EFFECTS OF A HYBRID RECOMBINANT HUMAN ALPHA-INTERFERON (A D) ON IN-VITRO CYTOTOXICITY AND IN-VIVO LOCALIZATION OF MONOCLONAL-ANTIBODY L6-CYTOSINE DEAMINASE CONJUGATE IN A COLON-CANCER MODEL/

L6 is an IgG2a murine monoclonal antibody which we have demonstrated b inds well to HT29 human colon carcinoma cells by flow cytometry, whole cell ELISA, and mixed hemadsorption. In vitro cytotoxicity studies re vealed that the monoclonal antibody L6-cytosine deaminase (L6-CD) immu noconjugate plus the nontoxic prodrug, 5-fluorocytosine (5-FC), is equ ivalent to 5-fluorouracil (5-FU) in its ability to kill HT29 cells. Hu man alpha-interferon (A/D) was able to enhance this cytotoxic effect. The I.C.(50)'s revealed that very small amounts of L6-CD are needed fo r this cytotoxic effect (approximately, 5 pg/ml resulted in 50% viabil ity). The limiting factor was the amount of 5-FC employed with L6-CD ( 3 mu M yielded 50% cell viability). alpha-Interferon (A/D) lowered the requirement of 5-FC to 1 mu M to achieve 50% cell lethality. In vivo biodistribution experiments indicated that 1 mu g of L6-CD is nonspeci fically taken up by the liver and spleen and cleared rapidly from the blood. Significant localization of L6-CD to HT29 tumors occurred only when 99 mu g of unlabeled L6-CD was added to 1 mu g of I-125-labeled i mmunoconjugate injected intravenously. Further augmentation of tumor/b lood ratios without reduction in percent injected dose per gram of tum or was possible with the intravenous injection of 100 mu g of anti-idi otypic monoclonal antibody 13B, 24 hours after L6-CD, which bound unre acted L6-CD and cleared it from the blood. The addition of 100,000 U o f alpha-interferon (A/D) given intraperitoneally every day increased t he clearance of L6-CD by the liver and spleen, but impaired tumor loca lization (percent injected dose per gram). These studies demonstrated that in vivo localization of the L6-CD conjugate to HT29 tumors could be optimized by injecting excess L6-CD followed by an equal amount of L6 anti-idiotype mAb 13B, 24 hours after L6-CD.