Project Summary We propose a pilot study to detect response to immunotherapy as early as 3 weeks after initiating therapy, using Doppler ultrasound and contrast-enhanced ultrasound (CEUS) to measure changes in tumor vascularity in patients with metastatic renal cell carcinoma (mRCC). Biomarkers to rapidly detect response to RCC therapy are urgently needed, because not all patients respond to first-line treatment with combined CTLA-4 immune checkpoint inhibitor (ipilimumab) plus anti-PD-1 immune checkpoint inhibitor (nivolumab). Yet, all patients endure the side effects of this combination treatment (fatigue, autoimmune hepatitis/thyroiditis/pneumonitis etc.) while awaiting standard-of-care computed tomography (CT) imaging. Currently, 12 weeks is required to assess response by measuring decreases in tumor diameters with CT because tumor size does not significantly change before 12 weeks of therapy. Rapid detection of response to RCC therapy would minimize use of ineffective drugs and allow patients to discontinue ineffective therapies and continue only effective therapy. We have found that imaging-based measurements of tumor vascularity, such as perfusion CT scans, can detect early response to therapy in mRCC. But these require potentially nephrotoxic iodine contrast agents and visits to the radiology department. We have developed highly sensitive, non-contrast, vascular imaging using advanced power Doppler ultrasound that can be performed at bedside in the oncology clinic to image vessels as small as 1 mm in diameter. Now, we propose to use advanced power Doppler ultrasound for a pilot study to assess changes in tumor vascularity, during routine oncology clinic visits for patients receiving combined therapy, and compare it to conventional power Doppler and contrast-enhanced ultrasound (CEUS, using non-nephrotoxic micro-/nanobubble contrast agents). We hypothesize that changes in tumor vascularity, measured by ultrasound, can detect response to treatment earlier than changes in tumor diameters. We will enroll 30 patients with mRCC, to be evaluated with Doppler ultrasound and CEUS before treatment and after 3 weeks and 6 weeks of combined ipilimumab and nivolumab therapy. Our pilot study aims to determine if 1) Doppler ultrasound and/or CEUS can detect changes as early as 3 and/or 6 weeks after initiating immunotherapy; 2) if changes detected by ultrasound correlate with response measured by standard- of-care CT scan after 12 weeks of therapy; and 3) which ultrasound time point (3 weeks or 6 weeks) better anticipates 12-week results. We will use ultrasound imaging to accelerate detection of response to combined ipilimumab + nivolumab. If successful, we can apply our approach to additional tumors and drugs, as changes in tumor vascularity are a key mechanism of response to most cancer therapies.