Localized Merkel cell carcinoma treatment considerations: a response to the forty-year experience at the Peter MacCallum cancer centre

Merkel cell carcinoma (MCC) is a rare but aggressive neuroendocrine tumour of the skin with poor prognosis and rising global incidence. A recently published article in BMC Cancer, titled "Merkel cell carcinoma: a forty-year experience at the Peter MacCallum Cancer Centre" (Wang et al.), provides a contemporary analysis of locoregional disease outcomes in Australia which highlights the comparative effectiveness of radiotherapy for excisions with involved margins versus wide local excision. There is a persistent lack of clear, well-defined guidelines to manage MCC in Australia despite experiencing the highest rates globally. The advanced age at onset also provides inherent challenges for optimal management and often, a case-by-case approach is necessary based on patient preferences, baseline function and fitness for surgery. This paper responds to the recently published article by Wang et al. and will expand the discourse regarding management of localized MCC. Specifically, we will discuss the surgical excision approaches; alternative treatment options for MCC including radiotherapy, Mohs micrographic surgery and novel immunotherapy agents being investigated through several clinical trials.

The Evolving Treatment Landscape of Merkel Cell Carcinoma

OPINION STATEMENT

Merkel cell carcinoma (MCC) has a high risk of recurrence and requires unique treatment relative to other skin cancers. The patient population is generally older, with comorbidities. Multidisciplinary and personalized care is therefore paramount, based on patient preferences regarding risks and benefits. Positron emission tomography and computed tomography (PET-CT) is the most sensitive staging modality and reveals clinically occult disease in ~ 16% of patients. Discovery of occult disease spread markedly alters management. Newly diagnosed, localized disease is often managed with sentinel lymph node biopsy (SLNB), local excision, primary wound closure, and post-operative radiation therapy (PORT). In contrast, metastatic disease is usually treated systemically with an immune checkpoint inhibitor (ICI). However, one or more of these approaches may not be indicated. Criteria for such exceptions and alternative approaches will be discussed. Because MCC recurs in 40% of patients and early detection/treatment of advanced disease is advantageous, close surveillance is recommended. Given that over 90% of initial recurrences arise within 3 years, surveillance frequency can be rapidly decreased after this high-risk period. Patient-specific assessment of risk is important because recurrence risk varies widely (15 to > 80%: Merkelcell.org/recur) depending on baseline patient characteristics and time since treatment. Blood-based surveillance tests are now available (Merkel cell polyomavirus (MCPyV) antibodies and circulating tumor DNA (ctDNA)) with excellent sensitivity that can spare patients from contrast dye, radioactivity, and travel to a cancer imaging facility. If recurrent disease is locoregional, management with surgery and/or RT is typically indicated. ICIs are now the first line for systemic/advanced MCC, with objective response rates (ORRs) exceeding 50%. Cytotoxic chemotherapy is sometimes used for debulking disease or in patients who cannot tolerate ICI. ICI-refractory disease is the major problem faced by this field. Fortunately, numerous promising therapies are on the horizon to address this clinical need.

Pembrolizumab and other immune checkpoint inhibitors in locally advanced or metastatic Merkel Cell Carcinoma: safety and efficacy

INTRODUCTION

Merkel Cell Carcinoma (MCC) is a rare aggressive skin cancer, mostly affecting elderly patients. Until recently, patients with advanced disease were treated with cytotoxic chemotherapies despite rapid chemoresistance and high toxicity. As with other cancers, immune checkpoint inhibitors (CPI), including pembrolizumab, allow durable responses with a manageable safety profile in these patients.

AREAS COVERED

This review describes the rationale for using PD-1/PD-L1 inhibitors in MCC, as well as efficacy and safety results from the three open-label trials investigating pembrolizumab or other PD-1/PD-L1 inhibitors in patients with advanced MCC. Real-life experience and predictive pre-treatment biomarkers are discussed to assess which patients are likely to be candidates for such strategies. Ongoing fields of research include the use of CPI in the adjuvant or neoadjuvant setting and combined strategies in refractory patients. Expert Opinion: CPI have become the standard of care for frontline treatment in patients with advanced MCC. Earlier introduction of CPI in the disease course, including neo-adjuvant and adjuvant settings, is likely to improve the outcomes further. Given the rarity of this cancer, we still need to harmonize efforts in order to conduct large-scale trials and efficiently identify best optimal care.

Immune Checkpoint Inhibitors and Beyond: An Overview of Immune-Based Therapies in Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is an aggressive skin cancer. Until 2017, patients with advanced disease were typically treated with conventional chemotherapies, with a median response duration of 3 months. Increased evidence of the role of the immune system in controlling this cancer has paved the way for immune-based therapies, with programmed cell death protein 1 (PD-1)/programmed cell death protein ligand 1 (PD-L1) inhibitors at the frontline. Avelumab, an anti-PD-L1 antibody, was the first-ever drug approved in advanced MCC after showing meaningful efficacy in a second-line setting. Objective responses were observed in one-third of patients and, most importantly, were durable with half of patients and one-third of patients still alive at 1 and 2 years, respectively. When used in a first-line setting, PD-1/PD-L1 inhibitors (avelumab, pembrolizumab, nivolumab) are even more promising as objective responses are observed in approximately 50-70% of patients within the first 4-8 weeks of treatment. Safety profiles are acceptable with 10-20% of patients experiencing adverse events grade ≥ 3. PD-1/PD-L1 inhibitors are considered the standard of care in advanced MCC and are currently being investigated in the adjuvant and neoadjuvant settings. However, innovative treatments are still needed in the metastatic setting, as approximately 50% of these patients will not persistently respond to currently available immunotherapies, and no predictors of response are available yet. Therefore, other immunotherapeutic strategies are now being investigated-ideally in combinations-to enhance the various aspects of the immune response against tumoral cells.

Product review: avelumab, an anti-PD-L1 antibody

Although immunotherapies have been employed for many decades, immune checkpoint inhibitors have only recently entered the oncologic landscape. Avelumab is a fully human monoclonal antibody that blocks the interaction between PD-L1 on tumor cells and PD-1 on T cells, thereby inhibiting immunosuppression in the tumor microenvironment and reducing tumor growth. Most early clinical trials of avelumab as monotherapy and in combination regimens were part of the international JAVELIN clinical trial program, which included more than 7000 patients in more than 30 trials with at least 15 tumor types. Avelumab has been approved by the U.S. FDA for the treatment of metastatic Merkel cell carcinoma and metastatic urothelial carcinoma that has progressed during or following treatment with a platinum-based regimen. Its acceptable safety profile and ability to induce durable responses in otherwise deadly tumors provide the rationale for its use in other tumor types and in combination with other therapies.