Evaluation of the systemic toxicity and pharmacokinetics of the immunoconjugate anti-B4-blocked ricin in non-human primates. Delivered by multiple bolus injections and by continuous infusion

The systemic toxicity of an immunoconjugate of blocked ricin and the anti-CD19 monoclonal antibody, anti-B4, was studied in cynomolgus monkeys to evaluate its safety for use in humans. Anti-B4-blocked Ricin (Anti-B4-bR) is a highly cytotoxic immunoconjugate which can kill up to 5 logs of antigen positive target cells at concentrations easily achievable in blood. Subacute toxicity studies with Anti-B4-bR were performed in 20 cynomolgus monkeys and 4 rhesus monkeys, which, unlike humans, do not express the CD19 epitope recognized by the anti-B4 antibody on their B-lymphocytes. Anti-B4-bR was administered to cynomolgus monkeys by 5 daily intravenous bolus injections of 10 or 100 μg/kg/day, and non-conjugated blocked ricin was administered by 5 daily intravenous bolus injections of 30μg/kg/day. Total doses of the conjugate of 200, 500, 1000 or 1500 μg/kg were also delivered to rhesus monkeys by continous intravenous infusion over seven days. The clinical signs of toxicity, clinical pathology parameters, and gross and microscopic tissue changes associated with Anti-B4-bR were minimal to moderate where present, and primarily hepatic. In monkeys treated with 5 × 10 μg/kg of Anti-B4-bR, lesions were noticeable on day 7 after the start of the treatment but were less severe or absent on day 14, suggesting that the toxic effects were reversible. Clearance of the conjugate from the serum after bolus injections of Anti-B4-bR was evaluated by ELISA and demonstrated an initial t12(α) of 1.4–2.0 h and a secondary t12(β) of about 14 h. Serum concentrations of Anti-B4-bR were about 10–20-fold lower at 24 h as compared to 1 h after each of the 5 bolus injections in monkeys. Continous infusion of Anti-B4-bR in primates acheived plateau levels of the immunotoxin in blood for almost the entire duration of the infusion. The therapeutic utility of the Anti-B4-bR is currently being evaluated in patients with B-cell malignancies.