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Biomedical subjects

H L Pearce

Publications and source records attributed to H L Pearce.

12 recordsLinked to original sources
PubMed2001

Arzoxifene, a new selective estrogen receptor modulator for chemoprevention of experimental breast cancer.

Arzoxifene ([6-hydroxy-3-[4-[2-(1-piperidinyl)-ethoxy]phenoxy]-2-(4-methoxyphenyl)]benzo[b]thiophene) is a selective estrogen receptor modulator (SERM) that is a potent estrogen antagonist in mammary and uterine tissue while acting as an estrogen agonist to maintain bone density and lower serum cholesterol. Arzoxifene is a highly effective agent for prevention of mammary cancer induced in the rat by the carcinogen nitrosomethylurea and is significantly more potent than raloxifene in this regard. Arzoxifene is devoid of the uterotrophic effects of tamoxifen, suggesting that, in contrast to tamoxifen, it is unlikely that the clinical use of arzoxifene will increase the risk of developing endometrial carcinoma.

Animals↗
PubMed1998

Effect of modulators on the ATPase activity and vanadate nucleotide trapping of human P-glycoprotein.

P-Glycoprotein (Pgp) is responsible for the energy-dependent efflux of many natural product oncolytics. Overexpression of Pgp may result in multidrug resistance (MDR). Modulators can block Pgp efflux and sensitize multidrug resistant cells to these oncolytics. To study the interaction of modulators with Pgp, Pgp-ATPase activity was examined, using plasma membranes isolated from the multidrug-resistant cell line CEM/VLB100. A survey of modulators indicated that verapamil, trifluoperazine, and nicardipine stimulated ATPase activity by 1.3- to 1.8-fold, whereas two others, trimethoxybenzoylyohimbine (TMBY) and vindoline, had no effect. Further evaluation showed that TMBY completely blocked the stimulation by verapamil of ATPase activity by competitive inhibition, with a Ki of 2.1 microM. When the effects of these two modulators on the formation of the enzyme-nucleotide complex important in the catalytic cycle were examined, verapamil increased the amount of vanadate-trapped 8-azido-[alpha-32P]ATP bound to Pgp by two-fold, whereas TMBY had no effect. Moreover, TMBY blocked the verapamil stimulation of vanadate-8-azido-[alpha-32P]ATP. Together, these data indicate that verapamil and TMBY bind to Pgp at a common site or overlapping sites, but only verapamil results in enhanced Pgp-ATP hydrolysis and formation of the vanadate-nucleotide-enzyme complex.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

Preclinical, pharmacologic, and phase I studies of gemcitabine.

Gemcitabine (2',2'-difluorodeoxycytidine) is a novel nucleoside analogue that exerts its antitumor activity via multiple mechanisms of action. These include (1) incorporation of gemcitabine into replicating DNA, which inhibits DNA replication and cell growth, (2) masked DNA chain termination, and (3) several self-potentiation mechanisms that serve to increase intracellular levels of the active compound. Preclinical experiments in various cell lines and animal models demonstrate a broad range of cytotoxic activity. Pharmacokinetic studies of gemcitabine delivered by its usual schedule (30-minute weekly infusion) reveal a short plasma half-life and a high clearance into central and peripheral compartments (two-compartment model). The drug is excreted almost completely in the urine as the parent compound and primary metabolite (difluorodeoxyuridine). Phase I trials demonstrate that pharmacokinetics are schedule dependent and that, in general, gemcitabine is well tolerated. Dose-limiting toxicities are primarily myelosuppression, with other toxicities being rash, flu-like symptoms, and transient elevations in liver function tests.

Animals↗
PubMed1995

Anticancer drug development at Lilly Research Laboratories.

BACKGROUND: The discovery and clinical development of new drugs to treat cancer at Lilly Research Laboratories has undergone significant change during the past 15 years. During the early 1980s drug discovery relied heavily on a panel of syngeneic murine solid tumour models to identify new agents for clinical trial. New classes of oncolytic agents identified by this methodology include the difluoronucleoside antimetabolites, diarylsulfonylureas, and a series of folate-based enzyme inhibitors. Within the folate-based discovery programme at Lilly, a broad understanding of the structure activity relationships of folate antimetabolites and the biochemical basis of folate transport, processing, and enzyme inhibition has enabled a more rational approach for drug discovery. CURRENT STUDIES: Folate receptor binding properties are being studied to predict tumour sensitivities and tissue toxicities. This information, together with knowledge of a compound's ability to undergo polyglutamation via the enzyme folylpolyglutamate synthase, assist in the more rational selection of agents with designed cellular selectivities. Ultimately, the complex metabolic pathways involving folate metabolism provide numerous targets for enzyme inhibition. Inhibitors of purine biosynthesis and thymidylate synthesis have demonstrated broad activity in preclinical models of disease including several human tumour xenografts, and are undergoing clinical testing. The folate-based drug discovery programme serves as a model for other biochemically based drug discovery programs including those based in drug resistance, signal transduction and cell cycle control.

Acyltransferases↗
PubMed1995

A structure-function relationship among reserpine and yohimbine analogues in their ability to increase expression of mdr1 and P-glycoprotein in a human colon carcinoma cell line.

We previously showed that there is a structure-function relationship among reserpine and yohimbine analogues in their ability to inhibit the function of P-glycoprotein (P-gp) and reverse multidrug resistance (MDR). Because some P-gp inhibitors (e.g., verapamil and nifedipine) can increase mdr1 and P-gp expression in human colon carcinoma cell lines, we used our reserpine/yohimbine analogues to determine whether there was a structural requirement for this induction. We found that 10 microM reserpine increased both mdr1 and P-gp expression by 4-10-fold in 48 hr in a human colon carcinoma cell line that expresses moderate levels of mdr1 (LS180-Ad50) but not in several other cell lines that expressed no mdr1. The reserpine/yohimbine analogues rescinnamine, trimethoxybenzoylyohimbine, and LY191401 (compound G), all of which contain the three structural elements used to describe the MDR pharmacophore, also increased both mdr1 and P-gp expression significantly. Despite some exceptions, we found that there was a good association between the ability of these analogues to induce mdr1 and P-gp expression and their ability to reverse vinblastine and doxorubicin resistance, revealing a structure-function relationship for this phenomenon. The increased P-gp expressed by these cells appeared to be functional, as determined by flow cytometric detection of rhodamine 123 retention. The increased expression was suppressed by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, an RNA synthesis inhibitor, whereas the protein synthesis inhibitor cycloheximide enhanced the expression several-fold, suggesting that induction of mdr1 by these analogues is regulated at both the transcriptional and post-transcriptional levels.(ABSTRACT TRUNCATED AT 250 WORDS)

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1990

Structural characteristics of compounds that modulate P-glycoprotein-associated multidrug resistance.

Multidrug resistance is mediated by a membrane-bound protein, P-gp, that functions as an energy dependent efflux system to reduce the intracellular concentration of anticancer drugs by binding to these drugs and actively exporting them from the cell. Compounds that interact with P-gp and compete with anticancer drug binding modulate the degree of drug resistance and therefore enhance the cytotoxicity of anticancer drugs against the resistant cell. Effective modulators share certain physical and chemical properties including octanol/water partitioning and molecular size, but the physical properties of size and shape seem to correlate best with modulator effectiveness. Using a photoactivatable analog of vinblastine as a probe, together with a semi-synthetic series of structurally homologous reserpine and yohimbine analogs, the need for two planar aromatic domains and a basic nitrogen atom was established within the structural context of these compounds. The use of three-dimensional comparisons was extended to examine important structural features in other modulator types such as the condensed-ring aromatics. This approach indicates that structural similarities between different classes of compounds are present in compounds recognized by the MDR phenotype. These studies emphasize the importance of a ligand-receptor relationship for modulators of MDR, and begin to define the P-gp-binding pharmacophore. It is likely that this approach will be useful in directing the de novo synthesis of compounds that modulate MDR and help to further define the requirements for molecular recognition by this system.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1990

Clinical oncology and chemical thermodynamics.

The development of oncolytic agents for cancer chemotherapy is often based on chance discovery and intensive structure modification. A mechanistic understanding of the essential biochemistry of many anticancer drugs remains elusive because of the biological complexity of drug-drug and drug-target interactions. The potential of computational science to analyze and quantify these interactions may provide a rational basis for drug modification and clinical trial design.

Antineoplastic Agents↗
PubMed1989

Thermodynamic analysis of the reaction of phosphoramide mustard with protector thiols.

The systemic use of thiol-containing uroepithelial protecting agents, e.g., N-acetylcysteine (NAC) or mesna, in conjunction with the alkylating agent cyclophosphamide is predicated on the assumption that the toxic metabolic by-products will be consumed by thiol without diminishing the cytotoxicity of the active alkylating intermediate, phosphoramide mustard. Studies in murine tumor systems have been with either a single dose or two equally divided doses of thiol, administered within 30 min of the addition of cyclophosphamide, without an observed adverse effect on antitumor activity; however, the relatively short serum half-life of thiol relative to alkylating agent in humans weakens the clinical relevance of these results. This study presents a thermodynamic model for the chemical reaction of phosphoramide mustard with either NAC or mesna. The gas phase thermodynamic parameters for these reactions, enthalpy (H) and entropy (S), were calculated using the semiempirical quantum mechanical method AM1 and were used to predict the free energy (delta G) for these processes. For the reaction of phosphoramide mustard with NAC or mesna, delta G = +3.82 and 2.29 kcal/mol, respectively. In the absence of enzyme catalysis, these results suggest that such reactions are not favored. In order to assess the validity of this gas phase thermodynamic model, the cellular cytotoxicity of phosphoramide mustard in the presence or absence of either NAC or mesna was studied using CCRF-CEM cells in culture. In these experiments the 50% effective dose of phosphoramide mustard was 1.7 micrograms/ml; this result was unchanged in the presence of 10 micrograms/ml concentration of either thiol. This study supports the conclusion that phosphoramide mustard and protector thiols are compatible.

Acetylcysteine↗
PubMed1989

Essential features of the P-glycoprotein pharmacophore as defined by a series of reserpine analogs that modulate multidrug resistance.

We have shown previously that reserpine is an effective "modulator" of P-glycoprotein-associated multidrug resistance (MDR). In addition to enhancing drug cytotoxicity in our multidrug-resistant human leukemia cell line, CEM/VLB100, reserpine strongly competes with a photoactivatible analog of vinblastine, N-(p-azido-3-[125I]iodosalicyl)-N'-(beta-aminoethyl)vindesine, for binding to P-glycoprotein. We also demonstrated previously that there are three substructural domains present in many compounds that modulate P-glycoprotein-associated MDR: a basic nitrogen atom and two planar aromatic rings. In the present study, we wished to test more rigorously the hypothesis that not only are these domains necessary for modulators of MDR but also they must exist in an appropriate conformation. Reserpine is a modulator of MDR in which these domains are present in a well-defined conformation. Accordingly, we prepared eight compounds that vary the spatial orientation of these domains, using either naturally occurring reserpine or yohimbine as chemical templates. When tested for their ability to enhance the cytotoxic activity of natural product antitumor drugs in CEM/VLB100 cells, five compounds that retained the pendant benzoyl function in an appropriate spatial orientation all modulated MDR. By contrast, compounds lacking this moiety failed to do so. These active modulators competed strongly with the 125I-labeled vinblastine analog for binding to P-glycoprotein in plasma membrane vesicles prepared from these cells. Conformational analysis using molecular mechanics revealed the structural similarities of the active modulators. Our results support the hypothesis that the relative disposition of aromatic rings and basic nitrogen atom is important for modulators of P-glycoprotein-associated MDR, and they suggest a ligand-receptor relationship for these agents. These results also provide direction for the definition of an MDR "pharmacophore."

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1988

Physical-chemical properties shared by compounds that modulate multidrug resistance in human leukemic cells.

Multidrug resistance (MDR), typified by resistance to Vinca alkaloids and anthracyclines, is a well characterized experimental phenomenon that may have some clinical correlates. Verapamil, chloroquine, and related drugs have been shown previously to be capable of enhancing anticancer drug cytotoxicity in multi-drug-resistant cells, but the mechanism(s) by which these agents do this is(are) unclear. Since these agents did not seem to have common features, we studied these and other compounds for their ability to "modulate" Vinca alkaloid resistance in order to determine whether they possessed any common chemical or physical features. In addition to verapamil, 24 compounds, consisting of indole alkaloids, lysosomotropic agents, and amines, were tested for their ability to enhance the cytotoxicity of vinblastine and/or vincristine in our human leukemic multidrug-resistant cell line, CEM/VLB100. Seventeen compounds that enhance the cytotoxicity of the Vinca alkaloids by more than 5-fold have been identified. These include quinolines (chloroquine, quinine, chinchonidine, and primaquine), acridines (acridine, acridine orange, and quinacrine), and indole alkaloids (yohimbine, corynanthine, reserpine, physostigmine, and the vindoline and catharanthine moieties of the Vinca alkaloids), as well as other alkaloids and amines (chlorpromazine, propranolol, atropine, and tryptamine). Vindoline, catharanthine, and quinacrine also enhanced the cytotoxicity of doxorubicin and teniposide in these cells, indicating that this "modulation" was not limited to Vinca alkaloids. We examined some well known lysosomotropic compounds (methylamine, epinephrine, suramin, and trypan blue) and found that they were not able to enhance the cytotoxicity of vincristine in the CEM/VLB100 cells, indicating that lysosomotropic activity per se is not required for modulator activity. Three-dimensional computer modeling permitted molecular comparisons of conformationally related congeners of vinblastine, vindoline, and verapamil and revealed three regions of structural homology. We measured the hydrophobicity (by oil/water partitioning) and calculated the molar refractivity (by the additive substituent constant method) of active and inactive compounds. We found that those cationic agents--verapamil, quinacrine, indole alkaloids, and quinolines--that were lipid soluble at physiologic pH and had similar molar refractivities were best able to enhance the cytotoxicity of the Vinca alkaloids in our multidrug-resistant cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Antibiotics, Antineoplastic↗