[Show abstract][Hide abstract] ABSTRACT: Chk1 kinase is a critical component of the DNA damage response checkpoint especially in cancer cells and targeting Chk1 is a potential therapeutic opportunity for potentiating the anti-tumor activity of DNA damaging chemotherapy drugs. Fragment elaboration by structure guided design was utilized to identify and develop a novel series of Chk1 inhibitors culminating in the identification of V158411, a potent ATP-competitive inhibitor of the Chk1 and Chk2 kinases. V158411 abrogated gemcitabine and camptothecin induced cell cycle checkpoints, resulting in the expected modulation of cell cycle proteins and increased cell death in cancer cells. V158411 potentiated the cytotoxicity of gemcitabine, cisplatin, SN38 and camptothecin in a variety of p53 deficient human tumor cell lines in vitro, p53 proficient cells were unaffected. In nude mice, V158411 showed minimal toxicity as a single agent and in combination with irinotecan. In tumor bearing animals, V158411 was detected at high levels in the tumor with a long elimination half-life; no pharmacologically significant in vivo drug-drug interactions with irinotecan were identified through analysis of the pharmacokinetic profiles. V158411 potentiated the anti-tumor activity of irinotecan in a variety of human colon tumor xenograft models without additional systemic toxicity. These results demonstrate the opportunity for combining V158411 with standard of care chemotherapeutic agents to potentiate the therapeutic efficacy of these agents without increasing their toxicity to normal cells. Thus, V158411 would warrant further clinical evaluation.
[Show abstract][Hide abstract] ABSTRACT: Background:
The myotonic dystrophy kinase-related CDC42-binding kinases MRCKα and MRCKβ regulate actin-myosin contractility and have been implicated in cancer metastasis. Along with the related ROCK1 and ROCK2 kinases, the MRCK proteins initiate signalling events that lead to contractile force generation which powers cancer cell motility and invasion. A potential strategy for cancer therapy is to reduce metastasis by blocking MRCK activity, either alone or in combination with ROCK inhibition. However, to date no potent small molecule inhibitors have been developed with selectivity towards MRCK.
Screening a kinase-focused small molecule chemical library resulted in the identification of compounds with inhibitory activity towards MRCK. Medicinal chemistry combined with in vitro enzyme profiling led to the discovery of 4-chloro-1-(4-piperidyl)-N-[5-(2-pyridyl)-1H-pyrazol-4-yl]pyrazole-3-carboxamide (BDP00005290; abbreviated as BDP5290) as a potent MRCK inhibitor. X-ray crystallography of the MRCKβ kinase domain in complex with BDP5290 revealed how this ligand interacts with the nucleotide binding pocket. BDP5290 demonstrated marked selectivity for MRCKβ over ROCK1 or ROCK2 for inhibition of myosin II light chain (MLC) phosphorylation in cells. While BDP5290 was able to block MLC phosphorylation at both cytoplasmic actin stress fibres and peripheral cortical actin bundles, the ROCK selective inhibitor Y27632 primarily reduced MLC phosphorylation on stress fibres. BDP5290 was also more effective at reducing MDA-MB-231 breast cancer cell invasion through Matrigel than Y27632. Finally, the ability of human SCC12 squamous cell carcinoma cells to invade a three-dimensional collagen matrix was strongly inhibited by 2 μM BDP5290 but not the identical concentration of Y27632, despite equivalent inhibition of MLC phosphorylation.
BDP5290 is a potent MRCK inhibitor with activity in cells, resulting in reduced MLC phosphorylation, cell motility and tumour cell invasion. The discovery of this compound will enable further investigations into the biological activities of MRCK proteins and their contributions to cancer progression.
Cell Communication and Signaling 10/2014; 12(1):54. DOI:10.1186/PREACCEPT-1467335606136283 · 3.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fragment-Based Drug Discovery (FBDD) is here to stay. Validated as a technology with the delivery of Zelboraf (Vemurafenib) for the treatment of mutant B-Raf(V600E) melanoma, it has become embedded within the pharmaceutical and biotechnology industries. FBDD has delivered clinical development candidates for a broad range of targets including some of the most challenging cases such as -secretase (BACE1) and protein-protein interactions. But the best is surely still to come.
[Show abstract][Hide abstract] ABSTRACT: The identification of high-quality hits during the early phases of drug discovery is essential if projects are to have a realistic chance of progressing into clinical development and delivering marketed drugs. As the pharmaceutical industry goes through unprecedented change, there are increasing opportunities to collaborate via pre-competitive networks to marshal multifunctional resources and knowledge to drive impactful, innovative science. The 3D Fragment Consortium is developing fragment-screening libraries with enhanced 3D characteristics and evaluating their effect on the quality of fragment-based hit identification (FBHI) projects.
Drug discovery today 07/2013; 18(23-24). DOI:10.1016/j.drudis.2013.07.011 · 6.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Inhibitors of the Hsp90 molecular chaperone are showing promise as anti-cancer agents. Here we describe a series of 4-aryl-5-cyanopyrrolo[2,3-d]pyrimidine ATP competitive Hsp90 inhibitors that were identified following structure-driven optimization of purine hits revealed by NMR based screening of a proprietary fragment library. Ligand-Hsp90 X-ray structures combined with molecular modeling led to the rational displacement of a conserved water molecule leading to enhanced affinity for Hsp90 as measured by fluorescence polarization, isothermal titration calorimetry and surface plasmon resonance assays. This displacement was achieved with a nitrile group, presenting an example of efficient gain in binding affinity with minimal increase in molecular weight. Some compounds in this chemical series inhibit the proliferation of human cancer cell lines in vitro and cause depletion of oncogenic Hsp90 client proteins and concomitant elevation of the co-chaperone Hsp70. In addition, one compound was demonstrated to be orally bioavailable in the mouse. This work demonstrates the power of structure-based design for the rapid evolution of potent Hsp90 inhibitors and the importance of considering conserved water molecules in drug design.
[Show abstract][Hide abstract] ABSTRACT: 78 kDa glucose-regulated protein (Grp78) is a heat shock protein (HSP) involved in protein folding that plays a role in cancer cell proliferation. Binding of adenosine-derived inhibitors to Grp78 was characterized by surface plasmon resonance and isothermal titration calorimetry. The most potent compounds were 13 (VER-155008) with K(D) = 80 nM and 14 with K(D) = 60 nM. X-ray crystal structures of Grp78 bound to ATP, ADPnP, and adenosine derivative 10 revealed differences in the binding site between Grp78 and homologous proteins.
[Show abstract][Hide abstract] ABSTRACT: Heat shock protein 90 (Hsp90) is a ubiquitously expressed molecular chaperone with ATPase activity involved in the conformational maturation and stability of key signaling molecules involved in cell proliferation, survival, and transformation. Through its ability to modulate multiple pathways involved in oncogenesis, Hsp90 has generated considerable interest as a therapeutic target. NVP-BEP800 is a novel, fully synthetic, orally bioavailable inhibitor that binds to the NH(2)-terminal ATP-binding pocket of Hsp90. NVP-BEP800 showed activity against a panel of human tumor cell lines and primary human xenografts in vitro at nanomolar concentrations. In A375 melanoma and BT-474 breast cancer cell lines, NVP-BEP800 induced client protein degradation (including ErbB2, B-Raf(V600E), Raf-1, and Akt) and Hsp70 induction. Oral administration of NVP-BEP800 was well tolerated and induced robust antitumor responses in tumor xenograft models, including regression in the BT-474 breast cancer model. In these tumor models, NVP-BEP800 modulated Hsp90 client proteins and downstream signaling pathways at doses causing antitumor activity. NVP-BEP800 showed in vivo activity in a variety of dosing regimens covering daily to weekly schedules, potentially providing a high degree of flexibility in dose and schedule within the clinical setting. Overall, given the mechanism of action, preclinical activity profile, tolerability, and pharmaceutical properties, NVP-BEP800 is an exciting new oral Hsp90 inhibitor warranting further development. Mol Cancer Ther; 9(4); 906-19. (c)2010 AACR.
Molecular Cancer Therapeutics 04/2010; 9(4):906-19. DOI:10.1158/1535-7163.MCT-10-0055 · 5.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The anti-apoptotic function of the 70 kDa family of heat shock proteins and their role in cancer is well documented. Dual targeting of Hsc70 and Hsp70 with siRNA induces proteasome-dependent degradation of Hsp90 client proteins and extensive tumor specific apoptosis as well as the potentiation of tumor cell apoptosis following pharmacological Hsp90 inhibition.
We have previously described the discovery and synthesis of novel adenosine-derived inhibitors of the 70 kDa family of heat shock proteins; the first inhibitors described to target the ATPase binding domain. The in vitro activity of VER-155008 was evaluated in HCT116, HT29, BT474 and MDA-MB-468 carcinoma cell lines. Cell proliferation, cell apoptosis and caspase 3/7 activity was determined for VER-155008 in the absence or presence of small molecule Hsp90 inhibitors.
VER-155008 inhibited the proliferation of human breast and colon cancer cell lines with GI(50)s in the range 5.3-14.4 microM, and induced Hsp90 client protein degradation in both HCT116 and BT474 cells. As a single agent, VER-155008 induced caspase-3/7 dependent apoptosis in BT474 cells and non-caspase dependent cell death in HCT116 cells. VER-155008 potentiated the apoptotic potential of a small molecule Hsp90 inhibitor in HCT116 but not HT29 or MDA-MB-468 cells. In vivo, VER-155008 demonstrated rapid metabolism and clearance, along with tumor levels below the predicted pharmacologically active level.
These data suggest that small molecule inhibitors of Hsc70/Hsp70 phenotypically mimic the cellular mode of action of a small molecule Hsp90 inhibitor and can potentiate the apoptotic potential of a small molecule Hsp90 inhibitor in certain cell lines. The factors determining whether or not cells apoptose in response to Hsp90 inhibition or the combination of Hsp90 plus Hsc70/Hsp70 inhibition remain to be determined.
Cancer Chemotherapy and Pharmacology 12/2009; 66(3):535-45. DOI:10.1007/s00280-009-1194-3 · 2.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Inhibitors of the Hsp90 molecular chaperone are showing considerable promise as potential molecular therapeutic agents for the treatment of cancer. Here we describe novel 2-aminothieno[2,3-d]pyrimidine ATP competitive Hsp90 inhibitors, which were designed by combining structural elements of distinct low affinity hits generated from fragment-based and in silico screening exercises in concert with structural information from X-ray protein crystallography. Examples from this series have high affinity (IC50 = 50-100 nM) for Hsp90 as measured in a fluorescence polarization (FP) competitive binding assay and are active in human cancer cell lines where they inhibit cell proliferation and exhibit a characteristic profile of depletion of oncogenic proteins and concomitant elevation of Hsp72. Several examples (34a, 34d and 34i) caused tumor growth regression at well tolerated doses when administered orally in a human BT474 human breast cancer xenograft model.
[Show abstract][Hide abstract] ABSTRACT: The design and synthesis of novel adenosine-derived inhibitors of HSP70, guided by modeling and X-ray crystallographic structures of these compounds in complex with HSC70/BAG-1, is described. Examples exhibited submicromolar affinity for HSP70, were highly selective over HSP90, and some displayed potency against HCT116 cells. Exposure of compound 12 to HCT116 cells caused significant reduction in cellular levels of Raf-1 and Her2 at concentrations similar to that which caused cell growth arrest.
[Show abstract][Hide abstract] ABSTRACT: We describe the biological properties of NVP-AUY922, a novel resorcinylic isoxazole amide heat shock protein 90 (HSP90) inhibitor. NVP-AUY922 potently inhibits HSP90 (K(d) = 1.7 nmol/L) and proliferation of human tumor cells with GI(50) values of approximately 2 to 40 nmol/L, inducing G(1)-G(2) arrest and apoptosis. Activity is independent of NQO1/DT-diaphorase, maintained in drug-resistant cells and under hypoxic conditions. The molecular signature of HSP90 inhibition, comprising induced HSP72 and depleted client proteins, was readily demonstrable. NVP-AUY922 was glucuronidated less than previously described isoxazoles, yielding higher drug levels in human cancer cells and xenografts. Daily dosing of NVP-AUY922 (50 mg/kg i.p. or i.v.) to athymic mice generated peak tumor levels at least 100-fold above cellular GI(50). This produced statistically significant growth inhibition and/or regressions in human tumor xenografts with diverse oncogenic profiles: BT474 breast tumor treated/control, 21%; A2780 ovarian, 11%; U87MG glioblastoma, 7%; PC3 prostate, 37%; and WM266.4 melanoma, 31%. Therapeutic effects were concordant with changes in pharmacodynamic markers, including induction of HSP72 and depletion of ERBB2, CRAF, cyclin-dependent kinase 4, phospho-AKT/total AKT, and hypoxia-inducible factor-1alpha, determined by Western blot, electrochemiluminescent immunoassay, or immunohistochemistry. NVP-AUY922 also significantly inhibited tumor cell chemotaxis/invasion in vitro, WM266.4 melanoma lung metastases, and lymphatic metastases from orthotopically implanted PC3LN3 prostate carcinoma. NVP-AUY922 inhibited proliferation, chemomigration, and tubular differentiation of human endothelial cells and antiangiogenic activity was reflected in reduced microvessel density in tumor xenografts. Collectively, the data show that NVP-AUY922 is a potent, novel inhibitor of HSP90, acting via several processes (cytostasis, apoptosis, invasion, and angiogenesis) to inhibit tumor growth and metastasis. NVP-AUY922 has entered phase I clinical trials.
Cancer Research 05/2008; 68(8):2850-60. DOI:10.1158/0008-5472.CAN-07-5256 · 9.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Heat shock protein 90 (HSP90) is a key component of a multichaperone complex involved in the post-translational folding of a large number of client proteins, many of which play essential roles in tumorigenesis. HSP90 has emerged in recent years as a promising new target for anticancer therapies.
The concentrations of the HSP90 inhibitor NVP-AUY922 required to reduce cell numbers by 50% (GI50 values) were established in a panel of breast cancer cell lines and patient-derived human breast tumors. To investigate the properties of the compound in vivo, the pharmacokinetic profile, antitumor effect, and dose regimen were established in a BT-474 breast cancer xenograft model. The effect on HSP90-p23 complexes, client protein degradation, and heat shock response was investigated in cell culture and breast cancer xenografts by immunohistochemistry, Western blot analysis, and immunoprecipitation.
We show that the novel small molecule HSP90 inhibitor NVP-AUY922 potently inhibits the proliferation of human breast cancer cell lines with GI50 values in the range of 3 to 126 nM. NVP-AUY922 induced proliferative inhibition concurrent with HSP70 upregulation and client protein depletion--hallmarks of HSP90 inhibition. Intravenous acute administration of NVP-AUY922 to athymic mice (30 mg/kg) bearing subcutaneous BT-474 breast tumors resulted in drug levels in excess of 1,000 times the cellular GI50 value for about 2 days. Significant growth inhibition and good tolerability were observed when the compound was administered once per week. Therapeutic effects were concordant with changes in pharmacodynamic markers, including HSP90-p23 dissociation, decreases in ERBB2 and P-AKT, and increased HSP70 protein levels.
NVP-AUY922 is a potent small molecule HSP90 inhibitor showing significant activity against breast cancer cells in cellular and in vivo settings. On the basis of its mechanism of action, preclinical activity profile, tolerability, and pharmaceutical properties, the compound recently has entered clinical phase I breast cancer trials.
Breast cancer research: BCR 02/2008; 10(2):R33. DOI:10.1186/bcr1996 · 5.49 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Heat shock protein (Hsp90) inhibitors are an increasingly interesting and important class of compounds where the first in class, natural product derived inhibitors such as 17-allylaminogeldanamycin (17-AAG), are entering late stage clinical development. Recently the emergence of synthetic, small molecule inhibitors has been described and both NVP-AUY922 and BIIB021 have entered clinical development. The medicinal chemistry of these and other published small molecule Hsp90 inhibitors is described in this review.
Current topics in medicinal chemistry 02/2008; 8(10):859-68. DOI:10.2174/156802608784911644 · 3.40 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Inhibitors of the Hsp90 molecular chaperone are showing considerable promise as potential chemotherapeutic agents for cancer. Here, we describe the structure-based design, synthesis, structure-activity relationships and pharmacokinetics of potent small-molecule inhibitors of Hsp90 based on the 4,5-diarylisoxazole scaffold. Analogues from this series have high affinity for Hsp90, as measured in a fluorescence polarization (FP) competitive binding assay, and are active in cancer cell lines where they inhibit proliferation and exhibit a characteristic profile of depletion of oncogenic proteins and concomitant elevation of Hsp72. Compound 40f (VER-52296/NVP-AUY922) is potent in the Hsp90 FP binding assay (IC50 = 21 nM) and inhibits proliferation of various human cancer cell lines in vitro, with GI50 averaging 9 nM. Compound 40f is retained in tumors in vivo when administered i.p., as evaluated by cassette dosing in tumor-bearing mice. In a human colon cancer xenograft model, 40f inhibits tumor growth by approximately 50%.
[Show abstract][Hide abstract] ABSTRACT: Virtual screening against a pCDK2/cyclin A crystal structure led to the identification of a potent and novel CDK2 inhibitor, which exhibited an unusual mode of interaction with the kinase binding motif. With the aid of X-ray crystallography and modelling, a medicinal chemistry strategy was implemented to probe the interactions seen in the crystal structure and to establish SAR. A fragment-based approach was also considered but a different, more conventional, binding mode was observed. Compound selectivity against GSK-3beta was improved using a rational design strategy, with crystallographic verification of the CDK2 binding mode.
[Show abstract][Hide abstract] ABSTRACT: Although the heat shock protein 90 (HSP90) inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) shows clinical promise, potential limitations encourage development of alternative chemotypes. We discovered the 3,4-diarylpyrazole resorcinol CCT018159 by high-throughput screening and used structure-based design to generate more potent pyrazole amide analogues, exemplified by VER-49009. Here, we describe the detailed biological properties of VER-49009 and the corresponding isoxazole VER-50589. X-ray crystallography showed a virtually identical HSP90 binding mode. However, the dissociation constant (Kd) of VER-50589 was 4.5 ± 2.2 nmol/L compared with 78.0 ± 10.4 nmol/L for VER-49009, attributable to higher enthalpy for VER-50589 binding. A competitive binding assay gave a lower IC50 of 21 ± 4 nmol/L for VER-50589 compared with 47 ± 9 nmol/L for VER-49009. Cellular uptake of VER-50589 was 4-fold greater than for VER-49009. Mean cellular antiproliferative GI50 values for VER-50589 and VER-49009 for a human cancer cell line panel were 78 ± 15 and 685 ± 119 nmol/L, respectively, showing a 9-fold potency gain for the isoxazole. Unlike 17-AAG, but as with CCT018159, cellular potency of these analogues was independent of NAD(P)H:quinone oxidoreductase 1/DT-diaphorase and P-glycoprotein expression. Consistent with HSP90 inhibition, VER-50589 and VER-49009 caused induction of HSP72 and HSP27 alongside depletion of client proteins, including C-RAF, B-RAF, and survivin, and the protein arginine methyltransferase PRMT5. Both caused cell cycle arrest and apoptosis. Extent and duration of pharmacodynamic changes in an orthotopic human ovarian carcinoma model confirmed the superiority of VER-50589 over VER-49009. VER-50589 accumulated in HCT116 human colon cancer xenografts at levels above the cellular GI50 for 24 h, resulting in 30% growth inhibition. The results indicate the therapeutic potential of the resorcinylic pyrazole/isoxazole amide analogues as HSP90 inhibitors.
Molecular Cancer Therapeutics 05/2007; 6(4):1198-211. DOI:10.1158/1535-7163.MCT-07-0149 · 5.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Finding novel compounds as starting points for optimization is a major challenge in drug discovery research. Fragment-based methods have emerged in the past ten years as an effective way to sample chemical diversity with a limited number of low molecular weight compounds. The structures of the fragments(s) binding to the protein can then be used to design new compounds with increased affinity, specificity and novelty. This article describes the Vernalis approach to fragment based drug discovery, called SeeDs (Structural exploitation of experimental Drug startpoints). The approach includes the design of a fragment library, identification of fragments that bind competitively to a target by ligand-based NMR techniques and protein crystal structures to characterize binding. Fragments that bind are then evolved to hits, either by growing the fragment or by combining structural features from a number of compounds. The process is illustrated with examples from recent medicinal chemistry programmes to discover compounds against the oncology targets Hsp90 and PDK1. In addition, we summarise our experience with using molecular docking calculations to predict fragment binding and anecdotes on the selectivity and binding modes for fragments seen against a range of targets.
Current topics in medicinal chemistry 02/2007; 7(16):1568-81. DOI:10.2174/156802607782341109 · 3.40 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Heat shock protein (Hsp)90 is a molecular chaperone that is responsible for the correct folding of a large number of proteins, which allows these proteins to achieve their functional conformation. Client proteins of Hsp90 include many overexpressed or mutated oncogenes that are known to be critical for the transformed phenotype observed in tumors. The compounds 17-AAG (Kosan Biosciences Inc/National Cancer Institute) and 17-DMAG (Kosan Biosciences Inc/National Cancer Institute) are Hsp90 inhibitors that are derived from the prototypical ansamycin natural product Hsp90 inhibitor geldanamycin. These compounds have demonstrated preclinical efficacy in mouse xenograft models, and are now undergoing phase II and I clinical trials, respectively. Preclinical efficacy studies of these compounds are collated and discussed in this review. More recent disclosures of small-molecule Hsp90 inhibitors include purine and resorcinol analogs, and the first small-molecule Hsp90 compounds showing oral efficacy have been described. Inhibition of Hsp90 not only results in the degradation of client proteins, but also results in the induction of another chaperone, Hsp70. Hsp70 is known to be anti-apoptotic, and therefore the induction of Hsp70 may ultimately limit the efficacy of Hsp90 inhibitors under certain circumstances. Histone deacetylase inhibitors have recently been demonstrated to exert some of their effect through modulation of Hsp90 chaperoning activity, and some mechanistic aspects of this control are also discussed herein.
Current opinion in drug discovery & development 08/2006; 9(4):483-95. · 5.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: CCT018159 was recently identified as a novel inhibitor of heat shock protein (Hsp) 90, a promising target for cancer therapy. Pharmacokinetic and metabolic properties are likely to be important for efficacy and need to be optimized during drug development. Here, we define the preclinical metabolism and pharmacokinetics of CCT018159 and some early derivatives. In addition, we assess in vitro metabolic stability screening and in vivo cassette dosing (simultaneous administration of several compounds to a single animal) as approaches to investigate these compounds. The plasma clearance following individual i.v. administration to mice was rapid (0.128-0.816 L/h), exceeding hepatic blood flow. For CCT066950 and CCT066952, this could be attributed in part to extensive (>80%) blood cell binding. Oral bioavailability ranged from 1.8% to 29.6%. Tissue distribution of CCT066952 was rapid and moderate, and renal excretion of the compounds was minimal (<1% of dose excreted). Compounds underwent rapid glucuronidation both in vivo and following incubation with mouse liver microsomes. However, whereas CCT066965 was metabolized to the greatest extent in vitro, this compound displayed the slowest plasma clearance. The rank order of the compounds from the highest to lowest area under the curve was the same following discrete and cassette dosing. Furthermore, pharmacokinetic variables were similar whether the compounds were dosed alone or in combination. We conclude that the pharmacokinetics of CCT018159 are complex. Cassette dosing is currently the best option available to assess the pharmacokinetics of this promising series of compounds in relatively high throughput and is now being applied to identify compounds with optimal pharmacokinetic properties during structural analogue synthesis.
Molecular Cancer Therapeutics 07/2006; 5(6):1628-37. DOI:10.1158/1535-7163.MCT-06-0041 · 5.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Novel piperazinyl, morpholino and piperidyl derivatives of the pyrazole-based Hsp90 inhibitor CCT018159 are described. Structure-activity relationships have been elucidated by X-ray co-crystal analysis of the new compounds bound to the N-terminal domain of human Hsp90. Key features of the binding mode are essentially identical to the recently reported potent analogue VER-49009. The most potent of the new compounds has a methylsulfonylbenzyl substituent appended to the piperazine nitrogen, possesses an IC50 of less than 600 nM binding against the enzyme and demonstrates low micromolar inhibition of tumour cell proliferation.