Clonal variation in the sensitivity of a murine mammary carcinoma to melphalan

The sensitivity to melphalan of clones derived from individual lung colonies produced by i.v. injection of cells of the MT murine mammary carcinoma (caMT) and its melphalan-resistant sub-line (MTME16) has been examined. A degree of clonal heterogeneity was observed which was greater than could be explained by experimental variation. The distribution of melphalan sensitivities in both wild-type caMT and MTME16 raises questions as to the validity of a two-compartment model of drug-resistance development in tumours. A more complex model, possibly involving a continuous spectrum of drug sensitivity, is required. Differences in the sensitivity of the clonal lines of wild-type caMT in various passages were observed and this would appear to be due to phenotypic instability in these lines. This suggests that to use survival data from clones which have been passaged many times for predicting the response of the parent tumour may be misleading.