Search

Background/Aims Recent research has increasingly focused on the role of the gastric microbiome in the development of gastric cancer. We aimed to investigate the changes in the microbiome during gastric carcinogenesis in structural and functional aspects, with a specific focus on the association between oral and gastric microbiomes. Methods We collected saliva, gastric juice, and gastric tissue samples from 141 patients at different stages of gastric carcinogenesis and processed them for microbiome analysis using 16S rRNA gene profiling. The alpha and beta diversities were analyzed, and the differences in microbiome composition and function profiles were analyzed among the groups, as well as the correlation between changes in the oral and gastric microbiomes during carcinogenesis. Results We observed significant differences in microbial diversity and composition between the disease and control groups, primarily in the gastric juice. Specific bacterial strains, including Schaalia odontolytica, Streptococcus cristatus, and Peptostreptococcus stomatis, showed a significant increase in abundance in the gastric juice in the low-grade dysplasia and gastric cancer groups. Notably, the correlation between the oral and gastric microbiota compositions, increased as the disease progressed. Predictive analysis of the metagenomic functional profiles revealed changes in functional pathways that may be associated with carcinogenesis (ABC transport and two-component systems). Conclusions During gastric carcinogenesis, the abundance of oral commensals associated with cancer increased in the stomach. The similarity in microbial composition between the stomach and oral cavity also increased, implying a potential role of oral-gastric bacterial interactions in gastric cancer development.

Abstract Human–machine interactive platforms that can sense mechanical stimuli visually and digitally are highly desirable. However, most existing interactive devices cannot satisfy the demands of tactile feedback and extended integration. Inspired by the mechanoluminescence (ML) function of cephalopod skin and the sensitive perception of microcracked slit‐organs, a bioinspired stretchable interactive platform is developed by designing a stretchable poly(styrene‐block‐butadiene‐block‐styrene)/fluorescent molecule (SFM) composite followed by the in situ polymerization of pyrrole (Py) and deposition of carbon nanotubes (CNTs), which possesses a simple multilayered structure and quantitatively senses the applied strains via the variations of digital electrical resistance and visual fluorescence intensity. Using the strain‐dependent microstructures derived from the synergistic interactions of the rigid PPy/CNTs functional layer and SFM, the SFM/PPy/CNTs‐based platforms exhibit excellent strain‐sensing performance manifested by a high gauge factor (GF = 2.64 × 104), wide sensing range (~270%), fast response/recovery time (~155/195 ms), excellent stability (~15,000 cycles at 40% strain), and sensitive ML characteristics under ultraviolet illumination. Benefiting from the novel fusion of digital data and visual images, important applications, including the detection of wrist pulses and human motions, and information dual‐encryption, are demonstrated. This study demonstrates the superiority of advanced structures and materials for realizing superior applications in wearable electronics.

Parkinson's disease (PD) is a complex neurodegenerative disorder with an unclear etiology. Despite significant research efforts, developing disease-modifying treatments for PD remains a major unmet medical need. Notably, drug repositioning is becoming an increasingly attractive direction in drug discovery, and computational approaches offer a relatively quick and resource-saving method for identifying testable hypotheses that promote drug repositioning. We used an artificial intelligence (AI)-based drug repositioning strategy to screen an extensive compound library and identify potential therapeutic agents for PD. Our AI-driven analysis revealed that efavirenz and nevirapine, approved for treating human immunodeficiency virus infection, had distinct profiles, suggesting their potential effects on PD pathophysiology. Among these, efavirenz attenuated α-synuclein (α-syn) propagation and associated neuroinflammation in the brain of preformed α-syn fibrils-injected A53T α-syn Tg mice and α-syn propagation and associated behavioral changes in the C. elegans BiFC model. Through in-depth molecular investigations, we found that efavirenz can modulate cholesterol metabolism and mitigate α-syn propagation, a key pathological feature implicated in PD progression by regulating CYP46A1. This study opens new avenues for further investigation into the mechanisms underlying PD pathology and the exploration of additional drug candidates using advanced computational methodologies.

Objective To determine correlations of alternation motor rate (AMR), sequential motor rate (SMR), and maximum phonation time (MPT) with the severity of dysphagia in subacute stroke patients. Methods This was a retrospective chart review study. Data of 171 subacute stroke patients were analyzed. Patient’s AMR, SMR, and MPT data were collected from their language evaluations. Video fluoroscopic swallowing study (VFSS) was done. Data of dysphagia scales including penetration-aspiration scale (PAS), American Speech-Language-Hearing Association National Outcomes Measurement System (ASHA-NOMS) scale, clinical dysphagia scale (CDS), and videofluoroscopic dysphagia scale (VDS) were obtained. AMR, SMR, and MPT were compared between a non-aspirator group and an aspirator group. Correlations of AMR, SMR, and MPT with dysphagia scales were analyzed. Results AMR ("ka"), SMR, and modified Rankin Scale were significant associated factors between non-aspirator group and aspirator group, while AMR ("pa"), AMR ("ta"), and MPT were not. AMR, SMR, and MPT showed significant correlations with PAS score, ASHA-NOMS scale, CDS, VDS oral, and VDS pharyngeal scores. The cut-off value for distinguishing non-aspirator group and aspiration group was 18.5 for AMR ("ka") (sensitivity of 74.4%, specificity of 70.8%) and 7.5 for SMR (sensitivity of 89.9%, specificity of 61.0%). AMR and SMR were significantly lower in before-swallow aspiration group. Conclusion Articulatory diadochokinetic tasks that can be easily performed at the bedside would be particularly helpful in determining the oral feeding possibility of subacute stroke patients who cannot undergo VFSS, which is the gold standard for dysphagia assessment.

Abstract Epidermal dry electrodes with high skin‐compliant stretchability, low bioelectric interfacial impedance, and long‐term reliability are crucial for biopotential signal recording and human–machine interaction. However, incorporating these essential characteristics into dry electrodes remains a challenge. Here, a skin‐conformal dry electrode is developed by encapsulating kirigami‐structured poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/polyvinyl alcohol (PVA)/silver nanowires (Ag NWs) film with ultrathin polyurethane (PU) tape. This Kirigami‐structured PEDOT:PSS/PVA/Ag NWs/PU epidermal electrode exhibits a low sheet resistance (≈3.9 Ω sq−1), large skin‐compliant stretchability (>100%), low interfacial impedance (≈27.41 kΩ at 100 Hz and ≈59.76 kΩ at 10 Hz), and sufficient mechanoelectrical stability. This enhanced performance is attributed to the synergistic effects of ionic/electronic current from PEDOT:PSS/Ag NWs dual conductive network, Kirigami structure, and unique encapsulation. Compared with the existing dry electrodes or standard gel electrodes, the as‐prepared electrodes possess lower interfacial impedance and noise in various conditions (e.g., sweat, wet, and movement), indicating superior water/motion‐interference resistance. Moreover, they can acquire high‐quality biopotential signals even after water rinsing and ultrasonic cleaning. These outstanding advantages enable the Kirigami‐structured PEDOT:PSS/PVA/Ag NWs/PU electrodes to effectively monitor human motions in real‐time and record epidermal biopotential signals, such as electrocardiogram, electromyogram, and electrooculogram under various conditions, and control external electronics, thereby facilitating human–machine interactions.

Abstract Gastric endoscopic resection (ER) is widely performed in Korea. This study aimed to investigate the overall status of gastric ER in Korea. We enrolled ESD or EMR cases performed for gastric cancer and adenoma from 2012 to 2017 by searching the NHIS database. The annual trend of gastric ER and the clinical characteristics were investigated. Institutions were classified into very high-, high-, low-, and very low volume centers (VHVC, HVC, LVC, and VLVC) by the procedure numbers, and institutional types, regional distributions, and medical resources were investigated accordingly. There were 175,370 ER cases during the study period, with an increasing trend over time. The average annual ESD procedure numbers were 3.9, 54.5, 249.5, and 540.3 cases in 131 VLVCs, 119 LVCs, 24 HVCs, and 12 VHVCs, respectively. Among ESD-performing institutions, 44.8% were located in the Seoul Capital Area. The distribution of medical resources showed a positive correlation with the procedural volume. Similar tendencies were also demonstrated in EMR, with some differences in hospital types and regional distribution. Gastric ER and ESD are increasing in Korea. There was a significant variance in the number of ER procedures and the distribution of types, regions, and medical resources according to the procedural volume.

Background/Aims Endoscopic submucosal dissection (ESD) for gastric neoplasms is a widely performed procedure. Local recurrence is rare, but various post-ESD scars are encountered during follow-up endoscopy. Therefore, we investigated atypical scar patterns and evaluated the associated factors. Materials and Methods Clinicopathologic and endoscopic reviews of gastric neoplasms treated with ESD from January 2009 to December 2015 were conducted. Atypical scar patterns were classified as irregular erythema, nodularity, or mucosal defect. Results A total of 264 patients with 274 gastric neoplasms, including 201 adenomas and 73 early gastric cancers, were enrolled. The key endoscopic findings at the resection scar were defined on the basis of gross morphology as follows: irregular erythema, mucosal defect (erosion or ulcer), and nodularity. An irregular erythema scar pattern was associated with male sex, a nodularity scar pattern with smoking, and a mucosal defect scar pattern with infra-angle location (angle and antrum) and cancer. An irregular erythema with nodularity scar pattern was also associated with male sex. An irregular erythema with nodularity and mucosal defect scar pattern was associated with liver disease and chronic kidney disease. Conclusions The atypical scar patterns after gastric ESD are associated with various clinicopathologic factors.

Korean guidelines for nonsteroidal anti-inflammatory drug (NSAID)-induced peptic ulcer were previously developed in 2009 with the collaboration of the Korean College of Helicobacter and Upper Gastrointestinal Research and Korean Society of Gastroenterology. However, the previous guidelines were based mainly upon a review of the relevant literature and expert opinion. Therefore, the guidelines need to be revised. We organized a guideline Development Committee for drug-related peptic ulcer under the auspices of the Korean College of Helicobacter and Upper Gastrointestinal Research in 2017 and developed nine statements, including four for NSAIDs, three for aspirin and other antiplatelet agents, and two for anticoagulants through a de novo process founded on evidence-based medicine that included a literature search and a meta-analysis, A consensus was reached through the application of the modified Delphi method. The primary target of these guidelines is adult patients undergoing long-term treatment with NSAIDs, aspirin or other antiplatelet agents and anticoagulants. The revised guidelines reflect the expert consensus and is intended to assist clinicians in the management and prevention of druginduced peptic ulcer and associated conditions.

Although high-risk gastrointestinal stromal tumors (GISTs) frequently recur, even after a complete resection and imatinib therapy, local recurrence at the suture line after complete resection is rare. The present case was an 88-year-old woman who was initially diagnosed with high-risk GIST without a distant metastasis. She underwent a complete surgical resection of the lesion and received adjuvant imatinib therapy for 18 months, which was discontinued due to severe drug-induced anemia. During the follow-up, an endoscopic examination performed 40 months after the initial surgery revealed local recurrence at the anastomosis site. Although a complete surgical resection was performed, repeated local recurrence was detected 18 months later, which progressed rapidly to metastatic disease. This paper reports a case of a completely resected gastric GIST with repeated local recurrence, despite the complete surgical resections and adjuvant imatinib therapy.

Abstract Cancer causes muscle mass loss, which is associated with a poor prognosis. Chemotherapy may also reduce muscle mass. We investigated skeletal muscle mass change during palliative chemotherapy for advanced gastric cancer (AGC) and its association with treatment outcomes. We retrospectively reviewed 111 consecutive AGC patients who underwent first-line palliative chemotherapy. Skeletal muscle area was measured before and after chemotherapy at the third lumbar vertebra level using computed tomography scans. We compared skeletal muscle index (SMI), body mass index (BMI), and body weight changes to chemotherapy response and survival. The 80 male and 31 female patients’ median age was 65 (range 31–87) years, and 46.8% had sarcopenia at baseline. Median pre-chemotherapy to post-chemotherapy SMI, BMI, and body weight decreases were − 4.5 cm2/m2 (− 11.3%) (P

Corrosive esophagitis and gastritis are characterized by caustic damage due to ingestion of chemical agents. Caustic agents cause tissue destruction through liquefaction or coagulation reactions. Here, we report a case of corrosive esophagitis and gastritis caused by accidental ingestion of glutaraldehyde in Korea. A 62-year-old man presented to the emergency department 8 hours after ingesting glutaraldehyde, which is widely used for the prevention of foot-and-mouth disease in pigs. Urgent endoscopic examination revealed severely damaged mucosae of the esophagus and stomach. With conservative treatment, the patient’s condition was improved, and he was discharged on the 35th day of admission.

The Korean guidelines for nonsteroidal anti-inflammatory drug (NSAID)-induced peptic ulcers were previously developed under co-work with the Korean College of Helicobacter and Upper Gastrointestinal Research and Korean Society of Gastroenterology at 2009. On the other hand, the previous guidelines were based mainly on a literature review and expert opinions. Therefore, the guidelines need to be revised. In this study, a guideline development committee for drug-induced peptic ulcers was organized under the Korean College of Helicobacter and Upper Gastrointestinal Research in 2017. Nine statements were developed, including four for NSAID, three for aspirin and other antiplatelet agents, and two for anticoagulants through de novo processes based on evidence-based medicine, such as a literature search, meta-analysis, and the consensus was established using the modified Delphi method. The primary target of this guideline was adult patients taking long-term NSAIDs, aspirin, or other antiplatelet agent and anticoagulants. The revised guidelines reflect the consensus of expert opinions and are intended to assist relevant clinicians in the management and prevention of drug-induced peptic ulcers and associated conditions.

Atrophic gastritis (AG) and intestinal metaplasia (IM) are considered the main precursor lesions of gastric cancer, and the risk of gastric cancer in the gastric mucosa increases in the presence of AG and IM. The development of intestinal-type gastric adenocarcinoma represents the last step of an inflammation-metaplasia-dysplasia-carcinoma sequence, called the Correa cascade of multistep gastric carcinogenesis. The incidences of both AG and IM tend to increase with age. Helicobacter pylori is regarded the most important factor in the development of IM; the progression of AG to IM is also affected by numerous environmental factors and individual genetic factors. Therefore, understanding the natural course of AG and IM is very important, especially in areas with a high prevalence of gastric cancer such as Korea.

Background/AimsAlthough acid suppressants are widely used for the prevention or treatment of drug-induced upper gastrointestinal bleeding (GIB), evidence regarding the prevention of anticoagulant-related GIB is scarce. The aim of this study was to evaluate the protective effect of acid suppressants against anticoagulant-related GIB.Methods : A systematic review was conducted of studies that evaluated the protective effect of acid suppressants against anticoagulant-related GIB found in PubMed, the Cochrane library, Embase, and KoreaMed from the date of database inception to April 2018. Random effect model meta-analyses with sensitivity analyses were conducted. The methodological quality of each included publication was evaluated using the Risk of Bias Assessment Tool for Nonrandomized Studies. Publication bias was assessed.Results : In total, six nested case-control or cohort studies were identified and analyzed. Proton-pump inhibitors (PPI) had a protective effect against upper GIB in patients on dicumarinics (risk ratio [RR], 0.56; 95% confidence interval [CI], 0.38 to 0.83; I2, 0%); however, the histamine-2 receptor antagonist did not have the same effect (RR, 0.97; 95% CI, 0.52 to 1.81; I2, 0%). Acid suppressants did not have a protective effect against GIB in patients on dabigatran (hazard ratio, 0.78; 95% CI, 0.44 to 1.37; I2, 81.8%).Conclusion : sThe protective effect of PPIs against dicumarinics-related upper GIB was clear, while there was no evidence supporting the protective effect of acid suppressants against dabigatran-related GIB. However, in the absence of randomized trials demonstrating a lack of bias, solid conclusions cannot be drawn.

One-step fabrication method for thin films and shells is developed with nature-derived eggshell membrane hydrolysates (ESMHs) and coffee melanoidins (CMs) that have been discarded as food waste. The nature-derived polymeric materials, ESMHs and CMs, prove highly biocompatible with living cells, and the one-step method enables cytocompatible construction of cell-in-shell nanobiohybrid structures. Nanometric ESMH-CM shells are formed on individual probiotic Lactobacillus acidophilus, without any noticeable decrease in viability, and the ESMH-CM shells effectively protected L. acidophilus in the simulated gastric fluid (SGF). The cytoprotection power is further enhanced by Fe3+-mediated shell augmentation. For example, after 2 h of incubation in SGF, the viability of native L. acidophilus is 30%, whereas nanoencapsulated L. acidophilus, armed with the Fe3+-fortified ESMH-CM shells, show 79% in viability. The simple, time-efficient, and easy-to-process method developed in this work would contribute to many technological developments, including microbial biotherapeutics, as well as waste upcycling.

Single-cell nanoencapsulation (SCNE) is a nanoarchitectonic strategy for creating cell-in-shell structures, in which the artificial shells, formed on individual living cells, protect the cells inside from otherwise lethal factors and also potentially provide them with advanced functions, such as exogenous biochemical reactions that are not attainable in wild-type cells. This work investigated enzymatic cascade systems for widening the substrate scope, beyond catecholamines, in the in-vitro formation of melanin-like films and shells and, ultimately, providing advanced building blocks and tools to the field of SCNE, inspired by the enzyme-derived structural diversification of melanin found in nature. The combination of glucose oxidase and horseradish peroxidase enabled the facilitated film formation of amine group-absent ortho-diphenols, such as protocatechuic aldehyde (PCA) and pyrocatechol. As a proof-of-demonstration, the developed reaction protocol was applied to the cytocompatible SCNE of Saccharomyces cerevisiae.

Background/AimsThis nationwide, multicenter prospective randomized controlled trial aimed to compare the efficacy and safety of 10-day concomitant therapy (CT) and 10-day sequential therapy (ST) with 7-day clarithromycin-containing triple therapy (TT) as first-line treatment for Helicobacter pylori infection in the Korean population.Methods : Patients with H. pylori infection were assigned randomly to 7d-TT (lansoprazole 30 mg, amoxicillin 1 g, and clarithromycin 500 mg twice daily for 7 days), 10d-ST (lansoprazole 30 mg and amoxicillin 1 g twice daily for the first 5 days, followed by lansoprazole 30 mg, clarithromycin 500 mg, and metronidazole 500 mg twice daily for the remaining 5 days), or 10d-CT (lansoprazole 30 mg, amoxicillin 1 g, clarithromycin 500 mg, and metronidazole 500 mg twice daily for 10 days). The primary endpoint was eradication rate by intention-to-treat (ITT) and per-protocol (PP) analyses.Results : A total of 1,141 patients were included. The 10d-CT protocol achieved a markedly higher eradication rate than the 7d-TT protocol in both the ITT (81.2% vs 63.9%) and PP analyses (90.6% vs 71.4%). The eradication rate of the 10d-ST protocol was superior to that of the 7d-TT protocol (76.3% vs 63.9%, ITT analysis; 85.0% vs 71.4%, PP analysis). No significant differences in adherence or serious side effects were found among the three treatment arms.Conclusion : sThe 10d-CT and 10d-ST regimens were superior to the 7d-TT regimen as standard first-line treatment in Korea.

The presence of protein inclusions, called Lewy bodies (LBs) and Lewy neurites (LNs), in the brain is the main feature of Parkinson’s disease (PD). Recent evidence that the prion-like propagation of α-synuclein (α-syn), as a major component of LBs and LNs, plays an important role in the progression of PD has gained much attention, although the molecular mechanism remains unclear. In this study, we evaluated whether neuronal ApoE regulates the cell-to-cell transmission of α-syn and explored its molecular mechanism using in vitro and in vivo model systems. We demonstrate that neuronal ApoE deficiency attenuates both α-syn uptake and release by downregulating LRP-1 and LDLR expression and enhancing chaperone-mediated autophagy activity, respectively, thereby contributing to α-syn propagation. In addition, we observed that α-syn propagation was attenuated in ApoE knockout mice injected with pre-formed mouse α-syn fibrils. This study will help our understanding of the molecular mechanisms underlying α-syn propagation.

Abstract Background Most international guidelines recommend triple-therapy regimens consisting of a proton pump inhibitor, clarithromycin, and amoxicillin/metronidazole for at least 7 days for the eradication of Helicobacter pylori. However, the efficacy of 7-day clarithromycin-based standard triple therapy for H. pylori infection is currently unacceptable in Korea. In this study, we will compare the efficacy and safety of 7-day standard triple therapy, 10-day sequential therapy, and 10-day concomitant therapy for the first-line treatment of H. pylori infection in Korea. Methods/design In this multicenter, investigator-blinded, randomized trial we are recruiting adult patients with H. pylori infection from 15 hospitals in Korea to determine whether sequential or concomitant treatment is superior to standard triple therapy. Patients are randomly assigned to receive either standard triple therapy (lansoprazole, amoxicillin, and clarithromycin) for 7 days, or sequential treatment (lansoprazole and amoxicillin for the first 5 days, followed by lansoprazole, clarithromycin, and metronidazole for another 5 days) for 10 days, or concomitant therapy (lansoprazole, amoxicillin, clarithromycin, and metronidazole) for 10 days. The primary outcome is the rate of H. pylori eradication in the intention-to-treat population. Discussion The results of this study will be crucial for determining the optimal regimen for the primary treatment of H. pylori infection in Korea. This study will produce vital evidence that will lead to revisions to guidelines concerning first-line treatment regimens for H. pylori infection. Trial registration Clinical Research Information Service (CRIS), Republic of Korea, KCT0001980. Registered on 25 July 2016.

Antimicrobial resistance is one of the major factors determining the efficacy of Helicobacter pylori eradication therapy. This study aimed to estimate the recent prevalence of multidrug resistance of H. pylori and its impact on eradication in Korea. A total of 174 patients were prospectively enrolled at Chung-Ang University Hospital from 2017 to 2019. H. pylori strains were isolated from the gastric body and antrum. The minimum inhibitory concentrations of antibiotics were determined by the serial twofold agar dilution method. Eradication results were reviewed and analyzed in connection with antibiotic resistance. The prevalence of H. pylori infection was 51.7% (90/174). The culture success rate was 77.8% (70/90). The resistance rates for clarithromycin, metronidazole, amoxicillin, tetracycline, levofloxacin, and moxifloxacin were 28.6% (20/70), 27.1% (19/70), 20.0% (14/70), 18.6% (13/70), 42.9% (30/70), and 42.9% (30/70), respectively. The multidrug resistance (resistance to two or more classes of antimicrobials) rate was 42.9% (30/70). Dual resistance to clarithromycin and metronidazole was confirmed in 8.6% (6/70). Eradication with a first-line treatment was successful in 75% (36/48), and those who received second-line treatment all achieved successful eradication. The rate of multidrug resistance is increasing, and standard triple therapy (STT) is no longer an acceptable first-line option for H. pylori eradication in Korea.

The efficacy of fluoroquinolone-based eradication therapy largely depends on the fluoroquinolone resistance of H. pylori. The aim of this study was to investigate the changes in the primary resistance rate of H. pylori to fluoroquinolone and the mechanism of resistance in Korea. A total of 153 strains and 48 strains of H. pylori were isolated at a tertiary hospital in 2005/2006 and 2017/2018, respectively. The minimum inhibitory concentrations (MICs) of fluoroquinolone were determined by the serial 2-fold agar dilution method. DNA sequences in the quinolone resistance-determining regions of gyrA/gyrB were analyzed in resistant strains. Subsequent natural transformation study was performed to determine the association between gyrase mutation and resistance. The resistance rates increased from 19.0% (29/153) to 43.8% (21/48) both for levofloxacin and moxifloxacin. The MIC values for resistant strains increased from 2–8 µg/mL to 4–16 µg/mL over time. Mutation of gyrA was detected in 93.1% (27/29) and 100% (21/21) among the resistant strains in both periods, respectively. A novel Gly-85 mutation of gyrA was found and confirmed to be associated with fluoroquinolone resistance. Fluoroquinolone resistance rate of H. pylori has markedly increased over time in Korea. The resistance is mostly due to the point mutation of gyrA. Fluoroquinolone-containing regimens should be carefully selected in Korea, considering the increasing fluoroquinolone resistance.

Inspired by the iron gall ink that has been used since the Middle Ages, we formulated a hair-dyeing solution for blackening hair. The ingredients in the formulation have been approved as cosmetic ingredients, including tannic acid, gallic acid, and Fe(d-gluconate)2. The formulation does not require any harmful oxidizing agents, such as hydrogen peroxide—the Fe(II) cations bound to tannins are oxidized spontaneously upon exposure to air and form the blackish Fe(III)-tannin nanocomplex that coats hair firmly. In our study, we show that the dyed color did not fade under sunlight exposure for at least three months and after shampooing. This natural formulation for black hair-dyeing can have great impact in the hair cosmetic industry.

Correction In the original publication [1] was an error in the grant number in the funding section. The correct version can be found in this Erratum.

We propose approximate solutions for pricing zero-coupon defaultable bonds, credit default swap rates, and bond options based on the averaging principle of stochastic differential equations. We consider the intensity-based defaultable bond, where the volatility of the default intensity is driven by multiple time scales. Small corrections are computed using regular and singular perturbations to the intensity of default. The effectiveness of these corrections is tested on the bond price and yield curve by investigating the behavior of the time scales with respect to the relevant parameters.

We propose approximate solutions to price defaultable zero-coupon bonds as well as the corresponding credit default swaps and bond options. We consider the intensity-based approach of a two-correlated-factor Hull-White model with stochastic volatility of interest rate process. Perturbations from the stochastic volatility are computed by using an asymptotic analysis. We also study the sensitive properties of the defaultable bond prices and the yield curves.

We present a simple numerical method to find the optimal exercise boundary in an American put option. We formulate an intermediate function with the fixed free boundary that has Lipschitz character near optimal exercise boundary. Employing it, we can easily determine the optimal exercise boundary by solving a quadratic equation in time-recursive way. We also present several numerical results which illustrate a comparison to other methods.