HIV drug resistance worsens in 25% of patients during wait for genotype results

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Australian researchers have reported that drug resistance is likely to worsen in only a quarter of patients during the interval between blood being drawn for a resistance test and delivery of the result.

The risk appears to be greatest in patients with extensive prior treatment experience – especially in relation to acquiring new reverse transcriptase mutations.

The results, reported in the January edition of the Journal of Acquired Immune Deficiency Syndromes, come from a sub-study of the CREST study, a randomised comparison of genotyping and Virtual Phenotype. Patients were eligible to join the CREST study if they had viral load above 1,000 copies/ml and were prepared to change therapy on the basis of a resistance test result.

Glossary

drug resistance

A drug-resistant HIV strain is one which is less susceptible to the effects of one or more anti-HIV drugs because of an accumulation of HIV mutations in its genotype. Resistance can be the result of a poor adherence to treatment or of transmission of an already resistant virus.

p-value

The result of a statistical test which tells us whether the results of a study are likely to be due to chance and would not be confirmed if the study was repeated. All p-values are between 0 and 1; the most reliable studies have p-values very close to 0. A p-value of 0.001 means that there is a 1 in 1000 probability that the results are due to chance and do not reflect a real difference. A p-value of 0.05 means there is a 1 in 20 probability that the results are due to chance. When a p-value is 0.05 or below, the result is considered to be ‘statistically significant’. Confidence intervals give similar information to p-values but are easier to interpret. 

nucleoside

A precursor to a building block of DNA or RNA. Nucleosides must be chemically changed into nucleotides before they can be used to make DNA or RNA. 

phenotype

The phenotype of an organism is all of its observable characteristics, defined by the genotype and the environment.  

reverse transcriptase

A retroviral enzyme which converts genetic material from RNA into DNA, an essential step in the lifecycle of HIV. Several classes of anti-HIV drugs interfere with this stage of HIV’s life cycle: nucleoside reverse transcriptase inhibitors and nucleotide reverse transcriptase inhibitors (NRTIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs). 

The sub-study recruited 30 patients, 18 receiving protease inhibitor based therapy, eight receiving NNRTI-based therapy, two receiving triple nucleoside regimens and two receiving regimens containing drugs from all three classes.

The sub-study evaluated the changes in resistance profiles during the period between two blood samples, taken a median of 37 days apart.

Thirty per cent of patients (n= 9) experienced changes in NRTI resistance during this period. In two cases some mutations emerged whilst others disappeared, but two patients lost mutations without gaining any new ones. There was no clear pattern to the disappearance of mutations, but those who gained resistance mutations (n=5) had greater treatment experience (8.7 years vs 5 years, p=0.02). The majority of those who gained resistance mutations accumulated thymidine analogue-associated mutations that might compromise future response to abacavir, zidovudine (AZT) and stavudine (d4T).

The authors say that mutations probably disappeared because they were associated with prior therapies rather than the current regimen. For example, a patient who lost the M184V mutation during rebound was receiving a combination of ddI, d4T and nelfinavir, none of which would maintain selective pressure on the M184V mutation.

Only one protease change was observed, in a patient receiving nelfinavir. This individual acquired the M46I mutation, having already developed the L90M mutation.

The authors conclude “it is more unlikely than likely that significant mutations will be generated if the time for testing and review is kept to within approximately 5 weeks.”

References

Birch C et al. Limited evolution of HIV antiretroviral drug resistance-associated mutations during the performance of drug resistance testing. JAIDS 32: 57-61, 2003.