Discovery of siRNA lipid nanoparticles to transfect suspension leukemia cells and provide in vivo delivery capability.

As a powerful research tool, siRNA's therapeutic and target validation utility with leukemia cells and long-term gene knockdown is severely restricted by the lack of omnipotent, safe, stable, and convenient delivery. Here, we detail our discovery of siRNA-containing lipid nanoparticles (LNPs) able to effectively transfect several leukemia and difficult-to-transfect adherent cell lines also providing in vivo delivery to mouse spleen and bone marrow tissues through tail-vein administration. We disclose a series of novel structurally related lipids accounting for the superior transfection ability, and reveal a correlation between expression of Caveolins and successful transfection. These LNPs, bearing low toxicity and long stability of >6 months, are ideal for continuous long-term dosing. Our discovery represents the first effective siRNA-containing LNPs for leukemia cells, which not only enables high-throughput siRNA screening with leukemia cells and difficult-to-transfect adherent cells but also paves the way for the development of therapeutic siRNA for leukemia treatment.

MDM2 antagonists boost antitumor effect of androgen withdrawal: implications for therapy of prostate cancer.

BACKGROUND: Hormone therapy is the standard of care for newly diagnosed or recurrent prostate cancers. It uses anti-androgen agents, castration, or both to eliminate cancer promoting effect of testicular androgen. The p53 tumor suppressor controls a major pathway that can block cell proliferation or induce apoptosis in response to diverse forms of oncogenic stress. Activation of the p53 pathway in cancer cells expressing wild-type p53 has been proposed as a novel therapeutic strategy and recently developed MDM2 antagonists, the nutlins, have validated this in preclinical models of cancer. The crosstalk between p53 and androgen receptor (AR) signaling suggest that p53 activation could augment antitumor outcome of androgen ablation in prostate cancer. Here, we test this hypothesis in vitro and in vivo using the MDM2 antagonist, nutlin-3 and the p53 wild-type prostate cancer cell line, LNCaP. RESULTS: Using charcoal-stripped serum as a cellular model of androgen deprivation, we show an increased apoptotic effect of p53 activation by nutlin-3a in the androgen-dependent LNCaP cells and to a lesser extent in androgen-independent but responsive 22Rv1 cell line. This effect is due, at least in part, to an enhanced downregulation of AR expression by activated p53. In vivo, androgen deprivation followed by two weeks of nutlin administration in LNCaP-bearing nude mice led to a greater tumor regression and dramatically increased survival. CONCLUSIONS: Since majority of prostate tumors express wild-type p53, its activation by MDM2 antagonists in combination with androgen depletion may offer an efficacious new approach to prostate cancer therapy.

Activation of p53 by MDM2 antagonists can protect proliferating cells from mitotic inhibitors.

Recent studies have shown that activation of cell cycle checkpoints can protect normal proliferating cells from mitotic inhibitors by preventing their entry into mitosis. These studies have used genotoxic agents that act, at least in part, by activation of the p53 pathway. However, genotoxic drugs are known also to have p53-independent activities and could affect the sensitivity of tumor cells to antimitotic agents. Recently, we have developed the first potent and selective small-molecule inhibitors of the p53-MDM2 interaction, the nutlins, which activate the p53 pathway only in cells with wild-type but not mutant p53. Using these compounds, we show that p53 activation leads to G1 and G2 phase arrest and can protect cells from mitotic block and apoptosis caused by paclitaxel. Pretreatment of HCT116 and RKO colon cancer cells (wild-type p53) or primary human fibroblasts (1043SK) with nutlins for 24 hours followed by incubation with paclitaxel for additional 48 hours did not increase significantly their mitotic index and protected the cells from the cytotoxicity of paclitaxel. Cancer cells with mutant p53 (MDA-MB-435) responded to the same treatment with mitotic arrest and massive apoptosis. These results have two major implications for cancer therapy. First, p53-activating therapies may have antagonistic effect when combined with mitotic poisons. Second, pretreatment with MDM2 antagonists before chemotherapy of tumors with mutant p53 may offer a partial protection to proliferating normal tissues.

3-Aminopyrrolidinone farnesyltransferase inhibitors: design of macrocyclic compounds with improved pharmacokinetics and excellent cell potency.

A series of macrocyclic 3-aminopyrrolidinone farnesyltransferase inhibitors (FTIs) has been synthesized. Compared with previously described linear 3-aminopyrrolidinone FTIs such as compound 1, macrocycles such as 49 combined improved pharmacokinetic properties with a reduced potential for side effects. In dogs, oral bioavailability was good to excellent, and increases in plasma half-life were due to attenuated clearance. It was observed that in vivo clearance correlated with the flexibility of the molecules and this concept proved useful in the design of FTIs that exhibited low clearance, such as FTI 78. X-ray crystal structures of compounds 49 and 66 complexed with farnesyltransferase (FTase)-farnesyl diphosphate (FPP) were determined, and they provide details of the key interactions in such ternary complexes. Optimization of this 3-aminopyrrolidinone series of compounds led to significant increases in potency, providing 83 and 85, the most potent inhibitors of FTase in cells described to date.

Potent N-(1,3-thiazol-2-yl)pyridin-2-amine vascular endothelial growth factor receptor tyrosine kinase inhibitors with excellent pharmacokinetics and low affinity for the hERG ion channel.

A series of N-(1,3-thiazol-2-yl)pyridin-2-amine KDR kinase inhibitors have been developed that possess optimal properties. Compounds have been discovered that exhibit excellent in vivo potency. The particular challenges of overcoming hERG binding activity and QTc increases in vivo in addition to achieving good pharmacokinetics have been acomplished by discovering a unique class of amine substituents. These compounds have a favorable kinase selectivity profile that can be accentuated with appropriate substitution.

Small-molecule inducer of cancer cell polyploidy promotes apoptosis or senescence: Implications for therapy.

Polyploidy results from deregulated cell division and has been considered an undesirable event leading to increased mutation rate and cancer development. However, polyploidy may also render cancer cells more vulnerable to chemotherapy. Here, we identify a small-molecule inducer of polyploidy, R1530, which interferes with tubulin polymerization and mitotic checkpoint function in cancer cells, leading to abortive mitosis, endoreduplication and polyploidy. In the presence of R1530, polyploid cancer cells underwent apoptosis or became senescent which translated into potent in vitro and in vivo efficacy. Normal proliferating cells were resistant to R1530-induced polyploidy thus supporting the rationale for cancer therapy by induced polyploidy. Mitotic checkpoint kinase BubR1 was found downregulated during R1530-induced exit from mitosis, a likely consequence of PLK4 inhibition. BubR1 knockdown in the presence of nocodazole induced an R1530-like phenotype, suggesting that BubR1 plays a key role in polyploidy induction by R1530 and could be exploited as a target for designing more specific polyploidy inducers.

Elevated MDM2 boosts the apoptotic activity of p53-MDM2 binding inhibitors by facilitating MDMX degradation.

The p53 tumor suppressor is a powerful growth suppressive and pro-apoptotic molecule frequently inactivated in human cancer. Many tumors overproduce its negative regulator MDM2, a specific p53 ubiquitin ligase and transcriptional inhibitor, to disable p53 function. Therefore, p53 activation by inhibiting MDM2 has been proposed as a novel strategy for cancer therapy in tumors expressing wild-type p53. Recently developed small-molecule p53-MDM2 binding inhibitors, the nutlins, selectively activate p53 function and induce cell cycle arrest and apoptosis in cancer cells. By stabilizing p53, nutlins also elevate the cellular level of its transcriptional target MDM2. Here, we present evidence that nutlin-induced MDM2 retains its ubiquitin ligase activity and contributes to the anti-tumor activity of p53-MDM2 binding inhibitors by facilitating the degradation of another p53 inhibitor, MDMX. MDM2 and MDMX levels were analyzed in a panel of 12 randomly selected solid tumor cell lines. In the presence of nutlin-3, MDM2 increased in all and MDMX decreased in most of the cell lines. MDMX was resistant to nutlin-induced degradation in 2/12 cell lines. In these cells, MDMX appears to be a major suppressor of the apoptotic response to p53 activation although this effect was only partially p53-dependent. Doxorubicin facilitated MDMX degradation through DNA damage response pathways and restored their sensitivity to nutlin, suggesting that combination therapy may be an effective way to overcome nutlin resistance in cancers with MDMX aberrations.

Selective small-molecule inhibitor reveals critical mitotic functions of human CDK1.

CDK1 is a nonredundant cyclin-dependent kinase (CDK) with an essential role in mitosis, but its multiple functions still are poorly understood at a molecular level. Here we identify a selective small-molecule inhibitor of CDK1 that reversibly arrests human cells at the G(2)/M border of the cell cycle and allows for effective cell synchronization in early mitosis. Inhibition of CDK1 during cell division revealed that its activity is necessary and sufficient for maintaining the mitotic state of the cells, preventing replication origin licensing and premature cytokinesis. Although CDK1 inhibition for up to 24 h is well tolerated, longer exposure to the inhibitor induces apoptosis in tumor cells, suggesting that selective CDK1 inhibitors may have utility in cancer therapy.

Small-molecule MDM2 antagonists reveal aberrant p53 signaling in cancer: implications for therapy.

The p53 tumor suppressor retains its wild-type conformation and transcriptional activity in half of all human tumors, and its activation may offer a therapeutic benefit. However, p53 function could be compromised by defective signaling in the p53 pathway. Using a small-molecule MDM2 antagonist, nutlin-3, to probe downstream p53 signaling we find that the cell-cycle arrest function of the p53 pathway is preserved in multiple tumor-derived cell lines expressing wild-type p53, but many have a reduced ability to undergo p53-dependent apoptosis. Gene array analysis revealed attenuated expression of multiple apoptosis-related genes. Cancer cells with mdm2 gene amplification were most sensitive to nutlin-3 in vitro and in vivo, suggesting that MDM2 overexpression may be the only abnormality in the p53 pathway of these cells. Nutlin-3 also showed good efficacy against tumors with normal MDM2 expression, suggesting that many of the patients with wild-type p53 tumors may benefit from antagonists of the p53-MDM2 interaction.

Kinesin spindle protein (KSP) inhibitors. Part 2: the design, synthesis, and characterization of 2,4-diaryl-2,5-dihydropyrrole inhibitors of the mitotic kinesin KSP.

The evolution of 2,4-diaryl-2,5-dihydropyrroles as inhibitors of KSP is described. Introduction of basic amide and urea moieties to the dihydropyrrole nucleus enhanced potency and aqueous solubility, simultaneously, and provided compounds that caused mitotic arrest of A2780 human ovarian carcinoma cells with EC(50)s<10nM. Ancillary hERG activity was evaluated for this series of inhibitors.

Kinesin spindle protein (KSP) inhibitors. Part 3: synthesis and evaluation of phenolic 2,4-diaryl-2,5-dihydropyrroles with reduced hERG binding and employment of a phosphate prodrug strategy for aqueous solubility.

2,4-Diaryl-2,5-dihydropyrroles have been discovered to be novel, potent and water-soluble inhibitors of KSP, an emerging therapeutic target for the treatment of cancer. A potential concern for these basic KSP inhibitors (1 and 2) was hERG binding that can be minimized by incorporation of a potency-enhancing C2 phenol combined with neutral N1 side chains. Aqueous solubility was restored to these, and other, non-basic inhibitors, through a phosphate prodrug strategy.

Biochemical and structural characterization of a novel class of inhibitors of the type 1 insulin-like growth factor and insulin receptor kinases.

The type 1 insulin-like growth factor receptor (IGF-1R) is often overexpressed on tumor cells and is believed to play an important role in anchorage-independent proliferation. Additionally, cell culture studies have indicated that IGF-1R confers increased resistance to apoptosis caused by radiation or chemotherapeutic agents. Thus, inhibitors of the intracellular kinase domain of this receptor may have utility for the clinical treatment of cancer. As part of an effort to develop clinically useful inhibitors of IGF-1R kinase, a novel class of pyrrole-5-carboxaldehyde compounds was investigated. The compounds exhibited selectivity against the closely related insulin receptor kinase intrinsically and in cell-based assays. The inhibitors formed a reversible, covalent adduct at the kinase active site, and treatment of such adducts with sodium borohydride irreversibly inactivated the enzyme. Analysis of a tryptic digest of a covalently modified IGF-1R kinase fragment revealed that the active site Lys1003 had been reductively alkylated with the aldehyde inhibitor. Reductive alkylation of the insulin receptor kinase with one of these inhibitors led to a similarly inactivated enzyme which was examined by X-ray crystallography. The crystal structure confirmed the modification of the active site lysine side chain and revealed details of the key interactions between the inhibitor and enzyme.

Phosphorylation of p53 on key serines is dispensable for transcriptional activation and apoptosis.

The p53 tumor suppressor is a key mediator of the cellular response to stress. Phosphorylation induced by multiple stress-activated kinases has been proposed to be essential for p53 stabilization, interaction with transcriptional co-activators, and activation of p53 target genes. However, genetic studies suggest that stress-activated phosphorylation may not be essential for p53 activation. We therefore investigated the role of p53 phosphorylation on six key serine residues (Ser(6), Ser(15), Ser(20), Ser(37), Ser(46), and Ser(392)) for p53 activation using nutlin-3, a recently developed small molecule MDM2 antagonist. We show here that nutlin does not induce the phosphorylation of p53. Comparison of the activity of unphosphorylated and phosphorylated p53 induced by the genotoxic drugs doxorubicin and etoposide in HCT116 and RKO cells revealed no difference in their sequence-specific DNA binding and ability to transactivate p53 target genes and to induce p53-dependent apoptosis. We conclude that p53 phosphorylation on six major serine sites is not required for activation of p53 target genes or biological responses in vivo.