Cancer combination chemotherapy with retinoids: experimental rationale.

Retinoids, cytokines as well as 1,25-dihydroxyvitamin D3 and its analogs are all classes of compounds with pleiotropic actions. They inhibit proliferation in human transformed epithelial cell lines and induce differentiation in human transformed hemopoietic cell lines. In a murine model of tumor cell-induced angiogenesis all three classes of compounds inhibit the formation of new blood vessels, necessary for supplying the growing tumor with oxygen and nutrients. Combinations of compounds from the three different classes lead to higher efficacy than the compounds administered as single agents. The effects of combinations vary depending on the individual representatives of the three classes and on the particular test models used. Additive, synergistic and potentiating effects have been observed. The results obtained in experimental systems raise hope that combination therapy might be useful in the treatment of certain human neoplastic diseases.