Tumor-Selective Radiopharmaceutical Targeting via Receptor-Mediated Endocytosis of Gallium-67-Deferoxamine-Folate

The receptor-mediated endocytosis uptake pathway for the vitamin folate was investigated as a target for tumor-selective radiopharmaceutical delivery. The molecular target for this delivery mechanism is a membrane-associated folate binding protein (FBP) that is overexpressed by a variety of malignant cell lines. Methods: The ability of a ⁶⁷Ga-labeled deferoxamine-folate conjugate (⁶⁷Ga-DF-folate) to target tumor cells in vivo was examined using an athymic mouse tumor model. Subcutaneous inoculation of ∼4 × 10⁶ folate-receptor-positive KB (human nasopharyngeal carcinoma) cells into athymic mice yielded ∼0.20 g tumors in 15 days, at which time either ⁶⁷Ga-DF-folate, ⁶⁷Ga-deferoxamine (⁶⁷Ga-DF) or ⁶⁷Ga-citrate was administered by intravenous injection. Results: The ⁶⁷Ga-DF-folate conjugate showed marked tumor-specific deposition in vivo, with 1.0 ± 0.3% of the injected dose (%ID) in tumor at 4 hr postinjection (equating to 5.2 ± 1.5 %ID/g tumor; n = 3 mice). Corresponding tumor-to-background ratios at 4 hr postinjection were: tumor/blood = 409 ± 195; tumor/muscle = 124 ± 47; tumor/liver = 11 ± 3; and tumor/kidney = 2.6 ± 0.9. Tumor uptake of ⁶⁷Ga-DF-folate conjugate was effectively blocked by co-injection of 2.4 ± 1.0 mg free folate. In control experiments, ⁶⁷Ga-citrate exhibited tumor uptake of 2.2 ± 0.4% of the injected dose (10.9 ± 0.2 %ID/g tumor), but very poor target-to-background contrast (tumor/blood = 0.84 ± 0.19; tumor/muscle = 5.4 ± 0.7; tumor/liver = 2.3 ± 0.2; and tumor/kidney = 2.4 ± 0.3). Unconjugated ⁶⁷Ga-deferoxamine showed no tumor affinity. Conclusion: Receptor-mediated endocytosis of radiolabeled folate-conjugates may offer a suitable mechanism for selectively delivering radiopharmaceuticals to tumors for diagnostic imaging and/or radiation therapy.