Moving Beyond the Standard of Care: Accelerate Testing of Radiation-Drug Combinations

Radiation therapy is a major treatment modality used in > 60% of cancer patients as definitive local treatment for inoperable locoregionally confined tumors and as palliative therapy. Although cytotoxic chemotherapy enhances the effectiveness of treatment, the benefit over radiation therapy alone is modest. There is a need to enhance the effectiveness of local tumor control over what sequentially or concurrently administered cytotoxic chemotherapy provides. Although many biological pathways are known to enhance the effectiveness of radiation therapy, there is currently a paucity of drugs approved for use in combination. Several clinical trials have tested the effectiveness of combining targeted agents or immunotherapies with radiation therapy, but the results of these trials have been negative, likely stemming from the relative lack of preclinical evidence using appropriate experimental standardization or model systems. Accelerating the identification of agents tested in an appropriate clinical context and experimental systems or models would greatly enhance the potential to bring forward early testing of drugs that would not only be safe but also more effective. This article provides an overview of the opportunities and challenges of developing therapeutics to combine with radiation therapy, and some guidance toward preclinical and early clinical testing to improve the chance that advanced phase testing of drug-radiation combinations would be successful in the long term.

Screening and Validation of Molecular Targeted Radiosensitizers

The development of molecular targeted drugs with radiation and chemotherapy are critically important for improving the outcomes of patients with hard-to-treat, potentially curable cancers. However, too many preclinical studies have not translated into successful radiation oncology trials. Major contributing factors to this insufficiency include poor reproducibility of preclinical data, inadequate preclinical modeling of inter-tumoral genomic heterogeneity that influences treatment sensitivity in the clinic, and a reliance on tumor growth delay instead of local control (TCD50) endpoints. There exists an urgent need to overcome these barriers to facilitate successful clinical translation of targeted radiosensitizers. To this end, we have employed 3D cell culture assays to better model tumor behavior in vivo. Examples of successful prediction of in vivo effects with these 3D assays include radiosensitization of head and neck cancers by inhibiting epidermal growth factor receptor or focal adhesion kinase signaling, and radioresistance associated with oncogenic mutation of KRAS. To address the issue of tumor heterogeneity we leveraged institutional resources that allow high-throughput 3D screening of radiation combinations with small molecule inhibitors across genomically characterized cell lines from lung, head and neck, and pancreatic cancers. This high-throughput screen is expected to uncover genomic biomarkers that will inform the successful clinical translation of targeted agents from the NCI CTEP portfolio and other sources. Screening "hits" need to be subjected to refinement studies that include clonogenic assays, addition of disease-specific chemotherapeutics, target/biomarker validation, and integration of patient-derived tumor models. The chemoradiosensitizing activities of the most promising drugs should be confirmed in TCD50 assays in xenograft models with/without relevant biomarker and utilizing clinically relevant radiation fractionation. We predict that appropriately validated and biomarker-directed targeted therapies will have a higher likelihood than past efforts to be successfully incorporated into the standard management of hard-to-treat tumors.

The Transcriptional Landscape of Radiation-Treated Human Prostate Cancer: Analysis of a Prospective Tissue Cohort

PURPOSE

The resistance of prostate cancer to radiation therapy (RT) is a significant clinical issue and still largely unable to be guided by patient-specific molecular characteristics. The present study describes the gene expression changes induced in response to RT in human prostate tissue obtained from a prospective tissue acquisition study designed for radiobiology research.

METHODS AND MATERIALS

A prospective cohort of 5 men with intermediate-risk and clinically localized tumors were treated with high-dose-rate brachytherapy with 2 × 10-Gy fractions. Image-guided transperineal biopsy specimens were taken immediately before and 14 days after the first high-dose-rate brachytherapy fraction. Using genome-wide 3' RNA sequencing on total RNA extracted from 10 biopsy specimens, we obtained quantitative expression data for a median of 13,244 genes. We computed the fold-change information for each gene and extracted high-confidence lists of transcripts with either increased or decreased expression (≥1.5-fold) after radiation in ≥4 of the 5 patients. Several gene ontology analyses were then used to identify functionally enriched pathways.

RESULTS

The predominant change in response to RT was elevation of the transcript levels, including that of DNA damage binding protein 2 and p21, and collagens, laminins, and integrins. We observed strong upregulation of the p53 pathway, without observable dysregulation of p53 itself. Interstitial remodeling, extracellular matrix proteins, and focal adhesion pathways were also strongly upregulated, as was inflammation. Functional network analysis showed clustering of the changes inherent in apoptosis and programmed cell death, extracellular matrix organization, and immune regulation.

CONCLUSIONS

In the present prospective study of matched clinical tissues, we successfully recognized known radiation-sensitive transcriptional pathways and identified numerous other novel and significantly altered genes with no current association with RT. These data could be informative in the development of future personalized therapeutic agents.