Volume and Infusion Rate Dynamics of Intraparenchymal Central Nervous System Infusion in a Large Animal Model

Abstract

Thalamic infusion of adeno-associated viral (AAV) vectors has been shown to have therapeutic effects in neuronopathic lysosomal storage diseases. Preclinical studies in sheep model of Tay-Sachs disease demonstrated that bilateral thalamic injections of AAV gene therapy are required for maximal benefit. Translation of thalamic injection to patients carries risks in that (1) it has never been done in humans, and (2) dosing scale-up based on brain weight from animals to humans requires injection of larger volumes. To increase the safety margin of this infusion, a flexible cannula was selected to enable simultaneous bilateral thalamic infusion in infants while monitoring by imaging and/or to enable awake infusions for injection of large volumes at low infusion rates. In this study, we tested various infusion volumes (200-800 lL) and rates (0.5-5 lL/min) to determine the maximum tolerated combination of injection parameters. Animals were followed for *1 month postinjection with magnetic resonance imaging (MRI) performed at 14 and 28 days. T1-weighted MRI was used to quantify thalamic damage followed by histopathological assessment of the brain. Trends in data show that infusion volumes of 800 lL (2 • the volume required in sheep based on thalamic size) resulted in larger lesions than lower volumes, where the long infusion times (between 13 and 26 h) could have contributed to the generation of larger lesions. The target volume (400 lL, projected to be sufficient to cover most of the sheep thalamus) created the smallest lesion size. Cannula placement alone did result in damage, but this is likely associated with an inherent limitation of its use in a small brain due to the length of the distal rigid portion and lack of stable fixation. An injection rate of 5 lL/min at a volume *1/3 of the thalamus (400-600 lL) appears to be well tolerated in sheep both clinically and histopathologically.

Current knowledge and challenges associated with targeted delivery of neurotrophic factors into the central nervous system: focus on available approaches

“…Approximately 100% of the injected agents can be delivered to the brain and spinal cord through CSF hence, the dose of an agent for ICV and IT is approximately programmable [ 178 ]. The CSF is invasive but the IT infusion is less invasive than the intraparenchymal route [ 179 , 180 ]. Using the CSF route, NTF-SCs, viral vectors, and NTF-loaded biomaterials can reach directly the CNS, without circulating in the bloodstream and transferring to other non-targeted organs [ 181 ].…”

“…To this end, an appropriate cannula is needed for straight injection of the agent into the brain and spinal tissues to minimize tissue irritation. The cannula can move in the injection site which can cause incorrect targeting [ 179 ]. Although the parenchyma of CNS restricts the diffusion of proteins [ 185 ], the NTFs can spread through neuronal internalization [ 158 ].…”

“…The direct-injected therapeutics have higher local concentration and longer half-life inside the CNS which can have dual aspects, upregulates their positive effects via presenting the agent directly to the injury site or in some cases can cause neurotoxicity [ 158 , 185 , 186 ]. In intraparenchymal injection, the therapeutic agent can reach deep brain regions like the striatum and hypothalamus that can be more efficient for regeneration [ 179 ]. One of the great merits of targeted parenchymal injection of therapeutics is the avoidance of CSF circulation which may deliver the therapeutic to the undesired area for example the spinal cord in the TBI model.…”

“…The parenchymal injection can deliver the agent to the specific site of the brain which is important in degenerative diseases like PD. Further, the infusion can be applied in the white matter to deliver the therapeutic agent to the connective tracts hence, can be effective in spread neurodegenerative like AD [ 179 ]. The infusion rate should be very slow because the parenchyma of CNS does not have space for receiving a high amount of agents in fast injection.…”

Empleo de Adeno Vectores Asociados para el Tratamiento de la Enfermedad de Tay Sachs

“…Los vectores recombinantes son inyectados vía intratalámica, o intratecal dependiendo del tamaño del encéfalo del animal testeado y las condiciones del estudio (Taghian et al, 2020). Hasta la fecha, los ensayos clínicos para la aplicación de estas terapias en el caso de Tay Sachs, sólo se han realizado en modelos animales y los estudios en seres humanos se encuentran aún en sus primeras fases.…”