pharmaceuticals Review Recent Advances in Hepatitis B Treatment Georgia-Myrto Prifti 1, Dimitrios Moianos 1 , Erofili Giannakopoulou 1, Vasiliki Pardali 1, John E. Tavis 2 and Grigoris Zoidis 1,* 1 Department of Pharmacy, Division of Pharmaceutical Chemistry, School of Health Sciences, National and Kapodistrian University of Athens, Panepistimiopolis Zografou, 15771 Athens, Greece; [email protected] (G.-M.P.); [email protected] (D.M.); [email protected] (E.G.); [email protected] (V.P.) 2 Molecular Microbiology and Immunology, Saint Louis University, Saint Louis, MO 63104, USA; [email protected] * Correspondence: [email protected]; Tel.: +30-210-7274809 Abstract: Hepatitis B virus infection affects over 250 million chronic carriers, causing more than 800,000 deaths annually, although a safe and effective vaccine is available. Currently used antivi- ral agents, pegylated interferon and nucleos(t)ide analogues, have major drawbacks and fail to completely eradicate the virus from infected cells. Thus, achieving a “functional cure” of the in- fection remains a real challenge. Recent findings concerning the viral replication cycle have led to development of novel therapeutic approaches including viral entry inhibitors, epigenetic control of cccDNA, immune modulators, RNA interference techniques, ribonuclease H inhibitors, and capsid assembly modulators. Promising preclinical results have been obtained, and the leading molecules under development have entered clinical evaluation. This review summarizes the key steps of the HBV life cycle, examines the currently approved anti-HBV drugs, and analyzes novel HBV treatment regimens. Citation: Prifti, G.-M.; Moianos, D.; Keywords: hepatitis B; hepatitis B virus; HBV; antiviral agents; HBV inhibitors; cccDNA; HBV life Giannakopoulou, E.; Pardali, V.; Tavis, cycle; HBV treatment; nucleoside analogues; antiviral therapy J.E.; Zoidis, G. Recent Advances in Hepatitis B Treatment. Pharmaceuticals 2021, 14, 417. https://doi.org/10.3390/ph14050417 1. Introduction Hepatitis B is a liver disease caused by the Hepatitis B Virus (HBV). HBV belongs to Academic Editor: Juan Carlos Saiz the Hepadnaviridae family and is classified into ten genotypes (A to J) [1]. It is transmitted by exposure to infectious blood or other body fluids (e.g., semen, vaginal secretions—sexual Received: 30 March 2021 intercourse) as well as perinatally from infected mothers to infants [2]. The acute phase Accepted: 23 April 2021 Published: 1 May 2021 of the infection can be either symptomatic or asymptomatic. Acute infections can either spontaneously resolve or proceed to chronic infections. Chronic HBV infection is among Publisher’s Note: MDPI stays neutral the leading causes of hepatic cirrhosis and is the single largest cause of hepatocellular with regard to jurisdictional claims in carcinoma (HCC). According to the World Health Organization (WHO), over 250 million published maps and institutional affil- people are chronically infected, and HBV caused 887,000 deaths in 2015 [3]. The highest iations. epidemic prevalence is present in SE Asian, African, and Western Pacific countries [4]. The hepatitis B surface antigen (HBsAg), originally known as “Australia antigen” (AusAg), was firstly identified in the serum of indigenous Australians by Baruch Samuel Blumberg in 1965 [5]. This antigen was later related with viral hepatitis [6]. The goal of the current therapeutic development is a “functional cure” defined as Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. sustained undetectable levels of HBsAg and HBV DNA in serum, with or without sero- This article is an open access article conversion to hepatitis B surface antibodies (anti-HBs) after the end of the treatment [7]. distributed under the terms and This reduction has been associated with an improved clinical condition and significantly conditions of the Creative Commons decreased the chance of infection rebound. Other important HBV biomarkers include Attribution (CC BY) license (https:// serum HBV DNA, hepatitis B core antigen (HBcAg), and its antibody anti-HBc, hepati- creativecommons.org/licenses/by/ tis B e antigen (HBeAg), and anti-HBe antibody [8–10]. HBeAg is a secreted variant of 4.0/). HBcAg, and viral infections are classified either as HbeAg-positive or HbeAg-negative, Pharmaceuticals 2021, 14, 417. https://doi.org/10.3390/ph14050417 https://www.mdpi.com/journal/pharmaceuticals Pharmaceuticals 2021, 14, x FOR PEER REVIEW 2 of 27 e antigen (HBeAg), and anti-HBe antibody [8–10]. HBeAg is a secreted variant of HBcAg, and viral infections are classified either as HbeAg-positive or HbeAg-negative, with HBeAg-positive patients having higher viral titers and a more frequent and rapid disease Pharmaceuticals 2021, 14, 417 progression [11]. These biomarkers are used to guide treatment decisions following2 of 27 guide- lines established by the major hepatology medical societies [12–14]. Despite the existence of a safe and effective vaccine, no therapeutic regimen that rou- tinelywith HBeAg-positiveinduces a “functional patients cure” having for higher chronic viral HBV titers has and been a more identified frequent yet. and rapidThis review summarizesdisease progression the HBV [11 replication]. These biomarkers cycle, the are existing used to guide treatment treatment options decisions and followingtheir significant disadvantages,guidelines established and novel by the therapeutic major hepatology approaches medical that societies are currently [12–14]. the subject of exten- sive scientificDespite theresearch, existence with of athe safe ultimate and effective goal of vaccine, achieving no therapeutica “functional regimen cure” thatof the dis- routinely induces a “functional cure” for chronic HBV has been identified yet. This review ease. summarizes the HBV replication cycle, the existing treatment options and their significant disadvantages, and novel therapeutic approaches that are currently the subject of extensive 2.scientific HBV Replication research, with Cycle the ultimate goal of achieving a “functional cure” of the disease. 2.1. Virion Structure and Genome 2. HBV Replication Cycle 2.1.HBV Virion particles, Structure and also Genome known as Dane particles (Figure 1A), were firstly identified by Dane and colleagues in 1970 [15]. Their shape is spherical, with a diameter of ∼42 nm. HBV particles, also known as Dane particles (Figure1A), were firstly identified by TheyDane consist and colleagues of an outer in 1970 envelope, [15]. Their which shape is a is host spherical,-derived with lipid a diameter bilayer ofcontaining∼42 nm. three differentThey consist-sized of HBV an outer surface envelope, antigens which (HBsAg is a host-derived or HBs)— lipidlarge bilayer (L-HBs), containing middle three (M-HBs) anddifferent-sized small (S-HBs) HBV— surfacesurrounding antigens the (HBsAg viral ornucleocaps HBs)—largeid. (L-HBs), The nucleocapsid middle (M-HBs) (∼27 and nm diam- eter)small is (S-HBs)—surroundingicosahedral and comprises the viral the nucleocapsid. HBV core protein The nucleocapsid (HBcAg), (as∼ 27well nm as diameter) the viral DNA genomeis icosahedral and the and viral comprises DNA polymerase the HBV core (P) protein [16,17]. (HBcAg), The virus as also well secretes as the viral a wide DNA range of defectivegenome andparticles the viral (Figure DNA 1B) polymerase, including (P) e [16nveloped,17]. The nucleocapsids virus also secretes that a are wide empty range or con- tainof defective defective particles immature (Figure genomes1B), including and subviral enveloped lipid nucleocapsids particles containing that are emptythe viral or surface antigens.contain defective The subviral immature particles genomes are secreted and subviral along lipidwith particlesthe infectious containing virions the at viral levels that surface antigens. The subviral particles are secreted along with the infectious virions at are thousands of times higher, and they play an important role in suppressing antibody levels that are thousands of times higher, and they play an important role in suppressing responsesantibody responsesto the virus to the [18]. virus [18]. FigureFigure 1. 1. HepatitisHepatitis B B Virus Virus particles. (AA)) InfectiousInfectious HBV HBV virion virion (Dane (Dane particle). particle). The The lipid lipid enve- envelope, bearinglope, bearing three threetypes types of surface of surface proteins proteins—small—small (S- (S-HBs),HBs), mid middledle (M (M-HBs)-HBs) and and large (L-HBs)—(L-HBs)—sur- roundssurrounds the nucleocapsid, the nucleocapsid, consisting consisting of ofHBV HBV relaxed relaxed circular circular DNA DNA (rcDNA), (rcDNA), the the viral viral DNA DNA poly- polymerase (P), and the core protein (HBcAg). (B) Non-infectious HBV particles; enveloped nucleo- capsids containing immature or defective DNA/RNA, subviral particles, and naked nucleocapsids. Pharmaceuticals 2021, 14, x FOR PEER REVIEW 3 of 27 merase (P), and the core protein (HBcAg). (B) Non-infectious HBV particles; enveloped nucle- ocapsids containing immature or defective DNA/RNA, subviral particles, and naked nucleocap- sids. The HBV genome is a 3.2 kb circular, partially double-stranded DNA (relaxed circu- lar DNA; rcDNA). The negative-sense, non-coding (−) DNA strand is complete and com- plementary to the mRNA transcripts, whereas the positive (+) DNA strand is incomplete Pharmaceuticals 2021, 14, 417 and has a fixed 5′-end and a variable-size 3′-end [19–321]. of 27 The former contains four over- lapping open reading frames (ORFs)—C, P, S,