Episode 334: Development of HIV entry inhibitors for therapy
Classical HIV antiviral therapy (ART) targets the three viral enzymes and combines nucleoside or nucleotide reverse transcriptase inhibitors (NRTI), non-nucleoside reverse transcriptase inhibitors (NNRTI), integrase strand transfer inhibitors (INSTI) and/or protease inhibitors (PI). There are various potent combinations and if the patient can adhere to the regimen, the chances on drug resistance are low, but with low adherence multi-drug resistance can emerge.
Inhibitors of entry and fusion have been developed as well and they can be very potent in vitro. The HIV Envelope, however, is much more variable than the viral enzymes. As a consequence, there are usually some viral strains which are naturally resistance and, upon monotherapy in vitro or in vivo, resistance to entry-inhibitors develops easily. This happens even with drugs targeting the human receptor CD4 or coreceptor CCR5: the virus mutates in such a way that it can interact with CD4 or CCR5, despite the bound inhibitor.
Some entry inhibitors have been tried as pre-exposure prophylaxis, but they have never reached an efficacy that approached the Truvada combination of RTI. In the field of therapy, two avenues have been explored:
- As maintenance treatment after full viral suppression. The example of several neutralizing monoclonal antibodies (mAbs) and of the CCR5-specific Leronlimab will be discussed in Par 5 and Par 8 respectively.
- As a “rescue” option, whenever the patients develop multi-drug resistance to RTI, PI and INSTI. In this case, the entry inhibitor will be combined with an optimized background therapy (OBT). To prevent or at least delay resistance, the best possible combination has to be carefully searched for individual patients, depending on their treatment history and resistance profile.
I will try to summarize the present state of the art on various entry inhibitors, including
- Peptides (such as T20 or Enfurvitide),
- Small organic compounds (such as Maraviroc and Fostemsavir)
- Monoclonal antibodies,
- either derived from HIV-infected patients (broadly neutralizing Ab)
- or humanized mAbs, directed against human CD4 (Ibalizumab) or human CCR5 (Leronlimab).
For each of those, I will discuss the mechanism of action, the most relevant data on efficacy, the resistance profile and the state of approval by the European Medicines Agency (EMA)
PAR 1 Introduction on viral entry and Env structure
C1-C4 = “constant” regions
V1-V5 = “variable” regions
= carbohydrate side chains
HIV ENTRY PROCESS
CCR5 = essential co-receptor for infection
PAR 2 Mechanism of action of various HIV entry inhibitors for treatment
2.1 Established entry inhibitors: Enfurvitide and Maraviroc
Enfurvitide = fusion inhibitor, binding to HIV gp41, approved since Oct 2008
Maraviroc = CCR5 inhibitor, binding to host CCR5, approved since Oct 2007
2.2. Novel entry inhibitors under development
2.2.1 Broadly Neutralizing antibodies (bNAbs) bind to HIV gp160
bNAbs from “elite cross-neutralizers” have 5 main targets
The image shows the five primary conserved epitope regions on the HIV Env surface proteins targeted by bNAbs.:
Three within gp120 (1) V3 glycan supersite–blue. (2) V2 apex–yellow; (3) CD4 binding site–green;
One on gp120/gp41 interface–purple and one at gp41 membrane proximal region (MPER)–red;
Several representative human monoclonal antibodies known to target each of these regions are listed.
2.2.2. FOS-TEMSAVIR binds HIV gp120 near CD4 binding site and prevents the interaction between gp120 and CD4
2.2.3. IBALIZUMAB: targets the human CD4 receptor at domain 2, which is in contact with glycan on gp120 V5 loop
2.2.4. LERONLIMAB (PRO-140) binds to human CCR5
(Kufel Int J Antimicr Agents 2020)
PAR 3 Enfurvirtide or T20 (Fuzeon®)
Ep 334-1: EMA on Enfurvitide 2023 update
Indication: Fuzeon is indicated in combination with other antiretroviral medicinal products for the treatment failure with protease inhibitors, non-nucleoside reverse transcriptase inhibitors and nucleoside reverse transcriptase inhibitors, or for patients who have intolerance to previous antiretroviral regimens
Dosage: 90 mg twice daily injected subcutaneously
Efficacy: blinded randomized phase 3 comparing optimized background (OB) with or without T20 after 48 weeks
Clear advantage of Enfurvitide over optimized background alone in terms of 0.85 log lower viral load and 46 higher CD4 T count, but less than 20 % of patients reach “undetectable” VL
Outcome better if lower baseline (BL) VL (< 100,000 copies) and higher number of remaining active ARV
Viral escape: mutations in the region of amino acids 36-45 of gp41. No cross-resistance to other classes
PAR 4 Maraviroc (Celsentri®)
Ep 334-2: EMA on Maraviroc (Celsentri) Oct 2022
- in combination with other optimized background therapy (OBT)
- for treatment-experienced patients of > 2 years of age, and weighing at least 10 kg
- infected with only CCR5-tropic HIV-1 detectable
Dosage: oral administration of two doses, either 150, 300 or 600 mg, depending on activity of co-medication on liver metabolism: many medicines have profound effects on maraviroc exposure due to drug-drug interactions
Efficacy Maraviroc + OBT versus OBT alone after 48 weeks in MOTOVATE-1 and -2 trials
Similar trends as with Enfurvitide, but overall efficacy seems somewhat better with Maraviroc.
Virologic escape via 2 routes:
- the emergence of pre-existing virus which can use CXCR4 as its entry co-receptor
- or the selection of virus that continues to use exclusively drug-bound CCR5: mutations in gp120 were not consistent across isolates
- No cross-resistance with other classes of anti-virals (including fusion-inhibitors).
PAR 5: Broadly neutralizing antibodies
Since a large number of bNab is becoming available, a lot of effort has been put in animal and clinical studies with the hope that combinations of bNab could either:
- Replace daily combined ART by infrequent injections of bNab (especially those mAbs that were modified to extend their in vivo live span)
- Prevent HIV infection (both sexually and mother-to-child
- Be part of an effective HIV Cure strategies
We will focus on the first aspect: trying to maintain viral control with bNab during analytical treatment interruption (ATI) in optimally treated subjects, after full viral suppression
mAbs used in the cited papers
Ep 334-3: Johannes Scheid Nature 2016: Single mAb 3BNC117 ( directed to CD4 binding site)
= proof of principle: 2 or 4 IV injections of 30 mg/kg separated by 2-3 weeks
→ Delay of viral rebound by on average 6.7 and 9.9 weeks (max 19 weeks)
HOWEVER: All patients rebounded with 3BNC117-resistant virus → combination needed
Ep 334-2: Michael Sneller Nature 2021 Phase 3 with combination of 2 bNab 3BNC117 (= CD4 binding site specific) and 10-1074 (= V3 glycan supersite specific)
Set-up: randomized, double-blind, placebo-controlled trial with participants who initiated antiretroviral therapy during the acute/early phase of HIV infection.
Viral load (< 200 copies) was much longer suppressed in the bNab arm, BUT:
- In all 7 patients ART had to be resumed (no permanent viral suppression)
- In 2/7 no sustained viral suppression and early reinitiating of ART:
- Participant 1 with baseline resistance to 3BCNC117 developed also resistance to 10-1074 after W16, with clear rebound of viral load (red triangles)
- Participant 14 baseline resistance to both mAb had to resume ART early after ATI (similar to placebo)
(Time 0 = ART stop ; shaded area = duration of ATI; red triangles = viral load)
Ep 334-5: Boris Julg Nat Med June 2022 Triple combination of VRC07-523LS (= CD4 binding site specific) + PGT121 (= V3 glycan-specific) + PGDM1400 (= V2 apex specific) in 5 viremic PLWH not on ART (single IV injection)
Plasma HIV-1 RNA levels after PGDM1400, PGT121 and VRC07-523LS infusion at 20 mg kg−1 each (group 3A) (a) and at 30 mg kg−1 each (group 3B) (b) in viremic participants with HIV not on ART.
The dotted line indicates the LLoQ for HIV-1 RNA levels (40 copies per ml).
Dots indicates when a sample was collected for sequencing.
If and when ART was started is indicated in the figures.
The symbol ‘&’ indicates the time point when participant 693–2290 was lost to follow-up.
- Quick viral rebound after single IV injection
- Rebound viruses demonstrated partial to complete resistance to PGDM1400 and PGT121 in vitro, whereas susceptibility to VRC07-523LS was preserved.
Ep 334-6: Laura Waters CID March 2023: bNab: broad in theory, narrow in reality
Conclusions: bNab have clear limitations for therapy, even in triple combinations:
- Resistance: need for primary resistance testing, low genetic barrier → easy resistance development
- Unknown ability to maintain long-term virological suppression systemically and in anatomical reservoirs.
Maybe useful in combination with “classical ART” for treatment?
However, since therapy nowadays relies on reverse transcriptase and integrase inhibitors, there is still a need for another class of drugs for prevention to avoid generating resistance to therapeutic agents. Therefore, bNabs may still have a place in prevention?
EP 334-7, -8, -9 Other interesting reviews with lots of data on animal testing, modification of bNab to enhance life span or improve efficacy, as well as list of ongoing clinical trials for prevention and treatment.
PAR 6: Fostemsavir (BMS-663068/GSK 3684934) (Rukobia®Viiv)
Fostemsavir mechanism of action
Temsavir, the active moiety of fostemsavir, binds the HIV-1 envelope protein gp120, close to the gp120-CD4+ binding site → it prevents the conformational change required for attachment of HIV-1 to the host cell.
Episode 334-10: Michael Kozal NEJM March 2020 BRIGHTE Fostemsavir in patients with multidrug resistant HIV
Set-up = complex patients with “heavy history”
Only subjects with no viable combination because drug failure to at least 4 classes of antivirals and viral load > 400
- 83% of the patients had received at least five antiretroviral regimens before enrollment,
- 85% had received a diagnosis of AIDS,
- 26% had a baseline CD4+ T-cell count of less than 20 cells per cubic millimeter,
- 29% had an HIV-1 RNA level of at least 100,000 copies per milliliter.
- Group 1: first randomization to fostemsavir or placebo for 8 days, then open label fostemsavir
- Group 2: no randomization and direct open label fostemsavir
Dosage: 600 mg twice per day orally
- Randomized placebo-controlled trial: VL decrease of 0.79 log in fostemsavir vs 0.17 log in placebo on day 8
- Open label effect of Fostemsavir + OBT combined on week 48:
- Viral load < 40 cp/ml: 62 % in originally randomized group 1 and 48 % in non-randomized group 2
- CD4 T increase: + 139 cells/ml in group 1 and + 64 cells/ml in group 2
- Response rates were reduced among the patients with high baseline viral load (>100,000 copies per milliliter) or a very low baseline CD4+T-cell count (<20 cells per cubic millimeter).
- Not very clear what the impact is of OBT versus fostemsavir
Ep 334-11: Nannan Zhou JAC 2014: Resistance associated mutations (RAM) for fostemsavir (BMS-626529, the active agent of the prodrug BMS-663068)
Major RAM: M426L or S375M
Contributing mutations: M434I and M475I
Ep 334-12: David Langley Proteins 2014 Homology model
RAM for fostemsavir (red dots), although remote in primary structure, cluster around the CD4 binding site of gp120 in the quaternary structure and are distinct from RAM, associated with other attachment inhibitors (grey dots).
Note: most CRF AE are naturally resistant because they have 375H and 475I substitutions
Ep 334-13: EMA approval file Feb 2021
Indication: Rukobia, in combination with other antiretrovirals, is indicated for the treatment of adults with multidrug resistant HIV-1 infection for whom it is otherwise not possible to construct a suppressive anti-viral regimen.
Dosage: 600 mg twice per day per os
Follow-up of BRIGHTE study until week 96 (see Ep 334-10)
(DTG = dolutegravir integrase inhibitor; DRV = darunavir protease inhibitor)
Stable viral suppression in 50-60 %
- Negative effect of high viral load and low CD4 T count at the start on outcome
- Favorable effect of higher number of active drugs and especially of Dolutegravir, but not Darunavir as part of ‘optimized background therapy (OBT)”
Ep 334-14: Margaret Gartland AAC 2022 Week 96 Genotypic and Phenotypic Results of the Fostemsavir phase 3 BRIGHTE study
The expected resistance-associated mutations (RAM) in amino-acids 375, 426, 434 and 475 are only present in half of the randomized and three quarters of the non-randomized cohort.
PAR 7: IBALIZUMAB or Trogarzo® (Tanox Taimed Biologics)
Ibalizumab targets the human CD4 receptor at domain 2, which is in contact with glycan on gp120 V5 loop
Ep 334-15: Brinda Emu NEJM 2018 Phase 3 Study of Ibalizumab for Multidrug-Resistant HIV-1
Set-up: Only 40 patients, who failed at least 3 drug classes and with viral load > 1000 copies.
- First a “functional monotherapy” phase of 1 week, maintaining their “failed” therapy, but with 2000 mg Ibalizumab IV added
- Then a 25 week Ibalizumab 800 mg IV / 14 days maintenance with optimized background therapy
Hence: non-randomized and not placebo controlled over entire course!
Clear effect of Ibalizumab in monotherapy after 1 week, sustained for 25 weeks under maintenance Ibalizumab with addition OBT, with better response if baseline CD4 T > 50 cells/µl
Similar to Fostemsavir trial not clear what the impact is of OBT versus Ibalizumab during maintenance period
Ep 334-16: Jonathan Toma J Virol 2011 Loss of Asparagine-Linked Glycosylation Sites in Variable Region 5 of HIV-1 gp 120 is Associated with Resistance to CD4 Antibody Ibalizumab
N = asparagine ; Q = glutamine ; MPI = maximum percent inhibition; PNGS = Asparagine (N)-glycosylation site
Mutation of asparagine to glutamine in amino-acids 460 and 464 of V5 gp120 abolishes N-linked glycosylation. These changes overcome the inhibitory action of Ibalizumab, because CD4 will now be able to interact with gp120, despite Ibalizumabs binding to CD4.
Ep 334-17: EMA authorized the use of Trogarzo in 2019, based on NEJM study of 2018
Ep 334-18: Withdrawal of Trogarzo on 1 Jan 2023, on request of Theratechnologies Europe, the marketing authorization holder…
PAR 8: PRO 140 or Leronlimab (CytoDyn Co)
Ep 334-19: Kush Dhody HIV Clin Trials 2018: CD01 study PRO-140 as maintenance monotherapy?
Set-up: 41 adult patients, infected exclusively with CCR5-tropic HIV-1 with viral loads <50 copies/mL, were switched from daily oral combination ART regimens to weekly PRO 140 monotherapy subcutaneously for 12 weeks.
Participants who completed 12 weeks of treatment without experiencing virologic rebound were allowed to self-administer PRO 140 as a 350 mg subcutaneous injection weekly, for up to an additional 160 weeks.
Results: Maintained viral suppression,
- 23/41 (56.1%) for 12 weeks
- 9/41 (22%) for 2 years
Ep 334-20: Xiao L. Chang PLoS Pathogens 2022 Long-term follow up of the CD01 study
Five (from 41 original participants) have successfully suppressed HIV with Leronlimab for over 7 years and are currently continuing this treatment.
Longitudinal plasma viremia in these participants was predominately below the limit of assay detection (40 copies). However, it is important to note that participants on Leronlimab monotherapy exhibited a higher frequency of viral blips (7.1%) than those on combinational oral ART regimens (2.0%).
Nevertheless, the ability of Leronlimab to sequentially control these viral blips demonstrated that Leronlimab-resistant variants did not arise during these seven years of Leronlimab monotherapy
Note: I could not find a publication on a phase 3 trial with Leronlimab + optimized background therapy. The website, on which the company CytoDyn had announced preliminary results in 2019 does not exist anymore….
The company has been exploring other possible indications in treatment of breast cancer and COVID, but has so far failed to get approval from FDA or EMA for any indication, as far as I can see…
Target + action
- Targeting HIV envelope
Peptide, based on HIV gp41
HV gp41 Fusion inhibitor
Small organic compound
Gp120 near CD4 binding
Broadly neutralizing antibodies
Five neutralizing supersites in gp120 or gp41
- Targeting human receptor CD4 or coreceptor CCR5
Yes (2019), withdrawn (2023)
Small organic compound
The trials, aiming to sustain viral control in well-treated and fully virally suppressed individuals after ATI with either a combination of Env-specific human mAbs or monotherapy with humanized anti-CCR5 Leronlimab have met with limited success. In most cases, virus rebounded and ART had to be resumed. More powerful combinations, with less emerging resistance will have to be developed before this strategy could enter the clinic.
Three medicines are available for patients with extensive treatment failure or intolerance: the fusion inhibitor Enfurvitide (subcutaneously), Maraviroc (orally) and Fostemsavir (orally), while Ibalizumab (intravenously) has recently been withdrawn from the European market.
The approval of the former two in the 2000’s was based on genuine double blind phase 3 trials, showing a clear benefit over optimized background treatment (OBT) only. Apparently, this well-controlled set-up was no more possible in the late 2010’s: a short monotherapy was followed by a combined treatment of Fostemsavir or Ibalizumab with OBT, , without OBT only control. It is very difficult to conclude which of these entry-inhibitors is most useful, because of the evolving characteristics of the patient population and the different set-up of trials in the mid 2000’s and those in the mid 2010’s .
In clinical practice these entry inhibitors should always be used in combination with an individually optimized background of ARV targeting RT, integrase and/or protease, based on the patient’s virus resistance profile. Nevertheless resistance development to all these entry inhibitors was still observed during the trials, because the patients viruses were no longer fully susceptible, even to the “optimized” background. Fortunately, there is no cross-resistance between these different entry inhibitors, because of their distinct mechanism of action.
I hope this overview was understandable also for non-experts and probably even useful.
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