BASIC SCIENCE
Europace (2015) 17, 1309–1315 doi:10.1093/europace/euu335
Optimal conditions for cardiac catheter ablation using photodynamic therapy Takehiro Kimura1*, Seiji Takatsuki 1, Shunichiro Miyoshi 1, Mei Takahashi 2, Emiyu Ogawa2, Yoshinori Katsumata 1, Takahiko Nishiyama 1, Nobuhiro Nishiyama 1, Yoko Tanimoto1, Yoshiyasu Aizawa1, Tsunenori Arai 2, and Keiichi Fukuda 1 1 Department of Cardiology, Keio University School of Medicine, 35 Shinanomachi Shinjuku-ku, Tokyo 160-8582, Japan; and 2School of Fundamental Science and Technology, Graduate School of Science and Technology, Keio University, Kanagawa, Japan
Received 26 July 2014; accepted after revision 17 October 2014; online publish-ahead-of-print 6 January 2015
Aims
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Ablation † Arrhythmia † Photodynamic therapy † Talaporfin sodium
Introduction We previously reported the usefulness of photodynamic therapy (PDT)-mediated cardiac catheter ablation using a four-polar laser catheter in a canine model.1 However, maximizing the benefits and minimizing the complications associated with future clinical applications of this technology are critically dependent on controlling the lesion size created. We therefore sought to determine the optimal conditions and critical factors involved in PDT-mediated lesion creation in vivo for cardiac ablation. Originally, PDT was used for treating malignant brain tumours,2 esophageal cancer,3 and lung cancer,4 with variations in the sensitivity to PDT among cancers dependent simply on the accumulated dose of
the photosensitizer.5 The advent of laser irradiation was thus a major advance for PDT to enable a targeted photosensitizer accumulation in tumour cells only and the disappearance of any accumulation in the surrounding normal tissue. However, in PDT-mediated cardiac ablation, the laser is irradiated during the capillary perfusion of the photosensitizer bound to plasma albumin and other heavy proteins,6 the so-called extra-cellular PDT, and no uptake occurs in the heart. Therefore, the ability to regulate the PDT reaction during cardiac catheter ablation is clearly critical for reducing the risk of injury to adjacent organs. Considering that PDT is based on the generation of tissue injury via singlet oxygen species using a combination of laser irradiation, photosensitizer administration, and oxygen,7 we evaluated the lesion size
* Corresponding author. Tel: +81 3 3353 1211 (ext. 61421); fax: +81 3 5363 3875, E-mail address: [email protected] Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2015. For permissions please email: [email protected].
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Photodynamic therapy (PDT) is based on non-thermal injury mediated by singlet oxygen species and is used clinically in cancer therapy. In our continuing efforts to apply this technology to cardiac catheter ablation, we clarified the optimal condition for creating PDT-mediated lesions using a laser catheter. ..................................................................................................................................................................................... Methods In a total of 35 canines, we applied a laser directly to the epicardium of the beating heart during open-chest surgery at and results 15 min after administration of a photosensitizer, talaporfin sodium. We evaluated the lesion size (depth and width) using hematoxylin-eosin staining under varying conditions as follows: laser output (5, 10, 20 W/cm2), irradiation time (0– 60 s), photosensitizer concentration (0, 2.5, 5 mg/kg), blood oxygen concentration (103.5 + 2.1 vs. 548.0 + 18.4 torr), and contact force applied during irradiations (low: ,20 g, high: .20 g). A laser irradiation at 20 W/cm2 for 60 s under 5 mg/kg (29 mg/mL) of photosensitizer induced a lesion 8.7 + 0.8 mm deep and 5.2 + 0.2 mm wide. The lesion size was thus positively correlated to the laser power, irradiation time, and photosensitizer concentration, and was independent of the applied contact force and oxygen concentration. In addition, the concentration of the photosensitizer strongly correlated with the changes in the pulse oximetry data and fluorescence of the backscattering laser, suggesting that a clinically appropriate condition could be estimated in real time. ..................................................................................................................................................................................... Conclusion Photodynamic therapy-mediated cardiac lesions might be controllable by regulating the photosensitizer concentration, laser output, and irradiation time.
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What’s new? † To the best of our knowledge, no previous reports have discussed the lesion size acquired by cardiac photodynamic therapy (PDT) in vivo, and this study elucidated the first prescribed conditions for a successful PDT-mediated cardiac catheter ablation. † The PDT-mediated cardiac lesion size was positively correlated with the laser power, irradiation time, and photosensitizer concentration, and was independent of the applied contact force and oxygen concentration. † The concentration of the photosensitizer strongly correlated with the changes in the pulse oximetry data and fluorescence of the backscattering laser, suggesting that a clinically appropriate condition could be estimated in real time.
Methods Our institutional ethics committee approved the study protocol, which also complied with the Declaration of Helsinki. Laser irradiation was applied directly onto the beating heart during open-chest surgery in a canine model under various conditions in order to elucidate the most appropriate conditions for PDT-mediated cardiac lesion creation.
Common procedure We used canines (HBD, Kitayama Company Limited), a crossbreeding of a Beagle, American fox hound and Labrador retriever. Normal saline was infused into the animals via the forelimbs to compensate for any fluid loss. The arterial oxygen saturation (SpO2) was monitored throughout the procedure. After sedation with propofol at a dose of 1 mL/kg, the canines were intubated and ventilated (Excel 110SE, Dartex/omeda, Model SN-480-3, Shinano Incorporated). The general anaesthesia was maintained during the procedure using 1.5% halothane. After isodine sterilization, the sixth left intercostal space was dissected in a side open-chest surgery and the pericardium was cut open. Irradiation points were decided in the anterior wall of the left ventricle and the operating room was kept dark to avoid any extra photosensitivity reactions after the administration of the talaporfin sodium photosensitizer (Meiji Seika Pharma Company Limited).10 Blood samples for assaying were centrifuged immediately after sampling and kept at 2208C until performing the high performance liquid chromatography (HPLC). The dose regimen was based on our previous pharmacokinetic report in a canine model.1 A laser with a wavelength of 663 nm (Optical FuelTM , Sony Company Limited) was used to excite the photosensitizer, which was irradiated from the tip of a laser catheter, as developed in the previous study.1
The tip lens diameter was 1.4 mm and the laser output was set to 300 mW for 20 W/cm2, 150 mW for 10 W/cm2, and 75 mW for 5 W/cm2, respectively. Irradiation was initiated 15 min after the administration of the photosensitizer according to the protocol described below. The chest wall and skin were sutured and the general anaesthesia was ceased after the study. Following the administration of antibiotics (mycillin 0.1 mL/kg), the animals remained alive for 2 – 4 weeks until sacrifice in a breeding facility. The animals were sacrificed by a bolus infusion of propofol at a dose of 2 mL/kg and a potassium infusion to induce ventricular fibrillation.
Study 1: Photosensitizer and laser A total of 27 canines weighing 11.9 + 1.2 kg were used to evaluate the relationships between the lesion size and photosensitizer concentration, laser power, and irradiation time. Blood sampling was performed 15 min after 0, 2.5 and 5 mg/kg administrations of the photosensitizer. The dosing regimen was decided according to a previous pharmacokinetic report, whereby the concentration of the photosensitizer remained constant at approximately 20– 30 mg/mL during the 4 – 6 h after the administration of 1.0 mg/kg in humans.6 The laser power was conditioned to 5, 10 and 20 W/cm2 for 5, 10, 20, 30, 40 and 60 s irradiation times. Each condition was repeated 3 times in different canines.
Study 2: Contact force A total of eight canines weighing 20.9 + 2.2 kg were used to evaluate the relationship between the lesion size and contact force applied during the irradiation. A digital force gauge (DPS-20, Imada Company Limited) was attached to the tip of the catheter to monitor the contact force between the tip of the catheter and the beating heart. The laser irradiation was initiated 15 min after the 2.5 mg/kg of photosensitizer administration followed by a 2.93 mg/kg/h continuous infusion.1 Data sampling was obtained with a rate of 15.63 Hz during 12 irradiations of 10 W/cm2 for 60 s. The catheter was handled manually with the intention to maintain the same contact force on the beating heart. In the low contact-force group, the tip of the catheter was held touching the surface of the heart, to minimize the respiratory and beating effects. In the high contact-force group, the lowest contact force was aimed to be above 20 g. Lesion depths were compared between the low contact-force (,20 g) and high contact-force (.20 g) groups. Blood sampling was performed prior to and following each procedure to measure the biomarkers (SRL, Incorporated).
Study 3: Oxygen A total of two canines weighing 21.3 + 1.8 kg were used to evaluate the relationship between the lesion size and oxygen condition. After the photosensitizer administration (2.5 mg/kg + 2.93 mg/kg/h), the animals were ventilated with room air (room air ventilation) followed by two irradiations of 10 W/cm2 for 60 s and then switched to 100% oxygen (O2 ventilation) with another two irradiations. Arterial blood gas sampling was obtained from the femoral artery through 5-french sheaths and the partial pressure of oxygen in the arterial blood (PaO2) was measured (GemTM Premier 3000TM , Instrumentation Laboratory) prior to the irradiations.
Study 4: Real-time condition monitoring Using eight canines weighing 21.3 + 1.8 kg, we monitored the relationships among the oxygen conditions measured using the SpO2 and PaO2, photosensitizer concentrations measured using HPLC, and backscattering of the laser catheter during the irradiation, to elucidate the feasibility of real-time condition monitoring. The backscattered light was measured
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created under various conditions. Specifically, we varied the laser output, irradiation time, photosensitizer concentration, and blood oxygen concentration in a canine heart in vivo. The method of estimating the photosensitizer concentration by using backscattered fluorescence was also elucidated as a tool for verifying the presence of a sufficient photosensitizer during a clinical procedure. We also evaluated the relationship between the contact force and lesion size because contact force monitoring was shown to be useful in radiofrequency (RF) catheter ablation.8, 9
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by a femtowatt photoreceiver (Model 2151, Newport Corporation) as shown previously.11 Each sample was obtained at baseline and 15 min after the administration of the photosensitizer with the same regimen at various intervals. Because the contact with the vessels or the positioning of the laser catheter might affect the fluorescence, the laser catheter remained in the same position during all measurements.
Histological analysis The canines were kept alive for 2 weeks in study 1 and for 1 month in studies 2 and 3. The tissue was fixed in 10% formalin and the irradiation point was sliced transversely to reveal the largest length of the lesion for staining with hematoxylin-eosin and Masson’s trichrome.
Statistical analysis The minimum sample size for detecting a difference in the measurements of 1 + 0.5 mm with an alpha error of 0.05 and a beta error of 0.2 was calculated as 4 in each group, 2 in case of 1 + 0.3 mm. We set the irradiation number for each condition based on this. The continuous variables are expressed as the mean + standard deviation, and a Mann– Whitney’ U test was used to compare the numerical data. A Pearson’s correlation test and simple linear regression analysis were used. A probability value of , 0.05 was considered to indicate statistical significance.
Results
The PDT-mediated lesion depths and widths had a positive dependency on the laser power, irradiation time, and photosensitizer concentration (Figure 1). The concentration of the photosensitizer after a 2.5 and 5 mg/kg administration was 15.3 + 1.9 and 28.8 + 8.0 mg/mL, respectively. The laser irradiation with 20 W/cm2 for 60 s induced a lesion 8.7 + 0.8 mm deep and 5.2 + 0.2 mm wide under 5 mg/kg of the photosensitizer (Figure 1C and F ). The laser energy by itself, without the photosensitizer (0 mg/kg), induced lesions with a maximum depth of 2.7 + 0.4 in depth and width of 1.5 + 1.1 mm under 20 W/cm2 for 60 s (Figure 1A and D). The depth had a linear correlation with the applied energy, while the width reached a plateau of 4.7 + 0.7 mm at 20 W/cm2 for 20 s under 5 mg/kg. The lesion size for each condition varied widely when the applied laser energy and photosensitizer concentration was insufficient to trigger the PDT reaction as shown by the standard deviation. Stable lesions with a depth of 5 mm and width of 4 mm were obtained with a sufficiently feasible condition, i.e. more than 60 s of 20 W/cm2 irradiation under 2.5 mg/kg, or more than 20 s of 10 W/cm2 irradiation under 5 mg/kg. A typical example of the histological evaluation exhibiting an enlargement of the lesion for each condition is shown in Figure 2A.
Contact force effect A typical example of the contact force monitoring during the laser irradiation on the beating heart is shown in Figure 3A. The average contact force in the low contact-force group was 2.8 + 4.3 g, as compared with 56.1 + 27.1 g in the high contact-force group. There was no significant correlation between the lesion size and average contact force at 10 W/cm2 of irradiation for 60 s (r ¼ 0.021, P ¼ 0.949). There was also no significant difference in lesion size between the
Oxygen condition The relationship between the oxygen level and lesion size was evaluated (Figure 2D). In the air ventilation group, the average PaO2 during irradiation was 103.5 + 2.1 torr, while in the O2 ventilation group, the PaO2 stabilized at 548.0 + 18.4 torr 15 min after 100% O2 ventilation, but SpO2 never reached 100%. The lesion size induced by irradiation with 10 W/cm2 for 60 s did not differ between the groups (4.9 + 0.5 vs. 5.0 + 0.4 mm, P ¼ 0.690, Figure 3C). The average concentration of the photosensitizer for each irradiation was 25.4 + 3.2 mg/mL and there was no significant difference (P ¼ 0.810). There might be no risk of creating a harmfully large lesion even in a situation with sufficient oxygen.
Safety There were no deaths or cardiac tamponades after the irradiation in all canine models. There was no blood charring or any other types of complications throughout the procedure. There was no macroscopic injury to any adjacent organs including the lungs.
Real-time photosensitizer monitoring Because the photosensitizer emits a green signal, the SpO2 data showed an opposite correlation with the photosensitizer concentration during stable PaO2 conditions (Figure 4A). The degree of the decreased SpO2 from the baseline (DSpO2) had a strong positive relation with the photosensitizer concentration (r ¼ 0.859, P , 0.001, Figure 4B); however, the PaO2 was unrelated (r ¼ 20.029, P ¼ 0.798). Thus, there was a mismatch between the SpO2 and PaO2 (r ¼ 0.462, P , 0.001), but the DSpO2 might still be helpful in estimating the photosensitizer concentration, and the SpO2 and PaO2 are regularly controlled by the oxygen supply during clinical circumstances. As it is important to monitor the concentration of the photosensitizer in the extra-cellular PDT, we demonstrated successful real-time monitoring of the concentration using the fluorescence of laser backscattering. The increase in the fluorescence after the administration (Dfluorescence) had a positive linear correlation with the concentration of the photosensitizer (r ¼ 0.950, P , 0.001, Dfluorescence ¼ 0.0007 × photosensitizer concentration, Figure 4C). A simple linear regression analysis revealed that the concentration was estimated by 2.0 + 1353.9 × Dfluorescence (adjusted R 2 ¼ 0.903).
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Conditions for the photosensitizer and laser
groups (4.3 + 1.1 mm in the low contact-force group vs. 4.4 + 0.5 mm in the high contact-force group, P ¼ 0.818, Figure 3B). The average concentration of the photosensitizer was 22.9 + 1.7 mg/mL and it did not significantly differ between the groups (P ¼ 0.836). A histological evaluation at 1 month after the procedure with hematoxylin-eosin and Masson’s trichrome staining showed a solid fibrotic scar lesion and no apparent difference with respect to the contact force (low contact in Figure 2B, high contact in Figure 2C). There were no remarkable changes in the biomarkers taken prior and immediately after the procedure (creatine kinase level: 83.7 + 22.8 vs. 64.3 + 50.8 IU/L, P ¼ 0.355; hemoglobin level: 12.2 + 3.2 vs. 11.7 + 2.6 g/dL, P ¼ 0.731; and lactate dehydrogenase level: 43.7 + 11.2 vs. 33.7 + 26.3 IU/L, P ¼ 0.389), suggesting that there was no acute photohemolysis.
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Discussion To the best of our knowledge, there have been no reports in the literature concerning the lesion size acquired by cardiac PDT in vivo, thus herein we have determined for the first time a suitable condition for PDT-mediated cardiac catheter ablation. Our data showed that the lesion size created was positively correlated with the laser power, irradiation time, and photosensitizer concentration, but was independent of the applied contact force and oxygen
concentration. Our study also revealed that real-time monitoring of the photosensitizer concentration was feasible and practical by measuring the backscattered laser fluorescence. In RF ablation, the parameters that determine the lesion size are the output, duration, and contact force, because the basic aspects of RF ablation depend on an effective tissue heating by an optimal contact of the electrodes.12 Temperature monitoring during RF ablation is no longer a useful parameter due to irrigated catheters13 and contact force has recently become measurable clinically. The optimal
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Figure 2 Histological findings. (A) Typical examples of the photosensitizer dose dependency presented by laser irradiation at 10 W/cm2 for 20 s are shown. The cross section was hematoxylin-eosin stained 2 weeks after the irradiation. (B and C ) Lesions 1 month after the laser irradiation are shown with Masson’s trichrome staining in the low and high contact-force groups (B: low contact, C: high contact). The lesions were replaced completely by fibrotic tissue. (D) Lesions acquired under room air ventilation and 100% O2 are shown for comparison. There was no remarkable difference. The black line indicates 1 mm.
contact force during RF catheter ablation to create sufficient lesions for therapy is .10 g, which was also shown to be important for controlling the risks of steam pops and thrombus formation in vivo 8 and clinical studies.14 – 16 In PDT ablation, the theoretical parameters for controlling the lesion size vary in terms of the reaction components, i.e. the laser parameters, photosensitizer concentration, and oxygen. We clearly demonstrated that the PDT-mediated lesion creation has both a laser-energy and photosensitizer-dose dependency. We fixed the administration regimen to minimize differences due to different metabolic speeds between canines and humans.6 However, the average photosensitizer concentration still varies among canines, and in a previous in vitro study, a concentration of .25 mg/mL was required for permanent cell lethality.17 Therefore, estimating the photosensitizer concentration reliably in the clinical setting is important. Our real-time monitoring might enable the concentration to be maintained above the sufficient level. Regarding the laser power, we also showed that the required energy for creating lesions was 12.3 J/cm2 under 2.5 mg/kg, and 3.1 J/cm2 under 5 mg/kg. Given
that a laser irradiation of less than 25 J/cm2 is reportedly harmless to the normal oesophagus,18 each irradiation condition was within the safety margin. Further, our data showed that there was no damage to the adjacent organs by the irradiations with direct contact to the heart. However, a varying sensitivity to PDT, different photosensitizer concentrations and variations in the distance to the neighbouring organs should be further evaluated, especially for the oesophagus and the phrenic and vagal nerves, because these tissues might be compromised by the RF ablation. The difference in the oxygen level and contact force did not affect the PDT-mediated lesion size, suggesting that the light penetration to the tissue determined the PDT reaction. In comparison to RF ablation, an interesting characteristic of PDT ablation is the needlessness of adequate contact force. Too much contact force might reduce the sensitivity of the myocardial effect when using PDT, and because of an insufficient photosensitizer or oxygen supply, experiments of light penetration during the intravascular application should be performed. As we were not able to identify the PDT-mediated acute lesion, we
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Figure 3 Lesion depth vs. contact force. (A) A contact force monitor was mounted on the tip of the catheter. Typical examples of tracing a low and high contact force during the irradiation are shown. (B) Lesion depth acquired by laser irradiation with high or low contact force was compared. There was no significant difference in the lesion depth due to the contact force applied during the irradiation. (C) Lesion depth acquired by laser irradiation under ventilation with room air and 100% O2 were compared. There was no significant difference in the lesion depth with respect to oxygen concentration.
measured the fibrotic scar lesion, which became thinner several weeks after the irradiation, and therefore the acute lesion depth might be larger than the measured depth. The lesion during the more chronic phase should be evaluated concerning the left ventricular function after multiple laser applications. Further parameters that were not investigated in this study might also be involved in controlling the mechanism of the PDT reactions. While PDT injury is mainly mediated by reactive oxygen-induced apoptosis,19 many other microscopic factors such as reperfusion, angiogenesis, vascular density, and vascular depth might contribute to the PDT lesion size. For instance, shutdown of the normal microvasculature was also reported as a player in the PDT mechanism,20
Figure 4 Real-time monitoring of the photosensitizer. (A) The pulse oximetry (SpO2) had an opposite correlation with the photosensitizer concentration, which was affected by the green photosensitizer fluorescence, even though the arterial blood gas sampling (PaO2) was stable. (B) The magnitude of the SpO2 decrease from baseline (DSpO2) had a positive correlation with the photosensitizer concentration. (C ) The difference in the backscattering laser between that before and after the administration of the photosensitizer (Dfluorescence) had a linear positive correlation with the photosensitizer concentration, suggesting that the photosensitizer concentration might be estimated in real time in a clinical situation.
and intravital microscopy of the rat cremaster muscle revealed a dose-related constriction of arterioles followed by venules.21 The magnitude of the direct cytotoxicity could also depend on both the amount of photosensitizer delivered and the delay between the administration and the laser irradiation.22 In a similar study, the photosensitizer was predominantly confined to the vasculature, and then subsequently redistributed throughout the extravascular regions with no difference in the extravasation rate between tumour and normal tissues.23 Treatment regimens should therefore be determined with consideration of both the vascular stasis and photosensitizer distribution. From a clinical viewpoint, it is quite important to evaluate the possible effect that is triggered because of the photosensitizer perfusion heterogeneity in cardiac ischaemia or remodelling
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due to tachycardias. After these microscopic factors affecting the lesion size for each organ are clarified, the PDT-mediated lesion size might be sufficiently calculated11 and controlled for a range of clinical applications.
Limitations The lesion size acquired during PDT by an intravascular approach might differ from that acquired in other clinical settings because of the blood flow between the catheter and tissue. The maximum depth of the lesion was limited by the thickness of the myocardium and thus the direction of the cross sections or laser irradiations might influence the lesion size; however, this was inevitable because the in vivo study was performed on a beating heart. In addition, lower oxygen conditions should be investigated, such as ischaemia; however, it was difficult to create a reproducible ischaemic model quantitatively and to measure the additional PDT lesion on that model. The relationship between the histological lesion and electrical change should also be performed in future studies.
Conclusions
Acknowledgements We thank Arisa Ito, PhD at Arai-Medphoton Research Laboratories Corporation for the technical advice. We also received generous support from Meiji Seika Pharma Company Limited. Conflict of interest: Takehiro Kimura reports grants from AraiMedphoton Research Laboratories Corporation. Takehiro Kimura has a patent regarding this technology pending.
Funding This work was supported by Keio University Grant-in-Aid for Encouragement of Young Medical Scientists [02-002-0002].
References 1. Kimura T, Takatsuki S, Miyoshi S, Fukumoto K, Takahashi M, Ogawa E et al. Nonthermal cardiac catheter ablation using photodynamic therapy. Circ Arrhythm Electrophysiol 2013;6:1025 –31. 2. Muragaki Y, Akimoto J, Maruyama T, Iseki H, Ikuta S, Nitta M et al. Phase II clinical study on intraoperative photodynamic therapy with talaporfin sodium and semiconductor laser in patients with malignant brain tumors. J Neurosurg 2013;119: 845 – 52. 3. Yano T, Muto M, Yoshimura K, Niimi M, Ezoe Y, Yoda Y et al. Phase I study of photodynamic therapy using talaporfin sodium and diode laser for local failure after chemoradiotherapy for esophageal cancer. Radiat Oncol 2012;7:113.
4. Kawakubo M, Eguchi K, Arai T, Kobayashi K, Hamblin MR. Surface layer-preserving photodynamic therapy (SPPDT) in a subcutaneous mouse model of lung cancer. Lasers Surg Med 2012;44:500–7. 5. Tsutsumi M, Miki Y, Akimoto J, Haraoka J, Aizawa K, Hirano K et al. Photodynamic therapy with talaporfin sodium induces dose-dependent apoptotic cell death in human glioma cell lines. Photodiagnosis Photodyn Ther 2013;10:103 –10. 6. Kessel D. Pharmacokinetics of N-aspartyl chlorin e6 in cancer patients. J Photochem Photobiol B 1997;39:81 –3. 7. McCaughan JS Jr. Photodynamic therapy: a review. Drugs Aging 1999;15:49 –68. 8. Yokoyama K, Nakagawa H, Shah DC, Lambert H, Leo G, Aeby N et al. Novel contact force sensor incorporated in irrigated radiofrequency ablation catheter predicts lesion size and incidence of steam pop and thrombus. Circ Arrhythm Electrophysiol 2008;1:354 –62. 9. Tilz RR, Makimoto H, Lin T, Rillig A, Metzner A, Mathew S et al. In vivo left-ventricular contact force analysis: comparison of antegrade transseptal with retrograde transaortic mapping strategies and correlation of impedance and electrical amplitude with contact force. Europace 2014;16:1387 –95. 10. Wang S, Bromley E, Xu L, Chen JC, Keltner L. Talaporfin sodium. Expert Opin Pharmacother 2010;11:133–40. 11. Takahashi M, Ito A, Kimura T, Takatsuki S, Fukuda K, Arai T. Myocardial necrosis depth prediction during extracellular photosensitization reaction of talaporfin sodium by defined index using fluorescence measurement. Lasers Med Sci 2014; 29:1173 –81. 12. Nath S, DiMarco JP, Haines DE. Basic aspects of radiofrequency catheter ablation. J Cardiovasc Electrophysiol 1994;5:863 –76. 13. Knecht S, Sacher F, Forclaz A, Verbeet T, Hocini M, Wright M et al. Is there a potential benefit to increased irrigation channels during radiofrequency ablation? Results from a two-center prospective randomized study. J Cardiovasc Electrophysiol 2011;22: 516 –20. 14. Kuck KH, Reddy VY, Schmidt B, Natale A, Neuzil P, Saoudi N et al. A novel radiofrequency ablation catheter using contact force sensing: Toccata study. Heart Rhythm 2012;9:18 –23. 15. Squara F, Latcu DG, Massaad Y, Mahjoub M, Bun SS, Saoudi N. Contact force and force-time integral in atrial radiofrequency ablation predict transmurality of lesions. Europace 2014;16:660 –7. 16. Provideˆncia R, Marijon E, Combes S, Bouzeman A, Jourda F, Khoueiry Z et al. Higher contact-force values associated with better mid-term outcome of paroxysmal atrial fibrillation ablation using the SmartTouchTM catheter. Europace 2015;17:56 –63. 17. Ogawa E, Ito A, Arai T. Detailed in vitro study of the photosensitization reaction of extracellular talaporfin sodium in rat myocardial cells. Lasers Surg Med 2013;45: 660 –7. 18. Horimatsu T, Muto M, Yoda Y, Yano T, Ezoe Y, Miyamoto S et al. Tissue damage in the canine normal esophagus by photoactivation with talaporfin sodium (laserphyrin): a preclinical study. PLoS One 2012;7:e38308. 19. Liu L, Zhang Z, Xing D. Cell death via mitochondrial apoptotic pathway due to activation of Bax by lysosomal photodamage. Free Radic Biol Med 2011;51: 53 – 68. 20. Moy WJ, Patel SJ, Lertsakdadet BS, Arora RP, Nielsen KM, Kelly KM et al. Preclinical in vivo evaluation of NPe6-mediated photodynamic therapy on normal vasculature. Lasers Surg Med 2012;44:158–62. 21. Fingar VH, Wieman TJ, Wiehle SA, Cerrito PB. The role of microvascular damage in photodynamic therapy: the effect of treatment on vessel constriction, permeability, and leukocyte adhesion. Cancer Res 1992;52:4914 –21. 22. McMahon KS, Wieman TJ, Moore PH, Fingar VH. Effects of photodynamic therapy using mono-L-aspartyl chlorin e6 on vessel constriction, vessel leakage, and tumor response. Cancer Res 1994;54:5374 –9. 23. Mitra S, Foster TH. In vivo confocal fluorescence imaging of the intratumor distribution of the photosensitizer mono-L-aspartylchlorin-e6. Neoplasia 2008;10: 429 – 38.
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Photodynamic therapy-mediated cardiac lesions might be sufficiently controllable by monitoring the photosensitizer concentration and laser energy.
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Europace (2015) 17, 1309–1315 doi:10.1093/europace/euu335
Optimal conditions for cardiac catheter ablation using photodynamic therapy Takehiro Kimura1*, Seiji Takatsuki 1, Shunichiro Miyoshi 1, Mei Takahashi 2, Emiyu Ogawa2, Yoshinori Katsumata 1, Takahiko Nishiyama 1, Nobuhiro Nishiyama 1, Yoko Tanimoto1, Yoshiyasu Aizawa1, Tsunenori Arai 2, and Keiichi Fukuda 1 1 Department of Cardiology, Keio University School of Medicine, 35 Shinanomachi Shinjuku-ku, Tokyo 160-8582, Japan; and 2School of Fundamental Science and Technology, Graduate School of Science and Technology, Keio University, Kanagawa, Japan
Received 26 July 2014; accepted after revision 17 October 2014; online publish-ahead-of-print 6 January 2015
Aims
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Ablation † Arrhythmia † Photodynamic therapy † Talaporfin sodium
Introduction We previously reported the usefulness of photodynamic therapy (PDT)-mediated cardiac catheter ablation using a four-polar laser catheter in a canine model.1 However, maximizing the benefits and minimizing the complications associated with future clinical applications of this technology are critically dependent on controlling the lesion size created. We therefore sought to determine the optimal conditions and critical factors involved in PDT-mediated lesion creation in vivo for cardiac ablation. Originally, PDT was used for treating malignant brain tumours,2 esophageal cancer,3 and lung cancer,4 with variations in the sensitivity to PDT among cancers dependent simply on the accumulated dose of
the photosensitizer.5 The advent of laser irradiation was thus a major advance for PDT to enable a targeted photosensitizer accumulation in tumour cells only and the disappearance of any accumulation in the surrounding normal tissue. However, in PDT-mediated cardiac ablation, the laser is irradiated during the capillary perfusion of the photosensitizer bound to plasma albumin and other heavy proteins,6 the so-called extra-cellular PDT, and no uptake occurs in the heart. Therefore, the ability to regulate the PDT reaction during cardiac catheter ablation is clearly critical for reducing the risk of injury to adjacent organs. Considering that PDT is based on the generation of tissue injury via singlet oxygen species using a combination of laser irradiation, photosensitizer administration, and oxygen,7 we evaluated the lesion size
* Corresponding author. Tel: +81 3 3353 1211 (ext. 61421); fax: +81 3 5363 3875, E-mail address: [email protected] Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2015. For permissions please email: [email protected].
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Photodynamic therapy (PDT) is based on non-thermal injury mediated by singlet oxygen species and is used clinically in cancer therapy. In our continuing efforts to apply this technology to cardiac catheter ablation, we clarified the optimal condition for creating PDT-mediated lesions using a laser catheter. ..................................................................................................................................................................................... Methods In a total of 35 canines, we applied a laser directly to the epicardium of the beating heart during open-chest surgery at and results 15 min after administration of a photosensitizer, talaporfin sodium. We evaluated the lesion size (depth and width) using hematoxylin-eosin staining under varying conditions as follows: laser output (5, 10, 20 W/cm2), irradiation time (0– 60 s), photosensitizer concentration (0, 2.5, 5 mg/kg), blood oxygen concentration (103.5 + 2.1 vs. 548.0 + 18.4 torr), and contact force applied during irradiations (low: ,20 g, high: .20 g). A laser irradiation at 20 W/cm2 for 60 s under 5 mg/kg (29 mg/mL) of photosensitizer induced a lesion 8.7 + 0.8 mm deep and 5.2 + 0.2 mm wide. The lesion size was thus positively correlated to the laser power, irradiation time, and photosensitizer concentration, and was independent of the applied contact force and oxygen concentration. In addition, the concentration of the photosensitizer strongly correlated with the changes in the pulse oximetry data and fluorescence of the backscattering laser, suggesting that a clinically appropriate condition could be estimated in real time. ..................................................................................................................................................................................... Conclusion Photodynamic therapy-mediated cardiac lesions might be controllable by regulating the photosensitizer concentration, laser output, and irradiation time.
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What’s new? † To the best of our knowledge, no previous reports have discussed the lesion size acquired by cardiac photodynamic therapy (PDT) in vivo, and this study elucidated the first prescribed conditions for a successful PDT-mediated cardiac catheter ablation. † The PDT-mediated cardiac lesion size was positively correlated with the laser power, irradiation time, and photosensitizer concentration, and was independent of the applied contact force and oxygen concentration. † The concentration of the photosensitizer strongly correlated with the changes in the pulse oximetry data and fluorescence of the backscattering laser, suggesting that a clinically appropriate condition could be estimated in real time.
Methods Our institutional ethics committee approved the study protocol, which also complied with the Declaration of Helsinki. Laser irradiation was applied directly onto the beating heart during open-chest surgery in a canine model under various conditions in order to elucidate the most appropriate conditions for PDT-mediated cardiac lesion creation.
Common procedure We used canines (HBD, Kitayama Company Limited), a crossbreeding of a Beagle, American fox hound and Labrador retriever. Normal saline was infused into the animals via the forelimbs to compensate for any fluid loss. The arterial oxygen saturation (SpO2) was monitored throughout the procedure. After sedation with propofol at a dose of 1 mL/kg, the canines were intubated and ventilated (Excel 110SE, Dartex/omeda, Model SN-480-3, Shinano Incorporated). The general anaesthesia was maintained during the procedure using 1.5% halothane. After isodine sterilization, the sixth left intercostal space was dissected in a side open-chest surgery and the pericardium was cut open. Irradiation points were decided in the anterior wall of the left ventricle and the operating room was kept dark to avoid any extra photosensitivity reactions after the administration of the talaporfin sodium photosensitizer (Meiji Seika Pharma Company Limited).10 Blood samples for assaying were centrifuged immediately after sampling and kept at 2208C until performing the high performance liquid chromatography (HPLC). The dose regimen was based on our previous pharmacokinetic report in a canine model.1 A laser with a wavelength of 663 nm (Optical FuelTM , Sony Company Limited) was used to excite the photosensitizer, which was irradiated from the tip of a laser catheter, as developed in the previous study.1
The tip lens diameter was 1.4 mm and the laser output was set to 300 mW for 20 W/cm2, 150 mW for 10 W/cm2, and 75 mW for 5 W/cm2, respectively. Irradiation was initiated 15 min after the administration of the photosensitizer according to the protocol described below. The chest wall and skin were sutured and the general anaesthesia was ceased after the study. Following the administration of antibiotics (mycillin 0.1 mL/kg), the animals remained alive for 2 – 4 weeks until sacrifice in a breeding facility. The animals were sacrificed by a bolus infusion of propofol at a dose of 2 mL/kg and a potassium infusion to induce ventricular fibrillation.
Study 1: Photosensitizer and laser A total of 27 canines weighing 11.9 + 1.2 kg were used to evaluate the relationships between the lesion size and photosensitizer concentration, laser power, and irradiation time. Blood sampling was performed 15 min after 0, 2.5 and 5 mg/kg administrations of the photosensitizer. The dosing regimen was decided according to a previous pharmacokinetic report, whereby the concentration of the photosensitizer remained constant at approximately 20– 30 mg/mL during the 4 – 6 h after the administration of 1.0 mg/kg in humans.6 The laser power was conditioned to 5, 10 and 20 W/cm2 for 5, 10, 20, 30, 40 and 60 s irradiation times. Each condition was repeated 3 times in different canines.
Study 2: Contact force A total of eight canines weighing 20.9 + 2.2 kg were used to evaluate the relationship between the lesion size and contact force applied during the irradiation. A digital force gauge (DPS-20, Imada Company Limited) was attached to the tip of the catheter to monitor the contact force between the tip of the catheter and the beating heart. The laser irradiation was initiated 15 min after the 2.5 mg/kg of photosensitizer administration followed by a 2.93 mg/kg/h continuous infusion.1 Data sampling was obtained with a rate of 15.63 Hz during 12 irradiations of 10 W/cm2 for 60 s. The catheter was handled manually with the intention to maintain the same contact force on the beating heart. In the low contact-force group, the tip of the catheter was held touching the surface of the heart, to minimize the respiratory and beating effects. In the high contact-force group, the lowest contact force was aimed to be above 20 g. Lesion depths were compared between the low contact-force (,20 g) and high contact-force (.20 g) groups. Blood sampling was performed prior to and following each procedure to measure the biomarkers (SRL, Incorporated).
Study 3: Oxygen A total of two canines weighing 21.3 + 1.8 kg were used to evaluate the relationship between the lesion size and oxygen condition. After the photosensitizer administration (2.5 mg/kg + 2.93 mg/kg/h), the animals were ventilated with room air (room air ventilation) followed by two irradiations of 10 W/cm2 for 60 s and then switched to 100% oxygen (O2 ventilation) with another two irradiations. Arterial blood gas sampling was obtained from the femoral artery through 5-french sheaths and the partial pressure of oxygen in the arterial blood (PaO2) was measured (GemTM Premier 3000TM , Instrumentation Laboratory) prior to the irradiations.
Study 4: Real-time condition monitoring Using eight canines weighing 21.3 + 1.8 kg, we monitored the relationships among the oxygen conditions measured using the SpO2 and PaO2, photosensitizer concentrations measured using HPLC, and backscattering of the laser catheter during the irradiation, to elucidate the feasibility of real-time condition monitoring. The backscattered light was measured
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created under various conditions. Specifically, we varied the laser output, irradiation time, photosensitizer concentration, and blood oxygen concentration in a canine heart in vivo. The method of estimating the photosensitizer concentration by using backscattered fluorescence was also elucidated as a tool for verifying the presence of a sufficient photosensitizer during a clinical procedure. We also evaluated the relationship between the contact force and lesion size because contact force monitoring was shown to be useful in radiofrequency (RF) catheter ablation.8, 9
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by a femtowatt photoreceiver (Model 2151, Newport Corporation) as shown previously.11 Each sample was obtained at baseline and 15 min after the administration of the photosensitizer with the same regimen at various intervals. Because the contact with the vessels or the positioning of the laser catheter might affect the fluorescence, the laser catheter remained in the same position during all measurements.
Histological analysis The canines were kept alive for 2 weeks in study 1 and for 1 month in studies 2 and 3. The tissue was fixed in 10% formalin and the irradiation point was sliced transversely to reveal the largest length of the lesion for staining with hematoxylin-eosin and Masson’s trichrome.
Statistical analysis The minimum sample size for detecting a difference in the measurements of 1 + 0.5 mm with an alpha error of 0.05 and a beta error of 0.2 was calculated as 4 in each group, 2 in case of 1 + 0.3 mm. We set the irradiation number for each condition based on this. The continuous variables are expressed as the mean + standard deviation, and a Mann– Whitney’ U test was used to compare the numerical data. A Pearson’s correlation test and simple linear regression analysis were used. A probability value of , 0.05 was considered to indicate statistical significance.
Results
The PDT-mediated lesion depths and widths had a positive dependency on the laser power, irradiation time, and photosensitizer concentration (Figure 1). The concentration of the photosensitizer after a 2.5 and 5 mg/kg administration was 15.3 + 1.9 and 28.8 + 8.0 mg/mL, respectively. The laser irradiation with 20 W/cm2 for 60 s induced a lesion 8.7 + 0.8 mm deep and 5.2 + 0.2 mm wide under 5 mg/kg of the photosensitizer (Figure 1C and F ). The laser energy by itself, without the photosensitizer (0 mg/kg), induced lesions with a maximum depth of 2.7 + 0.4 in depth and width of 1.5 + 1.1 mm under 20 W/cm2 for 60 s (Figure 1A and D). The depth had a linear correlation with the applied energy, while the width reached a plateau of 4.7 + 0.7 mm at 20 W/cm2 for 20 s under 5 mg/kg. The lesion size for each condition varied widely when the applied laser energy and photosensitizer concentration was insufficient to trigger the PDT reaction as shown by the standard deviation. Stable lesions with a depth of 5 mm and width of 4 mm were obtained with a sufficiently feasible condition, i.e. more than 60 s of 20 W/cm2 irradiation under 2.5 mg/kg, or more than 20 s of 10 W/cm2 irradiation under 5 mg/kg. A typical example of the histological evaluation exhibiting an enlargement of the lesion for each condition is shown in Figure 2A.
Contact force effect A typical example of the contact force monitoring during the laser irradiation on the beating heart is shown in Figure 3A. The average contact force in the low contact-force group was 2.8 + 4.3 g, as compared with 56.1 + 27.1 g in the high contact-force group. There was no significant correlation between the lesion size and average contact force at 10 W/cm2 of irradiation for 60 s (r ¼ 0.021, P ¼ 0.949). There was also no significant difference in lesion size between the
Oxygen condition The relationship between the oxygen level and lesion size was evaluated (Figure 2D). In the air ventilation group, the average PaO2 during irradiation was 103.5 + 2.1 torr, while in the O2 ventilation group, the PaO2 stabilized at 548.0 + 18.4 torr 15 min after 100% O2 ventilation, but SpO2 never reached 100%. The lesion size induced by irradiation with 10 W/cm2 for 60 s did not differ between the groups (4.9 + 0.5 vs. 5.0 + 0.4 mm, P ¼ 0.690, Figure 3C). The average concentration of the photosensitizer for each irradiation was 25.4 + 3.2 mg/mL and there was no significant difference (P ¼ 0.810). There might be no risk of creating a harmfully large lesion even in a situation with sufficient oxygen.
Safety There were no deaths or cardiac tamponades after the irradiation in all canine models. There was no blood charring or any other types of complications throughout the procedure. There was no macroscopic injury to any adjacent organs including the lungs.
Real-time photosensitizer monitoring Because the photosensitizer emits a green signal, the SpO2 data showed an opposite correlation with the photosensitizer concentration during stable PaO2 conditions (Figure 4A). The degree of the decreased SpO2 from the baseline (DSpO2) had a strong positive relation with the photosensitizer concentration (r ¼ 0.859, P , 0.001, Figure 4B); however, the PaO2 was unrelated (r ¼ 20.029, P ¼ 0.798). Thus, there was a mismatch between the SpO2 and PaO2 (r ¼ 0.462, P , 0.001), but the DSpO2 might still be helpful in estimating the photosensitizer concentration, and the SpO2 and PaO2 are regularly controlled by the oxygen supply during clinical circumstances. As it is important to monitor the concentration of the photosensitizer in the extra-cellular PDT, we demonstrated successful real-time monitoring of the concentration using the fluorescence of laser backscattering. The increase in the fluorescence after the administration (Dfluorescence) had a positive linear correlation with the concentration of the photosensitizer (r ¼ 0.950, P , 0.001, Dfluorescence ¼ 0.0007 × photosensitizer concentration, Figure 4C). A simple linear regression analysis revealed that the concentration was estimated by 2.0 + 1353.9 × Dfluorescence (adjusted R 2 ¼ 0.903).
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groups (4.3 + 1.1 mm in the low contact-force group vs. 4.4 + 0.5 mm in the high contact-force group, P ¼ 0.818, Figure 3B). The average concentration of the photosensitizer was 22.9 + 1.7 mg/mL and it did not significantly differ between the groups (P ¼ 0.836). A histological evaluation at 1 month after the procedure with hematoxylin-eosin and Masson’s trichrome staining showed a solid fibrotic scar lesion and no apparent difference with respect to the contact force (low contact in Figure 2B, high contact in Figure 2C). There were no remarkable changes in the biomarkers taken prior and immediately after the procedure (creatine kinase level: 83.7 + 22.8 vs. 64.3 + 50.8 IU/L, P ¼ 0.355; hemoglobin level: 12.2 + 3.2 vs. 11.7 + 2.6 g/dL, P ¼ 0.731; and lactate dehydrogenase level: 43.7 + 11.2 vs. 33.7 + 26.3 IU/L, P ¼ 0.389), suggesting that there was no acute photohemolysis.
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Figure 1 Lesion depth vs. photosensitizer concentration vs. laser energy. The lesion depths (A– C) and width (D –F ) acquired by laser irradiation at 5, 10, and 20 W/cm2 after the administration of 0 (A and D), 2.5 (B and E) and 5 (C and F ) mg/kg of the photosensitizer are shown. The lesion depth showed a positive relationship to the photosensitizer concentration and laser energy.
Discussion To the best of our knowledge, there have been no reports in the literature concerning the lesion size acquired by cardiac PDT in vivo, thus herein we have determined for the first time a suitable condition for PDT-mediated cardiac catheter ablation. Our data showed that the lesion size created was positively correlated with the laser power, irradiation time, and photosensitizer concentration, but was independent of the applied contact force and oxygen
concentration. Our study also revealed that real-time monitoring of the photosensitizer concentration was feasible and practical by measuring the backscattered laser fluorescence. In RF ablation, the parameters that determine the lesion size are the output, duration, and contact force, because the basic aspects of RF ablation depend on an effective tissue heating by an optimal contact of the electrodes.12 Temperature monitoring during RF ablation is no longer a useful parameter due to irrigated catheters13 and contact force has recently become measurable clinically. The optimal
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Figure 2 Histological findings. (A) Typical examples of the photosensitizer dose dependency presented by laser irradiation at 10 W/cm2 for 20 s are shown. The cross section was hematoxylin-eosin stained 2 weeks after the irradiation. (B and C ) Lesions 1 month after the laser irradiation are shown with Masson’s trichrome staining in the low and high contact-force groups (B: low contact, C: high contact). The lesions were replaced completely by fibrotic tissue. (D) Lesions acquired under room air ventilation and 100% O2 are shown for comparison. There was no remarkable difference. The black line indicates 1 mm.
contact force during RF catheter ablation to create sufficient lesions for therapy is .10 g, which was also shown to be important for controlling the risks of steam pops and thrombus formation in vivo 8 and clinical studies.14 – 16 In PDT ablation, the theoretical parameters for controlling the lesion size vary in terms of the reaction components, i.e. the laser parameters, photosensitizer concentration, and oxygen. We clearly demonstrated that the PDT-mediated lesion creation has both a laser-energy and photosensitizer-dose dependency. We fixed the administration regimen to minimize differences due to different metabolic speeds between canines and humans.6 However, the average photosensitizer concentration still varies among canines, and in a previous in vitro study, a concentration of .25 mg/mL was required for permanent cell lethality.17 Therefore, estimating the photosensitizer concentration reliably in the clinical setting is important. Our real-time monitoring might enable the concentration to be maintained above the sufficient level. Regarding the laser power, we also showed that the required energy for creating lesions was 12.3 J/cm2 under 2.5 mg/kg, and 3.1 J/cm2 under 5 mg/kg. Given
that a laser irradiation of less than 25 J/cm2 is reportedly harmless to the normal oesophagus,18 each irradiation condition was within the safety margin. Further, our data showed that there was no damage to the adjacent organs by the irradiations with direct contact to the heart. However, a varying sensitivity to PDT, different photosensitizer concentrations and variations in the distance to the neighbouring organs should be further evaluated, especially for the oesophagus and the phrenic and vagal nerves, because these tissues might be compromised by the RF ablation. The difference in the oxygen level and contact force did not affect the PDT-mediated lesion size, suggesting that the light penetration to the tissue determined the PDT reaction. In comparison to RF ablation, an interesting characteristic of PDT ablation is the needlessness of adequate contact force. Too much contact force might reduce the sensitivity of the myocardial effect when using PDT, and because of an insufficient photosensitizer or oxygen supply, experiments of light penetration during the intravascular application should be performed. As we were not able to identify the PDT-mediated acute lesion, we
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Figure 3 Lesion depth vs. contact force. (A) A contact force monitor was mounted on the tip of the catheter. Typical examples of tracing a low and high contact force during the irradiation are shown. (B) Lesion depth acquired by laser irradiation with high or low contact force was compared. There was no significant difference in the lesion depth due to the contact force applied during the irradiation. (C) Lesion depth acquired by laser irradiation under ventilation with room air and 100% O2 were compared. There was no significant difference in the lesion depth with respect to oxygen concentration.
measured the fibrotic scar lesion, which became thinner several weeks after the irradiation, and therefore the acute lesion depth might be larger than the measured depth. The lesion during the more chronic phase should be evaluated concerning the left ventricular function after multiple laser applications. Further parameters that were not investigated in this study might also be involved in controlling the mechanism of the PDT reactions. While PDT injury is mainly mediated by reactive oxygen-induced apoptosis,19 many other microscopic factors such as reperfusion, angiogenesis, vascular density, and vascular depth might contribute to the PDT lesion size. For instance, shutdown of the normal microvasculature was also reported as a player in the PDT mechanism,20
Figure 4 Real-time monitoring of the photosensitizer. (A) The pulse oximetry (SpO2) had an opposite correlation with the photosensitizer concentration, which was affected by the green photosensitizer fluorescence, even though the arterial blood gas sampling (PaO2) was stable. (B) The magnitude of the SpO2 decrease from baseline (DSpO2) had a positive correlation with the photosensitizer concentration. (C ) The difference in the backscattering laser between that before and after the administration of the photosensitizer (Dfluorescence) had a linear positive correlation with the photosensitizer concentration, suggesting that the photosensitizer concentration might be estimated in real time in a clinical situation.
and intravital microscopy of the rat cremaster muscle revealed a dose-related constriction of arterioles followed by venules.21 The magnitude of the direct cytotoxicity could also depend on both the amount of photosensitizer delivered and the delay between the administration and the laser irradiation.22 In a similar study, the photosensitizer was predominantly confined to the vasculature, and then subsequently redistributed throughout the extravascular regions with no difference in the extravasation rate between tumour and normal tissues.23 Treatment regimens should therefore be determined with consideration of both the vascular stasis and photosensitizer distribution. From a clinical viewpoint, it is quite important to evaluate the possible effect that is triggered because of the photosensitizer perfusion heterogeneity in cardiac ischaemia or remodelling
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due to tachycardias. After these microscopic factors affecting the lesion size for each organ are clarified, the PDT-mediated lesion size might be sufficiently calculated11 and controlled for a range of clinical applications.
Limitations The lesion size acquired during PDT by an intravascular approach might differ from that acquired in other clinical settings because of the blood flow between the catheter and tissue. The maximum depth of the lesion was limited by the thickness of the myocardium and thus the direction of the cross sections or laser irradiations might influence the lesion size; however, this was inevitable because the in vivo study was performed on a beating heart. In addition, lower oxygen conditions should be investigated, such as ischaemia; however, it was difficult to create a reproducible ischaemic model quantitatively and to measure the additional PDT lesion on that model. The relationship between the histological lesion and electrical change should also be performed in future studies.
Conclusions
Acknowledgements We thank Arisa Ito, PhD at Arai-Medphoton Research Laboratories Corporation for the technical advice. We also received generous support from Meiji Seika Pharma Company Limited. Conflict of interest: Takehiro Kimura reports grants from AraiMedphoton Research Laboratories Corporation. Takehiro Kimura has a patent regarding this technology pending.
Funding This work was supported by Keio University Grant-in-Aid for Encouragement of Young Medical Scientists [02-002-0002].
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4. Kawakubo M, Eguchi K, Arai T, Kobayashi K, Hamblin MR. Surface layer-preserving photodynamic therapy (SPPDT) in a subcutaneous mouse model of lung cancer. Lasers Surg Med 2012;44:500–7. 5. Tsutsumi M, Miki Y, Akimoto J, Haraoka J, Aizawa K, Hirano K et al. Photodynamic therapy with talaporfin sodium induces dose-dependent apoptotic cell death in human glioma cell lines. Photodiagnosis Photodyn Ther 2013;10:103 –10. 6. Kessel D. Pharmacokinetics of N-aspartyl chlorin e6 in cancer patients. J Photochem Photobiol B 1997;39:81 –3. 7. McCaughan JS Jr. Photodynamic therapy: a review. Drugs Aging 1999;15:49 –68. 8. Yokoyama K, Nakagawa H, Shah DC, Lambert H, Leo G, Aeby N et al. Novel contact force sensor incorporated in irrigated radiofrequency ablation catheter predicts lesion size and incidence of steam pop and thrombus. Circ Arrhythm Electrophysiol 2008;1:354 –62. 9. Tilz RR, Makimoto H, Lin T, Rillig A, Metzner A, Mathew S et al. In vivo left-ventricular contact force analysis: comparison of antegrade transseptal with retrograde transaortic mapping strategies and correlation of impedance and electrical amplitude with contact force. Europace 2014;16:1387 –95. 10. Wang S, Bromley E, Xu L, Chen JC, Keltner L. Talaporfin sodium. Expert Opin Pharmacother 2010;11:133–40. 11. Takahashi M, Ito A, Kimura T, Takatsuki S, Fukuda K, Arai T. Myocardial necrosis depth prediction during extracellular photosensitization reaction of talaporfin sodium by defined index using fluorescence measurement. Lasers Med Sci 2014; 29:1173 –81. 12. Nath S, DiMarco JP, Haines DE. Basic aspects of radiofrequency catheter ablation. J Cardiovasc Electrophysiol 1994;5:863 –76. 13. Knecht S, Sacher F, Forclaz A, Verbeet T, Hocini M, Wright M et al. Is there a potential benefit to increased irrigation channels during radiofrequency ablation? Results from a two-center prospective randomized study. J Cardiovasc Electrophysiol 2011;22: 516 –20. 14. Kuck KH, Reddy VY, Schmidt B, Natale A, Neuzil P, Saoudi N et al. A novel radiofrequency ablation catheter using contact force sensing: Toccata study. Heart Rhythm 2012;9:18 –23. 15. Squara F, Latcu DG, Massaad Y, Mahjoub M, Bun SS, Saoudi N. Contact force and force-time integral in atrial radiofrequency ablation predict transmurality of lesions. Europace 2014;16:660 –7. 16. Provideˆncia R, Marijon E, Combes S, Bouzeman A, Jourda F, Khoueiry Z et al. Higher contact-force values associated with better mid-term outcome of paroxysmal atrial fibrillation ablation using the SmartTouchTM catheter. Europace 2015;17:56 –63. 17. Ogawa E, Ito A, Arai T. Detailed in vitro study of the photosensitization reaction of extracellular talaporfin sodium in rat myocardial cells. Lasers Surg Med 2013;45: 660 –7. 18. Horimatsu T, Muto M, Yoda Y, Yano T, Ezoe Y, Miyamoto S et al. Tissue damage in the canine normal esophagus by photoactivation with talaporfin sodium (laserphyrin): a preclinical study. PLoS One 2012;7:e38308. 19. Liu L, Zhang Z, Xing D. Cell death via mitochondrial apoptotic pathway due to activation of Bax by lysosomal photodamage. Free Radic Biol Med 2011;51: 53 – 68. 20. Moy WJ, Patel SJ, Lertsakdadet BS, Arora RP, Nielsen KM, Kelly KM et al. Preclinical in vivo evaluation of NPe6-mediated photodynamic therapy on normal vasculature. Lasers Surg Med 2012;44:158–62. 21. Fingar VH, Wieman TJ, Wiehle SA, Cerrito PB. The role of microvascular damage in photodynamic therapy: the effect of treatment on vessel constriction, permeability, and leukocyte adhesion. Cancer Res 1992;52:4914 –21. 22. McMahon KS, Wieman TJ, Moore PH, Fingar VH. Effects of photodynamic therapy using mono-L-aspartyl chlorin e6 on vessel constriction, vessel leakage, and tumor response. Cancer Res 1994;54:5374 –9. 23. Mitra S, Foster TH. In vivo confocal fluorescence imaging of the intratumor distribution of the photosensitizer mono-L-aspartylchlorin-e6. Neoplasia 2008;10: 429 – 38.
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Photodynamic therapy-mediated cardiac lesions might be sufficiently controllable by monitoring the photosensitizer concentration and laser energy.
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