Hydroxamic acid (HA)-based histone deacetylase (HDAC) inhibitors, with 
trichostatin A (TSA) as the reference compound, are potential antitumoral drugs 
and show promise in the creation of long-term primary cell cultures. However, 
their metabolic properties have barely been investigated. TSA is rapidly 
inactivated in rodents both in vitro and in vivo. We previously found that 
5-(4-dimethylaminobenzoyl)aminovaleric acid hydroxyamide or 4-Me2N-BAVAH 
(compound 1) is metabolically more stable upon incubation with rat hepatocyte 
suspensions. In this study, we show that human hepatocytes also metabolize TSA 
more rapidly than compound 1 and that similar pathways are involved. 
Furthermore, structural analogs of compound 1 (compounds 2-9) are reported to 
have the same favorable metabolic properties. Removal of the dimethylamino 
substituent of compound 1 creates a very stable but 50% less potent inhibitor. 
Chain lengthening (4 to 5 carbon spacer) slightly improves both potency and 
metabolic stability, favoring HA reduction to hydrolysis. On the other hand, 
Calpha-unsaturation and spacer methylation not only reduce HDAC inhibition but 
also increase the rate of metabolic inactivation approximately 2-fold, mainly 
through HA reduction. However, in rat hepatocyte monolayer cultures, compound 1 
is shown to be extensively metabolized by phase II conjugation. In conclusion, 
this study suggests that simple structural modifications of amide-linked TSA 
analogs can improve their phase I metabolic stability in both rat and human 
hepatocyte suspensions. Phase II glucuronidation, however, can compensate for 
their lower phase I metabolism in rat hepatocyte monolayers and could play a yet 
unidentified role in the determination of their in vivo clearance.