In this study, we evaluated the activity of two novel pyrazolopyrimidine derivatives (Si 34 and Si 35) against ARO cells, a human anaplastic thyroid cancer cell line. ARO cells exposed to different concentrations of the drugs showed a reduced growth rate and an increase of mortality. After 72 h incubation, doses of 5 and 10 microM Si 34 determined a decrease of cell counts by approximately 25% and approximately 75% compared with those of control cells respectively. Similar findings were observed using Si 35. Treatment with both Si 34 and Si 35 at 10 microM increased cell mortality also ( approximately 29% and approximately 18% respectively). At these concentrations, a decrease in cyclin D1 levels was observed. To improve the biopharmaceutical properties, a liposome formulation was prepared. When entrapped in unilamellar liposomes, Si 34 exerted its cytotoxic effects even at lower doses (maximal inhibition at 5 microM) and after shorter incubation time (48 h) either in ARO or other thyroid cancer cell lines. The effects were associated with weak apoptotic death. Inhibition of epidermal growth factor-stimulated src and ERK phosphorylation, as well as reduction of migration properties of ARO cells was also observed. Moreover, the growth of tumor xenografts induced in severe combined immunodeficiency (SCID) mice was inhibited by i.v. administration of 25-50 mg/kg of the drug liposomal formulation. In conclusion, the liposomal preparation of this novel pyrazolopyrimidine derivative appears to be a promising tool for the treatment of anaplasic thyroid cancer.