Rapalink-1 Targets Glioblastoma Stem Cells and Acts Synergistically with Tumor Treating Fields to Reduce Resistance against Temozolomide

Abstract

Glioblastoma (GBM) is a lethal disease with limited clinical treatment options available. Recently, a new inhibitor targeting the prominent cancer signaling pathway mTOR was discovered (Rapalink-1), but its therapeutic potential on stem cell populations of GBM is unknown. We applied a collection of physiological relevant organoid-like stem cell models of GBM and studied the effect of RL1 exposure on various cellular features as well as on the expression of mTOR signaling targets and stem cell molecules. We also undertook combination treatments with this agent and clinical GBM treatments tumor treating fields (TTFields) and the standard-of-care drug temozolomide, TMZ. Low nanomolar (nM) RL1 treatment significantly reduced cell growth, proliferation, migration, and clonogenic potential of our stem cell models. It acted synergistically to reduce cell growth when applied in combination with TMZ and TTFields. We performed an in silico analysis from the molecular data of diverse patient samples to probe for a relationship between the expression of mTOR genes, and mesenchymal markers in different GBM cohorts. We supported the in silico results with correlative protein data retrieved from tumor specimens. Our study further validates mTOR signaling as a druggable target in GBM and supports RL1, representing a promising therapeutic target in brain oncology.

Combining mTOR Inhibitors and T Cell-Based Immunotherapies in Cancer Treatment

“…In particular, whereas orthotopic glioblastoma xenografts grow steadily under rapamycin or sapanisertib treatments, RapaLink-1 produces an initial tumor regression followed by tumor stabilization. In addition, RapaLink-1 targets glioblastoma stem cells and potentiates the anti-cancer efficacy of temozolomide, further supporting a therapeutic potential of RapaLink-1 in glioblastoma patients [52]. The anticancer benefit of RapaLink-1 is, however, not restricted to glioblastoma, as it also reduces the growth of sunitinibresistant renal cell carcinoma tumor xenograft and prostate cancer patient-derived tumor xenograft [53,54].…”

The Role of mTOR Signaling as a Therapeutic Target in Cancer

“…This dual binding may serve to increase affinity and stability, both of which likely contribute to efficacy. Rapalink-1 has shown increased and durable inhibitory action compared to the first and second-generation inhibitors in glioblastoma and follicular lymphoma in vitro and in vivo [174][175][176]. A resistance of cancer cell cultures to rapalogs and TORKIs was shown to be overcome by the use of Rapalink [143].…”

Permeabilizing Cell Membranes with Electric Fields

“…For example, the application of alternating electric fields has been shown to disrupt the membranes of subcellular compartments [ 85 ] as well as the plasma membrane of human glioblastoma cells [ 27 ]. The latter may have important clinical implications since enhanced membrane permeability of cancer cells via TTFields may render those cells more sensitive to chemotherapeutic treatments (e.g., temozolomide) when combined with TTFields [ 12 , 86 , 87 , 88 , 89 ]. Such a phenomenon may explain our recent findings of synergistic activity between TTFields and the potential chemotherapeutic agent, Withaferin A [ 26 ].…”