Searching for Synergy : Radiosensitization of 177Lu-DOTATATE

Cancers presents a major health challenge, and there is a pressing need to develop new therapeutic strategies. Surgery, chemotherapy and radiation are the most commonly used treatments for cancer today. Radiation can be given as targeted radionuclide therapy (TRT), i.e., systemic administration of a radiolabeled cancer-targeting molecule. This is especially suitable for inoperable and disseminated tumors. 177Lu-DOTATATE, a TRT directed against the somatostatin receptors (SSTRs), was recently approved for therapy of a subset of neuroendocrine tumors (NETs). Although it has prolonged the life of NET patients, complete remission is seldom achieved. Consequently, to increase the efficacy of the treatment, this thesis aimed to assess potential radiosensitizing strategies for 177Lu-DOTATATE. The two radiosensitization targets in focus were HSP90, a chaperone protein with numerous oncogenic client proteins, and p53, a central regulator of DNA damage. In papers I and II, we investigated the HSP90-inhibitor Onalespib, as a treatment for NETs, and as a potential radiosensitizer. The drugs were assessed in vitro and in vivo. We concluded that Onalespib reduced NET cell growth and acted synergistically with 177Lu-DOTATATE. Inhibition of EGFR, a HSP90 client protein, was suggested as a mediator of the observed synergy. Furthermore, the combination had a favorable toxicity profile. In paper III, we assessed the novel stapled peptide VIP116, which inhibits the p53 repressors MDM2 and MDM4, as a potentiator of 177Lu-DOTATATE in wildtype p53 neuroblastoma cells. Combination therapy exhibited growth-inhibitory effects, with resulting additive or synergistic effects. The treatment-mediated effects on p53 signaling were characterized, revealing a possible involvement of V-myc myelocytomatosis viral oncogene homolog, neuroblastoma derived (MYCN), a prognostic marker for poor survival in neuroblastoma. In paper IV, we aimed to improve targeted delivery of VIP116, with the use of lipid bilayer disks (lipodisks). VIP116 was successfully loaded onto epidermal growth factor receptor (EGFR)-targeting lipodisks, leading to specific delivery and reduction of viability of EGFR expressing tumor cells. The study provided a proof-of-concept for utilizing lipodisks as a drug delivery system for p53-stabilizing peptides. In conclusion, we have investigated, and found, suitable candidates for potentiating 177Lu-DOTATATE therapy. We have addressed the feasibility of the treatments, toxicity and targeted delivery. Moreover, the work has explored the biology of TRT. This is an area in need of more attention, as more and more radionuclide-based therapies are entering clinicals trials and reaching approval