Brain dysfunction in the era of combination antiretroviral therapy: Implications for the treatment of the aging population of HIV-infected individuals

Abstract

Improvements in the treatment of HIV infection and in the advancement of combination antiretroviral therapy (cART) have led to an increase in the number of individuals with HIV who are surviving to an older age. Preventing the development of neurocognitive abnormalities has become an increasingly important issue in this aging patient population, which is already at risk for cognitive impairment as a result of the neuropathological effects of HIV. cART has been critical in reducing the overall severity of HIV-associated neurocognitive disorders (HAND), but numerous challenges remain, as the prevalence of HAND continues to be high. There are several key areas in which treatment could be improved to reduce the incidence and severity of HAND. The use of well-tolerated cART medications that are able to penetrate the blood-brain barrier hold particular promise, as these agents may enable increased viral suppression in the parenchyma and may reduce neurocognitive dysfunction. In addition, the improved treatment of comorbid medical conditions that are common in patient populations with HIV (eg, HCV, liver failure and metabolic syndrome) is critical, as several of these conditions are known to have a significant effect on neural functions. Various research approaches indicate that the development of agents that control free radicals, neurotoxicity, proinflammatory processes and apoptosis may also have substantial potential in this field.

Introduction

Improvements in overall health caused by the increasing use of combination antiretroviral therapy (cART) have led to increased survival rates in patients infected with HIV [1], and in an associated increase in the number of older individuals living with HIV/AIDS [2]. In addition, the number of new HIV infections in older adults is also increasing; an estimate from 2007 suggests that individuals aged > 50 years account for 10% of all new HIV infection cases in the US [3]. This epidemiological change may have considerable impact on the observed effects of HIV infection, given that increased age at seroconversion is a significant risk factor for developing neurocognitive dysfunction [4].

Thus, the influences of aging processes and age-related cognitive changes have become important considerations in older individuals infected with HIV who are already at risk for neurocognitive impairments caused by the neuropathological effects of the virus. Concerns regarding the possible compounding effects of aging and HIV on neurocognitive processes appear to be justified; notably, there is evidence to suggest that HIV may exacerbate age-related brain dysfunction [5-7].

Although there is evidence to suggest that older patients infected with HIV are at greater risk for developing neurocognitive impairments, whether these impairments are directly related to HIV infection or whether additional age-related abnormalities may have a role has not been established. Most data suggest that the etiology of neurocognitive impairment in older patients infected with HIV is multifactorial, with the implicated processes including chronic inflammation, possible long- and short-term side effects of cART, increased neurovascular abnormalities, metabolic changes (eg, diabetes and insulin resistance), and concurrent age-related neurodegeneration [8]. Additional complications may arise secondary to the time elapsed between initial HIV infection and diagnosis, given that the average time between infection and diagnosis is longer in older adults [9-11]. This delay in diagnosis tends to lead to greater compromise of the immune system [12], as well as to increased proliferation of the virus in brain tissue in the absence of antiretroviral treatment, thereby increasing the potential for greater neuropathological effects. Furthermore, compared with younger patients, immune reconstitution following cART initiation appears to be diminished in older adults [13-15], and the resultant reduced effectiveness of therapy also increases the possibility of neurological damage. Thus, several factors may converge to increase the risk for cognitive dysfunction in older adults infected with HIV, and it is critically important to identify innovative ways to treat neurocognitive abnormalities in this population.

Neurocognitive and psychological impairments in HIV-infected individuals in the pre- and post-cART era

cART has significantly altered the prevalence and severity of HIV-associated neurocognitive disorders (HAND). Treatment with cART can improve neurocognition, whereas declines in cognition are observed in patients who do not receive such treatment [16]. Accordingly, the incidence of HIV-associated dementia, which is the most debilitating form of HAND, has declined significantly since the introduction of cART [17,18]. However, findings from postmortem neuropathological studies investigating the prevalence of HIV-related encephalopathy in the context of cART have been varied. Early studies suggested that the frequency of HIV-related encephalopathy in the post-cART era was either equal to or possibly higher than rates reported in the pre-cART era [19,20]. In contrast, more positive results were presented in a recent study [21], suggesting that cART may be more effective than initially believed at reducing the prevalence of parenchymal pathology in patients with HIV.

Nevertheless, neurocognitive and psychological impairments in patients receiving cART continue to be an issue, and the overall prevalence of HAND remains high [22-25]. Neurocognitive impairment has a demonstrated negative impact on the quality of life of patients infected with HIV [26]; up to 50% of patients are estimated to experience some form of neurocognitive impairment [27,28]. Increased rates of psychological impairment have also been reported [29,30]. These levels of neurocognitive and psychological impairment are not surprising, given that the brain continues to be a major target of infection in patients with HIV despite antiretroviral therapy [20]. Accordingly, there is significant evidence to suggest that disruptions in frontostriatal systems underlie HIV-related cognitive [31-33] and psychological [34] symptoms.

The progression of HAND has also changed as a result of the introduction of cART. Before such combination therapy became available, cognitive impairments were severe and chronic, with death following the onset of dementia in most cases [35]. In contrast, the milder forms of HAND that are currently more common are not generally the cause of death in patients. HAND may be classified as having either a progressive, stable, improving or fluctuating course [36]. The mechanisms underlying the variable presentation of HAND are not fully understood, but are believed to be related to fluctuations in the physiological response to the disease; these fluctuations could reflect changes in CD4 counts, viral load, comorbid medical conditions, metabolic states and inflammatory processes. Selected data regarding the contribution of each of these factors to the development of HAND and implications for current and future treatment strategies are discussed in the following sections.

Effects of viral load, CD4 nadir, and current CD4 counts on neurocognition and the brain

There has been significant research focus the relation between immune system functions and cognitive abilities in patients with HIV. Increases in plasma HIV-RNA levels (ie, viral load) and significant reductions in immune system functions (as measured by nadir and current CD4 lymphocyte counts) continue to result in brain abnormalities in patients receiving cART. These HIV-related disease factors correlate with the degree of cognitive impairment [24,37,38]. In particular, a notable relationship exists between CD4 counts and cognitive function for patients with CD4 counts of < 200 cells/μl [39]. Altogether, the data indicate that a history of advanced immunosuppression is detrimental to long-term neurocognitive function.

Several recent studies support the observation that advanced immune suppression is associated with increased pathological effects in the brains of patients receiving cART, as was also observed before the introduction of these therapies [21,40,41]. For example, a recent in vivo neuroimaging study by Cohen et al demonstrated that patients with HIV who had a history of severe immune suppression were at the greatest risk for brain volume loss [41]. Similarly, Cardenas et al reported that reduced CD4 lymphocyte counts and higher viral loads were associated with greater rates of brain tissue loss over a 2–year period in patients receiving a stable cART regimen [40]. Taken together, the data indicate that there may be an irreversible component of neural injury that reflects the historical severity of HIV-related immunosuppression. Neural injury also appears to be affected by current immunological status (eg, CD4 levels) [41]. Therefore, early and continuing treatment with cART following diagnosis is essential to help prevent the severe reductions in immune system functions that are associated with significant brain abnormalities. However, these findings also reveal that improved treatments are necessary for the complete prevention of HIV-related neuropathological effects, given that brain abnormalities continue to be observed in patients receiving cART. Two areas in which improvements in cART are needed to reduce the severity of HIV-related neural abnormalities relate to drug administration and to the overall effectiveness of this treatment strategy.

Challenges in cART administration

Long-term treatment with cART is often associated with increased toxicity, which can jeopardize adherence to the treatment regimen, potentially contributing to the development of viral resistance and increases in cognitive abnormalities. Even small reductions in adherence can result in viral rebound and reductions in immune function [42], both of which are risk factors for increasing HIV-related neuropathology and HAND. The adherence of patients to a cART regimen is a particular concern for older individuals, who are more susceptible to immunological decline in relation to reductions in adherence to treatment [43]. Furthermore, complex cART administration regimens and cognitive impairment have both been linked to reduced adherence [43-46]. Altogether, these data suggest that the development of antiretroviral agents with improved tolerability and simpler dosing schedules is critical, as such improvements may significantly increase adherence, particularly in older adults with cognitive impairments, and thereby help to maintain cognitive functions.

CNS penetration and the effectiveness of cART

Some evidence indicates that enhancements in the efficacy of cART could reduce neurocognitive impairment. For example, several studies have reported that agents with the ability to cross the blood-brain barrier (BBB) may be more effective at diminishing the severity of cognitive dysfunction in patients with HIV [47,48]. cART regimens that include multiple agents with the ability to penetrate the CNS (eg, stavudine, zidovudine, abacavir, efavirenz, nevirapine, indinavir and ritonavir-boosted lopinavir) have been associated with increased viral suppression [49], as well as improved neurocognitive outcomes [47]. Therefore, improved CNS penetration by antiretroviral agents might lead to greater HIV suppression in the brain. However, more recent findings indicate that the effects of increased CNS penetration of antiretroviral drugs are mixed [50], and there is some evidence that increased CNS penetration results in reductions in neurocognitive function [51]. These inconsistent findings likely reflect the different toxicities of the various classes of cART agents in the brain. For example, magnetic resonance spectroscopy (MRS) has been used to demonstrate that nucleoside reverse transcriptase inhibitors (NRTIs) may have specific neurotoxic effects [52]. Thus, the development of novel agents and administration methods to increase the penetration of antiretroviral agents into the CNS must avoid strategies that are associated with neurotoxicity. Furthermore, additional studies are necessary to characterize the cognitive benefits and potential toxicities associated with agents used in cART that have various levels of CNS penetration.

Recent research in the area of nanomedicine is also yielding exciting findings that may lead to significant advances in the ability to deliver cART across the BBB. Promising results have been obtained from in vitro and in vivo investigations of agents designed to inhibit the membrane-associated ATP-dependent efflux transporter P-glycoprotein (P-gp) and to increase the efficacy of drug delivery across the BBB [53-55]. P-gp, a transport protein that is expressed in brain microvascular endothelial cells, has been hypothesized to have a role in the regulation of drug entry into the CNS. Exposure to protease inhibitors (PIs) has been demonstrated to upregulate P-gp expression and functional activity in human brain microvessel endothelial cells (BMVECs) in vitro; however, treatment with the P-gp inhibitor valspodar (PSC-833; National Cancer Institute) can reverse this effect and can also lead to an increase in cellular PI uptake [55]. Studies with the P-gp inhibitor zosuquidar (LY-335979; Kanisa Pharmaceuticals Inc) in mice reported similar results [53]. Furthermore, studies in murine models indicated that the block copolymer P85, which has activity as a P-gp inhibitor, may have an independent effect on viral reproduction in neural tissue, and may also significantly improve the effects of antiretroviral agents on viral replication in the brain [54]. Such findings are promising regarding the potential for novel strategies to facilitate cART penetration into the CNS and to improve treatments for HAND through the inhibition of active efflux components, including P-gp.

Several nanotechnology-based methods that could potentially be used to facilitate the transmission of cART across the BBB and improve treatments for neurological disorders associated with HIV (neuroAIDS) are being investigated. For example, a series of studies has demonstrated that synthetic nanoparticles containing antiretroviral agents, termed nanoART, can be carried within monocytes/macrophages, which deliver these nanoparticles to virus-target tissues over a period of weeks [56-59]. Treating cells with nanoparticles of several antiretroviral agents (ie, several nanoformulations of single-drug nanoART particles), in a type of combination nanoART, appears to provide greater viral inhibition compared with monotherapy nanoART [60]. These findings suggest that cell-mediated delivery systems may have potential to deliver nanoART effectively across the BBB and to improve permeation of virus sanctuaries in the CNS. In addition to cell-mediated delivery systems, promising innovations in nanotechnology also include the development of nanoparticles with increased BBB permeability and novel methods to facilitate the uptake of cART-containing nanoparticles by BMVECs [61]. Overall, nanotechnology may result in significant advancements for the future treatment of HAND. However, additional research, particularly in the areas of safety and efficacy, will be necessary in order to progress these developments into the clinic.

Improving the management of neurocognitive impairment through the treatment of comorbid medical conditions

Patients with HIV are a diverse population. However, there are several frequently reported comorbid medical conditions that can impact neurocognitive function in many patients who are infected with the virus. The treatment of these conditions is essential, as better management could lead to cognitive improvements.

HCV and HCV-associated liver disease

Coinfection with HCV is common in patients with HIV, consistent with the fact that both of these viruses have similar routes of transmission. Conservative estimates of HCV coinfection rates in patients with HIV range from 16 to 23% [62-64]. However, rates of coinfection are significantly higher (73%) in patients with HIV who engage in high-risk behaviors (eg, intravenous drug use) [62].

The potential important role of HCV in the brain in the progression of HAND was established only recently [65]. HCV has been demonstrated to reside within CSF and brain tissue [65-67], and infection with the virus can result in cognitive impairments [68,69]. Patients coinfected with HIV and HCV exhibit greater neurocognitive abnormalities than monoinfected individuals [70-72], suggesting an additive effect of coinfection on neurocognitive functions.

The biological mechanisms that underlie HCV-related cognitive impairments in patients with HIV are not fully understood; however, several hypotheses have been proposed [73]. One interesting hypothesis relates to HCV-associated cytokine activity increases [74], which represent a pathway through which HCV might exacerbate HIV-related neurocognitive dysfunction. Notably, HCV is associated with proinflammatory markers in frontal white matter, as measured by MRS [75]. Targeting the activity of this proinflammatory cascade may provide several treatment strategies aimed at mitigating the negative effects of HCV on brain tissue. HCV may also increase cognitive impairment by inhibiting CD4 recovery associated with cART [76], reducing the tolerability to cART [77] and increasing the likelihood of AIDS-defining events [78]. Therefore, the improved treatment of neurocognitive disorders in patients coinfected with HIV and HCV may require the development of cART medications that are more robust in patients coinfected with HCV, yet are still well tolerated.

Chronic liver disease is an additional and important factor that affects neurocognition in patients coinfected with HIV and HCV. As well as being a major factor contributing to mortality among coinfected individuals [63], chronic liver disease can also cause metabolic encephalopathy with associated cognitive disturbances [79,80]. Ammonia, an intestinally derived neurotoxin that is not adequately removed by a damaged liver, contributes significantly to the development of neurocognitive dysfunction [81], changes in cerebral blood flow [82,83] and abnormalities in glucose metabolism in the brain [83]. Moreover, several pathophysiological mechanisms associated with chronic liver disease may lead to brain dysfunction [84-87]. In individuals with chronic liver disease (eg, cirrhosis), abnormalities in levels of nitric oxide and ammonia may contribute to metabolic encephalopathy [88]. Patients with liver disease may experience cytokine, lipid and neuropeptide disturbances; impaired liver function can also cause alterations in NADH/NAD⁺ that can influence the metabolism of carbohydrates, fatty acids, urea and other substances, and can affect cerebral calcium homeostasis and gene expression [84-87]. Disruptions in other neuroactive and neurotoxic substances, including glutamate and GABA, may occur as a result of the pathologies associated with liver failure, leading to neural dysfunction [89].

In the management of HCV-associated liver disease in coinfected patients, a combination of PEGylated IFN and ribavirin is a standard treatment approach [90]; however, the effectiveness of this regimen may be somewhat reduced for African-American patients compared with Caucasian patients, for reasons yet to be determined [91]. In addition, the treatment response in patients coinfected with HCV and HIV is reduced compared with the response in patients infected with HCV only [90]. Mouse models have also demonstrated that IFN may have negative effects on cognition and may cause neuronal dysfunction [92]. Furthermore, the standard treatment for HCV is not well tolerated by some patients, and often is not initiated in the early stages of infection.

In recent years, several alternative treatment approaches for patients coinfected with HCV and HIV have been explored. For example, novel nucleoside analog transcription inhibitors that have pro-apoptotic and anti-angiogenic properties have been demonstrated to reduce HCV replication in vitro [93]. siRNAs that impede RNA replication in mice also hold promise in this regard [94], although there is a challenge associated with achieving reductions in HCV replication without producing adverse effects. The potential utility of ammonia-lowering agents (eg, probiotics or lactulose) has also been examined in patients with minimal hepatic encephalopathy, and some evidence of cognitive improvement has been reported [95,96]. In addition, various investigational compounds are being developed to treat HCV, including several that are in phase III clinical trials. For example, Merck & Co Inc is developing several antiretroviral drugs: boceprevir, a HCV protease inhibitor for the treatment of chronic HCV, is in phase III trials, and an NDA to the FDA has been proposed for 2010 [97]. Another candidate drug for HCV therapy is telaprevir (VX-950; Vertex Pharmaceuticals Inc/Mitsubishi Tanabe Pharma Corp/Tibotec Pharmaceuticals Ltd/Janssen Pharmaceutica NV), which is in phase III development. Therefore, additional treatments for HCV in coinfected patients may become available in the near future. Nonetheless, there is a compelling need for a greater understanding of liver disease in coinfected patients and for a continuation of research efforts to identify new strategies to improve neurocognitive outcomes and overall survival.

Metabolic syndrome and cardiovascular disease

Metabolic syndrome is defined as the occurrence of three or more of the following abnormalities: hypertriglyceridemia, low HDL cholesterol, hypertension, abdominal obesity and high fasting glucose. The syndrome is associated with an increased risk of cardiovascular disease (CVD) and type 2 diabetes. Notably, metabolic syndrome has been associated with increased cognitive impairment [98] and white matter changes [99] in HIV-negative individuals. Diabetes can also contribute to cognitive decline [100]. Accordingly, insulin resistance, a known side effect of cART use [101], has been reported to contribute to neurocognitive impairments in patients with HIV [102]. In addition, CVD and arterial wall abnormalities, such as increased intima-media thickness, have also been associated with increased neurocognitive difficulties and brain abnormalities in samples from individuals who are not infected with HIV [103-106]. These observations suggest that improving the control of these conditions in patients with HIV could significantly affect neurocognitive outcomes.

Approximately 25% of patients with HIV meet the criteria for the diagnosis of metabolic syndrome, and the development of this syndrome appears to be related to HIV disease factors (eg, viral load) [107]. Moreover, there are concerns that the use of cART may promote the development of metabolic syndrome and elevate the risk of CVD through increases in hypercholesterolemia, hypertriglyceridemia and abnormal adiposity [108-111]. For example, data reported by Friis-Moller et al indicated a 26% relative increase in the rate of heart attacks per year of treatment in the first 4 to 6 years of cART administration [112]. HIV-disease factors and cART are associated with an increased risk for developing symptoms associated with CVD [113-115]. Patients with HIV exhibit arterial wall changes (ie, increased intima-media thickness) similar to those observed in patients with coronary artery disease [113]. In addition, greater arterial wall abnormalities have been observed in patients with HIV who have been infected for a longer duration and have higher viral loads, as well as in patients receiving cART (particularly PIs and NRTIs) [113,114]. Patients receiving cART also have higher total cholesterol, triglycerides and diastolic blood pressure levels compared with cART-naïve patients, and the duration of cART use appears to be positively correlated with the degree of arterial stiffness [115].

Postmortem studies indicate that the prevalence of cerebral infarction or intracranial hemorrhage in patients with HIV ranges from 6 to 34%; however, many of these cases were clinically asymptomatic [116-120]. Clinically diagnosed stroke is reported to occur in < 1 to 4% of patients with HIV [117]. In addition, an increase in the rate of thromboembolic disorders, including ischemic stroke, has been reported in these patients [121]. The available evidence suggests that acute neurovascular abnormalities may result from direct and indirect effects of HIV infection. For example, as noted, cART can be associated with high cholesterol and triglyceride levels [115], as well as insulin resistance [101]; however, whether these effects may ultimately translate into an increased risk for developing arterial thrombosis has not been established [121]. In addition, HIV may have independent effects on triglyceride, cholesterol and lipid levels, even in the absence of cART [122,123]; these independent effects may increase the risk of neurovascular events. For example, dilated cardiomyopathy and large artery atherosclerosis are responsible for a significant proportion of stroke events in patients with HIV; however, an increased frequency of atypical stroke mechanisms has also been reported, including infectious vasculitis and hypercoagulability [124]. Further investigation in the area of neurovascular abnormalities in patients with HIV is necessary, as gaining adequate control over the development of such pathologies is an important aspect of maintaining cognitive health in the aging population of patients with HIV.

Overall, the emerging data suggest that patients with HIV may be at risk for developing cognitive dysfunction over time as a result of vascular-related abnormalities associated with comorbid metabolic syndrome and CVD. The mechanisms underlying these processes are likely multifactorial. Viral replication may have a direct effect on lipid levels [123]. cART may affect lipid metabolism, endothelial and adipocyte cell function, and mitochondria [125]. Although direct effects of HIV on the inflammatory processes involved in the pathogenesis of metabolic syndrome have been suggested [123], the use of cART may increase proinflammatory cytokines, which may indirectly increase lipid accumulation and insulin resistance [125]. These observations are consistent with research implicating systemic inflammation in the development of CVD [126]. Accordingly, the use and further development of anti-inflammatory agents could be beneficial in reducing the incidence and severity of cardiovascular and neurovascular abnormalities that could exacerbate HAND. In addition, the development of novel PIs and NRTIs that have fewer metabolic side effects may also be necessary [113,114].

Efforts to improve cognitive functions in patients with HIV: Implications for drug development

Several approaches have been considered for the treatment of HAND and for the prevention of brain dysfunction in patients with HIV. Most of these approaches are in relatively early stages of development, but are important to consider given their potential value when used in conjunction with cART.

Neuroprotection and apoptosis

Maximally protecting the brain against the effects of HIV may ultimately depend on the availability of neuroprotective drugs that can be introduced soon after infection and that will augment the benefits achieved through viral load reduction and preserved immunological function. While such neuroprotective drugs are not yet available for routine clinical use, research directed at the mechanisms of HIV-associated neurotoxicity and apoptosis is rapidly evolving.

Substantial evidence from a variety of preclinical and clinical studies indicates that neuronal apoptosis occurs secondary to HIV infection [127-132]. While neurons are not directly infected by HIV, neuronal injury and apoptosis can occur secondary to a cascade of neurotoxic events that result from the viral infection of macrophages and microglia, including the release of HIV proteins (eg, gp120, gp41 and Tat), nitric oxide, glutamate, platelet-activating factor, chemokines and cytokines (eg, TNFα) [133,134]. Accordingly, there is strong justification for focusing on neuronal apoptosis pathways in the development of neuroprotective antiretroviral therapies. A potentially relevant pathway involves TNFα, the α chemokine receptor CXCR4 and HIV. Increased expression of TNFα has been observed in postmortem brain tissue samples from patients with HIV [135]. TNFα also contributes to apoptosis in differentiated human neuronal SK-N-MC cell cultures [136]. More recent findings indicate that the treatment of differentiated SK-N-MC cells with TNFα leads to an increased cell surface expression of CXCR4, and potentiates the ability of HIV to induce neuronal apoptosis [128]. The increase in CXCR4 expression can be reduced by treating cells with mAbs for CXCR4 and TNFα, suggesting that TNFα may increase the vulnerability of neuronal cells to the apoptotic effects of HIV through the increased cell surface expression of CXCR4 [128].

Neurotoxic proteins released as byproducts of HIV have received considerable research attention in recent years. Most notably, Tat and certain glycoproteins (eg, gp120) have been demonstrated to contribute to both neuronal apoptosis and excitotoxicity [130-132,137-142]. For example, exposing neuroblastoma cells to gp120 can result in an increase in the expression of Fas, as well as in the release of mitochondrial pro-apoptotic proteins, leading to cell death [143]. GABA-glutamate systems have also been demonstrated to contribute to calcium accumulation, which has a neurotoxic effect in the brain [144-148]. Soluble neurotoxins from HIV-infected brain macrophages cause excitation of NMDA receptors, which contribute to these effects. For example, O'Donnell et al observed that the susceptibility of hippocampal neurons to HIV-associated NMDA excitotoxicity varies by cell type in the hippocampus, but involves neuronal calpain activation, which leads to neuronal death. Antagonists of this excitotoxic activity appear to provide neuroprotection [149].

Exposure to Tat and gp120 can cause significant calcium disturbances in neurons [150,151]. Given that calcium homeostasis and signaling regulate many neuronal functions, including synaptic transmission, plasticity and cell survival, disturbances of calcium metabolism can affect neurons, particularly in the context of aging. As individuals age, the products of energy metabolism (eg, reactive oxygen free radicals) accumulate and, in interaction with oxidative stress, cause impaired calcium homeostasis; these factors ultimately cause a positive feedback loop, with each factor aggravating others [144]. Consequently, neurons become more vulnerable to additional stress, leading to neuronal degeneration. Neurodegenerative diseases, including Alzheimer's disease (AD) and Huntington's disease, may involve similar types of calcium dyshomeostasis, resulting in the aggregation of proteins such as β-amyloid (Aβ) [144,152]. Calcium disturbances in neurodegenerative diseases can lead to compromised mitochondrial and endoplasmic reticulum function, impaired calcium buffering, glutamate excitotoxicity and dysfunction of calcium entry into neurons [144,150]. Notably, a breakdown in the ability of astrocytes to buffer the effects of glutamate appears to have an important role in the pathophysiology of calcium dysregulation in HIV [153,154].

No effective treatment is available to prevent neuronal degeneration caused by the mechanisms described in the previous paragraphs. Nonetheless, drugs that inhibit neuronal death signaling pathways and stimulate cell survival pathways represent a potentially powerful approach for preventing HIV-associated brain dysfunction [134]. For example, p38 MAPK inhibitors have been explored for their potential value in inhibiting neuronal cell death signaling [155,156], and drugs that modulate Akt/PKB may be useful for stimulating cell survival [157,158]. NMDA receptor antagonists and agents acting on calcium and glutamate systems also represent important therapeutic options that are being investigated [134,157].

Reducing neuronal apoptosis may be accomplished using drugs that interfere with the pathophysiological cascade described in the previous paragraphs. However, these efforts are complicated by several issues: (i) many of the potential agents that affect calcium and glutamate metabolism are highly psychoactive and would be difficult for patients to tolerate; and (ii) there is likely considerable heterogeneity in the vulnerability of specific types of neurons to these effects. Therefore, newly developed drugs will need to be well tolerated and also be specific in their ability to affect particular classes of neurons or other cell types in the brain.

Astrocyte dysfunction

Astrocyte dysfunction has been implicated in HIV-associated brain abnormalities [159-169], as astrocytes have a critical role in CNS homeostasis and cerebral immunity. As well as causing a disruption in glutamate buffering, HIV-related astrocyte dysfunction may result in a poor regulation of BBB permeability [133]. In addition, there is a possibility that astrocytes might form an additional viral reservoir in individuals who have been infected with HIV for many years [170-172]; however, whether astrocytes serve as a source of productive infection has not been established [133]. Astrocytes have been demonstrated to produce TNFα and other proinflammatory cytokines following activation [173]. Interestingly, there is also evidence that substance abuse, particularly abuse of opiates and cocaine, may interact with HIV-associated astrocyte dysfunction to exacerbate the effects of HIV in the brain [174-176]. For example, morphine increases the expression of HIV entry coreceptor genes (eg, CCR3, CCR5 and CXCR2) and suppresses the production of HIV-protective chemokines (eg, IL-8 and MIP-1β) in astroglial cells [160]. These findings are relevant given the importance of substance abuse comorbidity in the development of HAND [177]. Further knowledge is required regarding the interaction of astrocytes, drug abuse and HIV neuropathology. Astrocytic infection clearly has an important role in the development of HAND, and improved methods of treatment will likely lead to decreased neuropathological effects in patients with HIV.

Cerebral metabolite disturbances

Evidence from various animal models indicates that HIV-associated cerebral metabolite disturbances [178-181] contribute to neurocognitive and brain abnormalities in patients with HIV [182-186], and that these disturbances coexist with alterations in both plasma and CSF cytokine and chemokine concentrations [187-192]. Cerebral metabolite and cytokine/chemokine disturbances can lead to the induction of proinflammatory processes, oxidative stress and neurotoxicity [162,178-181,193,194]; these effects may occur as a direct result of HIV infection and as an adverse consequence of cART [195,196]. As noted, one of the mechanisms underlying cerebral proinflammatory responses secondary to HIV infection is excitotoxicity caused by proteins such as Tat and gp120. Such proinflammatory processes are important because they exacerbate the direct effects of the virus and neurotoxicity on neuronal function [197-200]. Various examples of this type of exacerbation of proinflammatory processes in the brain have been observed in other diseases. For example, the volume of infarction caused by initial cerebral ischemia in stroke is often small compared with the eventual extent of the infarction caused by the biochemical cascade of proinflammatory processes, including the release of free radicals that occurs in response to the ischemia [201]. Agents aimed at controlling pathophysiological responses secondary to free radical and proinflammatory processes are an obvious area for consideration as potential novel therapeutics for brain dysfunction resulting from these effects. Drugs with antioxidant or free radical scavenging effects may have significant promise in this area [157].

Neurodegenerative processes

An aging population of patients with HIV is associated with additional concerns regarding the development of typical age-related neurodegenerative diseases, including AD. HIV infection may combine, either additively or synergistically, with underlying neurodegenerative processes to affect the development of HAND. Several studies have presented data that are consistent with this possibility. For example, a recent study reported that the neuropathology of HIV appears to be changing to involve subcortical as well as cortical structures [41], indicating that HIV in the cART era may progress to involve processes that have a greater resemblance to age-related neurodegenerative diseases.

AD neuropathology is characterized by the presence of amyloid plaques (containing amyloid fragments) and neurofibrillary tangles (containing hyperphosphorylated tau). There is substantial evidence that patients with HIV also exhibit AD-related neuropathology, including changes in Aβ [202-204] and hyperphosphorylated tau [205]. While there is some question whether Aβ is increased in HIV patients compared with uninfected individuals [205], data from one of the largest studies conducted to date that examined Aβ in postmortem brain samples from patients with HIV has revealed amyloid deposits in the frontal cortex, hippocampus and basal ganglia in individuals with HIV [203]. Unlike in AD, in which amyloid is mostly present in extracellular plaques, amyloid depositions in patients with HIV were present in both extracellular plaques and in the neuronal soma. Older patients and individuals carrying the ApoE 4 allele, which confers an increased risk of developing AD, appeared to exhibit greater accumulation of amyloid. Furthermore, patients treated with cART exhibited higher levels of amyloid than patients who did not receive such therapy – an increase that may have been mediated by inhibition of the insulin degradation enzyme [203]. There is also evidence that patients with HIV demonstrate elevated levels of hyperphosphorylated tau, particularly in the entorhinal cortex and the hippocampus [205]. Collectively, these neuropathological data provide evidence that AD-related pathologies occur frequently in patients with HIV, although AD is typically accompanied by a significantly greater burden of amyloid and tau neuropathology than has been observed in samples from patients with HIV. Nevertheless, these studies suggest the possibility of an accelerated neurological aging process, which could potentiate the development of AD-related neuropathology in individuals with HIV.

The BBB has been implicated in the regulation of brain amyloid levels. Poorly functioning clearance of amyloid by the BBB has been suggested to lead to neuronal dysfunction in AD [206]. Notably, BBB dysfunction has also been observed in HIV [207,208], and may contribute to the parenchymal amyloid abnormalities observed in patients with the virus. Exposure to HIV was demonstrated to increase the intracellular accumulation of amyloid in an in vitro model using human BMVECs [209]. Interestingly, the effects of HIV-induced accumulations in amyloid were reversed when cells were pretreated with simvastatin [209], suggesting that this agent may stimulate brain amyloid clearance across the BBB. These findings complement those reported in another study indicating that simvastatin and lovastatin can reduce intracellular and extracellular amyloid levels in hippocampal cell cultures [210]. Taken together, these results highlight the possibility that statins may possess clinical utility in protecting the brain against increased amyloid deposition in patients with HIV. Additional research into the use of statins to treat AD-related neuropathology in patients with HIV is needed, although the early results appear to be promising. In addition, the findings available to date are consistent with data suggesting that statin therapy may protect against the development of cognitive decline and dementia in the general population [211,212], effects that may be mediated through several known pleiotropic actions of statins [213].

Cognitive enhancement and adjunctive therapy approaches

Studies examining the use of cognitive-enhancing agents (eg, selegiline or memantine) and adjunctive therapies (eg, lithium) to treat HAND have reported mixed results [214,215]. Several of the challenges inherent in interpreting this literature relate to the wide variations in the dose and duration of the regimens tested across different clinical trials. In addition, the duration of the trials was sometimes shorter than the time that may be required to observe significant improvements in cognitive performance. This variability in methodology makes comparisons across studies challenging, and likely accounts for some of the inconsistencies in trial outcomes reported.

Memantine is an NMDA antagonist that is typically prescribed to treat cognitive impairments associated with moderate-to-severe AD. Results from a MRS study indicated that a 12-week regimen of memantine may help to prevent neuronal injury in patients with HIV who also have dementia [216]. These data are consistent with findings reported by Nath and colleagues, who demonstrated that memantine prevented gp120- and Tat-induced neuronal death in vitro [217]. A recent 60-week clinical trial in patients with HIV who exhibited mild-to-moderate cognitive impairment reported a possible positive effect on cognitive functions following 20 weeks of treatment with memantine. However, this positive effect was not apparent following 48 weeks of treatment; this lack of observed effect may have been related to reductions in sample size caused by attrition [218]. Although the potential for the widespread use of memantine in patients with HIV may be limited by questions related to the tolerability of this drug, these data suggest that memantine may be beneficial to patients with cognitive impairments. Moreover, the data reveal a critical need for further investigation into the efficacy of memantine in treating HAND.

Lithium appears to have emerged within the adjunctive therapy literature as an agent with potential benefit for patients with HAND. Lithium, a neuroprotective agent with known anti-apoptotic activity [219], has been hypothesized to prevent neuronal injury through several mechanisms, including stabilization of neuronal sodium and potassium ion channels, reduction of proinflammatory processes, and modulation of anti- and pro-apoptotic protein expression [220]. Lithium-induced inhibition of the pro-apoptotic molecule glycogen synthase kinase-3β (GSK-3β) led to a downstream blockade of β-catenin and reduced tau phosphorylation in a murine model of HIV-1 encephalitis [221]. Lithium has also been demonstrated to protect against gp120-mediated neurotoxicity in vitro [222]. Improvements in neuropsychological functions were also reported after a 12-week clinical trial of lithium in patients with HIV receiving cART [220]. Furthermore, a recent multimodal neuroimaging study in cART-treated patients with HIV demonstrated improved glutamate/glutamine metabolism and white matter integrity following 10 weeks of treatment with lithium [223]. Altogether, the data suggest that lithium may help to improve HIV-associated CNS injury and neurocognitive impairment.

The anticonvulsant valproic acid (VPA) is another GSK-3β inhibitor that has demonstrated some efficacy in reducing cognitive impairment in patients with HIV. Similar to lithium, VPA protected against apoptosis via the inhibition of GSK-3β in vitro [224]. VPA can inhibit in vitro gp120-induced neurotoxicity, resulting from the downregulation of GSK-3β and reductions in tau phosphorylation [225]. In a 10-week clinical trial, adjunctive therapy with VPA resulted in a trend toward improvement in neuropsychological functions in patients with HIV, as well as a significant increase in frontal lobe metabolism, as measured by MRS [226]. These findings suggest that additional studies of VPA are warranted; however, caution is needed, as VPA has the potential to increase HIV replication in monocytes [227,228]. The data available suggest that VPA-mediated changes in HIV replication could lead to increases in active viremia in the CNS despite control of peripheral viral loads by cART, thus increasing the potential for neurodegeneration. Further research into the clinical implications of these data are required.

Research in the area of adjunctive therapies for HAND represents a rapidly expanding and important field. In addition to the agents described in this section, several other therapies have demonstrated some benefit in early studies, including serotonin reuptake inhibitors [229], antioxidants (eg, REN-1189) [230], and other anticonvulsant drugs (eg, lorazepam) [231]. However, no drug that is an effective adjunctive treatment for HAND has yet been approved for this indication. There is an urgent need to identify novel target-specific agents that have fewer side effects and provide improved efficacy in treating HAND than existing therapies.

Biomarker development

The discovery and development of novel drugs for the treatment and prevention of HAND is strongly dependent on the development of new biomarkers that are sensitive to the specific neuropathological processes that underlie neurodegeneration [232]. The primary biomarkers routinely obtained from patients with HIV include measures of viral load and CD4 levels, both of which are typically obtained from plasma, although the measurement of viral activity in CSF has been determined to be more sensitive to the level of HIV activity in the brain [233-236]. Other types of measures that are not currently obtained as part of standard clinical care may also serve as useful biomarkers of emerging brain dysfunction. In particular, the quantification of plasma and CSF chemokines and cytokines is relatively easy with current multiplex processing systems, and can provide useful information regarding immunological states that may affect the brain [191,200,237-239]. In addition, a significant area of biomarker development during the past decade has involved the use of neuroimaging methods to study early changes in brain structure and function in patients infected with HIV [240-244]. MRS offers a particularly powerful tool for imaging cerebral metabolic abnormalities, and has the potential to detect disturbances that may result in subsequent neurodegeneration even in neuro-asymptomatic patients [182,216,243-247]. Clinical and metabolic biomarkers (eg, CD4 nadir and cerebral metabolites as measured by MRS) also appear to relate to structural and functional brain abnormalities among HIV-infected individuals [41,183,188].

Conclusion

Although significant advances have been made in the treatment of HIV infection, HAND remains a pervasive symptom. Concerns regarding neurocognitive dysfunction have become more relevant given that patients with HIV are living longer with the disease. Substantial evidence indicates that neural damage continues even in patients with well-controlled HIV, suggesting that the etiology of HAND is not only virus-driven in the era of cART; however, improvements in the control of viral loads and CD4 counts through cART may also have an effect on HAND. The relationship between historical markers of disease severity (eg, CD4 nadir) and the presence of brain abnormalities clearly indicates that early and continued treatment with cART following diagnosis with HIV is critical in helping to attenuate the progression of HAND through decreasing viral loads and sustaining immunological functions. Furthermore, there is some indication that improving the CNS penetration of cART in order to control levels of HIV in the brain more successfully may result in even greater reductions in the severity of HAND.

Nevertheless, the fact that neural injury continues to occur in patients whose infection is otherwise well controlled with cART indicates that additional etiologies may play a role in the development of HAND. The available data suggest that, in addition to controlling HIV itself, several other factors may be important for reducing neurocognitive dysfunction in patients. Improvements in the treatment of comorbid medical conditions will likely provide a greater level of protection against the development of risk factors (eg, vascular changes) that may exacerbate HIV-related neuropathological processes. In contrast, the effectiveness of available cognitive-enhancing drugs in preventing HAND is limited. Consequently, the development of novel agents that may be employed to halt the progression of HAND is a critical area of research. In particular, the new drugs that control free radicals, neurotoxicity, proinflammatory processes and apoptosis may hold substantial potential. For example, research is ongoing to investigate the potential utility of drugs with anti-inflammatory properties [248], as well as of agents that act on chemokine receptors, neurotrophic factors, and mitochondrial cells [214]. Such studies provide hope that several therapies to treat HAND may soon be available.

Despite the potential optimism generated by the numerous areas of novel drug development related to HAND, reducing the incidence and severity of such disorders currently depends primarily on several factors: (i) adequate cART treatment and adherence, including early intervention to prevent the brain from experiencing injury secondary to significant immune system compromise (ie, CD4 < 200 cells/μl); (ii) controlling major comorbid infections, most notably HCV; (iii) reducing behavioral risk factors that are associated with poor neurocognitive outcomes (eg, substance abuse) [177,249]; and (iv) reducing other risk factors that are associated with increased free radical levels and proinflammatory responses. Particular emphasis on treating HCV and managing comorbid liver disease is a priority, given the impact of these conditions on neurocognitive functions, mortality and morbidity.