I analyze the promised efficacy of Pembrolizumab, an immunotherapy regime under clinical trial for patients with metastatic colorectal cancer. Drawing on anthropological fieldwork with patients and health professionals in a gastrointestinal cancer clinic in London, UK, I tease out the dynamics through which scientists and clinicians assemble personalized technologies to halt cancer growth in patients' bodies; what patients undergo in order to participate in these innovations; and the constraints that restrict the efficacy of these treatments. Beyond examining the treatment possibilities that clinical trials offer, I illuminate some of the gaps made visible when personalization happens from below.