From Discovery to Development: Blocking PD-1 and its Ligands

Evan J. Lipson, MD
Department of Oncology

Suzanne L. Topalian, MD
Department of Surgery

Johns Hopkins University School of Medicine and Sidney Kimmel
Comprehensive Cancer Center
Baltimore, MD

On the heels of the recently approved anti-CTLA-4 drug ipilimumab, a new class of immune checkpoint therapies targeting the programmed death-1 (PD-1) receptor and its ligands is showing remarkable promise both as a monotherapy and in combination with other agents. Results from recent clinical trials hold the realistic hope of improved progression-free and overall survival for patients with advanced melanoma, with fewer toxicities to interfere with treatment.

The Pathway: PD-1 and its Ligands

Adaptive responses mediated by T and B lymphocytes play a critical role in antitumor immunity. The activity of the adaptive immune system is governed by tightly regulated molecular pathways that either promote or inhibit the full activation of immune cells. When a specific T or B cell receptor recognizes a target antigen, “immune checkpoints” that dampen immune activation are engaged. Ordinarily, these inhibitory pathways terminate immune cell activation at the appropriate time, preventing collateral damage to normal tissues. However, human cancers can commandeer these pathways, down-regulating antitumor immune activity and allowing unrestrained tumor growth.

The first immune checkpoint therapeutically targeted was cytotoxic Tlymphocyte antigen 4 (CTLA-4). After CTLA-4 blockade was shown to enhance antitumor immunity in preclinical studies,1 monoclonal antibodies (mAbs) blocking human CTLA-4 were developed, and a decade of clinical testing ensued.2-4 These trials culminated in FDA approval of ipilimumab (YervoyTM, Bristol-Myers Squibb, Princeton, NJ) in 2011 for the treatment of patients with metastatic melanoma. A mechanistically similar though functionally distinct pathway that plays a central role in immune inhibition is comprised of PD-1 (CD279) and its ligands PD-L1 (B7-H1/CD274) and PD-L2 (B7-DC/CD273).5 PD-1 is an inhibitory receptor expressed on activated T and B cells; its interaction with its primary ligand, PD-L1, inhibits the proliferation and survival of T cells.6 Like studies of murine anti-CTLA-4 that showed antitumor activity, preclinical studies of anti-PD-1’s effect on the PD-1/PD-L1 pathway also demonstrated its role in tumor immunosuppression.7

Just as the antitumor properties of anti-CTLA-4 and anti-PD-1 were predicted by their corresponding preclinical models, so too were their side effect profiles. Whereas CTLA-4 genetic knockout mice die at 3-4 weeks of age from massive multi-organ lymphocytic infiltration and tissue destruction,8,9 PD-1 knockout mice develop late-onset strain- and organ-specific autoimmunity.10,11 These observations are consistent with the distinct expression patterns of the ligands for CTLA-4 and PD-1. The ligands for CTLA-4 (B7.1 and B7.2) are expressed ubiquitously on antigen-presenting cells such as monocytes and dendritic cells; in contrast, PD-L1 is expressed primarily in peripheral tissues (including tumor deposits), so a less global immunological effect would be expected from PD-1 pathway blockade. Although adverse events with potential immune-related etiologies have been observed for both anti-CTLA-4 and anti-PD-1, a lower rate of grade 3 or 4 adverse events during prolonged drug administration was encountered with PD-1 pathway blockade.

Antibodies Blocking PD-1 and its Ligands

Several agents that block the interaction of PD-1 and its ligands are currently being evaluated in clinical trials [Table 1]. The drug that has undergone the most extensive clinical testing is nivolumab (BMS-936558/MDX-1106/ONO-4538), a fully human IgG4-blocking mAb specific for human PD-1. An initial glimpse of its tolerability and clinical activity was seen in a first-in-human, Phase I, intermittent-dose, dose-escalation trial involving 39 patients.12 In this study, drug-related toxicities were manageable, and clinical activity was observed in patients with advanced treatmentrefractory melanoma, renal cell carcinoma (RCC), colorectal cancer (CRC), and non-small cell lung cancer (NSCLC). The durability of tumor responses was demonstrated in a follow-up evaluation of three patients who experienced objective tumor regressions. Two patients, one with CRC and one with RCC, achieved complete responses, which were ongoing at last evaluation (three and five years, respectively). A melanoma patient experienced a partial response, which lasted 16 months after drug discontinuation. Following tumor progression, a partial response was again achieved with nivolumab re-induction.13

More recently, nivolumab was tested in a second Phase I trial with cohort expansion, intravenously administered biweekly to some 300 patients with treatment-refractory solid malignancies.14,15 Drug-related adverse events of special interest (i.e., immune-related events) occurred in about 45 percent of patients, though only 6 percent were grade 3 or 4. Three deaths associated with pneumonitis occurred, in two patients with NSCLC and one with CRC.

Objective responses (PR or CR, by RECIST) to nivolumab were observed in 31 percent of patients with melanoma, 17 percent with NSCLC, and 29 percent with RCC. As in the first-in-human trial, responses were durable: among 65 responding patients, 42 responses (65 percent) lasted >1 year. Stable disease lasting 24 weeks was observed in 7 percent, 10 percent, and 27 percent of patients withmelanoma, NSCLC and RCC, respectively [Figure 1]. Among 27 responding patients who discontinued treatment for reasons other than progression,19 (70%) maintained responses off-drug for 4 months. These findings demonstrate the potential for immune checkpoint blockade therapy to reset the equilibrium between tumor and host in favor of the immune system, leading to long-term, immune-based disease control. Median overall survival for melanoma patients receiving nivolumab therapy was 16.8 months.

The importance of the interaction between PD-1 and its ligands in cancer immunosuppression is further supported by results from a multicenter, Phase I study from Brahmer and colleagues, demonstrating that PD-L1 blockade also has antitumor effects.16 Escalating doses of BMS-936559, a fully human mAb blocking PD-L1, were administered to 207 patients with melanoma, NSCLC, RCC, CRC, or ovarian, pancreatic, gastric, or breast cancer. Grade 3 or 4 drugrelated toxicities occurred in 9 percent of patients, with no drug-related deaths. Overall, 9 of 52 melanoma patients (17 percent) experienced an objective response, five of which lasted =1 year. An additional 14 melanoma patients (27 percent) experienced stable disease that lasted 24 weeks. Objective responses and prolonged stable disease were also observed in patients with NSCLC, RCC, and ovarian cancer. Newer antibodies blocking PD-L1 are in the early stages of clinical testing and have demonstrated tolerable safety profiles as well as antitumor activity [Table 1].

Together, these studies suggest that blocking the PD-1/PD-L1 pathway ay become a significant component of the future management of patients with metastatic melanoma and other solid malignancies, some of which were not considered responsive to immunotherapyuntil recently.

Table 1: Agents Targeting PD-1 and its Ligands in Clinical Trials for Melanoma Patients *Detailed information available at

Research in Progress: Optimizing Immune Checkpoint Therapy

Current clinical studies centered on optimizing immune checkpoint modulation are generally focused on three broad areas: 1) the development of combinatorial therapeutic approaches [Table 2]; 2) the identification of biomarkers that predict and/or evaluate response to therapy; and 3) the implementation of immune-related clinical response criteria that may be more appropriate than conventional metrics for evaluating outcomes to therapy with anti-PD-1 and similar agents.

Table 2: Combinatorial Treatment Trials Using Anti-PD-1/PD-L1 Agents for Patients with Melanoma *Detailed information available at

Combinatorial approaches

Preclinical models that combine immune checkpoint-blocking drugs with other agents have shown antitumor synergy. These data suggest that combinatorial therapies might not only increase the percentage of treatment responders with the tumor types we have mentioned, but could also expand the spectrum of malignancies for which immunotherapy may be effective.17 For instance, one therapeutic strategy combines anti-PD-1 with blockade of additional immune checkpoints that may serve as dominant or co-dominant regulators of immune cell activation. Matsuzaki and colleagues demonstrated that human tumor-derived CD8+ T cells expressed both PD-1 and another immune inhibitory molecule, lymphocyte activation gene-3 (LAG-3). While blockade of either molecule alone was ineffective at restoring T cell function in vitro, combined application of mAbs blocking PD-1 and LAG-3 successfully bolstered T cell proliferation and cytokine production, consistent with findings from animal tumor models.18,19

Combinatorial strategies have also demonstrated success in the clinic. A recent trial from Wolchok and colleagues demonstrated a 40 percent objective response rate among 52 patients with melanoma who received ipilimumab and nivolumab concurrently.20 Other agents being tested in combination with immune checkpoint inhibitors include targeted drugs such as selective BRAF inhibitors, immune-based agents such as vaccines and cytokines, and chemotherapy drugs [Table 2].

Identifying molecular markers

Another active area of research is the evaluation of molecular markers predicting clinical response to PD-1 pathway blockade, such as the expression of PD-L1 in pretreatment tumor biopsies. Preliminary evidence of a correlation between tumor cell surface PD-L1 expression and the likelihood of response to anti-PD-1 therapy was observed in both the first-in-human trial12 and the larger Phase I trial14 of nivolumab. In the latter study, among a subset of 42 patients whose pre-treatment tumors were tested, 25 patients had specimens expressing PD-L1. Of those 25, 11 patients (44 percent) experienced an objective response to PD-1 blockade. Among the 17 patients whose tumors were PDL1 negative, there were no responders.

While these findings require further exploration in Phase II and III trials, defining molecular markers of response is important. It not only allows more elective drug administration, but provides a basis for the rational development of combination therapies based on PD-1 pathway blockade. For example, tumor biopsies obtained from 15 melanoma patients pre- and post-BRAF inhibitor therapy demonstrated a significant increase in tumor-infiltrating lymphocytes after treatment, which correlated with tumor regression and decreased tumor metabolic activity. These findings suggest that an immune response may play a role in the anti-melanoma activity of selective BRAF inhibitors, which could be augmented by the addition of an immune checkpoint inhibitor such as anti-PD-1.21

Needed: new metrics for evaluating response

Patterns of clinical response observed with immune checkpoint-blocking agents may differ from those seen with traditional cytotoxic therapies and small molecule inhibitors in three novel ways.22 First, responses to immune checkpoint-blocking agents can include “pseudo-progression” — apparent disease progression on radiologic imaging, as evidenced by the appearance of new lesions and/or apparent growth of existing lesions — followed by disease regression. Second, as exemplified by the recent studies of nivolumab and anti- PD-L1 (BMS-936559), patients often experience durable responses or stable disease even after therapy has been discontinued. Third, patients may demonstrate a pattern of mixed response, in which some tumors regress while others grow or new lesions appear. These responses are demonstrated in Figure 1, showing changes in disease burden in a cohort of 27 melanoma patients treated with nivolumab. Similar response patterns were observed in trials of ipilimumab, where the overall survival rates of patients with advanced melanoma exceeded the objective response rate. The response characteristics associated with immune checkpoint-blocking drugs present challenges for clinicians and regulators in accurately assessing clinical benefit as well as appropriate treatment management. Looking Forward: From Clinical Trials to Community Practice Twenty-four years passed between the cloning of the gene encoding CTLA-4 and FDA approval of ipilimumab for melanoma therapy. In the interval, the identification of the ligands for CTLA- 4; the demonstration of mechanism of action in preclinical models; the observation that anti-CTLA-4 therapy could mediate tumor regression in early-phase clinical trials; and the demonstration of overall survival benefit in randomized Phase III clinical trials occurred. If anti-PD-1 therapy follows a similar timeline, approval of a drug blocking the PD-1 pathway might be expected within the next few years.23

Figure 1: Response of metastatic melanoma to anti-PD-1 (nivolumab, BMS-936558)administered at 1mg/kg every two weeks. Shown is the percentage change in the sum of the longest diameters of lesions from baseline. Immune-related patterns of response are demonstrated, including long-term stable disease and an overall reduction in tumor burden despite the appearance of new lesions. (From Topalian, et al, Safety, activity, and immune correlates of anti-PD-1 antibody in cancer. New Engl J Med 366; 2443-54. Copyright © 2012 Massachusetts Medical Society. Reprinted with permission.)

Early evidence of the clinical activity of anti-PD-1 and anti-PD-L1 has caused a paradigm shift in the way we treat melanoma, and will likely change our approach to the treatment of other tumor types as well. Given the durability of responses to checkpoint blockade therapy and the ability of these agents to reorient the adaptive immune system away from tolerance and toward immune attack, the continued application of these therapies brings us ever closer to harnessing the immune system’s potential against cancer.


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