In vitro susceptibility of canine tumor stem cells to doxorubicin.

Thirty-four canine tumor specimens (13 sarcomas and 21 carcinomas) were grown in soft agar gel. Susceptibility after continuous exposure to doxorubicin at concentrations of 1, 5, and 10 micrograms/ml was compared with that of control cultures. Plating efficiency averaged 0.096%. Doxorubicin at a concentration of 1 microgram/ml was found to result in greater than 70% inhibition of colony formation in 3 of 13 sarcomas and 1 of 21 carcinomas tested, and greater than 50%, but less than 70%, inhibition in 3 carcinomas and 1 sarcoma. Twenty-six tumors had less than 50% reduction in colony formation and were considered resistant. Minor differences in responses to drug in primary and metastatic locations were observed and attributed to tumor heterogeneity and test variability.