Mitochondrial toxicity reports

17 October 2000. Related: Conference reports, Side effects, ICAAC 40th Toronto 2000.

Dr Mike Youle for HIV i-Base

It was fitting that the two days preceding the annual meeting of the American Society of Medicine in Toronto was given over to the adverse events caused by the current batch of HIV therapies. The abnormalities of fat metabolism and the multifarious presentations of mitochondrial damage were discussed in the suburban desolation of York Mills.

As is common for these workshop events much was then represented at the main ICAAC meeting downtown. However it is at the smaller meetings that often much of the ground work is undertaken for the next batch of studies both in the laboratory and in patients to more clearly define the problem and attempt to harness solutions. As at previous meetings dealing with these issues there were precious few of the latter. In this report news from both meetings will be amalgamated.

Whilst the lipid abnormalities and even those related to insulin resistance appear to be more clearly understood and studies with diet, exercise and the use of lipid lowering agents and drugs such as the glitazones hold hope for reversal of at least the fat accumulation and metabolic imbalance, the study of mitochondrial toxicity is in its infancy.

Professor Tony Schapira from the Royal Free Hospital, London, gave an excellent overview of mitochondria, their function and both primary and secondary dysfunction [1]. Earlier this year he had produced two articles in the Lancet that delineated both acquired and inherited mitochondrial dysfunction and in his lecture he showed that the study of these defects has increased in leaps and bounds in the past few years. What, however he also revealed was that our knowledge of the relative contributions of environmental and genetic factors in mitochondrial dysfunction is limited. Intriguing interactions between mitchondrial DNA (MtDNA) and the environment, including drugs are now emerging.

As an example the deafness due to the use of aminoglycoside antibiotics appears to only occur in those with a genetic predisposition for a particular mitochondrial damage in the ear. MtDNA mutations causing human disease include rearrangements of DNA with minor or major deletions of genetic material, depletion of the DNA and also damage to the RNA that does all the work within the cell. The spectrum of disease that can arise because of this is wide-ranging and includes diabetes, myopathy, encephalopathy, deafness, lipoma formation and cardiac damage.

The production of new MtDNA is by way of an enzyme called DNA-gamma polymerase that is inhibited by nucleoside reverse transcriptase inhibitors (NRTI’s). In fact, in his experiments Professor Schapira has used the available NRTI’s, specifically ddC (zalcitabine) and AZT (zidovudine) to create cells with little or no MtDNA to mimic disease state. He then went on to describe some of the potential studies that could be undertaken to clearly define the damage and defects induced by these agents. What is clear is that the area is awash with new techniques and technology that is both difficult to interpret and in its early stages of development.

Several presentations focused on the reduction of MtDNA in tissues that appear damaged in patients who have received antiretroviral treatment for HIV.

Walker and co-workers from Frankfurt examined subcutaneous fat from the buttocks of both treated and untreated patients and from HIV negative controls [AD06]. They quantified the MtDNA content of each sample using Southern blot technique. MtDNA content did not differ between HIV negative controls (N=8) and HIV patients never exposed to NRTI’s (N=4), p=0.9. However when this latter group was compared to patients treated with NRTI’s (N=19) there was a significant reduction in MtDNA content (p= 0.009). A significant reduction of 38% (p=0.04) of mtDNA was seen in those who had features of lipodystrophy.

Similar results were obtained by a US group where there was a reduction or absence of MtDNA in two thirds of subjects with clinical lipodystrophy whereas 20% of those with no lipodystrophic changes and HIV negative controls showed the reduction or absence of MtDNA [2].

However function of skeletal muscle did not seem to be compromised in a Danish study where oxidative and glycolytic capacity was assessed using ergometer cycling. Subjects with marked lipodystrophy were no more likely to increase lactate and pyruvate levels under exercise, although patients did have higher resting levels [3]. Measurement of MtDNA deletions in semen from men who had had greater than 18 months of HAART showed the presence of multiple deletions whereas those who had taken HAART for less than 18 months showed no deletions (p=0.01) [4]. Whilst MtDNA is inherited maternally this raises the question as to whether reduced fertility occurs in men on HAART and the functionality of the MtDNA in ova has yet to be assessed

A study of antioxidant rescue in mice treated with NRTI’s was presented by Jay Lenhard from Glaxo Wellcome, Research Triangle Park, NC, USA [5]. In this mice were treated with placebo or NRTI’s for 2 weeks with or without concomitant anti-oxidants, ascorbate and alpha-tocopherol. Whilst the presence of the NRTI’s clearly induced metabolic changes and damage to the oxidative phopshorylation pathway of energy production within the mitochondria, the presence of the antioxidants produced a marked 70% reduction in this damage.

The changes that occur in the levels of lactate in the blood in HIV disease are not yet well characterized with a reasonable proportion of all HIV patients having levels above the upper limit of normal. In one US study presented 14% of 68 consecutive patients tested showed this phenomenon [6]. However a small proportion of patients develop severe metabolic disorder denoted by raised lactate and a metabolic acidosis which leads to liver function abnormalities and respiratory failure. Brinkman and co-workers from the Netherlands showed information on 6 such subjects in whom they used B complex 4ml q12h and L-carnitine 1g q12h intravenously to try and reverse this condition [7]. Recovery of lactate to <3mmol/l was seen in all subjects between 3 and 30 days and there was only one death in a man who did not continue with these medications. Unfortunately there are no controlled data for these interventions but they are certainly worth considering in this life-threatening situation.

Lonergan and his group showed further information on the relative risk of developing lactic acidosis [8]. In 33 subjects who were characterized by raised lactate (median 4.5mmol/L), gastrointestinal symptoms 29/33 (25 nausea, 28 abdominal pain). The same proportion exhibited abnormal liver function, median alanine transaminase (ALT) 116mmol/L (normal <40mmol/L) although 7 had chronic hepatitis. One subjects ended up in intensive care but 20/22 without viral hepatitis showed normalization in a median of 52 days, whereas 32/33 had a serum lactate that returned to normal in a median of 49 days. The relative risk of a raised lactate when lamivudine/stavudine was the comparison was 3.2 for didanosine/stavudine (p=0.006) and 12.8 when all three agents had been taken (p<0.001).

References:

1. Schapira A et al. Primary and secondary causes of mitochondrial dysfunction. Abstract O5. 2nd Intl Workshop on Adverse Drug Interactions and Lipodystrophy, Toronto, Sep 2000.
2. Shikuma C et al. Subcutaneous adipose tissue mitochondrial DNA analysis from individuals with HAART-associated lipodystrophy. Abstract O7. 2nd Intl Workshop on Adverse Drug Interactions and Lipodystrophy, Toronto, Sep 2000.
3. Roge BT et al. Mitochondrial function in HIV-infected patients with lipodystrophy during exercise and recovery. Abstract P22. 2nd Intl Workshop on Adverse Drug Interactions and Lipodystrophy, Toronto, Sep 2000.
4. St John J. Multiple mtDNA deletions in human spermatozoa from long term HAART. Abstract P23. nd Intl Workshop on Adverse Drug Interactions and Lipodystrophy, Toronto, Sep 2000.
5. Paulik M et al. Anti-oxidants rescue NRTI-induced metabolic changes in AKR/J mice. Abstract O8. 2nd Intl Workshop on Adverse Drug Interactions and Lipodystrophy, Toronto, Sep 2000.
6. McCallister S et al. Clinical, virologic and immunologic significance of lactic acid levels in HIV+ adults. Abstract I-1301. 40th ICAAC, Toronto, Canada, Sep 2000.
7. Brinkman K et al. Treatment of lactic acidosis. Abstract P15. 2nd Intl Workshop on Adverse Drug Interactions and Lipodystrophy, Toronto, Sep 2000.
8. Lonergan JT. Hyperlactatemia associated with clinical manifestations in HIV-infected patients receiving nucleoside analogue combination regimens. Abstract P20. 2nd Intl Workshop on Adverse Drug Interactions and Lipodystrophy, Toronto, Sep 2000.