Blog – Page 2 – Drug Discovery Opinion

Blocking Faulty Replication Could Boost Response to Chemotherapy

Many chemotherapy drugs, including cisplatin, cause damage to DNA and kill cancer cells by interfering with DNA replication and cell division. The damage activates cellular DNA repair mechanisms but, if the damage is too extensive, the cell undergoes apoptosis. Unfortunately, although the initial response to cisplatin is generally good, the majority of tumours will eventually develop resistance to the drug. Resistance can develop when the cell is able to replicate DNA through damaged regions using a translesion synthesis (TSL) DNA polymerase. This type of DNA replication is highly error-prone, introducing mutations into the DNA which can drive drug resistance. Suppressing the ability of tumour cells to replicate damaged DNA using the translesion synthesis DNA polymerase, Polζ has been shown to block resistance to cisplatin in human cancer cells grown in culture and now, in two papers published in PNAS, researchers at the Massachusetts Institute of Technology have shown that the approach also works in mice.

The first paper describes a tumour transplantation approach to examine the effect of impaired translesion DNA synthesis on cisplatin response in aggressive late-stage lung cancers. The researchers used RNA interference to reduce levels of Rev3, an essential component of Polζ, and showed that a 60-70% reduction doubled survival time in cisplatin-treated animals. The team also showed that Rev3-deficient cells showed reduced cisplatin-induced mutations which have been suggested to contribute to secondary malignancies following chemotherapy.

In the second study, the researchers used a mouse model of B-cell lymphoma to show that suppressing Rev1, an essential TSL scaffold protein and dCMP transferase, inhibits both cisplatin- and cyclophosphamide-induced mutagenesis. By performing repeated cycles of tumor engraftment and treatment, the team were also able to show that Rev1 plays a critical role in the development of acquired cyclophosphamide resistance.

The studies show that chemotherapy can not only select for drug-resistant populations of tumour cells but can also directly promote the acquisition of resistance-causing mutations, suggesting that blocking translesion DNA polymerases may have dual anticancer effects by both increasing the sensitivity of tumours to chemotherapy as well as reducing the potential for emergence of drug resistance during treatment. The next challenge will be to identify inhibitors of the translesion DNA polymerases.

Stressful ‘Lifestyle’ is Bad for Neurons

Parkinson’s disease is characterised by loss of dopaminergic neurons in the area of the midbrain known as the substantia nigra. Although mitochondrial stress – an accumulation of damaging superoxide and free radicals – is believed to be the cause of cell death, it is not understood why this subset of neurons is especially vulnerable.

Researchers at Northwestern University have now suggested a possible answer: these neurons have an inherently stressful ‘lifestyle’. The cells in the substantia nigra act as pacemakers, releasing rhythmic bursts of dopamine. This activity is accompanied by an influx of calcium ions which must then be pumped back out of the cell in an energy-demanding process. The inflow of calcium ions is not essential for pacemaking activity so, if the energy needed to pump calcium ions out of the cell is adding extra stress, blocking the influx of calcium should help to alleviate this. Using mice engineered to express a redox-sensitive fluorescent protein in their mitochondria, the team showed that the opening of L-type calcium channels during normal pacemaking activity created an oxidant stress that was specific to dopaminergic cells of the substantia nigra. The oxidative stress, in turn, caused a defensive mild mitochondrial depolarization or uncoupling.

Although most cases of Parkinson’s disease have no known genetic cause, loss-of-function DJ-1 (PARK7) mutations can cause early-onset Parkinson’s disease in humans and transgenic mice lacking DJ-1 also show damage to dopaminergic cells in the substantia nigra. Knocking out DJ-1 down-regulates expression of two uncoupling proteins and increases oxidation of mitochondrial matrix proteins in dopaminergic neurons of the substantia nigra. Treatment of the transgenic animals with the L-type calcium channel blocker, isradipine, was found to protect the dopaminergic cells of the substantia nigra from oxidative damage.

The study, which is published in the journal Nature, builds on previous studies linking calcium channel blockade with protective effects in Parkinson’s disease.

A clinical trial is currently underway to examine the safety, tolerability and efficacy of isradipine – which is already approved for the treatment of high blood pressure – in patients with Parkinson’s disease. The hope is that the drug will slow disease progression and allow a broader window for existing symptomatic treatments.

New Insight into Reducing Stroke Damage

Stroke is the third most common cause of death in the developed world and is also the leading cause of serious long-term adult disability; many survivors never recover sufficient function to live independently. Although rapid intervention to restore blood flow to the affected area can improve outcomes, the brain has limited capacity for repair and there is currently no treatment that helps recovery. The zone immediately surrounding the damaged area is critically important for recovery since motor and sensory neurons in this region can make new connections and compensate for those killed by the stroke.

Immediately after a stroke, tonic inhibition in the affected area increases to reduce excitability and limit the extent of the damage, but this increased tonic inhibition also has the effect of reducing plasticity in surrounding areas. Researchers at UCLA and the University of Otago have now shown that the increased tonic inhibition can persist for weeks and eventually hinder recovery. In experimentally induced stroke in mice, tonic neural inhibition was found to be increased in the area surrounding the stroke damage and shown to be mediated by extrasynaptic GABA_A receptors. After a stroke in the motor cortex, six weeks treatment with L-655,708, a subtype-selective inverse agonist of the α5-subunit-containing extrasynaptic GABA_A receptor, restored tonic inhibition to pre-stroke levels and led to a sustained improvement of motor function. In keeping with a protective role of tonic inhibition immediately after a stroke, the treatment was only effective if delayed until three days after the stroke; initiating treatment too early increased the damage caused by the stroke.

The results suggest that reduction of tonic inhibition by reducing extrasynaptic GABA_A receptor function could be beneficial in promoting recovery after stroke and possibly other brain traumas. A treatment that is effective following delayed administration would offer a significant advantage over existing interventions which must be carried out within a few hours of the stroke occurring.

The study is published in the journal Nature.

No More (Involuntary) Laughing and Crying

Pseudobulbar affect (PBA) occurs in people with brain injury or underlying neurological conditions such as multiple sclerosis, amyotrophic lateral sclerosis and Parkinson’s disease and is characterised by unpredictable and uncontrollable fits of laughing or crying that may not reflect the individual’s underlying mood or may be inappropriate to the situation. Because many people feel embarrassed by their symptoms, PBA can have a significant effect on quality of life since sufferers tend to withdraw from social and professional activities. So far, there has been no specific treatment but, last week, the FDA approved Nuedexta™ as the first treatment of PBA.

Nuedexta™ is a combination of dextromethorphan, a component of some over-the-counter cough mixtures and the antiarrhythmic agent, quinidine, which acts as a metabolic inhibitor, allowing effective concentrations of dextromethorphan to be achieved.

Dextromethorphan acts on sigma-1 and NMDA receptors in the brain, although exactly how it exerts its therapeutic effects in patients with PBA is not known. Nuedexta™ has been shown to be safe and effective in patients with multiple sclerosis and amyotrophic lateral sclerosis but not in other groups of patients who may experience PBA, such as those with Alzheimer’s disease.

Nuedexta™ was developed by Avanir Pharmaceuticals, Inc. and is expected to be available by prescription in the US during the first quarter of 2011.

Asthma Treatment Could Get Tastier

Researchers at the University of Maryland School of Medicine in Baltimore have discovered functioning bitter taste receptors (TAS2Rs) on bronchial smooth muscle. Although identical to receptors on the tongue, the receptors in the bronchi are not clustered in buds and do not send signals to the brain. The lung receptors do, however, respond to substances that have a bitter taste. The team initially thought that the purpose of the lung receptors would be to reinforce the warning provided by those on the tongue against bitter substances, many of which are toxic. When the team tested such compounds on individual airway smooth muscle cells, or human and mouse airways, they found, however, that instead of causing contraction – which would lead to unpleasant feelings of chest tightness and coughing – the compounds had the opposite effect.

In laboratory tests, TAS2R agonists such as chloroquine and denatonium opened the airways more effectively than drugs currently used to treat asthma or COPD, and aerosols containing the bitter substances were also effective in a mouse model of asthma. At a cellular level, the effect of bitter compounds also provided a surprise: although the compounds cause the airway muscles to relax, they lead to an increase in intracellular Ca²⁺, which would usually be expected to cause muscle contraction.

Although the bitter tasting substances used in the current study may not be suitable for aerosol formulation, the team believe that it should be possible to discover other compounds with similar bronchodilating properties that could be formulated for delivery by an inhaler and used to treat patients with asthma and COPD.

The study is published in Nature Medicine.

FSH Receptors Outline Tumours

Researchers at Mount Sinai School of Medicine and France’s National Institute of Health and Medical Research (INSERM) have discovered a biomarker that could be applicable to a range of different cancers.

In a study of more than 1300 patients, the team found that the follicle stimulating hormone (FSH) receptor – which is normally present only at low levels in the blood vessels or the granulosa cells of the ovary and the Sertoli cells of the testis – is also expressed at higher levels in eleven different types of cancer. The receptor was absent in other normal tissues, including normal tissue from the organ bearing the tumour. Not only does the FSH receptor appear to be specific for endothelial cells in the vasculature surrounding tumour tissues, it is also present from the very early stages and is easily detectable using conventional imaging methods. In most cases, the blood vessels expressing FSH receptors were at the periphery of the tumour, in a layer about 10mm thick, making it a good target for improving cancer detection and also guiding surgery and radiation treatment. Blocking FSH receptor signalling, which stimulates angiogenesis via up-regulation of vascular endothelial growth factor, could also potentially provide a new strategy for developing anticancer drugs.

The study is published in The New England Journal of Medicine.

Targeting the Tangles

Although the relative importance of β-amyloid plaques and tau protein tangles in the progression of Alzheimer’s disease has been the subject of much debate, early emphasis was placed on the development of drugs to block production of β-amyloid. Although such compounds were shown to improve cognition in transgenic mice, unfortunately results from clinical trials have been more equivocal. Focus is now shifting to therapies that target tau pathology and, in a recent study, researchers from the University of Pennsylvania have identified a compound that reduced cognitive deficits in mutant human tau transgenic mice.

In healthy nerve cells, tau proteins interact with tubulin to stabilize axonal microtubules and promote tubulin assembly into microtubules. In Alzheimer’s disease and other ‘tauopathies’, hyperphosphorylated and misfolded tau proteins form insoluble neurofibrillary tangles that deplete levels of soluble tau and lead to destabilization of the microtubules and neuronal dysfunction. The team had previously proposed using microtubule-stabilising anti-cancer taxanes such as paclitaxel to treat tauopathies, but these do not penetrate the blood-brain barrier sufficiently well. The Penn team has now shown that once weekly treatment of tau transgenic mice with the brain-penetrant microtubule-stabilising agent, epothilone D, for three months significantly improved microtubule density and axonal integrity and also reduced cognitive deficits without notable side-effects.

The study, which is published in the Journal of Neuroscience, suggests that brain-penetrant microtubule-stabilising drugs could provide a new strategy for treating Alzheimer’s disease.

Towards a Safer, Sun-free Tan

deckchairs — Image: Flickr - Ali Harrison

Although many people believe that a suntan imparts a healthy glow, the truth is that tanning irreversibly damages the skin and can lead to skin cancer. Researchers from Massachusetts General Hospital have now shown that, in mice at least, inhibiting one of the phosphodiesterases, PDE-4D3, can activate the tanning process without exposure to harmful ultraviolet irradiation.

Using transgenic mice with melanocytes in their epidermis (which normal mice do not have), the group had already shown that inducing cyclic AMP production led to significant pigmentation. Further detailed analysis of the melanin expression pathway identified PDE-4D3 as the key regulator of cyclic AMP activity in melanocytes. Topical treatment of the transgenic mice with a PDE-4 inhibitor (rolipram or Ro 20-1724) for five days caused the skin to darken appreciably and the effect could be further enhanced by co-application of forskolin, which stimulates adenylate cyclase activity.

Although PDE4 isoforms play a key role in regulating intracellular cyclic AMP levels in a variety of tissues, the researchers believe that topical application of a selective PDE-4D3 inhibitor has potential to provide safe tanning and reduce the incidence of skin cancer.

The study is published in the journal Genes & Development.

11β-HSD1 Inhibitor Improves Spatial Memory in Ageing Mice

Inhibitors of 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), a microsomal enzyme that converts cortisone into cortisol, are being developed to treat diabetes and metabolic disorders and now, in a study supported by the Wellcome Trust, researchers at the University of Edinburgh have shown that such compounds may also help to reduce – or even reverse – age-related memory loss. Such memory loss has been linked to increased activity of 11β-HSD1 and higher levels of glucocorticoids in the hippocamus, an area of the brain associated with memory.

Ageing mice display deficits in memory and learning similar to those experienced by some elderly people and life-long partial deficiency of 11β-HSD1 prevents this decline in transgenic mice. More surprisingly, improvements in memory – as judged by performance in a Y maze – were seen in mice after only ten days treatment with a selective 11β-HSD1 inhibitor, UE1961.

The team had previously shown that a non-selective 11β-HSD1 inhibitor, carbenoxolone, improves memory in healthy elderly men and in patients with type II diabetes after only one month of treatment. They now hope to complete preclinical assessment of the new compound and begin clinical trials within a year. The study is published in the Journal of Neuroscience.

New (Host) Target for Treating HCV

Water droplet — Image: Flickr - Jack Newton

Hepatitis C virus (HCV) is one of the most important causes of chronic liver disease and infection can lead ultimately to cirrhosis and liver cancer. Current standard-of-care treatment – a combination of pegylated α-interferon and ribavirin – is unable to clear the virus in all patients and new antiviral agents designed to inhibit specific viral enzymes such as the protease, helicase and polymerase are being developed.

Researchers led by a team at the Gladstone Institute of Virology and Immunology (GIVI) have now identified a human enzyme that is also needed for viral infectivity, a discovery that may offer a new strategy for treatment. The enzyme, diacylglycerol acyltransferase 1 (DGAT1), is one of two DGAT enzymes that catalyse the final step in triglyceride synthesis. HCV infection is closely tied to lipid metabolism and the Gladstone team showed that infection and replication is severely impaired in liver cells that lack DGAT1 activity: either RNAi-mediated knockdown of DGAT1 or treatment with a DGAT1 inhibitor was effective in limiting production of infectious viral particles. The team went on to show that DGAT1 interacts with the viral nucleocapsid core protein and is required for the trafficking of the core protein to lipid droplets. Knockdown of the other enzyme involved in triglyceride synthesis, DGAT2, had no effect on viral replication.

DGAT1 inhibitors are already being developed as treatments for type II diabetes and obesity and the new study, which is published in Nature Medicine, suggests that they may also be useful for treating HCV infection.