Strategic Development of an Immunotoxin for the Treatment of Glioblastoma and Other Tumours Expressing the Calcitonin Receptor

Abstract

New strategies aimed at treatment of glioblastoma are frequently proposed to overcome poor prognosis. Recently, research has focused on glioma stem cells (GSCs), some quiescent, which drive expansion of glioblastoma and provide the complexity and heterogeneity of the tumour hierarchy. Targeting quiescent GSCs is beyond the capability of conventional drugs such as temozolomide. Here, we discuss the proposal that the calcitonin receptor (CT Receptor), expressed in 76–86% of patient biopsies, is expressed by both malignant glioma cells and GSCs. Forty-two percent (42%) of high-grade glioma (HGG; representative of GSCs) cell lines available from one source express CT Receptor protein in cell culture. The pharmacological calcitonin (CT)-response profiles of four of the HGG cell lines were reported, suggesting mutational/splicing inactivation. Alternative splicing, commonly associated with cancer cells, could result in the predominant expression of the insert-positive isoform and explain the atypical pharmacology exhibited by CT non-responders. A role for the CT Receptor as a putative tumour suppressor and/or oncoprotein is discussed. Both CT responders and non-responders were sensitive to immunotoxins based on an anti-CT Receptor antibody conjugated to ribosomal-inactivating proteins. Sensitivity was increased by several logs with the triterpene glycoside SO1861, an endosomal escape enhancer. Under these conditions, the immunotoxins were 250–300 times more potent than an equivalent antibody conjugated with monomethyl auristatin E. Further refinements for improving the penetration of solid tumours are discussed. With this knowledge, a potential strategy for effective targeting of CSCs expressing this receptor is proposed for the treatment of GBM.

Hsp70 and Calcitonin Receptor Protein in Extracellular Vesicles from Glioblastoma Multiforme: Biomarkers with Putative Roles in Carcinogenesis and Potential for Differentiating Tumor Types

“…The CTR isoforms present a different tissue distribution, as well as a different functional efficiency in the regulatory activity of downstream signal molecules. RT-PCR analysis demonstrated that CTR b represents the isoform that is mainly localized in the central nervous system and is primarily present in membranous structures close to the nucleus, while CTR a shows a broader distribution in vivo [ 34 , 111 , 112 , 113 ]. Importantly, the presence of the additional sequence in CTR b leads to a decrease in or loss of functions related to the intracellular calcium mobilization and the binding of downstream molecules, thus being less efficient than CTR a [ 34 , 112 , 113 , 114 ].…”

“…Information available in the Human Protein Atlas (https://www.proteinatlas.org/CALCR/brain, accessed on 1 September 2023) and immunohistochemistry analyses indicate that the CTR expression occurs only in the hypothalamus, limbic system, and circumventricular organs in the brain stem in physiological conditions [40,109], but it is not expressed in the frontal and temporal lobe sites of GBM at the onset [41]. Furthermore, high levels of CALCRL mRNA were found in human glioblastoma cancer stem-like cells [110,111]. The CACLR gene undergoes alternative splicing that leads to an upregulation of the positive insert isoform (CTR b ) with an unchanged total CALCR mRNA in GBM tissue; nonetheless, further studies are necessary to confirm this result [34].…”