AZD3355, a novel GABAB receptor agonist, inhibits transient lower esophageal sphincter relaxation through a peripheral mode of action
Anders Lehmann, Madeleine Antonsson, Ann Aurell Holmberg, L. Ashley Blackshaw, Lena Branden, Hans Brauner-Osborne, Bolette Christiansen, John Dent, Thomas Elebring, Britt-Marie Jacobson, Jorgen Jensen, Jan P. Mattsson, Karolina Nilsson, Simo S. Oja, Amanda J. Page, Pirjo Saransaari, and Sverker von Unge
jpet.109.153593
Supplemental Information
Effect of GABAB receptor agonists on various GABAB(1) splice variants
The human GABAB(1) splice variants GABAB(1a), GABAB(1b), GABAB(1e), GABAB(1g), GABAB(1m) and GABAB(1o), as well as GABAB(2), were cloned from a human brain cDNA and subcloned into pCI-Neo expression vectors. Transient co-transfections of GABAB(1) splice variants and GABAB(2) in CHO-Gqi5 cells were performed using lipofectamin (Life Technologies, Paisley, Scotland) according to the manufacturer’s protocol. In brief, for each transfection, CHO-Gqi5 cells (≈1.4 million cells) were seeded in T75 flasks 24 hours prior to the transfection to obtain 50–70% confluency. Cells were washed with 5 mL OptiMEM (Life Technologies) and thereafter incubated in 5 mL OptiMem for approximately 1 hour. Plasmid DNA (4 mg), 4 mL OptiMEM and 24 mL lipofectamine were mixed (transfection mixture) and incubated for 45 min at room temperature. Cells were incubated with transfection mixture at 37°C. After 5 hours, the transfection mixture was removed and 14 mL culture medium added (Ham’s F12 with Glutamax-1, supplemented with 10% fetal bovine serum, 100 U/mL penicillin, and 100 µg/mL streptomycin; Life Technologies) After 24 hours, cells were washed with 5 mL phosphate buffered saline (PBS) without calcium and magnesium (Life Technologies) followed by 6 mL PBS containing 500 BAEE units trypsin and 0.5 mM ethylenediaminetetraacetic acid (Trypsin-EDTA (T/E), Sigma-Aldrich, St. Louis, MO) to detach the cells. Cells from the same transfection were then used either for fluorescence imaging or saturation binding analysis. For the purposes of fluorescence imaging, cells were seeded (≈45,000 cells/well in 200 µL culture medium) in poly-D-lysine coated, black walled plates (Becton Dickson, Bedford, UK) for 24 hours. Results for agonistic activity of AZD3355 and baclofen for different GABAB receptor splice variants are summarized in Supplemental Table 1.
For saturation binding analysis on whole cells using [3H]CGP54626 (Tocris, Bristol, UK), cells (200,000–300,000 cells/well) were seeded in a 24-well plate and incubated for 24 hours. For cell counting, cells were washed with 0.5 mL PBS, detached with 0.5 mL PBS containing T/E. The cell suspension was then mixed with Trypan blue (Life Technologies) and counted in a Bürker chamber (excluding cells stained with Trypan blue). For saturation binding analysis, cells were washed 2 times with 0.8 mL Hank’s buffered salt solution (HBSS, Life Technologies) followed by the addition of 0.5 mL HBSS containing [3H]CGP546262 (0.1–30 nM) and 0.1% ethanol. Non-specific binding was determined by the inclusion of 3 mM GABA in the assay buffer. After 45 min incubation at 37°C, the cells were rapidly rinsed with 2 ´ 0.8 mL ice-cold HBSS. Cells were solubilized by addition of 250 µL 0.1 M NaOH and then transferred to a new 24-well plate (Wallac 1450-408). The wells in the original plate were washed with 180 µL H2O. The cells were mixed with 0.5 mL scintillation cocktail for 20 min and radioactivity determined in a Micro-b counter (Wallac, Turku, Finland). KD and Bmax values were calculated by fitting binding data to the equation B=Bmax/(1+KD/[L]), using Xlfit for Microsoft Excel. Findings for [3H]CGP54626 are summarized in Supplemental Table 2.
Uptake of radiolabeled GABAB receptor agonists in rat cerebrocortical slices
Rat cerebrocortical slices were prepared and incubated with 0.1, 1.0 or 10.0 mM of labeled compounds for 5, 10, and 30 min. The intracellular uptake of [3H]GABA (Amersham International, Bristol, UK, specific activity 3.40 TBq/mol, postitive control), [14C]baclofen (3.2 TBq/mol, radiochemical purity 99%), and [14C]AZD3355 (5.8 TBq/mol, radiochemical purity 96%) was calculated after subtraction of the amount of label in the extracellular spaces, estimated with the hydroxyl[14C-methyl]inulin (Amersham) method (Laakso and Oja, 1976).
In brief, superficial slices (0.4 mm thick) were manually cut from both cerebral hemispheres of 3-month-old Sprague-Dawley rats (weight about 300 g; Orion Ltd, Espoo, Finland) and immediately transferred to 4 mL of medium (composition in mM): NaCl 127, KCl 5, CaCl2 2.5, NaH2PO4 1.3, MgSO4 1.2, HEPES 10 and D-glucose 10, pH 7.4) and incubated at 37oC under O2 with 0.1, 1.0 or 10.0 mM of labeled compounds for 5, 30 or 60 min. The incubations were terminated by rapid filtration and rinsing of the slices with 5 mL of ice-cold medium. The slices were frozen in liquid nitrogen, weighed, and extracted with 5% (w/v) trichloracetic acid solution. The slice extracts and medium were subjected to scintillation counting. The amount of compound in the intracellular space was calculated by relating the radioactivity in the slice extracts to the specific radioactivity in medium after subtraction of the amount of radioactivity retained in the extracellular spaces. The size of the extracellular space was estimated with the hydroxy[14C-methyl]inulin (Amersham) method (Laakso and Oja, 1976). The results are given as mmol/kg wet weight of incubated slices.
[3H]GABA uptake assay and FLIPR® membrane potential assay in mammalian cells transiently expressing the human GATs
Cell culture and transfections – tsA201 cells (a transformed HEK293 cell line) were cultured in GlutaMAX-I DMEM supplemented with 10% fetal bovine serum, penicillin (100 U/mL), and streptomycin (100 µg/mL) at 37°C in a humidified atmosphere of 95% air and 5% CO2. The constructs encoding each of the human GABA transporters (termed hGAT-1, hBGT-1, hGAT-2 and hGAT-3) (Christiansen et al., 2007; Kvist et al., 2009; in press) inserted in the pcDNA5/FRT vector (Invitrogen) were transiently transfected into cells using PolyFect according to the protocol of the manufacturer (Qiagen, West Sussex, UK). Human EAAT3 was used as negative control, as previously described (Christiansen et al., 2007). The functional assays were performed 36–48 hours later.
[3H]GABA uptake assay – tsA201 cells transfected with each of the four human GABA transporters were split into poly-D-lysine-coated white 96-well plates (Perkin-Elmer, Boston, MA, USA). The next day, the medium was removed and cells were washed with 100 µL assay buffer (HBSS supplemented with 20 mM HEPES, 1 mM CaCl2, and 1 mM MgCl2, pH 7.4). Then 75 µL assay buffer supplemented with 30 nM [3H]GABA (or 30 nM D-[3H]Asp in the experiments with human EAAT3) and various concentrations of the test compounds was added to each well and the plate was incubated at 37°C for 3 min. Cells were then washed with 3 ´ 100 µL ice-cold assay buffer and 150 µL MicroscintTM20 scintillation fluid (Perkin-Elmer, Boston, MA, USA) was added to each well. The plate was shaken for at least 1 hour and counted in a Packard TopCount microplate scintillation counter. The [3H]GABA competition curves were constructed based on measurements obtained for eight different concentrations of the test compounds. The experiments were performed in triplicate in at least three independent experiments.
The FLIPR® membrane potential (FMP) assay – The test compounds were characterized functionally in the FMP assay (Molecular Devices, Crawley, UK) essentially as described previously (Jensen and Bräuner-Osborne, 2004). Briefly, tsA201 cells transfected with each of the four human GABA transporters were split into poly-D-lysine-coated black clear bottom 96-well plates (BD Biosciences, Bedford, MA, USA). The next day, the culture medium was removed, and the cells were washed with 100 µL assay buffer (same buffer as used in the [3H]GABA uptake assay). Then 100 µL assay buffer supplemented with FMP assay dye was added to each well, and the plate was incubated at 37°C for 30 min. The plate was assayed at 30°C in a NOVOstarTM plate reader (BMG Labtechnologies, Offenburg, Germany) measuring emission at 560 nm caused by excitation at 530 nm before and up to 1 min after addition of 25 µL substrate solution (the substrate was dissolved in assay buffer). The concentration-response curves were constructed based on measurements obtained for eight different concentrations of the test compounds. The experiments were performed in triplicate in at least three independent experiments.
Data analysis – All data were analyzed using Prism 5.0a (GraphPad Software, San Diego, CA, USA). The curves were fitted by nonlinear regression using the equation Y = Bottom + (Top – Bottom)/(1+10^((log IC50 – X) ´ Hill slope), where X is the logarithm of the concentration, Y is the response, Top is the response at the top plateau, Bottom is the response at the bottom plateau, log IC50 is the concentration when the response is half way between Top and Bottom. The Hill slope describes the steepness of the curve.
Transient lower esophageal sphincter relaxations (TLESRs) in ferrets
Experiments were performed on adult female ferrets (weight range 0.6–0.8 kg) obtained from the Institute for Medical and Veterinary Science, Adelaide, South Australia. In brief, chronic lateral cervical mucosa-to-skin esophagostomies were constructed under anesthesia. Manometric studies were begun at least a month after surgery. Manometry was performed with a micromanometric assembly (2.0-mm OD; Dentsleeve, Wayville, Australia) inserted through the esophagostomy. A pH electrode (0.8-mm OD; FlexilogTM Oakfield Instruments, Oxford, UK) was attached 10 mm above the proximal end of the sleeve sensor and referenced to an Ag-AgCl electrode. The combined pH/manometric assembly was introduced via the esophagostomy and held in place by a Neoprene harness around the animal’s upper thorax and neck. Swallows were detected with a microphone that was attached to the collar and positioned over the hyoid bone. After preamplification (Synectics Polygraf), manometric and pH outputs were acquired to disk and analyzed off-line with Labview-based software. Audio signals from the throat microphone were converted to a software time marker and integrated with other analog inputs.
TLESR in dogs dosed once daily with AZD3355 for 14 days
The inhibitory effect of baclofen on TLESR in the dog has been shown to remain stable after once daily administration for 14 days (Lehmann et al., 2000). However, since tolerance to GABAB receptor stimulation may be agonist-specific (Enna et al. 1998), it was deemed necessary to determine the effects of AZD3355 after repeated dosing. Six dogs were administered AZD3355 at 7 mmol/kg p.o. once a day for 14 days, and the effects on TLESR were assessed on day 1, 3, 7 and 14 using the same protocol as described for the other experiments. The effect on day 1 was lower than that established in the dose-response experiments, but there was no difference between the different experimental days (Supplemental Figure 2). This indicates that the effect of AZD3355 is maintained after repeated administration, at least in the short-term.
Hypothermia after s.c. administration of GABAB receptor agonists in mice
For each substance and time, an exponential model, given by
was fitted to data. The response was defined as the subject’s temperature change from baseline adjusted with the average change from baseline of all animals that were given dose zero. As baseline value, the average of all pre-drug administration measurements was used. In this model denotes the change from baseline for dose zero and denotes the change in when the dose increases with one unit, that is, when . In addition, this model was fitted under the assumption of homogeneous normal errors, . The fitted model was then used to calculate , the dose producing a 2ºC drop in temperature.
Whole body autoradiography in rats
A group of four male rats (Long Evans black hooded male rats, M&B A/S, Denmark) were each given 5.0 mL/kg of the test formulation corresponding to 10 µmol/kg (20 MBq/kg) body weight of [14C]baclofen. Following i.v. injection, individual rats were killed at varying times ranging from 5 min to 6 hours. [14C]AZD3355 (5.4 mL/kg) was administered at a nominal level of 20 µmol/kg body weight and the radioactive doses were 5.7MBq/kg body weight. Time of sacrifice ranged from 5 min to 48 hours. Both male and female rats were used (n=23; male Lister Hooded strain rats were from Charles River, Margate, Kent, UK, and pregnant female rats of the same strain, 18 days of gestation, were from Harlan, Bicester, Oxon, UK), and no sex-related differences in distribution of radioactivity was noted. Also, dosing was either i.v. or p.o. in males and i.v. in females, and again, there were no differences attributable to the route of administration in terms of distribution of radioactivity to the central nervous system (CNS). At the time of the experiment, the animals weighed about 200 g. The sectioning and visualization of distribution, with specific reference to the CNS, was done according to standard methods.
The animals were anesthetized and killed by complete immersion in hexane, cooled to -70°C. After removal of the limbs and tail, each carcass was embedded in a 2.5% aqueous solution of carboxymethyl cellulose and frozen for at least 10 minutes at -70°C. The animal blocks were stored at -20ºC until sectioning. Each block was subsequently mounted in a Leica CM3600 Cryostat (Leica Microsystems, Germany) maintained at approximately -20ºC. After initial trimming of the block, sagittal whole body sections (20-30 µm) were obtained at different levels through the carcass. Each section was attached to tape and numbered consecutively.
Five to six sections from each rat were chosen for phosphor imaging to best represent the different areas in brain and other organs of interest. The sections were talced and placed against phosphor imaging screens for 43 to 88 hours (baclofen experiments) or 7 days (AZD3355 experiments). The imaging plates were exposed in refrigerated light tight cassettes in a shielding box to protect from environmental radiation.
Following exposure the imaging plates were unloaded in subdued light and scanned using Fuji BAS 2500 or 1500 radioluminography. The latent images were visualized and stored in an electronic data file for subsequent analysis.
Chemicals
GABA and baclofen were purchased from Tocris, Bristol, UK. AZD3355, AR-H061719, and AR-H040551 were synthesized at AstraZeneca R&D, Mölndal Sweden or Albany Molecular Research, Albany, NY, USA. The radiolabeled compounds were synthesized by Roger Simonsson, AstraZeneca R&D Mölndal, Sweden. All other chemicals were of highest commercially available grade.
Extrapolation of results across species
The results in the present study are based on studies performed on recombinant human GABAB receptors, rat and dog brain membranes, as well as in vivo experiments in the rat, mouse, ferret, and dog. In order to be able to make any cross-species comparisons, the similarities of the GABAB receptor between species have to be considered. The GABAB receptor is one of the most highly conserved G-protein coupled receptors known, and the homology across the species used here is >98% on an amino acid level. It is therefore unlikely that there are any important species differences in terms of potency of isosteric agonists such as AZD3355 and baclofen. Further, the displacement of [3H]GABA from dog and rat brain membranes with AZD3355 was identical. Previous work has shown that baclofen has very similar potency in dogs (Lehmann et al., 1999), ferrets (Blackshaw et al., 1999), cats (Liu et al., 2002), and humans (Lidums et al., 2000; van Herwaarden et al., 2002; Zhang et al., 2002; Lee et al., 2003) as an inhibitor of TLESRs. It therefore seems reasonable to extrapolate results obtained in one species to the other species. Indeed, apart from expected differences in pharmacokinetics related to body mass (as illustrated by the different pharmacokinetic properties of AZD3355 in dogs and rats; see Supplemental Table 3 and Supplemental Figure 3), the only species differences that have been demonstrated in this regard pertain to the expression of certain splice variants in the rat but not in humans (Pfaff et al., 1999). However, these variants have a pharmacology similar to that of the other splice variants (Pfaff et al., 1999).