Using a standardised assay, in the current study we report the expression of PD-L1 in pre-treated STS tumour tissues. Furthermore, our study unravels the value of PD-L1 for predicting pazopanib efficacy. To our knowledge, this is the first study to evaluate the role of PD-L1 expression in pazopanib response in STS.
Pazopanib, an inhibitor of angiogenesis and tumour cell proliferation, has been recognized as a salvage treatment for STS[5]. The approval of this drug was based on the results of the randomised phase III PALETTE trial, which showed that pazopanib improved PFS in patients with previously treated STS compared to placebo treatment. However, despite the prolonged PFS, no significant difference in OS was observed, and our understanding of the factors mediating pazopanib sensitivity and resistance is still poor. There is an urgent need for the identification of predictive biomarkers which can be used to select subgroups of patients who will benefit from pazopanib treatment. In the PALETTE trial, it was noted that patients with leiomyosarcoma and synovial sarcoma exhibited better PFS upon pazopanib; however, a significant interaction with histological subtypes was not identified in predictive analysis. Other biological markers include post-treatment neutrophil-lymphocyte ratio, which was also suggested to be a robust predictive factor for pazopanib efficacy[7]. However, haematological parameters (such as neutrophil and lymphocyte counts) are unreliable because they can be easily influenced by other inflammatory conditions. Although a number of potential biomarkers have also been investigated, none of these are available as patient selection strategies for use in clinical practice.
Immune checkpoint inhibitors that block PD-L1 and PD-L1 have exhibited remarkable efficacy with respect to the treatment of refractory solid tumours including melanoma and RCC, and substantial attention has been paid to assessing PD-L1 expression in STS. PD-L1 expression has been studied in various STS subtypes[10], and a recent meta-analysis also reported a role for PD-L1 expression in the poor prognosis of bone and STS[15]. Interestingly, immunotherapy resulted in the generation of a response in certain sarcoma subtypes, mainly UPS, liposarcoma, and angiosarcoma[8, 9]. The Cancer Genome Atlas (TCGA) analysis have described the following immune classification based on the composition of the tumor microenvironment in STS: immune-low, immune-high, and vascularized group. In this study, immune-high group (mainly UPS and liposarcoma) exhibited high expression of PD1, PDL2, and CTLA-4[16]. Consistent with previous studies, we show that PD-L1 (+) was commonly observed in UPS, angiosarcoma, and leiomyosarcoma, and that PD-L1 (+) staining independently correlated with poor efficacy of pazopanib treatment. Furthermore, none of the synovial sarcoma tumours exhibited PD-L1 expression and this subtype exhibited a higher response to pazopanib treatment. Similarly, the synovial sarcoma subtype also showed favourable responses to pazopanib in the PALETTE trial[5]. Therefore, we propose that PD-L1 expression is associated with pazopanib resistance and that this hypothesis is worth further testing as a therapeutic strategy in future clinical trials.
A recent report described a correlation between PD-L1 expression on the tumour and the efficacy of pazopanib or sunitinib treatment in metastatic RCC. Patients whose tumours were TIL-positive and/or displayed high PD-L1 expression had poor survival outcomes in response to vascular endothelial growth factor (VEGF)-targeted agents[12]. A preclinical study showed that anti-PD-L1 therapy enhances tumour sensitivity to antiangiogenic therapy by inducing the development of endothelial venules that facilitate cytotoxic T cell activity and tumour lysis[17]. Therefore, on the basis that the VEGF receptor (VEGFR) plays a central role in immunosuppression, a strategy of combination treatments has been widely studied in RCC[18, 19]. Similarly, the combination of a VEGF inhibitor, axitinib, with pembrolizumab was shown to have promising efficacy in advanced sarcomas, particularly in patients with advanced soft-part sarcoma[20]. Taken together, the combination of pazopanib and PD-L1 blockade may be a promising future therapeutic option for STS, and the final results of an ongoing trial (A Study of Pazopanib and Durvalumab for Metastatic Soft Tissue Sarcoma; ClinicalTrials.gov Identifier: NCT03798106) may help to confirm this premise.
To clarify the exact characteristics and prognostic role of PD-L1 expression, we carefully assessed a large cohort of patients with STS who were treated with pazopanib. Furthermore, from a practical perspective, three different PD-L1 antibodies (22C3, 28 − 8, and SP263) showed comparable analytical performance with respect to assessing PD-L1 expression[21]. Therefore, we used a companion diagnostic marker for approved anti-PD-L1 antibodies, SP263 PD-L1, in accordance with a standard protocol (instrument platform, staining procedure, and scoring methods) to provide a practical guideline. However, because of the retrospective nature of this study, future studies should evaluate a larger, prospective cohort to address the question of whether PD-L1 expression can serve as a clinical prognostic marker for STS.