Nucleoside analogues and neuropathy in the era of HAART

Journal of Clinical Virology 26 (2003) 195 /207 www.elsevier.com/locate/jcv

Nucleoside analogues and neuropathy in the era of HAART Catherine L. Cherry a,b,*, Justin C. McArthur c, Jennifer F. Hoy a,b, Steven L. Wesselingh a,b a

Department of Infectious Diseases and Microbiology, The Alfred Hospital, Commercial Road, 3181 Prahran, Melbourne, Vic., Australia b Department of Medicine, Monash University, Melbourne, Australia c Departments of Neurology and Epidemiology, Johns Hopkins University, Baltimore, MD, USA

Abstract Background: Sensory neuropathies occur commonly in the setting of HIV infection. Sensory neuropathy (SN) is clearly associated with HIV itself, and in this context develops in association with increased macrophage activation in the peripheral nervous system. A clinically identical SN may also occur as a consequence of exposure to some HIV treatments. In this setting, impaired mitochondrial function is thought to play a role in the development of neurological dysfunction. Objective: This review explores the evidence for the neurotoxicity of HIV and HIV treatments, the effect of nucleoside reverse transcriptase inhibitors on mitochondria, and the likely associations between these. Conclusions: Dideoxynucleotide drugs are commonly associated with SN. The nucleoside reverse transcriptase inhibitors inhibit mitochondrial DNA synthesis and may thus exacerbate existing viral-induced nerve damage. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nucleoside analogues; Neuropathy; Mitochondrial DNA; HIV

1. Introduction

Abbreviations: AIDS, acquired immunodeficiency syndrome; AZT, zidovudine; ddC, zalcitabine; ddI, didanosine; d4T, stavudine; DDx, dideoxynucleoside; HAART, highly active antiretroviral therapy; HIV, human immunodeficiency virus; IL, interleukin; MAC, mycobacterium avium complex; MACS, multicenter AIDS cohort study; mtDNA, mitochondrial DNA; NRTIs, nucleoside reverse transcriptase inhibitors; PEP, post exposure prophylaxis; SN, sensory neuropathy; SRV, simian retrovirus; TNF, tumor necrosis factor. * Corresponding author. Tel.:
/61-3-9276-3955; fax:
/61-39276-2431 E-mail address: [email protected] (C.L. Cherry).

Sensory neuropathy (SN) has been recognised as one of the commonest neurological complications of HIV infection since the early 1980s. More recently, the prevalence of SN has increased in the developed world, at a time when improvements in HIV treatment have resulted in reductions in most other neurological manifestations of HIV (Sacktor et al., 2001). This rise has paralleled the emergence of a diverse range of treatment associated manifestations of HIV, including syndromes attributed to the effect of nucleoside reverse transcriptase inhibitors (NRTIs) on mitochondria. Here we review SN in the context of HIV and HIV

1386-6532/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 6 5 3 2 ( 0 2 ) 0 0 1 1 8 - X

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treatments, the effects of NRTIs on mitochondria, and the plausible associations between these.

2. HIV associated SN in the pre-HAART era SN was described as a complication of AIDS in the early 1980s, with initial descriptions written prior to the isolation and identification of HIV itself (Snider et al., 1983). Subsequently multiple groups have studied SN in people with HIV infection, including its prevalence and risk factors. Prevalence data have varied widely according to the population studied and the assessment tool used. Some groups have found SN present in more than 30% of patients studied (De La Monte et al., 1988; So et al., 1988), while others have described a prevalence as low as 1.5% (Barohn et al., 1993). The common finding among the various early studies of HIV-associated SN was that of a symmetrical, predominantly sensory, distal neuropathy that occurred most often as a manifestation of late-stage HIV/AIDS (Table 1).

3. Pathology Pathologically, SN is a length dependent process characterised by distal axonal degeneration (Tyor et al., 1995; Wulff et al., 2000). Loss of small, unmyelinated fibres occurs as a prominent feature, but large myelinated fibres are also eventually lost in more advanced cases. Infiltration by activated

macrophages is seen in peripheral nerves (Norton et al., 1997) and, to a lesser degree, in dorsal root ganglia. As with other types of SN, there is a reduction in epidermal nerve fibre density in the affected area (McCarthy et al., 1995; Holland et al., 1997; McArthur et al., 1998; Herrmann et al., 1999), allowing pathological diagnosis via the minimally invasive measurement of epidermal nerve fibre density on punch biopsy of skin (Fig. 1).

4. Pathogenesis Although the pathology of SN has been described, the pathogenesis of this common problem is not well understood. HIV infects the nervous system, with virus isolated from CSF and neural tissue (Ho et al., 1985). Direct infection of peripheral nerves by HIV may occur, with HIV DNA demonstrated in occasional dorsal root ganglia neurons from individuals with HIV-associated SN (Brannagan et al., 1997). However, the majority of HIV infection documented in neural tissue has been in cells of mononuclear lineage (An et al., 1999), and it is likely that HIV causes neurological disease, including SN, indirectly rather than by direct infection and destruction of neurons. Individuals with HIV associated neurological disease have increased macrophage numbers and evidence of activation in all areas of the nervous system, despite advanced immunosuppression

Table 1 Associations with HIV-associated SN in the pre-HAART era Population

Number studied

Associations with HIV SN

Reference

Inpatients with AIDS Air force recruits with HIV infection Outpatients with HIV infection Multi-centre AIDS Cohort study

37 798 94 1604

Longer systemic illness, Dementia Lower CD4 cell count MAC a infection b High viral load Lower CD4 cell count Older age b Prior AIDS b Use of antiretroviral drugs b

So et al., 1988 Barohn et al., 1993 Woolley et al., 1997 Bacellar et al., 1994

a b

MAC: Mycobacterium avium complex. Factors that remained significantly associated with HIV SN following multivariate analysis.

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myelopathy, and are not seen in HIV infected individuals without neurological disease (Tyor et al., 1995). Whatever the mechanism by which HIV causes SN, the early recognition of SN as a complication of HIV and the association of SN with prolonged HIV infection and advanced immunosuppression point to the importance of the virus itself in the development of this condition. This is supported by the occasional appearance of acute SN during primary HIV infection (Denning, 1988). Further evidence that HIV is important in the pathogenesis of SN is provided by studies showing abnormal peripheral nerve function, and even neuropathology in HIV infected individuals without overt evidence of neurological disease.

5. Sub-clinical neuronal dysfunction in the setting of HIV

Fig. 1. (a) Normal skin. Fifty mcm section stained with PGP 9.5 (a pan-axonal marker). Three nerve fibres are seen in the epidermis. b: Skin from a subject with severe HIV associated SN. Despite staining with PGP 9.5, no nerve fibres are visible in the epidermis.

(Wesselingh et al., 1994; Tyor et al., 1995). It is postulated that neuronal dysfunction may result from the excessive production of neurotoxic cytokines by activated macrophages. A recent study in cultured rat dorsal root ganglia cells demonstrated that chemokines as well as gp120 may act directly on chemokine receptors on nociceptive neurons, and thus potentially play a role in causing pain in HIV associated SN as well as other syndromes (Oh et al., 2001). Increased levels of TNFa and reduced levels of IL4 have been documented in patients with SN, as well as HIV dementia and vacuolar

Electrophysiological testing such as nerve conduction studies may be normal in the presence of clinically apparent SN from any cause, as these studies generally survey the function of large, myelinated fibres. Thermal threshold testing is a more sensitive diagnostic tool, with elevated thresholds typically demonstrated in the presence of most types of SN. In the setting of HIV the majority of individuals with symptoms suggestive of SN do have significantly elevated thermal thresholds. However HIV infected individuals without any symptoms or signs of SN perform less well on thermal threshold testing than HIV negative controls (Huengsberg et al., 1998; Bouhassira et al., 1999; Tagliati et al., 1999). The prognostic significance of the degree of functional neurological impairment (demonstrated by raised thermal thresholds) in asymptomatic individuals with HIV is not known. It is possible that this is a sub-clinical manifestation of early neuropathology, and that those with more abnormal thermal thresholds are at highest risk of subsequently developing SN. A longitudinal study examining this issue is currently underway. The association between SN and advanced HIV disease suggests that there may be a sub-clinical progression of neuropathology prior to the onset of

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symptoms. Post mortem data supports this, with evidence of varying degrees of peripheral nerve pathology present in 100% of those dying with AIDS, although only one third had been documented to have SN symptoms during life (Griffin et al., 1991). 5.1. The impact of antiretroviral therapy on SN The clear association between SN and advanced HIV infection raised early optimism that effective viral suppression and prevention of disease progression might also prevent the development of this disabling condition, or even allow existing cases of SN to regress. Anecdotal reports of clinical resolution of SN with the introduction of antiretroviral therapy supported this possibility (Dalakas et al., 1988; Markus and Brew, 1998). However, the introduction of effective antiretroviral agents has not led to a reduction in the incidence of HIV associated SN. The largest group of ambulatory patients with HIV to be longitudinally assessed for the presence of SN is men involved in the Multicenter AIDS Cohort Study (MACS) (Bacellar et al., 1994). Five year incidence data from this cohort demonstrated a significant and progressive rise in the incidence of SN between the years 1988 and 1992; a period during which highly active antiretroviral therapy (HAART) was not available, but many patients had had exposure to mono- or dual-therapy with zidovudine (AZT) and/or one or more of the dideoxynucleosides (DDx) (ddC, ddI and d4T). On univariate analysis, this study appeared to confirm previous findings of an increased risk of SN with lower CD4 cell counts and higher viral loads, as well as increasing age or a prior diagnosis of AIDS. However, on multivariate analysis, the only factors that independently predicted SN were age, AIDS and having used antiretroviral agents at any time. Thus, in the pre-HAART era there was evidence that the incidence of SN was increasing in the face of declining incidence of most other neurological complications of HIV infection, and that this increase might be attributable to the toxicity of HIV treatments (Bacellar et al., 1994). There was some prior evidence to point to SN as a potential complication of nucleoside analogue

treatment for other conditions. A dose limiting SN was seen in trials of vidarabine as a treatment for chronic hepatitis B (Garcia et al., 1987). SN may also occur when nucleoside analogues are used to treat haematological malignancies (Kornblau et al., 1993). Thus, despite the lack of excess SN with AZT exposure, prior experience with related compounds suggested that NRTIs might be associated with SN.

6. Zalcitabine Zalcitabine (ddC) was the second antiretroviral agent licensed for human use, and a toxic SN was noted as the dose limiting side effect of this agent in phase 1 clinical trials (Yarchoan et al., 1988; Moyle et al., 1998). Subsequent dose ranging studies demonstrated that toxic SN was universal with ddC in doses greater than 0.01 mg/kg 4 h, with a mean time to onset of 7/9 weeks, depending on the dose used. Even at lower doses (0.005 mg/ kg 4 h), SN evolved in one third of subjects at 26 weeks (Berger et al., 1993). The toxic SN seen with ddC therapy is often indistinguishable from HIV associated SN, but may be more abrupt in onset, and is often painful and rapidly progressive (Blum et al., 1996; Sadler and Nelson, 1997; Moyle et al., 1998). If ddC is ceased, resolution of pain occurs in around two thirds of patients (Blum et al., 1996; Moore et al., 2000), although abnormal clinical signs persist. In one study of patients with ddC induced SN, all abnormal clinical findings persisted at 14 months (Blum et al., 1996), suggesting that the neurological dysfunction associated with ddC related toxic SN is permanent, despite resolution of active symptoms in many patients.

7. Didanosine Didanosine (ddI) was the next DDx introduced into clinical studies. Initial trials of ddI were conducted using high doses of drug in patients with advanced HIV disease (Lambert et al., 1990; Valentine et al., 1990). SN was seen less frequently than had been observed in early trials of ddC, but

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remained a significant toxicity, occurring in more than 20% of all patients involved in one dose escalation study (Lambert et al., 1990) and in up to 51% of those given the highest doses. Subsequent reviews of ddI use have found that SN is much less common when therapeutic doses of ddI are used in patients with higher CD4 cell counts and without other underlying risk factors for SN, leading some authors to question whether ddI should be considered neurotoxic (Moyle et al., 1998; Kelleher et al., 1999). However, cumulative experience in large HIV treatment centres has found at least a modest increase in the prevalence of SN in patients who have received ddI (Moore et al., 2000; Cherry et al., 2001b) either singly or in combination.

8. Stavudine Stavudine (d4T) entered clinical trials in 1989 (Simpson et al., 1995). As with ddC, phase 1 trials in people with HIV demonstrated that SN was the dose limiting toxicity of d4T (Browne et al., 1993), and that the incidence of SN related to both the dose and duration of d4T used. High doses of d4T (around four times the currently recommended therapeutic dose) were associated with SN in more than 70% of study participants (Skowron, 1995). Studies utilizing doses of d4T similar to therapeutic doses (30 /40 mg bd) have found 1 year rates of SN of 12 /15% (Simpson et al., 1995) with more immunosuppressed individuals at highest risk. In most cases, the symptoms of SN improve soon after cessation of d4T (Petersen et al., 1995). As with other DDx related SN, improvement is often preceded by an initial period of worsening of symptoms, or ‘coasting’, that may last several weeks (Simpson et al., 1995; Sadler and Nelson, 1997; Moyle et al., 1998).

9. Use of combination DDx An initial report on the safety of combination use of ddI and d4T in varying doses in 86 patients suggested that SN was not more common than had previously been described with DDx monotherapy (Pollard et al., 1999). However, this study

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was carried out in-patients with high CD4 cell counts (between 200 and 500 cells per mm3) who were at low risk of developing SN. The available data from a non-trial clinical setting suggest that there is synergistic neurotoxicity when ddI and d4T are used in combination (Moore et al., 2000) (Fig. 2).

10. Use of DDx together with hydroxyurea The co-administration of hydroxyurea with ddI and d4T (aimed at increasing their short term antiretroviral activity by reducing the intracellular pool of deoxynucleotide derivatives) also increases the risk of SN (Moore et al., 2000; Rutschmann et al., 2000). This is most likely due to the increased cellular effect of DDx in the presence of hydroxyurea, as hydroxyurea itself has not been reported to cause SN despite its wide-spread use in cancer patients (Cepeda and Wilks, 2000).

11. Other associations with SN DDx are the only antiretroviral agents that have been associated with an increased risk of SN. With all DDx the risk of SN is increased in the presence of any underlying predisposition to neuropathy (Table 2), including diabetes mellitus, prior exposure to cytotoxic chemotherapy, alcoholism and vitamin B12 deficiency (Fichtenbaum et al., 1995; Simpson et al., 1995; Blum et al., 1996). Coinfection with hepatitis C, which has been associated with peripheral neuropathies either with or without cryoglobulinaemia (Tembl et al., 1999; Maisonobe et al., 2001), may also predispose to SN. The only HIV associated opportunistic infection that has repeatedly been associated with SN is Mycobacterium avium complex (Norton et al., 1997; Woolley et al., 1997). It seems likely that the well documented neurotoxicity of DDx may have contributed to the increasing prevalence of SN observed in the MACS cohort in the early 1990s, by which time ddC, ddI and d4T were all in use in some patients in the cohort (Bacellar et al., 1994). Individual drug regimens were not examined as risk factors

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Fig. 2. Rates of antiretroviral drug associated neuropathy seen at Johns Hopkins Hospital, according to dideoxynucleoside regimens (Moore et al., 2000)(ddI: didanosine, d4T: stavudine, HU: hydroxyurea).

for SN in the MACS cohort, but a systematic review of 1116 patients on the Johns Hopkins Hospital AIDS service covering the same period demonstrated a clear association between SN and DDx use (Moore et al., 2000) (Fig. 2). Data collected at the Alfred Hospital, Melbourne suggest that the prevalence of SN has risen even more dramatically in the last decade, and confirms that the prescription of DDx is independently associated with an increased risk of this condition (Cherry et al., 2001b).

infection may play a key role in the development of DDx associated SN. That is, exposure to DDx may unmask an otherwise mild or clinically silent HIV mediated SN. Although experience with DDx is limited in people without HIV, the absence of published reports of SN in the setting of DDx used in post exposure prophylaxis (PEP) lends some support to this concept. The 4 week recommended duration of PEP is somewhat less than the mean time to symptom onset with toxic SN. However, cases of SN were observed after only 1 week of DDx in trials of HIV treatment (Blum et al., 1996).

12. The role of HIV in DDx associated SN 13. Animal models of DDx associated SN Nucleoside analogues clearly have an association with SN, whether they are used to treat chronic viral infections (HIV and hepatitis B) or malignancies. However, the association between DDx use in the setting of HIV and SN is extraordinarily strong, particularly as these agents are otherwise much less cytotoxic than nucleosides used in oncological practice. This, together with the highest risk of toxic SN being confined to those with more advanced HIV infection (Table 2), raises the possibility that DDx may be only mildly neurotoxic in and of themselves, and that HIV

Attempts to find an animal model of toxic SN due to DDx have been almost universally unsuccessful. One group have described a rabbit model of ddC induced SN (Brinkman et al., 1998), but attempts to create a rodent model have been unrewarding. These have included a study in which 8 months of daily intraperitoneal injections of up to 100 mg/kg of ddC caused no behavioural or structural evidence of unmyelinated nerve fibre loss in rodents (unpublished data, Justin McArthur). Neither ddI nor d4T caused clinical,

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Table 2 Associations with an increased risk of SN in selected reviews and trials of DDx Population

Number Associations studied with SN

AIDS clinical trials group (ACTG) protocol 155 (ddC vs. AZT 79 vs. ddC/AZT in HIV-infected subjects with CD4 cell count B/300)

Retrospective review of SN in patients receiving various DDx regimens

Retrospective review of SN in patients exposed to either ddI or ddC

ACTG protocol 175 (double blind, placebo controlled trial of four different antiretroviral regimens)

Reference

ddC exposure

Blum et al., 1996

1116

Diabetes mellitus Weight loss while on ddC Use of combination

Moore et al., 2000

103

DDxa Use of hydroxyureaa Low CD4 cell counta Prior non-drug SNa Age /40 years (P /0.09)a Heavy ethanol Fichtenbaum et al., 1995

2467

Consumptiona Low vitamin B12 levela Prior symptoms of SNa Older agea

Simpson et al., 1998

Lower Karnofsky scorea Symptomatic HIV Diseasea Prior antiretroviral exposurea a

Factors that remained significantly associated with HIV SN following multivariate analysis.

electrophysiological or pathological evidence of peripheral neurotoxicity when administered in high doses to rabbits for 24 weeks (Warner et al., 1995). Pre-licensing studies of d4T included a study in which primates were exposed to d4T in doses as high as 600 mg/kg per day for 12 months and underwent detailed electrophysiological measurements and pathological studies to specifically assess neurotoxicity. No evidence of SN was seen (Schilling et al., 1995). While this may reflect species-specificity in susceptibility to DDx related neurotoxicity, it is also possible that DDx may be much less neurotoxic in the absence of HIV infection, macrophage activation, or immunosuppression. Further evidence for this latter possibility was seen in toxicity studies of ddC in macaques. Behaviour suggestive of SN was seen in three of six simian retrovirus (SRV) infected macaques after 12 days of high dose ddC therapy, but no evidence of SN was seen in any of six SRV negative

macaques treated with similar doses for up to 28 days (Tsai et al., 1989). 13.1. The effect of NRTIs on mitochondria Even before ddC had been licensed for human use outside clinical trials, this compound was noted to be cytotoxic in vitro, with delayed cell doubling of Molt-4F cells after 8 days of exposure to concentrations of ddC 5-fold lower than plasma levels reached in clinical trials (Chen and Cheng, 1989). Ongoing exposure of the cells to ddC resulted in a lowering of cellular mitochondrial DNA (mtDNA) content. Removal of ddC from the cell culture allowed normalisation of both the cell doubling time and mtDNA content. Subsequent studies have confirmed early hypotheses that NRTIs also inhibit DNA polymerase g-the enzyme responsible for replication of mtDNA (Lewis and Dalakas, 1995; Brinkman et

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al., 1999; Kakuda et al., 1999; Morris et al., 1999; Kakuda, 2000). Work using various cell lines has demonstrated a hierarchy of NRTIs in terms of potency of inhibition of polymerase g, with some differences seen depending on the cell line examined and the exposure concentration of NRTIs. ddC is the most potent inhibitor of this enzyme at therapeutic levels (Chen et al., 1991) and most authors have found both d4T and ddI to be significantly more potent in this regard than other NRTIs currently in use for the treatment of HIV (Kakuda, 2000). The in vivo effects of abnormal mitochondria are seen in a diverse range of inherited mitochondrial disorders. Features common to many of these syndromes include peripheral neuropathy, myopathies, pancreatic dysfunction and metabolic abnormalities including diabetes mellitus and lactic acidosis (Simon et al., 1999). The similarity between some of these syndromes and the observed side effects of the various NRTIs, combined with the known ability of NRTIs to inhibit mtDNA synthesis, has led many investigators to hypothesise that mitochondrial dysfunction underlies the pathogenesis of many NRTI toxicities (Brinkman et al., 1998, 1999; Kakuda et al., 1999; Morris et al., 1999; Simon et al., 1999). The SN seen with DDx is one such toxicity. It is certainly plausible that toxic SN is induced by mitochondrial dysfunction. Peripheral neuropathy is one of the most common features of the known mitochondrial disorders, and the NRTIs that have been associated with the development of toxic SN are ddC, d4T and ddI; the NRTIs that most potently inhibit mtDNA synthesis in vitro. However, the in vivo evidence to suggest that patients who develop SN when prescribed DDx are the same patients who have the greatest degree of mitochondrial dysfunction is conflicting. At least one group has found evidence of increased serum lactate levels in patients with DDx induced neuropathy (Brew et al., 2000), but results from a cross-sectional study in Melbourne did not confirm this finding (Cherry et al., 2001a). Others have described reduced serum levels of acetyl carnitine in the setting of DDx associated neuropathy (Famularo et al., 1997), and improvements in epidermal nerve fibre density as well as

the symptoms of neuropathy when patients are treated with high dose L-acetyl carnitine (Hart et al., 2000). Again, these findings were not confirmed in a more recent study that found normal levels of acetyl carnitine in HIV associated neuropathy patients (Simpson et al., 2001). 13.2. Measures of mitochondrial DNA in vivo There is significant interest in confirming the association between reduced tissue mtDNA and the subsequent development of clinical toxicities in patients treated with NRTIs. If reductions in mtDNA can be demonstrated prior to the onset of side effects, this would enable appropriate consideration of treatment changes before disabling drug toxicities occur. Plasma lactate measurements have traditionally been used as an indirect marker of mitochondrial dysfunction, and are useful in diagnosing the rare, serious metabolic complications of NRTI therapy (Carr et al., 2000). However, mild elevations of lactate are common in asymptomatic individuals on NRTIs (Lonergan et al., 2000), and were not predictive of subsequent toxicities in one cohort (John et al., 2001). Several laboratories have developed methods of directly quantifying cellular mtDNA content (Gahan et al., 2001) and are in the process of assessing the validity of these assays in the clinical context. Work undertaken at the Alfred Hospital, Melbourne, has demonstrated that mtDNA content in subcutaneous fat is significantly lower in patients currently prescribed at least one DDx compared with those who are either not currently on treatment, or those who are on treatment including only non-DDx NRTIs (Cherry et al., 2001a) (Fig. 3). Importantly, there was no reduction in mtDNA in subcutaneous fat in those who had prior exposure to DDx, suggesting that this effect may be reversible, at least in adipocytes. Another recent study examined mitochondria in nerve biopsies (Dalakas et al., 2001). Tissue from patients with HIV associated SN who had never been prescribed ddC was compared with tissue from those who had SN temporally related to ddC use. The authors found that both groups of patients had some degree of axonal degeneration,

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Fig. 3. Mitochondrial DNA (mtDNA) in fat according to current treatment status (Cherry et al., 2001a).

but that the ddC treated group additionally had abnormal appearing mitochondria in nerves and significantly reduced cellular mtDNA content. This lends some support to the hypothesis that mitochondrial toxicity is the mechanism of induction of SN by ddC, although studies on mitochondria in the nerves of ddC treated individuals without SN are needed to confirm this. The effect of NRTIs on neurons has been studied in vitro by exposing nerve growth factor primed PC-12 cells to a variety of these drugs. A marked, dose dependent inhibition of neurite regeneration was seen on exposure to any of ddI, d4T or ddC, but not with AZT or lamivudine (3TC) (Cui et al., 1997). An important observation in this study, however, was that the inhibition of neurite regeneration could not be explained solely by impaired mtDNA synthesis, because reduced mtDNA was seen with exposure to ddI and ddC but not with d4T. This raises the possibility that DDx may also affect other intracellular pathways.

14. Conclusions Overall, there is good evidence that HIV itself causes SN. Damage to peripheral nerves is probably an inevitable consequence of surviving several years with HIV infection, as demonstrated by the uniform finding of post-mortem neuropathology

in those who had died with AIDS at Johns Hopkins Hospital (Griffin et al., 1991). SN is extremely common with DDx therapy, particularly in high doses. Since the introduction of these drugs, SN occurs in more patients, and at higher CD4 cell counts (Cherry et al., 2001b). This can restrict the number of antiretroviral agents available in some patients, and symptoms may adversely affect adherence to prescribed medications in others. However, some patients tolerate years of DDx treatment without developing symptoms or signs of neuropathy. The fact that patients with more advanced immunosuppression are at increased risk of developing SN when prescribed DDx (Table 2) suggests that there may be an interaction between the virus and the drugs in causing this condition. NRTIs are known to inhibit mtDNA synthesis, with DDx being the most potent NRTIs in this respect. They may also act via other, as yet uncharacterised, intracellular pathways. Mitochondrial dysfunction may be an independent cause of neurological disease, as seen in inherited mitochondrial disorders, but it is also possible that a reduction in mtDNA ‘unmasks’ pre-existing subclinical nerve damage due to HIV. In this way, drugs that are otherwise well tolerated by the majority of people may appear to be profoundly neurotoxic because of the reduced neurological reserve of the population in which they are used.

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Fig. 4. Postulated interactions between HIV and DDx in the development of SN. 1 So et al. (1988), 2 Cherry et al. (2001b), 3 Pollard et al. (1999).

We propose that SN in the era of HAART is most often the product of the degree of existing viral induced nerve damage and the severity of drug induced mtDNA depletion (Fig. 4). This model suggests that the risk of a patient with HIV developing DDx associated SN may be estimated through detailed testing of peripheral nerve function prior to commencing DDx. When DDx are considered an essential component of HIV therapy in-patients with existing abnormalities of nerve function, it may be possible to monitor tissue mtDNA levels in order to more accurately define the individual’s risk of toxicity, enabling more informed treatment decisions. The serial collection of tissue for mtDNA quantification along with detailed clinical assessments (including quantitative sensory threshold testing) and lower limb epidermal nerve fibre density measurements is underway in a cohort of

patients at the Alfred Hospital to test this hypothesis.

Acknowledgements We are grateful to all the staff and patients who have contributed, and continue to take part in our work in this area. In particular, we would like to thank Michelle Gahan and Sharon Lewin for their roles in developing the mitochondrial DNA assay, and Katharine Costello, Anne Mijch, and Ian Woolley for their contributions to the design and implementation of our clinical studies of SN. CLC is supported by a Post Graduate Medical Research Scholarship from the NHMRC. Justin C. McArthur is supported by NS 26643 and NS 35619.

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