# Structural Biochemistry/Cell Signaling Pathways/Kinases Inhibitors
### Introduction of Protein Kinase Inhibitors

Signal transduction in cells occupies an essential position during the process of cellular metabolism, segmentation, differentiation, biological behaviors and cell death. Kinase inhibitors are used to restrain specific receptors, blocking the signal transductions and causing the disease cells to die.
Most protein kinase inhibitors were discovered in the past 10 years, in order to treat cancer and inflammatory disease, and most of them are multi- target kinase inhibitors, which can restrain multiple positions of kinase at same time, blocking the signal transduction more efficient.
There are three type of Protein Kinase Inhibitors:
Type I- a small molecule that binds to the active conformation of a kinase ATP pocket.
Type II- a small molecule that binds to the inactive conformation of a kinase ATP pocket.
Type III- a non-ATP competitive kinase inhibitor.

## Kinase inhibitors for cancer

Cancer chemotherapy has been one of the major medical advances in the last few decades. However, the drugs used for this therapy have a narrow therapeutic index, and often the results produced are only just palliative as well as unpredictable. In contrast, targeted therapy that has been introduced in recent years is directed against cancer-specific molecules and signaling pathways and thus has more limited nonspecific toxicities. Tyrosine kinases are an especially important target because they play an important role in the modulation of growth factor signaling.
Cell growth and cell cycle pathways are constitutively activated in cancer cells. The standard controls exerted by the kinase/phosphatase enzymes don’t function anymore. The main characteristic of cancer cells is their capability to replicate in the nonexistence of external signals such as growth factors.
Growth factors are involved in the initialization and regulation of cell cycles. The type of growth factor determines its effects on the cell. There are 3 primary growth factors that narrate to tyrosine kinase. The receptors of these growth factors are members of the Receptor Tyrosine Kinase family. Epidermal growth factors (EGF) help regulate cell growth and differentiation. Platelet-derived growth factor (PDGF) regulates cell growth and development. Vascular endothelial growth factors (VEGFR) are involved in the creation of blood vessels.
Tyrosine kinase inhibitors (TKIs) are a class of chemotherapy medications that inhibit, or block, the enzyme tyrosine kinase. TKIs were created out of modern genetics- the understanding of DNA, the cell cycle, and molecular signaling pathways- and thus represent a change from general to molecular methods of cancer treatment. This allows for targeted treatment of specific cancers, which reduces the risk of damage to healthy cells and increases treatment success.
Scientific research is being focused on Tyrosine kinase inhibitors because of their unique traits compared to previous methods. All medicines for chemotherapy seek to discontinue cells division and growth. They also attempt to kill cancerous cells without destroying the healthy cells. An inherent weakness in cancerous cells is that a failure of cell repair mechanisms is what turned the cells cancerous. The cell is therefore unable to repair damaged or changed DNA effectively.

### BUB1 and BUBR1 Protein Kinase

Multifaceted kinases that are vital to mitotic cell cycle include BUB1 and BUBR1. They primarily are known for their ability to construct the spindle assembly checkpoint. By doing so, the presence of protein kinases BUB1 and BUBR1 assist in allows for the high fidelity of chromosome separation by slowing down the onset of anaphase until the chromosomes are bi-oriented on the mitotic spindle. Even though both protein kinases are essential in the mitotic cycle, they serve different functions in the spindle assembly checkpoint (SAC).
BUB1 is important for chromosome congression and for maintaining the stability of bipolar attachment to spindle microtubules. If this protein kinase was removed, the rate of chromosome missegregation and possibly a slow growth of chromosome, whereas extreme cases include elevated chromosome loss entirely. BUBR1 is highly affiliated with the misattached kinetochores and provides stability to kinetochore-microtubule attachment in chromosome alignment. Moreover, BUBR1 monitors the Prophase 1 arrest, which is critical in the meiosis I in the production of fertilizable eggs. In doing so, it accumulates to chromatids that will generate unrepaired DNA double-strand breaks.
Many cancer-affiliated mutations are present throughout the human BUBR1 and BUB1 sequence. BUB1 and BUBR1 also have a very functional role in mutating cancer cells. Chromosome destabilization and cancer occurs with the existence of the BUB1 and BUBR1 sequences. BUB1 plays a significant role in oncogenesis; variations in mutations of the BUB1 gene and protein sequence can encourage the growth of cancer tissues and cells. Mutations in the BUBR1 sequence have identified to be related to a family of mosaic-variegated aneuploidy, in other words, a syndrome that is presented with microcephaly and mental retardation. Additionally, gastric cancer progressions have shown great relevance to excess BUBR1 expression. Such mutations in the sequences of the BUB1 and BUBR1 have led to the development of cancer treatments. In fact, recent research has discovered that BUBR1 could be used as an expression marker of inadequate survival of various types of human cancer. The protein kinases result in the impairment of mitotic checkpoint function. Studies have also shown that weakening the SAC gives leverage for the survival of cells that are associated with anticancer therapy.

### Protein Kinase Inhibitors in clinical use

Since last century 80s, key point of research of anticancer transfers from restraining syntheses of DNA to restraining catalysis activity of kinase. The following anticancer drugs are already used in clinical.
Imatinib, discovered by Novartis Company in 1990s, was approved by FDA in May 10, 2001. By using multiple hydrogen bond, Imatinib occupies ATP pockets of ABL protein kinase, preventing the combination of ATP and ABL kinase, which restrained the activity of protein kinase and stopped the lower signal transduction. Imatinib is used to treat chronic myelogenous leukemia Gastrointestinal stromal tumors. Imatinib also can restrain PDGFR kinase on membrane and c-Kit kinase. Imatinib is no harm to normal human cells.
Gefitinib, discovered by AstraZeneca Company, was approved by FDA in May 5, 2003. Gegitinib mainly focuses on EGFR which belongs to the HER receptor family. In normal cells, EGFR helps organ growing by adjusting the speed of forming and differentiation. However, in cancer cells, EGFR is overexpression. Compared to Imatinib, Gefitinib has a relatively narrow spectrum, which means that Gefitinib is more selective than Imatinib. Recent research proves that Gefitinib has really high combination ability of EGFR in nonsmall cell lung cancer cells.
Sorafenib, discovered by Bayer Company, was approved by FDA in December 20, 2005. Sorafenib uses to treat Renal cell carcinoma and Hepatocellular carcinoma. It uses both Hydrogen bonding and Van der Waal forces to restrain RAF kinase, occupying both ATP pockets and hydrophobic pockets of RAF. B-RAF kinase fails to phosphorylate under this condition, which loses its activity and restrains the signal transduction.
Erlotinib, discovered by OSI Company, was approved by FDA in November 18, 2004. Erlotinib is the kinase inhibitor for EGFR-TK, belonging to small molecule compound. Erlotinib occupies the ATP pockets of ErbB, stopping the phosphorylation process of ErbB Tyrosine. Erlotinib also has strong effects on LOK, ABL, FLT, AND SLK. Erlotinib uses to treat Pancreatic cancer.
Sunitinib, discovered by Pfizer Company, was approved by FDA in January 26, 2006. The process of inventing sunitinib is actually the process of transferring sample target to multi-target. Compare to other compound in the early stage of invention, the long chain of sunitinib increases the solubility. This could be the second reason why the effect of sunitinib is better than earlier drugs. In the experiment of measuring kinase spectrum, sunitinib shows broad spectrum level and strong restrainment of kinase activity. Sunitinib mainly restrain VEGFR2、PDGFRs、FLT3 and c-Kit protein kinases, controlling 3 lower signal transductions: PI-3K/AKT、Ras/Raf/MEK and PKCs. Sunitinib uses to treat patients who have Renal cell carcinoma or Imatinib-resistant gastrointestinal stromal tumor.
Nilotinib, discovered by Novartis Company, was approved by FDA in October 29, 2007. Nilotinib is the improvement of imatinib. Similar as imatinib, Nilotinib using multiple hydrogen bonds to occupied ATP pockets of ABL kinase, stopping the combination of ATP and ABL, restraining the activity of kinase, controlling the signal transductions. The different part of Nilotinib is that Nilotinib also reacts with hydrophobic pockets, which could be the reason why Nilotinib can restrain more abnormal ABL kinase than imatinib. Nilotinib uses to treat Imatinib-resistant chronic myelogenous leukemia.
Lapatinib, discovered by Glaxosmithkine Company, was approved by FDA in March 13, 2007. There are four different RTKs in human EGFR family: EGFR, HER2, HER3 and HER4. Lapatinib restrains both EGFR and HER2 Tyrosine kinase. EGFR is overexpression in HNSCC, nonsmall lung cancer, colon cancer, and breast cancer. At same time, it may help secret TGF-α, keeping activating signal transduction. HER2 also cause increasing signal transduction and its overexpression is related to women breast cancer. Lapatinib has a narrow spectrum level, but the activity of restrainment of kinase is stronger than other kinase inhibitors. Lapatinib controls 2 signal transductions: Ras/Raf/MEK and PI-3K/Akt. Lapatinib uses to treat HER2+ breast cancer.
Pazopanib was approved by FDA in 2009. Pazopanib is a multi-target kinase inhibitor, which retrains VEGFR-1, VEGFR-2,VEGFR-3,VEGFR-α/β and C-KIT kinase, stopping the signal transductions and slowing down the growth of tumor. Pazopanib uses to treat Advanced renal cell carcinoma.
Dasatinib, discovered by Bristol-Myers Squibb Company, was approved by FDA in June 8, 2006. By using hydrogen bond to combine with receptor kinase, Dasatinib can restrain multi-target at same time, which include ABL. In the experiment of measuring kinase spectrum, sunitinib shows very broad spectrum level, mainly restraining DDR、EPHA、EPHB kinase receptor families. Dasatinib has highest percentage of restraining kinases in whole drug market. During the process of signal transduction in the cell, Dasatinib restrains TCR receptor on the membrane, Src kinase in the cell and the activity of BCR-ABL Tyrosine to stop 3 signal transductions: Ras/Raf/ MEK、JAK/STAT and PI-3K/Akt, controlling formation of cancer. Dasatinib uses to treat Chronic myelogenous leukemia and Philadelphia chromosome positive acute lymphoblastic leukemia.

#### Raf-MEK-ERK Pathway

The three-tiered Raf-MEK-ERK is a common cytosolic kinase cascade triggered by the downstream of the small GTPase Ras and plays a critical role in cellular generation. The analysis of protein-protein interactions in the pathway has given sufficient information on temporal and spatial pathway regulation. This three-tiered array allows for a large increase in cumulative signal strength, and diversifies the signal needed for temporal modulation as it makes its way down the pathway.
Amongst other MAPK cascades that function in vertebrates, the Raf-MERK-ERK pathway was the first to be introduced and remains the one of most interest. Generally, the pathway is inflicted downstream with growth factor receptors through the trade of GTP for GDP occurring on the membrane-associated small G protein Ras. Ras that are GTP-bound drafts at the entrance of kinase Raf to the membrane, where it is then initiated by the complex. As a result, Raf phophorylates MEK, a dual specificity kinases which targets only the extracellular signal-regulated kinase, ERK. In contrast, ERK monitors targets that are distributed in different subcellular locations –such as metabolic enzymes, transcription factors, and structural proteins. Simply, regulation can be accomplished through direct binding of the components with other components within the pathway. This will cause the scaffolding of proteins and accommodation of localization signals.
The partnerships of dimers with the Raf-MEK-ERK have shown advantages and disadvantages. Dimerization is a regulatory mechanism in signal transduction; in this particular case, it can activate kinases (like in Raf), regulate negative feedback control (like in MEK), and permit concurrent binding to substrates (like in ERK). Dimerization in the Raf pathway promotes the heterodimerization of B-Raf with the protein scaffold KSR1, C- Raf, or A- Raf. The heterodimers contain high levels of MEK kinase activity and signify MEK activating unit. Phosphorylation of B-Raf that is ERK regulated cause the dissociation of Raf heterodimers and prevents the re-alliance of stable MEK1-MEK2 heterodimers.

## References

Wen Wu, Cheng Lu, Siyu Chen, Niefang Yu "The signal transduction pathway of multi-target kinase inhibitors as anticancer agents in clinical use or in phase III"
Arvin C. Dar1 and Kevan M. Shokat, "The Evolution of Protein Kinase Inhibitors from Antagonists to Agonists of Cellular Signaling"
Vascular endothelial growth factor stimulates protein kinase C{beta}II expression in chronic lymphocytic leukemia cells Blood June 3, 2010 115:4447-4454
http://www.cancerquest.org/kinase-inhibitors.html
http://inflacare.imbb.forth.gr/vml-mysite/arxeia/Wimmer.pdf