Can Radionuclide Therapy be the Solution for Hepatitis B Virus Infection?

Hepatitis B virus (HBV) infection continues to be a challenge to public health globally, causing serious complication such as chronic liver disease, cirrhosis, liver failure, hepatocellular carcinoma, and death, especially in Africa and the South East Asia [1]. Recent advancements in groundbreaking gene editing technologies such as CRISPR-Cas9 are currently under exploration for clinical application. However, to date, nucleos(t)ide analogues (NAs) with high genetic barrier to resistance remain the first-line therapy for HBV infection [2, 3]. For complete cure of HBV infection, targeting covalently closed circular DNA (cccDNA) is essential, which is yet to be achieved by NAs [4]. The current approach for managing chronic HBV infection effectively controls HBV replication. However, achieving a functional cure, as indicated by the disappearance of hepatitis B surface antigen (HBsAg), is rare. The presence of cccDNA often leads to almost universal viral relapse when treatment is stopped before the disappearance of HBsAg [4]. Persistent challenges to curing HBV include the presence of HBV-infected cells containing cccDNA and/or integrated viral sequences, as well as compromised innate and adaptive immune responses against HBV [5].

Cheng et al. approaches the problem with a novel radionuclide therapy utilizing ¹²⁵I labeled entecavir (¹²⁵I-ETV) derivative for the treatment of HBV infection, as a potential game-changer [6]. Most Auger-electron emitters, such as ¹²⁵I have low energy, but due to their very short-range travel within tissue at the nano-/micrometer level, they exhibit high linear energy transfer (LET) in the range of 1 ~ 23 KeV/µm. This characteristic allows the Auger electrons to target DNA or cell membranes, offering therapeutic effect once the target is within close reach [7, 8]. Synthesized via binding of vinylic halides, ¹²⁵I-ETV was very stable in both in vitro and in vivo in the current study. Moreover, ¹²⁵I-ETV synthesized by Cheng et al. demonstrated suitable characteristics for radiopharmaceutical therapy, with high radiochemical purity (over 95% until 48 h) and low radioactivity in critical organs including thyroid gland. As for the antiviral effect, ¹²⁵I-ETV decreased hepatitis B antigen secretion and reduced pregenomic RNA (pgRNA), totalDNA and cccDNA copies. Anti-HBV treatment efficacy of ¹²⁵I-ETV was superior to the antiviral efficacy of entecarvir alone with maximal inhibition effect being 36.3%, 22.6%, 33.0% for pgRNA, totalDNA, cccDNA, respectively. ¹²⁵I-ETV is promising as a therapeutic compound in that the major pharmacological functions of entecavir are maintained while the therapeutic effect is enhanced by the addition of the radiation of Auger electron from ¹²⁵I.

On account of the very short range of energy transfer, maximizing the proximity of Auger electrons to intranuclear DNA is crucial for achieving double strand breakage (DSB) of DNA. DSB of DNA can occur not only through direct effect but also through indirect effects caused by hydroxyl free radicals similar to those generated by α-particles, β-particles, γ-photons or X-rays. Auger electron emitters are also known to be capable of killing target cells through cell membrane damage. In addition, they can also eliminate non-target cells through the cross-dose effect on cells which are adjacent to target cells, and through bystander effect on distant cells. If decay occurs close to intranuclear DNA, Auger electrons from ¹²⁵I may exhibit superior cell-killing effects compared to the β-particles from ¹³¹I [8]. Several studies have shown that targeting nucleus rather than the cytoplasm or the whole cell results in significantly higher relative biological effectiveness (RBE) values [9, 10]. There have been experiments in antigene therapy using triplex-forming oligonucleotides (TFOs) for the sequence-specific delivery of DNA-damaging agents. Furthermore, antigene “radiotherapy” has been attempted by labeling Auger electron emitters on TFOs as the DNA damaging agents. Energy transfer in the subnanometer range is a characteristics of Auger electron emitters that enable them to target specific gene sequences and inflict damage while minimizing damage to the rest of the gene and cellular components [11, 12]. In a study observing the distribution of DNA strand breaks caused by ¹²⁵I-TFOs, there was a strong correlation between the yield of DNA strand breaks and the distance from the decay site, with the majority of breaks occurring within 10 base pairs of the decay site [13]. Instead of ¹²⁵I with relatively long half-life (60 days), Karamychev et al. utilized ¹²³I and ¹¹¹In as Auger electron emitters in an in vitro study of synthetic oligodeoxynucleotides. The efficiency of DNA strand breaks induced by ¹²³I and ¹¹¹In were compared with those by ¹²⁵I, and the results indicated that ¹²³I and ¹¹¹In caused DNA strand breaks less efficiently than ¹²⁵I [14].

The authors succeeded in synthesizing a new compound, ¹²⁵I-ETV, which exhibited a significantly increased antiviral effect compared to entecavir [6]. Moreover, it demonstrated appropriate in vivo biodistribution and low toxicity profile in normal tissue, indicating its potential as a treatment modality for HBV infection. However, to achieve the complete removal of intracellular cccDNA for the complete control of HBV, improved targeting of cccDNA is required. The authors are currently conducting further research to enhance targeting of hepatocytes and cccDNA. The authors are also planning additional research to investigate whether the targeting carrier using the short guide RNA (sgRNA) component of CRISPR-Cas9 for delivery can degrade cccDNA [2], as well as whether the effect of entecavir in blocking HBV replication in the cytoplasm can be enhanced by Auger electrons. We anticipate the results of these future studies and the successful translation of this novel compound from laboratory and small animal models to human clinical trials.

In this study, the aspects that required further clarification and validation are as follows. First, it is necessary to explain in more detail the mechanism by which ¹²⁵I-ETV reduces the amount of cccDNA in HBV-infected hepatocytes. Second, since patients with HBV infection usually take antiviral drugs for a long time until there is HBsAg loss, meticulous validation is needed regarding the treatment duration and safety of ¹²⁵I-ETV in the future. Third, we recommend that the authors compare the efficacy of ¹²⁵I-ETV with other orally available cccDNA-targeting agents, such as recently reported ccc_R80 in subsequent research [15].

Cheng et al’s preclinical study demonstrated the potential of a new treatment modality for HBV by successfully synthesizing a novel compound that combines an antiviral drug with an Auger electron emitter, thereby increasing the therapeutic effectiveness against HBV. Further validation and studies could pave the way to clinical translation for treatment of chronic HBV infection, and diverse approaches and innovations as exemplified by this study will positively influence the field of radionuclide therapy.

Author Contributions

Conceptualization: IR Yoo. Writing-original draft: IR Yoo and PS Sung. Writing-review and editing: IR Yoo.

Declarations

Ethical Approval

This work does not contain any studies with human participants or animals performed by any of the authors.

Informed Consent

Not applicable.

Consent for Publication

Not applicable.

Conflict of Interest

Ie Ryung Yoo and Pil Soo Sung declare that they have no conflict of interest.