Biological characterization of a novel class of toll-like receptor 7 agonists designed to have reduced systemic activity

BACKGROUND AND PURPOSE Toll-like receptor 7 (TLR7) agonists have potential in the treatment of allergic diseases. However, the therapeutic utility of current low molecular weight TLR7 agonists is limited by their systemic activity, resulting in unwanted side effects. We have developed a series of TLR7-selective antedrugs, including SM-324405 and AZ12441970, which contain an ester group rapidly cleaved in plasma to reduce systemic exposure. EXPERIMENTAL APPROACH Agonist activity at TLR7 of the parent ester and acid metabolite was assessed in vitro in reporter cells and primary cells from a number of species. Pharmacokinetics following a dose to the lungs was assessed in mice and efficacy evaluated in vivo with a mouse allergic airway model. KEY RESULTS Compounds were selective agonists for TLR7 with no crossover to TLR8 and were metabolically unstable in plasma with the acid metabolite showing substantially reduced activity in a number of assays. The compounds inhibited IL-5 production and induced IFN-alpha, which mediated the inhibition of IL-5. When dosed into the lung the compounds were rapidly metabolized and short-term exposure of the 'antedrug' was sufficient to activate the IFN pathway. AZ12441970 showed efficacy in a mouse allergic airway model with minimal induction of systemic IFN-alpha, consistent with the low plasma levels of compound. CONCLUSIONS AND IMPLICATIONS The biological and metabolic profiles of these TLR7-selective agonist 'antedrug' compounds are consistent with a new class of compound that could be administered locally for the treatment of allergic diseases, while reducing the risk of systemic side effects.