PV QA 1 - Poster Viewing Q&A 1
Purpose/Objective(s): Radiotherapy is a mainstay in prostate cancer (PCa) treatment, and administered when androgen levels are at their nadir. Recent studies have shown that androgen receptor (AR)-induced DNA double strand breaks (DSB) can be used to enhance radiosensitivity during castration. While this strategy is supported by clinical trials in castrate-resistant metastatic disease, concern remains about the appropriateness and safety of this strategy in local or locally advanced PCa. To address this concern and exploit AR signaling-induced DNA damage, we showed that the antiandrogen hydroxyflutamide (FLU) similarly leads to radiosensitivity in PCa cells by virtue of its ability to engage the AR in castrate conditions. Here we investigated the safety of using FLU as a radiosensitizer as well as the nature of the DNA damage induced by FLU and radiation. We suggest FLU could be used safely in patients with locally advanced disease, leading to radiosensitivity while preventing AR-mediated pro-growth programs in PCa.
Materials/Methods: Human PCa cell lines were androgen-deprived prior to 6 hour exposures to AR agonists or antiandrogens. Cell growth was measured by Incucyte live cell analysis following addition of AR agonists or antiandrogens. Expression of AR target genes were measured following exposure by qRT-PCR. The neutral comet assay was used to quantify DNA DSB in cells grown in castrate conditions and subsequently exposed to AR agonists or antiandrogens and various doses of ionizing radiation (IR). Colony formation was assessed under similar conditions, and formal synergy of DSB or clonogenic potential was assessed using Bliss Independence analysis.
Results: Androgen-deprived PCa cells were exposed to either dihydrotestosterone (DHT) or FLU, resulting in AR accumulation in the nucleus and increased DSBs. However, only DHT led to an increase in cell growth. Treatment with FLU led to no increase in growth despite inducing AR translocation to the nucleus. Similarly, DHT exposure led to significant increases in AR target gene mRNA while FLU exposure failed to induce such changes. Bliss Independence calculations show formal synergy in the extent of DSB induced by the combination of FLU or DHT exposure prior to IR doses of 0.5Gy (FLU p<0.0001; DHT p<0.01) or 1Gy (FLU p<0.05; DHT p<0.05), compared to either agent alone. Synergy was also observed with respect to clonogenic survival in PCa cells exposed to DHT (p<0.001) or FLU (p<0.001) prior to IR compared to either agent alone.
Conclusion: Here we show that the antiandrogen FLU leads to radiosensitivity similar in magnitude to DHT, yet prevents increases in AR target gene expression or cell growth. Moreover, we report formal synergy in the action of FLU and IR. We therefore suggest AR signaling can be safely exploited to radiosensitize PCa cells without pro-growth effects. Future work is directed toward testing the ability of FLU to cause DSB in PCa tissues in patients undergoing androgen deprivation to support a clinical trial of FLU-induced radiosensitization.
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