by ecancer reporter Janet Fricker

Five studies reveal a growing list of cancers responding to checkpoint blockade and start to define the characteristics of patients responding to therapy.

The studies, which range from phase 1 trials to experimental work in mice, open the way for using predictive markers.

Two of the studies furthermore suggest that cancer mutations that do not contribute to cancer initiation and progression may have an important  role to play in tumour immunity.

In an accompanying article, Jedd Wolchok and Timothy A Chan, from Memorial Sloan Kettering Cancer Center, New York, wrote, “These five papers, together with other recent studies, support the hypothesis that immune responses to tumour-specific mutations are central to both natural anti-tumour immunity and to the anti-tumour activity generated by checkpoint blockade therapy.”

Checkpoint blockade, which ‘reawakens’ the immune system’s response to tumours, involves use of antibodies to block the immune-inhibitory pathways switched on by cancer cells.

Systems evolved to prevent hyperactivity of T cells and autoimmunity have been hijacked by cancer cells to evade immune control and elimination.

CTLA-4 and PD-1 are two cell-surface receptors that, when bound by their ligands, trigger inhibitory pathways and dampen T-cell activity.

In the case of the PD-1 pathway expression of ligands, such as the programmed death-ligand 1 (PD-L1) on tumour cells, can directly lead to the death of T cells expressing PD-1.

Although antibodies that block CTLA-4 (ipilimumab) and PD-1 (pembrolizumab and nivolumab) have been approved to treat patients with melanoma and renal-cell carcinoma, little in the way of efficacy has been noted for other tumours.

In the first letter Thomas Powles and colleagues, from Queen Mary University of London, report on a phase 1 study testing MPDL3280A, a high-affinity engineered human monoclonal antibody inhibiting interaction of PD-L1 with PD-1 and B7.1 (both negative regulators of T-lymphocyte activation), in metastatic urothelial bladder cancer (UBC).

“There is an urgent need for efficacious and well-tolerated therapies in metastatic UBC, as even first-line chemotherapy is poorly tolerated in a large proportion of individuals,” wrote the authors.

A hallmark of UBC, they explained, is the presence of high rates of somatic mutations, which may enhance the ability of host immune systems to recognise tumour cells as foreign owing to increased numbers of antigens.

Such cancers may elude immune surveillance and eradication through the expression of PD-L1 in tumour microenvironments.

Between March 2013 and January 2014, 68 UBC patients who had not responded previously to platinum based chemotherapy were treated with MPDL3280A.

Using immunohistochemistry (IHC) techniques subjects were tested for PD-L1 expression, and 30 found to have PD-L1 positive tumours.

After six weeks of treatment results showed tumours had shrunk for 43% (13/30) of PD-L1 positive patients compared to 11% of PD-L1 negative patients.

Complete response (disappearance of visible tumours on scans) was demonstrated for 7% (2/30) PD-L1 positive patients.

Treatment-related Grade 3 adverse events (AEs) occurred in 4% of patients, and there were no life-threatening or fatal grade 4-5 treatment related AEs.

“This study provides striking preliminary efficacy and safety results with MPDL3280A for the treatment of UBC. Additionally, our data demonstrate the potential of immune cell PD–L1 levels as a biomarker,” wrote the authors.

Further work to evaluate the frequency of somatic mutations at baseline, they add, will help elucidate the relationship between mutational frequency and response to PD-L1 blockade.

In the second letter Roy Herbst and colleagues, from Yale School of Medicine, evaluated 277 patients with advanced incurable melanoma or cancers of the lung, kidney, colon, GI tract or head and neck treated with intravenous MPDL3280A every three weeks.

The trial incorporated serial biopsies of patients before and during treatment to identify tumour profiles that predict response to treatment.

Results showed that of the 175 evaluable patients, 21% (11 of 53) showed partial or complete response to MPDL3280A, with some responses being rapid and durable.

Across all tumour types, 46% of patients with high PD-L1 expression on non-tumour cells showed a partial or complete response.

“We knew that the expression of PD-L1 in tumour cells is critical in blocking the immune system so we were intrigued to find that the expression of PD-L1 in non-tumour cells such as macrophages also predicted drug response,” said Herbst in a press release.

“Together, these data suggest that MPDL3280A is most effective in patients in which pre-existing immunity is suppressed by PD-L1, and is re-invigorated on antibody treatment,” wrote the authors.

Understanding the profiles of non-responders, they add, will provide even more valuable information, possibly revealing the diversity of mechanisms controlling anti tumour immunity.

In a third letter Antoni Ribas and colleagues, from the University of California, Los Angeles, identified biomarkers to predict treatment response by analysing tumour tissue samples taken from 46 patients with metastatic melanoma obtained before and during treatment with pembrolizumab (an antibody blocking the PD-1 receptor).

To examine CD8 expression (a T cell subset that directly kills target cells) the team undertook qualitative and quantitative IHC analysis both in the invasive tumour margin and inside the tumour parenchyma.

Patients who responded to therapy, they found, had the highest number of CD8 T-cells at the invasive margins of tumours, while those who progressed had the lowest numbers.

Based on CD8 expression at the invasive margin the team developed a predictive model using multivariate analysis that they were able to validate in an independent cohort of 15 patients.

“Releasing the PD-1 immune checkpoint results in clinically relevant anti-tumour activity when there is a greater density of pre-existing tumour-antigen-restricted CD8 T cells that are negatively regulated by PD-1/PD-L1 interactions,” wrote the authors.

Two further letters, by Lelia Delamarre and Robert Schreiber, suggest that ‘passenger’ mutations – cancer-cell mutations that do not directly contribute to cancer initiation and progression – play an important role in tumour immunity.

“Although it is increasingly evident that the new antigens generated by such mutations are targeted by antitumour T cells, identifying which of these neo-antigens are functionally important has been a challenge,” wrote Wolchok and Chan.

Delamarre and colleagues, from Genentech, South San Francisco, sequenced the exomes (protein coding regions of the genome) of two mouse tumour-cell lines and compared these with the reference mouse exome to predict candidate neo-antigens in tumour cells.

The team went on to identify which of the neo-antigens could potentially elicit immune responses by isolating those that bind to major histocompatibility complex (MHC) proteins, and then analysing bound peptides by mass spectrometry.

Schreiber and colleagues, from Washington University School of Medicine, St Louis, showed that tumours that have become resistant to immune-mediated rejection have lost the mutant spectrin-Beta 2 protein.

They identified two mutations in the Alg8 and Lama4 genes that created neo-antigens mediating these effects.

Vaccinating mice with these antigens, they found, induced tumour rejection at a level comparable to that of checkpoint blockade therapy.

“These results reveal that tumour-specific mutant antigens are not only important targets of checkpoint blockade therapy, but they can also be used to develop personalised cancer-specific vaccines and to probe the mechanistic underpinnings of different checkpoint blockade treatments,” wrote the authors.

Source: Nature