Auger electron therapy of glioblastoma using [125I]5-iodo-2'-deoxyuridine and concomitant chemotherapy - Evaluation of a potential treatment strategy

Abstract

Introduction: Treatment of glioblastomas (GBM) using the Auger electron emitting compound [¹²⁵I]5-Iodo-2'-deoxyuridine ([¹²⁵I]I-UdR), combined with the thymidylate synthase inhibitor methotrexate (MTX) and concomitant chemotherapy with temozolomide (TMZ) has recently shown very promising therapeutic effects in vitro and in vivo in animals. The aim of the current study was to investigate if the therapeutic effects of this multimodal treatment strategy could be further increased by the thymidylate synthase inhibitor, 5-fluoro-2'-deoxyuridine (F-UdR), in comparison to MTX, and if the co-treatment should be given in a neoadjuvant or adjuvant setting.

Methods: A patient-derived GBM cancer stem cell (CSC)-enriched cell line, grown as neurospheres, was employed to evaluate DNA-incorporation of [¹²⁵I]I-UdR, determined by a DNA precipitation assay, using either pre-treatment or co-treatment with MTX or F-UdR. The therapeutic effects in the CSC-enriched cell line after exposure to various combinations of MTX, F-UdR, TMZ and [¹²⁵I]I-UdR were also investigated by a CellTiter-Blue assay.

Results: The highest general increase in [¹²⁵I]I-UdR incorporation was observed with F-UdR co-treatment, which resulted in approx. 2.5-fold increase in the DNA-associated activity. Also the cell viability was significantly decreased when F-UdR was combined with [¹²⁵I]I-UdR compared to [¹²⁵I]I-UdR alone at all activity concentrations tested. MTX was redundant when combined with 400 and 500 Bq/ml [¹²⁵I]I-UdR. TMZ was effective in combination with either [¹²⁵I]I-UdR alone or with both thymidylate synthase inhibitors combined with 50-100 Bq/ml [¹²⁵I]I-UdR.

Conclusions: Overall, our study revealed a higher incorporation and therapeutic effect of [¹²⁵I]I-UdR when GBM cells were co-treated with F-UdR compared to MTX. The therapeutic effects were further increased when TMZ was combined with [¹²⁵I]I-UdR in combination with the thymidylate synthase inhibitors.

Advances in knowledge and implications for patient care: Auger electron therapy in combination with thymidylate synthase inhibition and concomitant chemotherapy has the potential to become a future therapeutic treatment option for patients with glioblastoma.

Keywords: Auger electrons; Chemotherapy; Glioblastoma; Radionuclide therapy; [(125)I]I-UdR.