(391a) Targeting Translocator Protein 18kDa (TSPO) Improves Mitochondrial and Tumor-Specific Targeting of PAMAM Dendrimers in Glioblastoma

Authors: 
Liaw, K., Johns Hopkins University School of Medicine
Sharma, A., Johns Hopkins University School of Medicine
Kannan, S., Johns Hopkins University School of Medicine
Kannan, R., Johns Hopkins University School of Medicine
Introduction:

Translocator protein 18kDa (TSPO) has been identified as both a biomarker and potential therapeutic target in brain cancers, which are among the most severe types of cancers due to barriers associated with delivering therapeutics to the brain in addition to traditional solid tumor delivery challenges. Due to its location on the outer mitochondrial membrane, TSPO plays a central role in cell proliferation, survival, and immune responses and has been found to be highly overexpressed in both glioma cells and tumor-associated macrophages (TAMs). As a result, TSPO ligands have been explored as mitochondrial- and tumor-specific carriers. However, many TSPO ligands such as PK11195 suffer from poor penetration of the blood brain barrier (BBB). A nano-platform that carries therapies through the BBB, into the brain tumor, and leverages TSPO overexpression to effectively target mitochondria in glioma cells and TAMs for targeted drug delivery provides an optimal clinical strategy.

Methods:

Our group has extensively characterized hydroxyl-terminated polyamidoamine (PAMAM) dendrimers as promising nano-carriers for targeted drug delivery in many in vivo models of neurodegenerative disorders. Here we present dendrimers surface modified with a PK11195 analog DPA (D-DPA) to target TSPO via non-cleavable ether linkers. In vitro experiments were carried out in murine microglia to assess mitochondrial targeting via fluorescence imaging and immune-modulation via mRNA expression of inflammatory markers. In vivo mitochondrial and tumor targeting was assessed in a mouse model of glioblastoma (GBM). C57BL/6 mice were inoculated with GL261 murine GBM cells by intracranial injection, systemically administered with unmodified dendrimer or D-DPA, and analyzed via confocal microscopy.

Results:

We first demonstrate that TSPO binding affinity of PK11195 is maintained at nanomolar IC50 when conjugated to PAMAM dendrimers. In vitro fluorescence analyses show that D-DPA significantly improves mitochondrial targeting compared to unmodified dendrimer. D-DPA also suppressed pro-tumor cytokines in activated microglia while inducing the expression of the tumor-killing immune response. Upon systemic administration in GBM-bearing mice, D-DPA crossed the BBB and penetrated throughout the solid GBM tumor. D-DPA exhibited co-localization with mitochondria in TAMs. Unlike with unmodified dendrimer, D-DPA also exhibited significant localization with tumor cells directly.

Conclusions: Here we present a novel TSPO-targeting dendrimer construct for enhanced targeting of brain tumors with systemic therapies. By leveraging the overexpression of TSPO on the mitochondria in TAMs and tumor cell surfaces, we are able to achieve dual functionality on the dendrimers to both stimulate the anti-tumor immune response and deliver anti-cancer therapies directly to cancer cells.