OXS-3550 is a single-chain, trispecific scFv recombinant fusion protein conjugate composed of the variable regions of the heavy and light chains of anti-CD16 and anti-CD33 antibodies and a modified form of IL-15. When the NK stimulating cytokine human IL-15 is used as a crosslinker between the two scFvs, it provides a self-sustaining signal that activates NK cells and enhances their ability to kill. We intend to study this anti-CD16-IL-15-anti-CD33 tri-specific killer engager, or TriKE, in CD33 positive leukemias, a marker expressed on tumor cells in acute myelogenous leukemia, or AML, myelodysplastic syndrome, or MDS, and other hematopoietic malignancies. CD33 is primarily a myeloid differentiation antigen with endocytic properties broadly expressed on AML blasts and, possibly, some leukemic stem cells. CD33 or Siglec-3 (sialic acid binding Ig-like lectin 3, SIGLEC3, SIGLEC3, gp67, p67) is a transmembrane receptor expressed on cells of myeloid lineage. It is usually considered myeloid-specific, but it can also be found on some lymphoid cells. The anti-CD33 antibody fragment that will be used for these studies was derived from the M195 humanized anti-CD33 scFV and has been used in multiple human clinical studies. It has been exploited as target for therapeutic antibodies for many years. Improved survival seen in many patients when the antibody-drug conjugate gemtuzumab was added to conventional chemotherapy validates this approach.

The OXS-3550 IND will focus on AML, the most common form of adult leukemia with 21,000 new cases expected in 2017 alone (American Cancer Society). These patients will require frontline therapy, usually chemotherapy including cytarabine and an anthracycline, a therapy that has not changed in over 40 years. Also, about half will have relapses and require alternative therapies. In addition, about 13,000 new cases of MDS are diagnosed each year and there are minimal treatment options (Siegel et al, 2014). At a minimum, OXS-3550 can be expected to serve as a relatively safe, inexpensive, and easy to use therapy for resistant/relapsing AML. From a biologic standpoint, it could also be combined with chemotherapy as frontline therapy.