['The folate receptor (FR) is frequently overexpressed in tumors and can be targeted with folate-based (radio)pharmaceuticals. However, significant accumulation of radiofolates in FR-positive kidneys represents a drawback. We have shown that preadministration of the antifolate pemetrexed (PMX) significantly improved the tumor-to-kidney ratio of radiofolates in mice. The aim of this study was to investigate the dose dependence of these effects and whether the same results could be achieved with folic acid (FA) or 5-methyl-tetrahydrofolate (5-Me-THF).', 'Biodistribution was assessed 4 h postinjection of the organometallic (99m)Tc-picolylamine monoacetic acid folate in nude mice bearing FR-positive KB tumor xenografts. PMX (50-400 microg/mouse) was injected 1 h previous to radioactivity. The effects of FA and 5-Me-THF (0.5-50 microg/mouse) were investigated likewise. Tissues and organs were collected and counted for radioactivity and the values tabulated as percentage of injected dose per gram tissue (% ID/g).', 'PMX administration reduced renal retention (<1.6% ID/g vs. control: >10% ID/g), while the tumor uptake (average 1.35%+/-0.40% ID/g vs. control: 1.79%+/-0.49% ID/g) was only slightly affected independent of the PMX dose. Replacement of PMX by FA or 5-Me-THF (50 microg/mouse) resulted in a significant renal blockade (<0.1% ID/g) but at the same time in an undesired reduction of tumor uptake (<0.2% ID/g).'] Do dose-dependent effects of ( anti ) folate preinjection on 99mTc-radiofolate uptake in tumors and kidneys?