ABSTRACT
Primary penile melanomas are rare tumors that represent less than 0.1% of all melanomas. We report a case of a 60-year-old Japanese male with a mucosal penile melanoma and describe an increased CD8⁺ T cell infiltration in brain after dacarbazine (DTIC) administration. After partial penectomy and left inguinal lymphadenectomy, he developed multiple lung, bone, spleen, brain and skin metastases. He was treated with interferon-β, DTIC and nivolumab. However, the metastases were not reduced in size. Immunohistochemistry showed an increase of CD8⁺ T cell infiltration and programmed death-ligand 1 (PD-L1) expression after the administration of DTIC, but the expression of programmed cell death protein 1 (PD-1) was negative. We speculate that DTIC exerted immunostimulatory effects, but nivolumab was ineffective due to the negative expression of PD-1 and/or an insufficient infiltration of CD8⁺ T cells. Although this is only one case, this case report could be the first step to discuss the development of effective therapies against melanoma to take advantage of the increased CD8⁺ T cell infiltration elicited by chemotherapeutic agents. It would be beneficial to pay more attention to the relationship between DTIC and immune checkpoint modulators.