Discovery of the First Potent and Selective Inhibitor of Centromere-Associated Protein E: GSK923295.

Inhibition of mitotic kinesins represents a novel approach for the discovery of a new generation of anti-mitotic cancer chemotherapeutics. We report here the discovery of the first potent and selective inhibitor of centromere-associated protein E (CENP-E) 3-chloro-N-{(1S)-2-[(N,N-dimethylglycyl)amino]-1-[(4-{8-[(1S)-1-hydroxyethyl]imidazo[1,2-a]pyridin-2-yl}phenyl)methyl]ethyl}-4-[(1-methylethyl)oxy]benzamide (GSK923295; 1), starting from a high-throughput screening hit, 3-chloro-4-isopropoxybenzoic acid 2. Compound 1 has demonstrated broad antitumor activity in vivo and is currently in human clinical trials.

Comparative preclinical drug metabolism and pharmacokinetic evaluation of novel 4-aminoquinoline anti-malarials.

The disposition of three 4-aminoquinoline leads, namely isoquine (ISO), des-ethyl isoquine (DEI) and N-tert-butyl isoquine (NTBI), were studied in a range of in vivo and in vitro assays to assist in selecting an appropriate candidate for further development. Analogous to amodiaquine (ADQ), ISO undergoes oxidative N-dealkylation to form DEI in vivo. Blood clearance of DEI was as much as 10-fold lower than that of ISO in animals and after oral administration, metabolite exposure exceeded that of parent by as much as 14-fold. Replacement of the N-ethyl with an N-tert-butyl substituent substantially reduced N-dealkylation as blood clearance of NTBI was approximately 2 to 3-fold lower than DEI in mouse, rat, dog and monkey. Mean NTBI oral bioavailability was generally higher than the other leads (>/=68%). Blood cell association was substantial for NTBI, particularly in dog and monkey, where blood to plasma concentration ratios >4 were observed. Human plasma protein binding was similar for NTBI, DEI, and des-ethyl amodiaquine (DEA). Allometric scaling predicted human blood clearance (CL) for NTBI to be low ( approximately 12% liver blood flow). All the 4-aminoquinolines inhibited recombinant human cytochrome P450 2D6 with similar potency; DEI also inhibited 1A2. On balance, NTBI appeared the most promising lead to progress towards full development.

Involvement of intestinal uptake transporters in the absorption of azithromycin and clarithromycin in the rat.

Macrolide antibiotics azithromycin (AZI) and clarithromycin (CLARI) are large molecular weight compounds and are substrates for apically polarized efflux transporters such as P-glycoprotein, which can potentially restrict intestinal absorption. However, despite these undesired physicochemical and biopharmaceutical properties, AZI and CLARI exhibit moderate to excellent p.o. bioavailability in preclinical species and humans. Intestinal uptake transporters, such as organic anion transporting polypeptides (OATPs), can facilitate the uptake of drugs that are substrates and hence increase p.o. absorption. The present study was designed to determine whether the intestinal Oatps are involved in absorption of these macrolides. AZI or CLARI was dosed p.o. to Sprague-Dawley rats after p.o. administration with vehicle or rifamycin SV (RIF), an OATP inhibitor. The p.o. exposures of AZI and CLARI were reduced 65 and 45%, respectively, when coadministered with an optimized RIF regimen. The p.o. RIF had no affect on the total blood clearance of these macrolides and most likely did not cause induction of metabolizing enzymes and/or transporters. Therefore, the results suggest that inhibition of an RIF-sensitive uptake transporter such as Oatp along the rat gastrointestinal tract was responsible for reduced p.o. exposure of AZI and CLARI. In addition, AZI and CLARI caused inhibition of taurocholate uptake in rat Oatp1a5-transfected Madin-Darby canine kidney cell monolayers. The in vitro and in vivo results suggest that the intestinal Oatps are involved in the p.o. absorption of AZI and CLARI in the rat.

Novel ATP-competitive kinesin spindle protein inhibitors.

Kinesin spindle protein (KSP), an ATPase responsible for spindle pole separation during mitosis that is present only in proliferating cells, has become a novel and attractive anticancer target with potential for reduced side effects compared to currently available therapies. We report herein the discovery of the first known ATP-competitive inhibitors of KSP, which display a unique activity profile as compared to the known loop 5 (L5) allosteric KSP inhibitors that are currently under clinical evaluation. Optimization of this series led to the identification of biphenyl sulfamide 20, a potent KSP inhibitor with in vitro antiproliferative activity against human cells with either wild-type KSP (HCT116) or mutant KSP (HCT116 D130V). In a murine xenograft model with HCT116 D130V tumors, 20 showed significant antitumor activity following intraperitoneal dosing, providing in vivo proof-of-principle of the efficacy of an ATP-competitive KSP inhibitor versus tumors that are resistant to the other known KSP inhibitors.

Demonstration of a genetic therapeutic index for tumors expressing oncogenic BRAF by the kinase inhibitor SB-590885.

Oncogenic BRAF alleles are both necessary and sufficient for cellular transformation, suggesting that chemical inhibition of the activated mutant protein kinase may reverse the tumor phenotype. Here, we report the characterization of SB-590885, a novel triarylimidazole that selectively inhibits Raf kinases with more potency towards B-Raf than c-Raf. Crystallographic analysis revealed that SB-590885 stabilizes the oncogenic B-Raf kinase domain in an active configuration, which is distinct from the previously reported mechanism of action of the multi-kinase inhibitor, BAY43-9006. Malignant cells expressing oncogenic B-Raf show selective inhibition of mitogen-activated protein kinase activation, proliferation, transformation, and tumorigenicity when exposed to SB-590885, whereas other cancer cell lines and normal cells display variable sensitivities or resistance to similar treatment. These studies support the validation of oncogenic B-Raf as a target for cancer therapy and provide the first evidence of a correlation between the expression of oncogenic BRAF alleles and a positive response to a selective B-Raf inhibitor.

Preclinical drug metabolism and pharmacokinetic evaluation of GW844520, a novel anti-malarial mitochondrial electron transport inhibitor.

GW844520 is a potent and selective inhibitor of the cytochrome bc1 complex of mitochondrial electron transport in P. falciparum, the parasite primarily responsible for the mortality associated with malaria worldwide. GW844520 is fully active against the parasite including resistance isolates, showing no cross resistance with agents in use. To evaluate full potential of this development candidate, we conducted drug metabolism and pharmacokinetic studies of this novel anti-malarial. GW844520 had low blood clearance of about 0.5-4% of hepatic blood flow and a steady-state volume of distribution of 2-4 times total body water in mouse, rat, dog, and monkey. Oral bioavailability was high (51-100%). Consistent with the in vivo data, GW844520 had low intrinsic clearance in liver microsomes and hepatocytes of animal and human origin, high passive cellular permeability and was not a P-glycoprotein substrate. GW844520 did not associate appreciably with blood cells but was highly bound to plasma proteins (>99%) in all species. GW844520 was a substrate and inhibitor of human CYP2D6 but not of CYP1A2, 2C9, 2C19, and 3A4. This conjunctive analysis supports continued evaluation of this compound in definitive pre-IND studies and exemplifies our strategy supporting the discovery of novel agents to treat diseases of the developing world.

Automated analysis of polyethylene glycol-induced inhibition of P-glycoprotein activity in vitro.

Previous studies in our laboratories have shown that commonly used polyethoxylated pharmaceutical excipients inhibit P-glycoprotein activity in cell culture models of the intestinal mucosa. The results presented in this technical note show that the TECAN Genesis robotic workstation can be utilized to automate cellular transport studies for evaluating excipient effects on P-glycoprotein activity in vitro and for estimating the permeation of drug-like molecules across cell monolayers.

Effects of poly(ethylene glycol) on efflux transporter activity in Caco-2 cell monolayers.

Poly(ethylene glycol) (PEG) is an excipient commonly used in pharmaceutical formulations to increase the aqueous solubility of drugs intended for oral administration. High concentrations of PEG are often used to solubilize drug candidates for in vitro experiments in cell culture (e.g., Caco-2 cell permeability studies) and/or for in vivo pharmacokinetic and safety studies in animals. Although PEG is often deemed safe in these studies based on gross morphological studies, changes on a molecular level may be overlooked. The purpose of this study was to determine the possible effects of PEG on efflux transporter activity in Caco-2 cell monolayers, an in vitro model of the intestinal mucosa. In these studies, relatively high, yet clinically achievable, concentrations of PEG-300 did not significantly change the passive paracellular or transcellular permeation of model solutes across Caco-2 cell monolayers. More importantly, PEG-300 inhibited efflux transporter activity in Caco-2 cell monolayers, which is probably mediated by P-gp and/or MRP. Such PEG-induced inhibition of efflux transporter activity is most likely caused by changes in the microenvironment of the Caco-2 cell membranes, which perturbs the ability of these transporters to efflux substrates such as taxol and doxorubicin.

A comparison of commonly used polyethoxylated pharmaceutical excipients on their ability to inhibit P-glycoprotein activity in vitro.

P-glycoprotein (P-gp), a multidrug resistance (MDR) protein encoded by the MDR1 gene in humans, is responsible for the efflux of structurally diverse drugs. Previous studies in our laboratory have shown that excipients such as poly(ethylene)glycol (PEG)-300, Cremophor EL, and Tween 80 inhibit P-gp activity in Caco-2 cell monolayers. The objective of this study was to determine the effects of these excipients in an MDR1- transfected Madin Darby Canine Kidney (MDR1-MDCK) cell line and to compare the results with those obtained from Caco-2 cells. The results presented herein show that PEG-300 (20%, v/v) causes almost complete inhibition of P-gp activity in both Caco-2 and MDR1-MDCK cell monolayers, whereas Cremophor EL (0.1%, w/v) and Tween 80 (0.05%, w/v) only partially inhibit P-gp activity in Caco-2 cells. Cremophor EL (0.1%, w/v) and Tween 80 (0.05%, w/v) were inactive as P-gp inhibitors in MDR1-MDCK cell monolayers. This inability of Tween 80 and Cremphor EL to inhibit P-gp activity in MDR1-MDCK cells may be related to differences in the interactions of the surfactants with these different cell membranes. PEG-induced changes in P-gp activity are probably related to changes in the fluidity of the polar head group regions of cell membranes.