Thermochemical Ablation: A Novel Technique for Solid Tumor Therapy

To overcome the limitations of existing ablation techniques, we propose a novel combinatorial approach that would utilize the thermal and chemical destructive effects of exothermic chemical reactions, such as an acid/base neutralization reaction, to treat solid tumors. Thermochemical ablation is a potential technique for percutaneous probe-based tumor therapy. It involves simultaneous intratumoral delivery of multiple reagents resulting in thermal energy released by an exothermic reaction to ablate tumor tissue with concurrent generation of a hyperosmolar byproduct that could accentuate tumor destruction. Besides the benefit of synergistic thermal and chemical effects for tumor tissue destruction, this technique is potentially highly cost-effective, easy to implement, and able to treat larger sized tumors. Our hypothesis is that thermochemical ablation can create an evenly distributed zone of coagulation in tumor tissue without systemic toxicity. A prototype device assembled using off-the-shelf components is being investigated in our lab for concurrent intraparenchymal delivery of an acid and a base. The distal portion of the multi-lumen device allows for passive mixing of the reagents before entering the tissue. The prototype device also satisfies other desirable design criteria such as rigidity to penetrate body tissue, reduced diameter, chemical stability to reagents, etc. However, the device can be improved upon by incorporating additional characteristics such as optimized imaging characteristic for real-time visualization and localization within tumor tissue, MRI compatibility, thermal insulation, improved mixing at the tip, etc. Our lab is currently working on improving the design of the infusion device as well as assessing the feasibility of the thermochemical ablation technique in vitro and in vivo. While currently being targeted conservatively for palliative therapy of unresectable or late-stage aggressive malignancies such as hepatocellular carcinoma, thermochemical ablation has potential use in the therapy of a majority of solid tumors such as breast cancer, lung cancer, prostate cancer, renal cancer, sarcomas, etc.

Molecular Enhancement of Thermal Ablation Therapies Using TNF-α-Coated Gold Nanoparticles in a Translational Model of Renal Tumors

Thermal ablation therapies are currently used for the treatment of select renal masses. Such treatments are limited to tumors that are small (<3 cm diameter), exophytic, and away from vital structures such as ureter or intestine. Novel treatment approaches are geared towards increasing the size of the thermal lesion created, limiting damage to collateral normal tissues, reducing local recurrence and distant metastases as well as improving the imaging potential of the therapy. Previous studies have demonstrated the enhancement of thermal therapies in pre-clinical murine models of solid tumors by intravenously infusing 33 nm TNF-α and PEG coated gold nanoparticles (CYT-6091, Cytimmune Sciences Inc.) prior to ablation. This study investigates the enhancement of thermal ablation therapy by CYT-6091 in a translational animal model of renal tumors. New Zealand White rabbits (37 for radiofrequency ablation (RFA), 20 for cryoablation) had VX-2 tumors implanted into their bilateral kidneys. The tumors were allowed to grow for 14 days to a size of ∼1 cm. For RFA, the rabbits were split into 3 treatment groups of 10 rabbits each and a sham group of 7 rabbits. The groups were treated with CYT-6091 (200 μg/kg) only, RFA only, or CYT-6091 (200 μg/kg) followed 4 hours later by RFA. For cryoablation, 2 treatment groups of 10 rabbits each were used. The groups were treated with cryoablation only or CYT-6091 (200 μg/kg) followed 4 hours later by cryoablation. The kidneys were harvested 3 days later for RFA and 7 days later for cryoablation. Gross and microscopic measurements of the ablation size as well as histological analysis using H&E staining were performed. The RFA plus CYT-6091 group had a larger zone of complete cell death than the RFA only group when measured both on gross sectioning (0.32±0.03 vs. 0.22±0.07cm3, p=0.015) and on microscopic examination (0.30±0.07 vs. 0.23±0.03cm3, p=0.03). Overall this was a 23% increase in ablation volume. This difference in ablation size was due to a replacement of partially ablated tissue at the periphery in the RFA only group by completely ablated tissue in the RFA plus CYT-6091 group. Thus this zone of partially ablated tissue was smaller in the RFA plus CYT-6091 group than the RFA only group (0.08±0.02cm3 vs. 0.13±0.05cm3, p=0.01). Excessive tumor growth into the ablation lesion at day 7 following cryoablation prevented accurate measurements in these groups; however, a significant decrease in the rate of peritoneal carcinomatosis (metastases) was obtained in the cryo plus CYT-6091 group compared to the cryoablation alone group (1/10 vs. 8/10, p=0.04). We have shown that use of CYT-6091 prior to thermal ablation therapy in a rabbit kidney tumor model can minimize the zone of partial treatment at the periphery of the thermal lesion and thus maximize the complete kill zone in RFA while significantly decreasing the rate of metastases in cryoablation. These data provide preliminary evidence for the efficacy of adjuvant use of CYT-6091 for thermal ablation therapies in a large animal translational tumor model.

CT Visualization of Cryoablation in Pulmonary Veins

Over 2 million adults in the United States are affected by atrial fibrillation (AF), a common cardiac arrhythmia that is associated with decreased survival, increased cardiovascular morbidities, and a decrease in quality of life. AF can be initiated by ectopic beats originating in the myocardial sleeves surrounding the pulmonary viens. Pulmonary vein (PV) isolation via radio frequency ablation is the current gold standard for treating patients with drug-refractory AF. However, cryoablation is emerging as a new minimally-invasive technique to achieve PV isolation. Cryoablation is fast gaining acceptance due to its minimal tissue disruption, decreased thrombogenicity, and reduced complications (RF can lead to low rate of stenosis). One important question in regard to this technology is whether the PV lesion is transmural and circumferential and to what extent adjacent tissues are involved in the freezing process. As ice formation lends itself to image contrast in the body, we hypothesized that intraprocedural CT visualization of the iceball formation would allow us to predict the extent of the cryolesion and provide us with a measure of the adjacent tissue damage. Cryoablation was performed using a prototype balloon catheter cryoablation system (Boston Scientific Corporation). CT visualization of iceball formation was assessed both in vitro and in vivo. Initial in vitro studies were performed in agarose gel phantoms immersed in a 37°C water bath. Subsequently, in vivo cryoablations were performed in 5 PV ostia in 3 crossbred farm swine. The catheters were positioned in the ostia under fluoroscopic guidance. CT scans of the thoracic region were obtained every 2.5 minutes. Animals were sacrified 6 days after the procedures. Gross pathology and histology of tissues in the region of interest were evaluated. Significant metal artifacts from the catheter and edge artifacts from the tissues surrounding the cryoballoon were observed under CT imaging both in vitro and in vivo. In vitro, it was found that the size of the iceball was comparable to that observed visually during freezing of agarose gel phantoms. In vivo, contrast change consistent with iceball formation was observed during the ablation in two out of five veins. The most clearly delineated iceball also yielded the clearest morbidity. In this case, esophageal injury on the anterior side proximal to the cryoablation site was noticed during necropsy of the animal in which the iceball was visualized. Transmural and circumferential lesions were obtained in all PVs ablated. We have shown that CT can be used to visualize iceball formation in vitro and in vivo (with limitations) using our cryoablation system. While the iceball in vitro is easily visualized, iceball growth in vivo is most evident once the iceball has grown beyond the PV into the adjacent tissues. This suggests that while CT cannot easily visualize iceball growth in the PV wall itself, it may still be an important tool to guide clinicians and reduce potential morbidities in adjacent tissues. The authors acknowledge Dan Busian (Fairview University Medical Center, Minneapolis, MN) and Dr. Erik Cressman for assistance with CT imaging.

The nitric oxide synthesis inhibitor nitro-L-arginine (L-NNA) attenuates nicotine abstinence syndrome in the rat

Nitric oxide synthesis contributes to opiate tolerance and dependence. Nicotine dependence and abstinence syndrome in the rat appear to involve opiate mechanisms. Therefore, it was postulated that nitric oxide synthase (NOS) activity might be essential for the expression of nicotine abstinence syndrome. Twenty-one rats were rendered dependent by SC infusion of 9 mg/kg per day nicotine tartrate via Alzet osmotic minipump. Rats were pretreated SC with vehicle alone, or with 18 or 30 mg/kg of the NOS inhibitor L-NNA (nitro-L-arginine). Thirty minutes later, rats were challenged by 1 mg/kg of the nicotinic antagonist mecamylamine SC and observed for 30 additional minutes. Rats pretreated with vehicle displayed a total of 68.7+/-8.0 mecamylamine-precipitated abstinence signs (mean+/-SEM), while those receiving 18 or 30 mg/kg L-NNA had 12.7+/-2.0 and 5.1+/-1.7 signs, respectively. All three groups differed significantly from one another according to Dunn's post-hoc procedure. Rats pretreated with L-NNA combined with an excess of the NOS substrate L-arginine had significantly more mecamylamine-precipitated abstinence signs than rats receiving L-NNA combined with D-arginine. Also, D-NNA, which does not selectively bind to NOS, was significantly less effective than L-NNA in preventing mecamylamine-precipitated abstinence syndrome. Additional studies determined the effect of L-NNA on spontaneous nicotine abstinence syndrome. Rats were assessed for abstinence signs at 17 and 20 h after termination of nicotine infusion. They received injections of 9, 18, or 30 mg/kg L-NNA SC or vehicle alone immediately before the 20-h observation; all rats were observed for 30 min. Signs at 20 h (post-injection) as a percentage of signs at 17 h (pre-injection) declined significantly as a function of L-NNA dose. Once again, this effect was attenuated significantly more by co-administration of L-arginine than by D-arginine. The overall pattern of results suggests that nitric oxide synthesis is critical to the expression of nicotine abstinence syndrome.

Nicotine abstinence syndrome precipitated by the competitive nicotinic antagonist dihydro-beta-erythroidine

Rats infused subcutaneously with 9 mg/kg/day nicotine tartrate for 7 days exhibit behavioral abstinence signs following either termination of nicotine infusion or injection of the noncompetitive nicotinic antagonists mecamylamine (s.c.) or hexamethonium (ic.c.v.). This study examined the abstinence precipitating effects of dihydro-beta-erythroidine (DHbetaE), a competitive nicotinic antagonist. Twenty-four nicotine-dependent rats were injected in the third ventricle with 10, 18, or 25 microg DHbetaE in 20 microl saline or with saline alone and observed for abstinence signs over a 20-min period. There was a significant positive linear trend of overall abstinence signs as a function of dose, p < 0.01. In 12 nondependent rats, the high dose of DHbetaE did not induce more abstinence-like signs than saline alone. In a second experiment, 18 nicotine-dependent rats were injected s.c. with 1 or 6 mg/kg of the muscarinic antagonist scopolamine or with saline alone. Few abstinence signs were observed in any group: there was no significant drug effect. The results suggest that nicotine abstinence signs observed in the rat are specific to reduced stimulation of previously overstimulated nicotinic receptors.

Nicotine abstinence syndrome precipitated by central but not peripheral hexamethonium

A rodent model of nicotine dependence has been developed based on continuous subcutaneous (s.c.) infusion of nicotine tartrate. Nicotine abstinence syndrome was precipitated by s.c. injection of the nicotinic antagonist mecamylamine, which freely crosses the blood-brain barrier. In contrast, the nicotinic antagonist hexamethonium crosses the blood-brain barrier very poorly. This study determined whether central or peripheral administration of hexamethonium could precipitate nicotine abstinence. In the first experiment, 26 nicotine-dependent rats were injected s.c. with 0.5, 5 or 10 mg/kg hexamethonium dichloride or saline alone and observed for 20 min. Few abstinence signs were observed in any group; there was no significant drug effect. In the second experiment, 18 rats were cannulated in the third ventricle and rendered nicotine dependent. One week later, rats were injected through the cannula with 12 or 18 ng hexamethonium or saline alone and observed for 20 min. Both dose groups differed significantly from the saline-injected group, and there was a significant positive linear trend of signs as a function of dose. The high dose had no significant effect in 14 nondependent rats. We conclude that hexamethonium is much more potent by the central route, and there is a major central nervous system component in nicotine dependence.