Genmab Petosemtamab Phase 3 Colorectal Cancer Trial Starts September 15, 2026: Second-Line RAS/BRAF Wild-Type Study Design

Genmab A/S (GMAB) has a Phase 3 colorectal cancer trial reaching its expected start date of September 15, 2026, according to trial registration data that entered BioSniper's catalyst tracking on September 14, 2026 — one day before the expected start. The study evaluates petosemtamab combined with chemotherapy against two approved-antibody comparator arms in second-line metastatic colorectal cancer. This preview sets out the verified design, the institutional holdings our data shows, and how the trial extends a petosemtamab program that already has a first-line Phase 3 on our calendar.

The event

The trial is registered in our catalyst dataset as:

"A Randomized, Open-label, Phase 3 Trial of Petosemtamab + FOLFIRI/mFOLFOX6 Versus Cetuximab or Bevacizumab + FOLFIRI/mFOLFOX6 as Second-line Treatment in Participants With RAS and BRAF Wild-type, Recurrent, Unresectable or Metastatic Colorectal Cancer"

The expected start date is September 15, 2026 — today — recorded with "exact" date confidence. As of publication our data contains no actual-start confirmation, so the verified fact is the registered plan.

What's verified

• Company: Genmab A/S, ticker GMAB, active listing status.
• Event type: Phase 3 trial start.
• Expected date: September 15, 2026, "exact" confidence.
• First tracked: September 14, 2026 — this registration is one day old in our data, making it among the freshest events on the entire calendar.
• Design elements from the registered title: randomized; open-label; Phase 3; petosemtamab plus FOLFIRI or mFOLFOX6; comparators cetuximab or bevacizumab each plus FOLFIRI or mFOLFOX6; second-line setting; RAS and BRAF wild-type population; recurrent, unresectable or metastatic colorectal cancer.
• Company scale: Market capitalization of $20,021,227,520 in our snapshot.

What our 13F data shows

Our institutional holdings snapshot for Genmab (as of quarter-end March 31, 2026) records three institutions holding positions filed in 13F reports, totaling $8,825,303:

• UBS Group AG — 298,132 shares, $7,998,882
• KeyBank National Association/OH — 30,530 shares, $819,128
• Cornerstone Planning Group LLC — 265 shares, $7,293

We note this plainly: Genmab is a Danish-listed company, and US 13F filings capture only US-institutional positions, so the small absolute number reflects coverage of the filing system, not the company's full institutional base. Our data shows 12 quarterly trend points for the name.

What remains unknown

• Enrollment size: UNKNOWN in our data.
• Primary endpoint: the title describes the comparison but does not name formal endpoints such as overall survival or progression-free survival; our structured endpoint fields are empty.
• Actual start and readout timing: not recorded.
• Mechanism details for petosemtamab: our event record does not include target or mechanism information, and we will not describe it beyond the registered title.

Platform context: petosemtamab's first-line sibling and Genmab's engine

This is the second petosemtamab Phase 3 start on Genmab's calendar in five weeks. On August 11, 2026, our tracker recorded a first-line counterpart: a randomized, open-label Phase 3 trial of petosemtamab in combination with mFOLFOX6 or FOLFIRI versus cetuximab-based therapy in RAS and BRAF wild-type, unresectable or metastatic, left-sided colorectal cancer. Two Phase 3 starts in the same indication — one first-line, one second-line — sketch a program investing across the metastatic treatment sequence.

Beyond petosemtamab, our calendar holds 17 Genmab catalyst records, including:

• Rinatabart sesutecan (Rina-S) — six separate tracked studies: Phase 3 in platinum-resistant ovarian cancer (expected June 26, 2026), Phase 3 maintenance after second-line platinum therapy in recurrent platinum-sensitive ovarian cancer (expected April 3, 2026), Phase 3 in endometrial cancer after platinum and PD(L)-1 therapy (expected November 28, 2025), Phase 3 second-line with or without bevacizumab in platinum-sensitive ovarian cancer (expected August 28, 2026), Phase 2 in advanced gastrointestinal cancers (expected May 11, 2026), and Phase 2 in non-small cell lung cancer (expected January 30, 2026).
• Epcoritamab — a trial readout record dated July 31, 2026: the EPCORE DLBCL-3 primary analysis of epcoritamab monotherapy or epcoritamab with lenalidomide as first-line therapy for diffuse large B-cell lymphoma (a phase 2 trial). Our outcome field for this readout remains empty — the result is not yet in our data.
• Early pipeline — first-in-human Phase 1 starts recorded for GEN1119 in solid tumors (expected August 13, 2026), GEN1106 in solid tumors (expected April 14, 2026), GEN1079 in advanced malignant solid tumors (expected April 7, 2026), and GEN3018 in relapsed or refractory acute myeloid leukemia or higher-risk myelodysplastic syndrome (expected February 16, 2026).

How to track it

• Genmab company page — full catalyst calendar: https://biosniper.co/company/GMAB
• This trial-start event page: https://biosniper.co/calendar/event/5799060

Bottom line

A Phase 3 second-line colorectal cancer trial of petosemtamab plus chemotherapy versus cetuximab- or bevacizumab-based regimens in RAS and BRAF wild-type disease reaches its registered start date of September 15, 2026 — one day after the registration entered our tracking. It joins a first-line petosemtamab Phase 3 recorded in August, making colorectal cancer the program's visible center of gravity. Enrollment size, formal endpoints, and results remain unknown in our data; the design skeleton above is exactly what the registration states, no more.