Ineffective Use of Oral Pre-exposure Prophylaxis in Women
Ineffective Use of Oral Pre-exposure Prophylaxis in Women
There were five cases (7%) of FTC-resistant infections among 68 seroconverters in the FEM-PrEP study detected using clinical assays, representing an overall prevalence that is similar to the prevalence of primary or transmitted resistance in Africa (1 to 12%). Analysis using highly sensitive assays identified one more case in the active arm, and three additional cases in the placebo arm. As with trials that showed evidence of oral FTC/TDF PrEP efficacy, this trial reveals no excess in the rate of occurrence of FTC resistance even when more sensitive assays are used.
Drug resistance can be transmitted between people or selected within a person by antiretroviral medications used for treatment or prophylaxis. One of the clinical FTC-resistant infections had several characteristics associated with recent drug selection: RT M184I was present and typically indicates recent selection by FTC or 3TC, the drug levels were among the highest observed in seroconverters, and the resistant mutant waned quickly in blood plasma (within 4 weeks) after PrEP was stopped. A second FTC-resistant infection also had moderate concentrations of drug and resistance isolated only to FTC, although the month-long delay in the waning of her RT M184V mutant is more typical of primary or transmitted resistance. The two other infections with clinical FTC resistance occurred with no recent drug exposure at the time of seroconversion, although one woman had missed her visit prior to seroconversion, so drug exposure during the infection window could not be ruled out. One infection had evidence of transmitted drug resistance to NNRTIs, indicating that these mutants were likely to have been transmitted rather than selected. There were no cases of TFV resistance.
Other PrEP trials indicated that the risk of drug resistance was limited to those in the viral RNA-positive/antibody-negative window period before starting PrEP. In the FEM-PrEP study, both FTC-resistant infections that occurred in the presence of detectable PrEP drug concentrations had detectable infection by week 4 of PrEP use. In these two participants, the very rapid appearance of systemic infection in the face of detectable concentrations of PrEP drugs suggests that infection had been incubating at the time that PrEP was started. In contrast to other studies, retrospective use of sensitive standard and investigational assays for viral RNA and DNA in blood specimens did not identify incubating infection in these cases. The sensitivity of such assays may have been impaired by processing, cryopreservation, and shipment required for remote testing. Alternatively, the infections may have been in the eclipse phase between viral exposure and the first expression of virus in the blood, thought to last for 4 to 11 days.
The clinical implications of the observed FTC resistance are not yet known. The RT M184V/I mutations primarily affect 3TC and FTC while increasing susceptibility to ZDV and TFV, and have no direct impact on the activity of protease inhibitors, NNRTIs, integrase inhibitors, entry inhibitors, or fusion inhibitors. The FTC-resistant viral mutants had diminished replication capacity, which has been associated with better long-term immunological responses to therapy and decreased rates of vertical transmission. The FTC-resistant mutations are present in untreated infections, as discerned by their rapid out-growth after starting 3TC or by deep sequencing that revealed their prevalence between 0.07 and 0.09%. Selective enrichment of these pre-existing mutants by PrEP, even if for a few months, may increase their eventual fitness and their prevalence in the virus population after PrEP is stopped. If so, the prior circulation of M184V/I mutants in these women, whether they were acquired by transmission or enriched by PrEP, could require use of additional active agents to assure the best chance for a treatment response. Use of such second-line regimens could increase costs of treatment and side effects for the patient.
The low frequency of drug resistance in the setting of PrEP failure contrasts with the experience from nevirapine (NVP) monotherapy to prevent mother to child transmission, in which resistance is detected in 25% of women using standard clinical assays that characterize the majority viral variant, and in nearly 90% of infants when sensitive assays for minor resistant variants are used. The relatively low occurrence of drug resistance in the setting of oral FTC/TDF PrEP reflects its use in HIV-uninfected people, its high efficacy when used consistently, and the high fitness barrier to TFV and FTC resistance. Seroconversions among those offered PrEP are associated with no or low level of exposure to the drugs; such low drug concentrations appear to be insufficient to select for FTC or TFV resistance in most people. Unlike NNRTI resistance which preserves viral replication capacity and persists after therapy is stopped, selected resistance to FTC impairs viral replication capacity and wanes after drug exposure ends, although likely it remains archived in tissue reservoirs.
The concentrations of drug in this trial were not sufficient to select for TFV resistance; TFV resistance was also not observed in nonhuman primate models. This suggests that the concentration of drug required to select for TFV resistance is higher than the level required for prevention of infection. In contrast, the trend toward more frequent appearance of clinical FTC resistance among women with moderate drug concentrations suggests that risk of clinical FTC resistance occurs primarily with higher levels of oral FTC/TDF use, especially if infection is incubating when PrEP is started (as previously observed). This finding is also consistent with nonhuman primate studies showing M184V mutations appearing when in some animals dosed daily with FTC, but not in animals receiving two doses per week.
Drug resistance was rare in the context of PrEP trials, even when adherence was low and sporadic, as in this study. In practice settings where follow-up and HIV testing may be less well controlled, drug resistance may occur more frequently. Ultimately, the risks of FTC resistance need to be weighed against the benefits of HIV prevention and engagement in care, and monitored during clinical practice.
Discussion
There were five cases (7%) of FTC-resistant infections among 68 seroconverters in the FEM-PrEP study detected using clinical assays, representing an overall prevalence that is similar to the prevalence of primary or transmitted resistance in Africa (1 to 12%). Analysis using highly sensitive assays identified one more case in the active arm, and three additional cases in the placebo arm. As with trials that showed evidence of oral FTC/TDF PrEP efficacy, this trial reveals no excess in the rate of occurrence of FTC resistance even when more sensitive assays are used.
Drug resistance can be transmitted between people or selected within a person by antiretroviral medications used for treatment or prophylaxis. One of the clinical FTC-resistant infections had several characteristics associated with recent drug selection: RT M184I was present and typically indicates recent selection by FTC or 3TC, the drug levels were among the highest observed in seroconverters, and the resistant mutant waned quickly in blood plasma (within 4 weeks) after PrEP was stopped. A second FTC-resistant infection also had moderate concentrations of drug and resistance isolated only to FTC, although the month-long delay in the waning of her RT M184V mutant is more typical of primary or transmitted resistance. The two other infections with clinical FTC resistance occurred with no recent drug exposure at the time of seroconversion, although one woman had missed her visit prior to seroconversion, so drug exposure during the infection window could not be ruled out. One infection had evidence of transmitted drug resistance to NNRTIs, indicating that these mutants were likely to have been transmitted rather than selected. There were no cases of TFV resistance.
Other PrEP trials indicated that the risk of drug resistance was limited to those in the viral RNA-positive/antibody-negative window period before starting PrEP. In the FEM-PrEP study, both FTC-resistant infections that occurred in the presence of detectable PrEP drug concentrations had detectable infection by week 4 of PrEP use. In these two participants, the very rapid appearance of systemic infection in the face of detectable concentrations of PrEP drugs suggests that infection had been incubating at the time that PrEP was started. In contrast to other studies, retrospective use of sensitive standard and investigational assays for viral RNA and DNA in blood specimens did not identify incubating infection in these cases. The sensitivity of such assays may have been impaired by processing, cryopreservation, and shipment required for remote testing. Alternatively, the infections may have been in the eclipse phase between viral exposure and the first expression of virus in the blood, thought to last for 4 to 11 days.
The clinical implications of the observed FTC resistance are not yet known. The RT M184V/I mutations primarily affect 3TC and FTC while increasing susceptibility to ZDV and TFV, and have no direct impact on the activity of protease inhibitors, NNRTIs, integrase inhibitors, entry inhibitors, or fusion inhibitors. The FTC-resistant viral mutants had diminished replication capacity, which has been associated with better long-term immunological responses to therapy and decreased rates of vertical transmission. The FTC-resistant mutations are present in untreated infections, as discerned by their rapid out-growth after starting 3TC or by deep sequencing that revealed their prevalence between 0.07 and 0.09%. Selective enrichment of these pre-existing mutants by PrEP, even if for a few months, may increase their eventual fitness and their prevalence in the virus population after PrEP is stopped. If so, the prior circulation of M184V/I mutants in these women, whether they were acquired by transmission or enriched by PrEP, could require use of additional active agents to assure the best chance for a treatment response. Use of such second-line regimens could increase costs of treatment and side effects for the patient.
The low frequency of drug resistance in the setting of PrEP failure contrasts with the experience from nevirapine (NVP) monotherapy to prevent mother to child transmission, in which resistance is detected in 25% of women using standard clinical assays that characterize the majority viral variant, and in nearly 90% of infants when sensitive assays for minor resistant variants are used. The relatively low occurrence of drug resistance in the setting of oral FTC/TDF PrEP reflects its use in HIV-uninfected people, its high efficacy when used consistently, and the high fitness barrier to TFV and FTC resistance. Seroconversions among those offered PrEP are associated with no or low level of exposure to the drugs; such low drug concentrations appear to be insufficient to select for FTC or TFV resistance in most people. Unlike NNRTI resistance which preserves viral replication capacity and persists after therapy is stopped, selected resistance to FTC impairs viral replication capacity and wanes after drug exposure ends, although likely it remains archived in tissue reservoirs.
The concentrations of drug in this trial were not sufficient to select for TFV resistance; TFV resistance was also not observed in nonhuman primate models. This suggests that the concentration of drug required to select for TFV resistance is higher than the level required for prevention of infection. In contrast, the trend toward more frequent appearance of clinical FTC resistance among women with moderate drug concentrations suggests that risk of clinical FTC resistance occurs primarily with higher levels of oral FTC/TDF use, especially if infection is incubating when PrEP is started (as previously observed). This finding is also consistent with nonhuman primate studies showing M184V mutations appearing when in some animals dosed daily with FTC, but not in animals receiving two doses per week.
Drug resistance was rare in the context of PrEP trials, even when adherence was low and sporadic, as in this study. In practice settings where follow-up and HIV testing may be less well controlled, drug resistance may occur more frequently. Ultimately, the risks of FTC resistance need to be weighed against the benefits of HIV prevention and engagement in care, and monitored during clinical practice.