Xencor Inc. (XNCR) brought updated Phase 1 results for its bispecific T-cell engager XmAb819 to the ESMO Congress 2026, and the market liked what it saw. The data cover advanced clear cell renal cell carcinoma, or ccRCC, a tough-to-treat kidney cancer. By Monday afternoon, the stock was up more than 20%.

What the Data Showed

The Phase 1 trial evaluated 119 participants across 11 intravenous dose cohorts. Within the 68-patient expansion group, confirmed objective response rates climbed alongside dose increases, rising from 30% at 100 mcg/kg to 45.5% at 400 mcg/kg.

Across those expansion groups, 25 patients achieved confirmed responses, including one complete response and 24 partial responses. At the September data cut, 19 of those responses were still ongoing.

Safety looked manageable too. Patients tolerated XmAb819 well throughout the expansion cohorts, with a low 5.9% treatment discontinuation rate linked to adverse events.

Then came the October follow-up data, which added more color. Patients receiving the 400 mcg/kg target dose achieved an unconfirmed overall response rate of 55% and an 82% progression-free survival rate at six months. The study cohort has not yet reached the median progression-free survival estimate.

The Path to Late-Stage Testing

Xencor has designated 400 mcg/kg as its proposed intravenous dose for late-stage testing and outlined registration-enabling plans for late 2027. The company expects data from at least 20 evaluable patients at this dose level during early 2027 to solidify its selection.

On the regulatory front, Xencor plans to discuss potential accelerated approval pathways with the FDA for patient populations who have exhausted standard clinical options. In pooled analyses, patients with prior exposure to checkpoint inhibitors and multiple VEGFR inhibitors achieved a 45% confirmed response rate.

Further out, Xencor aims to initiate a global randomized trial evaluating XmAb819 against tivozanib to pursue full regulatory approval as a monotherapy. Ongoing sub-studies continue testing subcutaneous delivery and combination therapies for earlier-stage disease.

What the Analysts Think

William Blair weighed in on Monday, and the tone was bullish. "Overall, we believe XmAb819 is showing an unprecedented profile in ccRCC," analyst Matt Phipps wrote.

Phipps also pointed to the ESMO discussant's take on the data. "We continue to believe the discussant of the data at the ESMO conference summed it up appropriately in saying the results presented at the conference are likely the 'floor' in terms of efficacy for XmAb819, and can further improve as management of CRS improves and the therapy advances to less heavily pretreated patient populations."

In other words, the efficacy numbers could get better from here, not worse, as doctors get more comfortable managing cytokine release syndrome and the drug moves earlier in the treatment sequence.

Market Reaction

Xencor shares were up 20.25% at $30.27 at the time of publication on Monday. The stock is trading near its 52-week high of $30.61, according to market data.