Overexpression of PD-L1 has been shown to be associated with better clinical responses to PD-1/PD-L1 blockade in melanoma. However, the utility of PD-L1 immunostaining as a predictive biomarker for anti-PD-1 treatment remains unclear, especially in melanoma of acral/mucosal origin. Materials and methods We collected and reviewed the medical records of 37 patients with metastatic melanoma who were treated with the anti-PD-1 antibodies pembrolizumab or nivolumab between January and December 2015. Patients with histologically diagnosed malignant melanoma and whose pretreatment tumor specimens were available for immunohistochemical staining of PD-L1 expression in tumor or immune cells were included. Results Of 37 patients, 26 patients had either acral or mucosal melanoma. The overall response rate was 10.8 % (95 % CI, 0.8-20.8 %). The response rate to PD-1 inhibitor was 11.5 % (95 % CI, 0-23.8 %) in acral/mucosal melanoma and that for cutaneous melanoma was 9.1 % (95 % CI, 0-26.1 %). Of these 37 patients, 18 had pre-treatment tumor specimens available for PD-L1 staining. Of 18 patients, 10 (55.5 %) were of acral/mucosal origin. In all patients with acral melanoma, the overall response rate (ORR) was 16.7 % (1 of 6 patients) and disease control rate (DCR) was 50 % (3 of 6 patients). In the PDL-1(+) melanoma group (1 % cut-off value), ORR was 20 % (2/10) and DCR was 80 %; for PDL-1 (-) group, ORR was 12.5 % (1/8) and DCR of 37.5 %. In the PDL-1 (+) group by 5 % cut-off value, ORR was 33.3 % (2/6) and DCR was 83.3 %; for patients with PDL-1 (-), ORR was 8.3 % (1/12) and DCR was 50 %. The median PFS was 6.8 months in PDL-1(+) group and 1.9 months in PDL-1(-) group (p = 0.149). Anti-PD-1 treatment was very well tolerated without serious adverse events of grade 3 or 4 in all patients. Conclusions The treatment outcome to PD-1 antibody was not different in acral/mucosal melanoma when compared with cutaneous melanoma. The immunohistochemical PD-L1 expression seemed to be correlated with better clinical outcomes of anti-PD-1 treatment in limited cases.
Keywords: Acral melanoma; Immune checkpoint inhibitor; Immunohistochemistry; PD-L1; Predictive marker.