The development of neurotrophic factors for treatment of Alzheimer disease (AD) and other neurodegenerative disorders has proceeded in fits and starts, in no small part due to the difficulty of delivering peptide factors where they are needed in the brain. Now, a small phase I trial of nerve growth factor (NGF) has shown that gene therapy can safely deliver the neurotrophic factor to a precisely chosen group of cholinergic neurons, and that doing so may slow cognitive decline in a handful of early AD patients.
Results from the trial, which was started in 1999 (see ARF related news story) by Mark Tuszynski and colleagues from the University of California at San Diego with collaborators at the University of California at Irvine and the University of Chicago, appeared online April 24 in Nature Medicine. The publication follows on the presentation of preliminary trial data at the Society for Neuroscience meeting in 2002.
NGF, a trophic factor for cholinergic neurons, has been studied extensively both for its role in the loss of cholinergic neurons that occurs in AD, and as a potential therapy to preserve neuronal function (see coverage of Sorrento meeting). The earliest clinical studies using NGF showed safe delivery to the CNS to be a major challenge. As a peptide, the factor didn’t cross the blood-brain barrier, and when it was directly infused into the ventricles of the brain, severe side effects ensued, including pain, weight loss, and Schwann cell migration into the spinal cord (see Tuszynski, 2002).
Tuszynski and his colleagues turned to gene therapy in attempts to deliver sufficient quantities of NGF in a local, controlled manner. Their trial involved surgical implantation of patients’ own fibroblasts which had been genetically engineered to produce human nerve growth factor. The modified cells were injected into the cholinergic nucleus basalis region of the forebrain, a region that provides input to broad areas of the cortex and has been implicated in injury-induced plasticity and cholinergic-dependent brain repair (see below).
After some initial and serious problems with the surgical procedure, the researchers safely implanted six out of eight early-stage AD patients with NGF-producing cells, and followed their progress for a mean of 22 months. The first two surgeries resulted in brain hemorrhages after the subjects, who were alert but sedated, moved during the operation, and one of the patients subsequently died. For the rest of the trial, surgery was done under general anesthesia and the patients’ heads were immobilized. Under these conditions, the surgery was successful, and no adverse effects attributed to NGF were seen.
During follow-up, two common clinical measures, the Mini-Mental Status Exam (MMSE) and the AD Assessment Scale-Cognitive subcomponent (ADAS-Cog) were used to gauge cognitive state. The rate of cognitive decline over the entire follow-up period, as measured by the MMSE, was cut in half after implantation of the NGF-producing cells compared to the year before the surgery. In the period from 6 to 18 months postsurgery, when NGF production was expected to be at its peak, the results looked even better: Two patients showed improved cognitive scores and three showed little or no decline. For the ADAS-Cog test, the researchers did not have pretreatment data for the patients, but the rate of decline in scores in the six- to 18-month postoperative period was lower compared to average rates observed in other studies. Despite the caveats that the study was small and not placebo-controlled, the authors point out that the observed reduction in disease progression was far in excess of that seen with cholinesterase inhibitors currently approved for AD.
Imaging and morphological data demonstrated that NGF was in fact exerting a neurotrophic effect on cholinergic circuits. PET scans revealed broad increases in glucose uptake by cortical neurons after surgery in four subjects who received bilateral cell implants, a reversal of the normal decline seen with time in AD patients. Increases were seen all over the cortex in regions known to be innervated by projections from the nucleus basalis, suggesting that the projections were being activated by NGF. Histochemical analysis of the brain of one subject who died 5 weeks postoperatively, showed robust NGF expression by the transplanted fibroblasts and sprouting of cholinergic axons in and around the transplant site. There was little evidence of inflammation.
In summary, the authors write that their study, along with recent clinical success with GDNF for Parkinson disease (Slevin et al., 2005; Gill et al., 2003) provides “early but suggestive” evidence that long-term growth factor treatment is well-tolerated and has the potential to improve symptoms and affect disease progression, when administered into the CNS in therapeutic doses and in a regionally restricted manner.
The logic in targeting the nucleus basalis for NGF therapy is borne out by another recent study from the Tuszynski lab, showing the importance of this region for the recovery from traumatic brain injury in rats. In a publication in Neuron on April 21, James Conner, Andrea Chiba, and Tuszynski describe the process by which rats can regain their ability to retrieve food pellets with a paw after a small motor cortex lesion impairs their reach. When the researchers damaged the cholinergic cells of the nucleus basalis with bilateral injections of the toxin 192-IgG-saporin before introducing the cortical lesion, the rats were far less able to recover paw mobility after a course of rehabilitation compared to animals with an intact nucleus basalis. By mapping the motor organization of the cortex in each group of rats, the researchers showed that the rats lacking a nucleus basalis failed to reorganize areas of cholinergic innervation around the lesioned area normally. The results show that the cholinergic function of the nucleus basalis is central to the synaptic remodeling that allows recovery from brain injury. Cholinergic deficits in the nucleus basalis are seen with normal aging and in AD, and perhaps contribute to the decreased ability of the brain to compensate for damage, whether from trauma or from the toxic processes of AD.—Pat McCaffrey