Drug Database: Islatravir

What is islatravir?What is islatravir?

What is islatravir?

Islatravir is an investigational drug that is being studied to treat HIV.³

Islatravir belongs to a group of HIV drugs called nucleoside reverse transcriptase translocation inhibitors (NRTTIs). NRTTIs use several different methods to block an HIV enzyme called reverse transcriptase. By blocking reverse transcriptase, NRTTIs prevent HIV from multiplying and can reduce the amount of HIV in the body.⁸

Islatravir may be effective against certain HIV strains that are resistant to other HIV drugs.⁹

To learn about how investigational drugs are tested during clinical trials, read the HIVinfo  What is an Investigational HIV Drug? and HIV and AIDS Clinical Trials fact sheets.

Select clinical trials of islatravirSelect clinical trials of islatravir

Select clinical trials of islatravir

On December 13, 2021, the U.S. Food and Drug Administration (FDA) placed clinical holds on studies of islatravir for HIV treatment and prevention. The FDA’s decision was based on reports of decreases in total lymphocyte and CD4 counts in some participants receiving islatravir in trials. Please refer to the drug developer’s December 13, 2021 press release for more information about the islatravir clinical hold.¹⁰

Subsequently, on September 20, 2022, the drug developer announced initiation of a new Phase 3 program evaluating a once-daily oral combination of doravirine and a lower dose of islatravir (DOR/ISL) for HIV treatment. Once-daily oral treatment studies of doravirine/islatravir that use doses higher than what will be studied in the new Phase 3 program remain under a partial clinical hold. The development of once-monthly oral islatravir for pre-exposure prophylaxis (PrEP) has been discontinued. Please refer to this September 20, 2022 press release for more information.⁵

Islatravir for HIV treatment

Study Names: MK-8591-011; NCT03272347

Phase: 2b
Status: This study has been completed.
Locations: Chile, France, United Kingdom, United States
Purpose: The purpose of this clinical trial was to evaluate the safety and effectiveness of a treatment regimen consisting of islatravir (given at three different dose levels) plus doravirine (brand name: Pifeltro) and lamivudine (brand name: Epivir) over 24 weeks in treatment-naive adults with HIV. After 24 weeks, investigators evaluated the safety and effectiveness of islatravir (given at three different dose levels) plus doravirine.¹¹
Selected Study Results: Results published in Lancet HIV (2021) showed that treatment regimens containing islatravir and doravirine were highly effective in reducing participants’ viral load levels.¹² Safety results through Week 144 of the study presented at EAC 2021 showed that there were few reports of discontinuations due to side effects associated with islatravir plus doravirine. Additionally, drug-related side effects occurred less frequently in the islatravir plus doravirine arms than in the control arm.¹³

Study Names: IMAGINE-DR; MK-8591-013; NCT04564547

Phase: 2b
Status: This study has been completed. (See note below.)
Locations: United States, France, Switzerland
Purpose: The purpose of this study was to evaluate the safety of oral weekly islatravir plus the investigational non-nucleoside reverse transcriptase inhibitor (NNRTI) MK-8507 (also known as ulonivirine) based on review of accumulated safety data in participants who had viral suppression on bictegravir/emtricitabine/tenofovir alafenamide (brand name: Biktarvy).^14,15
Note: The developers of islatravir announced in a November 18, 2021 press release that they were stopping dosing of participants in the MK-8591-013 trial. This decision was based on findings of reduced total lymphocyte and CD4 counts in study participants receiving islatravir and MK-8507.¹⁵ As of December 1, 2021, the MK-8591-013 study protocol was updated and all participants discontinued study treatment.¹⁴

Study Names: GS-US-563-6041; NCT05052996

Phase: 2
Status: This study is ongoing, but not recruiting participants.
Location: United States
Purpose: The purpose of this study is to evaluate the efficacy of oral weekly islatravir in combination with the capsid inhibitor lenacapavir in participants with viral suppression on Biktarvy.¹⁶
Selected Study Results: Results presented at CROI 2024 showed that participants switching to weekly oral islatravir plus lenacapavir maintained a high rate of viral suppression at Week 24, which was comparable to the rate of viral suppression in participants who remained on Biktarvy. Only one participant in the islatravir plus lenacapavir group had a detectable viral load measurement (above 50 copies/mL) at Week 24. Notably, this participant resuppressed at Week 30 on islatravir plus lenacapavir.¹⁷

Additional Published Material:

•  HIV Glasgow  2024: Once-weekly islatravir plus lenacapavir in virologically suppressed PWH: Week 48 safety, efficacy, and metabolic changes

Study Names: ILLUMINATE SWITCH A; MK-8591A-017; NCT04223778

Phase: 3
Status: This study has been completed.
Locations: Multiple countries, including United States
Purpose: The purpose of this trial was to evaluate the safety and effectiveness of a switch to a once-daily doravirine/islatravir fixed-dose combination tablet versus containing on a current ART regimen.¹⁸
Selected Study Results: Results presented at CROI 2023 and published in Lancet HIV (2024) indicated that doravirine/islatravir was as effective as current ART regimens in maintaining suppression of participants’ viral load levels through 48 weeks of treatment. The safety profile of doravirine/islatravir was similar to that of the current ART regimens. However, due to decreases in CD4 cell counts and total lymphocyte counts from baseline to Week 48, the development of the doravirine/islatravir fixed-dose combination tablet at the 100 mg/0.75 mg dose level was stopped.^19,20
Additional Published Material:

• EACS  2023: Switch to fixed-dose doravirine (100 mg) with islatravir (0.75 mg) once daily in adults with HIV-1 virologically suppressed on antiretroviral therapy: Week 96 results of a randomized, open-label, Phase 3 trial
• CROI 2024: Switching to doravirine/islatravir maintains viral suppression regardless of archived mutations

Study Names: ILLUMINATE SWITCH B; MK-8591A-018; NCT04223791

Phase: 3
Status: This study has been completed.
Locations: Multiple countries, including United States
Purpose: The purpose of this trial is to evaluate the safety and effectiveness of a switch to a once-daily doravirine/islatravir fixed-dose combination tablet versus continuing Biktarvy.²¹
Selected Study Results: Results presented at CROI 2023 and published in Lancet HIV (2024) indicated that doravirine/islatravir was as effective as Biktarvy in maintaining suppression of participants’ viral load levels through 48 weeks of treatment. The safety profile of doravirine/islatravir was similar to that of Biktarvy. However, due to decreases in CD4 cell counts and total lymphocyte counts from baseline to Week 48, the development of the doravirine/islatravir fixed-dose combination tablet at the 100 mg/0.75 mg dose level was stopped.^22,23
Additional Published Material:

• EACS 2023: Switch to fixed-dose doravirine/islatravir (100/0.75 mg) once daily in adults with HIV-1 virologically suppressed on bictegravir/emtricitabine/tenofovir alafenamide: Week 96 results of a Phase 3, randomized, double-blind, non-inferiority trial
• CROI 2024: Switching to doravirine/islatravir maintains viral suppression regardless of archived mutations

Study Names: ILLUMINATE HTE; MK-8591A-019; NCT04233216

Phase: 3
Status: This study has been completed.
Locations: Multiple countries, including United States
Purpose: The purpose of this trial was to evaluate the safety and efficacy of islatravir, doravirine, and a fixed-dose combination tablet containing doravirine/islatravir, each compared to placebo, in heavily treatment-experienced participants.²⁴
Selected Study Results: Results presented at EACS 2023 showed that virologic response from baseline to Day 8 was greatest among participants in the doravirine/islatravir plus ART group. Notably, enrollment in the MK-8591A-019 study was stopped early because of findings of decreased CD4 cell counts and lymphocyte counts in other islatravir studies. Doravirine/islatravir was generally well tolerated through Week 49.²⁵

Study Names: ILLUMINATE NAIVE;  MK-8591A-020; NCT04233879

Phase: 3
Status: This study has been completed.
Locations: Multiple countries, including United States
Purpose: The purpose of this trial is to evaluate the safety and efficacy of a once-daily doravirine/islatravir fixed-dose combination tablet versus Biktarvy in treatment-naive participants.²⁶
Selected Study Results: Week 48 results presented at IAS 2023 showed that doravirine/islatravir was as effective as Biktarvy in suppressing viral load in treatment-naive participants. Treatment-related side effects occurred with similar frequency in both groups. More participants receiving doravirine/islatravir discontinued treatment due to a side effect than participants receiving Biktarvy.²⁷

Study Names: MK-8591A-051; NCT05631093

Phase: 3
Status: This study is ongoing, but not recruiting participants. 
Locations: Multiple countries, including United States
Purpose: The purpose of this trial is to evaluate the safety and efficacy of a switch to a once-daily doravirine/islatravir fixed-dose combination tablet versus continuing on a current ART regimen.²⁸
Selected Study Results: Week 48 results released in a company press release (December 2024) and presented at CROI 2025 indicated that doravirine/islatravir was as effective as current ART regimens in controlling viral load levels in virologically suppressed adults. The safety profile of doravirine/islatravir was generally comparable to current ART regimens.^29,30

Study Names: MK-8591A-052; NCT05630755

Phase: 3
Status: This study is ongoing, but not recruiting participants. 
Locations: Multiple countries, including United States
Purpose: The purpose of this trial is to evaluate the safety and efficacy of a switch to a once-daily doravirine/islatravir fixed-dose combination tablet versus continuing Biktarvy.³¹
Selected Study Results: Week 48 results released in a company press release (December 2024) and presented at CROI 2025 indicated that doravirine/islatravir was as effective as Biktarvy in controlling viral load levels in virologically suppressed adults. Doravirine/islatravir, however, was not superior over Biktarvy. The safety profile of doravirine/islatravir was generally comparable to Biktarvy.^29,32

Study Names: MK-8591A-053; NCT05705349

Phase: 3
Status: This study is ongoing, but not recruiting participants. 
Locations: Multiple countries, including United States
Purpose: The purpose of this trial is to evaluate the safety and efficacy of a once-daily doravirine/islatravir fixed-dose combination tablet versus Biktarvy in treatment-naive participants.³³

Study Names: MK-8591A-054; NCT05766501

Phase: 3
Status: This study is ongoing, but not recruiting participants.
Locations: Multiple countries, including United States
Purpose: The purpose of this trial is to evaluate the safety and tolerability of a once-daily doravirine/islatravir (lower dose) fixed-dose combination tablet in participants who have previously been treated in earlier clinical studies with doravirine/islatravir (higher dose).³⁴ 

Study Names: ISLEND-1; GS-US-563-5925; NCT06630286

Phase: 3
Status: This study is currently recruiting participants.
Locations: Multiple countries, including United States
Purpose: The purpose of this trial is to evaluate the efficacy of a switch to a once-weekly islatravir/lenacapavir fixed-dose combination tablet versus continuing Biktarvy.⁶

Study Names: ISLEND-2; GS-US-563-5926; NCT06630299

Phase: 3
Status: This study is currently recruiting participants.
Locations: Multiple countries, including United States
Purpose: The purpose of this trial is to evaluate the efficacy of a switch to a once-weekly islatravir/lenacapavir fixed-dose combination tablet versus continuing standard-of-care ART.⁷

For more details on the studies listed above, see the Health Professional version of this drug summary.

Additional trials evaluating doravirine/islatravir for HIV treatment have been or are being conducted, including:

• ILLUMINATE YOUTH (MK-8591A-028; NCT04295772): A Phase 2 study that evaluated once-daily doravirine/islatravir as an HIV treatment in pediatric participants who were virologically suppressed on ART or treatment-naive. This study has been completed.³⁵
• MK-8591A-033 (NCT04776252): A Phase 3 rollover study evaluating the safety of once-daily doravirine/islatravir in adult and pediatric participants who received doravirine/islatravir in a previous clinical trial. This study is ongoing, but not recruiting participants. (In December 2021, this study was placed on partial clinical hold.)^10,36

What side effects might islatravir cause?What side effects might islatravir cause?

What side effects might islatravir cause?

One goal of HIV research is to identify new drugs that have fewer side effects. The following side effects were observed in some of the studies of islatravir listed above.

MK-8591-011 (NCT03272347)

In this Phase 2b study, 8% of participants who received islatravir had a drug-related side effect. No serious drug-related side effects occurred with islatravir treatment. Two participants who received the highest dose of islatravir stopped treatment early because of a side effect; one participant experienced diarrhea, nausea, and vomiting, and one participant had an existing hepatitis B virus infection reactivate. Headache was more common in the combined islatravir groups than in the  control arm. Most of the cases of headache were mild, temporary, and unrelated to treatment.^11,12

GS-US-563-6041 (NCT05052996)

In this Phase 2 study, adults with viral suppression on Biktarvy were randomized to either weekly oral islatravir plus lenacapavir or continue daily Biktarvy. Treatment-related side effects, all of which were mild or moderate, occurred more frequently in participants receiving islatravir plus lenacapavir than in participants receiving Biktarvy. The most common treatment-related side effects with islatravir plus lenacapavir were dry mouth and nausea. There were no severe or life-threatening treatment-related side effects in either group.^16,17

ILLUMINATE SWITCH A (MK-8591A-017; NCT04223778)

In this Phase 3 trial, participants continued their current ART regimen or switched to a fixed-dose combination (FDC) containing doravirine/islatravir. Results through Week 48 showed that doravirine/islatravir had a similar safety profile to that of current ART regimens. Drug-related side effects occurred in 19.6% of doravirine/islatravir participants. The most common drug-related side effects associated with doravirine/islatravir were insomnia, abnormal dreams, headache, nausea, itching skin, and weight gain. A drug-related serious side effect—paranoia—occurred in one participant receiving doravirine/islatravir. Five participants discontinued doravirine/islatravir because of a drug-related side effect.^18,19

After Week 48, all participants received doravirine/islatravir. Most of the treatment-related side effects and discontinuations due to side effects that occurred after Week 48 were attributed to protocol-defined decreases in CD4 or total lymphocyte counts. Investigators determined that the decreases in CD4 and/or lymphocyte counts were generally not clinically significant.³⁷

ILLUMINATE SWITCH B (MK-8591A-018; NCT04223791)

In this Phase 3 trial, participants continued Biktarvy or switched to an FDC containing doravirine/islatravir. Results through Week 48 showed that doravirine/islatravir had a similar safety profile to that of Biktarvy. Drug-related side effects occurred in 10% of doravirine/islatravir participants. The most common drug-related side effect associated with doravirine/islatravir was nausea. Six participants discontinued doravirine/islatravir because of a drug-related side effect.^21,22

Results at Week 96 showed that a greater proportion of participants receiving doravirine/islatravir than participants receiving Biktarvy had treatment-related side effects and discontinued treatment because of side effects. Most of the treatment-related side effects and discontinuations due to side effects in the doravirine/islatravir group were attributed to protocol-defined decreases in CD4 and/or total lymphocyte counts. The decreases in CD4 and/or lymphocyte counts were not considered clinically significant.³⁸

ILLUMINATE HTE (MK-8591A-019; NCT04233216)

In Part 1 of this Phase 3 study, participants received either islatravir, doravirine, an FDC containing doravirine/islatravir, or placebo added on to a failing ART regimen. In Part 2 of the trial, all participants received doravirine/islatravir plus optimized background therapy. Results at Week 49 showed that 48.6% of participants in the study had a drug-related side effect, and the majority of these side effects were associated with doravirine/islatravir. One participant receiving doravirine/islatravir plus ART discontinued treatment due to a drug-related side effect. No drug-related serious side effects were reported. CD4 counts were only modestly different between treatment groups.^24,25,39 

ILLUMINATE NAIVE (MK-8591A-020; NCT04233879)

In this Phase 3 study, treatment-naive participants received either an FDC containing doravirine/islatravir or Biktarvy. Drug-related side effects occurred in 26% of doravirine/islatravir participants. The most common drug-related side effects that occurred with doravirine/islatravir included decreased lymphocyte count, headache, and diarrhea. There were no drug-related serious side effects associated with doravirine/islatravir. A higher proportion of participants discontinued treatment due to a side effect in the doravirine/islatravir group compared with the Biktarvy group. The higher rate of side effect-related discontinuations seen with doravirine/islatravir was due to protocol-required withdrawals for decreased CD4 counts or total lymphocyte counts.^27,40

MK-8591A-051 (NCT05631093)

This Phase 3 study evaluated a switch to a doravirine/islatravir FDC tablet versus continuing on a current ART regimen. Results at Week 48 showed a similar safety profile for doravirine/islatravir and current ART regimens. Drug-related side effects occurred more frequently in participants on doravirine/islatravir than in participants on current ART regimens. The most common drug-related side effects were diarrhea, fatigue, dizziness, swollen abdomen, increased weight, and headache. No drug-related serious side effects or discontinuations due to serious side effects occurred. The number of participants discontinuing treatment due to drug-related side effects was low in both groups. There were no discontinuations due to decreases in CD4 or total lymphocyte counts.³⁰

MK-8591A-052 (NCT05630755)

In this Phase 3 study, participants received either a doravirine/islatravir FDC tablet or continued Biktarvy. The safety profiles of doravirine/islatravir and Biktarvy were comparable through Week 48. Drug-related side effects occurred in 10.2% of doravirine/islatravir participants. In each group, 1.2% of participants discontinued treatment due to a drug-related side effect. One participant receiving doravirine/islatravir discontinued treatment because of a drug-related serious side effect—immune thrombocytopenic purpura. The percentage of participants discontinuing treatment due to declines in CD4 or total lymphocyte counts was the same in the doravirine/islatravir and Biktarvy groups.³² 

Because islatravir is still being studied, information on possible side effects of the drug is not complete. As testing of islatravir continues, additional information on possible side effects will be gathered.

Where can I get more information about clinical trials studying islatravir?Where can I get more information about clinical trials studying islatravir?

Where can I get more information about clinical trials studying islatravir?

More information about islatravir-related research studies is available from ClinicalTrials.gov. (The ClinicalTrials.gov search can be modified so that you can get results that better match your interests.)

Some clinical trials may be looking for volunteer participants. Your health care provider can help you decide whether participating in a clinical trial is right for you. For more information, visit NIH Clinical Research Trials and You.

ReferencesReferences

References

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6. Gilead Sciences. A Phase 3, randomized, double-blind, active-controlled study to evaluate a switch to an oral weekly islatravir/lenacapavir regimen in people with HIV-1 who are virologically suppressed on bictegravir/emtricitabine/tenofovir alafenamide (B/F/TAF). In: ClinicalTrials.gov. Bethesda (MD): National Library of Medicine (US). Registered on October 4, 2024. NLM Identifier: NCT06630286. Accessed April 25, 2025
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14. Merck Sharp & Dohme LLC. A Phase 2b, randomized, active-controlled, double-blind, dose-ranging clinical study to evaluate a switch to islatravir (ISL) and MK-8507 once-weekly in adults with HIV-1 virologically suppressed on bictegravir/emtricitabine/tenofovir alafenamide (BIC/FTC/TAF) once-daily. In: ClinicalTrials.gov. Bethesda (MD): National Library of Medicine (US). Registered on September 21, 2020. NLM Identifier: NCT04564547. Accessed April 25, 2025
15. Merck: Press release, dated November 18, 2021. Merck provides update on Phase 2 clinical trial of once-weekly investigational combination of MK-8507 and islatravir for the treatment of people living with HIV-1. Accessed April 25, 2025
16. Gilead Sciences. A Phase 2 randomized, open-label, active-controlled study evaluating the safety and efficacy of an oral weekly regimen of islatravir in combination with lenacapavir in virologically suppressed people with HIV. In: ClinicalTrials.gov. Bethesda (MD): National Library of Medicine (US). Registered on September 13, 2021. NLM Identifier: NCT05052996. Accessed April 25, 2025
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21. Merck Sharp & Dohme LLC. A Phase 3, randomized, active-controlled, double-blind clinical study to evaluate a switch to doravirine/islatravir (DOR/ISL) once-daily in participants with HIV- 1 virologically suppressed on bictegravir/emtricitabine/tenofovir alafenamide (BIC/FTC/TAF). In: ClinicalTrials.gov. Bethesda (MD): National Library of Medicine (US). Registered on January 8, 2020. NLM Identifier: NCT04223791. Accessed April 25, 2025
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24. Merck Sharp & Dohme LLC. A Phase 3, randomized, clinical study in HIV-1-infected heavily treatment-experienced participants evaluating the antiretroviral activity of blinded islatravir (ISL), doravirine (DOR), and doravirine/islatravir (DOR/ISL), each compared to placebo, and the antiretroviral activity, safety, and tolerability of open-label DOR/ISL. In: ClinicalTrials.gov. Bethesda (MD): National Library of Medicine (US). Registered on January 15, 2020. NLM Identifier: NCT04233216. Accessed April 25, 2025
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28. Merck Sharp & Dohme LLC. A Phase 3, randomized, active-controlled, open-label clinical study to evaluate a switch to doravirine/islatravir (DOR/ISL 100 mg/0.25 mg) once-daily in participants with HIV-1 who are virologically suppressed on antiretroviral therapy. In: ClinicalTrials.gov. Bethesda (MD): National Library of Medicine (US). Registered on November 18, 2022. NLM Identifier: NCT05631093. Accessed April 25, 2025
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