Proteinated Elemental Selenium Potentiates Anti-Tumor Effect of Ionizing Radiation and Chemotherapeutic Agents but Is Only Minimally Affected by Drug Resistance Mechanisms.

Elemental selenium generated by the photobleaching of selone photosensitizers combined with serum albumin and lipoproteins to generate conjugates that were highly cytotoxic to leukemia and selected solid tumor cells but well tolerated by normal CD34-positive hematopoietic cells. Selenium-protein conjugates were internalized by an endocytotic mechanism and localized in lysosomes. Internalized conjugates caused an extremely rapid and extensive (up to 80% in 1 hour) depletion of intracellular glutathione (GSH), the oxidation of 2′,7′-dichlorofluorescin, a loss of plasma membrane asymmetry, a breakdown of mitochondrial potential, and the activation of several caspases. Additive or synergistic anti-tumor effects were obtained when Se-protein conjugates were used in combination with ionizing radiation, melphalan, amifostine, 4-hydroperoxycyclophosphamide, edelfosine, buthionine sulfoximine, or moderate doses of arsenic (III) oxide. For example, pretreatment with a nontoxic dose of amifostine enhanced the depletion of L1210 leukemia cells from 7 log to 12 log (depletions in excess of 6 log estimated from linear extrapolations of survival curves) while the yield of normal CD34-positive cells remained at 100%. Antagonistic interactions were obtained with simultaneously applied cisplatin or very high doses of arsenic (III) oxide. The antagonistic interaction with cisplatin probably reflected a competition of cisplatin and selenium for the same binding site at or near cysteine-34 of albumin. Melphalan-resistant L1210/L-PAM1 leukemia cells (elevated intracellular GSH) were more sensitive to Se-protein conjugates than wild-type cells because of an unexpected increase in conjugate uptake by mutant cells. Cisplatin-resistant H69/CDDP cells (elevated GSH, metallothioneine, glutathione-S-transferase-pi) were slightly more sensitive than wild-type cells. Cisplatin-resistant PC14/CDDP cells (reduced drug uptake) and adriamycin-resistant HL-60/ADR cells (MRP-mediated efflux) were slightly less sensitive than the corresponding wild-type cells. In the latter cases, the reduced sensitivity of mutant tumor cells correlated with reduced conjugate uptake and/or higher intracellular GSH levels. Taken together, the above results suggest that compounds modeled after our Se-protein conjugates may prove useful for the systemic therapy of leukemia and selected solid tumors. Supported by CA77387, MACC Fund, Children’s Hospital Foundation, and Wisconsin Breast Cancer Showhouse for a Cure.