- Top of page
- Discussion and conclusions
- Conflict of interest
Overactive bladder (OAB) is a common, chronic and debilitating disorder characterized by urinary urgency often accompanied by incontinence (involuntary urine leakage), frequency (≥8 micturition in 24 h) and nocturia (≥1 awakening per night to void; Lam and Hilas, 2007). OAB has a significant negative effect on the patient's quality of life and emotional well-being. It leads to withdrawal from social activities and isolation, increased risks of falls and fractures (especially in elderly patients), depression, sleep disturbances and associated fatigue (Ouslander, 2004; Chu and Dmochowski, 2006).
OAB is abnormal and involuntary contraction of the detrusor muscle in the bladder during the filling phase. Its pathophysiology appears to be complex and believed to involve peripheral as well as central mechanisms. The first-line pharmacotherapy for OAB involves several muscarinic receptor antagonists, including oxybutynin and tolterodine (Lam and Hilas, 2007). While these drugs have been shown to produce some improvement in OAB symptoms, they are also linked to significant side effects, including dry mouth, constipation, tachycardia, sedation, impaired cognitive function and blurred vision that cause non-compliance and discontinuation of treatment (Erdem and Chu, 2006). Thus, alternative pharmacological strategies are needed to deliver more effective and better tolerated compounds compared to those available at present.
There is a wealth of experimental, preclinical and clinical evidence suggesting that the GABAB receptor is a viable target for the discovery of new pharmacotherapies for OAB. GABAB receptors play an important role in the regulation of bladder activity at several sites in the CNS and periphery (Santicioli et al., 1984; Maggi et al., 1985; Chen et al., 1992; Malcangio and Bowery, 1996; Coggeshall and Carlton, 1997). In several animal models of OAB, baclofen, the orthosteric agonist of the GABAB receptor, reduced signs of bladder overactivity. For example, intrathecal baclofen abolished bladder overactivity induced in female rats by intravesical oxyhaemoglobin administration (Pehrson et al., 2002). Also, baclofen dose-dependently inhibited bladder contractions and decreased micturition pressure in female rats showing bladder overactivity as a result of spinal cord injury (Miyazato et al., 2008). Importantly, baclofen has been shown to alleviate the symptoms of OAB in patients with idiopathic detrusor instability (Taylor and Bates, 1979), neurogenic voiding disturbances (Haubensak, 1977) and non-neurogenic dysfunctional voiding (Xu et al., 2007). However, while the efficacy of baclofen in OAB has been shown in several clinical studies, the drug is not widely prescribed due to its short half-life and long list of side effects.
Here, we evaluated the effects of a novel GABAB positive allosteric modulator (PAM), ADX71441, in experimental models of OAB. GABAB PAMs have long been recognized as a powerful pharmacological approach to achieve activity-dependent efficacy in drug action while offering the potential of being devoid of side effects associated with GABAB receptor agonists (Pin and Prézeau, 2007). ADX71441 was discovered through the lead optimization of a chemical series acting as PAMs, which was identified following a high throughput screening campaign of the corporate chemical library of Addex Therapeutics (L. Tang et al. unpubli. obs.). A series of in vitro and in vivo studies identified ADX71441 as a potent, selective, reversible and orally bioavailable GABAB PAM, which recently received approval for Phase I clinical studies (Addex Therapeutics Web page). In rodents, ADX71441 showed efficacy in the marble burying and elevated plus maze tests relevant for anxiety-like reactivity (M. Kalinichev et al., unpubl. data), supporting and expanding earlier evidence of the broad anxiolytic-like profiles of GABAB PAMs CGP7930 and GS39783 (Cryan et al., 2004; Mombereau et al., 2004; Jacobson and Cryan, 2008). Acute ADX71441 reduced visceral pain in the acetic acid (AA) writhing test and also reduced chronic pain in the monosodium iodoacetate (MIA)-induced model of osteoarthritis, after acute and subchronic administration regimens (M. Kalinichev, unpubl. obs.). Also, ADX71441 was found to markedly and specifically reduce alcohol consumption in mouse models of binge drinking and chronic alcoholism (Hwa et al., 2013). Here, we tested the effects of ADX71441 and baclofen in two rodent models of OAB: (i) bladder overactivity in conscious mice undergoing water overload and (ii) overactivity induced by intravesicle administration of diluted AA in anaesthetized guinea pigs. ADX71441 reduced the signs of bladder overactivity in both models of OAB, suggesting that the GABAB PAM approach has potential for the pharmacological treatment of OAB.
Discussion and conclusions
- Top of page
- Discussion and conclusions
- Conflict of interest
This is the first study in which a GABAB PAM was tested in models of OAB. Previously, baclofen, a GABAB orthosteric agonist, showed efficacy in rodent models of OAB (Pehrson et al., 2002; Miyazato et al., 2008) as well as in several clinical studies (Haubensak, 1977; Taylor and Bates, 1979; Xu et al., 2007). Here, ADX71441, a novel, selective and orally bioavailable GABAB PAM, showed a robust and dose-dependent ability to reverse the signs of OAB in conscious mice and anaesthetized guinea pigs.
The diuretic stress-induced overactive bladder model in conscious mice has been used to evaluate the efficacy of potential therapeutics for OAB. This model has been used increasingly as an in vivo screening assay, as it shows sensitivity to cholinergic antagonists, such as oxybutynin (Yoshida et al., 2010). Here, ADX71441 reduced the number of urinary events, total and average urinary volumes, normalizing the OAB values to those seen in untreated, intact controls. ADX71441 also increased urinary latency, restoring it to that seen in intact controls. The in vitro effects of ADX71441 on urinary parameters showed a good correlation with those in vivo, indicative of a target-related, OAB-reducing efficacy. The magnitudes of the changes in micturition parameters seen in animals treated with 10 mg kg−1 ADX71441 were similar to those seen in oxybutynin-treated animals. In fact, ADX71441 was greater than 10-fold more potent than oxybutynin. Among the micturition parameters monitored in the study, urinary latency and the number of urinary events are likely to be the most clinically relevant and translatable variables. Baclofen was less active in the mouse OAB model, showing only trends of activity at the highest dose administered (6 mg kg−1). It is unlikely that somewhat smaller absolute bladder activity values shown by the reference group in this experiment made it difficult to detect the effect of baclofen, as in all urinary parameters the relative changes in comparison to intact controls were either comparable to the study with ADX71441 (urinary latency) or even larger (number of urinary events, total and average urinary volumes).
As a follow-up study, ADX71441 and baclofen were tested in a model of OAB in anaesthetized guinea pigs. The guinea pig has been considered as one of the most appropriate species to study urinary bladder function since its bladder physiology and neural control are similar to those in humans (McMurray et al., 2006). Moreover, as in humans, bladder voiding in urethane-anaesthetized guinea pigs is largely dependent on cholinergic neurotransmission (Maggi et al., 1988). Recently, a model of AA-induced bladder overactivity in anaesthetized guinea pigs was validated using an antimuscarinic drug, tolterodine, and an NK1 receptor antagonist, netupitant (Palea et al., 2010).
In all experimental groups, AA elicited bladder overactivity characterized by increases in MF paralleled by marked decreases in ICI, BC and a similar trend in ThP. These results are in agreement with previous data obtained in our laboratories using the same experimental model of OAB (Palea et al., 2010). In the present study, 1 and 3 mg kg−1 ADX71441 had a similar effect, increasing ICI and BC and reducing MF, normalizing them to the levels seen following saline treatment (i.e. control values). As the effects of 1 and 3 mg kg−1 on all variables were near maximal (approaching control values), the minimal effective dose of ADX71441 in this model is likely to be less than 1 mg kg−1 (i.v.). All the effects of ADX71441 on the cystometry measures were seen during the first 15 min after its administration, while they were absent at later time points. In an independent pharmacokinetic experiment, plasma concentrations of ADX71441 declined rapidly after i.v. administration of 1 mg kg−1 to guinea pigs. Although the pharmacokinetic data were obtained in conscious animals, plasma concentrations measured in this experiment and the plasma concentrations evaluated at the end of the cystometric measurements were comparable, indicating that pharmacokinetic data can be used in support of the interpretation of the time course of efficacy. The short-lived effect of ADX71441 in the cystometric guinea pig model can therefore be explained by the fast decline in plasma concentrations observed after its i.v. administration, suggesting that concentrations sufficient to produce an effect were present only during the first 15 min, but not later. Indeed, concentrations of ADX71441 at the end of the test, 67 and 195 ng mL−1 (at 1 and 3 mg kg−1, respectively), were below the minimal concentration (∼350 ng mL−1) needed to obtain an in vivo effect (M. Kalinichev et al., unpubl. data). It should be noted that the use of distinct vehicles in the guinea pig OAB study (PEG400/saline for ADX71441 and saline for baclofen) could have contributed to the differences observed for the time profiles of the two drugs.
In five out of 10 animals, 3 mg kg−1 ADX71441 completely blocked the micturition reflex and induced dribbling incontinence almost immediately after its administration. In addition, ADX71441 (3 mg kg−1, i.v.) significantly increased ThP, which accords with the finding that 50% of the animals tested presented urinary leakage, characterized by a strong increase in intravesical pressure during bladder filling as well as dribbling incontinence. These results support and expand those obtained previously, which showed that intrathecal baclofen (0.5 μg) produced dribbling incontinence in 58% of rats (Watanabe et al., 1997). Whether or not ADX71441 at certain doses will induce in incontinence in human patients with OAB remains to be seen.
In guinea pigs, baclofen had modest, albeit significant effects on ICI, MF, BC, while having no effect on ThP and BP. In accord with this, administration of racemic baclofen, 8–16 mg kg−1, to conscious rats resulted in progressive increases in BC, as assessed by cystometry, while causing dribbling incontinence in some animals (Igawa et al., 1993). In another study, baclofen 4 mg kg−1, i.v., attenuated bladder overactivity induced by intravesical administration of oxyhaemoglobin in conscious rats (Pehrson et al., 2002), but was devoid of effect on BC in normal, conscious rats at 1 mg kg−1, i.v. (Kontani and Ueda, 2005). Therefore, modest effects of 1 mg kg−1 baclofen (i.v.) on cystometric parameters were anticipated. The weak activity of baclofen suggests that direct GABAB agonism might be less effective than positive modulation in the models of OAB used in the present study. Indeed, these data indicate that, unlike spinal injury models, diuretic-induced stress models do not elicit changes in GABAergic transmission that can be overcome by direct stimulation of the GABAB receptor. However, enhancing GABAergic transmission by increasing GABAB receptor responsiveness and thus enhancing the effects of normal levels of GABA through the use of PAMs, such as ADX71441, may be effective in this model of urinary incontinence. Alternatively, baclofen may be more effective in guinea pigs than in mice because of the different pharmacokinetic profiles of the compound in the two species. Finally, urinary incontinence induced by AA may be more responsive to GABAB agonism than diuretic-induced stress.
At this point, we can only speculate about the site of action of ADX71441 as various GABAB receptor populations localized on different central and peripheral sites have been implicated in the control of voiding in mammals. From results obtained in previous investigations it was concluded that GABA inhibits the micturition reflex by acting on GABAB receptors localized in several distinct sites, including the pontine micturition centre in the brain, the sacral parasympathetic neurons in the spinal cord, the pelvic ganglions and the urinary bladder (Maggi et al., 1988). Since ADX71441 can easily cross the blood-brain barrier and has a balanced central-peripheral distribution (M. Kalinichev et al., unpubl. data), the effects we observed in the mouse and guinea pig studies could involve any or all of the sites described above.
We hypothesize that the inhibitory effects of ADX71441 in models of OAB are mediated through inhibition of cholinergic neurotransmission, at the central and maybe also the peripheral level, since activation of GABAB receptors was reported to inhibit electrical field stimulation-induced contractions in the guinea pig isolated urinary bladder (Maggi et al., 1985). In the clinic, administration of baclofen is associated with improvements in OAB symptoms in patients with idiopathic detrusor instability (Taylor and Bates, 1979), neurogenic voiding disturbances (Haubensak, 1977) and non-neurogenic dysfunctional voiding (Xu et al., 2007). Taken together, these findings confirm the role of the GABAB system in controlling the micturition reflex and for the first time demonstrate that GABAB receptor PAMs may represent a novel approach to treat OAB. However, to confirm this hypothesis, further studies in conscious animals, using a validated model of bladder overactivity, like rats with chronic bladder outlet obstruction (Igawa et al., 1993), are needed to further support the therapeutic potential of ADX71441 for the treatment of OAB.
The possibility that the muscle-relaxant properties of ADX71441 contributed to its efficacy in our models of OAB in mice and guinea pigs cannot be excluded from the present results. In mice, the muscle-relaxant effects of ADX71441 were probably mild, as it has previously been found that 10 mg kg−1 causes a mild reduction in the rotarod activity in mice in an accelerated setup, while being inactive in a constant speed setup (M. Kalinichev et al., unpubl. data). A small reduction in rotarod activity was also seen in rats with 10 mg kg−1 ADX71441 (M. Kalinichev et al., unpubl. data). In addition, ADX71441 has been shown to dose-dependently reduce spontaneous locomotor activity (sLMA) in mice and rats. These effects may not be centrally-mediated sedative effects of the compound per se, as ADX71441 and pentobarbital have very distinct sleep/wake EEG and neck muscle EMG profiles, while resulting in similar reductions in sLMA in rats (M. Kalinichev, unpubl. data). Also, ADX71441 can dose-dependently and transiently reduce body temperature in mice and rats (M. Kalinichev et al., unpubl. data). However, it is unlikely that the hypothermic effects of the compound had any effect on its efficacy in models of OAB. In guinea pigs, the hypothermic effects can be fully excluded, as the body temperature of the anaesthetized animals was maintained at 37°C by automated heating pads. In mice, the hypothermic effects of ADX71441, if any, were probably mild, as no shivers or piloerection were noticed in the treated animals.
Furthermore, ADX71441 has been shown to reduce the number of buried marbles in mice [minimum effective dose [MED] 3 mg kg−1−1) and increase open arm exploration in the elevated plus maze test in mice and rats (both MED 3 mg kg−1) showing a good in vitro/in vivo correlation, indicative of target-related, anxiolytic-like efficacy (M. Kalinichev et al., unpubl. data). Also, ADX71441 reduced acute visceral pain in the AA-induced writhing test in mice (MED 3 mg kg−1) and alleviated chronic osteoarthritic-like pain in the MIA-induced osteoarthritis model in the rat (MED 15 mg kg−1) after acute and chronic treatment (M. Kalinichev, unpubl. obs.).
In conclusion, ADX71441, a novel, selective and orally bioavailable GABAB PAM, showed efficacy in two models of OAB in mice and guinea pigs with a good pharmacokinetic/pharmacodynamic relationship. These results provide strong evidence that an orally active GABAB PAM can be considered as viable approach for the treatment of various forms of OAB in humans.