Systemic toxicity of diphtheria toxin-related fragments (CRM26, CRM45), a hormone-toxin hybrid protein (TRH-CRM45), and ricin A.

As a preliminary step in evaluating the use of thyrotropin releasing hormone (TRH) linked to toxin fragments for systemic treatment of pituitary disease, we have examined the metabolic degradation, tissue distribution, renal excretion, and toxicity in rats of TRH-CRM45 which consists of TRH coupled to CRM45, a diphtheria toxin-related polypeptide. For comparison, we have similarly studied CRM45, CRM26 (a smaller diphtheria toxin-related fragment), and ricin A. All four proteins were found to concentrate in the kidney and liver relative to blood (in comparison to bovine serum albumin), tissue:plasma ratios for the kidney being much higher than those observed for the liver. Radioiodinated CRM45 and ricin A were rapidly cleared from the circulation with similar patterns. Systemic toxicity studies showed that at doses greater than 2 micrograms/100 g body wt (bw), CRM45 caused a decrease in growth rate and caused renal damage. CRM45 modified with 2-pyridyldithio(propionate) groups as well as TRH-CRM45 was significantly less toxic than CRM45, as was TRH-CRM45. CRM26 had no discernible effect on the growth rate of the animals. Ricin A, at a dose of 50 micrograms/100 g bw slowed the growth rate of rats, but specific liver or kidney damage could not be detected. These findings define an upper range of doses for possible therapeutic use.