OXS-1550 is a bispecific scFv recombinant fusion protein-drug conjugate composed of the variable regions of the heavy and light chains of anti-CD19 and anti-CD22 antibodies and a modified form of diphtheria toxin as its cytotoxic drug payload.  CD19 is a membrane glycoprotein present on the surface of all stages of B lymphocyte development, and is also expressed on most B-cell mature lymphoma cells and leukemia cells.1  CD22 is a glycoprotein expressed on B-lineage lymphoid precursors, including precursor B acute lymphoblastic leukemia, and often is co-expressed with CD19 on mature B-cell malignancies such as lymphoma.2

OXS-1550 targets cancer cells expressing the CD19 receptor or CD22 receptor or both receptors.  When OXS-1550 binds to cancer cells, the cancer cells internalize OXS-1550 and are killed due to the action of drug's cytotoxic payload.  OXS-1550 has demonstrated success in early human clinical trials in patients with relapsed/refractory B-cell lymphoma or leukemia.

Twenty-five patients with advanced B-cell lymphoid malignancies expressing CD19 and/or CD22 were enrolled in a clinical study to evaluate OXS-1550 in a phase 1 FDA study. Patients were enrolled that had previous failed chemotherapy, immunotherapy, and/or hematopoietic (bone marrow or stem cell) transplantation. Patients received a single course of OXS-1550 according to study rules. Adverse events were successfully managed and included weight gain, low albumin, transaminitis, and fever were transient grade 1–2 and occurred in patients at the higher doses tested (>/=40µg/kg/day). Durable objective responses occurred in 2 patients at these higher doses. A complete response occurred in the only patient given a second cycle of therapy that had a 70% cancer reduction after the first cycle of therapy... The patient has been in remission for a year now. Correlative studies showed a surprisingly low incidence of neutralizing antibody (30%) production that could be related to the drug's ability to suppress antibody responses.  For further information about the clinical trial, see Bachanova, V., et. al.Clin Cancer Res; 21(6) March 15, 2015.