Successful anticancer strategies require a differential response between tumor and normal tissue (i.e., a therapeutic ratio). In fact, improving the effectiveness of a cancer therapeutic is of no clinical value in the absence of a significant increase in the differential response between tumor and normal tissue. Although radiation dose escalation with the use of intensity modulated radiation therapy has permitted the maximum tolerable dose for most locally advanced cancers, improvements in tumor control without damaging normal adjacent tissues are needed. As a means of increasing the therapeutic ratio, several new approaches are under development. Drugs targeting signal transduction pathways in cancer progression and more recently, immunotherapeutics targeting specific immune cell subsets have entered the clinic with promising early results. Radiobiological research is underway to address pressing questions as to the dose per fraction, irradiated tumor volume and time sequence of the drug administration. To exploit these exciting novel strategies, a better understanding is needed of the cellular and molecular pathways responsible for both cancer and normal tissue and organ response, including the role of radiation-induced accelerated senescence. This review will highlight the current understanding of promising biologically targeted therapies to enhance the radiation therapeutic ratio.
The concept of a radiation therapeutic ratio for cancer treatment has evolved over the years and generally centers around efforts to maximize the radiation response of cancer cells to achieve local control, while minimizing the potential for radiation-induced acute and late morbidity on normal tissues [
The interaction between targeted therapies and RT is of clinical importance for at least two reasons. Targeted cancer therapies are often combined with conventional approaches such as RT, so the effect of targeted therapies on radiation effectiveness and vice-versa is important. Additionally, targeted therapies act on specific molecular targets that are associated with cancer, whereas radiation (and most standard chemotherapies) acts on all rapidly dividing normal and cancerous cells. Consequently, the effects of targeted therapies are often cytostatic, whereas radiation (and conventional chemotherapy agents) generally kill rapidly dividing cancer cells. Specific inhibitors have been developed targeting the signal transduction pathways. Among them, three pathways are of particular interest when combined with radiation. The PI3K/mTOR pathway has been extensively studied in conjunction with radiation [
The PI3K/AKT/mTOR pathway is an intracellular signaling pathway important in regulating the cell cycle. It is directly related to cellular quiescence, proliferation, cancer, and longevity. The protein complex mTORC1 is a convergence point for multiple signaling pathways that control cell growth and proliferation. mTOR is a nutrient-sensing enzyme that regulates cell growth in nearly all eukaryotic species. Since the PI3K/mTOR pathway is frequently activated or dysregulated in many tumors, several classes of agents targeting this pathway are in under clinical development (
Since PI3K/mTOR pathways are altered in many human cancers and the increased activation of these signaling pathways have been implicated in the tumor radioresistance, numerous studies have been carried out to determine the extent of radiation modification using
At the cellular level, other investigators showed that mTOR inhibitors (e.g., temsirolimus) could overcome the radioresistance of hypoxic tumors, since the upregulation of mTOR regulates the hypoxia-inducible factor-1α (HIF-1α) pathway [
What is clear is that the mechanisms by which mTOR inhibitor enhances tumor radiosensitivity are multi-factorial. Currently, several clinical phase II studies are underway in which everolimus is administered in combination with radiation therapy for the treatment of lung and brain tumors.
In recent years, metformin, an oral anti-diabetic biguanide medication for type II diabetes, has attracted interest as a cancer therapeutics. The widespread use of metformin as an anti-diabetic drug, its expected safety profile, and encouraging pre-clinical data led to the several clinical trials evaluating the anti-cancer properties of metformin in combination with chemotherapeutic drugs and fractionated radiotherapy.
Though metformin has been in use for decades to reduce blood sugar levels, the exact mechanisms by which it exerts this anti-diabetic effect is not completely understood. It is widely accepted that metformin decreases hepatic gluconeogenesis and lowers insulin levels, with strong experimental evidence suggesting that it decreases ATP production by acting on mitochondrial respiratory complex I, ultimately activating AMP kinase with secondary inhibition of protein synthesis [
Mechanistic studies to date have demonstrated that metformin enhances the therapeutic gain of RT in multiple ways. Some researchers [
Data suggest that the mechanism of radiosensitization by metformin may derive not only from re-oxygenation of the hypoxic tumor but from other cellular mechanism as well, especially when metformin is combined with high dose per fraction (more than 8–10 Gy per fraction) radiation. High doses of irradiation is known to shut down the tumor vasculature and the subsequent radiation-induced hypoxia and low glucose would preferentially favor tumor necrosis [
Arsenic trioxide (ATO) is a US Food and Drug Administration approved anti-tumor drug (e.g., acute promyelocytic leukemia), that inhibits the mitochondrial complex III activity in the electron transport chain. Similar to results with combined metformin and radiation, a strong synergistic action of ATO has been shown when the radiation is delivered shortly after the drug administration. Mitochondrial inhibitors such as metformin or ATO, have a marginal cytotoxic effect when administered alone, whereas combining with these inhibitors with radiation has a dramatic cytotoxic effect [
The integrity of DNA replication and repair during cell proliferation is normally tightly controlled by several key cell cycle checkpoint proteins, including cyclin-dependent kinase (CDK), checkpoint kinase, and WEE1 kinase (WEE1). It is well known that ionizing radiation induces both doubleand single-strand DNA breaks which disrupt cell proliferation. These aberrant DNA structures induce signaling pathways involved in the DNA damage response (DDR), which regulate these cell cycle checkpoints and DNA repair. Two key kinases involved in this response are ataxia telangiectasia-mutated (ATM), activated primarily by DNA double-strand breaks, and ATM- and Rad3-related (ATR), activated by a broad spectrum of DNA lesions. ATM and ATR work in a coordinated fashion to effect homologous recombination (HR) repair, involving a gradual switch from ATM to ATR activation [
Several inhibitors of CHK1 and CHK2 are being investigated as radiosensitizers, both in pre-clinical and clinical settings. The DDR in tumor cells differs significantly from that in normal cells, with tumor cells often having defective DNA damage signaling through loss of ATM or p53 mutations. Since p53 mutations compromise efficient G1 checkpoint signaling, the DDR in tumor cells carrying these mutations depends more heavily on the ATR-CHK1-activated G2/M checkpoint for cell cycle arrest. Thus, checkpoint inhibitors selectively targeting the ATR-CHK1 pathway have the potential to sensitize tumor cells to radiation-induced DNA damage without sensitizing normal cells with wild type p53. Such compounds are also being investigated in this context and appear to be potent and selective radiosensitizers of cancer cells [
CHK1 functions in HR repair, stabilizing replication forks, and inhibiting apoptosis. Many CHK1 inhibitors exhibit radiosensitization in clinical models [
Many human tumors exhibit overexpression of WEE1, which selectively regulates the G2/M checkpoint in response to DNA damage by inhibiting CDK1. Pre-clinical studies have shown that WEE1 also participates in DNA repair by stabilizing DNA replication forks. WEE1 inhibitors, therefore, have the potential to remove the G2/M block and to compromise WEE1-mediated DNA repair. MK-1775, a selective, small molecule inhibitor of WEE1, has been shown to radiosensitize multiple human cancer cell lines [
Ever since the early 1960s, when presence of tumor specific antigens was convincingly demonstrated [
CTLA4 is a CD28 homolog that acts the early stages of immune response by affecting the interaction between T cells and antigen-presenting cells in the lymph nodes. PD-1/PDL1 suppress T cells later on during the effector phase of the immune response, especially in the periphery. Anti-CTLA4 therapies increase T cell activation by enhancing proliferation and reducing Treg immunosuppression. On the other hand, anti-PD-1/PD-L1 therapies act to restore T cells that are exhausted due to chronic exposure to carcinogen and the accumulation of mutations over time [
Like many other therapeutic regimen used in combination with RT (e.g., cytotoxic chemotherapy, targeted therapy), key radiobiological parameters play important roles when combined with immune checkpoint inhibitors [
The tumor microenvironment harbors several immune suppressors such as myeloid derived macrophages, regulatory T cells, and TGFβ, to name a few [
In this context, it would be highly desirable to determine whether the depletion of TAM with CSF1-R inhibitor would enhance the tumor radiation response or not. If it shows an enhancing effect, this approach would be another novel strategy for enhancing the therapeutic ratio. Furthermore, a combination with immune checkpoint inhibitors and the depletion of TAM with CSF1-R inhibitors would be a potentially exciting new approach.
TGFβ is a multifunctional and pleiotropic cytokine affecting many cellular processes including epithelial cell growth, mesenchymal cell proliferation, and extracellular matrix production. Irradiation, even at low doses, is one of the few exogenous factors known to induce TGFβ activation and TGFβ is believed to play a central role in mediating radiationinduced anti-tumor immunity [
The use of immunosupressive approaches other than M2 macrophages, such as myeloid-derived-suppressor cells, MDSC, is worth considering since these immunosuppressive myeloidderived cells also infiltrate tumors after RT. For example, STING-dependent MDSC infiltrate tumors after radiation exposure providing an immunosuppressive environment and their blockade enhances the effectiveness of RT [
Recent advances in our understanding of the cellular and molecular pathways leading to tissue and organ damage have provided a novel insight into the mechanisms of pathogenesis. Strategies aimed at reducing or counteracting oxidative stress and the resulting excessive production of reactive oxygen and nitrogen chemical species (ROS and RNS, respectively) and pro-inflammatory cytokines have been covered in detail in this journal [
We have discussed the role of rapamycin and its analogs as a radiosensitizer in the foregoing section on the PI3K/mTOR pathway. The radiosensitizing effect was shown in mostly tumor tissues. Interestingly, similar inhibitors have been shown to protect or mitigate the radiation induced normal tissue injury [
Inhibition of the HIF-1α pathway has been shown to mitigate radiation-induced gastrointestinal toxicity [
Cellular senescence, which is a normal consequence of aging, can result from DNA damage such as that found after radiation exposure as well as oxidative stress, and chronic inflammation. Senescent cells lose the proliferative potential normally found in replication-competent cells and becomes resistant to apoptosis, with an increase in metabolic activity. These changes are often accompanied by the development of a phenomenon known as senescence-associated secretory phenotype (SASP) (
Laboratory studies have confirmed the importance of senescence as a cause of radiation toxicity in bone marrow, skin and lung and of toxicity from DNA damaging chemotherapy [
Multiple pharmacological strategies are under investigation to remove senescent cells from non-genetically modified animals, using so-called ‘senolytic agents’, including small molecules, peptides, and antibodies with varying degrees of success [
Recent advances in the cellular and molecular pathways of tumor growth and the pathogenesis of tissue and organ injury provide novel strategies to enhance the radiation therapeutic ratio. Radiobiological research is aimed to optimize combined therapeutic regimens of radiation and targeted therapies and immunotherapeutics. Efforts to minimize or mitigate the risk of radiation injury constitute a promising direction of future study.
No potential conflict of interest relevant to this article are reported.
The studies were support in part by the National Institutes of Health under awards: No. U19-A1067734 (PI: John Moulder, Medical College of Wisconsin; Program Director Jae Ho Kim, Mitigating and treating radiation-induced CNS injury); No. R-21-CA205660 (PI: Jae Ho Kim, Improving the radiation therapeutic ratio by inhibiting pro-inflammatory cytokines), and No. RO1-CA218596 (PI: James Ewing and Stephen Brown, MRI Signatures of response to high-dose radiotherapy in rat models of cerebral tumor). We thank Andrew Kolozsvary and Karen Lapanowski, our long time collaborators for expert technical assistance.
The abbreviated signaling pathways regulating mTORC1. The protein complex mTORC1 is a convergence point for multiple signaling pathways that control cell growth and proliferation. Rapamycin and its analogs inhibit mTOR. Metformin activates AMPK, which in turn inhibits mTOR. Activated mTOR increases eIF4E through eIF4G complex. Increased eIF4E is correlated to the radioresistance.
Diagram illustrating where immune check point inhibitors block specific check point proteins. Ipilimumab inhibits CTLA4 (cytotoxic T-lymphocyte associated protein 4). Nivolumab and pembrolizumab inhibit PD-1 (programmed death protein). Atezolizumab and durvalumab inhibit PD-L1 (programmed death ligand).
Radiation induces accelerated senescence. Depending on the dose of radiation, normal tissues sustain sub-lethal damage, unable to divide but metabolically active. Senescent cells elaborate a complex mixture of cytokines and chemokines known as the senescence-associated secretory phenotype, SASP. This in turn would be source of increased, sustained source of ROS, chronic inflammation and modify immune phenotypes and stimulate fibrotic tissue remodeling. Ultimately, the presence of senescent cells impede the recovery and repair of remaining irradiated normal tissues and organs. ROS, reactive oxygen species; RNS, reactive nitrogen species.