Effects of combined antigrowth factor receptor treatment on in vitro growth of multiple myeloma

Although passive serotherapy for cancer with monoclonal antibodies is an attractive concept, it has unfortunately had limited efficacy in clinical trials. An alternative approach to passive serotherapy is targeting cell surface growth factor receptors with monoclonal antibodies. With some limitations, anti-growth factor receptor antibodies can limit cell growth by blocking stimulatory or trophic growth factor receptors and by marshaling in vivo antitumor immune responses.

The purpose of our study was to determine the extent to which anti-interleukin-6 (IL-6) and anti-transferrin (Tf) receptor antibodies, when used individually or combined, could limit myeloma cell growth.

The four myeloma cell lines studied varied in IL-6 responses from factor independence (myeloma cell lines 8226 and U266) to strict factor dependence (OCI-My4 myeloma cells and human acute myelogenous leukemia [AML] cell line UCSD/AML1). IL-6 RNA was detected using reverse transcriptase-polymerase chain reaction. IL-6 protein was detected in U266 supernatant by growth stimulation of UCSD/AML1 cells and by enzyme-linked immunosorbent assay. For cell growth assays, cell lines were plated with various concentrations of IL-6 and anti-receptor antibodies and [3H]thymidine uptake determined after 3 days. Cells were grown in varying concentrations of IgG1 monoclonal anti-Tf receptor antibodies E2.3 and A27.15 or antibodies PM1, AUK 146-15, AUK 64-7, or AUK 12-20 to the human IL-6 receptor-alpha protein. Tf and IL-6 receptors were detected by immunofluorescence staining.

Using short-term proliferation assays, anti-Tf receptors and anti-IL-6 antibodies caused dose-dependent growth inhibition of varying degrees, and, in one of three cell lines, a combination of anti-Tf and anti-IL-6 antibodies showed supra-additive growth inhibition. IL-6-independent cells were inhibited by anti-Tf receptor antibodies, while IL-6-dependent cells were resistant to these antibodies but sensitive to anti-IL-6 receptor. Factor-dependent myeloma cells exposed to either anti-Tf or anti-IL-6 receptor antibodies for 48 hours lost colony-forming capability. A combination of anti-Tf and anti-IL-6 antibodies increased elimination of colony-forming cells at 24 hours.

Anti-receptor antibodies have distinct patterns of myeloma cell growth inhibition and inhibit in vitro growth of factor-dependent myeloma cells. Combinations of anti-growth factor receptor antibodies also increase toxicity for IL-6-dependent myeloma colony-forming units.