Ga-DOTATATE avid lesions were found in two patients. One of them had focal intense uptake in thyroid gland and frontal cerebrum. The other one had intense uptake in bone and adrenal mass. Four patients were operated for unilateral primary CBP. Last patient was treated with peptide receptor radionuclide therapy (¹⁷⁷Lu-DOTATATE) for both metastatic pheochromocytoma and CBP.

Conclusions

⁶⁸Ga-DOTATATE PET–CT is a valuable imaging modality for staging of CBPs, detecting unknown lesions and changing the management of patients. It is also useful in demonstrating expression of SSTRs for PRRT opportunity.

     

PET imaging of α<Subscript>7</Subscript> nicotinic acetylcholine receptors: a comparative study of [<Superscript>18</Superscript>F]ASEM and [<Superscript>18</Superscript>F]DBT-10 in nonhuman primates,and further evaluation of [<Superscript>18</Superscript>F]ASEM in humans

Purpose

The α₇ nicotinic acetylcholine receptor (nAChR) is implicated in many neuropsychiatric disorders, making it an important target for positron emission tomography (PET) imaging. The first aim of this work was to compare two α₇ nAChRs PET radioligands, [¹⁸F]ASEM (3-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-6-([¹⁸F]fluorodibenzo[b,d]thiophene 5,5-dioxide) and [¹⁸F]DBT-10 (7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-([¹⁸F]fluorodibenzo[b,d]thiophene 5,5-dioxide), in nonhuman primates. The second aim was to assess further the quantification and test-retest variability of [¹⁸F]ASEM in humans.

Methods

PET scans with high specific activity [¹⁸F]ASEM or [¹⁸F]DBT-10 were acquired in three rhesus monkeys (one male, two female), and the kinetic properties of these radiotracers were compared. Additional [¹⁸F]ASEM PET scans with blocking doses of nicotine, varenicline, and cold ASEM were acquired separately in two animals. Next, six human subjects (five male, one female) were imaged with [¹⁸F]ASEM PET for 180 min, and arterial sampling was used to measure the parent input function. Different modeling approaches were compared to identify the optimal analysis method and scan duration for quantification of [¹⁸F]ASEM distribution volume (V _T). In addition, retest scans were acquired in four subjects (three male, one female), and the test-retest variability of V _T was assessed.

Results

In the rhesus monkey brain [¹⁸F]ASEM and [¹⁸F]DBT-10 exhibited highly similar kinetic profiles. Dose-dependent blockade of [¹⁸F]ASEM binding was observed, while administration of either nicotine or varenicline did not change [¹⁸F]ASEM V _T. [¹⁸F]ASEM was selected for further validation because it has been used in humans. Accurate quantification of [¹⁸F]ASEM V _T in humans was achieved using multilinear analysis with at least 90 min of data acquisition, resulting in V _T values ranging from 19.6?±?2.5 mL/cm³ in cerebellum to 25.9?±?2.9 mL/cm³ in thalamus. Test-retest variability of V _T was 11.7?±?9.8%.

Conclusions

These results confirm [¹⁸F]ASEM as a suitable radiotracer for the imaging and quantification of α₇ nAChRs in humans.

     

PET imaging evaluation of [<Superscript>18</Superscript>F]DBT-10, a novel radioligand specific to α<Subscript>7</Subscript> nicotinic acetylcholine receptors,in nonhuman primates

Purpose

Positron emission tomography (PET) radioligands specific to α₇ nicotinic acetylcholine receptors (nAChRs) afford in vivo imaging of this receptor for neuropathologies such as Alzheimer’s disease, schizophrenia, and substance abuse. This work aims to characterize the kinetic properties of an α₇-nAChR-specific radioligand, 7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-[¹⁸F]-fluorodibenzo[b,d]thiophene 5,5-dioxide ([¹⁸F]DBT-10), in nonhuman primates.

Methods

[¹⁸F]DBT-10 was produced via nucleophilic substitution of the nitro-precursor. Four Macaca mulatta subjects were imaged with [¹⁸F]DBT-10 PET, with measurement of [¹⁸F]DBT-10 parent concentrations and metabolism in arterial plasma. Baseline PET scans were acquired for all subjects. Following one scan, ex vivo analysis of brain tissue was performed to inspect for radiolabeled metabolites in brain. Three blocking scans with 0.69 and 1.24 mg/kg of the α₇-nAChR-specific ligand ASEM were also acquired to assess dose-dependent blockade of [¹⁸F]DBT-10 binding. Kinetic analysis of PET data was performed using the metabolite-corrected input function to calculate the parent fraction corrected total distribution volume (V _T/f _P).

Results

[¹⁸F]DBT-10 was produced within 90 min at high specific activities of 428?±?436 GBq/μmol at end of synthesis. Metabolism of [¹⁸F]DBT-10 varied across subjects, stabilizing by 120 min post-injection at parent fractions of 15–55 %. Uptake of [¹⁸F]DBT-10 in brain occurred rapidly, reaching peak standardized uptake values (SUVs) of 2.9–3.7 within 30 min. The plasma-free fraction was 18.8?±?3.4 %. No evidence for radiolabeled [¹⁸F]DBT-10 metabolites was found in ex vivo brain tissue samples. Kinetic analysis of PET data was best described by the two-tissue compartment model. Estimated V _T/f _P values were 193–376 ml/cm³ across regions, with regional rank order of thalamus?>?frontal cortex?>?striatum?>?hippocampus?>?occipital cortex?>?cerebellum?>?pons. Dose-dependent blockade of [¹⁸F]DBT-10 binding by structural analog ASEM was observed throughout the brain, and occupancy plots yielded a V _ND/f _P estimate of 20?±?16 ml/cm³.

Conclusion

These results demonstrate suitable kinetic properties of [¹⁸F]DBT-10 for in vivo quantification of α₇-nAChR binding in nonhuman primates.

     

Positron emission tomography imaging of adrenal masses: <Superscript>18</Superscript>F-fluorodeoxyglucose and the 11β-hydroxylase tracer <Superscript>11</Superscript>C-metomidate

Purpose ¹¹C-metomidate (MTO), a marker of 11-hydroxylase, has been suggested as a novel positron emission tomography (PET) tracer for adrenocortical imaging. Up to now, experience with this very new tracer is limited. The aims of this study were (1) to evaluate this novel tracer, (2) to point out possible advantages in comparison with ¹⁸F-fluorodeoxyglucose (FDG) and (3) to investigate in vivo the expression of 11-hydroxylase in patients with primary aldosteronism.Methods Sixteen patients with adrenal masses were investigated using both MTO and FDG PET imaging. All patients except one were operated on. Five patients had non-functioning adrenal masses, while 11 had functioning tumours(Cushings syndrome, n=4; Conns syndrome, n=5; phaeochromocytoma, n=2). Thirteen patients had benign disease, whereas in three cases the adrenal mass was malignant (adrenocortical cancer, n=1; malignant phaeochromocytoma, n=1; adrenal metastasis of renal cancer, n=1).Results MTO imaging clearly distinguished cortical from non-cortical adrenal masses (median standardised uptake values of 18.6 and 1.9, respectively, p<0.01). MTO uptake was slightly lower in patients with Cushings syndrome than in those with Conns syndrome, but the difference did not reach statistical significance. The expression of 11-hydroxylase was not suppressed in the contralateral gland of patients with Conns syndrome, whereas in Cushings syndrome this was clearly the case. The single patient with adrenocortical carcinoma had MTO uptake in the lower range.Conclusion MTO could not definitely distinguish between benign and malignant disease. FDG PET, however, identified clearly all three study patients with malignant adrenal lesions. We conclude: (1) MTO is an excellent imaging tool to distinguish adrenocortical and non-cortical lesions; (2) the in vivo expression of 11-hydroxylase is lower in Cushings syndrome than in Conns syndrome, and there is no suppression of the contralateral gland in primary aldosteronism; (3) for the purpose of discriminating between benign and malignant lesions, FDG is the tracer of choice.     

Molecular imaging of σ receptors: synthesis and evaluation of the potent σ<Subscript>1</Subscript> selective radioligand [<Superscript>18</Superscript>F]fluspidine

Purpose  

Neuroimaging of σ₁ receptors in the human brain has been proposed for the investigation of the pathophysiology of neurodegenerative and psychiatric diseases. However, there is a lack of suitable ¹⁸F-labelled PET radioligands for that purpose.     

Comparison of <Superscript>99m</Superscript>Tc-labeled PR81 and its F(ab′)<Subscript>2</Subscript> fragments as radioimmunoscintigraphy agents for breast cancer imaging

Objective  

We digested anti-MUC1 monoclonal antibody PR81 to produce F(ab′)₂ fragments. A comparison was performed between the two radiolabeled PR81 and F(ab′)₂ fragments for breast tumor imaging in a mouse model.     

Intra-individual comparison of p-[<Superscript>123</Superscript>I]-iodo-L-phenylalanine and L-3-[<Superscript>123</Superscript>I]-iodo-α-methyl-tyrosine for SPECT imaging of gliomas

Objectives Radioactive amino-acids accumulate in gliomas even with an intact blood-brain-barrier. L-3-[¹²³I]-iodo-α-methyl-tyrosine (IMT) is well established for SPECT imaging of gliomas. Recently, we introduced p-[¹²³I]-iodo-L-phenylalanine (IPA) for the characterisation of brain lesions. This study compares both tracers in glioma patients. Methods Eleven patients with gliomas (1 WHO grade 1, 5 grade 2, 1 grade 3, 2 grade 4 gliomas, 1 unconfirmed upgrading and 1 post-therapeutic non-neoplastic lesion) underwent SPECT imaging with IPA (early and delayed acquisitions at 30 min and 3 h) and IMT (early only). Maximum tumour-to-brain ratios (TBR) were calculated using region-of-interest analysis to assess uptake of IMT and IPA. Imaging results were compared to histopathological findings. Results Early TBRs of IMT and IPA were strongly correlated (r = 0.828, p = 0.002). TBRs were higher for IMT than IPA (1.95±0.50 versus 1.79±0.42; p < 0.05), but independent from tumour cell density (p > 0.1). Visual interpretation by different observers was more concordant for IMT-SPECT than IPA-SPECT (kappa 1.0 versus 0.774). No differences in early TBRs were observed between low-grade and high-grade gliomas for IMT (1.97±0.53 versus 2.21±0.44, p > 0.5) or IPA (1.70±0.23 versus 2.21±0.56, p = 0.167) with a trend to higher TBRs in low-grade tumours for IMT (p = 0.093). In contrast to the known wash-out of IMT, we observed persistent accumulation of IPA in gliomas. Conclusions IPA shows lower TBRs than IMT, especially in low-grade tumours, so IMT should be preferred for the delineation of low-grade gliomas by SPECT imaging. Due to its prolonged retention, however, IPA remains promising for therapeutic use in gliomas after labelling with I-131. This work was supported by a grant from the “Deutsche Krebshilfe” (70-3024-He 1).