Proof of principle for the use of 11C-labelled peptides in tumour diagnosis with PET

Copper-Mediated Radiocyanation of Unprotected Amino Acids and Peptides

This report describes a copper-mediated radiocyanation of aryl halides that is applicable to complex molecules. This transformation tolerates an exceptionally wide range of functional groups, including unprotected amino acids. As such, it enables the site-specific introduction of [¹¹C]CN into peptides at an iodophenylalanine residue. The use of a diamine-ligated copper(I) mediator is crucial for achieving high radiochemical yield under relatively mild conditions, thus limiting racemization and competing side reactions of other amino acid side chains. The reaction has been scaled and automated to deliver radiolabeled peptides, including analogues of adrenocorticotropic hormone 1-27 (ACTH) and nociceptin (NOP). For instance, this Cu-mediated radiocyanation was leveraged to prepare >40 mCi of [¹¹C]cyano-NOP to evaluate biodistribution in a primate using positron emission tomography. This investigation provides preliminary evidence that nociceptin crosses the blood-brain barrier and shows uptake across all brain regions (SUV > 1 at 60 min post injection), consistent with the known distribution of NOP receptors in the rhesus brain.

Gallium-68 DOTATOC PET/CT in vivo characterization of somatostatin receptor expression in the prostate

AIM

The aim was to investigate somatostatin receptor (sstr) expression in normal prostate by determining the maximum standardized uptake value (SUVmax) of (68)Ga-DOTATOC PET/CT in neuroendocrine tumor (NET) patients, without NET involvement of the prostate gland, for establishing the reference standard.

METHODS

Sixty-four NET patients underwent (68)Ga-DOTATOC PET/CT. SUVmax of the prostate gland, normal liver, testes, and gluteus muscles were evaluated. The prostate gland size was measured. Statistical analysis was performed using dedicated software (SPSS13).

RESULTS

Mean/median (68)Ga-DOTATOC SUVmax values were as follows: normal prostate 2.6 ± 0.0, slightly enlarged prostate 4.2 ± 1.6, prostatic hypertrophy 4.9 ± 1.6, prostatic hyperplasia 5.0 ± 1.5, prostate cancer 9.5 ± 2.1, normal liver 7.3 ± 1.8, testes 1.8 ± 0.5, and gluteus 1.0 ± 0.2. The normal prostate gland had three times less sstr expression than normal liver tissue. Strong correlation was found between patient age and sstr expression in prostate/prostate size. No significant difference existed in sstr expression between prostatic hypertrophy and hyperplasia. Much higher sstr expression was found in prostatic cancer compared with normal prostate.

CONCLUSION

(68)Ga-DOTATOC PET/CT defines the baseline sstr uptake in prostate not affected by NET (significantly lower than in the liver). Higher values were established in prostatic hyperplasia and hypertrophy. Only concomitant prostate cancer was associated with higher SUVmax in comparison with non-neoplastic liver.

Accelerating preclinical PET-screening: reductive amination with [11C]methoxybenzaldehydes

We report, herein, a simple and efficient labelling strategy for multiple PET tracer preparation using a common intermediate, which has the potential to accelerate preclinical PET radiotracer screening.

Carbon-11 radiolabeling of an oligopeptide containing tryptophan hydrochloride via a Pictet-Spengler reaction using carbon-11 formaldehyde

A procedure for the synthesis of a(11)C-labeled oligopeptide containing [1-(11)C]1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid ([1-(11)C]Tpi) from the corresponding Trp•HCl-containing peptides has been developed involving a Pictet-Spengler reaction with [(11) C]formaldehyde. The synthesis of [1-(11)C]Tpi from Trp and [(11)C]formaldehyde was examined as a model reaction with the aim of developing a facile and effective method for the labeling of peptides with carbon-11. The Pictet-Spengler reaction of Trp and [(11)C]formaldehyde in acidic media (TsOH or HCl) afforded the desired [1-(11)C]Tpi in a moderate radiochemical yield. Herein, the application of a Pictet-Spengler reaction to an aqueous solution of Trp•HCl gave the desired product with a radiochemical yield of 45.2%. The RGD peptide cyclo[Arg-Gly-Asp-D-Tyr-Lys] was then selected as a substrate for the labeling reaction with [(11)C]formaldehyde. The radiolabeling of a Trp•HCl-containing RGD peptide using the Pictet-Spengler reaction was successful. Furthermore, the remote-controlled synthesis of a [1-(11)C]Tpi-containing RGD peptide was attempted by using an automatic production system to generate [(11)C]CH3 I. The radiochemical yield of the [1-(11) C]Tpi-containing RGD at the end of synthesis (EOS) was 5.9 ± 1.9% (n = 4), for a total synthesis time of about 35 min. The specific activity was 85.7 ± 9.4 GBq/µmol at the EOS.

Small animal tumour imaging with MRI and the ECAT EXACT scanner: application of partial volume correction and comparison with microPET data

OBJECTIVE

Partial volume effects caused by limited spatial resolution of conventional positron emission tomography (PET) scanners result in an underestimation of the activity concentration in small tumours. The aim of the study was to evaluate the feasibility of small animal tumour imaging with the clinical PET scanner ECAT EXACT after partial volume correction based on MRI calculations. The same tumour model was examined additionally with the small animal PET system, microPET focus 120.

METHODS

Before the ECAT EXACT studies recovery coefficients for different sphere volumes were generated with phantom experiments. For the following in-vivo study DS-sarcoma cells were implanted on both hind foot dorsum of male Sprague-Dawley rats. In-vivo tumour volume calculations were done with the high-resolution MRI system, Magnetom Vision Experimental. Dynamic F-fluorodeoxyglucose (FDG) PET was performed with the scanner ECAT EXACT (5 MBq intravenous, two-dimensional mode, n = 16 tumours) or with the microPET focus 120 (20 MBq intravenous, two-dimensional mode, n = 10 tumours). The animals were then killed, the tumours rapidly explanted, weighed and homogenized. The concentration of F-FDG was measured with a gamma counter and decay corrected; the ex-vivo F-FDG concentration was compared with the mean tumour activity concentration of the PET data.

RESULTS

Using the ECAT EXACT mean underestimation of actual tumour F-FDG concentration was 35.4%, for partial volume-corrected data this error decreased to 1.7%. In addition, after partial volume correction congruence and linear correlation between the regions of interest-based activity concentration and ex-vivo measurements were excellent (r = 0.98). These results were quite similar to the microPET experiments without partial volume correction: r = 0.99.

CONCLUSION

These data indicate that partial volume correction might allow use of the clinical PET system, ECAT EXACT, for the metabolic assessment of small animal tumours >/=10 mm with sufficient accuracy if no dedicated animal PET is available.

Preparation and radiosynthesis of [18F]FE@CFN (2-[18F]fluoroethyl 4-[N-(1-oxopropyl)-N-phenylamino]-1-(2-phenylethyl)-4-piperidinecarboxylate): a potential μ-opioid receptor imaging agent

PET imaging of the μ-opioid receptor (OR) is still restricted to [11C]carfentanil ([11C]CFN) but its use is limited due to its short half-life and high agonistic potency. Recently, the radiosynthesis of [18F]fluoroalkyl esters of CFN was proposed, unfortunately yielding products not suitable for human PET due to their low specific activities. Therefore, our rationale was to develop a reliable radiosynthesis of a [18F]fluoroethylated CFN derivative overcoming these drawbacks.

The [18F]fluoroethyl ester of carfentanil, [18F]FE@CFN (2-[18F]fluoroethyl 4-[N-(1-oxopropyl)-N-phenylamino]-1-(2-phenylethyl)-4-piperidinecarboxylate), and its corresponding inactive standard compound were prepared. Purification of [18F]FE@CFN was achieved via a simple solid phase extraction method. [18F]FE@CFN was prepared with excellent purity (> 98%) and sufficient yields. Specific activity surpassed the level required for safe administration. We therefore conclude that our simplified synthesis of [18F]FE@CFN, for the first time, overcomes the shortcomings of [11C]CFN and the previously suggested alternatives, namely, (1) longer half-life; (2) easy production and (3) adequate specific activity, should make a wider application possible. Hence, [18F]FE@CFN may become a valuable PET tracer for the imaging of the μ-OR in human brain and heart.

Tumor receptor imaging

Tumor receptors play an important role in carcinogenesis and tumor growth and have been some of the earliest targets for tumor-specific therapy, for example, the estrogen receptor in breast cancer. Knowledge of receptor expression is key for therapy directed at tumor receptors and traditionally has been obtained by assay of biopsy material. Tumor receptor imaging offers complementary information that includes evaluation of the entire tumor burden and characterization of the heterogeneity of tumor receptor expression. The nature of the ligand-receptor interaction poses a challenge for imaging--notably, the requirement for a low molecular concentration of the imaging probe to avoid saturating the receptor and increasing the background because of nonspecific uptake. For this reason, much of the work to date in tumor receptor imaging has been done with radionuclide probes. In this overview of tumor receptor imaging, aspects of receptor biochemistry and biology that underlie tumor receptor imaging are reviewed, with the estrogen-estrogen receptor system in breast cancer as an illustrative example. Examples of progress in radionuclide receptor imaging for 3 receptor systems--steroid receptors, somatostatin receptors, and growth factor receptors-are highlighted, and recent investigations of receptor imaging with other molecular imaging modalities are reviewed.

An orthotopic model of pancreatic somatostatin receptor (SSTR)-positive tumors allows bimodal imaging studies using 3T MRI and animal PET-based molecular imaging of SSTR expression

Somatostatin receptor (SSTR) scintigraphy is currently used as one standard imaging modality in neuroendocrine tumors (NETs). However, future optimization of NET imaging may be achieved with positron emission tomography based methods utilizing more sensitive and specific tracers in combination with computed tomography or magnetic resonance imaging. Here we established an orthotopic mouse model that reflects relevant aspects of human pancreatic NETs such as SSTR expression, dense vascularization and metastatic disease. This model was then utilized to test the feasibility of combined magnetic resonance imaging and animal positron emission tomography. Orthotopic implantation of amphicrine, SSTR-positive pancreatic AR42J cells resulted in rapidly growing tumors, with concomitant metastatic spread into abdominal lymph nodes and peritoneal cavity. Primary tumors as well as their metastases expressed the neuroendocrine markers chromogranin A and synaptophysin. For imaging experiments, the SSTR ligands (68)Ga-DOTATOC or (68)Ga-DOTANOC were injected intravenously, and animals were subsequently examined in an animal positron emission tomography scanner and a clinical 3T (tesla) magnetic resonance imager. All animals showed radionuclide accumulation in the primary tumor. Definite anatomical correlation was achieved using digital image fusion of the positron emission tomography and magnetic resonance imaging data. (68)Ga-DOTANOC strongly accumulated in the tumor tissue (mean 6.6-fold vs. control tissues) when compared to (68)Ga-DOTATOC, which showed a higher renal clearance. In good agreement with the biodistribution data, the kidney-to-tumor ratio was higher for (68)Ga-DOTATOC (2.43-fold vs. 1.75-fold). Consequently, (68)Ga-DOTANOC achieved better signal enhancement in the primary tumor and allowed for detection of metastatic lesions. In summary, we established a novel orthotopic pancreatic SSTR-positive tumor model and used this model to provide proof of principle for the diagnostic combination of SSTR-based molecular imaging and magnetic resonance imaging. Specifically, the animal model allowed the comparative evaluation of (68)Ga-DOTANOC and (68)Ga-DOTATOC, with (68)Ga-DOTANOC providing better tumor-specific accumulation and renal activity. We conclude that this animal model will be of innovative value for further investigation in the imaging of NETs.

Are radiogallium-labelled DOTA-conjugated somatostatin analogues superior to those labelled with other radiometals?

PURPOSE

Gallium-68 is a metallic positron emitter with a half-life of 68 min that is ideal for the in vivo use of small molecules, such as [68Ga-DOTA,Tyr3]octreotide, in the diagnostic imaging of somatostatin receptor-positive tumours. In preclinical studies it has shown a striking superiority over its 111In-labelled congener. The purpose of this study was to evaluate whether third-generation somatostatin-based, radiogallium-labelled peptides show the same superiority.

METHODS

Peptides were synthesised on solid phase. The receptor affinity was determined by in vitro receptor autoradiography. The internalisation rate was studied in AR4-2J and hsst-HEK-transfected cell lines. The pharmacokinetics was studied in a rat xenograft tumour model, AR4-2J.

RESULTS

All peptides showed high affinities on hsst2, with the highest affinity for the Ga(III)-complexed peptides. On hsst3 the situation was reversed, with a trend towards lower affinity of the Ga(III) peptides. A significantly increased internalisation rate was found in sst2-expressing cells for all 67Ga-labelled peptides. Internalisation into HEK-sst3 was usually faster for the 111In-labelled peptides. No internalisation was found into sst5. Biodistribution studies employing [67Ga-DOTA,1-Nal3]octreotide in comparison to [111In-DOTA,1-Nal3]octreotide and [67Ga-DOTA,Tyr3]octreotide showed a significantly higher and receptor-mediated uptake of the two 67Ga-labelled peptides in the tumour and somatostatin receptor-positive tissues. A patient study illustrated the potential advantage of a broad receptor subtype profile radiopeptide over a high-affinity sst2-selective radiopeptide.

CONCLUSION

This study demonstrates that 67/68Ga-DOTA-octapeptides show distinctly better preclinical, pharmacological performances than the 111In-labelled peptides, especially on sst2-expressing cells and the corresponding animal models. They may be excellent candidates for further development for clinical studies.

Chemoselective hydrazone formation between HYNIC-functionalized peptides and (18)F-fluorinated aldehydes

INTRODUCTION

Since the demand for (18)F-fluorinated peptides for quantitative in vivo receptor imaging using PET has increased, a new chemoselective two-step (18)F-labeling strategy based on hydrazone formation between an unprotected hydrazine-functionalized peptide and an (18)F-labeled aldehyde was developed.

METHODS

First, 4-[(18)F]fluorobenzaldehyde ([(18)F]FB-CHO) was prepared from 4-formyl-N,N,N-trimethylanilinium triflate via direct no-carrier-added (18)F-fluorination (dimethyl sulfoxide, 90 degrees C, 5 min) and purified by RP-HPLC. Hydrazone formation between [(18)F]FB-CHO and 6-hydrazinonicotinic acid (HYNIC) and the unprotected HYNIC-functionalized peptides (HYNIC-d-Phe(1))-Tyr(3)-Thr(8)-octreotide and (HYNIC-Arg(1))-substance P was evaluated with respect to the dependence of radiochemical yield on pH, precursor concentration and temperature. The stability of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)(NH(2))-octreotide in aqueous solution at various pH (4.0, 5.5 and 7.5) as well as the in vivo stability of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)-octreotide in mouse blood (30 min p.i.) was investigated.

RESULTS

Yields of the hydrazone formation were independent of pH between pH 0.5 and 5.5. Optimal labeling yields of 85% were obtained with a precursor concentration of 2.1 mM at 70 degrees C for 10 min. The labeling products were stable at pH 7.5 at 37 degrees C, while in more acidic media (pH 4.0) the product slowly decomposed to form up to 31+/-2% [(18)F]FB-CHO within 5 h. Metabolite studies showed no detectable degradation of [(18)F]FB-CH=N-HYNIC-Tyr(3)-Thr(8)-octreotide in mouse blood (30 min p.i.).

CONCLUSIONS

In conclusion, chemoselective hydrazone formation between unprotected HYNIC-functionalized peptides and [(18)F]FB-CHO is a fast and straightforward radiolabeling method leading to high yields under mild acidic conditions. In addition, it represents a powerful and versatile radiolabeling strategy that is applicable to a variety of radionuclides and peptide precursors already available for (99m)Tc labeling.

Small animal positron emission tomography in food sciences

Positron emission tomography (PET) is a 3-dimensional imaging technique that has undergone tremendous developments during the last decade. Non-invasive tracing of molecular pathways in vivo is the key capability of PET. It has become an important tool in the diagnosis of human diseases as well as in biomedical and pharmaceutical research. In contrast to other imaging modalities, radiotracer concentrations can be determined quantitatively. By application of appropriate tracer kinetic models, the rate constants of numerous different biological processes can be determined. Rapid progress in PET radiochemistry has significantly increased the number of biologically important molecules labelled with PET nuclides to target a broader range of physiologic, metabolic, and molecular pathways. Progress in PET physics and technology strongly contributed to better scanners and image processing. In this context, dedicated high resolution scanners for dynamic PET studies in small laboratory animals are now available. These developments represent the driving force for the expansion of PET methodology into new areas of life sciences including food sciences. Small animal PET has a high potential to depict physiologic processes like absorption, distribution, metabolism, elimination and interactions of biologically significant substances, including nutrients, 'nutriceuticals', functional food ingredients, and foodborne toxicants. Based on present data, potential applications of small animal PET in food sciences are discussed.

18F-labeled sufentanil for PET-imaging of mu-opioid receptors

The synthesis of an (18)F-labeled sufentanil analogue with apparent high mu-opioid receptor selectivity is reported. Intravenous injection of N-[4-(methoxymethyl)-1-[2-(2-thienyl)ethyl]-4-piperidinyl]-N-phenyl-2-(+/-)-[(18)F]fluoropropan-amide in mice resulted in high brain uptake and a regional brain activity distribution corresponding to the mu-opioid receptor expression pattern. The developed ligand is a promising tracer for extended protocols in mu-opioid receptor mapping and quantitation with positron emission tomography.

New functional imaging modalities for chromaffin tumors, neuroblastomas and ganglioneuromas

Nuclear medicine modalities use radiolabeled ligands that either follow metabolic pathways or act on cellular receptors. Thus, they permit functional imaging of physiological processes and help to localize sites such as tumors that harbor pathological events. The application of positron emission tomography (PET) ligands to the specific pathways of synthesis, metabolism and inactivation of catecholamines found in chromaffin tumors, neuroblastomas and ganglioneuromas can be used to provide a more thorough localization of these types of tumor. Recent advances have been made in functional imaging to localize pheochromocytomas, paragangliomas, neuroblastomas and ganglioneuromas, including approaches based on PET with [(18)F]fluorodopamine, [(18)F]fluorohydroxyphenylalanine, [(11)C]epinephrine or [(11)C]hydroxyephedrine. Such functional imaging can complement computed tomography or magnetic resonance imaging and other scintigraphic techniques to localize these tumors before surgical or medical therapeutic approaches are considered.