Measurement of compound toxicity and cell viability

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The ATP values without addition of any inhibitor or test component were used as positive control. The negative control was calculated from the data obtained by preincubation with a standard antagonist. The concentration was selected high enough to completely block any receptor activity. When it was not possible to apply a standard antagonist on the assay plate, the buffer control was taken as the negative control instead. The Z’-factors from all experiments were averaged for each receptor and the standard error of the mean was calculated. At a Z’-factor higher than 0.5, the separation band between positive and negative control is large and the assay is deemed as suitable for high-throughput screening.

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The dilutions of test compounds were prepared as discussed in chapter 2.2.2.1. The pipetting schemes can be found in chapters 2.1.6.6, 2.1.6.7 and 2.1.6.8. The dilutions for POMs were prepared in cell culture medium containing 10 % DMSO. All other compounds were diluted in pure DMSO. 5-Fluorouracil, a known cytotoxic cancer drug, was included in each experiment as a positive control. Cells were then supplemented with 20 µl (POMs) or 1 µl test compound, or pure DMSO (negative control), respectively, and then incubated for 72 hours at 37°C and 5 % CO2. The final DMSO concentration in the well never exceeded 1 %. Each compound concentration was measured in duplicates in three independent experiments.

Day 5

One hour prior to the end of the 72 hour incubation period, each well was supplemented with 40 µl (POMs) or 20 µl (all other compounds) of freshly prepared MTT solution 5 mg/ml in PBS.

Incubation was then continued to the end at 37°C and 5 % CO2, so that still alive cells could transform the yellow tetrazolium salt to purple formazan. The medium was removed and 100 µl of DMSO was added to each well to lyse the cells. The plate was shaken at 200 rpm for 2 minutes to dissolve the formazan crystals produced in mitochondria. UV absorbance of the purple formazan was measured using FlexStation 3 at 570 nm. A second measurement was conducted directly afterwards to eliminate background absorbance. The full settings of the measurement are summarized in Table 2.2.

Table 2.2: Advanced settings of FlexStation3 for measurement UV absorbance of produced purple Formazan during MTT cell viability experiment

Measurement parameter Settings

Read mode Absorbance

Wavelength LM1: 570 nm

LM2: 690 nm

Assay plate 96 well standard clear bottom

Wells to read Entire plate

Compound transfer None

Automix No

Calibration On

Settle time Off

Priority: wavelength or column column

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Data analysis was performed analog to results of calcium influx measurement using Microsoft®

Excel and GraphPad® Prism 4.02. The EC50 value was determined using sigmoidal dose-response analysis with variable slope. No constraints were defined. The results of each well obtained via UV measurement at 690 nm were subtracted from the respective results obtained at 570 nm to eliminate background fluorescence of residual medium and cell components. The data was then transferred to GraphPad® Prism 4.02. The EC50 value is here defined as the concentration of test compound, at which half of the cells are dead after 72 hours exposure.

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3 Characterization of the S15V mutant of rat P2X3 receptor

3.1 Introduction

Anthraquinones are chemical derivatives of anthracene and represent a class of natural products. They are either produced by the polyketide or the shikimate pathway. Anthraquinones synthesized by the latter often carry hydroxyl residues in positions 1 and 8 as well as methyl in position 3. The hydroxyl residues also represent glycosylation sites. Anthraquinones and dianthrones are important contents of plants including senna, frangula, aloe and rhubarb, and have been known for their laxative purposes for a long time.¹⁹⁹ Naturally glycosylated, the glycosides behave as prodrugs and are metabolized in the colon. This leads to the release of the aglycones that are responsible for the laxative effect by causing an increase of secretion and motility in the colon.

Anthraquinone derivatives are also used as anticancer drugs. The most prominent representative is mitoxantrone, an alkaline substituted anthracendione, which is utilized in the therapy of lymphoma, leukemia, breast and prostate cancer.²⁰⁰ It acts as an intercalate substance and inhibits the activity of topoisomerase II. Alongside cancer, it is also approved for the treatment of multiple sclerosis.²⁰¹ The use in both indications is limited by cardiotoxicity.^{200, 202}

The anthraquinone derivative RB-2 is a weak antagonist and allosteric modulator at P2 receptors.^{66, 203} RB-2 is an anthraquinone-chlorotrizinyl reactive dye and was formerly used in textile industry and in the purification process of proteins by affinity chromatography.²⁰⁴ A truncated version of RB-2 was selected as a lead structure for the development of new potent antagonists of P2 receptors. The library contains 165 amino and 26 desamino derivatives. It was originally developed for the pharmacological testing at the P2Y2 and the P2Y12 receptors.^{205, 206} The aim was the identification of a new and potent ligand with antithrombotic efficacy. P2Y12, a Gi-coupled GPCR, plays an important role in blood platelet function and regulation.²⁰⁷ All investigated compounds display an aminoanthraquinone- or desaminoanthraquinone core with a sulfonic acid group in position 2 and various residues with great structural differences in position 4 linked via an amino bridge. The synthesis of the compounds has been described in detail.^208-210 Briefly, 4-bromo-substituted anthraquinone derivatives are coupled with the appropriate amine by microwave-assisted Ullmann reaction in phosphate buffer (pH 6-7) in the presence of Cu⁰. The synthesis scheme is presented in Figure 3.1, and Figure 3.2 gives an overview of the different basic structures of the tested compounds.

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Figure 3.1: Microwave-assisted Cu(0)-catalyzed synthesis of 4-phenylamino-substituted 1-amino-2-sulfoanthraquinone derivatives in phosphate buffer.²⁰⁸

A variety of different substitutions is available, which facilitates the study of structure-activity relationships. This compound library has previously been identified to contain potent antagonists of P2X2, P2Y2, P2Y12 receptors, ecto-5’-nucleotidase and ectonucleoside triphosphate diphosphohydrolases. They display high inhibitory potency with nanomolar IC50

and Ki concentrations, respectively.103, 205, 206, 211-214

A total of 26 compounds are available in their deaminated form, where the amino residue in position 1 was replaced with hydrogen. Most of them are derivatives displaying scaffold C. The purpose of eliminating the amino function in position 1 was to characterize its influence on compound binding and efficacy.

Hausmann et al. succeeded in creating a stable non-desensitizing rat P2X3 receptor mutant by exchanging serine in position 15 to valine, as is described in detail in chapter 1.2.8.¹¹⁸ The receptor was transfected retrovirally into 1321N1 astrocytoma cells, creating a stably expressing cell line. The aim of this study was to evaluate the created cell line and determine its suitability for high-throughput screening by testing the anilinoanthraquinone library via measurement of calcium influx. Since Cibacron Blue 3GA is an identified PAM of the P2X3 receptor, the effect of structural derivatization on compound activity was also evaluated.¹²⁷

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Figure 3.2: Basic scaffolds of tested aminoanthraquinone derivatives. Residues are presented in chapters 3.4.1 and 3.4.2.