People live longer these days, and with increasing age an unwelcome visitor often tags along: a greater tendency to develop degenerative diseases. A new drug discovery strategy—minimizing amyloid deposition—is now being pursued to help fight these debilitating conditions.
In degenerative diseases like Alzheimer's, insoluble amyloid deposits form from proteins gone awry in the brain or other organs. The misbehaving proteins misfold and aggregate to form fibrous amyloid deposits.
Some drugs are available to manage symptoms of amyloid conditions, but they don't alter the underlying process in these diseases: the accumulation of misfolded and aggregated proteins. Now, researchers are developing a new generation of agents, such as tafamidis and CPHPC, that might have a better chance of completely preventing, halting, or even reversing degenerative diseases by attacking amyloid accumulation directly.
Protein misfolding specialist Jeffery W. Kelly of Scripps Research Institute places these new anti-amyloid strategies into four categories: stabilizing the native state of an aggregation-prone protein so it doesn't form amyloid, reducing the concentration of amyloid-forming protein, inhibiting amyloid formation, and removing toxic oligomers or amyloid fibrils once they've formed.
Kelly's group has demonstrated that the first strategy arrests progression of amyloid disease symptoms in some patients (Acct. Chem. Res. 2005, 38, 911). They've focused on transthyretin, a tetrameric protein that can dissociate, misfold, and aggregate, apparently causing three fatal degenerative diseases: senile systemic amyloidosis, familial amyloid cardiomyopathy, and familial amyloid polyneuropathy. They identified a small molecule that binds to transthyretin's native state in hopes it might prevent the protein from aggregating. The researchers reported the structure of the agent, now called tafamidis, in 2003. Transthyretin is stabilized by the binding of tafamidis to at least one of two largely unoccupied thyroxine-binding sites in a central region between the tetramer's four monomers, studies of the agent's mechanism have shown.
Human clinical trials of tafamidis have been carried out by FoldRx Pharmaceuticals, a firm in Cambridge, Mass., cofounded by Kelly and Whitehead Institute for Biomedical Research biology professor Susan Lindquist. A Phase II/III trial showed that tafamidis halts progression and improves symptoms of familial amyloid polyneuropathy. It also "appears to be safe and well tolerated," according to the company. The only treatment currently available for this disease is liver transplantation. FoldRx is also conducting a Phase II trial of tafamidis in patients with familial amyloid cardiomyopathy, for which heart transplantation is the lone therapy currently available.
According to FoldRx President and Chief Executive Officer Richard Labaudinière, the firm anticipates filing marketing applications for tafamidis by later this year. The small molecule has orphan drug status in the U.S. and Europe and has been granted fast-track development status in the U.S.
"It has been a 20-year journey from basic research to an approvable drug," Kelly says. "FoldRx Pharmaceuticals performed brilliantly in that they developed tafamidis with a staff of 30 people and about $60 million dollars"—a modest level of resources for drug development, he notes.
Work by Kelly's group and FoldRx researchers "shows that a rational approach to therapies against this class of disease is not just possible but has now been achieved in a specific and important case," says protein folding and misfolding researcher Christopher M. Dobson of the University of Cambridge, in England.
Dobson and coworkers are also developing agents to stabilize native states of problematic proteins. In 2003, they found that newly discovered antibodies from camels and related humped animals could stabilize the native structure of lysozyme. Misfolding and aggregation of mutated lysozyme in humans causes non-neuropathic systemic amyloidosis, another fatal amyloid disease. The researchers found that a highly stable and soluble single-chain antibody fragment raised against human lysozyme bound the mutated enzyme and prevented it from aggregating.
Dobson's team is currently extending its studies by generating antibodies that bind α-synuclein, the protein responsible for the pathogenic amyloid-like deposits in patients with Parkinson's disease. In related work, Lindquist and coworkers recently identified several small molecules that prevent the accumulation of α-synuclein deposits in cells and protect against the development of traits associated with Parkinson's (Dis. Model. Mech., DOI: 10.1242/dmm.004267).
Other scientists are investigating the stabilization of superoxide dismutase as a therapy for amyotrophic lateral sclerosis, a fatal neurodegenerative condition characterized by superoxide dismutase-based aggregates. A recent study demonstrated that small molecules can stabilize the native superoxide dismutase dimer, preventing it from dissociating and aggregating.
Protein stabilization using small molecules could also provide an effective therapy for lysosomal storage diseases such as Gaucher's and Fabry's and for cystic fibrosis. In these conditions, proteins are unable to fold in the endoplasmic reticulum and to traffic to their proper location in cells. Small molecules called pharmacological chaperones can stabilize such variant proteins, allowing them to be transported to where they are needed. Amicus Therapeutics, in Cranbury, N.J., has conducted clinical trials on small-molecule pharmacological chaperones to treat Gaucher's and Fabry's diseases and related conditions.
But protein stabilization isn't the only game in town. The other three strategies for fighting amyloid formation are also under investigation.
A number of pharmaceutical and biotech companies are trying to treat Alzheimer's disease by inhibiting secretase enzymes to decrease the production of amyloid-forming proteins. Secretase enzymes cleave amyloid precursor protein to form amyloid β peptide, which aggregates to form "plaques" in the brains of Alzheimer's patients. Secretase inhibitors lower the production of amyloid β peptide, an effect researchers hope will slow the death of brain cells in Alzheimer's patients. For example, a γ-secretase inhibitor called LY450139 is currently in Phase III clinical trials sponsored by Eli Lilly & Co.
Inhibiting aggregation is also being explored as a strategy to deter amyloid formation. For instance, ProteoTech, in Kirkland, Wash., identified the small molecule Exebryl-1 in an antiaggregation screen and is developing it as a potential Alzheimer's treatment. In vitro assays showed that Exebryl-1 prevents amyloid formation and helps dissolve existing fibrils. The agent improved memory in animal trials and is currently in Phase I trials for humans.
Exebryl-1's ability to break up fibrils puts it partly in yet another antiaggregation category—last-resort agents that promote the clearance of toxic oligomers or fibrils once they've already formed. One pure-play approach in this area is a bis-d-proline-based small molecule called CPHPC.
"CPHPC has the unique pharmacological property of totally depleting circulating serum amyloid P [SAP] component," a universal constituent of all amyloid deposits, says Mark B. Pepys, director of the Centre for Amyloidosis & Acute Phase Proteins at University College London Medical School. CPHPC-based removal of SAP from the circulation "enables us to then administer anti-SAP antibodies, which target SAP remaining in amyloid deposits, leading to clearance of the deposits by macrophages. This takes place rapidly, safely, and completely. Such clearance of amyloid deposits from the major organs in systemic amyloidosis is unprecedented," he says.
Studies of CPHPC have so far been carried out in mice in which human SAP has been introduced or expressed transgenically. GlaxoSmithKline has licensed the agent from Pepys' company, a University College London spin-off called Pentraxin Therapeutics, and CPHPC in combination with anti-SAP antibodies will go into human clinical trials in 2012, Pepys says.
Meanwhile, findings disclosed last month add a new level of complexity to scientists' understanding of the way proteins aggregate and thus could influence future strategies for designing drugs for amyloid disease. A group led by Dobson and Mark E. Welland, head of Cambridge University's Nanoscience Centre, reported finding an analytical solution to the complex equation that describes the kinetics of amyloid self-assembly (Science 2009, 326, 1533). Amyloid fibril formation is accompanied by occasional fragmentation of the growing chains. The study shows that the rate-limiting step in fibril growth is commonly fragmentation, not nucleation of polymerization by small oligomers, as has been widely assumed.
"This mechanism is crucial, not just in vitro," Dobson says, noting that "analysis of data from transgenic mouse models shows that fragmentation of fibrils can be the critical feature that determines the onset of amyloid diseases."
The study has "real implications for strategic approaches to the design of therapeutics," Dobson adds. "It makes it possible to assess with greater confidence the step in the aggregation process that is influenced by a potential inhibitor, and it gives deeper insight into how amyloid conditions develop."
Efforts to find new drugs to stop degeneration offer hope for the future. "The current breadth of a large number of discovery and clinical-stage programs suggests that we will soon see breakthrough results that will transform therapy for patients in desperate need," says Mark A. Findeis, senior vice president of research at Satori Pharmaceuticals, in Cambridge, Mass., a neurodegenerative drug company. "The positive results at FoldRx are the beginning of this change."