CUDC-101 – AN INHIBITOR WITH MULTIPLE TARGETS

INTRODUCTION

An elaborate research on cancers has shown that they are mostly an outcome of the up regulated MAPK pathway or due to the variations in the levels of HDAC enzymes. Inhibitors which target these pathways either check the uncontrolled progression of this pathway or increase the acetylation of the histone proteins. A single inhibitor which can regulate all these effects would be highly beneficial. CUDC-101 was discovered to be a single inhibitor with multiple effects.

CUDC-101 AND ITS DISCOVERY

A pharmacophore having EGFR and HER2 inhibitor properties was selected and HDAC inhibitor functionality was integrated into it. This invention - CUDC-101, was further subjected to clinical development. It inhibited HDAC at a concentration of 4.4 nM, EGFR at 2.4 nM, and HER2 at 15.7 nM. It was more potent when compared to vorinostat and other RTK inhibitors. It interferes with multiple pathways and checked the proliferation of various cancer cell lines [1].

MULTIPLE INHIBITORY EFFECTS OF CUDC-101

Even during the course of treatment many tumors develop new mis-regulated survival and growth pathways. Hence a combination of inhibitors or a single inhibitor which can check the growth of tumor and control the process of metastasis are very beneficial. When multiple inhibitors are used there are chances of adverse side effects. A single inhibitor with multiple targets can solve this problem. During the first phases of research the molecule which can inhibit HER (human epidermal growth factor receptor) kinases and contains HDAC inhibitor properties was designed. Later on the novel molecule CUDC-101 was discovered which targeted HER2, HDACs belonging to class I and II and EGFR (epidermal growth factor receptor). Due to its multiple effects it suppressed the growth of a large variety of tumors. It indirectly regulates the HER3, Akt and MET pathways also [2].

CUDC-101 IS EFFECTIVE IN CELLS RESISTANT TO RTK INHIBITORS

The receptors of the HER family are inhibited by inhibitors like gefitinib, erlotinib and lapatinib, which are also known as RTK inhibitors. These inhibitors are highly efficient in case of solid tumors. But the heterogeneity of tumors poses a problem in treatment using the above mentioned inhibitors. The efficacy of these inhibitors is further reduced due to the resistance offered during the course of treatment. CUDC-101 is effective even in those cells which are resistant to erlotinib and lapatinib [2].

EFFECT OF BLOCKING EGFR IN COLON CANCER CELLS

Colon cancer cells show a coexpression of EGFR and when they heterodimerize, the signal transduction pathways get diversified. GW572016 is a dual inhibitor which inhibits both ErbB2 and EGFR kinases. AG1478 is a selective EGFR inhibitor and AG879 acts specifically on ErbB2 kinase. A combination of these inhibitors checked the growth and stimulated the process of apoptosis. GW572016 is a single inhibitor which shows more potent changes at lower concentrations. This research shows that a single agent is more effective against EGFR and ErbB2 rather than a combination of two inhibitors [3]. Breast cancers are usually associated with abnormal signaling by EGF receptor. In fact a constitutive phosphorylation of HER2 is seen in most cases of breast cancers. Hence inhibitors which can target EGFR can also modulate the expression of HER2 [4].

CONCLUSION

In summary CUDC-101 is a single inhibitor which modulates multiple pathways by targeting multiple kinases. This inhibitor has a high therapeutic potential.

 REFERENCES

1. Cai X, Zhai HX, et al. Discovery of 7-(4-(3-ethynylphenylamino)-7-methoxyquinazolin-6-yloxy)-N-hydroxyheptanamide (CUDc-101) as a potent multi-acting HDAC, EGFR, and HER2 inhibitor for the treatment of cancer. J Med Chem 2010 Mar 11; 53(5):2000-9.

2. Bao R, Tao X, et al. CUDC-101, a Multitargeted Inhibitor of Histone Deacetylase, Epidermal Growth Factor Receptor, and Human Epidermal Growth Factor Receptor 2, Exerts Potent Anticancer Activity. Cancer Res 2010 May 1; 70; 3647

3. Zhou Y, Li S, et al. Blockade of EGFR and ErbB2 by the novel dual EGFR and ErbB2 tyrosine kinase inhibitor GW572016 sensitizes human colon carcinoma GEO cells to apoptosis. Cancer Res 2006 Jan 1; 66(1):404-11.

4. Moulder SL, Yakes FM, et al. Epidermal Growth Factor Receptor (HER1) Tyrosine Kinase Inhibitor ZD1839 (Iressa) Inhibits HER2/neu (erbB2)-overexpressing Breast Cancer Cells in Vitro and in Vivo1. Cancer Res 2001 Dec 15; 61(24):8887-95.

ROLE OF HDAC INHIBITORS IN THE FIGHT AGAINST CANCER

The deadly disease cancer is associated with an uncontrolled and unregulated growth of cells within our body. This disease is claiming a large number of deaths across the world. According to the statistics provided by WHO (World Health Organization), the number of deaths due to cancer have increased by 45%. This disease can affect almost any part within our body. Initially it appears as a small lump or mass but proves to be deadly when it spreads all over the body through blood or lymphatic system.

Causative agents for cancer are many, like unhealthy lifestyle habits, exposure to carcinogenic pollutants or radiations, few viral infections etc. These causative agents ultimately stimulate genetic defects within our cells. The genetic defects appear in the form of chromosomal aberrations or gene mutations (deletion or insertion of genes). The ultimate effect of these genetic effects is the suppression of tumor suppressor genes or hyperactivation of oncogenes.

The expression of genes into proteins within eukaryotes is controlled at different stages in a large number of ways. This control mechanism starts right from the chromatin stage. Histone acetyltransferases (HAT) and histone deacetylases (HDAC) are two sets of enzymes which show opposing effect on the chromatin modifications and hence regulate the expression of genes. Under the action of HATs, the chromatin gets more relaxed increasing the accessibility of transcription factors to DNA. This stimulates the transcription of genes whereas HDACs make the chromatin more condensed and repress the process of transcription. An increased activity of HDACs or inactivity of HATs, has been noticed in large number of tumors. It is difficult to induce an enzyme under physiological conditions through pharmacological agents. Hence inducing the activity of HATs is rather difficult in comparison to inhibition of the activity of HDACs pharmacologically. This makes HDACs a potential target in clinical studies. HDACs have a potential to alter the epigenetic status of a cell. Apart from histones, HDACs also target certain non-histone proteins like transcription factors, heat-shock proteins etc. As a result they can modulate various cellular processes also [1].

Histone deacetylases are a group of enzymes which are classified into 4 different groups. Amongst them the HDACs belonging to classes I, II and IV are also known as classical HDACs whereas the HDACs which belong to class III are known as sirtuins [2]. The compounds which target these enzymes and inhibit their action are known as HDAC inhibitors (HDACi). These inhibitors are either obtained after extraction from natural sources or are chemically synthesized. The classification of HDAC inhibitors is based upon their chemical structure and its potency to inhibit a particular HDAC enzyme. Almost all the HDACi possess a common pharmacophore. This pharmacophore unit consists of a zinc binding group which helps in the chelation of the cation to the catalytic domain of HDAC. Apart from this a pharmacophore also contains cap, connecting unit and a linker.

HDACi show multiple biological activities within a cancerous cell like:

STIMULATION OF APOPTOSIS

HDACi have an inherent capacity to induce apoptosis within the tumor cells. An added benefit of these inhibitors is that they selectively stimulate this apoptotic process within the tumor cells and leave the normal cells unaffected. Some side effects like nausea, fatigue and thrombocytopaenia have been noticed but can be clinically managed. HDACi show different actions depending on the cell type. On the other hand different HDAC inhibitors which vary in their structures show different effects within the same cell type. For example SAHA or Vorinostat shows a widespread activity in comparison to Tubacin [3].

STIMULATION OF DEATH LIGANDS (EXTRINSIC DEATH PATHWAY)

In vitro studies using human tumor cell lines have shown that HDACi induced apoptosis is largely through stimulation of the death receptor pathway. In vivo studies were performed in case of transgenic mice which developed AML. Upon administration of Valaporate, death ligands like FAS and TRAIL were induced, hence stimulating the process of apoptosis. However clinical trials in this line are yet to be done [3].

STIMULATION OF MITOCHONDRIAL OR INTRINSIC DEATH PATHWAY

HDAC inhibitors regulate the expression of pro and anti-apoptotic genes. It stimulates the expression of pro-apoptotic proteins which in turn activate the apoptosis through the intrinsic death pathway. In vitro studies have proved this fact but in vivo studies are yet to be done [3]. 

REGULATION OF ROS ACTIVITY

HDAC inhibitors elevate the levels of reactive oxygen species followed by the changes in the mitochondrial membrane potential. Various free-radical scavengers have the potential to reverse this effect. However the exact mechanism by which the free radicals are increased is still not well understood. Free radicals can either be produced by active process which gets further enhanced by the increased ROS production or due to alterations in the expression of ROS-regulatory proteins (thioredoxin and TBP2) [3]. Further studies are yet to be done in this line.

ARRESTING CELL CYCLE

HDAC inhibitors promote the cellular differentiation by arresting the cell cycle at the Gap1 phase. This arrest of the cell cycle is known to be mediated by the retinoblastoma proteins. In fact all the HDAC inhibitors except Tubacin have the potential to arrest the cell cycle. The underlying mechanism behind G1 arrest has been found to be the transcriptional activation of CDKN1A. HDAC inhibitors also activate the G2 phase check point. However the mechanism behind the HDACi stimulated G2 arrest is not well defined [3].

ANTI-ANGIOGENIC AND ANTI-INVASIVE PROPERTIES OF HDACi

Results from in vitro and in vivo studies show that HDAC inhibitors can control the process of angiogenesis (cut the supply of nutrients) and metastasis within tumor cells. This checks the tumor development and prevents it from getting spread. The mechanism behind this action is HDACi induced expression of pro-angiogenic genes. The process of metastasis is controlled by HDACi induced suppression of matrix metalloproteinases [3].

MODULATE THE IMMUNE SYSTEM

HDAC inhibitors modify the malignant cells in such a way that they become potent immune targets. They can also alter the cytokine production. The reason behind the increased immunogenicity of HDACi induced tumor cells has been related to the increased expression of MHC class I and II proteins along with the increased expression of co-stimulatory molecules like  CD86, CD80, ICAM1, and CD40 [3].

Initially it was understood that HDACi can regulate the gene expression through histone acetylation. However now its is well known that HDACi can stimulate more diverse biological effects by affecting various molecular processes like DNA replication, mitosis, DNA repair etc. They have shown promising results when administered alone. However their combination with other agents proved to be more successful. It has been tested in combination with conventional chemotherapeutic agents, transcriptional modulators, death receptor ligands, proteasomal degradation regulators and kinase inhibitors. A significant success achieved during clinical studies makes the oncologists equipped with a new weapon to fight against the deadly disease, cancer.

REFERENCES

1. Kouraklis G, Theocharis S. Histone deacetylase inhibitors as novel anticancer therapeutics. Oncol Rep, 2006 Feb, 15(2), 489-94.

2. Dickinson M, et al. Histone deacetylase inhibitors: potential targets responsible for their anti-cancer effect. Invest New Drugs, 2010 Dec, 28 Suppl 1, S3-20.

3. Bolden JE, et al. Anticancer activities of histone deacetylase inhibitors. Nat Rev Drug Discov, 2006 Sep, 5(9), 769-84.