Administration of certain
antifolates before radiofolate application has previously proven to have a positive effect on undesired kidney uptake of radiofolates in mice bearing human
tumor xenografts. The aims of this study were to (i) test the effects of the
antifolates,
pemetrexed and
CB3717, on tissue distribution of the clinically investigated radiofolate, (99m)Tc-EC20, and (ii) to determine if
pemetrexed's kidney-selective blocking effect also functions in mice bearing syngeneic
tumors. Relative binding affinities of
pemetrexed and
CB3717 were determined in
folate receptor (FR)-positive KB cells at 0 and 37 degrees C using (3)H-folic
acid. In vivo studies were performed in nude mice with KB
tumor xenografts (A) and in Balb/c mice bearing FR-positive M109
tumor grafts (B). (99m)Tc-EC20 was prepared via a kit formulation. The
antifolates pemetrexed and
CB3717 (20 mumol/kg
body weight) were administered intravenously 1 h before injection of (99m)Tc-EC20 (67 nmol/kg
body weight). Similar to previously published data we found that FR-binding affinities of
pemetrexed and
CB3717 at 0 degrees C were in the same range as that of
folic acid. Interestingly, experiments performed at 37 degrees C showed that
pemetrexed has a nearly approximately 700-fold lower FR-affinity than
CB3717. Tissue distribution of (99m)Tc-EC20 was largely comparable in both animal models (A and B). Radiofolate accumulation was found in FR-positive
tumors (A, 8.92 +/- 2.14% ID/g; B, 15.02 +/- 0.95% ID/g) and FR-positive kidneys (A, 59.10 +/- 8.03% ID/g; B, 69.44 +/- 4.66% ID/g, 4 h p.i.). Preinjection of
pemetrexed resulted in a significant decrease of (99m)Tc-EC20 uptake in kidney (A, 18.80 +/- 2.73% ID/g; B, 15.27 +/- 2.64% ID/g; 4 h p.i), whereas uptake in the
tumors was unaltered. However, administration of the
CB3717 resulted in a reduction of (99m)Tc-EC20 uptake in both the kidney and
tumor (<1% ID/g, 4 h p.i.). We have thus demonstrated that
pemetrexed effectively reduces kidney uptake of radiofolates not only in xenografted mice but also in a syngeneic
tumor mouse model, thereby indicating that the kidney-specific blocking effect is not based on differences between human and murine FRs that are expressed in xenografts and kidneys, respectively. This effect was not observed with the
antifolate,
CB3717, which targets the FR selectively in contrast to
pemetrexed that is predominantly transported into cells through carrier systems.