Effects of antifolate drugs on the cellular uptake of radiofolates in vitro and in vivo.
Targeting the folate receptor (alpha-FR) with radiolabeled folates for the noninvasive diagnosis and therapy of alpha-FR-overexpressing neoplastic tissue is of great interest. However, the tumor uptake of folate-based radiotracers was shown to be low compared with the high renal retention of radioactivity attributable to alpha-FR expression in the proximal tubule cells. In order to increase the tumor uptake of radiofolates, we wanted to stimulate alpha-FR expression or transport through coapplication of the antifolates methotrexate (MTX), raltitrexed (RTX), and pemetrexed (PMX).


METHODS
(99m)Tc-picolylamine monoacetic acid folate ((99m)Tc-PAMA-folate) was used for these studies. The in vitro experiments with antifolates were performed with alpha-FR-positive KB cancer cells. In vivo experiments were performed with KB tumor-bearing athymic nude mice. In vivo images were acquired with a small-animal SPECT/CT scanner.


RESULTS
KB cells incubated with solutions (10 micro mol/L) of MTX, RTX, or PMX for 24 h displayed twice as much (99m)Tc-PAMA-folate uptake as untreated cells. In contrast, KB tumor-bearing mice that received MTX intravenously 24 h before (99m)Tc-PAMA-folate showed significantly lower uptake of the radiofolate in tumors (1.35 +/- 0.33 percentage injected dose per gram of tissue [%ID/g] [mean +/- SD]) and the alpha-FR-positive kidneys (9.35 +/- 1.73 %ID/g) than did control mice (2.33 +/- 0.36 and 18.48 +/- 0.72 %ID/g, respectively, at 4 h after injection). When the antifolate PMX and (99m)Tc-PAMA-folate were injected 1 h apart, the tumor uptake of the radiotracer was unaffected (2.21 +/- 0.34 %ID/g at 4 h after injection), whereas radioactivity in the kidneys was significantly decreased (1.14 +/- 0.18 %ID/g at 4 h after injection). In vivo SPECT/CT studies demonstrated the specific accumulation of (99m)Tc-PAMA-folate in tumors and almost a complete absence of radioactivity in the renal tissue of mice preinjected with PMX.


CONCLUSIONS
Our data suggest that the preadministration of antifolates improves tumor-to-kidney ratios of radiofolates and opens a "therapeutic window" for folates radiolabeled with particle-emitting nuclides, which could otherwise be nephrotoxic.
Müller
Cristina
C
Center for Radiopharmaceutical Science ETH-PSI-USZ, Paul Scherrer Institute, Villigen, Switzerland.
Brühlmeier
Matthias
M
Schubiger
P August
PA
Schibli
Roger
R
D000818:Animals; D045744:Cell Line, Tumor; D004305:Dose-Response Relationship, Drug; D005260:Female; D005492:Folic Acid / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D005493:Folic Acid Antagonists / Q000008:administration & dosage*; D006801:Humans; D008657:Metabolic Clearance Rate / Q000187:drug effects; D051379:Mice; D008819:Mice, Nude; D009303:Nasopharyngeal Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism*; D009928:Organ Specificity / Q000187:drug effects; D015609:Organotechnetium Compounds / Q000493:pharmacokinetics*; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D014018:Tissue Distribution / Q000187:drug effects
D005493:Folic Acid Antagonists; D015609:Organotechnetium Compounds; D019275:Radiopharmaceuticals; C517544:technetium 99m picolylamine monoacetic acid folate; D005492:Folic Acid