Abstract

Nucleoside- and nucleotide-analogue reverse-transcriptase inhibitors (NRTIs) require intracellular phosphorylation for anti–human immunodeficiency virus (HIV) activity and toxicity. Long-term toxicities associated with NRTIs may be related to overactivation of this process. In vitro experiments have shown increased rates of NRTI and endogenous nucleoside phosphorylation to be associated with cellular activation. Patients with advanced HIV disease often have overexpression of cytokines, which corresponds to an elevated cellular activation state. These patients also have higher rates of NRTI phosphorylation and NRTI toxicity, suggesting an interaction between a proinflammatory biological state, NRTI phosphorylation, and toxicity. Studies suggest that women may have higher rates of NRTI phosphorylation than do men, as well as increased risk for NRTI-induced toxicity. Future research is needed to understand the NRTI activation process and improve the long-term toxicity profile of NRTIs. Such research should include comparisons of NRTI phosphorylation according to sex and cellular activation state (i.e., elevated vs. low).

Treatment of HIV infection with combination antiretroviral therapy (ART) is a long-term undertaking [1, 2]. Although ART has been proven to reduce morbidity and mortality, drug toxicity issues have dampened enthusiasm for these accomplishments [3–5]. Nucleoside- and nucleotide-analogue reverse-transcriptase inhibitors (NRTIs) play a central role in the treatment of HIV. Eight NRTIs are currently available: abacavir, didanosine, emtricitabine, lamivudine, stavudine, tenofovir disoproxil fumarate (nucelotide analogue), zalcitabine, and zidovudine. Several other NRTIs are in various stages of clinical development. Long-term toxicities attributed to NRTIs include hyperlactatemia and lactic acidosis, hepatomegaly with steatosis, peripheral neuropathy, myopathy and/or cardiomyopathy, ototoxicity, cytopenias, pancreatitis, and lipoatrophy [3, 4, 6–10]. Cross-sectional studies, summarized in table 1, identify recognized risk factors for the most common of these events. Limitations of the analyses—such as study design, lack of a uniform toxicity definition (particularly in the case of lipodystrophy), and evaluations during ART compared with those during monotherapy or dual NRTI therapy [10, 27, 35, 38]—should be considered when evaluating these risk factors.

Table 1

Risk factors associated with major nucleotide- and nucleoside-analogue reverse-transcriptase inhibitor (NRTI) toxicities.

Table 1

Risk factors associated with major nucleotide- and nucleoside-analogue reverse-transcriptase inhibitor (NRTI) toxicities.

Although NRTI toxicities are a major problem for patients infected with HIV, pharmacological research in this area is limited, in large part because measuring the active intracellular NRTI triphosphate concentrations in patients is difficult. The objective of this review is to address cellular NRTI activation as it relates to NRTI toxicity and to identify focus areas for future research. Information was included from epidemiological studies of NRTI toxicity, descriptive studies of NRTI phosphorylation in patients, and in vitro experiments.

Nrti Pharmacology

For all NRTIs, it is essential to consider drug exposures in terms of intracellular NRTI triphosphate concentrations, because these are the moieties that exert antiretroviral and toxic activities. This creates a distinctive therapeutic index for NRTIs, as depicted in figure 1, in which cellular NRTI activation can be one factor that influences antiretroviral effects and drug toxicity.

Figure 1

Therapeutic index of nucleotide- and nucleoside-analogue reverse-transcriptase inhibitors (NRTIs). The probabilities (y axis) of drug efficacy (solid-line curve) and toxicity (dashed-line curve) are analyzed with respect to drug exposure (x axis.) A narrow horizontal distance between the curves signifies a narrow therapeutic index. Biological diversity in the patient and in the disease influences the probability of drug effects (dashed horizontal arrows).

Figure 1

Therapeutic index of nucleotide- and nucleoside-analogue reverse-transcriptase inhibitors (NRTIs). The probabilities (y axis) of drug efficacy (solid-line curve) and toxicity (dashed-line curve) are analyzed with respect to drug exposure (x axis.) A narrow horizontal distance between the curves signifies a narrow therapeutic index. Biological diversity in the patient and in the disease influences the probability of drug effects (dashed horizontal arrows).

The cellular activation of NRTIs produces at least 2 distinct sets of pharmacokinetic dispositions, one for the biologically inactive drug in plasma and the other for the active NRTI phosphate in cells [43]. Although underlying relationships probably exist between plasma NRTI concentrations and intracellular NRTI phosphate concentrations, these relationships are currently unpredictable in patients [44, 45]. This is likely the result of rate-limiting or saturated-phosphorylation steps and the overall biological complexity of the system. A simplified diagram of the intracellular activation of the currently available NRTIs is shown in figure 2 [46–48].

Figure 2

Host-cell-mediated sequential enzymatic phosphorylation steps required for activating the nucleotide- and nucleoside-analogue reverse-transcriptase inhibitors (NRTIs) to the triphosphate moiety. ABV, abacavir; AMPD, adenosine monophosphate deaminase; AMPK, adenosine monophosphate kinase (adenylate kinase); APT, adenosine phosphotransferase; CBV, carbovir; dCK, deoxycytidine kinase; dCMPK, deoxycytidine monophosphate kinase; ddA, 2′,3′-dideoxyadenosine; ddI, didanosine; DP, diphosphate; d4T, stavudine; FTC, emtricitabine; gK, guanylate kinase; MP, monophosphate; PMPA, tenofovir (PMPA DP is a triphosphate analogue); TDF, tenofovir disoproxil fumarate; TP, triphosphate; ZDV, zidovudine; 3TC, lamivudine; 5′NDPK, 5′ nucleoside diphosphate kinase; 5′NT, 5′ nucleotidase.

Figure 2

Host-cell-mediated sequential enzymatic phosphorylation steps required for activating the nucleotide- and nucleoside-analogue reverse-transcriptase inhibitors (NRTIs) to the triphosphate moiety. ABV, abacavir; AMPD, adenosine monophosphate deaminase; AMPK, adenosine monophosphate kinase (adenylate kinase); APT, adenosine phosphotransferase; CBV, carbovir; dCK, deoxycytidine kinase; dCMPK, deoxycytidine monophosphate kinase; ddA, 2′,3′-dideoxyadenosine; ddI, didanosine; DP, diphosphate; d4T, stavudine; FTC, emtricitabine; gK, guanylate kinase; MP, monophosphate; PMPA, tenofovir (PMPA DP is a triphosphate analogue); TDF, tenofovir disoproxil fumarate; TP, triphosphate; ZDV, zidovudine; 3TC, lamivudine; 5′NDPK, 5′ nucleoside diphosphate kinase; 5′NT, 5′ nucleotidase.

Most of the clinical manifestations of NRTI toxicities resemble mitochondrial diseases, and histologic evidence demonstrates abnormal mitochondria and/or mtDNA depletion in affected tissues [22, 47, 49–51]. Studies show that NRTI triphosphates competitively inhibit mtDNA polymerase γ in vitro [52, 53]. This, in turn, may decrease the number of mitochondrial respiratory chain proteins, inhibit aerobic respiration, induce oxidative stress, increase mutation in mtDNA, and result in mitochondrial and/or tissue failure [54]. This mechanism of toxicity is a selectivity/specificity problem, and, therefore, toxicity will strongly depend on drug dose and concentration, as shown in figure 1. As such, elevations in NRTI triphosphate concentrations will significantly impact the mitochondrial toxicity of NRTIs.

To date, the cellular activation state for a given cell type has been determined to be the most influential factor for increased generation of intracellular NRTI phosphate concentrations, although this was elucidated in vitro. Cells treated with phytohemagglutinin or granulocyte-macrophage colony-stimulating factor (GM-CSF) generated 2- to >150-fold higher triphosphate concentrations of zalcitabine, lamivudine, stavudine, zidovudine, and didanosine (2′,3′-dideoxyadenosine triphosphate) than resting cells in vitro [55–59]. Elevated cell activation results in high nucleic acid synthesis and an upregulation of kinases that phosphorylate NRTIs [57, 60]. It must be noted that zalcitabine, lamivudine, and didanosine are more active virologically in resting cells than in activated cells, which has been attributed to a more favorable ratio of NRTI triphosphate to endogenous nucleoside triphosphate (e.g., lamivudine triphosphate to 2′-deoxycytidine-5′ triphosphate ratio) in resting cells [57, 58]. Nevertheless, the actual concentrations of lamivudine, zalcitabine, and 2′,3′-dideoxyadenosine triphosphates were ∼2–4-fold higher in activated versus resting cells [55–58]. It is not known whether the same cytoplasmic (whole cell) ratio is as relevant to NRTI toxicities in the mitochondrial compartment as it is to antiviral effects in the cytoplasmic/nuclear compartment.

The higher incidences of pancreatitis and peripheral neuropathy observed in patients receiving hydroxyurea—a ribonucleotide reductase inhibitor—were hypothesized to be caused by decreased endogenous deoxynucleotide pools in cytoplasm and, thus, a ratio favoring the NRTI triphosphates, particularly didanosine (2′,3′-dideoxyadenosine triphosphate) [24]. However, a study in patients treated with hydroxyurea could not detect changes in endogenous deoxynucleotide pools, including deoxyadenosine triphosphate pools [61]. Hydroxyurea may also upregulate salvage pathways of nucleoside phosphorylation, and it arrests cells in the G1-S phase, which may upregulate generation of NRTI phosphates and could also explain increased toxicity [62]. These multiple possibilities of hydroxyurea effects make it difficult to resolve the importance of the cytoplasmic ratio for NRTI toxicities in the mitochondrial compartment.

In the mitochondrial compartment, there are kinases that potentially phosphorylate NRTIs, but a nucleotide carrier protein may also shuttle nucleotides from cytoplasm into the mitochondrion [47, 63]. This nucleotide carrier has a stronger affinity for many NRTI phosphates than the corresponding endogenous nucleotides, which may influence the importance of the ratio with cytoplasmic endogenous nucleotides [63]. As a possible illustration of the activity of this carrier, one in vitro study with zidovudine could not generate mitochondrial toxicity during exposure of isolated mitochondria to zidovudine. Instead, zidovudine was extremely toxic to mitochondria in activated whole cells, compared with resting whole cells, which may indicate significant NRTI phosphate shuttling into mitochondria to elicit the toxicity [64].

In Vivo Versus in Vitro NRTI Triphosphate Concentrations and Toxicity

Thus far, only 1 study has addressed a possible relationship between NRTI phosphate concentrations in patients and NRTI toxicity: higher total zidovudine phosphate concentrations (mono-, di-, and tri-) in PBMCs were associated with reduced hemoglobin levels during zidovudine monotherapy, whereas no such relationships were found with plasma zidovudine concentrations [65]. Table 2 compares the typical NRTI triphosphate pharmacological data descriptively measured in patient's PBMCs, along with the NRTI triphosphate concentrations that inhibit mtDNA polymerase γ in vitro. Intracellular concentrations of 2′,3′-dideoxyadenosine, emtricitabine, lamivudine, and stavudine triphosphates reach levels near the in vitro binding affinity for polymerase γ. Table 2 also shows the in vitro therapeutic index of each NRTI; a low number indicates a narrow therapeutic index. Lamivudine values may need to be viewed in light of the exonuclease function of polymerase γ. Lamivudine monophosphate is excised 750-fold more rapidly than zalcitabine monophosphate, which presumably lessens the toxicity of lamivudine relative to that of zalcitabine [52]. The other NRTIs are excised at rates between those of these 2 agents [52].

Table 2

Cellular pharmacological data for nucleotide- and nuceloside-analogue reverse-transcriptase inhibitor (NRTI) triphosphates (TPs) in patients and in vitro and the relationship to toxicity.

Table 2

Cellular pharmacological data for nucleotide- and nuceloside-analogue reverse-transcriptase inhibitor (NRTI) triphosphates (TPs) in patients and in vitro and the relationship to toxicity.

Clinically, zidovudine, didanosine, stavudine, and zalcitabine all underwent dose de-escalations during development to approximately one-half of the present-day dose, because the probability of toxicity was unacceptably high (mainly cytopenia and myopathy were associated with zidovudine, and pancreatitis and peripheral neuropathy were associated with didanosine, stavudine, and zalcitabine) [6, 74–78]. This narrow margin of drug exposures that elicited the increased risk of clinical toxicity is indicative of a low therapeutic index, as depicted in figure 1. Zidovudine seems to have more clinical toxicity than would be predicted on the basis of the values in table 2. Zidovudine may exert activity on cellular and mitochondrial processes other than the inhibition of polymerase γ, such as inhibition of adenylate kinase, impairment of the mitochondrial ADP-ATP translocator, and uncoupling of the electron transport chain [47, 79].

Of importance, the exact biochemical events that elicit clinical toxicities are not well understood, and specific pharmacological questions remain: Are concentrations of NRTI mono- or diphosphates important? Do changes in endogenous nucleotide pools in cytoplasm versus those in mitochondria contribute to toxicity? How do NRTI phosphates appear and disappear in the cytoplasmic versus mitochondrial cell compartments? Why do different tissues have different NRTI toxicity profiles?

Intracellular NRTI Phosphate Levels and Clinical Toxicities in Women

In small descriptive studies of NRTI phosphorylation, one characteristic that correlates with higher intracellular PBMC concentrations is female sex. One study found median lamivudine triphosphate and zidovudine triphosphate concentrations that were 1.6- and 2.3-fold higher, respectively, in a group of 4 HIV-infected women than those in a group of 29 men who initiated zidovudine, lamivudine, and indinavir (P < .01), although zidovudine or lamivudine plasma concentrations did not differ according to sex [44]. A second study reported mean total zidovudine phosphate levels that were 2-fold higher in a group of 5 women than those in a group of 16 men (P = .004), and another report described carbovir triphosphate (the active triphosphate for abacavir) levels in a single woman that were 2–8-fold higher than the levels in a group of 4 men [43, 66]. These data provide pharmacological insight that is consistent with epidemiological studies that showed 4-fold more lipodystrophy during dual therapy with NRTIs, disproportionately high rates of lactic acidosis and pancreatitis, and stronger antiviral responses to NRTIs in women versus men, respectively [27, 11–13, 80]. Of interest, women with cancer experienced 1.5-fold–more severe toxicities with the use of the anticancer nucleoside analogue fluorouracil than did men, after adjustment for study, dose, body size, and age (P < .0001) [81]. Fluorouracil requires cellular activation for biological activity, using steps similar to those of zidovudine and stavudine [82].

Intracellular NRTI Phosphate Levels in Patients with Advanced HIV Disease

It has also been found that the presence and severity of HIV disease correlates with higher concentrations of intracellular NRTI phosphates in PBMCs [83, 84]. Among persons who received zidovudine monotherapy, the lowest intracellular zidovudine phosphate (mono-, di-, and tri-) concentrations were in healthy volunteers, followed by 5- and 12-fold higher concentrations in HIV-infected patients with CD4 cell counts of >100 and <100 cells/mm3, respectively [85]. One small study also found high zidovudine monophosphate concentrations in patients with low CD4 cell counts, although the zidovudine triphosphate levels were lower [86].

A relationship between HIV disease severity and NRTI phosphorylation was also manifested in rates of NRTI phosphorylation in patients with advanced disease that were higher during the early stages of treatment relative to rates during later stages. In ART-naive patients initiating therapy with zidovudine, lamivudine, and indinavir, zidovudine triphosphate levels were 2.5-fold higher early in therapy (at week 2) in patients with a CD4 cell count of <100 cells/mm3 than they were in patients with higher CD4 cell counts [44, 87]. However, after 1 year of therapy, the zidovudine triphosphate levels in patients with advanced disease were reduced to the concentration range observed in patients with mild disease [44, 87]. In another zidovudine monotherapy study, the highest zidovudine phosphate concentrations were observed on day 1 of therapy; these concentrations decreased to ∼30% of this value when measured again 6 months later [43]. These data suggest that the presence of HIV infection and advanced HIV disease status are associated with higher NRTI phosphate concentrations, particularly just after initiation of therapy.

This provides pharmacological insight that is consistent with the advanced disease risk factor for NRTI toxicity. For example, a CD4 cell count of <100 cells/mm3 was the strongest predictor of lipoatrophy for NRTI-treated subjects in the HIV Outpatient Study (HOPS) cohort [30]. NRTI-associated peripheral neuropathy is ∼2-fold more common in persons with CD4 cell counts of <100 cells/mm3 than it is in persons with higher CD4 cell counts [6]. Neuropathy risk in the HOPS cohort was highest in patients with CD4 cell counts of <100 cells/mm3 during the first months after initiating NRTI therapy, but it subsequently decreased over time. This finding is similar to the initially elevated and later decreased NRTI phosphate concentrations described in patients with advanced disease (Lichtenstein et al., unpublished data). In the dose de-escalation studies described above, patients with CD4 cell counts of <100 cells/mm3 experienced significantly increased NRTI toxicities [77, 88].

Some investigators hypothesized that the genetic sequence of the polymerase γ enzyme may vary among individuals, resulting in increased or decreased affinity for NRTI triphosphates. Specific sequence variants of the mtDNA polymerase γ gene obtained from 14 patients with NRTI-associated lactic acidosis or peripheral neuropathy were compared with sequences from 45 patients without NRTI toxicity. The investigators found no correlations between variations in the DNA polymerase γ gene sequence and these NRTI-associated toxicities. Instead, low CD4 cell counts were found to be predictive of neuropathy [25].

Relationship Between Cellular Activation and NRTI Phosphorylation in Patients

Just as cellular activation increases NRTI phosphorylation in vitro, an elevated state of cellular activation in patients with advanced disease may be a biological mechanism for increased NRTI phosphorylation in vivo. HIV infection and advanced HIV disease are associated with highly elevated concentrations of proinflammation and cellular activation markers, such as proinflammatory serum cytokines and molecules, IFN, TNF, and soluble TNF receptor type 2 (sTNFrII), and lymphocyte activation markers, such as CD38+ cells and HLA-DR+ [89–93]. For instance, compared with healthy volunteers, TNF and sTNFrII levels were 4–25-fold higher in patients with Centers for Disease Control and Prevention (CDC) class A and B HIV infection and 10–40-fold higher in patients with CDC class C HIV infection [89]. As described in the previous section, these findings are similar to the apparent rates of NRTI phosphorylation in the same patient groups [85]. In addition, treatment of advanced disease is known to significantly reduce the concentration of cell activation markers; this finding also corresponds to the reduction in the intracellular level of NRTI phosphate observed in patients with advanced disease after the disease had been controlled [44, 94]. Together, this points to a possible relationship between proinflammatory cellular activation and intracellular NRTI pharmacology and toxicity, but this possibility has not been adequately studied. One clinical study assessed total levels of zidovudine phosphate in PBMCs of patients initially treated with zidovudine alone followed by the cytokine GM-CSF plus zidovudine. GM-CSF is known to increase cell activation in patients, as shown by increased expression of HLA-DR+ on monocytes [95]. An overall trend toward higher zidovudine phosphate levels was observed during combination GM-CSF–zidovudine therapy in the entire cohort (P = .07), and higher doses of GM-CSF led to higher total zidovudine phosphate levels (P = .01) [96].

Table 3 presents relationships among proinflammatory cellular activation, NRTI triphosphate concentrations, and the epidemiological NRTI toxicity risk factors identified in table 1—including female sex, white race, age, hepatitis B and C coinfection, nadir CD4 cell count, and concomitant protease inhibitors. To provide some additional considerations, in animal models, NRTI toxicity could not be generated unless experimentally induced HIV infection was also present [116]. In phase 1–2 studies with fialuridine—an investigational anti–hepatitis B virus nucleoside analogue that was urgently halted during the developmental phase because 7 of 15 patients died or required liver transplantation—analysis of liver biopsy specimens obtained before initiation of treatment showed higher inflammation scores for the 7 patients who died or underwent liver transplantation than for patients who averted serious fialuridine toxicity [117, 118].

Table 3

Associations between epidemiologically identified risk factors for nucleotide- and nucleoside-analogue reverse-transcriptase inhibitor (NRTI) toxicity, proinflammation/cellular activation, and elevated NRTI triphosphate (TP) levels.

Table 3

Associations between epidemiologically identified risk factors for nucleotide- and nucleoside-analogue reverse-transcriptase inhibitor (NRTI) toxicity, proinflammation/cellular activation, and elevated NRTI triphosphate (TP) levels.

Coinciding with the possibility that advanced HIV disease and the corresponding cellular activation may increase NRTI triphosphate concentrations and thereby increase toxicity, inflammation may also cause biological harm that increases susceptibility to NRTI toxicities in an additive way [119, 120]. For illustration, ART-naive HIV-infected patients already show evidence of mtDNA depletion in PBMCs, compared with healthy volunteers, and, thus, NRTI-induced mtDNA depletion could be adding to preexisting mtDNA depletion [121]. In addition, the inflammation associated with HIV infection itself may harm bystander tissues, which occurs in HIV-associated peripheral neuropathy. Pathological analyses of nerve specimens obtained from these subjects indicate that activated macrophages infiltrate nerve tissue and may secrete destructive molecules [122]. Consistent with this finding, HIV-associated peripheral neuropathy occurs most frequently in the later stages of HIV infection. Patients with a history of non–drug-related neuropathy are at increased risk for NRTI-associated neuropathy [24, 122].

These observations identify a need to investigate how inflammation and cytokines affect NRTI cellular pharmacology and toxicity. One avenue for study should include determining how certain cytokines activate certain tissues and how this may stimulate high rates of NRTI phosphorylation in that tissue or cell type, which could contribute to the tissue selectivity of NRTI toxicities. For example, patients with ART-associated lipoatrophy and/or lipodystrophy express more TNF mRNA in subcutaneous fat and exhibit higher serum levels of sTNFrII and TNF than do control subjects [100, 101, 104, 123]. TNF exerts activity on fat [91, 124], and elevated TNF levels could specifically upregulate NRTI phosphorylation and/or susceptibility to NRTI toxicity in fat cells and tissue.

Special Considerations for Cellular NRTI Activation with Concomitant Anti–Hepatitis C Virus Therapy

In the mid-1990s, studies evaluated the use of the proinflammatory cytokine IFN in combination with NRTIs to treat HIV infection. Increased NRTI toxicities were observed. One randomized study of IFN-α with or without zalcitabine-zidovudine demonstrated significantly more peripheral neuropathies (P = .02) and cytopenias (P = .03) in the combination IFN-α arm than in the zidovudine-zalcitabine only arm [26]. In a phase 1–2 study of didanosine alone followed by IFN-α–didanosine, excess clinical pancreatitis or elevations of serum amylase/lipase was observed during IFN-α–didanosine combination therapy (didanosine doses of ⩽250 mg b.i.d. were associated with a pancreatitis incidence of 24%) [23].

Present concerns for drug-drug interactions between IFN-ribavirin and ARTs are focused on intracellular interactions between ribavirin and NRTIs, because in vitro studies showed that ribavirin enhanced conversion of didanosine to the active 2′,3′-dideoxyadenosine triphosphate [108, 125]. In vitro studies with ribavirin also showed that zidovudine and stavudine were less efficiently converted to their triphosphates [108, 126, 127]. Clinically, the risk of lactic acidosis and/or pancreatitis was 5% among patients coinfected with HIV/hepatitis C virus and cotreated with ART plus IFN-ribavirin, and the toxicity risk was significantly higher for didanosine recipients than for patients who received other NRTIs (OR, 18.3; P = .0002) [17]. However, the individual effects of ribavirin versus those of IFN on the increased risk of didanosine toxicity are not separable.

An increased number of cases of hyperlactatemia have been reported among patients treated with stavudine combined with IFN-ribavirin, which is counterintuitive on the basis of the in vitro ribavirin data [128]. This raises concern about the possible pharmacological interaction between IFN and stavudine. A small study of intracellular stavudine triphosphate concentrations in 15 patients before and after initiating IFN-ribavirin therapy found no statistical change in median peak concentrations 1 month after initiation of therapy, compared with baseline concentrations, although the range of peak concentrations increased (from a median of 26.4 and a range of 0.9–47.9 fmol per 106 PBMCs at baseline to a median of 14.7 and a range of 1.1–148.3 fmol per 106 PBMCs at month 1) [71]. Future in vivo studies are needed to investigate intracellular interactions between NRTI phosphates and ribavirin, and such studies should also investigate the effects of the proinflammatory cytokine IFN.

Conclusions and Clinical Implications

First, this review underscores the pressing need for more research in the area of cellular NRTI pharmacology as it relates to NRTI toxicity, because these adverse events severely affect many patients. Research activity may be stimulated by new mass spectrometry methods that could simplify the process for measuring NRTI phosphate concentrations in patients [129]. Although PBMCs are currently the only biological specimens in which NRTI phosphate concentrations can reasonably be measured, this should not discourage research because NRTI toxicities presumably occur at the mitochondrial level in tissues of nerves, the liver, fat, the pancreas, and so on. The data in this review were based on measurement of NRTI phosphate levels in PBMCs, and possible research avenues were identified on the basis of these data.

Second, the observations in this review highlight specific research questions that warrant high priority, because the clinical implications are important. For example, the strategy of postponing administration of ART to ART-naive patients as long as possible to circumvent toxicities—as is recommended by the current national guidelines—may in fact predispose these patients with a state of elevated biological activation to increased NRTI toxicity [1]. Investigations are needed to address a possible association between inflammatory cytokines and cellular NRTI pharmacology in patients. Drug-drug interaction studies involving HIV and hepatitis C virus treatments should also investigate whether IFN upregulates NRTI phosphorylation in patients. Studies are needed to determine whether NRTI triphosphate levels are higher in women than in men, which could also provide pharmacological insights into the predisposition to fluorouracil toxicity in women. If NRTIs can be widely supplied to developing countries, these same issues may become more important, because women constitute the majority of the HIV-infected population of sub-Saharan Africa, and ARTs would presumably be rationed to patients with more-advanced disease on the basis of need [130]. Ultimately, to provide the safest, most informed, and rational use of NRTIs for patients, additional studies of cellular NRTI pharmacology will be needed.

Acknowledgments

We thank Dr. Carlos Catalano and Dr. Dorie Hoody for helpful discussions.

References

1
US Public Health Service
Guidelines for the use of antiretroviral agents in HIV-infected adults and adolescents
  
2
Yeni
PG
Hammer
SM
Carpenter
CC
, et al.  . 
Antiretroviral treatment for adult HIV infection in 2002: updated recommendations of the International AIDS Society—USA Panel
JAMA
 , 
2002
, vol. 
288
 (pg. 
222
-
35
)
3
Powderly
WG
Long-term exposure to lifelong therapies
J Acquir Immune Defic Syndr
 , 
2002
, vol. 
29
 
Suppl 1
(pg. 
28
-
40
)
4
Brinkman
K
Kakuda
TN
Mitochondrial toxicity of nucleoside analogue reverse transcriptase inhibitors: a looming obstacle for long-term antiretroviral therapy?
Curr Opin Infect Dis
 , 
2000
, vol. 
13
 (pg. 
5
-
11
)
5
Lenert
LA
Feddersen
M
Sturley
A
Lee
D
Adverse effects of medications and trade-offs between length of life and quality of life in human immunodeficiency virus infection
Am J Med
 , 
2002
, vol. 
113
 (pg. 
229
-
32
)
6
Moyle
G
Clinical manifestations and management of antiretroviral nucleoside analog-related mitochondrial toxicity
Clin Ther
 , 
2000
, vol. 
22
 (pg. 
911
-
36
discussion, 898
7
Brinkman
K
Smeitink
JA
Romijn
JA
Reiss
P
Mitochondrial toxicity induced by nucleoside-analogue reverse-transcriptase inhibitors is a key factor in the pathogenesis of antiretroviral-therapy-related lipodystrophy
Lancet
 , 
1999
, vol. 
354
 (pg. 
1112
-
5
)
8
Carr
A
Cooper
DA
Adverse effects of antiretroviral therapy
Lancet
 , 
2000
, vol. 
356
 (pg. 
1423
-
30
)
9
Dalakas
MC
Peripheral neuropathy and antiretroviral drugs
J Peripher Nerv Syst
 , 
2001
, vol. 
6
 (pg. 
14
-
20
)
10
Chen
D
Misra
A
Garg
A
Lipodystrophy in human immunodeficiency virus-infected patients
J Clin Endocrinol Metab
 , 
2002
, vol. 
87
 (pg. 
4845
-
56
)
11
Dieterich
DT
Long-term complications of nucleoside reverse transcriptase inhibitor therapy
AIDS Read
 , 
2003
, vol. 
13
 (pg. 
176
-
84
187
12
Moyle
GJ
Datta
D
Mandalia
S
Morlese
J
Asboe
D
Gazzard
BG
Hyperlactataemia and lactic acidosis during antiretroviral therapy: relevance, reproducibility and possible risk factors
AIDS
 , 
2002
, vol. 
16
 (pg. 
1341
-
9
)
13
Boxwell
DE
Styrt
BA
Lactic acidosis (LA) in patients receiving nucleoside reverse transcriptors (NRTIs) [abstract 1284]
Program and abstracts of the 39th Interscience Conference on Antimicrobial Agents and Chemotherapy (San Francisco)
 , 
1999
Washington, DC
American Society for Microbiology
pg. 
496
 
14
John
M
Mallal
S
Hyperlactatemia syndromes in people with HIV infection
Curr Opin Infect Dis
 , 
2002
, vol. 
15
 (pg. 
23
-
9
)
15
Fortgang
IS
Belitsos
PC
Chaisson
RE
Moore
RD
Hepatomegaly and steatosis in HIV-infected patients receiving nucleoside analog antiretroviral therapy
Am J Gastroenterol
 , 
1995
, vol. 
90
 (pg. 
1433
-
6
)
16
Coghlan
ME
Sommadossi
JP
Jhala
NC
Many
WJ
Saag
MS
Johnson
VA
Symptomatic lactic acidosis in hospitalized antiretroviral-treated patients with human immunodeficiency virus infection: a report of 12 cases
Clin Infect Dis
 , 
2001
, vol. 
33
 (pg. 
1914
-
21
)
17
Hor
T
Deshayes
J
Banisadr
F
, et al.  . 
Concomitant ddI/d4T and IFN (standard or pegylated)/ribavirin treatments may induce a high risk of mitochondrial toxicity in HIV/HCV infected patients (ANRS HCO2-RIBAVIC Study) [abstract 1735]
Program and abstracts of the 42nd Interscience Conference on Antimicrobial Agents and Chemotherapy (San Diego)
 , 
2002
Washington, DC
American Society for Microbiology
18
Lafeuillade
A
Hittinger
G
Chadapaud
S
Increased mitochondrial toxicity with ribavirin in HIV/HCV coinfection
Lancet
 , 
2001
, vol. 
357
 (pg. 
280
-
1
)
19
Sauleda
S
Juarez
A
Esteban
JI
, et al.  . 
Interferon and ribavirin combination therapy for chronic hepatitis C in human immunodeficiency virus-infected patients with congenital coagulation disorders
Hepatology
 , 
2001
, vol. 
34
 (pg. 
1035
-
40
)
20
Harris
M
Tesiorowski
A
Chan
K
, et al.  . 
Lactic acidosis complicating antiretroviral therapy: frequency and correlates [abstract 34]
Antivir Ther
 , 
2000
, vol. 
5
 pg. 
31
 
21
Bonnet
F
Bonarek
M
Abridj
A
, et al.  . 
Severe lactic acidosis in HIV-infected patients treated with nucleosidic reverse transcriptase analogs: a report of 9 cases [in French]
Rev Med Interne
 , 
2003
, vol. 
24
 (pg. 
11
-
6
)
22
Bonnet
F
Bonarek
M
Morlat
P
, et al.  . 
Risk factors for lactic acidosis in HIV-infected patients treated with nucleoside reverse-transcriptase inhibitors: a case-control study
Clin Infect Dis
 , 
2003
, vol. 
36
 (pg. 
1324
-
8
)
23
Kovacs
JA
Bechtel
C
Davey
RT
Jr
, et al.  . 
Combination therapy with didanosine and interferon-α in human immunodeficiency virus-infected patients: results of a phase I/II trial
J Infect Dis
 , 
1996
, vol. 
173
 (pg. 
840
-
8
)
24
Moore
RD
Wong
WM
Keruly
JC
McArthur
JC
Incidence of neuropathy in HIV-infected patients on monotherapy versus those on combination therapy with didanosine, stavudine and hydroxyurea
AIDS
 , 
2000
, vol. 
14
 (pg. 
273
-
8
)
25
Chen
X
Goudsmit
J
van der Kuyl
AC
Lack of correlation between length variation in the DNA polymerase γ gene CAG repeat and lactic acidosis or neuropathy during antiretroviral treatment
AIDS Res Hum Retroviruses
 , 
2002
, vol. 
18
 (pg. 
531
-
4
)
26
Fischl
MA
Richman
DD
Saag
M
, et al.  . 
Safety and antiviral activity of combination therapy with zidovudine, zalcitabine, and two doses of interferon-α2a in patients with HIV. AIDS Clinical Trials Group Study 197
J Acquir Immune Defic Syndr Hum Retrovirol
 , 
1997
, vol. 
16
 (pg. 
247
-
53
)
27
Galli
M
Ridolfo
AL
Adorni
F
, et al.  . 
Body habitus changes and metabolic alterations in protease inhibitor-naive HIV-1-infected patients treated with two nucleoside reverse transcriptase inhibitors
J Acquir Immune Defic Syndr
 , 
2002
, vol. 
29
 (pg. 
21
-
31
)
28
Martinez
E
Mocroft
A
Garcia-Viejo
MA
, et al.  . 
Risk of lipodystrophy in HIV-1-infected patients treated with protease inhibitors: a prospective cohort study
Lancet
 , 
2001
, vol. 
357
 (pg. 
592
-
8
)
29
Currier
J
Carpenter
C
Daar
E
Kotler
D
Wanke
C
Identifying and managing morphologic complications of HIV and HAART
AIDS Read
 , 
2002
, vol. 
12
 (pg. 
114
-
9
124–5
30
Lichtenstein
KA
Delaney
KM
Armon
C
, et al.  . 
Incidence of and risk factors for lipoatrophy (abnormal fat loss) in ambulatory HIV-1-infected patients
J Acquir Immune Defic Syndr
 , 
2003
, vol. 
32
 (pg. 
48
-
56
)
31
Mallal
SA
John
M
Moore
CB
James
IR
McKinnon
EJ
Contribution of nucleoside analogue reverse transcriptase inhibitors to subcutaneous fat wasting in patients with HIV infection
AIDS
 , 
2000
, vol. 
14
 (pg. 
1309
-
16
)
32
Lichtenstein
KA
Ward
DJ
Moorman
AC
, et al.  . 
Clinical assessment of HIV-associated lipodystrophy in an ambulatory population
AIDS
 , 
2001
, vol. 
15
 (pg. 
1389
-
98
)
33
Chene
G
Angelini
E
Cotte
L
, et al.  . 
Role of long-term nucleoside-analogue therapy in lipodystrophy and metabolic disorders in human immunodeficiency virus-infected patients
Clin Infect Dis
 , 
2002
, vol. 
34
 (pg. 
649
-
57
)
34
Cossarizza
A
Mussini
C
Vigano
A
Mitochondria in the pathogenesis of lipodystrophy induced by anti-HIV antiretroviral drugs: actors or bystanders?
Bioessays
 , 
2001
, vol. 
23
 (pg. 
1070
-
80
)
35
Boufassa
F
Dulioust
A
Lascaux
AS
, et al.  . 
Lipodystrophy in 685 HIV-1-treated patients: influence of antiretroviral treatment and immunovirological response
HIV Clin Trials
 , 
2001
, vol. 
2
 (pg. 
339
-
45
)
36
Wurtz
R
Ceaser
S
Adipose redistribution in human immunodeficiency virus-seropositive patients: association with CD4 response
Clin Infect Dis
 , 
2000
, vol. 
31
 (pg. 
1497
-
8
)
37
Heath
KV
Hogg
RS
Chan
KJ
, et al.  . 
Lipodystrophy-associated morphological, cholesterol and triglyceride abnormalities in a population-based HIV/AIDS treatment database
AIDS
 , 
2001
, vol. 
15
 (pg. 
231
-
9
)
38
Thiebaut
R
Daucourt
V
Mercie
P
, et al.  . 
Lipodystrophy, metabolic disorders, and human immunodeficiency virus infection: Aquitaine cohort, France, 1999. Groupe d'Epidemiologie Clinique du Syndrome d'Immunodeficience Acquise en Aquitaine
Clin Infect Dis
 , 
2000
, vol. 
31
 (pg. 
1482
-
7
)
39
Mauss
S
Corzillius
M
Wolf
E
, et al.  . 
Risk factors for the HIV-associated lipodystrophy syndrome in a closed cohort of patients after 3 years of antiretroviral treatment
HIV Med
 , 
2002
, vol. 
3
 (pg. 
49
-
55
)
40
John
M
Nolan
D
Mallal
S
Antiretroviral therapy and the lipodystrophy syndrome
Antivir Ther
 , 
2001
, vol. 
6
 (pg. 
9
-
20
)
41
Rakotoambinina
B
Medioni
J
Rabian
C
Jubault
V
Jais
JP
Viard
JP
Lipodystrophic syndromes and hyperlipidemia in a cohort of HIV-1-infected patients receiving triple combination antiretroviral therapy with a protease inhibitor
J Acquir Immune Defic Syndr
 , 
2001
, vol. 
27
 (pg. 
443
-
9
)
42
Zylberberg
H
Nalpas
B
Pol
S
Brechot
C
Viard
JP
Is there a relationship between hepatitis C virus infection and antiretroviral-associated lipoatrophy?
AIDS
 , 
2000
, vol. 
14
 pg. 
2055
 
43
Stretcher
BN
Pesce
AJ
Frame
PT
Stein
DS
Pharmacokinetics of zidovudine phosphorylation in peripheral blood mononuclear cells from patients infected with human immunodeficiency virus
Antimicrob Agents Chemother
 , 
1994
, vol. 
38
 (pg. 
1541
-
7
)
44
Anderson
PL
Kakuda
TN
Kawle
S
Fletcher
CV
Antiviral dynamics and sex differences of zidovudine and lamivudine triphosphate concentrations in HIV-infected individuals
AIDS
 , 
2003
, vol. 
17
 (pg. 
2159
-
68
)
45
Hoggard
PG
Back
DJ
Intracellular pharmacology of nucleoside analogues and protease inhibitors: role of transporter molecules
Curr Opin Infect Dis
 , 
2002
, vol. 
15
 (pg. 
3
-
8
)
46
Stein
DS
Moore
KH
Phosphorylation of nucleoside analog antiretrovirals: a review for clinicians
Pharmacotherapy
 , 
2001
, vol. 
21
 (pg. 
11
-
34
)
47
Kakuda
TN
Pharmacology of nucleoside and nucleotide reverse transcriptase inhibitor-induced mitochondrial toxicity
Clin Ther
 , 
2000
, vol. 
22
 (pg. 
685
-
708
)
48
Van Rompay
AR
Johansson
M
Karlsson
A
Phosphorylation of nucleosides and nucleoside analogs by mammalian nucleoside monophosphate kinases
Pharmacol Ther
 , 
2000
, vol. 
87
 (pg. 
189
-
98
)
49
Dalakas
MC
Semino-Mora
C
Leon-Monzon
M
Mitochondrial alterations with mitochondrial DNA depletion in the nerves of AIDS patients with peripheral neuropathy induced by 2′,3′-dideoxycytidine (ddC)
Lab Invest
 , 
2001
, vol. 
81
 (pg. 
1537
-
44
)
50
Dalakas
MC
Illa
I
Pezeshkpour
GH
Laukaitis
JP
Cohen
B
Griffin
JL
Mitochondrial myopathy caused by long-term zidovudine therapy
N Engl J Med
 , 
1990
, vol. 
322
 (pg. 
1098
-
105
)
51
Shikuma
CM
Hu
N
Milne
C
, et al.  . 
Mitochondrial DNA decrease in subcutaneous adipose tissue of HIV-infected individuals with peripheral lipoatrophy
AIDS
 , 
2001
, vol. 
15
 (pg. 
1801
-
9
)
52
Johnson
AA
Ray
AS
Hanes
J
, et al.  . 
Toxicity of antiviral nucleoside analogs and the human mitochondrial DNA polymerase
J Biol Chem
 , 
2001
, vol. 
276
 (pg. 
40847
-
57
)
53
Martin
JL
Brown
CE
Matthews-Davis
N
Reardon
JE
Effects of antiviral nucleoside analogs on human DNA polymerases and mitochondrial DNA synthesis
Antimicrob Agents Chemother
 , 
1994
, vol. 
38
 (pg. 
2743
-
9
)
54
Lewis
W
Copeland
WC
Day
BJ
Mitochondrial DNA depletion, oxidative stress, and mutation: mechanisms of dysfunction from nucleoside reverse transcriptase inhibitors
Lab Invest
 , 
2001
, vol. 
81
 (pg. 
777
-
90
)
55
Robbins
BL
Wilcox
CK
Fridland
A
Rodman
J
Metabolism of tenofovir and didanosine in quiescent or stimulated human peripheral blood mononuclear cells
Pharmacotherapy
 , 
2003
, vol. 
23
 (pg. 
695
-
701
)
56
Perno
CF
Cooney
DA
Gao
WY
, et al.  . 
Effects of bone marrow stimulatory cytokines on human immunodeficiency virus replication and the antiviral activity of dideoxynucleosides in cultures of monocyte/macrophages
Blood
 , 
1992
, vol. 
80
 (pg. 
995
-
1003
)
57
Gao
WY
Shirasaka
T
Johns
DG
Broder
S
Mitsuya
H
Differential phosphorylation of azidothymidine, dideoxycytidine, and dideoxyinosine in resting and activated peripheral blood mononuclear cells
J Clin Invest
 , 
1993
, vol. 
91
 (pg. 
2326
-
33
)
58
Gao
WY
Agbaria
R
Driscoll
JS
Mitsuya
H
Divergent anti-human immunodeficiency virus activity and anabolic phosphorylation of 2′,3′-dideoxynucleoside analogs in resting and activated human cells
J Biol Chem
 , 
1994
, vol. 
269
 (pg. 
12633
-
8
)
59
Robbins
BL
Srinivas
RV
Kim
C
Bischofberger
N
Fridland
A
Anti-human immunodeficiency virus activity and cellular metabolism of a potential prodrug of the acyclic nucleoside phosphonate 9-R-(2-phosphonomethoxypropyl)adenine (PMPA), Bis(isopropyloxymethylcarbonyl)PMPA
Antimicrob Agents Chemother
 , 
1998
, vol. 
42
 (pg. 
612
-
7
)
60
Soler
C
Garcia-Manteiga
J
Valdes
R
, et al.  . 
Macrophages require different nucleoside transport systems for proliferation and activation
FASEB J
 , 
2001
, vol. 
15
 (pg. 
1979
-
88
)
61
Hoggard
PG
Kewn
S
Maherbe
A
, et al.  . 
Time-dependent changes in HIV nucleoside analogue phosphorylation and the effect of hydroxyurea
AIDS
 , 
2002
, vol. 
16
 (pg. 
2439
-
46
)
62
Kewn
S
Hoggard
PG
Sales
SD
Johnson
MA
Back
DJ
The intracellular activation of lamivudine (3TC) and determination of 2′-deoxycytidine-5′-triphosphate (dCTP) pools in the presence and absence of various drugs in HepG2 cells
Br J Clin Pharmacol
 , 
2000
, vol. 
50
 (pg. 
597
-
604
)
63
Dolce
V
Fiermonte
G
Runswick
MJ
Palmieri
F
Walker
JE
The human mitochondrial deoxynucleotide carrier and its role in the toxicity of nucleoside antivirals
Proc Natl Acad Sci U S A
 , 
2001
, vol. 
98
 (pg. 
2284
-
8
)
64
Sales
SD
Hoggard
PG
Sunderland
D
Khoo
S
Hart
CA
Back
DJ
Zidovudine phosphorylation and mitochondrial toxicity in vitro
Toxicol Appl Pharmacol
 , 
2001
, vol. 
177
 (pg. 
54
-
8
)
65
Stretcher
BN
Pesce
AJ
Frame
PT
Greenberg
KA
Stein
DS
Correlates of zidovudine phosphorylation with markers of HIV disease progression and drug toxicity
AIDS
 , 
1994
, vol. 
8
 (pg. 
763
-
9
)
66
Harris
M
Back
D
Kewn
S
Jutha
S
Marina
R
Montaner
JS
Intracellular carbovir triphosphate levels in patients taking abacavir once a day
AIDS
 , 
2002
, vol. 
16
 (pg. 
1196
-
7
)
67
Kewn
S
Hoggard
PG
Sales
SD
, et al.  . 
Development of enzymatic assays for quantification of intracellular lamivudine and carbovir triphosphate levels in peripheral blood mononuclear cells from human immunodeficiency virus-infected patients
Antimicrob Agents Chemother
 , 
2002
, vol. 
46
 (pg. 
135
-
43
)
68
Becher
F
Landman
R
Mboup
S
, et al.  . 
Monitoring of didanosine and stavudine intracellular triphosphorylated anabolite concentrations in HIV-infect patients
AIDS
 , 
2004
, vol. 
18
 (pg. 
181
-
7
)
69
Rousseau
FS
Kahn
JO
Thompson
M
, et al.  . 
Prototype trial design for rapid dose selection of antiretroviral drugs: an example using emtricitabine (Coviracil)
J Antimicrob Chemother
 , 
2001
, vol. 
48
 (pg. 
507
-
13
)
70
Rodriguez
JF
Rodriguez
JL
Santana
J
Garcia
H
Rosario
O
Simultaneous quantitation of intracellular zidovudine and lamivudine triphosphates in human immunodeficiency virus-infected individuals
Antimicrob Agents Chemother
 , 
2000
, vol. 
44
 (pg. 
3097
-
100
)
71
Salmon-Ceron
D
Lassalle
R
Pruvost
A
, et al.  . 
Interferon-ribavirin in association with stavudine has no impact on plasma human immunodeficiency virus (HIV) type 1 level in patients coinfected with HIV and hepatitis C virus: a CORIST-ANRS HC1 trial
Clin Infect Dis
 , 
2003
, vol. 
36
 (pg. 
1295
-
304
)
72
Pruvost
A
Theodoro
F
Hascoet
S
, et al.  . 
Development and validation of a direct LC/MS/MS assay for intracellular tenofovir-diphosphate in human PBMCs [abstract 837]
Antivir Ther
 , 
2003
, vol. 
8
 
Suppl 1
pg. 
420
 
73
Traut
TW
Physiological concentrations of purines and pyrimidines
Mol Cell Biochem
 , 
1994
, vol. 
140
 (pg. 
1
-
22
)
74
Broder
S
Pharmacodynamics of 2′,3′-dideoxycytidine: an inhibitor of human immunodeficiency virus
Am J Med
 , 
1990
, vol. 
88
 (pg. 
2
-
7
)
75
Yarchoan
R
Pluda
JM
Thomas
RV
, et al.  . 
Long-term toxicity/activity profile of 2′,3′-dideoxyinosine in AIDS or AIDS-related complex
Lancet
 , 
1990
, vol. 
336
 (pg. 
526
-
9
)
76
Fischl
MA
Parker
CB
Pettinelli
C
, et al.  . 
A randomized controlled trial of a reduced daily dose of zidovudine in patients with the acquired immunodeficiency syndrome. The AIDS Clinical Trials Group
N Engl J Med
 , 
1990
, vol. 
323
 (pg. 
1009
-
14
)
77
Skowron
G
Biologic effects and safety of stavudine: overview of phase I and II clinical trials
J Infect Dis
 , 
1995
, vol. 
171
 
Suppl 2
(pg. 
113
-
7
)
78
Browne
MJ
Mayer
KH
Chafee
SB
, et al.  . 
2′,3′-Didehydro-3′-deoxythymidine (d4T) in patients with AIDS or AIDS-related complex: a phase I trial
J Infect Dis
 , 
1993
, vol. 
167
 (pg. 
21
-
9
)
79
Moyle
G
Toxicity of antiretroviral nucleoside and nucleotide analogues: is mitochondrial toxicity the only mechanisms
Drug Saf
 , 
2000
, vol. 
23
 (pg. 
467
-
81
)
80
Currier
JS
Spino
C
Grimes
J
, et al.  . 
Differences between women and men in adverse events and CD4+ responses to nucleoside analogue therapy for HIV infection. The AIDS Clinical Trials Group 175 Team
J Acquir Immune Defic Syndr
 , 
2000
, vol. 
24
 (pg. 
316
-
24
)
81
Sloan
JA
Goldberg
RM
Sargent
DJ
, et al.  . 
Women experience greater toxicity with fluorouracil-based chemotherapy for colorectal cancer
J Clin Oncol
 , 
2002
, vol. 
20
 (pg. 
1491
-
8
)
82
Grem
JL
Cancer chemotherapy principles and practice
 , 
1990
Philadelphia
JB Lippincott
83
Barry
MG
Khoo
SH
Veal
GJ
, et al.  . 
The effect of zidovudine dose on the formation of intracellular phosphorylated metabolites
AIDS
 , 
1996
, vol. 
10
 (pg. 
1361
-
7
)
84
Hoggard
PG
Lloyd
J
Khoo
SH
, et al.  . 
Zidovudine phosphorylation determined sequentially over 12 months in human immunodeficiency virus-infected patients with or without previous exposure to antiretroviral agents
Antimicrob Agents Chemother
 , 
2001
, vol. 
45
 (pg. 
976
-
80
)
85
Barry
M
Wild
M
Veal
G
, et al.  . 
Zidovudine phosphorylation in HIV-infected patients and seronegative volunteers
AIDS
 , 
1994
, vol. 
8
 (pg. 
1
-
5
)
86
Rodman
JH
Robbins
B
Flynn
PM
Fridland
A
A systemic and cellular model for zidovudine plasma concentrations and intracellular phosphorylation in patients
J Infect Dis
 , 
1996
, vol. 
174
 (pg. 
490
-
9
)
87
Anderson
PL
Brundage
RC
Weller
D
Kawle
SP
Bushman
L
Fletcher
CV
The pharmacokinetics of zidovudine-triphosphate (ZDV-TP) in HIV-infected adults [abstract 6.3]
Program and abstracts of the 3rd International Workshop on Clinical Pharmacology of HIV Therapy (Washington, DC)
 , 
2002
Utrecht, The Netherlands
Virology Education
pg. 
6.3
 
88
Richman
DD
Fischl
MA
Grieco
MH
, et al.  . 
The toxicity of azidothymidine (AZT) in the treatment of patients with AIDS and AIDS-related complex: a double-blind, placebo-controlled trial
N Engl J Med
 , 
1987
, vol. 
317
 (pg. 
192
-
7
)
89
Hestdal
K
Aukrust
P
Muller
F
, et al.  . 
Dysregulation of membrane-bound tumor necrosis factor-α and tumor necrosis factor receptors on mononuclear cells in human immunodeficiency virus type 1 infection: low percentage of p75-tumor necrosis factor receptor positive cells in patients with advanced disease and high viral load
Blood
 , 
1997
, vol. 
90
 (pg. 
2670
-
9
)
90
Godfried
MH
van der Poll
T
Jansen
J
, et al.  . 
Soluble receptors for tumour necrosis factor: a putative marker of disease progression in HIV infection
AIDS
 , 
1993
, vol. 
7
 (pg. 
33
-
6
)
91
Finck
BN
Johnson
RW
Tumor necrosis factor-α regulates secretion of the adipocyte-derived cytokine, leptin
Microsc Res Tech
 , 
2000
, vol. 
50
 (pg. 
209
-
15
)
92
Martin-Romero
C
Santos-Alvarez
J
Goberna
R
Sanchez-Margalet
V
Human leptin enhances activation and proliferation of human circulating T lymphocytes
Cell Immunol
 , 
2000
, vol. 
199
 (pg. 
15
-
24
)
93
Kestens
L
Vanham
G
Vereecken
C
, et al.  . 
Selective increase of activation antigens HLA-DR and CD38 on CD4+ CD45RO+ T lymphocytes during HIV-1 infection
Clin Exp Immunol
 , 
1994
, vol. 
95
 (pg. 
436
-
41
)
94
Behbahani
H
Landay
A
Patterson
BK
, et al.  . 
Normalization of immune activation in lymphoid tissue following highly active antiretroviral therapy
J Acquir Immune Defic Syndr
 , 
2000
, vol. 
25
 (pg. 
150
-
6
)
95
Pluda
JM
Yarchoan
R
Smith
PD
, et al.  . 
Subcutaneous recombinant granulocyte-macrophage colony-stimulating factor used as a single agent and in an alternating regimen with azidothymidine in leukopenic patients with severe human immunodeficiency virus infection
Blood
 , 
1990
, vol. 
76
 (pg. 
463
-
72
)
96
Scadden
DT
Pickus
O
Hammer
SM
, et al.  . 
Lack of in vivo effect of granulocyte-macrophage colony-stimulating factor on human immunodeficiency virus type 1
AIDS Res Hum Retroviruses
 , 
1996
, vol. 
12
 (pg. 
1151
-
9
)
97
Corica
F
Allegra
A
Corsonello
A
, et al.  . 
Relationship between plasma leptin levels and the tumor necrosis factor-α system in obese subjects
Int J Obes Relat Metab Disord
 , 
1999
, vol. 
23
 (pg. 
355
-
60
)
98
Pernerstorfer-Schoen
H
Jilma
B
Perschler
A
, et al.  . 
Sex differences in HAART-associated dyslipidaemia
AIDS
 , 
2001
, vol. 
15
 (pg. 
725
-
34
)
99
Olsen
NJ
Kovacs
WJ
Gonadal steroids and immunity
Endocr Rev
 , 
1996
, vol. 
17
 (pg. 
369
-
84
)
100
Mynarcik
DC
McNurlan
MA
Steigbigel
RT
Fuhrer
J
Gelato
MC
Association of severe insulin resistance with both loss of limb fat and elevated serum tumor necrosis factor receptor levels in HIV lipodystrophy
J Acquir Immune Defic Syndr
 , 
2000
, vol. 
25
 (pg. 
312
-
21
)
101
Ledru
E
Christeff
N
Patey
O
de Truchis
P
Melchior
JC
Gougeon
ML
Alteration of tumor necrosis factor-α T-cell homeostasis following potent antiretroviral therapy: contribution to the development of human immunodeficiency virus-associated lipodystrophy syndrome
Blood
 , 
2000
, vol. 
95
 (pg. 
3191
-
8
)
102
Christeff
N
Melchior
JC
de Truchis
P
Perronne
C
Gougeon
ML
Increased serum interferon alpha in HIV-1 associated lipodystrophy syndrome
Eur J Clin Invest
 , 
2002
, vol. 
32
 (pg. 
43
-
50
)
103
Maher
B
Alfirevic
A
Vilar
FJ
Wilkins
EG
Park
BK
Pirmohamed
M
TNF-α promoter region gene polymorphisms in HIV-positive patients with lipodystrophy
AIDS
 , 
2002
, vol. 
16
 (pg. 
2013
-
8
)
104
Bastard
JP
Caron
M
Vidal
H
, et al.  . 
Association between altered expression of adipogenic factor SREBP1 in lipoatrophic adipose tissue from HIV-1-infected patients and abnormal adipocyte differentiation and insulin resistance
Lancet
 , 
2002
, vol. 
359
 (pg. 
1026
-
31
)
105
Ziccardi
P
Nappo
F
Giugliano
G
, et al.  . 
Reduction of inflammatory cytokine concentrations and improvement of endothelial functions in obese women after weight loss over one year
Circulation
 , 
2002
, vol. 
105
 (pg. 
804
-
9
)
106
Hohler
T
Kruger
A
Gerken
G
Schneider
PM
Meyer zum Buschenefelde
KH
Rittner
C
A tumor necrosis factor-alpha (TNF-α) promoter polymorphism is associated with chronic hepatitis B infection
Clin Exp Immunol
 , 
1998
, vol. 
111
 (pg. 
579
-
82
)
107
Hohler
T
Kruger
A
Gerken
G
Schneider
PM
Meyer zum Buschenfelde
KH
Rittner
C
Tumor necrosis factor alpha promoter polymorphism at position -238 is associated with chronic active hepatitis C infection
J Med Virol
 , 
1998
, vol. 
54
 (pg. 
173
-
7
)
108
Balzarini
J
Lee
CK
Herdewijn
P
De Clercq
E
Mechanism of the potentiating effect of ribavirin on the activity of 2′,3′-dideoxyinosine against human immunodeficiency virus
J Biol Chem
 , 
1991
, vol. 
266
 (pg. 
21509
-
14
)
109
Kimball
P
Elswick
RK
Shiffman
M
Ethnicity and cytokine production gauge response of patients with hepatitis C to interferon-α therapy
J Med Virol
 , 
2001
, vol. 
65
 (pg. 
510
-
6
)
110
Fletcher
CV
Acosta
EP
Henry
K
, et al.  . 
Concentration-controlled zidovudine therapy
Clin Pharmacol Ther
 , 
1998
, vol. 
64
 (pg. 
331
-
8
)
111
Rea
IM
McNerlan
SE
Alexander
HD
CD69, CD25, and HLA-DR activation antigen expression on CD3+ lymphocytes and relationship to serum TNF-α, IFN-γ, and sIL-2R levels in aging
Exp Gerontol
 , 
1999
, vol. 
34
 (pg. 
79
-
93
)
112
Fletcher
CV
Kawle
SP
Kakuda
TN
, et al.  . 
Zidovudine triphosphate and lamivudine triphosphate concentration-response relationships in HIV-infected persons
AIDS
 , 
2000
, vol. 
14
 (pg. 
2137
-
44
)
113
Carr
A
Samaras
K
Burton
S
, et al.  . 
A syndrome of peripheral lipodystrophy, hyperlipidaemia and insulin resistance in patients receiving HIV protease inhibitory
AIDS
 , 
1998
, vol. 
12
 (pg. 
51
-
8
)
114
Jones
S
Janneh
O
Maher
B
Khoo
S
Back
D
Pirmohamed
M
Altered TNF-α and IL-6 levels and antiadipogenic effects of anti-retorivirals on cultured adipocytes: possible mechanisms for their role in lipodystrophy in HIV-infected patients
Antivir Ther
 , 
2002
, vol. 
7
 pg. 
2
 
115
Hoggard
PG
Manion
V
Barry
MG
Back
DJ
Effect of protease inhibitors on nucleoside analogue phosphorylation in vitro
Br J Clin Pharmacol
 , 
1998
, vol. 
45
 (pg. 
164
-
7
)
116
Lewis
W
Haas
CP
Raidel
SM
, et al.  . 
Combined antiretroviral therapy causes cardiomyopathy and elevates plasma lactate in transgenic AIDS mice
Lab Invest
 , 
2001
, vol. 
81
 (pg. 
1527
-
36
)
117
Kleiner
DE
Gaffey
MJ
Sallie
R
, et al.  . 
Histopathologic changes associated with fialuridine hepatotoxicity
Mod Pathol
 , 
1997
, vol. 
10
 (pg. 
192
-
9
)
118
McKenzie
R
Fried
MW
Sallie
R
, et al.  . 
Hepatic failure and lactic acidosis due to fialuridine (FIAU), an investigational nucleoside analogue for chronic hepatitis B
N Engl J Med
 , 
1995
, vol. 
333
 (pg. 
1099
-
105
)
119
Cherry
CL
Wesselingh
SL
Nucleoside analogues and HIV: the combined cost to mitochondria
J Antimicrob Chemother
 , 
2003
, vol. 
51
 (pg. 
1091
-
3
)
120
Cherry
CL
McArthur
JC
Hoy
JF
Wesselingh
SL
Nucleoside analogues and neuropathy in the era of HAART
J Clin Virol
 , 
2003
, vol. 
26
 (pg. 
195
-
207
)
121
Cote
HC
Brumme
ZL
Craib
KJ
, et al.  . 
Changes in mitochondrial DNA as a marker of nucleoside toxicity in HIV-infected patients
N Engl J Med
 , 
2002
, vol. 
346
 (pg. 
811
-
20
)
122
Pardo
CA
McArthur
JC
Griffin
JW
HIV neuropathy: insights in the pathology of HIV peripheral nerve disease
J Peripher Nerv Syst
 , 
2001
, vol. 
6
 (pg. 
21
-
7
)
123
Kotler
DP
HIV infection and lipodystrophy
Prog Cardiovasc Dis
 , 
2003
, vol. 
45
 (pg. 
269
-
84
)
124
Fantuzzi
G
Faggioni
R
Leptin in the regulation of immunity, inflammation, and hematopoiesis
J Leukoc Biol
 , 
2000
, vol. 
68
 (pg. 
437
-
46
)
125
McGovern
B
Bica
I
Risk of HAART therapy in hepatitis C
Hepatology
 , 
2002
, vol. 
35
 pg. 
730
 
126
Hoggard
PG
Veal
GJ
Wild
MJ
Barry
MG
Back
DJ
Drug interactions with zidovudine phosphorylation in vitro
Antimicrob Agents Chemother
 , 
1995
, vol. 
39
 (pg. 
1376
-
8
)
127
Hoggard
PG
Kewn
S
Barry
MG
Khoo
SH
Back
DJ
Effects of drugs on 2′,3′-dideoxy-2′,3′-didehydrothymidine phosphorylation in vitro
Antimicrob Agents Chemother
 , 
1997
, vol. 
41
 (pg. 
1231
-
6
)
128
Brau
N
Update on chronic hepatitis C in HIV/HCV-coinfected patients: viral interactions and therapy
AIDS
 , 
2003
, vol. 
17
 (pg. 
2279
-
90
)
129
Becher
F
Pruvost
A
Goujard
C
, et al.  . 
Improved method for the simultaneous determination of d4T, 3TC and ddl intracellular phosphorylated anabolites in human peripheral-blood mononuclear cells using high-performance liquid chromatography/tandem mass spectrometry
Rapid Commun Mass Spectrom
 , 
2002
, vol. 
16
 (pg. 
555
-
65
)
130
Dabis
F
Ekpini
ER
HIV-1/AIDS and maternal and child health in Africa
Lancet
 , 
2002
, vol. 
359
 (pg. 
2097
-
104
)
Financial support: National Institutes of Health (grant RO1 AI33835 10).

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