Intra-Abdominal Photodynamic Therapy in a Rabbit Model of Perforated Appendicitis.

Importance: Perforated appendicitis is a common and morbid pediatric surgical condition frequently requiring prolonged antibiotic therapy. Adjunctive intraoperative strategies for local antimicrobial therapy are limited, and photodynamic therapy (PDT) may offer a targeted, resistance-independent approach. Objective: To evaluate the feasibility, safety, and preliminary efficacy of laparoscopic methylene blue photodynamic therapy (MB-PDT) for intra-abdominal disinfection in a rabbit model of perforated appendicitis. Design: Experimental preclinical animal study conducted between 2023 and 2025. Setting: Single-center laboratory-based animal research facility affiliated with an academic medical center. Participants: Nineteen New Zealand White rabbits underwent surgically induced perforated appendicitis with peritonitis via appendiceal ligation and electrocautery perforation. Interventions: Rabbits received laparoscopic MB-PDT (n=9) or control conditions (methylene blue only, no light; n=10) 24 - 40 hours after appendiceal perforation. MB-PDT involved peritoneal lavage with 300 µg/mL methylene blue, followed by 665 nm laser illumination to 25 J/cm 2 . Peritoneal aspirates were collected before and 24h after intervention. Bacterial burden was quantified, and isolated bacteria were submitted to in vitro PDT. Tissue samples from intraperitoneal organs were collected 24 hours post-intervention. Main Outcomes and Measures: Primary outcomes included feasibility of PDT delivery, safety (histologic evidence of off-target injury), and preliminary in vivo efficacy (change in bacterial burden pre-vs. post-intervention). Secondary outcomes included clinical parameter changes and in vitro MB-PDT efficacy. Results: Of the 14 rabbits that developed peritonitis, 10 (age 5-6 months, weight 3.1-3.8 kg) completed follow-up. Laparoscopic MB-PDT delivery was feasible and caused no off-target histologic injury. Bacterial burden increased in MB-PDT animals relative to controls, though not significantly (p=0.18). Post-intervention, body temperature showed a trend toward reduction in MB-PDT animals (-0.37 ± 1.32°C) vs controls (0.26 ± 0.76°C; p=0.27). In vitro MB-PDT significantly reduced bacterial burden for all tested species (p<0.001), with greater efficacy in Gram-positive bacteria. Conclusions and Relevance: Laparoscopic MB-PDT was feasible and safe in a rabbit model of perforated appendicitis, with high in vitro antimicrobial efficacy. Although in vivo bacterial reduction was not demonstrated, these results support further investigation of MB-PDT as a novel intraoperative adjunct for treating intra-abdominal infection.