Initially, OVB3-PE was described as a disulfide conjugate with an ID50 for OVCAR-3 cells of 10-20 ng/ml, an LD50 for mice of 4 microgram and antitumor activity that prolonged survival of tumor- bearing mice by 40 days. The same conjugate made by thioether linkage (OVB3-PE/s-c) is 10- to 20-fold more cytotoxic (ID50 1 ng/ml) for OVCAR-3 cells and has an LD50 of 2 microgram for mice. OVBm-PE/s-c, given to nude mice bearing the OVCAR-3 tumor increased_the median survival by greater than 100 days. This result was seen when the immunotoxin was given 3-5 days after tumor cells had been implanted. Increased survival was also noted when mice were treated at much later times. To obtain approval for Phase 1 clinical testing, OVB3-PE/s-c was evaluated for toxicity in monkeys. Some elevation in liver enzymes was noted. OVB3-PE/s-c has been administered to four patients with recurrent ovarian cancer. Post-treatment sampling of peritoneal fluid showed that active immunotoxin was recovered from two of these patients. By measuring the recoverable immunotoxin activity over time, the kinetics of immunotoxin egress from the abdominal cavity was determined. The other two patients had neutralizing activity to PE. Because of this, a sensitive PE-neutralizing assay was developed to screen sera from all prospective patients. The OVB3 antibody binds some colon and breast cancer cell lines. HT-29 and MCF-7 cell lines were killed by OVB3-PE/s-c. OVB3-PE/s- c had antitumor activity against HT-29 cells growing in the peritoneal cavity of nude mice. When used alone, OVB3-PE/s-c had a modest increase in surviva. But when used in combination with chemotherapy much greater survival was achieved. Immunotoxins were also made using recombinant forms of PE such as PE40 and LysPE40. AntiTac-PE40 and HB21-PE40 were about 10-fold less active than the corresponding PE conjugates; however, antiTac- LysPE40 had equal potency to antiTac-PE.
