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Biomedical subjects

M E Wall

Publications and source records attributed to M E Wall.

At least 55 records · Page 3Linked to original sources
PubMed1995

High-resolution macromolecular structure determination using CCD detectors and synchrotron radiation.

BACKGROUND: Synchrotron radiation sources have made impressive contributions to macromolecular crystallography. The delay in development of appropriate X-ray detectors has, however, been a significant limitation to their efficient use. New technologies, based on charge-coupled devices (CCDs), provide capabilities for faster, more accurate, automated data collection. RESULTS: A CCD-based X-ray detector has been developed for use in macromolecular crystallography and has been in operation for about one and a half years at the Cornell High Energy Synchrotron Source. It has been used for a variety of crystallographic projects, including a number of high-resolution structural studies. The statistical quality of the data, the detector's ease and efficiency of use, and the growing number of structural results illustrate the practical utility of this new detector system. CONCLUSIONS: The new detector has enhanced capabilities for measuring diffraction patterns from crystals of macromolecules, especially at high resolution, when the X-ray intensities are weak. The survey of results described here ranges from virus crystallography to weakly diffracting small-molecule structure determination and demonstrates the potential of CCD detectors when combined with synchrotron radiation sources.

Amino Acid Sequence↗
PubMed1995

Camptothecin and taxol: discovery to clinic--thirteenth Bruce F. Cain Memorial Award Lecture.

Camptothecin and taxol are secondary metabolites found, respectively, in the wood bark of Camptotheca acuminata, a native of China, and Taxus brevifolia, found in the northwest Pacific coastal region of the United States. The compounds were isolated guided by bioassay on various extracts and chromatographic fractions. Their unique and hitherto unknown structures were elucidated by nuclear magnetic resonance, mass spectrometry, and X-ray analysis. Both compounds have unique mechanisms of antitumor activity; camptothecin uniquely inhibits an enzyme, topoisomerase I, involved in DNA replication. Taxol binds to a protein, tubulin, thus inhibiting cell division. Taxol has been called the best new anticancer agent developed from natural products, showing particular efficacy against ovarian cancer. Camptothecin and analogues singly or combined with cisplatin show efficacy against solid tumors, breast, lung, and colorectal, which hitherto have been unaffected by most cancer chemotherapeutic agents.

Animals↗
PubMed1995

Isolation and structural elucidation of pentacyclic triterpenoids from Maprounea africana.

Pentacyclic triterpenoids based on the taraxer-14-ene skeleton with a C-28 attached carboxylic acid group have been isolated from the roots of Maprounea africana. These compounds were identified as 1 beta,2 alpha-dihydroxyaleuritolic acid 2,3-bis-p-hydroxybenzoate [1], 2 alpha-hydroxyaleuritolic acid 3-p-hydroxybenzoate [2], 2 alpha-hydroxyaleuritolic acid 2,3-bis-p-hydroxybenzoate [4], aleuritolic acid 3-p-hydroxybenzoate [5], aleuritolic acid [6], and aleuritolic acid 3-acetate [7]. Compounds 1 and 2 are new triterpene esters. Compound 3 was previously reported as 7 beta-hydroxymaprounic acid 3-p-hydroxybenzoate [13]. However, based on detailed nmr studies, its structure has been revised.

Magnetic Resonance Spectroscopy↗
PubMed1995

A reinvestigation of Maprounea triterpenes.

Anti-HIV activity and the inhibition of phorbol ester receptor binding activity in two species of Maprounea were traced to small amounts of highly potent phorbol esters of the daphnane type. The triterpenes previously isolated from this genus were found to be devoid of biological activity when scrupulously purified. Four new triterpene esters were elucidated; two [3,4] were found in M. africana, while three [4,6,7] were found in M. membranacea. Nmr assignments have also been made for two previously known compounds [2,5] in this group.

Acetylation↗
PubMed1995

Gummiferol, a cytotoxic polyacetylene from the leaves of Adenia gummifera.

A new polyacetylenic diepoxide compound, gummiferol [1] was isolated from the leaves of Adenia gummifera by KB cytotoxicity-guided fractionation. Compound 1 exhibited significant activity against the KB human cell line and a broad cytotoxic spectrum against other human cancer cell lines. The structure of 1 was established primarily on the basis of its spectral data.

Acetates↗
PubMed1995

Isolation and structure identification of an active DNA strand-scission agent, (+)-3,4-di-hydroxy-8,9-methylenedioxypterocarpan.

A new pterocarpan, (+)-3,4-dihydroxy-8,9-methylenedioxypterocarpan [1], was isolated from the flowers of Petalostemon purpureus by a DNA strand-scission assay-guided fractionation procedure. Compound 1 demonstrated activity in a standard in vitro DNA strand-scission assay, and cytotoxicity toward a KB tumor cell line. Two other related pterocarpans [2, 3] isolated from same plant were found to be moderately active for KB cells, but were inactive in the DNA strand-scission assay. (+)-4-Hydroxy-3-methoxy-8,9-methylenedioxypterocarpan [2] has not been reported previously as a natural product, while (+)-maackiain [3] has been isolated only as an optically inactive racemate along with its optical antipode, the (-)-isomer.

Antineoplastic Agents↗
PubMed1994

Plant antitumor agents. 31. The calycopterones, a new class of biflavonoids with novel cytotoxicity in a diverse panel of human tumor cell lines.

Three new biflavonoids to which we have accorded the trivial names calycopterone (1), isocalycopterone (2), and 4-demethylcalycopterone (3) and the known flavone 4',5-dihydroxy-3,3',6,7-tetramethoxyflavone (4) were isolated as cytotoxic constituents from the flowers of Calycopteris floribunda Lamk. (Combretaceae). Compounds 1-3 showed a wide range of activity against a panel of solid tumor cell lines. Among the biflavonoids, calycopterone (1) is the major constituent.

Animals↗
PubMed1993

Plant antitumor agents. 30. Synthesis and structure activity of novel camptothecin analogs.

A large number of camptothecin (CPT) analogs have been prepared in the 20S, 20RS, and 20R configurations with a number of ring A substituents. Topoisomerase I (T-I) inhibition data (IC50) have been obtained by standard procedures. In general, substitution at the 9 or 10 positions with amino, halogeno, or hydroxyl groups in compounds with 20S configuration results in compounds with enhanced T-I inhibition. Compounds in the 20RS configuration were less active in vitro and in vivo and those in the 20R configuration were inactive. Compounds with 10,11-methylenedioxy substitution on ring A displayed a marked increase in potency in the T-I inhibition assay. The activities of some of the analogs as determined in a variety of in vivo assays including the L-1210 mouse leukemia assay were, in general, in accord with T-I inhibition. A number of water-soluble analogs such as 20-glycinate esters, 9-glycinamides, or hydrolyzed lactone salts were prepared and tested in in vitro and in vivo assays. In general, these compounds were less active than CPT both in terms of T-I inhibition and life prolongation in the L-1210 assay. However, certain 20-glycinate esters showed good in vivo activity after iv administration.

Animals↗
PubMed1993

Lipid bilayer partitioning and stability of camptothecin drugs.

The intense intrinsic fluorescence emissions from several clinically relevant camptothecin drugs have been exploited in order to determine (1) the structural basis of drug binding to lipid bilayers, (2) the lipid bilayer stability of each drug's alpha-hydroxylactone moiety, a pharmacophore which is essential for antitumor activity, and (3) the site of drug binding in the bilayer. Equilibrium affinities of camptothecin and related congeners for small unilamellar vesicles composed of electroneutral dimyristoylphosphatidylcholine (DMPC) or negatively-charged dimyristoylphosphatidylglycerol (DMPG) were determined using the method of fluorescence anisotropy titration. Experiments were conducted in phosphate-buffered saline (PBS) at 37 degrees C and overall association constants (K values) were determined. Of the seven compounds studied, the new compound 9-chloro-10,11-methylenedioxy-(20S)-camptothecin (CMC) was found to display the highest membrane affinities (KDMPC = 400 M-1, KDMPG = 320 M-1), followed by 10,11-methylenedioxy-camptothecin and camptothecin, which exhibited KDMPC and KDMPG values of 100 M-1 or greater. Topotecan displayed markedly reduced binding to lipid bilayers (KDMPC = 10 M-1, KDMPG = 50 M-1). HPLC assays were subsequently employed to assess the relative stabilities of the lactone ring of membrane-bound drugs. Our results clearly indicate that lipid bilayer interactions stabilize the lactone moiety of camptothecin drugs. In comparison to half-lives in PBS (37 degrees C) of 17 and 19 min for camptothecin and CMC, respectively, DMPC- or DMPG-bound drugs were found to be stable even for periods up to 72 h. Iodide quenching data indicate that membrane-bound camptothecin intercalates between the lipid acyl chains, in a protected environment well removed from the aqueous interface. In this manner lipid bilayer interactions stabilize the lactone ring structure of camptothecins and thereby conserve the biologically active form of each medication.

Binding Sites↗
PubMed1992

Antimutagenic agents from natural products.

Certain secondary metabolites found in terrestrial and marine plants and organisms have evinced the capability for inhibiting the mutagenicity toward Salmonella typhimurium of a number of mutagens. These include 2-aminoanthracene (2AN), ethylmethanesulfonate (EMS), and benzo-[a]pyrene(B[alpha]P). The sensitivity of the antimutagenicity assay is such that crude extracts can be evaluated and purification of extracts readily followed. Major classes of antimutagenic compounds that have been isolated include flavonoids, coumarins, and cymopols.

Animals↗
PubMed1991

Camptothecin overcomes MDR1-mediated resistance in human KB carcinoma cells.

In order to understand the high efficacy of camptothecin derivatives against human colon tumor xenografts in nude mice, we have studied the transport properties of camptothecin derivatives across cellular membranes of MDR1-overexpressing cells. MDR1 overexpression was shown to have little effect on camptothecin cytotoxicity; camptothecin was equally cytotoxic to both the drug-sensitive parental cell line, KB 3-1, and its multidrug-resistant derivative, KB V1. The ability of camptothecin to overcome MDR1-mediated resistance is most likely due to unimpaired accumulation of camptothecin in MDR1 cells as suggested from the following experiments: (a) cytotoxicity of camptothecin against KB V1 cells was not altered by the known MDR1-reversing agent, verapamil; (b) camptothecin was ineffective as compared with vinblastine in competing with [3H]azidopine for photoaffinity labeling of MDR1; (c) camptothecin was equally efficient in trapping cellular topoisomerase I molecules on chromosomal DNA in the form of cleavable complexes in both KB 3-1 and KB V1 cells. The mechanism by which camptothecin overcomes MDR1-mediated resistance has been further studied using a number of uncharged and charged camptothecin derivatives. In contrast to the uncharged camptothecin derivatives, such as 9-amino-camptothecin and 10,11-methylenedioxy-camptothecin, the charged camptothecin derivative, topotecan, showed reduced cytotoxicity against MDR1-overexpressing KB V1 cells. The reduced cytotoxicity of topotecan in KB V1 cells was due to the overexpression of MDR1 in KB V1 cells since verapamil restored both topotecan accumulation and cytotoxicity. These results suggest that the charge on camptothecin can affect the drug's sensitivity to MDR1. The possible effect of membrane permeability in determining drug selectivity of MDR1 is discussed.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1990

Plant antitumor agents. 29. Synthesis and biological activity of ring D and ring E modified analogues of camptothecin.

The total synthesis of the pentacyclic camptothecin analogues 3 and 4 in 11 steps from p-tolualdehyde is described. The overall shape of compound 3 is the same as that of potent, naturally occurring camptothecin (1a). Despite the near spatial identity of 3 and 1b (racemic, (20RS)-camptothecin) from a three-dimensional standpoint, the 9KB and 9PS cytotoxicity assays indicate at least a 40-60-fold decrease in activity of 3 compared to that of 1b, and the isomer 4 was inactive. Similarly, studies of the inhibition of topoisomerase I activity indicated only slight activity for 3 and no activity for 4. It is evident that the pyridone ring D is essential for antitumor activity. Three E ring modified analogues of camptothecin, 2d-f, are described in which the net change is replacement of O by N in ring E. Compared to (20S)-camptothecin (1a) or (20RS)-camptothecin (1b), the ring E modified analogues 2d-f display little or no cytotoxic activity, greatly reduced effect on the inhibition of topoisomerase I, and total loss of life prolongation in the in vivo L-1210 mouse leukemia assay, indicative of the highly restricted structural and electronic requirements of ring E for biological activity in camptothecin.

Animals↗
PubMed1989

DNA topoisomerase I--targeted chemotherapy of human colon cancer in xenografts.

Drug development is needed to improve chemotherapy of patients with locally advanced or metastatic colon carcinoma, who otherwise have an unfavorable prognosis. DNA topoisomerase I, a nuclear enzyme important for solving topological problems arising during DNA replication and for other cellular functions, has been identified as a principal target of a plant alkaloid 20(S)-camptothecin. Significantly increased concentrations of this enzyme, compared to that in normal colonic mucosa, were found in advanced stages of human colon adenocarcinoma and in xenografts of colon cancer carried by immunodeficient mice. Several synthetic analogs of camptothecin, selected by tests with the purified enzyme and tissue-culture screens, were evaluated in the xenograft model. Unlike other anticancer drugs tested, 20(RS)-9-amino-camptothecin (9-AC) induced disease-free remissions. The overall drug toxicity was low and allowed for repeated courses of treatment.

Adenocarcinoma↗
PubMed1989

DNA topoisomerase I-mediated DNA cleavage and cytotoxicity of camptothecin analogues.

20(S)-Camptothecin, the 20(S)-camptothecin sodium salt, and 12 analogues with substituents on the A ring differ widely in their effectiveness in the treatment of murine L1210 lymphoblastic leukemia in vivo. The drugs were screened in the following systems: System 1, the cleavage of DNA in the presence of purified topoisomerase I; System 2, drug-induced trapping of topoisomerase I in a covalent complex with DNA; and System 3, the induction of protein-associated DNA breaks in drug-treated L1210 leukemia cells. 9-Amino-20(S), 10-amino-20(RS), and 10,11-methylenedioxy-20(RS), drugs effective against murine L1210 leukemia in vivo, stabilize topoisomerase I-DNA cleavable complexes in a purified system and in cultured L1210 cells. Other analogues, inactive against L1210 leukemia in vivo, were totally ineffective in topoisomerase I-directed screens. The rest of the analogues were intermediate in terms of their antitumor and topoisomerase I-directed activities. The study shows that the drug-induced accumulation of enzyme-DNA cleavable complexes is directly proportional to drug cytotoxicity and antitumor activity.

Animals↗
PubMed1989

Structure-activity study of the actions of camptothecin derivatives on mammalian topoisomerase I: evidence for a specific receptor site and a relation to antitumor activity.

Twenty-two compounds related to camptothecin, a known inhibitor of eukaryotic topoisomerase I, were studied. The following effects on the actions of topoisomerase I were observed and were well correlated among most of the compounds studied: (a) inhibition of the first-order rate of relaxation of supercoiled DNA; (b) conversion of supercoiled DNA to nicked circles; and (c) single-strand cleavage of linear DNA at specific sites. The locations of the stimulated cleavage sites were the same for all of the active derivatives. Stereochemistry and the positions of substituents were found to be crucial for the presence or absence of effects on topoisomerase I, indicating that the compounds interact with an asymmetrical receptor site on the enzyme or enzyme-DNA complex. From the structure-activity relations, the regions of interaction between the camptothecin ring system and the receptor site were inferred. Striking correlations were observed between activity against topoisomerase I and reported activity against murine leukemias, indicating that an action on topoisomerase I is responsible for the antitumor activity of the camptothecins.

Animals↗