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 binds well to HT29 human colon carcinoma cells by flow cytometry, whole cell ELISA, and mixed hemadsorption. In vitro cytotoxicity studies revealed that the monoclonal antibody L6-cytosine deaminase (L6-CD) immunoconjugate plus the nontoxic prodrug, 5-fluorocytosine (5-FC), is equivalent to 5-fluorouracil (5-FU) in its ability to kill HT29 cells. Human α-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 for this cytotoxic effect (approximately, 5 pg/ml resulted in 50% viability). The limiting factor was the amount of 5-FC employed with L6-CD (3 μM yielded 50% cell viability). α-Interferon (A/D) lowered the requirement of 5-FC to 1 μM to achieve 50% cell lethality.

In vivo biodistribution experiments indicated that 1 μg of L6-CD is nonspecifically 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 μg of unlabeled L6-CD was added to 1 μg of ¹²⁵I-labeled immunoconjugate injected intravenously. Further augmentation of tumor/blood ratios without reduction in percent injected dose per gram of tumor was possible with the intravenous injection of 100 μg of anti-idiotypic monoclonal antibody 13B, 24 hours after L6-CD, which bound unreacted L6-CD and cleared it from the blood. The addition of 100,000 U of α-interferon (A/D) given intraperitoneally every day increased the clearance of L6-CD by the liver and spleen, but impaired tumor localization (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.