A phase II trial of stereotactic radiation therapy and in situ oncolytic virus therapy in metastatic triple-negative breast cancer (mTNBC) patients followed by pembrolizumab (STOMP).

1079 Background: Pembrolizumab, stereotactic body radiotherapy (SBRT), and viral vector-based gene therapy such as adenovirus-mediated expression of herpes simplex virus thymidine kinase (ADV/HSV-tk) plus ganciclovir have each shown antitumor immune activity. The combination of those modalities may represent a window of opportunity to enhance pembrolizumab efficacy in mTNBC patients. Methods: In this single-arm, open-label phase II trial, mTNBC patients were treated with in situ oncolytic ADV/HSV-tk (5 x 10 11 vp) intratumoral injection, followed by SBRT to the injected tumor site, then pembrolizumab until progression or intolerable toxicity. Response was assessed in non-irradiated metastatic sites. The primary end point was clinical benefit rate (CR, PR and SD per RECIST version1.1). Secondary endpoints included duration on treatment (DoT) and safety. Immune correlative analysis with peripheral blood CYTOF (D1 baseline, D7 and D38), and tissue imaging mass cytometry (IMC) with 35 cell surface markers was performed on paired biopsies (D1 baseline and D17). Results: 28 mTNBC patients were enrolled; 18 (64.3%) had PD-L1 negative tumors. Median age was 54 years (range 34-78). Median prior lines of chemotherapy were 2 (range 0-6), with 8 (28.6%) having received >3. 3 (10.7%) had brain metastases. Clinical benefit was seen in 6 (21.43%) patients; 2 CR (7.1%),1 PR (3.57%) and 3 SD (10.7%). Patients who had clinical benefits had durable responses, with median DoT of 383 days (range 195-1195). One patient who had CR, but discontinued pembrolizumab due to Grade 3 pneumonitis, has remained disease free without any systemic therapy for 39 months. The combination was well tolerated; 9 (32.1%) patients had Grade 3- 4 AEs. Pre- and post-therapy CYTOF analysis showed significant association between immune biomarkers with clinical responses (AUC 0.75, Cohen’s Kappa 0.364). Tumor PDL1 was independently associated with response (AUC 0.70, Cohen’s Kappa 0.347); AUC for PDL1 together with immune biomarkers is 0.85. Conclusions: ADV/HSV-tk gene therapy followed by radiation therapy and then pembrolizumab is a well-tolerated promising treatment in heavily pretreated mTNBC patients. Early detection of increased effector and effector memory CD8 T cells and nonclassical monocytes correlates with response and non-response respectively. Clinical trial information: NCT03295916 .[Table: see text]