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Evaluation of the Antitumor Efficacy, Pharmacokinetics, and Pharmacodynamics of the Histone Deacetylase Inhibitor Depsipeptide in Childhood Cancer Models In vivo

Authors' Affiliations: Departments of ¹ Molecular Pharmacology, ² Biostatistics, ³ Hematology-Oncology, and ⁴ Pharmaceutical Sciences, St. Jude Children's Research Hospital, Memphis, Tennessee

Requests for reprints: Peter J. Houghton, Department of Molecular Pharmacology, St. Jude Children's Research Hospital, 332 North Lauderdale Street, Memphis, TN 38105-2794. Phone: 901-495-3440; Fax: 901-495-4290; E-mail: peter.Houghton{at}stjude.org.

Purpose: Histone acetyltransferases and histone deacetylases (HDAC) control the acetylation state of histones and other proteins regulating transcription and protein function. Several structurally diverse HDAC inhibitors have been developed as cancer therapeutic agents and in vitro have been shown to cause differentiation, cell cycle arrest, or apoptosis. Here, we have evaluated depsipeptide, a natural tetrapeptide HDAC inhibitor, against a panel of pediatric solid tumor models in vivo and evaluated pharmacokinetic and pharmacodynamic variables with tumor sensitivity.

Experimental Design: Depsipeptide was administered at the maximum tolerated dose (4.4 mg/kg administered every 7 days x 3 i.v. repeated q21d for a total of two cycles) to scid mice bearing 39 independently derived childhood tumors (9 brain tumors, 11 kidney cancers, 9 rhabdomyosarcomas, 3 neuroblastomas, and 7 osteosarcomas). Pharmacokinetic variables were determined, as were changes in histone and p53 acetylation, induction of p53 and p53 genotype, and alterations in Akt phosphorylation.

Results: Of 39 tumors evaluated, three showed objective tumor regressions [two brain tumors (primitive neuroectodermal tumor and atypical teratoid malignant rhabdoid tumor) and one Wilms' tumor]. Depsipeptide inhibited growth of many tumor lines but achieved stable disease (<25% increase in volume during treatment cycle 1) in only two tumor models (anaplastic astrocytoma, two rhabdomyosarcomas, and a Wilms' tumor). Pharmacokinetic analysis showed that the population estimated AUC_0-24 was 1,123 ng h/mL, similar to the exposure following 13 mg/m² in ongoing phase I trials. Pharmacodynamic changes in histone acetylation (H2A, H2B, H3, and H4) in three depsipeptide-sensitive and three intrinsically resistant tumors followed a similar pattern; maximal increases in histone acetylation occurred at 8 hours and were elevated for up to 96 hours. In two sensitive tumor lines, IRS56 and BT27 (both wild-type p53) p53 increased in treated tumors being maximal at 8 hours and associated with induction of p21^cip1, whereas p53 was stable in tumors with mutant p53. Sensitivity to depsipeptide did not correlate with p53 genotype, p53 acetylation, cleaved poly(ADP-ribose) polymerase, or phosphorylation of Akt (Ser⁴⁷³).

Conclusions: Our results show that depsipeptide inhibits its target in vivo causing increased histone acetylation; however, this does not correlate with drug sensitivity. The relatively low objective response rate [3 of 39 (8%) tumor lines showing greater than or equal to partial response and 4 (10%) stable disease] administered at dose levels that give clinically relevant drug exposures suggests that as a single agent depsipeptide may have limited clinical utility against pediatric solid tumors in a first-line setting.

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