**3. Pharmacology and Safety of Current and Investigational Therapies of Hepatitis B and D**

#### *3.1. Myrcludex B (Myr)*

Myr targets the hepatocytes exclusively, and this might allow subcutaneous administration of low drug doses [18]. Phase III clinical trials have established a subcutaneous injection of 10 mg as the optimal dose to reach more than 80% saturation of the NTCP receptor for at least 15 h [19]. The raised concern that NTCP blockage might cause an elevated plasma bile acid levels-related adverse reaction [20] is now insubstantial because, while the inhibition of HBV/HDV infection is reached with an inhibitory concentration (IC) 50 of 80 pM [21], the increase in bile acid transportation is impaired with an IC 50 of 47 nM, therefore significantly higher. Hence, Myr effectively inhibits HBV/HDV infection at concentrations where the NTCP-mediated transport of substrates is not yet affected. However, whether NTCP inhibition can also affect drug exposure is unknown. Conversely, plasma bile acid levels might work as the drug's marker. A study by Blank et al. recently investigated the pharmacokinetic data of Myr, and its effects on TDF 300 mg in 12 healthy volunteers after administration of a 10 mg SC dose. The authors noted that the steady-state AUC and the Cmax were significantly higher compared with those following the first dose, thus indicating an accumulation [19]. A further major consideration for clinical practice concerns the drug's excretion, and renal excretion resulted as a negligible route of elimination of Myr [22].

A critical issue is definitely the combination of antiviral therapies for hepatitis B infection. The reason behind it is precisely to achieve the HBsAg loss, acting at different stages of the disease, and simultaneously decrease HBV attachment and entry, ccc DNA formation, nucleocapsid and core assembly. For example, although Myr blocks viral entry, HDV and HBV can still propagate undisturbed through cell division, which is, conversely, efficiently restricted by IFN. IFN-α inhibits HBV transcription and replication in cell culture and in humanized mice by targeting the epigenetic regulation of the nuclear cccDNA minichromosome [23,24]. In the published first results of a phase 2 trial, a benefit of the antiviral combination of PegIFNα-2a and myrcludex was definitely observed [13]. However, follow-up showed a viral rebound after treatment cessation. Myr may thus be combined with current HBV drugs to improve HBV or HBV-HDV infected patient outcome; however, despite a decrease in HDV-RNA in a dose-dependent manner, only 10% of patients treated with Myr showed a definite virological response (defined as a 2 log10 reduction in HDV-RNA). The optimal duration of treatment to clear HDV RNA permanently is still unknown, since studies of 2 to 3-year duration are being planned, while the suggestion of potential benefit of a higher dose of Myr has been investigated by Loglio et al. [25].
