Long-acting approaches for delivery of antiretroviral drugs for prevention and treatment of HIV: a review of recent research

Abstract

Introduction:

Despite significant advances in treatment and prevention of HIV-1 infection, poor adherence to daily combination antiretroviral therapy (ART) regimens remains a major obstacle towards achieving sustained viral suppression and prevention. Adherence to ART could also be compromised by adverse drug reactions and societal factors that limit access to therapy. Therefore, medicines that aim to improve adherence by limiting ART side effects, frequency of dosing and socially acceptable regimens are becoming more attractive.

Areas covered:

This review highlights recent advances and challenges in the development of long-acting drug delivery strategies for HIV prevention and treatment. Approaches for extended oral and transdermal deliveries, microbicides, broadly neutralizing antibodies, and long-acting implantable and injectable deliveries are reviewed.

Expert opinion:

Emerging approaches on long-acting antiretroviral therapies and broadly neutralizing antibody technologies are currently at various stages of development. Such efforts, if successful and become broadly accepted by clinicians and users, will provide newer and simpler options for prevention and treatment of HIV infection.

1. Long-Acting Antiretroviral Therapies for HIV Treatment and Prevention

Combined antiretroviral therapy (cART) has been remarkably effective in the treatment and prevention of human immunodeficiency virus (HIV) infection[1]. cART has greatly reduced disease morbidity and mortality and as a result has prolonged the lifespan of those living with HIV. Further, oral pre-exposure prophylaxis (PrEP) of uninfected individuals has been shown to effectively reduce the chances of acquiring HIV to near zero when taken correctly and consistently[2, 3]. Therapeutic effectiveness of cART and PrEP, however, is limited by lack of consistent use and adherence to the required strict daily dosing regimens[4–6], leading to development of viral resistance, treatment failure and lack of prevention. To address poor regimen adherence, long-acting formulations, offer a promising alternative to traditional regimens and thus could enhance the success rates for prevention and treatment and help lower rates of HIV-1 acquisition and transmission [7–9]. Longer-acting oral formulations, topical exposure strategies, microbicides and neutralizing antibodies, implantable devices, as well as long-acting injectable formulations are being developed (see Table 1). Strategies for improving the pharmacokinetics of oral formulations are focused on enhancing gastric residence and promoting bioavailability in order to reduce dosing frequency. Transdermal delivery approaches can provide sustained drug release that bypasses first-pass metabolism and is quickly reversible if needed. Longer-acting approaches for delivery of HIV microbicides to prevent infection are also being studied. , Broadly neutralizing antibodies that can remain in circulation longer than antiretroviral drugs (ARVs) could reduce levels of virus. Implants that provide sustained release of ARVs over months are being developed. Injectable formulations of ARVs currently in clinical trials can provide therapeutically effective concentrations of drug for two months for treatment and prevention. Finally, newly developed injectable ARV prodrug formulations have been shown to provide therapeutic plasma and tissue drug levels for up to one year in experimental animals. This review will discuss these strategies to extend dosing intervals for current ART and provide long-acting therapeutic regimens using new technologies. Strategies fitting the definition of “long-acting” proposed by Owen et al[10] will be included in the review. Specifically, technologies meeting dosing intervals of ≥ 1 week for oral and transdermal approaches, ≥ 1 month for microbicide and injectable approaches, or ≥ 6 months for implantable approaches have been selected for discussion[10].

Table 1.

Advantages and limitations of long-acting approaches to HIV-1 treatment and prevention

Approach	Advantages	Limitations
Oral delivery (nanoparticle and nano-emulsifying formulations, long acting ARVs)	• Potential for weekly dosing  • Convenience of self-administration	• Compared to daily dosing, weekly dosing could be more difficult to remember  • HIV-1 treatment requires drug partners with similar PK properties
Gastric residence devices	• Potential for weekly dosing  • Convenience of self-administration  • Administration of drug combinations possible	• Device failure could result in adverse events such as drug overdose or GI obstruction  • Compared to daily dosing, weekly dosing could be more difficult to remember  • Combination product that may be subject to additional testing and/ or regulations
Suspension and reservoir transdermal patches	• Potential of weekly dosing for PrEP  • Avoids first-pass metabolism  • Convenience of self-administration  • Easy to discontinue  • Can combine with contraceptives to prevent unintended pregnancy	• Subject to user error  • Device failure could result in drug overdose  • May be affected by daily activities such as bathing and swimming  • Combination product that may be subject to additional testing and/ or regulations
Transdermal microneedle patch	• Potential for weekly dosing for PrEP  • Avoids first-pass metabolism  • Convenience of self-administration  • Can combine with contraceptives to prevent unintended pregnancy	• Subject to user error  • Irreversible following microneedle dissolution  • May be affected by daily activities such as bathing and swimming  • Combination product that may be subject to additional testing and/ or regulations
Vaginal rings	• Once-monthly or greater potential dosing interval for PrEP  • Avoids first-pass metabolism  • Convenience of self-administration  • Can combine with contraceptives	• Subject to user error  • Combination product that may be subject to additional testing and/ or regulations
Broadly neutralizing antibodies	• Discreet  • Unique mechanism of action offers treatment alternative  • Can be used in both treatment and prevention regimens	• Combination approach required to avoid escape mutations  • Not a user controlled- method, therefore dosing may be less convenient  • Potentially difficult to administer in regions with limited access to healthcare
Subcutaneous implants	• Once every six months or greater dosing interval  • Avoids first-pass metabolism  • Discreet  • Reservoir devices are removable	• Device failure could result in drug overdose  • Biodegradable implants are not fully removable following polymer-dissolution  • Potentially difficult to administer in regions with limited access to healthcare  • Combination product that may be subject to additional testing and/ or regulations  • Use of organic solvents to dissolve hydrophobic drugs maybe challenging during scale up
Long acting cabotegravir and rilpivirine (cabenuva)	• Discreet  • Once every month or two months dosing interval  • Avoids first-pass metabolism	• Not reversible  • Not a user controlled- method, therefore dosing may be less convenient  • Monthly injections and clinical visits could be potentially challenging to implement in regions with limited access to healthcare
Long acting capsid inhibitor (GS-6207)	• Discreet  • Once every 3-6 months dosing interval  • Unique mechanism of action offers treatment alternative  • Avoids first-pass metabolism	• Not reversible  • Not a user controlled- method, therefore dosing may be less convenient
LASER ART	• Offers possibility of once-yearly dosing for PrEP  • Discreet  • Avoids first-pass metabolism  • Drug delivery to sites of infection  • Potential role in HIV-1 elimination strategies	• Not reversible  • Not a user controlled- method, therefore dosing may be less convenient

2. Long-Acting Oral Delivery

Oral combination antiretroviral drug therapy has had impressive success in both suppressing viral replication in HIV-infected individuals and preventing acquisition of HIV. For HIV-1 infected individuals on ART, side effects such as incomplete recovery of the CD4 T-cell population, mild neurocognitive dysfunction, and persistent immune activation remain. Despite these issues, oral ART regimens have greatly reduced HIV morbidity and mortality. Since the introduction of ART, the complexity and toxicity of oral regimens has reduced over time. Currently, once-daily single tablet regimens (STRs) have proven successful in both the treatment of HIV-1 infection, and for prevention of HIV-1 infection when utilized in PrEP regimens[1]. Though these STRs represent a significant advantage over early ART protocols[11] daily dosing still remains a burden to the patient, which may impair the high levels of adherence necessary for complete and long-term viral suppression and avoidance of drug resistance. Additionally, adolescents, a population at risk for HIV-1 acquisition, often have difficulties adhering to daily oral medication[12]. Thus, the development of oral sustained release dosage forms is of high interest due to the potential for ease of administration, enhanced pharmacokinetics, reduced dosing frequency, and improved adherence.

Although once-daily administration of oral ARVs is the current standard of care, less frequent administration of existing and investigational oral ARV’s may be a possibility. Tenofovir (TFV), whose intracellular active metabolite tenofovir-diphosphate has a half-life of 60–100 hours, could easily be given less often than once per day while maintaining antiviral effects [13]. With an elimination half-life of ~40 days after oral administration, cabotegravir (CAB) may also be a candidate for reduced dosing[14]. Efavirenz (EFV), with a plasma half-life of 45–60 hours, is also a good candidate for less frequent dosing. However, regimens involving administration every other or every 3rd day could be difficult to remember, if not more difficult, to adhere to than an every-day regimen[15]. Therefore, one possibility might be to aim for a once-weekly dosing, which offers greater convenience, and possibly adherence compared to daily therapy[16, 17].

2.1. Long-Acting Islatravir

Islatravir (ISL) is a novel investigational potent nucleoside reverse transcriptase–translocation inhibitor that could potentially enable weekly oral administration. When assessed as a PrEP agent in simian-human immunodeficiency virus (SHIV)-challenged rhesus macaques, once-weekly oral administration of ISL at a dose of either 1.3 mg/kg or 3.9 mg/kg prevented SHIV infection in all treated animals[18]. In humans, single doses of ISL as low as 0.5 mg suppressed HIV-1 RNA plasma levels by more than 1.0 log at day 7 in treatment-naive adults[19]. The prolonged efficacy observed for ISL distinguishes it from currently approved oral ARV agents. Its 3′-hydroxyl, 4′-ethynyl, and 2-fluoro groups are thought to contribute to its distinct pharmacologic attributes. Specifically, the 4′-ethynyl group binds to a conserved pocket in HIV-1 reverse transcriptase, interfering with extended primer translocation and resulting in chain termination[20, 21]. The 3′-hydroxyl group, a feature shared with natural nucleotides, binds to reverse transcriptase. Finally, the 2-fluoro on the adenine base slows deamination of the drug by adenosine deaminase, contributing to its long intracellular half-life of over 72 hours [22]. The structure-function relationship of ISL has been thoroughly reviewed by Markowitz, et al[23]. These features, combined with the promising results from macaque and human studies, suggest the potential for once-weekly dosing of ISL. In all, design of potent drug candidates with unique structural features may offer opportunities to extend the half-life of ARV agents, making infrequent oral administration feasible.

While modification of compound structural features and enhancement of drug potency may prove an effective approach to extending drug half-life[24, 25] many challenges remain. For instance, while drug monotherapy may be suitable for PrEP regimens, in the context of long-term HIV treatment, drug monotherapy is associated with viral rebound and emergence of viral resistance[26]. Therefore, one of the major challenges with weekly dosing of ISL (or any other long-acting oral agent) for treatment of HIV-1 infection would be finding suitable drug partners with similar pharmacokinetic properties. Given their longer half-lives co-administration with EFV, CAB, or TFV prodrugs are possibilities [13,14] but further work will be required to demonstrate the feasibility of such an approach.

2.2. Enhanced Gastric Residence Approach

Gastric residence devices could also reduce the dosing frequency of ARVs and improve patient adherence. In recent years there a number of systems have been designed to prolong gastric residence of drugs. Strategies such as high-density sedimentation, low-density flotation, mucoadhesion, and expansion have all prolonged in vivo gastric residence to varying degrees[27, 28]. In the case of ARVs, a modular drug delivery system capable of enhancing the gastric residence of up to six different drugs has recently been described. The device, which unfolds after capsule dosing, retains its star-like structure in the stomach, and has demonstrated sustained release of dolutegravir (DTG), CAB, and RPV for a week after a single dose in a swine model[29]. However, device complexity remains a major hurdle for translation; large-scale manufacture of these multi-component, multi-drug devices could prove both difficult and costly.

2.3. Oral Nanoparticle Formulations

Nanoparticle formulations have also been employed to enhance oral bioavailability and reduce dosing frequency. In the case of maraviroc (MVC), whose oral bioavailability in humans is estimated to be approximately 33% for a 300 mg dose, solid drug nanoparticle (SDN) formulations produced by emulsion template freeze drying (ETFD) have been employed to overcome MVC’s low oral bioavailability[30]. More specifically, a SDN MVC formulation demonstrated a 2.5-fold increase in AUC compared to conventional MVC treatment. Improvements for LPV and EFV formulations were also achieved through ETFD. Similar nanosuspensions have also improved the oral bioavailability of LPV and EFV[31, 32]. Augustine, et al., describe a nanoparticle system capable of enhancing the solubility of darunavir/ritonavir (DRV/r)[33]. The reported Nanoparticle-in-Microparticle Delivery System (NiMDS) contains nanoparticles of DRV/r encapsulated within film-coated microparticles. A series of polymethacrylate copolymers that coat the DRV/r nanoparticles ensure stability at gastric pH. In rats, the DRV/r-loaded NiMDS increased the oral bioavailability of DRV by 2.3-fold. The administration of antiretrovirals formulated as copolymer-stabilized nanoemulsions may also enhance oral ART bioavailability. Self-emulsifying systems are isotropic mixtures that contain an oil phase, the drug compound and a combination of surfactants that spontaneously emulsify on contact with the gastrointestinal fluids[34]. Lipid-based nanoemulsions have been reported to enhance drug absorption through a combination of mechanisms including: increasing the aqueous drug solubility, facilitating absorption through bile secretion, and inhibition of enzymes such as P-glycoprotein and cytochrome P450[35, 36]. These nanoemulsifying drug delivery systems have demonstrated enhanced bioavailability of a range of poorly-water soluble drugs, such as EFV, lopinavir (LPV), and DRV, in a Caco-2 gut epithelium model[36]. Altogether, nanoparticles designed for oral administration may be a strategy that could improve oral bioavailability and reduce the dosing interval of antiretrovirals.

In summary, while oral administration faces many challenges in terms of patient adherence, it offers advantages over other delivery routes. For example, unlike intramuscular injectables or implants, which require administration by a healthcare provider, oral delivery is painless, can be self-administered, and can be discontinued quickly to ensure drug is reduced quickly from the body should adverse reactions occur. By reducing dosing frequency, long-acting oral formulations could potentially improve regimen adherence. However, the consequence of a missed dose may be greater with intermittent dosing compared to daily dosing. For example, there is an increased risk of therapeutic ineffectiveness and emergence of resistant virus due to suboptimal drug levels, and an increased risk of toxicity should patients take additional doses to make up for those missed[37].

3. Long-Acting Transdermal Therapies

Transdermal systems of current interest include: (1) matrix patches, where the drug is incorporated into an adhesive polymer matrix and continuously released into the skin, (2) reservoir patches, which suspend the drug in a gel or solution and release is controlled by a microporous membrane, and (3) microneedles, which are patches composed of arrayed micron-size needles that are coated or impregnated with drug³¹. Transdermal drug delivery systems offer several benefits that include overcoming hepatic first pass metabolism and gastrointestinal (GI) degradation of drug. Additionally, these approaches are painless, easy to apply without the intervention of a healthcare provider, and could therefore if successful, facilitate patient acceptance and adherence to ART. Moreover, microneedle (MN) patches, which are designed to provide transdermal delivery by bypassing the outer layer of skin, could provide a means of long-acting ARV delivery. Antiretrovirals could benefit from delivery via MN patches. Because MNs deliver drug to the lymphatic system through the skin, they may enhance the efficacy of antiretrovirals by avoiding first pass metabolism and allowing for focused delivery to HIV viral reservoirs[38]. Additionally, lengths and array density of MNs can be tuned to maximize drug loading, possibly increasing plasma concentrations and allowing for weekly dosing intervals. However, unlike reservoir and matrix transdermal patches that can be easily discontinued if adverse reactions arise, microneedle patches are not reversible following microneedle dissolution.

3.1. Transdermal Patches and Microneedles

From a transdermal delivery perspective, the choice of antiretroviral agent is important as chemical and physical properties of the drug effect skin permeation. For instance, drugs with low molecular weight (<500 Da), melting point below 250°C, and moderate log P are ideal for passive permeation through skin[39]. Fitting within these parameters, tenofovir alafenamide fumarate (TAF) would be a good potential candidate for transdermal patch formulation given its molecular weight of 476.47 g/mol, melting point of 279°C, and a logP =1.8[40]. A recent study aimed to develop a transdermal patch for sustained release of TAF for approximately one week. In order to reach clinically relevant concentrations of TAF, it was determined that a 50 cm² transdermal patch should release a TAF dose of 8 mg/day, making the target permeation flux 7 μg/cm²/h. The authors developed a silicone-based patch composed of 15% (w/w) TAF demonstrated permeation of 7.24 ± 0.47 μg/cm²/h, demonstrating the feasibility of developing transdermal patches that can successfully achieved the target release and duration profile for TAF suitable for weekly drug dosing[40].

Microneedles have been employed in transdermal systems to enhance drug delivery. Recently, MN patches comprised of long-acting rilpivirine (RPV LA) nanosuspension were developed as a means to address imperfect ART adherence[41]. The patches were capable of penetrating skin in vitro and delivering RPV intradermally⁴⁰. In rats, the mean plasma concentration of RPV at day 7 post-application was found to be 431 ng/ml, a concentration approximately ten-fold greater than the trough concentration observed after a single intramuscular dose of RPV LA in human clinical studies⁴⁰. Therefore, the described MN array patches could potentially be used to deliver translatable human doses of RPV. Of note, the authors also detected RPV in rat lymph nodes, indicating the potential to deliver this ARV agent to a major site of viral replication. Overall, these studies provide proof of principle for the use transdermal patches, specifically MN patches, for HIV-1 treatment and prevention.

Overall, transdermal delivery of ARVs offers several advantages over other routes of administration (see Table 1). Transdermal patches may also address unmet needs for women. For example, women in low- and middle-income countries are at high risk of HIV infection and unintended pregnancy. There is an unmet need for products that provide long-acting protection against HIV and contraception, and transdermal patches may provide a means of addressing both needs simultaneously[42]. Current transdermal contraceptive patches require a once-weekly application[43]. Thus, if a once-weekly dosing interval could be achieved with ARV patches, a transdermal system that combines ARVs and contraceptive hormones could improve both adherence to PrEP and access to long-acting contraceptives.

However, transdermal systems face multiple challenges. For instance, to maintain consistent drug delivery, transdermal patches must strongly adhere to skin under all conditions such as sweating, bathing, and during physical activities. Host skin characteristics could also affect drug delivery from the patch, therefore comprehensive clinical pharmacokinetic testing should involve patients of different races, sex, ages, and body mass indexes to ensure that patches provide consistent drug exposures under varying conditions[44]. Additionally, due to prolonged physical adherence, transdermal patches are less discreet than other ARV delivery systems. Furthermore, current studies have only demonstrated transdermal delivery of single ARV drugs, however ARV drug combinations are required for treatment of HIV-1 to prevent viral rebound and resistance. Therefore, current transdermal technologies may be better suited for PrEP. However, further work would be required to demonstrate adequate drug biodistribution to rectal and cervicovaginal tissues for successful implementation in HIV-1 PrEP regimens. There are also disadvantages from a regulatory perspective. A combination of a drug and a device, patches could be subject to additional regulatory requirements and/or testing such as human factor and in vivo adhesion studies[45][46].

4. HIV-1 Microbicides

HIV microbicides are topical products generally comprised of an ARV drug, that when applied to the vagina or rectum has the potential to prevent the sexual acquisition of HIV in women and men. Topical microbicides may meet the needs of populations with adherence issues to oral daily PrEP. Adherence is central to PrEP effectiveness, thus developing products that support high adherence is important. There is a need for on-demand microbicide products because not all individuals who need an HIV prevention product would choose a long-acting product; for some HIV protection may only be desired during times of sexual activity. Microbicides can provide discreet and personal control over HIV prevention and offer the possibility of on-demand use. Today, a pipeline of microbicidal gels, films, inserts, and rings has been developed for evaluation in at-risk populations of men and women. Designed for on-demand use, most microbicidal products offer short-acting protection against HIV-1 infection. However, vaginal rings may provide a means of long-acting HIV-1 PrEP. Overall, microbicide strategies that would enable sustained release of therapeutic concentrations of ART for months at sites of viral exposure could potentially improve patient acceptability and adherence.

4.1. Vaginal Microbicides

The use of microbicides for HIV PrEP that women can self-administer to reduce the risk of infection is of great interest. Despite the incidence of new HIV infections declining for many populations, young women in Africa account for a fifth of new HIV infections[47], and are eight times more likely to be infected with HIV than their male peers[48]. Therefore, microbicides, such as gels, films and rings are potentially important female-controlled options for HIV prevention. Tenofovir gels were found to reduce the risk of HIV-1 acquisition in women if the product was used correctly and applied regularly as prescribed[49]. However, these products offer only short-acting protection from HIV infection. Vaginal film formulations may provide an attractive alternative to microbicide gels by delivering ARV drug to sites of viral exposure without the leakage associated with gel products. Once applied, vaginal film formulations dissolve in situ and release ARV drugs directly into vaginal fluids. In addition to providing a more acceptable format of PrEP to women, this technology may also offer longer duration of action. However, vaginal films have a low overall mass that compromises drug-loading capacity. Other alternatives include vaginal rings (VR), which have been successfully used for birth control, and are currently being evaluated for sustained delivery of ART (see Table 2). Compared to gels and films, VRs are longer acting and can provide monthly sustained ARV release. Therefore, by reducing the dosing interval and simplifying the use of antiretroviral medications VRs may enhance HIV-1 protection by facilitating PrEP adherence. Multiple phase 3, randomized, double blind, placebo-controlled trials of a monthly VR containing dapivirine, have demonstrated device safety and reduced risk of HIV-1 acquisition in a cohort of African women[50, 51]. Elsewhere, a VR aimed at preventing both HIV-1 infection and unintended pregnancy has been reported. The VR, which delivers TFV and levonorgestrel (LNG), was evaluated in a randomized, placebo controlled, phase I study (CONRADA13-128)[52]. The study demonstrated the safety of the TFV/ LNG rings, and observed higher anovulation rates in the participants randomized to the TFV/LNG VR compared to controls. Additionally, users of the TFV containing VR exhibited cervicovaginal aspirate TFV concentrations above 100,000 ng/mL 4 hours post insertion and TFV-DP concentrations in vaginal tissue above 1000 fmol/mg even 3 days after removal. However, more clinical evaluations are required to determine the efficacy of the dual contraceptive/ HIV-1 PrEP ring. Despite the potential of VR, not all ARVs are well suited for delivery in this format. For instance, a Phase I study evaluating vaginal rings containing dapivirine and MVC found that only dapivirine, but not MVC, demonstrated concentration-dependent inhibition of HIV-1 infection in cervical tissue[53]. During the study MVC concentrations were consistently detectable in CVF, but not plasma. Therefore, selection of ARV drug is important for the clinical translation of VRs for PrEP. In all, VRs offer a longer-acting mode of HIV PrEP. Recently, a yearly contraceptive ring gained FDA approval and these advances in VR technology may translate into VRs for HIV PrEP with even longer duration of action[54].

Table 2.

Long-acting approaches for the treatment and prevention of HIV

Agent	Route of Administration	Potential Indication	Stage of Development	References
Islatravir	Oral	HIV PrEP	Phase 1b	18–23
Gastric residence device	Oral	HIV treatment and PrEP	Preclinical	29
Maraviroc, efavirenz, and lopinavir solid drug nanoparticle formulations	Oral	HIV PrEP	Preclinical	30–32
Darunavir/ ritonavir nanoparticle-in-microparticle delivery system (NiMDS)	Oral	HIV PrEP	Preclinical	33
EFV, lopinavir, and darunavir nanoemulsifying systems	Oral	HIV PrEP	Preclinical	34–36
Tenofovir alafenamide fumarate patch	Transdermal	HIV PrEP	Preclinical	40
Rilpivirine microneedle patch	Transdermal	HIV PrEP	Preclinical	41
Dapivirine ring	Vaginal	HIV PrEP	Phase 3	50, 51
Tenofovir and levonorgestrel ring	Vaginal	HIV PrEP	Phase 1	52
3BNC117	Intravenous infusion	HIV PrEP	Phase 1	67
VRC01LS	Intravenous infusion or subcutaneous injection	HIV PrEP	Phase 1	68
3BNC117 + 10-1074	Intravenous infusion	HIV treatment and PrEP	Phase 1b	71, 72
Nevirapine implant	Subcutaneous	HIV PrEP	Preclinical	79
Dolutegravir implant	Injectable (subcutaneous)	HIV PrEP	Preclinical	82
Islatravir implant	Subcutaneous	HIV PrEP	Phase 1	84, 85
Tenofovir alafenimde fumarate implant	Subcutaneous	HIV PrEP	Preclinical	86–88
Refillable emtricitabine and tenofovir alafenamide fumarate implant	Subcutaneous	HIV PrEP	Preclinical	89
Cabotegravir nanofluidic implant	Subcutaneous	HIV PrEP	Preclinical	90
Long acting cabotegravir and rilpivirine (cabenuva)	Injectable (intramuscular)	HIV treatment and PrEP	Phase 3- USA Approved-Canada	96–99, 101,102
Long acting capsid inhibitor (GS-6207`	Injectable (subcutaneous)	HIV treatment	Phase 1	103, 104
Dolutegravir, lamivudine, emtricitabine, abacavir, rilpivirine, and cabotegravir LASER-ART	Injectable (intramuscular)	HIV PrEP	Preclinical	107–113, 120

4.2. Rectal Microbicides

The rectum is particularly vulnerable to HIV transmission having only a single protective layer of columnar epithelial cells overlying tissue rich in activated lymphoid cells. Therefore, unprotected anal intercourse in both sexes carries a higher risk of infection than other sexual routes. In the absence of effective vaccines, increasing attention has been given to the use of rectal microbicides as a PrEP modality. As with vaginal products, the focus has been on directly acting antiretroviral active pharmaceutical ingredients. In clinical trials microbicide gels[54–58] and films[59] reduce the risk of HIV-1 acquisition in men given correct product use and adherence[48, 49]. However, these products offer only short-acting protection from HIV infection. To date, no rectally-applied microbicide designed for long-acting PrEP has been described.

Overall, microbicides offer advantages over many other administration routes for PrEP. Unlike implants and injectables, microbicides are a self-administered and painless method of ARV delivery. Like oral and transdermal delivery, microbicide therapy can be easily ceased in the case of adverse events. Unlike other approaches to PrEP, topical microbicides may have the potential to be dosed intermittently. Additionally, because microbicides are applied to the vagina or rectum, ARV drug is delivered to sites of viral challenge. Compared to systemic PrEP agents, locally applied microbicides may also have fewer side effects. However, microbicides still face challenges in their implementation. For example, tenofovir was found to be three times as effective among women with a Lactobacillus-dominant vaginal microbiome compared to those with a Gardnerella vaginalis-dominant microbiome due to drug metabolism and inactivation by Gardnerella vaginalis [55]. Therefore, TFV microbicides may provide variable protection in women. Additionally, for female interventions such as VRs, factors such as male partner influence and menstruation underlie imperfect ring adherence and premature device removal [56].

5. Broadly Neutralizing Antibodies

HIV-1 broadly neutralizing antibodies (bNAbs) possess the ability to neutralize HIV-1 strains from diverse genetic and geographic backgrounds through the recognition of highly conserved epitopes. In contrast to current oral ARV regimens which offer highly effective, but short-lasting antiviral activity, bNAbs could remain in circulation longer, reducing the frequency of dosing. Currently, clinical development of bNAbs for the treatment and prevention of HIV-1 is of high interest. Second-generation bNAbs, isolated through cloning of antigen-specific antibodies from B cells of HIV-1 infected patients, target conserved sites of HIV-1 Env such as: N-glycan associated epitopes of V1/V2 (PG9)[57] and V3 regions (PGT121)[58], gp120/gp41 interface (35O22)[59], fusion peptide (VRC34, ACS202)[60, 61], CD4bs (VRC01, 3BNC117)[62], MPER on gp41 (10E8)[63], and silent face center (VRCPG05, SF12)[64]. To date, roughly a dozen second-generation bNABs have reached clinical development[65, 66] for HIV-1 treatment and prevention, demonstrating safety, tolerability, suppression of viremia, and enhanced immune function[67, 68]. Of interest, NCT02716675 and NCT02568215 are trials currently ongoing to evaluate the safety and efficacy of bimonthly intravenous administration of VRC01 in preventing HIV-1 infection in uninfected women, men, and transgender persons who have sex with men. However, the emergence of escape mutants highlights a major limitation of bNAb monotherapy[69, 70] and underscores the need for a combination approach for the treatment of HIV-1.

Indeed, combinations of second-generation bNAbs have demonstrated effective viral suppression and reduced viral escape in individuals with sensitive viruses[71, 72]. Additionally, although clinical studies have demonstrated suppression of viremia following administration of bNAbs in HIV-1 infected individuals, the effect is transient, further emphasizing the need for combinatorial strategies to achieve long-acting treatment of HIV-1[73]. Currently one clinical trial (NCT03739996) investigating the combination of long-acting CAB plus the bNAb VRC07–523LS in HIV-1 infected adults is underway (see Table 2). bNAbs generally exhibit short half-lives, however, antibody modifications such as Met428Leu and Asn434Ser (LS) substitutions within crystallizable fragment domains and paratope engraftment have resulted in bNAbs with prolonged in vivo half-lives, reduced autoreactivity, and increased potency[74, 75]. Altogether, these recent advances provide the framework for implementation of bNAbs in long-acting HIV-1 treatment and PrEP regimens.

6. Long-Acting Implants

Another technology being explored for sustained release of ART to improve patient adherence is implantable ART[8, 76]. The first and most notable successful clinical use of implants is that for hormonal therapy to prevent pregnancies, typically given as intrauterine devices and subcutaneous implants[77, 78]. Based upon the successful use of such devices, numerous antiretroviral implant technologies have been designed, however, they have met with little clinical success. In 2005 an implantable version of the non-nucleoside reverse transcriptase inhibitor (NNRTI) nevirapine was developed[79]. Nevirapine was shaped into 2.0 or 4.5 mm granular pellets coated with 5% poly(vinyl alcohol)(PVA) and subcutaneously implanted into rats for three months. However, the amount of active pharmaceutical compound released in blood was lower than the therapeutic drug concentration and a large drug burst release upon implantation was observed. Additional NNRTI work includes reports on a clinically-tested removable implant consisting of dapivarine loaded into a circular silicone-based matrix vaginal ring[80, 81]. Recent works with biodegradable removable implants have used DTG co-formulated with poly(lactic-co-glycolic acid) (PLGA) and N-methyl-2-pyrrolidone (NMP). The formulation solidifies upon administration, allowing removal, and reportedly provides therapeutic drug levels for up to nine months in mice[82]. Long-acting iterations of up to six drugs, based upon solubility with NMP, have been reported using similar technologies[83]. While erodible implants are promising, drug release rates, amounts of organic solvents required to solubilize hydrophobic drugs during scale-up, and implant degradation kinetics need to be optimized for safety and efficacy.

ISL (MK-8591) implants were created that were loaded with 40% weight of drug and coated with biocompatible polymers such as PVA or poly(caprolactone) (PCA). When implanted subcutaneously in rats and monkeys drug release was observed for 6 months[84]. ISL implants using non-erodible poly (ethylene vinyl acetate) were also described. Raman imaging was used to show distribution of drug within the implant prepared by solvent-free holt melt extrusion. The implant was a cylindrical silicone tube with pre-slit pores that determined drug release rate. An initial safety and tolerability study with a removal subdermal implant in humans reported therapeutic drug levels for four weeks and was well-tolerated[85]. Long-term safety concerns with degradation driven diffusion and implant size remain obstacles in these works.

Implantable devices containing TAF are of increased interest due to the drug’s potency and long-lasting active metabolite. A PCA membrane incorporating formulated TAF with a polyethylene glycol polymer or castor oil provided controllable in vitro release[86, 87]. Tests in beagle dogs demonstrated zero order drug release kinetics (the release rate is based off number of size of pores within PVA membrane) with substantially higher amounts of the active pharmacologically metabolite, tenofovir diphosphate (TFV-DP), detected in peripheral blood mononuclear cells than is required for PrEP for 40 days[88]. This device was non-degradable and constructed by punching 1 mm holes in silicone tubing, coated by a thin heat-annealed PVA membrane, and loaded with TAF free base. Other studies have described the development of refillable implants with the intent to limit the need for surgical removal and implantation. A silicon-coated nanomembrane channel medical grade titanium device was fabricated for both TAF and emtricitabine (FTC). Drug release rates were tuned using an algorithm to predict optimal channel size and physicochemical features of the ARV used. Therapeutic levels of TFV-DP and FTC-triphosphate (FTC-TP) were observed for three months when the devises were implanted in rhesus macaques[89]. A similar positive result was obtained with a 2-hydroxypropyl-β-cyclodextrin modified CAB using the same reservoir system[90]. Despite this promise, release rates from the devices were not uniform in vivo, and certain animals did not respond well to the implants used. A recent report described the development of an implant prepared by sealing TAF into a pellet with NaCl and magnesium stearate and inserting the pellets into a medical-grade polyurethane tube. When inserted subcutaneously in rabbits and rhesus macaques therapeutic TFV-DP levels were observed throughout the study, however substantial histological changes were observed in tissues surrounding the implant site[91].

In conclusion, implantable ART technology addresses common adherence issues, with multiple works showing the ability to deliver highly soluble compounds with release profiles that extend to several months in duration[84, 90, 92]. Further, release rates are tailorable according to drug and polymer selection. Implants are also discreet and can remove some of the stigma of taking ART from patients. Depending on polymer and device composition, implantable devices can be surgically removed or be biodegradable. The ability to remove a device can be advantageous in the case of adverse reactions or when a change in therapy is needed due to development of resistance mutations or upon patient request. However, removal requires invasive surgical procedures that may be costly and not practical in rural areas or settings where cost is of importance. The use of placebos to assess the safety of these devices needs further study as initial reports indicate substantial tissue necrosis after 5 and 12 weeks in animals given NRTI-loaded implantable devices[91]. Biodegradable devices require only one insertion, but drawbacks include the inability to remove them in the case of adverse events. The future of implantable ART will need to address long-term drug toxicities and more controlled release rates in vivo.

7. Long-Acting Injectables

Despite significant advances in treatment and prevention of HIV-1 infection, poor adherence to the daily cART regimen remains a major obstacle towards achieving sustained viral suppression. Adherence to cART could also be compromised by adverse drug reactions. Therefore, medicines that aim to improve adherence by limiting ART side effects and frequency of dosing are becoming more attractive. Long-acting (LA) ART injectables, implants, patches and vaginal rings have gained considerable interest and are at various stages of preclinical or clinical development [93]. Significant interest in LA ART delivery technologies has set a potential new standard for HIV/AIDS care. The most extensively studied LA ART dosage forms are injectable nanosuspensions of rilpivirine (RPV) and CAB which are being developed for monthly or every other month administration for HIV treatment and prevention. Considering the proven successful application of nanocrystal technologies to pharmaceutical products for other chronic conditions, novel LA ART depot injectable formulations that apply similar manufacturing processes are discussed. Specifically, nanosuspension approaches by either high-pressure homogenization or wet milling technologies have been successfully used to overcome obstacles associated with dissolution and delivery of small hydrophobic molecules[94].

7.1. Long-Acting Cabotegravir (CAB) and Rilpivirine (RPV)

Long-acting RPV and CAB are investigational once-every-month injectable formulations for the treatment of HIV infection. Intramuscular administration of CAB and RPV LA creates nanocrystal depots at the injection site that slowly dissociate to provide sustained release of each compound over extended periods of time[95]. Two phase III clinical trials - the Antiretroviral Therapy as Long-Acting Suppression (ATLAS) and First Long-Acting Injectable Regimen (FLAIR) are evaluating CAB and RPV LA in virologically suppressed patients who were switched from a standard oral three drug regimen. The patients in both studies were started on an oral CAB and RPV lead-in regimen before randomly switching to the monthly regimen. Of significance, the two-drug injectable regimen demonstrated comparable antiretroviral activity to the standard daily oral three-drug regimen for maintenance therapy at 48 weeks[96, 97]. Also, even though the incidence of injection-site reactions was high in both studies, more than 90% of those surveyed preferred the monthly injectable over daily oral therapy. However, limitations of CAB and RPV LA include the requirement for large injection volumes, the risk for emergence of drug resistant virus strains during sub-therapeutic exposure after treatment discontinuation and limited access of native CAB and RPV to cellular and tissue reservoirs of infection[98–100]. To achieve once every two months dosing of CAB and RPV, an ongoing phase III clinical trial, the Antiretroviral Therapy as Long-Acting Suppression every two Months (ATLAS-2M), is evaluating the efficacy and tolerability of the two formulations at higher drug doses in virally suppressed patients. At week 48, the ATLAS-2M study demonstrated comparable efficacy and safety profiles to monthly dosing[101]. The HIV Prevention Trials Network (HPTN) 083 study is a phase 2b/3 double-blind study designed to evaluate the safety and efficacy of CAB LA for HIV prevention administered every eight weeks compared with the daily oral two drug regimen of tenofovir disoproxil fumarate and FTC (TDF/FTC) that is currently indicated for PrEP. A recent announcement from HPTN 083 reported that fewer participants using CAB LA became HIV positive compared to those using the daily oral TDF/FTC regimen[102]. These encouraging findings underscore the potential important role of long-acting single drugs for PrEP. Extended release parenteral dosage forms that would overlap patient dosing schedules with routine once every three to six months CD4+ T cell count and viral load tests could potentially have a major impact on the effectiveness of HIV treatment and prevention efforts.

7.2. Long-acting HIV-1 capsid inhibitor

A novel potent investigational HIV-1 capsid inhibitor GS-6207 is currently being evaluated for treatment of HIV infection[103]. The inherent hydrophobicity and metabolic stability of GS-6207 enabled development of a long-acting surfactant stabilized aqueous suspension of the capsid inhibitor for subcutaneous administration. A single dose of the capsid inhibitor in HIV negative patients demonstrated sustained therapeutic drug concentrations with the potential to be administered once every three months. Importantly, the study recorded no serious adverse events[104]. An ongoing phase 1 clinical trial (NCT03739866) is evaluating the efficacy and safety profiles of injectable suspensions of GS-6207 at doses of 150, 450 and 900 mg in HIV patients. The development of such long-acting formulations with a unique mechanism of action will provide treatment alternatives to existing ART.

7.3. Long-Acting Slow Effective Release (LASER) ART

HIV treatment strategies have been stalled in part by limited therapeutic access to anatomical reservoirs of infection. Attempts to overcome these obstacles have led to transformation of native ARVs into long-acting slow effective release antiretroviral therapy (LASER ART). LASER ART is comprised of homogeneous dispersions of solid prodrug nanocrystals stabilized by aqueous surfactant solutions [8, 105, 106]. Reversible chemical modifications of existing ARVs into lipophilic and hydrophobic prodrugs facilitated the application of nanocrystal technology to water insoluble and hydrophilic compounds to affect formulation and drug transport across physiological barriers[100, 107–113]. In addition to high drug loading, LASER ART extends the apparent half-lives of ARVs by altering drug metabolism and solubility profiles[114–116]. Recent efficacy studies in humanized mice demonstrated the importance of drug delivery to intracellular and tissue sites of infection[117]. Delivery and storage of long-acting formulations of ARVs was enhanced by co-treatment with URMC-099, a mixed lineage kinase 3 inhibitor, suggesting that adjunctive therapies could be used to facilitate the activity of long-acting ARVs[118, 119]. Also, under preclinical evaluation are optimized injectable cabotegravir prodrug nanocrystals with significantly improved efficacy and pharmacokinetic profiles demonstrating the feasibility of once-yearly dosing of CAB[120]. Longer intervals between doses will potentially improve ART adherence and could also facilitate implementation of formulation strategies that require dosing in clinical settings. In addition to enabling less frequent dosing, efficient delivery and sustained release of therapeutic concentrations of ART at intracellular and tissue sites of infection could be combined with other approaches to potentially facilitate HIV elimination as was recently demonstrated in a subset of humanized mice using LASER ART and CRISPR-Cas9 technologies[117].

8. Expert Opinion

HIV treatment and prevention with a goal of viral eradication is a challenge that needs to be met. Current oral therapies, while effective in preventing disease and reducing disease morbidity and mortality cannot meet this goal alone. Lack of adherence to daily oral dosing regimens leads to viral resistance, reduced treatment effectiveness and lack of prevention. Long-acting formulations and devices for ART delivery provide a means of meeting these challenges. These strategies that included oral formulations with enhanced pharmacokinetics, extended delivery of microbicide products, combination use of bNAbs and ARVs, implantable delivery devices, and long-acting formulations for extended parenteral delivery build upon already proven ARV therapies. Recent success in eliminating viral infection in an experimental model of infection using a combination of long-acting ART and CRISPR-Cas9 highlights the benefits of long-acting intracellular and tissue targeted drug delivery approaches[117]. However, to be truly effective the medicines must be safe, bioavailable, have better or comparable efficacy to existing daily oral medicines, provide sustained therapeutic drug concentrations at sites of action, reach reservoirs of HIV replication, be accessible and be affordable. For these goals to be met and for these strategies to have a positive impact on HIV treatment and prevention, several challenges need to be realized and overcome. Production of long-acting drug delivery systems will require careful evaluation of the resources and equipment needed for cost-effective production under good manufacturing practices guidelines[121]. Development of simple, scalable, efficient and cost-effective synthesis and production processes would enable the use of these technologies in resource-limited areas where PrEP can have an immediate impact[9, 121, 122]. Generic companies could be engaged during early stages of development to facilitate cost-effective production and distribution in countries where the medications are utilized[47]. Also, ARV and delivery system stability would need to be optimized for shipment and long-term storage at ambient temperature in low and middle income and tropical and subtropical regions[9, 47]. Finally, and importantly, the at-risk and patient populations that can most benefit from these formulations should be identified and access for those who are willing to accept such formulation regimens will need to be facilitated.

Article Highlights:

• Desirable features for long-acting drug delivery strategies

• Long-acting strategies for HIV treatment and prevention

• Long-acting slow effective release prodrug approaches for improved intracellular and tissue drug delivery

• Potential role of long-acting formulations in HIV eradication strategies

• Potential implementation challenges for long-acting drug delivery strategies