Can IDO activity predict primary resistance to anti-PD-1 treatment in NSCLC?

In NSCLC, nivolumab showed a long-term benefit in a significant proportion of pretreated patients with 2-year overall survival (OS) of 23 and 29% in squamous and non-squamous histology, respectively, over-performing standard chemotherapy [1, 2]. However, more than two-thirds of NSCLC patients demonstrate primary or acquired resistance to PD-1 inhibition resulting in treatment failure [3]. Moreover, recent evidence suggested the existence of a novel pattern of disease progression referred to as hyper-progressive and characterized by a two-fold increase in tumour burden at first instrumental assessment following anti-PD-1 treatment and very poor OS [4]. Therefore, there is an urgent need for biomarkers able to predict treatment responsiveness and to identify those patients unlikely to respond to PD-1 blockade in order to candidate them to different treatment approaches. While some predictive factors of response to immune checkpoint inhibitors (ICI) have been proposed, among which PD-L1 expression/amplification, high tumor mutational burden, and mismatch repair gene defects, reliable biomarkers for treatment resistance are still lacking [3]. Indoleamine 2,3-dioxygenase (IDO) is a key enzyme catalyzing the first and rate-limiting step along the kynurenine (kyn) pathway of tryptophan (trp) metabolism outside the liver, which converts the essential amino acid l-trp to the main metabolite kyn [5]. Indoleamine 2,3-dioxygenase has been shown to act as immune checkpoint involved in peripheral immune tolerance since it is able to inhibit T-cells proliferation by starving them from trp to sensitize T-cells to apoptosis [6‐11]. In NSCLC, augmented trp catabolism, resulting in higher kyn serum concentration, has been linked to more advanced stage at diagnosis, poorer prognosis and to a lesser likelihood of response to chemotherapy [12‐14]. More interestingly, growing preclinical evidence suggests that increase in IDO activity can be involved in resistance to checkpoint inhibition [15] In the present study, we aimed to investigate whether IDO activity, expressed as kyn/trp ratio, can inform about response to nivolumab, particularly identifying those patients resistant to PD-1 blockade in a cohort of previously treated NSCLC.

In our preliminary analysis we found that baseline serum kyn/trp ratio, expression of IDO activity, could represent both a prognostic and predictive value in advanced NSCLC patients treated with second-line nivolumab. In fact, lower baseline level of kyn/trp ratio were significantly associated with better PFS and OS in our cohort. More interestingly and conversely, higher level of kyn/trp ratio correlated with early progression retaining an independent statistically significant value in multivariate analysis. In this study, patients were selected in accordance with eligibility criteria from registrative clinical trials of nivolumab in pretreated NSCLC (CheckMate 017 and CheckMate 057) and median PFS was consistent with historical data reported in the literature [16, 17].

Given the evidence that only a minority of cancer patients can benefit from ICI, a large amount of research is currently focused on defining predictive biomarkers able to select responsive or resistant patients in order to maximize the cost-effectiveness of treatment.

Over last few years, several predictive factors have been proposed that rely on patients’ characteristics as well as tumour and microenvironment profiles. PD-L1, which is expressed both on tumour and inflammatory cells, is arguably the most extensively studied biomarker for ICIs responsiveness, although it has showed some limitations [3]. More recently, the understanding of the correlation of microsatellite instability status with immunotherapy response has led to the FDA cancer site/histology-agnostic approval of the anti-PD-1 pembrolizumab [18]. Accordingly, tumour mutational burden (TMB) and mRNA profile are being considered promising response indicators [19]. In particular TMB has been used to select patients in CHECKMATE 227 trial, demonstrating a benefit of immunotherapy in patients with mutational load higher than 10 mutations per megabase. Regarding circulating biomarkers, some encouraging results have been reported for IL 8 [20] and circulating T lymphocytes [21]. However, all these putative biomarkers have strong limitations and they have not shown clinical usefulness for the different settings.

In this context, IDO has been identified as a marker of activation of immunological pathways induced by IFN-gamma, representing one of the main immune escape mechanism of tumor cells. IDO activity would also appear as negative prognostic factor in stage III-IV lung cancer [22‐27].

Furthermore, Allison et al. demonstrated that IDO knockout mice treated with anti CTLA-4, anti PD-1 or anti GITR presented delay in B16 melanoma tumour growth and increased OS compared with wild-type mice [28]. Notably, tissue expression of IDO has been lately associated with a better response to nivolumab in patients with renal cell carcinoma [29].

Our preliminary results showed that kyn/trp ratio serum levels are statistically higher in early progressors allowing to identify a proportion of NSCLC patients with intrinsic resistance to anti PD-1, and with a poor prognosis both in terms of PFS and OS. In particular, the median PFS of the whole cohort is not reached, while the median PFS of “high kyn/trp ratio patients “was 3 months, very similar to that of hyperprogressive patients from the study by Champiat et al. [4]. Interestingly, in our cohort, the IDO activity does not seem to be correlated with clinical parameters such as brain metastases, pleural effusion, age and ECOG PS.

Moreover, quinolinic acid levels are strongly associated with response to nivolumab in our study. Quinolinic acid reflects the metabolism of kynurenine and it is involved in tumour progression not only diminishing immune activation but also increasing the synthesis of NAD, through quinolinic acid phosphoribosyltransferase, that is necessary to prevent apoptosis of neoplastic cells [30].

Our findings suggest a possible role to IDO in mediating resistance to anti PD-1 agents and provide a rationale for an early administration of anti-IDO compounds in those patients with high baseline kyn/trp ratio or high quinolinic acid levels. In fact, patient selection based on this biomarker could most probably avoid negative results of clinical trials designed without considering the immunological background of patients, i.e. the extent of the immunosuppressive milieu.

In conclusion we found that the kyn/trp ratio and quinolinic acid level in the peripheral blood of NSCLC patients may be a potential predictive biomarker of resistance to anti-PD-1. The kyn/trp ratio and quinolinic acid are associated with early progression, PFS and OS. Despite the small sample, our study suggests that IDO could represent an easy-assessable and cost-effective biomarker particularly useful in identifying patients more likely to have a benefit with a combination of anti-PD-1 and anti-IDO. Future clinical trials are warranted to confirm its role as predictive biomarkers as well as therapeutic target, introducing immuno-oncology in the scenario of precision immuno-oncology.