An investigational antibody-drug conjugate labeled SGN-CD70A showed signs of efficacy against relapsed or refractory non-Hodgkin lymphomas in a phase 1 trial, but its future is clouded by a high incidence of treatment-associated thrombocytopenia, investigators reported.
Among 20 patients with diffuse large B-cell lymphoma (DLBCL), mantle cell lymphoma, and other histologies, SGN-CD70A was associated with one complete remission (CR) and three partial remissions (PR), two of which were ongoing at nearly 43 weeks of follow-up.
However, 15 of the 20 patients (75%) had treatment-related thrombocytopenias, and 13 of these adverse events (AEs) were grade 3 or greater in severity, reported, of the University of Michigan, Ann Arbor, and his colleagues.
Notwithstanding the antibody-drug conjugate’s apparent efficacy in this early trial, “the applicability of SGN-CD70A is limited by the frequency and severity of thrombocytopenia, despite the long-term of response with limited drug exposure. Given that we are currently unable to mitigate this AE, the rationale for further investigation of SGN-CD70A remains limited and is, therefore, not planned,” they wrote in the journal.
SGN-CD70A consists of an antibody directed against the plasma membrane protein CD70, a protease-cleavable linker, and a DNA-crosslinking pyrrolobenzodiazepine dimer drug. Its mechanism of action is via double-strand DNA breaks in CD70-positive cells that eventually cause programmed cell death.
Dr. Phillips and his colleagues reported on the high-risk non-Hodgkin lymphoma cohort in the phase 1 trial. The cohort included nine patients with DLBCL, five with mantle cell lymphoma, two with transformed DLBCL, one with T- cell/histocyte–rich large B cell lymphoma, and three with unspecified NHL histologies.
The patients had undergone a median of 3.5 prior lines of systemic therapy, and all had relatively good performance status, with Eastern Cooperative Oncology Group scores of 0 or 1.
Patients were started on intravenous SGN-CD70A at a dose of 8 mcg/kg on day 1 of each 3-week cycle, with a planned dose escalation to 200 mcg/kg, The protocol was amended to dosing every 6 weeks, however, after the investigators observed prolonged thrombocytopenias in some patients. A total of 12 patients were treated every 3 weeks, and 8 were treated every 6 weeks.
The most common treatment-related AEs were thrombocytopenias, which occurred in three-quarters of all patients, and were largely grade 3 or greater in severity. Other treatment-related AEs of grade 3 or greater occurring in more than one patient include neutropenia in six patients; anemia in five patients; and congestive heart failure, Clostridium difficile infections, dyspnea, and decreased forced expiratory volume in two patients each.
Other common AEs were nausea and fatigue.
The investigators noted that the cause of the deep and durable thrombocytopenias could not be determined, despite assessment of known biomarkers for this complication.
The duration of the thrombocytopenia and the fact that some of the few responses that did occur were also durable after the end of treatment suggest that the dimer drug, the cytotoxic “payload” of the antibody-drug conjugate, was responsible for the effects they observed, the authors said.
The study was funded by Seattle Genetics. Dr. Phillips reported advisory board membership with the company, and four of the coauthors are employees of the company with equity interests.
SOURCE: Phillips T et al. Invest New Drugs. 2018 Aug 22. .