Patients who had tried everything
The people in this trial were not newly diagnosed. They had advanced clear cell kidney cancer that had already pushed through multiple rounds of treatment. That is the group where new drugs usually disappoint, because the easy wins are long gone.
So the headline from the Phase 1 ARC-20 trial, published in Nature on July 1, 2026, is a genuine surprise. An experimental once-daily pill called casdatifan shrank tumors in about a third of these heavily pre-treated patients, and the shrinkage tended to last. It is early data with real caveats, but for a population this far along, “a third responded and kept responding” is not a small thing.
The numbers, in plain terms
Across the 127 patients, the trial reports:
- 35% saw their tumors shrink in the subgroup on the recommended dose. A confirmed objective response, in trial language, means measurable shrinkage that held up on a follow-up scan.
- 31% shrank across the whole trial, including patients on other dose levels.
- More than 80% had their disease controlled, meaning it either shrank or at least stopped growing.
- About 3 months was the typical wait for a first response, and tumors kept shrinking past that point instead of stalling.
The team was led by Toni K. Choueiri, Jamie Merchan, and Amita Patnaik, across several institutions. The trial is registered as NCT05536141.
Why blocking one protein matters here
Clear cell renal cell carcinoma is about 75% of kidney cancers, and a lot of it runs on a single faulty switch. Normally, cells read oxygen levels and adjust their growth. In these tumors, a broken gene (the VHL tumor-suppressor) lets a protein called HIF-2a pile up unchecked.
Think of HIF-2a as an accelerator jammed to the floor. It tells the tumor to build new blood vessels, tough out low-oxygen conditions, and spread. Existing drugs come at kidney cancer from other angles, immune checkpoint inhibitors and vessel-blocking VEGF-TKIs, and they help, but tumors often learn to resist. Casdatifan takes a more direct route: it binds HIF-2a itself and tries to switch the accelerator off. It was built to soak into tumor tissue better than the first drug of this type, belzutifan (sold as Welireg).
A blood test that showed the drug was landing
One neat detail: the researchers could tell the drug was actually hitting its target by watching a hormone in the blood called erythropoietin, which HIF-2a controls. When casdatifan works, that hormone drops.
And the drop tracked with benefit. Patients whose erythropoietin fell the most tended to have better tumor responses, slower progression, and longer time before the cancer advanced. Tumors that started with more HIF-2a activity were also more likely to shrink. It is the kind of biological breadcrumb trail that suggests the drug is working the way its designers hoped, not by accident. ARC-20 built on an earlier study in healthy volunteers (ARC-14) that had worked out the basic dosing.
Safety, and what the investigators said
On safety, casdatifan looked manageable. The most common problems were anemia, fatigue, and low blood oxygen, all handled with supportive care and dose adjustments. Few patients quit over side effects, and no deaths were tied to the treatment.
Dr. Jamie Merchan, a study co-author and director of the Phase 1 program at the University of Miami’s Sylvester Comprehensive Cancer Center, put it plainly: “These results are notable in a heavily pretreated population.” He added that the findings “are encouraging and suggest we may be able to better understand which patients benefit from targeting this pathway,” calling it “an important step in understanding how tumor biology may help guide treatment decisions.”
Read it as a lead, not a verdict
The excitement is fair, but so is the fine print, and it is worth being straight about it.
This was a Phase 1, single-arm study. Everyone got casdatifan and there was no control group, which means nobody can yet say how it stacks up head-to-head against standard care or against belzutifan. Comparing across separate trials is unreliable because the patients and designs differ. The report also did not name the exact milligram dose in the recommended-dose group.
And casdatifan is still investigational. No regulator has approved it, and it is not in any treatment guideline. The next round of research is already pointed at combinations, testing casdatifan alongside other drugs. For now, the honest summary is that a stubborn cancer met a pill that a third of very sick patients responded to, and that is a reason to keep going, not a reason to claim victory.
Related Coverage
- Why Kidney Cancer Learns to Shrug Off Its Own Drugs
- The Cancer Cell Has a Tow Truck for Stuck Drugs. Scientists Just Found It.
Sources:
- Technology Networks, “Phase 1 Trial Results Highlight Casdatifan’s Potential in Kidney Cancer” (https://www.technologynetworks.com/tn/news/phase-1-trial-results-highlight-casdatifans-potential-in-kidney-cancer-415111)
- EurekAlert!, “Study: Casdatifan demonstrates durable tumor responses in heavily pretreated patients with advanced kidney cancer” (https://www.eurekalert.org/news-releases/1137816)
- Nature, “Casdatifan shows durable response linked to HIF-2a biology in kidney cancer” (https://www.nature.com/articles/s41586-026-10718-x)
Disclaimer: This article is for general information only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider about any medical condition or before making health decisions.