An ASTCT, CIBMTR, EBMT, and APBMT Consensus Statement Defining Response Criteria for Hematopoietic Cell Transplantation Associated Thrombotic Microangiopathy (TA-TMA) Directed Therapy: Consensus Response Criteria for TA-TMA.

Background: Transplant associated thrombotic microangiopathy (TA-TMA) confers significant morbidity and mortality in hematopoietic cell transplant recipients. The safety and efficacy of multiple TA-TMA directed therapeutic agents are being tested in ongoing clinical trials. In the absence of approved drugs, several treatments are used off-label. Response definitions to TA-TMA directed therapy from retrospective studies and ongoing interventional clinical trials vary widely, limiting cross study comparisons. An expert panel from multiple international blood and marrow transplant societies (ASTCT, CIBMTR, EBMT, APBMT) who initially convened to harmonize diagnostic and risk stratification criteria for TA-TMA extended its mandate to review response criteria.

Objective: Our objective was to propose clinically meaningful response criteria for TA-TMA directed therapy to enhance consistent evaluation of therapeutic agents in clinical practice, interventional trials, and registry studies.

Methods: After a relevant literature review, the Delphi method was used to achieve consensus on proposed response criteria.

Results: The panel focused on the three key concepts. First, due to ongoing concurrent co-morbidities and severity of illness, the complete resolution of TA-TMA manifestations may be difficult to achieve immediately after initiating treatment, making the definition of clinically meaningful partial responses essential to assess early efficacy of TA-TMA-directed therapies. Second, because hematologic manifestations may resolve faster than organ damage, we suggest assessing hematologic/biochemical and organ manifestations independently, in addition to having an overall response assessment. Finally, using the previously established diagnostic criteria as a framework, we propose objective response definitions for each TA-TMA criterion and organ manifestation. While consensus was achieved on response definitions, due to the lack of evidence, there was no agreement on standardized time points for assessing response or when to consider alternative therapies for patients unresponsive to initial treatments. Hematologic and biochemical response assessments include anemia and thrombocytopenia, with criteria accounting for transfusion dependence or independence at the time of treatment, schistocytes, lactate dehydrogenase, and soluble C5b-9. Patients with other established etiologies of cytopenias (i.e. poor graft function or relapsed hematologic malignancy) should be considered unevaluable for hematologic response. Responses for involved organ manifestations were also proposed. In an overall assessment, the best overall response is limited by the lowest hematologic/biochemical or organ response. NR of either hematologic/biochemical or organ is considered an overall NR.

Conclusion: The consensus response criteria proposed by this expert panel are a step towards standardizing the assessment of treatment responses of TA-TMA directed agents for future studies and interventional clinical trials. Adoption of these criteria will enhance consistency of response assessment and facilitate the comparison of TA-TMA treatments. Since achieving an early overall CR may be challenging/protracted in patients with organ injury; establishment of criteria for clinically meaningful PR is important and may be a more useful early endpoint in studies. Given the complexity of these patients and assessment of response, these definitions may need be revised in the future after application in large cohorts diverse in age, HCT approaches, and interventional agents.