New urologic frontiers explored for neurotoxins

March 1, 2013

OnabotulinumtoxinA (Botox) has earned FDA approval for the treatment of overactive bladder in adults who cannot use or don't adequately respond to anticholinergic drugs, and its potential uses in urology may not end there.

OnabotulinumtoxinA (Botox) has earned FDA approval for the treatment of overactive bladder in adults who cannot use or don’t adequately respond to anticholinergic drugs, and its potential uses in urology may not end there. In this interview, Paulo Dinis, MD, PhD, discusses new research on onabotulinumtoxinA as well as resiniferatoxin, and how they may benefit patients with various urologic disorders. Dr. Dinis is associate professor of urology at Hospital de São João in Porto, Portugal. He was interviewed at the International Society for the Study of Bladder Pain Syndrome in Porto by Urology Times Editorial Consultant Philip M. Hanno, MD, MPH, professor of urology at the University of Pennsylvania, Philadelphia. Dr. Dinis has served as an adviser for Allergan.

 

You and your department have been very interested in urologic aspects of resiniferatoxin (RTX), which is considered investigational in the U.S., and botulinum toxin. In 2004, you showed the beneficial effects of RTX in a rat model of urinary inflammation. Do you have any thoughts as to why the randomized controlled trial of RTX in bladder pain syndrome failed, and will we see another trial in the foreseeable future?

RTX is a very unstable compound. Once you prepare it, you should not expose it to light, and you should not wait more than 1 and a half to 2 hours before applying it. In addition, it degrades very quickly, and it adheres to plastic. My guess is that something happened to destabilize the compound between the lab and the patients’ bladders.

Based on my experience-I worked personally on the experimental models of this agent-RTX blocks the TRPV1 receptor, and this in turn blocks action potentials in afferent C-fibers. This is documented. And it worked in my pilot study. That alone doesn’t mean a lot, but there are other studies with similar results.In my view, RTX should be stabilized and it should be retried, because it’s very easy to apply and if it works, it will potentially offer patients months of well-being.

 

In 2004, you studied 12 patients with BPH and associated storage lower urinary tract symptoms with results that suggested that intravesical desensitization of bladder C-fibers with RTX administered intravesically might be useful in the treatment of this condition. Three years later, your group reported on the use of RTX in 23 patients with overactive bladder. You found that 69% of patients wanted repeat instillation after 3 months, compared to 17% in the placebo group. Has there been further work with this compound in voiding dysfunction, and are we likely to see further studies leading to regulatory approval in the future?

Unfortunately, after the placebo-controlled trials, we had difficulty, medicolegally and ethically, in continuing to use RTX. That’s why we are not using it clinically at this point. In my department, we are more than eager to see other, perhaps larger, randomized clinical trials with a stabilized form of RTX and a uniform method of application.

 

Let’s turn to the use of botulinum toxin. Your group has reported very good results for intraprostatic injection of Botox for chronic urinary retention (which is considered an off-label use in the U.S.). You had a 76% success rate at 1 month and an 81% success rate at 3 months in restoring micturition with acceptable urinary residuals. Efficacy lasted 18 months in long-term follow-up studies. Can you comment on these data?

As you know, botulinum toxin inhibits exocytosis or any other intracellular mechanism dependent on the SNARE protein complex, such as intracellular molecule traffic or the liberation of neuromediators. And when I refer to neuromediators, I’m not just referring to acetylcholine as was discovered in striated muscle. I’m also including adrenaline and mediators of sensory fibers, such as substance P, CGRP (calcitonin gene-related peptide), and glutamate, release of which is also hindered, especially the peripheral neuropeptides.

In the case of the prostate gland, our goal was to explain in the laboratory what we saw in the clinic. Botulinum toxin virtually paralyzes the peripheral nerves, regardless of what type they are, resulting in a loss of autonomic tonus to this organ. It’s worse than the prostate in a paraplegic patient, because paraplegic patients still have working sensory fibers. These patients, as the urologic community well knows, have a very small prostate. This is also the case with some long-term diabetics or juvenile diabetics.

What we think is happening is that loss of tonus induces shrinkage of the organ through apoptosis of cells, and this is mainly due to lack of sympathetic tonus. In fact, if we give our laboratory model phenylephrine systemically along with local onabotulinumtoxinA, the prostate does not shrink.

When we transferred this knowledge to patients in the clinic, we injected 100 units of onabotulinumtoxinA transrectally at four sites in the prostate. Our cohort was elderly and very frail, suffered from urinary retention, and had general health that was so poor that surgery was not possible. All patients had indwelling catheters. A month after the injections, the results were fantastic. Eighty-one percent of the patients were able to resume spontaneous voiding at 3 months. At the same time, the patients’ prostates had significant volume decrease.

 

Many years ago, Dr. Rick Schmidt postulated that this sympathetic activity in the prostate was trophic. Do you feel your research confirms this?

Yes, I do. If you gave phenylephrine to our rat model, the prostate would not diminish in size. In addition, in the immunohistochemical studies we undertook, you would see more density of and more impact of the toxin on one type of targeted fibers. These fibers expressed markers of the adrenergic (sympathetic) arm of the autonomic system.

 

Are we likely to see botulinum toxin used for outlet obstruction in the future, and are you using it in select patients?

Yes, we still use it for this indication. We expanded its use to non-catheterized patients and our results show a benefit. Mainly, however, we use it as a compassionate treatment for very frail, catheterized patients.

 

Are there sexual side effects associated with its use in men?

We looked at sexual side effects in a study, and they were not significant. When you treat BPH with other surgical or medical modalities, you always have some degree of sexual side effects. Here, we found virtually none.

 

Can you comment on the main targets of botulinum toxin when injected into the urinary bladder with regard to SV2 (synaptic vesicle protein 2) and SNAP-25 (synaptosomal-associated protein 25)?

SV2 is the receptor molecule localized at the neuronal cellular membrane, to which botulinum toxin adheres when injected. The toxin is then internalized into the cytoplasm. There, the toxin’s light chain will cleave SNAP-25, a protein that is part of the SNARE complex. The latter is paramount in docking and fusion of neuromediator-containing vesicles with the cellular membrane of neurons at the synapse. This step will lead in turn to neuromediator release in the synaptic cleft.

If you cleave SNAP-25, the docking mechanism is paralyzed for months, because cleaved SNAP-25 stays in the axon for a long time. So you have the attachment as well as the ­extrusion mechanism, exocytosis, compromised, paralyzing the nerve affected by the toxin. This is how botulinum toxin A works: It precludes liberation of all vesicle-­dependent neuromediators, not just ­acetylcholine.

 

Please discuss your work on intra-trigonal injection of botulinum toxin and its efficacy and potential advantages over pan-urothelial injection.

That is my current research focus and my favorite subject, because it’s just a simple idea. We started out to study innervation, both in the rat bladder and prostate and in cadaveric organ transplant donors, and we observed a very rich sensory innervation of the prostatic urethra. There were already publications stating that most sensory fibers of the bladder are at the base (trigone) because nerves enter the bladder at the bladder neck and not at the bladder dome.

With this idea in mind, we asked, what is bladder pain syndrome? Bladder pain syndrome or BPS in our minds is pain, and the other symptoms develop as the result of pain. When we’re presented with pain and don’t know the etiology, we annul the pain. Pain is generated where you have sensory afferent fibers conveying pain, so we were determined to knock out these fibers with neurotoxins, which could paralyze or at least temporarily paralyze the sensory afferent fibers. This was the rationale for application at the trigone.

In addition-and this is controversial-if you apply neurotoxin to the trigone, you greatly lower the chances of post-op urinary retention. Of course, we still have to do it under anesthesia because it’s very painful for the patient with BPS to be manipulated while he or she is awake.

 

So it appears that injecting into the trigone does limit the amount of retention that you’re getting.

We have at present performed several dozen injections in these patients. This data has not been published yet. We had one case of temporary urinary retention, which is also in accordance with our laboratory findings. Dr. Ana Coelho from our group published a study where with a single toxin injection, you can find cleaved SNAP-25 throughout the bladder (Eur Urol 2012; 61:1178-84).

 

Can you tell us where your research on BPS is taking you now or what you see happening in the next few years?

In the field of BPS, I am most curious about what is happening in cell signaling. In my view, BPS is a disorder in which there are transcriptional and post-transcriptional alterations in urothelial cells that stay on whatever the triggering mechanism is. In turn, they will have implications on the molecules that contribute to molecular adhesion and intercellular signaling, so I suspect that urothelial cells are not communicating with each other as in a normal situation. It would be fantastic to see this aspect of BPS.

The urothelium today is not viewed as only a barrier; it’s like the retina to the eye but just more peripheral. It’s part of the bladder ­afferent system. It’s the sensor and transducer of bladder status to afferent nerves through interaction with the peripheral sensory fibers.UT

 

Europe’s debt crisis has wide-­ranging impact on medicine

When asked how the debt crisis in Europe is affecting the practice of medicine in Portugal, Dr. Dinis explained that its effect is felt on multiple levels.


“We do have a crisis, there’s no doubt about it,” Dr. Dinis said. “My patients are missing appointments. This, I am guessing, is because they either have no money for transportation or they don’t have the money to pay the so-called moderating tax. This is a comparatively small fee paid at public health care institutions initially conceived to curtail excessive health care demand.


“In terms of research,” he added, “funding at the national level has gone way down. If you were to seek funding for 10 high-quality projects, you might get two funded and even then maybe not to the extent of your request.


“On a personal level, as a public servant, I saw a pay cut of 30.5% from 2011 to 2012. But I am still happy, I am still working, and everybody has this attitude. Portugal has been here for 900 years, and Europe has been here even longer, so we will survive,” he said.