Summit Therapeutics Inc. (Summit Therapeutics (SMMT)) shares are down on Monday, even after the company released some pretty impressive clinical trial results over the weekend. The stock dropped about 10.7% to $15.66 as of Monday afternoon, which might seem counterintuitive given the news. But markets can be funny that way — sometimes good news is already priced in, or investors are taking profits after a run-up.

The big news came from the Phase 3 HARMONi-6 trial, which tested ivonescimab in combination with chemotherapy against BeOne Medicines Inc.'s (BeOne Medicines (ONC)) Tevimbra (tislelizumab) plus chemo in patients with non-small cell lung cancer. The results showed a 34% reduction in the risk of death — a hazard ratio of 0.66 — which is a pretty big deal in oncology. The 24-month overall survival rate was 64.7% for the ivonescimab group versus 48.6% for the tislelizumab group. That's a meaningful gap, and the benefit was seen across subgroups, including patients with PD-L1-negative tumors.

The findings were published in The Lancet, which adds some extra credibility. The median follow-up was 21.4 months, so the data is reasonably mature.

But Summit didn't stop there. On Saturday, the company also presented results from a Phase 2 study of ivonescimab in first-line metastatic colorectal cancer (mCRC). This was a small study — 48 evaluable patients — but the numbers look encouraging. The objective response rate was 70.8% when ivonescimab was combined with standard chemotherapy (mFOLFOX6). That's higher than what you'd typically see with standard-of-care regimens like bevacizumab plus FOLFOX.

The higher dose of ivonescimab (20 mg/kg) seemed to produce more durable responses. At 9 months, 79.1% of patients on the higher dose still had a response, compared to 41.5% on the lower dose. Progression-free survival data is still early, but the 9-month PFS rate for the 20 mg/kg group was 76.1%.

So why is the stock down? It could be a classic "buy the rumor, sell the news" situation. Summit shares had rallied ahead of the data release, and some investors may have decided to cash out. Or maybe the market was hoping for even more dramatic results. Either way, the data itself is solid and positions ivonescimab as a potential contender in both lung and colorectal cancers.

Summit is co-developing ivonescimab with Akeso, and the drug is also being studied in other indications. The lung cancer data alone could support regulatory filings, and the colorectal cancer data adds another arrow to the quiver. For now, the stock's dip might be a buying opportunity for those who believe in the drug's potential — but as always, clinical development is a long and uncertain road.