⁴⁴Sc: An Attractive Isotope for Peptide-Based PET Imaging

Abstract

The overexpression of integrin αvβ3 has been linked to tumor aggressiveness and metastasis in several cancer types. Because of its high affinity, peptides containing the arginine–glycine–aspartic acid (RGD) motif have been proven valuable vectors for noninvasive imaging of integrin αvβ3 expression and for targeted radionuclide therapy. In this study, we aim to develop a 44Sc-labeled RGD-based peptide for in vivo positron emission tomography (PET) imaging of integrin αvβ3 expression in a preclinical cancer model. High quality 44Sc (t1/2, 3.97 h; β+ branching ratio, 94.3%) was produced inexpensively in a cyclotron, via proton irradiation of natural Ca metal targets, and separated by extraction chromatography. A dimeric cyclic-RGD peptide, (cRGD)2, was conjugated to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and radiolabeled with 44Sc in high yield (>90%) and specific activity (7.4 MBq/nmol). Serial PET imaging of mice bearing U87MG tumor xenografts showed elevated 44Sc-DOTA-(cRGD)2 uptake in the tumor tissue of 3.93 ± 1.19, 3.07 ± 1.17, and 3.00 ± 1.25 %ID/g at 0.5, 2, and 4 h postinjection, respectively (n = 3), which were validated by ex vivo biodistribution experiments. The integrin αvβ3 specificity of the tracer was corroborated, both in vitro and in vivo, by competitive cell binding and receptor blocking assays. These results parallel previously reported studies showing similar tumor targeting and pharmacokinetic profiles for dimeric cRGD peptides labeled with 64Cu or 68Ga. Our findings, together with the advantageous radionuclidic properties of 44Sc, capitalize on the relevance of this isotope as an attractive alternative isotope to more established radiometals for small molecule-based PET imaging, and as imaging surrogate of 47Sc in theranostic applications.

Separation of cyclotron-produced 44Sc from a natural calcium target using a dipentyl pentylphosphonate functionalized extraction resin

“…The percentage impurities at EoB and 9 h, the timeframe of maximum uptake for many radiolabeled peptides (Hernandez et al, 2014), are presented in Table 4.…”

“…Plotting the percentage of chelated 44 Sc against mass of chelator, a sigmoid curve is obtained from which the reactivity or effective specific activity is calculated by dividing the activity in each vial over the amount of mass with which 50% of the radioactive scandium is chelated, and then multiplying this value times two. When the solution is loaded onto the UTEVA column at a concentration of ~10.5 M, the reactivity was 18.1 ± 6.7 GBq/μmol, which was enough for radiolabeling the DOTA conjugated cyclic arginine-glycine-aspartate (RGD) dimer E[c(RGDyK)] 2 (Hernandez et al, 2014) as well as the DTPA derivative cyclohexyldiethylenetriaminepentaacetic acid ligand (CHX-A″-DTPA) conjugated to an antibody fragment at a nmol scale with > 90% yield. The rest of the results are presented in Table 7.…”

“…In spite of this, only a handful of small molecules have been radiolabeled with 44 Sc, and even less tested in a preclinical setting (Hernandez et al, 2014; Koumarianou et al, 2012; Koumarianou et al, 2011; Muller et al, 2013; Pruszynski et al, 2012). This is a result of the non-optimal current production and separation methods, which have limited the broad availability of this isotope.…”

Cyclotron production of 44Sc: From bench to bedside

“…In a number of preclinical in vivo studies 44 Sc was successfully employed in combination with DOTA-derivatized biomolecules, including dimeric cyclic RGD (arginine-glycineaspartic acid) peptides [10], bombesin analogs [11], puromycin [12], folic acid conjugates [8] and antibody fragments [13]. The 44 Sc activity which was necessary for the performance of these in vivo studies was very low and, hence, a production yield of <350 MBq 44 Sc always sufficient.…”

Evaluation of two novel 64Cu-labeled RGD peptide radiotracers for enhanced PET imaging of tumor integrin αvβ3

“…Following our previously reported method, the integrin α v β 3 specificity and binding affinity of NOTA-c(RGDfK), NOTA-(PEG) 2 -c(RGDfK), and NOTA-PEG 4 -SAA 4 -c(RGDfK) were evaluated via a competitive binding assay using 125 I-echistatin (PerkinElmer, Waltham, MA) as the integrin-specific radio-ligand [15]. Briefly, 1 × 10 5 U87MG cells where seeded into 96-well filter plates (EMD Millipore Corp., Billerica, MA) and incubated with 125 I-Echistatin (~10,000 cpm) for 2 h at room temperature in the presence of increasing concentration of the RGD-based peptides.…”