A renewed regulatory opening and a clinical setback sent two biotech stocks in opposite directions in October. Biohaven gained ground after the FDA agreed to review its rare disease resubmission, while ArriVent BioPharma plunged after its lead candidate missed in a Phase 3 cancer study, then suffered another selloff despite encouraging early results for a separate asset.
Here, BioSpace analyzes these two sizeable stock shifts and the developments driving the activity. This monthly series is not intended to be a comprehensive review of all public biotech movement.
Who’s hot: Biohaven (BHVN)
The FDA has accepted Biohaven’s twice-rebuffed rare disease drug for review, a regulatory decision that sent the biotech’s stock up at least 25% in late October.
It’s been a rocky road for troriluzole, Biohaven’s investigational treatment for spinocerebellar ataxia (SCA), a group of rare diseases that lead to the progressive loss of control over movement. In 2023, the FDA refused to even review Biohaven’s new drug application based on a Phase 3 trial that missed its primary endpoint.
More recently, in November 2025—after a three-month delay—the FDA rejected troriluzole, also known as Vyglxia, in SCA. In the complete response letter, the regulator cited concerns “inherent to real-world evidence and external control studies.” At the time, Biohaven’s stock halved and, on the heels of the rebuff, the company launched a sweeping cost-optimization initiative to maximize its resources.
But now the FDA has accepted the company’s resubmission for review and assigned a new target action date sometime in the first quarter of 2027, according to an Oct. 22 SEC filing. The agency also plans to hold an advisory committee meeting for the application, with an adcomm date expected to be set “in the near term,” according to the document.
“Ultimately, we would expect Biohaven shares to re-rate if there is a clear path to approval here for Vyglxia given a blockbuster orphan disease opportunity across all SCA genotypes,” William Blair analysts wrote Oct. 22.
Who’s not: Arrivent BioPharma (AVBP)
ArriVent BioPharma’s stock halved after its tyrosine kinase inhibitor (TKI) failed to improve progression-free survival (PFS) rates in a Phase 3 lung cancer trial. The biotech’s stock has yet to recover, falling further even after early “promising” data for a separate asset at this year’s European Society for Medical Oncology (ESMO) meeting in Spain.
In the first week of October, Arrivent shared open-label data for its most advanced asset, firmonertinib, an EGFR TKI designed to penetrate the brain. The Allist Pharmaceuticals–partnered candidate demonstrated a median PFS of 11 months in certain non-small cell lung cancer (NSCLC) populations, as compared to 9.5 months for patients on chemotherapy—a finding that wasn’t statistically significant.
Truist analysts viewed the miss as largely driven by the stronger-than-expected PFS seen in the control group, up from an expected seven months.
The trial results sent ArriVent’s stock plummeting nearly 50% on Oct. 6, from $28.46 to $12.09 at market close.
Analysts have turned their focus to a late-stage test of firmonertinib in a different type of lung cancer and the biotech’s antibody-drug conjugate (ADC) pipeline, including ARR-217, an ADC ArriVent is developing alongside Lepu Biopharma for colorectal and gastrointestinal cancers.
Early data for the asset were presented as a late-breaker at ESMO last week. The Phase 1 data slice demonstrated a 44.8% preliminary objective response rate among patients with metastatic colorectal cancer (mCRC).
While Truist found the early findings “encouraging,” displaying “emerging durability, with favorable tolerability,” it didn’t stop investors from sending the biotech’s stock down about 27% that day. Truist deemed the stock drop “disproportionate” and “disconnected” to the ARR-217 data.
“We believe today’s weakness instead reflects plausible profit-taking following the stock’s ~39% recovery from its October 6 close through Friday,” Truist wrote in an Oct. 26 note, while also citing “a high differentiation bar in the crowded mCRC ADC landscape.”
ArriVent is expected to share Phase 1b data for the asset in the first half of next year, which will provide clarity on its development path, according to Truist. The analyst modeled peak sales of $850 million for ARR-217 and has started including it in the company’s valuation.
“Firmonertinib clarity is expected in 1Q27, but ARR-217 begins to rebalance the story,” the analysts wrote.
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