The farnesyl transferase inhibitor R115777 has been found to have clinical activity in diverse hematopoietic tumors. Clinicalefficacy, however, doesnot correlate with Rasmutationstatus orinhibition of farnesyl transferase. To further elucidate the mechanisms by which R115777 induces apoptosis and to investigate drug resistance, we have identified and characterized a R115777-resistant human myeloma cell line. 8226/R5 cells were found to be at least 50 times more resistant to R115777 compared with the parent cell line 8226/S. K-Ras remained prenylated in both resistant and sensitive cells after R115777 treatment; however, HDJ-2 farnesylation was inhibited in both lines, implying that farnesyl transferase (the drug target) has not been mutated. Whereas many 8226 lines that acquire drug resistance have elevated expression of P-glycoprotein, we found that P-glycoprotein expression is not increased in the 8226/R5 line and intracellular accumulation of R115777 was not reduced. In fact, 8226/R5 cells were insensitive to a diverse group of antitumor agents including PS-341, and multidrug resistance didnot correlate with the expressionof heat shock proteins.Comparison of gene expression profiles between resistant and sensitive cells revealed expression changes in several genes involved in myeloma survival and drug resistance. Future experiments will attempt to identify genes that are directly linked to the resistant phenotype. Identification of molecules associated with R115777 and PS-341resistance is clinically relevant because both compounds are being tested in solid tumors and hematopoietic malignancies.