Free-radical toxicity and antioxidant medications in Parkinson's disease.

Biosynthesis and Effects of Free Radicals

A free radical is any chemical species that contains one or more unpaired electrons.[10] The presence of an unpaired electron often causes the free radical to be highly reactive because the free radical acts as an electron acceptor and essentially steals electrons from other molecules.[10] This loss of electrons is called oxidation, and free radicals often are referred to as oxidizing agents because they tend to cause other molecules to donate their electrons to free radicals.[10]

The most common cellular free radicals are hydroxyl radical ([OH.sup..]), superoxide radical ([O.sub.2.sup.-.]), and nitric oxide ([NO.sup..]) (Fig. 1).[7,9] Other molecules, such as hydrogen peroxide ([H.sub.2] [O.sub.2]) and peroxynitrate ([ONOO.sup.-]), are not free radicals but can lead to the generation of free radicals through various chemical reactions (outlined in Fig. 1). Free radicals and related molecules (eg, [H.sub.2] [O.sub.2] and [ONOO.sup.-]) often are classified together as reactive oxygen species (ROS) to signify their ability to lead to oxidative changes within the cell.[9]

Free radicals and other ROS are by-products of cellular metabolism, and cells normally have a number of mechanisms to defend against damage induced by free radicals.[5,9] Problems occur, however, when the production of ROS exceeds the ability of cells to defend against these substances. This imbalance between cellular production of ROS and the ability of cells to defend against them is referred to as oxidative stress.[4,7,9] Oxidative stress can cause cellular damage and subsequent cell death because the ROS oxidize critical cellular components, such as lipids, proteins, and DNA.[9] It follows that cells such as substantia nigra neurons would undergo degeneration and death if subjected to damage induced by free radicals, and this idea is explored here.

Role of Oxidative Stress in Parkinson's Disease 

There is evidence that oxidative stress may be present in the region of the substantia nigra of people with Parkinson's disease. For example, membrane lipids in nigral cells typically show signs of oxidative damage suggesting free-radical -- induced injury (lipid peroxidation).[6.7,9] Nigral cells from people with Parkinson's disease also seem to lack adequate amounts of antioxidant substances, such as glutathione.[4,6] As illustrated in Figure 1D, glutathione peroxidase catalyzes the conversion of [H.sub.2] [O.sub.2] to oxygen and water, and adequate amounts of glutathione are needed to participate in this reaction. A deficit in cellular glutathione levels will prevent nigral cells from eliminating excess [H.sub.2] [O.sub.2]. Indeed, a reduction in glutathione levels seems to be one of the earliest changes that occurs in the substantia nigra of people with Parkinson's disease.[7] Furthermore, the levels of reactive (ferrous) iron appear to be increased in the substantia nigra of people with Parkinson's disease.[8.9,11] Ferrous iron facilitates the conversion of [H.sub.2] [O.sub.2] to [OH.sup..] (Fig. 1A), thus contributing to oxidative stress by generating a free radical that can directly damage nigral cells.

This evidence of oxidative stress also has generated considerable debate about the possible source of free-radical production in people with Parkinson's disease. Excess free radicals may be produced when dopamine is degraded enzymatically (Fig. 2).[12-14] As illustrated in Figure 2, normal by-products of dopamine metabolism, such as [H.sub.2] [O.sub.2], may he converted to harmful free radicals if cells are subjected to oxidative stress because of inadequate glutathione or excess reactive iron levels. In addition, dysfunction of certain mitochondrial enzymes has been implicated as a possible factor in oxidative stress and nigral cell degeneration.[15,16] Mitochondria are a major source of cellular free-radical biosynthesis, and dysfunction of certain enzyme pathways in nigral cells may initiate a vicious cycle whereby mitochondrial dysfunction leads to free-radical production, which causes further mitochondrial damage, and so on.[16,17]

The idea that free radicals are the definitive cause of nigral cell degeneration in people with Parkinson's disease is theoretical at the present time.[18] There is growing evidence, however, that nigral cells may be subjected to oxidative stress and that oxidative damage may accelerate the neuronal degeneration characteristic of Parkinson's disease. Pharmacologic efforts to prevent oxidative stress therefore might be helpful in preventing free radical-induced damage and subsequent degeneration of nigral cells. Several pharmacologic strategies that may help provide neuroprotection in people with Parkinson's disease currently are being investigated.

Use of Selegiline in Persons With Parkinson's Disease

Selegiline (Eldepryl) is a drug that has been studied extensively for possible neuroprotective benefits in persons with Parkinson's disease.[19,20] This agent (also known as deprenyl) was developed originally to help increase dopaminergic influence in the brains of people with Parkinson's disease. Selegiline inhibits the monoamine oxidase type B (MAO-B) enzyme in the basal ganglia. This enzyme inactivates dopamine (Fig. 2), thus terminating action of this neurotransmitter. By Inhibiting MAO-B, selegiline can provide symptomatic benefits in people with Parkinson's disease by prolonging the activity of any dopamine that is present in the basal ganglia.[2] Selegiline therefore has been used to decrease symptoms in people with Parkinson's disease for about 20 years, and this drug often is used in conjunction with classic anti-Parkinson's disease medications, such as L-dopa, and dopamine agonists, such as bromocriptine and pergolide.[2,20]

More recently, it has been suggested that selegiline also may be able to delay the neuronal degeneration that characterizes Parkinson's disease. For example, selegiline may directly decrease the production of free radicals through its inhibitory effect on MAO-B.[19,20] As discussed earlier, MAO-B is responsible for the normal destruction of dopamine in the basal ganglia, and free radicals may be generated as a by-product of this enzymatic destruction. These free radicals usually are not harmful because they are processed by cellular protective mechanisms, including antioxidant proteins and enzymes that convert free radicals into harmless substances (Fig. 2). In Parkinson's disease, however, the production of free radicals in the area of' the substantia nigra may exceed the ability of these neurons to process these potentially harmful substances.[19,20] By inhibiting MAO-B, selegiline may reduce free-radical formation because less dopamine is being degraded.

Some of the biochemical effects of selegiline, however, may be unrelated to MAO-B inhibition. For example, selegiline also appears to induce the synthesis of superoxide dismutase (Fig. 1C) and similar enzymes that help detoxify free radicals.[21-23] Selegiline also may act directly as a free radical scavenger and may increase the effects of other neuroprotective substances, such as cell-derived nerve growth factors.[24-26]

Selegiline appears to have the potential to reduce oxidative stress in nigral cells by decreasing free-radical production (through MAO-B inhibition) and by increasing the ability of cells to process excess free radicals (through mechanisms unrelated to the effect on MAO-B).[24,27] Evidence of the neuroprotective benefits of selegiline, however, is based largely on animal studies and in vitro experiments. The critical question is whether changes that occur on a cellular level will lead to clinically meaningful results in people with Parkinson's disease.

Clinical Trials of Selegiline

The effect of selegiline on people with Parkinson's disease was investigated originally in a controlled clinical trial known as the Deprenyl and Tocopherol Antioxidant Therapy of Parkinsonism (DATATOP) study.[28,29] This large multicenter study was initiated in 1987 and consisted of data obtained from 800 people with idiopathic Parkinson's disease of less than 5 years' duration. People were randomly assigned to one of four treatment groups: group 1 received selegiline (deprenyl), group 2 received tocopherol (vitamin E, an antioxidant with a putative ability to trap excess free radicals), group 3 received a combination of selegiline and tocopherol, and group 4 received a placebo.

All treatments were administered in a double-blind fashion, and people were reevaluated periodically for approximately 2 years.[28] The primary outcome (end point) of this study was the length of time before it was necessary to begin administering L-dopa. This end point was selected because it was thought to represent the onset of disability that was severe enough to warrant the initiation of traditional anti-Parkinson's disease medications (L-dopa). The investigators hypothesized that any neuroprotective effects produced by selegiline, tocopherol, or a combination of these two medications would be reflected by a delay in the onset of disability; that is, people taking the active medications would reach the end point later than would people taking a placebo.[29]

Results from the DATATOP study suggested that tocopherol did not have any appreciable effect on the onset of disability but that the onset of disability was delayed in people taking selegiline.[28,29] People taking selegiline alone or in combination with tocopherol took longer to reach the end point (the time at which L-dopa was needed) than did people taking tocopherol alone or the placebo. It was estimated, for example, that the median times tO reach the end point were 719 days for people receiving selegiline but only 454 days for people who did not receive selegiline.[28] This difference suggested that the onset of disability occurred approximately 9 months later in people receiving selegiline than in people who did not receive selegiline.[28]

Results from the original DATATOP study were interpreted cautiously, however.[20,30] As indicated earlier, selegiline also can produce symptomatic improvements in persons with Parkinson's disease by inhibiting MAO-B and thus prolonging the activity of dopamine present in the basal ganglia. In the DATATOP study, symptoms in patients taking selegiline might have progressed more slowly simply because of the dopamine-sparing effect of selegiline rather than an actual delay in the degeneration of dopamine-secreting neurons.[20] Likewise, there was no evidence that selegiline had any direct effect on the survival of nigral neurons. The decision to start L-dopa treatment was the primary end point of the DATATOP trial; neuronal viability never was measured directly. The fact that selegiline delayed the need to start L-dopa treatment was encouraging because it suggested that this drug may have some ability to delay the progression of disability in early stages of Parkinson's disease. The DATATOP investigators therefore were interested in determining whether more prolonged treatment with selegiline would continue to produce beneficial effects in these patients.

To obtain more information about the long-term effects of selegiline, the DATATOP study was extended beyond the 2-year period planned for the initial trial.[31] The investigators were particularly interested in determining whether selegiline actually had neuroprotective effects or whether the apparent delay in the onset of disability was caused by temporary (symptomatic) effects of this drug. One aspect of this extended study focused on a subgroup of 310 patients who had not reached the end point (the time at which L-dopa administration was required).[31] Some of these patients (n = 189) previously had received selegiline during the initial DATATOP trial, whereas others (n=121) had vet to receive selegiline. These patients were removed from all the experimental treatment groups for a 2-month washout period and subsequently were all placed on daily selegiline treatment. The rationale for this intervention was that if selegiline slowed the degeneration of dopaminergic neurons, patients who had received selegiline during the original trial may have retained a larger percentage of functioning dopaminergic neurons. These patients therefore may have continued to experience a delay in the onset of disability (the need to start L-dopa administration) relative to patients who had not received selegiline during the initial trial.

Results from this extended trial indicated that the onset of disability was not delayed in people who previously had received selegiline.[31] In fact, patients who had received selegiline during the original trial actually reached the end point (the time at which L-dopa was needed) sooner than did patients who were not assigned to the selegiline groups during the original study.[31] The investigators suggested that patients who had received selegiline earlier may have had more severe impairments at the beginning of the original study so that they investigators substantial benefits from early selegiline treatment but eventually needed L-dopa sooner than did patients who had not received selegiline during the initial trial.[31] Regardless of the reason for this finding, it was clear that the initial benefits seen with early selegiline treatment were not sustained and that prior selegiline administration did not seem to confer any appreciate protection against the onset of disability later in the course of Parkinson's disease.

The original DATATOP study also was extended to determine whether prior treatment with selegiline would decrease the incidence and severity of L-dopa -- related side effects in patients who eventually required L-dopa treatment.[32] Certain L-dopa side effects such as dyskinesia and diminished motor responses toward the end of a dosing interval (wearing off) are associated with more advanced stages of Parkinson's disease; that is, these side effects seem to occur more often as neuronal degeneration becomes more severe.[2] If early selegiline treatment could provide neuroprotective effects, patients who had received selegiline during the initial study might have experienced fewer L-dopa -- related side effects after being placed on L-dopa treatment. Results indicated., however, that prior treatment with selegiline did not reduce the incidence or type of side effects associated with subsequent L-dopa treatment.[32] This finding placed further doubts on whether early selegiline treatment actually can delay neuronal degeneration in people with Parkinson's disease.

Another study, conducted in the United Kingdom, focused on examining whether combining selegiline with traditional L-dopa treatment would be more successful than L-dopa therapy alone.[33] Patients with early Parkinson's disease were randomly divided into two primary groups: group 1 (n=249) received traditional L-dopa therapy (L-dopa combined with benserazide, a peripheral dopa decarboxylase inhibitor that prevents the premature conversion of L-dopa to dopamine), and group 2 (n=271) received traditional L-dopa therapy combined with selegiline. These patients were monitored for an average of 5.6 years and were evaluated periodically for increasing disability, frequency of side effects, and deaths from all causes. Results indicated that disability was not different between the two groups but that adding selegiline to traditional L-dopa treatment was associated with an increase in the frequency of severe disabling side effects.[33] More importantly, the mortality rate was higher in patients taking selegiline combined with L-dopa therapy than in patients receiving L-dopa therapy alone.[33] These findings suggested that the addition of selegiline to L-dopa therapy did not provide any benefits and that the prolonged administration of these medications in combination actually might have increased mortality in patients with Parkinson's disease.[33]

Results from these extended clinical trials therefore have cast doubt on whether selegiline has neuroprotective effects in people with Parkinson's disease. Early treatment with selegiline may delay the need for traditional anti-Parkinson's disease medications (L-dopa), but this effect seems to be mediated by temporary symptomatic effects rather than an actual decrease in neuronal degeneration. Benefits from early selegiline treatment do not seem to be sustained into the later stages of Parkinson's disease, and there is evidence that prolonged use of selegiline with L-dopa actually may have detrimental effects on disease progression and mortality. Current evidence, therefore, suggests that selegiline may be somewhat useful in the early treatment of persons with Parkinson's disease, especially when it is worthwhile to delay the start of L-dopa therapy.[31] It seems doubtful, however, that selegiline provides sufficient protection of nigral neurons to actually delay the clinical progression of Parkinson's disease.

Other Neuroprotective Agents

Several other pharmacologic strategies might provide neuroprotection in persons with Parkinson's disease. Most of these strategies focus on some tv e of antioxidant effect to prevent free radical toxicity. For example, [Alpha]-tocopherol (vitamin E) is a common free-radical scavenger that has been considered a potential agent for delaying the progression of symptoms in people with Parkinson's disease.[4] As indicated earlier, the DATATOP clinical trial that described the initial benefits of selegiline also investigated whether tocopherol could produce a similar delay in the progression of symptoms.[28] Regrettably, no benefits were observed with tocopherol at the dosages used in that study (2,000 IU per day).[28] It is unclear, however, whether the tocopherol used in the DATATOP study actually crossed the blood-brain barrier and ultimately reached the basal ganglia. Likewise, tocopherol is a general antioxidant and may not have been very effective in controlling the specific type of oxidative stress that seems to occur in nigral cells.[28] Consequently, research to determine whether tocopherol and similar antioxidants have the potential to delay the progression of Parkinson's disease is ongoing.

Members of another group of free-radical scavengers known as lazaroid antioxidants also are being considered for their neuroprotective effects.[34] These compounds appear to directly with oxygen-based free radicals and to prevent free-radical -- induced damage to lipids and other structures.[35] The use of lazaroids still is fairly experimental, however, and it may be some time before their role in clinical conditions such as Parkinson's disease can be defined.

Another potential source of neuroprotection involves drugs that already are being used to enhance dopamine activity in the brain. Bromocriptine (Parlodel) and pergolide (Permax) are dopamine agonists that bind to striatal dopamine receptors and help decrease symptoms in people with Parkinson's disease.[1] Preliminary evidence from animal studies suggests that these drugs also scavenge free radicals, such as [OH.sup..], [O.sub.2.sup.-.], and [NO.sup.sub..][36-38] It will be interesting to determine whether the antioxidant properties of these drugs can help delay the progression of Parkinson's disease, and clinical trials to determine the neuroprotective effects of these dopamine agonists hopefully will be forthcoming.

Clinical implications

Physical therapists should be aware that there are ongoing efforts to develop pharmacologic methods to control the progression of Parkinson's disease rather than merely to provide symptomatic benefits through dopamine replacement. The goal of current research is the early use of neuroprotective medications to enable patients to remain in the early stages of Parkinson's disease for longer periods. Delayed disease progression also would allow therapists to implement more aggressive rehabilitation regimens to help patients retain optimum neuromuscular and cardiovascular functions well into advanced age. Therapists must realize, however, that the exact role of oxidative stress in people with Parkinson's disease still is speculative. Likewise, additional clinical research is needed to establish whether various medications actually can provide neuroprotection and delay the progression of Parkinson's disease. Clinical trials of potential neuroprotective medications are ongoing, and it is hoped that physical therapists can play an active role in these trials by helping to assess the effects of these medications in patients with Parkinson's disease.

Conclusion

Evidence suggests that degeneration of neurons in the substantia nigra of people with Parkinson's disease may be accelerated because these neurons are subjected to oxidative stress. Oxidative stress occurs because of an imbalance between free-radical production and the ability cells to process these cytotoxic free radicals. Thus, pharmacologic strategies that help control oxidative stress and prevent free-radical-induced neuronal degeneration may be effective in delaying the progression of Parkinson's disease. Selegiline, an agent with multiple antioxidant properties, initially showed promise in delaying the onset of disability in people with early Parkinson's disease. Extended clinical trials of selegiline, however, were not able to document any long-term effects that would suggest that this medication actually provided neuroprotection or delayed the progression of Parkinson's disease. Other antioxidant medications currently are being studied, and future clinical trials may help establish whether these agents have neuroprotective effects. Although the use of neuroprotective agents still is experimental, this idea has generated hope that medications may be used in the future to prevent the progressive deterioration that is often so devastating in people with Parkinson's disease.

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