Azenosertib in metastatic triple-negative breast cancer: ZAP-IT closes after Zentalis pulls the program over safety

What was tried

The ZAP-IT study (NCT06351332) was a Phase 1/2 single-arm, investigator-initiated trial led by Filipa Lynce at Dana-Farber Cancer Institute. The protocol combined azenosertib (ZN-c3, ChEMBL5095036), an oral WEE1 kinase inhibitor, with carboplatin and pembrolizumab in patients with metastatic triple-negative breast cancer who had progressed on or after standard first-line therapy. Azenosertib was dosed at 25 or 100 mg orally per the dose-finding schedule. Carboplatin and pembrolizumab were delivered intravenously on standard institutional schedules. The Phase 1 portion was designed to define the maximum tolerated dose and the dose-limiting toxicity rate; the Phase 2 portion was designed to estimate objective response rate by RECIST 1.1. Actual enrollment closed at 7 participants when sponsor Zentalis Pharmaceuticals terminated the supply agreement, citing serious adverse events reported in other azenosertib trials. The official primary completion date is 24 February 2026. No results were posted.

The biological hypothesis

WEE1 is a G2/M checkpoint kinase that phosphorylates CDK1 and CDK2 on tyrosine 15 to prevent premature mitotic entry. In cancers with high replication stress, defective p53 function, or cyclin E1 (CCNE1) amplification, WEE1 inhibition forces premature mitosis with unrepaired DNA damage and triggers mitotic catastrophe. Triple-negative breast cancer carries near-universal TP53 mutation (Cancer Genome Atlas), high baseline replication stress, and a subset with CCNE1 amplification. The mechanistic rationale for combining azenosertib with platinum was straightforward: carboplatin elevates DNA damage, and WEE1 inhibition removes the checkpoint that would otherwise allow tumor cells to repair it. Pembrolizumab was added on the premise that WEE1 inhibition activates endogenous retroviruses and the dsRNA-cGAS-STING pathway (J Exp Med 2022), and would prime the tumor microenvironment for immune checkpoint blockade. Open Targets v25.06 places the WEE1 to triple-negative breast cancer association score at 0.099, which is weak; the strongest WEE1 disease association is non-malignant (neurodegenerative disease at 0.517), and within oncology the top signal is ovarian cancer at 0.121. The biological case for azenosertib in TNBC was built on replication-stress logic rather than on a TNBC-specific genetic anchor.

What actually happened

Zentalis terminated the contract before the ZAP-IT trial reached its first dose-expansion readout. The publicly cited reason on the ClinicalTrials.gov record is concern for toxicity from serious adverse events seen in earlier azenosertib programs. Across the azenosertib portfolio, the dominant emergent safety pattern has been hematologic, in particular thrombocytopenia, neutropenia, and anemia, consistent with the WEE1 inhibitor class profile (Crit Rev Oncol Hematol 2024). OpenFDA FAERS holds 2 azenosertib case reports through April 2026, reporting platelet count decreased, neutrophil count decreased, hemoglobin decreased, white blood cell count decreased, nausea, vomiting, diarrhoea, and abdominal pain. The closest class comparator, adavosertib (AZD1775), accumulated 25 reaction reports in FAERS with hypotension (5), abdominal pain (4), atrial fibrillation (4), duodenitis (4), lactic acidosis (4), neutropenia (4), and respiratory failure (4) leading the profile. AstraZeneca discontinued adavosertib in 2023 after a series of mixed Phase 2 readouts. Zentalis announced in early 2026 that it was reprioritizing the azenosertib program away from investigator-initiated combinations, effectively ending external trial supply.

Failure mechanism, best guess

The most parsimonious explanation is class-level overlapping hematologic toxicity stacked on top of a chemotherapy and immunotherapy backbone that already has its own marrow burden. Carboplatin produces dose-dependent thrombocytopenia in roughly 30 percent of patients at standard AUC dosing. Pembrolizumab adds immune-related cytopenias in 1 to 3 percent. Adding a WEE1 inhibitor that drives platelet and neutrophil decline in 40 to 60 percent of single-agent recipients (Mol Cancer Ther 2025) created a triplet whose hematologic ceiling was too low to support efficacy-relevant exposure. The biomarker rationale for TNBC was also thin: the ZAP-IT protocol did not enrich for CCNE1 amplification, which the Phase 2 ADAGIO data and other gynecologic readouts identified as the strongest single predictor of WEE1 inhibitor response (NPJ Precis Oncol 2025). Without enrichment, the trial would have needed an unrealistic effect size to overcome the toxicity burden in the first 6 to 12 patients of the dose-expansion.

How to prevent this next time

Investigator-initiated combinations of any WEE1 inhibitor with platinum and immune checkpoint blockade should require explicit hematologic stopping rules and CCNE1 enrichment before opening enrollment. With a prior on Grade 3 or higher thrombocytopenia centered at 0.45 from the azenosertib monotherapy package and a tolerability ceiling of 0.30 for the triplet, a Bayesian sequential design can compute the posterior probability of safety success after each enrolled patient:

PP(success)=∫θ​P(final success∣θ,interim data)⋅π(θ∣interim data)dθ

The trial should close on safety once the posterior probability of staying below the 0.30 ceiling drops under 0.10. Biomarker enrichment by CCNE1 amplification, FBXW7 mutation, or RB1 loss should be mandatory at screening, because the Phase 2 historical objective response rate for unselected solid tumors on WEE1 inhibition is approximately 5 to 10 percent versus 25 to 35 percent in CCNE1-amplified cancers (J Clin Oncol 2023). A historical base-rate adjustment using ADAGIO, the GOG-3066 adavosertib platinum combinations, and the SY-4835 program would have anchored the prior on triplet toxicity. The single highest leverage change would have been a sponsor-mandated CCNE1 amplification screening gate and a triplet thrombocytopenia stopping rule before opening dose-expansion.

What this means for similar programs

The WEE1 inhibitor class now has a documented pattern: thrombocytopenia and neutropenia are the rate-limiting toxicities in any combination, and unenriched solid tumor cohorts do not generate response rates that justify the safety burden. Adavosertib was discontinued at AstraZeneca in 2023, azenosertib has been narrowed by Zentalis, debio 0123 is recruiting against CCNE1-amplified disease, and IMP7068 is in early Phase 1. Combination strategies should pivot to lower-dose WEE1 inhibition with synthetic-lethal partners (PARP, ATR, KRAS G12C) in biomarker-enriched tumors rather than to chemotherapy plus immunotherapy backbones. Investigator-initiated trials that depend on a single sponsor's drug supply remain a fragile structure: when the sponsor reprioritizes, the trial dies regardless of scientific merit.

Open questions

What were the specific serious adverse events Zentalis cited in its termination letter, and were they grade 4 or 5? Did any of the 7 enrolled ZAP-IT patients experience dose-limiting toxicity? Will Dana-Farber publish a patient-level safety summary on the partial enrollment? Does Zentalis plan to continue azenosertib in CCNE1-enriched ovarian and uterine indications, where the ADAGIO data signaled benefit? Will the WEE1 class consolidate around once-weekly dosing schedules that preserve hematologic recovery between cycles?

Sources

1. ClinicalTrials.gov, NCT06351332, full record retrieved 2026-05-23. ChEMBL CHEMBL5095036 (azenosertib / ZN-c3), small molecule, max phase 2. Canonical SMILES C=CCn1c(=O)c2cnc(Nc3ccc(N4CCN(C)CC4)cc3)nc2n1-c1ccc2c(n1)C@@(CC)CC2. ChEMBL CHEMBL5491 (Wee1-like protein kinase, single protein, target). Open Targets Platform v25.06, target WEE1 (ENSG00000166483), top disease associations: neurodegenerative disease 0.517, actinic keratosis 0.438, cutaneous squamous cell carcinoma 0.410, ovarian cancer 0.121, glioblastoma multiforme 0.108, triple-negative breast cancer 0.099. Retrieved 2026-05-23. OpenFDA FAERS azenosertib reaction counts (all 2 reports): platelet count decreased, neutrophil count decreased, haemoglobin decreased, white blood cell count decreased, nausea, vomiting, diarrhoea, abdominal pain upper, myelodysplastic syndrome, incorrect route of administration. OpenFDA FAERS adavosertib (AZD1775) top reactions: hypotension 5, abdominal pain 4, atrial fibrillation 4, duodenitis 4, lactic acidosis 4, neutropenia 4, respiratory failure 4. Retrieved 2026-05-23. Sun L et al. Azenosertib Is a Potent and Selective WEE1 Kinase Inhibitor with Broad Antitumor Activity Across a Range of Solid Tumors. Molecular Cancer Therapeutics 2025,Cuneo KC et al. Adavosertib and beyond: Biomarkers, drug combination and toxicity of WEE1 inhibitors. Critical Reviews in Oncology/Hematology 2024,Au-Yeung G et al. Cyclin E1/CDK2 activation defines a key vulnerability to WEE1 kinase inhibition in gynecological cancers. NPJ Precision Oncology 2025,Liu JF et al. Multicenter Phase II Trial of the WEE1 Inhibitor Adavosertib in Refractory Solid Tumors Harboring CCNE1 Amplification. Journal of Clinical Oncology 2023,Sun L et al. WEE1 inhibition induces anti-tumor immunity by activating ERV and the dsRNA pathway. Journal of Experimental Medicine 2022,Yap TA et al. Targeting WEE1 Kinase for Breast Cancer Therapeutics: An Update. International Journal of Molecular Sciences 2025,Chen X et al. The Selective WEE1 Inhibitor Azenosertib Shows Synergistic Antitumor Activity with KRAS G12C Inhibitors in Preclinical Models. Cancer Research Communications 2025,. Available from: https://clinicaltrials.gov/study/NCT06351332.