Biomedical subjects
Publications and source records attributed to Paul Workman.
The first golden era of cancer drug development was initiated in the 1940s and gave rise to the cytotoxic agents that dominate current cancer medicine. The second golden era is now underway in which cancer genomics will direct drug development.
High-throughput screening of chemical libraries and the subsequent rapid progress of hit compounds through an iterative developmental test cascade are essential parts of modern molecular mechanism-based drug discovery. These processes depend on the use of efficient assay technologies and equipment. Enzyme-linked immunosorbent assays have historically been carried out in 96-well microtitre plates. Improvements in reagents and assay technologies mean that solid-phase immunoassays can be adapted for higher throughput to play an important role in modern drug discovery. The molecular chaperone heat-shock protein (Hsp) 90 is an important anticancer drug target because it maintains the conformation, stability, and function of many important oncogenic client proteins, including those involved with signal transduction, cell proliferation, survival, differentiation, motility angiogenesis, and metastasis. Using the standard inhibitors of the adenosine triphosphatase (ATPase) activity of Hsp90, geldanamycin (GA) and 17-allylamino-17- demethoxygeldanamycin (17AAG), novel solid-phase immunoassays have been validated using a time-resolved fluorescence (TRF) end point. Their utility for confirming the mechanism of action of Hsp90 inhibition in secondary cell-based assays has been shown and applied to the novel Hsp90 inhibitor CCT018159. Adaptation of these assays for later studies using human tumour xenografts and samples obtained from a Phase 1 trial of 17AAG is also described. Finally, comparison is made between the use and applicability of this type of immunoassay and other techniques such as western blotting, immunohistochemistry, and flow cytometry analysis.
The small-molecule compound pifithrin-alpha (PFT-alpha) has been reported to inhibit p53 function and protect against a variety of genotoxic agents. We show here that PFT-alpha is unstable in tissue culture medium and is rapidly converted to its condensation product PFT-beta. Both compounds showed limited solubility with PFT-alpha precipitating out of tissue culture medium at concentrations >30 micromol/L. PFT-alpha and -beta exhibited cytotoxic effects in vitro towards two human wild-type p53-expressing tumor cell lines, A2780 ovarian and HCT116 colon (IC(50) values for both cell lines were 21.3 +/- 8.1 micromol/L for PFT-alpha and 90.3 +/- 15.5 micromol/L for PFT-beta, mean +/- SD, n = 4). There was no evidence of protection by clonogenic assay with either compound in combination with ionizing radiation. Indeed, there was some evidence that PFT-alpha enhanced cytotoxicity, particularly at higher concentrations of PFT-alpha. Neither compound had any effect on p53, p21, or MDM-2 protein expression following ionizing radiation exposure and there was no evidence of any abrogation of p53-dependent, ionizing radiation-induced cell cycle arrest. Similarly, there was no evidence of cellular protection, or of effects on p53-dependent gene transcription, or on translation of MDM-2 or p21 following UV treatment of these human tumor cell lines. In addition, there was no effect on p53 or p21 gene transactivation or p38 phosphorylation after UV irradiation of NIH-3T3 mouse fibroblasts. In conclusion, neither PFT-alpha nor -beta can be regarded as a ubiquitous inhibitor of p53 function, and caution should be exercised in the use of these agents as specific p53 inhibitors.
Histone acetylation plays an important role in regulating the chromatin structure and is tightly regulated by two classes of enzyme, histone acetyltransferases (HAT) and histone deacetylases (HDAC). Deregulated HAT and HDAC activity plays a role in the development of a range of cancers. Consequently, inhibitors of these enzymes have potential as anticancer agents. Several HDAC inhibitors have been described; however, few inhibitors of HATs have been disclosed. Following a FlashPlate high-throughput screen, we identified a series of isothiazolone-based HAT inhibitors. Thirty-five N-substituted analogues inhibited both p300/cyclic AMP-responsive element binding protein-binding protein-associated factor (PCAF) and p300 (1 to >50 micromol/L, respectively) and the growth of a panel of human tumor cell lines (50% growth inhibition, 0.8 to >50 micromol/L). CCT077791 and CCT077792 decreased cellular acetylation in a time-dependent manner (2-48 hours of exposure) and a concentration-dependent manner (one to five times, 72 hours, 50% growth inhibition) in HCT116 and HT29 human colon tumor cell lines. CCT077791 reduced total acetylation of histones H3 and H4, levels of specific acetylated lysine marks, and acetylation of alpha-tubulin. Four and 24 hours of exposure to the compounds produced the same extent of growth inhibition as 72 hours of continuous exposure, suggesting that growth arrest was an early event. Chemical reactivity of these compounds, as measured by covalent protein binding and loss of HAT inhibition in the presence of DTT, indicated that reaction with thiol groups might be important in their mechanism of action. As one of the first series of small-molecule inhibitors of HAT activity, further analogue synthesis is being pursued to examine the potential scope for reducing chemical reactivity while maintaining HAT inhibition.
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R-roscovitine (seliciclib, CYC202) is a cyclin-dependent kinase inhibitor currently in phase II clinical trials in patients with cancer. Here, we describe its mouse metabolism and pharmacokinetics as well as the identification of the principal metabolites in hepatic microsomes, plasma, and urine. Following microsomal incubation of R-roscovitine at 10 microg/mL (28 micromol/L) for 60 minutes, 86.7% of the parent drug was metabolized and 60% of this loss was due to formation of one particular metabolite. This was identified as the carboxylic acid resulting from oxidation of the hydroxymethyl group of the amino alcohol substituent at C2 of the purine core present in R-roscovitine. Identification was confirmed by chemical synthesis and comparison of an authentic sample of the R-roscovitine-derived carboxylate metabolite (COOH-R-roscovitine). Other minor metabolites were identified as C8-oxo-R-roscovitine and N9-desisopropyl-R-roscovitine; these accounted for 4.9% and 2.6% of the parent, respectively. The same metabolic pattern was observed in vivo, with a 4.5-fold lower AUC(infinity) for R-roscovitine (38 micromol/L/h) than for COOH-R-roscovitine (174 micromol/L/h). Excretion of R-roscovitine in the urine up to 24 hours post-dosing accounted for an average of only 0.02% of the administered dose of 50 mg/kg, whereas COOH-R-roscovitine represented 65% to 68% of the dose irrespective of the route of administration (i.v., i.p., or p.o.). A partially deuterated derivative (R-roscovitine-d9) was synthesized to investigate if formation of COOH-R-roscovitine could be inhibited by replacement of metabolically labile protons with deuterium. After 60 minutes of incubation of R-roscovitine-d9 or R-roscovitine with mouse liver microsomes, formation of COOH-R-roscovitine-d9 was decreased by approximately 24% compared with the production of COOH-R-roscovitine. In addition, the levels of R-roscovitine-d9 remaining were 33% higher than those of R-roscovitine. However, formation of several minor R-roscovitine metabolites was enhanced with R-roscovitine-d9, suggesting that metabolic switching from the major carbinol oxidation pathway had occurred. Synthetic COOH-R-roscovitine and C8-oxo-R-roscovitine were tested in functional cyclin-dependent kinase assays and shown to be less active than R-roscovitine, confirming that these metabolic reactions are deactivation pathways.
Tumor resistance to current drugs prevents curative treatment of human colon cancer. A pressing need for effective, tumor-specific chemotherapies exists. The non-receptor-tyrosine kinase c-Src is overexpressed in >70% of human colon cancers and represents a tractable drug target. KM12L4A human metastatic colon cancer cells were stably transfected with two distinct kinase-defective mutants of c-src. Their response to oxaliplatin, to SN38, the active metabolite of irinotecan (drugs active in colon cancer), and to activation of the death receptor Fas was compared with vector control cells in terms of cell cycle arrest and apoptosis. Both kinase-defective forms of c-Src co-sensitized cells to apoptosis induced by oxaliplatin and Fas activation but not by SN38. Cells harboring kinase-defective forms of c-Src carrying function blocking point mutations in SH3 or SH2 domains were similarly sensitive to oxaliplatin, suggesting that reduction in kinase activity and not a Src SH2-SH3 scaffold function was responsible for the observed altered sensitivity. Oxaliplatin-induced apoptosis, potentiated by kinase-defective c-Src mutants, was dependent on activation of caspase 8 and associated with Bid cleavage. Each of the stable cell lines in which kinase-defective mutants of c-Src were expressed had reduced levels of Bcl-x(L.) However, inhibition of c-Src kinase activity by PP2 in vector control cells did not alter the oxaliplatin response over 72 h nor did it reduce Bcl-x(L) levels. The data suggest that longer term suppression of Src kinase activity may be required to lower Bcl-x(L) levels and sensitize colon cancer cells to oxaliplatin-induced apoptosis.
The molecular chaperone heat-shock protein 90 (HSP90) plays a key role in the cell by stabilizing a number of client proteins, many of which are oncogenic. The intrinsic ATPase activity of HSP90 is essential to this activity. HSP90 is a new cancer drug target as inhibition results in simultaneous disruption of several key signaling pathways, leading to a combinatorial approach to the treatment of malignancy. Inhibitors of HSP90 ATPase activity including the benzoquinone ansamycins, geldanamycin and 17-allylamino-17-demethoxygeldanamycin, and radicicol have been described. A high-throughput screen has been developed to identify small-molecule inhibitors that could be developed as therapeutic agents with improved pharmacological properties. A colorimetric assay for inorganic phosphate, based on the formation of a phosphomolybdate complex and subsequent reaction with malachite green, was used to measure the ATPase activity of yeast HSP90. The Km for ATP determined in the assay was 510+/-70 microM. The known HSP90 inhibitors geldanamycin and radicicol gave IC(50) values of 4.8 and 0.9 microM respectively, which compare with values found using the conventional coupled-enzyme assay. The assay was robust and reproducible (2-8% CV) and used to screen a compound collection of approximately 56,000 compounds in 384-well format with Z' factors between 0.6 and 0.8.
The molecular chaperone Hsp90 is not only of major current interest in fundamental biological research but also recognised as an exciting new target for the treatment of cancer and other diseases. In addition to playing an important role in response to proteotoxic heat shock and others stresses, Hsp90 is also critical for maintaining normal cellular homeostasis. Hsp90 is responsible for ensuring the conformational stability, shape and function of a selected range of key proteins, including many kinases and transcription factors. Furthermore, recent studies show that Hsp90 plays a key role in development and evolution. Hsp90 is overexpressed in cancer cells and is thought to be involved in dealing with the cellular stress associated with malignancy, as well as being essential for a range of key oncogenic proteins, including ErbB2, Raf-1, Akt/PKB, mutant p53 and many others. A major attraction of Hsp90 inhibitors is their potential to inhibit a range of 'mission critical' cancer pathways, thereby blocking all of the 'hallmark traits' of malignancy and exhibiting broad-spectrum antitumour activity. The first-in-class Hsp90 inhibitor 17AAG has entered clinical trials with promising early results and a range of other agents is under investigation and preclinical development. This article reviews the current status and future prospects for the exploitation of Hsp90 as a new molecular target for cancer treatment.
PURPOSE: The impact of the presence of a germ-line BRCA1 mutation on gene expression in normal breast fibroblasts after radiation-induced DNA damage has been investigated. EXPERIMENTAL DESIGN: High-density cDNA microarray technology was used to identify differential responses to DNA damage in fibroblasts from nine heterozygous BRCA1 mutation carriers compared with five control samples without personal or family history of any cancer. Fibroblast cultures were irradiated, and their expression profile was compared using intensity ratios of the cDNA microarrays representing 5603 IMAGE clones. RESULTS: Class comparison and class prediction analysis has shown that BRCA1 mutation carriers can be distinguished from controls with high probability (approximately 85%). Significance analysis of microarrays and the support vector machine classifier identified gene sets that discriminate the samples according to their mutation status. These include genes already known to interact with BRCA1 such as CDKN1B, ATR, and RAD51. CONCLUSIONS: The results of this initial study suggest that normal cells from heterozygous BRCA1 mutation carriers display a different gene expression profile from controls in response to DNA damage. Adaptations of this pilot result to other cell types could result in the development of a functional assay for BRCA1 mutation status.
Deregulation of the cell cycle commonly occurs during tumorigenesis, resulting in unrestricted cell proliferation and independence from mitogens. Cyclin-dependent kinase inhibitors have the potential to induce cell cycle arrest and apoptosis in cancer cells. CYC202 (R-roscovitine) is a potent inhibitor of CDK2/cyclin E that is undergoing clinical trials. Drugs selected to act on a particular molecular target may exert additional or alternative effects in intact cells. We therefore studied the molecular pharmacology of CYC202 in human colon cancer cells. Treatment of HT29 and KM12 colon carcinoma cell lines with CYC202 decreased both retinoblastoma protein phosphorylation and total retinoblastoma protein. In addition, an increase in the phosphorylation of extracellular signal-regulated kinases 1/2 was observed. As a result, downstream activation of the mitogen-activated protein kinase pathway occurred, as demonstrated by an increase in ELK-1 phosphorylation and in c-FOS expression. Use of mitogen-activated protein kinase kinases 1/2 inhibitors showed that the CYC202-induced extracellular signal-regulated kinases 1/2 phosphorylation was mitogen-activated protein kinase kinases 1/2 dependent but did not contribute to the cell cycle effects of the drug, which included a reduction of cells in G(1), inhibition of bromodeoxyuridine incorporation during S-phase, and a moderate increase in G(2)-M phase. Despite activation of the mitogen-activated protein kinase pathway, cyclin D1 protein levels were decreased by CYC202, an effect that occurred simultaneously with loss of retinoblastoma protein phosphorylation and inhibition of cell cycle progression. The reduced expression of cyclin D1 protein was independent of the p38(SAPK) and phosphatidylinositol 3-kinase pathways, which are known regulators of cyclin D1 protein. Interestingly, CYC202 caused a clear reduction in cyclins D1, A, and B1 mRNA, whereas c-FOS mRNA increased by 2-fold. This was accompanied by a loss of RNA polymerase II phosphorylation and total RNA polymerase II protein, suggesting that CYC202 was inhibiting transcription, possibly via inhibition of CDK7 and CDK9 complexes. It can be concluded that although CYC202 can act as a CDK2 inhibitor, it also has the potential to inhibit CDK4 and CDK1 activities in cancer cells through the down-regulation of the corresponding cyclin partners. This provides a possible mechanism by which CYC202 can cause a reduction in retinoblastoma protein phosphorylation at multiple sites and cell cycle arrest in G(1), S, and G(2)-M phases. In addition to providing useful insights into the molecular pharmacology of CYC202 in human cancer cells, the results also suggest potential pharmacodynamic end points for use in clinical trials with the drug.
Inhibition of the ATPase activity of the chaperone protein HSP90 is a potential strategy for treatment of cancers. We have determined structures of the HSP90alpha N-terminal domain complexed with the purine-based inhibitor, PU3, and analogs with enhanced potency both in enzyme and cell-based assays. The compounds induce upregulation of HSP70 and downregulation of the known HSP90 client proteins Raf-1, CDK4, and ErbB2, confirming that the molecules inhibit cell growth by a mechanism dependent on HSP90 inhibition. We have also determined the first structure of the N-terminal domain of HSP90beta, complexed with PU3. The structures allow a detailed rationale to be developed for the observed affinity of the PU3 class of compounds for HSP90 and also provide a structural framework for design of compounds with improved binding affinity and drug-like properties.
We review in detail how gene expression microarray technology is benefiting all phases of the discovery, development and subsequent use of new cancer therapeutics. Global gene expression profiling is valuable in cancer classification, elucidation of biochemical pathways and the identification of potential targets for novel molecular therapeutics. We exemplify the value in tissue culture and animal models of cancer, as well as in clinical studies. The power of expression profiling alongside gene knockout or knockdown methods such as RNA interference is illustrated. The use of basal or constitutive gene expression profiling to understand and predict drug sensitivity or resistance is described. The ability of expression profiling to define detailed molecular signatures of drug action is emphasised. The approach can identify on-target and off-target effects. It can be used to identify molecular biomarkers for proof of concept studies, pharmacodynamic endpoints and prognostic markers for predicting outcome and patient selection.
The molecular chaperone Hsp90 is an exciting cancer drug target. The first Hsp90 inhibitor to enter clinical trials--the geldanamycin derivative 17AAG--has recently demonstrated proof-of-concept for successful target modulation, with sighs of therapeutic benefit. An important property of Hsp90 inhibitors is their ability to cause simultaneous, combinatorial blockade of multiple cancer-causing pathways by promoting the degradation of many oncogenic client proteins. However, the reason for therapeutic selectivity in cancer cells versus normal cells is unclear. New research now shows that Hsp90 exists in cancer cells in a heightened, activated state that is highly susceptible to inhibition by 17AAG.
STK15/Aurora2 is a centrosome-associated serine/threonine kinase, the protein levels and kinase activity of which rise during G2 and mitosis. STK15 overexpression induces tumorigenesis and is amplified in various human cancers and tumor cell lines. Thus, STK15 represents an important therapeutic target for small molecule inhibitors that would disrupt its activity and block cell proliferation. The availability of a robust and selective small molecule inhibitor would also provide a useful tool for identification of the potential role of STK15 in cell cycle regulation and tumor development. The authors report the development of a novel, fast, simple microplate assay for STK15 activity suitable for high-throughput screening. In the assay, gamma-(33)P-ATP and STK15 were incubated in a myelin basic protein (MBP)-coated FlashPlate(R) to generate a scintillation signal. The assay was reproducible, the signal-to-noise ratio was high (11) and the Z' factor was 0.69. The assay was easily adapted to a robotic system for drug discovery programs targeting STK15. The authors also demonstrate that STK15 is regulated by phosphorylation and the N-amino terminal domain of the protein. Treatment with phosphatase inhibitors (okadaic acid) or deletion of the N-amino terminal domain results in a significant increase in the enzymatic activity.
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In eukaryotes, genomic DNA is packaged with histone proteins into the cell nucleus as chromatin, condensing the DNA > 10,000-fold. Chromatin is highly dynamic and exerts profound control on gene expression. Localised chromatin decondensation facilitates access of nuclear machinery. Chromatin displays epigenetic inheritance, in that changes in its structure can pass to the next generation independently of the DNA sequence itself. It is now clear that the post-translational modification of histones, for example, acetylation, methylation and phosphorylation, plays a crucial role in the regulation of nuclear function through the 'histone code'. There has been significant progress in identifying and understanding the enzymes that control these complex processes, in particular histone acetyltransferases and histone deacetylases. The exciting discovery that compounds inhibiting histone deacetylase activity also have antitumour properties has focused attention on their use as anticancer drugs. As a consequence, there is ongoing evaluation of several histone deacetylase inhibitor compounds in Phase I and II clinical trials with promising early results. It is likely that many of the enzymes involved in the control of histone modification will provide therapeutic opportunities for the treatment of cancer, including histone methyltransferases and Aurora kinases.
Determination of pharmacokinetic properties in the intact animal remains a major bottleneck in drug discovery. Cassette dosing involves administration of a cocktail of drugs to individual animals. Here we describe the cassette dosing properties of a 107-membered library of 2,6,9-trisubstituted purine cyclin-dependent kinase 2 (CDK2) inhibitors. A three-step parallel synthesis approach produced compounds with purity ranging from 63% to 100%. Cassette dosing was validated by comparing the pharmacokinetic parameters obtained following i.v. administration of a mixture of olomoucine, R-roscovitine (CYC202), and bohemine, each at 16.6 mg/kg, with results for administration of single agents at 50 mg/kg. No significant difference was observed between the pharmacokinetic parameters of agents when dosed in combination compared with those of individual compounds. CYC202 showed the highest area under the curve (AUC) and the longest elimination half-life (t(1/2)). Further cassettes evaluated the library of trisubstituted purines with CYC202 and purvalanol A included as pharmacokinetic standards in a validated limited sampling strategy. The ratios of pharmacokinetic parameters to that of CYC202 [AUC, maximum concentration (C(max)), and t(1/2)] remained similar when compounds were tested in two different cassettes or as individual compounds. Following dosing of the same cassette on three different days, there was less than 20% variation in pharmacokinetic parameters between days. The structure-pharmacokinetics relationship showed that the favored purine substituents are benzylamine and veratrylamine at position 6, amino-2 propanol at position 2, and methylpropyl or hydroxyethyl at position 9. Without cassette dosing, this study would have used 3 times as many animals and would have taken 4 times longer, illustrating the power of this method in lead optimization.