I-DXd BLA Withdrawal Exposes FDA's Rising Accelerated Approval Bar; Phase 3 Now the Only Path

Daiichi Sankyo and Merck Withdraw BLA for Ifinatamab Deruxtecan

Daiichi Sankyo and Merck have voluntarily withdrawn the Biologics License Application (BLA) for ifinatamab deruxtecan (I-DXd) in the U.S. for adult patients with previously treated extensive-stage small cell lung cancer (ES-SCLC). This decision follows discussions with the U.S. FDA, which indicated that data from the Phase 2 IDeate-Lung01 trial did not meet the requirements for accelerated approval for the proposed indication. Despite this withdrawal, the companies are continuing the global clinical development program for I-DXd, including the ongoing Phase 3 IDeate-Lung02 trial for ES-SCLC, with enrollment nearing completion.

• The Biologics License Application for ifinatamab deruxtecan in extensive-stage small cell lung cancer was voluntarily withdrawn after discussions with the U.S. FDA. The agency determined that the data, particularly from the Phase 2 IDeate-Lung01 trial, did not satisfy the requirements necessary to support an accelerated approval for the specified indication.
• Despite the BLA withdrawal, the development of ifinatamab deruxtecan continues with the ongoing Phase 3 IDeate-Lung02 trial for relapsed ES-SCLC. Enrollment for this trial is nearing completion, and the companies anticipate assessing its results for a potential future filing with the FDA and other global regulatory authorities.
• Ifinatamab deruxtecan is an investigational B7-H3 directed antibody-drug conjugate (ADC) designed with Daiichi Sankyo’s proprietary DXd ADC Technology. It has received Orphan Drug Designation from multiple regulatory bodies for SCLC and esophageal cancer, and its global clinical program includes two additional Phase 3 trials for castration-resistant prostate cancer and esophageal squamous cell carcinoma.

The IDeate-Lung01 Data and ES-SCLC Regulatory Requirements

Several pivotal trials have established and refined the standard of care for extensive-stage small cell lung cancer (ES-SCLC), spanning first-line chemo-immunotherapy combinations to later-line novel agents. The table below summarizes the design parameters and endpoints across the key trials identified in the literature.

For KEYNOTE-604, the prespecified efficacy boundaries were one-sided P = .0048 for PFS and .0128 for OS. Pembrolizumab plus EP significantly improved PFS (HR, 0.75; 95% CI, 0.61 to 0.91; P = .0023), though the OS significance threshold was not met (HR, 0.80; 95% CI, 0.64 to 0.98; P = .0164). In the DeLLphi-301 Asia subgroup, tarlatamab achieved an ORR of 46.3% (95% CI, 30.7–62.6), median DOR of 7.2 months (95% CI 3.9 to not estimable), median PFS of 5.4 months (95% CI 3.0–8.1), and median OS of 19.0 months (95% CI 11.4 to not estimable). In the quavonlimab plus pembrolizumab study, confirmed ORR was 18% (95% CI 7–33) among all patients, median PFS was 2.0 months, and median OS was 11.0 months.

Why New Options Are Still Crucial for ES-SCLC Patients

Despite meaningful advances with immune checkpoint inhibitors, ES-SCLC remains one of oncology's most intractable challenges, with a 5-year survival rate below 10% even following integration of PD-L1 inhibitors with chemotherapy. Resistance — both primary and acquired — driven by tumor microenvironment immunosuppression, impaired antigen presentation, and aberrant signaling, continues to limit long-term efficacy. The disease's biological complexity, including molecular plasticity and heterogeneity, further complicates the development of durable treatment strategies.

• Modest survival benefit from immunotherapy: ICIs have delivered only a 2–3 month improvement in median survival in SCLC, a paradox given the disease's characteristically high tumor mutational burden, pointing to complex, multifactorial resistance mechanisms including MHC class I downregulation and epigenetic repression of immune pathways.

• Absence of validated predictive biomarkers: Existing biomarkers such as PD-L1 expression and tumor mutational burden face limitations due to insufficient standardization and spatiotemporal heterogeneity, leaving clinicians without reliable tools to identify which patients will benefit from specific regimens. Exploratory frameworks such as the DDR-Immune Fitness (DDR-IF) score — integrating homologous recombination deficiency, TMB, and STING pathway activity — represent promising but as yet unvalidated approaches requiring confirmation in prospective multicenter trials.

• Chemotherapy-induced myelosuppression: Myelosuppression and its sequelae cause significant toxicity and harm to quality of life across first- and second-line chemotherapy regimens, necessitating supportive interventions, hospitalizations, and occasionally treatment discontinuation.

• Immune-related adverse events (irAEs): ICI-based regimens introduce irAEs across multiple organ systems that require vigilant clinical management and can necessitate treatment discontinuation, adding complexity to an already difficult therapeutic landscape.

• Tumor plasticity and molecular subtype evolution: SCLC can undergo dynamic transitions in neuroendocrine differentiation during disease evolution — including shifts in expression of lineage transcription factors such as ASCL1, NEUROD1, POU2F3, and YAP1 — rendering initial molecular subtype classifications potentially unreliable over the course of treatment and undermining subtype-directed therapeutic strategies.

• Chemoresistance and limited second-line options: Standard chemotherapy frequently results in chemoresistance, and second-line agents such as topotecan demonstrate modest efficacy, with median progression-free survival of only 2.0–2.2 months even in sensitive relapse populations.

Beyond SCLC: Ifinatamab Deruxtecan's Expanding Pipeline

The IDeate-PanTumor01 phase 1/2 trial established ifinatamab deruxtecan's activity across a broad range of advanced solid tumours beyond ES-SCLC, with a confirmed objective response rate of 34% (95% CI 23–47) across 70 evaluable patients. The trial enrolled patients with treatment-refractory disease spanning ten tumour types, providing early evidence of pan-tumour applicability for this B7-H3-directed antibody-drug conjugate.

• Oesophageal squamous cell carcinoma (ESCC): Included as a target indication in IDeate-PanTumor01, a two-part, multicentre, open-label, first-in-human phase 1/2 study conducted across ten hospitals and cancer centres in the USA and Japan. Patients received ifinatamab deruxtecan at doses of 0.8–16.0 mg/kg intravenously every 3 weeks.

• Castration-resistant prostate cancer (CRPC): Also enrolled within IDeate-PanTumor01 under the same open-label, dose-escalation intervention model. Preclinical research further supports B7-H3 as a druggable target in this setting, including in RB1-deficient CRPC and neuroendocrine prostate cancer, where DS-7300a (ifinatamab deruxtecan) demonstrated potent single-agent antitumour activity in models with high B7-H3 expression.

• Squamous non-small-cell lung cancer, head and neck squamous cell carcinoma, bladder cancer, sarcoma, endometrial cancer, melanoma, and breast cancer: All included as target tumour types within IDeate-PanTumor01, evaluated under the same multicentre, open-label, phase 1/2 intervention model with intravenous dosing every 3 weeks. Antitumour activity was assessed per Response Evaluation Criteria in Solid Tumours version 1.1 in patients receiving doses of 4.8 mg/kg or higher.

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I-DXd's SCLC Path: FDA Withdrawal and Phase 3 Focus

Small cell lung cancer (SCLC) remains one of oncology's most formidable challenges, characterized by aggressive disease progression and a dire prognosis, particularly in the extensive-stage setting where treatment options beyond initial platinum-etoposide chemotherapy and immune checkpoint inhibitors are severely limited. The emergence of antibody-drug conjugates (ADCs) like ifinatamab deruxtecan (I-DXd), which targets the B7-H3 protein highly expressed in SCLC, has offered a beacon of hope, signaling a shift towards more targeted precision medicine approaches.

I-DXd's journey has been closely watched, especially after its Phase 2 IDeate-Lung01 trial demonstrated encouraging activity in heavily pretreated patients, including a notable objective response rate and intracranial activity. However, the recent voluntary withdrawal of its Biologics License Application for accelerated approval in the U.S. for previously treated ES-SCLC patients underscores the rigorous standards of regulatory bodies. This decision, following discussions with the FDA, suggests that while the Phase 2 data were promising, they did not fully satisfy the requirements for an accelerated pathway, potentially due to concerns regarding the durability of response (median PFS of 4.9 months) or the overall risk-benefit profile, including the observed rates of interstitial lung disease (12.4% overall, 4.4% Grade ≥3).

For Daiichi Sankyo and Merck, this means a recalibration of their strategy, with full attention now turning to the ongoing Phase 3 IDeate-Lung02 trial. This pivotal study, comparing I-DXd against physician's choice in patients with relapsed SCLC after one prior line of therapy, is critical. Its success will be essential to definitively establish I-DXd's efficacy and safety, paving the way for a potential full approval. The outcome of IDeate-Lung02 will not only determine the future of I-DXd but also provide crucial insights into the role of B7-H3-targeted ADCs in reshaping the therapeutic landscape for this aggressive and challenging cancer.