Preclinical evaluation of novel organometallic 99mTc-folate and 99mTc-pteroate radiotracers for folate receptor-positive tumour targeting.

The folate receptor (FR) is a valuable tumour marker, since it is frequently overexpressed on various cancer types. The purpose of the present study was to pre-clinically evaluate novel site-specifically modified (99m)Tc(CO)(3) folate (gamma-derivative 4, alpha-derivative 5) and pteroate (6) conjugates for FR targeting. The (99m)Tc(CO)(3) radiotracers 4-6 were prepared by a kit-like procedure. In vitro characterisation (K (D) and B (max)) of the radiotracers was performed with FR-positive KB cells. Tissue distribution was studied in tumour-bearing mice. SPECT/CT experiments were performed with a dedicated small animal SPECT/CT scanner. The complexes 4-6 were formed in high yields (>92%). Binding constants of the radiotracers (K (D) in nM: 4: 2.09; 5: 2.51; 6: 14.52) were similar to those of (3)H-folic acid (K (D) in nM: 7.22). In vivo the folate derivatives showed significantly better tumour uptake (4: 2.3+/-0.4% ID/g and 5: 1.2+/-0.2% ID/g, 4 h p.i.) than the pteroate derivative (6: 0.4+/-0.2% ID/g, 4 h p.i.). Clearance of all radiotracers from the blood pool and from non-targeted tissues was efficient (tumour to blood ratio approx. 200-350, 24 h p.i.). FR-positive tissue and organs were successfully visualised via small animal SPECT/CT. Radiotracers 4-6 are the first (99m)Tc(CO)(3) tracers prepared via a kit formulation which exhibit full biological activity in vitro and in vivo. Folate derivatives 4 and 5 revealed significantly better pharmacokinetic properties than the pteroate derivative 6. Promising pre-clinical SPECT results warrant further assessment of (99m)Tc(CO)(3) radiofolates for detection of FR-positive tumours.