PAP-IN-1 (compound 28) is an inhibitor of purple acid phosphatases ( PAPs ), a ubiquitous binuclear metallohydrolase. PAP-IN-1 inhibits mammalian PAP with K i value of 168 nM. PAP-IN-1 targets to pig PAP with K ic value of 0.17 μM. PAP inhibitors can be used for research of anti-osteoporotic drugs.