Numerous antivirals approved for the treatment of HSV-1 and HSV-2 infections are acyclic nucleoside and nucleotide analogs that interfere with the elongation of viral genome during replication, which is carried out by the viral DNA polymerase (UL30). Acyclovir (ACV) is an acyclic guanosine analog discovered in the sponge Cryptotethya crypta and is at present the most frequently used compound to treat HSV-1 and HSV-2, mainly because of its low price, tolerability and safety (Hassan et al., 2015). Importantly, acyclovir needs to be activated intracellularly by its phosphorylation into acyclovir triphosphate for exerting its antiviral activity. This process is carried out by the viral thymidine kinase (TK, UL23 gene), which catalyzes acyclovir into acyclovir monophosphate, thus increasing the concentration of acyclovir within infected cells by reducing its exit from the cell (Reusser, 1996; Kukhanova et al., 2014). Additional phosphorylations are carried out by cellular kinases and once in its triphosphate form, acyclovir becomes a substrate for the viral DNA polymerase interfering with DNA synthesis (Reusser, 1996; De Clercq, 2013). Importantly, inhibition of the synthesis of new viral genome copies, which translates lesser formation of novel infectious viral particles, does not affect latent virus within host neurons and hence, does not cure infection (Poole and James, 2018). Other limitations related to the treatment with ACV also exist. For instance, oral intake of ACV has an absorption efficiency of only 15–30% (Bean, 1992), and previous reports indicated that senior patients that had renal problems could experience significant neurotoxicity, because they were not able to properly excrete this drug (Chau, 2018). Another important limitation associated to ACV is the ability of HSV-1 and HSV-2 to mutate and generate variants that are resistant to this drug by acquiring point mutations in the gene encoding for the viral thymidine kinase (TK), which decreases enzyme expression or modifies substrate specificity abrogating acyclovir phosphorylation, or by acquiring mutations in the gene encoding for the viral DNA polymerase (UL30), which may enable HSV-1 and HSV-2 to replicate in the presence of ACV (Reusser, 1996). Such ACV-resistant variants occur mainly in immunosuppressed individuals, as they are otherwise generally attenuated in immunocompetent individuals (Field, 2001; Morfin and Thouvenot, 2003).

Oral intake of ACV for treating skin lesions produced by HSV-1 and HSV-2 only reduces the healing process in little more than 2 days (time to loss of scab), from 7.9 days (placebo group) to 5.8 days, if taken as soon as signs of the prodrome or erythema are detected (Spruance et al., 1990). Because these differences are moderate, although statistically significant, the clinical benefit of these antiviral drugs for the treatment of herpetic lesions has been somewhat questioned (Kroon, 1990; Mindel, 1991). Importantly, when the drug is taken at the stage of papule, a significant effect for ACV is not observed (Spruance, 1993). Furthermore, within this scenario, the recovery time of cutaneous lesions was found to be larger in the treated group (antiviral) than in the placebo group (8 days vs. 7.2 days, respectively). Regretfully, nearly 50% of patients fail to perceive initial stages of the prodrome and the erythema phases before papule formation and therefore these individuals will not be in time to start an effective oral treatment with ACV in such a way to significantly reduce the time of the herpetic lesions (Spruance et al., 1990; Spruance, 1993). Thus, treatment with ACV has poor benefits under these circumstances (Spruance et al., 1990).

As an alternative to oral intake, ACV can be applied topically as a cream. Although topical application of ACV over herpetic lesions at the papule stage has beneficial effects, these benefits are relatively weak, since they only reduce healing time in approximately one or two days during herpes labialis (Bean, 1992; Evans et al., 2000) and three days in genital herpes infections (Thin et al., 1983; Leflore et al., 2000).

Besides ACV, there are other alternatives to treat skin lesions caused by herpes simplex viruses, such as valacyclovir, penciclovir, and famciclovir, which are also considered first-line drugs to treat HSV-1 and HSV-2 and thus, are frequently used (Kimberlin and Whitley, 2007).

These drugs are nucleic acid analogs, similar to ACV with a shared mechanism of action that interferes with the function of the viral DNA polymerase (De Clercq, 2013). These compounds differ from each other mainly in their bioavailability, half-life in the body and dosing (Wagstaff and Bryson, 1994), yet similar to ACV they reduce herpetic lesions and associated pain in approximately 1–3 days, as compared to untreated groups when used topically (Emmert, 2000).

To increase the bioavailability of acyclovir, an L-valine ester of acyclovir was developed. Valacyclovir is a prodrug of ACV with enhanced absorption at the intestinal level (54%) (Soul-Lawton et al., 1995). Later, penciclovir was developed with the aim of being phosphorylated more rapidly than ACV, and consequently has a higher half-life than acyclovir (Hodge and Perkins, 1989). While acyclovir has a half-life of 0.7 h for HSV-1 and 1 h for HSV-2, penciclovir has a half-life of 10 h for HSV-1 and 20 h for HSV-2 (Luber and Flaherty, 1996). Famciclovir is a prodrug that derives into penciclovir and has increased oral bioavailability (Hodge et al., 1989). Clinical benefits granted by these drugs have also generated discussions on the recommendation of their use to treat skin lesions caused by herpes simplex virus infections (Chi et al., 2015; Chen et al., 2016).

Valacyclovir has also been approved for the treatment of HSV-1 and HSV-2 infections and clinical manifestations produced by HSV-1 and HSV-2, such as cold sores and recurrent genital herpes, as well as VZV and cytomegalovirus (Ormrod et al., 2000). Furthermore, famciclovir is approved for treating herpes viruses, such as HSV-1 and HSV-2 (genital herpes and orolabial herpes), as well as VZV (Simpson and Lyseng-Williamson, 2006).

Resistance to valacyclovir, penciclovir, and famciclovir can occur. Furthermore, there is cross-resistance between valacyclovir- and acyclovir-resistant HSV isolates (Reusser, 1996), because valacyclovir is derived from acyclovir (Moomaw et al., 2003). On the other hand, cross resistance to penciclovir and the prodrug famciclovir may arise in acyclovir-resistant HSV-1 isolates in immunocompromised patients (Boyd et al., 1993). Some studies have reported HSV-1 and HSV-2 resistance to penciclovir in cell cultures and in immunocompromised patients related to TK-deficiency (Boyd et al., 1993; Sarisky et al., 2002, 2003; Bacon et al., 2003).

In the last decade, alternatives to ACV for herpesvirus treatment have emerged and become commercially available as therapeutic drugs. Ganciclovir is a nucleic acid analog that does not require viral proteins for its activation in the cell (i.e., viral TK) (Markham and Faulds, 1994; Prichard et al., 2011; Vere Hodge and Field, 2013; Poole and James, 2018). These compounds differ from each other either, in their molecular processing into an active form of an acyclic guanosine analog, or bioavailability which significantly determines the frequency of administration (De Clercq, 2013).

Ganciclovir is indicated for the treatment of CMV, particularly for systemic and ocular infections in immunosuppressed patients (Buhles et al., 1988; Poole and James, 2018). However, ganciclovir has also been reported to have antiviral activity against HSV-1 and HSV-2 and may be used for the treatment of herpetic keratitis (Smee et al., 1983; Matthews and Boehme, 1988; Chou and Hong, 2014). At present, there is an ongoing clinical study that is recruiting patients for assessing the effects of oral ganciclovir, together with femtosecond laser-assisted corneal debridement in the treatment of herpes simplex virus epithelial keratitis (ClinicalTrials.gov NCT03217474). Of note, ganciclovir has been reported to produce considerable adverse side effects in a high percentage of individuals (Kimberlin and Whitley, 2007), such as nephrotoxicity, neutropenia, myelosuppression, confusion, altered mental status, anxiety, ataxia, tremors, convulsions, fever, abnormal levels of liver enzymes in serum, diarrhea, and nausea, among others (Kimberlin and Whitley, 2007).

Other alternative drugs that have emerged in the last decades as second-line drugs that are commercially available are cidofovir (an acyclic nucleotide analog) (Ashley et al., 1988; Blot et al., 2000; James and Prichard, 2014) and foscarnet (a pyrophosphate analog) (Crumpacker, 1992; Wagstaff and Bryson, 1994). When compared to each other, these two compounds affect different steps in the viral replication cycle. Cidofovir acts as a nucleotide analog and polymerase inhibitor with a high affinity for the viral DNA polymerase, with cidofovir-diphosphate having 25–50 fold higher affinity for the viral DNA polymerase than the host DNA polymerase, causing thus a more effective block in the replication of viral DNA than acyclovir (Ho et al., 1991). Importantly, cidofovir has a phosphonate group that does not require an initial phosphorylation step by HSV proteins (Ho et al., 1991). Cidofovir has shown to be efficacious against acyclovir-resistant isolates of HSV-1 or HSV-2 in vitro (Chilukuri and Rosen, 2003). However, this drug has not been approved for the treatment of herpes simplex viruses in humans. Yet, a clinical study was carried out to evaluate the effectivity of topical cidofovir for refractory mucocutaneous HSV-1 and HSV-2 in AIDS; However, the results of this clinical study have not been reported (ClinicalTrials.gov Identifier: NCT00002116). On the other hand, foscarnet inhibits the viral DNA polymerase of herpesviruses by binding near to the pyrophosphate binding site that is needed for polymerase activity (Crumpacker, 1992; Poole and James, 2018). In contrast to nucleoside analogs, resistance to foscarnet only occurs because of mutations in the viral DNA polymerase gene (Reusser, 1996).

Regretfully, these two drugs also produce numerous adverse effects in patients, such as nephrotoxicity, azotemia, proteinuria, crystalluria, interstitial nephritis, acute tubular necrosis, increases in the concentrations of creatinine up to 50%, hypo- and hypercalcemia, hypo- and hyper-phosphatemia, and the formation of urogenital ulcers, among others (Deray et al., 1989; Sauerbrei, 2016). Because of these effects, it is recommended that patients receiving foscarnet be monitored clinically to control abnormalities in metabolites and electrolytes that may result in the alterations indicated above.

Resistance to foscarnet has been observed in immuno- compromised individuals, particularly in patients having undergone bone marrow transplants. On the other hand, there are only few studies reporting cidofovir-resistant HSV isolates. In one case, three patients with bone marrow transplants received cidofovir as a therapy, but nevertheless showed HSV-related diseases symptoms. Cidofovir resistance was confirmed in one of the three cases with the isolate showing mutations that truncated the C-terminal of the viral DNA polymerase (Wyles et al., 2005). Notably, cidofovir resistance has also been reported in children, particularly in three patients with hematopoietic stem cell transplants that received, in a prophylactic manner acyclovir and cidofovir together, because ganciclovir produced adverse effects. Unfortunately, these children showed HSV-related stomatitis during cidofovir treatment and the authors suggested that the treatment with cidofovir did not prevent HSV-1 reactivation in the patients (Dvorak et al., 2009). Nevertheless, cidofovir is considered a good option when encountering HSV isolates that express reduced amounts of enzymes that are related to the phosphorylation of nucleoside analogs, or resistance to foscarnet (Reusser, 1996). For example, Blot et al. (2000) reported a clinical case of a child affected by a variant of HSV-1 resistant to ACV and foscarnet (in vitro), in which case the treatment with cidofovir was effective against this drug-resistant HSV-1. Another study reporting an HSV-1 isolate resistant to both, ACV and foscarnet in a girl with lymphatic leukemia indicated that only cidofovir treatment was successful at helping avoid recurrent oral stomatitis (Bryant et al., 2001).

Due to the somewhat reduced clinical benefits of the drugs mentioned above in the treatment of skin lesions caused by herpes simplex viruses, new therapeutic alternatives have emerged in the most recent years. One of these alternatives is a combination of acyclovir and hydrocortisone for topical use (Xerese^®, Medivir). This formulation reduces the duration of herpetic lesions by 1.6 days (compared to 1.0 days by acyclovir when applied alone as a topic in that study) and reduces the size of the lesion area by 50% (Strand et al., 2012). While this approach yields a statistically significant improvement in the treatment of herpetic lesions, it still evidences the need for identifying new drugs or drug combinations that have even better effectiveness.

Another relatively new drug to treat skin lesions caused by HSV-1 is docosanol 10% formulated as a topical cream (Abreva^®, Avanir), which is the only FDA approved formulation available over the counter (OTC) to treat HSV-1 symptoms. This drug consists of an aliphatic chain (hydrophobic linear chain) with an alcohol group at one of its ends and has emulsifying properties, which is why it has been used in both, the cosmetic and food industries. Currently, docosanol 10% cream is approved for the treatment of HSV-1 lesions. However, the literature available on its effectiveness is somewhat scarce and more studies seem to be required to compare its effectiveness side by side with other compounds such as acyclovir 5% cream and Xerese^® (Woo and Challacombe, 2007). One study indicates that docosanol 10% cream reduces the healing time of oral lesions due to HSV-1 and HSV-2 by 18 h (Sacks et al., 2001). The mechanism of action of docosanol would be mediated by the inhibition of the fusion of the virus to the cell membrane (Pope et al., 1998).

On the other hand, another drug marketed to treat skin lesions due to HSV-2 infection is Viroxyn^® (Quadex Pharmaceuticals), which consists of benzalkonium chloride, a Category III antiseptic, that is also used for various applications, mainly as a biocidal preservative. This compound would act as a virucidal agent over herpes simplex viruses (Bélec et al., 2000). Although Viroxyn has been sold for more than 16 years, there are only limited studies that have evaluated its effectiveness, some proposing that it is more effective than Abreva^® (McCarthy et al., 2012). However, in 2016 the FDA announced a ban on the sale of numerous bactericidal ingredients, leaving benzalkonium chloride in a “stand-by” status until obtaining clinical results that prove its safety in humans, so it may eventually be recalled (“Safety and Effectiveness of Consumer Antiseptics; Topical Antimicrobial Drug Products for Over-the-Counter Human Use.” 2016-09-06. Retrieved October 05, 2016) (Wolf, 2017).

Finally, another compound marketed for treating skin lesions caused by HSV-1 is Novitra^®, which consists of a zinc oxide-based cream. This compound has been shown to reduce HSV-1 skin lesions by up to 1.5 days, compared to untreated individuals (Godfrey et al., 2001). Other aspects, such as pain formation and itching, were also shown to be improved with its use. Table 1 summarizes the antiviral compounds discussed above.

Other anti-HSV compounds, are the nucleoside analogs idoxuridine and vidarabine, which are currently discontinued as there are at present better and more effective treatments available. Trifluridine, which is also a nucleoside analogs is mainly used to treat herpetic keratitis.

Idoxuridine is a thymidine (pyrimidine) analog that was identified as the first effective topical agent against HSV infection (Chou and Hong, 2014; Wilhelmus, 2015), and was mainly used topically as an ointment for treating epithelial keratitis caused by HSV-1 infection of the corneal epithelium (Roozbahani and Hammersmith, 2018). However, its efficacy was clouded by its toxicity to the corneal epithelium of the eye and poor hydrosolubility and thus, has been currently replaced in favor of more effective, better-tolerated and less-toxic compounds (Wilhelmus, 2015).

On the other hand, vidarabine is a purine analog with fewer side-effects than idoxuridine, yet it is also poorly soluble and therefore its use is limited to topical formulations, being less preferable than other current drugs available (Chou and Hong, 2014).

Trifluridine on the other hand is a synthetic pyrimidine nucleoside that is frequently used for the treatment of herpetic keratitis as a topical formulation. This drug was approved by the FDA in 1980 for its use as a 1% solution treatment for HSV-related keratitis and is at present one of the most common used topical antiviral for this type of affection in the United States (Chou and Hong, 2014), with considerable effectivity reported (Wilhelmus, 2015). However, local side effects have been described, some particularly severe. Table 1 summarizes the antiviral activity of these compounds.

Finally, another anti-HSV agent is brivudine (BVDU), a pyrimidine analog that acts as a prodrug, phosphorylated by viral thymidine kinase only and thus targeting the viral DNA polymerase (Wilhelmus, 2015). This compound has been proven to be efficacious against HSV-1 and at least as effective as acyclovir in the treatment of HSV-1 infection (Chou and Hong, 2014). Currently, is mainly used in different countries for treatment of VZV infections (De Clercq, 2019).

Currently, several new anti-herpetic drugs are being assessed in clinical trials, such as brincidofovir (Quenelle et al., 2010; Prichard et al., 2011), amenamevir (Chono et al., 2010), pritelivir (Wald et al., 2014), and nelfinavir mesylate (Kalu et al., 2014).

Brincidofovir is an acyclic nucleotide phosphonate, similar to cidofovir, yet it is conjugated to a lipid (Jiang et al., 2016). When brincidofovir enters the cell, the lipid sidechain is cleaved and the compound is phosphorylated, acting as a substrate inhibitor for the viral DNA polymerase. Noteworthy, brincidofovir accumulates within the cell significantly more than cidofovir, and has up to 1,000-fold higher antiviral activity as compared to the latter (Hostetler, 2009). Brincidofovir was evaluated in phase III clinical trial that has concluded, yet to our knowledge the results have not been reported (ClinicalTrials.gov Identifier: NCT01143181).

On the other hand, amenamevir and pritelivir target the viral DNA helicase/primase complex (H/P) (Kleymann et al., 2002; Poole and James, 2018). There are three finished clinical studies for pritelivir, yet similar to brincidofovir, the results have not been reported to the best of our knowledge (ClinicalTrials.gov Identifier: NCT01047540, NCT01658826, and NCT02871492). There is also one clinical study that is currently ongoing and is in the recruitment phase for immunocompromised subjects with acyclovir-resistant mucocutaneous HSV-1 or HSV-2 infection (ClinicalTrials.gov Identifier: NCT03073967). Amenamevir is an oxadiazolephenyl derivate that belongs to the helicase-primase group of inhibitors and has been evaluated in at least three clinical trials, although the results have not been published (ClinicalTrials.gov Identifier: NCT02209324, NCT01959295, and NCT02852876) (Chono et al., 2010). Regretfully, a study indicates that amenamevir displayed adverse events in an early clinical phase against HSV-1 and HSV-2 (Chono et al., 2010).

Finally, nelfinavir mesylate, the mesylate salt of the antiviral drug nelfinavir which has been characterized as a HIV-1 protease inhibitor (Patick et al., 1996; Markowitz et al., 1998), was found to have antiviral activity against HSV-1 and other herpesviruses and to inhibit the maturation and export of viral particles (Kalu et al., 2014). As a consequence, nelfinavir mesylate is currently being assessed in a clinical study for the treatment of patients with Kaposi’s sarcoma, as well as its potential effectivity activity against HSV-1 and HSV-2, which is a tertiary goal in this study (ClinicalTrials.gov Identifier: NCT03077451).

Many studies have reported the existence of algae with bioactive compounds that display potent antiviral activity against numerous viruses, such as dengue (Lee et al., 2006; Pujol et al., 2012), avian influenza (Gerber et al., 1958), HIV (Lee et al., 1999; Thuy et al., 2015), human papillomavirus (HPV) (Buck et al., 2006), and picornavirus (Lee et al., 2009). Also, numerous studies have reported algae with antiviral activity against herpes simplex viruses (Faral-Tello et al., 2012; Ribeiro et al., 2012; Hassan et al., 2015). A study performed in Brazil analyzed more than 36 species of algae from its coasts and reported that four of them had significant antiviral activity against both, HSV-1 and HSV-2. In their study, the authors suggested that the antiviral activity of extracts of green alga Stypopodium zonale (Ochrophyta) against HSV-1 was related to the secondary metabolite atomaric acid (Figure 1) (Ribeiro et al., 2012). On the other hand, the antiviral activity of Ulva fasciata and Codium decorticatum against HSV-1 were shown to be mediated by fatty acids present in high concentrations in the extracts, yet the precise molecules involved were not identified or reported (Ribeiro et al., 2012). For the red alga Laurencia dendroidea, the antiviral activity against HSV-1 was likely mediated by sesquiterpenes (Ribeiro et al., 2012).

Another example of an alga with antiviral effects against HSV is Hypnea musciformis, a red seaweed present in Italy, which has shown strong antiviral activity against HSV-1, even in different aqueous fractions obtained from its processing. Several mechanisms of action for these preparations were identified, such as virucidal activity and the inhibition of virus binding into the cell (Mendes et al., 2012). Other reports have also described antiviral effects for different algae extracts against HSVs, such as Xalas, a derivative of Scinaia hatei that has antiviral activity against HSV-1 and HSV-2, which inhibits the entry of the virus into the cell, and is likely mediated by sulfated xylans present in the extract (Figure 1) (Damonte et al., 2009). On the other hand, Osmundaria obtusiloba, an algae obtained from the Brazilian coast was reported to have antiviral activity against both, HSV-1 and HSV-2, which was suggested to be mediated by algae glycolipids interacting with viral glycoproteins (Souza et al., 2012). Padina pavonia, another algae that inhibits HSV-1 replication, has been reported to have a bioactive compound consisting of sulfated polysaccharides (fucoidan) that hampers the binding of the virus to the surface of the cell (Figure 1) (Hayashi et al., 2008).

Extracts derived from the green microalga Haematococcus pluvialis have also shown anti-herpetic activity. Extracts obtained from this microalga through pressurized liquid extraction displayed inhibitory action against HSV-1 replication, which was suggested to be mediated by an inhibition in the attachment of the virus to the host cell, the virus-cell fusion process and/or virus entry into the cell (Santoyo et al., 2012). Another example of an alga extract with antiviral activity against HSV-1 was obtained from Cystoseira myrica, which strongly inhibits the replication of this virus (Zandi et al., 2007).

Interestingly, researchers have isolated and studied a chemically modified polysaccharide from the green alga Enteromorpha compressa with anti HSV-1 activity (Figure 1). Notably, this study reported total viral inhibition when the evaluated compound was added to human Hep-2 cells infected with a clinical isolate of HSV-1 in a time-of-addition assay. Because the effect was maintained when applied post-treatment, the authors suggested that the antiviral activity might be mediated by the inhibition of virus replication and/or viral protein synthesis (Lopes et al., 2017).

Another alga which has been studied is Eucheuma gelatinae, a red alga that is widespread in tropical and subtropical regions. Polysaccharides obtained from this organism were tested for their antiviral activity against HSV in vitro using Vero cells infected with ACV-sensitive or ACV-resistant HSV-1 and HSV-2 isolates (Figure 1). In this study, strong antiviral activity against HSV was observed in early stages of infection affecting the attachment of HSV. Additionally, it was shown experimentally that viral protein synthesis was affected through the evaluation of the expression of the viral protein VP5, as well as the cellular localization of this protein which is normally found in the nucleus. After the treatment with the alga extract, VP5 was mainly found in the cytoplasm (Jin et al., 2015). Table 2 summarizes the antiviral activity of these compounds.

Griffithsin is a lectin extracted from the red alga Griffithsia sp., which has been shown to be capable of inhibiting HSV infection in a murine model of HSV-2 infection (Figure 1) (Nixon et al., 2013). The mechanism of action of Griffithsin consists on its binding to mannose N-glycosylations that blocks the infection process of HSV-2 and inhibits cell-to-cell spread of the virus. The results with this compound have been promising and its effectiveness is expected to be evaluated in humans in the near future. Additionally, Griffithsin has been evaluated as an antiviral in combination with carrageenan both, in cell cultures and in animals to evaluate the potential synergic effect of the two compounds against HSV-2 in a murine model of infection. The study showed a strong reduction in HSV-2 infection when applied together as a prophylactic, namely between 10 min and 1 h prior to infection (Levendosky et al., 2015). A more recent article reported the efficacy of Griffithsin in combination with carrageenan against HSV-2 infections together with HPV and HIV-1, which was evaluated in both, a murine and rhesus macaque model using a vaginal fast-dissolving insert (Derby et al., 2018). This study showed that a fast-dissolving vaginal insert with low moisture content is able to protect against SHIV in macaques, while in mice it showed promising results protecting against HSV-2 and HPV.

Notably, Symphyocladia latiuscula is a red macroalga that has been reported to produce compounds with virucidal antiviral activity against HSV-1 (Figure 1). Vero cells infected with HSV-1 and treated with extracts from this microalga showed reduced plaque formation. The antiviral effect of these compounds has also been evaluated in a murine model skin infection, which showed decreased skin lesions compared to controls when administrated 4 h before infection and then three times per day for 6–10 days. Also, skin obtained from these treated and infected animals showed lesser plaque forming units than the control (Park et al., 2014).

On the other hand, Hayashi and colleagues reported that fucoidan from the brown macroalga Undaria pinnatifida, which are sulfated polysaccharide, have antiviral activity against HSV-1 and HSV-2 that is mediated by hampering the binding of the virus to the cell surface (Figure 1) (Lee et al., 2004). When the effect of fucoidan was tested in another study against corneal infection with HSV-1, a reduction in herpetic lesions was found in those animals that received pretreatments with fucoidan during 1 week (Hayashi et al., 2008). Finally, a study performed by Alboofetileh et al. (2019), showed that fucoidans extracted from the brown seaweed Nizamuddinia zanardinii exert strong antiviral activity against HSV-2 infection. They found that an algal extract containing this compound inhibited the attachment of HSV-2 to Vero cells, inhibiting the early phase of HSV-2 infection (Alboofetileh et al., 2019). Table 2 summarizes the antiviral activity of these compounds.

Another study reported that chemically modified polysaccharides from the green algae E. compressa of the Ulvaceae family, has antiviral activity against HSV-1 infection. In this work the green algae was processed and chemically modified polysaccharides were purified and tested in plaque reduction assays, determining an antiviral effect mediated after virus penetration (Lopes et al., 2017).

Interestingly, a recent article showed antiviral activity against HSV-2 for two purified sulfated polysaccharides isolated from the brown alga Sargassum henslowianum, a species found in southeastern China and Asia (Sun et al., 2019). In this work, algae extracts were treated to purify the polysaccharides named SHAP-1 and SHAP-2, which were later evaluated in viral plaque formation assay for both HSV-1 and HSV-2 virus. However, following experiments in this research article focused on HSV-2 rather than HSV-1, arguing greater clinical value for the former (Sun et al., 2019). Time-of-addition assays, as well and adsorption and penetration assays suggest an antiviral effect at early stages of infection (Sun et al., 2019).

Fungus-derived compounds have also been explored for identifying novel molecules with antiviral activity against HSV-1 and HSV-2. The fungus Aspergillus versicolor has been shown to produce secondary metabolites, such as anthraquinones with anti-herpetic activity (Figure 1). In the study by Huang et al. (2017), three anthraquinones were found to have antiviral effects against HSV-1 in vitro using Vero cells. Another fungus of the Aspergillus genus showed antiviral activity against HSV-1, with two secondary metabolites flavoglaucin and isodihydroauroglaucin derived from A. ruber being assessed, and two compounds showing anti-HSV-1 effects (Figure 1) (Liang et al., 2018). In another study, peptides produced by the marine-derived fungus Scytalidium were found to have antiviral activity against HSV-1 and HSV-2, particularly by a peptide named Halovir, which was suggested to have virucidal activity when in direct contact with HSV-1 and HSV-2 (Figure 1) (Rowley et al., 2003).

Furthermore, an aqueous extract from Inonotus obliquus (AEIO) was shown to inhibit HSV-1 infection in Vero cells (Pradeep et al., 2019). the antiviral activity was detected at early times during viral infection, suggesting AEIO blocks viral entry, particularly membrane fusion (Pradeep et al., 2019).

Interestingly, fungus proteins that inhibit HSV infection have also been identified. Two proteins that bind polysaccharides from the mushroom Ganoderma lucidum were found to have antiviral effects against HSV-1 and HSV-2. One was named the neutral protein bound to polysaccharide (NPBP) and the other the acidic protein bound to polysaccharide (APBP). Although APBP had more potent antiviral activity than NPBP, both inhibited plaque formation by both types of HSV. Interestingly, it was found that the mechanism of action of APBP was mediated by the inhibition of the attachment and penetration of the virus into Vero cells (Eo et al., 2000). Table 2 summarizes the antiviral activity of these compounds.

Sulfated compounds isolated from a polysaccharide (MI-S) derived from Agaricus brasiliensis have shown antiherpetic activity against HSV-1 and HSV-2 (De Sousa Cardozo et al., 2014). MI-S inhibited viral adsorption and penetration into Vero cells and had a synergistic effect with acyclovir (De Sousa Cardozo et al., 2014). Interestingly, this compound was also shown to reduce the severity of HSV-2 disease in a murine genital infection model with one single application (Cardozo et al., 2013).

On the other hand, Qing et al., reported that a protein from Grifola frondosa (GFAHP) had antiviral activity against HSV-1, which was shown to have virucidal effects in cell cultures and suppressed viral entry into Vero cells (Gu et al., 2007). Furthermore, GFAHP also showed antiviral activity against HSV-1 in animals when applied topically to the cornea of mice. Mice treated with GFAHP had a significant reduction in blepharitis, vascularization and stromal disease, as well as reduced viral replication in the cornea (Gu et al., 2007). Notably, the antiviral protein RC28 obtained from the fungus Rozites caperata showed antiviral activity against HSV-1 in Vero cells (Pradeep et al., 2019). Moreover, the authors evaluated the antiviral effect of RC28 in an animal model and observed that this peptide decreased the severity of stromal keratitis (Pradeep et al., 2019). Table 2 summarizes the antiviral activity of these compounds.

Plant extracts have received particular attention when searching for new molecules with anti-herpetic activity (Li et al., 2017; Akram et al., 2018). Interestingly, numerous plant-derived extracts and compounds have been reported to inhibit HSV replication. For instance, organic extracts belonging to the Peganum harmala species have been described to have antiviral activity against HSV-2 and to interfere with virus entry (Benzekri et al., 2018).

Essential oils extracted from plants belonging to the Labiatae and Verbenaceae families have also been shown to have antiviral activity against HSV. Vero cells incubated with HSV and plant-extracted essential oils for 48–72 h significantly reduced HSV-1 and HSV-2 viral titers. Interestingly, their mechanisms of action were found to be related to the pre-infective stages (Brand et al., 2016). Another study that also assessed essentials oils, but extracted from Glechon spathulata and Glechon marifolia identified antiviral activity against HSV-1, which was effective after the infection of Vero cells (Figure 1) (Venturi et al., 2015). Eucalyptus essential oils are used to treat symptoms during cough and bronchitis, in numerous presentations, such as ointments, liniments, in oral form and in vapor baths as inhalants (Kaur et al., 2013). The main components of the essential oils from the Eucalyptus species are cineole and alpha-pinene (Sebei et al., 2015). One study evaluated the Australian tea tree oil and Eucalyptus oil over HSV and reported antiviral activity against both, HSV-1 and HSV-2. Treatments before, during or post-infection determined that these compounds had virucidal activity affecting the virus before or during adsorption, before the virus had entered the cells (Schnitzler et al., 2001). Ethanolic extracts from the leaves of E. camaldulensis were shown to inhibit HSV-1 and HSV-2 infection when the extracts were added to Vero cells during- and post-infection. Additionally, in this study a synergistic effect between acyclovir and the ethanolic extracts was reported in cell cultures (Abu-Jafar and Mahmoud, 2017). In another study, researchers reported 24 new metabolites in the leaves of E. sideroxylon and four new metabolites within the genus Eucalyptus that have antiviral activity using an ultra-performance liquid chromatography coupled to photodiode-array and electrospray ionization mass spectrometer (UPLC/PDA/ESI-qTOF-MS). Within the antiviral activities detected, there were compounds that also inhibited hepatitis A, coxsackie and adenoviruses, besides HSV-1 and HSV-2. Interestingly, the highest antiviral activity was observed against HSV-2, with the antiviral effect acting pre-treatment (virucidal), thus inhibiting virus entry and subsequent infection processes, while the antiviral effect against HSV-1 was only observed when the extract was incubated with the virus previous to the cell infection (Okba et al., 2017). In addition, twelve compounds isolated from the leaves and twigs of E. globulus were found to have antiviral activity against HSV-1 and HSV-2. In this study, Tereticornate A was identified to have the greatest activity against HSV-1, which was higher than acyclovir. Cypellocarpin C displayed the strongest antiviral activity against HSV-2, greater than that observed for acyclovir (Brezáni et al., 2018).

Other essential oils, particularly those obtained from the leaves of Melissa officinalis, which is better known as lemon balm, have also been shown to have antiviral activity against both, HSV-1 and HSV-2 with the antiviral activity attributed to tannins and non-tannin polyphenolic fractions within the extract (Mazzanti et al., 2008). Another study determined that volatile oils from M. officinalis Lamiaceae had antiviral activity against HSV-2 (Allahverdiyev et al., 2004). On the other hand, extracts from Aglaia odorata, Moringa oleifera, and Ventilago denticulate have also been shown to have antiviral activity against wild-type and drug-resistant HSV-1 isolates, yet the mechanisms of action seems to not have been identified yet, or reported (Lipipun et al., 2003).

Plants used in traditional Chinese medicine have also been tested and found to have antiviral activity (Ganjhu et al., 2015). Recently, a study reported that leaves from Mulberry (Morus alba L.), a plant that is common in Asia, has antiviral properties against HSV-1 and HSV-2. The active compound within this plant has been reported to be Kuwanon X, a stilbene polyphenol derivative, which has antiviral activity over HSV at multiple steps of the infection process, inhibiting cellular adsorption and penetration, as well as the expression of HSV-1 immediate early and late genes, and the synthesis of HSV-1 DNA (Figure 1) (Ma et al., 2016). Another study showed that aqueous extracts from Houttuynia cordata, a Chinese herbal medicine, blocks HSV-2 infection by inhibiting NF-κB activation, a host transcription factor that has been reported to be required for effective HSV infection (Amici et al., 2006). In order to identify the compounds with antiviral activity within H. cordata, several flavonoid compounds in this plant were evaluated individually to determine their capacity to block the replication cycle of HSV-2. Quercetin, quercitrin, and isoquercitrin, the major flavonoid compounds found within H. cordata were found to be strong inhibitors of HSV-2 activity (Figure 1) (Chen et al., 2011). A subsequent study from another group determined that the mechanism of action behind the anti-herpetic activity of H. cordata occurred at multiple levels, such as at the adsorption level, entry, post-infection acting over NF-κB and had virucidal activity (Hung et al., 2015). Another stilbene compound that inhibits NF-κB activation is resveratrol, which was isolated from Veratum grandiflorum (Chen et al., 2012), and it is the main bioactive compound found in berries, peanuts, legumes and other plant-derived matrices, as well as red wine. Several studies have reported antiviral properties for this compound over the replication cycles of ACV-resistant and wild-type HSV-1 and HSV-2, both in cell cultures and in animal models (Figure 1) (Faith et al., 2006; Leyton et al., 2015).

Aqueous and hydroalcoholic extracts derived from native plants of Chile have also been investigated for their antiviral activity against HSV-1 and HSV-2. Hydroalcoholic extracts of Cassia stipulacea and Escallonia illinita displayed antiviral activity against HSV-1, while hydroalcoholic extracts from Aristotelia chilensis, Drymis winteri, Elytropus chilensis, as well as an aqueous extract from Luma apiculata showed antiviral activity against HSV-2. The active antiviral compounds within these preparations have not been identified or reported yet (Pacheco et al., 1993). In addition, another group determined that aqueous extracts from Quillaja saponaria, a Chilean soapbark tree that is endemic in the central zone of Chile, has antiviral activity against HSV-1 and other viruses. This extract, which is currently used in food and beverages was reported to have virucidal activity by blocking the attachment of viruses to the cell surface (Roner et al., 2007). Table 2 summarizes the antiviral activity of these compounds.

Several studies have assessed the antiviral effects of plant-derived compounds against herpes simplex virus in animal models either, alone or combined with acyclovir. One of these studies found that each of the following, Geum japonicum Thunb., Rhus javanica L., Syzygium aromaticum (L.), or Terminalia chebula Retzus displayed increased antiviral activity against HSV-1 when combined with acyclovir, as compared to acyclovir alone (Kurokawa et al., 1995). On the other hand, extracts from A. odorata, M. oleifera, and V. denticulate were shown to have antiviral effects against HSV-1 upon cutaneous infections in BALB/c mice. Here, the plant extracts combined with ACV and orally administered to the mice were shown to hamper the development and progression of HSV-1 skin lesions and increased the mean survival times of the animals (Lipipun et al., 2003). Chikusetsusaponin IV, a compound extracted from Alternanthera philoxeroides has shown to have antiviral activity against HSV-2 when the compound is added to the inoculum for 1 h before viral infection or immediately after viral infection (Rattanathongkom et al., 2009). The authors suggested that the mechanism of action of the antiviral activity action of Chikusetsusaponin IV was virucidal (Rattanathongkom et al., 2009). Moreover, Chikusetsusaponin IV showed antiviral activity against HSV-2 genital infection in mice when administrated three times per day three days before infection and up to 7 days after infection (Rattanathongkom et al., 2009). Finally, Meliacine (MA) a glycopeptide obtained from Melia azedarach has been reported to have antiviral activity against acyclovir-sensitive and acyclovir-resistant HSV-1 (Barquero et al., 1997). Furthermore, MA showed favorable results against HSV-2 in a mouse model of infection when applied topically immediately after infection with HSV-2 (Petrera and Coto, 2009). Table 2 summarizes the antiviral activity of these compounds.