Development of a PET tracer for MS

Objectives Multiple sclerosis (MS) is a common neurological disease for which no PET tracers are available. Tracers are in development for brain inflammation (e.g. TSPO ligands) and myelin but no tracers for demyelinated axons -the hallmark of MS- exist. Upon demyelination, K+ channels that are normally buried under the myelin sheath become exposed and leaky. The recently approved drug: 4-aminopyridine (4-AP, Ampyra) blocks these channels enhancing conduction and ameliorating the symptoms. We hypothesized that this drug acted selectively on demyelinated axons and could be turned into a tracer. Our goals: 1. Confirm that 4-AP preferentially acts on demyelinated axons. 2. Synthesize fluorinated derivatives compatible with F18 labeling. 3. Demonstrate that 4-AP and derivatives localize to demyelinated areas in mouse models of MS.

Methods We synthesized four fluorinated derivatives of 4-AP. Using electrophysiology we measured their affinity to Kv1 channels and effect on conduction of dys/myelinated mouse optic nerves. We performed PK studies and compared CNS permeability and stability. We injected 14C-labeled drugs into live mice with focal and widespread demyelination and evaluated the distribution of the drug in their brains using ex-vivo autoradiography. We quantified uptake values and correlated them with histological analysis for myelin.

Results 4-AP does not localize to white matter areas unless there is demyelination. Using a [C14] 4-AP we can accurately detect regions of demyelination in mouse models. We developed fluorinated derivatives that have similar affinity and greater brain permeability and stability. These compounds are excellent candidates for PET. F18 labeling and PET imaging is ongoing.

Conclusions In this project we have identified the first compound whose uptake increases with demyelination, which is also brain permeable, metabolically stable and safe. We have developed fluorinated analogs compatible with F18 labeling. These compounds are potential PET tracers for MS and other diseases with white matter damage such as traumatic brain injury.

Research Support NMSS