Piping in Passive Immunization Spares Blood Vessels

Could a new twist to anti-amyloid immunotherapy—one that slowly drips the antibodies directly into the ventricles of the brain—solve some of the controversial issues in passive immunization That is the implication of new work by Lisa Shafer and colleagues at the medical device company Medtronic in Minneapolis, Minnesota. The scientists report that prolonged infusion of low-dose anti-Aβ antibodies into the intracerebroventricular space of aged AD transgenic mice cleared established plaques and improved performance in behavior tests while at the same time avoiding accumulation of amyloid in blood vessel walls and associated microhemorrhage. These are two unwanted side effects that have been reported to be associated with passive immunization through peripheral routes. The work appears in this week’s PNAS online edition.

With passive immunization, mobilization of parenchymal plaques by Aβ antibodies has in some studies increased vascular Aβ deposition and led to the appearance of microbleeds (see ARF news story on Pfiefer et al., 2002; Wilcock et al., 2004; Racke et al., 2005). More recently, a study suggested that at least part of the vascular complication may stem from giving high concentrations of antibody, since lower doses given over six months were found to clear plaque and vascular amyloid without triggering bleeding in mice (see ARF related news story).

In the new study, lead author Deepak Thakker and colleagues looked to see what would happen if they bypassed the circulation altogether and delivered antibodies directly into the intracerebroventricular (ICV) space. They compared the effects of the 6E10 antibody, which binds the N-terminus of Aβ, in aged Tg2576 mice, administered for five weeks either by an implanted osmotic mini-pump into the ventricular space, or by repeated intraperitoneal (IP) injections. (Medtronic markets numerous pumps for peripheral and CNS applications.) At the end of the study, both groups of mice showed similar and extensive clearance of amyloid plaque from the cortex and hippocampus, and a reduction in astrocyte clusters and dystrophic neurites around plaques. These histological changes were accompanied by improvement to the levels of normal mice in the cued fear-conditioning test. The results were striking because the central dosing regimen used 10 times less antibody than the injection route (0.2 mg total dose vs. 2 mg for IP injections).

In contrast to their similar effects on plaques, the two regimens had a different impact on vascular amyloid. Tg2576 mice develop age-dependent CAA (as do many people with AD). This vascular pathology grew by more than half in response to systemic antibody treatment, coupled with a doubling of the frequency of cerebral microhemorrhages. The ICV antibodies, on the other hand, reduced CAA by 40-45 percent, and did not increase the number of hemorrhages.

With both treatments, antibodies infiltrated the brain tissue and similarly activated microglia around plaques. This suggests that the difference in vascular amyloid was not due to microglia-dependent mechanisms, the authors write. What did differ was peripheral clearance. Antibody levels and plasma Aβ were both significantly elevated by systemic antibody treatment, but not by ICV infusion. The results suggest that antibody-mediated clearance of brain Aβ through blood vessels may exacerbate CAA and associated microhemorrhages. This idea jibes with a recent study showing that people who received a now-discontinued Elan Aβ vaccine showed a transient worsening of CAA (Boche et al., 2008). Current human immunotherapy trials monitor patients closely for signs of microhemorrhage.

The lack of evidence for peripheral clearance after ICV antibody administration suggests that this treatment does not engage the peripheral sink clearance mechanism. Instead, the authors speculate, antibodies in the CSF might represent a CSF sink, and clear Aβ via a pathway that bypasses the cerebral vasculature. The observed slower rate of clearance could be another reason why low intracerebroventricular doses did not increase CAA, the authors hypothesize. They bolstered this idea by showing that a single high dose of antibody (12 micrograms) given via the ICV route rapidly cleared plaques, but also caused a transient increase in CAA and microhemorrhages.

The net result of ICV delivery is to increase the safety profile of passive immunization at later stages of disease, the authors said. Preliminary data from recent Phase 1/2 Elan/Wyeth trials, along with recent preclinical work in AD mouse models, suggest that immunotherapy may be more effective if initiated in early stages of the disease, when the amyloid accumulation is less extensive and the cerebral vasculature is not highly compromised, Thakker told ARF. With the dosing paradigm and delivery strategy described in the paper, we are able to see a significant decrease in the vascular amyloid pathology, parenchymal amyloid pathology, and associated neuropathology, and a reversal of the behavioral deficits at a later disease stage in the transgenic mouse model.

From a safety perspective, Shafer told ARF, Preclinical and clinical work with systemic administration of anti-Aβ antibodies indicates an increased risk of accumulating amyloid in the vasculature and development of microhemorrhages. Our data replicates the increase in vascular amyloid and associated microhemorrhages with systemic administration of antibodies in the mouse model, and extends to show that these undesirable pathological indices are actually decreased with ICV delivery of antibodies.

As for the feasibility of translating this delivery method to humans, the technology is in early clinical testing. NeuroNova of Sweden and Medtronic have started a Phase 1 trial using a programmable infusion pump linked to an ICV catheter to deliver vascular endothelial growth factor for the treatment of ALS.

This same system could be applicable to an Alzheimer disease indication, Shafer told ARF. In addition, she noted that surgical placement of an ICV shunt is a common procedure in elderly patients to treat normal pressure hydrocephalus, which causes a form of dementia that reverses when excess CSF is drained. Medtronic markets such a shunt. There you are pulling from the ventricular space and here you’d be pushing in, but as far as a neurosurgical approach, the shunt provides some precedent, Shafer said. Other routes of entry to the CSF are also possible. For that, Medtronic already has an FDA-approved device for chronic intrathecal infusion of drugs for the treatment of pain and spasticity. Furthermore, if delivery to the CSF does not work, Shafer says, infusion directly into brain tissue could be a possibility, as was done in a trial of the neurotrophic factor GDNF for Parkinson disease (see ARF related news story).

But, Shafer cautioned, the research is at an early stage. We are encouraged by the results and especially the safety profile in transgenic mice, but we don’t know how that will translate. We are still determining the next step. She declined to say if her group has tested any other antibodies or other models, or whether Medtronic plans to collaborate with other companies to deliver existing antibodies to humans.—Pat McCaffrey.

Thakker DR, Witherspoon MR, Harrison J, Keene TE, Lane DS, Kaemmerer WF, Stewart GR, Shafer LL. Intracerebroventricular amyloid-b antibodies reduce cerebral amyloid angiopathy and associated micro-hemorrages in aged Tg2576 mice. PNAS. 2009 Feb 23. Abstract

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