IDO Pathway Analysis for Identifying Novel Biomarkers in Multiple Myeloma

Background: Indoleamine 2,3-dioxygenase (IDO) is an intracellular enzyme that catalyzes the initial rate-limiting step in Tryptophan (Trp) degradation along the Kynurenine pathway. Trp can be catabolized by tryptophan hydroxylase into serotonin (Ser) or alternatively, by IDO and tryptophan-2,3-dioxygenase (TDO) into Kynurenine (Kyn). A variety of human tumors have been shown to express an active IDO which leads to the induction of immune tolerance and malignant cell survival (Mellor et al, 1999). Elevated IDO activity within tumor microenvironment impairs the function of antigen specific T cells and suppresses the anti-tumor immune response, while in tumor-draining lymph nodes an active IDO inhibits the antigen presentation reducing the population of antigen-specific T cells and stimulates regulatory T cells (T_reg) activation (Curti et al. 2009). IDO overexpression has been correlated with poor prognosis in a number of human solid tumors including ovarian, endometrial and colon carcinoma (Okamoto et al. 2005, Ino et al. 2006, Brandacher et al. 2006). In hematological malignancies, such as AML and DLBCL, IDO expression has also shown significant prognostic value (Folgiero et al. 2014, Ninomiya et al. 2011). Currently, our understanding of the role of IDO in multiple myeloma (MM) is limited to a few existing studies correlating the abundance of selected IDO pathway metabolites with the disease (Bonanno et al. 2012, Zdzisinska et al. 2010). We hypothesize that IDO activity correlates with the disease onset and progression in MM patients.

Methods: We quantified the levels of Trp, Kyn, and Ser in human serum by mass spectrometry using the Biocrates AbsoluteIDQ p180 kit. Analysis was performed using MetaboAnalyst 3.0 and Graphpad prism software. The serum samples derived from healthy donors and MM patients were obtained commercially.

Results: We found significantly reduced levels of Trp (n=13, p<0.001) and elevated levels of Kyn (n=13, p<0.05) in serum samples from MM patients compared to matching (sex and age paired) healthy donor samples, while Ser levels did not differ significantly. Similarly, samples obtained from Newly Diagnosed (ND) MM patients showed a significant reduction in Trp (n=16, p<0.01) and a trend towards increased Kyn. The overall results in Relapsed/Refractory (R/R) MM patients were similar to the ND setting, although we only detected a trend towards Trp reduction, and observed a significant increase in Kyn levels (n=26, p<0.01) when compared to healthy donors. These results were confirmed when analyzing the data using the widely employed Kyn/Trp ratio, a potential marker for assessing IDO activity. A significantly elevated Kyn/Trp ratio was observed in all of the MM malignant populations (paired: n=13, p<0.001; ND: n=16, p<0.01; R/R: n=26, p<0.01) as compared to healthy individuals.

Conclusions: Our study suggests that increased activity of IDO, and/or the production of downstream metabolites is one of the features of MM, and could potentially define a valuable set of novel biomarkers. Here we show a consistent pattern of increased Kyn and decreased Trp levels as well as an elevated Kyn/Trp ratio, all of which are hallmarks of high IDO activity in MM subjects when compared to healthy individuals. We did not detect any significant differences in Ser levels between any of the groups which may suggest that the catabolic pathway is not dependent on tryptophan hydroxylase. Additional studies are being initiated to further investigate the role of IDO/Trp pathway in MM pathogenesis as well as better understand its potential prognostic and predictive impact in currently used MM therapies.