Editor’s Briefing: September 28, 2026

In Short. Overnight, Merck and Daiichi Sankyo withdrew their accelerated-approval filing for the lung-cancer antibody-drug conjugate ifinatamab deruxtecan after the FDA said the phase 2 data were not enough, Mirum’s pill for a soft-tissue-to-bone disease cleared the agency, Adicet’s off-the-shelf CAR-T reported early lupus remissions, Ultragenyx won a first treatment nod for Sanfilippo syndrome type A, and Merck’s Welireg plus Eisai’s Lenvima gained a kidney-cancer combo label.

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Merck and Daiichi pull a small-cell lung filing

Antibody-drug conjugates (ADCs) are antibodies that deliver a toxic payload to cells that display a chosen surface marker. Merck and Daiichi Sankyo had sought accelerated approval of ifinatamab deruxtecan (I-DXd), which aims at B7-H3, for extensive-stage small-cell lung cancer after platinum chemotherapy, based on the phase 2 IDeate-Lung01 study in 187 previously treated patients. On Monday the partners said they voluntarily withdrew that filing after the FDA told them the dataset fell short of what accelerated approval requires. Merck said it was disappointed and pointed to the phase 3 IDeate-Lung02 trial, whose enrollment is nearing completion, as a path to return with more data. The setback hits a 2023 pact that began with a 4 billion upfront payment across three ADCs and has already seen manufacturing and survival disappointments on a sister HER3 program. (Fierce Biotech)

Mirum’s FOP pill wins FDA clearance

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare genetic disease in which muscles, tendons, and ligaments gradually turn into bone. On Friday the FDA approved Mirum Pharmaceuticals’ Atebrioz (zilurgisertib), an oral ALK2 inhibitor licensed from Incyte, for people aged 12 and older to reduce the volume of new heterotopic ossification. In the 63-patient PROGRESS study, mean new bone volume fell 3.2 cubic centimeters on Atebrioz at week 24 versus a 24.6 cubic centimeter rise on placebo. It is the third approved FOP treatment after Regeneron’s Pasatru and Ipsen’s Sohonos. Mirum expects U.S. availability in October. (Healio; pdufa.bio)

Adicet’s off-the-shelf CAR-T shows early lupus remissions

CAR-T therapies reprogram immune cells to attack a chosen target; most approved products are made from each patient’s own cells. Adicet Bio reported phase 1 results for prula-cel, an off-the-shelf product built from gamma-delta T cells that also targets CD19 on B cells. One year after a single dose, 12 of 22 evaluated patients met remission of lupus symptoms, and 8 of 16 with lupus nephritis met kidney-symptom remission, with patients off standard immunosuppressants. The company reported no IEC-HS (a dangerous hyperinflammatory syndrome that recently led to deaths in a Novartis autoimmune CAR-T program), no ICANS neurotoxicity, and mostly grade 1 cytokine release. Adicet says the FDA has agreed a single-arm pivotal path in lupus nephritis that could start next year. Early numbers in a small study are a signal, not a label. (Fierce Biotech)

Ultragenyx gets a Sanfilippo type A approval

Sanfilippo syndrome type A is a rare childhood neurodegenerative disease in the mucopolysaccharidosis family. BioSpace reported that the FDA approved Ultragenyx’s Fayuvi for the condition, a milestone CEO Emil Kakkis called particularly personal after decades working on MPS diseases. The nod follows an August gene-therapy clearance for glycogen storage disease type Ia (Genglycos) and sits beside a costly phase 3 miss for the Angelman antisense apazunersen, after which the company said it must cut expenses and may set Angelman aside for now. Each of the two recent approvals came with a priority review voucher that can be kept or sold. (BioSpace)

Welireg plus Lenvima expands in kidney cancer

Clear-cell renal cell carcinoma is the most common form of kidney cancer. On September 24 the FDA approved Merck’s Welireg (belzutifan), a HIF-2α inhibitor, with Eisai’s Lenvima (lenvatinib) for adults with advanced clear-cell disease after a PD-1 or PD-L1 inhibitor. In the phase 3 LITESPARK-011 trial of 747 patients, median progression-free survival was 14.6 months on the combo versus 10.6 months on cabozantinib; overall survival improved numerically but was not statistically significant. Objective responses were 53% versus 40%. Recommended dosing is 120 mg belzutifan plus 20 mg lenvatinib once daily. (FDA; Merck)

What to watch

pdufa.bio still lists Merck/Daiichi ifinatamab deruxtecan for October 10; that date is now stale after Monday’s voluntary withdrawal of the ES-SCLC accelerated-approval filing. Still ahead: Roche Tecentriq adjuvant MSI-H / stage III colon (October 9), Roche Enspryng in thyroid eye disease (October 15), Viatris/Opus MR-141 phentolamine ophthalmic (October 17). Mirum/Incyte Atebrioz has flipped to Approved (dated September 25 on the calendar).