Synthesis and preclinical evaluation of BOLD-100 radiolabeled with ruthenium-97 and ruthenium-103

BOLD-100 (formerly IT-139, KP1339), a well-established chemotherapeutic agent, is currently being investigated in clinical trials for the treatment of gastric, pancreatic, colorectal, and bile duct cancer. Despite numerous studies, the exact mode of action is still the subject of discussions. Radiolabeled BOLD-100 could be a powerful tool to clarify pharmacokinetic pathways of the compound and to predict therapy responses in patients using nuclear molecular imaging prior to the therapy. In this study, the radiosyntheses of carrier-added (c.a.) [97/103Ru]BOLD-100 were performed with the two ruthenium isotopes ruthenium-103 (103Ru; β-, γ) and ruthenium-97 (97Ru; EC, γ), of which in particular the latter isotope is suitable for imaging by single-photon emission computed tomography (SPECT). To identify the best tumor-to-background ratio for diagnostic imaging, biodistribution studies were performed with two different injected doses of c.a. [103Ru]BOLD-100 (3 and 30 mg kg-1) in Balb/c mice bearing CT26 allografts over a time period of 72 h. Additionally, ex vivo autoradiography of the tumors (24 h p.i.) was conducted. Our results indicate that the higher injected dose (30 mg kg-1) leads to more unspecific accumulation of the compound in non-targeted tissue, which is likely due to an overload of the albumin transport system. It was also shown that lower amounts of injected c.a. [103Ru]BOLD-100 resulted in a relatively higher tumor uptake and, therefore, a better tumor-to-background ratio, which are encouraging results for future imaging studies using c.a. [97Ru]BOLD-100.

Straightforward Synthesis of DFO* - An Octadentate Chelator for Zirconium-89

DFO* is an octadentate chelator able to form highly stable chelates with Zirconium-89 (89 Zr) for nuclear medicinal applications in Positron Emission Tomography (PET).[1,2] The synthesis of DFO* and its scale-up remains challenging by reported synthetic protocols. For this reason, we set out to develop a de novo synthesis of a hydroxamate-containing building block suitable for the coupling to the commercially available DFO (desferrioxamine B, mesylate salt) yielding, after deprotection, the desired chelator DFO* in a more efficient procedure. Highlights of the new synthesis of DFO* reported herein are less synthetic steps and the isolation of the desired product DFO* by using solid phase extraction (SPE), thus avoiding tedious HPLC purification. DFO* is obtained in excellent purity (92-98 %) and an overall yield of approximately 29 %. In addition, the isolated trifluoroacetic acid (TFA)-salt of DFO* displays an improved solubility in organic solvents (DMSO, DMF, methanol), which will facilitate its use for the preparation of structurally diverse derivatives suitable for bioconjugation chemistry and the development of 89 Zr-labeled radiotracers.

Quality Assurance Investigations and Impurity Characterization during Upscaling of [177Lu]Lu-PSMAI&T

[177Lu]Lu-PSMAI&T is widely used for the radioligand therapy of metastatic castration-resistant prostate cancer (mCRPC). Since this kind of therapy has gained a large momentum in recent years, an upscaled production process yielding multiple patient doses in one batch has been developed. During upscaling, the established production method as well as the HPLC quality control were challenged. A major finding was a correlation between the specific activity and the formation of a pre-peak, presumably caused by radiolysis. Hence, nonradioactive reference standards were irradiated with an X-ray source and the formed pre-peak was subsequently identified as a deiodination product by UPLC-MS. To confirm the occurrence of the same deiodinated side product in the routine batch, a customized deiodinated precursor was radiolabeled and analyzed with the same HPLC setup, revealing an identical retention time to the pre-peak in the formerly synthesized routine batches. Additionally, further cyclization products of [177Lu]Lu-PSMAI&T were identified as major contributors to radiochemical impurities. The comparison of two HPLC methods showed the likelihood of the overestimation of the radiochemical purity during the synthesis of [177Lu]Lu-PSMAI&T. Finally, a prospective cost reduction through an optimization of the production process was shown.

Synthesis and Preclinical Evaluation of Radiolabeled [103Ru]BOLD-100

The first-in-class ruthenium-based chemotherapeutic agent BOLD-100 (formerly IT-139, NKP-1339, KP1339) is currently the subject of clinical evaluation for the treatment of gastric, pancreatic, colorectal and bile duct cancer. A radiolabeled version of the compound could present a helpful diagnostic tool. Thus, this study investigated the pharmacokinetics of BOLD-100 in more detail to facilitate the stratification of patients for the therapy. The synthesis of [103Ru]BOLD-100, radiolabeled with carrier added (c.a.) ruthenium-103, was established and the product was characterized by HPLC and UV/Vis spectroscopy. In order to compare the radiolabeled and non-radioactive versions of BOLD-100, both complexes were fully evaluated in vitro and in vivo. The cytotoxicity of the compounds was determined in two colon carcinoma cell lines (HCT116 and CT26) and biodistribution studies were performed in Balb/c mice bearing CT26 allografts over a time period of 72 h post injection (p.i.). We report herein preclinical cytotoxicity and pharmacokinetic data for BOLD-100, which were found to be identical to those of its radiolabeled analog [103Ru]BOLD-100.

Theranostics with photodynamic therapy for personalized medicine: to see and to treat

Photodynamic Therapy (PDT) is an approved treatment modality, which is presently receiving great attention due to its limited invasiveness, high selectivity and limited susceptibility to drug resistance. Another related research area currently expanding rapidly is the development of novel theranostic agents based on the combination of PDT with different imaging technologies, which allows for both therapy and diagnosis. This combination can help to address issues of suboptimal biodistribution and selectivity through regional imaging, while therapeutic agents enable an effective and personalized therapy. In this review, we describe compounds, whose structures combine PDT photosensitizers with different imaging probes - including examples for near-infrared optical imaging, magnetic resonance imaging (MRI) and nuclear imaging (PET or SPECT), generating novel theranostic drug candidates. We have intentionally focused our attention on novel compounds, which have already been investigated preclinically in vivo in order to demonstrate the potential of such theranostic agents for clinical applications.

Towards Ruthenium(II)-Rhenium(I) Binuclear Complexes as Photosensitizers for Photodynamic Therapy

The search for new metal-based photosensitizers (PSs) for anticancer photodynamic therapy (PDT) is a fast-developing field of research. Knowing that polymetallic complexes bear a high potential as PDT PSs, in this study, we aimed at combining the known photophysical properties of a rhenium(I) tricarbonyl complex and a ruthenium(II) polypyridyl complex to prepare a ruthenium-rhenium binuclear complex that could act as a PS for anticancer PDT. Herein, we present the synthesis and characterization of such a system and discuss its stability in aqueous solution. In addition, one of our complexes prepared, which localized in mitochondria, was found to have some degree of selectivity towards two types of cancerous cells: human lung carcinoma A549 and human colon colorectal adenocarcinoma HT29, with interesting photo-index (PI) values of 135.1 and 256.4, respectively, compared to noncancerous retinal pigment epithelium RPE1 cells (22.4).

Automated solid-phase synthesis of metabolically stabilized triazolo-peptidomimetics

The use of 1,4-disubstituted 1,2,3-triazoles as trans-amide bond surrogates has become an important tool for the synthesis of metabolically stabilized peptidomimetics. These heterocyclic bioisosters are generally incorporated into the peptide backbone by applying a diazo-transfer reaction followed by CuAAC (click chemistry) with an α-amino alkyne. Even though the manual synthesis of backbone-modified triazolo-peptidomimetics has been reported by us and others, no procedure has yet been described for an automated synthesis using peptide synthesizers. In order to efficiently adapt these reactions to an automated setup, different conditions were explored, putting special emphasis on the required long-term stability of both the diazo-transfer reagent and the Cu(I) catalyst in solution. ISA·HCl is the reagent of choice to accomplish the diazo-transfer reaction; however, it was found instable in DMF, the most commonly used solvent for SPPS. Thus, an aqueous solution of ISA·HCl was used to prevent its degradation over time, and the composition in the final diazo-transfer reaction was adjusted to preserve suitable swelling conditions of the resins applied. The CuAAC reaction was performed without difficulties using [Cu (CH3 CN)4 ]PF6 as a catalyst and TBTA as a stabilizer to prevent oxidation to Cu(II). The optimized automated two-step procedure was applied to the synthesis of structurally diverse triazolo-peptidomimetics to demonstrate the versatility of the developed methodology. Under the optimized conditions, five triazolo-peptidomimetics (8-5 amino acid residues) were synthesized efficiently using two different resins. Analysis of the crude products by HPLC-MS revealed moderate to good purities of the desired triazolo-peptidomimetics (70-85%). The synthesis time ranged between 9 and 12.5 h.

Evaluation of a Radiolabelled Macrocyclic Peptide as Potential PET Imaging Probe for PD-L1

The interaction between the immune checkpoint PD‐1 and PD−L1 promotes T‐cell deactivation and cancer proliferation. Therefore, immune checkpoint inhibition therapy, which relies on prior assessment of the target, has been widely used for many cancers. As a non‐invasive molecular imaging tool, radiotracers bring novel information on the in vivo expression of biomarkers (e. g., PD−L1), enabling a personalized treatment of patients. Our work aimed at the development of a PD−L1‐specific, peptide‐based PET radiotracer. We synthesized and evaluated a radiolabeled macrocyclic peptide adapted from a patent by Bristol Myers Squibb. Synthesis of [⁶⁸Ga]Ga‐NJMP1 yielded a product with a radiochemical purity>95 % that was evaluated in vitro. However, experiments on CHO−K1 hPD−L1 cells showed very low cell binding and internalization rates of [⁶⁸Ga]Ga‐NJMP1 in comparison to a control radiopeptide (WL12). Non‐radioactive cellular assays using time‐resolved fluorescence energy transfer confirmed the low affinity of the reported parent peptide and the DOTA‐derivatives towards PD−L1. The results of our studies indicate that the macrocyclic peptide scaffold reported in the patent literature is not suitable for radiotracer development due to insufficient affinity towards PD−L1 and that C‐terminal modifications of the macrocyclic peptide interfere with important ligand/receptor interactions.

Unexpected transformation of n.c.a. [111In]InCl3 in stock solutions into an unreactive [111In]In-species

While performing multiple indium-111 labeling of DOTA-modified peptides from a single batch of [¹¹¹In]InCl₃, inconsistent radiochemical yields were observed. We found that the formation of a radioactive impurity in the [¹¹¹In]InCl₃ stock solution hampered the reactivity of the indium-111 during radiolabeling reactions. The formation of this unknown ¹¹¹In-species could be successfully suppressed by increasing the concentration of chloride ions in the stock solution and [¹¹¹In]InCl₃ was "recovered". Radiolabeling of DOTA-peptides with the stabilized [¹¹¹In]InCl₃ resulted again in acceptable radiochemical yields. In addition, we report convenient iTLC systems that allow distinguishing between [¹¹¹In]InCl₃, the formed unknown ¹¹¹In-species, radiocolloids, and radiolabeled peptides (DOTANOC).

Development and in vitro evaluation of new bifunctional 89Zr-chelators based on the 6-amino-1,4-diazepane scaffold for immuno-PET applications

INTRODUCTION

Combination of hydroxamate bearing side chains with the 6-amino-1,4-diazepane scaffold provides a promising strategy for fast and stable ⁸⁹Zr-labeling of antibodies. Following this approach, we hereby present the development, labeling kinetics and in vitro complex stability of three resulting bifunctional chelator derivatives both stand-alone and coupled to a model protein in comparison to different linear deferoxamine (DFO) derivatives.

METHODS

The novel ⁸⁹Zr-chelator Hy₃ADA⁵ was prepared via amide-coupling of separately synthesized 6-amino-1,4-diazepane-6-pentanoic acid and hydroxamate-containing side chains. Two further bifunctional derivatives were synthesized by extending the resulting system with either a squaramide- or p-isothiocyanatophenyl moiety for simplified binding to proteins. After coupling to a model antibody and purification, the resulting immunoconjugates as well as the unbound chelator derivatives were ⁸⁹Zr-labeled at room temperature (RT) and neutral pH. For comparison, different DFO derivatives were analogously coupled, purified and radiolabeled. In vitro complex stability of the resulting radioconjugates was investigated in phosphate buffered saline (PBS) and human serum at 37 °C over a period of 7 days.

RESULTS

⁸⁹Zr-labeling of the novel unbound Hy₃ADA⁵ derivatives indicated rapid complexation kinetics resulting in high radiochemical conversions (RCC) of 84-94% after 90 min. Similar or even faster radiolabeling with slightly increased maximum yields was obtained using the DFO-analogues. Initially, [⁸⁹Zr]Zr-DFO*-p-Ph-NCS showed a delayed formation, nevertheless reaching almost quantitative complexation. Radiolabeling of the corresponding immunoconjugates Hy₃ADA⁵-SA-mAb and Hy₃ADA⁵-p-Ph-NCS-mAb resulted in 82.0 ± 1.1 and 89.2 ± 0.7% RCC, respectively after 90 min representing high but slightly lower labeling efficiency compared to the DFO- and DFO*-functionalized analogues. All examined radioimmunoconjugates showed very high in vitro complex stability both in human serum and PBS, providing no significant release of the radiometal. In the case of unbound chelators, however, the p-Ph-NCS-functionalized derivatives indicated considerable instability in human serum already after 1 h.

CONCLUSION

The novel chelator derivatives based on hydroxamate-functionalized 6-amino-1,4-diazepane revealed fast and high yielding ⁸⁹Zr-labeling kinetics as well as high in vitro complex stability both stand-alone and coupled to an antibody. Therefore, Hy₃ADA⁵ represents a promising tool for radiolabeling of biomolecules such as antibodies at mild conditions for immuno-PET applications.

1,5-Disubstituted 1,2,3-Triazoles as Amide Bond Isosteres Yield Novel Tumor-Targeting Minigastrin Analogs

1,5-Disubstituted 1,2,3-triazoles (1,5-Tz) are considered bioisosteres of cis-amide bonds. However, their use for enhancing the pharmacological properties of peptides or proteins is not yet well established. Aiming to illustrate their utility, we chose the peptide conjugate [Nle15]MG11 (DOTA-dGlu-Ala-Tyr-Gly-Trp-Nle-Asp-Phe-NH2) as a model compound since it is known that the cholecystokinin-2 receptor (CCK2R) is able to accommodate turn conformations. Analogs of [Nle15]MG11 incorporating 1,5-Tz in the backbone were synthesized and radiolabeled with lutetium-177, and their pharmacological properties (cell internalization, receptor binding affinity and specificity, plasma stability, and biodistribution) were evaluated and compared with [Nle15]MG11 as well as their previously reported analogs bearing 1,4-disubstituted 1,2,3-triazoles. Our investigations led to the discovery of novel triazole-modified analogs of [Nle15]MG11 with nanomolar CCK2R-binding affinity and 2-fold increased tumor uptake. This study illustrates that substitution of amides by 1,5-disubstituted 1,2,3-triazoles is an effective strategy to enhance the pharmacological properties of biologically active peptides.

Guideline on current good radiopharmacy practice (cGRPP) for the small-scale preparation of radiopharmaceuticals

This guideline on current good radiopharmacy practice (cGRPP) for small-scale preparation of radiopharmaceuticals represents the view of the Radiopharmacy Committee of the European Association of Nuclear Medicine (EANM). The guideline is laid out in the format of the EU Good Manufacturing Practice (GMP) guidelines as defined in EudraLex volume 4. It is intended for non-commercial sites such as hospital radiopharmacies, nuclear medicine departments, research PET centres and in general any healthcare establishments. In the first section, general aspects which are applicable to all levels of operations are discussed. The second section discusses the preparation of small-scale radiopharmaceuticals (SSRP) using licensed generators and kits. Finally, the third section goes into the more complex preparation of SSRP from non-licensed starting materials, often requiring a purification step and sterile filtration. The intention is that the guideline will assist radiopharmacies in the preparation of diagnostic and therapeutic SSRP’s safe for human administration.

Head-to-head comparison of DFO* and DFO chelators: selection of the best candidate for clinical 89Zr-immuno-PET

Purpose

Almost all radiolabellings of antibodies with ⁸⁹Zr currently employ the hexadentate chelator desferrioxamine (DFO). However, DFO can lead to unwanted uptake of ⁸⁹Zr in bones due to instability of the resulting metal complex. DFO*-NCS and the squaramide ester of DFO, DFOSq, are novel analogues that gave more stable ⁸⁹Zr complexes than DFO in pilot experiments. Here, we directly compare these linker-chelator systems to identify optimal immuno-PET reagents.

Methods

Cetuximab, trastuzumab and B12 (non-binding control antibody) were labelled with ⁸⁹Zr via DFO*-NCS, DFOSq, DFO-NCS or DFO*Sq. Stability in vitro was compared at 37 °C in serum (7 days), in formulation solution (24 h ± chelator challenges) and in vivo with N87 and A431 tumour-bearing mice. Finally, to demonstrate the practical benefit of more stable complexation for the accurate detection of bone metastases, [⁸⁹Zr]Zr-DFO*-NCS and [⁸⁹Zr]Zr-DFO-NCS-labelled trastuzumab and B12 were evaluated in a bone metastasis mouse model where BT-474 breast cancer cells were injected intratibially.

Results

[⁸⁹Zr]Zr-DFO*-NCS-trastuzumab and [⁸⁹Zr]Zr-DFO*Sq-trastuzumab showed excellent stability in vitro, superior to their [⁸⁹Zr]Zr-DFO counterparts under all conditions. While tumour uptake was similar for all conjugates, bone uptake was lower for DFO* conjugates. Lower bone uptake for DFO* conjugates was confirmed using a second xenograft model: A431 combined with cetuximab. Finally, in the intratibial BT-474 bone metastasis model, the DFO* conjugates provided superior detection of tumour-specific signal over the DFO conjugates.

Conclusion

DFO*-mAb conjugates provide lower bone uptake than their DFO analogues; thus, DFO* is a superior candidate for preclinical and clinical ⁸⁹Zr-immuno-PET.

Electronic supplementary material

The online version of this article (10.1007/s00259-020-05002-7) contains supplementary material, which is available to authorized users.

1,4-Disubstituted 1,2,3-Triazoles as Amide Bond Surrogates for the Stabilisation of Linear Peptides with Biological Activity

Peptides represent an important class of biologically active molecules with high potential for the development of diagnostic and therapeutic agents due to their structural diversity, favourable pharmacokinetic properties, and synthetic availability. However, the widespread use of peptides and conjugates thereof in clinical applications can be hampered by their low stability in vivo due to rapid degradation by endogenous proteases. A promising approach to circumvent this potential limitation includes the substitution of metabolically labile amide bonds in the peptide backbone by stable isosteric amide bond mimetics. In this review, we focus on the incorporation of 1,4-disubstituted 1,2,3-triazoles as amide bond surrogates in linear peptides with the aim to increase their stability without impacting their biological function(s). We highlight the properties of this heterocycle as a trans-amide bond surrogate and summarise approaches for the synthesis of triazole-containing peptidomimetics via the Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC). The impacts of the incorporation of triazoles in the backbone of diverse peptides on their biological properties such as, e.g., blood serum stability and affinity as well as selectivity towards their respective molecular target(s) are discussed.

Single Peptide Backbone Surrogate Mutations to Regulate Angiotensin GPCR Subtype Selectivity

Mutating the side-chains of amino acids in a peptide ligand, with unnatural amino acids, aiming to mitigate its short half-life is an established approach. However, it is hypothesized that mutating specific backbone peptide bonds with bioisosters can be exploited not only to enhance the proteolytic stability of parent peptides, but also to tune its receptor subtype selectivity. Towards this end, four [Y]⁶ -Angiotensin II analogues are synthesized where amide bonds have been replaced by 1,4-disubstituted 1,2,3-triazole isosteres in four different backbone locations. All the analogues possessed enhanced stability in human plasma in comparison with the parent peptide, whereas only two of them achieved enhanced AT₂ R/AT₁ R subtype selectivity. This diversification has been studied through 2D NMR spectroscopy and unveiled a putative more structured microenvironment for the two selective ligands accompanied with increased number of NOE cross-peaks. The most potent analogue, compound 2, has been explored regarding its neurotrophic potential and resulted in an enhanced neurite growth with respect to the established agent C21.

Radiolabelling of the octadentate chelators DFO* and oxoDFO* with zirconium-89 and gallium-68

In recent years, clinical imaging with zirconium-89 (⁸⁹Zr)-labelled monoclonal antibodies (Ab) by positron emission tomography (immunoPET) has been gaining significant importance in nuclear medicine for the diagnosis of different types of cancer. For complexation of the radiometal ⁸⁹Zr and its attachment to the Ab, chelating agents are required. To date, only the hexadentate chelator desferrioxamine (DFO) is applied in the clinic for this purpose. However, there is increasing preclinical evidence that the [⁸⁹Zr]Zr-DFO complex is not sufficiently stable and partly releases the radiometal in vivo due to the incomplete coordination sphere of the metal. This leads to unfavourable unspecific uptake of the osteophilic radiometal in bones, hence decreasing the signal-to-noise-ratio and leading to an increased dose to the patient. In the past, several new chelators with denticities > 6 have been published, notably the octadentate DFO derivative DFO*. DFO*, however, shows limited water solubility, wherefore an oxygen containing analogue, termed oxoDFO*, was developed in 2017. However, no data on the suitability of oxoDFO* for radiolabelling with ⁸⁹Zr has yet been reported. In this proof-of-concept study, we present the first radiolabelling results of the octadentate, water-soluble chelator oxoDFO*, as well as the in vitro stability of the resulting complex [⁸⁹Zr]Zr-oxoDFO* in comparison to the analogous octadentate, but less water-soluble derivative DFO* and the current "standard" chelator DFO. In addition, the suitability of DFO* and oxoDFO* for radiolabeling with the short-lived PET metal gallium-68 is discussed. The water-soluble, octadentate chelator oxoDFO* provides stable complexes with the positron emitter Zirconium-89. The radiolabelling can be performed at room temperature and neutral pH and thus, oxoDFO* represents a promising chelator for applications in immunoPET.

Design of Radiolabeled Analogs of Minigastrin by Multiple Amide-to-Triazole Substitutions

The insertion of single 1,4-disubstituted 1,2,3-triazoles as metabolically stable bioisosteres of trans-amide bonds (triazole scan) was recently applied to the ¹⁷⁷Lu-labeled tumor-targeting analog of minigastrin, [Nle¹⁵]MG11. The reported novel mono-triazolo-peptidomimetics of [Nle¹⁵]MG11 showed either improved resistance against enzymatic degradation or a significantly increased affinity toward the target receptor but never both. To enhance further the tumor-targeting properties of the minigastrin analogs, we studied conjugates with multiple amide-to-triazole substitutions for additive or synergistic effects. Promising candidates were identified by modification of two or three amide bonds, which yielded both improved stability and increased receptor affinity of the peptidomimetics in vitro. Biodistribution studies of radiolabeled multi-triazolo-peptidomimetics in mice bearing receptor-positive tumor xenografts revealed up to 4-fold increased tumor uptake in comparison to the all-amide reference compound [Nle¹⁵]MG11. In addition, we report here for the first time a linear peptidomimetic with three triazole insertions in its backbone and maintained biological activity.

Triazolo-Peptidomimetics: Novel Radiolabeled Minigastrin Analogs for Improved Tumor Targeting

MG11 is a truncated analog of minigastrin, a peptide with high affinity and specificity toward the cholecystokinin-2 receptor (CCK2R), which is overexpressed by different tumors. Thus, radiolabeled MG11 derivatives have great potential for use in cancer diagnosis and therapy. A drawback of MG11 is its fast degradation by proteases, leading to moderate tumor uptake in vivo. We introduced 1,4-disubstituted 1,2,3-triazoles as metabolically stable bioisosteres to replace labile amide bonds of the peptide. The "triazole scan" yielded peptidomimetics with improved resistance to enzymatic degradation and/or enhanced affinity toward the CCK2R. Remarkably, our lead compound achieved a 10-fold increase in receptor affinity, resulting in a 2.6-fold improved tumor uptake in vivo. Modeling of the ligand-CCK2R complex suggests that an additional cation-π interaction of the aromatic triazole moiety with the Arg³⁵⁶ residue of the receptor is accountable for these observations. We show for the first time that the amide-to-triazole substitution strategy offers new opportunities in drug development that go beyond the metabolic stabilization of bioactive peptides.

Manganese in PET imaging: Opportunities and challenges

Several radionuclides of the transition metal manganese are known and accessible. Three of them, ⁵¹Mn, ^52mMn, and ^52gMn, are positron emitters that are potentially interesting for positron emission tomography (PET) applications and, thus, have caught the interest of the radiochemical/radiopharmaceutical and nuclear medicine communities. This mini‐review provides an overview of the production routes and physical properties of these radionuclides. For medical imaging, the focus is on the longer‐living ^52gMn and its application for the radiolabelling of molecules and other entities exhibiting long biological half‐lives, the imaging of manganese‐dependent biological processes, and the development of bimodal PET/magnetic resonance imaging (MRI) probes in combination with paramagnetic ^natMn as a contrast agent.

Comparison of desferrioxamine and NODAGA for the gallium-68 labeling of exendin-4

INTRODUCTION

Radiolabeled exendin-4 (Ex4) derivatives are used to target the glucagon-like peptide-1 receptor (GLP-1R) for the clinical diagnosis of insulinomas, a rare type of neuroendocrine tumor. Gallium-68 is an ideal diagnostic nuclide for this application and a study evaluating an exendin-4-NODAGA conjugate is currently underway. However, in complexion with the chelator DFO, its in vivo stability has been a matter of dispute. The aim of this work was to directly compare [⁶⁸Ga]Ga-Ex4NOD with [⁶⁸Ga]Ga-Ex4DFO in vitro and in vivo.

METHODS

In our approach, we directly compared N'-[5-(acetyl-hydroxy-amino)pentyl]-N-[5-[3-(5-aminopentyl-hydroxy-carbamoyl)propanoylamino]pentyl]-N-hydroxy-butane diamide (desferriox-amine B, DFO) and 2-(4,7-bis (carboxymethyl)-1,4,7-triazonan-1-yl) pentanedioic acid (NODAGA) conjugated to exendin-4 in vitro and in vivo. We radiolabeled the peptides with gallium-68, followed by HPLC quality control. In vitro characterization was performed in CHL cells overexpressing the GLP-1R and in vivo studies were conducted with CD1 nu/nu mice carrying tumors derived from these cells.

RESULTS

We found that both peptides could be radiolabeled with a molar activity of about 9.33 MBq/nmol without further purification. They internalized equally well into GLP-1R-expressing cells and their IC₅₀ was similar with 15.6 ± 7.8 nM and 18.4 ± 3.0 nM for [^natGa]Ga-Ex4NOD and [^natGa]Ga-Ex4DFO, respectively. In vivo, [⁶⁸Ga]Ga-Ex4NOD accumulated more in all tissue, while [⁶⁸Ga]Ga-Ex4DFO exhibited a more favorable target-to-kidney ratio.

CONCLUSION AND RELEVANCE

DFO is a suitable chelator for the radiolabeling of exendin-4 derivatives with gallium-68 for in vitro and preclinical in vivo studies. DFO performed better in vivo due to its significantly lower kidney accumulation (p < 0.0001). It was also found to be stable in vivo in mice, contrary to earlier reports. Based on our results, the DFO chelating system in combination with exendin-4 would be an interesting option for clinical imaging of insulinomas.

Mini-review: Targeted radiopharmaceuticals incorporating reversible, low molecular weight albumin binders

The combination of low molecular weight, reversible human serum albumin (HSA) binders with targeted radiopharmaceuticals in dual-targeted radioconjugates holds great promise, in particular for endoradiotherapy. Attachment of HSA-binders to radiopharmaceuticals extends their blood circulation time and results in an enhanced tumour uptake as well as often in an improved pharmacokinetic profile. In this mini-review, an overview of currently pursued approaches of this novel strategy is provided.

Consensus nomenclature rules for radiopharmaceutical chemistry - Setting the record straight

Over recent years, within the community of radiopharmaceutical sciences, there has been an increased incidence of incorrect usage of established scientific terms and conventions, and even the emergence of 'self-invented' terms. In order to address these concerns, an international Working Group on 'Nomenclature in Radiopharmaceutical Chemistry and related areas' was established in 2015 to achieve clarification of terms and to generate consensus on the utilisation of a standardised nomenclature pertinent to the field. Upon open consultation, the following consensus guidelines were agreed, which aim to.

Amide-to-triazole switch vs. in vivo NEP-inhibition approaches to promote radiopeptide targeting of GRPR-positive tumors

INTRODUCTION

Radiolabeled bombesin (BBN)-analogs have been proposed for diagnosis and therapy of gastrin-releasing peptide receptor (GRPR)-expressing tumors, such as prostate, breast and lung cancer. Metabolic stability represents a crucial factor for the success of this approach by ensuring sufficient delivery of circulating radioligand to tumor sites. The amide-to-triazole switch on the backbone of DOTA-PEG₄-[Nle¹⁴]BBN(7-14) (1) was reported to improve the in vitro stability of resulting ¹⁷⁷Lu-radioligands. On the other hand, in-situ inhibition of neutral endopeptidase (NEP) by coinjection of phosphoramidon (PA) was shown to significantly improve the in vivo stability and tumor uptake of biodegradable radiopeptides. We herein compare the impact of the two methods on the bioavailability and localization of ¹⁷⁷Lu-DOTA-PEG₄-[Nle¹⁴]BBN(7-14) analogs in GRPR-positive tumors in mice.

METHODS

The 1,4-disubstituted [1-3]-triazole was used to replace one (2: Gly¹¹-His¹²; 3: Ala⁹-Val¹⁰) or two (4: Ala⁹-Val¹⁰ and Gly¹¹-His¹²) peptide bonds in 1 (reference) and all compounds were labeled with ¹⁷⁷Lu. Each of [¹⁷⁷Lu]1-[¹⁷⁷Lu]4 was injected without (control) or with PA in healthy mice. Blood samples collected 5min post-injection (pi) were analyzed by HPLC. Biodistribution of [¹⁷⁷Lu]1-[¹⁷⁷Lu]4 was conducted in SCID mice bearing human prostate adenocarcinoma PC-3 xenografts at 4h pi. Groups of 4 animals were injected with radioligand, alone (controls), or with coinjection of PA, or of a mixture of PA and excess and [Tyr⁴]BBN to determine GRPR-specificity of uptake (Block).

RESULTS

The in vivo stability of the radioligands was: [¹⁷⁷Lu]1 (25% intact), [¹⁷⁷Lu]2 (45% intact), [¹⁷⁷Lu]3 (30% intact) and [¹⁷⁷Lu]4 (40% intact). By PA-coinjection these values notably increased to 90%-93%. Moreover, treatment with PA induced an impressive and GRPR-specific uptake of all radioligands in the PC-3 xenografts at 4h pi: [¹⁷⁷Lu]1: 4.7±0.4 to 24.8±4.9%ID/g; [¹⁷⁷Lu]2: 8.3±1.2 to 26.0±1.1%ID/g; [¹⁷⁷Lu]3: 6.6±0.4 to 21.3±4.4%ID/g; and [¹⁷⁷Lu]4: 4.8±1.6 to 13.7±3.8%ID/g.

CONCLUSIONS

This study has shown that amide-to-triazole substitutions in ¹⁷⁷Lu-DOTA-PEG₄-[Nle¹⁴]BBN(7-14) induced minor effects on bioavailability and tumor uptake in mice models, whereas in-situ NEP-inhibition(s) by PA impressively improved in vivo profiles.

Methoxinine - an alternative stable amino acid substitute for oxidation-sensitive methionine in radiolabelled peptide conjugates

Radiolabelled peptides with high specificity and affinity towards receptors that are overexpressed by tumour cells are used in nuclear medicine for the diagnosis (imaging) and therapy of cancer. In some cases, the sequences of peptides under investigations contain methionine (Met), an amino acid prone to oxidation during radiolabelling procedures. The formation of oxidative side products can affect the purity of the final radiopharmaceutical product and/or impair its specificity and affinity towards the corresponding receptor. The replacement of Met with oxidation resistant amino acid analogues, for example, norleucine (Nle), can provide a solution. While this approach has been applied successfully to different radiolabelled peptides, a Met → Nle switch only preserves the length of the amino acid side chain important for hydrophobic interactions but not its hydrogen-bonding properties. We report here the use of methoxinine (Mox), a non-canonical amino acid that resembles more closely the electronic properties of Met in comparison to Nle. Specifically, we replaced Met¹⁵ by Mox¹⁵ and Nle¹⁵ in the binding sequence of a radiometal-labelled human gastrin derivative [d-Glu¹⁰ ]HG(10-17), named MG11 (d-Glu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH₂ ). A comparison of the physicochemical properties of ¹⁷⁷ Lu-DOTA[X¹⁵ ]MG11 (X = Met, Nle, Mox) in vitro (cell internalization/externalization properties, receptor affinity (IC₅₀ ), blood plasma stability and logD) showed that Mox indeed represents a suitable, oxidation-stable amino acid substitute of Met in radiolabelled peptide conjugates. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

A solid phase-assisted approach for the facile synthesis of a highly water-soluble zirconium-89 chelator for radiopharmaceutical development

The solid-phase synthesis of a water-soluble bifunctional chelator for ⁸⁹Zr is reported.

Glycated 99m Tc-Tricarbonyl-Labeled Peptide Conjugates for Tumor Targeting by "Click-to-Chelate"

Attaching polar pharmacological modifiers to molecular imaging probes is a common strategy to modulate their pharmacokinetic profiles to improve such parameters as the clearance rate of radiotracers and/or metabolites, and to enhance signal-to-background ratios. We combined the tumor-targeting peptide sequence of bombesin (BBN) with glucuronic acid and the single-photon emission computed tomography (SPECT) radionuclide ^99m Tc by the "click-to-chelate" methodology. The ^99m Tc-tricarbonyl-labeled glucuronated BBN conjugate was compared with a reference compound lacking the carbohydrate. The radiolabeled conjugates displayed similar characteristics in vitro (cell internalization, receptor affinity), but the hydrophilicity of the glycated version was significantly increased. While the tumor uptake of the two radioconjugates in xenografted mice was similar, the glycated peptide exhibited unexpected higher uptake in organs of the hepatobiliary excretion pathway than the more lipophilic reference compound. Control experiments suggest that this may be the result of unspecific accumulation of metabolites in which the glucuronic acid moiety does not act as an innocent pharmacological modifier.

Toward the Optimization of Bombesin-Based Radiotracers for Tumor Targeting

The peptide bombesin (BBN) is a peptide with high affinity for the gastrin-releasing peptide receptor (GRPr), a receptor that is overexpressed by, for example, breast and prostate cancers. Thus, GRPr agonists can be used as cancer-targeting vectors to shuttle diagnostic and therapeutic agents into tumor cells. With the aim of optimizing the tumor targeting properties of a radiolabeled [Nle(14)]BBN(7-14) moiety, novel BBN(7-14)- and BBN(6-14)-based radioconjugates were synthesized, labeled with Lu-177, and fully evaluated in vitro and in vivo. The effect of residue and backbone modification on several parameters such as the internalization of the radiolabeled peptides into PC3 and AR42J tumor cells, their affinity toward the human GRPr, metabolic stability in blood plasma, and biodistribution in mice bearing GRPr-expressing PC3 xenografts was studied. As a result of our investigations, a novel radiolabeled GRPr agonist with a high tumor uptake and a high tumor-to-kidney ratio was identified.

Novel chemoselective (18)F-radiolabeling of thiol-containing biomolecules under mild aqueous conditions

We report a novel prosthetic group based on a heterocyclic methylsulfone derivative for the rapid, stable, and chemoselective (18)F-labeling of thiol-containing (bio)molecules under mild aqueous reaction conditions. Compared to established maleimide approaches, the new methodology displays some clear advantages for imaging probe development.

Position paper on requirements for toxicological studies in the specific case of radiopharmaceuticals

This is a position paper of the Radiopharmacy Committee of the EANM (European Association of Nuclear Medicine) addressing toxicology studies for application of new diagnostic and therapeutic radiopharmaceuticals (RP) that are not approved (i.e., not having a marketing authorization or a monograph in the European Pharmacopoeia), excluding endogenous and ubiquitous substances in humans. This paper discusses the requirements for clinical trials with radiopharmaceuticals for clinical research applications, not necessarily intended to aim at a marketing authorization. If marketing authorization is intended, scientific advice of the competent authorities is mandatory and cannot be replaced by this position paper. The position paper reflects the view of the Radiopharmacy Committee of the EANM and can be used as a basis for discussions with the responsible authorities.

Radiolabeled analogs of neurotensin (8–13) containing multiple 1,2,3-triazoles as stable amide bond mimics in the backbone

Substitution of multiple amide bonds by metabolically stable 1,2,3-triazoles yields novel tumour-targeting neurotensin-based peptidomimetics with interesting biological properties.

Structure-Activity Relationship Studies of Amino Acid Substitutions in Radiolabeled Neurotensin Conjugates

Radiolabeled derivatives of the peptide neurotensin (NT) and its binding sequence NT(8-13) have been studied as potential imaging probes and therapeutics for NT-1-receptor-positive cancer. However, a direct comparison of reported NT analogues, even if radiolabeled with the same radionuclide, is difficult because different techniques and models have been used for preclinical evaluations. In an effort to identify a suitable derivative of NT(8-13) for radiotracer development, we herein report a side-by-side in vitro comparison of radiometallated NT derivatives bearing some of the most commonly reported amino acid substitutions in their sequence. Performed investigations include cell internalization experiments, determinations of receptor affinity, measurements of the distribution coefficient, and blood serum stability studies. Of the [(177)Lu]-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-labeled examples studied, analogues of NT(8-13) containing a short hydrophilic tetraethylene glycol (PEG4 ) spacer between the peptide and the radiometal complex, and a minimum number of substitutions of amino acid residues, exhibited the most promising properties in vitro.

Development of 68Ga- and 89Zr-Labeled Exendin-4 as Potential Radiotracers for the Imaging of Insulinomas by PET

Clinical studies have demonstrated the potential of radiometallated exendin-4 derivatives for the imaging of glucagonlike peptide-1 receptor-overexpressing insulinomas. Recently investigated exendin-4 derivatives were radiolabeled with the SPECT isotopes 99mTc or 111In. Despite promising results, the low spatial resolution associated with SPECT and the occasional need to perform imaging several days after injection for the demarcation of insulinomas from the kidneys represent current limitations. The aim of this work was the development of exendin-4 derivatives for the imaging of insulinomas by high-resolution PET at early or late time points after injection of the radiotracer.

METHODS

An exendin-4 derivative conjugated to desferrioxamine (DFO) was used for radiolabeling with the PET isotopes 68Ga and 89Zr. Both radiotracers were evaluated in vitro with RIN-m5F cells for their cell internalization properties as well as affinities and specificities toward the glucagonlike peptide-1 receptor. Serum stabilities of the radiopeptides were assessed in blood serum, and their distribution coefficient was determined by the shake-flask method. Biodistribution experiments were performed with nude mice bearing RIN-m5F xenografts. For all experiments, clinically evaluated [Lys40-(AHX-DTPA-111In)NH2]exendin-4 was used as a reference compound.

RESULTS

[Lys40-(AHX-DFO)NH2]exendin-4 was labeled with 89Zr and 68Ga in high radiochemical yield and purity. In vitro experiments showed favorable cell uptake and receptor affinity for [Lys40-(AHX-DFO-68Ga)NH2]exendin-4, and [Lys40-(AHX-DFO-89Zr)NH2]exendin-4 and [Lys40-(AHX-DTPA-111In)NH2]exendin-4 performed similarly well. In biodistribution experiments, [Lys40-(AHX-DFO-68Ga)NH2]exendin-4 exhibited a significantly enhanced tumor uptake 1 h after injection in comparison to the other 2 radiotracers. Tumor uptake of [Lys40-(AHX-DFO-89Zr)NH2]exendin-4 was comparable to that of [Lys40-(AHX-DTPA-111In)NH2]exendin-4 at 1-48 h after injection. All compounds showed a fast blood clearance and low accumulation in receptor-negative organs and tissue with the exception of the kidneys, a known characteristic for exendin-4-based radiotracers.

CONCLUSION

68Ga- and 89Zr-radiolabeled [Lys40-(AHX-DFO)NH2]exendin-4 exhibit characteristics comparable or superior to the clinically tested reference compound [Lys40-(AHX-DTPA-111In)NH2]exendin-4 and, thus, represent potential new tracers for the imaging of insulinomas by PET.

EANM guideline for the preparation of an Investigational Medicinal Product Dossier (IMPD)

The preparation of an Investigational Medicinal Product Dossier (IMPD) for a radiopharmaceutical to be used in a clinical trial is a challenging proposition for radiopharmaceutical scientists working in small-scale radiopharmacies. In addition to the vast quantity of information to be assembled, the structure of a standard IMPD is not well suited to the special characteristics of radiopharmaceuticals. This guideline aims to take radiopharmaceutical scientists through the practicalities of preparing an IMPD, in particular giving advice where the standard format is not suitable. Examples of generic IMPDs for three classes of radiopharmaceuticals are given: a small molecule, a kit-based diagnostic test and a therapeutic radiopharmaceutical.

An octadentate bifunctional chelating agent for the development of stable zirconium-89 based molecular imaging probes

(89)Zr-based imaging agents hold great promise as novel radio-tracers in nuclear medicine. However, insufficient stability of currently used radiometal complexes in vivo is a safety concern for clinical applications. We herein report the first octadentate bifunctional chelating agent for the development of (89)Zr-labelled (bio)conjugates with improved stability.

Radiolabeled antagonistic bombesin peptidomimetics for tumor targeting

The replacement of amide bonds in the backbone of peptides by proteolytically stable 1,2,3-triazole isosteres can provide novel peptidomimetics with promising properties for the development of tumor-targeting radiopeptides. On the basis of our previous work with radiolabeled agonistic bombesin (BBN) derivatives of the sequence [Nle(14) ]BBN(7-14), we substituted selected amide bonds of the structurally closely related antagonistic peptide analog JMV594. With the exception of the C-terminal modification, amide-to-triazole substitutions tolerated by [Nle(14) ]BBN(7-14) without loss of biological function led to abolished receptor affinity in the case of JMV594. These findings provide an additional piece of evidence for the currently disputed differences in the modes of action of agonistic and antagonistic gastrin-releasing peptide receptor (GRPR)-targeting radiopeptides.