Biomedical subjects

Malcolm F G Stevens

Publications and source records attributed to Malcolm F G Stevens.

At least 37 records · Page 2Linked to original sources

PubMed2004

Structural studies on bioactive compounds. 39. Biological consequences of the structural modification of DHFR-inhibitory 2,4-diamino-6-(4-substituted benzylamino-3-nitrophenyl)-6-ethylpyrimidines ('benzoprims').

Benzimidazole-N-oxide modifications of potent lipophilic dihydrofolate reductase (DHFR) inhibitors (e.g., methylbenzoprim 1 and dichlorobenzoprim 2) have been prepared by base-promoted cyclization of the nitrophenylbenzylamino groups to explore the possibility that abrogation of DHFR-inhibitory activity might reveal clues to an alternative anti-ras mechanism. Examples of the new series had only low growth inhibitory activities (GI(50) generally >50 microM) against colon HCT116 and lung HT29 cell lines but, unlike methylbenzoprim, this activity was unaffected by hypoxanthine/thymidine rescue.

Antineoplastic Agents↗

PubMed2004

Structure and K+ ion-dependent stability of a parallel-stranded DNA quadruplex containing a core A-tetrad.

The c-MYC oncogene, containing the DNA 5'-GGAGG repeat sequence, is emerging as an attractive target for anti-cancer therapeutics with evidence to suggest that DNA quadruplex formation is involved in regulating gene transcription. We describe NMR studies and molecular dynamics (MD) simulations on intermolecular quadruplex structures containing the purine-rich GGAGG motif which we show readily assembles to form stable parallel-stranded quadruplexes containing A- and G-tetrads with evidence for weak sequence-specific K(+) ion binding at the 5'-GA step.

Antineoplastic Agents↗

PubMed2004

The experimental antitumor agents Phortress and doxorubicin are equiactive against human-derived breast carcinoma xenograft models.

Phortress (the dihydrochloride salt of the lysylamide prodrug of 2-(4-amino-3-methylphenyl)-5-fluoro-benzothiazole (5F 203)) is an experimental antitumor agent with potent and selective activity against human-derived carcinomas of breast, ovarian and renal origin. UK clinical trials of Phortress are scheduled to begin in 2004. The mechanism of action of Phortress is distinct from all classes of chemotherapeutic agents currently in the clinic, and involves metabolic activation by cytochrome P450 (CYP) 1A1 to electrophilic species, which generate DNA adducts in sensitive tumors only. In the present study, the antitumor efficacy of Phortress has been compared with that of doxorubicin (Dox) in nine human-derived mammary carcinoma xenograft models, cultivated subcutaneously in the flanks of nude mice. In addition, cyp1a1 mRNA expression was measured in tumors of control and treated animals. Phortress compared favorably with Dox: significant activity, independent of estrogen receptor (ER) status, was established in 7/9 xenografts; in one xenograft model, Phortress elicited superior antitumor activity; no model demonstrated complete resistance to Phortress. In accordance with this observation, all xenografts available for examination (8) displayed clear induction of cyp1a1 expression upon treatment of mice with Phortress whereas Dox failed to induce cyp1a1 expression in all models. Prolonged viability of tumor fragments, recovered for treatment ex vivo could not be sustained; thus correlations between tumor cells' response to Phortress and cyp1a1 or cyp1b1 inducibility following 5F 203 treatment could not be determined with confidence.

Animals↗

PubMed2004

In vitro, in vivo, and in silico analyses of the antitumor activity of 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazoles.

Phortress is a novel, potent, and selective experimental antitumor agent. Its mechanism of action involves induction of CYP1A1-catalyzed biotransformation of 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole (5F 203) to generate electrophilic species, which covalently bind to DNA, exacting lethal damage to sensitive tumor cells, in vitro and in vivo. Herein, we investigate the effects of DNA adduct formation on cellular DNA integrity and progression through cell cycle and examine whether a relevant pharmacodynamic end point may be exploited to probe the clinical mechanism of action of Phortress and predict tumor response. Single cell gel electrophoresis (SCGE) was applied to quantify DNA damage and cell cycle analyses conducted upon 5F 203 treatment of benzothiazole-sensitive MCF-7 and inherently resistant MDA-MB-435 breast carcinoma cells. Following treatment of xenograft-bearing mice and mice possessing hollow fiber implants containing MCF-7 or MDA-MB-435 cells with Phortress (20 mg/kg, i.p., 24 hours), tumor cells and xenografts were recovered for analyses by SCGE. Dose- and time-dependent DNA single and double strand breaks occurred exclusively in sensitive cells following treatment with 5F 203 in vitro (10 nmol/L-10 micromol/L; 24-72 hours). In vivo, Phortress-sensitive and Phortress-resistant tumor cells were distinct; moreover, DNA damage in xenografts, following treatment of mice with Phortress, could be determined. Interrogation of the mechanism of action of 5F 203 in silico by self-organizing map-based cluster analyses revealed modulation of phosphatases and kinases associated with cell cycle regulation, corroborating observations of selective cell cycle perturbation by 5F 203 in sensitive cells. By conducting SCGE, tumor sensitivity to Phortress, an agent currently undergoing clinical evaluation, may be determined.

Animals↗

PubMed2003

Synthesis and properties of bioactive 2- and 3-amino-8-methyl-8H-quino[4,3,2-kl]acridine and 8,13-dimethyl-8H-quino[4,3,2-kl]acridinium salts.

Cyclisation of 9-(benzotriazol-1-yl)acridine to the pentacycle 8H-quino[4,3,2-kl]acridine in a range of low-boiling solvents is mechanistically distinct from previously published photochemical (carbene) and thermolytic (radical) cyclisations. Fragmentation of the triazole ring of to a diazonium intermediate, and its subsequent heterolysis (-N(2)) and cyclisation is facilitated by solvation of intermediate zwitterionic species. Derivatives of 2- and 3-aminoquinoacridines methylated in the 8-position can be converted to 8,13-dimethylquino[4,3,2-kl]acridinium iodide salts with methyl iodide and were required for biological examination as potential telomerase inhibitors. The chloro group in 3-chloro-8-methyl-8H-quino[4,3,2-kl]acridine can be replaced efficiently by benzylamino, 4-morpholinyl and cyano substituents in palladium(0) mediated reactions.

Acridines↗

PubMed2003

Drug recognition and stabilisation of the parallel-stranded DNA quadruplex d(TTAGGGT)4 containing the human telomeric repeat.

The NMR structure of the parallel-stranded DNA quadruplex d(TTAGGGT)(4), containing the human telomeric repeat, has been determined in solution in complex with a fluorinated pentacyclic quino[4,3,2-kl]acridinium cation (RHPS4). RHPS4 has been identified as a potent inhibitor of telomerase at submicromolar levels (IC(50) value of 0.33(+/-0.13)microM), exhibiting a wide differential between telomerase inhibition and acute cellular toxicity. All of the data point to RHPS4 exerting its chemotherapeutic potency through interaction with, and stabilisation of, four-stranded G-quadruplex structures. RHPS4 forms a dynamic interaction with d(TTAGGGT)(4), as evident from 1H and 19F linewidths, with fast exchange between binding sites induced at 318 K. Perturbations to DNA chemical shifts and 24 intermolecular nuclear Overhauser effects (NOEs) identify the 5'-ApG and 5'-GpT steps as the principle intercalation sites; a structural model has been refined using NOE-restrained molecular dynamics. The central G-tetrad core remains intact, with drug molecules stacking at the ends of the G-quadruplex. The partial positive charge on position 13-N of the acridine ring appears to act as a "pseudo" potassium ion and is positioned above the centre of the G-tetrad in the region of high negative charge density. In both ApG and GpT intercalation sites, the drug is seen to converge to the same orientation in which the pi-system of the drug overlaps primarily with two bases of each G-tetrad. The drug is held in place by stacking interactions with the G-tetrads; however, there is some evidence for a more dynamic, weakly stabilised A-tetrad that stacks partially on top of the drug at the 5'-end of the sequence. Together, the interactions of RHPS4 increase the t(m) of the quadruplex by approximately 20 degrees C. There is no evidence for drug intercalation within the G-quadruplex; however, the structural model strongly supports end-stacking interactions with the terminal G-tetrads.

Acridines↗

PubMed2003

Induction of apoptosis without redox catastrophe by thioredoxin-inhibitory compounds.

The dithiol-reducing thioredoxin/thioredoxin reductase system normally maintains the reduced state of key enzymes responsible for the cell's anti-oxidant defences. We therefore addressed the question of whether AW 464--a novel thioredoxin inhibitor--as well as broad spectrum dithiol ligands diamide and phenylarsine oxide are able to induce and execute a regular apoptotic sequence of events without overwhelming the cell's ability to detoxify reactive oxygen species. All three agents were found to target the thioredoxin system in a cell-free assay. In HL-60 leukaemia cells, they were also found to induce Bak activation, cytochrome c release from mitochondria, decreasing Delta Psi m, chromatin condensation, phosphatidyl serine exposure and Tdt-sensitive DNA nicks. At the onset of apoptosis there was no evidence of increases in oxygen free radicals or peroxide in cells treated with AW 464 or diamide. Phenylarsine oxide induced both free radicals and hydrogen peroxide, but this did not appear to interfere with apoptosis. We conclude that pharmacological targeting of thioredoxin can induce a well-orchestrated apoptotic programme.

Amino Acid Chloromethyl Ketones↗

PubMed2003

Antitumour polycyclic acridines. Palladium(0) mediated syntheses of quino[4,3,2-kl]acridines bearing peripheral substituents as potential telomere maintenance inhibitors.

Pd(0) mediated couplings between substituted 2-(pivaloylamino)benzeneboronic acids and 3,6-disubstituted-10-methylacridones 13 bearing a bromo or trifluoromethylsulfonyloxy substituent in the 1-position yield intermediate 1-arylacridones 16 which can be can be cyclised to new 8-methylquino[4,3,2-kl]acridines 17 with phosphorus oxychloride or 6 M HCI in EtOH. Heck reactions between triflate-substituted substrates 17 and acrylic acid derivatives afforded quinoacridines with unsaturated side-chains in the 6-position. Alkylboranes, prepared by interaction of 9-borabicyclo[3,3,1]nonane (9-BBN) and allyl acetate or N-allyltrifluoroacetamide, participated in Suzuki-Miyaura reactions with chloro-substituted 8-methylquinoacridines to form derivatives bearing functionalised propyl groups in the 6- and 10-positions. Representative 8-methylquinoacridines were methylated with methyl iodide to yield telomerase-inhibitory 8,13-dimethylquinoacridinium iodides 24.

Acridines↗

PubMed2003

Metabolic activation of temozolomide measured in vivo using positron emission tomography.

The purpose of this research was to quantitate and confirm the mechanism of in vivo metabolic activation of temozolomide. The secondary aims were to evaluate the tumor, normal tissue, and plasma pharmacokinetics of temozolomide in vivo, and to determine whether such pharmacokinetics resulted in tumor targeting. [(11)C]temozolomide kinetics were studied in men using positron emission tomography (PET). It has been postulated that temozolomide undergoes decarboxylation and ring opening in the 3-4 position to produce the highly reactive methyldiazonium ion that alkylates DNA. To investigate this, a dual radiolabeling strategy, with [(11)C]temozolomide separately radiolabelled in the 3-N-methyl and 4-carbonyl positions, was used. We hypothesized that (11)C in the C-4 position of [4-(11)C-carbonyl]temozolomide would be converted to [(11)C]CO(2) if the postulated mechanism of metabolic conversion was true resulting in lower [(11)C]temozolomide tumor exposure. Paired studies were performed with both forms of [(11)C]temozolomide in 6 patients with gliomas. Another PET scan with (11)C-radiolabelled bicarbonate was performed and used to account for the metabolites of temozolomide using a data-led analytical approach. Plasma was analyzed for [(11)C]temozolomide and [(11)C]metabolites throughout the scan duration. Exhaled air was also sampled throughout the scan for [(11)C]CO(2). The percentage ring opening of temozolomide over 90 min was also calculated to evaluate whether there was a differential in metabolic breakdown among plasma, normal tissue, and tumor. There was rapid systemic clearance of both radiolabelled forms of [(11)C]temozolomide over 90 min (0.2 liter/min/m(2)), with [(11)C]CO(2) being the primary elimination product. Plasma [(11)C]CO(2) was present in all of the studies with [4-(11)C-carbonyl]temozolomide and in half the studies with [3-N-(11)C-methyl]temozolomide. The mean contributions to total plasma activity by [(11)C]CO(2) at 10 and 90 min were 12% and 28% with [4-(11)C-carbonyl]temozolomide, and 1% and 4% with [3-N-(11)C-methyl]temozolomide, respectively. There was a 5-fold increase in exhaled [(11)C]CO(2) sampled with [4-(11)C-carbonyl]temozolomide compared with [3-N-(11)C-methyl]temozolomide (P < 0.05). A decrease in tissue exposure [area under the curve between 0 and 90 min (AUC(0-90 min))] to [(11)C]temozolomide was also observed with [4-(11)C-carbonyl] temozolomide compared with [3-N-(11)C-methyl]temozolomide. Of potential therapeutic advantage was the higher [(11)C]radiotracer and [(11)C]temozolomide exposure (AUC(0-90 min)) in tumors compared with normal tissue. [(11)C]temozolomide ring opening over 90 min was less in plasma (20.9%; P < 0.05) compared with tumor (26.8%), gray matter (29.7%), and white matter (30.1%), with no differences (P > 0.05) between tumor and normal tissues. The significantly higher amounts of [(11)C]CO(2) sampled in plasma and exhaled air, in addition to the lower normal tissue and tumor [(11)C]temozolomide AUC(0-90 min) observed with [4-(11)C-carbonyl]temozolomide, confirmed the postulated mechanism of metabolic activation of temozolomide. A higher tumor [(11)C]temozolomide AUC(0-90 min) in tumors compared with normal tissue and the tissue-directed metabolic activation of temozolomide may confer potential therapeutic advantage in the activity of this agent. This is the first report of a clinical PET study used to quantify and confirm the in vivo mechanism of metabolic activation of a drug.

Adult↗

PubMed2003

4-Substituted 4-hydroxycyclohexa-2,5-dien-1-ones with selective activities against colon and renal cancer cell lines.

The synthesis and antitumor evaluation of a series of new heteroaromatic- and aromatic-substituted hydroxycyclohexadienones ("quinols"), and their imine counterparts, are described. The quinols were synthesized via the addition of a lithiated aromatic moiety to a quinone ketal followed by deprotection. When the aromatic portion of the molecule is a fused heterobicyclic structure (e.g., benzothiazole derivative 7a), potent in vitro antitumor activity was observed in HCT 116 (GI50 = 40 nM) and HT 29 (GI50 = 380 nM) human colon as well in as MCF-7 and MDA 468 human breast cancer cell lines. When examined on the NCI Developmental Therapeutics Screening Program in vitro screen (60 human cancer cell lines), active compounds in this series consistently displayed a highly unusual pattern of selectivity; cytotoxicity (LC50) was concentrated in certain colon and renal cell lines only. Analogue 7a also showed in vivo antitumor activity against human RXF 944XL renal xenografts in nude NMRI mice and is the focus of further study.

Algorithms↗

PubMed2003

Antitumour benzothiazoles. Part 20: 3'-cyano and 3'-alkynyl-substituted 2-(4'-aminophenyl)benzothiazoles as new potent and selective analogues.

The synthesis of a new series of antitumour 2-(4-aminophenyl)benzothiazole analogues, substituted in the 3'-position by cyano or alkynyl groups, is described. Several of the analogues, notably the 5-fluorinated compounds 7c and 9c, were found to possess potent in vitro activity against MCF-7 and MDA 468 human cancer cell lines. More comprehensive in vitro analysis (NCI 60-cell line) established compound 7c as a particularly potent and selective 2-(4-aminophenyl)benzothiazole analogue.

Antineoplastic Agents↗

PubMed2002

Antitumor imidazotetrazines. 40. Radiosyntheses of [4-11C-carbonyl]- and [3-N-11C-methyl]-8-carbamoyl-3-methylimidazo[5,1-d]-1,2,3,5-tetrazin-4(3H)-one (temozolomide) for positron emission tomography (PET) studies.

8-Carbamoyl-3-methylimidazo[5,1-d]-1,2,3,5-tetrazin-4(3H)-one (temozolomide, 1) is an anticancer prodrug. As part of investigations to probe its postulated mode of action using PET we have developed two rapid radiosynthetic routes for the preparation of temozolomide labeled with the short-lived positron emitter, carbon-11 (t(1/2) = 20.4 min). Reaction of 5-diazoimidazole-4-carboxamide (7) with the novel labeling agent [(11)C-methyl]methyl isocyanate (8) gave [3-N-(11)C-methyl]temozolomide (9) in 14-20% radiochemical yield from [(11)C-methyl]methyl isocyanate (8) (decay corrected). The position of radiolabeling in the 3-N-methyl group was confirmed by [(11/13)C]colabeling and subsequent carbon-13 NMR spectroscopy. Similarly, the reaction of 5-diazoimidazole-4-carboxamide (7) with [(11)C-carbonyl]methyl isocyanate (10) gave [4-(11)C-carbonyl]temozolomide (11) in 10-15% radiochemical yield from [(11)C-carbonyl]methyl isocyanate (10) (decay corrected). Apyrogenic samples of [3-N-(11)C-methyl]temozolomide (9) and [4-(11)C-carbonyl]temozolomide (11), with good chemical and radiochemical purities, have been prepared and used in human PET studies.

Antineoplastic Agents↗

PubMed2002

Antitumor imidazotetrazines. 41. Conjugation of the antitumor agents mitozolomide and temozolomide to peptides and lexitropsins bearing DNA major and minor groove-binding structural motifs.

Carboxylic acids derived from the amido groups of the antitumor agents mitozolomide and temozolomide have been conjugated to simple amino acids and peptides by carbodiimide coupling. Solid-state peptide synthesis has been applied to link the acids to DNA major groove-binding peptidic motifs known to adopt alpha-helical conformations. Attachment of the acids to pyrrole and imidazole polyamidic lexitropsins gave a series of potential DNA minor groove-binding ligands. In vitro biological evaluation of a limited number of these novel conjugates failed to demonstrate any enhanced growth-inhibitory activity compared to the unconjugated drugs; sites of alkylation at tracts of multiple guanines were also unaffected. Attachment of additional residues at C-8 of the imidazotetrazines did not perturb the chemistry of activation of the bicyclic nucleus, and biological sequelae can be rationalized by invoking the liberation of a common, diffusible, reactive chemical intermediate, the methanediazonium ion.

Amino Acids↗

PubMed2002

Antitumor polycyclic acridines. 8.(1) Synthesis and telomerase-inhibitory activity of methylated pentacyclic acridinium salts.

Two short routes to novel methylated pentacyclic quinoacridinium salts have been devised. New compounds display telomerase-inhibitory potency (<1 microM) in the TRAP assay. 3,11-Difluoro-6,8,13-trimethyl-8H-quino[4,3,2-kl]acridinium methosulfate (12d, RHPS4, NSC 714187) has a higher selectivity for triplex and quadruplex DNA structures than the 3,6,8,11,13-pentamethyl analogue (12c, RHPS3, NSC 714186) and a low overall growth-inhibitory activity in the NCI 60 cell panel (mean GI(50) 13.18 microM); in addition, the activity profile of 12d does not COMPARE with agents of the topoisomerase II class. Compound 12d is soluble in water, stable in the pH range of 5-9, efficiently transported into tumor cells, and is currently the lead structure for further elaboration in this new class of telomerase inhibitor.

Acridines↗

PubMed2002

Antitumor benzothiazoles. 16. Synthesis and pharmaceutical properties of antitumor 2-(4-aminophenyl)benzothiazole amino acid prodrugs.

A series of water-soluble L-lysyl- and L-alanyl-amide prodrugs of the lipophilic antitumor 2-(4-aminophenyl)benzothiazoles has been synthesized to address formulation and bioavailability issues related to the desired parenteral administration of the chosen clinical candidate. The prodrugs exhibit the required pharmaceutical properties of good water solubility (in weak acid) and stability at ambient temperature and degradation to free base in vivo. The lysyl-amide of 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole (NSC 710305, 6d) has been selected for phase 1 clinical evaluation.

Alanine↗

PubMed2002

Structural studies on bioactive compounds. Part 36: design, synthesis and biological evaluation of pyrimethamine-based antifolates against Pneumocystis carinii.

As part of a research effort to improve the quality of current chemotherapy of Pneumocystis carinii pneumonia, we report a structure-based design project to optimise activity, species selectivity and pharmaceutical properties of the triazenyl-pyrimethamine TAB (4) (IC(50)=0.17 microM; rat liver DHFR IC(50)/P. carinii DHFR IC(50)=114). This has led us to design, synthesise and evaluate four new series of pyrimethamine derivatives bearing triazole, triazolium, triazinium and amino moieties at the 3'-position of the p-chlorophenyl ring. Such stabilised 'triazene' derivatives address the potentially compromised pharmaceutical profile of TAB and the 3'-amine substituted agents afford conformationally flexible substitutes. The benzylamino-pyrimethamine derivative (24a) (IC(50)=0.12 microM, rat liver DHFR IC(50)/P. carinii DHFR IC(50): 5.26) was the most potent and the only P. carinii-selective antifolate of the new series.

Animals↗

PubMed2002

Aryl hydrocarbon receptor mediates sensitivity of MCF-7 breast cancer cells to antitumor agent 2-(4-amino-3-methylphenyl) benzothiazole.

2-(4-Amino-3-methylphenyl) benzothiazole (NSC 674495; DF 203) demonstrates drug uptake and metabolism by tumor cells sensitive to the antiproliferative activity of the drug [J Med Chem 1999;42:4172-4184]. In insensitive cells, little metabolism occurs. Because CYP1A1 can metabolize DF 203, the aryl hydrocarbon receptor (AhR) may mediate drug action. We demonstrate here that DF 203 increases CYP1A1 and CYP1B1 transcription in sensitive MCF-7 cells, accompanied by AhR translocation to the nucleus, increase in xenobiotic-responsive element (XRE)-driven luciferase activity, and induction of protein/DNA complexes on the XRE sequence of the CYP1A1 promoter. MDA-MB-435 and PC3 cells, resistant to DF 203, did not show drug-induced CYP1A1 and CYP1B1 gene expression. AhR was observed to be constitutively localized in the nucleus, with no induction of XRE-driven luciferase activity in transiently transfected cells and weak or no induction of protein/DNA complexes on the XRE sequence of CYP1A1. Taken together, these data elucidate a novel basis for antitumor drug action: induction in sensitive cells of a metabolizing system for the drug itself. These results suggest that clarification of the basis for differential engagement of AhR-related signaling in different tumor cell types may aid in further preclinical development and perhaps early clinical studies.

Active Transport, Cell Nucleus↗

PubMed2002

Preclinical evaluation of amino acid prodrugs of novel antitumor 2-(4-amino-3-methylphenyl)benzothiazoles.

Novel 2-(4-aminophenyl)benzothiazoles (e.g., compounds 1 and 2) possess highly selective, potent antitumor properties in vitro and in vivo. Elucidation of the mechanism of action of this structurally simple class of compounds has occurred in parallel with selection of a candidate clinical agent. Antitumor benzothiazoles induce and are biotransformed by cytochrome P 450 1A1 to putative active, as well as inactive metabolites. Metabolic inactivation of the molecule has been thwarted by isosteric replacement of hydrogen with fluorine atoms at positions around the benzothiazole nucleus. Amino acid conjugation to the exocyclic primary amine function of 2-(4-aminophenyl)benzothiazoles has been used to overcome limitations posed by drug lipophilicity. Water soluble, chemically stable prodrugs rapidly and quantitatively revert to their parent amine in mice, rats, and dogs in vivo. Plasma concentrations of 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole (2) regenerated from the lysylamide prodrug (2b), sufficient to elicit cytocidal activity against ZR-75-1 and T47D human mammary carcinoma cell lines persist > 6 h. The growth of breast (MCF-7) and ovarian (IGROV-1) xenograft tumors is significantly retarded by 2b. Manageable toxic side effects are reported from preclinically efficacious doses of 2b. Cytochrome P 450 1A1 protein expression, selectively induced in sensitive carcinoma cells, was detected in MCF-7 and IGROV-1 tumors 24 h after treatment of mice with 2b (20 mg/kg). The lysyl amide prodrug of 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole is potentially suitable for clinical evaluation.

Amino Acids↗