HDAC9 and rheumatoid arthritis: Bone erosion is associated with RA severity and poor functional outcomes.7 A recent study showed that a combination of class I and II HDAC inhibitors synergistically inhibited osteoclast activity, and this combination was more effective than a class I or class II inhibitor alone,24 suggesting that a pan-HDAC inhibitor may be more effective in suppressing the osteoclast activity.