Sishc, B. J. | bioRxiv Science

bioRxiv2024

Defective Homologous Recombination and Genomic Instability Predict Increased Responsiveness to Carbon Ion Radiotherapy in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is inherently resistant to conventional chemo-and radiation-therapy. However, clinical trials showed that carbon ion radiotherapy (CIRT) with concurrent gemcitabine can be effective for treating unresectable locally advanced PDAC. In this study, we aimed to determine features that could identify patients who would benefit most from CIRT. A panel of human PDAC cell lines with various genetic backgrounds was leveraged to determine whether a subset could be identified that preferentially responds to CIRT. The cell lines displayed a differential response to CIRT as compared to {gamma}-rays as measured by relative biological effectiveness (RBE) calculated at 10% survival, which ranged from 1.96 to 3.04. Increased radiosensitivity correlated with decreased DNA double strand break (DSB) repair as measured by {gamma}H2AX foci resolution. We determined that the cell lines most sensitive to CIRT are defective in the homologous recombination (HR) DSB repair pathway and/or have high genomic instability due to elevated replication stress. Next, this knowledge was utilized to assess whether the HR pathway could be targeted to potentiate CIRT in vitro. It was determined that pretreating a radioresistant PDAC cell line with the HR inhibitor, B02, resulted in a marked increase in sensitivity to CIRT when treated with high linear energy transfer (LET) radiation in the spread-out Bragg peak (74.1-89.3 keV/m) but not at the entry LET (13.0-16.4 keV/m) in vitro as opposed to that seen with a NHEJ inhibitor. These data suggest a greater therapeutic index with the combination therapy in the tumor over normal tissues based on LET distribution. These data support the notion that PDAC tumors with defects in HR and/or those with high inherent replication stress respond to CIRT without the concern for excessive normal tissue toxicity. With the advent of agents targeting HR, the difference in tumor cell response in the entry region vs within the SOBP based upon the respective LETs of those regions of the beam profile, will be superior to agents that target NHEJ.

cancer biology↗

bioRxiv2020

The superoxide dismutase mimetic GC4419 enhances tumor killing when combined with stereotactic ablative radiation

The penta-aza macrocyclic manganese compound GC4419 is in phase 3 clinical trials as a modifier of mucositis in H&N cancer treated by radio-chemotherapy based upon its properties as a superoxide dismutase mimetic. In studies to address the potential for tumor radioprotection, a significant anti-tumor effect was identified in tumors generated from the non-small cell lung cancer (NSCLC) cell line H1299, when GC4419 was combined with radiation. This effect was directly related to the size of the radiation dose as demonstrated by greater efficacy in tumor growth delay when biologically equivalent irradiation regimens using a limited number of dose fractions was substantially more effective compared to regimens where the fraction number was higher and dose per fraction decreased. Furthermore, a TCD50 assay using H1299 tumors that tested the combination of GC4419 with radiation revealed a Dose Enhancement Factor of 1.67. Based upon these results the hypothesis that GC4419 was generating cytotoxic levels of hydrogen peroxide during the superoxide dismutation process. Peroxide flux did increase in cells exposed to GC4419 as did the expression of the oxidative stress markers 4-HNE and 3-NT. H1299 cells that overexpressed catalase were then challenged as tumors by the combination of radiation and GC4419 and the tumoricidal effect was nearly eliminated. The enhanced radiation response was not specific to NSCLC as similar findings were observed in human head and neck squamous cell carcinoma and pancreatic ductal adenocarcinoma xenografts. RNA sequencing analysis revealed that GC4419, in addition to generating high levels of hydrogen peroxide in irradiated cells, alters inflammatory and differentiation signaling in the tumor following irradiation. Together, these findings provide abundant evidence that the radioprotector GC4419 has dual functionality and will increase tumor response rates when combined with agents that generate high levels of superoxide like stereotactic ablative body radiotherapy (SAbR). Combining SAbR with GC4419 may be an effective strategy to enhance tumor response in general but may also allow for fully potent radiation doses to tumors that might not necessarily be able to tolerate such doses. The potential for protection of organs at risk may also be exploitable.

cancer biology↗