When better still might not be good enough

Drug Conjugates for Targeting Eph Receptors in Glioblastoma

Glioblastoma (GBM) is a complex and heterogeneous tumor that warrants a comprehensive therapeutic approach for treatment. Tumor-associated antigens offer an opportunity to selectively target various components of the GBM microenvironment while sparing the normal cells within the central nervous system. In this study, we conjugated a multivalent vector protein, QUAD 3.0, that can target four receptors: EphA3, EphA2, EphB2, and also IL-13RA2, spanning virtually 100% of the GBM microenvironment, to doxorubicin derivatives. The conjugates effectively bound to all four receptors, although to varying degrees, and delivered cytotoxic loads to both established and patient-derived GBM cell lines, with IC50 values in the low nM range. The conjugates were also non-toxic to animals. We anticipate that the QUAD 3.0 Dox conjugates will be further used in preclinical models and possibly clinics in the foreseeable future.

Multireceptor targeting of glioblastoma

BACKGROUND

Treatment for glioblastoma (GBM) remains an unmet need in medicine. Novel therapies that address GBM complexity and heterogeneity in particular are warranted. To this end, we target 4 tumor-associated receptors at a time that span virtually all of the GBM microenvironment including bulk tumor cells, infiltrating tumor cells, neovasculature, and tumor-infiltrating cells with one pharmaceutical agent delivering a cytotoxic load.

METHODS

We engineered multivalent ligand-based vector proteins termed QUAD with an ability to bind to 4 of the following GBM-associated receptors: IL-13RA2, EphA2, EphA3, and EphB2. We conjugated QUAD with a modified bacterial toxin PE38QQR and tested it in vitro and in vivo.

RESULTS

The QUAD variants preserved functional characteristics of the respective ligands for the 4 receptors. The QUAD 3.0 variant conjugate was highly cytotoxic to GBM cells, but it was nontoxic in mice, and the conjugate exhibited strong antitumor effect in a dog with spontaneous GBM.

CONCLUSION

The QUAD addresses, to a large extent, the issues of intra- and intertumoral heterogeneity and, at the same time, it targets several pathophysiologically important tumor compartments in GBM through multiple receptors overexpressed in tumors allowing for what we call "molecular resection." QUAD-based targeted agents warrant further pre- and clinical development.

Receptor-Targeted Glial Brain Tumor Therapies

Among primary brain tumors, malignant gliomas are notably difficult to manage. The higher-grade tumors represent an unmet need in medicine. There have been extensive efforts to implement receptor-targeted therapeutic approaches directed against gliomas. These approaches include immunotherapies, such as vaccines, adoptive immunotherapy, and passive immunotherapy. Targeted cytotoxic radio energy and pro-drug activation have been designed specifically for brain tumors. The field of targeting through receptors progressed significantly with the discovery of an interleukin 13 receptor alpha 2 (IL-13RA2) as a tumor-associated receptor over-expressed in most patients with glioblastoma (GBM) but not in normal brain. IL-13RA2 has been exploited in novel experimental therapies with very encouraging clinical responses. Other receptors are specifically over-expressed in many patients with GBM, such as EphA2 and EphA3 receptors, among others. These findings are important in view of the heterogeneity of GBM tumors and multiple tumor compartments responsible for tumor progression and resistance to therapies. The combined targeting of multiple receptors in different tumor compartments should be a preferred way to design novel receptor-targeted therapeutic approaches in gliomas.