As the applications for peptide-based radiopharmaceuticals continue to increase in number, so does the need for appropriate chelators that will enable new radiolabeling strategies for a “theranostic” approach. The aim of theranostics is to combine methods used for diagnostic imaging with therapeutic modalities.
Theranostics involves, for example, exchanging the radionuclide 68Ga used for diagnostic purposes for 177Lu, as a therapeutic agent for the same targeting vector, such as a peptide. In this way, diagnostic molecular characterization is translated into an opportunity for treatment.
However, the different labeling properties of radionuclides make this approach challenging. The chelators currently used are limited in terms of providing different coordinations for different metals and by the fact that radiolabeling requires high temperatures.
A new alternative bifunctional chelator for applications in radiopharmacy is 6-[Bis(carboxymethyl)amino]-1,4-bis(carboxymethyl)-6-methyl-1,4-diazepane (AAZTA), a heptadentate ligand that allows for fast labeling under mild conditions with trivalent metals. So far, only few data have been published on the properties of AAZTA-conjugated peptides.
In a study by Clemens Decristoforo (Innsbruck Medical University, Austria,) and colleagues, a bifunctional AAZTA derivative conjugated to the minigastrin analogue, MGLL, was evaluated as a model peptide and potential theranostic agent.
MG11 was chosen since gastrin has diagnostic and therapeutic applications in some rare forms of cancer. Minigastrin is also sensitive to extensive heating, but in the case of AAZTA, radiolabeling does not require any heating. Minigastrin analogues are also interesting because they can target the cholecystokinin 2 (CCK2) receptor, which is over expressed in some rare malignant diseases.
As well as focusing on AAZTA as a bifunctional chelator that allows radiolabeling with 68Ga for positron emission tomography (PET), the researchers also wanted to label the trivalent radiometals 111In for single-photon emission computed tomography (SPECT) and 177Lu for theranostic purposes.
The team found that the AAZTA chelator demonstrated excellent labeling ability, resulting in specific activities for all three radionuclides and showing high radiochemical yields, of more than 95% for 68GA and more than 98% for 177Lu and 111In.
In vitro studies showed that the distribution coefficient (logD) was −3.6 for [68Ga] AAZTA-MG and −3.73 for [177Lu] AAZTA-MG, indicating that both are highly hydrophilic, which would usually result in renal excretion of these peptides.
When the half maximal inhibitory concentration (IC50) for AAZTA-MG was determined, there was no significant difference between the three radionuclides, indicating high affinity of AAZTA-MG to the CCK2 receptor, irrespective of the radiometal introduced.
After 60 minutes, the specific cell uptake for 68GA-AAZTA-MG and 177Lu-AAZTA-MG was more than 7.5% and more than 9.5%, respectively, which is comparable to the uptake of currently used DOTA-MG analogues.
Further analysis looking at the biodistribution of 68Ga-AAZTA-MG and 111In-AAZTA-MG in a tumor xenograft mouse model showed comparable uptake in CCK2-positive tumor tissue that was about four times higher than uptake in the CCK2-negative tumor tissue, indicating specific uptake in tumor tissue expressing the CCK2 receptor.
Using Bruker BioSpin’s Albira PET/SPECT/CT small animal imaging system, the researchers acquired PET and CT images that showed high selective accumulation in A431-CCK2R positive tumors of 68GA-labelled AAZTA-MG.
Decristoforo et al. concluded that AAZTA “showed interesting properties as a bifunctional chelator for peptides providing mild radiolabeling conditions for both 68Ga and trivalent metals, having advantages over the currently used chelator DOTA.”
Additional studies are ongoing, to enable further investigation of in vivo targeting properties, stability issues and AAZTA biodistribution.
Decristoforo C, et al. Influence of a novel, versatile bifunctional chelator on theranostic properties of a minigastrin analogue. EJNMMI Research 2015 5:74 DOI 10.1186/s13550-015-0154-7