Preclinical Optimization of Magnetotactic Bacteria Therapy for the Treatment of Pancreatic and Rectal Cancer.

Background/Objectives: Magnetotactic bacteria therapy is an emerging active intratumoral drug delivery platform in which drug-loaded, magnetically responsive bacteria are injected into a tumor and navigated through its microenvironment toward defined targets using an external magnetic field, fundamentally distinct from passive intratumoral injection. No endoscopic ultrasound (EUS)- or ultrasound (US)-guided delivery workflow for MTBT has been described, and five critical technical prerequisites for clinical translation remain unaddressed: three-body image registration, minimum contrast concentration for cone-beam computed tomography (CBCT) bolus localization, needle repositioning accuracy, optimal fiducial strategy, and EUS/US procedural feasibility within the magnetic guidance apparatus. Methods: In three healthy female swine, we tested an image-guided intratumoral workflow at pancreatic and rectal sites. Gold rod fiducials were implanted under EUS in two animals and via transabdominal/endorectal US in one animal. A needle was navigated toward the implanted fiducial group using real-time gold-fiducial shine-through, and 2 mL of saline mixed with iodine contrast (Isovue-370) at 15%, 10%, or 5% v/v (one concentration per animal) was injected. Scan with CBCT was acquired before and after injection; fiducials, needle tip, and contrast bolus were segmented in 3D Slicer and their centroids compared to quantify targeting accuracy. Contrast visibility and artifact were scored by an expert radiologist on a five-point Likert scale. EUS feasibility within the magnetic apparatus was assessed using a full-scale cardboard replica of the CuraDrone PolarTrak. Results: Five percent v/v Isovue-370 was the minimum concentration for reliable CBCT bolus identification (280-360 HU; 1:1 injection-to-volume ratio). Needle repositioning accuracy was ≤2 cm in 5/6 sites using fiducial shine-through guidance. Gold rod fiducials served a dual role: CBCT tumor registration and real-time EUS/US navigation, not previously described for intratumoral injection. EUS was feasible within the PolarTrak with defined workflow adaptations. Conclusions: This pilot study demonstrates the technical feasibility of an EUS/US-CBCT workflow for MTBT, yielding quantitative, actionable parameters for first-in-human trial design. These feasibility findings, obtained in a small healthy animal cohort, are intended to inform the design of subsequent clinical studies.