Systematic Review of PD-1/PD-L1 Inhibitors in Oncology: From Personalized Medicine to Public Health.

BACKGROUND: To review and summarize all U.S. Food and Drug Administration (FDA) approvals of programmed death (PD)-1 and PD-ligand 1 blocking antibodies (collectively referred to as PD-[L]1 inhibitors) over a 6-year period and corresponding companion/complementary diagnostic assays. MATERIALS AND METHODS: To determine the indications and pivotal trials eligible for inclusion, approval letters and package inserts available on Drugs@FDA were evaluated for approved PD-[L]1 inhibitors to identify all new indications granted from the first approval of a PD-[L]1 inhibitor on September 4, 2014, through September 3, 2020. The corresponding FDA drug and device reviews from the marketing applications for the approved indications were identified through FDA internal records. Two reviewers independently extracted information for the endpoints, efficacy data, basis for approval, type of regulatory approval, and corresponding in vitro diagnostic device test. The results were organized by organ system and tumor type. RESULTS: Of 70 Biologic Licensing Application or supplement approvals that resulted in new indications, 32 (46%) were granted based on response rate (ORR) and durability of response, 26 (37%) on overall survival, 9 (13%) on progression-free survival, 2 (3%) on recurrence-free survival, and 1 (1%) on complete response rate. Most ORR-based approvals were granted under the accelerated approval provisions and were supported with prolonged duration of response. Overall, 21% of approvals were granted with a companion diagnostic. Efficacy results according to tumor type are discussed. CONCLUSION: PD-[L]1 inhibitors are an effective anticancer therapy in a subset of patients. This class of drugs has provided new treatment options for patients with unmet need across a wide variety of cancer types. Yet, the modest response rates in several tumor types signal a lack of understanding of the biology of these diseases. Further preclinical and clinical investigation may be required to identify a more appropriate patient population, particularly as drug development continues and additional treatment alternatives become available. IMPLICATIONS FOR PRACTICE: The number of PD-[L]1 inhibitors in drug development and the associated companion and complementary diagnostics have led to regulatory challenges and questions regarding generalizability of trial results. The interchangeability of PD-L1 immunohistochemical assays between PD-1/PD-L1 drugs is unclear. Furthermore, robust responses in some patients with low levels of PD-L1 expression have limited the use of PD-L1 as a predictive biomarker across all cancers, particularly in the setting of diseases with few alternative treatment options. This review summarizes the biomarker thresholds and assays approved as complementary and companion diagnostics and provides regulatory perspective on the role of biomarkers in oncology drug development.

FDA Approval Summary: Pembrolizumab for Recurrent Locally Advanced or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma Expressing PD-L1.

On September 22, 2017, the U.S. Food and Drug Administration (FDA) granted accelerated approval for pembrolizumab (Keytruda, Merck & Co., Inc., Whitehouse Station, NJ) for the treatment of patients with recurrent, locally advanced or metastatic, gastric or gastroesophageal junction (GEJ) adenocarcinoma with disease progression on or after two or more systemic therapies, including fluoropyrimidine- and platinum-containing chemotherapy and, if appropriate, HER2/neu-targeted therapy, and whose tumors express programmed death-ligand 1 (PD-L1), as determined by an FDA-approved test. Approval was based on demonstration of durable overall response rate (ORR) in a multicenter, open-label, multicohort trial (KEYNOTE-059/Cohort 1) that enrolled 259 patients with locally advanced or metastatic gastric or GEJ adenocarcinoma. Among the 55% (n = 143) of patients whose tumors expressed PD-L1 based on a combined positive score ≥1 and either were microsatellite stable or had undetermined microsatellite instability or mismatch repair status, the confirmed ORR as determined by blinded independent central review was 13.3% (95% CI, 8.2-20.0); 1.4% had complete responses. Response durations ranged from 2.8+ to 19.4+ months; 11 patients (58%) had response durations of 6 months or longer, and 5 patients (26%) had response durations of 12 months or longer. The most common (≥20%) adverse reactions of pembrolizumab observed in KEYNOTE-059/Cohort 1 were fatigue, decreased appetite, nausea, and constipation. The most frequent (≥2%) serious adverse drug reactions were pleural effusion, pneumonia, dyspnea, pulmonary embolism, and pneumonitis. Pembrolizumab was approved concurrently with the PD-L1 immunohistochemistry 22C3 pharmDx test (Dako, Agilent, Santa Clara, CA) for selection of patients with gastric cancer for treatment with pembrolizumab based on PD-L1 tumor expression. IMPLICATIONS FOR PRACTICE: This report presents key information on the basis for Food and Drug Administration approval of pembrolizumab for the treatment of patients with locally advanced or metastatic gastric or GEJ adenocarcinoma whose tumors express PD-L1. The report discusses the basis for limiting the indication to patients with PD-L1-expressing tumors and the basis for recommending that PD-L1 status be assessed using a fresh tumor specimen if PD-L1 expression is not detected in an archival gastric or GEJ cancer specimen.

Anti-semaphorin 4D immunotherapy ameliorates neuropathology and some cognitive impairment in the YAC128 mouse model of Huntington disease.

Huntington disease (HD) is an inherited, fatal neurodegenerative disease with no disease-modifying therapy currently available. In addition to characteristic motor deficits and atrophy of the caudate nucleus, signature hallmarks of HD include behavioral abnormalities, immune activation, and cortical and white matter loss. The identification and validation of novel therapeutic targets that contribute to these degenerative cellular processes may lead to new interventions that slow or even halt the course of this insidious disease. Semaphorin 4D (SEMA4D) is a transmembrane signaling molecule that modulates a variety of processes central to neuroinflammation and neurodegeneration including glial cell activation, neuronal growth cone collapse and apoptosis of neural precursors, as well as inhibition of oligodendrocyte migration, differentiation and process formation. Therefore, inhibition of SEMA4D signaling could reduce CNS inflammation, increase neuronal outgrowth and enhance oligodendrocyte maturation, which may be of therapeutic benefit in the treatment of several neurodegenerative diseases, including HD. To that end, we evaluated the preclinical therapeutic efficacy of an anti-SEMA4D monoclonal antibody, which prevents the interaction between SEMA4D and its receptors, in the YAC128 transgenic HD mouse model. Anti-SEMA4D treatment ameliorated neuropathological signatures, including striatal atrophy, cortical atrophy, and corpus callosum atrophy and prevented testicular degeneration in YAC128 mice. In parallel, a subset of behavioral symptoms was improved in anti-SEMA4D treated YAC128 mice, including reduced anxiety-like behavior and rescue of cognitive deficits. There was, however, no discernible effect on motor deficits. The preservation of brain gray and white matter and improvement in behavioral measures in YAC128 mice treated with anti-SEMA4D suggest that this approach could represent a viable therapeutic strategy for the treatment of HD. Importantly, this work provides in vivo demonstration that inhibition of pathways initiated by SEMA4D constitutes a novel approach to moderation of neurodegeneration.

SEMA4D compromises blood-brain barrier, activates microglia, and inhibits remyelination in neurodegenerative disease.

Multiple sclerosis (MS) is a chronic neuroinflammatory disease characterized by immune cell infiltration of CNS, blood-brain barrier (BBB) breakdown, localized myelin destruction, and progressive neuronal degeneration. There exists a significant need to identify novel therapeutic targets and strategies that effectively and safely disrupt and even reverse disease pathophysiology. Signaling cascades initiated by semaphorin 4D (SEMA4D) induce glial activation, neuronal process collapse, inhibit migration and differentiation of oligodendrocyte precursor cells (OPCs), and disrupt endothelial tight junctions forming the BBB. To target SEMA4D, we generated a monoclonal antibody that recognizes mouse, rat, monkey and human SEMA4D with high affinity and blocks interaction between SEMA4D and its cognate receptors. In vitro, anti-SEMA4D reverses the inhibitory effects of recombinant SEMA4D on OPC survival and differentiation. In vivo, anti-SEMA4D significantly attenuates experimental autoimmune encephalomyelitis in multiple rodent models by preserving BBB integrity and axonal myelination and can be shown to promote migration of OPC to the site of lesions and improve myelin status following chemically-induced demyelination. Our study underscores SEMA4D as a key factor in CNS disease and supports the further development of antibody-based inhibition of SEMA4D as a novel therapeutic strategy for MS and other neurologic diseases with evidence of demyelination and/or compromise to the neurovascular unit.

Translating Immuno-oncology Biomarkers to Diagnostic Tests: A Regulatory Perspective.

The rapid development of effective immunotherapy using immune-checkpoint inhibitors (ICIs) against many different cancer types opened a new front in cancer treatment. Immunotherapy is undoubtedly one of the biggest breakthroughs in cancer therapy within the past decade. The identification of predictive biomarkers to select the patients most likely to respond to ICI monotherapies or emerging combination therapies remains one of the major unmet needs for the oncology community.This chapter provides an overview of existing and emerging biomarkers associated with ICI response. Additionally, using several case studies of FDA approved or authorized in vitro diagnostic oncology devices, this chapter also provides an overview of analytical and clinical validation considerations of diagnostic tests for immuno-oncology biomarkers.

Saccharomyces cerevisiae flap endonuclease 1 uses flap equilibration to maintain triplet repeat stability.

Flap endonuclease 1 (FEN1) is a central component of Okazaki fragment maturation in eukaryotes. Genetic analysis of Saccharomyces cerevisiae FEN1 (RAD27) also reveals its important role in preventing trinucleotide repeat (TNR) expansion. In humans such expansion is associated with neurodegenerative diseases. In vitro, FEN1 can inhibit TNR expansion by employing its endonuclease activity to compete with DNA ligase I. Here we employed two yeast FEN1 nuclease mutants, rad27-G67S and rad27-G240D, to further define the mechanism by which FEN1 prevents TNR expansion. Using a yeast artificial chromosome system that can detect both TNR instability and fragility, we demonstrate that the G240D but not the G67S mutation increases both the expansion and fragility of a CTG tract in vivo. In vitro, the G240D nuclease is proficient in cleaving a fixed nonrepeat double flap; however, it exhibits severely impaired cleavage of both nonrepeat and CTG-containing equilibrating flaps. In contrast, wild-type FEN1 and the G67S mutant exhibit more efficient cleavage on an equilibrating flap than on a fixed CTG flap. The degree of TNR expansion and the amount of chromosome fragility observed in the mutant strains correlate with the severity of defective flap cleavage in vitro. We present a model to explain how flap equilibration and the unique tracking mechanism of FEN1 can collaborate to remove TNR flaps and prevent repeat expansion.

Antibody Blockade of Semaphorin 4D Promotes Immune Infiltration into Tumor and Enhances Response to Other Immunomodulatory Therapies.

Semaphorin 4D (SEMA4D, CD100) and its receptor plexin-B1 (PLXNB1) are broadly expressed in murine and human tumors, and their expression has been shown to correlate with invasive disease in several human tumors. SEMA4D normally functions to regulate the motility and differentiation of multiple cell types, including those of the immune, vascular, and nervous systems. In the setting of cancer, SEMA4D-PLXNB1 interactions have been reported to affect vascular stabilization and transactivation of ERBB2, but effects on immune-cell trafficking in the tumor microenvironment (TME) have not been investigated. We describe a novel immunomodulatory function of SEMA4D, whereby strong expression of SEMA4D at the invasive margins of actively growing tumors influences the infiltration and distribution of leukocytes in the TME. Antibody neutralization of SEMA4D disrupts this gradient of expression, enhances recruitment of activated monocytes and lymphocytes into the tumor, and shifts the balance of cells and cytokines toward a proinflammatory and antitumor milieu within the TME. This orchestrated change in the tumor architecture was associated with durable tumor rejection in murine Colon26 and ERBB2(+) mammary carcinoma models. The immunomodulatory activity of anti-SEMA4D antibody can be enhanced by combination with other immunotherapies, including immune checkpoint inhibition and chemotherapy. Strikingly, the combination of anti-SEMA4D antibody with antibody to CTLA-4 acts synergistically to promote complete tumor rejection and survival. Inhibition of SEMA4D represents a novel mechanism and therapeutic strategy to promote functional immune infiltration into the TME and inhibit tumor progression.

Analysis of DNA replication intermediates suggests mechanisms of repeat sequence expansion.

We previously developed a system to investigate the mechanism of repeat sequence expansion during eukaryotic Okazaki fragment processing. Upstream and downstream primers were annealed to a complementary template to overlap across a CAG repeat region. Annealing by the competing primers lead to structural intermediates that ligated to expand the repeat segment. When an equal number of repeats overlapped on the upstream and downstream primers, a 2-fold expansion was expected, but no expansion occurred. We show here that such substrates do not expand irrespective of their repeat length. To reveal mechanism, we tested different hairpin loop intermediates expected to form and facilitate ligation. Substrates configured to form large loops in either the upstream or downstream primer alone allowed expansion. Large or small fixed position single loops allowed expansion when located at least six nucleotides up- or downstream of the nick. Fixed loops in both primers, simulating a double loop intermediate, allowed expansion as long as each loop was nine nucleotides from the nick. Thus, neither the double loop configuration required to form with equal length overlaps nor the large single loop configuration are fundamental structural impediments to expansion. We propose a model for the expansion mechanism based on the relative stabilities of single loop, double loop, hairpin, and flap intermediates that is consistent with the observed expansion efficiency of equal and unequal overlap substrates. The model suggests that the equilibrium concentration of double loop intermediates is so vanishingly small that they are not likely contributors to sequence expansion.

On the roles of Saccharomyces cerevisiae Dna2p and Flap endonuclease 1 in Okazaki fragment processing.

Short DNA segments designated Okazaki fragments are intermediates in eukaryotic DNA replication. Each contains an initiator RNA/DNA primer (iRNA/DNA), which is converted into a 5'-flap and then removed prior to fragment joining. In one model for this process, the flap endonuclease 1 (FEN1) removes the iRNA. In the other, the single-stranded binding protein, replication protein A (RPA), coats the flap, inhibits FEN1, but stimulates cleavage by the Dna2p helicase/nuclease. RPA dissociates from the resultant short flap, allowing FEN1 cleavage. To determine the most likely process, we analyzed cleavage of short and long 5'-flaps. FEN1 cleaves 10-nucleotide fixed or equilibrating flaps in an efficient reaction, insensitive to even high levels of RPA or Dna2p. On 30-nucleotide fixed or equilibrating flaps, RPA partially inhibits FEN1. CTG flaps can form foldback structures and were inhibitory to both nucleases, however, addition of a dT(12) to the 5'-end of a CTG flap allowed Dna2p cleavage. The presence of high Dna2p activity, under reaction conditions favoring helicase activity, substantially stimulated FEN1 cleavage of tailed-foldback flaps and also 30-nucleotide unstructured flaps. Our results suggest Dna2p is not used for processing of most flaps. However, Dna2p has a role in a pathway for processing structured flaps, in which it aids FEN1 using both its nuclease and helicase activities.

DNA ligase I competes with FEN1 to expand repetitive DNA sequences in vitro.

Repeat sequences in various genomes undergo expansion by poorly understood mechanisms. By using an oligonucleotide system containing such repeats, we recapitulated the last steps in Okazaki fragment processing, which have been implicated in sequence expansion. A template containing either triplet or tandem repeats was annealed to a downstream primer containing complementary repeats at its 5'-end. Overlapping upstream primers, designed to strand-displace varying numbers of repeats in the downstream primer, were annealed. Human DNA ligase I joined overlapping segments of repeats generating an expansion product from the primer strands. Joining efficiency decreased with repeat length. Flap endonuclease 1 (FEN1) cleaved the displaced downstream strand and together with DNA ligase I produced non-expanded products. However, both expanded and non-expanded products formed irrespective of relative nuclease and ligase concentrations tested or enzyme addition order, suggesting the pre-existence and persistence of intermediates leading to both outcomes. FEN1 activity decreased with the length of repeat segment displaced presumably because the flap forms structures that inhibit cleavage. Increased MgCl(2) disfavored ligation of substrate intermediates that result in expansion products. Examination of expansion in vitro enables dissection of substrate and replication enzyme dynamics on repeat sequences.