HT-29 human colon
tumor cells growing as spheroids have been evaluated as a model system for measuring the response of human colon
tumor cell to
antineoplastic agents. HT-29 cells have been capacity to form spheroids up to 1 mm or more in diameter when grown in spinner culture. The multicellular HT-29 spheroids develop hypoxic centers reflecting the cellular conditions found in human
cancer treatment, i.e., nutritionally deficient hypoxic cells that are felt to be a significant source of both radiation and
chemotherapy clinical treatment failures. Spheroids of increasing size were radiated and then dispersed into single cells for colony survival assay. Compared with irradiated single cell
suspensions, the spheroid cells demonstrated a significant increase in radioresistance. Growing spheroids developed a complex radiation survival curve which was variable with respect to size of the spheroid. The
drug 5-Fu was studied to examine in a preliminary fashion its interaction with these resistant cell fractions. In direct cytotoxicity assay,
5-fluorouracil (5-FU) exhibited both cytotoxic and
cytostatic effects when the
drug was present at a concentration greater than 0.4 microgram/ml. The interaction of
5-FU with x-rays in the HT-29 spheroids was complex and dependent on the type of assay employed (spheroid size versus clonogenicity). The effect of
allopurinol, an agent that protects cells from
5-FU toxicity was examined.
Allopurinol at a concentration of 100 microgram/ml was found to protect these human colonic
carcinoma cells from the cytotoxic effects of
5-FU under conditions resembling those found in vivo. Overall, this HT-29 spheroid system appears to b an interesting model for studying a variety of
drug/x-ray interactions in vitro and may prove capable of answering specific questions of preclinical and clinical relevance.