177Lu‑DOTATATE was recently approved for the treatment of
somatostatin receptor (SSTR)‑positive
neuroendocrine tumors (NETs). However, despite impressive response rates, complete responses are rare.
Heat shock protein 90 (HSP90) inhibitors have been suggested as suitable therapeutic agents for NETs, as well as a potential radiosensitizers. Consequently, the aim of this study was to investigate whether the HSP90‑inhibitor
onalespib could reduce NET cell growth and act as a radiosensitizer when used in combination with 177Lu‑DOTATATE. The NET cell lines BON, NCI‑H727 and NCI‑H460, were first characterized with regards to 177Lu‑DOTATATE uptake and sensitivity to
onalespib treatment in monolayer cell assays. The growth inhibitory effects of the monotherapies and combination treatments were then examined in three‑dimensional multicellular
tumor spheroids. Lastly, the molecular effects of the treatments were assessed. 177Lu‑DOTATATE uptake was observed in the BON and NCI‑H727 cells, while the NCI‑H460 cells exhibited no detectable uptake. Accordingly, 177Lu‑DOTATATE reduced the growth of BON and NCI‑H727 spheroids, while no effect was observed in the NCI‑H460 spheroids.
Onalespib reduced cell viability and spheroid growth in all three cell lines. Furthermore, the combination of
onalespib and 177Lu‑DOTATATE exerted synergistic
therapeutic effects on the BON and NCI‑H727 spheroids. Western blot analysis of BON spheroids revealed the downregulation of
epidermal growth factor receptor (EGFR) and the upregulation of γ H2A
histone family member X (γH2AX) following combined treatment with
onalespib and 177Lu‑DOTATATE. Moreover, flow cytometric analyses revealed a two‑fold increase in
caspase 3/7 activity in the combination group. In conclusion, the findings of this study demonstrate that
onalespib exerts antitumorigenic effects on NET cells and may thus be a feasible treatment option for NETs. Furthermore,
onalespib was able to synergistically potentiate 177Lu‑DOTATATE treatment in a SSTR‑specific manner. The radiosensitizing mechanisms of
onalespib involved the downregulation of EGFR expression and the induction of apoptosis. Consequently, the combination of
onalespib and 177Lu‑DOTATATE may prove to be a promising strategy with which to improve therapeutic responses in patients with NETs. Further studies investigating this strategy in vivo regarding the
therapeutic effects and potential toxicities are warranted to expand these promising findings.