Scandium-44

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 59.1 a) from a ⁴⁴Ti/⁴⁴Sc generator or produced by the nuclear reaction ⁴⁴Ca(p,n)⁴⁴Sc in small cyclotrons. This isotope is of potential interest for clinical PET imaging.

Applications

Titanium-44 generation

⁴⁴Ti with its long half-life provides a cyclotron-independent source of ⁴⁴Sc for decades. It is obtained by the nuclear reaction ⁴⁵Sc(p,2n)⁴⁴Ti and transforms, through electron capture, into excited states of ⁴⁴Sc and emits gamma rays 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 about 128,000, which can be treated as virtually infinite. Consequently, half the saturation activity is generated every four hours (⁴⁴Sc half-life), and identical ⁴⁴Sc batch activities very close to saturation may be eluted each day. One can attain >97% elution efficacy for ⁴⁴Sc and very low breakthrough of <5*10^{−5} % of ⁴⁴Ti.

⁴⁴Ti is adsorbed onto a 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.

Scandium-44 as a PET tracer

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. 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.

See also

• Isotopes of scandium

References

• 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