Dopaminergic transplantation for parkinson's disease Current status and future prospects.код для вставкиСкачать
POINT OF VIEW Dopaminergic Transplantation for Parkinson’s Disease: Current Status and Future Prospects C. Warren Olanow, MD, FRCPC,1,2 Jeffrey H. Kordower, PhD,3 Anthony E. Lang, MD,4 and Jose A. Obeso, MD5 Cell-based therapies that involve transplantation into the striatum of dopaminergic cells have attracted considerable interest as possible treatments for Parkinson’s disease (PD). However, all double-blind, sham-controlled, studies have failed to meet their primary endpoints, and transplantation of dopamine cells derived from the fetal mesencephalon is associated with a potentially disabling form of dyskinesia that persists even after withdrawal of levodopa (off-medication dyskinesia). In addition, disability in advanced patients primarily results from features such as gait dysfunction, freezing, falling, and dementia, which are likely due to nondopaminergic pathology. These features are not adequately controlled with dopaminergic therapies and are thus unlikely to respond to dopaminergic grafts. More recently, implanted dopamine neurons have been found to contain Lewy bodies, suggesting that they are dysfunctional and may have been affected by the PD pathological process. Collectively, these findings do not bode well for the short-term future of cell-based dopaminergic therapies in PD. Ann Neurol 2009;66:591–596 Parkinson’s disease (PD) is characterized by degeneration of dopamine neurons in the substantia nigra pars compacta (SNc), coupled with the presence of intracellular proteinaceous inclusions known as Lewy bodies. Current treatment is primarily based on a dopamine replacement strategy using the dopamine pro-drug, levodopa.1 While levodopa remains the most effective therapy for the classic motor features of the illness, chronic treatment is complicated by wearing off and dyskinesia. Further, disease progression is associated with the development of features such as freezing, falling, and dementia that are not satisfactorily controlled with current medical or surgical therapies. The possibility that transplantation of dopaminergic neurons derived from sources such as the fetal mesencephalon or stem cells might be a solution to these problems has attracted considerable interest in both the scientific and lay communities. However, the failure of double-blind, sham-controlled trials testing transplantation of fetal nigral cells,2,3 fetal porcine nigral cells, and retinal pigmented epithelial (RPE) cells (C.W.O., personal obser- vations), plus reports describing the emergence of a potentially disabling form of dyskinesia in some transplantation patients,3– 6 has slowed clinical progress. More recently, we and others have found that fetal dopamine neurons transplanted 11 to 14 years earlier had decreased staining for the dopamine transporter (DAT) and contained intracellular inclusions identical to Lewy bodies (Fig 1), suggesting that they may have been affected by the PD pathologic process.7–9 These findings warrant a reexamination of the potential for dopaminergic cell-based therapies to offer a viable treatment for PD. From the Departments of 1Neurology and 2Neuroscience, Mount Sinai School of Medicine, New York, NY; 3Department of Neurological Sciences, Rush Medical Center, Chicago, IL; 4Division of Neurology, University of Toronto, Toronto, Ontario, Canada; and 5 Department of Neurology, and Neuroscience Division, Clinica Universitaria and Medical School and CIMA, University of Navarra and CIBERNED, Pamplona, Spain. Potential conflicts of interest: Dr C. Warren Olanow, Dr Jeffrey Kordower, and Dr Anthony Lang have served as consultants to Ceregene, Inc.; Dr Lang has also served as a consultant for BoerhingerIngelheim, Novartis, Solvay, Teva; and Dr Olanow has also served as a consultant to Novartis/Orion, Teva, Solvay, Merck Serono, and Boehringer Ingleheim. Address correspondence to Dr C. Warren Olanow, Department of Neurology, Mount Sinai School of Medicine, Annenberg 14-94, One Gustave L. Levy Place, New York, NY 10029. E-mail: email@example.com Received Mar 24, 2009, and in revised form May 13. Accepted for publication May 18, 2009. Published online, in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ana.21778 Dopamine Cell Transplantation Transplantation of dopaminergic cells into the striatum has been investigated as a possible therapy for PD based on their potential to replace those cells that are lost as a result of the neurodegenerative process in a more physiologic manner than can be accomplished with oral therapies, so as to maximize clinical benefits while avoiding motor complications. Laboratory stud- © 2009 American Neurological Association 591 Fig 1. Ubiquitin-stained sections from a PD patient who died 14 years after an intraputamenal fetal nigral transplant.7 (A) Lowpower and (B) high-power photomicrographs of illustrating a Lewy body in a grafted dopamine neuron that is indistinguishable from (C) low-power and (D) high-power photomicrographs of a Lewy body in a dopamine neuron in the substantia nigra pars compacta of the same individual. Scale bar shown in (D) represents the following magnifications: (A, C) ⫽ 25m; (B, D) ⫽ 10m. It is noteworthy that inclusions in both graft and host nigra also stained comparably for alpha synuclein and thioflavin-S, providing further evidence that inclusions in both nigra and implanted neurons are Lewy bodies. ies have demonstrated that implanted dopaminergic cells can survive, reinnervate the striatum, and improve motor function in rodent and primate models of PD.10 Open-label trials have reported clinical benefit with transplantation of dopaminergic cells derived from fetal mesencephalon, carotid body, and RPE cells in patients with advanced PD.11–15 Further evidence of increased dopaminergic activity following transplantation has been demonstrated by positron emission tomography16 and postmortem studies showing evidence of robust graft survival with extensive reinnervation of the striatum.17 However, double-blind, sham-controlled trials of fetal nigral transplantation,2,3 and double-blind, sham-controlled trials of fetal porcine nigral transplantation and RPE cells, which have not yet been formally published (C.W.O., personal observations), each failed to demonstrate superiority over placebo with respect to their primary endpoints. Long-term open-label follow-up studies suggest that individual transplantation patients have done very well and in some instances can even be maintained with minimal or even no levodopa.18 Further, post hoc analyses performed in the double-blind fetal nigral trials have reported significant benefits in subgroups of patients who were less than 60 years of age2 or had milder disease at baseline.3 In addition, postmortem studies (see below) have shown evidence of activated microglia with T-cells and B-cells in grafted regions,19 raising the possibility that some de- 592 Annals of Neurology Vol 66 No 5 November 2009 gree of immune rejection may have contributed to the lack of efficacy in these patients who received no immunosuppression, or only received cyclosporine for 6 months, and that patients might do better with more prolonged immunosuppression.20 It is thus possible that superior results might be attained with modifications in the transplant protocol.21 However, to date no dopamine cell-based therapy has as yet been demonstrated to provide benefits for patients with PD in a double-blind, controlled trial. Transplant-Related Dyskinesias Transplant procedures have generally been well tolerated. However, as many as 50% of transplantation patients develop a novel and previously unreported form of involuntary movement referred to as “off-medication dyskinesia.”4 – 6 Abnormal involuntary movements or dyskinesias are a common complication of levodopa therapy. Most often, they coincide with the peak levodopa plasma concentration and the period of maximal clinical benefit, and disappear within 2 to 3 hours as the levodopa dose wears off (peak-dose dyskinesia). Less commonly, short-lasting (usually 5–10 minutes) dyskinesias emerge in association with the rise and fall of the plasma levodopa concentration following individual doses (diphasic dyskinesia). Graft-related dyskinesias have been described with similarities to peakdose dyskinesia5 and diphasic dyskinesias,4 but differ from each of these in that they can persist for prolonged periods of time (days to weeks) after dose reduction or even complete withdrawal of levodopa; for this reason, they have been referred to as “offmedication dyskinesia.”4 These involuntary movements can be severe and disabling, and may necessitate an additional neurosurgical procedure (deep brain stimulation).5 The precise mechanism responsible for graftinduced dyskinesias is not known, but their presence suggests that transplantation of dopamine cells using current transplant protocols does not restore dopamine in a physiological manner. At present, we lack an understanding of how to prevent off-medication dyskinesia and this side effect remains an obstacle to further clinical testing of dopamine cell-based therapies in PD. Neuropathologic Changes in Transplanted Dopamine Cells Initial neuropathologic studies performed 18 months after the transplantation procedure demonstrated robust survival of healthy-appearing tyrosine hydroxylaseimmunoreactive (TH-ir) grafted dopamine neurons with extensive reinnervation of the host striatum.2,3,17 However, approximately 11 to 14 years after the transplant procedure, grafted dopamine neurons were found to contain inclusion bodies that stained positively for alpha synuclein, ubiquitin, and thioflavin-S, and were identical to Lewy bodies (Fig 1B).7–9 Graft sites did not stain for dopamine transporter, and in more affected areas there was also reduced staining for tyrosine hydroxylase. These findings are consistent with the possibility that the implanted neurons had been adversely affected by the disease process. It thus appears that following transplantation, even young, healthy, genetically independent dopamine cells can be affected by the PD pathological process, which might limit their ability to provide sustained benefit. Nondopaminergic Features of PD While most clinical and pathological attention in PD has focused on the dopamine system, it is important to appreciate that cell loss and Lewy body pathology can also be seen in multiple other sites, including cholinergic, norepinephrine, and serotonin neurons in selected regions of the cerebral cortex, olfactory system, basal forebrain, brain stem, spinal cord, and peripheral autonomic nervous system.22 This nondopaminergic pathology is thought to underlie clinical features such as freezing, falling, autonomic dysfunction, mood disturbances, and dementia, which are not well controlled with levodopa and represent the primary source of disability and nursing home placement for patients with advanced PD.23 There is presently no data or scientific basis to consider that transplantation of dopaminergic cells into the striatum will relieve or modify these nondopaminergic features of the disease (Fig 2).24 Stem Cells and Gene Therapies Stem cells have attracted considerable interest as a possible therapy for PD because of their potential to provide an unlimited supply of optimized dopamine neurons for transplantation.25 Numerous studies have reported that dopamine neurons suitable for transplantation can be derived from mice, monkey, or human embryonic stem cells, and that they can provide motor benefits in rodent and nonhuman primate models of PD.26 –28 Dopamine-producing cells can also be induced to differentiate from autologous stem cells generated from the umbilical matrix, bone marrow, and reprogrammed fibroblasts.29 –32 Autologous stem cells offer the advantage of avoiding the immunological and societal concerns associated with the use of foreign embryonic tissue. To date, however, cell survival following transplantation of dopamine neurons derived from stem cells has been limited, and motor benefits in animal models do not exceed that which can be obtained with fetal nigral cells, which have not as yet been shown to provide benefits for PD patients in double blind trials. The safety of stem cells has also not yet been adequately assessed in preclinical studies. Specifically, tumor formation has been observed with transplanted embryonic stem cells in rodents26 and in a patient with ataxia telangiectasia who received a stem cell transplant,33 and represents a major theoretical concern for PD patients. Finally, there is no reason to believe that dopaminergic stem cells will be any more likely to address the nondopaminergic features of PD than other dopaminergic therapies. While enthusiasm for the potential of stem cells remains high, it is clear that there are many issues that remain to be resolved before clinical trials in PD patients can proceed, and there is no assurance that this approach will prove to be superior to fetal nigral transplantation or more effective than other available dopaminergic or surgical therapies. Gene therapies offer a broad range of possibilities for enhancing dopaminergic function, with the potential of delivering proteins such as aromatic amino acid decarboxylase (AADC) that promote the conversion of levodopa to dopamine34; glutamic acid decarboxylase (GAD) that forms GABA and might suppress firing in overactive glutamatergic neurons35; and trophic factors such as neurturin or glial-derived neurotrophic factors (GDNF) that might enhance survival and protect against degeneration in targeted regions.36 While preliminary open-label studies have been promising with each of these approaches, the only double-blind study that has been performed to date showed no significant benefit of adeno-associated virus type 2 (AAV2) delivery of neurturin in comparison to a sham procedure (C.W.O., personal observations). In addition, each of the studies that have been performed to date have targeted the nigrostriatal dopamine system or its connections, and it is not clear how these approaches would Olanow et al: DA Cell-Based Therapy for PD 593 Fig 2. (A) Schematic representation of the normal nigrostriatal dopaminergic system (shown in red). Fibers from dopamine neurons in the substantia nigra pars compacta project to the striatum (putamen and caudate nucleus). (B) Schematic representation of the pattern of degeneration that occurs in Parkinson’s disease. Note that in addition to the degeneration in the nigrostriatal dopamine system, there is degeneration in nondopaminergic regions, including the dorsal motor nucleus of the vagus (DMN), pedunculopontine nucleus (PPN), locus coeruleus (LC), nucleus basalis of Meynert (NBM), olfactory tract, and cerebral hemisphere. Degeneration can also affect neurons in the spinal cord and peripheral autonomic nervous system (not shown). (C) Schematic representation of dopaminergic graft deposits (black dots) transplanted into the striatum with dopaminergic reinnervation of the putamen and caudate nucleus (red areas). Note that it would not be anticipated, nor is there scientific data to suggest, that dopamine transplants would reinnervate or restore function to areas of nondopaminergic neurodegeneration. influence the important nondopaminergic features of the disease. Conclusions and Future Directions What does all of this mean for the future of dopamine cell-based therapies in PD? We believe it is realistic to expect that with modifications in the transplant protocol, dopamine cell transplantation may one day be able to restore striatal dopamine innervation in a physiological manner and provide clinical benefits comparable to levodopa, but without motor complications. However, we can already largely accomplish this goal with deep brain stimulation,37 and it is very possible that we will soon be able to achieve comparable results with medical therapies such as long-acting formulations of levodopa.38 The more important question is whether dopamine cell transplants can provide benefits that are superior to what can be obtained with levodopa, and more specifically, will they be able to provide benefits for the nondopaminergic features of the illness? At present there is no data to indicate that this will be the case. It is 594 Annals of Neurology Vol 66 No 5 November 2009 theoretically possible that physiologic restoration of the nigrostriatal dopamine system could have consequences that extend to nondopaminergic neurons. For example, physiologic restoration of dopamine could inhibit overactivity in neurons of the subthalamic nucleus, and thereby prevent glutamate-mediated excitotoxic damage in its targeted nondopaminergic structures.39 Transplantation into alternate targets such as the substantia nigra pars compacta might physiologically restore dopamine innervation to extrastriatal regions and thereby improve deficits related to cortical and brain stem dopamine deficiency that might not benefit from traditional levodopa therapy. It is also possible that transplantation of different cell types (eg, glia), or the use of implanted cells to deliver molecules such as trophic factors or proteins that are deficient or exist in a mutant form in hereditary forms of PD (eg, Parkin) might provide more widespread benefits than are currently contemplated. Indeed, it is possible that certain selected patient subtypes manifesting a pure nigrostriatal dopamine deficiency, such as patients with early-onset autosomal recessive PD, might remain ex- cellent candidates for a dopamine cell transplant. However, all of these concepts are theoretical and presently lack empirical confirmation. In summary, cell-based dopaminergic therapies using current transplant designs have not as yet met expectations. The failure of dopaminergic cell-based therapies to achieve efficacy in double blind clinical trials, the development of unanticipated and occasionally disabling side effects, evidence that implanted cells themselves can develop the pathological changes of PD, and the likelihood that these treatments will not address the nondopaminergic features of the disease do not bode well for the near-term future of cell-based therapies as a clinically meaningful treatment for the majority of patients with PD. 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