ErbB2-overexpressing human cancers represent potentially sensitive targets for therapy by candidate histone deacetylase (HDAC) inhibitors as we have shown that HDAC inhibitors can selectively reduce ErbB2 expression by repressing the ErbB2 promoter and accelerating the decay of cytoplasmic ErbB2 transcripts. To extend these in vitro findings and enhance the in vivo pharmacodynamic properties of HDAC inhibitors, we stably encapsulated a potent hydroxamate-based HDAC inhibitor (LAQ824) within long-circulating liposomes (Ls-LAQ824) and immunoliposomes (ILs-LAQ824) bearing >10,000 LAQ824 molecules per nanovesicle. Liposomal LAQ824 exhibits prolonged in vivo stability and, unlike free LAQ824, circulates with a half-life of 10.8 hours following a single i.v. injection. Three weekly i.v. injections of 20 to 25 mg/kg Ls-LAQ824 in nude mice with ErbB2 overexpressing BT-474 breast tumor xenografts significantly impairs tumor growth, and administration of ErbB2-targeted ILs-LAQ824 may further improve this antitumor activity. Studies of tumor-bearing mice 24 hours after single treatment indicate that: (a) >10% of injected liposomal LAQ824 is still circulating (whereas free LAQ824 is undetectable in the blood after 15 minutes); and (b) tumor uptake of Ls-LAQ824 and ILs-LAQ824 is >3% injected drug per gram of tumor, producing levels of acetylated tumor histones that are 5- to 10-fold increased over those following free LAQ824 or saline treatments and resulting in concordantly reduced levels of tumor ErbB2 mRNA. These preclinical results support the clinical evaluation of HDAC inhibitors against ErbB2-overexpressing malignancies, and further indicate that encapsulation into targeted and nontargeted liposomes substantially improves the in vivo pharmacokinetics, tumor uptake, and antitumor properties of hydroxamate-based HDAC inhibitors.