Protein Kinase

InhibitorsBy Omer Bayazei

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Protein Kinase: A protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them (phosphorylation).

Phosphorylation usually results in a functional change of the target protein (substrate) by changing enzyme activity, cellular location, or association with other proteins.

The human protein kinase family is divided into the following groups:

1. AGC kinases - containing PKA, PKC and PKG.

2. CaM kinases - containing the calcium/calmodulin-dependent protein kinases.

3. CK1 - containing the casein kinase 1 group.4. CMGC - containing CDK, and CLK kinases. 5. STE - containing the homologs of yeast Sterile 7,11

and 20 kinases.

6.TK - containing the tyrosine kinases. 7. TKL - containing the tyrosine-kinase like group of

kinases.

(a) Protein kinase C (PKC)

Protein kinase C:

Protein Kinase C (PKC), a specific type of kinase enzyme, is composed of a family of single polypeptide enzymes that help mediate signal transduction cascades by hydrolyzing lipids.

  PKC plays a role in:

1. Modulating membrane structure.2. Regulating transcription.3. Mediating immune responses.4. Regulating cell growth.

Generally, PKC's function is mediated by two mechanisms:

1)Phosphorylations that support PKC for catalysis and localize it to the cytosol.

2) Binding of substrates or ligands that free the PS from the core binding site and activating the enzyme.

Protein kinase C Inhibitors

Role of PKC in cancer The PKC family is undoubtedly an attractive target for

therapeutic intervention given its role in tumorigenesis and the potential for enhancing cytotoxicity of existing drugs.

The existence of different isozymes provides an opportunity to develop pharmacological agents that target specific PKC functions.

However, there is no doubt that the complex nature of the many secondary messenger systems that involve PKC makes selective drug action difficult.

The microbial alkaloid staurosporine was identified 30 years ago as an anti-proliferative agent and potent inhibitor of PKC.

It probably acts by competing at PKC’s conserved ATP-binding sites.

Drugs such as PKC412 (N-benzoyl-staurosporine) and UCN01 (7-hydroxystaurosporine) exhibit improved selectivity, with a potentially better therapeutic index in vivo and have been reported to enhance the effects of other cytotoxic agents.

Few of the currently available pharmacological agents exhibit a high degree of selectivity for a specific PKC isoform, and the majority also act on other PKs.

The bryostatins are a family of at least 20 naturally occurring macrocyclic lactones derived from the marine bryozoan.

They have been shown to have promising ant-ineoplastic and immuno-modulatory activity in preclinical models.

Bryostatin I is the prototype of this novel class of agents.

Bryostatin

They are potent activators of cPKC and nPKC subfamilies. However, in the presence of activating ligands, such as the tumour-promoting phorbol esters, bryostatins act as antagonists.

Furthermore, bryostatin I potently downregulates cPKC/nPKC activity, probably via degradation of the enzyme.

Bryostatin may also activate effector cells of the immune system and stimulate cytokine production.

Bryostatin 1 has been widely investigated in a series of phase I trials with infusion times varying from 1 to 24 h

The dose-limiting toxicity in all studies has been myalgia with localised phlebitis a problem using the shorter infusion times.

Antitumour activity has been demonstrated in patients with melanoma, epithelial ovarian carcinoma.

The future for PKC inhibitors

The current PKC inhibitors that have reached the clinic are relatively non-specific in their actions, do not fully exploit the potential for differential inhibition of PKC functions or specific isozymes and have encountered PK problems such as Alfa-1 acid glycoprotien binding.

In the future it may be possible to develop agents that target a single isoform, different activating pathways or specific membrane interactions.

(b)Tyrosine Kinase Inhibitor

A tyrosine kinase is an enzyme that can transfer a phosphategroup from ATP to a protein in a cell. It functions as an "on" or "off“ switch in many cellular functions Tyrosine kinases are a subclass of protein kinase. The phosphate group is attached to the amino acid tyrosine on the protein.

Tyrosine kinases are a subgroup of the larger classof protein kinases that attach phosphate groups toother amino acids (serine and threonine).

Tyrosine kinase:

Phosphorylation of proteins by kinases is an important mechanism in communicating signals within a cell (signal transduction) and regulating cellular activity, such as cell division.

Protein kinases can become modified, stuck in the "on" position, and cause unregulated growth of the cell, which is a necessary step for the development of cancer.

Therefore, kinase inhibitors, such as imatinib, are often effective cancer treatments.

Most tyrosine kinases have an associated protein tyrosine phosphatase, which removes the phosphate group.

The term kinase describes a large family of enzymes that are responsible for catalyzing the transfer of a phosphoryl group from a nucleoside triphosphate donor, such as ATP, to an acceptor molecule.

Tyrosine kinases catalyze the phosphorylation of tyrosine residues in proteins.

The phosphorylation of tyrosine residues in turn causes a change in the function of the protein that they are contained in.

Function:

Phosphorylation at tyrosine residues controls a wide range of properties in proteins such as enzyme activity, subcellular localization, and interaction between molecules.

Furthermore, tyrosine kinases function in many signal transduction cascades wherein extracellular signals are transmitted through the cell membrane to the cytoplasm and often to the nucleus, where gene expression may be modified.

Finally mutations can cause some tyrosine kinases to become constitutively active, a nonstop functional state that may contribute to initiation or progression of cancer.

A tyrosine-kinase Inhibitor

(TKI)

A tyrosine-kinase inhibitor (TKI):

Is a pharmaceutical drug that inhibits tyrosine kinases.

They are also called Tyrphostins, the short name for “tyrosine phosphorylation inhibitor”, originally made in 1988 ,which was the first description of compounds inhibiting the catalytic activity of the epidermal growth factor receptor (EGFR).

The 1988 study was the first demonstration of a systematic search and discovery of small molecular weight inhibitors of tyrosine phosphorylation, which do not inhibit protein kinases that phosphorylate serine or threonine residues and can discriminate between the kinase domains of the EGFR and that of the insulin receptor.

TKIs operate by four different mechanisms:

1. They can compete with adenosine triphosphate (ATP),

2. The phosphorylating entity, 3. The substrate or both or4. Can act in an allosteric fashion, namely bind to

a site outside the active site, affecting its activity by a conformational change.

Signal transduction therapy can in principle also apply for non-cancer proliferative diseases and for inflammatory conditions.

Mechanism:

Tasigna Nilotinib

It is FDA- (29 October 2007) for use as a treatment for Philadelphia Chromosome (Ph+)-positive Chronic myelogenous leukaemia.

In June 2006, a Phase I clinical trial found nilotinib has a relatively favorable safety profile and shows activity in cases of leukemia resistant to treatment with imatinib, another tyrosine kinase inhibitor currently used as a first-line treatment.

In that study 92% of patients (already resistant or unresponsive to imatinib) achieved a normal white blood cell counts after five months of treatment.in this setting.

The drug carries a black box warning for possible heart complications.

The use of low doses of nilotinib is being investigated for use for Parkinson's and Alzheimer's disease, as well as for dementia and Huntington's disease.

Novartis announced on April 11, 2011 that it is discontinuing a Phase III trial of Tasigna (nilotinib) for investigational use in the first-line treatment of gastrointestinal stromal tumor based on the recommendation of an independent data monitoring committee.

Mechanism of Action: Nilotinib is a tyrosine-kinase inhibitor. Nilotinib binds to and stabilizes the inactive conformation of the kinase domain of Abl protein.

In vitro, nilotinib inhibited Bcr-Abl mediated proliferation of murine leukemic cell lines and human cell lines derived from patients with Ph+ CML.

Under the conditions of the assays, nilotinib was able to overcome imatinib resistance resulting from Bcr-Abl kinase mutations, in 32 out of 33 mutations tested.

In vivo, nilotinib reduced the tumor size in a murine Bcr-Abl xenograft model.

Nilotinib inhibited the autophosphorylation of the following kinases at IC50 values as indicated: Bcr-Abl (20-60 nM), PDGFR (69 nM), c-Kit (210 nM), CSF-1R (125-250 nM) and DDR (3.7 nM).

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