On September 8, Inhibrx Biosciences presented primary endpoint results from its INBRX-106 Phase 2 study in head and neck squamous cell carcinoma. Confirmed overall response rate: 48.3% with INBRX-106 plus pembrolizumab, versus 26.5% for pembrolizumab alone. In HPV-positive patients: 80% versus 33.3%. Six-month progression-free survival: 72.4% versus 42.8%.
The stock opened above $121 and sold off 5.6% intraday. Profit-taking. Sell the news. The usual.
Three insiders bought into it.
Mark Lappe, the CEO, bought 25,000 shares at $105.82 — near the intraday low. He put $2.6 million into his own stock at the bottom of a data-day selloff. Directors Jon Faiz Kayyem ($568K) and Douglas Forsyth ($287K) bought as the stock recovered toward the close. Combined: $3.5 million in a single session.
The Kayyem Arc
Kayyem is not a casual buyer. He is a serial accumulator who has built a 424,360-share position — 7.9% of the company — entirely through open-market purchases.
Both Kayyem and Lappe filed Schedule 13D/A amendments in February 2026, declaring activist stakes — 7.9% and 6.4% respectively. These are not passive holders trimming positions on data-day volatility. They are principals, and they are adding.
What the Data Said
The INBRX-106 Phase 2 HexAgon study tested a hexavalent OX40 agonist combined with pembrolizumab versus pembrolizumab alone in first-line PD-L1-positive head and neck squamous cell carcinoma:
| INBRX-106 + Pembro | Pembro Alone | |
|---|---|---|
| Confirmed ORR | 48.3% | 26.5% |
| 6-Month PFS | 72.4% | 42.8% |
| Median PFS | 9.6 months | — |
| HPV+ ORR | 80.0% | 33.3% |
| HPV+ Complete Response | 30.0% | 0% |
The HPV-positive subgroup is the number that matters most. Eighty percent response rate versus thirty-three. Complete response in 30% of patients versus none. Inhibrx is expanding the HPV-positive cohort by approximately 50 patients as a potential path to accelerated approval.
The Short Side
Twenty-two percent of the float is short. Days to cover: 14.4. The stock surged 45% in the month prior to data day, and shorts barely moved — trimming only 6% from a peak of 3.43 million shares. Cost to borrow actually declined during the rally, suggesting shorts added rather than panicked.
Three insiders bought $3.5 million on data day. Twenty-two percent of the float is positioned against them. One side is wrong.
Context
Inhibrx also has ozekibart — a DR5 agonist for chondrosarcoma with a BLA accepted by the FDA in June and a PDUFA date of April 14, 2027. If approved, it would be the first-ever FDA-approved treatment for conventional chondrosarcoma. Analyst consensus price target: $267, roughly 133% above the September 8 close. Four Buy ratings.
Data-day insider buying is one of the cleaner signals in Form 4 analysis. No earnings blackout to confound it. No pre-scheduled 10b5-1 plan. Three people with full visibility into the underlying science looked at the response rates, looked at the selloff, and wired money to their brokers. The CEO bought the bottom.
After-hours: $118.99, up 4.07% from the close.