A small molecule designed to bind to the adenine nucleotide pocket of Hsp90 causes Her2 degradation and the growth arrest and differentiation of breast cancer cells.

BACKGROUND: The Hsp90s contain a conserved pocket that binds ATP/ADP and plays an important role in the regulation of chaperone function. Occupancy of this pocket by several natural products (geldanamycin (GM) and radicicol) alters Hsp90 function and results in the degradation of a subset of proteins (i.e. steroid receptors, Her2, Raf). We have used the structural features of this pocket to design a small molecule inhibitor of Hsp90. RESULTS: The designed small molecule PU3 competes with GM for Hsp90 binding with a relative affinity of 15-20 microM. PU3 induces degradation of proteins, including Her2, in a manner similar to GM. Furthermore, PU3 inhibits the growth of breast cancer cells causing retinoblastoma protein hypophosphorylation, G1 arrest and differentiation. CONCLUSIONS: PU3 is representative of a novel class of synthetic compounds that binds to Hsp90 and inhibits the proliferation of cancer cells. These reagents could provide a new strategy for the treatment of cancers.

LY294002-geldanamycin heterodimers as selective inhibitors of the PI3K and PI3K-related family.

Several LY294002-GM heterodimers were synthesized with the intent of modulating their activity in the presence of hsp90 and thereby creating selective inhibitors of PI3K and PI3K-related family.

Farnesyl:protein transferase inhibitors as potential agents for the management of human prostate cancer.

The effects of farnesyl:protein transferase inhibitors (FTIs) were evaluated against hormone-dependent and hormone-independent prostate cancer cell lines harboring mutant and wild type Ras. The combinations of the FTI with hormones and chemotherapy were explored. The effect of FTI on the growth of human prostate cancer lines was examined under anchorage-dependent and -independent conditions. Changes in Ras processing and cellular localization were examined by immunoblotting and immunocytochemistry. Hormone-dependent (LNCaP) and -independent (TSU-Pr1, PC3 and DU145) human prostate cancer cell lines were growth-inhibited by the FTI L-744,832 at concentrations ranging from 100 nM to 20 &mgr;M. The inhibition was accompanied by loss of protein farnesylation and with the accumulation of Ha-Ras as its unprocessed, cytosolic form. No effect on N- and Ki-Ras processing was observed. The transformed phenotype of TSU-Pr1 cells, which possess a Ha-Ras Gly-12-Val activating mutation, reverted following FTI treatment. Enhanced antitumor effects were observed when the FTI was combined with gamma-radiation, etoposide, doxorubicin, cisplatin, estramustine and the antihormone bicalutamide. In particular, the combination of taxol and FTI was synergistic for DU145 cells, a cell line that is only marginally sensitive to the FTI alone. The sensitivity of human prostate cancer cell lines to the FTI is independent of the presence of mutations of tumor suppressors, cell cycle regulators and of the activation of a variety of oncogenes, including Ras. A cell line expressing mutated Ha-Ras is particularly sensitive. Enhanced antitumor effects were observed with an anti-androgen, gamma-irradiation, and several chemotherapeutic agents. These findings support the clinical evaluation of FTIs alone or in combination as treatment for this disease. Prostate Cancer and Prostatic Diseases (2001) 4, 33-43

92nd Annual Meeting of the American Association for Cancer Research. 24-28 March, 2001, New Orleans, Louisiana, USA.

The 92nd Annual Meeting of the AACR comprised over 5000 abstracts, 12 plenary and award lectures and numerous talks in educational sessions, symposia and mini-symposia. Given the wealth of information presented, we narrowed our coverage to the area of prenyltransferase and protein kinase inhibitors. Many rationally designed drugs are now in clinical trials and exciting results were presented for the Bcr-Abl inhibitor STI-571. The cancer community is beginning to envision new ways to evaluate and administer these well-tolerated drugs which do not fit the traditional anticancer drug profile. There is an emphasis in developing surrogate markers for evaluating the mechanism-based effectiveness as well as identifying off-target toxicities. In addition, there is a large effort in investigating effective drug combinations and the use of these new agents as radiosensitisers. Here we present specific examples of these issues as applied to prenylation and protein kinase inhibitors.

A peptidomimetic inhibitor of ras functionality markedly suppresses growth of human prostate tumor xenografts in mice. Prospects for long-term clinical utility.

PURPOSE: These studies sought to evaluate the antitumor properties of an inhibitor of ras functionality, L-744,832, which acts at the level of its associated protein farnesyltransferase. METHODS: Studies were carried out to measure the effects of L-744,832 alone and in combination with paclitaxel (PTXL) against TSU-PR1, DU-145 and PC-3 human prostate tumors xenografted to NCR-nul (AT) mice. Tumor-bearing mice were treated on a schedule of daily for 5 days x2 or 3 with the MTD of L-744,832 and every 3-4 days x4 with the MTD of PTXL starting 3-5 days after tumor implantation. Tumor volume in millimeters (4/3pir3) was measured 3 5 days after cessation of treatment and the increase in tumor volume in treated and control groups compared. Statistical analysis was carried out by the Chi-squared test. RESULTS: L-744,832 at its MTD markedly inhibited the growth of all three tumors (TIC for increase in tumor mass varied from 11% to 15% and inhibition of growth had a rapid onset (within 1-2 days) and was independent of ras gene status. Estimated tumor doubling times were 8-12-fold greater in treated animals than in control animals. Treatment with L-744,832 for as long as 3 weeks had no untoward effects on the mice as determined by gross examination or necropsy. Administration of L-744,832 with this same dose and schedule potentiated the growth-inhibitory effect of PTXL at its MTD and induced some regression of TSU-PR1 with no obvious deleterious effects on the mice. CONCLUSIONS: L-744,832 could be safely administered over a protracted period of time to mice at doses which were markedly inhibitory to the growth of three human prostate tumor xenografts and in combination with PTXL was also well tolerated and brought about some regression of the TSU-PR1 tumor. Overall, these results suggest that L-744,832 could be clinically useful for long-term treatment of early-stage prostate cancer in patients and as an adjunct to cytotoxic therapy for late stages of this disease.

Synthesis and evaluation of geldanamycin-testosterone hybrids.

Geldanamycin (GDM) binds to the Hsp90 chaperone protein resulting in the degradation of several important signaling proteins. A series of GDM-testosterone linked hybrids has been synthesized and evaluated for activity against prostate cancer cell lines. The hybrid with the greatest activity exhibits potent and selective cytotoxicity against prostate cancer cells containing the androgen receptor.

Identification of a geldanamycin dimer that induces the selective degradation of HER-family tyrosine kinases.

Geldanamycin (GM) is a natural antibiotic that binds Hsp90 and induces the degradation of receptor tyrosine kinases, steroid receptors, and Raf. It is a potent inhibitor of cancer cells that overexpress HER-kinases, but its effects on other important proteins may cause significant toxicity and limit its clinical use. We report the synthesis and identification of a GM dimer, GMD-4c, which had selective activity against HER-kinases. Selectivity was a function of linker length and required two intact GM moieties. GMD-4c is a potent inducer of G1 block and apoptosis of breast cancer cell lines that overexpress HER2, but does not appreciably inhibit the growth of 32D cells that lack HER-kinases. GMD-4c could be useful in the treatment of carcinomas dependent on HER-kinases.

Angiogenesis-research frontiers. A basic science conference of the New York Academy of Medicine.

Angiogenesis, the development of new blood vessels, is essential for both tumour growth and metastasis. Recent advances in our understanding of the molecular mechanisms underlying the angiogenic process and its regulation have led to the discovery of a variety of targets for therapeutic intervention. The potential application of these angiogenic inhibitors is currently under intense preclinical and clinical investigation. Compelling evidence suggests that vascular endothelial growth factors (VEGFs) and their receptors play critical roles in tumour-associated angiogenesis. Tumour homing factors will drive the growth of new vessels, neoangiogenesis, to satisfy the demands of the growing tumour. By attacking the angiogenic process the tumour will he starved for oxygen and nutrients, thus impairing its growth. This has been demonstrated in a variety of animal tumour models in which disabling the function of VEGF or its receptor was shown to inhibit both tumour growth and metastasis. The New York Academy of Medicine organised a day-long meeting to discuss emerging ideas, currently available in vitro and animal models and evaluation of these therapies during their preclinical development and in clinical trials.

Synthesis and evaluation of geldanamycin-estradiol hybrids.

Geldanamycin (GDM) binds to the Hsp90 chaperone protein and causes the degradation of several important signalling proteins. A series of novel estradiol-geldanamycin hybrids has been synthesized and evaluated for their ability to induce the selective degradation of the estrogen receptor (ER). The hybrid compounds are active and more selective than the parent causing degradation of ER and HER2, but not other GDM targets.

The microtubule-stabilizing agents epothilones A and B and their desoxy-derivatives induce mitotic arrest and apoptosis in human prostate cancer cells.

Epothilones are a new class of natural products that bind to tubulin and prevent the depolymerization of microtubules, although they have no structural similarity to paclitaxel. Taxanes are only marginally effective in the treatment of disseminated prostate cancer, although they may have useful activity when administered in combination with estramustine. Unlike paclitaxel, epothilones are not substrates for P-glycoprotein and are active in multidrug resistant cells. Epothilones A and B (EA, EB) have recently been synthesized in toto. In this report, we examine the effects of synthetic epothilones and their desoxy derivatives, as well as paclitaxel, on prostate cancer cell lines. EB was the most active of these compounds in tissue culture (IC(50): 50-75 pM), four to ten-fold more potent than paclitaxel. EA and the desoxyderivatives of EA and EB (dEA, dEB) were also active, but less potent than EB. Each of these compounds causes mitotic block followed by apoptotic cell death. The relative potencies for cell cycle arrest and cytotoxicity directly correlate with the ability of the drugs to bind microtubules, stabilize mitotic spindles and induce the formation of interphase microtubule bundles. Therefore, synthetic epothilones are potent inhibitors of prostate cancer cell lines and work in a fashion similar to paclitaxel. Recently, we showed that farnesyl transferase inhibitors sensitize tumor cells to paclitaxel-induced mitotic arrest. We now have extended these observations to show that paclitaxel and the epothilones synergize with FTI to arrest the growth of prostate cancer cells. Moreover, this occurs in DU145, a cell line that is not particularly sensitive to the FTI. The combination of FTI and epothilone represent a new potential clinical strategy for the treatment of advanced prostatic cancer.

A farnesyl-protein transferase inhibitor induces p21 expression and G1 block in p53 wild type tumor cells.

Farnesylation is required for the membrane partition and function of several proteins, including Ras. Farnesyl-protein transferase inhibitors (FTIs) were developed to prevent Ras processing and thus to be effective agents for the treatment of cancers harboring mutated ras. However, FTIs inhibit the growth of most tumor cells and several xenograft models, irrespective of whether they possess mutated ras. Furthermore, the antiproliferative effect is not correlated with inhibition of Ki-Ras processing; tumors with wild type ras are inhibited, and FTIs are not particularly toxic. These data suggest that the mechanism of FTI action is complex and may involve other targets besides Ras. To begin to understand how FTIs work, we investigated the mechanism of growth inhibition. FTI causes G1 arrest in a subset of sensitive lines. This is accomplished by transcriptional induction of p21, which mediates the inhibition of cyclin E-associated protein kinase activity, pRb hypophosphorylation and inhibition of DNA replication. Induction of p21 is p53-dependent; it does not occur in cells with mutant p53 or in cells expressing human papillomavirus E6. However, neither p53 nor p21 are required for inhibition of cell proliferation. FTI still blocks the growth of cells deficient in these proteins. In the absence of p21, G1 block is relaxed, DNA replication is not affected, and cells become polyploid and undergo apoptosis. These results suggest that farnesylated protein(s) may be involved in regulating p53 function and in coordinating entrance into S, and that the consequences of FTI treatment are a function of the other mutations found in the tumor cell.

Farnesyl transferase inhibitors cause enhanced mitotic sensitivity to taxol and epothilones.

An important class of cellular proteins, which includes members of the p21ras family, undergoes posttranslational farnesylation, a modification required for their partition to membranes. Specific farnesyl transferase inhibitors (FTIs) have been developed that selectively inhibit the processing of these proteins. FTIs have been shown to be potent inhibitors of tumor cell growth in cell culture and in murine models and at doses that cause little toxicity to the animal. These data suggest that these drugs might be useful therapeutic agents. We now report that, when FTI is combined with some cytotoxic antineoplastic drugs, the effects on tumor cells are additive. No interference is noted. Furthermore, FTI and agents that prevent microtubule depolymerization, such as taxol or epothilones, act synergistically to inhibit cell growth. FTI causes increased sensitivity to induction of metaphase block by these agents, suggesting that a farnesylated protein may regulate the mitotic check point. The findings imply that FTI may be a useful agent for the treatment of tumors with wild-type ras that are sensitive to taxanes.

Structure and function of the insulin-like growth factor I receptor.

Insulin-like growth factors I and II (IGF-I, IGF-II) were originally identified as potent mitogens and as the mediators of growth hormone action. Besides being mitogenic, however, these polypeptide growth factors play a crucial role in cell survival, and contribute to transformation and to maintenance of the malignant phenotype. Here we will discuss signaling by the IGFs, focusing specifically on the structure and function of the IGF-I receptor and the domains of this receptor responsible for distinct IGF functions: mitogenesis, transformation, and protection from apoptosis. We will also compare the structural domains of the related but functionally distinct receptor for insulin.

Targeted therapy for prostate cancer: the Memorial Sloan-Kettering Cancer Center approach.

Carcinoma of the prostate represents a wide range of diseases with differing prognoses. A key to selecting treatment depends on the ability to predict the natural history of the disease for the individual. Thus far, non-hormonal approaches have not demonstrated a survival advantage in randomized comparisons and, clearly, innovative approaches are needed. The clinical trials program developed at Memorial Sloan-Kettering Cancer Center is based on specific manifestations and specific targets of the disease and the predicted prognosis, using prostate-specific antigen and acid phosphatase changes as biomarkers of progression and response. In patients with minimal disease who have received local treatments but progressed systemically, we are studying methods aimed at stimulating their immune systems either by nonspecific immunopotentiation or specific immunization to specific glycoprotein or carbohydrate targets on the cancer cells, or to anti-growth factor receptor antibody aimed at blocking the specific signalling pathways that contribute to hormonal failure. These and other approaches provide an opportunity to treat this disease while maintaining an acceptable quality of life for patients.

Pharmacologic shifting of a balance between protein refolding and degradation mediated by Hsp90.

The role of the abundant stress protein Hsp90 in protecting cells against stress-induced damage is not well understood. The recent discovery that a class of ansamycin antibiotics bind specifically to Hsp90 allowed us to address this problem from a new angle. We find that mammalian Hsp90, in cooperation with Hsp70, p60, and other factors, mediates the ATP-dependent refolding of heat-denatured proteins, such as firefly luciferase. Failure to refold results in proteolysis. The ansamycins inhibit refolding, both in vivo and in a cell extract, by preventing normal dissociation of Hsp90 from luciferase, causing its enhanced degradation. This mechanism also explains the ansamycin-induced proteolysis of several protooncogenic protein kinases, such as Raf-1, which interact with Hsp90. We propose that Hsp90 is part of a quality control system that facilitates protein refolding or degradation during recovery from stress. This function is used by a limited set of signal transduction molecules for their folding and regulation under nonstress conditions. The ansamycins shift the mode of Hsp90 from refolding to degradation, and this effect is probably amplified for specific Hsp90 substrates.

Signal transduction pathways induced by heregulin in MDA-MB-453 breast cancer cells.

Heregulins (HRGs) induce tyrosine phosphorylation of several members of the erb-B family of receptors. Although originally isolated as the ligands for p185c-erb-2, recent evidence suggests that other receptors of the erbB family, including p180erbB-3 and p180erbB-4, are their true cognate receptors. Stimulation of MDA MB-453 cells with HRG beta 2 resulted in the tyrosine phosphorylation of p185c-erbB-2 and p180erbB-4 in a time- and dose-dependent fashion. This event was accompanied by the formation of multimeric complexes between the activated receptors and SH2-containing proteins. Ligand caused p120-rasGTPase activating protein (GAP), SHC and the p85 subunit of phosphatidylinositol-3'-kinase (PI3K) to be associated with both p185c-erbB-2 and p180erbB-4. In addition, tyrosine phosphorylation of p85-PI3K and SHC, but not of GAP or of its associated p62 and p190 proteins, was also detected. HRG also induced the association of GRB2 with tyrosine phosphorylated p185c-erbB-2, p180erbB-4 and SHC. Activation of mitogen-activated protein kinase (MAPK) ( > 30-fold over untreated controls) was observed upon receptor(s) activation, as it was the induction of the immediate early gene c-fos ( > 200-fold). These observations suggest that p21ras activation plays a role in the HRG pathway. Furthermore, comparative analysis of the binding of p85-PI3K to 185c-erbB-2 and p180erbB-4, revealed a preferential association with activated p180erbB-4. These findings might suggest a model of HRG action in which the relative expression of the various erb-B family members and the partitioning of signal transduction molecules between each type of receptor might determine the nature of the signal elicited by the ligand and the biological response attained.

Farnesyltransferase inhibitors and anti-Ras therapy.

The oncoprotein encoded by mutant ras genes is initially synthesized as a cytoplasmic precursor which requires posttranslational processing to attain biological activity; farnesylation of the cysteine residue present in the CaaX motif located at the carboxy-terminus of all Ras proteins is the critical modification. Once farnesylated and further modified, the mature Ras protein is inserted into the cell's plasma membrane where it participates in the signal transduction pathways that control cell growth and differentiation. The farnesylation reaction that modifies Ras and other cellular proteins having an appropriate CaaX motif is catalyzed by a housekeeping enzyme termed farnesyl-protein transferase (FPTase). Inhibitors of this enzyme have been prepared by several laboratories in an effort to identify compounds that would block Ras-induced cell transformation and thereby function as Ras-specific anticancer agents. A variety of natural products and synthetic organic compounds were found to block farnesylation of Ras proteins in vitro. Some of these compounds exhibit antiproliferative activity in cell culture, block the morphological alterations associated with Ras-transformation, and can block the growth of Ras-transformed cell lines in tumor colony-forming assays. By contrast, these compounds do not affect the growth or morphology of cells transformed by the Raf or Mos oncoproteins, which do not require farnesylation to achieve biological activity. The efficacy and lack of toxicity observed with FPTase inhibitors in an animal tumor model suggest that specific FPTase inhibitors may be useful for the treatment of some types of cancer.