The invention provides a novel class of compounds, pharmaceutical compositions comprising such compounds and methods of using such compounds to treat or prevent diseases or disorders associated with Toll-Like Receptors, including TLR7 and TLR8. In one aspect, the compounds are useful as adjuvants for enhancing the effectiveness of a vaccine (formula I) wherein: X3 is N; X4 is N Or CR3; X5 is —CR4═CR5.