Updated: Sep 19, 2021
The small molecule macrocyclic lactone ivermectin, approved by the US Food and Drug Administration for parasitic infections, has received renewed attention in the last eight years due to its apparent exciting potential as an antiviral. It was identified in a high-throughput chemical screen as inhibiting recognition of the nuclear localizing Human Immunodeficiency Virus-1 (HIV-1) integrase protein by the host heterodimeric importin (IMP) α/β1 complex and has since been shown to bind directly to IMPα to induce conformational changes that prevent its normal function in mediating nuclear import of key viral and host proteins. Excitingly, cell culture experiments show robust antiviral action towards HIV-1, dengue virus (DENV), Zika virus, West Nile virus, Venezuelan equine encephalitis virus, Chikungunya virus, Pseudorabies virus, adenovirus, and SARS-CoV-2 (COVID-19). Phase III human clinical trials have been completed for DENV, with >50 trials currently in progress worldwide for SARS-CoV-2. This mini-review discusses the case for ivermectin as a host-directed broad-spectrum antiviral agent for a range of viruses, including SARS-CoV-2.
Keywords: ivermectin; antiviral; SARS-CoV-2; COVID-19; flavivirus; dengue virus; Zika virus
The 2015 Nobel Prize for medicine recognizes the seminal contribution of Campbell and Ōmura in terms of the “wonder drug” ivermectin, a macrocyclic lactone 22,23-dihydro-avermectin B produced by the bacterium Streptomyces avermitilis , as a novel therapeutic against “infections caused by roundworm parasites”; this was alongside Tu Youyou for her seminal work on artemisinin and malaria ]. Discovered in 1975, ivermectin was marketed successfully from 1981 for parasitic infection indications in animals and then approved for human use for activity against onchocerciasis (river blindness) in 1987. It has since been used successfully to treat several human parasitic worm infestations causing river blindness/filariasis, strongyloidiasis/ascariasis, ectoparasites causing scabies, pediculosis, and rosacea. More recent applications include controlling insect mediators of infection, such as malaria. Ivermectin is on the World Health Organization’s Model List of Essential Medicines.
From 2012 onwards, there have multiple reports that ivermectin has antiviral properties  towards a growing number of RNA viruses, including human immunodeficiency virus (HIV)-1, influenza, flaviviruses such as dengue virus (DENV) and Zika virus (ZIKV) and, most notably, SARS-CoV-2 (COVID-19) . Evidence for activity against DNA viruses is more limited but encompasses Pseudorabies, polyoma, and adenoviruses . The basis of ivermectin’s broadspectrum antiviral activity appears to relate to the fact that ivermectin binds to, and inhibits, the nuclear transport role of the host importin α (IMPα) protein , which is known to mediate nuclear import of various viral proteins and key host factors, but other possible antiviral actions of ivermectin have been proposed (e.g., ), including in the case of SARS-CoV-2 (e.g., ]). This mini-review will summarise the weight of evidence for ivermectin’s broad-spectrum antiviral activity and the basis of its IMPα-directed activity in light of the possibility that ivermectin could be a critically useful antiviral in the current SARS-CoV-2 crisis .
Ivermectin as an IMPα Targeting Agent with Antiviral Activity
Transport into and out of the nucleus is central to eukaryotic cell and tissue function, with a key role to play in viral infection, where a common strategy used by viruses is to antagonize the cellular antiviral response . The targeting signal-dependent mediators of this transport are the members of the IMP superfamily of proteins, of which there are multiple α and β forms . The pathway mediated by the IMPα/β1 heterodimer is the best-characterized pathway by which host proteins, including members of the signal transducers and activators of transcription (STATs) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) transcription factor families, enter the nucleus through nuclear envelope-embedded nuclear pores. A large number of viral proteins (e.g., [27,28]) also use this pathway (see Figur1), where IMPα within the IMPα/β1 heterodimer performs the adaptor role of specific targeting signal recognition, while IMPβ1 performs the main nuclear roles of binding to/translocation through the nuclear pores, and release of the nuclear import cargo within the nucleus (Figure 1) 
Schematic showing IMPα’s role in nuclear transport of host and viral proteins, and mechanism of inhibition by ivermectin. (a) Host proteins, such as members of the STAT or NF-κB transcription factor families, localize in the nucleus through the action of the IMPα/β1 heterodimer, where the “IBB” (IMPβ-binding) region of IMPα (green curved line) is bound by IMPβ1 to enable cargo recognition by IMPα within the heterodimer; IMPβ1 subsequently mediates transport of the trimeric complex through the nuclear pore (NPC, nuclear pore complex) embedded within the nuclear envelope (NE) into the nucleus. This is followed by release within the nucleus to enable the transcription factors to carry out the normal function in transcriptional regulation, including in the antiviral response. IMPα can only mediate nuclear import within the heterodimer with IMPβ1. (b) In a viral infection, specific viral proteins (e.g., NS5 in the case of DENV, ZIKV, WNV) able to interact with IMPα utilize the IMPα/β1 heterodimer to access the nucleus and antagonize the antiviral response . This is critical to enable optimal virus production as shown by mutagenic and inhibitor studies. Which SARS-CoV-2 proteins may access the nucleus in infected cells has not been examined (see ). (c) The IMPα targeting compound ivermectin binds to IMPα (binding site shown as a red lozenge) both within the IMPα/β heterodimer to dissociate it and to free IMPα to prevent it binding to IMPβ1, thereby blocking NS5 nuclear import . GW5074 (see Table 1) has been shown to exhibit a similar mechanism .
Ivermectin as an Antiviral in the Clinic
One of the greatest challenges in antiviral research, as in many other disciplines, is to transition from laboratory experiments to preclinical/clinical studies, with the question of dosing challenging ]. However, it is important to stress the obvious in this context: that the antiviral activities of ivermectin documented in Table 1 have been derived from laboratory experiments that largely involve high, generally non-physiological, multiplicities of infection, and cell monolayer cultures, often of cell lines such as Vero cells (African green monkey kidney, impaired in interferon α/β production) that are not clinically relevant. The results in Table 1 for low μM EC50 values should not be interpreted beyond the fact that they reveal robust, dose-dependent antiviral activity in the cell model system used, and it would be naïve to strive for μM concentrations of ivermectin in the clinic based on them.
A key consideration in any clinical intervention using ivermectin is its host-directed (IMPα-directed) mechanism of action. Host-directed agents that impact cellular activities that are essential to healthy function must be tested with caution; although ivermectin has an established safety profile in humans [23,25], and is US Food and Drug Administration-approved for several parasitic infections [, it targets a host function that is unquestionably important in the antiviral response, and titration of a large proportion of the IMPα repertoire of a cell/tissue/organ likely to lead to toxicity. With this in mind, where a host-directed agent can be a “game-changer” in treating a viral infection may well be in the initial stages of infection or even prophylactically (see Section 6) to keep the viral load low so that the body’s immune system has an opportunity to mount a full antiviral response.
Ivermectin’s real potential as an antiviral to treat the infection can, of course, only be demonstrated in preclinical/clinical studies. Preclinical studies include a lethal Pseudorabies (PRV) mouse challenge model which showed that dosing (0.2 mg/kg) 12 h post-infection protected 50% of mice, which could be increased to 60% by administering ivermectin at the time of infection . Apart from the many clinical trials currently running for SARS-CoV-2 (see below), the only other study thus far reported relates to a phase III trial for DENV infection . Almost 70% of the world’s population in over 120 countries are currently threatened by mosquito-borne flaviviral infections, with an estimated 100 million symptomatic DENV infections and up to 25,000 deaths each year from dengue hemorrhagic fever [41,42], despite sophisticated large-scale vector control programs. As for the closely related ZIKV (cause of large outbreaks in the Americas in 2015/2016), the dearth of antiviral treatments and challenges in developing efficacious vaccines hamper disease control. Clinical data published in preliminary form for the phase III trial in Thailand  indicate antiviral activity; daily dosing (0.4 mg/kg) was concluded to be safe, and have virological efficacy, but the clear clinical benefit was not reported, potentially due to the timing of the intervention. The authors concluded that dosing regimen modification was required to ensure clinical benefit . This study both underlines ivermectin’s potential to reduce viral load in a clinical context and highlights the complexities of timely intervention and effective dosing regimens to achieve real clinical benefit in the field.
A Viable Treatment for SARS-CoV-2?
Despite efforts in multiple domains, the current SARS-CoV-2 pandemic has now eclipsed the porcine flu epidemic in terms of numbers of infections (rapidly nearing 30 million) and deaths (>930,000) worldwide. The search for antivirals for SARS-CoV-2 through repurposing existing drugs has proved challenging (e.g., see ), one important aspect of repurposing being the perceived need to achieve therapeutic levels in the lung. Published pharmacokinetic modeling based on both the levels of ivermectin achievable in human serum from standard parasitic treatment dosing and robust large animal experiments where lung levels of ivermectin can be measured, indicates that concentrations of ivermectin 10 times higher than the c. 2.5 μM EC50 indicated by in vitro experiments (Table 1) are likely achievable in the lung in the case of SARS-CoV-2 ; modeling based on different assumptions predicts lower values but stresses the long-term stability of ivermectin in the lung (for over 30 days) based on data from animals . It should also be noted that liquid formulations for intravenous administration of long-acting ivermectin have been described, with aerosol administration also in development, to enable ivermectin administration to achieve even higher concentrations to tackle SARS-CoV-2, whilst the use of ivermectin in combination with other agents may enhance efficacy at lower doses.
There are currently more than 50 trials worldwide testing the clinical benefit of ivermectin to treat or prevent SARS-CoV-2 (see Table 2).
An instinctive response in developing antiviral agents is to strive for high specificity, making the idea of virus-targeted agents specific to a particular viral component or function attractive, since, ideally, they circumvent the possibility of impacting host function. However, the high propensity of viral genomes, and particularly those of RNA viruses, to mutate and evolve means that selection for resistance can be all too prevalent (e.g., for HIV). Importantly, the high specificity of an agent to a particular virus also inevitably means that its utility against a distinct virus may be limited or non-existent. Thus, it is not surprising that repurposed antivirals active against influenza or HIV, for example, may prove efficacious against distantly related flaviviruses or coronaviruses.
In contrast, host-directed antivirals can be repurposed more easily, as long as the viruses in question rely on the same host pathway/functions for robust infection, simply because the host pathway/function targeted is the same . Although potential complication here is the viral tissue tropism (e.g., blood or lung in the case of systemic or respiratory viral infections) and accompanying pharmacokinetic considerations, selection for viral resistance is largely circumvented in this scenario. As long as toxicity is not an issue, host-directed agents thus have the potential to be genuinely broad-spectrum agents against various viruses that rely on a common host pathway. The fact that so many viruses rely on IMPα/β1-dependent nuclear import for robust infection ( and see above) means that agents targeting this pathway have true potential to be broad-spectrum antivirals. After decades of use in the field, ivermectin clearly “fits the bill” here in terms of human safety, but whether it turns out to be the molecule that proves this principle will only begin to be established unequivocally, one way or another, in the ensuing months concerning SARS-CoV-2.
Credited to MDPI