Insulin-like growth factor-I receptor signaling pathway induces resistance to the apoptotic activities of SCH66336 (lonafarnib) through Akt/mammalian target of rapamycin-mediated increases in survivin expression.

Although preclinical studies have suggested that farnesyltransferase inhibitors (FTI) have promising antitumor activity, clinical trials have shown that FTI activity in patients is actually limited. The mechanism that induces resistance to FTI treatment is still not fully understood. The FTI SCH66336 has been shown to induce apoptotic and antiangiogenic activities in a subset of head and neck squamous cell carcinoma (HNSCC) and non-small cell lung cancer (NSCLC) cell lines. We therefore investigated the mechanisms mediating resistance to the therapeutic activities of SCH66336 in HNSCC and NSCLC. Our various analyses showed that insulin-like growth factor-I receptor (IGF-IR) activation interferes with the antitumor activity of SCH66336 in HNSCC and NSCLC cells. Treatment with SCH66336 activated the IGF-IR/phosphatidylinositol 3-kinase/Akt pathway, leading to increased mammalian target of rapamycin (mTOR)-mediated protein synthesis of survivin in a subset of HNSCC and NSCLC cell lines that were insensitive to the apoptotic activities of the drug. Inhibition of IGF-IR, Akt, or mTOR or the knockdown of survivin expression abolished resistance to SCH66336 and induced apoptosis in the cells. Overexpression of survivin by the use of adenoviral vector protected SCH66336-sensitive HNSCC cells from the apoptotic activities of the drug. Our results suggest that expression of phosphorylated IGF-IR, phosphorylated Akt, phosphorylated mTOR, and survivin serves as biological markers of SCH66336 responsiveness in HNSCC and NSCLC cells and that SCH66336 induces survivin expression through an IGF-IR/Akt/mTOR-dependent pathway. Thus, combining inhibitors of IGF-IR, phosphatidylinositol 3-kinase/Akt, mTOR, or survivin with SCH66336 may be an effective anticancer therapeutic strategy for patients with HNSCC or NSCLC.     

A multiplicity of anti-invasive effects of farnesyl transferase inhibitor SCH66336 in human head and neck cancer

Metastasis is a critical event in the progression of head and neck squamous cell carcinoma (HNSCC) and closely correlates with clinical outcome. We previously showed that the farnesyl transferase inhibitor SCH66336 has antitumor activities in HNSCC by inducing the secretion of insulin-like growth factor binding protein 3 (IGFBP-3), which in turn inhibits tumor growth and angiogenesis. In our study, we found that SCH66336 at a sublethal dose for HNSCC inhibited the migration and invasion of HNSCC cells. The inhibitory effect of SCH66336 was associated with the blockade of the IGF-1 receptor (IGF-1R) pathway via suppressing IGF-1R itself and Akt expression. Consistent with previous work, induction of IGFBP-3 by SCH66336 also contributed in part to the anti-invasive effect. SCH66336 treatment also reduced the expression and activity of the urokinase-type plasminogen activator (uPA) and matrix metalloproteinase 2 (MMP-2), both important regulators of tumor metastasis. The effect of SCH66336 on uPA activity was inhibited partly by knockdown of IGFBP-3 using small interfering RNA. The inhibitory effect of SCH66336 on migration or invasion was attenuated partly or completely by knockdown of IGFBP-3, Akt or IGF-1R expression, respectively. Our results demonstrate that the IGF-1R pathway plays a major role in the proliferation, migration and invasion of HNSCC cells, suggesting that therapeutic obstruction of the IGF-1R pathway would be a useful approach to treating patients with HNSCC.     

Farnesyl transferase inhibitor SCH66336 is cytostatic,pro-apoptotic and enhances chemosensitivity to cisplatin in melanoma cells

The constitutive activity of a number of growth and cell survival pathways are thought to contribute to the inherent resistance of melanoma to chemotherapy and radiotherapy. Many of these pathways are driven through the small GTPase Ras. Novel drugs such as the farnesyl transferase inhibitors (FTIs) and farnesyl thiosalicylic acid (FTS) interfere with the signaling of oncogenic Ras. The aim of our study was to assess the anti-tumour activity of the FTI SCH66336 in melanoma and to assess whether SCH66336 and FTS could modulate chemoresistance in melanoma cells. SCH66336 had marked anti-proliferative activity in both human and mouse melanoma cell lines, but not in non-transformed NIH 3T3 cells. The anti-proliferative activity of SCH66336 was due to G1-phase cell cycle arrest and retinoblastoma protein inactivation, followed by apoptosis. Cisplatin, when administered alone, induced little apoptosis. In combination with cisplatin, both FTS and SCH66336 markedly enhanced the level of cisplatin-induced apoptosis, an effect that was associated with enhanced G2/M cell cycle arrest. Pharmacological inhibitors of either ERK or PI-3 kinase/Akt did not mimic the chemosensitising activity of either SCH66336 or FTS. In summary, our study demonstrates that SCH66336 has good in vitro anti-tumour activity in both human and mouse melanoma cell lines, and suggests that Ras antagonists could be useful in overcoming chemoresistance to cisplatin in melanoma.     

Identification of insulin-like growth factor binding protein-3 as a farnesyl transferase inhibitor SCH66336-induced negative regulator of angiogenesis in head and neck squamous cell carcinoma.

The farnesyl transferase inhibitor (FTI) SCH66336 has been shown to have antitumor activities in head and neck squamous cell carcinoma (HNSCC) in vitro and in vivo. However, its mechanism of action has not been well defined. Here, we report that the insulin-like growth factor (IGF) binding protein (IGFBP)-3 mediates antitumor activities of SCH66336 in HNSCC by inhibiting angiogenesis. SCH66336 significantly suppressed HNSCC tumor growth and angiogenesis via mechanisms that are independent of H-Ras and RhoB. By inducing IGFBP-3 secretion from HNSCC cells, this compound suppresses angiogenic activities of endothelial cells, including vessel formation in chorioallantoic membranes of chick, endothelial cell sprouting from chick aorta, and capillary tube formation of human umbilical vascular endothelial cells (HUVEC). Knockdown of IGFBP-3 expression in HNSCC cells by RNA interference or depletion of IGFBP-3 in HUVECs by neutralizing antibody effectively blocked the effects of IGFBP-3 secreted from SCH66336-treated HNSCC cells on HUVECs. These findings suggest that IGFBP-3 could be a primary target for antitumor activities of FTIs and that IGFBP-3 is an effective therapeutic approach against angiogenesis in HNSCC.     

Farnesyltransferase inhibitor SCH66336 induces rapid phosphorylation of eukaryotic translation elongation factor 2 in head and neck squamous cell carcinoma cells

Farnesyltransferase inhibitors (FTI) are a class of therapeutic agents designed to target tumors with mutations of the ras oncogene. However, the biological effect of FTIs is often independent of ras mutation status, which suggests the existence of additional mechanisms. In this study, we investigated the molecular effects of SCH66336, an FTI, in head and neck squamous cell carcinoma cells using proteomic approaches. We showed that SCH66336 induced phosphorylation (inactivation) of eukaryotic translation elongation factor 2 (eEF2), an important molecule for protein synthesis, as early as 3 hours after SCH66336 administration. Protein synthesis was subsequently reduced in the cells. Paradoxically, activation of eEF2 kinase (eEF2K), the only known kinase that regulates eEF2, was observed only at 12 hours after SCH66336 treatment. Consistent with this observation, the inhibition of phosphorylated-MEK and phosphorylated-p70S6K, the two key signaling molecules responsible for activation of eEF2K, also occurred at least 12 hours after SCH66336 administration. Our data suggest that inhibition of protein synthesis through inactivation of eEF2 is a novel mechanism of SCH66336-mediated growth inhibition and that this effect is independent of ras-MEK/p70S6K-eEF2K signaling cascades.     

In vitro study of farnesyltransferase inhibitor SCH 66336, in combination with chemotherapy and radiation, in non-small cell lung cancer cell lines

K-ras alterations have been reported in 20-30% of non-small cell lung cancer (NSCLC) and represent a suitable target for the development of novel anticancer agents, such as Farnesyl transferase inhibitors (FTi), a new class of agents inhibiting the post-translational modification of the K-ras proteins. The effectiveness of FTi SCH66336 in inhibiting cell proliferation and deranging cell cycle of NSCLC cell lines as well as its interaction with chemotherapy or radiation have been evaluated. The activity of FTi SCH66336, alone or in combination with paclitaxel, gemcitabine, and radiotherapy, was examined in 3 cell lines, A-549, LX-1 and CaLu-6, by colorimetric MTT assay. Cell cycle perturbation and apoptosis were also assessed by cytofluorimetric analysis. The activity of SCH 66336 was found to be concentration- and time-dependent. The effect of SCH 66336, as demonstrated by cell growth recovery experiments, resulted cytostatic and it was superimposable in both cell lines bearing 2 different K-ras mutations (A-549 and LX-1) and in K-ras wild-type Ca-Lu-6. In all cell lines the combination of SCH 66336 and paclitaxel resulted in a synergism of action when SCH 66336 followed paclitaxel treatment, whereas, antagonism was found when SCH 66336 preceded paclitaxel treatment. No significant synergism or addition with SCH 66336 followed by radiation treatment was noted. Different cell cycle phase blocks at various drug concentrations were observed. In conclusion, SCH 66336 displays concentration-dependent cytostatic antitumour activity and schedule-dependent synergy with 2 commonly used anticancer agents in NSCLC cell lines. Further clinical testing of these combinations is warranted.     

A farnesyltransferase inhibitor increases survival of mice with very advanced stage acute lymphoblastic leukemia/lymphoma caused by P190 Bcr/Abl.

Treatment of chronic myelogenous leukemia with a specific inhibitor of the Bcr/Abl tyrosine kinase, imatinib, has shown great promise. However, acute lymphoblastic leukemias that express Bcr/Abl only transiently respond to imatinib. Therefore, alternative treatments for this type of leukemia are urgently needed. Here, we examined the activity of the farnesyltransferase inhibitor SCH66336 as a single chemotherapeutic agent in a nude mouse model representative of very advanced stage Bcr/Abl P190-positive lymphoblastic leukemia/lymphoma. Our results show that oral administration of the inhibitor was able to significantly increase the survival of these mice compared to controls treated with vehicle (P<0.005), and caused marked regression of the tumor burden in the treated mice. Upon prolonged treatment, lymphomas re-emerged and a subset of cells from two of such lymphomas tested was able to survive in the presence of increased concentrations of SCH66336. The same cells, however, remained sensitive towards imatinib. A combination of the two drugs, preceded by a therapy to reduce the initial tumor burden, could be very effective in the treatment of Ph-positive ALL. We conclude that SCH66336, on its own, is remarkably effective in eradicating large numbers of lymphoblastic lymphoma cells and causing visible reduction in tumor size, with minimal toxicity.     

Synergy of the protein farnesyltransferase inhibitor SCH66336 and cisplatin in human cancer cell lines.

The enzyme protein farnesyltransferase, which catalyzes the first step in the posttranslational modification of ras and a number of other polypeptides, has emerged as an important target for the development of anticancer agents. SCH66336 is one of the first farnesyltransferase inhibitors to undergo clinical testing. In the present study, we examined the effect of combining SCH66336 with several classes of antineoplastic drugs in various human tumor cell lines. Flow cytometry indicated that SCH66336 had no effect on the cell cycle distribution of treated cells. Nonetheless, colony-forming assays revealed that the antiproliferative effects of SCH66336 and 5-fluorouracil were less than additive. In contrast, the effects of SCH66336 and melphalan were additive. Moreover, the combination of SCH66336 + cisplatin produced antiproliferative effects that were additive or synergistic over a broad range of clinically achievable concentrations in A549 non-small cell lung cancer cells and T98G human glioblastoma cells, but less than additive in MCF-7 breast, HCT116 colon, or BxPC-3 pancreatic adenocarcinoma cells. Examination of the effect of drug sequencing in A549 cells revealed synergism when cells were exposed to SCH66336 and then cisplatin and antagonism when drugs were administered in the opposite order. The additive and synergistic effects resulted in enhanced apoptosis with the SCH66336 + cisplatin combination. Additional studies failed to show any effect of SCH66336 on the formation or removal of platinum-DNA adducts, raising the possibility that SCH66336 is affecting survival of cisplatin-treated cells downstream of the DNA lesions. These observations suggest that SCH66336 exhibits additive or synergistic effects when combined with cisplatin in a sequence- and cell line-dependent fashion. Additional preclinical and clinical study of this combination appears warranted.     

The farnesyl protein transferase inhibitor SCH66336 synergizes with taxanes in vitro and enhances their antitumor activity in vivo

Purpose: SCH66336 is an orally active, farnesyl protein transferase inhibitor. SCH66336 inhibits ras farnesylation in tumor cells and suppresses tumor growth in human xenograft and transgenic mouse cancer models in vivo. The taxanes, paclitaxel (Taxol) and docetaxel (Taxotere) block cell mitosis by enhancing polymerization of tubulin monomers into stabilized microtubule bundles, resulting in apoptosis. We hypothesized that anticancer combination therapy with SCH66336 and taxanes would be more efficacious than single drug therapy. Methods: We tested the efficacy of SCH66336 and taxanes when used in combination against tumor cell proliferation in vitro, against NCI-H460 human lung tumor xenografts in nude mice, and against mammary tumors in wap-ras transgenic mice. Results: SCH66336 synergized with paclitaxel in 10 out of 11 tumor cells lines originating from breast, colon, lung, ovary, prostate, and pancreas. SCH66336 also synergized with docetaxel in four out of five cell lines tested. In the NCI-H460 lung cancer xenograft model, oral SCH66336 (20 mg/kg twice daily for 14 days) and intraperitoneal paclitaxel (5 mg/kg once daily for 4 days) caused a tumor growth inhibition of 56% by day 7 and 65% by day 14 compared to paclitaxel alone. Male transgenic mice of the wap-ras/F substrain [FVB/N-TgN(WapHRAS)69LlnYSJL] spontaneously develop mammary tumors at 6–9 weeks of age which have been previously shown to be resistant to paclitaxel. Paclitaxel resistance was confirmed in the present study, while SCH66336 inhibited growth of these tumors. Most importantly, SCH66336 was able to sensitize wap-ras/F mammary tumors to paclitaxel chemotherapy. Conclusion: Clinical investigation of combination therapy using SCH66336 and taxanes in cancer patients is warranted. Further, SCH66336 may be useful for sensitizing paclitaxel-resistant tumors to taxane treatment. Received: 30 November 1999 / Accepted: 10 May 2000     

Combination therapy with the farnesyl protein transferase inhibitor SCH66336 and SCH58500 (p53 adenovirus) in preclinical cancer models

SCH66336 is a p.o.-active, farnesyl protein transferase inhibitor. SCH66336 inhibits farnesylation of RAS and other proteins in tumor cells and suppresses tumor growth in human xenograft and transgenic mouse cancer models in vivo. SCH58500 is a replication-deficient, recombinant adenovirus, which expresses the human p53 tumor suppressor. In preclinical models, SCH58500 has therapeutic efficacy against a wide range of human tumor types containing nonfunctional p53 and enhanced activity in combination with many chemotherapeutic drugs. Here we report that combination therapy with SCH66336 and SCH58500 has synergistic or additive antiproliferative effects on a panel of tumor cells lines in vitro. The efficacy of the three-drug combination of SCH66336, SCH58500, and paclitaxel was also examined in vitro. Each two-drug interaction displayed such marked synergy, the addition of a third drug to the statistical model could only yield additivity. Greater combined efficacy for SCH66336 and SCH58500 was also observed in vivo in the DU-145 human prostate and wap-ras/F transgenic mouse cancer models.     

Akt/protein kinase B is constitutively active in non-small cell lung cancer cells and promotes cellular survival and resistance to chemotherapy and radiation

To evaluate the role of Akt/PKB in non-small cell lung cancer (NSCLC) survival, we analyzed NSCLC cell lines that differed in tumor histology as well as p53, Rb, and K-ras status. Constitutive Akt/protein kinase B (PKB) activity was demonstrated in 16 of 17 cell lines by maintenance of S473 phosphorylation with serum deprivation. Additional analysis of five of 2these NSCLC lines revealed that phosphorylation of S473 and T308 correlated with in vitro kinase activity. Akt/PKB activation was phosphatidylinositol 3-kinase-dependent and promoted survival because the phosphatidylinositol 3 inhibitors LY294002 and wortmannin inhibited Akt/PKB phosphorylation, Akt/PKB activity, and increased apoptosis only in cells with active Akt/PKB. To test whether Akt/PKB activity promoted therapeutic resistance, LY294002 was added with individual chemotherapeutic agents or irradiation. LY294002 greatly potentiated chemotherapy-induced apoptosis in cells with high Akt/PKB levels, but did not significantly increase chemotherapy-induced apoptosis in cells with low Akt/PKB levels. Combined with radiation in cells with active Akt/PKB, LY294002 additively increased apoptosis and inhibited clonogenic growth. These results were extended with transiently transfected Akt/PKB mutants. Transfecting dominant negative Akt/PKB decreased Akt/PKB activity and increased basal apoptosis as well as chemotherapy- and irradiation-induced apoptosis only in cells with high Akt/PKB activity. Conversely, transfecting constitutively active Akt/PKB into cells with low Akt/PKB activity increased Akt/PKB activity and attenuated chemotherapy- and radiation-induced apoptosis. We therefore identify Akt/PKB as a constitutively active kinase that promotes survival of NSCLC cells and demonstrate that modulation of Akt/PKB activity by pharmacological or genetic approaches alters the cellular responsiveness to therapeutic modalities typically used to treat patients with NSCLC.     

Inducible NO synthase confers chemoresistance in head and neck cancer by modulating survivin

The dual role of the inducible NO synthase (iNOS) and NO signaling in head and neck squamous cell carcinoma (HNSCC) is a complex and can both promote or inhibit tumor progression. However, the underlying molecular mechanisms are not yet resolved in detail. We show for the first time that conditions, favoring low NO levels conferred resistance against cisplatin/taxol‐induced apoptosis in HNSCC cell lines. Cytoprotection was mediated by survivin, because we observed its upregulation subsequent to low doses of the NO donors S‐nitroso‐N‐acetyl‐penicillamine (SNAP) and sodium nitroprusside (SNP) or ectopic expression of physiologic amounts of iNOS. Also, RNAi‐mediated depletion of survivin blocked NOs anti‐apoptotic effects. Induction of survivin involves activation of the phosphatidylinositol‐3‐kinase/Akt (PI3K/Akt) pathway, which was antagonized by the PI3K‐inhibitor LY294002. Importantly, application of the iNOS‐specific inhibitor 1400W combined with RNAi‐mediated downregulation of survivin cooperatively enhanced drug‐induced cell death. The iNOS/survivin‐axis appears to be also of clinical relevance since immunohistochemistry revealed that iNOS expression correlated with enhanced survivin levels in HNSCC specimens. In contrast, high NO concentrations suppressed survivin levels in HNSCC but also in non‐malignant cells resulting in apoptosis. Cell death induced by high amounts of SNAP/SNP or by strong overexpression of iNOS involved activation of p38MAP‐kinase, which was counteracted by the p38MAP‐kinase inhibitor SB202190. Here, we provide evidence for a novel molecular mechanism how NO signaling may contribute to therapy resistance in HNSCC by modulating survivin expression. Our data further suggest pursuing pharmacogenetic iNOS/survivin‐targeting approaches as potential therapeutic strategies in head and neck cancer. © 2008 Wiley‐Liss, Inc.     

Multicentre EORTC study 16997: feasibility and phase II trial of farnesyl transferase inhibitor & gemcitabine combination in salvage treatment of advanced urothelial tract cancers

In this study, the feasibility and activity of combined chemotherapy of the farnesyl transferase inhibitor SCH66336 and gemcitabine was evaluated. This therapy was used as second-line treatment in patients with advanced urothelial tract cancer and the influence of SCH66336 exposure on the pharmacokinetics of gemcitabine was also determined. Patients who had received one previous chemotherapy regime for advanced urothelial cancer were treated with a combination of SCH66336 (150 mg in the morning and 100 mg in the evening) and Gemcitabine (1000 mg/m2 on day 1, 8 and 15 per 28-day cycle). Dosages of gemcitabine and its metabolite dFdU were performed on day one of cycle 1 before exposure to SCH66336 and day one of cycle 2. A total of 152 cycles were administered in 33 patients (median 3, range: 1-15). No patients had severe hematological toxicity, defined as Grade 4 thrombocytopenia or febrile neutropenia. Nine partial responses and one complete response were achieved in 31 assessable patients and corresponded to an overall response rate of 32.3% [95% CI:17%-51%]. There was no influence of exposure to SCH66336 on the level of gemcitabine or dFdU in 11 assessable patients. In conclusion, a combination of SCH66336 and gemcitabine is feasible in terms of toxicity and active as second-line treatment in patients with advanced urothelial tract cancer. SCH66336 had no effect on the pharmacokinetics of gemcitabine. Randomised trials should be undertaken to clarify the role of SCH66336 in combination with gemcitabine in cancer treatment.     

A Phase I trial of the farnesyl transferase inhibitor SCH66336: evidence for biological and clinical activity

Farnesyl protein transferase (FT), an enzyme that catalyzes the first step in the posttranslational modification of ras and a number of other polypeptides, has emerged as an important target for the development of anticancer agents. SCH66336 is one of the first FT inhibitors to undergo clinical testing. We report a Phase I trial to assess the maximum tolerated dose, toxicities, and biological effectiveness of SCH66336 in inhibiting FT in vivo. Twenty patients with solid tumors received 92 courses of escalating SCH66336 doses given orally twice a day (b.i.d.) for 7 days out of every 3 weeks. Gastrointestinal toxicity (nausea, vomiting, and diarrhea) and fatigue were dose-limiting at 400 mg of SCH66336 b.i.d. Moderate reversible renal insufficiency, secondary to dehydration from gastrointestinal toxicity, was also seen. Inhibition of prelamin A farnesylation in buccal mucosa cells of patients treated with SCH66336 was demonstrated, confirming that SCH66336 inhibits protein farnesylation in vivo. One partial response was observed in a patient with previously treated metastatic non-small cell lung cancer, who remained on study for 14 months. This study not only establishes the dose for future testing on this schedule (350 mg b.i.d.) but also provides the first evidence of successful inhibition of FT in the clinical setting and the first hint of clinical activity for this class of agents.     

Inhibitors of the mevalonate pathway as potential therapeutic agents in multiple myeloma

Clinical studies have suggested that bisphosphonates may prolong the survival of sub-sets of myeloma patients. Newer nitrogen containing bisphosphonates such as zoledronate act, at least in part, by inhibiting farnesyl diphosphate synthase and subsequent protein prenylation, furthermore, limited data suggests that zoledronate exerts a direct anti-tumour effect against human myeloma cell lines. We therefore investigated the anti-myeloma potential of zoledronate in comparison to, and in combination with, two other inhibitors of the mevalonate pathway: the HMGCoA reductase inhibitor fluvastatin and the farnesyl transferase inhibitor SCH66336. We found that fluvastatin was able to inhibit the proliferation of myeloma cells more effectively than zoledronate or SCH66336 and that combinations of zoledronate and fluvastatin, but not zoledronate and SCH66336 acted synergistically. Our data indicated that the anti-proliferative effect of mevalonate pathway inhibitors is mediated principally via prevention of geranylgeranylation and is the result of both cell cycle arrest and apoptosis induction. Microarray and quantitative real-time PCR analyses further demonstrated that genes related to apoptosis, cell cycle control, and the mevalonate pathway were particularly affected by zoledronate and fluvastatin, and that some of these genetic effects were synergistic. We conclude that the mechanisms of geranylgeranylation inhibition mediated anti-myeloma effects warrant further evaluation and may provide novel targets for future therapeutic development.     

The farnesyl protein transferase inhibitor SCH66336 is a potent inhibitor of MDR1 product P-glycoprotein

P-glycoprotein (Pgp)-mediated drug efflux is a major factor contributing to the variance of absorption and distribution of many drugs, particularly cancer chemotherapeutics. Multidrug resistance (MDR) is caused largely by the efflux of therapeutics out of the tumor cell by Pgp, resulting in reduced efficacy of chemotherapy. SCH66336, a farnesyl transferase inhibitor in development for cancer therapy, was examined in the present study for its ability to inhibit Pgp. In a test system consisting of a NIH-G185 cell line presenting an overexpressed amount of the human transporter Pgp, known Pgp inhibitors, such as cyclosporin A, paclitaxel, verapamil, tamoxifen, and vinblastine, were demonstrated to inhibit the Pgp-mediated efflux of daunorubicin. SCH66336 significantly inhibited daunorubicin transport with an IC50 of about 3 microM and similarly affected the transport of rhodamine 123 with a potency similar to cyclosporin A. Additionally, by an ATP-hydrolysis assay, SCH66336 was shown to decrease Pgp-mediated ATP hydrolysis by >70% with a Km of 3 microM. This observation indicates that SCH66336 directly interacts with the substrate binding site of Pgp, a quality unique to SCH66336 and its analogues, although not inherent to farnesyl transferase inhibitors in general. Moreover, low concentrations of SCH66336 exhibit synergy with the Pgp substrate/inhibitors paclitaxel, tamoxifen, and vinblastine respectively by significantly potentiating their inhibition of Pgp. Treatment with SCH66336 would be predicted to be synergistic with coadministered cancer therapeutics that are substrates of Pgp. A further benefit of coadministration of SCH66336 could be reduced chemotherapy dosage, hence, lower exposure to normal cells and, therefore, less undesired toxicity.