A phase I/II study found that IMGN632, a novel CD123-targeting antibody-drug conjugate, was tolerable and resulted in a 29% overall response rate in patients with relapsed or refractory blastic plasmacytoid dendritic cell neoplasm (BPDCN), a rare but aggressive form of leukemia. Treatment with the agent also produced a 31% overall response rate in patients with BPDCN previously treated with tagraxofusp. Early results from the study were presented by Naveen Pemmaraju, MD, Associate Professor of Leukemia at The University of Texas MD Anderson Cancer Center, at the 2020 American Society of Hematology (ASH) Annual Meeting & Exposition (Abstract 167).

BPDCN is known as a disease with historically poor survival and limited treatment options. The first targeted therapy, tagraxofusp, was approved for BPDCN in December 2018. However, poor outcomes persist after patients experience relapse or the disease stops responding to treatment.

Naveen Pemmaraju, MD

“There’s still an urgent, unmet need for patients with relapsed/refractory BPDCN, so it’s important and necessary to have other therapies,” said Dr. Pemmaraju. “Our study shows IMGN632 provides a novel, safe, favorable treatment profile thus far for this notable patient population.”

Study Details

Researchers estimate that BPDCN affects 500 to 1,000 Americans each year. With 28 adult patients enrolled, this ongoing multicenter clinical trial is the largest study to date of prospectively treated patients with relapsed/refractory BPDCN. One patient with previously untreated BPDCN was also enrolled. The majority (52%) of patients in the study received prior intensive chemotherapy regimens, and 24% had undergone prior allogeneic stem cell transplant.

Thirteen patients (45%) had previously received tagraxofusp, which also targets the CD123 marker present in BPDCN. As an antibody-drug conjugate, IMGN632 works differently than tagraxofusp by combining a high-affinity anti-CD123 antibody with a novel DNA-alkylating payload.

BPDCN tends to affect older adults, with nearly of half of new patients diagnosed over the age of 70, and the majority of patients are male. Study demographics reflected these disease characteristics, with 76% men participating and a median age of 72 years (range = 19–82 years).

Results

Eight of the 28 relapsed/refractory patients (29%) had an objective response, with 5 patients achieving a complete response or clinical complete response. Among the 13 patients who were previously treated with tagraxofusp, 4 (31%) had an objective response to IMGN632. Nine of the 15 patients (60%) who entered the study with bone marrow disease achieved bone marrow clearance of blasts (< 5%), and 7 of these 9 patients achieved an overall response.

IMGN632 is given intravenously once every 3 weeks and can be administered in an outpatient setting. The drug had a favorable safety profile. Nausea, peripheral edema, and infusion-related reactions were the most commonly reported treatment-emergent adverse events. The most common grade 3 or higher events were thrombocytopenia, febrile neutropenia, and hyperglycemia (10% each). No cases of capillary leak syndrome were reported, and no treatment-related deaths occurred. Based on initial findings, the U.S. Food and Drug Administration granted Breakthrough Therapy designation to IMGN632 in October 2020.

“While tagraxofusp is the only approved therapy for BPDCN so far, it also has the potential for serious toxicity in the form of capillary leak syndrome, so the favorable safety data seen thus far for IMGN632 is good news for CD123 agents,” explained Dr. Pemmaraju. “Having a clinical trial focused on a rare disease is a promising development in and of itself because it’s an opportunity for patients to access a potential therapy. As the trial continues to enroll, we are expanding this program to newly diagnosed patients as well.”

Disclosure: For full disclosures of the study authors, visit ash.confex.com.