Safety of Indocyanine Green Microdosing for Clinical Imaging of CSF Ventricular Dynamics and Extracranial Outflow

Intravenous administration of indocyanine green (ICG) has been approved in brain surgeries for decades, yet concerns about neurotoxicity prevent its direct administration into the cerebrospinal fluid (CSF). Armed with prior animal studies demonstrating the feasibility of using ICG microdosing into the CSF, we sought to evaluate its nonclinical safety profile and obtain surrogate measures in adults prior to its use in human neonates.

Evaluation of ICG toxicity was conducted in mixed primary CNS cell cultures and in an extended safety study of juvenile rat pups deploying intraventricular injections of saline (as control) or ICG. Analysis of animal behavior included Novel Object Place Recognition Test and rotarod behavioral tests. Immunohistochemical analysis of tumor necrosis factor-alpha (TNF-α), oxidative deoxyribonucleic acid damage, microglial activation, and neuronal density was performed on collected brains. We measured ICG levels (before and after intravenous administration) in collected CSF from external ventricular drain catheters of 10 brain-injured adults.

TNF-α and lactate dehydrogenase assay for cytotoxicity showed transient elevations after 1 h of incubation with 1291 µM ICG, but none at or below 322 µM ICG, even after 24 h of incubation. Behavioral tests and immunohistochemical analyses showed no differences between ICG-administered animals and controls. Intraventricular concentrations of ICG in collected human CSF ranged between 0.17 and 7.93 µM, with no adverse events associated with intravenous administration.

With intraventricular microdosing of 100 µg ICG, maximal ICG concentrations in neonatal CSF range from 1.3 to 6 5 µM. CNS cell culture, rat safety studies, and surrogate measures in adults evidence the safety of microdosing ICG directly into the CSF.