⁶⁴Cu-labeled folate-conjugated shell cross-linked nanoparticles for tumor imaging and radiotherapy: Synthesis, radiolabeling, and biologic evaluation

Long-circulating nanoparticles functionalized with ligands for receptors overexpressed by tumor cells have promising applications for active and passive tumor targeting. The purpose of this study was to evaluate ⁶⁴Cu- radiolabeled folate-conjugated shell cross linked nanoparticles (SCKs) as candidate agents to shuttle radionuclides and drugs into tumors overexpressing the folate receptor (FR). Methods: SCKs were obtained by cross-linking the shell of micelles obtained from amphiphilic diblock copolymers. SCKs were then functionalized with folate, fluorescein thiosemicarbazide (FTSC), and 1,4,8,11-tetraazacyclotetradecane-N,N′,N″,N‴-tetraacetic acid (TETA). The specific interaction of SCK-folate with the FR was investigated on KB cells. The biodistributions of ⁶⁴Cu-TETA-SCK and ⁶⁴Cu-TETA-SCK-folate were evaluated in athymic mice bearing small-size KB cell xenografts (10-100 mg), whereas the intratumor distributions were investigated by autoradiography in 0.3 to 0.6-g KB cell xenogratts. Results: A global solution-state functionalization strategy has been introduced for attaching optimum numbers of targeting and imaging agents onto the SCKs for increasing the efficiency of interaction with cell-surface receptors. Epifluorescence microscopy confirmed the specific interaction of FTSC-SCK-folate with the FR in vitro. ⁶⁴Cu labeling of TETA-SCKs led to the radiolabeled compounds with 15%-20% yield and >95% radiochemical purity. The biodistribution results demonstrated high accumulation of ⁶⁴Cu- labeled SCKs in organs of the reticuloendothelial system (RES) (56.0 ± 7.1%ID/g and 45.7 ± 3.5 %ID/g [percentage injected dose per gram] in liver at 10 min after injection for folated and nonfolated SCKs, respectively) and a prolonged blood circulation. No increase of SCK tumor uptake deriving from folate conjugation was observed (5.9 ± 2.8 %ID/g and 6.0 ± 1.9%ID/g at 4 h after injection for folated and nonfolated SCKs, respectively). However, tumor accumulation was higher in small-size tumors, where competitive block of SCK-folate uptake with excess folate was observed. Autoradiography results confirmed the extravasation of radiolabeled SCKs in vascularized areas of the tumor, whereas no diffusion was observed in necrotic regions. Conclusion: Despite high RES uptake, the evaluated ⁶⁴Cu-labeled SCKs exhibited long circulation in blood and were able to passively accumulate in tumors. Furthermore, SCK-folate uptake was competitively blocked by excess folate in small-size solid tumors, suggesting interaction with the FR. For these reasons, functionalized SCKs are promising drug-delivery agents for imaging and therapy of early stage solid tumors.