Addition of Metastasis-Directed Therapy to Standard of Care for Oligometastatic Solid Tumors: Primary Analysis of All Tumor-Histology Baskets of the Phase II Randomized EXTEND Trial

Purpose We tested the hypothesis that adding metastasis-directed therapy (MDT) to standard of care (SOC) systemic therapy improves progression-free survival (PFS) among patients with oligometastatic disease. Methods EXTEND was a multicenter randomized phase II trial. Patients with 1-5 metastases were randomized to MDT+SOC vs SOC in 1 of 6 baskets (breast, pancreas, kidney, two prostate baskets, and an “Other” basket) with basket-specific stratification and powering. PFS, the primary endpoint, was pre-specified in the per-protocol set within each basket, across all baskets, and across all baskets excluding the prostate baskets. Exploratory endpoints included circulating tumor DNA (ctDNA) and immune profiling. Results From 2018 through 2023, 521 patients were screened, 350 were randomized, and 334 were analyzed per protocol (MDT+SOC, n=166; SOC, n=168). Radiotherapy was used as MDT for 98% of metastases (370/379). Overall, after median follow-up of 53 months, PFS was improved with MDT+SOC (HR 0.54, 95% CI 0.41 to 0.72, p < 0.001). Similarly, PFS was improved when excluding the prostate baskets (HR, 0.60; 95% CI: 0.40 to 0.89). Within each basket, PFS superiority was identified for the pancreas, prostate, and “Other” baskets, whereas the breast and kidney baskets were inconclusive. At enrollment, detectable ctDNA correlated with shorter PFS and survival; by contrast, ctDNA clearance 3-months post-enrollment correlated with improved survival. MDT+SOC-induced systemic immune activation was most pronounced among baskets demonstrating PFS superiority. Conclusion The phase II EXTEND trial supports the addition of MDT to SOC for oligometastatic disease. Histology-specific efficacy signals were identified for phase III testing. Translational insights suggest the potential for optimizing the definition of oligometastasis using ctDNA, and point to systemic immune responses as a possible mechanism of benefit from MDT.