Treatment of castration-resistant prostate cancer: updates on therapeutics targeting the androgen receptor signaling pathway

Androgens play a critical role in the progression of castration-resistant prostate cancer through androgen receptor (AR)-regulated signaling pathways. Progress has been made in the development of potent agents designed to suppress androgen function by blocking the AR, inhibiting the synthesis of androgens, or targeting downstream AR signaling pathways. This review summarizes the development of novel therapies based on current insights into AR signaling pathways in castration-resistant prostate cancer.

Histone deacetylase inhibitors as anti-neoplastic agents

Histone deacetylase inhibitors (HDACIs) constitute a novel class of targeted drugs that alter the acetylation status of histones and other important cellular proteins. These agents modulate chromatin structure leading to transcriptional changes, induce pleiotropic effects on functional pathways and activate cell death signaling in cancer cells. Anti-neoplastic activity in vitro was shown in several experimental models of cancer, but the exact mechanism of cytotoxicity and responses are not clearly understood. Phase I/II clinical trials of various HDACIs as single agents conducted to date have shown substantial activity in cutaneous T cell lymphoma (CTCL), preliminary activity in Hodgkin's disease and modest activity in myeloid neoplasms. Responses have been rare in solid tumors. Several agents are being tested in combination therapy clinical trials, either as chemosensitizers for cytotoxic chemotherapy or radiation therapy, or in association with DNA methylation inhibitors based on in vitro synergy. In this review, we focus on recent basic and clinical data that highlight the anti-neoplastic role of HDACIs.

Evaluation of the pharmacodynamic effects of MGCD0103 from preclinical models to human using a novel HDAC enzyme assay

PURPOSE

The pharmacodynamic properties of MGCD0103, an isotype-selective inhibitor of histone deacetylase (HDAC), were evaluated in preclinical models and patients with a novel whole-cell HDAC enzyme assay.

EXPERIMENTAL DESIGN

Boc-Lys(epsilon-Ac)-AMC, a HDAC substrate with fluorescent readout, was found to be cell permeable and was used to monitor MGCD0103-mediated HDAC inhibition in cultured cancer cells in vitro, in peripheral WBC ex vivo, in mice in vivo, and in human patients.

RESULTS

MGCD0103 inhibited HDAC activity in several human cancer cell lines in vitro and in human peripheral WBC ex vivo in a dose-dependent manner. Unlike suberoylanilide hydroxamic acid, the HDAC inhibitory activity of MGCD0103 was time dependent and sustained for at least 24 hours following drug removal in peripheral WBC ex vivo. Inhibitory activity of MGCD0103 was sustained for at least 8 hours in vivo in mice and 48 hours in patients with solid tumors. HDAC inhibitory activity of MGCD0103 in peripheral WBC correlated with induction of histone acetylation in blood and in implanted tumors in mice. In cancer patients, sustained pharmacodynamic effect of MGCD0103 was visualized only by dose-dependent enzyme inhibition in peripheral WBC but not by histone acetylation analysis.

CONCLUSIONS

This study shows that MGCD0103 has sustained pharmacodynamic effects that can be monitored both in vitro and in vivo with a cell-based HDAC enzyme assay.

The discovery of 6-amino nicotinamides as potent and selective histone deacetylase inhibitors

This communication highlights the development of a nicotinamide series of histone deacetylase inhibitors within the benzamide structural class. Extensive exploration around the nicotinamide core led to the discovery of a class I selective HDAC inhibitor that possesses excellent intrinsic and cell-based potency, acceptable ancillary pharmacology, favorable pharmacokinetics, sustained pharmacodynamics in vitro, and achieves in vivo efficacy in an HCT116 xenograft model.

Histone Deacetylase Inhibitors and Demethylating Agents: Clinical Development of Histone Deacetylase Inhibitors for Cancer Therapy

The histone deacetylase inhibitors are a new class of agents that are currently in various stages of clinical development. Clinical trials have demonstrated activity, urging further investigation. At the same time, it has been discovered that these agents have their own challenges. In this review, we discuss clinical data gathered to date, combination therapies designed to increase efficacy, and toxicities attributed to this new class of agents.