Nucleoside reverse transcriptase inhibitor-induced mitochondrial toxicity as an etiology for lipodystrophy

Mitochondrial expression of the human equilibrative nucleoside transporter 1 (hENT1) results in enhanced mitochondrial toxicity of antiviral drugs

Many antiviral drugs (e.g. fialuridine; FIAU) produce clinically significant mitochondrial toxicity that limits their dose or prevents their use in the clinic. Because the majority of nucleoside drugs is too hydrophilic to cross the highly impermeable mitochondrial membrane, we have hypothesized that they must be transported into the mitochondria to produce their toxicity. To test this hypothesis, we have sought to determine whether the nucleoside transporters, human equilibrative nucleoside transporter 1 (hENT1) or human concentrative nucleoside transporter 1 (hCNT1), when stably expressed in Madin-Darby canine kidney cells as yellow fluorescent fusion protein (YFP), are localized to the mitochondria. By using organelle-selective dyes and confocal microscopy, we have found that hENT1-YFP is localized to the mitochondria as well as the plasma membrane, whereas hCNT1-YFP was found predominantly on the plasma membrane. hENT1-YFP was not localized to the nuclear envelope, endosomes, lysosomes, or Golgi complex. Western blotting confirmed the presence of hENT1-YFP or endogenous hENT1 in mitochondria isolated from hENT1-YFP-expressing cells and human livers, respectively. In agreement with these localization data, [14C]FIAU was efficiently transported into the mitochondria of cells expressing hENT1-YFP but not of cells expressing hCNT1-YFP. The mitochondrial toxicity of FIAU to Madin-Darby canine kidney cells was enhanced by hENT1-YFP, even when hENT1 activity on the plasma membrane was selectively blocked by 10 nm nitrobenzylthioinosine. Moreover, FIAU (50 microm) produced significant mitochondrial toxicity ( approximately 70% decrease in mitochondrial DNA synthesis) when it was directly incubated with mitochondria isolated from hENT1-expressing cells. In conclusion, we have identified for the first time that hENT1 is expressed on the mitochondrial membrane and that this expression enhances the mitochondrial toxicity of nucleoside drugs such as FIAU. Mitochondrial expression of hENTs may explain the clinically significant mitochondrial toxicity caused by the anti-HIV nucleoside drugs such as zidovudine, stavudine, and didanosine.

Mitochondrial dysfunction and nucleoside reverse transcriptase inhibitor therapy: experimental clarifications and persistent clinical questions

Nucleoside reverse transcriptase inhibitors (NRTIs) in combination with other antiretrovirals (HAART) are critical in current AIDS therapy, but mitochondrial side effects have come to light with the increased use of these compounds. Clinical experience, pharmacological, cell and molecular biological evidence links altered mitochondrial (mt-) DNA replication to the toxicity of NRTIs in many tissues, and conversely, mtDNA replication defects and mtDNA depletion in specific target tissues are observed. The shared features of mtDNA depletion and energy depletion became key observations and related the clinical and in vivo experimental findings to inhibition of mtDNA replication by NRTI triphosphates in vitro. Subsequent to those findings, other observations suggested that mitochondrial energy deprivation is concomitant with or the result of mitochondrial oxidative stress in AIDS (from HIV, for example) or from NRTI therapy itself. With increased use of NRTIs, mtDNA mutations may become increasingly important pathophysiologically. One important future goal is to prevent or attenuate the side effects so that improved efficacy is achieved.

Site-specific differences in the action of NRTI drugs on adipose tissue incubated in vitro with lymphoid cells, and their interaction with dietary lipids

Existing theories of the origin of HIV-related adipose tissue redistribution syndrome cannot adequately explain simultaneous hypertrophy of certain depots and atrophy of others, or its occasional occurrence in untreated HIV infection. These experiments explore the hypothesis that hypertrophy of lymphoid tissue-containing adipose depots arises from drug-induced disruption to local interactions between perinodal adipocytes and activated lymphoid cells. Guinea pigs were fed on plain or lipid-supplemented (10% suet, sunflower or fish oil) chow ad libitum or restricted, and the popliteal lymph nodes were activated by repeated injection of lipopolysaccharide. Explants of perinodal and other samples from popliteal, mesentery, omentum and nodeless perirenal and epididymal depots were incubated with lymphoid cells and zidovudine, didanosine, lamivudine or stavudine at physiological concentrations (0.1-1 microg/ml) or interleukin-10 and interleukin-6, and basal and maximum lipolysis was measured. All drugs increased lipolysis from perinodal adipocytes, especially mesenteric, though less than exogenous cytokines. Effects on adipocytes from non-perinodal sites and nodeless depots were minimal. The sunflower-oil diet enhanced, and the fish-oil and restricted diets reduced, these effects. We conclude that these NRTI antiretroviral drugs modulate the local interactions between perinodal adipocytes and activated lymphoid cells. Local interactions, and hence the selective hypertrophy of node-containing adipose depots, may be curtailed by dietary manipulation.

Drug-induced steatohepatitis

Drugs rarely cause steatohepatitis, but amiodarone, perhexiline, and DH, have unequivocally been found to independently induce the histologic picture of alcoholic liver disease or NASH. All three agents have similar pathogenetic mechanisms of hepatotoxicity, targeting mitochondrial ATP production and fatty acid catabolism. Other drugs that occasionally cause steatohepatitis, most importantly steroid hormones, likely exacerbate the pathogenetic mechanisms leading to NASH. Similar to NASH, lipid peroxidation resulting from mitochondrial injury may account for all of the histologic findings in drug-induced steatohepatitis. Further research should determine the mechanisms by which drug-induced steatosis, a benign lesion, evolves to steatohepatitis and progressive fibrosis.

Nutritional aspects of HIV-infected children receiving highly active antiretroviral therapy

The presentation of the nutritional problems of HIV-infected children is changing over time with improved antiretroviral regimens. Early reports of HIV infection in the 1980s, included such problems as malnutrition and wasting. However, as treatment and prophylactic regimens improve, the current nutritional problems of HIV-infected children in developed countries include truncal obesity and insulin resistance in addition to malnutrition. Background data on the wasting syndrome, etiology of malnutrition, nutritional effects of highly active antiretroviral therapies, and nutritional intervention strategies for HIV-infected children will be presented.

Use of the transgenic mouse in models of AIDS cardiomyopathy

Heart disease in AIDS, particularly cardiomyopathy (CM), is an increasingly recognized clinical problem with as yet undefined pathogenetic mechanisms. Among the potential etiologies of AIDS CM are HIV-1 infection of cardiac myocytes and subsequent cardiac dysfunction, opportunistic infection, inflammatory reactions, cytokine effects, and cardiotoxicity of prescribed or illicit drugs. It seems probable that multiple factors may impact on the development of CM in AIDS. Transgenic mice (TG) are useful biological tools to explore mechanisms of cardiac function and disease. In AIDS models, TG offer novel ways to elucidate mechanisms of AIDS CM through combined in vivo and in vitro studies. With targeted and non-targeted TG, structural and functional effects of specific HIV-1 gene products on heart tissue may be addressed. The impact of environmental agents including therapeutics or cardiotoxins may also be defined. To address the complexity of AIDS CM using TG, an experimental approach has been employed in our laboratories to model the clinical condition. We utilize AIDS TG with generalized expression of HIV-1 gene products in CM models with combined antiretroviral regimens to define the cardiovascular effects of AIDS and its therapy on the structure and function of the murine heart. We are developing a series of cardiac specific TG bearing selected HIV-1 genes. These TG target the selected HIV-1 genes expressed in cardiac ventricular myocytes. Tissue-specific targeting of this type enables us to define structural and functional effects of specific HIV-1 gene products on the cardiac myocyte.

Mitochondrial DNA replication, nucleoside reverse-transcriptase inhibitors, and AIDS cardiomyopathy

Nucleoside reverse-transcriptase inhibitors (NRTIs) in combination with other antiretrovirals (HAART) are the cornerstones of current AIDS therapy, but extensive use brought mitochondrial side effects to light. Clinical experience, pharmacological, cell, and molecular biological evidence links altered mitochondrial (mt-) DNA replication to the toxicity of NRTIs in many tissues, and conversely, mtDNA replication defects and mtDNA depletion in target tissues are observed. Organ-specific pathological changes or diverse systemic effects result from and are frequently attributed to HAART in which NRTIs are included. The shared features of mtDNA depletion and energy depletion became key observations and related the clinical and in vivo experimental findings to inhibition of mtDNA replication by NRTI triphosphates in vitro. Subsequent to those findings, other observations suggested that mitochondrial energy deprivation is concomitant with or the result of mitochondrial oxidative stress in AIDS (from HIV, for example) or from NRTI therapy itself.

Incidence of and risk factors for lipoatrophy (abnormal fat loss) in ambulatory HIV-1-infected patients

To identify clinical factors associated with the incidence of HIV-1-associated lipoatrophy, HIV-1-infected patients in the HIV Outpatient Study (HOPS) were prospectively evaluated for clinical signs of lipoatrophy at two visits about 21 months apart. Development of lipoatrophy was analyzed in stratified and multivariate analyses for its relationship to immunologic, virologic, clinical, and drug treatment information for each patient. Of 337 patients with no lipoatrophy at Survey 1, 44 (13.1%) developed moderate or severe lipoatrophy between the two surveys. In multivariate analyses, significant risk factors for incident lipoatrophy were white race (OR = 5.2; 95% CI: 1.9-17.1; =.003), CD4 T-lymphocyte count at Survey 2 less than 100 cells/mm3 (OR = 4.2; 95% CI: 1.3-13.1; =.013), and body mass index (BMI) less than 24 kg/m2 (OR = 2.4; 95% CI: 1.1-5.4; =.024). Analyses that controlled for the severity of HIV illness demonstrated no significant association with use of or time on any antiretroviral agent or class of agents and the development of lipoatrophy. Some host factors and factors associated with previous or current severity of HIV infection, especially CD4 T-lymphocyte cell count, appeared to have the strongest association with incidence of lipoatrophy.

Increased risk of lipoatrophy under stavudine in HIV-1-infected patients: results of a substudy from a comparative trial

OBJECTIVES

To compare the incidence of clinical lipodystrophy in HIV-1-infected patients receiving zidovudine or stavudine, in combination with indinavir and lamivudine, in a randomized trial.

METHODS

NOVAVIR was a randomized multicentre trial comparing stavudine/lamivudine/indinavir and zidovudine/lamivudine/indinavir in 170 patients pretreated with zidovudine, didanosine or zalcitabine (> 6 months), but naive for lamivudine, stavudine and protease inhibitors. The incidence of clinical lipodystrophy and metabolic abnormalities was assessed in a subgroup of 101 patients after 30 months of follow-up.

RESULTS

The incidence of lipoatrophy was increased in the stavudine arm versus the zidovudine arm, as followed: facial atrophy: 48 versus 22% of patients, P = 0.011, lower limb atrophy: 49 versus 22% of patients, P = 0.006, buttock atrophy: 47 versus 20% of patients, P = 0.009, venomegaly: 57 versus 24% of patients, P = 0.001. There was no significant difference in the incidence of clinical signs of central fat accumulation nor in fasting metabolic parameters at month 30 between the two arms. In multivariate analyses, the stavudine arm, previous therapy with didanosine, and a lower CD4 cell count at study entry were associated with an increased risk of lipoatrophy, whereas older patients and women had an increased risk of lipohypertrophy.

CONCLUSION

Patients receiving stavudine/lamivudine/indinavir had a greater rate of clinical lipodystrophy, mainly lipoatrophy, than those treated with zidovudine/lamivudine/indinavir.

HIV disease and advanced age: an increasing therapeutic challenge

The mean age of patients at both first HIV detection and AIDS diagnosis is progressively rising over time. However, reliable epidemiological estimates, clinical data or controlled therapeutic and outcome figures are lacking for elderly patients, especially with regard to laboratory and clinical response to antiretroviral therapy, treatment tolerability, drug-drug interactions, short- and long-term toxicity, and interactions with underlying illnesses and concurrent pharmacological treatment. In fact, the large majority of randomised, controlled trials evaluating and comparing new antiretroviral drugs or anti-HIV therapeutic strategies, as well as antimicrobial treatment or chemoprophylaxis of HIV-related complications, either excluded patients with advanced age and/or concurrent disorders or did not offer substudies or detailed data analysis focusing on older patients compared with younger ones. The life expectancy of HIV-infected persons receiving highly active antiretroviral therapy (HAART) is now extended (approaching that of the general population), so that the definition of AIDS has lost its epidemiological and clinical significance thanks to the immune reconstitution resulting from potent antiretroviral therapy. However, an ever-increasing number of individuals aged > or =50 years with HIV infection is expected in the coming years, as a result of both increased survival of patients with treated disease and delayed recognition of individuals with occult HIV disease. The limited data available about combined antiretroviral therapy in the elderly seem to show an overlapping virological success rate but a slower and blunted immune recovery compared with younger patients. Thymic output, however, seems somewhat preserved even in adulthood and may contribute to the reconstitution of most of the quantitative and functional T cell abnormalities caused by HIV disease. More attention must be paid to underlying end-organ disorders, as well as expected pharmacological interactions and combined drug toxicity that may interfere with HAART efficacy and patients' compliance with recommended regimens and could lead to increased adverse effects. The available guidelines for antiretroviral treatment and therapy and prophylaxis of AIDS-related illnesses should be regularly updated and should include problems related to HIV disease in an aging population. Specific trials or substudies focusing on older people are warranted to obtain controlled data on all issues of antiretroviral therapy in the elderly, including time and mode of initiation, and modification and salvage HAART regimens. Antiretroviral drug dosage adjustment to take into account underlying pathological conditions or other pharmacological treatments is another emerging issue.

Effect of highly active antiretroviral therapy (HAART) and hepatitis C Co-infection on hyperlipidemia in HIV-infected patients: a retrospective longitudinal study

BACKGROUND

Lipid abnormalities are among the most frequent treatment-limiting adverse events during HAART in HIV-infected individuals. Lipid disturbances have also been associated with hepatitis C virus (HCV) chronic infection in HIV-uninfected participants. HAART-induced lipid abnormalities may then have peculiar features in HIV-HCV co-infected individuals.

PURPOSE

To estimate the prevalence and incidence rates of hypertriglyceridemia and hypercholesterolemia and to identify associated factors in a large clinic population of HIV patients after HAART has been initiated.

DESIGN

We performed a retrospective longitudinal follow-up study in a large cohort of HIV patients on their first HAART.

PATIENTS AND METHOD

The clinical databases of two major clinical centers in Italy participating in the MASTER study were merged. Treatment-emerging metabolic disorders in patients on their first HAART regimen (PI-based or NNRTI-based) who were stable for at least 4 months were prospectively analyzed by baseline parameters, drug regimens, and viroimmunological outcome of therapy. Follow-up was continued for 24 months or until drug discontinuation, whichever came first.

RESULTS

Two hundred and eighty two (282) HIV-infected patients undergoing HAART (203 PI + 79 NNRTI; 65 including stavudine [d4T]) met inclusion criteria and were enrolled in the study from 1997 to 2001. Mean follow-up was 18.5 +/- 6.7 months. After HAART had been initiated, a statistically significant mean increase in total cholesterol over time was observed in comparison to baseline (p <.0001), without difference between treatment groups (PI vs. NNRTI, with or without d4T). In the univariate analysis, predictive factors for HAART-induced hypercholesterolemia were baseline total plasma cholesterol and triglycerides values and CD4+ cell count differential increase over time, while a negative correlation was found with zalcitabine-including regimens and baseline HCV seropositivity. At multivariate analysis, only high baseline total plasma cholesterol and triglycerides values retained their predictive value and baseline HCV seropositivity was significantly associated with lower increase in total cholesterol values under HAART, regardless of treatment groups (p <.001).

CONCLUSION

HCV co-infection is an independent factor preventing the emergence of treatment-limiting total cholesterol increase under any HAART regimen, possibly reflecting impaired total cholesterol synthesis in the liver or total cholesterol hypercatabolism. On the contrary, no HCV influence on triglycerides plasma levels was noted. Our data do not suggest any favoring role of specific treatment or drugs (PI and/or d4T) on total cholesterol and triglycerides increase under HAART.

Management of metabolic complications associated with antiretroviral therapy for HIV-1 infection: recommendations of an International AIDS Society-USA panel

OBJECTIVE

Alterations in glucose and lipid metabolism, lactic acidemia, bone disorders, and abnormal body fat distribution have been recognized recently as frequent complications associated with HIV-1 infection and potent antiretroviral therapy, but limited data are available regarding the appropriate management of these disorders. These recommendations were developed to guide physicians actively involved in HIV care in the management of metabolic complications that occur primarily within the context of potent antiretroviral therapy.

PARTICIPANTS

A 12-member panel representing international expertise in HIV-1 patient care, antiretroviral therapy, and endocrine and metabolic disorders was selected in the spring of 2000 by the International AIDS Society-USA, a not-for-profit physician education organization. Panel members met in closed meetings beginning in May 2000. All work was funded by the International AIDS Society-USA; the panel members are not compensated for their participation.

EVIDENCE

The panel reviewed published results of clinical, epidemiologic, and basic science studies and data and abstracts presented at research conferences, primarily from 1997 to 2002. The panel also considered studies of the pathophysiology and treatment of similar metabolic abnormalities in noninfected persons. Emphasis was placed on results from prospective, randomized, controlled clinical trials when available.

PROCESS

For each metabolic complication, 1 or more member(s) reviewed and presented all available evidence to the panel, and then wrote a summary of the evidence and preliminary recommendations. Final recommendations were determined by full group consensus. The summaries were combined into a single working document and all panel members edited and approved all subsequent drafts.

CONCLUSIONS

Carefully controlled studies to determine the incidence, etiology, risk factors, and most appropriate treatments for metabolic complications in HIV-1 infection are urgently needed. In the absence of these data, and to prevent acute illness and mitigate long-term risks, the panel recommends routine assessment and monitoring of glucose and lipid levels and assessment and monitoring of lactic acidemia and bone abnormalities if clinical signs or symptoms are detected. With the exception of body fat distribution abnormalities, specific treatments for these complications are also recommended. Successful long-term antiretroviral therapy will require diligent monitoring and preemptive treatment of metabolic complications to optimize the risk-benefit ratio of antiretroviral therapies.

High-density lipoprotein cholesterol is low in HIV-infected patients with lipodystrophic fat expansions: implications for pathogenesis of fat redistribution

OBJECTIVE

To demonstrate relationships between plasma high-density lipoprotein (HDL) cholesterol and fat expansions in the fat redistribution syndrome.

DESIGN

Patients who had significant buffalo humps or intra-abdominal fat (IAF) expansions were identified and their HDL levels were measured. Control patients were all those undergoing a single trial of antiretroviral treatment. Some patients answered a self-administered questionnaire concerning self-perceived fat expansions, and their responses were related to their HDL levels. In other patients, relationships were studied between IAF measured by cross-section computerized tomography scans and HDL levels. Finally, patients who had IAF > 70 cm(2), were administered niacin, 3000 mg/day for > or = 6 months, in order to test whether raising HDL induced a decrease in IAF.

RESULTS

Twenty-three patients with buffalo humps had mean HDL of 30.4 mg/dl; 47 HIV-positive controls had mean HDL of 41.9 mg/dl (P = 0.001). In 27 patients, IAF area and HDL were negatively correlated (r, 0.40; P = 0.04). Among these 27, the 17 patients with IAF area > 100 cm(2) had mean HDL of 35.3 mg/dl; the 10 patients with IAF area < 100 cm(2) had mean HDL of 51 mg/dl (P < 0.05). The 24 patients who indicated in a questionnaire that they had self-perceived IAF expansion had a median HDL of 36.0 mg/dl; the 20 who indicated that they did not have IAF expansion had a median HDL of 44.5mg/dl (P = 0.06). IAF decreased by 26.9% in 13 (81%) of the 16 patients who took niacin for 1 year; and the decrease in IAF was associated with a significant (P = 0.002) increase in HDL.

Exposure to dideoxynucleosides is reflected in lowered mitochondrial DNA in subcutaneous fat

OBJECTIVES

Nucleoside reverse transcriptase inhibitors (NRTIs), particularly dideoxynucleoside analogs (ddNs), used in the treatment of HIV, inhibit mitochondrial DNA polymerase gamma in vitro. Mitochondrial DNA (mtDNA) depletion is proposed as the underlying mechanism of many of the in vivo side effects of these agents. A reliable and valid laboratory test to detect this is not yet available. The objective of this study was to correlate tissue mtDNA quantification in HIV-infected patients with exposure to nucleoside analogs.

METHODS

60 HIV-infected adults underwent detailed clinical assessment and blood and tissue sampling. Clinical and antiretroviral treatment details were correlated with results of plasma lactate assays, and real-time polymerase chain reaction quantification of mtDNA in peripheral blood mononuclear cells (PBMCs) and subcutaneous fat from the lower limb.

RESULTS

Forty-nine (82%) subjects were on combination antiretroviral therapy. Of these, 33 (55%) were currently receiving one or more ddNs (stavudine, didanosine, or zalcitabine). mtDNA in subcutaneous fat was lower in subjects currently on ddNs than in those not taking ddNs (mean [log10] 2.47 vs. 2.74, p =.002). Plasma lactate was somewhat higher in subjects currently taking ddNs than those on no antiretroviral treatment (median 1.5 vs. 1.0, p =.03), but was not significantly different in either of these groups compared with subjects on other NRTIs. mtDNA in PBMCs did not vary with treatment status.

CONCLUSIONS

mtDNA in subcutaneous fat was significantly reduced in patients currently taking ddNs. mtDNA in PBMCs was independent of patient exposure to NRTIs.

L-carnitine reduces lymphocyte apoptosis and oxidant stress in HIV-1-infected subjects treated with zidovudine and didanosine

Apoptosis is critical to the progression of human immunodeficiency virus-1 (HIV-1) infection. It appears reasonable that antiretroviral therapies may not achieve a full control of the infection in the absence of an impact on apoptosis. We assigned 20 asymptomatic HIV-infected subjects with advanced immunodeficiency to receive either zidovudine (AZT), and didanosine (DDI) or the same regimen plus L-carnitine, a known antiapoptotic drug, for 7 months. Immunologic and virologic parameters were measured at baseline and after 15, 60, 120, and 210 days of treatment. We assessed on each time point the following: (a) the frequency of peripheral blood apoptotic CD4 and CD8 lymphocytes, CD4 and CD8 cells with disrupted mitochondrial membrane potential, and CD4 and CD8 cells undergoing oxidant stress; (b) the expression of the molecular markers of apoptosis Fas and caspase-1; and (c) the expression of p35/cdk-5 regulatory subunit that is involved in regulating cell survival and apoptosis. Absolute CD4 and CD8 counts and plasma viremia were also measured. Apoptotic CD4 and CD8 cells, lymphocytes with disrupted mitochondrial membrane potential, and lymphocytes undergoing oxidant stress were greatly reduced in subjects treated with AZT and DDI plus L-carnitine compared with those who did not receive L-carnitine. Fas and caspase-1 were down-expressed and p35 over-expressed in lymphocytes from patients of the L-carnitine group. No difference was found in CD4 and CD8 counts and viremia between the groups. No toxicity of L-carnitine was recognized. The addition of L-carnitine is safe and allows apoptosis and oxidant stress to be greatly reduced in lymphocytes from subjects treated with AZT and DDI.

Lipotoxic diseases

I review evidence that leptin is a liporegulatory hormone that controls lipid homeostasis in nonadipose tissues during periods of overnutrition. When adipocytes store excess calories as triacylglycerol (TG), leptin secretion rises so as to prevent accumulation of lipids in nonadipose tissues, which are not adapted for TG storage. Whenever leptin action is lacking, whether through leptin deficiency or leptin resistance, overnutrition causes disease of nonadipose tissues with generalized steatosis, lipotoxicity, and lipoapoptosis. Examples of such disorders of liporegulation include generalized lipodystrophies, mutations of leptin and leptin receptor genes, and diet-induced obesity. Lipotoxicity of pancreatic beta-cells, myocardium, and skeletal muscle leads, respectively, to type 2 diabetes, cardiomyopathy, and insulin resistance. In humans this constellation of abnormalities is referred to as the metabolic syndrome, a major health problem in the United States. When lipids overaccumulate in nonadipose tissues during overnutrition, fatty acids enter deleterious pathways such as ceramide production, which, through increased nitric oxide formation, causes apoptosis of lipid-laden cells, such as beta-cells and cardiomyocytes. Lipoapoptosis can be prevented by caloric restriction, by thiazolidinedione treatment, and by administration of nitric oxide blockers. There is now substantial evidence that complications of human obesity may reflect lipotoxicity similar to that described in rodents.

Changes in mitochondrial DNA as a marker of nucleoside toxicity in HIV-infected patients

BACKGROUND

Nucleoside analogues can induce toxic effects on mitochondria by inhibiting the human DNA polymerase gamma. The toxic effects can range from increased serum lactate levels to potentially fatal lactic acidosis. We studied changes in mitochondrial DNA relative to nuclear DNA in the peripheral-blood cells of patients with symptomatic, nucleoside-induced hyperlactatemia.

METHODS

Total DNA was extracted from blood cells. A nuclear gene and a mitochondrial gene were quantified by real-time polymerase chain reaction. Three groups were studied: 24 controls not infected with the human immunodeficiency virus (HIV), 47 HIV-infected asymptomatic patients who had never been treated with antiretroviral drugs, and 8 HIV-infected patients who were receiving antiretroviral drugs and had symptomatic hyperlactatemia. The patients in the last group were studied longitudinally before, during, and after antiretroviral therapy.

RESULTS

Symptomatic hyperlactatemia was associated with marked reductions in the ratios of mitochondrial to nuclear DNA, which, during therapy, averaged 68 percent lower than those of non-HIV-infected controls and 43 percent lower than those of HIV-infected asymptomatic patients never treated with antiretroviral drugs. After the discontinuation of antiretroviral therapy, there was a statistically significant increase in the ratio of mitochondrial to nuclear DNA (P=0.02). In the patients followed longitudinally, the decline in mitochondrial DNA preceded the increase in venous lactate levels.

CONCLUSIONS

Mitochondrial DNA levels are significantly decreased in patients with symptomatic, nucleoside-related hyperlactatemia, an effect that resolves on the discontinuation of therapy.

Role of long-term nucleoside-analogue therapy in lipodystrophy and metabolic disorders in human immunodeficiency virus-infected patients

The role of nucleoside analogues (NAs) in lipodystrophy (LD) syndrome in human immunodeficiency virus (HIV)-infected patients remains controversial. We studied the prevalence of LD in previously untreated patients randomized to receive different NA combinations (in the ALBI-ANRS 070 trial) for 6 months. At month 30 of follow-up, 37 (31%) of 120 patients had >/=1 morphologic change, and 21 (57%) of 37 had isolated peripheral lipoatrophy; corresponding values for the patients who received only NAs throughout follow-up were 20 (30%) of 66 and 14 (67%) of 21, respectively. In multivariate analysis, factors associated with presence of LD at month 30 were initial assignment to the group receiving stavudine and didanosine (odds ratio [OR], 6.7; P=.02), age (OR for being 10 years older, 3.6; P=.002), and HIV RNA level at month 30 (OR, 0.4; P=.007). No difference was observed in cholesterol and glucose levels as a function of any pattern of antiretroviral exposure. Exposure to stavudine and didanosine was associated with LD syndrome (predominantly lipoatrophy).

Hyperlactatemia syndromes in people with HIV infection

Hyperlactatemia associated with use of nucleoside analogue reverse transcriptase inhibitors (NRTIs) is not a single entity but a spectrum of abnormalities. The spectrum reflects varying degrees of derangement in systemic homeostasis in the face of primary drug effects on lactate load. Lactic acidosis, characterized by metabolic acidosis, blood lactate above 5 mmol/l, hepatic steatosis and high mortality, represents the extreme end of this spectrum where there is complete decompensation. Partially compensated states of lactate excess have now been described, ranging from less fulminant symptomatic hyperlactatemia with hepatic steatosis to chronic or intermittent low-grade hyperlactatemia without acidosis, steatosis or any symptoms. At a population level, average venous lactate concentrations do rise following treatment with NRTIs but stabilize long term in the majority of cases. The average increase in systemic lactate turnover that is required to maintain such compensated blood levels is not known and research into this may provide insights into the extent of incipient mitochondrial toxicity associated with chronic NRTI use. At a tissue-specific level, it is not known which tissues or organs (liver, fat, other) are the predominant contributors to an increase in systemic lactate load, nor whether the primary defect is one of increased production, decreased elimination or both.

The evaluation of metabolic function and fat redistribution in clinical trials

Changes in lipid and carbohydrate metabolism and in body composition associated with the most potent and effective therapies available are being reported with increasing frequency. These changes could potentially lead to increased risks of cardiovascular disease. Clinical trials set up to investigate new antiretroviral therapies need to explore the therapies' impact on these potentially serious adverse outcomes, at least in a subset of patients. The measurements that are feasible to include for all patients clearly differ from those required by a metabolic substudy. We propose the following: firstly, a minimal set of parameters that should be included in all trials; secondly, a desirable set of parameters that should be included whenever possible; and thirdly, a list of exploratory measures that should be considered. These exploratory measures are classified by the different mechanisms for changes in body composition or weight: endocrinal, cardiovascular, sterol and chemokine/cytokine pathways. Standardized instruments for evaluating patients' reports of body changes and potential methods for assessing the risk of cardiovascular disease are described. Minimum and desirable standards for methods of measurement are also proposed. The choice of parameters is based on expert clinical opinion. The experts consulted include investigators from four large ongoing clinical trials with substudies specifically designed to investigate lipid and carbohydrate metabolism and changes in body composition, together with standard parameters for measurement within individual mechanistic pathways. The parameters proposed should be kept under review as the body of knowledge about metabolic function and fat redistribution in HIV infection increases.

Mitochondrial Toxicity Associated with Nucleoside Reverse Transcriptase Inhibitor Therapy

Nucleoside reverse transcriptase inhibitors result in a wide range of toxic side effects. These include lactic acidosis syndrome, myopathy, cardiomyopathy, pancreatitis, peripheral neuropathy, and possibly lipodystrophy. Despite the seemingly diverse nature of these side effects, all of these toxicities may be mediated by a common pathophysiologic mechanism, namely, mitochondrial toxicity resulting from nucleoside reverse transcriptase inhibitor-induced inhibition of DNA polymerase g. This article reviews the relevant mitochondrial biology and mechanism underlying nucleoside reverse transcriptase inhibitor-induced mitochondrial toxicity. Clinical manifestations of this toxicity are reviewed followed by a discussion of clinical management.

Sterol regulatory element-binding proteins and reactive oxygen species: potential role in highly-active antiretroviral therapy (HAART)-associated lipodystrophy

OBJECTIVES

To summarize the existing pathophysiological concepts and to hypothesize new mechanisms involving sterol regulatory element-binding proteins (SREBP) and reactive oxygen species (ROS), in highly-active antiretroviral therapy (HAART)-associated lipodystrophy.

CONCLUSIONS

The widespread use of HAART has dramatically reduced AIDS-related deaths in the developed world. Unfortunately, long-term HAART has been associated with a unique and unexpected syndrome of lipodystrophy manifested by fat wasting in the subcutaneous adipose tissue of the face and extremities, and accumulation of fat in the viscera and neck, often accompanied by hyperlipidemia and insulin resistance. Despite intensive study of this syndrome over the past three years, the pathophysiologic mechanism(s) underlying HAART-associated lipodystrophy syndrome remains elusive. A continued attempt to elucidate pathophysiological mechanisms involved in HAART-associated lipodystrophy remains critically important to improving the treatment strategies for this epidemic condition. In this review, we suggest two new hypotheses that may explain the pathogenesis and pathophysiology of HAART-associated lipodystrophy that warrant further investigations. First, we hypothesize that upregulation and/or increase in the mature form of SREBP-1 caused by HAART may lead to perturbations in synergistic regulation of genes involved in maintenance of cholesterol homeostasis and synthesis of fatty acids, that may explain the accumulation of fat which is a hallmark of this syndrome. Second, we hypothesize that the generation of reactive oxygen species in adipocytes may be an early and critical event in HAART-associated toxicity leading to cell death, partially explaining the mechanism underlying lipoatrophy.

Mitochondrial DNA decrease in subcutaneous adipose tissue of HIV-infected individuals with peripheral lipoatrophy

OBJECTIVE

To determine whether the peripheral fat wasting (lipodystrophy), which is seen in association with highly active antiretroviral therapy (HAART) that includes a nucleoside reverse transcriptase inhibitor (NRTI), is associated with a decrease in subcutaneous adipose tissue mitochondrial DNA (mtDNA) content or with large mtDNA deletions or insertions.

DESIGN

A four cohort cross-sectional study.

METHODS

The mtDNA content of subcutaneous fat tissue from the neck, abdomen and thigh was determined by polymerase chain reaction utilizing the amplification of three different mtDNA fragments. The results from HIV-infected patients with peripheral fat wasting following more than 6 months of NRTI-containing HAART were compared with the results from three different control cohorts: HIV-infected patients with a similar treatment history without lipodystrophy; HIV-infected patients naive to antiretroviral therapy and HIV sero-negative participants.

RESULTS

A decrease in mtDNA content was found in HAART-treated HIV-infected patients with peripheral fat wasting in comparison with subjects in the control cohorts. No large mitochondrial deletions or insertions were found.

CONCLUSIONS

Lipodystrophy with peripheral fat wasting following treatment with NRTI-containing HAART is associated with a decrease in subcutaneous adipose tissue mtDNA content.

Mitochondrial involvement in antiretroviral therapy-related lipodystrophy

OBJECTIVES

The management of HIV infection has greatly improved during recent years essentially because of the appearance of new antiretroviral drugs. Highly active antiretroviral therapy (HAART) has achieved important reductions of viraemia and significant recoveries of CD4(+) cell counts in HIV-infected patients. Nonetheless, cases of HIV-infected individuals experiencing lipodystrophy (LD) are being increasingly reported. The purpose of this work was to analyse whether the presence of mitochondrial abnormalities is a frequent feature in LD, since we previously detected mitochondrial abnormalities in an HIV-patient. The second main objective was to study whether LD could be associated with a specific drug.

DESIGN

Seven HIV patients presenting LD and five HIV non-LD controls participated in the study. LD patients met the following criteria: (1) LD was their only clinical abnormality, (2) LD was clinically relevant, (3) compliance with antiretroviral treatment was higher than 90% and (4) patients did not have personal or familial history suggestive of mitochondrial disease or neuromuscular disorder.

METHODS

Histological stainings, histo-enzymatic reactions, enzymatic and respiratory activities of mitochondrial respiratory chain complexes, and mitochondrial DNA (mtDNA) depletion and rearrangements were examined on muscle mitochondria.

RESULTS

Structural muscle abnormalities, mitochondrial respiratory chain dysfunction or mtDNA deletions were detected in all HIV lipodystrophic patients.

CONCLUSIONS

The mitochondrial abnormalities found suggest that mitochondrial dysfunction could play a role in the development of antiretroviral therapy-related lipodystrophy.

Metabolic complications associated with antiretroviral therapy

Mortality rates in the HIV-infected patient population have decreased with the advent of highly active antiretroviral therapy (HAART) for the treatment of AIDS. Due to the chronic nature of HAART, long-term metabolic complications are associated with therapy, such as hyperlipidemia, fat redistribution and diabetes mellitus. Currently, all of these symptoms are classified as the lipodystrophy (LD) syndrome(s). However, hyperlipidemia and fat redistribution occur independently, indicating there may be multiple syndromes associated with HAART. Although fat gain/loss and dyslipidemia occur in protease inhibitor (PI) naïve patients treated with nucleoside reverse transcriptase inhibitors (NRTIs), combination therapies (PI and NRTI) accelerate the syndrome. Recent clinical trials, cell culture and animal studies indicate that these effects are not drug class specific and select PIs, NRTIs and non-nucleoside reverse transcriptase inhibitors (NNRTIs) can be associated with metabolic complications. Moreover, the effects can vary between various members of the same class of antiretroviral agents (i.e. not all PIs cause the same adverse reactions) and may be influenced by duration of infection, genetics and environmental factors. Although HAART increases the risk of metabolic complications, this does not outweigh the benefits of survival. In this review, we summarize the latest clinical and scientific information on these metabolic complications, examine current hypotheses explaining the syndromes and comment on the existing methods available to manage these metabolic side effects.

Mitochondrial toxicity and HIV therapy

Nucleoside reverse transcriptase inhibitors (NRTIs) remain the cornerstone of highly active antiretroviral therapy (HAART) combination regimens. However, it has been known for some time that these agents have the potential to cause varied side effects, many of which are thought to be due to their effects on mitochondria. Mitochondria, the key energy generating organelles in the cell, are unique in having their own DNA, a double stranded circular genome of about 16 000 bases. There is a separate enzyme present inside the cell that replicates mitochondrial DNA, polymerase gamma. NRTIs can affect the function of this enzyme and this may lead to depletion of mitochondrial DNA or qualitative changes. The study of inherited mitochondrial diseases has led to further understanding of the consequences of mutations or depletion in mitochondrial DNA. Key among these is the realisation that there may be substantial heteroplasmy among mitochondria within a given cell, and among cells in a particular tissue. The unpredictable nature of mitochondrial segregation during cellular replication makes it difficult to predict the likelihood of dysfunction in a given tissue. In addition, there is a threshold effect for the expression of mitochondrial dysfunction, both at the mitochondrial and cellular level. Various clinical and in vitro studies have suggested that NRTIs are associated with mitochondrial dysfunction in different tissues, although the weight of evidence is limited in many cases. The heterogeneity in the tissues affected by the different drugs raises interesting questions, and possible explanations include differential distribution or activation of these agents. This article reviews the major recognised toxicities associated with NRTI therapy and evidence for mitochondrial dysfunction in these complications. Data were identified through searching of online databases including Medline and Current Contents for relevant articles, along with abstracts and posters from recent conferences in the HIV and mitochondrial fields.

Increased risk of lipodystrophy when nucleoside analogue reverse transcriptase inhibitors are included with protease inhibitors in the treatment of HIV-1 infection

BACKGROUND

Changes in body fat distribution are an adverse effect of therapy with HIV protease inhibitors (PI). It has been suggested that nucleoside analogue reverse transcriptase inhibitors (NRTI) may also contribute to this so-called lipodystrophy syndrome, but the relative contribution of the two drug classes is unclear as they are usually administered concomitantly.

METHOD

The occurrence of lipodystrophy, as reported by physicians using no standardized criteria, was followed in patients randomly assigned to treatment with either a PI alone or a PI combined with an NRTI. The patients were part of a multicenter, open-label, randomized comparison of ritonavir (RTV)/saquinavir (SQV) with or without the addition of stavudine (d4T) in HIV-1-infected patients without prior PI and d4T experience (the Prometheus study).

RESULTS

Lipodystrophy was reported in 29 of 175 (17%) patients during 96 weeks of follow up. Overall, it was reported significantly more frequently in patients who were randomized to RTV/SQV/d4T (22/88; 25%), than in patients randomized to RTV/SQV alone (7/87; 8%) (P = 0.003). When the analysis was limited to patients without any prior antiretroviral experience, lipodystrophy likewise was significantly more frequent in patients randomized to RTV/SQV/d4T (12/50; 24%) than in those randomized to RTV/SQV (2/44; 5%) (P = 0.008).

CONCLUSION

This randomized clinical trial, in spite of not having been blinded, supports a contributory role of NRTI in the development of antiretroviral therapy-associated lipodystrophy. The low incidence of lipodystrophy in patients with no or limited NRTI exposure supports further evaluation of NRTI-sparing regimens as alternatives to current antiretroviral regimens.

Chronic hyperlactatemia in HIV-infected patients taking antiretroviral therapy

OBJECTIVE

To determine the prevalence, course and risk factors for hyperlactatemia in HIV-infected patients.

DESIGN

A prospective, longitudinal study of venous lactate concentrations over an 18-month period in 349 participants of the Western Australian HIV Cohort Study.

RESULTS

In 516 patient-years of observation, two patients experienced severe fulminant lactic acidosis (lactate > 5 mmol/l) and hepatic steatosis attributable to nucleoside analogue reverse transcriptase inhibitors (NRTI). A further five patients with lesser elevations of lactate (2.8-4.1 mmol/l) but with symptoms of nausea or abdominal discomfort and evidence of hepatic steatosis had NRTI therapy revised, with relief of symptoms and a fall in lactate levels. Most remaining patients on highly active antiretroviral therapy (HAART) had mild, chronic, asymptomatic hyperlactatemia, with mean lactate level between 1.5 mmol/l and 3.5 mmol/l most commonly. Longitudinal data was analysed in a non-linear mixed effects growth model which indicated that average lactate levels rose after the start of HAART but tended to stabilise at low-grade elevation, with an average 0.23 mmol/l greater long term level in stavudine users compared with zidovudine users (p < 0.01). A multiple linear regression model showed that the association between stavudine and higher lactate level was not confounded by longer duration of total NRTI exposure. Risk of hyperlactatemia was not significantly associated with use of other NRTIs, protease inhibitors, non-nucleoside analogue reverse transcriptase inhibitors or multiple immunological and virological factors in multivariate analyses.

CONCLUSIONS

Chronic, compensated, asymptomatic hyperlactatemia is common in patients taking HAART. Decompensated, life-threatening lactic acidosis/hepatic steatosis is rare. Treatment with stavudine appears to be the predominant risk factor for development of chronic hyperlactatemia.

Disorders of lipid metabolism in patients with HIV disease treated with antiretroviral agents: frequency, relationship with administered drugs, and role of hypolipidaemic therapy with bezafibrate

OBJECTIVES

To assess the correlation between antiretroviral treatment and dyslipidaemia in HIV-infected patients, and the role of bezafibrate as a lipid-lowering agent.

METHODS

We retrospectively compared serum lipid levels of five groups of 40 patients, each of them treated with either saquinavir hard gel, indinavir, or ritonavir (associated with two nucleoside analogues), or dual nucleoside reverse transcriptase inhibitors (NRTI) with or without a non-nucleoside reverse transcriptase inhibitor (NNRTI), or not treated with antiretrovirals, randomly selected from nearly 1000 HIV-infected patients followed-up for >or= 12 months, while on the relevant therapy. Hypertriglyceridaemia was defined by triglyceride levels >or= 172 mg/dl, and hypercholesterolaemia by cholesterol levels >or= 200 mg/dl. All patients with triglyceridaemia > 300 mg/dl and cholesterolaemia > 220 mg/dl for at least 6 months, and unresponsive to a >or= 3-month diet, started bezafibrate (400 mg/day), and were prospectively followed-up at a <or= 3-month interval, evaluating both efficacy and tolerability of the hypolipidaemic treatment, provided that they did not change their protease inhibitor treatment for reasons other than metabolic abnormalities.

RESULTS

Hypertrygliceridaemia occurred in 75 patients out of 200 (37.5%), but was significantly more frequent and severe with ritonavir vs. indinavir (P<0.001), and in subjects given indinavir vs. all remaining patients (either treated or not) (P<0.001), while isolated saquinavir use was associated with higher tri glyceride levels than NRTI-NNRTI treatment alone, or no antiretroviral therapy (P<0.03). Hypercholesterolaemia was found in 27 subjects (13.5%), and a significantly higher frequency and severity was shown in patients treated with indinavir and ritonavir vs. saquinavir, NRTI-NNRTI, and no anti-HIV therapy (P<0.05 to P<0.001). No appreciable difference was found between patients undergoing NRTI-NNRTI and untreated controls, for all evaluated variables. Bezafibrate was administered once daily for 6-18 months to 49 patients with elevated and diet-resistant hyperlipidaemia due to ritonavir or indinavir (27 and 22 subjects, respectively), and reduced triglyceride and cholesterol levels by 35% and 25%, respectively over 6 months, without differences between the underlying protease inhibitor regimen. Thirty-three patients (67.3%) reached a normal triglyceridaemia after 6-9 months, and normal cholesterol levels were obtained in all subjects. Bezafibrate proved safe and well tolerated.

CONCLUSIONS

Careful monitoring of the serum lipid profile is needed during antiretroviral therapy, including protease inhibitors, to identify the need for a diet and/or an hypolipidaemic treatment, and to prevent clinical sequelae related to long-term dyslipidaemia. Specific guidelines for the management of disorders of lipid metabolism in HIV-infected patients are needed.

Lipodystrophy Syndrome in Patients with HIV Infection

Current antiretroviral therapy has lead to longer survival in patients infected with HIV, but it is also associated with new and important problems. Body fat redistribution and metabolic abnormalities, the so-called lipodystrophy syndrome, are among the most prevalent and worrisome ones. While an increasing number of patients infected with HIV are becoming affected by this syndrome, the pathogenesis of this syndrome and how to prevent and treat the problem all remain largely unknown. Body fat changes stigmatise the bodies of patients infected with HIV giving them a similar look to that seen in patients some years ago when the wasting syndrome was more prevalent and HIV infection was ultimately fatal. The psychological impact of body fat changes may be severe enough to affect a patients' desire to continue with antiretroviral therapy. Metabolic abnormalities, probably with the exception of symptomatic diabetes mellitus and hypertriglyceridaemia-induced pancreatitis, do not have an immediate impact on the quality of the lives of patients with HIV. However, their potential long term cardiovascular and bone consequences may increase the morbidity and the mortality of patients infected with HIV through noninfectious diseases. The impact of lipodystrophy on patients infected with HIV is not readily captured with the classic instruments used to measure quality of life and hence it is necessary to modify them urgently. Though treating lipodystrophy seems fully justified, there is no proven treatment for this problem, although a number of treatments have been used with varying success. Despite the recognition that lipodystrophy may have important psychological repercussions, the best psychological approach for this problem is not known at present. Although lipodystrophy has its own peculiarities, existing knowledge about how to psychologically help other patients with deforming body changes might be of help for patients infected with HIV, or at least may act as a starting point.

Significant Toxicities Associated with Antiretroviral Therapy

Use of at least three potent antiretroviral agents has become the standard of care in the management of HIV infection. The potential toxicities associated with highly active antiretroviral therapy (HAART) however, may limit a patient’s ability to adhere to and tolerate these agents. Although a comprehensive discussion of all toxicities associated with HAART is beyond the scope of this article, selected short-term and long-term significant toxicities will be reviewed. Short-term toxicities that will be discussed include abacavir-induced hypersensitivity reactions, efavirenz-associated central nervous system side effects and rash associated with the non-nucleoside reverse transcriptase inhibitors (NNRTIs) and the protease inhibitor (PI) amprenavir. Several long-term toxicities associated with the nucleoside reverse transcriptase inhibitors (NRTIs) are hypothesized to be due to mitochondrial toxicity. These toxicities include myositis and lactic acidosis with hepatic steatosis, pancreatitis and peripheral neuropathy. Some experts also hypothesize that mitochondrial toxicity is responsible for the lipodystrophy syndrome, which includes hyperglycemia, abnormal fat redistribution and dyslipidemia. Finally, indinavir-associated nephrolithiasis, which may present with either short term or long term use will be discussed. This article will provide the practicing pharmacist with a review of these significant toxicities, the implicated agents, incidence, usual clinical presentation, and recommendations for management.

Management of dyslipidemia in patients with HIV disease

The epidemiological, etiopathogenetic, laboratory and clinical features of serum lipid abnormalities occurring in the course of HIV disease are still poorly understood (especially when the supporting role of single antiretroviral compounds is considered), while limited literature data are to date available regarding the management of HIV-related dyslipidemia, as well as the efficacy and safety of dietary-exercise programs, and that of selected hypolipidemic agents. At this time, a careful monitoring of serum lipid profile is needed during combination antiretroviral therapy including protease inhibitors, in order to suggest a diet and hypolipidemic treatment when applicable, and to prevent clinical sequelae related to long-term dyslipidemia. The selection of an appropriate hypolipidemic agent is difficult, since no controlled studies are available in this field, and possibly increased risks of pharmacologic interactions, toxicity and impaired patient's adherence should be taken into consideration. Waiting for specific guidelines for the treatment of hypertriglyceridemia and hypercholesterolemia in the setting of HIV infection, all available literature reports dealing with the management of HIV-associated hyperlipidemia are briefly discussed, on the basis of personal clinical experience.

Clinical manifestations and management of antiretroviral nucleoside analog-related mitochondrial toxicity

OBJECTIVES

This article reviews the clinical manifestations of mitochondrial toxicity associated with the use of nucleoside analog reverse transcriptase inhibitors (NRTIs) and outlines strategies to manage these sequelae.

BACKGROUND

NRTIs are the key components of the antiretroviral combinations used in the management of patients infected with HIV. The available NRTIs differ in their convenience of administration, frequency of dosing, resistance profiles, and side-effect profiles. NRTIs act as competitive inhibitors of the RNA/DNA polymerase reverse transcriptase of HIV and cause chain termination in the growing viral DNA chain. Many of the important and treatment-limiting side effects of NRTIs may be related to the effect of these agents on human DNA polymerases, in particular, mitochondrial DNA polymerase gamma. Depletion of mitochondrial DNA during chronic NRTI therapy may lead to cellular respiratory dysfunction and generalized and tissue- and drug-specific toxicities, including myopathy, peripheral neuropathy, and lactic acidosis. Recently, it has been proposed that the fat redistribution syndrome, or lipodystrophy, reported during chronic antiretroviral therapy is a manifestation of the differential impact of at least some NRTIs on peripheral and visceral adipocytes. Management of potential mitochondrial toxicity during NRTI therapy remains a challenge. A range of nutritional supplements, both as treatments and prophylaxes, have been proposed, and some have been investigated in vitro; no in vivo studies have yet been conducted.

METHODS

The information in this review was compiled using MEDLINE and AIDSLINE searches of the literature, including conference abstracts.

CONCLUSIONS

At present, interruption of NRTI therapy or substitution of the probable causative agent with alternative NRTIs that appear to be better tolerated represents the mainstay of management for mitochondrial toxicity and its clinical manifestations.

Absence of association between individual thymidine analogues or nonnucleoside analogues and lipid abnormalities in HIV-1-infected persons on initial therapy

Changes in levels of triglycerides and cholesterol during antiretroviral therapy raise concerns regarding an increased future risk of atherogenic disease and may precede the appearance of fat redistribution. Hypotheses regarding the impact of nucleoside analogues on adipocytes provide a possible explanation for metabolic and clinical fat disturbances. It is unclear whether the choice of nucleoside analogue combination or coadministration of nonnucleoside agents influences change in lipids. We performed a cross-sectional analysis of 135 persons receiving their first nucleoside analogue plus nonnucleoside-based combination antiretroviral regimen for at least 1 month and for whom cholesterol and triglyceride values were available on therapy. Univariate and multivariate regression models were used to explore the relation between cholesterol and triglycerides, as continuous variables with other variables. Both significant and nonsignificant variables from univariate analyses were evaluated in multivariate models to limit possible confounders. No association with drug choice was observed, either when comparing thymidine analogues (stavudine or zidovudine), all nucleoside analogue combinations or choice of either efavirenz or nevirapine as nonnucleoside. Age and triglyceride levels were found in a multivariate analysis to be associated with higher cholesterol. Only higher cholesterol was associated with higher triglyceride levels. In conclusion, no differences were observed between choice of drug or combination on cholesterol or triglyceride values during therapy. Older individuals may be more likely to have elevated cholesterol values.

Pharmacology of nucleoside and nucleotide reverse transcriptase inhibitor-induced mitochondrial toxicity

OBJECTIVE

This paper reviews the function of the mitochondria and the mechanisms by which nucleoside and nucleotide reverse transcriptase inhibitors (NRTIs) cause mitochondrial toxicity.

BACKGROUND

Highly active antiretroviral therapy (HAART) reduces rates of morbidity and mortality due to HIV disease. However, long-term treatment with these drugs may be associated with adverse effects. Nucleoside and nucleotide analogues are potent inhibitors of HIV reverse transcriptase and have become the cornerstone of HAART. Unfortunately, these drugs have also been shown to inhibit cellular polymerases, most notably mitochondrial DNA polymerase gamma.

RESULTS

Studies of the NRTIs in enzyme assays and cell cultures demonstrate the following hierarchy of mitochondrial DNA polymerase gamma inhibition: zalcitabine > didanosine > stavudine > lamivudine > zidovudine > abacavir. In vitro investigations have also documented impairment of the mitochondrial enzymes adenylate kinase and the adenosine diphosphate/adenosine triphosphate translocator. Inhibition of DNA polymerase gamma and other mitochondrial enzymes can gradually lead to mitochondrial dysfunction and cellular toxicity. The clinical manifestations of NRTI-induced mitochondrial toxicity resemble those of inherited mitochondrial diseases (ie, hepatic steatosis, lactic acidosis, myopathy, nephrotoxicity, peripheral neuropathy, and pancreatitis). Fat redistribution syndrome, or HIV-associated lipodystrophy, is another side effect attributed in part to NRTI therapy. The morphologic and metabolic complications of this syndrome are similar to those of the mitochondrial disorder known as multiple symmetric lipomatosis: suggesting that this too may be related to mitochondrial toxicity. The pathophysiology of less common adverse effects of nucleoside analogue therapy, such as diabetes, ototoxicity, and retinal lesions, may be related to mitochondrial dysfunction but have not been adequately studied.

CONCLUSION

NRTls can block both HIV reverse transcriptase and mitochondrial DNA polymerase gamma. Inhibition of the latter enzyme is the most likely cause of the adverse effects associated with these drugs.

An Introduction to Nucleoside and Nucleotide Analogues

The introduction of newer and more potent agents has diverted attention away from the importance of nucleoside analogue reverse transcriptase inhibitors (NRTIs) in modern antiretroviral drug regimens. As a class, these proviral chain terminators lack the virological potency of either non-nucleoside reverse transcriptase inhibitor (NNRTI) or protease inhibitor (PI) drugs, due largely to their competitive mode of inhibition and requirement for metabolic activation. However, neither NNRTIs nor PIs alone can maintain the complete suppression of HIV replication required for extended therapy, and both suffer from serious class cross-resistance on therapeutic failure. Thus, the NRTIs will remain essential components of highly active antiretroviral therapy (HAART) for the foreseeable future, both for their contribution to a regimen's virological potency and the subsequent preservation of the more potent drug classes used with them. However, it has become apparent in recent years that the current NRTIs exhibit duration-dependent adverse events as a class, which may limit the length of time for which they can be safely used. An independent contribution to peripheral fat wasting in lipodystrophy syndrome has been established for the use of NRTI drugs. Of greater clinical concern is their established association with potentially fatal lactic acidaemia and hepatic steatosis. Both these class events, as well as several individual drug events, such as peripheral neuropathy, can be linked to progressive mitochondrial destruction with a greater or lesser degree of confidence. Mitochondrial toxicity, due in large part to the high affinity of several NRTI agents for uptake by mitochondrial DNA polymerase γ, has been demonstrated both in vitro and in vivo. New chain-terminating agents are urgently needed that address issues of improved virological potency, greater efficacy in NRTI-experienced individuals, and greater long-term safety. The nucleotide class of reverse transcriptase inhibitor (NtRTI), currently under clinical development, addresses improved potency by abbreviating the intracellular activation pathway to allow a more rapid and complete conversion to the active agent. These nucleoside monophosphate analogues are taken as masked prodrugs bearing labile lipophilic groups to facilitate penetration of target cell membranes. Subsequent unmasking by endogenous chemolytic enzymes releases a partially activated nucleoside analogue metabolite. The NtRTI furthest along the developmental process is tenofovir disoproxil fumarate (TDF), an orally available acyclic adenine phosphonate analogue, currently in Phase III clinical trials. This agent has shown high potency and an unusually durable response in trials of single-agent therapy intensification in highly treatment-experienced individuals, and its active metabolite, tenofovir diphosphate, exhibits a long intracellular half-life in both resting and activated peripheral blood mononuclear cells that permits once daily dosing. Tenofovir diphosphate also exhibits a very low affinity for DNA polymerase γ in vitro, suggesting a low degree of in vivo mitochondrial toxicity may be observed on long-term follow-up, although clinical data to support this inference are not yet available. The introduction of TDF and other NtRTIs as ‘second-generation’ nucleoside analogues carefully evaluated for potential long-term toxicity, can be expected to significantly improve the therapeutic options for both those currently on HAART and those yet to begin.

Mitochondrial toxicity of nucleoside analogue reverse transcriptase inhibitors: a looming obstacle for long-term antiretroviral therapy?

Nucleoside reverse transcriptase inhibitors suppress HIV replication by blocking reverse transcriptase, an RNA-dependent DNA polymerase. These drugs can also affect cellular and mitochondrial DNA polymerases. Mitochondrial DNA polymerase gamma is particularly sensitive to nucleoside reverse transcriptase inhibitors, and the majority of adverse effects caused by nucleoside reverse transcriptase inhibitors are most likely caused by mitochondrial dysfunction. This article reviews the recent clinical implications of nucleoside reverse transcriptase inhibitor-induced mitochondrial toxicity and discusses options for management.