Structure–Activity Relationship Studies in Benzothiadiazoles as Novel Vaccine Adjuvants

Extracellular
vesicles (EVs) can transfer antigens and immunomodulatory
molecules, and such EVs released by antigen-presenting cells equipped
with immunostimulatory functions have been utilized for vaccine formulations.
A prior high-throughput screening campaign led to the identification
of compound 634 (1), which enhanced EV release
and increased intracellular Ca2+ influx. Here, we performed
systematic structure–activity relationship (SAR) studies to
investigate the scaffold for its potency as a vaccine adjuvant. Synthesized
compounds were analyzed in vitro for CD63 reporter activity (a marker
for EV biogenesis) in human THP-1 cells, induction of Ca2+ influx, IL-12 production, and cell viability in murine bone-marrow-derived
dendritic cells. The SAR studies indicated that the ester functional
group was requisite, and the sulfur atom of the benzothiadiazole ring
replaced with a higher selenium atom (9f) or a bioisosteric
ethenyl group (9h) retained potency. Proof-of-concept
vaccination studies validated the potency of the selected compounds
as novel vaccine adjuvants.