The findings of this study suggest novel applications for ZnPP in clinical oncology based on its IDO inhibitory properties. Antitumor effects reported previously for ZnPP have been interpreted to reflect its ability to inhibit HO-1, the catabolites of which (bilirubin and biliverdin) are powerful scavengers of reactive oxygen species (ROS) that might explain the cytoprotective action of HO-1. Evidence offered in this report suggest that ZnPP can exert antitumor effects through disruption of IDO-mediated immune escape in cancer. ZnPP might be applied against earlier stage tumors or to leverage the effect of chemotherapy against larger tumors; further experiments to examine dose and route or schedule of administration may optimize these possible applications. Like the IDO enzymes, HO-1 may support the activation of regulatory T cell functions that mediate immune escape. However, whether HO-1 inhibition participates in immune responses in cancer has not been addressed.
We found that the antitumor properties of ZnPP were largely abolished in the absence of IDO1 and that ZnPP could inhibit IDO activity in a HO-1-independent manner. These findings speak genetically and biochemically to the concept that the antitumor properties of ZnPP may be based to a significant degree on a disruption of IDO signaling. Our findings do not rule out the possibility that HO-1 inhibition by ZnPP could play some role in facilitating an immune response or decreasing levels of cytoprotective HO-1 reaction products bilirubin and biliverdin. Nevertheless, while HO-1 inhibition may be involved in certain therapeutic settings, in the absence of IDO1 our results indicate that HO-1 inhibition may not be sufficient. In contrast to another report (11
), we did not observe any correlation between antitumor activity and cytotoxic activity, the latter of which was minimal for ZnPP in both the B16-F10 melanoma cells and HeLa cells we used in this study. While the basis is unknown, melanoma cells have not been examined by others, and differences in cellular susceptibility to ZnPP cytotoxicity are readily conceivable. Nevertheless, at minimum, our findings establish that the antitumor properties of ZnPP extend beyond cytotoxicity to involve immune-based and IDO-targeted effects that are efficacious on their own. Thus, ZnPP may exert at least two anti-tumor activities, one related to cytotoxicity and a second to relieving IDO-mediated immune escape.
Targeting tumoral immune suppression represents a novel approach to cancer treatment, but this area has yet to be vigorously pursued in part due to a paucity of suitable agents to advance. Relative to other IDO inhibitory compounds that have been described, ZnPP has unique and appealing properties for clinical evaluation. It has a unique mechanism of action that targets both IDO enzymes as well as HO-1. It is closely related to clinical agent SnPP, suggesting mimicry for the clinical development pathway and cues for safety monitoring. Lastly, as a heme precursor it occurs naturally in vivo
as the first clinically translatable inhibitor of both IDO1 and IDO2. ZnPP has been used widely as a tool in pediatric and newborn testing for lead paint poisoning and hyperbilirubinema, respectively (25
). Evaluating ZnPP on the basis of its IDO inhibitory properties offers an opportunity to reposition this compound as an immunochemotherapeutic agent that limits immune escape in cancer. Our findings offer initial support for the notion that ZnPP might be applicable where IDO is expressed in either the tumor or the host immune system. Preclinical evidence supports the combination of IDO inhibitors with ‘immunogenic’ chemotherapeutic drugs (15
). While one study has reported negative findings on the combinatorial use of ZnPP with chemotherapy (11
), this issue should be reconsidered further given the opportunity ZnPP may provide to improve outcomes in specific settings of combinatorial immunochemotherapy where IDO inhibitors are effective (17
). In considering these properties, we also propose ZnPP as an immunochemotherapeutic agent to treat diseases beyond cancer which are characterized by local or systemic immune suppression, where IDO dysregulation occurs and has been implicated in persistent pathology (e.g. chronic infections).