kinase – Page 4 – Drug Discovery Opinion

Virus Helps in Fight against Cancer

Reoviruses (Respiratory Enteric Orphan virus) are found in sewage and water supplies and, although infection in humans is quite common, most cases do not cause any clinical symptoms and go unnoticed. Over a decade ago, it was discovered that, whilst reoviruses are harmless to normal cells, they selectively kill cancer cells that have a constitutively activated Ras pathway. It is thought that the cycle of infection, replication and cell death is repeated until no cells with an activated Ras pathway remain. Cells with an activated Ras pathway are unable to mount a normal antiviral response mediated by the double-stranded RNA activated protein kinase, PKR. Activating mutations of Ras and mutations along the Ras pathway occur in approximately two-thirds of all tumours.

Now the Canadian company, Oncolytics, have announced that they intend to start a phase II/III study examining the effects of REOLYSIN®, the company’s proprietary formulation of the human reovirus, in combination with paclitaxel/carboplatin in refractory patients with head and neck cancers. In earlier studies, eight out of nine head and neck patients reported on to date had either a partial response or stabilization of disease, a response that exceeds the current standard of care treatment for this patient group. In a separate study, REOLYSIN® was found to be well tolerated and show promise for the treatment of bone/soft tissue sarcoma metastatic to the lung.

CDK5 Involved in APP Membrane Relocalisation

The amyloid plaques characteristic of Alzheimer’s disease are made up of small peptides formed by cleavage of amyloid precursor protein (APP). Although the primary function of APP is poorly understood, it is believed to regulate synapse formation and neural plasticity. Synaptic activity of APP involves membrane microdomains containing syntaxin whereas amyloidogenic cleavage occurs primarily in cholesterol-rich lipid raft regions containing one of the proteases needed for APP cleavage, BACE. APP preferentially associates with syntaxin microdomains, but neuronal stimulation causes APP to associate instead with BACE-containing microdomains. It is unclear why this trafficking occurs, but a recently published study in the Journal of Cell Biology showed that movement of APP between the two membrane microdomains is an active process, involving cyclin-dependent kinase 5 (CDK5).

The study also showed that treatment of neurons with the CDK5 inhibitor, roscovitine, which is currently undergoing clinical trials as a treatment for cancer, reduced the association of APP with BACE-rich microdomains, and reduced cleavage.

Chemical Biology used to Probe Apoptotic Pathways

The journal Nature Chemical Biology defines chemical biology as ‘both the use of chemistry to advance a molecular understanding of biology and the harnessing of biology to advance chemistry’.

Scientists at the Karolinska Institute, Stockholm, have exploited the principles of chemical biology in the study of apoptotic pathways. In a recent publication, the scientists identified a set of 40 chemical agents (‘bioprobes’) that induce apoptosis from screening of a chemical library.

helenalin Using a variety of reporter cell lines, they were able to establish that the ‘bioprobes’ induced different patterns of signalling. Experiments using a calcium chelator, BAPTAAM, showed that Ca²⁺ was involved in induction of apoptosis by the majority of the ‘bioprobes’ and that Ca²⁺ was in general required several hours into the apoptosis process. Further studies showed that the calmodulin pathway was an important mediator of the apoptotic response. Inhibition of calmodulin kinase II (CaMKII) resulted in more effective inhibition of apoptosis compared to inhibition of calpain, calcineurin/PP2B or DAP kinase. One of the ‘bioprobes’, the plant alkaloid helenalin, was used to study the role of CaMKII in apoptosis. Helenalin induced CaMKII, ASK1 and Jun-N-terminal kinase (JNK) activity, and inhibition of these kinases inhibited apoptosis.

The study shows that calcium signalling is generally not an early event during the apoptosis process and suggests that a CaMKII/ASK1 signalling mechanism is important for sustained JNK activation and apoptosis by some types of stimuli.

PKC Inhibitor: New Option for Treatment of Psoriasis

AEB071 The precise cause of psoriasis is not known but a number of factors, such as skin injury and infection, are thought to trigger outbreaks. T-cells become activated, leading to an acceleration of the normal replacement processes of the skin and an accumulation of skin cells as plaques on the surface of the skin. First line treatments include emollients and topical application of drugs such as vitamin D derivatives, coal tar preparations, steroids, vitamin A derivatives and dithranol. For refractory cases, retinoids or immunosuppressants may be prescribed. A recent study shows that the selective PKC inhibitor, AEB071, may become a new therapeutic option for the treatment of psoriasis. At a dose of 300 mg bid, AEB071 was well tolerated and improved the symptoms of psoriasis within a 2-week treatment period.

AEB071 is also undergoing clinical trials in kidney and liver transplant patients.

New Drugs Could Make Tumour Cells More Sensitive to Radiotherapy

In the rare inherited human disease, ataxia-telangiectasia (A-T), a mutation is present in a gene encoding a protein that normally activates cellular responses to DNA damage. The mutation in the ATM gene leads to decreased ability to repair damaged DNA, and an increased sensitivity to ionising radiation and other DNA damaging agents. This highlights the ATM pathway as a potential target to increase the sensitivity of tumour cells to radiotherapy or chemotherapy. The ATM protein demonstrates kinase activity and a selective, small molecule inhibitor of this kinase, CP466722, has now been shown to enhance the sensitivity of tumour cells grown in vitro to ionising radiation.

Inhibition of ATM kinase activity is rapid, and is completely and rapidly reversed on wash-out; further experiments suggested that inhibition of ATM for a short period of time may be sufficient to sensitise tumour cell to radiotherapy. Because CP466722 is effective in murine cells as well as human cells, it may be possible to use mouse models to further explore the potential of using ATM inhibitors to increase the effectiveness of radiotherapy.

mTORC1 and MAPK Inhibitors for Treatment of Cancer

Rapamycin is a macrolide antibiotic used as an immunosuppressant to prevent organ rejection in transplant patients. Rapamycin and analogues have also been found to have anti-proliferative properties and their effects have been studied in a variety of cancers. Despite early promise, however, clinical tests have proved less successful than had been hoped.

Rapamycin

A report in the Journal of Clinical Investigation now suggests a reason for this lack of success. The anti-tumour effects of rapamycin are brought about by inhibition of the mTORC1 (mammalian target of rapamycin complex 1) pathway which is activated in many cancers, but the new study shows that this inhibition leads to activation of the mitogen-activated protein kinase(MAPK) cascade which stimulates the growth of cancer cells. The authors showed that the MAPK inhibitor, PD0325901, enhanced the effect of rapamycin or an analogue, RAD001 in cancer cell lines, and a xenograft mouse model of cancer.

The results suggest that patient stratification based on molecular pathways and combined use of these drug families, both of which are currently used as single agents in the clinic, will provide more effective treatments for cancer.

Cell Receptor for HCMV Entry Identified

Human Cytomegalovirus (HCMV) is a member of the Herpes virus family and infects 50-80% of adults in the US. After infection, the virus remains latent in the body for the rest of the person’s life. Most people who are infected with HCMV after birth remain asymptomatic unless their immune system is compromised by drugs (transplant recipients), HIV infection or old age. In such individuals, HCMV infections can become life-threatening.

The route by which the virus enters cells is not well understood, but a report in the journal Nature now shows that the HCMV glycoprotein B binds to cell-surface platelet-derived growth factor receptor-α(PDGFR-α), which acts as an entry receptor for HCMV. PDGFR-α is a receptor tyrosine kinase, and both laboratory and clinical strains of HCMV trigger autophosphorylation of the receptor, resulting in activation of downstream signalling pathways. Cells in which PDGFR-α was genetically deleted were non-permissive to HCMV entry, viral gene expression or infectious virus production. An HCMV glycoprotein B neutralizing antibody, an antibody to PDGRF-α and Gleevec, an inhibitor of the receptor kinase activity, also completely blocked viral entry. These data suggest that PDGFR-α is critical for HCMV infection and that the receptor provides a novel target for anti-HCMV therapies.
Gleevec
PDGFRs also play an important role in angiogenesis and have been linked to the pathogenesis of some tumors. Gleevec is marketed by Novartis for treating certain types of cancer.

Fitness without Effort?

AICAR Researchers at the Salk Institute have shown that agonists of both AMP-activated protein kinase (AMPK ) and a peroxisome proliferator-activated receptor (PPAR) can mimic some of the beneficial effects of exercise in mice. In a treadmill running test, the PPAR-β/δ agonist, GW 1516 (GW 501516), acted synergistically with exercise to increase running endurance after 4 weeks. The AMPK agonist, AICAR, surprisingly enhanced running endurance even in sedentary mice, also after 4 weeks dosing. PPAR-δ and AMPK agonists have the potential to treat diseases such as diabetes, where exercise has been shown to be beneficial and to offer protection against obesity, but also have the more controversial potential to increase endurance in athletes.
GW1516
Like exercise, AICAR and GW1516 trigger a variety of changes that contribute improved endurance and the ability of muscle cells to burn fat. A phase II clinical trial of GW1516 for the potential treatment of dyslipidemia has been completed.

New Target in Fight Against Obesity

A recent report in the journal Cell Metabolism (Cell Metab. 2008,7(5):377-388) identifies the serine/threonine kinase, calcium/calmodulin-dependent protein kinase kinase 2 (CAMKK2), as a key component of the ghrelin signalling pathway. Ghrelin, which is produced in the stomach, is a polypeptide that promotes food intake by increasing production of the appetite-stimulating neurotransmitter, neuropeptide Y, by the hypothalamus. The authors established the role of CAMKK2 in appetite control and glucose tolerance both by experiments with CAMKK2-null mice and by administering the CAMKK2 inhibitor, STO-609, to normal mice. CAMKK2 is expressed at low levels in peripheral tissue and the effects of inhibition are likely to be brain-specific. The results suggest that blocking CAMKK2 has the potential to promote weight loss and improve glucose tolerance.
STO-609

STO-609

Other approaches to the management of obesity targeting the ghrelin pathway are being investigated. A group at the Scripps Institute has developed an anti-obesity vaccine that is directed against ghrelin and a number of groups are investigating small molecule modulators of the ghrelin receptor.

Plk1 in Oncology – the Case Strengthens

Polo-like kinase 1 (Plk1) has received attention in recent years as a potential target for intervention in cancer. It is known to be important in regulation of cell cycle progression during M-phase and has been shown to be overexpressed in certain tumours. Now scientists at the NYU Cancer Institute and Howard Hughes Medical Institute have shown that Plk1 is involved in a new pathway associated with the cellular response to DNA damage. In the July 25th issue of Cell, the authors describe targeting of the phosphatase Cdc14B to APC/C, a protein that marks other proteins for destruction. The resultant activated APC/C then tags Plk1 for destruction. If Plk1 remains active, the cell continues to divide despite the DNA damage.

Tekmira Pharmaceuticals, a specialist in delivery of RNA interference molecules, has just announced plans to advance an siRNA product targeting Plk1 into Phase 1 clinical trials in the second half of 2009. Meanwhile, the search for small molecule inhibitors of Plk1 continues. Scientists at Pfizer deposited x-ray coordinates of the catalytic domain of a mutant Plk1(complexed with BI2536) in the protein data bank earlier this year. Additional structure factors have been deposited by Sunesis scientists, although the coordinates have not yet been released.