The finding that EP elicits IDO-directed, immune-based antitumor responses reinforces emerging concepts about the important role of inflammatory processes in supporting cancer pathophysiology. Prompted by observations of Fink and colleagues in rodent models of ischemia/reperfusion injury, hemorrhagic shock and sepsis (26
), EP treatment has been investigated for several years in a variety of animal models of acute and chronic inflammatory disorders (17
), while clinical testing thus far has been limited to the evaluation of EP as an intravenous agent for the prevention of single and multiple organ dysfunction in patients undergoing cardiopulmonary bypass surgery (42
). Early studies of IDO have noted its elevation in response to bacterial infections or LPS exposure (32
), and more recently it has been reported to be a mediator of endotoxic shock-associated lethality (44
). It is intriguing to speculate that diseases as seemingly disparate as cancer and sepsis may be linked at some underlying level of pathophysiology through a shared dysregulation of IDO. In this light, the interaction of cancer cells with the host may bear some similarity to unresolved infections that result in the clinical manifestations of sepsis. Survival following the onset of severe sepsis as modeled in the mouse can be dramatically improved by the administration of EP (26
), and it will be important to assess whether this benefit is also linked to the ability of EP to inhibit IDO.
Many studies have suggested that correcting imbalances in NFκB signaling in cancer may have important benefits in the context of both the tumor cell and the inflammatory tumor microenvironment, but the concept of correcting immune escape via this signaling pathway has received relatively little attention. Given the centrality of NFκB as a signal transduction node, the degree to which EP antitumor activity was found to rely specifically upon IDO targeting in the host might be considered somewhat surprising. However, this outcome aligns with a concept we have termed ‘tolerance addiction’, proposed as a result of previous studies of IDO inhibitory compounds (14
). As noted previously, B16-F10 melanomas are illustrative of a class of tumors that appear to preferentially use IDO as an immune escape mechanism, such that, once a tumor is established, continued IDO activity must be sustained to maintain the immunoprivileged state of the tumor. In this context, acute disruption of IDO activity, as with a pharmacological agent, causes an immunological unmasking that promotes rejection. However, if upregulation of IDO activity is not an available option (as in the IDO-deficient animal), alternate immune escape mechanisms can apparently be accessed by the developing tumor in which case IDO-targeting compounds are ineffectual. The specific target of EP, p65 RelA, is an important component of canonical NFκB signaling but plays no apparent role in the non-canonical pathway (22
). This specifically implicates canonical NFκB signaling as the regulatory pathway controlling IDO expression, a conclusion that is further supported by the demonstration that the NFκBα (IkBα) super-repressor, which also selectively interferes with canonical NFκB signaling, also effectively suppresses induction of IDO promoter activity. These results appear to run counter to evidence that non-canonical NFκB signaling is important for IDO induction mediated through GITR signaling (45
). Our data do not, however, necessarily contradict these findings, but rather indicate that the regulation of IDO expression is likely to be complex and that the relative importance of canonical versus non-canonical NFκB signaling in controlling this process may be contextual.
Although our findings are consistent with published evidence of EP as an NFκB inhibitor, they do not rule out alternative mechanisms that may be germane to its in vivo
effects. EP has also been reported to exert anti-inflammatory effects through ROS scavenging (46
) and through blocking HMGB1 release (26
), and it is not inconceivable that elevated ROS or HMGB1 release may support dysregulated expression of IDO. One group has recently reported that EP can exert antitumor activity in a liver metastasis model and has suggested that EP may produce anti-inflammatory and pro-apoptotic effects responsible for its antitumor activity though mechanistic validation was lacking (47
). From our studies it is clear that direct cytotoxicity is not sufficient to account for the antitumor efficacy of EP against B16-F10 tumors in vivo
, as no evidence of antitumor activity was observed when this compound was administered to athymic, tumor-bearing mice. Furthermore, it is clear from the loss of EP efficacy in Ido1
-deficient mice that the relevant regulatory pathway targeted by EP directs an immune escape mechanism that is predominantly orchestrated through the elevation of IDO activity.
While direct inhibition of the IDO enzyme is presently being explored by many groups as an interventional approach, EP may offer an alternative, low-cost, readily accessible tool to indirectly block IDO for therapeutic purposes. It is likely that IDO inhibitors will prove most effective when combined with other cancer treatment modalities (7
), and EP, as a safe and inexpensive food additive, could readily be evaluated as an adjuvant to standard of care treatments with minimal risk of adverse side effects. Given accumulating evidence that elevated IDO activity may have a pathophysiological role in other diseases such as chronic infections and autoimmune disorders (25
), EP may find other clinical applications as an IDO inhibitory strategy as well. Fink and colleagues have described a simple formulation to administer EP (49
) by substituting it for lactate in Ringer’s lactate solution that is usually given intravenously for fluid resuscitation after blood loss or as a conduit for drug delivery. Insofar as IDO inhibitors have been shown to cooperate with different types of cancer therapy in mouse tumor models, we suggest the same Ringer’s formulation as a route to administer EP with standard i.v. chemotherapeutics. Repositioning EP for an oncology study in this manner would be a straightforward strategy to clinically evaluate EP’s potential as a cutting edge immunochemotherapeutic agent that could address the acute need in developing countries for simple, low cost advances in cancer treatment.