Merck Announces Positive Data from Phase 3 Trials that Show the Investigational, Once-Daily, Oral, Two-Drug Regimen of Doravirine/Islatravir (DOR/ISL) Maintained HIV-1 Viral Suppression at Week 48

DOR/ISL demonstrated non-inferiority and a similar safety profile to comparator antiretroviral therapies in adults with virologically suppressed HIV-1

RAHWAY, N.J.--(BUSINESS WIRE)-- Merck (NYSE: MRK), known as MSD outside of the United States and Canada, today announced the presentation of positive results from two pivotal Phase 3 trials of the investigational, once-daily, oral, two-drug regimen of doravirine/islatravir [DOR/ISL (100mg/0.25mg)] in adults with HIV-1 infection that is virologically suppressed on bictegravir/emtricitabine/tenofovir alafenamideⁱ [BIC/FTC/TAF (50mg/200mg/25mg)] in trial MK-8591A-052) or antiretroviral therapy [baseline antiretroviral therapy (bART)] in trial MK-8591A-051. In both trials, DOR/ISL met the primary efficacy success criterion for non-inferiority to comparator antiretroviral therapies and primary safety objectives at Week 48. The findings will be shared in late-breaking oral presentations at the 32nd Conference on Retroviruses and Opportunistic Infections (CROI) being held in San Francisco and were featured in a CROI press conference. Merck plans to begin submitting applications for marketing authorization to regulatory agencies by mid-2025.

In the double-blind trial MK-8591A-052 (Abstract #204A), results for the primary endpoint (HIV-1 RNA ≥50 copies/mL) showed that 1.5% of participants who switched to DOR/ISL had a viral load of ≥50 copies/mL at Week 48, compared to 0.6% on BIC/FTC/TAF (treatment difference 0.9%, 95% CI -1.9, 2.9). At Week 48, 91.5% of participants who switched to DOR/ISL maintained viral suppression (HIV-1 RNA <50 copies/mL) compared to 94.2% of participants who continued receiving BIC/FTC/TAF (treatment difference -2.6%, 95% CI -7.1, 2.6; secondary endpoint).

In the open-label trial MK-8591A-051 (Abstract #204B), results for the primary endpoint (HIV-1 RNA ≥50 copies/mL) showed that 1.4% of participants who received DOR/ISL had a viral load of ≥50 copies/mL at Week 48, compared to 4.9% on bART (treatment difference -3.6%, 95% CI -7.8, -0.8. At Week 48, 95.6% of participants who switched to DOR/ISL maintained viral suppression (HIV-1 RNA <50 copies/mL) compared to 91.9% of participants who continued on bART (treatment difference 3.7%, 95% CI -0.3, 8.9; secondary endpoint).

Across both trials, the safety profile of DOR/ISL was generally comparable to the comparator antiretroviral regimens, including BIC/FTC/TAF in MK-8591A-052. At Week 48, the mean percent change in total lymphocyte and CD4 counts were similar for DOR/ISL and comparator regimens. No treatment-emergent resistance to DOR or ISL was observed in either trial.

“Despite the availability of multiple daily antiretroviral therapies, the needs of people living with HIV are evolving. Many people living with HIV are older and also managing comorbidities, making it important to have daily treatment options that can help meet each person’s unique health needs,” said Professor Chloe Orkin, Dean for Healthcare Transformation, Queen Mary University of London, United Kingdom. “I’m excited to see that DOR/ISL has potential as a new daily treatment option for people living with HIV who may benefit from this two-drug regimen.”

Islatravir, Merck’s investigational nucleoside reverse transcriptase translocation inhibitor (NRTTI), blocks HIV-1 replication by multiple mechanisms including inhibition of reverse transcriptase translocation resulting in immediate chain termination and delayed chain termination from structural changes induced in the viral DNA.

“We are excited that DOR/ISL is the first two-drug regimen without an integrase inhibitor to demonstrate comparable efficacy and safety to the three-drug InSTI-based regimen, BIC/FTC/TAF, in a Phase 3 pivotal trial,” said Dr. Eliav Barr, senior vice president, head of global clinical development and chief medical officer, Merck Research Laboratories. “Merck has been a research pioneer in HIV for decades. These data and our work on the longer-acting islatravir-based therapies in our pipeline show our continued commitment to help find new options that address the evolving needs of people living with HIV.”

About the Phase 3 data from MK-8591A-052
MK-8591A-052 is a Phase 3, double-blind randomized, active-controlled, clinical trial to evaluate the efficacy and safety of a switch to investigational, oral, once-daily DOR/ISL (100mg/0.25mg) in adults with HIV-1 infection that has been virologically suppressed on BIC/FTC/TAF (50mg/200mg/25mg). The primary efficacy endpoint was the percentage of participants with HIV-1 RNA ≥50 copies/mL at Week 48 (non-inferiority margin 4%). In this trial, 513 adults with HIV-1 who had virologic suppression for three months or more on BIC/FTC/TAF, no history of treatment failure and no known resistance to DOR were randomized (2:1) and switched to DOR/ISL (n= 342) or continued treatment with BIC/FTC/TAF (n=171). The median age of participants was 47 years; 21.4% were assigned female sex at birth, 30.8% were Black or African American, and 22.8% were Hispanic or Latine. The median duration of BIC/FTC/TAF treatment prior to trial enrollment was 3.4 years (IQR 2.0-5.0).

Results for the primary efficacy endpoint showed that five participants (1.5%) treated with DOR/ISL and one participant (0.6%) in the BIC/FTC/TAF group had a viral load of ≥50 copies/mL at Week 48, demonstrating non-inferiority of DOR/ISL to BIC/FTC/TAF (treatment difference 0.9%, 95% CI -1.9, 2.9). The superiority criteria were not met. Results for a secondary endpoint, the proportion of individuals with HIV-1 RNA <50 copies/mL at Week 48, showed that participants who switched to treatment with DOR/ISL or continued BIC/FTC/TAF maintained comparable rates of viral suppression at Week 48 (91.5% on DOR/ISL vs. 94.2% on BIC/FTC/TAF (treatment difference -2.6%, 95% CI -7.1, 2.6). No treatment-emergent resistance to DOR or ISL was observed.

At Week 48, the mean percent change in total lymphocyte and CD4 counts were similar for DOR/ISL and BIC/FTC/TAF. There were identical rates of discontinuation for protocol-specified decreases in total lymphocyte and/or CD4 counts (two participants (0.6%) in the DOR/ISL group and one participant (0.6%) in the BIC/FTC/TAF group).

Drug-related adverse events (AEs) and discontinuations due to drug-related AEs were similar between groups (n=35, 10.2% for DOR/ISL and n=16, 9.4% for BIC/FTC/TAF; n=4, 1.2% for DOR/ISL and n=2, 1.2% for BIC/FTC/TAF, respectively). Rates of toxicity grade 3 or 4 AEs and serious AEs were similar for DOR/ISL and BIC/FTC/TAF (n=25, 7.3% for DOR/ISL and n=13, 7.6% for BIC/FTC/TAF; n=15, 4.4% for DOR/ISL and n=11, 6.4% for BIC/FTC/TAF, respectively). Mean change in weight from baseline to Week 48 was minimal (-0.03 kg for DOR/ISL versus 0.28 kg for BIC/FTC/TAF; difference -0.30 kg, 95% CI -1.13, 0.53). The most common AEs (>6% in either study arm) were arthralgia, COVID-19, nasopharyngitis, and fatigue. One participant on DOR/ISL discontinued due to a drug-related serious AE (immune thrombocytopenia). There were two cases of low-level hepatitis B (HBV) viremia (HBV DNA <50 IU/mL) with no antigenemia or elevated transaminases in the DOR/ISL group and no cases in the BIC/FTC/TAF group; there were no cases of clinical HBV reactivation.

About the Phase 3 data from MK-8591A-051
MK-8591A-051 is a Phase 3, open-label randomized, active-controlled, clinical trial evaluating the efficacy and safety of a switch to investigational, oral, once-daily DOR/ISL (100mg/0.25mg) in adults with HIV-1 infection that has been virologically suppressed using ART. The primary efficacy endpoint was percentage of participants with HIV-1 RNA ≥50 copies/mL at Week 48 (non-inferiority margin 4%). In this trial, 551 adults with HIV-1 RNA <50 copies/mL for three months or more on oral 2- or 3-drug ART, with no history of treatment failure and no known virologic resistance to DOR were randomized 2:1 and switched to DOR/ISL (n=366) or continued baseline antiretroviral therapy (bART) (n=185), stratified by bART regimen. The median age of participants was 51 years; 39.7% were assigned female sex at birth, 45.4% Black or African American and 14.5% Hispanic or Latine. At baseline, 64.2% were treated with an InSTI-based regimen, 30.3% with an NNRTI-based regimen, and 5.4% with a protease inhibitor (PI)-based regimen, with median duration on current ART of 3.8 years (IQR 2.0-6.3).

Results for the primary efficacy endpoint showed that five participants (1.4%) in the DOR/ISL group and nine participants (4.9%) in the bART group had a viral load of ≥50 copies/mL at Week 48, demonstrating non-inferiority of DOR/ISL to bART (treatment difference -3.6%, 95% CI -7.8, -0.8). Results for a secondary endpoint, the proportion of individuals with HIV-1 RNA <50 copies/mL at Week 48, showed that participants who switched to treatment with DOR/ISL or continued bART maintained comparable rates of viral suppression at Week 48 (95.6% on DOR/ISL vs. 91.9% on bART; treatment difference 3.7%, 95% CI -0.3, 8.9). No treatment-emergent resistance to DOR or ISL was observed. Two participants discontinued DOR/ISL early after virologic failure at Week 4 with multiple resistance-associated mutations that were also present in baseline proviral DNA. These two participants were later found to be not eligible for the trial due to history of prior virologic failure and exclusionary DOR resistance.

At Week 48, the mean percent change in total lymphocyte and CD4 counts were similar for DOR/ISL and bART. No participants discontinued treatment due to decrease in total lymphocyte and/or CD4 counts.

In this open-label study, drug-related AEs were more commonly reported with DOR/ISL (n=44; 12.0%) than bART (n=9; 4.9%). Rates of toxicity grade 3 or 4 AEs and serious AEs were similar for DOR/ISL and bART (n=39, 10.7% for DOR/ISL and n=18, 9.7% for bART and n=23, 6.3% for DOR/ISL and n=9, 4.9% for bART, respectively). There were no drug-related serious AEs and there were no discontinuations due to serious AEs in the DOR/ISL group; there was one drug-related serious AE and two discontinuations due to serious AEs in the bART group. The most common drug-related AEs were diarrhea (DOR/ISL 3.3%, bART 0%), fatigue (1.9%, 0.5%), dizziness (1.9%, 0.5%), abdominal distention (1.6%, 0%), weight increased (1.6%, 0%), and headache (1.6%, 1.1%).

Change in lipid parameters from baseline were similar between treatment groups for all bART strata. Mean change in weight from baseline to Week 48 was 0.94 kg for DOR/ISL and -0.18 kg for bART (difference -1.13 kg, 95% CI 0.31, 1.94). For baseline regimens without EFV or TDF, the difference in weight between DOR/ISL and bART was 0.82 kg (95% CI -0.22, 1.87). There was one case of low-level HBV viremia with no antigenemia or elevated transaminases in the DOR/ISL group and no cases in the bART group; there were no cases of clinical HBV reactivation.

Indications and usage for PIFELTRO^® (doravirine) and DELSTRIGO^® (doravirine, lamivudine, and tenofovir disoproxil fumarate) in the U.S.
PIFELTRO is indicated in combination with other antiretroviral (ARV) agents for the treatment of HIV-1 infection in adult patients with no prior ARV treatment history or to replace the current ARV regimen in those who are virologically suppressed (HIV-1 RNA less than 50 copies per mL) on a stable ARV regimen with no history of treatment failure and no known substitutions associated with resistance to doravirine.

DELSTRIGO is indicated as a complete regimen for the treatment of HIV-1 infection in adult patients with no prior ARV treatment history or to replace the current ARV regimen in those who are virologically suppressed (HIV-1 RNA less than 50 copies per mL) on a stable ARV regimen with no history of treatment failure and no known substitutions associated with resistance to the individual components of DELSTRIGO.

Selected Safety Information

Warning: Posttreatment Acute Exacerbation of Hepatitis B Virus (HBV) for DELSTRIGO
All patients with HIV-1 should be tested for the presence of HBV before initiating ARV therapy. Severe acute exacerbations of HBV have been reported in people with concomitant HIV-1 and HBV who have discontinued products containing lamivudine or tenofovir disoproxil fumarate (TDF), which are components of DELSTRIGO. Patients coinfected with HIV-1 and HBV who discontinue DELSTRIGO should be monitored with both clinical and laboratory follow-up for at least several months after stopping DELSTRIGO. If appropriate, initiation of anti-HBV therapy may be warranted.

Contraindications
PIFELTRO and DELSTRIGO are contraindicated when coadministered with drugs that are strong cytochrome P450 (CYP)3A enzyme inducers (including the anticonvulsants carbamazepine, oxcarbazepine, phenobarbital, and phenytoin; the androgen receptor inhibitor enzalutamide; the antimycobacterials rifampin and rifapentine; the cytotoxic agent mitotane; and the herbal product St. John’s wort (Hypericum perforatum)), as significant decreases in doravirine plasma concentrations may occur, which may decrease the effectiveness of DELSTRIGO and PIFELTRO.

DELSTRIGO is contraindicated in patients with a previous hypersensitivity reaction to lamivudine.

Warnings and Precautions
Severe Skin Reactions
Severe skin reactions, including Stevens-Johnson syndrome (SJS)/toxic epidermal necrolysis (TEN), have been reported during the postmarketing experience with doravirine-containing regimens. Discontinue PIFELTRO or DELSTRIGO, and other medications known to be associated with severe skin reactions, immediately if a painful rash with mucosal involvement or a progressive severe rash develops. Clinical status should be closely monitored, and appropriate therapy should be initiated.

New or Worsening Renal Impairment
Renal impairment, including cases of acute renal failure and Fanconi syndrome, have been reported with the use of TDF. DELSTRIGO should be avoided with concurrent or recent use of a nephrotoxic agent (eg, high-dose or multiple NSAIDs). Cases of acute renal failure after initiation of high-dose or multiple NSAIDs have been reported in people living with HIV with risk factors for renal dysfunction who appeared stable on TDF.

Prior to or when initiating DELSTRIGO, and during treatment, assess serum creatinine, estimated creatinine clearance, urine glucose, and urine protein in all patients. In patients with chronic kidney disease, also assess serum phosphorus. Discontinue DELSTRIGO in patients who develop clinically significant decreases in renal function or evidence of Fanconi syndrome. Discontinue DELSTRIGO if estimated creatinine clearance declines below 50 mL/min.

Bone Loss and Mineralization Defects
In clinical trials in adults living with HIV, TDF was associated with slightly greater decreases in bone mineral density (BMD) and increases in biochemical markers of bone metabolism. Serum parathyroid hormone levels and 1,25 Vitamin D levels were also higher. Cases of osteomalacia associated with proximal renal tubulopathy have been reported with the use of TDF. The effects of TDF-associated changes in BMD and biochemical markers on long-term bone health and future fracture risk in adults are unknown.

Immune Reconstitution Syndrome
Immune reconstitution syndrome can occur, including the occurrence of autoimmune disorders with variable time to onset, which may necessitate further evaluation and treatment.

Drug Interactions
Because DELSTRIGO is a complete regimen, coadministration with other antiretroviral medications for the treatment of HIV-1 infection is not recommended.

Coadministration of PIFELTRO with efavirenz, etravirine, or nevirapine is not recommended.

If DELSTRIGO is coadministered with rifabutin, take one tablet of DELSTRIGO once daily, followed by one tablet of doravirine (PIFELTRO) approximately 12 hours after the dose of DELSTRIGO.

If PIFELTRO is coadministered with rifabutin, increase PIFELTRO dosage to one tablet twice daily (approximately 12 hours apart).

Consult the full Prescribing Information prior to and during treatment for more information on potential drug-drug interactions.

Dosage and Administration/Specific Populations
Renal Impairment
Because DELSTRIGO is a fixed-dose combination tablet and the dosage of lamivudine and TDF cannot be adjusted, DELSTRIGO is not recommended in patients with estimated creatinine clearance less than 50 mL/min.

Adverse Reactions
The most common adverse reactions with DELSTRIGO (incidence ≥5%, all intensities) were dizziness (7%), nausea (5%), and abnormal dreams (5%). The most common adverse reactions with PIFELTRO (incidence ≥5%, all intensities) were nausea (7%), dizziness (7%), headache (6%), fatigue (6%), diarrhea (6%), abdominal pain (5%), and abnormal dreams (5%).

By week 96 in DRIVE-FORWARD, 2% of adult participants in the PIFELTRO group and 3% in the darunavir+ritonavir (DRV+r) group had adverse events leading to discontinuation of study medication.

By week 96 in DRIVE-AHEAD, 3% of adult participants in the DELSTRIGO group and 7% in the efavirenz (EFV)/emtricitabine (FTC)/TDF group had adverse events leading to discontinuation of study medication.

In DRIVE-FORWARD, mean changes from baseline at week 48 in LDL-cholesterol (LDL-C) and non-HDL-cholesterol (non-HDL-C) were pre-specified. LDL-C: -4.6 mg/dL in the PIFELTRO group vs 9.5 mg/dL in the DRV+r group. Non-HDL-C: -5.4 mg/dL in the PIFELTRO group vs 13.7 mg/dL in the DRV+r group. The clinical benefits of these findings have not been demonstrated.

In DRIVE-AHEAD, mean changes from baseline at week 48 in LDL-C and non-HDL-C were pre-specified. LDL-C: -2.1 mg/dL in the DELSTRIGO group vs 8.3 mg/dL in the EFV/FTC/TDF group. Non-HDL-C: -4.1 mg/dL in the DELSTRIGO group vs 12.7 mg/dL in the EFV/FTC/TDF group. The clinical benefits of these findings have not been demonstrated.

In DRIVE-SHIFT, mean changes from baseline at week 24 in LDL-C and non-HDL-C were pre-specified. LDL-C: -16.3 mg/dL in the DELSTRIGO group vs -2.6 mg/dL in the PI + ritonavir group. Non-HDL-C: -24.8 mg/dL in the DELSTRIGO group vs -2.1 mg/dL in the PI + ritonavir group. The clinical benefits of these findings have not been demonstrated.

In DRIVE-AHEAD, neuropsychiatric adverse events were reported in the three pre-specified categories of sleep disorders and disturbances, dizziness, and altered sensorium. Twelve percent of adult participants in the DELSTRIGO group and 26% in the EFV/FTC/TDF group reported neuropsychiatric adverse events of sleep disorders and disturbances; 9% in the DELSTRIGO group and 37% in the EFV/FTC/TDF group reported dizziness; and 4% in the DELSTRIGO group and 8% in the EFV/FTC/TDF group reported altered sensorium.

The safety of DELSTRIGO in virologically-suppressed adults was based on week 48 data from participants in the DRIVE-SHIFT trial. Overall, the safety profile in virologically-suppressed adult participants was similar to that in participants with no ARV treatment history.

Serum ALT and AST Elevations: In the DRIVE-SHIFT trial, 22% and 16% of participants in the immediate switch group experienced ALT and AST elevations greater than 1.25 X ULN, respectively, through 48 weeks on DELSTRIGO. For these ALT and AST elevations, no apparent patterns with regard to time to onset relative to switch were observed. One percent of participants had ALT or AST elevations greater than 5 X ULN through 48 weeks on DELSTRIGO. The ALT and AST elevations were generally asymptomatic, and not associated with bilirubin elevations. In comparison, 4% and 4% of participants in the delayed switch group experienced ALT and AST elevations of greater than 1.25 X ULN through 24 weeks on their baseline regimen.

Pregnancy/Breastfeeding
There is a pregnancy exposure registry that monitors pregnancy outcomes in individuals exposed to PIFELTRO or DELSTRIGO during pregnancy. Healthcare providers are encouraged to register patients by calling the Antiretroviral Pregnancy Registry (APR) at 1-800-258-4263.

Inform individuals with HIV-1 infection of the potential risks of breastfeeding, including: (1) HIV-1 transmission (in HIV-1–negative infants), (2) developing viral resistance (in HIV-1–positive infants), and (3) serious adverse reactions in a breastfed infant similar to those seen in adults.

About Islatravir (MK-8591) and Merck’s HIV Research
Islatravir (MK-8591) is Merck’s investigational nucleoside reverse transcriptase translocation inhibitor (NRTTI) under evaluation in multiple ongoing early and late-stage clinical trials in combination with other antiretrovirals for the treatment of HIV-1. Trials with islatravir are designed to offer different dosing options as potential daily and once-weekly treatments. In addition to the MK-8591A-051 and MK-8591A-052 trials, ongoing Phase 3 trials of daily DOR/ISL (100mg /0.25mg) include MK-8591A-053 in people with HIV who had not previously received treatment (treatment-naïve), and MK-8591A-054 evaluating open-label DOR/ISL (100 mg/0.25 mg) in individuals who participated in earlier Phase 3 trials of DOR/ISL (100 mg/0.75 mg). For an overview of Merck’s HIV treatment and prevention clinical development program, please click here.

Merck’s Commitment to HIV
For more than 35 years, Merck has been committed to scientific research and discovery in HIV leading to scientific breakthroughs that have helped change HIV treatment. Our work has been pioneering in the development of new options across multiple drug classes to help those impacted by HIV. Today, we are developing a series of antiviral options designed to help people manage HIV and protect people from HIV, with the goal of reducing the growing burden of infection worldwide. We want to ensure people are not defined by HIV and our work focuses on transformational innovations, collaborations with others in the global HIV community, and access initiatives aimed at the goal of helping to end the HIV epidemic for everyone.

About Merck
At Merck, known as MSD outside of the United States and Canada, we are unified around our purpose: We use the power of leading-edge science to save and improve lives around the world. For more than 130 years, we have brought hope to humanity through the development of important medicines and vaccines. We aspire to be the premier research-intensive biopharmaceutical company in the world – and today, we are at the forefront of research to deliver innovative health solutions that advance the prevention and treatment of diseases in people and animals. We foster a diverse and inclusive global workforce and operate responsibly every day to enable a safe, sustainable and healthy future for all people and communities. For more information, visit www.merck.com and connect with us on X (formerly Twitter), Facebook, Instagram, YouTube and LinkedIn.

Forward-Looking Statement of Merck & Co., Inc., Rahway, N.J., USA
This news release of Merck & Co., Inc., Rahway, N.J., USA (the “company”) includes “forward-looking statements” within the meaning of the safe harbor provisions of the U.S. Private Securities Litigation Reform Act of 1995. These statements are based upon the current beliefs and expectations of the company’s management and are subject to significant risks and uncertainties. There can be no guarantees with respect to pipeline candidates that the candidates will receive the necessary regulatory approvals or that they will prove to be commercially successful. If underlying assumptions prove inaccurate or risks or uncertainties materialize, actual results may differ materially from those set forth in the forward-looking statements.

Risks and uncertainties include but are not limited to, general industry conditions and competition; general economic factors, including interest rate and currency exchange rate fluctuations; the impact of pharmaceutical industry regulation and health care legislation in the United States and internationally; global trends toward health care cost containment; technological advances, new products and patents attained by competitors; challenges inherent in new product development, including obtaining regulatory approval; the company’s ability to accurately predict future market conditions; manufacturing difficulties or delays; financial instability of international economies and sovereign risk; dependence on the effectiveness of the company’s patents and other protections for innovative products; and the exposure to litigation, including patent litigation, and/or regulatory actions.

The company undertakes no obligation to publicly update any forward-looking statement, whether as a result of new information, future events or otherwise. Additional factors that could cause results to differ materially from those described in the forward-looking statements can be found in the company’s Annual Report on Form 10-K for the year ended December 31, 2024 and the company’s other filings with the Securities and Exchange Commission (SEC) available at the SEC’s Internet site (www.sec.gov).

Please see Prescribing Information for PIFELTRO (doravirine) at: https://www.merck.com/product/usa/pi_circulars/p/pifeltro/pifeltro_pi.pdf and Patient Information for PIFELTRO at: https://www.merck.com/product/usa/pi_circulars/p/pifeltro/pifeltro_ppi.pdf

Please see Prescribing Information for DELSTRIGO (doravirine, lamivudine, and tenofovir disoproxil fumarate) at: https://www.merck.com/product/usa/pi_circulars/d/delstrigo/delstrigo_pi.pdf and Patient Information for DELSTRIGO at: https://www.merck.com/product/usa/pi_circulars/d/delstrigo/delstrigo_ppi.pdf

ⁱbictegravir/emtricitabine/tenofovir alafenamide (BIKTARVY) is a registered trademark of Gilead Sciences, Inc.

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