The rapamycin ester, CCI-779, potently inhibits cell growth in vitro, inhibits tumor growth in vivo, and is currently in Phase I clinical trials. To further understand the relationship between plasma systemic exposure and inhibition of the target Ser/Thr kinase, mTOR/FRAP, two assays have been developed. The first assay involves determination of the 4E suppressor protein (4E-BP1) bound to eukaryotic initiation factor 4E (eIF4E), and the second is direct Western analysis of phosphorylation of residue Thr(70) of 4E-BP1. Under normal growth conditions in vitro, rapamycin caused rapid association of 4E-BP1 with eIF4E within 1 h in Rh30 and GC(3) human tumor cells. Association was persistent up to 16 h. In mice, administration of rapamycin (5 or 20 mg/kg) caused rapid association of 4E-BP1 with eIF4E within 4 h in both human colon adenocarcinoma GC(3) and rhabdomyosarcoma Rh30 xenografts. Using phospho-specific antibody against Thr(70) of 4E-BP1, rapid and persistent dephosphorylation within 30 min of exposure to rapamycin was detected in Rh18 rhabdomyosarcoma cells. Evaluation of CCI-779 against Rh18 xenografts showed this tumor to be growth inhibited at daily dose levels of > or =8.7 mg/kg. Because immunoblotting may be more suitable for assaying tumor biopsy tissue, a "blinded" comparison between the effect of CCI-779 on Thr(70) phosphorylation and growth inhibition of human tumor xenografts was undertaken. Mice were treated daily for 5 days with CCI-779 (20 mg/kg/day) or with drug vehicle, and tumor diameters were measured. Tumors were excised 1 h after the final administration and frozen, and phospho Thr(70) was determined by Western blot analysis. The correlation coefficient for decreases in Thr(70) phosphorylation and growth inhibition was high (r(2), 0.99). The results indicate that an assay of decreases in phosphorylation of Thr(70) of 4E-BP1 may be a useful surrogate for determining the inhibition of mTOR activity in tumor specimens.