APN421 is based on a similar approach as pursued with APN411, but is based on a different chemical structure, and a different mode of action. The project is not yet partnered. Extensive medicinal chemistry is ongoing with the Lead Discovery Center (LDC, Germany) to get insight into structure/activity relationship. Modified compounds are already identified with markedly improved activity in in-vitro assays for stimulation of human (and murine) immune cells.