Structure-Activity Relationship of Fluorinated Sialic Acid Inhibitors for Bacterial Sialylation

Abstract

Bacterial pathogens such as Nontypeable Haemophilus influenzae (NTHi) can evade the immune system by taking up and presenting host-derived sialic acids. Herein, we report a detailed structure-activity relationship of sialic acid-based inhibitors that prevent the transfer of host sialic acids to NTHi. We report the synthesis and biological evaluation of C-5, C-8, and C-9 derivatives of the parent compound 3-fluorosialic acid (SiaNFAc). Small modifications are tolerated at the C-5 and C-9 positions, while the C-8 position does not allow for modification. These structure-activity relationships define the chemical space available to develop selective bacterial sialylation inhibitors.

Figure 1

Sialic acid utilization in NTHi. Host-derived sialic acids are taken up by the SiaPQM transporter system. Intracellular sialic acid is either used as a carbon source (via NanA) or used to sialylate the lipooligosaccharide (LOS). To this end, sialic acid is CMP activated by SiaB and incorporated by sialyltransferases Lic3A, Lic3A2, and/or LsgB.

Scheme 1. Synthesis of C-5 Modified SiaFNAc Inhibitors

(i) The synthesis of 1a–1j has been described previously;⁻1k: MeOH, NaOMe, TEA, methyl formate; (ii) Sodium fluoropyruvate, H₂O, Neu₅Ac aldolase, 37 °C; (iii) 1. MsCl, DIPEA, DCM, 0 °C; 2. NaOMe, MeOH; (iv) 1. Selectfluor, DMF, H₂O; 2. NaOH, H₂O.

Figure 2

Inhibitory potency and EC₅₀ curves of C-5 modified SiaFNAc inhibitors on the incorporation of 100 μM SiaNAz by NTHi. EC₅₀ values were calculated based on the fitted S-curves, see the Supporting Information for more details.

Scheme 2. Synthesis of C-8 and C-9 Modified SiaFNAc Inhibitors

(i) 1. NaIO₄, MeOH; 2. NaBH₄, MeOH; (ii) 1. Selectfluor, DMF, H₂O; 2. NaOH, MeOH; (iii) 1. TsCl, pyridine; 2. NaN₃, acetone, H₂O.

Figure 3

Inhibitory potency of SiaFNAc inhibitors modified at the glycerol side chain.