In the central nervous system, the ATP-gated Purinergic receptor P2X ligand-gated ion channel 7 (P2X7) is expressed in glial cells and modulates neurophysiology via release of gliotransmitters, including the proinflammatory cytokine interleukin (IL)-1β. In this study, we characterized JNJ-42253432 [2-methyl-N-([1-(4-phenylpiperazin-1-yl)cyclohexyl]methyl)-1,2,3,4-tetrahydroisoquinoline-5-carboxamide] as a centrally permeable (brain-to-plasma ratio of 1), high-affinity P2X7 antagonist with desirable pharmacokinetic and pharmacodynamic properties for in vivo testing in rodents. JNJ-42253432 is a high-affinity antagonist for the rat (pKi 9.1 ± 0.07) and human (pKi 7.9 ± 0.08) P2X7 channel. The compound blocked the ATP-induced current and Bz-ATP [2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate tri(triethylammonium)]–induced release of IL-1β in a concentration-dependent manner. When dosed in rats, JNJ-42253432 occupied the brain P2X7 channel with an ED50 of 0.3 mg/kg, corresponding to a mean plasma concentration of 42 ng/ml. The compound blocked the release of IL-1β induced by Bz-ATP in freely moving rat brain. At higher doses/exposure, JNJ-42253432 also increased serotonin levels in the rat brain, which is due to antagonism of the serotonin transporter (SERT) resulting in an ED50 of 10 mg/kg for SERT occupancy. JNJ-42253432 reduced electroencephalography spectral power in the α-1 band in a dose-dependent manner; the compound also attenuated amphetamine-induced hyperactivity. JNJ-42253432 significantly increased both overall social interaction and social preference, an effect that was independent of stress induced by foot-shock. Surprisingly, there was no effect of the compound on either neuropathic pain or inflammatory pain behaviors. In summary, in this study, we characterize JNJ-42253432 as a novel brain-penetrant P2X7 antagonist with high affinity and selectivity for the P2X7 channel.
Purinergic receptor P2X ligand-gated ion channel 7 (P2X7), a member of the purinergic family of receptors, is an ATP-gated ion channel expressed predominantly on cells of hematopoietic lineage, including macrophages and monocytes in the periphery and microglia and astrocytes in the central nervous system (CNS). ATP-mediated P2X7 activation leads to opening of the channel pore, followed by nonselective cation flux. Prolonged exposure to ATP leads to formation of a large opening (termed a macropore) in vitro, although the physiologic significance of this macropore formation is not clearly understood. One of the hallmark features of P2X7 activation is release of the proinflammatory cytokine interleukin (IL)-1β (Solle et al., 2001), which in turn may contribute to neuroinflammation and pain (Chessell et al., 2005; North and Jarvis, 2013), mood disorders (Iwata et al., 2013; Chrovian et al., 2014), and neurodegenerative disorders, such as Alzheimer’s disease (Diaz-Hernandez et al., 2012), multiple sclerosis (Sharp et al., 2008), and epilepsy (Engel et al., 2012; Mesuret et al., 2014). In addition to IL-1β, P2X7-dependent release of glutamate (Andó and Sperlágh, 2013; Ficker et al., 2014), cathepsins (Clark et al., 2010), and chemokines (Shiratori et al., 2010) have been reported, and all of these mediators can serve as “gliotransmitters” aiding in the neuroimmune cross-talk and modulating synaptic plasticity.
Within the CNS, and in particular within the context of brain neuro(patho)physiology, there is growing interest in the role of ATP and P2X ion channels in modulating neurophysiology (Burnstock, 2008, 2014; Abbracchio et al., 2009; Burnstock et al., 2011a,b; Khakh and North, 2012; Sperlagh et al., 2012; Chrovian et al., 2014). Specifically, the P2X7 ion channel in the brain has attracted interest due to the genetic association with depression and bipolar disorder, coupled with emerging science on the role of the P2X7–IL-1β pathway in central neuroinflammation (Bennett, 2007; Sperlagh et al., 2012; Bhattacharya et al., 2013; Iwata et al., 2013). There is growing evidence that strengthens the role of P2X7 in mood disorders. Several human genetic studies have associated the highly polymorphic P2RX7 gene with both bipolar disorder and depression (McQuillin et al., 2009; Backlund et al., 2011; Soronen et al., 2011) and some of these mutations have been linked to modulation of P2X7 channel function in vitro (Roger et al., 2010; Stokes et al., 2010). The progress in the field has been somewhat hampered by the absence of a suitable brain-penetrant P2X7-selective antagonist, because most of the P2X7 compounds described in the literature are either human-specific (low affinity for rodent P2X7), or suffer from poor brain penetration or are nonselective and may exert polypharmacology in vivo. In this article, we pharmacologically characterize a novel P2X7 antagonist JNJ-42253432 [2-methyl-N-([1-(4-phenylpiperazin-1-yl)cyclohexyl]methyl)-1,2,3,4-tetrahydroisoquinoline-5-carboxamide]. The compound is a potent P2X7 antagonist, penetrates into the CNS with a brain-to-plasma ratio of 1, and exhibits excellent pharmacokinetic and pharmacodynamic properties. In the process of pharmacologic characterization, we discovered that the compound also demonstrates affinity for the serotonin transporter (SERT), although JNJ-42253432 is more potent at engaging P2X7 than SERT; moreover, the low affinity for SERT does not compromise our interpretation that JNJ-42253432 is a P2X7 antagonist and in vivo efficacy is due to P2X7 occupancy. The compound is orally bioavailable, exhibits good drug-like properties, and was used in several animal models to better understand the role of P2X7 in depression, mania, and pain.
Materials and Methods
1321N1 cells expressing the P2X7 channels (human, mouse) were cultured in Dulbecco’s modified Eagle’s medium/high-glucose medium supplemented with 10% fetal bovine serum and 500 µg/ml G418. 1321N1 cells expressing the rat P2X7 were grown in the same media supplemented with 10% fetal bovine serum and 100 µg/ml Zeocin (Invivogen, San Diego, CA). For isolation of primary astrocytes, rat brains were dissected from 3-day-old neonatal rats as described by Bhattacharya et al. (2013).
1321N1 cells expressing P2X7 orthologs were dissociated 18–24 hours prior to the assay using 0.05% trypsin/EDTA (Invitrogen, Carlsbad, CA), and plated at density of 25,000 cells/well into poly-d-lysine–coated, 96-well, black-walled, clear-bottom plates (Becton-Dickinson, Bedford, MA). On the day of the experiment, cell plates were washed with assay buffer, containing the following: 130 mM NaCl, 2 mM KCl, 1 mM CaCl2, 1 mM MgCl2, 10 mM HEPES, and 5 mM glucose; pH 7.40. After the wash, dye loading was achieved by adding a 2× Calcium-4 (Molecular Devices, Sunnyvale, CA) dye solution in the assay buffer. Cells were stained with the Calcium-4 dye in staining buffer for 30 minutes at room temperature in the dark. Test compounds were prepared at 250× the final test concentration in neat dimethylsulfoxide. Intermediate 96-well compound plates were prepared by transferring 1.2 µl of the compound into 300 µl of assay buffer. A further 3× dilution occurred when transferring 50 µl/well of the compound plate to 100 µl/well in the cell plate. Cells were incubated with test compounds and dye for 30 minutes. Calcium flux was monitored in Fluorometric Imaging Plate ReaderTetra as the cells were challenged by adding 50 µl/well of Bz-ATP [2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate tri(triethylammonium)]. The final concentration of Bz-ATP was 250 μM (human, rat) or 600 μM (mouse) and 100 µM (rat primary astrocytes).
[3H]A-804598 (N-cyano-N′′-[(1S)-1-phenylethyl]-N′-5-quinolinyl-guanidine) was used as the radioligand (Donnelly-Roberts et al., 2009). P2 membranes were prepared from recombinant cells; for one 96-well assay, cells were harvested from 10 T225 confluent flasks and frozen at −80°C. On the day of the experiment, cells were thawed or rat cortex was homogenized for membrane (P2) preparation. Fifty millimoles Tris-HCl (pH 7.4) was added to the cells and homogenized for approximately 30 seconds at high speed. The homogenate was centrifuged at 1500 rpm for 5 minutes followed by careful decanting of the supernatant, which was centrifuged at 32,000g for 30 minutes. Six milliliters of ice-cold assay buffer (50 mM Tris-HCl + 0.1% bovine serum albumin) was added to the cell pellet. The assay volume of 100 μl was composed of the following: (a) 10 μl compound (10×) + (b) 40 μl tracer (2.5×) + 50 μl membrane (2×). The reaction was incubated for 1 hour at 4°C. The assay was terminated the by filtration (GF/B filters presoaked with 0.3% polyethylenimine) and washed with washing buffer (Tris-HCl 50 mM) repeatedly. After drying, the plate Microscint 0 was added to the filters and radioactivity was counted.
Human blood, isolated human monocytes, or mouse blood were primed with lipopolysaccharide (LPS) followed by addition of the test antagonist or vehicle with the final P2X7 stimulus of Bz-ATP. For the human blood and monocytes, priming concentration of LPS was 30 ng/ml. Test compounds were added and incubated for an additional 30 minutes. The P2X7 agonist Bz-ATP (1 mM for blood, 0.5 mM for monocytes) was finally added and incubated for 1.5 hours at 37°C. After incubation, the plates were centrifuged (low-speed spin) and the supernatant was collected for IL-1β enzyme-linked immunosorbent assay analysis as per the manufacturer’s protocol (human IL-1β; Thermo Scientific, Waltham, MA).
1321N1 cells stably expressing hP2X7 were maintained in culture as previously described. Twenty-four hours prior to experimentation, cells were washed with divalent-free phosphate-buffered saline (Hyclone, Rockford, IL), dissociated with 0.05% trypsin (Gibco, Grand Island, NY) and plated on poly(d-lysine)–coated cover slips (BD Biosciences, San Jose, CA) in a 24-well plate at a density of 10,000 cells/well. On the day of experimentation, cells were transferred to a recording chamber mounted on a Nikon Eclipse TE2000-U microscope (Nikon Inc., Melville, NY) and continuously perfused (1 ml/min) with a low divalent physiologic saline solution (137 mM NaCl, 5.4 mM KCl, 0.5 mM MgCl2, 0 mM CaCl2, 5 mM glucose, and 10 mM HEPES, pH 7.4). Patch pipettes (2 to 3 MΩ) were visually guided to the surface of individual cells. Patch pipettes contained 135 mM CsF, 10 mM CsCl, 5 mM EGTA, 5 mM NaCl, and 10 mM HEPES, osmolarity adjusted to 290 mOsm·l−1 with sucrose, pH 7.3. Voltage-clamp recordings were made with an Axopatch 200B, and Clampex 9.2 software (Molecular Devices). Data were acquired at 5 kHz and filtered at 2 kHz. Pipette capacitance was cancelled in all recordings. Whole-cell responses to ATP and Bz-ATP were measured by rapidly transitioning the cell from being bathed in physiologic saline to ATP or Bz-ATP (5 mM and 300 μM, respectively) for 5 seconds in successive trials using a large-bore drug applicator (a linear array of glass tubes driven by a linear motor, SF-77B Perfusion Fast-Step; Warner Instruments Corp., Hamden, CT). For experiments examining channel block, JNJ-42253432 was applied in the presence of agonist. Channel block at each concentration of JNJ-42253432 was allowed to proceed to apparent steady state, and then increased in 1/2 log intervals until complete block of the agonist-evoked current had been achieved. All responses were quantified using Clampfit 9.2 software (Molecular Devices) to calculate the mean current amplitude during the last 100 milliseconds of an agonist application or drug application interval.
Ex Vivo Receptor Binding Autoradiography.
All animal work described in this article was in accordance with the Guide for the Care Use of Laboratory Animals adopted by the US National Institutes of Health. Animals were allowed to acclimate for 7 days after receipt. They were group housed in accordance with institutional standards, received food and water ad libitum, and were maintained on a 12-hour light/dark cycle. Male Sprague-Dawley rats approximately 300–400 g in body weight were used. The animals were euthanized using carbon dioxide and decapitated at different time points after drug administration. Brains were rapidly frozen on powdered dry ice and stored at −80°C before sectioning. Plasma samples were also collected for bioanalysis. Twenty-micron-thick tissue sections at the level of the hippocampus were prepared for autoradiography. P2X7 radioligand binding autoradiography was determined at room temperature with 30 nM [3H]A-804598 in 50 mM Tris-HCl incubation buffer containing 0.1% bovine serum albumin as previously described (Bhattacharya et al., 2013). Nonspecific binding was measured with 100 μM A-740003 (N-[1-[[(cyanoamino)(5-quinolinylamino)methylene]amino]-2,2-dimethylpropyl]-3,4-dimethoxybenzeneacetamide). Ex vivo receptor labeling was expressed as the percentage of receptor labeling in corresponding brain areas (i.e., hippocampus) of saline-treated animals. The percentage of receptor occupancy was plotted against time or dosage using GraphPad Prism software (GraphPad Software Inc., La Jolla, CA). SERT binding autoradiography was conducted as previously described (Barbier et al., 2007). Briefly, tissue sections at the level of prefrontal cortex were incubated with 1 nM [3H]citalopram in 50 mM Tris buffer with 120 mM NaCl for 1 minute at room temperature to minimize dissociation. Sections were allowed to dry before acquisition with β-Imager (Biospace Laboratory, Paris, France). Quantitative analysis was performed using M3 Vision software (Biospace Laboratory).
In Vivo Microdialysis.
For serotonin microdialysis, male Sprague-Dawley rats (280–350 g) were implanted with a guide cannula (Eicom, Kyoto Japan) in the prefrontal cortex. Dialysis probes (4-mm active membrane length; Eicom) were perfused with artificial cerebral spinal fluid at a flow rate of 1 μl/min. Experimentation was conducted from 9:00 AM through the following day and samples were collected every 60 minutes. After 3 hours of baseline collection, animals received either a vehicle or subcutaneous injection of JNJ-42253432 at 3 or 10 mg/kg. Samples were collected into a 96-well plate maintained at 4°C containing 15 µl of antioxidant (0.1 M acetic acid, 1 mM oxalic acid, and 3 mM l-cysteine in sterile water). Dialysis samples were analyzed for serotonin using high-performance liquid chromatography with electrochemical detection. Dialysate was injected by a refrigerated autosampler (Alcott 719) at a volume of 10 µl and separation was achieved using an Eicompak PP-ODS column (4.6 mm i.d. × 30 mm; Eicom) with the potential of the graphite electrode set to +400 mV against the Ag/AgCl reference electrode. The mobile phase consisted of 100 mM sodium phosphate buffer, pH 6.0, 500 mg/l decanesulfonic acid, 50 mg/l EDTA, and 1% (v/v) methanol. For IL-1β microdialysis, male Sprague-Dawley rats of similar weights were implanted with a hippocampal guide cannula. Experimental apparatus and sample analysis were previously described (Bhattacharya et al., 2013). After 2 hours of baseline collection, animals received vehicle, 1.0, 3.0, or 10 mg/kg subcutaneous injection of JNJ-42253432. One hour after drug injection, each animal was challenged with Bz-ATP at 100 mM administered locally via reverse dialysis for 2 hours. The lower limits of detection were 0.01 pg/μl for serotonin and 15 pg/ml for IL-1β. Graphical and statistical analysis was done using GraphPad Prism software (version 5.01). An area under the curve (AUC) value was calculated from the baseline for each animal. A one-way analysis of variance (ANOVA) with Dunnett’s multiple comparison test was used to determine significance between treatment groups.
Pharmacoelectroencephalography Study in Rats.
Experiments were carried out in male adult Sprague-Dawley rats, supplied by Harlan Laboratories (Horst, The Netherlands) and weighing about 250 g at the time of surgery. Animals were housed in full-view Plexiglas cages (25 × 33 cm, 18 cm high) that integrate into racks of individually ventilated cages located in a sound-attenuated chamber. Animals were maintained under controlled environmental conditions throughout the study as follows: 22°C ± 2°C ambient temperature, the relative humidity at 60%, 12-hour light/dark cycle (lights off 6:59 AM to 6:59 PM; light intensity: approximately 100 lux,) and food and water available ad libitum. The surgery was performed under isoflurane inhalation anesthesia. A mixture of 30% O2, 70% N2O, and 5% isoflurane was administered to animals as an initial induction for 2 minutes. Then, the animals were mounted in a stereotaxic apparatus and were given a continuous constant mixture of O2, N2O, and 2% isoflurane. An analgesic Piritramide (dipidolor) 0.025 mg/kg (0.1 ml/100 g body weight) was administered before the incision over the total length of the head. The oval area of the scalp was removed, and the uncovered skull was cleared of the periosteum. Animals were equipped with electroencephalography (EEG) and electromyography (EMG) electrodes as described in Fig. 8A. Eight stainless steel electrode screws (diameter 1 mm) were inserted bilaterally in the left and right hemispheres along the anteroposterior axes at the locations (frontal left, parietal left, and occipital left, and frontal right, parietal right, and occipital right, respectively). Electrodes were placed stereotaxically (AP, +2 mm; L, ±2 mm; AP, −2 mm; L ±2 mm; AP, −6 mm; L, ±2 mm; from Bregma) and referenced to the same ground electrode place midline above the cerebellum, while the incisor bar was −5 mm under the center of the ear bar. Additional electrodes were placed in the muscles of the neck to record EMG activity. These electrodes (stainless steel wire, 7N51465T5TLT, and 51/46 Teflon; Bilaney, Dusseldorf, Germany) were connected to a pin (Future Electronics, Pointe-Claire, QC, Canada) with a small insert (track pins; Dataflex, Rotterdam, The Netherlands) and fitted into a 10-hole connector, after which the whole assembly was fixed with dental cement to the cranium. Two weeks after surgery, the animals were habituated to the recording procedure. EEG recordings occurred under a vigilance controlled waking condition. After a stable baseline recording session of 30 minutes, signals from six brain areas (frontal left, frontal right, parietal left, parietal right, occipital left, and occipital right) were recorded for 2 hours after drug administration (n = 8 for each condition). Continuous EEG and EMG field potentials were acquired at a 2-kHz sample rate with an input range of ±500 mV through a Biosemi ActiveTwo system (Biosemi, Amsterdam, The Netherlands), which replaces the conventional ground electrodes by two separate electrodes: the common mode sense active electrode and the driven right leg passive electrode. This common mode reference for online data acquisition and impedance measures is a feedback loop driving the average potential across the montage close to the amplifier zero. The signals were digitized with 24-bit resolution, amplified, and analog band-pass filtered between 1 and 100 Hz. Spectral density estimates were calculated using fast Fourier transform with the Hanning window function; block size of 512 data points, giving 1.0-Hz resolution, and power was expressed as percentage of total power over 1–100 Hz. The average spectral density in each frequency oscillation was normalized across animals to obtain the full power spectrum constructed from overlapping windows of 4-second EEG data and plotted for baseline periods and postdrug periods windowing over 1–100 Hz and also averaged for the following frequency windows: δ band (1–4 Hz), Θ band (Θ1: 4–6.5 Hz; Θ2: 6.5–8 Hz), α band (α1: 8–11 Hz; α2: 11–14 Hz), β band (β1: 14–18 Hz; β2: 18–32 Hz), and γ band (γ1: 32–48; γ2: 52–100 Hz).
In the dose-response experiment, values of consecutive 4-second epochs were averaged over a 30-minute period (baseline recording), followed by the administration of vehicle or test compound and averaging over consecutive 15-minute periods for the remaining 2 hours after administration (treatment recording; see Fig. 8A). In the pharmacologic reversal challenge experiments, EEG oscillations were averaged for 30 minutes before and 30 minutes after test compound administration (drug-free and test compound only baseline, respectively), followed by administration of the amphetamine challenge. Next, averaging was done over consecutive 15-minute periods to measure combined effects of the challenge and test compound. Changes in EEG spectral power were calculated in those blocks of 15 minutes for a total of 2 hours as the ratio of mean spectral power obtained after the administration of test drug versus the mean spectral power obtained during the 30-minute drug-free baseline period. This within-subject procedure allows for assessment of drug-induced changes in EEG power expressed in each frequency band as a percentage of the subjects’ original power, before comparing values with the vehicle condition (i.e., between subjects).
Amphetamine Hyperactivity Model.
Adult male Sprague-Dawley rats (Harlan) maintained on a normal 12-hour light/dark cycle (lights on at 6:00 AM) under standard housing conditions with food and water provided ad libitum (except during testing) were used for this study. Experiments took place during the light cycle. Animals were single housed with enrichment (sunflower seeds and nylon bone) and were allowed to acclimate to housing conditions for at least 5 days after arrival. On day 1 of the experiment, animals were acclimated to the procedure room for at least 1 hour and then each cage was placed onto a MotorMonitor rack (Kinder Scientific, San Diego, CA), which surrounded each animal cage by a grid of photobeams (7 beams along the width of the cage and 15 beams along the length). The photobeams allowed for tracking the distance traveled by each animal. After 1 hour, rats were injected subcutaneously with either vehicle (30% sulfobutylether β-cyclodextrin) or JNJ-42253432 (3 mg/kg) and locomotor activity was recorded for an additional hour. All animals then received an injection of d-amphetamine sulfate (2 mg/kg i.p., weight of salt) and locomotor activity was recorded for 2 additional hours. On days 2–5, the experiment was repeated in the same manner, with the same treatments provided each day.
Stress-Induced Suppression of Social Interaction.
Adult male Sprague-Dawley rats (Harlan Laboratories) maintained on a 12-hour light/dark cycle (lights on at 7:00 AM) under standard housing conditions with food and water provided ad libitum (except during testing) were used for this study. All experiments took place during the light cycle. Animal testing procedures complied with the Guide for the Care and Use of Laboratory Animals and were approved by the Institutional Animal Care and Use Committee at Binghamton University-State University of New York (Binghamton, NY). Rats were given 1 to 2 weeks to acclimate to colony conditions prior and during this period they were pair-housed. The experiment conducted here used a 2 × 2 between-subjects design in which the stress condition (control versus foot-shock) was crossed with the drug condition (0 or 10 mg/kg of JNJ-42253432). Four days prior to experimentation, test subjects were isolate-housed to produce mild social deprivation and thereby increase spontaneous interactions with a social partner observed in subsequent tests. On day 1 of testing, all experimental subjects were habituated to the two-chamber test apparatus (30 cm × 30 cm × 20 cm) for 30 minutes followed immediately by 10 minutes of baseline social behavior testing as described elsewhere (Varlinskaya et al., 2013). The next day, rats (n = 8 per group; N = 32) received 0 or 10 mg/kg of JNJ-42253432 administered intragastrically immediately prior to a 2-hour session of intermittent foot-shock (80 shocks, 90 seconds between shocks, 5 seconds each) as previously described (Arakawa et al., 2009). Three and one-half hours later, rats were transferred to a test apparatus for a 30-minute acclimation period followed by a 10-minute exposure to a nonstressed, drug-naïve social partner. These time points were selected because prior work has demonstrated a robust suppression in social investigation after footshock exposure, an effect that was blocked by prior intracerebroventricular injection of IL-1 receptor antagonist (Arakawa et al., 2009). Weight differences between test subjects and social partners were minimized prior to experimentation, with test partners always being lighter than experimental animals. All subjects were randomly assigned to experimental conditions. Behavioral sessions were video recorded and later scored by an observer blind to experimental conditions. Three dependent measures were scored and analyzed to provide a comprehensive characterization of the social interaction session. First, an aggregate measure (composite variable) of overall social activity was calculated as the broadest assessment of social behavior. Overall social activity was scored as the sum of the frequencies of the following social behaviors: social investigation (sniffing of any part of the body of the partner), contact (crawling over and under the partner and social grooming), and play behavior (pouncing or playful nape attack, chasing, and pinning). Second, social preference/avoidance was assessed by separately measuring the number of crossovers between compartments demonstrated by the experimental subject toward as well as away from the social partner and was indexed by means of a coefficient of preference/avoidance [coefficient (%) = (crossovers to the partner − crossovers away from the partner)/(total number of crosses both to and away from the partner) × 100]. Social preference was defined as positive values of the coefficient, whereas social avoidance was associated with negative values. Finally, the total number of crossings was analyzed as a measure of gross activity changes incurred by experimental treatment.
Complete Freund’s Adjuvant Model of Inflammatory Pain.
This study was conducted as a fee for service at Algos Preclinical Services. Rats were housed three per cage and were maintained on a 12-hour light/dark schedule for the duration of the study. Rats were housed in standard rodent solid bottom caging with Harlan Teklad, one-eighth-inch corncob bedding, in a well ventilated, environmentally monitored housing room. The mean (± S.E.M.) body weight of animals on the day of testing was 204 ± 1.1 g with a body weight range from 190 to 225 g. Inflammatory pain was induced by an intraplantar injection of 75 µl of 100% complete Freund’s adjuvant (CFA) into the left hind paw under isoflurane anesthesia (3–5% in oxygen). JNJ-42253432 was formulated in 0.5% hydroxypropyl methyl cellulose and dosed via oral gavage in a dose volume of 5 ml/kg. Preinjury, baseline, and postinjury values for mechanical hyperalgesia were evaluated using a digital Randall-Selitto device (IITC Life Sciences, Woodland Hills, CA). Animals were allowed to acclimate to the testing room for a minimum of 30 minutes before testing. Animals were placed in a restraint sling that suspends the animal, leaving the hind limbs available for testing. The paw compression threshold was measured once at each time point for the ipsilateral and contralateral paws. The stimulus was applied to the plantar surface of the hind paw by a cone-shaped tip placed between the third and fourth metatarsus, and pressure was applied gradually over approximately 10 seconds. Measurements were taken from the first observed nocifensive behavior of vocalization, struggle, or withdrawal. A cut-off value of 300 g was used to prevent injury to the animal. The mean and S.E.M. were determined for each paw for each treatment group. The animals were tested before CFA injection, prior to compound administration, and 1, 2, and 4 hours after administration.
Chung Model of Neuropathic Pain.
For creating the neuropathic preparation, the surgical procedure previously described by Kim and Chung (1992) was performed to induce an allodynic state. Briefly, the left L-5 and L-6 spinal nerves were isolated adjacent to the vertebral column and ligated with 6-0 silk suture distal to the dorsal root ganglion while under isoflurane anesthesia. The rats were allowed a 10- to 14-day postoperative recovery period and allowed adequate time to develop the neuropathy before being placed in the study. Animals that did not meet a tactile allodynia 50% threshold of <3.0 g were not used. For each drug, six animals were prepared with the Chung model of spinal nerve ligation. Test articles were injected by one individual with all behavioral observations made by a second individual who was unaware of the article received by each animal. Tactile thresholds were assessed using Von Frey hairs and the up/down method as described in the initial study protocol at baseline, 30 minutes, 60 minutes, and 120 minutes after compound dosing. Summary statistics were computed and include group means and standard deviations and numbers of animals per group. For time course curves, two-way repeated measures were undertaken and a post hoc analysis performed comparing with time zero. The area under the percent hyperalgesic effect curve (not shown) was calculated and referred to as the AUC. Statistical comparison of the AUC was carried out with a one-way ANOVA and a post hoc analysis was carried out comparing all value to vehicle. All statistical analyses were performed on spreadsheets using Prism statistical software (version 5.0; GraphPad Software Inc.).
In Vitro Pharmacology
The structure of JNJ-42253432 is shown in Fig. 1. The potency (pIC50 ± S.E.M) of JNJ-42253432 to attenuate Bz-ATP–induced calcium flux for the three P2X7 orthologs is depicted in Table 1 [human (7.7 ± 0.07), rat (7.8 ± 0.1), and mouse (7.1 ± 0.2)]. As shown in Fig. 2, JNJ-42253432 attenuated both ATP- and Bz-ATP–induced currents from hP2X7-1321N1 cells with similar potencies (for ATP: pIC50 = 6.93 ± 0.05; for Bz-ATP: pIC50 = 7.00 ± 0.04). In addition, JNJ-42253432 did not block calcium flux via human P2X1, P2X2, P2X3, P2X2/P2X3, and P2X4 up to 10 μM concentrations (Table 2). In an effort to understand the affinity of the compound under true equilibrium (both Fluorometric Imaging Plate Reader and electrophysiology are in pseudo-equilibrium), JNJ-42253432 was tested in a radioligand binding assay. The affinity (pKi ± S.E.M) of JNJ-42253432 was 7.9 ± 0.08 and 9.1 ± 0.07, at the recombinant human P2X7, and rat P2X7, respectively (Table 1). To assess a broader selectivity profile of JNJ-42253432, the compound was tested at 1 μM against a panel of receptors, channels, and transporters (Bhattacharya et al., 2013); >50% displacement was observed for human NK2, the human dopamine transporter, and the rat brain sodium channel (data not shown).
The next step in the characterization cascade of JNJ-42253432 was to test the pharmacology in native systems known to express P2X7 ion channels. Rat primary cultures of astrocytes and human blood cells express functional P2X7 channels (Bhattacharya et al., 2013). JNJ-42253432 blocked Bz-ATP–induced calcium influx in a concentration-dependent manner with a potency of 7.0 ± 0.05 in rat astrocytes (Table 1). The affinity of JNJ-42253432 for P2X7 in rat brain cortical membranes as measured by displacement of [3H]A-804598 was 8.5 ± 0.07, similar to the affinity at the recombinant rat P2X7 channel (Table 1). Because P2X7 activation leads to release of the proinflammatory cytokine IL-1β, we also characterized JNJ-42253432 in human whole blood and in freshly isolated monocytes. As shown in Fig. 3, JNJ-42253432 attenuated LPS-primed Bz-ATP–induced IL-1β release in a concentration-dependent manner with full block achieved at 1-μM concentrations. The potency of JNJ-42253432 to attenuate IL-1β release was 6.7 ± 0.1 in the human whole blood. Likewise, the pIC50 of JNJ-42253432 was 7.4 ± 0.1 when tested in freshly isolated human monocytes from the blood, with the difference in potency probably due to protein binding of JNJ-42253432 in whole blood. Antagonism was surmountable by increasing concentrations of Bz-ATP (Fig. 3B), a functional phenomenon that would relate to a similar/overlapping site of action with Bz-ATP. In addition to IL-1β block, the compound also blocked IL-18 release, but did not block IL-6 and tumor necrosis factor-α release, under identical conditions (of LPS and BZ-ATP incubation times and concentrations) used for IL-1β and IL-18 release (data not shown).
Pharmacokinetic Properties of JNJ-42253432
To evaluate the pharmacokinetic properties of JNJ-42253432, the compound was dosed orally and intravenously in rats and mice and followed for 24 hours (Fig. 4). The compound was orally bioavailable in both species and the in vivo plasma clearance was 16.2 ml/min per kilogram and 50 ml/min per kilogram in mice and rats, respectively. In mice, after a single oral dose of 10 mg/kg, Cmax in the plasma of 716.4 ng/ml (1.6 μM) was achieved at 0.75 hours. The t1/2 of the compound in the mouse plasma was between 7.7 and 8.8 hours. The kinetic profile of the compound was identical in the mouse plasma and mouse brain tissue with brain to plasma ratio of 1.26. In rats, after a single oral dose of 5 mg/kg, Cmax in the plasma of 225 ng/ml (0.5 μM) was achieved at 0.8 hours. JNJ-42253432 was orally bioavailable with a %F of 56.4 ± 6.7. The t1/2 of the compound in rat plasma was between 2.7 and 4 hours, in line with a faster clearance number. The kinetic profile of the compound was also similar in the rat plasma and rat brain tissue with a brain-to-plasma ratio of 1. These data suggest that JNJ-42253432 is a good in vivo tool compound to probe the role of central P2X7 in CNS animal models of disease.
Pharmacodynamic Properties of JNJ-42253432: Target Engagement
Autoradiography is an excellent means of assessing binding of a ligand to brain targets. For P2X7, we have used this technique to measure occupancy of central P2X7 channels (Bhattacharya et al., 2013). Here we report the pharmacology of JNJ-42253432 in rats dosed either subcutaneously or orally. Fig. 5A depicts the exposure and P2X7 brain occupancy of JNJ-42253432 at various time points after a dose of 10 mg/kg s.c. The compound demonstrated a time- and concentration-dependent occupancy with a time for maximal occupancy at 2 hours. There was excellent correlation of occupancy and exposure of the compound in the brain; as seen from the exposure-time graph, JNJ-42253432 partitioned effectively between the plasma and brain compartment with a brain to plasma ratio of unity. JNJ-42253432 was dosed from 0.03 to 10 mg/kg s.c., followed by ex vivo receptor occupancy at 2 hours postdose. The compound exhibited a dose-dependent increase in channel occupancy with an ED50 of 0.3 mg/kg (Fig. 5B). Likewise, a dose-dependent linear change in exposure was observed (data not shown) covering the entire dose range, with a brain to plasma ratio of unity. The plasma EC50 of JNJ-42253432 was approximately 42 ng/ml (0.094 μM). To assess brain occupancy after an oral dose, JNJ-42253432 was tested for duration of action at 30 mg/kg p.o. As shown in Fig. 5C, the compound maintained reasonable high occupancy for the entire time course of 1 day after an oral dose of 30 mg/kg. The compound engaged brain P2X7 sites with high occupancy for 8 hours and was deemed suitable for once- or twice-daily dosing regimens for efficacy studies in rats.
Brain IL-1β Microdialysis.
In an effort to better understand the consequence of P2X7 binding to function, we established an in vivo microdialysis assay to measure brain IL-1β levels in freely moving rats challenged with the P2X7 agonist Bz-ATP (see Materials and Methods). As depicted in Fig. 6, Bz-ATP (100 mM) produced a robust increase in measurable IL-1β from hippocampal dialysates. JNJ-42253432 dose dependently attenuated the IL-1β release in freely moving rat brains. JNJ-42253432 significantly attenuated IL-1β release at 10 mg/kg (P < 0.05 versus vehicle one-way ANOVA Dunnett’s post hoc analysis of AUC). The effect of IL-1β suppression at the 3 mg/kg group was statistically not significant; nonetheless, a strong trend was observed as depicted in Fig. 6. At 1 mg/kg, JNJ-42253432 was clearly without any effect.
Serotonin Microdialysis and SERT Autoradiography.
Because SERT inhibition is a common mechanism of antidepressant efficacy, we wanted to counterscreen JNJ-42253432 in a microdialysis assay for serotonin. At the 10 mg/kg dose, significant (P < 0.001 versus vehicle one-way ANOVA Dunnett’s post hoc analysis of AUC) elevations of serotonin were observed in the rat prefrontal cortex (Fig. 7A). There was no significant elevation of serotonin at the 3 mg/kg dose; likewise, the compound did not cause any significant changes to either norepinephrine or dopamine brain levels at similar doses (data not shown). This robust and sustained increase of serotonin was limited to this particular compound; other P2X7 compounds did not elevate brain serotonin levels (data not shown) and as such it was concluded that the serotonin increase was not mediated via P2X7 antagonism; rather it was probably a direct off-target activity of JNJ-42253432 at the SERT. The next step was to profile JNJ-42253432 in an ex vivo assay for SERT, similar to the one described for P2X7. The goal was to compare dose response of JNJ-42253432 for SERT and P2X7 occupancy. As shown in Fig. 7B, JNJ-42253432 did bind to rat SERT at 10 mg/kg, thus supporting the increases of serotonin observed in the microdialysis study. It was also apparent that JNJ-42253432 was P2X7 selective over SERT, at doses below 10 mg/kg, where there was no significant SERT occupancy (Fig. 7B). Thus, JNJ-42253432 was SERT sparing at doses below 10 mg/kg and consequently all efficacy studies to address the role of P2X7 generally never exceeded 10 mg/kg dose. In vivo effects of JNJ-42253432 at doses significantly higher than 10 mg/kg (where occupancy for both P2X7 and SERT occupancy is high) will probably be contaminated by SERT blockade and caution should be exerted in interpretation of efficacy data.
Efficacy of JNJ-42253432 in Pharmaco-EEG Recordings from Rat Brain
To test whether acute JNJ-42253432 exerted central functional effects, quantified EEG was studied, assessing changes in spectral power in a dose-response study and in an amphetamine-challenge study (see Fig. 8A). Findings show that the compound demonstrated clear dose-dependent effects on cortical spectral power (Fig. 8B), most strikingly a reduction in the α-1 band, accompanied by (to a lesser extent) a reduction in Θ-2 and an increase in Θ-1 power, These latter two effects on Θ bandings were only clear at the highest dose tested of 40 mg/kg. The changes in α-1 band power reached significance (P < 0.05) at about 30 minutes after administration and lasted throughout the 2-hour recording period for the 10 and 40 mg/kg doses (see Fig. 8B, bottom line graphs). These effects showed up most prominently at the posterior cortical recording sites.
The amphetamine-challenge study showed the compounds’ potential to normalize the changes induced by amphetamine and its dopaminergic mechanism, known to particularly induce a clear increase in power of the α-1 band (see Fig. 8C, second row). From Fig. 8C (bottom row), it can be seen that at the high dose of 40 mg/kg s.c. JNJ-42253432 is able to augment the amphetamine-induced α-1 power, while seemingly “slowing” the EEG oscillations, resulting in an increased Θ-2 power.
Efficacy of JNJ-42253432 in Models of Mania, Depression, and Pain
Locomotor activity from the acute phase (day 1) and sensitized phase (day 5) are shown in Fig. 9. Amphetamine (2 mg/kg) administration (fat arrow) caused changes in locomotor activity that sensitized between day 1 and day 5 in rats. Amphetamine-treated animals displayed a sensitized response to amphetamine on day 5 compared with day 1, with significantly more hyperactivity at 10–20 minutes (P < 0.001). On day 1, there were no significant differences between the groups (vehicle and JNJ-42253432 treated, thin arrow) at any time point, indicating that JNJ-42253432 did not have any effect on basal locomotor activity nor did the compound modulate amphetamine-induced changes in locomotion. On day 5, JNJ-42253432 (3 mg/kg) pretreatment attenuated amphetamine sensitization (compare day 1 and day 5 vehicle locomotor activity), such that significant differences between drug-pretreated and vehicle-pretreated groups were revealed on day 5. A three-way repeated-measures ANOVA (for treatment × day × time), with repeated measures on both day and time, analyzing the 1-hour habituation, 1-hour post-pretreatment, and 1 hour postamphetamine treatment, on day 1 and day 5, showed a significant treatment × day × time interaction [F(23,483) = 1.59, P = 0.04].
Stress-Induced Suppression of Social Interaction.
Data from this experiment were examined using separate analyses for each behavioral measure in a 2 (stress condition) × 2 (drug treatment) ANOVA, with post hoc Fisher’s analyses performed in which significant main effects or interaction terms were observed. Analysis of overall social activity revealed significant main effects of both stress condition [F(1,28) = 15.96, P < 0.001] and drug treatment [F(1,28) = 8.30, P < 0.01]. These findings not only recapitulate our previous studies showing that foot-shock exposure led to reduced social interactions observed 4 hours after stress cessation (Arakawa et al., 2009), but also demonstrate that administration of JNJ-42253432 increased overall social activity. Note, however, that the interaction between stress and drug conditions was not significant [F(1,28) = 0.005, P > 0.05], suggesting that the action of JNJ-42253432 was independent of the influence of prior foot-shock exposure (Fig. 10). These findings were supported by the analysis of the derived variable, coefficient of social preference, which also demonstrated a significant main effect of drug condition [F(1,28) = 9.42, P < 0.01], but no effect of stress exposure [F(1,28) = 0.67, P > 0.05] and no interaction between stress and drug exposures [F(1,28) = 1.71, P > 0.05]. These data suggest that JNJ-42253432 significantly increased motivation to interact with social partners irrespective of whether experimental subjects had been exposed to stress. Terminal (approximately 6.25 hours postdose) plasma exposures of JNJ-42253432 was 141 ± 10 ng/ml. To assess the specificity of both stress and drug effects on general activity, total crossings were analyzed and are presented in Fig. 10B. As expected, foot-shock exposure led to a significant suppression in total crossings [F(1,28) = 6.97, P < 0.05], whereas drug condition had no significant effect on total crossings [F(1,28) = 0.51, P > 0.05]. The interaction between stress and drug condition was also nonsignificant [F(1,28) = 0.02, P > 0.05]. Overall, these findings suggest a very selective socially facilitating effect of JNJ-42253432, as indexed by increases in overall social activity and the preference to interact with a partner irrespective of prior stress history. This latter effect recapitulates our recent work showing that administration of an alternative P2X7 receptor antagonist, A-804598, had no effect on the foot-shock–induced suppression of exploratory activity in a novel environment (Catanzaro et al., 2014).
Chung Model of Neuropathic Pain and CFA-Induced Inflammatory Pain.
Animals prepared with Chung lesions demonstrated a prominent tactile allodynia at 10–14 days postoperatively (Fig. 11A). Animals that exhibited a tactile threshold of 3 g or less were considered significantly allodynic and were selected in the study. Subcutaneous delivery of gabapentin (200 mg/kg) resulted in a significant reversal of the allodynia that lasted in excess of 240 minutes. JNJ-42253432 (10 mg/kg s.c.) did not produce any significant reversal of mechanical allodynia; terminal plasma (after 4.25 hours) exposure of the compound was approximately 199 ± 19.3 ng/ml. No effect on contralateral thresholds was noted for any treatment. These results show that gabapentin had an antiallodynic action of an expected magnitude and duration, whereas JNJ-42253432 did not exhibit an efficacy. The inflammatory stimulus CFA caused mechanical hyperalgesia of the rat hind paw as seen in Fig. 11B. Oral administration of diclofenac (10 mg/kg) significantly increased mean paw compression thresholds 2 and 4 hours after administration compared with baseline values. Oral administration of JNJ-42253432 (10 mg/kg) had no significant effect on mean paw compression thresholds at any time point tested after administration compared with vehicle-treated animals. Again, analysis of terminal plasma samples revealed exposure of 163 ± 7 ng/ml of JNJ-42253432.
JNJ-42253432 is a high-affinity P2X7 antagonist that penetrates into the brain efficiently with a brain to plasma ratio of 1 in rats. This compound exhibits significant improvements over the previous compound described by this group, JNJ-47965567 (N-((4-(4-phenyl-piperazin-1-yl)tetrahydro-2H-pyran-4-yl)methyl)-2-(phenyl-thio) nicotinamide; Bhattacharya et al., 2013), both in terms of oral bioavailability, CNS penetration, and physicochemical, pharmacokinetic, and pharmacodynamic properties. JNJ-42253432 exhibits a different and unique profile from JNJ-47965567. JNJ-42253432 engages rat brain P2X7 for a longer duration than JNJ-47965567; even though the affinity for rat P2X7 and rat plasma EC50 are comparable between the two compounds, JNJ-42253432 produces efficacy at a significantly lower dose than JNJ-47965567, probably due to increased partitioning into the brain tissue coupled with increased residence time at the rat brain P2X7. For example, whereas a 30 mg/kg dose of JNJ-47965567 did not attenuate IL-1β release in the rat brain, a 10 mg/kg dose of JNJ-42253432 blocked the same pharmacodynamic effect. Likewise, at a 10-fold-lower dose (to JNJ-47965567), JNJ-42253432 attenuated amphetamine hyperactivity in rats. Overall, JNJ-42253432 is a more optimal P2X7 compound that will help delineate the role of P2X7 in the CNS (patho)physiology.
As noted in Results, JNJ-42253432 increased serotonin levels that were later confirmed to be due to SERT binding at a 10 mg/kg dose or higher. The increase in serotonin after JNJ-42253432 administration was not due to a direct effect of P2X7 antagonism because other P2X7 compounds devoid of SERT activity did not cause a similar increase in serotonin (L. Aluisio and P. Bonaventure, unpublished data). There was a clear separation of P2X7 and SERT (Fig. 7B) occupancy; for this reason, the compound was not tested at higher doses except for the EEG study (Fig. 8). At the time of the pharmaco-EEG study, we were not aware of the effect of JNJ-42253432 at SERT. Since then, we have been prudent and cautious in dose selection for in vitro studies with JNJ-42253432, because our main goal was to understand the role of P2X7 in the models used in this study. From a therapeutic standpoint, and from the angle of drug discovery, adding a SERT blockade to P2X7 antagonism for CNS diseases, such as depression, is not necessarily a bad approach. It will indeed be very interesting to use this compound in various stress-induced models of depression (chronic unpredictable stress or chronic mild stress) at both low and high doses to address the role of P2X7 alone or in combination with SERT. Interestingly, at a 40 mg/kg dose (should be high for P2X7 + SERT occupancy), JNJ-42253432 did not have any effect in an acute model of depression (DRL-72) (Bhattacharya et al., 2014). As seen in Fig. 7B, doses of 3 mg/kg are SERT sparing but still high for P2X7 occupancy; as such, the efficacy observed for attenuating amphetamine hyperactivity is probably a result of P2X7 blockade. This will be the third publication indicating a role of the P2X7 pathway in amphetamine hyperactivity (Bhattacharya et al., 2013; Csölle et al., 2013). It will be an interesting avenue of science to follow to elucidate the mechanism of the interaction of amphetamine and P2X7; preliminary data indicate that the effect of P2X7 antagonists on attenuating amphetamine hyperactivity is independent of dopamine (L. Aluisio and P. Bonaventure, unpublished data).
The main effect of JNJ-42253432 in the present pharmaco-EEG studies appears in the decrease of power in the α-1 band: EEG α oscillations have a comparable, “translational” frequency range definition between rodents and humans (Jobert et al., 2012). The α rhythms are believed to be modulated by thalamocortical and corticocortical interactions facilitating or inhibiting the transmission of sensorimotor information between subcortical pathways and the retrieval of information from cortical storage. These thalamocortical circuits generate a large network oscillatory activity in the α frequency range during cortical operations. Functionally, α oscillations in humans are seen as an indication of relaxation (occur with eyes closed from mainly occipital origin) and are thought to reflect a mode of information transfer within the networks and are thus predictive of cognitive performance (Babiloni et al., 2013). As such, the appearance of α waves has been associated with lapses of attention: this rhythm may serve to functionally disengage and reduce the processing capabilities of a given brain region. The α activity is decreased in engaged brain regions, whereas it increased in disengaged regions (Klimesch et al., 2007). In general, Θ and γ activities are accompanied by reduced synchrony in network α rhythm. The findings with JNJ-42253432, at the doses of 2.5 and 10 mg/kg, most relevant for P2X7-specific activity, point toward a reduction of α-1 power, although without effects on the Θ and γ bands (some effects on Θ and γ were seen at 40 mg/kg, but it cannot be excluded that these effects were exerted via the compound’s SERT activity). Correspondingly, in rats, we have seen in earlier studies that increases in α-1 band power are mainly associated with stereotypic behaviors as can be seen after dopaminergic manipulations such as with amphetamine. Although only tested at the (in retrospect) high dose of 40 mg/kg, JNJ-42253432 seems to have the potential to augment the typical amphetamine-induced α rhythms.
The social interaction test is a behavioral model that is exquisitely sensitive to the influence of a variety of situational characteristics and physiologic influences. In particular, and most relevant to the present context, is the utility of this paradigm for the assessment of sickness-like behavior, which is known to be mediated by central cytokines (Dantzer, 2004). Because exposure to stressful procedures has been shown to activate neuroimmune processes and, in particular, increase IL-1 mRNA and protein expression, we sought to test the ability of JNJ-42253432 1) to reverse the foot-shock–induced suppression of social investigation, which was reversed by intracerebroventricular administration of an IL-1 receptor antagonist in prior studies (Arakawa et al., 2009); and 2) the direct influences of JNJ-42253432 on social behavior, because basal fluctuations in cytokines have been suggested as key mediators of the depth and degree of social interactions even under unchallenged circumstances (Hennessy et al., 2014). When JNJ-42253432 was tested in our rodent model of social interactions, it produced a robust increase in overall social activity and social preference, but did not alter the foot-shock–induced suppression in social interactions. These promising effects are commensurate with a growing body of literature implicating a key role for IL-1 in both consequences of stress and the propensity to engage in social interactions (Arakawa et al., 2009; Hennessy et al., 2014). Moreover, the lack of aberrant side effects in the form of social avoidance, reduced social behavior, and no indications of nonspecific actions on either general locomotor activity or levels or the plasma corticosterone response (data not shown) evoked by the test situation are strongly encouraging and support further research and development efforts with this compound.
However, the question remains that if foot-shock–induced alterations in social behavior are mediated by IL-1β (as our prior studies indicate), then what does this tell us about the role of P2X7 as a putative mediator of IL-1β release during foot-shock. Unfortunately, studies demonstrating the functional release of IL-1 via microdialysis during foot-shock will be necessary to verify that increased tissue content of IL-1β mRNA and protein coincides with increased functional release of IL-1β. Such studies would then permit the assessment of whether JNJ-42253432 is capable of blocking foot-shock–induced IL-1β release. These studies are planned in the near future, while in the meantime we are reliant on behavioral assays that are sensitive to the influence of recent stress history. When comparing the procedures for social behavior assessment used in our prior work to the present studies, it should be noted that Arakawa et al. (2009) used a slightly different procedure that assessed social investigation under conditions in which rats were in close proximity but not able to interact directly. A more detailed description and validation of the restricted-access social investigation model can be found in our recent publications (Deak et al., 2009; Arakawa et al., 2011). Briefly, the limited-access test used previously sustains much higher levels of social investigation initially that then dissipate precipitously because there is no reinforcement provided through actual physical contact. By contrast, this study used full access to a social partner for 10 minutes (a much briefer test). In addition, rats were tested in the present experiment against a nonmanipulated partner, which yields a more specific and subtle effect of foot-shock exposure relative to test situations in which both rats in the social dyad receive identical treatments. Furthermore, Arakawa et al. (2009) exposed both rats in the dyad to identical treatments for both stress and drug conditions, which likely amplified the effect of both foot-shock and the IL-1 receptor antagonist in those studies. The modified social interaction test employed in this study was selected because it produces a more comprehensive behavioral profile of the animal (and thus stress and drug treatments) through detailed scoring and analysis of rodent behavior directed toward a peer than automated testing chambers. Although future studies will clearly be necessary to fully evaluate the efficacy of JNJ-42253432 to reverse stress-induced alterations in behavior, these findings raise the novel and intriguing possibility that P2X7 antagonism may provide a novel therapeutic intervention for ameliorating social deficits associated with a range of neuropsychiatric conditions.
The lack of efficacy observed in both neuropathic and inflammatory pain models with JNJ-42253432 is surprising given that the P2X7 knockout mice are protected against both forms of pain (Chessell et al., 2005) and several publications exist on this mechanism in models of pain, as reviewed elsewhere (Alves et al., 2013; Chrovian et al., 2014). Our previous publication with the P2X7 antagonist JNJ-47965567 demonstrated a transient but modest efficacy in neuropathic pain (Bhattacharya et al., 2013). Our data (modest efficacy with JNJ-47965567 to no effect in this study with JNJ-42253432), along with the two failed clinical trials in rheumatoid arthritis (Keystone et al., 2012; Stock et al., 2012) using P2X7 antagonists, raise this dilemma of the utility of P2X7 antagonists as analgesics (Bhattacharya et al., 2011). Whether P2X7 antagonism has therapeutic benefits in CNS disorders, such as in neuropsychiatry and neurodegeneration, remains clinically untested. The emergence of the role of P2X7 in neuroinflammation, coupled to the role of activated glia in various CNS disorders, makes P2X7 an attractive CNS drug target in which modulation of glial function may exert clinical benefits to CNS pathology.
The authors thank Jason Rech, Hong Ao, Qi Wang, Natalie Welty, Ian Fraser, Abdel Ahnaou, and Heidi Huysmans for assistance during the course of the project.
Participated in research design: Ceusters, Lovenberg, Carruthers, Bonaventure, Letavic, Deak, Drinkenburg, Bhattacharya.
Conducted experiments: Lord, Aluisio, Shoblock, Neff, Varlinskaya.
Performed data analysis: Lord, Aluisio, Shoblock, Neff, Varlinskaya, Deak, Drinkenburg, Bhattacharya.
Wrote or contributed to the writing of the manuscript: Lord, Aluisio, Varlinskaya, Deak, Drinkenburg, Bhattacharya.
- Received July 21, 2014.
- Accepted September 29, 2014.
B.L. and L.A. contributed equally to this work.
This research was supported by Janssen Research & Development, LLC. All authors except T.D. and E.I.V. are full-time employees of Janssen Research & Development, LLC.
- analysis of variance
- area under the curve
- 2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate tri(triethylammonium)
- complete Freund’s adjuvant
- central nervous system
- N-((4-(4-phenyl-piperazin-1-yl)tetrahydro-2H-pyran-4-yl)methyl)-2-(phenyl-thio) nicotinamide
- purinergic receptor P2X ligand-gated ion channel 7, subtype 7
- serotonin transporter
- Copyright © 2014 by The American Society for Pharmacology and Experimental Therapeutics