Potential Targets in the Treatment of Urinary Incontinence

Abstract

Urgency and involuntary urine loss are distressing problems for both men and women. Attempts to block the primary cholinergic excitatory input to the bladder have led to a number of anticholinergic agents, but treatment with anticholinergics, the current first-line therapy, is not always effective. Metabolic and growth factor receptor targets are being investigated as a way to control the signal transduction process that leads to bladder contraction as well as the release of calcium that triggers this process. Because of the redundancy in mechanisms that promote bladder contraction, it is necessary to investigate multiple targets. Medical research is also focusing on purinergic receptors, the targeting of normally silent C-fibers that become activated due to neuroplasticity in the bladder, nerve-growth-factor blockade, and the blockade of neurotransmitters that control the bladder. Moreover, a strong correlation appears to exist between depression and the occurrence of incontinence, and there is evidence that points to a genetic link. However, the problem, with attacking this multiplicity of sites lies in establishing therapeutic efficacy and a high degree of specificity.

In both men and women, urgency and involuntary urine loss can be distressing. In addition to the loss of urine itself, symptoms of urgency are disturbing to the patient. Therefore, current research seeks to target mechanisms that will control urgency. The bladder contracts and empties by default. It is designed to eliminate waste, and will continue to empty even in the case of a neural disorder. If the bladder was not able to empty itself, urine would create back-pressure in the kidneys, and death would ensue. Therefore, if one mechanism is blocked, another mechanism takes over to help keep the bladder functioning. As a result, it is very difficult to turn off a detrusor contraction.

The bladder is the only internal viscera, except for the diaphragm, that relies on input from the central nervous system (CNS). The heart, kidneys, and other organs can be transplanted because neural input only plays a regulatory role for these organs, but nerve input is essential for the bladder, and many of its redundancy mechanisms ultimately rely on calcium. Yet it is difficult to determine which of the many potential targets will be effective in the development of treatment of urinary disorders. Specificity is a key issue with regard to CNS targets because these targets potentially affect all autonomic functions. Investigation of risk factors and genomics is expected to lead to linkage studies to determine the mechanisms behind the high prevalence of overactive bladder (OAB).

Current Incontinence Literature

At the World Health Organization meeting in 1998, members of an expert panel from around the world reviewed evidence-based research on drugs for the treatment of incontinence. They found little evidence to support the use of anticholinergic agents.¹ The literature on incontinence revealed an approximate 15% success rate in curing the disorder and a 30% to 50% improvement rate with the use of oxybutynin, propantheline and tolterodine, the gold standards in incontinence treatment. Other muscle relaxants have been examined, as well as nonsteroidal anti-inflammatory drugs (NSAIDs). Unfortunately, much of the literature on the treatment of bladder disease consists of anecdotal reports without placebo controls. While some drugs may have demonstrable efficacy in certain patients, evidence-based research in this area is sparse. The literature does not provide answers to the question of why certain drugs work in some patients and not in others.

Mechanisms of Bladder Contraction

When evaluating agents for treating incontinence, it is necessary to consider both physiology and pharmacology. There are two mechanisms whereby the bladder smooth muscle contracts. One involves excitationcontraction coupling, and the other is pharmacomechanical. Smooth muscle is an intrinsically excitable tissue. When stretched, it responds by contracting (actual tone); its activity can also result through a nerve signal from the CNS. Bladder contraction through either of these two routes leads to urine loss.

The excitation-contraction mechanism generates an action potential without neural input. When a strip of bladder is stretched, it quivers and shakes, generating force. These contractions rely on the intracellular release of calcium from the endoplasmic reticulum (Figure 1). Certain inward currents can be blocked by inhibiting channel activation, thereby preventing intracellular calcium release. Alternatively, the pharmacomechanical mechanism and signal transduction amplification can be blocked with anticholinergics to inhibit this bladder activity. The latter approach has been widely used to combat urge incontinence.

Figure 1.

Distention of the bladder activates voltage-gated channels, resulting in a rise in cytosolic Ca⁺², which initiates interaction between actin and myosin, producing phasic contractions. If sufficient activation of bladder afferents occurs, a micturition reflex allows release of acetylcholine, which binds to muscarinic receptors and opens receptor-operating Ca+2 channels. Influx of Ca⁺², activation of the inositol phosphate pathway, and release of Ca⁺² from intracellular stores cause the large increase in Ca⁺² resulting in detrusor contraction and bladder emptying.

ATP

Adenosine triphosphate

NGF

Nerve growth factor

OAB

Overactive Bladder

PDE

Phosphodiesterase

SCI

Spinal cord injury

TTX

Tetrodotoxin

Receptor Blockade

The primary excitatory input to the bladder is cholinergic. It should be possible to block that input and hence block the nerve signal responsible for contraction and urge incontinence. This rationale has lead to a plethora of anticholinergic agents, primarily nonselective M2 and M3 antagonists. Three selective M3 antagonists and an M1 agonist are now in trials. The recent focus has been on reducing side effects. None of these agents appears to be superior in terms of efficacy, and it is possible that we have reached maximum efficacy with anticholinergics.

It should be noted, that if one of the two muscarinic receptors expressed by the bladder is altered, the other is altered as well. For example, if the M3 receptor is blocked, the M2 receptor, which turns off muscle relaxation, is also affected. Normally, activation of M3 triggers contraction. In contrast, M2 activation with acetylcholine, working through an adenylate cyclase mechanism, promotes relaxation. Even with gene therapy to alter M-receptor expression, it may not be possible to gain specificity because of the interaction of these two receptors.

Receptor blockade on bladder smooth-muscle cells blocks neurotransmitter action, and there are agents that prevent neurotransmitter release. For example, M1 agents and the neuropeptide galanin block acetylcholine release. Alternatively, interfering with the signal transduction mechanisms responsible for raising calcium in the cell and causing contraction could be used to inhibit bladder contractions. These drugs are probably not bladder-specific, but administration intravesically may provide the needed specificity.

Metabolic and Growth Targets

The signal transduction process, which relies on intracellular messengers and leads to contraction, contains redundancy in transduction pathways. This has been exploited in treatments for malignancy and cellular growth. Thus many processes related to growth and metabolism could theoretically be exploited for treatment of OAB. Another approach is to attack the processes that signal calcium release. Agents are being developed that affect metabolism, especially in the mitochondria which control the storage of calcium necessary for the bladder muscle to contract. Like a low gas gauge in a car, a sensor signals that calcium is low so that more is released. Mitochondria might act as that sensor.

Multiple Targets

With so many agents available and several targets to focus on, it is necessary to determine which of these actors show promise. Work on purinergic receptors has shown that transgenic knock-outs of the P2X3 receptor result in bladder enlargement due to blocked sensory signaling. There is significant excitatory purinergic transmission to the bladder in animals, although human contraction of the bladder does not normally rely on triphosphate (ATP). In disease processes, however, ATP-induced purinergic contraction is more prevalent. A purinergic antagonist, suramin, has been evaluated for treatment of prostate cancer.² Although it does not cause bladder dysfunction, it blocks ATPase and could raise ATP levels. This result casts some doubt on the value of nonselective P2X antagonists for overactive bladder conditions.

Other approaches may rely on blocking stretch-gated channels, endothelin, histamine, or bradykininreceptor antagonists. With the advent of Viagra, recent interest has centered around phosphodiesterase (PDE) inhibition. The problem with utilizing strategies relying on PDE, especially the phosphodiesterase type III inhibitors, is the potential for cardiac toxicity. Although PDE inhibition may cure their incontinence, patients could die of arrhythmias. Work by Uckert and colleagues in Germany has revealed PDE type I inhibitors that have very weak effects on the bladder.³ It is not likely that these agents will play a major role in controlling OAB, but they may provide some value in combination with anticholinergics.

Ca⁺² channels offer another target to prevent bladder excitation—both calcium antagonists and potassium channel openers. But calcium antagonists lack efficacy. There has been some research done with drugs that open potassium channels, especially at Zeneca Pharmaceuticals.⁴ If potassium channels are opened, the result is relaxation, because bladder muscle becomes hyperpolarized and calcium levels cannot rise. However, the vasculature is very sensitive to potassium channel openers, especially calciumgated potassium channel openers, which exert a profound action on the vasculature, and hypotension is a very likely outcome. Selectivity is difficult to achieve in potassium channels acting in the bladder, and administration of these drugs intravesically has resulted in a very weak effect. Other drugs that target potassium channels and are used for diabetes do not appear to have an effect on the overactive bladder. Other agents, such as minoxidil, the agent used for hair loss, relax the bladder only at doses that could lead to hypotension. These mixed results suggest that although many strategies make sense theoretically, anecdotal data and side-effect profiles fail to support the notion that these classes of drugs prevent detrusor overactivity.

Pharmacological research has focused on growth processes, cell interactions, and malignancy, especially targeting G-proteins. A drug targeting G-protein coupling might be very useful, because M2 and M3 receptors act through G-protein. Unfortunately, the data for all these approaches fail to suggest any beneficial effects, even in combination with anticholinergics. In addition, as is seen with the use of terodiline, toxicity in the form of cardiac side effects (cardiac arrhythmias following blockade of muscarinic receptors and calcium channels) could be a limiting factor.

Calcium channel blockade could be a useful strategy for the aging bladder. In the elderly, the bladder muscle starts wearing out, resulting in poor muscle contraction. This may be due to apoptosis and fibrosis, two independent processes associated with aging. Fibrosis and denervation are not likely to lead to OAB. By preventing changes in the handling of calcium (inappropriate elevations), muscle cell apoptosis may be prevented.

Neuroplasticity

The theory that nerves are static and do not change over time is incorrect. There are many examples in neurobiology of what is known as neuroplasticity. During development, from the neonatal period to adult, transmitter phenotypes can change. Neuroplasticity can also occur in disease states. Plasticity occurs when, for example, one remembers recent activity, such as reading the words on this page—a phenomenon known as long-term potentiation of the hippocampus. Strengthening of synapses occurs in a process referred to as “plasticity,” and in fact, it is easier to form synapses in the CNS than it is to break them. There is plasticity in the urinary tract, which can be demonstrated by the occurrence of voiding reflexes. When human infants are tickled in the perineal region during the first two days after birth, they urinate. Although that reflex soon stops, it returns when the spinal cord is cut in spinal cord injury (SCI). Patients with SCI often demonstrate the same “trigger” voiding.

SCI and inflammation unmask several reflexes. Change can occur in nerves called silent C-fibers that become activated. Molecular changes develop that can potentially be targeted. Reorganization of nerves also occurs if bladder nerves are cut. In the past, some physicians have proposed cutting nerves to the overactive bladder. This is, however, an ineffective approach, because other nerves take over the function of those that have been cut. If the parasympathetic nerves in the bladder are cut, sympathetic nerves, which normally do not contract the bladder except following sacral rhizotomy, become active. Although initially the bladder becomes quiescent, it eventually becomes hypertonic. In addition, plasticity causes unpredictable neural arrangements that often result in conditions that may be worse than the one being treated. The lower urinary tract and sacral part of the spinal cord are recognized as the most plastic area of the CNS.⁵ This plasticity may be related to the fact that most of the organs innervated in this region have access to the outside world—that is, through the bladder and sex organs.

Examples of OAB states illustrate plasticity. The causes of these states may be very different, yet there may be a common molecular basis. Irritants in the bladder cause inflammatory states, as does bladder infection or interstitial cystitis—all leading to OAB. Obstruction of a woman’s bladder after anti-incontinence surgery, or obstruction in a man’s bladder after prostate surgery, results in OAB characterized by frequency or urination, urgency, and even urge incontinence. Nocturia develops after suprasacral SCI. These different stimuli cause a similar result. In all of these conditions, however, C-fiber afferents are activated, and the effect is found in both humans and animals. Hypertrophy of the afferent neurons occurs in the dorsal root ganglia. Electrophysiology experiments indicate that a spinal reflex becomes enhanced. In the spinal cord, growth-associated protein (GAP-43) surrounds afferent projections in the dorsal horn, showing that specific genes are turned on. GAP-43 expression occurs whether there is a central or peripheral stimulus, or a stimulus to the bladder. The time course varies with these states. Some subtle changes that occur here may be very important for targeting therapy. If these changes occurred only in reawakened C-fibers that are not functional in normal states, one might argue that it might be possible to affect bladder function without changing bowel and other visceral functions.

Nerve Growth Factor

The common denominator for human and animal data in these conditions is nerve growth factor (NGF). NGF is elevated in the bladder following obstruction, inflammation, and SCI. In animals, blockade of NGF prevents overactive bladder, reflex plasticity, and neuronal hypertrophy. The events that have been found in the laboratory causing NGF levels to increase are stretching and inflammation. Stretching the bladder causes calcium release, triggering NGF production. Inflammation will also affect Ca⁺² levels. Persson and colleagues report on producing this result during both in vitro and in vivo studies.⁶ Protein kinase-C in bladder smooth muscle cells is involved. Moreover, of all the known tissue examined, bladder smooth muscle has the highest constituent levels of nuclear transcription factor beta (NFKβ). NFKβ causes the immediate early genes fos and jun to combine to form AP1, which is on the promoter of the NGF gene. NGF from the cell is taken up by sensory and sympathetic nerves and transported through the CNS in retrograde fashion.

When the level of NGF increases, it induces a number of changes. During inflammation, the number of tetrodotoxin (TTX)-resistant sodium channels increases. Sodium channels are responsible for generating an action-potential in nerves. Na+ channels consist of 3 α subunits and two β subunits. The α subunits change with certain disorders and alter the properties of the Na+ channel. Blockade of TTX-sensitive sodium channels leads to respiratory arrest. Knock out transgenic mice for the TTX-R Na+ channels exhibit reduced sensitivity to pain with no other major abnormalities. Many pharmaceutical companies are now looking for drugs that block the TTX-R Na+, because postulating such an agent could be useful to treat chronic pain states. One antagonist has been developed by Glaxo Wellcome for pain and inflammation.⁷

SCI causes TTX-sensitive sodium channels to increase in number, but not all neurons express the type III and the SNS α subunits. Therefore, it is possible to have C-fibers that change with respect to one subunit of the Na+ channel that confers unique electrophysiologic properties, such as lowered thresholds for firing, or spontaneous and burst firing. In the bladder C-fibers are activated after obstruction. Likewise, following cystitis C-fibers are also activated. In this latter case these nerves appear to express the TTX-resistant channel.⁸ The use of antisense oligonucleotides targeting the TTX-R channels prevents bladder overactivity due to cystitis, although the effect is very weak. In the absence of a selective TTX-R Na+ channel antagonist, intrathecal antisense oligonucleotides are currently the only viable approach.

Neurotransmitter Control of the Bladder

Several neurotransmitters of the CNS control the bladder. Some are accelerators and some act as brakes. Currently we have only a limited understanding of their role. Certain areas of the brain, especially the prefrontal cortex, are altered in many disease states associated with OAB-Alzheimer’s disease and depression are two examples—and OAB itself is associated with certain neurotransmitter changes. Dopamine and glutamate, for example, are accelerators to the bladder. Thus, administering antagonists to these might actually block OAB. Serotonin, however, is an inhibitor, and currently 5-HT-1A and serotonin reuptake inhibitors are being evaluated.

Specificity is a key factor, therefore it is necessary to identify which patients these agents will affect. There are many potential targets, starting with the bladder, glutamate receptors, and the sacral cord, among others, for which there are pharmacologic data for both animals and humans. These targets include NMDA receptors, alpha-2, 2A and alpha-1 receptors, delta-opiate receptors, GABA, and bradykinin receptors (Figure 2). Theoretically, it is possible to target all these agents.

Figure 2.

Potential receptor targets for control of incontinence.

Genomics

Two studies from Scandinavia show that 100% of children with enuresis develop OAB and urge incontinence as adults.⁹ There is also data indicating that lower urinary tract symptoms (LUTS), storage symptoms, and benign prostatic hyperplasia (BPH) are highly correlated with hypertension and overactivity of the sympathetic nervous system. Hypertension is inherited, and the children of hypertensives tend to manifest autonomic dysregulation. These children have accentuated elevations in blood pressure with bladder distension.

The spontaneously hypertensive rat (SHR) is an excellent screening model and is being used by two pharmaceutical companies to screen their drugs for treatment of OAB. In this animal, NGF is elevated and sympathetics are overexpressed. When SHR are bred with normal rats a bell-shaped distribution for blood pressure is obtained. Although urine specific gravities and volumes are the same in normotensive and hypertensive rats, urinary frequency (overactive bladders) correlates with elevated blood pressure (Figure 3). SHR demonstrate urinary frequency and unstable bladder contractions compared to genetic controls (WKY). However, bladder smooth muscle activity as assessed with in vitro muscle bath experiments in SHR is similar to the muscle activity witnessed in the control population. Bladders from SHR manufacture more NGF than the genetic controls. Because both noradrenergic sympathetic nerves and visceral afferents express the receptors for NGF and can respond to this neurotrophin, these nerves may play a role in the induced neural plasticity leading to OAB. Unfortunately, destruction of noradrenergic sympathetic nerves in SHR with 6 hydroxydopamine fails to reduce bladder overactivity. However reminiscent of the cystitis model of OAB, intrathecal antisense OGN against the TTX-R Na+ raises the volume threshold for micturition and reduces unstable bladder contractions in the SHR. Therefore in obstruction, inflammation in the SHR models of OAB, NGF is elevated. In cystitis and SHR, downregulation of the TTX-R Na+ channel, thought to be expressed because of increased NGF, inhibits bladder overactivity.

Figure 3.

Linear association between blood pressure and urinary frequency.

Anxiety Disorders and Depression

A recent paper from Columbia University discusses isolation of a gene contributing to panic/anxiety disorders.¹⁰ Based on linkage studies of families with high levels of anxiety, this paper reported that 70% of the subjects had mitral valve prolapse and 100% of the probands had OAB. Bed-wetting occurs during rapid eye movement sleep (REM) when there is decreased firing of neurons in the raphe nuclei in the brainstem. The raphe nuclei supplies descending serotonergic input to to the spinal cord. REM sleep is when serotonin levels are lowest.

Fifteen percent of depression is due to factors of inheritance.¹¹ Although there are a lot of conflicting data, a significant factor is that the expression of the serotonin transporter gene is apparently lower in individuals with depression. Moreover, autopsy studies show that women produce 50% less serotonin in the brain when compared with men.¹² It is postulated that when the level of serotonin is low, women become depressed, and women have higher rates of bipolar depression and anxiety disorders than men. The question then is the following: If serotonin levels are lower in women, and serotonin is normally an inhibitor of OAB, are women predisposed to OAB? In our incontinence clinic we counted depressed patients in one of two ways: either by scoring with a depression index, or by examining the history of psychiatric treatment for depression. We compared 100 consecutive patients attending our incontinence clinic with 100 patients who presented with other urological disorders but not incontinence or bladder complaints. Of the cohort group, 17% had a history of depression. Those with stress incontinence had the same incidence. In contrast, among those with urge and mixed incontinence, over 60% had a statistically significant history of depression or tested positively for depression.¹³ The data suggests that depression is a significant factor in urge incontinence, and studies indicate that the use of antidepressant drugs is high in patients with OAB.¹⁴ The association of depression with lowered serotonin levels and bladder overactivity has been demonstrated in chlorimipamide treated rats (unpublished observations). Depression, manifested as reduced feeding, motor and sexual activity, is correlated with urinary frequency and non-micturition bladder contractions (Figure 4). Chlorimipramine treated rats have reduced firing of raphe neurons and lower levels in 5-HT in autonomic centers of the CNS.¹⁵ Similar bladder overactivity has been demonstrated in rats treated with reserpine and p-chlorophenylalanine, which also reduce 5-HT in the CNS.^16,17 This suggests that there may be a neurochemical basis for the early onset of OAB in certain individuals and families. It also suggests that research into developing markers for these altered neurochemical responses might be fruitful. As a whole, the data on depression suggests that there may be something pharmacologically different, from a genomic perspective, with those individuals who develop depression.

Figure 4.

Awake CMGs two months after neonatal treatments with chlorimipramine (22.5mg/kg sc twice daily postnatal days 8–22). Unstable bladder contractions develop coinciding with urinary frequency.

Conclusion

There is not a shortage of sites for the potential reduction of bladder activity to benefit OAB and urge incontinence. The challenge remains in finding useful agents that have not only therapeutic efficacy but an excellent site profile, that is, improved specificity, especially newer agents that can act on the sensory nerves. The greatest potential will be from agents directly targeting either the bladder or the sensory nerves. Agents that influence bladder afferents have the potential to reduce bladder activity and painful bladder conditions such as interstitial cystitis.

Main Points.

• Specificity is a key issue with regard to CNS targets because they potentially have effects on all autonomic functions.

• A WHO review of evidence-based research on drugs for treatment of incontinence found little evidence to support the use of anticholinergics.

• Recent interest has centered around phosphodiesterase (PDE) inhibition, which may not play a major role in controlling OAB but may have some value in combination with anticholinergics.

• NGF is elevated in the bladder following obstruction, inflammation, and spinal cord injury.

• NGF induces changes in C-fibers such as the expression of TTX-resistant Na+ channels that confer lowered threshold for nerve activation as well as burst or spontaneous firing. That blockade of NGF or the TTX-R Na+ channels represents a potential therapy for OAB.

• Depression appears to be a significant factor in urge incontinence, and the use of antidepressant drugs is high in patients with OAB.

• Research has shown that bladder hyperactivity developed and depression was induced as a result of lowered serotonin activity in the CNS.