Primary extramammary Paget disease (EMPD) is an adnexal carcinoma of the apocrine gland ducts that presents as an erythematous patch on cutaneous sites rich with apocrine glands.1 Primary EMPD can be in situ or invasive with the potential to become metastatic.2 Treatment of primary EMPD is challenging due to the difficulty of achieving clear surgical margins, as the tumor has microscopic spread throughout the epidermis in a skipping fashion.3 Mohs micrographic surgery is the treatment of choice; however, there is a clinical need to identify additional treatment modalities, especially for patients with unresectable, invasive, or metastatic primary EMPD,4 which partly is due to lack of data to understand the pathogenesis of primary EMPD. Recently, there have been studies investigating the genetic characteristics of EMPD tumors. The interaction between the programmed cell death receptor 1 (PD-1) and its ligand (PD-L1) is one of the pathways recently studied and has been reported to be a potential target in EMPD.5-7 Programmed cell death receptor 1 signaling constitutes an immune checkpoint pathway that regulates the activation of tumor-specific T cells.8 In several malignancies, cancer cells express PD-L1 on their surface to activate PD-1 signaling in T cells as a mechanism to dampen the tumor-specific immune response and evade antitumor immunity.9 Thus, blocking PD-1 signaling widely is used to activate tumor-specific T cells and decrease tumor burden.10 Given the advances of immunotherapy in many neoplasms and the paucity of effective agents to treat EMPD, this article serves to shed light on recent data studying PD-1 signaling in EMPD and highlights the potential clinical use of immunotherapy for EMPD.
EMPD and Its Subtypes
Extramammary Paget disease is a rare adenocarcinoma typically affecting older patients (age >60 years) in cutaneous sites with abundant apocrine glands such as the genital and perianal skin.3 Extramammary Paget disease presents as an erythematous patch and frequently is treated initially as a skin dermatosis, resulting in a delay in diagnosis. Histologically, EMPD is characterized by the presence of single cells or a nest of cells having abundant pale cytoplasm and large vesicular nuclei distributed in the epidermis in a pagetoid fashion.11
Extramammary Paget disease can be primary or secondary; the 2 subtypes behave differently both clinically and prognostically. Although primary EMPD is considered to be an adnexal carcinoma of the apocrine gland ducts, secondary EMPD is considered to be an intraepithelial extension of malignant cells from an underlying internal neoplasm.12 The underlying malignancies usually are located within dermal adnexal glands or organs in the vicinity of the cutaneous lesion, such as the colon in the case of perianal EMPD. Histologically, primary and secondary EMPD can be differentiated based on their immunophenotypic staining profiles. Although all cases of EMPD show positive immunohistochemistry staining for cytokeratin 7, carcinoembryonic antigen, and epithelial membrane antigen, only primary EMPD will additionally stain for GCDFP-15 (gross cystic disease fluid protein 15) and GATA.11 Regardless of the immunohistochemistry stains, every patient newly diagnosed with EMPD deserves a full workup for malignancy screening, including a colonoscopy, cystoscopy, mammography and Papanicolaou test in women, pelvic ultrasound, and computed tomography of the abdomen and pelvis.13
The first-line treatment of EMPD is surgery; however, obtaining clear surgical margins can be a challenge, with high recurrence rates due to the microscopic spread of the disease throughout the epidermis.4 In addition, anatomic location affects the surgical approach and patient survival. Recent studies on EMPD mortality outcomes in women show that mortality is higher in patients with vaginal EMPD than in those with vulvar/labial EMPD, partly due to the sensitive location that makes it difficult to perform wide local excisions.13,14 Assessing the entire margins with tissue preservation using Mohs micrographic surgery has been shown to be successful in decreasing the recurrence rate, especially when coupled with the use of cytokeratin 7 immunohistochemistry.4 Other treatment modalities include radiation, topical imiquimod, and photodynamic therapy.15,16 Regardless of treatment modality, EMPD requires long‐term follow-up to monitor for disease recurrence, regional lymphadenopathy, distant metastasis, or development of an internal malignancy.
The pathogenesis of primary EMPD remains unclear. The tumor is thought to be derived from Toker cells, which are pluripotent adnexal stem cells located in the epidermis that normally give rise to apocrine glands.17 There have been few studies investigating the genetic characteristics of EMPD lesions in an attempt to understand pathogenesis as well as to find druggable targets. Current data for targeted therapy have focused on HER2 (human epidermal growth factor receptor 2) hormone receptor expression,18 ERBB (erythroblastic oncogene B) amplification,19 CDK4 (cyclin-dependent kinase 4)–cyclin D1 signaling,20 and most recently PD-1/PD-L1 pathway.5-7