The response of tumour cells to radiation and cytotoxic drugs--a comparison of clonogenic and isotope uptake assays.

We have carried out a series of experiments to compare the response to radiation and drugs of cells disaggregated from solid tumours as assayed by clonogenic survival and by an isotope incorporation method. This latter assay consisted of measuring the 24 h uptake of tritium labelled thymidine into cells plated in liquid medium upon a layer of semi-solid agar. The isotope was administered 4 days after plating. For cells from the RIF-1 mouse tumour, good agreement was seen between response to radiation, adriamycin, vincristine and CCNU as measured by the two assays. The two curves for radiation response, for example, showed similar shoulders and subsequent exponential regions. For cells from xenografts of the NCI-H69 human small cell lung cancer line, the response to radiation was dose-related for both assays, but the curve for clonogenic assay was about twice as steep as that for isotope uptake. For a range of five cytotoxic drugs, good agreement was seen between the two assays over the first 1 1/2 decades of response but with a tendency for the isotope uptake curve to plateau with further increasing drug dose. It appears that, at least for these two well-defined experimental tumour systems, the isotope uptake assay can provide a rapid quantitative assessment of cellular drug and radiation sensitivity comparable to that provided by clonogenic assay but in a much shorter period of time.