Fig 1.
Inhibition of RSV replication by ALS-8112 and ALS-8176.
(A) Chemical structure of 2'-fluoro-4'-chloromethyl (2'F-4'ClCH2) cytidine, or ALS-8112. (B) In vitro inhibition potency of ALS-8112 against the RSV A2 and B1 strains grown in HEp-2 cells. The viral RNA level was measured by qRT-PCR, and reported as percentage of the uninhibited condition (n = 3). The effect of ALS-8112 on the viability of human epithelial HEp-2 cells was also evaluated. The highest concentration of ALS-8112 used to measure the concentration resulting in 50% cytotoxicity (CC50) was 100 μM (n = 3). (C) In vitro efficacy of ALS-8112 in a three-dimensional lung model. Primary human tracheal/bronchial epithelial cells from three individual human donors were infected on the apical side with the RSV A2 strain, while increasing concentrations of ALS-8112 were added to the basal medium. (D) Fluorescence microphotographs of HEp-2 cells infected with recombinant RSV-mKate2, in the presence of DMSO, 10 μM ALS-8112, or 10 μM ALS-8176. (E) African Green monkey efficacy model. ALS-8176 was administered BID for a total of 6 days. At the end of treatment (Day 5 post-infection), RSV RNA titers were measured in bronchoalveolar lavage (BAL) and nasopharyngeal (NP) swab samples for each group (vehicle and drug) containing four animals. Limit of detection (LOD) for qRT-PCR analysis was 50 copies/mL (dashed line).
Fig 2.
Identification of RSV polymerase as the molecular target for ALS-8112.
(A) Selection of resistance mutations associated with the prolonged culture of RSV A2 in the presence of increasing concentrations of ALS-8112. All four mutations detected from full-genome sequence analysis mapped to motif B, a region of the RdRp domain (residues ~500–1100) of the RSV L protein just upstream of the CRIII region containing the catalytic motif 810-GDNQ-813 (red) responsible for nucleotide incorporation by the RSV polymerase. (B) In vitro inhibition potency of ALS-8112 against the RSV minigenome luciferase-based reporter assay. HEp-2 cells were co-transfected to transiently express the RSV N, P, M2-1 and L proteins containing either the wild-type or the QUAD mutated sequence (n = 3) *P < 0.05 (Student's t test). (C) Inhibitory effect of ALS-8112-TP on the RdRp activity of the crude RSV RNP complex. The RNP complex containing the RSV L protein was extracted from virus-infected cells (RSV+), and uninfected cells were used as negative control for RdRp activity (RSV-). The enzymatic reaction was conducted in the presence of ALS-8112, ALS-8112-MP, or ALS-8112-TP. Labeled transcripts were separated from the initial radiolabeled CTP substrate by urea PAGE prior to phosphor-imaging. (D) Effect of increasing concentration of ALS-8112-TP on the RdRp activity of the RNP complex. The intensity of RNA product after gel electrophoresis was reported for each nucleotide concentration (n = 3). (E) In vitro inhibition potency of ALS-8112-TP against RSV wild-type (n = 4) and QUAD (n = 2) RdRp activity **P < 0.005 (Student's t test).
Fig 3.
Competitive inhibition of RSV RNP complex by ALS-8112-TP.
(A) Effect of low (1 μM) and high (100 μM) concentration of ATP, UTP, CTP, or GTP, on inhibition of RSV RNP by ALS-8112-TP used at a single concentration of 30 μM. (n = 2) *P < 0.05 (Student's t test). (B) Effect of increasing concentration of ALS-8112-TP on the RdRp activity of the RNP complex. RNA products were treated with RNase H prior to high-resolution electrophoresis in order to visualize individual gene transcripts. Lane 1 had no inhibitor, and ALS-8112-TP concentration ranged from 0.0003 μM (lane 2) to 300 μM (lane 8) by 10-fold increments.
Table 1.
Inhibition potency of ALS-8112 against RSV and other RNA viruses.
Table 2.
Inhibition potency of ALS-8112-TP against RSV and other viral RNA polymerases.
Fig 4.
Chain termination of RNA synthesis by ALS-8112-TP.
(A) SDS PAGE of recombinant RSV L-P polymerase complex. (B) Principle of the single nucleotide incorporation assay with RSV L-P polymerase: The 11-mer template contains a single G at the +4 position. In presence of radiolabled GTP (G*), the primer can be extended by one base (+1). GTP+ATP allows for a +3 extension, while the addition of CTP enables full-length RNA product synthesis (+7). (C) Wild-type recombinant RSV L-P polymerase complex (all lanes except 3 and 4) was incubated with primer/template (P/T) and GTP* (lane 1), or GTP* + 10 μM ATP (lane 2). Lanes 3 and 4: same as 1 and 2, except that the lethal N812A mutation was introduced within the L subunit. Lane 5: enzyme + GTP* + 10 μM CTP. Lane 6: enzyme + GTP* + 10 μM ALS-8112-TP. Lane 7: enzyme + GTP* + ATP + 10 μM CTP. Lane 8: enzyme + GTP* + ATP + 10 μM ALS-8112-TP. (D) RSV L-P incubated in the presence of GTP* + ATP and increasing concentrations of either ALS-8112-TP or mericitabine-TP: lane 1, 0.021 μM; lane 2, 0.062 μM; lane 3, 0.19 μM; lane 4, 0.56 μM; lane 5, 1.7 μM; and lane 6, 5.0 μM. (E) Product formation was quantified and expressed as % primer extension from the +3 position (see calculation in Fig D in S1 Text). CTP Km = 0.057±0.009 μM (n = 4), and ALS-8112-TP Km = 0.74±0.08 μM (n = 4).
Table 3.
Inhibition potency of cytidine analogs against RSV versus HCV replicon.
Fig 5.
Identification of dual RSV/HCV polymerase inhibitors.
(A) Incorporation by recombinant RSV polymerase of CTP analogs with a ribose containing either an OH, F, diF, or F-Me at the 2'-position, and either an H, N3, or ClCH2 at the 4'-position. Immediate chain terminators form a product at the +4 position, while natural substrate (CTP) fully extends the primer to the +7 position. (B) Comparative analysis of IC50 values of CTP analogs tested against RSV RNP complex versus HCV polymerase. Inhibitors of RSV polymerase are circled in green, and inhibitors of HCV polymerase are circled in red.
Fig 6.
Rational design of ALS-8112 as a selective RSV inhibitor.
(A) The nucleotide analog 2'-F-CTP is a substrate for both RSV and HCV polymerase, but it does not cause any inhibition by immediate chain termination. The addition of a 4'ClCH2 group (ALS-8112-TP) makes the molecule a selective inhibitor of RSV polymerase. The addition of a 2'Me group favors recognition by HCV polymerase, and the addition of a 4'N3 group causes dual RSV/HCV polymerase inhibition. (B) X-ray structure of natural CDP in the active site of HCV polymerase (PDB 4WTC, [28]). (C, D, and E) Docked binding modes of 2'F-2'Me-, 2'F-4'ClCH2-, and 2'F-4'N3-CDP, respectively.
Fig 7.
Contribution of the 4'ClCh2 group to the QUAD-mutant resistance to ALS-8112-TP.
(A) The RSV L-P proteins (WT and QUAD) were incubated in the presence of GTP* + ATP and increasing concentrations of either CTP or ALS-8112-TP. Product formation was quantified and expressed as % primer extension from the +3 position (see calculation in Fig D in S1 Text). QUAD Km CTP = 0.056±0.010 μM (n = 2), and QUAD Km ALS-8112-TP = 1.74±0.34 μM (n = 2). Km values for the WT enzyme were reported in Fig 4. (B) Fold discrimination for each enzyme was calculated as Km CTP analog / Km CTP. (C and D) The RSV L-P proteins (WT and QUAD) were incubated in the presence of GTP* + ATP and increasing concentrations of 2'F-CTP. WT Km 2'F-CTP = 0.12±0.014 μM (n = 2), and QUAD Km 2'F-CTP = 0.07±0.017 μM (n = 2). (E and F) The RSV L-P proteins (WT and QUAD) were incubated in the presence of GTP* + ATP and increasing concentrations of 4'ClCH2-CTP. WT Km 4'ClCH2-CTP = 16±2.7 μM (n = 2), and QUAD Km 4'ClCH2-CTP = 198±18 μM (n = 2).