HBsAg Isoforms as Innovative Biomarkers in Predicting Virological Response to Bulevirtide in Patients With Chronic Hepatitis D

Background and Aims: HDV exploits HBV surface-protein (HBsAg) for entering into hepatocytes. HBsAg consists of 3 isoforms: Large- (L-HBs, predominantly present in virions and mediating binding to NTCP-receptor), Middle- (M-HBs) and Small-HBsAg (S-HBs). Here, we investigated the kinetics of HBs isoforms under bulevirtide treatment (BLV). Methods: 67 consecutive patients with HDV-related compensated cirrhosis starting BLV 2 mg/day were enrolled. L-HBs, M-HBs and S-HBs were quantified by ad-hoc ELISAs in baseline and week 48 (W48) samples. Results: At baseline, median (IQR) HDV-RNA was 5.1 (4.3-5.7) log IU/mL while median (IQR) S-HBs, M-HBs and L-HBs levels were 3801 (1401-7462), 743 (211-1710) and 5 (1-13) ng/mL. At W48, virological responses (VR) were observed in 72% (48/67) of patients, while 25.4% (17/67) achieved undetectable HDV-RNA (11/17 with ALT-normalisation). A decline of S-HBs, M-HBs and L-HBs levels was observed in 51%, 63% and 31% of patients (median [IQR] decline: 961 [461-1985], 258 [68-626] and 4 [2-12] ng/mL). Notably, patients with undetectable HDV-RNA at W48 had baseline L-HBs and S-HBs levels lower than patients not achieving this end-point (1 [0.3-7] vs. 6 [2-13] ng/mL, p = 0.04 and 1570 [369-5185] vs. 4015 [1646-8687] ng/mL, p = 0.002). By AUROC, patients with baseline L-HBs < 3 ng/mL or S-HBs < 3400 ng/mL were more likely to achieve HDV-RNA undetectability at W48 (39.3% vs. 15.8%, p = 0.04 and 38.7% vs. 13.9%, p = 0.03). Furthermore, the combination of pre-treatment L-HBs < 3 ng/mL + HDV-RNA < 5logIU/mL and S-HBs < 3400 ng/mL + HDV-RNA < 5logIU/mL was the best predictor for achieving undetectable HDV-RNA at W48 (56.3% vs. 15.7%, p = 0.002 and 60% vs. 11%, p < 0.001). Conclusions: Quantification of L-HBs and of S-HBs, along with HDV-RNA, may reflect the burden of circulating infectious virions in HBV/HDV co-infection, providing a promising tool to identify patients more likely to respond to BLV.