Recent work identifies the AKT kinase as a potential mediator of tumor expansionin multiple myeloma. The finding of PTEN mutations in several myeloma cell lines suggests that loss of PTEN function may be one mechanism by which AKT activity is increased in this disease. Because PTEN-deficient myeloma cells may have up-regulated activity of the mammalian target of rapamycin (mTOR), downstream of AKT, they may be particularly sensitive to mTOR inhibition. To test this hypothesis, we challenged myeloma cell lines with CCI-779, a newly developed analogue of rapamycin and an efficient inhibitor of mTOR. Three of four PTEN-deficient cell lines with constitutively active AKT were remarkably sensitive to cytoreduction and G₁ arrest induced by CCI-779 with ID₅₀ concentrations of <1 nm. In contrast, myeloma cells expressing wild-type PTEN were >1000-fold more resistant. Acute expression of a constitutively active AKT gene in CCI-779-resistant myeloma cells containing wild-type PTEN and quiescent AKT did not convert them to the CCI-779-sensitive phenotype. Conversely, expression of wild-type PTEN in CCI-779-sensitive, PTEN-deficient myeloma cells did not induce resistance. Differential sensitivity did not appear to be due to differences in the ability of CCI-779 to inhibit mTOR and induce dephosphorylation of p70S6kinase or 4E-BP1. However, CCI-779 inhibited expression of c-myc in CCI-sensitive PTEN-null myeloma cells but had no effect on expression in CCI-resistant cells. In contrast, cyclin D1 expression was not altered in either sensitive or resistant cells. These results indicate that PTEN-deficient myeloma cells are remarkably sensitive to mTOR inhibition. Although the results of transfection studies suggest that the level of PTEN and AKT function per se does not regulate sensitivity, PTEN/AKT status may be a good predictive marker of sensitivity.