key: cord-1022653-4j8i5nl2
authors: Chen, Ran; Wang, Tingting; Song, Jie; Pu, Daojun; He, Dan; Li, Jianjun; Yang, Jie; Li, Kailing; Zhong, Cailing; Zhang, Jingqing
title: Antiviral Drug Delivery System for Enhanced Bioactivity, Better Metabolism and Pharmacokinetic Characteristics
date: 2021-07-22
journal: Int J Nanomedicine
DOI: 10.2147/ijn.s315705
sha: 2b365a2547783ec79feb086d8162caf0994f469d
doc_id: 1022653
cord_uid: 4j8i5nl2

Antiviral drugs (AvDs) are the primary resource in the global battle against viruses, including the recent fight against corona virus disease 2019 (COVID-19). Most AvDs require multiple medications, and their use frequently leads to drug resistance, since they have poor oral bioavailability and low efficacy due to their low solubility/low permeability. Characterizing the in vivo metabolism and pharmacokinetic characteristics of AvDs may help to solve the problems associated with AvDs and enhance their efficacy. In this review of AvDs, we systematically investigated their structure-based metabolic reactions and related enzymes, their cellular pharmacology, and the effects of metabolism on AvD pharmacodynamics and pharmacokinetics. We further assessed how delivery systems achieve better metabolism and pharmacology of AvDs. This review suggests that suitable nanosystems may help to achieve better pharmacological activity and pharmacokinetic behavior of AvDs by altering drug metabolism through the utilization of advanced nanotechnology and appropriate administration routes. Notably, such AvDs as ribavirin, remdesivir, favipiravir, chloroquine, lopinavir and ritonavir have been confirmed to bind to the severe acute respiratory syndrome-like coronavirus (SARS-CoV-2) receptor and thus may represent anti-COVID-19 treatments. Elucidating the metabolic and pharmacokinetic characteristics of AvDs may help pharmacologists to identify new formulations with high bioavailability and efficacy and help physicians to better treat virus-related diseases, including COVID-19.

Many infectious diseases have rapid propagation and high infection rates with potential for human pandemics. Emerging infectious diseases caused by such viruses as severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS) and Ebola virus, present major threats to public health. 1 The current outbreak of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to spread. 2 COVID-19 may cause severe cytokine storm and aggravate immunopathological damage, which make antiviral treatment more complicated. The related cytokines include but not limited to TNF-α, IL-1 and IL-6. [3] [4] [5] With the explosive growth of confirmed cases, the World Health Organization (WHO) declared the outbreak to be a public health emergency of international concern on January 30, 2020. 6 At present, COVID-19 has spread to more than 200 countries and regions around the world, and the outbreak of COVID-19 has caused serious damage worldwide. As of 14:00 on July 19, 2021, there were a total of 191,229,635 diagnosed cases of COVID-19 diagnosed, and 4,105,799 deaths globally. Compared with SARS and MERS, the spread rate of COVID-19 is more rapid, probably due to the increased globalization and virus adaptability to various environments. 7 At present, drugs employed to fight viral infections include natural medicines [8] [9] [10] [11] (eg, liquorice, Scutellaria baicalensis and forsythia), chemical drugs 12 (eg, ribavirin, remdesivir, favipiravir) and biotechnology-derived drugs 3 (eg, IFN-α, IFN-β and peptide). Natural antiviral medicines have multiple targets and moderate effects, and they usually contain complex ingredients but exhibit only limited efficacy. 13 Biotechnology-derived AvDs have high curative effects and induce low drug resistance, but they usually need to be administered by injection due to their poor stability and bioavailability, and they are very easily inactivated in vivo. 14 Chemical AvDs inhibit viruses quickly and strongly, and as most are administered orally, their use and storage are convenient. Chemical AvDs are the main treatment employed in antiviral therapy. 15 The physicochemical properties of these drugs, as well as their metabolism, affect their efficacy to varying degrees. [16] [17] [18] To improve the antiviral efficacy of AvDs, we need to comprehensively understand the characteristics of AvDs. In this review, we systematically investigated the structural, physicochemical, metabolic, kinetic and bioactive characteristics and pharmacological effects of popular nucleoside analogs (NA-AvDs) and non-nucleoside analogs of AvDs (NN-AvDs) currently on the market ( Figure 1 ).

The scientific community is urged to explore and develop novel potent antiviral agents. Considering that vaccine development is time-consuming and that viruses mutate quickly, antivirals remain the main treatment for viral infections. To date, there are α-interferon, lopinavir/ ritonavir, ribavirin, chloroquine phosphate, and arbidol have been proven to possess effectiveness in the general treatment of COVID-19, and tocilizumab (biological oseltamivir) had low permeability, probably due to the hydroxyl groups in their molecular structures, which increase their polarity. Also, 25% of NA-AvDs (such as remdesivir, favipiravir) and 88% of NN-AvDs (such as chloroquine, lopinavir, arbidol, ritonavir, peramivir, baloxavir marboxil and letermovir) had low solubility due to their hydrophobic macromolecular structures.

Oral drugs require sufficient solubility and intestinal absorption to enable drug molecules to reach action sites and exert therapeutic effects. 25 Solubility is a prerequisite to confirm drug absorption and clinical response for drugs given orally. 23 Permeability represents the speed and extent of oral drug diffusion through the mucus layer and then through the submucosa and epithelial cell barriers into the blood or lymphatic circulation. 26 Currently, numerous advanced pharmaceutical technologies have been applied to increase the solubility and permeability of AvDs, including adding auxiliary ingredients (such as latent solvents and penetration enhancers) and applying cutting-edge preparation methods (such as inclusion technology, solid dispersion technology and micronization technology and nanotechnology).

Lopinavir solid dispersions were developed by using Soluplus as a polymeric solubilizer. 27 An in vitro characterization study showed that Soluplus solubilized lopinavir in water almost linearly as a function of concentration by creating H-bonds of water with the drug carbonyl group and forming micelles in water. A Caco-2 cell transport study demonstrated that Soluplus significantly enhanced the permeability of lopinavir through the rat intestine via H-bond or micelle formation and P-glycoprotein (P-gp) inhibition. The bioavailability of lopinavir in Soluplus matrixed extrudate was 3.70-fold that of lopinavir crystal. Soluplus (polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol grafted copolymer) is a new amphiphilic nonionic medicinal polymer material that not only changes the interface state of the solution system but also increases the solubility of poorly soluble drugs. 28 

The polymeric micelles enhanced the solubility and permeability of acyclovir. 29 The apparent solubility value (1.39 mg/mL) of acyclovir polymeric micelles was 1.36-fold that of acyclovir. The amount of acyclovir in the polymeric micelles that passed through the cornea in 6 h was approximately 10 times greater and the lag time was apparently shorter than that of aqueous solution. Polymeric micelles are colloidal structures of block copolymers: hydrophobic fragments form the spherical inner core, encapsulating poorly water-soluble drugs, while hydrophilic fragments form the outer shell. 30 

A lyophilized milk-based solid dispersion was developed to enhance the solubility and permeability of ritonavir. 31 Ritonavir was dispersed in an amorphous polymer matrix and existed primarily in a molecularly dispersed state. 32 This formulation (drug:carrier mass ratio of 1:4) exhibited higher dissolution efficiency (~55.26 ± 1.29%, representing a 10-fold increase compared to pure ritonavir). Ex vivo 

permeation research indicated that the permeation extent of ritonavir formulation (33~75% w/w) was ~1.5~3.7-fold greater than that of pure ritonavir (~20%).

The use of γ-cyclodextrin (a cyclodextrin derivatization) resulted in an 87-fold increase in lopinavir solubilization. 33 Cyclodextrin is a class of cyclic oligosaccharides of α (1→4) glucopyranosides, having a hydrophilic outer surface and a considerably less hydrophilic central cavity that enable it to form complexes with drug molecules, thereby enhancing the aqueous solubility and bioavailability of the drug. 34 

The use of ritonavir nanosuspension increased up the dissolution rate of ritonavir. The area under the plasma concentration-time curve (AUC) values of ritonavir nanosuspension were 1.40-, 6.16-and 12.40-fold higher, and the maximum plasma concentration (C max ) values were 1.90-, 3.23-and 8.91-fold higher, than those of the commercial product, physical mixture and coarse powder of ritonavir, respectively. 35 Nanosuspensions containing surfactants or polymers as stabilizers may be employed to address drug delivery issues. 36 Preparing a self-microemulsifying drug delivery system (SMEDDS). The solid SMEDDS tablets (completely dissolved in 60 min) markedly enhanced the drug dissolution rate (30%), C max (160.63%) and oral bioavailability (196.46%) compared to free ritonavir. 37 SMEDDS is a promising approach to deliver lipophilic drugs due to their self-dispersion characteristics. The small droplet sizes observed upon dispersion have shown that drug absorption benefited from the large interfacial area. 38 Pharmaceutical Technology to Increase Permeability Alone Adding Penetration Enhancer Caco-2 cell permeation studies showed that the permeability of acyclovir increases by 30 to 40 times in the presence of chitosan. 39 Chitosan is an unbranched binary heteropolysaccharide consisting of the two units N-acetyl-d-glucosamine and d-glucosamine, which can increase or accelerate drug penetration. 17, 40 Adding an Absorption Enhancer Gelucire 44/14 (at a concentration of 0.05% or 0.1% w/v) is able to increase the apparent permeability coefficient (P app ) by 6.47-fold and promote ocular bioavailability by 5.40-fold compared to free ribavirin. 41 Gelucires are a series of amphiphilic pharmaceutical excipients that are widely used as powerful solubilization agents and bioavailability enhancers via oral and topical routes.

Oseltamivir carboxylate formed a valyl amino acid prodrug via an isopropyl-methylenedioxy linker. This oseltamivir prodrug had a 9-fold enhanced P app value in Caco-2 cells compared to that of the parent drug. 42 The ethanol and butanol prodrugs of lamivudine increased permeability 2-and 10-fold, respectively. 43

AvDs are converted into active ingredients or metabolites with high polarity and high-water solubility, mainly in the liver and intestine. Phase I biotransformation involves oxidation, reduction or hydrolysis reaction of the functional group of drug molecules. Phase II biotransformation (combination reaction) combines the polar groups produced by phase I with endogenous components in the body through covalent bonds. 23, 44 The produced combination, possessing high polarity, is easy to dissolve in water and discharge from the body. Renal clearance is mainly controlled by membrane transport proteins and is an important elimination pathway for antiviral agents.

The main metabolic enzymes of AvDs are microsomal enzymes existing in the liver, lung, kidney, small intestine, placenta and skin, mainly in intestinal epithelial cells and hepatocytes. Cytochrome P450 enzyme system (CYP450) mainly exists on the smooth endoplasmic reticulum and mitochondria of hepatocytes and in the small intestinal epithelia or the proximal tubules of the kidneys to a lesser extent. CYP450 is mainly involved in phase I reactions of AvDs. 45 Approximately 40-70% of all clinical drugs are subjected to glucuronidation reactions metabolized by uridine diphosphate-glucuronosyl transferases (UGTs) in humans. 46 UGTs are a superfamily of membrane-bound enzymes that catalyze the formation of https://doi.org/10.2147/IJN.S315705

International Journal of Nanomedicine 2021:16 4964 a chemical bond between a nucleophilic O-, N-, S-, or C atom and uridine-5'-diphosphate-α-D-glucuronic acid (UDPGA). 47 

Metabolic Reaction of NA-AvDs NA-AvDs enter cells through specific plasma membrane nucleoside transporters, such as the SLC22, SLC15, SLC28 and SLC29 gene family. 48 After entering the cells, NA-AvDs are phosphorylated by cellular nucleoside kinase to form nucleoside monophosphate, diphosphate and triphosphate. Nucleoside triphosphates are the active form of NA-AvDs, which work by inhibiting cellular or viral enzymes (such as DNA/RNA polymerase). 49 Most NA-AvDs are activated in vivo through 5'-phosphorylation. NA-AvDs are subjected to phosphorylation (7/8, ie, 7 of a total of 8), glucuronidation (1/8), oxidation (1/8), and reduction (1/8) ( Table 2) .

Purine NA-AvDs: (1) Phosphorylation. Adefovir with hydroxyl groups was phosphorylated to the diphosphate compound, which competes with deoxyadenosine triphosphate kinase for incorporation by HBV reverse transcriptase to exert its antiviral effect. 55 Entecavir undergoes intracellular phosphorylation to form di-and triphosphate metabolites by the natural substrate deoxyguanosine triphosphate and is then incorporated into HBV DNA polymerase to inhibit replication. 56 (2) Oxidation. Acyclovir with the primary hydroxyl group in the side chain underwent oxidation, resulting in the formation of active carboxy-acyclovir. 62 Pyrimidine NA-AvDs: (1) Phosphorylation. Zidovudine undergoes intracellular phosphorylation to form monophosphate, diphosphate and active triphosphate compounds by thymidine, thymidylate kinase and nucleoside diphosphate kinase, respectively. 53 Lamivudine formed monophosphate, diphosphate and active 5'-triphosphate by deoxycytidine kinase, deoxycytidylate kinase and nucleoside diphosphate kinase, in turn. 54 (2) Glucuronidation. Zidovudine underwent glucuronidation by UGT2B7 to form inactive metabolites excreted in urine. 53 (3) Reduction. CYP450s and CYP450 reductase have been applied in the reduction of the azido moiety of zidovudine. The metabolite 30-amino-3'-deoxythymidine of zidovudine has been shown to be approximately 5-to 7-fold more toxic to human hematopoietic progenitor cells. 53 Other NA-AvDs: Phosphorylation. Favipiravir first forms ribonucleoside 5'-monophosphate and then forms ribonucleosides 5'-diphosphate and 5'-triphosphate under the action of guanine phosphoribosyltransferase. In addition, another active metabolite, nicotinamide adenosine favipiravir, was formed by nicotinamide mononucleotide adenylyltransferase. 52 Remdesivir is metabolized through the following 3 pathways: forming nucleoside triphosphate by phosphorylation, forming alanine metabolite by dephenylization, and forming nucleoside monophosphate by deamination. 51 Ribavirin forms 5'-phosphorylation compounds by adenosine kinase catalysis. 63 Phosphorylated ribavirin metabolites have exhibited broad antiviral activity. 50

The whole 8 NN-AvDs investigated in this article are subjected to oxidation (3/8), reduction (2/8), hydrolysis (1/8) and conjugation (1/8).

Oxidation: Lopinavir is mainly catalyzed by CYP3A to undergo oxidative metabolism. The predominant metabolic site was carbon-4 of the cyclic urea moiety, with subsequent secondary metabolism occurring on the diphenyl core moiety. 58 Arbidol contains sulfide, which forms sulfinylarbidol by sulfoxidation. The oxidation was catalyzed by flavin-containing monooxygenases. 60 Ritonavir undergoes several oxidations, including hydroxylation at the isopropyl group by CYP2D6 to form the major metabolite and the oxidation of the thiazole rings on the eastern and western side of the molecule catalyzed by CYP2J2. 61 CYP3A4 was the major isoform involved in arbidol metabolism in the liver and intestines. 60 Reduction: Chloroquine was rapidly dealkylated into the pharmacologically active desethylchloroquine, bisdesethylchloroquine and 7-chloro-4-aminoquinoline. The main metabolic enzymes were two major isoforms, CYP3As and CYP2D6. 57 Ritonavir underwent dealkylation by CYP3A4 to form demethylation metabolites. 61 Hydrolysis: Oseltamivir was metabolized to GS4071, (3R,4R,5S)-3-(1-ethylpropyloxy)-4-acetamido-5-aminocyclohexene-1-carboxylate (an active neuraminidase inhibitor) through ester hydrolysis. 59 Conjugation: Arbidol was conjugated with glucuronide and sulfate via free hydroxyl groups on the indole ring by UGT1A9. 64 The obtained conjugates were major metabolites in human urine.

Pharmacological Activity of AvDs 

Remdesivir ---Phosphorylation at 1:

Favipiravir ---Phosphorylation at 1:

Glucuronidation at 1:

Zidovudine -CYP-450/P450 reduction at 2:

Glucuronidation at 1:

Adefovir ---Phosphorylation at 1: [55] (Continued) 

Entecavir ---Phosphorylation at 1:

Chloroquine -Dealkylate at Ribavirin interfered with the replication of DNA and RNA viruses and boosted the antiviral Th1 arm of the immune system to regulate T cells. In addition, ribavirin was proven to be effective for COVID-19 in several clinical trials. 70 Recently, remdesivir in the triphosphate form was found to compete with the natural counterpart ATP and cause SARS-CoV RNA synthesis arrest at a specific position; thus, remdesivir might resist COVID-19. 71 Favipiravir was effective in reducing the SARS-CoV-2 infection in vitro. 12 Favipiravir stopped the disease progression of COVID-19 by inhibiting and clearing SARS-CoV-2 virus to achieve good treatment outcomes in COVID-19 patients. 72 Chloroquine was a potential drug for COVID-19, since it altered the crosstalk of SARS-CoV-2 molecules with target cells because the inhibition capability of p38 mitogen-activated protein kinase (MAPK) interfered with proteolytic processing of the M protein and altered virion assembly and budding. 73 Both lopinavir and ritonavir bound well to the SARS-CoV 3C-like protease. 74 The combination of lopinavir and ritonavir represent a potential treatment for COVID-19. Arbidol effectively acted against SARS-CoV-2 in vivo and in vitro study. 75 (1) Five AvDs exhibited broad-spectrum antiviral effects. Three NA-AvDs (ribavirin, 99 remdesivir 100 and favipiravir 101 ) are inhibitors of RNA-dependent RNA polymerase. NN-AvD chloroquine 69 inhibited phosphorylation of MAPK in THP-1 (a human monocytic leukemia) cells, as well as caspase-1, and then blocked the virus replication cycle. 102 The NN-AvD arbidol 103 had broadspectrum activity, since it interacted with both membranes and with viral/cellular proteins. 104 (2) Three AvDs exhibited anti-HBV effects. Three NA-AvDs (lamivudine, 85 adefovir, 86 entecavir 87 ) effectively inhibited the replication of HBV virus. Lamivudine also reduced viral load and reversed fibrosis, and entecavir was also a highly selective inhibitor of HBV DNA polymerase. 105 (3) Two AvDs exhibited anti-HIV effects. Two NA-AvDs (zidovudine 106 and lamivudine 107 are inhibitors of reverse transcriptase enzymes. Zidovudine also inhibited 

viral replication by interfering with chain elongation of the viral DNA. 108 Lamivudine also exerted antiviral effects by acting as a DNA chain terminator. 107 (4) Two AvDs exhibited anti-CoV effects. Two NN-AvDs (lopinavir 109 and ritonavir 82 ) were combined to treat coronavirus infections. Lopinavir was a protease inhibitor which mainly inhibited the 3C-like protease of CoV-virus and modulated apoptosis in human cells. 110 While ritonavir inhibited the CYP3A mediated metabolism of lopinavir and thereby potentiated the serum level of lopinavir. 111 (5) Two AvDs exhibited anti-influenza effects. Two NN-AvDs contained oseltamivir 112 and peramivir 96 ). The former mainly targeted a glycoprotein neuraminidase on the surface of influenza virus. 113 Peramivir is an efficacious nucleoside analog inhibitor to treat influenza. 96 (6) 1 AvDs exhibited anti-HSV effects. NA-AvD acyclovir 88 was activated by viral thymidine kinase and then di-and tri-phosphorylated by cellular kinases. The active tri-phosphorylated forms of acyclovir specifically interfered with viral DNA polymerase and caused chain termination. 88 

The in vivo metabolization of AvDs had a large influence on viral replication processes and pharmacological function. (1) Generation of active metabolites. Eight inactive or low-activity AvDs were transformed into active or highactivity forms through metabolism in the body.

Typically, NA-AvDs are prodrugs that must be converted to nucleotide triphosphate metabolites to exert antiviral activity. Ribavirin was phosphorylated to form monophosphate by adenosine kinase and triphosphate by nucleoside mono-and diphosphate kinases. 114 The triphosphate metabolite of remdesivir markedly increased the plasma half-life time (t 1/2 ) value (14 h versus 0.39 h for remdesivir) and antiviral activity (> 40 times) compared to the free drug. 115 Favipiravir showed no inhibitory effect on influenza, whereas its active metabolite (favipiravir triphosphate) strongly inhibited influenza virus RNA polymerase. 116 Zidovudine was phosphorylated at the intracellular site, and zidovudine triphosphate was the inhibitor of reverse transcriptase activity required for viral replication. 117 Lamivudine was phosphorylated to lamivudine triphosphate, which was able to inhibit viral reverse transcriptase and terminate proviral DNA chain extension due to the lack of the 3'-hydroxyl group required for nucleic acid replication. 118 Entecavir-formed triphosphate inside cells inhibited replication of the hepatitis B virus. 119 In the case of NN-AvDs, desethylchloroquine was an active metabolite of chloroquine. Both chloroquine and its metabolite acted against Zika virus at low concentrations (ie, at the micromolar level). 120 Oseltamivir was converted into oseltamivir carboxylate (a potent neuraminidase inhibitor). 121 The active metabolites of arbidol, sulfinylarbidol and sulfonylarbidol, increased the t 1/2 values (25 h, 26 h versus 15.7 h for arbidol) and peak time (T max ) values (13 h, 19 h versus 1.38 h), respectively. 122 (2) Generation of inactive/low-active metabolites. Zidovudine was metabolized quickly to the inactive form glucuronide with a t 1/2 value of 1 h. 123 Lopinavir was primarily mediated by CYP3A enzymes and yielded metabolites that were less potent as protease inhibitors. 124 (3) Generation of toxic metabolites. Zidovudine's metabolite (30-amino-3'-deoxythymidine) was approximately 5-to 7-fold more toxic to human hematopoietic progenitor cells. 53 Delivery System to Improve Pharmacological Activity of AvDs Most AvDs had poor solubility/permeability and low oral bioavailability. Appropriate drug delivery systems might be able to overcome the shortcomings of AvDs and improve pharmacological activity.

(1) Preparing nanocarriers to lessen the side effects of AvDs. Ribavirin caused hemolytic anemia due to the accumulation inside red blood cells. Poly(glycerol-adipate) nanoparticles (NPs) delivered ribavirin to the targeted liver and subsequently decreased the red blood cell uptake rate, which overcame the side effects of ribavirin (hemolytic anemia caused by ribavirin due to its accumulation inside red blood cells). 125 Lamivudine has low bioavailability in the brain (0.05-1.14%) and cannot kill viruses completely. Mannosylated polymeric NPs improved brain bioavailability and targeted mannose receptors on the macrophage surface to improve the therapeutic outcome and reduce toxicity. 126 Oseltamivir loaded into the surface of selenium NPs improved antiviral activity and increased the viability of cells infected with a virus to 83.2%. 127 (2) Preparing nanocarriers to extend the dose interval of AvDs. Zidovudine had low bioavailability and biological t 1/2 value that led to dose-dependent anemia and firstpass metabolism. Amide-functionalized alginate NPs were used to encapsulate zidovudine to realize slow and sustained drug release. 123 (3) Other delivery systems to improve pharmacological activity of AvDs. Acyclovir was loaded onto activated carbon particles. The highly porous carbon structures trapped virions, blocked infection and thus improved efficacy with acyclovir. 128 Ribavirin was coupled to macromolecular carriers and delivered the drug to the targeted site liver, which may reduce systemic complications. Hemoglobin-ribavirin conjugates had greater antiviral activity on both isolated hepatocytes and macrophages, and they significantly reduced viral replication at 1 μM, while free ribavirin was ineffective at the same concentration. 129

Most AvDs belong to BCS II, III or IV types with low solubility/permeability. Therefore, their AUC, C max , t max , t 1/2 or mean retention time (MRT) values are low (Table 4 ). These adverse pharmacokinetic behaviors are often unfavorable to the pharmacological action of AvDs. The bioavailabilities of orally administered adefovir were 1% in monkeys and 8-11% in rats. The low bioavailability was mainly attributed to the low passive permeability 

across the intestinal membrane. 130 It is crucial to apply appropriate drug delivery systems to improve the bioavailability and enhance the pharmacological effects of AvDs.

Iron-catechol-based nanoscale coordination polymers with antiretroviral ligand functionalization served as a promising strategy for the bioavailability enhancement of zidovudine. These polymers offered long-lasting drug release and improved colloidal stabilities, as well as enhanced cellular uptake (ie, an increase of up to 50fold). 144 The proliposomes increased the MRT of adefovir dipivoxil in the liver by approximately 3-fold compared with the adefovir suspension. 145 The poly (lactic acid)-poly (ethylene glycol)-ligand NPs targeted the intestinal transporter PepT1 and increased permeability by 2.7-fold compared with free acyclovir. 146 The triglyceride-mimetic superficially modified mesoporous silica NPs increased the AUC, C max and MRT values of lopinavir by 9.65-, 3.87-and 2.70-fold, respectively. The NPs improved poor solubility and avoided the first-pass metabolism of oral lopinavir to achieve high oral bioavailability with no side effects. 147 The lopinavir-loaded bioadhesive protein NPs increased the oral bioavailability by 4-and 1.5-fold compared to the free lopinavir suspension and lopinavir/ritonavir formulations, respectively. 140 The poly (lactic-co-glycolic acid) NPs increased the permeability and oral bioavailability of lopinavir by 3.04-and 13.9-fold in rats. 139 The lopinavir in hydrophobically modified pullulan NPs was metabolized to a lesser extent in the gut. The NPs increased the bioavailability twofold. 148 

Prodrugs usually offer a versatile strategy to overcome the flaws of antiviral drugs. The prodrugs strategy improved the pharmacokinetic properties, efficacy and safety profile of many viable drugs. 149 Effective prodrug strategies include ester prodrugs, targeted delivery prodrugs, macromolecular prodrugs and nucleoside conjugates. 16 Zidovudine prodrug (the ester conjugation of zidovudine with ursodeoxycholic acid) permeated considerably more efficiently, and remained in murine macrophages longer, than the parent drug. The MRTs for zidovudine and its prodrug were 6.5 min and 19.6 min, respectively. 150 Entecavir ester prodrugs prolonged the therapeutic period. After subcutaneous injection of the entecavir prodrug in beagle dogs, the plasma drug concentration was markedly protracted (T 1/2 is 129.3 h) with a lower maximum plasma concentration (C max is 4.7 ng/mL) compared to entecavir (oral administration, T 1/2 is 4.09 h and C max is 15.4 ng/ mL). 136 The chloroquine prodrug hydroxychloroquine had higher accumulation in cells and a longer elimination halflife, resulting in a more effect against SARS-CoV-2 infection. 151

The current primary administration route for AvDs is oral administration, which has the advantages of convenience, safety and cost-effectiveness. However, the oral bioavailability of most AvDs was not satisfactory due to poor solubility and permeability. Therefore, it was very important to choose the appropriate administration route 152 according to the treatment needs and safety assessment to maximize the efficacy of AvDs (Table 5) .

Ribavirin nasal spray consisting of spray-dried excipient particles was suitable for nasal deposition. This method provided effective mucosal adhesion and penetration enhancement. In vivo results confirmed that the agglutination rate was nearly 6 times higher than conventional intravenous administration, suggesting that the preparation has the potential to deliver a highly brain-targeted antiviral from the nose to the brain. 160 Adefovir suspension after intravenous administration increased the AUC 5.45-fold compared with oral administration. 135 After subcutaneous injection of entecavir in beagle dogs, the C max (4.70 ng/ mL) was protracted, and the t 1/2 value was lengthened (129.30 h) compared to oral administration (15.40ng/mL, 4.1 h). 136 After intravenous administration of acyclovir solution, there was 90-fold higher C max (~26.23 μg/mL) and considerably shorter T max (~8.00 min) compared to that of acyclovir suspension by oral administration (C max was ~0.29 μg/mL and T max was ~26.00 min). 137 After intravenous administration of oseltamivir solution 92 

As the main resource in the effort to treat viruses, AvDs have advantages such as convenient use, accurate dosage, clear targets and strong antiviral efficacy. However, insufficient solubility/permeability of AvDs (most belong to BCS II and  III) , has a great impact on their oral absorption and results in lower oral bioavailability and requires multiple medications. A better understanding of the in vivo metabolism and pharmacokinetic process of AvDs may help researchers to develop new formulations to overcome these problems. In this review, we investigated the structure-based metabolic reactions of AvDs mainly in the liver and intestine. Phosphorylation (one conjugation reaction) is the most common reaction of NA-AvDs; other reactions, such as oxidation and reduction, also occur, but no hydrolysis occurs. For NN-AvDs, in addition to oxidation, reduction and conjugation reactions, hydrolysis also occurs. Enzymatic systems (CYPs and phosphokinase) are mainly produced by epithelial cells.

Most AvDs are activated, and a small number are inactivated or maintain activity through metabolic reactions. The pharmacological activity and pharmacokinetics of AvDs are improved by loading into suitable delivery systems with nanotechnology, prodrug strategy and administration route consideration. The mechanisms of action of AvDs are generally to inhibit the key enzymes (such as RNA-dependent RNA polymerase, reverse transcriptase and nucleoside reverse transcriptase) of virus synthesis and consequently block viral synthesis. Notably, some AvDs (such as ribavirin, remdesivir, favipiravir, chloroquine, lopinavir and ritonavir) exhibit potential as treatments for COVID-19. Several new antiviral drugs have been approved for marketing, such as bictegravir sodium, emtricitabine and tenofovir alafenamide fumarate tablets (Biktarvy ® , a nucleotide reverse transcriptase inhibitor); baloxavir marboxil tablets (Xofluza ® , a polymerase acidic endonuclease inhibitor) and letermovir injections (Prevymis ® , viral terminase inhibitor) have been developed rapidly, and several antiviral drugs that are undergoing clinical trials may also contribute to the global management of antiviral infections (such as albuvirtide and elsulfavirine). In addition, Regeneron's REGN-COV2 antibody cocktail and Eli Lilly's LY-CoV555 are being used in the treatment of the president of the US, and their clinical trials have proven that the drugs have anti-COVID-19 efficacy. In the future, new antiviral drugs in suitable delivery systems are expected to perform better than old ones. 

AvD, antiviral drug; BCS, biopharmaceutical classification system; COVID-19, coronavirus disease 2019; CYP450, cytochrome P450 enzyme system; dATP, deoxyadenosine triphosphate; D 0 , dose number; log P, oil-water partition coefficient; MAPK, p38 mitogen-activated protein kinase; MERS, Middle East respiratory syndrome; NA-AvD, nucleoside analog of antiviral drug; NN-AvD, non-nucleoside analog of antiviral drug; NP, nanoparticle; SARS, severe acute respiratory syndrome; SMEDDS, semisolid selfmicroemulsifying drug delivery system; P app , apparent permeability coefficient; P-gp, P-glycoprotein; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; T max , peak time; t 1/2 , plasma half-life time; UDPGA, uridine-5'diphosphate-α-D-glucuronic acid; UGT, uridine diphosphate-glucuronosyl transferase; WHO, world health organization.

This research is partially supported by grants from Chongqing Science and Technology Association (2020-11), Chongqing Science and Technology Committee (cstc2015jcyjBX0027, cstc2017shmsA130028) and Chongqing Education Committee (CYS20212, CY160 406).

Daojun Pu is an employee of Southwest Pharmaceutical Limited Company. The authors declare no other potential conflicts of interest for this work and no financial support.

A new coronavirus associated with human respiratory disease in china

Covid-19, a pandemic or not?

Induction of pro-inflammatory cytokines (il-1 and il-6) and lung inflammation by coronavirus-19 (covi-19 or sars-cov-2): anti-inflammatory strategies

Il-1 induces throboxane-a2 (txa2) in covid-19 causing inflammation and micro-thrombi: inhibitory effect of the il-1 receptor antagonist (il-1ra)

Mast cells contribute to coronavirus-induced inflammation: new anti-inflammatory strategy

Covid-19: epidemiology, evolution, and cross-disciplinary perspectives

Covid-19: a promising cure for the global panic

Liquorice (glycyrrhiza glabra): a phytochemical and pharmacological review

Baicalein and baicalin as zika virus inhibitors

Lignan dimers from forsythia viridissima roots and their antiviral effects

Natural oral anticancer medication in small ethanol nanosomes coated with a natural alkaline polysaccharide

Remdesivir and chloroquine effectively inhibit the recently emerged novel coronavirus (2019-ncov) in vitro

Development of anti-influenza agents from natural products

Dissecting distinct proteolytic activities of fmdv lpro implicates cleavage and degradation of rlr signaling proteins, not its deisgylase/dub activity, in type i interferon suppression

Discovering small-molecule therapeutics against sars-cov-2

Advanced prodrug strategies in nucleoside and non-nucleoside antiviral agents: a review of the recent five years

Biomimetic membrane-structured nanovesicles carrying a supramolecular enzyme to cure lung cancer

Biomimetic polysaccharide-cloaked lipidic nanovesicles/microassemblies for improving the enzymatic activity and prolonging the action time for hyperuricemia treatment

Biowaiver monographs for immediate release solid oral dosage forms: ribavirin

Oseltamivir in seasonal, avian h5n1 and pandemic 2009 a/h1n1 influenza: pharmacokinetic and pharmacodynamic characteristics

The influence of donor and reservoir additives on caco-2 permeability and secretory transport of hiv protease inhibitors and other lipophilic compounds

The delivery of arbidol by salt engineering: synthesis, physicochemical properties and pharmacokinetics

Toward a better understanding of metabolic and pharmacokinetic characteristics of low-solubility, low-permeability natural medicines

The evolution of nucleoside analogue antivirals: a review for chemists and non-chemists. Part 1: early structural modifications to the nucleoside scaffold

Advances in cell-based permeability assays to screen drugs for intestinal absorption

Nanoformulation strategies for improving intestinal permeability of drugs: a more precise look at permeability assessment methods and pharmacokinetic properties changes

Solubility and bioavailability enhancement study of lopinavir solid dispersion matrixed with a polymeric surfactant -soluplus

Parenteral nanosuspensions: a brief review from solubility enhancement to more novel and specific applications

Soluplus micelles for acyclovir ocular delivery: formulation and cornea and sclera permeability

Nanomedicine formulations for the delivery of antiviral drugs: a promising solution for the treatment of viral infections

Enhancement of the aqueous solubility and permeability of a poorly water soluble drug ritonavir via lyophilized milk-based solid dispersions

Fundamental aspects of solid dispersion technology for poorly soluble drugs

Cyclodextrin solubilization and complexation of antiretroviral drug lopinavir: in silico prediction; effects of derivatization, molar ratio and preparation method

Cyclodextrins as excipients in tablet formulations

Evaluation of improved oral bioavailability of ritonavir nanosuspension

Emerging role of nanosuspensions in drug delivery systems

Solid self-microemulsifying drug delivery system of ritonavir

Colloidal aspects of dispersion and digestion of self-dispersing lipid-based formulations for poorly water-soluble drugs

The effect of chitosan on the bioaccessibility and intestinal permeability of acyclovir

Chitosanmodified lipid nanovesicles for efficient systemic delivery of l-asparaginase

Gelucire44/14 as a novel absorption enhancer for drugs with different hydrophilicities: in vitro and in vivo improvement on transcorneal permeation

Carrier-mediated prodrug uptake to improve the oral bioavailability of polar drugs: an application to an oseltamivir analogue

Intestinal permeability of lamivudine (3tc) and two novel 3tc prodrugs. Experimental and theoretical analyses

Uricase alkaline enzymosomes with enhanced stabilities and anti-hyperuricemia effects induced by favorable microenvironmental changes

Drug metabolism in the liver

Improved delivery of natural alkaloids into lung cancer through woody oil-based emulsive nanosystems

Implications of genetic variation of common drug metabolizing enzymes and abc transporters among the pakistani population

Predictive value of microdose pharmacokinetics

Nucleoside analogs as a rich source of antiviral agents active against arthropod-borne flaviviruses

Targeting eif4e signaling with ribavirin in infant acute lymphoblastic leukemia

Therapeutic efficacy of the small molecule gs-5734 against ebola virus in rhesus monkeys

Cell line-dependent activation and antiviral activity of t-1105, the non-fluorinated analogue of t-705 (favipiravir)

Pharmgkb summary

The potential of long-acting, tissue-targeted synthetic nanotherapy for delivery of antiviral therapy against hiv infection

Effective metabolism and long intracellular half life of the anti-hepatitis b agent adefovir in hepatic cells

Immunomodulatory mechanism of acyclic nucleoside phosphates in treatment of hepatitis b virus infection

Chloroquine for sars-cov-2: implications of its unique pharmacokinetic and safety properties

Mechanism-based pharmacokinetic (mbpk) models describe the complex plasma kinetics of three antiretrovirals delivered by a long-acting anti-hiv drug combination nanoparticle formulation

Synthesis and biological evaluation of nh2-sulfonyl oseltamivir analogues as influenza neuraminidase inhibitors

Arbidol and other low-molecular-weight drugs that inhibit lassa and ebola viruses

Investigating the contribution of cyp2j2 to ritonavir metabolism in vitro and in vivo

Oxidation of the antiviral drug acyclovir and its biodegradation product carboxy-acyclovir with ozone: kinetics and identification of oxidation products

Identification of a nucleoside analog active against adenosine kinase-expressing plasma cell malignancies

Metabolic mapping of a3 adenosine receptor agonist mrs5980

Triple combination of interferon beta-1b, lopinavir-ritonavir, and ribavirin in the treatment of patients admitted to hospital with covid-19: an openlabel, randomised, Phase 2 trial

Compassionate use of remdesivir for patients with severe covid-19

Effect of favipiravir and an anti-inflammatory strategy for covid-19

Is hydroxychloroquine beneficial for covid-19 patients?

The broad-spectrum antiviral recommendations for drug discovery against covid-19

Novel coronavirus treatment with ribavirin: groundwork for an evaluation concerning covid-19

The antiviral compound remdesivir potently inhibits rna-dependent rna polymerase from middle east respiratory syndrome coronavirus

Experimental treatment with favipiravir for covid-19: an open-label control study. Engineering (Beijing)

New insights on the antiviral effects of chloroquine against coronavirus: what to expect for covid-19?

Why are lopinavir and ritonavir effective against the newly emerged coronavirus 2019? Atomistic insights into the inhibitory mechanisms

The anti-influenza virus drug, arbidol is an efficient inhibitor of sars-cov-2 in vitro

Ribavirin is effective against drug-resistant h7n9 influenza virus infections

Ribavirin antagonizes the in vitro anti-hepatitis c virus activity of 2'-c-methylcytidine, the active component of valopicitabine

Differential antiviral activities of respiratory syncytial virus (rsv) inhibitors in human airway epithelium

Efficacy of favipiravir alone and in combination with ribavirin in a lethal, immunocompetent mouse model of lassa fever

Evaluation of antiviral efficacy of ribavirin, arbidol, and t-705 (favipiravir) in a mouse model for Crimean-Congo hemorrhagic fever

The effectiveness of antiviral agents with broad-spectrum activity against chikungunya virus varies between host cell lines

Comparative therapeutic efficacy of remdesivir and combination lopinavir, ritonavir, and interferon beta against mers-cov

Coronavirus susceptibility to the antiviral remdesivir (gs-5734) is mediated by the viral polymerase and the proofreading exoribonuclease. mBio

Testing anti-hiv activity of antiretroviral agents in vitro using flow cytometry analysis of cem-gfp cells infected with transfection-derived hiv-1 nl4-3

Inhibition of hepatitis b virus replication by targeting ribonucleotide reductase m2 protein

Prodrugs of phosphonates and phosphates: crossing the membrane barrier

An expert review on the use of tenofovir alafenamide for the treatment of chronic hepatitis b virus infection in asia

Omeprazole increases the efficacy of acyclovir against herpes simplex virus type 1 and 2

Chloroquine is a potent inhibitor of sars coronavirus infection and spread

Screening of an fda-approved compound library identifies four small-molecule inhibitors of middle east respiratory syndrome coronavirus replication in cell culture

Chloroquine, an endocytosis blocking agent, inhibits zika virus infection in different cell models

Comparison of anti-influenza virus activity and pharmacokinetics of oseltamivir free base and oseltamivir phosphate

The antiviral activity of arbidol hydrochloride against herpes simplex virus type ii (hsv-2) in a mouse model of vaginitis

Inhibition of the infectivity and inflammatory response of influenza virus by arbidol hydrochloride in vitro and in vivo (mice and ferret)

The synthetic antiviral drug arbidol inhibits globally prevalent pathogenic viruses

Peramivir conjugates as orally available agents against influenza h275y mutant

In vitro characterization of baloxavir acid, a first-in-class cap-dependent endonuclease inhibitor of the influenza virus polymerase pa subunit

Letermovir for the management of cytomegalovirus infection

Broad-spectrum coronavirus antiviral drug discovery

Broad-spectrum antiviral gs-5734 inhibits both epidemic and zoonotic coronaviruses

Favipiravir (t-705), a broad spectrum inhibitor of viral rna polymerase

The possible mechanisms of action of 4-aminoquinolines (chloroquine/hydroxychloroquine) against sars-cov-2 infection (covid-19): a role for iron homeostasis?

Arbidol combined with lpv/r versus lpv/r alone against corona virus disease 2019: a retrospective cohort study

Candidate drugs against sars-cov-2 and covid-19

Hematological malignancies and hbv reactivation risk: suggestions for clinical management

Do antiretroviral drugs protect from multiple sclerosis by inhibiting expression of ms-associated retrovirus? Front Immunol

Twenty-five years of lamivudine: current and future use for the treatment of hiv-1 infection

The safety and efficacy of zidovudine (azt) in the treatment of subjects with mildly symptomatic human immunodeficiency virus type 1 (hiv) infection. A double-blind, placebo-controlled trial. The aids clinical trials group

Factors associated with prolonged viral shedding and impact of lopinavir/ritonavir treatment in hospitalised non-critically ill patients with sars-cov-2 infection

Treatment with lopinavir/ ritonavir or interferon-beta1b improves outcome of mers-cov infection in a nonhuman primate model of common marmoset

Comparison of hydroxychloroquine, lopinavir/ ritonavir,and standard of care in critically ill patients with sars-cov-2 pneumonia: an opportunistic retrospective analysis

Salinomycin inhibits influenza virus infection by disrupting endosomal acidification and viral matrix protein 2 function

Design, in silico studies, synthesis and in vitro evaluation of oseltamivir derivatives as inhibitors of neuraminidase from influenza a virus h1n1

Mutagenic effects of ribavirin on hepatitis e virus-viral extinction versus selection of fitness-enhancing mutations

Development and validation of a uhplc-ms/ms method for quantification of the prodrug remdesivir and its metabolite gs-441524: a tool for clinical pharmacokinetics of sars-cov-2/covid-19 and ebola virus disease

Favipiravir, an anti-influenza drug against life-threatening rna virus infections

Azt acts as an anti-influenza nucleotide triphosphate targeting the catalytic site of a/pr/8/34/h1n1 rna dependent rna polymerase

Structural insights into the recognition of nucleoside reverse transcriptase inhibitors by hiv-1 reverse transcriptase: first crystal structures with reverse transcriptase and the active triphosphate forms of lamivudine and emtricitabine

An evaluation of entecavir for the treatment of chronic hepatitis b infection in adults

Antimalarial drugs and their metabolites are potent zika virus inhibitors

Simulation of the pharmacokinetics of oseltamivir and its active metabolite in normal populations and patients with hepatic cirrhosis using physiologically based pharmacokinetic modeling

Pharmacokinetics, metabolism, and excretion of the antiviral drug arbidol in humans

Encapsulation of zidovudine in pf-68 coated alginate conjugate nanoparticles for anti-hiv drug delivery

Effects of cytochrome p450 3a (cyp3a) and the drug transporters p-glycoprotein (mdr1/abcb1) and mrp2 (abcc2) on the pharmacokinetics of lopinavir

Viramidine-loaded galactosylated nanoparticles induce hepatic cancer cell apoptosis and inhibit angiogenesis

Targeted delivery of mannosylated-plga nanoparticles of antiretroviral drug to brain

Functionalized selenium nanoparticles enhance the anti-ev71 activity of oseltamivir in human astrocytoma cell model

Drug-encapsulated carbon (decon): a novel platform for enhanced drug delivery

Targeted delivery of ribavirin improves outcome of murine viral fulminant hepatitis via enhanced anti-viral activity

Reduction sensitive lipid conjugates of tenofovir: synthesis, stability, and antiviral activity

Pharmacokinetics and metabolism of [(14)c]ribavirin in rats and cynomolgus monkeys

Alterations in favipiravir (t-705) pharmacokinetics and biodistribution in a hamster model of viral hemorrhagic fever

Improved safety, bioavailability and pharmacokinetics of zidovudine through lactoferrin nanoparticles during oral administration in rats

Exploring dried blood spot sampling technique for simultaneous quantification of antiretrovirals: lamivudine, stavudine and nevirapine in a rodent pharmacokinetic study

Solid lipid nanoparticles and nanosuspension of adefovir dipivoxil for bioavailability improvement: formulation, characterization, pharmacokinetic and biodistribution studies

Microsuspension of fatty acid esters of entecavir for parenteral sustained delivery

Semisolid self-microemulsifying drug delivery systems (smeddss): effects on pharmacokinetics of acyclovir in rats

A hybrid design to optimize preparation of lopinavir loaded solid lipid nanoparticles and comparative pharmacokinetic evaluation with marketed lopinavir/ritonavir coformulation

Bioavailability enhancement, caco-2 cells uptake and intestinal transport of orally administered lopinavir-loaded plga nanoparticles

Bioadhesive food protein nanoparticles as pediatric oral drug delivery system

Hplc-ms/ms analysis of peramivir in rat plasma: elimination of matrix effect using the phospholipid-removal solid-phase extraction method

Population pharmacokinetics of baloxavir marboxil in japanese pediatric influenza patients

Pharmacokinetics and safety of letermovir, a novel anti-human cytomegalovirus drug, in patients with renal impairment

Versatile iron-catecholbased nanoscale coordination polymers with antiretroviral ligand functionalization and their use as efficient carriers in hiv/aids therapy

Adefovir dipivoxil loaded proliposomal powders with improved hepatoprotective activity: formulation, optimization, pharmacokinetic, and biodistribution studies

Functionalized pla-peg nanoparticles targeting intestinal transporter pept1 for oral delivery of acyclovir

Chylomicron-pretended nano-bio self-assembling vehicle to promote lymphatic transport and galts target of oral drugs

Modified pullulan nanoparticles for oral delivery of lopinavir: formulation and pharmacokinetic evaluation

The expanding role of prodrugs in contemporary drug design and development

Bile salt-coating modulates the macrophage uptake of nanocores constituted by a zidovudine prodrug and enhances its nose-to-brain delivery

In vitro antiviral activity and projection of optimized dosing design of hydroxychloroquine for the treatment of severe acute respiratory syndrome coronavirus 2 (sars-cov-2)

Azithromycin cationic non-lecithoid nano/microparticles improve bioavailability and targeting efficiency

A novel anti-hiv dextrin-zidovudine conjugate improving the pharmacokinetics of zidovudine in rats

Nanogel-conjugated reverse transcriptase inhibitors and their combinations as novel antiviral agents with increased efficacy against hiv-1 infection

Lipid-based nanocarrier efficiently delivers highly water soluble drug across the blood-brain barrier into brain

Preclinical pharmacokinetics of lamivudine and its interaction with schisandra chinensis extract in rats

Injectable plga adefovir microspheres; the way for long term therapy of chronic hepatitis-b

Entecavir-loaded poly (lactic-co-glycolic acid) microspheres for long-term therapy of chronic hepatitis-b: preparation and in vitro and in vivo evaluation

Integration of computational and experimental approaches to elucidate mechanisms of first-pass lymphatic drug sequestration and long-acting pharmacokinetics of the injectable triple-hiv drug combination tlc-art 101

In vivo nose-to-brain delivery of the hydrophilic antiviral ribavirin by microparticle agglomerates

and the Elsevier Bibliographic databases. The manuscript management system is completely online and includes a very quick and fair peer-review system