Acrivon Therapeutics Inc. (ACRV) delivered a mixed bag on Thursday: solid clinical trial data that would normally send biotech investors reaching for champagne, paired with a stock price that decided to take the stairs down instead. Shares tumbled 28.81% to $2.10, proving once again that good news in oncology trials doesn't always translate to good days for shareholders.

Strong Response Rates in Endometrial Cancer

The company shared updates from its Phase 2b ACR-368 program targeting endometrial cancer, and the numbers look genuinely encouraging. In Arm 1 of the registrational-intent trial, the overall response rate hit 39%. That's the percentage of patients whose tumors responded meaningfully to treatment. Among patients who'd already been through two or more prior therapies—the toughest crowd, essentially—the response rate climbed to 44%.

Here's where it gets interesting: Acrivon noticed that patients with serous endometrial cancer, a particularly aggressive subtype also known as uterine serous carcinoma, were responding significantly better than others. So they made a strategic pivot. Arm 3 of the trial will now focus exclusively on serous cancer patients who've received two or fewer prior treatments.

The data justifying that decision is pretty compelling. Among biomarker-positive (BM+) subjects with serous endometrial cancer, the confirmed response rate reached 67% in a small group of 12 patients. Looking at a broader set of 23 serous cancer patients (both biomarker-positive and biomarker-negative), the response rate was 52%, compared to just 22% in the 37 patients with non-serous endometrial cancer who received up to two prior therapies.

The company plans to kick off patient enrollment in Europe during the first quarter of 2026, while continuing to enroll at already-active U.S. sites.

Early Lung Cancer Program Shows Promise

Acrivon also shared initial data from its ACR-2316 Phase 1 study, which is still in the dose-finding stage. Thirty-three patients have been dosed across two weekly oral schedules in the ongoing monotherapy trial.

The study established two weekly dosing regimens—160 mg once daily on a 3 days on/4 days off schedule, and 240 mg once daily on a 2 days on/5 days off schedule—both showing favorable tolerability. The company has also initiated a cohort testing a bi-weekly 2 days on/12 days off regimen, aiming for enhanced single-agent activity and more flexibility for potential combination studies down the road.

Clinical activity emerged at the 120 mg dose level and above, with tumor shrinkage observed in 9 out of 20 evaluable patients with small cell lung cancer and squamous non-small cell lung cancer. For an early-stage study, that's a promising signal.

Pipeline Expansion and Financial Runway

Beyond the clinical updates, Acrivon announced its next preclinical development candidate: ACR-6840, a potential first-in-class oral CDK11 inhibitor developed using the company's AP3 platform. The company expects to submit an investigational new drug application in the fourth quarter of 2026.

On the cash front, the company reported preliminary, unaudited cash, cash equivalents, and investments of approximately $119 million as of December 31, 2025. Management expects this to fund operating expenses and capital expenditures into the second quarter of 2027, giving the company roughly 18 months of runway.

So why did the stock crater on what appears to be decent news? Welcome to biotech, where investor expectations, trial timelines, and statistical significance often create their own mysterious calculus that defies straightforward explanation.