Metallofullerene-based nanoplatform for brain tumor brachytherapy and longitudinal imaging in a murine orthotopic xenograft model

Purpose: To demonstrate in an orthotopic xenograft brain tumor model that a functionalized metallofullerene (f-Gd₃N@C₈₀) can enable longitudinal tumor imaging and, when radio-labeled with lutetium 177 ( ¹⁷⁷Lu) and tetraazacyclododecane tetraacetic acid (DOTA) ( ¹⁷⁷Lu-DOTA-f-Gd₃N@C₈₀), provide an anchor to deliver effective brachytherapy. Materials and Methods: All experiments involving the use of mice were carried out in accordance with protocols approved by the institutional animal care and use committee. Human glioblastoma U87MG cells were implanted by using stereotactic procedures into the brains of 37 female athymic nude-Foxn1nu mice and allowed to develop into a tumor for 8 days. T1- and T2-weighted magnetic resonance (MR) imaging was performed in five mice. Biodistribution studies were performed in 12 mice at four time points over 7 days to evaluate gadolinium content. Survival studies involved 20 mice that received infusion of a nanoplatform by means of convection-enhanced delivery (CED) 8 days after tumor implantation. Mice in survival studies were divided into two groups: one comprised untreated mice that received f-Gd ₃N@C₈₀ alone and the other comprised mice treated with brachytherapy that received 1.11 MBq of ¹⁷⁷Lu-DOTA-f-Gd ₃N@C₈₀. Survival data were evaluated by using Kaplan-Meier statistical methods. Results: MR imaging showed extended tumor retention (25.6% ± 1.2 of the infused dose at 52 days, confirmed with biodistribution studies) of the f-Gd₃N@C₈₀ nanoplatform, which enabled longitudinal imaging. Successful coupling of ¹⁷⁷Lu to the f-Gd ₃N@C₈₀ surface was achieved by using a bifunctional macrocyclic chelator. The extended tumor retention allowed for effective brachytherapy, as indicated by extended survival time (>2.5 times that of the untreated group) and histologic signs of radiation-induced tumor damage. Conclusion: The authors have developed a multimodal nanoplatform and have demonstrated longitudinal tumor imaging, prolonged intratumoral probe retention, biodistribution, and extended survival in an orthotopic xenograft brain tumor model.