Measurement of the survival of human tumor cells after implantation in athymic nude mice

A method has been developed to determine the survival of human tumor cells after implantation in athymic nude mice. The method involves the labelling of cells of the prostatic cancer cell line PC‐3 with tritiated thymidine after attachment of the cells to Millipore filter disks. The amount of radioactive label incorporated in the cells remained constant for at least 48 hr during in vitro incubation. After s.c. implantation of the filter disks in nude mice, however, the fraction of surviving cells, reflected by the amount of radioactivity persisting on the filters, rapidly decreased with time. Only a relatively small fraction (approx. 10%) of the cells survived for more than 24 hr under these conditions. Pretreatment of the mice with cyclophosphamide (100 mg/kg, 24 hr prior to implantation) resulted in a 2‐fold increase of tumor‐cell survival. Likewise, the frequency of tumor takes after injection of PC‐3 cells was enhanced following cyclophosphamide pretreatment. These results indicate that immune defense mechanisms might be partly (but not completely) responsible for tumor‐cell rejection in nude mice. The technique described here might be a valuable tool for the rapid evaluation of possible effects of various types of pretreatment of the host animal on tumor cell survival and, thereby, on tumor growth.