DATATOC: a novel conjugate for kit-type 68Ga labelling of TOC at ambient temperature

Background The widespread acceptance and application of 68Ga-PET depends on our ability to develop radiopharmaceuticals that can be prepared in a convenient and suitable manner. A kit-type labelling protocol provides such characteristics and requires chelators that can be radiolabelled under exceptionally mild conditions. Recently the DATA chelators have been introduced that fulfil these requirements. In continuing their development, the synthesis and radiolabelling of the first DATA bifunctional chelator (BFC) and peptide conjugate are described. Results A BFC derived from the DATA ligand (2,2’-(6-((carboxymethyl)amino)-1,4-diazepane-1,4-diyl)diacetic acid) has been synthesised in five steps from simple building blocks, with an overall yield of 8 %. DATAM5-3tBu (5-[1,4-Bis-tert-butoxycarbonylmethyl-6-(tert-butoxycarbonylmethyl-methyl-amino)-[1, 4]diazepan-6-yl]-pentanoic acid) has been coupled to [DPhe1][Tyr3]-octreotide (TOC) and the resulting peptide conjugate (DATATOC) radiolabelled with purified 68Ga derived via four different 68Ge/68Ga generator post-processing (PP) methods. The stability and lipophilicity of the radiotracer have been assessed and a kit-type formulation for radiolabelling evaluated. 68Ga-DATATOC has been prepared with a > 95 % radiochemical yield (RCY) within 1 (fractionated and acetone-PP) and 10 min (ethanol- and NaCl-PP) at 23 °C (pH 4.2–4.9, 13 nmol). The radiolabelled peptide is stable in the presence of human serum. Lipophilicity of 68Ga-DATATOC was calculated as logP = −3.2 ± 0.3, with a HPLC retention time (t R = 10.4 min) similar to 68Ga-DOTATOC (logP = −2.9 ± 0.4, t R = 10.3 min). Kit-type labelling from a lyophilised solid using acetone-PP based labelling achieves > 95 % RCY in 10 min at 23 °C. Conclusions The favourable labelling properties of the DATA chelators have been retained for DATATOC. High radiochemical purity can be achieved at 23 °C in less than 1 min and from a kit formulation. The speed, reliability, ease, flexibility and simplicity with which 68Ga-DATATOC can be prepared makes it a very attractive alternative to current standards. Electronic supplementary material The online version of this article (doi:10.1186/s41181-016-0007-3) contains supplementary material, which is available to authorized users.


Background
The positron emitter 68 Ga has a number of characteristics which make it a very attractive and promising radionuclide for PET imaging of disease and infection (Smith et al. 2013;Fani et al. 2008). In spite of this and numerous publications which provide support for its superiority, more established radionuclides and imaging modalities stand in its path (Buchmann et al. 2007;Tran et al. 2015). Interest in 68 Ga-PET in Europe has grown over the last years, highlighted by the recent promotion of 68 Ga -DOTATATE and -DOTATOC to orphan drug status in the United States (FDA Grants Orphan Drug Designation for 68 Ga-DOTATOC. J Nucl Med 2014). 99m Tc became the work-horse of nuclear medicine in part due to the development of SPECT radiopharmaceuticals which can be prepared in a simple kit-type manner (Roesch 2013). The development of chelators that permit a similar protocol for 68 Ga complement the inherent advantages of the 68 Ge/ 68 Ga-generator, paving the way for realisation of its full potential and delivery of advanced imaging diagnostics worldwide (Mukherjee et al. 2014;Velikyan et al. 2008).
The last decade of 68 Ga-radiopharmaceutical chemistry has been dominated by studies with DOTATOC and its derivatives for the diagnosis of neuroendocrine tumours (NETs) (Frilling et al. 2010). The omnipresence of DOTA derivatives in 68 Ga-PET arose from its success in other imaging applications (MRI and optical imaging with lanthanides) and its ability to provide an acceptable labelling profile and sufficient complex stability with 68 Ga (Notni et al. 2011;Boros et al. 2010). Optimisation of radiolabelling protocols and the development of labelling modules have simplified radiopharmaceutical preparation, but the desire for further development remains (Ocak et al. 2010). The major inherent disadvantage of DOTA derivatives is the relatively harsh conditions required for radiolabelling (80-95°C, 5-10 min, pH 3-4) that impose a number of limitations (Notni et al. 2011;Boros et al. 2010). Chelators which can be labelled quickly at room temperature would simplify labelling further and offer the potential for kit-type labelling akin to the prestigious 99m Tc kits. Temperature/pH sensitive targeting vectors (TVs) would benefit from these new chelators in particular, and widen the portfolio of 68 Ga-based diagnostics. In recent times, there has been greater focus on the development of more efficient hexadentate chelators, and bifunctional derivatives of NOTA, TRAP, NOPO, DEDPA, CP256, HBED have been described that chelate 68 Ga(III) rapidly at room temperature (Notni et al. 2011;Fani et al. 2012;Simeček et al. 2012;Boros et al. 2012;Berry et al. 2011;Eder et al. 2008).
The potential for kit-type labelling of biomolecule conjugates has been alluded to on numerous occasions, but a protocol which can achieve acceptable yields from a lyophilised solid at ambient temperature remains 'the final frontier' (Mukherjee et al. 2014;Velikyan et al. 2008Asti et al. 2015Waengler et al. 2011). The DATA chelators rapidly form stable complexes with 68 Ga under exceptionally mild conditions befitting kit-type labelling . The ligand DATA M (Fig. 1) for instance, can enable radiochemical yields (RCYs) greater than 97 % at 23°C in under 1 min (Seemann et al. 2015a). Earlier work showed that it is possible to label DATA chelators over the pH range 4-7, however for the purposes of this work it was decided that only optimum labelling conditions would be used in line with the desire to develop a room temperature kit-type labelling protocol Seemann et al. 2015a).
The main objective of this work has been to demonstrate the potential of a DATA conjugate towards development of kit-type labelling in a setting that holds considerable practical interest. The peptide TOC has been extensively studied with other chelators (DOTA, NODAGA, DTPA, DFO) in both imaging and therapeutic modalities, and has significant clinical and commercial importance (Lin et al. 2013;Eisenwiener et al. 2002;Dumont et al. 2011;Fani et al. 2011;Ugur et al. 2002). Hence, DATA conjugates were envisaged to allow comparative analysis.
The first step was to develop and synthesise a novel DATA bifunctional prochelator (DATA M5 -3 t Bu) that permits convenient amide conjugation to the N-terminus of TOC. A bifunctional derivative of DATA M has been synthesised and conjugated to protected TOC using standard methods (Fig. 1). An initial radiolabelling evaluation of the conjugate (DATATOC, Fig. 1) has been carried out at ambient temperature, and a kit-type formulation tested.

Methods
Reagents were purchased from Sigma-Aldrich® or Merck® and used without further purification. Purite® water used was filtered through a Millex® Millipore filter membrane (0.54 μm). Reaction progress was monitored using silica TLC-plates (silica 60 F 254 4.5 × 4.5 cm, Merck) and visualised with UV 254nm and/or KMnO 4 . Column chromatography was performed with silica gel 60 (Fisher Scientific®; 0.04-0.063 nm). NMR spectra ( 1 H, 13 C, HSQC, HMBC) were recorded on an Avance III HD 400 (Bruker, United States). Chemical shifts are given in ppm. MS (ESI) were performed with a Thermo Quest Navigator Instrument (Thermo Electron). Mass spec results are given as m/z in g/mol. HPLC was performed with a metal-free Dionex ICS-5000 system with a quaternary pump, an AS-50 auto sampler, UV/vis detector and automated fraction collector AFC-3000.

Synthesis
The chemical identity of synthesised compounds has been confirmed by 1 H-, 13 C-NMR and HR MS with the exception of TOC conjugates, which have been characterised by HPLC and HRMS. 1 H-, 13 C-NMR and HRMS data for compounds prior to TOC conjugation are provided in the S.I. (Fig. 2).

Radiochemistry
All radiochemical evaluations were conducted using chemicals of the highest available purity grade. Volumes were measured using an Eppendorf pipette. A TiO 2 -based 68 Ge/ 68 Ga generator (Cyclotron Co., Obninsk, Russia) was used for all radiochemical evaluations. Four post-processing (PP) methods were used to purify and preconcentrate the radioactive eluate: fractionation, acetone-, ethanol-and NaCl-based. Procedures were carried out, and solutions required prepared, as detailed in the relevant publication (Zhernosekov et al. 2007;Mueller et al. 2012;Eppard et al. 2014;Breeman et al. 2005). pH was measured using a Mettler-Toledo, SevenEasy pH. Radio-TLC was performed using silica-60 TLC plates (Merck F254, 4.5 × 4.5 cm), and eluted using 0.1 M citrate buffer (pH 4). Eluted radio-TLC plates were analysed using a flat-bed imaging scanner (Instant Imager, Canberra Packard).

Radiolabelling
Experiments were carried out in triplicate, and the amount of DATATOC used was constant in each case (13 nmol) taken from a 1 mg/mL stock solution. The volume of eluate differed according to the post-processing used, but was diluted as necessary so that~100 MBq 68 Ga was used for radiolabelling in each instance. Radiolabelling experiments were maintained at 23°C by means of a heater-shaker device (DITABIS MHR 11), which was also used to agitate (400 rpm) radiolabelling solutions. TLC samples (1 μL) were taken at 1, 3, 5 and 10 min. The optimised labelling conditions vary according to the PP method applied. The important differences and approach used in each case are summarised in Table 1.

Kit-type labelling
Formulations for labelling following fractionation and acetone-PP were used to investigate the possibility of kit-type labelling. Acetone-PP: The precursor solution was prepared as per Table 1 and lyophilised into a vial suitable for labelling. The post-processed 68 Ga was diluted to the labelling volume (1.4 mL) using H 2 O and added to the lyophilised solid. The mixture was agitated on a test-tube vortex for 2 s and left to stand at ambient temperature.
Fractionated 68 Ga: owing to the nature of the labelling media used, it was not possible to prepare a lyophilised solid. After addition of 68 Ga, the labelling solution was agitated on a test-tube vortex for 2 s and left to stand at ambient temperature. After 10 min a 1 μL sample was extracted and analysed by radio-TLC.

Cold-Synthesis
The synthesis of DATA M5 -3 t Bu was successfully carried out in five-steps in an 8 % yield. A further two steps were required to afford DATATOC in a yield of 31 %.

Radiochemical evaluations
The optimised labelling of DATATOC using 68 Ga PP by four common procedures is shown in Fig. 4. The labelling volume varies depending on the PP method used, and as a result the precursor concentration is not the same in each case. Radiochemical yields (RCYs given with ± SD, n = 3) greater than 95 % were achieved with each type of PP eluate within 10 min, and from a kit formulation. Radio-HPLC confirmed that the radiolabelled product was the same in each instance. In human serum 98.7 % of the initially present 68 Ga-DATATOC remained intact after 2 h. The lipophilicity of 68 Ga-DATATOC and 68 Ga-DOTATOC, determined using the shake-flask method, are logP = −3.22 ± 0.32 and −2.93 ± 0.37 respectively. The respective retention times from radio-HPLC are 10.4 and 10.3 min.

Discussion
The synthesis of DATA M5 -3 t Bu followed the same synthetic pathway as that of DATA M , with the only modification being the type of reagent used in the first step. A similar synthetic route was reported for an AAZTA conjugate with the same linker functionality, and the product labelled with 68 Ga (Manzoni et al. 2012). In our experience, the heptadentate AAZTA ligand itself is unsuitable for chelation of 68 Ga due to the formation of multiple radiolabelled species (Baranyai et al. 2009). The spacer and linker moieties for conjugation are incorporated in the first step during formation of the 7-membered diazepine ring. A five-carbon spacer, attached to the quaternary carbon of the ring, possesses an acid which is orthogonally protected as an ester relative to the chelating acids, to ensure selective conjugation of TOC. The key step is the Nmethylation (step 4), necessary to prevent an internal cyclisation with an adjacent acetate, where care is required to prevent formation of a quaternary amine. A suitably protected TOC derivative was conjugated in solution, with subsequent deprotecting of the chelator and peptide occurring simultaneously.
There are four commonly used 68 Ga generators and four main PP methods (acetone-, ethanol-, NaCl-based and fractionation) (Zhernosekov et al. 2007;Mueller et al. 2012;Eppard et al. 2014;Breeman et al. 2005). Each generator and PP method has particular advantages and the combination used varies from group to group. This variety adds a layer of complication because the optimum labelling conditions and labelling efficiency of a given precursor can vary depending on the combination applied (Seemann et al. 2015a). A precursor which can be labelled reliably using any combination of the available technology is desirable, and may facilitate easier translation of labelling between institutions. The labelling experiments have been performed using an Obninsk (EZAG) generator eluted with 0.1 M HCl. The setup is intended to serve as an example to demonstrate the versatility of labelling procedures which can be applied to DATA-TOC. Further validation of any kit-type labelling protocol requires implementation with a market authorised 68 Ga-generator. Radiolabelling of DATATOC was evaluated at ambient temperature using 68 Ga PP by each of the four methods, optimised in terms of the labelling pH. Remarkably, RCYs > 95 % were achieved in only 1 min with fractionated and acetone-PP 68 Ga, and > 98 % in 10 min regardless of the PP method applied. The precursor concentration increases along the series NaCl-, ethanol-, acetone-and fractionation PP, and is most likely the reason for the increasing rate of labelling along the same series. According to European Pharmacopeia governing 68 Ga-DOTA-TOC a radiochemical purity of at least 91 % is required (measured by HPLC and TLC) prior to in vivo administration, which may require the inclusion of a postlabelling purification (Virgolini et al. 2010). Based on the results gathered such a procedure to improve the RCY is redundant for the preparation of 68 Ga-DATATOC. This is analogous to the very successful 99m Tc-kits and not only simplifies the process but also saves timea valuable commodity considering the short half-life of 68 Ga compared to 99m Tc and 18 F. The influence of lower precursor concentrations has not been tested, but based on the radiolabelling profile it is apparent that there is scope for reduction. Nevertheless, the concentration used compares well with other 68 Ga-labelled peptide preparations (Notni et al. 2011;Berry et al. 2011;Lin et al. 2013;Eisenwiener et al. 2002;Dumont et al. 2011;Fani et al. 2011;Ugur et al. 2002;Virgolini et al. 2010;Wester et al. 1997). Based on the results it is evident that the favourable properties of the original DATA M chelator have not been negatively affected to any significant extent by conjugation to TOC. Surprisingly, with NaCl-and ethanol-PP 68 Ga, DATATOC shows superior labelling kinetics to the unfunctionalised chelator -DATA M (Seemann et al. 2015b).
There is no evidence for radiolysis of 68 Ga-DATATOC on radio-HPLC following labelling. In human serum there was no appreciable release of 68 Ga over 2 h, indicating a high metabolic and kinetic stability (98.7 %).
The relative lipophilicities 68 Ga-DATATOC and 68 Ga-DOTATOC determined by radio-HPLC and shake-flask methods are consistent and show that the former is marginally more lipophilic. Publications involving 68 Ga-labelled TOC with different chelators have highlighted the importance of the chelator in terms of in vivo performance, suggesting that the BFC could be tailored to the TV used (Lin et al. 2013;Eisenwiener et al. 2002;Dumont et al. 2011;Fani et al. 2011;Ugur et al. 2002;Wester et al. 1997). Previous work has shown that is possible to modify the lipophilicity of DATA chelators without disturbing the radiolabelling characteristics, offering the potential to look at this relationship more closely (Seemann et al. 2015b).
The feasibility of a kit-type formulation has been assessed using fractionated and acetone-PP 68 Ga and produced excellent results. Virtually quantitative yields were obtained in less than 10 min (shorter intervals not analysed) from a lyophilised solid formulation with acetone-PP 68 Ga. A similar result was achieved with fractionated 68 Ga (97 % RCY). However, it was not possible to lyophilise the precursor formulation due to the tendency of ammonia acetate to sublime under the reduced pressure required to lyophilise the labelling media. This issue can be avoided if the ammonium acetate required for labelling is incorporated into the water used to dilute activity prior to labelling rather than as part of the lyophilised solid.
To the best of our knowledge, this is the first example of a 68 Ga-radiopharmaceutical that can be prepared from a lyophilised solid at ambient temperature in less than 10 min. There are examples of 68 Ga kits requiring elevated temperatures, but we are only aware of a single ambient temperature kit where labelling of a NOTA-peptide conjugate occurred from a pre-dissolved formulation (Mukherjee et al. 2014;Velikyan et al. 2008). Beyond the development of kit-type formulations, these favourable characteristics can also be exploited through the development of previously inaccessible temperature sensitive biomolecules.
It could be argued that the PP procedures currently available do not compliment the advantages of kit-type labelling because they are manual procedures with a sequence of steps. At this stage this part of the manufacturing process then does not correspond to a kit-type preparation. However, the PP protocols are still simple and routine such that there remains substantial benefits to a very simple, fast and reliable labelling method. Future improvements to the generator and elution procedures may lend themselves better to kit-type labelling, but in the meantime there are sophisticated modules capable of postprocessing the eluate with minimal input from the user which would benefit from kit-type labelling.

Conclusion
The first DATA bifunctional chelator has been synthesised and conjugated to TOC in a short seven-step synthesis using affordable starting materials. DATATOC displays remarkable radiolabelling characteristics with > 95 % RCYs possible within 1 min at ambient temperature (pH 4.9, 13 nmol) providing a radiotracer with high human serum stability. 68 Ga-DATATOC can be efficiently prepared using 68 Ga post-processed by the full range of commonly used methods. The labelling protocols are facile, reliable, robust and do not require non-standard equipment and reagents. Initial efforts towards the development of a kit-type formulation analogous to 99m Tc have been successful, and are now being shared with other research groups to assess performance in different settings. In vivo and in vitro studies comparing 68 Ga-labelled DATATOC, DOTATOC and NODAGATOC are underway. 68 Ga-DATA conjugated radiopharmaceuticals may meet the five main definitions of kit-like preparations: A radiopharmaceutical which can be prepared in a (i) sufficient radiochemical yield (ii) from a lyophilised solid (iii) at room temperature (iv) within a short time (v) that does not require post-labelling purification to meet pharmacopeia standards. The radiochemical performance of DATATOC highlights the potential of DATA-conjugates to carry 68 Ga-PET into widespread application through the availability of a true 'kit-type' formulation, a facile and cost effective synthesis as well as the preparation of previously inaccessible radiotracers.

Ethical approval
This article does not contain any studies with human participants or animals performed by any of the authors.