Influence of a novel, versatile bifunctional chelator on theranostic properties of a minigastrin analogue

Abstract

Background6-[Bis(carboxymethyl)amino]-1,4-bis(carboxymethyl)-6-methyl-1,4-diazepane (AAZTA ) is a promising chelator with potential advantages over 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) for radiopharmaceutical applications. Its mesocyclic structure enables fast radiolabelling under mild conditions with trivalent metals including not only 68Ga for positron emission tomography (PET) but also 177Lu and 111In for single-photon emission computed tomography (SPECT) and radionuclide therapy. Here, we describe the evaluation of a bifunctional AAZTA derivative conjugated to a model minigastrin derivative as a potential theranostic agent.MethodsAn AAZTA derivative with an aliphatic C9 chain as linker was coupled to a minigastrin, namely [AAZTA0, D-Glu1, desGlu2–6]-minigastrin (AAZTA-MG), and labelled with 68Ga, 177Lu and 111In. The characterisation in vitro included stability studies in different media and determination of logD (octanol/PBS). Affinity determination (IC50) and cell uptake studies were performed in A431-CCK2R cells expressing the human CCK2 receptor. μPET/CT and ex vivo biodistribution studies were performed in CCK2 tumour xenograft-bearing nude mice and normal mice.ResultsAAZTA-MG showed high radiochemical yields for 68Ga (>95 %), 177Lu (>98 %) and 111In (>98 %). The logD value of −3.7 for both [68Ga]- and [177Lu]-AAZTA-MG indicates a highly hydrophilic character. Stability tests showed overall high stability in solution with some degradation in human plasma for [68Ga]- and transchelation towards DTPA for and [177Lu]-AAZTA-MG. An IC50 value of 10.0 nM was determined, which indicates a high affinity for the CCK2 receptor. Specific cell uptake after 60 min was >7.5 % for [68Ga]-AAZTA-MG and >9.5 % for [177Lu]-AAZTA-MG, comparable to other DOTA-MG-analogues. μPET/CT studies in CCK2 receptor tumour xenografted mice not only revealed high selective accumulation in A431-CCK2R positive tumours of 68Ga-labelled AAZTA-MG (1.5 % ID/g in 1 h post injection) but also higher blood levels as corresponding DOTA-analogues. The 111In-labelled peptide had a tumour uptake of 1.7 % ID/g. Biodistribution in normal mice with the [177Lu]-AAZTA-MG showed a considerable uptake in intestine (7.3 % ID/g) and liver (1.5 % ID/g).ConclusionOverall, AAZTA showed interesting properties as bifunctional chelator for peptides providing mild radiolabelling conditions for both 68Ga and trivalent metals having advantages over the currently used chelator DOTA. Studies are ongoing to further investigate in vivo targeting properties and stability issues and the influence of spacer length on biodistribution of AAZTA.Electronic supplementary materialThe online version of this article (doi:10.1186/s13550-015-0154-7) contains supplementary material, which is available to authorized users.

AAZTA5-squaramide ester: promising tool for 177Lu-labeling of monoclonal antibodies under mild conditions

“…Radiolabeling of DOTA-SA with 177 Lu Radiolabeling of AAZTA 5 -SA-bevacizumab and DOTA-SAbevacizumab with 177 Lu and puri cation of [ 177 (44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55) MBq in 14-50 µL 0.04 M HCl) was diluted with 0.5 M HEPES-buffer (pH 7) to a total volume of 550 µL. Afterwards a solution of either AAZTA 5 -SA-bevacizumab (125-900 µg, 0.8-6.0 nmol) or DOTA-SA-bevacizumab (272-900 µg, 1.8-6.0 nmol) in PBS (450 µL) was added and the mixture (1 mL) was shaken for 60-90 min at 550 rpm and either at room temperature or 37 °C via thermomixer.…”

New Insights in the Design of Bioactive Peptides and Chelating Agents for Imaging and Therapy in Oncology

“…More recently, a new chelator, the 6-[Bis(carboxymethyl)amino]-1,4-bis(carboxymethyl)-6-methyl-1,4-diazepane (AAZTA) ( Figure 4 ), showed better properties than 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) for radiopharmaceutical applications. The radiolabeling is carried out under mild conditions for short time with trivalent metals including 68 Ga for PET and 177 Lu or 111 In for SPECT and radionuclide therapy [ 103 ]. Pfister at al.…”

“…The radiolabeled AAZTA-MG has shown good tumor targeting although was observed some degradation in human plasma and a considerable uptake by intestine and liver in healthy mice. Additional modifications of chelator structure or linker could improve the tumor targeting and pharmacokinetic properties [ 103 ].…”

Bifunctional Gallium-68 Chelators: Past, Present, and Future

“…17 AAZTA and DATA are chimeric chelators, because they combine a macrocyclic 1,4-diazapam ring with a free acyclic arm. Bifunctional derivates of AAZTA have been applied in small animal 68 Ga-PET studies, using minigastrin 18 -or RGD peptidomimetic D58 83 target vectors. Both studies required mild conditions for labeling (10 minutes, RT), but evaluations also indicate that AAZTA is not completely stable against human serum and chelators such as EDTA and DTPA.…”

“…Both studies required mild conditions for labeling (10 minutes, RT), but evaluations also indicate that AAZTA is not completely stable against human serum and chelators such as EDTA and DTPA. 18 An auspiciously derivate of AAZTA is DATA. 175 It was successfully labeled under mild conditions and had shown no loss of gallium in human serum or DTPA solution.…”