The in vivo anti-tumor activity of immunotoxins containing two versus one deglycosylated ricin A chains

We have compared the in vitro and in vivo activities of three deglycosylated ricin A chain (dgA)-containing immunotoxins (ITs) with one versus two molecules of dgA per molecule of IgG. Two of the ITs (anti-CD19-dgA₂ and anti-CD25-dgA₂) had 3-0-fold improvements over their dgA₁ counterparts in vitro. However, the in vivo anti-tumor activity of both constructs of each IT was the same. In contrast, anti-CD22-dgA₂ was 7-fold more potent in vitro and significantly more potent in vivo than its dgA₁ counterpart. These studies suggest that monoclonal antibodies must be individually tested to determine whether the addition of two versus one dgA will improve cytotoxic activity in vivo.