Phase II study of maintenance pembrolizumab (pembro) in extensive stage small cell lung cancer (ES-SCLC) patients (pts)

8504 Background: The median progression free survival (PFS) and overall survival (OS) following initial chemotherapy in ES-SCLC pts are 2 and 7 months, respectively (Ready N, J Clin Oncol 2015). We evaluated the benefits of maintenance pembro in ES-SCLC pts who had response/stable disease after 4-6 cycles of platinum/etoposide. Methods: Pts were required to begin pembro within 8 weeks of completion of chemotherapy, with restaging scans no more than 3 weeks prior to start of pembro. Prophylactic cranial radiation was permitted. Pts were treated with pembro 200 mg I.V. every 3 weeks for a maximum of 2 years. Disease assessment was done every 2 cycles for the first 6 cycles and then as per investigator discretion. Primary end point of the study was PFS. PFS according to immune related response criteria (irPFS) and OS were also assessed. Tumor tissue was analyzed for PD-L1 expression by the DAKO 22C3 antibody. Any level of expression was considered as positive for PDL1. Blood for circulating tumor cells (CTCs) was collected prior to first, second and third cycle of pembro. Results: Of the 45 pts enrolled, 55% were males and 22% had brain metastases. Median age was 66 years. The median time from end of chemotherapy to start of pembro was 5 weeks. Median number of pembro cycles was 4 (1-20 cycles). 35 pts had measurable disease at study entry. The disease control rate with pembro was 42% (1 CR, 3 PR, 15 SD). At a median follow up of 6 months, the median PFS was 1.4 months (90% CI-1.3-4.0) and the irPFS was 4.7 months (90% CI- 1.8-6.7). The median OS was 9.2 months (90% CI-6.1-15.2). 11 pts are still on therapy (3-20 cycles). The median CTC prior to pembro was 1 (0-256, n=37 pts). Each unit increase in baseline CTC correlated with worse PFS (p = 0.052; adjusted for brain mets, age and sex). PDL1 could be assessed in 35 pts and was positive in 1 pt. Most common adverse events were fatigue, nausea, cough and dyspnea. One pt developed atrio-ventricular conduction block and 1 pt type 1 diabetes. Conclusions: Maintenance pembro did not improve PFS in these patients but favorable OS suggests that some SCLC patients can benefit from maintenance pembro. Biomarkers to identify patients most likely to benefit from pembro need to be defined. Clinical trial information: NCT02359019.