The Next Immune-Checkpoint Inhibitors: PD-1/PD-L1 Blockade in Melanoma

Abstract

Purpose: Blocking the interaction between the programmed cell death (PD)-1 protein and one of its ligands, PD-L1, has been reported to have impressive antitumor responses. Therapeutics targeting this pathway are currently in clinical trials. Pembrolizumab and nivolumab are the first of this anti-PD-1 pathway family of checkpoint inhibitors to gain accelerated approval from the US Food and Drug Administration (FDA) for the treatment of ipilimumab-refractory melanoma. Nivolumab has been associated with improved overall survival compared with dacarbazine in patients with previously untreated wild-type serine/threonine-protein kinase B-raf proto-oncogene BRAF melanoma. Although the most mature data are in the treatment of melanoma, the FDA has granted approval of nivolumab for squamous cell lung cancer and the breakthrough therapy designation to immune- checkpoint inhibitors for use in other cancers: nivolumab, an anti-PD-1 monoclonal antibody, for Hodgkin lymphoma, and MPDL-3280A, an anti-PD-L1 monoclonal antibody, for bladder cancer and non-small cell lung cancer. Here we review the literature on PD-1 and PD-L1 blockade and focus on the reported clinical studies that have included patients with melanoma.

Methods: PubMed was searched to identify relevant clinical studies of PD-1/PD-L1-targeted therapies in melanoma. A review of data from the current trials on clinicaltrial.gov was incorporated, as well as data presented in abstracts at the 2014 annual meeting of the American Society of Clinical Oncology, given the limited number of published clinical trials on this topic.

Findings: The anti-PD-1 and anti-PD-L1 agents have been reported to have impressive antitumor effects in several malignancies, including melanoma. The greatest clinical activity in unselected patients has been seen in melanoma. Tumor expression of PD-L1 is a suggestive, but inadequate, biomarker predictive of response to immune-checkpoint blockade. However, tumors expressing little or no PD-L1 are less likely to respond to PD-1 pathway blockade. Combination checkpoint blockade with PD-1 plus cytotoxic T-lymphocyte antigen (CTLA)-4 blockade appears to improve response rates in patients who are less likely to respond to single-checkpoint blockade. Toxicity with PD-1 blocking agents is less than the toxicity with previous immunotherapies (eg, interleukin 2, CTLA-4 blockade). Certain adverse events can be severe and potentially life threatening, but most can be prevented or reversed with close monitoring and appropriate management.

Implications: This family of immune-checkpoint inhibitors benefits not only patients with metastatic melanoma but also those with historically less responsive tumor types. Although a subset of patients responds to single-agent blockade, the initial trial of checkpoint-inhibitor combinations has reported a potential to improve response rates. Combination therapies appear to be a means of increasing response rates, albeit with increased immune-related adverse events. As these treatments become available to patients, education regarding the recognition and management of immune-related effects of immune-checkpoint blockade will be essential for maximizing clinical benefit.

Keywords: MPDL3280A; PD-1; PD-L1; melanoma; nivolumab; pembrolizumab; pidilizumab; programmed cell death 1.

Figure 1

The interaction of PD-1 and PD-L1 reduces T-lymphocyte function. APC = antigen presenting cell; CTLA = cytotoxic T-lymphocyte antigen; ITIM = immunoreceptor tyrosine-based inhibitory motif; ITSM = immunoreceptor tyrosine-based switch motif; MHC = major histocompatibility complex; P = phosphoryation site; PD = programmed cell death protein; SHP = Src homology 2 domain–containing phosphatase; TCR = T cell receptor.

Figure 2

Tumor-infiltrating lymphocytes and natural killer cells can express multiple co-stimulatory and co-inhibitory receptors, which may be potential therapeutic targets. The butyrophilin gene family consists of ~30 B7-like proteins, associated with some autoimmune diseases, and some appear to negatively regulate lymphocyte activation. To avoid confusion, B7-H5 has been omitted because it has been assigned to both proteins VISTA and HHLA-2. APC = antigen presenting cell; BTLA = B- and T-lymphocyte attenuator; DNAM = DNAX Accessory Molecule-1; GITR = glucocorticoid-induced tumor necrosis factor receptor; HHLA = HERV–H LTR-associating protein 2; HVEM = herpesvirus entry mediator; ICOS = inducible co-stimulator; KIR = killer Ig-like receptors; L = ligand; LAG-3 = Lymphocyte-activation gene 3; MHC = major histocompatibility complex; NK = natural killer; PD = programmed cell death protein; PtdSer = phosphotidylserine; PVR = poliovirus receptor; TCR = T cell receptor; TIGIT = T cell immunoreceptor with Ig and ITIM domains; TIM = T-cell immunoglobulin and mucin domain; TL1A = TNF-like cytokine; a TNF-like ligand for DR3 and TR6/DcR3; TMIGD = Transmembrane and Immunoglobulin Domain-containing Protein 1; TNFRSF = TNF receptor superfamily; VISTA = V-domain Ig suppressor of T cell activation.