Towards authentically labelled bi-modal PET(SPECT)/MR-probes

Application of radiolabelled, existing MRI probes using a suitable reporter group for multimodal PET(SPECT)/MRI imaging is limited due to the required alteration of the molecular structure and thus changing their in vivo properties. Radiolabelling of existing MRI contrast agents with PET(SPECT) isotopes of paramagnetic elements offers a simple way to address this issue. Therefore, new routes to the production of SPECT/PET-radionuclides ^147,149Gd and ^52gMn were examined which can be applied for n.c.a. labelling of Gd(III) and Mn(II) MRI contrast agents. Additionally, Mn(II)-based complexes stable for in vivo application are to be synthesized.

Reaction cross sections and experimental thick target yields were measured by irradiation of ^natCr or Eu₂O₃. Integral yields were calculated from measured excitation functions. A radiochemical separation of Mn from Cr was developed based on cation-exchange chromatography [1].

Cross section data of the ^natEu(d,x) and ^natEu(p,x) reactions were measured up to 70.9 MeV and 44.8 MeV, respectively. Integral yields of up to 177.3 MBq/μAh and 81.6 MBq/μAh for ^natEu(d,x)^147,149Gd reactions and up to 43.3 MBq/μAh and 61.8 MBq/μAh for ^natEu(p,x)^147,149Gd reactions, respectively, were calculated. Those were several times higher than for α- or ³He induced reactions on highly enriched ¹⁴⁴Sm [2, 3].

With n.c.a. ⁵²Mn, also cross sections of co-produced ⁴⁸V, ^48,49,51Cr, ^52gMn were determined in the energy range of 7.6 to 45 MeV. The production rates of ^52g,mMn were measured from 8.2 to 16.9 MeV with up to 13.1 MBq/μAh which was separated from ^natCr by column chromatography.

Production data of the SPECT nuclides ^147,149Gd and the PET nuclide ^52gMn were established. Different to Mn a practical isolation procedure for Gd is still required. Current work focuses on the radiolabelling of stable complexes of manganese (II) with the goal to develop PET/MRI tracers addressing molecular targets.