Global burden of human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS)

HIV/AIDS has profoundly shaped the landscape of global public health since the disease’s first clinical descriptions in the early 1980s. Despite four decades of scientific progress, HIV/AIDS endures as one of the most consequential infectious pandemics in recorded history. The World Health Organization estimated that approximately 39.0 million people were living with HIV at the end of 2022 (range: 33.1 to 45.7 million), with roughly two-thirds of this population residing in the WHO African region, where healthcare infrastructure and access to treatment remain the most constrained [1, 2]. Transmission continues in every country, and several settings that had previously registered declining incidence trends are now reporting renewed upward trajectories [1].

HIV is a retrovirus that progressively dismantles the host’s adaptive immune defences by targeting CD4-positive T lymphocytes, the orchestrators of the cellular immune response. In its most advanced clinical manifestation, AIDS renders the individual profoundly vulnerable to opportunistic infections, including Pneumocystis pneumonia, disseminated tuberculosis, and invasive fungal disease, as well as AIDS-defining malignancies such as Kaposi sarcoma and certain lymphomas [1–3].

The transformative impact and persistent limitations of antiretroviral therapy

No curative intervention for HIV/AIDS currently exists. Nevertheless, the development and widespread adoption of combination antiretroviral therapy (ART) since the mid-1990s has been transformative, converting a once-fatal diagnosis into a manageable chronic condition for millions worldwide [4, 5]. The overarching goals of ART are to suppress viral replication, preserve and reconstitute immune function, reduce HIV-associated morbidity and mortality, improve quality of life, and prevent onward transmission [1].

Despite these achievements, the clinical management of HIV infection remains attended by formidable challenges. Adherence to lifelong multi-drug regimens is undermined by pill burden, dosing frequency, drug-drug interaction potential, and adverse effect profiles [5, 6]. Of particular urgency is the emergence and spread of multidrug-resistant (MDR) HIV-1, which arises from years of suboptimal drug exposure, successive treatment failure, or transmitted resistance. Patients harbouring MDR HIV-1 can rapidly exhaust all conventional treatment options, placing them at high risk of progressive immune deterioration and AIDS-related mortality [5, 6].

Monoclonal antibodies as a novel therapeutic class in HIV

The pursuit of antiretroviral agents with novel mechanisms has driven extensive investigation into biologically derived agents, particularly monoclonal antibodies (mAbs) directed against host-cell entry machinery rather than viral enzymatic targets. This approach offers the theoretical advantages of high target specificity, a mechanistically distinct resistance pathway relative to small-molecule antiretrovirals, and the potential for long-acting administration [7, 8]. The clinical development of ibalizumab represents the culmination of this conceptual approach [8, 9].

Ibalizumab (Trogarzo; TaiMed Biologics), a recombinant humanised IgG4 mAb, was approved by the FDA on 6 March 2018 as the first mAb for HIV-1 treatment [8, 9]. It is indicated in combination with other antiretroviral agents for heavily treatment-experienced adults with MDR HIV-1 infection who are failing their current antiretroviral regimen [8]. In 2019, the European Medicines Agency (EMA) approved ibalizumab for adults with MDR HIV-1 in whom it is otherwise difficult to construct a suppressive antiviral regimen [10]. This review provides a comprehensive and critical appraisal of ibalizumab, synthesising the full body of evidence from its discovery through to its present and future clinical applications.

Structural basis of the ibalizumab-CD4 interaction

Ibalizumab is a recombinant humanised IgG4 mAb, with murine-derived complementarity-determining regions (CDRs) grafted onto a human IgG4 framework, that binds to the extracellular surface of the CD4 receptor, specifically at the interface of domain 1 (D1)–D2 [8, 11, 12]. CD4 is expressed on the surface of T helper lymphocytes and macrophages, the principal cellular targets of HIV-1. Viral entry into these cells is initiated when the gp120 surface glycoprotein of the HIV-1 envelope engages D1 of CD4, triggering a sequence of conformational rearrangements that progressively expose the co-receptor binding site and ultimately culminate in gp41-mediated membrane fusion [11, 12].

Ibalizumab’s binding epitope at the D1–D2 interface is spatially distinct from the gp120-CD4 contact site at D1. Consequently, ibalizumab does not prevent the initial attachment of gp120 to CD4. Instead, once the virus has bound to the cell surface, ibalizumab sterically occludes the downstream conformational transitions in gp120 that are essential for productive engagement with either the CCR5 or CXCR4 co-receptor, thereby arresting the entry process before membrane fusion can occur [11, 12]. This post-attachment mechanism of inhibition is characterised by an exceptionally high binding affinity for CD4, with a dissociation constant (K_d) of approximately 8.25 × 10⁻¹¹ M [11, 12].

The D2 binding site confers a crucial immunological advantage: because CD4-MHC class II interaction occurs through D1 rather than D2, ibalizumab does not interfere with antigen presentation or T-helper cell activation [11, 12]. This selectivity distinguishes ibalizumab from earlier anti-CD4 antibodies that bound D1 and produced unacceptable immunosuppression, and it is precisely what made ibalizumab the first anti-CD4 agent suitable for therapeutic use in HIV. Of additional immunological importance, ibalizumab was intentionally engineered with a human IgG4 Fc region. The IgG4 isotype has intrinsically diminished Fc-mediated effector functions compared to IgG1 or IgG3, meaning ibalizumab does not trigger antibody-dependent cellular cytotoxicity or complement-dependent cytotoxicity directed at CD4-expressing T cells. This design choice substantially reduces the risk of CD4 T-cell depletion and represents a deliberate and clinically significant safety feature [12].

Resistance mechanisms

Resistance to ibalizumab has been characterised at the molecular level and is associated principally with conformational alterations in the gp120 V5 loop of the viral envelope glycoprotein. Specifically, the loss of N-linked glycosylation sites within this region is believed to modify the spatial geometry of the gp120-CD4 complex in a manner that circumvents ibalizumab’s steric obstruction, permitting co-receptor engagement and viral fusion to proceed despite antibody occupancy [13]. This resistance pathway emerges with alarming rapidity, within one to two weeks, during ibalizumab monotherapy, which provides a compelling mechanistic rationale for the drug’s exclusive use in combination with a fully active optimised background therapy (OBT) [14, 15]. At the clinical level, resistance has also been documented following a single missed maintenance infusion, underscoring the importance of strict adherence to the biweekly dosing schedule [15].

Comparison with other entry and post-attachment inhibitors

Ibalizumab occupies a mechanistically distinct position relative to all other approved antiretroviral agents. CCR5 antagonists such as maraviroc block the co-receptor directly but are inactive against CXCR4-tropic (X4) variants, a limitation ibalizumab does not share because its mechanism operates upstream of co-receptor engagement regardless of tropism. Fostemsavir (Rukobia), a first-in-class attachment inhibitor that binds gp120 directly and prevents its interaction with CD4, is mechanistically related to ibalizumab as both agents act at the entry stage; notably, the two drugs are not infrequently used concurrently in heavily treatment-experienced patients with limited options [16]. Table 2 provides a comparative overview of ibalizumab alongside other mAbs currently under clinical investigation for HIV treatment. The table distinguishes between host-targeted agents such as ibalizumab, which acts on the CD4 receptor, and virus-targeted agents including broadly neutralising antibodies (bNAbs) that act directly on HIV-1 envelope glycoproteins [17, 18]. Whereas ibalizumab binds host CD4, the majority of bNAbs in clinical development, including VRC07-523LS, N6LS (lotivibart), 10-1074 (zinlirvimab), 3BNC117 (teropavimab), 10E8V4/5R, and multispecific constructs such as SAR441236, target diverse epitopes on the viral surface. A comprehensive review of bNAbs in clinical development was published in 2025 [19].

In vitro studies

Laboratory studies conducted prior to and during clinical development established ibalizumab’s broad antiviral potency against a diverse panel of primary HIV-1 clinical isolates, including strains carrying resistance mutations to all major conventional antiretroviral drug classes [25, 26]. The antibody demonstrated effective inhibition of both CCR5-tropic (R5) and X4 viral variants, a property of considerable clinical relevance given that tropism switching from R5 to X4 or dual-tropic variants is common in heavily treatment-experienced patients [12, 26]. Structural binding studies confirmed that ibalizumab’s epitope at the D1–D2 interface did not overlap with the gp120 contact site, in alignment with its post-attachment mechanism [25].

In vivo animal studies and humanised mouse models

Preclinical in vivo studies in humanised mouse models provided critical proof-of-concept evidence [27, 28]. Humanised mice engrafted with human peripheral blood lymphocytes or haematopoietic stem cells and subsequently challenged with HIV-1 showed significant reductions in plasma viral RNA levels following ibalizumab treatment, with preservation of human CD4 T-cell counts compared to untreated controls [27, 28]. These models were instrumental in characterising in vivo pharmacokinetics, tissue distribution, and the durability of CD4 receptor occupancy under physiological conditions [22, 28].

Immunogenicity studies in animal models provided reassurance regarding the tolerability profile of the humanised antibody [22]. Cynomolgus monkey enhanced pre- and postnatal development (ePPND) studies identified no structural teratogenicity or reproductive toxicity. However, reversible neonatal immunosuppression was observed in infant monkeys born to ibalizumab-exposed mothers, with lymphocyte counts returning to near-normal levels by three months of age [29]. These preclinical reproductive toxicity findings have direct implications for clinical practice and are discussed further in the Special populations section.

Phase I: safety, tolerability, and dose-ranging

The first-in-human Phase I studies evaluated multiple ascending intravenous doses from 0.3 to 25 mg/kg in treatment-experienced HIV-1-infected adults [7]. No dose-limiting toxicities or severe adverse events were identified at any dose level [7]. Antiviral activity was minimal at the lowest doses (0.3 and 1.0 mg/kg) but became clearly discernible at 3.0 mg/kg and above, with peak median reductions in plasma HIV-1 RNA of 0.56 log₁₀ (3.0 mg/kg), 1.33 log₁₀ (10 mg/kg), and 1.11 log₁₀ (25 mg/kg) copies per mL [30]. These results established an acceptable safety profile and provided the dose-response rationale for Phase IIb advancement.

Phase IIb: dose-response and long-term durability

The Phase IIb TMB-202 study (NCT00784147) was a randomised dose-response evaluation of ibalizumab combined with an optimised background regimen (OBR) in 82 treatment-experienced adults over 48 weeks [30]. Results confirmed that ibalizumab in combination with OBR produced viral load suppression superior to OBR alone, with responses deepening at higher ibalizumab doses [30]. The investigator-sponsored TMB-311 continuation protocol (NCT01056393) extended follow-up of Phase IIb completers, ultimately demonstrating remarkable durability: at the final study visit, 11 of 12 evaluable participants had achieved viral loads below 200 copies/mL, all 12 had viral loads below 50 copies/mL, and patients had accumulated a mean CD4 T-cell gain of 99 cells/μL above baseline [31].

Phase III: pivotal trial (TMB-301)

The Phase III TMB-301 trial (NCT02707861) enrolled 40 heavily treatment-experienced adults with MDR HIV-1 resistant to at least three antiretroviral drug classes who were failing their current regimen [8, 9]. Participants received ibalizumab at a loading dose of 2,000 mg IV on day 1, followed by 800 mg IV every two weeks, in combination with individually optimized OBT [8, 32].

The primary efficacy endpoint, assessed at week 24, was a viral load reduction of at least 0.5 log₁₀ copies/mL from baseline. This threshold was achieved in 83% of participants, substantially exceeding the pre-specified regulatory benchmark [8, 32]. Approximately 43% of participants attained viral loads below 200 copies/mL, and the mean CD4 T-cell count increased by 48 cells/μL from baseline [32]. Health-economic modelling based on TMB-301 data estimated that ibalizumab with OBT conferred 0.95 additional years during which patients maintained CD4 counts above 200 cells/μL, representing a 28% improvement over OBT alone [21]. Table 3 presents a consolidated summary of key outcomes across all clinical development phases.

Virological response

Ibalizumab’s most compelling clinical contribution is its ability to produce meaningful viral load suppression in patients with MDR HIV-1 who have exhausted conventional treatment options [8, 9]. Across Phase I through Phase III trials, consistent and durable reductions in plasma HIV-1 RNA were observed in treatment-experienced patients, with the depth of response correlated with the extent of virological activity provided by the accompanying background regimen [30–32]. The long-term durability of virological suppression was most compellingly demonstrated in the TMB-311 extended follow-up cohort, where full viral suppression below 50 copies/mL was maintained in all evaluable participants at their last study visit [31].

Immunological recovery

Alongside virological suppression, ibalizumab treatment was associated with meaningful and progressive immunological recovery. CD4 T-cell counts rose from baseline in both Phase II and Phase III settings, with average gains of 48 to 99 cells/μL depending on the study population and duration of follow-up [21, 31, 32]. These CD4 gains carry direct clinical significance: reconstitution above 200 cells/μL is associated with a substantial reduction in the risk of AIDS-defining opportunistic infections and a demonstrable improvement in overall survival prognosis [1, 4].

Resistance and virological escape

Resistance to ibalizumab warrants careful clinical consideration, particularly because it emerges rapidly in the absence of combination therapy. The principal resistance mechanism involves conformational alterations in the V5 loop of the gp120 envelope glycoprotein, specifically the loss of N-linked glycosylation sites, that modify the spatial geometry of the post-attachment complex in a manner that permits co-receptor engagement despite antibody occupancy [13]. This escape can occur within one to two weeks of monotherapy exposure [14, 15]. Additionally, breakthrough viraemia has been documented following a single missed infusion under the less frequent 2,000 mg every-four-week dosing schedule, emphasising the clinical importance of strict adherence to the approved biweekly maintenance regimen and the concurrent provision of a fully active OBT [15].

Paediatric population

Ibalizumab is not approved for use in children or adolescents under 18 years of age, reflecting the absence of paediatric safety and efficacy data [36]. The drug’s orphan drug designation for MDR HIV-1 confers an exemption from the requirements of the Paediatric Research Equity Act [37]. No dosing guidance is available, and dedicated paediatric pharmacokinetic and safety studies have not been conducted. Ibalizumab cannot be recommended in this population until such evidence is available.

Pregnancy

No FDA pregnancy category has been assigned to ibalizumab, and the manufacturer provides no formal recommendation for its use in pregnancy [29, 38]. Pharmacokinetic studies in human pregnancy are absent, and the available clinical data are insufficient to characterise the risk of congenital anomalies associated with in utero exposure [29]. Animal reproductive toxicology studies in cynomolgus monkeys using an ePPND design identified no structural teratogenicity; however, reversible neonatal immunosuppression was observed in infants of exposed mothers, with lymphocyte counts normalising within three months [29]. In response to an FDA requirement, the manufacturer has established a pregnancy registry to collect prospective data on maternal and neonatal outcomes [36]. Women of childbearing potential receiving ibalizumab are recommended to use effective contraception throughout treatment.

Breastfeeding

No published data characterise ibalizumab transfer into human breast milk. As a large protein molecule of approximately 150,000 Da, ibalizumab is expected to achieve only very low milk concentrations, consistent with the general pharmacokinetic behaviour of therapeutic immunoglobulins [39, 40]. Significant gastrointestinal proteolytic degradation would further limit systemic absorption in the nursing infant [40]. Despite these reassuring pharmacokinetic considerations, the absence of human lactation data necessitates caution, particularly for neonates and preterm infants whose gastrointestinal barrier and immune systems are functionally immature [37].

Geriatric population

Pharmacokinetic data from patients aged 65 years and above remain limited, deriving from only five individuals in the clinical development programme [10]. The available data suggest that ibalizumab’s pharmacokinetic profile in older adults is broadly comparable to that in the general adult population, but the very small sample precludes definitive conclusions [10]. In clinical practice, the management of elderly patients receiving ibalizumab should account for age-related changes in immune function, comorbidity burden, and polypharmacy, all of which may modulate therapeutic response and adverse event risk.

HIV-related stigma and treatment adherence

HIV/AIDS continues to be attended by a pervasive social stigma that undermines HIV testing, treatment-seeking behaviour, and adherence across all age groups, ethnic backgrounds, and socioeconomic strata [41, 42]. Patients with high stigma concerns are estimated to be 3.3 times more likely to report non-adherence to ART than those with low stigma concerns [43]. For patients requiring ibalizumab, the intersection of HIV stigma with the practical demands of biweekly intravenous infusion at a healthcare facility can create compounding barriers to sustained therapeutic engagement. Addressing HIV stigma at structural, clinical, and community levels is therefore not merely a matter of social justice but a fundamental precondition for the therapeutic success of ibalizumab.

Access, affordability, and health equity

Ibalizumab is largely absent from the therapeutic landscape of sub-Saharan Africa and other high-burden low- and middle-income country (LMIC) regions, where the global HIV epidemic is most acute [44]. Multiple interlocking factors account for this inequity: the high cost of a proprietary biologic without a generic equivalent, the infrastructural demands of intravenous cold-chain logistics and infusion clinic capacity, the paucity of diagnostic services capable of characterising MDR HIV-1, and the near-absence of structured programmes for managing treatment-resistant HIV in most LMIC health systems [35, 44].

The World Health Organization’s 95-95-95 targets for 2025 frame the global ambition within which ibalizumab must be assessed [2]. Achieving 95% viral suppression among all people receiving treatment demands that effective salvage therapies reach those with MDR HIV-1, a goal that remains aspirationally distant for the majority of the world’s treatment-failing patients [45, 46]. The HIV scientific and policy community has emphasised that injectable HIV medications must be both clinically effective and practically deliverable within resource-constrained settings if they are to contribute meaningfully to global epidemic control [44].

Patient autonomy and informed consent

Obtaining meaningful informed consent for ibalizumab therapy requires transparent communication about the drug’s distinctive administration route, dosing schedule, and the evolving state of long-term safety data [32]. The overall tolerability profile observed in trials and real-world settings supports a constructive, patient-centred consent discussion in which the therapeutic benefits are weighed against the practical demands of biweekly clinic attendance [34, 35]. Clinicians bear the responsibility of ensuring that patients understand both the limitations of current long-term safety evidence and the absence of comparable alternatives for highly treatment-experienced MDR HIV-1 disease.

Long-acting formulations

The single most practically limiting attribute of ibalizumab is its requirement for biweekly intravenous infusion, confining patients to healthcare settings and generating adherence and accessibility challenges [45]. Subcutaneous and intramuscular administration routes under investigation represent the most clinically consequential near-term development for ibalizumab. Phase III evaluation of intramuscular ibalizumab (NCT03913195) and Phase I data on subcutaneous administration at 480 mg weekly for four weeks, which demonstrated full CD4 receptor occupancy persisting for at least 14 days after the last dose, collectively suggest that extended-interval, self-administered formulations may be achievable [23, 24]. If validated, such formulations would transform the drug’s practical utility in community-based settings and potentially within LMIC health programmes.

Combination and bispecific antibody strategies

A scientifically exciting frontier involves the engineering of bispecific antibodies that incorporate ibalizumab-like CD4-targeting activity alongside a second, independently neutralising specificity. The 10E8.4/iMab bispecific antibody, which combines ibalizumab’s post-attachment mechanism with 10E8.4-mediated neutralisation of the gp41 membrane-proximal external region (MPER), is currently under Phase I evaluation in combination with the bNAb VRC07-523LS (NCT05890963) [17, 23]. Separately, Combinectin (GSK3732394), a multi-domain fusion protein incorporating one adnectin domain targeting CD4, a second adnectin domain targeting gp41, and an enfuvirtide-analogous helical peptide also targeting gp41, offers a conceptually parallel multi-site entry inhibition strategy [17, 18]. These combination approaches are intended to achieve a high genetic barrier to resistance, broad cross-clade activity, and long-acting pharmacokinetics in a single agent.

Prevention applications

Ibalizumab has not been evaluated for pre-exposure or post-exposure prophylaxis (PrEP or PEP), nor for prevention of mother-to-child transmission, and the current intravenous route renders it unsuitable for these indications in its present formulation [8]. The development of long-acting subcutaneous formulations could, in principle, create a pathway toward preventive applications in high-risk individuals unable to maintain adherence to oral PrEP. This would require dedicated clinical investigation including safety studies in HIV-negative populations and pharmacokinetic modelling of the concentrations needed to achieve prophylactic receptor occupancy [45].

The broader landscape of MDR HIV-1 therapeutics

Ibalizumab’s approval and clinical maturation reflect a wider evolution in HIV therapeutics toward long-acting agents, biologics, and mechanistically novel classes [45–47]. Lenacapavir, a first-in-class capsid inhibitor with a twice-yearly dosing interval, was approved for heavily treatment-experienced adults in 2022 and, alongside ibalizumab, is reshaping the salvage therapy landscape [47, 48]. The increasing pipeline of bNAbs, therapeutic vaccines, and gene-editing-based strategies collectively defines the horizon toward which HIV biomedical research is advancing [17, 18]. Ibalizumab’s role in this evolving ecosystem is as an established, clinically validated agent whose mechanistic distinctiveness and compatibility with diverse antiretroviral combinations ensure its continued relevance in the management of MDR HIV-1.