Pharmacokinetic and pharmacodynamic analysis of gastric cancer patients treated with telatinib

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Background: The soluble form of the VEGFR2 receptor (sVEGFR2) neutralizes circulating VEGF, and functions as a negative feedback mechanism to enable partial inhibition of VEGF-stimulated endothelial cell migration and proliferation. In response to inhibition of VEGFR2 tyrosine kinase activity, up-regulation of VEGF expression and down-regulation of sVEGFR2 expression levels have been observed (Murukesh et al., 2010). Telatinib (tel) is a novel orally available kinase inhibitor that is highly selective for the VEGFR, PDGFR, and KIT tyrosine kinases at nanomolar concentrations with potent antiangiogenic activity. Correlation between telatinib exposure and reduction in plasma sVEGFR2 levels from baseline has previously been demonstrated in patient serum samples in phase I studies.

Methods: TEL0805, a phase II study administered tel with capecitabine (X) and cisplatin (P) in previously untreated metastatic or unresectable gastric or GEJ adenocarcinoma pts. Patient serum samples were obtained on day -7, [6 plasma samples (pre-dose, and at 30 min, 1, 2, 3 and 4 hours)] and on day 1, [4 plasma samples (pre-dose and at 1, 2 and 3 hours)] and evaluated for the levels of telatinib and its M2 metabolite (M2). Additional serum samples were collected every 42 days and evaluated for VEGF and sVEGFR2 levels.

Results: Measurement of tel and M2 levels confirmed previous pharmacokinetic findings and demonstrated no drug accumulation following continuous daily dosing. Baseline plasma sVEGFR2 levels have previously not been reported for advanced gastric cancer patients and displayed a wide range at disease presentation. Reduction in sVEGFR2 levels were noted for nearly all treated patients and correlated with the duration of stay on tel therapy.

Conclusions: Telatinib continuous dosing is possible in combination with XP with little effect on tel exposure or accumulation. Reduction in sVEGFR2 levels may be useful in identifying patients who may benefit from tel treatment in this combination setting.

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Cell cycle and biochemical effects of PD 0183812. A potent inhibitor of the cyclin D-dependent kinases CDK4 and CDK6

Progression through the G1 phase of the cell cycle requires phosphorylation of the retinoblastoma gene product (pRb) by the cyclin D-dependent kinases CDK4 and CDK6, whose activity can specifically be blocked by the CDK inhibitor p16(INK4A). Misregulation of the pRb/cyclin D/p16(INK4A) pathway is one of the most common events in human cancer and has lead to the suggestion that inhibition of cyclin D-dependent kinase activity may have therapeutic value as an anticancer treatment. Through screening of a chemical library, we initially identified the [2,3-d]pyridopyrimidines as inhibitors of CDK4. Chemical modification resulted in the identification of PD 0183812 as a potent and highly selective inhibitor of both CDK4 and CDK6 kinase activity, which is competitive with ATP. Flow cytometry experiments showed that of the cell lines tested, only those expressing pRb demonstrated a G1 arrest when treated with PD 0183812. This arrest correlated in terms of incubation time and potency with a loss of pRb phosphorylation and a block in proliferation, which was reversible. These results suggest a potential use of this chemical class of compounds as therapeutic agents in the treatment of tumors with functional pRb, possessing cell cycle aberrations in other members of the pRb/cyclin D/p16(INK4A) pathway.

Analyses of single-amino-acid substitution mutants of adenovirus type 5 E1B-55K protein

The E1B-55K protein plays an important role during human adenovirus type 5 productive infection. In the early phase of the viral infection, E1B-55K binds to and inactivates the tumor suppressor protein p53, allowing efficient replication of the virus. During the late phase of infection, E1B-55K is required for efficient nucleocytoplasmic transport and translation of late viral mRNAs, as well as for host cell shutoff. In an effort to separate the p53 binding and inactivation function and the late functions of the E1B-55K protein, we have generated 26 single-amino-acid mutations in the E1B-55K protein. These mutants were characterized for their ability to modulate the p53 level, interact with the E4orf6 protein, mediate viral late-gene expression, and support virus replication in human cancer cells. Of the 26 mutants, 24 can mediate p53 degradation as efficiently as the wild-type protein. Two mutants, R240A (ONYX-051) and H260A (ONYX-053), failed to degrade p53 in the infected cells. In vitro binding assays indicated that R240A and H260A bound p53 poorly compared to the wild-type protein. When interaction with another viral protein, E4orf6, was examined, H260A significantly lost its ability to bind E4orf6, while R240A was fully functional in this interaction. Another mutant, T255A, lost the ability to bind E4orf6, but unexpectedly, viral late-gene expression was not affected. This raised the possibility that the interaction between E1B-55K and E4orf6 was not required for efficient viral mRNA transport. Both R240A and H260A have retained, at least partially, the late functions of wild-type E1B-55K, as determined by the expression of viral late proteins, host cell shutoff, and lack of a cold-sensitive phenotype. Virus expressing R240A (ONYX-051) replicated very efficiently in human cancer cells, while virus expressing H260A (ONYX-053) was attenuated compared to wild-type virus dl309 but was more active than ONYX-015. The ability to separate the p53-inactivation activity and the late functions of E1B-55K raises the possibility of generating adenovirus variants that retain the tumor selectivity of ONYX-015 but can replicate more efficiently than ONYX-015 in a broad spectrum of cell types.

Pyrido[2,3-d]pyrimidin-7-one inhibitors of cyclin-dependent kinases

The identification of 8-ethyl-2-phenylamino-8H-pyrido[2, 3-d]pyrimidin-7-one (1) as an inhibitor of Cdk4 led to the initiation of a program to evaluate related pyrido[2, 3-d]pyrimidin-7-ones for inhibition of cyclin-dependent kinases (Cdks). Analysis of more than 60 analogues has identified some clear SAR trends that may be exploited in the design of more potent Cdk inhibitors. The most potent Cdk4 inhibitors reported in this study inhibit Cdk4 with IC(50) = 0.004 microM ([ATP] = 25 microM). X-ray crystallographic analysis of representative compounds bound to the related kinase, Cdk2, reveals that they occupy the ATP binding site. Modest selectivity between Cdks is exhibited by some compounds, and Cdk4-selective inhibitors block pRb(+) cells in the G(1)-phase of the cell division cycle.

CDK inhibition and cancer therapy

The cell-division cycle is a tightly controlled process that is regulated by the cyclin/CDK family of protein kinase complexes. Stringent control of this process is essential to ensure that DNA synthesis and subsequent mitotic division are accurately and coordinately executed. There is now strong evidence that CDKs, their regulators, and substrates are the targets of genetic alteration in many human cancers. As a result of this, the CDKs have been targeted for drug discovery and a number of small molecule inhibitors of CDKs have been identified.

Human Myt1 is a cell cycle-regulated kinase that inhibits Cdc2 but not Cdk2 activity

Activation of the Cdc2.cyclin B kinase is a pivotal step of mitotic initiation. This step is mediated principally by the dephosphorylation of residues threonine 14 (Thr14) and tyrosine 15 (Tyr15) on the Cdc2 catalytic subunit. In several organisms homologs of the Wee1 kinase have been shown to be the major activity responsible for phosphorylating the Tyr15 inhibitory site. A membrane-bound kinase capable of phosphorylating residue Thr14, the Myt1 kinase, has been identified in the frog Xenopus laevis and more recently in human. In this study, we have examined the substrate specificity and cell cycle regulation of the human Myt1 kinase. We find that human Myt1 phosphorylates and inactivates Cdc2-containing cyclin complexes but not complexes containing Cdk2 or Cdk4. Analysis of endogenous Myt1 demonstrates that it remains membrane-bound throughout the cell cycle, but its kinase activity decreased during M phase arrest, when Myt1 became hyperphosphorylated. Further, Cdc2. cyclin B1 was capable of phosphorylating Myt1 in vitro, but this phosphorylation did not affect Myt1 kinase activity. These findings suggest that human Myt1 is negatively regulated by an M phase-activated kinase and that Myt1 inhibits mitosis due to its specificity for Cdc2.cyclin complexes.

New functional activities for the p21 family of CDK inhibitors

The association of cdk4 with D-type cyclins to form functional kinase complexes is comparatively inefficient. This has led to the suggestion that assembly might be a regulated step. In this report we demonstrate that the CDK inhibitors p21(CIP), p27(KIP), and p57(KIP2) all promote the association of cdk4 with the D-type cyclins. This effect is specific and does not occur with other cdk inhibitors or cdk-binding proteins. Both in vivo and in vitro, the abundance of assembled cdk4/cyclin D complex increases directly with increasing inhibitor levels. The promotion of assembly is not attributable to a simple cell cycle block and requires the function of both the cdk and cyclin-binding domains. Kinetic studies demonstrate that p21 and p27 lead to a 35- and 80-fold increase in K(a), respectively, mostly because of a decrease in K(off). At low concentrations, p21 promotes the assembly of active kinase complexes, whereas at higher concentrations, it inhibits activity. Moreover, immunodepletion experiments demonstrate that most of the active cdk4-associated kinase activity also associates with p21. To confirm these results in a natural setting, we examine the assembly of endogenous complexes in mammary epithelial cells after release from a G(0) arrest. In agreement with our other data, cyclin D1 and p21 bind concomitantly to cdk4 during the in vivo assembly of cdk4/cyclin D1 complexes. This complex assembly occurs in parallel to an increase in cyclin D1-associated kinase activity. Immunodepletion experiments demonstrate that most of the cellular cyclin D1-associated kinase activity is also p21 associated. Finally, we find that all three CIP/KIP inhibitors target cdk4 and cyclin D1 to the nucleus. We suggest that in addition to their roles as inhibitors, the p21 family of proteins, originally identified as inhibitors, may also have roles as adaptor proteins that assemble and program kinase complexes for specific functions.

Myt1: a Wee1-type kinase that phosphorylates Cdc2 on residue Thr14

Most somatic cell division cycles contain a gap period (G2 phase) between the completion of DNA synthesis and the initiation of mitosis. This delay of mitotic entry is controlled, at least in part, by the repression of Cdc2 kinase activity by the phosphorylation of two conserved residues (Thr14 and Tyr15) within the ATP-binding pocket of the Cdc2 catalytic subunit. The kinases responsible for these two phosphorylation events include the Myt1 and Wee1 kinases, which phosphorylate Cdc2 on Thr14 and Tyr15, respectively. In this discussion, we summarise our current knowledge of the Myt1 kinase and its regulation of Cdc2 kinase activity during the G2-to -M phase transition.

An adenovirus mutant that replicates selectively in p53-deficient human tumor cells

The human adenovirus E1B gene encodes a 55-kilodalton protein that inactivates the cellular tumor suppressor protein p53. Here it is shown that a mutant adenovirus that does not express this viral protein can replicate in and lyse p53-deficient human tumor cells but not cells with functional p53. Ectopic expression of the 55-kilodalton EIB protein in the latter cells rendered them sensitive to infection with the mutant virus. Injection of the mutant virus into p53-deficient human cervical carcinomas grown in nude mice caused a significant reduction in tumor size and caused complete regression of 60 percent of the tumors. These data raise the possibility that mutant adenoviruses can be used to treat certain human tumors.

Reverse two-hybrid and one-hybrid systems to detect dissociation of protein-protein and DNA-protein interactions

Macromolecular interactions define many biological phenomena. Although genetic methods are available to identify novel protein-protein and DNA-protein interactions, no genetic system has thus far been described to identify molecules or mutations that dissociate known interactions. Herein, we describe genetic systems that detect such events in the yeast Saccharomyces cerevisiae. We have engineered yeast strains in which the interaction of two proteins expressed in the context of the two-hybrid system or the interaction between a DNA-binding protein and its binding site in the context of the one-hybrid system is deleterious to growth. Under these conditions, dissociation of the interaction provides a selective growth advantage, thereby facilitating detection. These methods referred to as the "reverse two-hybrid system" and "reverse one-hybrid system" facilitate the study of the structure-function relationships and regulation of protein-protein and DNA-protein interactions. They should also facilitate the selection of dissociator molecules that could be used as therapeutic agents.

Direct interaction of human TFIID with the HIV-1 transactivator tat

The tat gene of the human immunodeficiency virus (HIV) plays a central role in the activation and life cycle of HIV. The tat protein (Tat) specifically transactivates HIV transcription in vivo and in vitro, exerting its effects at the level of transcriptional initiation and elongation. Here we report that Tat binds directly to the basal transcription factor TFIID. The transcriptional activity of HeLa extracts was depleted after chromatography on a Tat affinity column, which specifically retained the polymerase II-specific factor TFIID. Direct interaction of Tat with holo-TFIID, composed of TATA-binding protein (TBP) and associated factors (TAFs), was observed. Tat binds, through amino acids 36-50, directly to the TBP subunit of TFIID. Our results suggest that Tat may transduce upstream or downstream regulatory signals by direct interaction with the basal transcription factor TFIID.

Analysis of p107-associated proteins: p107 associates with a form of E2F that differs from pRB-associated E2F-1

The binding of viral oncogenes to cellular proteins is thought to modulate the activities of these cellular targets. The p107 protein is targeted by many viral proteins, including adenovirus E1A, simian virus 40 large T antigen, and human papillomavirus type 16 E7 protein. A panel of monoclonal antibodies against p107 was raised and used to identify cellular proteins that interact with the p107 protein in vivo. p107-associated proteins included cyclin A, cyclin E, and cdk2. In addition, p107 was found to associate with 62- to 65- and 50-kDa phosphoproteins in ML-1 cells, a human myeloid leukemia cell line. The 62- to 65-kDa proteins have many of the properties of the transcription factor E2F but were distinguished from pRB-associated E2F-1 by both immunologic and biochemical properties.

Inhibition of E2F-1 transactivation by direct binding of the retinoblastoma protein

Loss of a functional retinoblastoma tumor suppressor gene product, pRB, is a key step in the development of many human tumors. pRB is a negative regulator of cell proliferation and appears to participate in control of entry into the S phase of the cell cycle. The recent demonstration that pRB binds to transcription factor E2F has provided a model for the mechanism of pRB-mediated growth regulation. Since adenovirus E1A proteins dissociate the pRB-E2F complexes and stimulate E2F-dependent transcription, it has been suggested that pRB inhibits E2F transactivation. Although some evidence for this hypothesis has been provided, it has not been possible to determine the mechanism of pRB-mediated inhibition of E2F transactivation. In this study, we constructed mutants of E2F-1 that do not bind to pRB yet retain the ability to transactivate the adenovirus E2 promoter through E2F DNA-binding sites. We demonstrated that transactivation mediated by the wild-type E2F-1 protein was inhibited by overexpression of wild-type pRB but not by a naturally occurring mutant of pRB. Transactivation mediated by mutants of E2F-1 which do not bind to pRB was not affected by overexpression of wild-type pRB. Furthermore, when the E2F-1 transactivation domain was fused to the GAL4 DNA-binding domain, pRB inhibited GAL4-E2F-1 transactivation through GAL4 sites. Expression of pRB did not inhibit transactivation mediated by GAL4-E2F-1 mutant constructs which were devoid of pRB binding. In conclusion, these data demonstrate that pRB inhibits E2F-dependent transactivation by direct protein-protein interaction.

Inhibition of cell proliferation by p107, a relative of the retinoblastoma protein

The cellular protein p107 shares many structural and biochemical features with the retinoblastoma gene product, pRB. We have isolated a full-length cDNA for human p107 and have used this clone to study the function of p107. We show that, like pRB, p107 is a potent inhibitor of E2F-mediated trans-activation, and overexpression of p107 can inhibit proliferation in certain cell types, arresting sensitive cells in G1. Several experiments, however, showed that growth inhibition by pRB and p107 did not occur through the same mechanism. First, in the cervical carcinoma cell line C33A, p107 was able to block cell proliferation, whereas pRB could not, even though both proteins were potent inhibitors of E2F-mediated transcription in this cell line. Second, growth arrest by pRB and p107 was rescued differentially by various cell cycle regulators. Third, some mutants of p107 that cannot associate with adenovirus E1A were still able to inhibit cell proliferation, whereas analogous mutants in pRB are known to be unable to block cell growth. Together, these results suggest a biological role of p107 that is related, but not identical, to that of pRB.

Transcriptional inhibition by the retinoblastoma protein

The retinoblastoma protein, pRB, appears to play a key role in coordinating the regulation of cell cycle position and transcriptional events. pRB undergoes specific cell-cycle-dependent phosphorylation, being underphosphorylated in G1 and heavily phosphorylated in S, G2, and M. The underphosphorylated form is able to interact with the E2F transcription factor. Recently, we have cloned a cDNA for E2F-1. By using this clone and a series of non-pRB binding mutants, we have been able to show that the binding of pRB to E2F-1 causes inhibition of E2F-mediated transactivation. pRB's inhibition of E2F-mediated transcription would be lost by mutation in the retinoblastoma gene in human tumours, by pRB's interaction with DNA tumour virus oncoproteins, or by phosphorylation during the cell cycle.

A cDNA encoding a pRB-binding protein with properties of the transcription factor E2F

The retinoblastoma protein (pRB) plays an important role in the control of cell proliferation, apparently by binding to and regulating cellular transcription factors such as E2F. Here we describe the characterization of a cDNA clone that encodes a protein with properties of E2F. This clone, RBP3, was identified by the ability of its gene product to interact with pRB. RBP3 bound to pRB both in vitro and in vivo, and this binding was competed by viral proteins known to disrupt pRB-E2F association. RBP3 bound to E2F recognition sequences in a sequence-specific manner. Furthermore, transient expression of RBP3 caused a 10-fold transactivation of the adenovirus E2 promoter, and this transactivation was dependent on the E2F recognition sequences. These properties suggest that RBP3 encodes E2F, or an E2F-like protein.

Production and characterization of monoclonal antibodies to budgerigar fledgling disease virus major capsid protein VP

Eleven hybridoma cell lines producing monoclonal antibodies (MAbs) against intact budgerigar fledgling disease (BFD) virions were produced and characterized. These antibodies were selected for their ability to react with BFD virions in an enzyme-linked immunosorbent assay. Each of these antibodies was reactive in the immunofluorescent detection of BFD virus-infected cells. These antibodies immunoprecipitated intact virions and specifically recognized the major capsid protein, VP1, of the dissociated virion. The MAbs were found to preferentially recognize native BFD virus capsid protein when compared with denatured virus protein. These MAbs were capable of detecting BFD virus protein in chicken embryonated cell-culture lysates by dot-blot analysis.