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New Frontiers In Medicine: RNAi Therapeutics

The discovery of RNA interference (RNAi), awarded of the Nobel Price for Physiology or Medicine in 2006, represents a remarkable milestone in biology and opens the way for innovative applications in pharmaceutical science and therapy. Research in this field advances rapidly and recent findings published in the Proceedings of the National Academy of Sciences (PNAS) by Alnylam scientists and collaborators demonstrate significant therapeutic effects of RNAi treatment in an animal model of Huntington's disease.

Huntington's Disease

The inherited neurodegenerative disorder Huntington's disease (HD) results from genetically programmed degeneration of neurons in specific areas of the brain. Common symptoms are uncontrolled body movements, lack of coordination in addition to mental disabilities and emotional disturbance. The mean age of adult-onset is about 40 years with onset and disease progression depending on the severity of the huntingtin (Htt) mutation. Defective Htt gene produces an atypical form of Htt protein which accumulates in the cell nucleus and might trigger the process that kills neurons in the corpus striatum. The precise correlation between the microscopic nuclear aggregates, the symptoms and the pathogenicity of the disorder remains controversial and additional studies are required to confirm this hypothesis [1].

According to the US National Institute of Neurological Disorders and Stroke (NINDS) the prevalence of the disease is one out of every 10,000 in the US and in the major European countries. Each child of an HD parent has one-in-two chance of inheriting the gene that causes the disorder. A child that does not inherit the HD gene will not develop the disease and will not pass it to the next generations, although in 1-3% of patients there is no family history of HD [2]. This autosomal dominant disease is caused by an abnormal number of CAG repeats in the Htt gene [3], located on chromosome four, which results in the expansion of polyglutamine tracts in the disease protein. A predictive DNA blood-test, available since the mid '80s, helps physicians to diagnose the disease in asymptomatic individuals with family history of HD. The DNA test result is considered normal up to 30 CAG repeats and abnormal with 39 or more repeats [4].

There is currently no proven cure for HD and patients are destined to an irreversible deterioration of neuronal function. Available drugs are prescribed to alleviate the symptoms associated with the pathology which can vary from patient to patient. Among the major marketed drugs, neuroleptics such as clozapine (Clozaril) and haloperidol (Haldol) are used to control movements and hallucinations and clonazepam (Klonopin) is used as an anxiolytic. Fluoxetine (Prozac, Sarafem) and nortriptyline (Aventyl, Pamelor) are used to control depression and obsessive-compulsive rituals.

New Hopes In A Risky Market

The lack of valid treatment in HD therapy reflects both the complexity of the pathology and the scarce involvement of major pharmaceutical firms in research and development of new drugs. As for other rare diseases, the relatively small number of patients affected translates in a predictable limited market. Nevertheless, the efforts of public research institutions and charity foundations have been continuous during the past three decades and the understanding of the disease etiology advances rapidly. By instance, the identification of the Htt gene in 1993 by the Huntington's Disease Collaborative Research Group [3] improved the accuracy of genetic testing which is since then based on the identification of the specific mutation. The existence of a genetic link to HD and the availability of a reliable diagnostic test induce analysts to predict a significant involvement of healthcare key players in the incoming years. Moreover, the NIND approximates that in US the number of HD cases might be 2.5 times higher than the currently diagnosed number including all the pre-symptomatic cases that could be identified by genetic testing among individuals at risk. By extrapolation, across the six major HD pharmaceutical markets of US, France, Germany, Spain, Italy and the UK the total number of individuals affected would correspond to approximately 150,000 cases. A development of the marketplace will certainly give more confidence to investors and pharmaceutical firms and is vital to progress rapidly in the treatment amelioration of HD patients.

An example that reflects this tendency is given by a partnership initiated in February 2005 between Alnylam Pharmaceuticals, Inc. and Medtronic, Inc. With different medical expertise the two leading healthcare companies have a common goal: develop a novel therapeutic approach to cure a major neurodegenerative disease. The success of this alliance in the battle against HD relies upon their joint forces aimed to develop a new drug-device combination. Medtronic, a world leader in medical technology such as diagnostic equipment and drug delivery devices, will develop an implantable infusion pump for the delivering of a novel RNAi therapeutic to precise areas within the brain. The RNAi therapeutic engineered by Alnylam, a biopharmaceutical company leader in the RNAi technology, is expected to target the Htt gene in patients affected by HD. A positive outcome from this collaboration could represent an unprecedented advance in the treatment of HD and more importantly, it would be a significant validation of the approach adopted, which could be exploited for the treatment of other neurodegenerative disorders.

RNAi Therapeutics

The discovery of RNAi, a natural mechanism of gene silencing that is now extensively studied in humans and a number of other organisms, represents an exceptional advance in biology and facilitates the understanding of complex regulatory mechanisms in gene expression [5]. Moreover, RNAi therapeutics is an innovative approach to drug discovery and development and has the potential to revolutionize medical therapy in the near future. This new class of medicines targets directly the cause of diseases by silencing specific messenger RNAs (mRNAs) and consequently preventing the synthesis of the disease-causing protein. Although in vivo delivery of RNAi remains a major complication for treatment, different technologies are under development and a number of preclinical studies have demonstrated the feasibility of the RNAi approach for more than one neurodegenerative disease [6].

Alnylam announced at the end of October 2007 the publication of a pre-clinical study in a mouse model of HD disease by Alnylam scientists and collaborators [7]. A single injection of chemically synthesized small interfering RNAs (siRNAs), the molecules that mediate RNAi, was able to attenuate the neuronal pathology by targeting the Htt gene product. Moreover, the abnormal behavioral phenotype normally observed in the transgenic mouse model of HD was delayed. The RNAi therapeutic, silencing the expression of mutant Htt in the brain, attenuated the pathological changes associated with HD. Moreover, significant improvement in motor behavior was observed in the animals for more than one week. In preliminary studies, the RNAi therapeutic was found to be well tolerated in the brain after direct administration to the central nervous system (CNS). These results suggest that lowering mutant Htt gene expression in the brain may represent an appropriate treatment for HD.

Challenges And Expectations

The blood-brain barrier represents a supplementary obstacle for the delivery of drugs to target any part of the brain. In addition, to accomplish a long-term effect in the treatment of CNS chronic disorders the novel RNAi therapeutic must be delivered continuously. The Medtronic implantable infusion pump might represent an appropriate delivery system for this purpose and the late-on development will be certainly followed with interest. Before that, supplementary preclinical studies to assess safety and effectiveness of the most promising RNAi therapeutics are awaited. An achievement in HD therapy would be a remarkable demonstration of the possibility to treat serious neurodegenerative disorders through an RNAi approach.

Bibliography

[1] Sisodia, S. Nuclear inclusions in glutamine repeat disorders: Are they pernicious, coincidental or beneficial? Cell (1998) 95:1–4.

[2] Nance MA, Westphal B, Nugent S. Diagnosis of patients presenting to a Huntington disease (HD) clinic without a family history of HD. Neurology (1996) 47(6):1578-80.

[3] Huntington’s Disease Collaborative Research Group. Cell (1993) 72:971–983.

[4] Harper P, Lim C, Craufurd D. Ten years of presymptomatic testing for Huntington's disease: the experience of the U.K. Huntington's Disease Prediction Consortium. Journal of Medical Genetics (2000) 37:567-571.

[5] Zamore PD. RNA interference: big applause for silencing in Stockholm. Cell (2006) 127(6):1083-6.

[6] Gonzalez-Alegre P, Paulson HL. Technology insight: therapeutic RNA interference--how far from the neurology clinic? Nat Clin Pract Neurol (2007) 3(7):394-404. Review.

[7] DiFiglia M, Sena-Esteves M, Chase K, Sapp E, Pfister E, Sass M, Yoder J, Reeves P, Pandey RK, Rajeev KG, Manoharan M, Sah DW, Zamore PD, Aronin N. Therapeutic silencing of mutant huntingtin with siRNA attenuates striatal and cortical neuropathology and behavioral deficits. Proc Natl Acad Sci U S A (2007) 104(43):17204-9.