Irreversible Electroporation Balloon Therapy for Palliative Treatment of Obstructive Urethral Transitional Cell Carcinoma in Dogs
ABSTRACT
Progression of transitional cell carcinoma (TCC) in dogs often leads to urinary obstruction. This observational pilot study aimed to evaluate the safety and efficacy of irreversible electroporation (IRE) balloon therapy for the palliative treatment of TCC with partial urethral obstruction. Three client-owned dogs diagnosed with TCC causing partial urethral obstruction were enrolled. After ultrasonographic and cystoscopic examination, IRE pulse protocols were delivered through a balloon catheter device inflated within the urethral lumen. After the procedure, the patients were kept overnight for monitoring and a recheck was planned 28 days later. No complication was observed during the procedure and postprocedural monitoring. After 28 days, one dog had a complete normalization of the urine stream, one dog had stable stranguria, and one dog was presented with a urethral obstruction secondary to progression of the TCC. On recheck ultrasound, one dog had a 38% diminution of the urethral mass diameter whereas the other two dogs had a mass stable in size. IRE balloon therapy seems to be a feasible and apparently safe minimally invasive novel therapy for the palliative treatment of TCC causing urethral obstruction. Further studies are needed to better characterize the safety, efficacy, and outcome of this therapy.
Introduction
Bladder cancer comprises ∼1.5–2% of all naturally occurring cancers in dogs.1 Although this rate is similar to that reported in humans,2 intermediate- to high-grade invasive transitional cell carcinoma (TCC) diagnosed in ∼20% of human bladder cancer represents the vast majority of bladder cancer in dogs (>90% of cases).3,4 Also, bladder cancer in dogs is found primarily in the trigone region with an extension down the urethra reported in >50% of cases, whereas human bladder cancer is more evenly distributed across the bladder.3,4 These differences are significant for the management of bladder cancer in our canine population. Dogs are poor surgical candidates because complete surgical excision is often impossible, and disease progression can lead to urinary obstruction. Primary tumor has been shown to be the cause of death in 61% of cases in one study, whereas only 14% of cases died of metastatic disease.5 Because urinary obstruction occurs before the development of lethal metastasis in many dogs, palliative treatment for the rapid restoration of urethral patency can improve canine TCC outcomes. Urethral stent placement is currently the most successful technique to restore luminal patency in dogs, with a median survival time after stent placement of 78 days. 6,7 However, complications such as urinary incontinence (described in 25–39% of cases), stent migration, and reobstruction due to further tumor growth can occur.6–8 Transurethral laser ablation often applied to superficial tumors in humans has also been described in dogs.9–11 Compared with humans, however, TCC in most dogs is unfortunately invasive, and the benefit of this technique is not well defined because the published cases were also receiving chemotherapy and/or radiation therapy.9–11 Reported complications for this technique included urethral stenosis, local tumor seeding, and urethral perforation.9,10
Electroporation is based on the principle in which ultrashort electric pulses are used to create nanoscale defects in the cell membrane.12,13 Above a certain electrical threshold,14 the membrane defects become irreversible, causing cell death owing to the inability to maintain homeostasis.12,13 Three different medical applications of electroporation have been developed in human medicine: anticancer electrochemotherapy (electroporation to transfer cytotoxic drugs that do not freely cross the plasma membrane), electrogenetherapy (electroporation to transfer nucleic acids inside the cells), and irreversible electroporation (IRE, electroporation for local ablative treatment of the tissue exposed).
IRE is a new minimally invasive therapy first described in 2005 for the treatment of cancer.12 The fundamental rationale of this focal therapy is to spare the organ, by aiming on the cancerous lesion and thus sparing the healthy tissue and neighboring organs such as the neurovascular bundles, external sphincter, rectum, and urethra, thus permitting treatment in regions unsuitable for surgical resection or thermal therapies.15,16 Human medicine has been using a canine model for IRE ablation characterization in multiple organs, such as prostate,15,17,18 kidney,19 heart,20–22 and brain.23–27 IRE therapy has also been used in canine patients to treat gliomas,28–30 meningiomas,31 and a large soft tissue sarcoma.32 Two main precautions need to be taken during IRE therapy to prevent complications from the high-intensity electrical pulses administered. First, IRE procedures are performed under general anesthesia with additional muscle relaxation to prevent severe muscle contractions.13,33 Second, synchronization of the IRE pulses with the cardiac rhythm is advised because administered IRE pulses could potentially cause cardiac arrhythmia.13,34 In veterinary medicine, a recent case report described the clinical outcomes of three dogs with vesical TCC treated with electrochemotherapy using bleomycin or bleomycin in combination with intratumorally administered cisplatin.35
In the aforementioned studies, electroporation therapy consists of monopolar needle electrodes inserted in the targeted tissue in all but one study, describing IRE balloon therapy for intrapulmonary vein ablation.21 IRE balloon therapy in dogs has the potential to provide a new therapy for canine obstructive urethral TCC and could be used as an animal model for treatments in humans with similar types of neoplasia. Previously, the novel bipolar balloon catheter device, prototyped by Boston Scientific, was evaluated using internal research and translational safety trials approved by the Institutional Animal Care and Use Committee (protocol no. 17-017N, approved February 27, 2018)a in the healthy canine male prostatic urethra across four subjects with the goal to assess tolerability of IRE treatment without adverse reactions. Positive results were obtained across all four canines, clearly showing that the balloon urethral delivery and device therapy was tolerated and inherently safe as measured by immediate demonstration of urinary function with equivalent void volume and flow after the procedure without catheterization or pain. The postnecropsy tissue evaluation also showed healing from nonthermal ablation to the urethral lining.
The objectives of the pilot study reported here are to demonstrate and assess the feasibility and efficacy of the IRE balloon device in a natural disease model of transitional cell carcinoma causing urethral obstruction.
Materials and Methods
Animals
Three client-owned dogs diagnosed with urethral transitional cell carcinoma causing partial obstruction of the urethra were enrolled in the study. Before study enrollment, each dog had a complete physical examination and routine laboratory screening tests (complete blood count, serum chemistry profile, and urinalysis and urine culture) without significant changes. Absence of metastatic disease was also confirmed before enrollment with thoracic radiographs and abdominal ultrasound. A written informed consent was obtained from each owner and the study design was approved by the Tufts University Institutional Animal Care and Use Committee (protocol no. G2018-56).
Study Protocol
Each patient was presented twice, day 0 and day 28. Day 0 corresponded to the day of treatment with irreversible electroporation therapy. Cystoscopy with biopsies of the abnormal tissue were performed before the treatment and the dog was kept hospitalized overnight after the procedure to ensure appropriate comfort and to assess the ability to urinate. The patient was continued on piroxicam or meloxicam started before enrollment. No other chemotherapy treatment was instituted until day 28, the endpoint of the study. The recheck included anamnesis from the owner, recheck of complete physical examination, complete blood count, serum chemistry profile, urinalysis and urine culture, abdominal ultrasound, and cystoscopy with biopsies of tumor tissue. The response to therapy was measured on ultrasound and described using the new canine response evaluation criteria in solid tumors.36
IRE Balloon Therapy
The IRE device is a 6.0 × 25 mm noncompliant transcatheter polyethylene terephthalate balloonb that uses four flexible, high-voltage electrode paired circuits attached to the balloon body to create a bipolar field strength spanning a length of approximately 12 mm (Figure 1). Each of the four electrode pairs is spaced roughly 90 degrees to form opposing bipolar circuits, which can be configured to activate 180 degrees apart. The balloon catheter supplies irreversible, nonthermal electroporation energy using short microsecond electrical field pulses combined into a series of burst trains to induce cell death and local tissue response. In this manner, each individual burst is made up of multiple short, monophasic pulses separated by intrapulse delays.
Citation: Journal of the American Animal Hospital Association 58, 5; 10.5326/JAAHA-MS-7160
Five dermal heart electrocardiogram (ECG) leads were placed on the dog for connection to an ECG Trigger Monitorc. A hydrophilic guide wired (Figure 2) was placed through the cystoscope5 into the bladder and then the cystoscope was removed, leaving the wire in place. The IRE balloon was threaded over the guide wire along the length of the involved urethra. The balloon has echogenic electrodes that were used for accurate visualization with ultrasound guidance. Once positioned, the balloon was inflated with 0.9% saline up to 3 atm to provide good tissue apposition to the urethral wall.
Citation: Journal of the American Animal Hospital Association 58, 5; 10.5326/JAAHA-MS-7160
The IRE balloon catheter was connected to the external pulsed-electric field generatorf, using electrically rated extension cables. The generator pulse delivery was gated by an ECG unitc to detect the R wave of the QRT complex with precision and reliability and delay 50 ms before delivering a burst packet. This was done in order to synchronize delayed treatment bursts with the heart rhythm and avoid any undue arrythmia complications.
Based on the balloon electrode design, computational modeling using a commercial COMSOL multiphysics code was used to solve the electrical and thermal field distributions to estimate the ablative volume range for both reversible and irreversible field thresholds. The model solutions also predicted an ablative volume target estimate for the intended electrode configurations between 2 and 8 mm of urethral tissue depth depending on waveform parameters, such as voltage, pulse width, and number as well as number of total bursts. Based on IRE mechanisms and modeling predictions, the tissue response can be monitored in situ by carefully tracking the electrical impedance from current and voltage changes during pulse delivery, providing a real-time diagnostic from several key indicators. First, good electrode contacts for tissue apposition was confirmed by a single test pulse at 500 V such that electrical continuity can be confirmed across expected tissue impedance ranges of 100–300 Ohms. Second, based on intracellular leakage, there is a local increase in electrical conductivity, presenting as a complex tissue impedance decay. During IRE delivery, this gradual decay can be tracked over successive bursts to monitor treatment efficacy. Finally, based on modeling output for target volumes and previous correlations over preclinical safety studies in the urethra, an overall impedance decay of 40% is expected to show when the irreversible electroporation treatment reached a therapeutic plateau. Also, additional electrical energy can cause electrode heating resulting in tissue damage, thereby increasing impedance due to tissue coagulation or charring, which could also be monitored. Based on these expectations, a procedural plan was put in place to ensure safe delivery with a targeted goal to complete the therapy until there was minimal impedance change across successive bursts. At such time, the treatment would stop and the electrode configuration pairs could be switched to the opposing pattern in order to complete a uniform circumferential delivery.
For the treatment setup, high-voltage monophasic electrical bursts, with total burst time not exceeding 80 μsecs, made up of separate 2 μsecs pulses at a frequency of 100 kHz were delivered for each measured heartbeat. The electric field was delivered at between 1500 and 2000 V/cm, which was deemed clinically relevant for IRE field thresholds depending on cancer invasion and animal size.14 The total pulse power and frequency was chosen to accumulate high enough energy per burst without increasing the thermal temperature near the electrodes. Finally, delivered pulse peak current was sensed and maintained below a safety threshold of 30 amps to prevent tissue breakdown or arcing. Monitoring the transient change in tissue impedance, a burst train of between 50 and 100 individual bursts was delivered across a pair of opposing electrode pairs for one therapy cycle. Based on the bipolar radial design of the balloon, electrode pairs were opposed 180° and activated in two different combinations to form a complete circumferential ablated lesion for each urethral segment. Depending on impedance response, either the same burst train cycle could be repeated or the electrode pairs could be switched to treat the other half-lesion circumference. In order to cover the entire length of the invasive lesion via urethra access, the balloon could be deflated and repositioned under ultrasound guidance to repeat the treatment cycle. Once the entire procedure was completed, the cystoscope was advanced along the urethra to the bladder to confirm the urethral wall integrity.
Anesthesia and Analgesia
The general anesthesia protocol with appropriate analgesia was determined by the anesthesiologist on duty as dictated by individual patient requirements. Each patient was placed under artificial ventilation and paralyzed with IV atracurium muscle blockade at a baseline dose of 0.25 mg/kg IV to eliminate muscle contraction; the dosage was adjusted based on the extent of visible muscle contraction during the IRE balloon therapy to avoid as much as possible muscle spasming secondary to nerve stimulation. Before each therapeutic treatment, one test burst, at between 500 and 1000 V, was applied to each patient to test the degree of muscle stimulation (as well as baseline tissue impedance to ensure device continuity and tissue-to-balloon apposition).
A defibrillator was present for each procedure in case of ventricular fibrillation secondary to electric pulses created by the treatment, but it was not needed. Each patient received a periprocedural treatment of cefazolin IV at 22 mg/kg.
Side Effects
During the procedure, the vitals and degree of muscle contractions were closely monitored. The potential complications after treatment were suspected to be urethritis (with worsened pollakiuria, dysuria, and potential functional obstruction due to urethral spasm), urethral tear, or urethral stricture. The postprocedural care included close follow-up of the patient vitals, comfort, and urinations every 4 hr. The level of comfort was assessed using the Colorado State University Canine Acute Pain Scale.37 During the 28 days following the procedure, the owners were requested to write down and note any changes in their animals’ general behavior and urinations.
Results
Animals
A 10 yr old female spayed West Highland white terrier (first patient), a 13 yr old female spayed Labrador retriever (second patient), and an 11 yr old female spayed dachshund cross (third patient) were included in the study.
The first patient presented to the primary care veterinarian because she had been taking significantly longer to urinate for about a week. A thickening of the urethra was noted on rectal examination. Her complete blood work was unremarkable, as was her urinalysis except for the presence of transitional epithelial cells with frequent multinucleation and moderate to marked anisocytosis/anisokaryosis. Her ultrasound showed a urethral mass extending to the bladder and the BRAF mutation present in TCC was detected in her urineg.
The second patient had a 2 yr history of recurrent urinary tract infection and lower urinary tract signs, with stranguria and hematuria that started 7 mo before final diagnosis of TCC on cystoscopic biopsies. Her most recent ultrasound revealed a thickened bladder and urethra, although no BRAF mutation was detected in her urineg 1 mo earlier.
The third patient had a 5 mo history of pollakiuria and dysuria. An ultrasound 2 mo before presentation to our referral center demonstrated a thickening of the urethra, and surgical bladder wall biopsies performed by the primary care veterinarian did not show evidence of neoplasia. The clinical signs had first improved with meloxicam but were now back as previously, with mild hematuria also noted in the last few days before consultation visit. Cystoscopic biopsies confirmed a TCC.
Diagnostic Tests Before IRE Balloon Therapy
Before enrollment, no significant changes were seen on complete blood work, urinalysis, and urine culture for all three patients. On ultrasound, they all had a normal urinary bladder wall thickness and layering but a heterogeneous hypoechoic mass within the proximal urethra. The cystoscopic examination before the IRE treatment confirmed that all patients had marked changes along the entire urethra: the mucosa was thickened, irregular, and proliferate in the urethra lumen (Figure 3). The advancement of the rigid cystoscope demonstrated significant resistance, but no site of complete obstruction was present. None of the patients had evidence of metastatic disease on thoracic radiographs and abdominal ultrasound.
Citation: Journal of the American Animal Hospital Association 58, 5; 10.5326/JAAHA-MS-7160
Transitional cell carcinoma was diagnosed 14, 20, and 19 days, respectively, and piroxicam started 16 days, 7 mo, and 1 mo, respectively, before IRE balloon therapy.
IRE Balloon Therapy
The IRE balloon device was placed without complication in the urethra. The balloon catheter and, more importantly, the electrodes’ location were easily confirmed with ultrasound guidance (Figure 4) during either tracking or deployment, except for the treatments in the caudal part of the urethra, where the most caudal part of the balloon was intrapelvic and thus not visible. No concern with balloon device migration during the treatments was noted. A small increase in heart rate was observed during the treatments, but the maximum heart rate stayed below 130 bpm, except for our second patient, who developed a mild tachycardia at 160 bpm after the first treatment that rapidly resolved with a bolus of lidocaine (1 mg/kg IV) and ketamine (2 mg/kg IV). During the IRE pulses, no cardiac arrhythmias were noted and the muscle contractions were limited to a mild twitch of the posterior legs. Table 1 shows the applied monophasic (direct current) IRE targeted pulse settings applied to each of the three patients. The total number of circumferential bursts were averaged across the two opposing radial electrode positions. In cases with consecutive serial burst trains, an intermediate pause of between 30 and 90 s was deployed to allow some cell recovery before a subsequent retreatment to the electrosensitive lesion. The tissue impedance decay was monitored by burst-to-burst measurements as well as comparison with initial baseline levels (Figure 5). The therapy for each urethral segment was deemed complete when impedance measurements plateaued over the desired burst train, optimally culminating in a 20–40% impedance reduction from baseline, although the location, degree of infiltration, and electrode contact area could be variable for each application. The gradual impedance decay with applied IRE treatment bursts demonstrated a local increase in tissue conductivity from electroporated cells. This result is expected and in line with literature values indicative of in situ tissue changes that can be used to plan and monitor therapeutic outcomes during the IRE application phase. After each segment was deemed complete, the balloon was deflated and repositioned to a new location until all diseased length was treated and the balloon was withdrawn from the urethra.
Citation: Journal of the American Animal Hospital Association 58, 5; 10.5326/JAAHA-MS-7160
Citation: Journal of the American Animal Hospital Association 58, 5; 10.5326/JAAHA-MS-7160
All the patients had an uneventful recovery and stayed comfortable and stable with vitals within the normal ranges. They all had stable stranguria and hematuria compared with their clinical signs before their treatment. None of them required catheterization, so they were all discharged the following day with antimicrobial therapy (cephalexin 26 mg/kg per os [PO] q 12 hr for the first and third patient and amoxicillin/clavulanate 16 mg/kg PO q 12 hr for the second patient) for 5 days; they were continued on their nonsteroidal anti-inflammatory (meloxicam 0.1 mg/kg PO q 24 hr for the first patient and piroxicam 0.33 mg/kg PO q 24 hr and 0.37 mg/kg PO q 24 hr for the second and third patients) and started on prazosin at a dose of 0.06–0.1 mg/kg PO q 12 hr.
One-Month Recheck
For the first patient, no lower urinary signs were reported; her urine stream was completely back to normal. She showed one episode of very mild hematuria the day after she was discharged, which resolved the same day. The recheck complete blood work, urinalysis, and culture were unremarkable. On ultrasound, the infiltrative disease showed a partial response with a maximal diameter of the urethral mass measured at 5.5 mm, compared with the 8.9 mm in diameter before IRE balloon therapy (38% reduction). The urinary bladder was still normal. At the level of the urethra, along the ventral aspect of the rectal wall, there was now a focal thickening of the muscularis layer on the midline that measured up to 2 mm in thickness and about 3 cm in length.
The second patient returned 3 wk after the IRE balloon therapy because of lethargy, hematuria, vomiting, and diarrhea with melena. The dog was found to be severely azotemic (blood urea nitrogen 127 mg/dL, creatinine 9.1 mg/dL); the urinalysis and culture were unremarkable. The ultrasound initially revealed a severely distended urinary bladder as well as mild to moderate bilateral renal pelvis distention, which resolved after urinary catheterization. However, the ultrasound revealed stable disease with an overall unchanged urethral diameter (7.2 mm, previously 7.3 mm). The patient’s azotemia resolved after 3 days of hospitalization with IV fluid therapy.
The third patient was reported to have prolonged urinations since the IRE balloon therapy. No other concerns were reported. The recheck complete blood work, urinalysis, and culture were unremarkable. The ultrasound revealed progressive disease. The urethral diameter was stable in size (19 mm), but cranial to this mass, the bladder wall now had proliferative tissue extending at the level of the trigone, without reaching the ureterovesical junctions.
Except for the second patient, the cystoscope was advanced with significantly less resistance. For all patients, mucosal changes were persistent but less tissue was protruding into the urethral lumen, which was significantly more open in comparison with the cystoscopy before IRE balloon therapy (Figure 6).
Citation: Journal of the American Animal Hospital Association 58, 5; 10.5326/JAAHA-MS-7160
Discussion
The results of the present study did not reveal any clinically significant complications during IRE balloon therapy or during the close follow-up and 1 mo recheck. The two main complications anticipated during the treatment itself were severe muscle contractions and cardiac arrhythmias, both due to the high-intensity electrical pulses administered. For all three patients, the amount of muscle contraction during the treatment was maintained minimal and limited to the hind legs with an injection of atracurium 0.25 mg/kg IV before the first treatment and 0.1 mg/kg IV before the third treatment. No cardiac arrhythmias were seen. No complications were seen during the procedure and the overnight stay: the patients stayed comfortable and were all discharged the following day. At 1 mo recheck, the only suspected complication was the rectal wall changes in our first patient. The owner did not report any large bowel clinical signs, and the changes were no longer visible on ultrasound 2 mo after IRE balloon therapy.
Regarding the efficacy of this therapy, some of our results are promising. Our first patient, using a fixed number of bursts (n = 100) per urethral segment location, had a normalized urine stream at 1 mo recheck, whereas our third patient, using a higher number of segmental bursts leading to greater impedance decay, showed a notable clinical improvement in the stranguria and pollakiuria. Because these patients were already on an anti-inflammatory medication for 2 and 4 wk, respectively, the improvement seems less likely to be associated with the anti-inflammatory treatment. One could argue that this improvement may have been related to the prazosin started after the balloon therapy. Although this medication definitely could have helped, the decreased resistance of the cystoscope passage along the urethra and the decreased amount of tissue proliferation in the urethral lumen are unlikely to be related to the prazosin. Although a stent would have been discussed at the time when these two patients presented, our first patient required a stent because of disease progression 178 days after the IRE balloon therapy. Our third patient is still alive at the time of writing without the need of urethral stenting 298 days after the IRE balloon therapy. The owner of our second dog declined further therapeutic intervention and the patient was euthanized 58 days after IRE balloon therapy.
Our study aimed to serve as a pilot study for further investigation of IRE balloon therapy in dogs with TCC causing urethral obstruction. Our main limitation was evidently our number of cases. This study also included only one IRE treatment for each patient, whereas a protocol with multiple treatments may potentially be as safe and more effective.
Regardless of these limitations, IRE balloon therapy in canine urethra seems to be safe and has the potential to be effective in improving malignant obstruction. These preliminary results support future investigations into IRE balloon therapy for the palliative treatment of canine TCC causing urethral obstruction.
Conclusion
Progression of TCC in dogs often leads to urinary obstruction. IRE balloon therapy was delivered in three client-owned dogs under ultrasonographic guidance through a balloon catheter device inflated within the urethra. No complication was observed during the procedure and postprocedural monitoring. After 28 days, one dog had a complete normalization of the urine stream with partial response on ultrasound (38% diminution of the urethral mass diameter on ultrasound), one dog had stable stranguria but progressive disease (proliferative tissue extending at the level of the trigone), and one dog was presented with a urethral obstruction despite stable disease on ultrasound. IRE balloon therapy seems to be a safe, minimally invasive novel therapy for the palliative treatment of TCC causing urethral obstruction. Further studies are needed to better characterize the efficacy and outcome of this therapy.
6.0 × 25 mm noncompliant transcatheter polyethylene terephthalate balloonb with four flexible, high-voltage electrode paired circuits attached to the balloon body spanning a length of ∼12 mm. Deflated (A) and inflated (B) balloon threaded over the guide wire.
Material required: guide wire ZipWire, Straight, 0.035” × 260 cm (A), Phoenix IRE balloon 6 × 25 mm, 0.035” (B), and Encore inflation device (C) that inflates the IRE balloon with 0.9% saline up to 3 atm. IRE, irreversible electroporation.
Cystoscopic examination of the urethra before irreversible electroporation balloon therapy in the first (A), second (B) and third (C) patient. All patients had marked changes along the entire urethra: the mucosa was thickened, irregular, and proliferative in the urethral lumen.
Balloon with irreversible electroporation electrodes inserted in the urethra under ultrasound guidance and inflated with saline to 3 atm. The tip of the balloon (arrowhead) and the most cranial port of the electrodes’ location (thin arrow) was easily confirmed with ultrasound guidance.
Visual change in tissue impedance at one location (mid-urethra) during pulsed electric field balloon treatment with opposing electrode pairs (Elec 1_3 and 2_4) in the first (P1), second (P2), and third (P3) patient.
Mucosal changes were persistent but with less tissue protruding into the urethral lumen in comparison with the cystoscopy before irreversible electroporation balloon therapy in the first (A), second (B), and third (C) patient.
Contributor Notes
