ACTIVATED MAPK PATHWAY LIMITS EFFICACY OF ROMIDEPSIN
The Problem with HDAC Inhibitors in Solid Tumors
Histone deacetylase (HDAC) inhibitors (HDIs) are a newer class of epigenetic anti-tumor agents that have shown promise treating hematologic malignancies. Most cell lines tested in vitro are sensitive to HDIs, and drug combinations often show synergy. Yet Phase I and II trials of HDIs have consistently disappointed in solid tumors — even preclinical models where HDIs showed strong anti-tumor effects in vivo haven’t translated to success in the clinic. Understanding the mechanisms of resistance to HDIs in detail could help researchers design strategies to improve clinical efficacy.
Studies have proposed several resistance mechanisms, including increased expression of the P-glycoprotein (Pgp)-encoding multidrug-resistance gene ABCB1, increased expression of the reactive oxygen species (ROS) scavenger protein thioredoxin, elevated expression of anti-apoptotic proteins Bcl-2 and Bcl-xL, increased expression of HDAC enzymes, and activation of several signaling pathways including MAPK, phosphoinositide 3-kinase, and signal transducer and activator of transcription.
Romidepsin Resistance in T-Cell Lymphoma
The FDA approved the HDAC inhibitor romidepsin (Istodax®) to treat cutaneous T-cell lymphoma (CTCL) in 2009, and peripheral T-cell lymphoma (PTCL) in 2011. Some patients who initially responded to therapy later saw disease progression, while others never responded at all — suggesting both de novo and acquired resistance to romidepsin occurred during the trials. This makes understanding the mechanisms of romidepsin resistance in T-cell lymphoma an important research priority.
Looking Beyond Pgp-Mediated Resistance
Several studies have identified Pgp overexpression in vitro as a mechanism of drug resistance to romidepsin. Pgp is easy to select for in these studies partly because romidepsin is a substrate for Pgp, and partly because ABCB1 gene induction is a consistent cellular response to HDIs. Induction of ABCB1 has been observed in both normal and malignant peripheral blood mononuclear cells from romidepsin-treated patients. But no evidence of Pgp-mediated resistance showed up in clinical samples taken at the time of disease progression from CTCL or PTCL patients — meaning non-Pgp resistance mechanisms for romidepsin need to be identified.
A study published in the peer-reviewed journal Blood by Chakraborty et al. (Blood. 2013 Mar 26) found that activation of the mitogen-activated protein kinase (MAPK) pathway causes resistance to romidepsin by degrading the pro-apoptotic BH-3-only protein Bim.
MAPK Activation Degrades a Key Pro-Apoptotic Protein
To explore Pgp-independent resistance mechanisms, the researchers used a CTCL model built from the HuT78 cell line and its romidepsin-selected sublines. These sublines were selected in romidepsin along with the Pgp inhibitors verapamil or valspodar (PSC833), specifically to avoid overexpression of Pgp. The resulting cell lines were resistant to romidepsin, and a Pgp inhibitor could only reverse resistance in cells that hadn’t been selected alongside a Pgp inhibitor. Since a Pgp inhibitor couldn’t reverse resistance in the romidepsin-selected cells, this pointed to a different, previously unidentified mechanism.
A gene microarray study found increased expression of the insulin receptor in the romidepsin-selected cells compared to the parental HuT78 cells. The romidepsin-resistant cells also showed increased activation of MEK protein, a downstream component of the MAPK signaling pathway. Treating these resistant cells with allosteric MEK inhibitors made them highly sensitive to treatment, while the parental HuT78 cells didn’t respond to MEK inhibition at all. MEK inhibition also restored the pro-apoptotic protein Bim in the romidepsin-resistant cells — a protein that had otherwise been degraded in these cells. Combining a MEK inhibitor with romidepsin further increased death of the resistant cells.
Confirmation in Patient Samples
Chakraborty and colleagues also found a loss of Bim in skin biopsy samples from CTCL patients whose disease progressed after romidepsin treatment. They also reported disruption of MAPK-regulated genes in CTCL patients treated with romidepsin.
What This Means for Future Trials
Together, these findings suggest that MAPK pathway activation may limit the effectiveness of the HDI romidepsin by degrading the pro-apoptotic protein Bim. Combining romidepsin with a MEK inhibitor in future clinical trials may produce better results.
References
1. Fantin VR, Richon VM. Mechanisms of resistance to histone deacetylase inhibitors and their therapeutic implications. Clin Cancer Res. 2007;13(24):7237-7242.
2. Chakraborty AR, Robey RW, Luchenko VL, et al. MAPK pathway activation leads to Bim loss and histone deacetylase inhibitor resistance: rationale to combine romidepsin with a MEK inhibitor. Blood. 2013.
