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Islatravir and ART
Will Islatravir Add Value to Current ART Options?

Released: October 10, 2019

Expiration: October 08, 2020

Babafemi Taiwo
Babafemi Taiwo, MBBS
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The HIV treatment armamentarium currently includes many effective ARV agents and strategies, making the bar for introducing a new agent higher. Future agents will need to have unique properties and potential for novel applications, such as two-drug regimens or long-acting agents that may offer the potential for less frequent dosing.

Three examples of agents in phase II or III development with less frequent dosing are injectable cabotegravir and rilpivirine (an INSTI + NRTI combination), injectable leronlimab (an anti-CCR5 monoclonal antibody in the same class as ibalizumab), and parenteral or subdermal islatravir (MK-8591, in a new class of nucleoside reverse transcriptase translocation inhibitors NRTTIs).

Unique MoA
As it is the first NRTTI, what I find intriguing about islatravir is that both the 3′-OH and 4’-ethynyl groups allow unique multiple mechanisms for inhibiting the reverse transcriptase enzyme. HIV inhibition by islatravir occurs at subnanomolar concentrations, and preclinical in vitro studies showed that the drug’s potency is greater than 10 times that of any currently available ARV drug. The active triphosphate of islatravir has an intracellular half-life of up to 128 hours.

Early studies showed that a single 10-mg oral dose of islatravir produced antiviral effects for 10 days and that parenteral dosing as infrequently as once per year may be feasible. These promising results have been extended in small studies of islatravir for HIV prevention and treatment that were presented at IAS 2019.

  • Prevention? In one study, in which 2 doses of islatravir implant were inserted in the upper arm for 12 weeks and then removed, pharmacokinetic modeling showed levels of the active compound stayed above the level needed for HIV prevention (based on animal studies) for a year.
  • Treatment? In the Protocol 011 Study, participants were randomized to 0.25, 0.75, or 2.25 mg of islatravir or tenofovir disoproxil fumarate, and all participants received doravirine and lamivudine. Islatravir recipients who achieved HIV-1 RNA < 50 copies/mL by Week 24 stopped lamivudine and continued the two-drug regimen of islatravir plus doravirine. At Week 48, the study showed good efficacy at all 3 doses of islatravir.

Islatravir also has a high barrier against HIV resistance—it is more potent against some NRTI-resistant strains than some other NRTIs are against wild-type virus. And the dose of islatravir being investigated for daily oral administration is much lower than the dose of other daily ARV drugs, suggesting that single-tablet, two-drug regimens with islatravir may have an advantage in pill size.

What dont we know about islatravir? A lot. Like every new drug, the adverse event profile has yet to be fully characterized. In both studies that I mentioned, islatravir was generally well tolerated but adverse events were experienced by study participants. Regarding prevention, it is unknown whether patients will prefer a surgical implant over other potential modalities. In reality, an implant could turn out to be paradoxically stigmatizing—if visible, it could be erroneously interpreted as a physical marker of high-risk sexual behavior. Nevertheless, it is exciting to see islatravir and other new compounds in the HIV prevention and treatment pipeline.

Your Thoughts?
Do you foresee patient interest in less frequent dosing ART in the future? I invite you to join the discussion by posting a comment. Answer the polling question and add your thoughts to the discussion section.

Then, to hear me and other HIV experts discussing long-acting ART and other innovative paradigms for ART, see our CME-certified video and read our responses to frequently asked questions.

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