Antibody Response to DT–GM, a Novel Fusion Toxin Consisting of a Truncated Diphtheria Toxin (DT) Linked to Human Granulocyte–Macrophage Colony Stimulating Factor (GM), during a Phase I Trial of Patients with Relapsed or Refractory Acute Myeloid Leukemia

We are conducting a Phase I trial of a fusion toxin (DT–GM) for the treatment of relapsed or refractory acute myeloid leukemia (AML). The fusion toxin consists of a truncated diphtheria toxin (DT) linked to human granulocyte–macrophage colony stimulating factor (GM). Prior to beginning the Phase I trial, our first goal was to determine whether healthy controls and adult AML patients had preexisting antibodies able to inhibit DT–GM. Sera from 5 of the 9 controls completely neutralized DT–GM by an in vitro bioassay to assess the inhibition of DT–GM. Sera from 43 patients with AML were tested by bioassay and a specific enzymoimmunoassay (EIA) for anti-DT–GM antibodies. Forty-two of 43 samples were positive by EIA, and 5 patients (11.6%) showed complete neutralization of DT–GM in the bioassay. Anti-DT–GM concentrations were significantly higher in samples demonstrating neutralization than in samples demonstrating no neutralization (P = 0.003). In the Phase I trial of DT–GM prior to therapy, none of 28 patients exhibited neutralization by bioassay, but 89% were positive by EIA. After the first course of DT–GM, 23% developed neutralizing antibodies by the bioassay, and 64% of patients exhibited an increase in their anti-DT–GM antibody concentrations by EIA. Further studies are needed to determine the clinical impact of the anti-DT–GM antibodies and whether the neutralization bioassay can be replaced by our EIA.