Usefulness of abrin as a positive control for the human tumor clonogenic assay

A series of approaches were tested in order to develop a simple technique for introducing a routine positive control into the Human Tumor Clonogenic Assay. Of the various techniques tested, the best proved to be the addition of the toxic plant lectin abrin to the culture system. When added to the agar underlayer of the culture system so that the final abrin concentration in culture was 10 micrograms/ml, survival of human tumor colony-forming units (TCFU) was reduced to less than 1% of control in 16/34 (47%) of human tumors, to less than 10% in 33/34 (97%) and to less than 30% in all tumors tested (100%). A clear dose-response relationship to fractional survival was observed for individual tumors tested at multiple dose levels of abrin. When the dosage of abrin was reduced to 1 micrograms/ml, survival of TCFU was reduced to less than 30% in 25/28 (89.2%) of experiments. Inclusion of abrin controls in clonogenic assays thus provides an excellent and reproducible positive control to which cytotoxic effects of a variety of therapeutic agents can be compared.