Even very low levels of pre-existing resistance increase risk of efavirenz failure

Very small amounts of transmitted drug resistance can increase the risk of efavirenz treatment failure, US investigators report in the March 1^st edition of the Journal of Infectious Diseases (now online).

The investigators used a testing technique called allele-specific PCR testing, which can detect resistant virus that comprises below 0.1% of the total viral population.

They found that when the Y181C mutation comprised just 0.03% of the total viral population, the risk of virological failure was increased threefold for adherent patients starting who started antiretroviral therapy with an efavirenz (Sustiva) containing regimen.

“More-sensitive resistance assays could improve the clinical management of HIV-infected subjects”, comment the investigators.

Antiretroviral treatment guidelines recommend the non-nucleoside reverse transcriptase (NNRTI) efavirenz (Sustiva, also in the combination pill Atripla) for patients starting HIV treatment.

A major drawback of efavirenz is its low barrier to resistance.

Approximately 10% of patients are infected with virus that is resistant to one or more anti-HIV drug. It is therefore recommended that a resistance test should be performed before a patient starts antiretroviral therapy.

However, current genotypic tests cannot detect resistance that comprises below 15%-20% of the total viral population.

Newer testing technology, such as the allele-specific PCR assay can detect much smaller populations of resistant virus.

However, it is uncertain if such low levels of resistance have an impact on the virological efficacy of HIV therapy.

To answer this question, investigators from the US ACTG 5095 study conducted a case-controlled study, using the allele-specific PCR assay to monitor for very low levels of resistance in efavirenz-treated patients with or without virological failure.

All the patients were taking antiretroviral treatment for the first time, with a combination that included efavirenz and two or three NRTIs.

Virological failure was defined as two consecutive viral load measurements above 200 copies/ml at least 16 weeks after treatment was started.

The allele-specific PCR test was used to check for minority populations of three NNRTI-associated resistance mutations. In each case, the assay had the capability of detecting very small minority populations: K103 N (AAC), 0.003%; K103N (AAT), 0.001%; and Y181C, 0.03%.

The study population comprised 322 patients, 178 of whom had experienced virological failure.

Amongst the patients in whom minority resistance populations were detected, the median level of K103N (AAC allele) was 0.012%; K103N (AAT allele), 0.013%; and Y181C, 0.06%.

Low levels of the Y181C mutation were found in 58% of those experiencing virological failure compared to 29% of those who did not, a significant difference (p = 0.01).

Detection of minority populations of Y181C was associated with an increased risk of virological failure (HR = 2.54; 95% CI, 1.53-4.20).

Unsurprisingly, poor adherence was also associated with lack of HIV control (HR = 2.30; 95% CI, 1.40-3.78).

Small minority populations of K103N were not associated with lack of virological control.

The investigators conducted further statistical analysis that took into account patient adherence to therapy. This showed that the presence of low levels of Y181C were associated with a significant increase in the risk of virological failure for adherent patients (HR = 3.45; 95% CI, 1.90-6.26, p < 0.001), but not amongst individuals with poorer adherence (p = 0.46).

“Detection of pre-existing minority Y181C mutants encoding NNRTI resistance was associated with a greater than three-fold increased risk of virological failure of initial antiretroviral therapy with efavirenz-based regimens in antiretroviral-naïve HIV-1-infected subjects with perfect treatment adherence”, write the investigators.

They add that this risk “persisted across subjects with diverse baseline characteristics…the risk magnitude was considerable and clinically significant.”

However, the investigators note that the use of allele-specific PCR assays in routine care “will require further technical improvement, a better understanding of the role of low-abundance resistant variants in different clinical scenarios, and refinement of assay thresholds that identify patients at increased risk of virologic failure.”