Abstract: GSK1265744, commonly known as cabotegravir (CAB), is a potent integrase strand transfer inhibitor (INSTI) that has emerged as a transformative agent for HIV pre-exposure prophylaxis (PrEP). Formulated as a long-acting injectable (LA) nanosuspension, CAB-LA overcomes the significant adherence challenges associated with daily oral PrEP regimens. Clinical trials, notably HPTN 083 and HPTN 084, have demonstrated the superiority of CAB-LA over daily oral tenofovir disoproxil fumarate/emtricitabine (TDF/FTC) in preventing HIV acquisition across diverse populations, including men who have sex with men, transgender women, and cisgender women. Despite its high efficacy, the deployment of CAB-LA faces unique clinical challenges. These include injection site reactions, the prolonged pharmacokinetic "tail" phase that risks the selection of INSTI-resistant mutations (such as G140S and G148H) if HIV is acquired, and the phenomenon of Long-Acting Early Viral Inhibition (LEVI) syndrome, which complicates and delays acute HIV diagnosis. This review synthesizes the pharmacological properties, molecular mechanisms, clinical efficacy, current limitations, and future perspectives of cabotegravir as a long-acting injectable for HIV prevention.
1. Introduction
The global effort to end the HIV epidemic relies heavily on effective antiretroviral (ARV)-based prevention strategies. While daily oral pre-exposure prophylaxis (PrEP) with tenofovir disoproxil fumarate and emtricitabine (TDF/FTC) has been highly effective, its real-world success is heavily dependent on strict daily adherence [1] [11]. Variable rates of protection in clinical trials have been directly correlated with drug nonadherence, particularly among vulnerable populations such as young cisgender women [1] [12]. To address these adherence barriers, long-acting injectable ARVs have been developed to provide sustained therapeutic exposures without the need for daily dosing [1].
GSK1265744 (cabotegravir) is a novel HIV-1 integrase strand transfer inhibitor (INSTI) that possesses unique physicochemical properties allowing it to be formulated as a long-acting injectable nanosuspension [1] [3]. Administered intramuscularly every 8 weeks, long-acting cabotegravir (CAB-LA) represents a paradigm shift in HIV prevention, offering a highly efficacious, coitally independent alternative to daily oral pills [2] [11].
2. Pharmacological Activity
The prophylactic efficacy of cabotegravir was initially established in robust nonhuman primate models. In macaque studies, long-acting cabotegravir provided complete protection against repeated intrarectal and intravaginal challenges with simian-human immunodeficiency virus (SHIV), maintaining protective plasma concentrations comparable to those achieved in humans [1] [3] [7].
Clinically, the efficacy of CAB-LA for PrEP was definitively proven in two landmark phase III, randomized, double-blind trials: HPTN 083 and HPTN 084. HPTN 083 evaluated CAB-LA in cisgender men who have sex with men (MSM) and transgender women, demonstrating a 66% lower risk of incident HIV infection compared to daily oral TDF/FTC [2] [11]. HPTN 084 evaluated the regimen in at-risk cisgender women in sub-Saharan Africa, where CAB-LA showed an even more profound 89% reduction in HIV acquisition risk compared to the oral TDF/FTC arm [2] [11]. Both trials were stopped early due to the overwhelming superiority of the injectable regimen [2].
3. Molecular Mechanism of Action
Cabotegravir is a potent integrase strand transfer inhibitor (INSTI) [1] [6]. The HIV-1 integrase enzyme is responsible for inserting the viral DNA into the host cell genome, a critical step in the viral replication cycle. Cabotegravir binds to the integrase active site and blocks the integrase-catalyzed viral cDNA strand transfer process [1]. It exhibits subnanomolar antiviral activity, with an in-vitro half-maximal inhibitory concentration (IC50) of 0.22 to 0.34 nmol/L against wild-type HIV-1 strains (such as BAL and NL432) in peripheral blood mononuclear cells [1] [6].
4. Structure-Activity Relationship (SAR)
Cabotegravir is a structural analogue of the widely used INSTI dolutegravir; both are carbamoyl pyridone analogues [1] [11]. The molecule was specifically designed with structural elements that prevent the rapid selection of resistance and chemical features that enable long-acting formulation [11]. The free acid of cabotegravir has very low water solubility (<10 mg/mL), which is a critical property exploited for its injectable formulation [1].
The long-acting nanosuspension is created by wet-milling the crystalline active drug to a median particle size of 200 nm, suspended in an aqueous vehicle with surfactants and polymers [1] [3]. Unlike matrix nanoparticles that encase drugs in lipids or polymers, CAB-LA nanoparticles are essentially 100% active drug, allowing for a high drug loading capacity (200 mg/mL) and reduced injection volumes [1]. Following intramuscular or subcutaneous injection, the drug exhibits a slow absorption rate from the poorly soluble tissue depot, resulting in a prolonged apparent terminal half-life of 21 to 50 days [1] [6]. Cabotegravir is highly bound to plasma proteins (>99%) and is primarily metabolized by uridine diphosphate glucuronosyltransferase (UGT) 1A1, with a minor contribution from UGT1A9, and is eliminated largely in the feces [3] [9].
5. Current Limitations
Despite its high efficacy, CAB-LA has several notable limitations. The most common adverse events are injection site reactions (ISRs), including pain and erythema, though these are generally mild to moderate and rarely lead to discontinuation [1] [6].
A significant pharmacological challenge is the pharmacokinetic "tail." After discontinuation, sub-therapeutic concentrations of cabotegravir can persist in the plasma for over a year (median 43 weeks for men and 67 weeks for women) [11]. If an individual acquires HIV during this tail phase, the low drug levels provide selective pressure for the emergence of INSTI-resistant mutations [11]. In vitro and clinical data indicate that high-level resistance (>1000-fold) requires the accumulation of multiple mutations, such as T97A/G140S/G148H, which can confer cross-resistance to dolutegravir, compromising future first-line antiretroviral therapy options [11].
Furthermore, breakthrough infections on CAB-LA often present with suppressed viral loads and delayed antibody responses, a clinical presentation termed Long-Acting Early Viral Inhibition (LEVI) syndrome [5] [12]. This phenomenon complicates standard HIV diagnostic algorithms, necessitating the use of sensitive HIV-1 RNA nucleic acid testing to detect acute infections before administering subsequent injections, which poses a significant implementation barrier in resource-limited settings [11] [12].
6. Future Perspectives
The future of CAB-LA for PrEP relies on overcoming implementation and accessibility barriers. Research is actively exploring alternative injection sites, such as the vastus lateralis (thigh muscle), to provide more privacy and convenience compared to the current gluteal administration [2]. Additionally, the necessity of the oral lead-in phase is being re-evaluated, with "direct-to-inject" strategies showing promise for simplifying initiation [2].
Expanding access to priority populations is also a major focus. Ongoing studies, such as the open-label extension of HPTN 084, are gathering crucial safety and pharmacokinetic data on CAB-LA use during pregnancy and breastfeeding, as initial data suggest that PK parameters in pregnant women remain similar to those in non-pregnant individuals [10] [11]. Finally, the cost-effectiveness of CAB-LA compared to generic oral PrEP remains a critical factor for global scale-up. Ensuring equitable access through voluntary licensing and optimized delivery models in low- and middle-income countries will be essential to realizing the full public health potential of long-acting injectable PrEP [10] [12].
