In vitro test-system for chemo- and thermosensitivity: an analysis of survival fractions and cell-cycle distributions in human Ewing's sarcomas as a modelfor tumors in pediatric oncology.

BACKGROUND

Tumor cell resistance to anticancer drugs is the primary reason for treatment failure in childhood cancer. Resistance can exist at the onset of treatment or can become clinically apparent under selective pressure of drug exposure. In vitro predictive tests are important for the experimental study of drug resistance. Although in vitro studies appear to be fairly good for predicting drug resistance, they are rarely used in the routine management of individual cases. An exception that proves the rule is the MTT- (3-[4,5-dimethylthiazol-2-yl]-2,5- diphenyltetrazoliumbromide) assay in children with acute lymphoblastic leukemias (ALL), which can be correlated with the clinical outcome in this group of patients. In the present study we used a predictive test-system to evaluate the synergistic cytotoxic effects of chemotherapy +/- hyperthermia with respect to cell cycle disturbance.

METHODS

As a tumor model two well defined human Ewing's sarcoma cell lines VH64 and SK-ES-1 were treated for 1 h with cis-diamminedichloroplatinum II (cDDP) (0.1, 0.5, 1, 3, 5 micro g/ml) or 4'-demethyl-epipodophyllotoxin-5-(4,6-0-)-ethylidene-beta-D-glycopyranoside (VP-16) (1, 5, 10, 20, 50 micro g/ml) +/- hyperthermia (42 degrees C, 43 degrees C); control: 37 degrees C, without chemotherapy. Cell survival was tested using the XTT- (2,3-bis[2-Methoxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxanilide) assay. Assay conditions were optimized for each tumor cell line, extinction was measured 72 h post treatment at 450 nm in an ELISA-reader. Cell cycle fractions (G0/G1-, S-, G2/M-phase) were determined immediately, 12 h and 24 h after treatment by labeling proliferating tumor cells with bromodeoxyuridine (BrdU) and measuring DNA-content with propidium-iodide (PI) and analyzed by flow cytometry.

RESULTS

Survival fractions: Hyperthermia alone at 43 degrees C reduced tumor cell survival to 51 % in SK-ES-1 and 74 % in VH64. cDDP (5 micro g/ml): reduction of survival fraction to 23 % in SK-ES-1 and 33 % in VH64. cDDP (5 micro g/ml) + hyperthermia (43 degrees C): enhanced reduction of tumor cell survival compared to 37 degrees C to 11 % in SK-ES-1 and 8 % in VH64. VP-16 (50 micro g/ml): survival fraction of 18 % in SK-ES-1 and of 31 % in VH64. In contrast to cDDP, chemosensitivity of the tumor cells to VP-16 could not synergistically be enhanced by using hyperthermia. Cell cycle analysis: Hyperthermia alone at 43 degrees C induced an accumulation in G2/M and a slight reduction in G0/G1-phase 24 h after treatment, whereas the S-phase was not markedly affected. cDDP (5 micro g/ml) alone led to a prominent S-phase arrest and a G0/G1 decrease 24 h after treatment. Simultaneous application of cDDP (5 micro g/ml) + hyperthermia (43 degrees C) however significantly reduced S-phase cells. VP-16 (50 micro g/ml) alone induced a temporary S-phase arrest 12 h after treatment and a delayed G2/M-arrest after 24 h. Additional hyperthermia at 43 degrees C did not show further effects on VP-16 induced cell cycle disturbances.

CONCLUSIONS

Test-system discloses treatment-specific alterations in tumor cell survival and cell cycle distribution, e. g. synergistic enhancement of cDDP cytotoxicity by heat application, which might predict chemo- and thermosensitivity.