44Sc-DOTA-BN[2-14]NH2 in comparison to 68Ga-DOTA-BN[2-14]NH2 in pre-clinical investigation. Is 44Sc a potential radionuclide for PET?

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Abstract

Aim

In the present study we demonstrate the in vitro and in vivo comparison of the 44Sc and 68Ga labeled DOTA-BN[2-14]NH2. 44Sc is a positron emitter with a half life of 3.92 h. Hence it could be used for PET imaging with ligands requiring longer observation time than in the case of 68Ga.

Methods

The binding affinity of natSc-DOTA-BN[2-14]NH2 and natGa-DOTA-BN[2-14]NH2 to GRP receptors was studied in competition to [125I-Tyr4]-Bombesin in the human prostate cancer cell line PC-3. A preliminary biodistribution in normal rats was performed, while first microPET images were assessed in male Copenhagen rats bearing the androgen-independent Dunning R-3327-AT-1 prostate cancer tumor.

Results

The affinity to GRP receptors in the PC-3 cell line was higher for natGa-DOTA-BN[2-14]NH2 (IC50(nM)=0.85±0.06) than that of natSc-DOTA-BN[2-14]NH2 (IC50 (nM)=6.49±0.13). The internalization rate of 68Ga labeled DOTA-BN[2-14]NH2 was slower than that of 44Sc, but their final internalization percents were comparable. 68Ga-DOTA-BN[2-14]NH2 was externalized faster than 44Sc-DOTA-BN[2-14]NH2. The biodistribution of 44Sc-DOTA-BN[2-14]NH2 and 68Ga-DOTA-BN[2-14]NH2 in normal rats revealed a higher uptake in target organs and tissues of the first one while both excreted mainly through urinary tract. In microPET images both tracers were accumulated in the tumor with similar uptake patterns.

Conclusions

Despite the differences in the receptor affinity both the 68Ga- and the 44Sc-labeled DOTA-BN[2-14]NH2 tracers showed comparable distribution and similar time constants of uptake and elimination. Moreover no differences in tumor accumulation (neither in the overall uptake nor in the dynamics) were observed from the microPet imaging. From that perspective the use of either 44Sc or 68Ga for detecting tumors with GRP receptors is equivalent.

Highlights

In vitro and in vivo evaluation of 44Sc- and 68Ga-DOTA-BN[2-14]NH2 in reference to published data. ► Higher in vitro affinity to GRP receptors (PC-3 cells) for natGa-DOTA-BN[2-14]NH2. ► Both showed similar internalization rates, however the efflux rate of the 44Sc analog was lower. ► 68Ga- and 44Sc-DOTA-BN[2-14]NH2 showed no differences in tumor accumulation. ► Hence the use of either 44Sc or 68Ga for detecting tumors with GRPR is equivalent.

Introduction

44Sc (Scandium-44) is a positron emitter (Eβ+ 1475.4 keV (94.34%) with a gamma radiation component of 1157 keV (99.9%). Due to its half life (T1/2=3.92 h), which is almost 4 times as long as the half life of 68Ga (Gallium-68) (T1/2=67.71 min), it may be considered as an interesting radionuclide for PET imaging. It can be produced by 44Ca(p, n)44Sc nuclear reaction in cyclotrons or from the decay of long lived 44Ti (T1/2=60.4 yr) (Welch and McCarthy, 2000). Although several generator systems have been described in literature (Welch and McCarthy, 2000, Rösch and (Russ) Knapp., 2003), including a 44Ti/44Sc generator, practically there were no sources of this radionuclide available so far. Recent developments by Filosofov et al. (2010) resulted in the high-performance, 5mCi44Ti/44Sc radionuclide generator, which is currently available at the Institute of Nuclear Chemistry, University of Mainz. It has been shown by Pruszynski et al. (2010) and Loktionova et al. (2009), that the eluate of this generator can be effectively used for labeling of a somatostatin analogue, DOTATOC. Additionally, there is another radionuclide of scandium - 47Sc (T1/2=3.35 d) emitting γ radiation of 159.4 keV (63.3%) and β−radiation with maximum energy 0.600 MeV (31.6%) and 0.439 MeV (68.4%), which can be utilized in radiotherapy using the same vector molecules as for 44Sc. Practical aspects of 47Sc production both in the nuclear reactor and in cyclotron have been described by Mausner et al. (1998). The same group proved also the therapeutic potential of this radionuclide (Kolsky et al., 1998, Mausner et al., 1995). The combination of 44Sc/47Sc labeled radiopharmaceuticals for diagnostic PET imaging and for therapy, could provide a unique opportunity for patient qualification, dosimetry, therapy and therapy follow up. Both radionuclides create a matched pair and their clinical application may bring additional value, particularly in combination with ligands requiring longer observation time than the one which can be reached in the case of 18F or 68Ga labeled molecules. In recent years 68Ga is experiencing its renaissance in PET imaging (Roesch and Riss, 2010). The 68Ga labeled DOTA-chelated peptides (DOTA (1,4,7,10-teraazacyclododecane-N,N′,N″,N″′-teraacetic acid) such as somatostatin analogues DOTATOC, DOTATATE and DOTANOC have been proved to be useful in the imaging of neuroendocrine tumors. DOTA can also form stable complexes with Sc (Majkowska-Pilip and Bilewicz, 2011). The chemistry of Sc+3 is similar to that of Lanthanides. Due to its small ionic radius scandium is also chemically similar to aluminum and gallium. The thermodynamic stability of Sc complexes with DOTA is similar to that of Ga (Viola-Villegas and Doyle, 2009). However, there is no data published so far on the in vitro and in vivo receptor affinity and uptake of 44Sc labeled DOTA-chelated peptides.

We have previously demonstrated that the affinity of a Bombesin (BN) analog, DOTA-BN[2-14]NH2 (DOTA-QRLGNQWAVGHLMCONH2) to the Gastrin Releasing Peptide Receptors (GRPR) in prostate cancer cell line PC3 varies, depending on the coupled radiometal (Koumarianou et al., 2009). Moreover these differences were reflected in the in vivo biodistribution of either 90Y or 177Lu labeled DOTA-BN[2-14]NH2 in mice. Based on these findings we decided to extend our study using the same model peptide to assess the influence of Sc on the GRPR affinity in comparison to Ga, both in vitro and in vivo.

Section snippets

Chemicals and quality control techniques

DOTA-BN[2-14]NH2 was synthesized by standard Fmoc solid phase synthesis on Rink Amide Resin as described previously (Koumarianou et al., 2009, Gourni et al., 2006). Briefly, starting from α-fluorenyl-methoxycarbonyl (Fmoc) the amino acids [Met, Gln, Arg, Leu, Gly, Asn, Trp, Ala, Val, Gly, His, Leu] were coupled and then the terminal DOTA-tris (t-Bu-ester) (Macrocyclics) was conjugated. The purity and identity of the peptide was confirmed by HPLC and Electron Spray Ionization–Mass Spectroscopy

68Ge/68Ga and 44Ti/44Sc generators processing

The eluted radioactivity of 68Ga ranged from 100 to 150 MBq in 0.4 mL 0.05 M HCl/acetone (2:98), pH 2, the respective eluted radioactivity for 44Sc varied from 150 to 200 MBq in 3 mL ammonium acetate 0.25 M, pH 4.

Radiolabeling of DOTA-BN[2-14]NH2 with 68Ga and 44Sc

The radiolabeling yield was higher than 80% for both 68Ga-DOTA-BN[2-14]NH2 and 44Sc-DOTA-BN[2-14]NH2. TLC and HPLC quality control were in good agreement. The HPLC analysis revealed one peak of non-bound 44Sc at 2.82 min (6.0%) while the retention time for 44Sc-DOTA-BN[2-14]NH2 was about 5.55 

Discussion

The increasing availability of new radionuclides with diagnostic and therapeutic properties offers new possibilities for individualized nuclear medicine options. This is especially relevant in case of matched pairs of radionuclides such as 44Sc and 47Sc. Grignon et al. (2007) reported that 44Sc is the most interesting radionuclide for nuclear medicine imaging using β+, γ coincidences. 44Sc PET can be used for a pre-therapeutic imaging while the same ligand labeled with 47Sc could be used for

Acknowledgements

This work was supported by COST ACTION BM0607:Targeted Radionuclide therapy (TRNT) and COST ACTION D38:Metal Based Systems for Molecular Imaging. We are thankful to Dr. R. Postina and Dr. V. Metz from the Institute of Biochemistry at the Johannes Gutenberg University of Mainz, Germany for kindly provide the facilities for the cell culturing of PC-3 cells which were used for the in vitro studies and Dr. C. Zikos of the Institute R-RP, N.C.S.R “Demokritos”, Athens, Hellas for the synthesis of

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