Scandium-44

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Scandium-44 (⁴⁴Sc) is a radioactive isotope of scandium that decays by positron emission to stable ⁴⁴Ca with a half-life of 4.042 hours.

⁴⁴Sc can be obtained as a daughter radionuclide of long-lived ⁴⁴Ti (t_1/2 60.4 a) from ⁴⁴Ti /⁴⁴Sc generator or can be produced by nuclear reaction ⁴⁴Ca ( p, n)⁴⁴Sc in small cyclotrons. This isotope is of potential interest for clinical PET imaging.

⁴⁴Ti with its long half-life of ca. 60 years provides a cyclotron-independent source of ⁴⁴Sc for several decades. ⁴⁴Ti is obtained in the nuclear reaction ⁴⁵Sc(p,2n)⁴⁴Ti and it transforms directly into the ground state of ⁴⁴Sc via electron capture, emitting low energetic photons at 67.9 keV and 78.3 keV.

The ⁴⁴Ti /⁴⁴Sc generator represents a secular equilibrium system with a half-life ratio between parent and daughter of ca. 130 000. Consequently, 50% of saturation activity is generated every 3.97 hours, and identical ⁴⁴Sc batch activities very close to saturation may be eluted each day. >97% elution efficacy for ⁴⁴Sc and very low breakthrough of < 5*10⁻⁵ % of ⁴⁴Ti.

⁴⁴Ti is adsorbed onto column filled with anion-exchange resin. ⁴⁴Sc is eluted with 20 mL of 0.005M H₂C₂O₄/0.07M HCl solution. The eluate is directly post-processed on miniaturized column filled with cation-exchange resin where ⁴⁴Sc is quantitatively adsorbed online and successively eluted using 2–3 mL of 0.25 M ammonium acetate buffer (pH 4.0). This ⁴⁴Sc solution of small volume and free of competing oxalates can be used for further labelling studies. small cyclotrons.

⁴⁴Sc complexates with DOTA, a well-established bifunctional chelators conjugated to peptides or other molecular targeting vectors, its half-life of 4 h, a high positron branching, stable and non-toxic decay product and being generator-produced makes it an appropriate candidate in PET/CT diagnosis. ⁴⁴Sc has an almost 4-times longer half-life and higher β⁺ branching than commonly used ⁶⁸Ga, therefore it can be used for more accurate planning and dosimetric calculations being able to cover imaging periods of more than one day.^[3] A specific field might be application of diagnostic ⁴⁴Sc tracers for matching therapeutic analog compounds labelled with another radioisotope such as ⁹⁰Y, ¹⁷⁷Lu or with the β⁻ emitter ⁴⁷Sc.

Recent studies with ⁴⁴Sc-DOTA-conjugated tumor targeting vectors such as octreotides, showed in vitro and in vivo stability and revealed pharmacological parameters adequate to long-term (up to one day) molecular imaging. Initial human studies with ⁴⁴Sc-DOTATOC PET/CT imaging of somatostatin receptor positive liver metastases in a patient at early time-points (40 min p.i.) showed comparable results to ⁶⁸Ga-DOTATATE(90 min. p.i.) in the same patient.

• Isotopes of scandium

• Scandium-44: Benefits of a Long-Lived PET Radionuclide Available from the 44Ti/44Sc Generator System. F.Rösch
• Radiolabeling of DOTATOC with the long-lived positron emitter 44Sc. M.Pruszynski, A.Majkowska-Pilip, N.Loktionova, E.Eppard, F.Roesch