The HSP90 inhibitor Onalespib exerts synergistic anti-cancer effects when combined with radiotherapy: an in vitro and in vivo approach

Abstract

Oncogenic client-proteins of the chaperone Heat shock protein 90 (HSP90) insure unlimited tumor growth and are involved in resistance to chemo-and radiotherapy. The HSP90 inhibitor Onalespib initiates the degradation of oncoproteins, and might also act as a radiosensitizer. The aim of this study was therefore to evaluate the efficacy of Onalespib in combination with external beam radiotherapy in an in vitro and in vivo approach. Onalespib downregulated client proteins, lead to increased apoptosis and caused DNA-double-strands. Monotherapy and combination with radiotherapy reduced colony formation, proliferation and migration assessed in radiosensitive HCT116 and radioresistant A431 cells. In vivo, a minimal treatment regimen for 3 consecutive days of Onalespib (3 × 10 mg/kg) doubled survival, whereas Onalespib with radiotherapy (3 × 2 Gy) caused a substantial delay in tumor growth and prolonged the survival by a factor of 3 compared to the HCT116 xenografted control group. Our results demonstrate that Onalespib exerts synergistic anti-cancer effects when combined with radiotherapy, most prominent in the radiosensitive cell models. We speculate that the depletion and downregulation of client proteins involved in signalling, migration and DNA repair mechanisms is the cause. Thus, individually, or in combination with radiotherapy Onalespib inhibits tumor growth and has the potential to improve radiotherapy outcomes, prolonging the overall survival of cancer patients.

Priming of Anti-tumor Immune Mechanisms by Radiotherapy Is Augmented by Inhibition of Heat Shock Protein 90

“…Mechanistically, our results together with prior reports reveal that HSP90i-mediated apoptosis induction upon ionizing irradiation derives from targeted disintegration of crucial DNA damage response regulators and is executed via the intrinsic apoptosis pathway as indicated by its clear dependence on functional Bax (37,39). The observed accelerated transit into secondary necrosis interestingly was dependent on hyperactive Kras.…”

“…Furthermore, HSP90 in tumor cells is largely organized in multi-chaperone complexes which exhibit higher affinity for HSP90i than "normal" HSP90 complexes in non-malignant cells, thus ensuring a relevant degree of tumor specificity for HSP90i-based targeted radiosensitization (68). In this context, we and others have previously shown the synergistic therapeutic efficacy of HSP90 inhibition in combination with radiotherapy in different models of CRC in vitro and in vivo (34,37,39).…”

“…We have previously shown that the second generation HSP90i NW457 exhibits reduced hepatotoxicity and potently sensitizes CRC cells toward ionizing irradiation in vitro and in vivo by interfering with the DNA damage response ( 36 – 38 ). The underlying mechanisms of radiation-induced cell death in the presence of HSP90i are currently being dissected ( 39 ). However, the immunological potential of HSP90i-mediated radiosensitization has not yet been examined, although seminal data suggest that HSP90i may augment the stimulation of anti-tumor immune mechanisms ( 19 ).…”

Overcoming Limitations of Cisplatin Therapy by Additional Treatment With the HSP90 Inhibitor Onalespib

“…Among them, onalespib (AT13387) is a potent second-generation compound, currently undergoing phase II studies in advanced solid tumors ( 13 , 17 ). Studies have demonstrated potent radiosensitizing effects of onalespib both in vitr o and in vivo , an effect likely mediated by impairment of the DNA damage response ( 13 , 21 , 22 ). Here, combination therapy of onalespib and radiotherapy resulted in a substantial increase in DNA double breaks (DSBs) as well as delay in DNA repair measured by the DSB markers γH2AX and 53BP1 ( 22 ).…”

“…Studies have demonstrated potent radiosensitizing effects of onalespib both in vitr o and in vivo , an effect likely mediated by impairment of the DNA damage response ( 13 , 21 , 22 ). Here, combination therapy of onalespib and radiotherapy resulted in a substantial increase in DNA double breaks (DSBs) as well as delay in DNA repair measured by the DSB markers γH2AX and 53BP1 ( 22 ). These findings raise the question whether HSP90 inhibition may also enhance the cytotoxic effect of cisplatin, due to similarities between the effects of cisplatin and ionizing radiation on tumor cells.…”