Human antibody-ribonuclease (RNase) fusion proteins, referred to as immunoRNases, have been proposed as an alternative to heterologous immunotoxins, without their immunogenicity and unspecific toxicity issues. In this study, we investigated if human pancreatic RNase will be suitable as effector component in a therapeutic antibody development platform. We generated several fusion proteins consisting of tumor-specific human immunoglobulins (IgGs) and human pancreatic RNase. Transient mammalian cell production was efficient and IgG-RNases were purified to homogeneity. Antigen binding was comparable to the parental antibodies and RNase catalytic activity was retained even in the presence of 50-fold molar excess of human cytosolic RNase inhibitor (RI). Serum stability, cell binding and internalization of IgG-RNases were comparable to the parental IgGs. Despite these promising properties, none of the IgG-RNases revealed significant inhibition of tumor cell growth in vitro even when targeting different antigens putatively employing different endocytotic pathways. The introduction of different linkers containing endosomal protease cleavage sites into the IgG-RNase did not enhance cytotoxicity. Similarly, RI evasive human pancreatic RNase variants mediated only small inhibiting effects on tumor cell growth at high concentrations, potentially reflecting inefficient cytosolic translocation. Taken together, human pancreatic RNase and variants did not prove to be generally suitable as effector component for a therapeutic antibody drug development platform.
Keywords: IgG; RNase inhibitor; antibodies; cancer therapy; human pancreatic RNase; immunoRNase; immunoglobulin.