COMPARTMENTAL DISTRIBUTION OF TUMOR-SPECIFIC MONOCLONAL-ANTIBODIES INHUMAN-MELANOMA XENOGRAFTS

Monoclonal antibodies (MAb) are attractive for tumor therapy because o f their exquisite specificity. Although a majority of tumor cells in s mall (less than or equal to 20 mg) solid tumors can be labeled followi ng systemic administration of antitumor cell MAbs, little quantitative information is available as to the distribution of these MAbs within the several compartments that comprise solid tumors. Our goal was to p rovide such data in a well-characterized melanoma xenograft system. In accord with earlier work, i.v.-injected, melanoma-specific MAbs 436 a nd IND1, directed, respectively, against the 125 kD and HMW-melanoma-a ssociated antigens, accumulated in M21 and SK-MEL-2 tumor xenografts i n amounts of similar to 20 % of injected dose/g. However, only 20-24% of the MAbs present in tumor xenografts was bound to tumor cells; the great majority (76-80%) was in the tumor extracellular fluid (ECF) and collagenous residue fractions. These results could not be accounted f or by MAb degradation or release of MAbs from tumor cells during xenog raft dissociation. Rather, they reflected in large part interactions o f MAbs with antigens which tumors had shed into the ECF. Thus, 48 h af ter i.v. injection of 20 mu g of melanoma-specific, biotin-tagged MAb4 46-66% of that present in the tumor ECF was complexed with melanoma-a ssociated antigens. Overall, 61-73 % of the MAbs recovered from tumor xenografts were bound to tumor antigens (either to tumor cells themsel ves or to tumor-shed antigens). In contrast, only similar to 4% of a m elanoma-nonspecific MAb (B72.3) accumulated per g tumor after i.v. inj ection and nearly all of this was free in the ECF. Consistent with the se data, fluorescence microscopy revealed that i.v. injected, fluoresc ein-tagged MAbs achieved highest concentrations in tumor stroma, parti cularly at the tumor-host interface. Flow cytometry of dissociated sol id tumors revealed that both the fraction of MAb-labeled tumor cells a nd the amount of MAb/tumor cell could be increased by increasing the a dministered i.v. dose of melanoma-specific MAb. Nonetheless, even at t he highest i.v. injected dose (300 mu g), 15-37% of tumor cells lacked detectable MAb labeling. Taken together, the data indicate that deliv ery of tumor cell-specific MAbs to solid tumors cannot be equated with their delivery to tumor cells. This distinction is important for immu notherapeutic approaches that require MAb contact with tumor cells.