Conference Coverage

Immune checkpoint inhibition in SCLC: Modest outcomes, many questions


 

EXPERT ANALYSIS FROM WCLC 2019

– Immune checkpoint inhibitors demonstrate activity in small cell lung cancer (SCLC), but achieving more durable disease control and better survival requires improved understanding of biomarkers and the immune microenvironment.

That was the overarching message from experts speaking at a minisymposium on immunotherapy in SCLC at the World Conference on Lung Cancer.

“None of us are disputing that immunotherapy is clearly active in this space, but I think that we can all agree that the outcomes have been somewhat modest in an unselected population, and there is certainly room to grow,” said Dr. Stephen V. Liu, MD. “Moving forward, while we will look for any advances we can, we also feel strongly that these incremental gains are probably not enough.”

The state of the art

Hints that immunotherapy could be clinically efficacious in SCLC emerged in 2016 when interim findings from the CheckMate 032 study showed that the programmed cell death-1 (PD-1) inhibitor nivolumab, either alone or in combination with the anti-CTLA4 antibody ipilimumab, had efficacy in recurrent SCLC. Efficacy was seen regardless of programmed death-ligand 1 (PD-L1) status, which is “a good thing since PD-L1 is expressed much less frequently in SCLC than in non-SCLC,” Scott J. Antonia, MD, of Duke Cancer Institute, Durham, N.C., who was the first author on that study, said during the symposium.

A particularly encouraging finding was that responders included patients with platinum-refractory SCLC for whom treatments in the relapse setting are lacking, Dr Antonia said.

An exploratory analysis of CheckMate 032 also showed better responses among patients in the highest tumor mutation burden (TMB) tertile, especially in the combination therapy group, leading to the hypothesis-generating finding that TMB may predict response, he said.

Another suggestion of nivolumab’s potential came from the randomized CheckMate 331 study comparing the checkpoint inhibitor with chemotherapy in relapsed SCLC patients. As reported in 2018 at the European Society for Medical Oncology (ESMO), no overall survival (OS) benefit was apparent at 12 months (37% vs. 34%), but a separation of the curves at 36 months suggested a possible OS benefit with nivolumab, Dr. Antonia noted, adding that the difference was “obviously small” and requires “a lot more work related to that.”

Subgroup analyses in that study also were “perhaps revealing” in that patients without liver metastases derived benefit (hazard ratio, 0.75), as did those who were platinum resistant (HR, 0.71), he said.

The phase 1b KEYNOTE-028 study showed that the anti–PD-1 monoclonal antibody pembrolizumab also has activity in PD-L1–positive SCLC patients in the relapsed setting, and pooled data from that study and KEYNOTE-158, which included both PD-L1–positive and –negative patients, showed promising antitumor activity and durable responses with pembrolizumab. The pooled data, as presented at the 2019 annual meeting of the American Association for Cancer Research, showed an objective response rate (ORR) of 19%, including complete and partial response rates of 2% and 17%, respectively.

“And there appears to be, at least preliminarily, some durability to the responses,” he said, noting that 9 of 16 patients experienced at least an 18-month response. “Progression-free survival was 2 months, and overall survival was 7.7 months.”

The IMpower133 study showed significantly longer OS and progression-free survival (PFS) with the addition of atezolizumab to chemotherapy in the first-line treatment of extensive-stage SCLC (HR, 0.70). Some late merging of the survival curves was apparent, but the data haven’t matured.

“Hopefully there will be some evidence of a lifting of the tail of the survival curve with some durability of responsiveness like we see in non–small cell lung cancer,” he said.

When it comes to “making the next leap” toward improved clinical efficacy with immunotherapy for SCLC, “we need to think about three general categories of how it is that tumors evade rejection by the immune system,” he said.

One category involves SCLC patients with an insufficient numbers of T cells generated within the lymphoid compartment; in those patients, an immunotherapeutic approach directed at the tumor microenvironment won’t lead to a response. Another category includes patients who generate enough T cells within the lymphoid compartment but in whom those cells aren’t driven into the tumor parenchyma. The third involves those whose T cells may make it into the tumor parenchyma, but are inhibited in the tumor microenvironment, he explained.

Strategies to increase the number of T cells generated in the lymphoid compartment – such as vaccines, radiation, adoptive cell therapy with chimeric antigen receptors, to name a few – were a focus of research efforts more than a decade ago, but the pendulum swung more toward addressing the tumor microenvironment.

“I think that the pendulum needs to swing back to the middle, and we do need to develop combination immunotherapies paying attention to the lymphoid compartment as well as the tumor microenvironment,” Dr. Antonia said, listing these “guiding principles” for the development of effective SCLC immunotherapy:

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