Human Toll-like Receptor (TLR) 8‑Specific Agonistic Activity in Substituted Pyrimidine-2,4-diamines

Activation of human toll-like receptor-8
(TLR8) evokes a distinct
cytokine profile favoring the generation of Type 1 helper T cells.
A multiplexed high-throughput screen had led to the identification
of N4-butyl-5-iodo-6-methylpyrimidine-2,4-diamine
as a pure TLR8 agonist, and a detailed structure–activity relationship
study of this chemotype was undertaken. A butyl substituent at N4 was optimal, and replacement of the 5-iodo
group with chloro, bromo, or fluoro groups led to losses in potency,
as did the introduction of aromatic bulk. Drawing from our previous
structure-based design, several 5-alkylamino derivatives were evaluated.
Significant enhancement of potency was achieved in 5-(4-aminobutyl)-N4-butyl-6-methylpyrimidine-2,4-diamine. This
compound potently induced Th1-biasing IFN-γ and IL-12 in human
blood, but lower levels of the proinflammatory cytokines IL-1β,
IL-6, and IL-8. These results suggest that the inflammatory and reactogenic
propensities of this compound could be considerably more favorable
than other TLR8 agonists under evaluation.