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Updated: Dec 13, 2025
Ex Vivo Treatment Response of Primary Tumors and/or Associated Metastases for Preclinical and Clinical Development of Therapeutics
Published on: October 2, 2014
Low-Dose Hsp90 Inhibitor Selectively Radiosensitizes HNSCC and Pancreatic Xenografts
Ranjit K Mehta1, Sanjima Pal1, Koushik Kondapi1
1Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan.
Purpose:
Treatment approaches using Hsp90 inhibitors at their maximum tolerated doses (MTDs) have not produced selective tumor toxicity. Inhibition of Hsp90 activity causes degradation of client proteins including those involved in recognizing and repairing DNA lesions. We hypothesized that if DNA repair proteins were degraded by concentrations of an Hsp90 inhibitor below those required to cause nonspecific cytotoxicity, significant tumor-selective radiosensitization might be achieved.
Experimental Design:
Tandem mass tagged-mass spectrometry was performed to determine the effect of a subcytotoxic concentration of the Hsp90 inhibitor, AT13387 (onalespib), on global protein abundance. The effect of AT13387 on in vitro radiosensitization was assessed using a clonogenic assay. Pharmacokinetics profiling was performed in mice bearing xenografts. Finally, the effect of low-dose AT13387 on the radiosensitization of three tumor models was assessed.
Results:
A subcytotoxic concentration of AT13387 reduced levels of DNA repair proteins, without affecting the majority of Hsp90 clients. The pharmacokinetics study using one-third of the MTD showed 40-fold higher levels of AT13387 in tumors compared with plasma. This low dose enhanced Hsp70 expression in peripheral blood mononuclear cells (PBMCs), which is a biomarker of Hsp90 inhibition. Low dose monotherapy was ineffective, but when combined with radiotherapy, produced significant tumor growth inhibition.
Conclusions:
This study shows that a significant therapeutic ratio can be achieved by a low dose of Hsp90 inhibitor in combination with radiotherapy. Hsp90 inhibition, even at a low dose, can be monitored by measuring Hsp70 expression in PBMCs in human studies.
Insights
Low-dose Hsp90 inhibitors selectively degrade DNA repair proteins, enhancing radiotherapy. This approach achieves significant tumor growth inhibition and can be monitored via Hsp70 expression in PBMCs.
Area of Science:
- Oncology
- Molecular Biology
- Radiotherapy
Background:
- Hsp90 inhibitors at maximum tolerated doses (MTDs) lack tumor selectivity.
- Hsp90 inhibition degrades DNA repair proteins, potentially sensitizing tumors to radiation.
- Hypothesis: Sub-cytotoxic Hsp90 inhibition may achieve tumor-selective radiosensitization.
Purpose of the Study:
- To investigate the effect of sub-cytotoxic Hsp90 inhibitor concentrations on DNA repair proteins.
- To assess the radiosensitizing potential of low-dose Hsp90 inhibition in tumor models.
Main Methods:
- Global protein abundance analysis using mass spectrometry.
- In vitro radiosensitization assays (clonogenic assay).
- Pharmacokinetics studies in tumor-bearing mice.
- Evaluation of low-dose Hsp90 inhibitor combined with radiotherapy in three tumor models.
Main Results:
- Sub-cytotoxic AT13387 (onalespib) reduced DNA repair proteins without broad Hsp90 client degradation.
- Pharmacokinetics revealed higher drug concentrations in tumors than plasma.
- Low-dose AT13387 combined with radiotherapy significantly inhibited tumor growth.
- Hsp70 expression in PBMCs served as a biomarker for Hsp90 inhibition.
Conclusions:
- A low dose of Hsp90 inhibitor combined with radiotherapy yields a significant therapeutic ratio.
- Hsp90 inhibition monitoring via Hsp70 in PBMCs is feasible for human studies.
