Back HIV Prevention Pre-exposure (PrEP) CROI 2016: Almost-Certain Case of Truvada PrEP Failure Due to Drug Resistance Reported

CROI 2016: Almost-Certain Case of Truvada PrEP Failure Due to Drug Resistance Reported

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A case report of a man in Toronto who became infected with a multidrug-resistant strain of HIV despite apparently very consistent adherence to PrEP using tenofovir/emtricitabine (Truvada) was presented at the 2016 Conference on Retroviruses and Opportunistic Infections this week in Boston.

[Produced in collaboration with Aidsmap.com]

David Knox, a doctor at Toronto’s Maple Leaf Medical clinic, said the patient was a 43-year-old gay man who had been on PrEP for 2 years. He had an HIV-positive partner who was undetectable on antiretroviral therapy, but also had casual sexual contacts involving a risk of HIV exposure.

The man was a regular attender at the clinic and tested for HIV on average every 3 months. It was suggested to him that he start Truvada PrEP in April 2013, and he appeared to have good adherence to it on the basis of the frequency of pharmacy refills.

In April 2015, 2 years later, he started having symptoms after a period of exposure to HIV with multiple partners. The symptoms were not classic HIV seroconversion symptoms, but involved an episode of fever with abdominal pain severe enough for him to go for hospital investigation, where a scan revealed an inflamed colon.

During this time he came in for his regular HIV test and it showed he had acute HIV infection, with a negative test for HIV antibodies but a positive test for the HIV p24 antigen, which shows up sooner. His HIV viral load 3 days later was 28,000 copies/mL -- rather low for acute HIV infection and suggesting that either his PrEP had "blunted" viral replication without stopping infection, or that the highly drug-resistant virus turned out to have paid the price for its resistance by replicating weakly.

The man was adamant that he maintained excellent adherence to PrEP, so Knox, by now concerned that he might be seeing a case of genuine PrEP failure, ordered more tests. One was a resistance test for all antiretroviral drug classes for the patient’s virus, from a sample taken a week after his HIV diagnosis.

The patient was treated within a cash-poor public health setting and his old blood samples had not been saved. So there was no way to directly prove that he had drug levels consistent with high adherence around the time of HIV exposure.

There was an indirect way to determine this, however. Knox analyzed a so-called dried blood spot (DBS) test from the patient taken 20 days after he was diagnosed with HIV. The point of a DBS test is that it measures drug levels inside red blood cells, rather than inside the white blood cells that HIV infects or in blood plasma.

Drug levels rise much more slowly inside red blood cells, taking 17 days to reach half of their steady-state levels, and a full 8 weeks to reach the drug-saturated steady state completely. These drug levels also rise steadily and are less susceptible to short-term peaks and troughs. Drug levels in the patient’s DBS were actually 47% higher than the average figure, suggesting consistent PrEP adherence for most of the period covering his exposure to HIV. If he had only been taking the drugs since he learned of his diagnosis, the drug levels would only be 47% of the average steady state level, or 31% of their actual level in this patient.

This is an indirect way of measuring drug levels. The onset of symptoms occurred 4 weeks before the patient’s HIV diagnosis, the dried blood test was taken over 3 weeks after, and the period of risk according to the patient started 2 weeks before the onset of symptoms, thus leaving 9 weeks for the drug to accumulate, so this test did not entirely rule out the possibility that he had been off PrEP at the time he took a risk and that this had prompted him to start taking it again, though the patient insists this is not the case.

There was a small blood sample left over from his diagnostic test, taken 3 days before the patient learned he had HIV. This revealed high levels of tenofovir and levels of emtricitabine so high they were above the test’s limit of quantification. However, this was not a test of long-term drug levels and again cannot completely rule out the possibility that the man had had a lapse in adherence around the time he was exposed to HIV.

Resistance testing showed that the man's HIV had no significant resistance to the protease inhibitor class of antiretrovirals. He had 1 resistance mutation to the first generation NNRTI drug nevirapine, and complete resistance to emtricitabine. He also had extensive resistance to the first-generation NRTI drugs like zidovudine (AZT) and stavudine (d4T), and these mutations also confer some resistance to tenofovir.

However he did not have the so-called K65R mutation that confers high-level resistance to tenofovir, and it was estimated that the resistance pattern he did have only confers 1.3-fold resistance to tenofovir, meaning that drug levels 30% higher than those needed for non-resistant virus should have been enough to prevent infection -- and he had much higher drug levels than this in the tests. Resistance, however is a complex process and some combinations of mutations can catalyze higher levels of resistance than they would produce alone.

Not relevant to the apparent PrEP failure, but to the spread of drug resistant virus was the fact that this patient also had 2 resistance mutations to the integrase inhibitor drugs and complete resistance to the drug elvitegravir, even though he presumably had never been exposed to these drugs.

Transmission of HIV with integrase inhibitor resistance is very rare, and especially resistance to drugs other than raltegravir (Isentress), the first drug in this class. The pattern of resistance observed is compatible with the unnamed person who passed on the virus being on a failing regimen of Stribild (the 2-class, 4-drug combination pill containing tenofovir, emtricitabine, elvitegravir, and cobicistat).

Given that 4 out of the 5 first-line HIV drug regimens recommended by the U.S. Department of Health and Human Services are integrase inhibitor-based, and that this drug class is being investigated for use as PrEP, it would be of concern if more integrase inhibitor-resistant virus started to circulate.

The patient himself was put on a potent 3-class regimen of dolutegravir (Tivicay), rilpivirine, and boosted darunavir (Prezista) and achieved undetectable viral load only 3 weeks after starting it.

In conclusion, this is probably not an absolutely clinching case -- one would need drug level samples taken at the time of infection for that. But on the balance of probabilities, with 3 different measures all supporting the patient’s self-report, this is probably the first documented case of the failure of Truvada PrEP despite high adherence and more-than-adequate drug levels, though 2 cases of HIV infection despite solo tenofovir were recently published.

It is not unexpected that there would be occasional cases of PrEP failure, but the fact that this is the first case report among the tens of thousands of people now taking Truvada PrEP shows that it is very rare.

2/26/16

Reference

DC Knox, DH Tan, PR Harrigan, et al. HIV-1 Infection With Multiclass Resistance Despite Preexposure Prophylaxis (PrEP). Conference on Retroviruses and Opportunistic Infections. Boston, February 22-25, 2016. Abstract 169aLB.