AstraZeneca’s Ultomiris HSCT-TMA Expansion Stumbles as Adult Trial Failure Clouds Pediatric Survival Signal

AstraZeneca Reports Mixed Phase III Results for Ultomiris and Positive Data for Sone-Ve

AstraZeneca announced mixed Phase III results for Ultomiris (ravulizumab) and positive Phase III results for sonesitatug vedotin (Sone-Ve). Ultomiris failed to achieve statistical significance for event-free survival in adults and adolescents with HSCT-TMA in the ALXN1210-TMA-313 trial. However, it showed clinically meaningful overall survival rates of 87.2% at 26 weeks and 73.4% at 52 weeks in pediatric HSCT-TMA patients in the ALXN1210-TMA-314 study. Separately, Sone-Ve demonstrated a statistically significant and clinically meaningful overall survival improvement in CLDN18.2-positive advanced gastric, GEJ, or oesophageal adenocarcinoma patients in the CLARITY-Gastric01 trial.

• Ultomiris's Mixed Outcomes in Adult/Adolescent HSCT-TMA: In the global ALXN1210-TMA-313 Phase III trial, Ultomiris (ravulizumab) did not meet its primary endpoint of event-free survival through 26 weeks in adults and adolescents with HSCT-TMA, failing to achieve statistical significance compared to placebo. Despite a trend towards treatment benefit, it did not reach the pre-specified level, prompting ongoing discussions with health authorities regarding data interpretation.
• Positive Pediatric HSCT-TMA Data for Ultomiris: Conversely, Ultomiris showed promising results in pediatric patients with HSCT-TMA in the ALXN1210-TMA-314 open-label Phase III study, demonstrating a clinically meaningful overall survival rate of 87.2% at 26 weeks and 73.4% at 52 weeks. These positive findings are supporting ongoing regulatory filings for this specific indication, alongside existing orphan drug and breakthrough therapy designations.
• Sone-Ve's Significant Survival Benefit in Advanced Gastric Cancer: The CLARITY-Gastric01 global Phase III trial for sonesitatug vedotin (Sone-Ve) successfully met its dual primary endpoint, showing a statistically significant and highly clinically meaningful improvement in overall survival. This benefit was observed in CLDN18.2-positive advanced gastric, GEJ, or oesophageal adenocarcinoma patients in the second and later-line treatment settings, compared to investigator’s choice of therapy.

Ultomiris Phase III Results in HSCT-TMA: Mixed Efficacy, Consistent Safety

Recent literature provides varied insights into the management and incidence of hematopoietic stem cell transplant-associated thrombotic microangiopathy (HSCT-TMA). Studies have evaluated specific complement inhibitors like eculizumab in pediatric populations, assessed the incidence of endothelial complications in cohorts receiving post-transplant cyclophosphamide, and identified TMA as a safety signal in novel preconditioning regimens. These findings highlight diverse efficacy outcomes, key risk factors, and the significant mortality burden associated with this complication.

Addressing Unmet Needs in Paediatric HSCT-TMA with Ultomiris

Paediatric HSCT-TMA (TA-TMA) remains a diagnostically elusive and often fatal complication of allogeneic transplantation, with no universally accepted diagnostic criteria and, until recently, no approved therapy. Ultomiris (ravulizumab) is being positioned to address longstanding gaps in complement-directed treatment, particularly for children who face distinct risk profiles and unmet clinical needs compared with adult populations. The evidence base highlights several converging challenges that define the opportunity for targeted intervention.

• Diagnostic uncertainty and underdiagnosis: In the absence of tissue biopsy, TA-TMA is diagnosed provisionally against clinical criteria; retrospective analyses show only 2.6% of patients were diagnosed by treating physicians versus 20–36% who retrospectively met the Cho or Jodele criteria, underscoring a persistent lack of standardisation that delays intervention.

• High-risk paediatric subgroups requiring targeted management: Elevated TA-TMA risk is concentrated in patients receiving reduced-intensity conditioning (OR 1.96), those with grade 3/4 acute GVHD, patients undergoing ≥2 HCT procedures, and specific inherited metabolic disease populations such as Hurler syndrome, where 78% of TA-TMA cases also presented with concurrent bloodstream infection.

• Substantial mortality burden despite treatment: TA-TMA is independently associated with markedly worse overall survival and transplant-related mortality (HR 10.9), with additional risk conferred by elevated LDH, need for ≥2 antihypertensives, and acute kidney injury — reinforcing the urgency for effective, targeted complement inhibition in this population.

• Inadequate response to existing complement inhibition and lack of paediatric dosing standards: Eculizumab produces variable responses, and no standard paediatric dosing regimen exists specifically for TA-TMA (doses are extrapolated from aHUS), highlighting a clear need for optimised, long-acting complement inhibitors such as ravulizumab with defined paediatric posology.

• Early real-world paediatric eculizumab experience signals unmet need for improved regimens: In a small paediatric cohort (ages 1–17), three of four children with exclusive renal involvement achieved TA-TMA resolution after prolonged eculizumab courses (40.6–65 weeks), while a patient with multisystemic disease died despite treatment — pointing to the need for earlier diagnosis, faster-acting agents, and better stratification of multisystem versus renal-limited disease.

• Emerging complement/interferon pathophysiology supports mechanism-based therapy: Discovery of an "interferon-complement loop" and activation of classical, alternative, and lectin complement pathways (distinct from the alternative-pathway-dominant profile in aHUS) provides rationale for long-acting C5 inhibitors like ravulizumab as part of a broader shift toward mechanism-based, rather than empirical supportive, care in paediatric TA-TMA.

Navigating Challenges in HSCT-TMA Treatment Landscape

Managing TA-TMA remains fraught with diagnostic ambiguity, inconsistent treatment responses, and persistently high mortality, despite the emergence of complement-targeted therapies. The absence of standardized diagnostic criteria and treatment algorithms continues to complicate clinical decision-making, while real-world outcomes with agents such as eculizumab reveal substantial gaps between therapeutic promise and actual patient benefit.

• Diagnostic inconsistency and delayed recognition: The lack of a uniform definition for TA-TMA drives extreme variability in reported incidence (0–74%, or 0.5–76% across series), with diagnostic criteria alone accounting for the majority of interstudy heterogeneity (I² = 98%). Studies relying on clinician diagnosis rather than laboratory criteria report incidence as low as 3%, and TA-TMA is frequently diagnosed late in disease progression, delaying intervention.

• Absence of a standardized treatment strategy: No established first-line treatment exists for TA-TMA. Withdrawal of calcineurin inhibitors combined with therapeutic plasma exchange (TPE) produces response in only 50–63% of patients, many of whom require additional therapy. TPE itself has not demonstrated high response rates and, absent definitive trials, cannot be considered standard of care.

• Suboptimal and variable response to eculizumab: In a large pediatric cohort (n=176 across seven centers, 2015–2023), only 24% of patients achieved complete response and 19% partial response at six months, while 57% had no response. Non-relapse mortality was markedly lower among responders versus non-responders (2.3% vs. 24.2%). Patients refractory to eculizumab (rTA-TMA) face mortality rates exceeding 80%, with no established alternative therapies.

• Predictors of poor response and high-risk subgroups: Multi-organ dysfunction at diagnosis independently predicts non-response across transplant types, and allogeneic HCT recipients are more likely to be non-responders than autologous recipients (HR 1.84). Grade 3–4 acute GVHD (present in 46% of allogeneic recipients), renal or neurological dysfunction, and time to TA-TMA onset are independent risk factors for overall survival. Failure to achieve at least a partial response by day 45 sharply reduces the likelihood of subsequent response (≤25%) and confers a 3.74-fold higher mortality risk—identifying day 45 as a critical decision point for reassessing therapy.

• Limited access to pharmacologic alternatives: While rituximab, vincristine, defibrotide, pravastatin, and eculizumab show broadly similar response rates (69–80%), treatment costs vary considerably. Defibrotide remains investigational in the US and is not readily available, narsoplimab data in children are limited, and complement blockers are not universally accessible—factors that keep PE clinically relevant despite its unproven efficacy.

• Persistently high mortality and complexity of complications: Reported mortality associated with TA-TMA can reach 100%, and the post-transplant course is often complicated by concurrent acute/chronic GVHD, viral reactivation, and secondary hemophagocytic lymphohistiocytosis, further obscuring management decisions.

• Unresolved questions warranting further research: Larger prospective studies are needed to validate pharmacologic agents as upfront or TPE-refractory therapy, establish more valid diagnostic criteria, risk-stratify patients likely to benefit from specific interventions, and clarify optimal biomarkers (e.g., neutrophil extracellular traps, circulating endothelial cells, complement activation products such as Ba) for early detection and monitoring of treatment response.