Synergistic Effects of Novel Thiosemicarbazones and Selected Anticancer Drugs on Cells of Rare Pediatric Solid Tumors

Introduction:

Multidrug resistance is a major therapeutic obstacle for a wide variety of advanced and resistant tumor types, including pediatric solid tumors. In this study, we examined the combined treatment of cell lines derived from some types of pediatric solid tumors with selected compounds already used in metronomic chemotherapy and with novel thiosemicarbazones, patented chelators with potent and selective anti-tumor activity.

Materials and methods:

Three established cell lines were included in this study: SH-SY5Y neuroblastoma cell line, DAOY medulloblastoma cell line, and Saos-2 osteosarcoma cell line. For combined treatment, vinblastine, temozolomide, etoposide and celecoxib were chosen as therapeutic agents with different mechanisms of action on cancer cells. These compounds were combined with chelator Dp44mT and its analogue DpC. After this treatment, the changes in cell proliferation activity were evaluated by MTT assay and the synergy quantification was analyzed using Chou Talalay method. Finally, the immunoblotting was employed to assess the possible influence of thiosemicarbazones on target molecules of chemotherapeutics included in this study.

Results and discussion:

Both Dp44mT and DpC enter lysosomes where they induce lysosomal membrane permeabilization and the release of the stored drug. In SH-SY5Y neuroblastoma cells, only vincristine showed slight synergism in combination with both thiosemicarbazones. This finding was expected according the fact that vincristine undergoes lysosomal sequestration. In contrast, no similar effect via lysosomes was found in DAOY medulloblastoma cells, and only etoposide showed synergism with thiosemicarbazones. Surprisingly, Saos-2 osteosarcoma cells were the most sensitive to the combined treatment, where vincristine, temozolomide as well as celecoxib showed moderate to strong synergism in combinations with both Dp44mT and DpC. Mechanistically, the synergistic effect of the combined treatment of these thiosemicarbazones with temozolomide could be based on the ability of the thiosemicarbazones to chelate iron and decrease MGMT protein levels. Notably, MGMT is an iron-containing enzyme, which is known to be involved in resistance to temozolomide via its DNA repair activity.

Conclusions:

Our results showed that novel thiosemicarbazones are able to potentiate the cytotoxic effects of chemotherapeutics used in metronomic regimens in pediatric oncology. Furthermore, the synergism between these drugs is probably based on several different mechanisms. For example, with temolozomide, the ability of thiosemicarbazones to decrease the levels of the DNA repair enzyme, MGMT.

Supported by grant AZV MZCR 17-33104A.