oncology – Page 7 – Drug Discovery Opinion

Hitting Cancer with an Iron Fist?

The energy demands of rapidly proliferating cancer cells require high levels of nutrients, including iron. Since cells are sensitive to free iron, they utilise iron storage proteins to protect themselves. Scientists at the German Cancer Research Center (DFKZ) and University Medical Center Mannheim, studying Sézary’s disease, have now shown that manipulation of free intracellular iron can induce apoptosis in T-cell lymphomas.

iron-rich river Sézary’s disease is an aggressive and ultimately fatal type of cutaneous T-cell lymphoma that is resistant to currently available treatments. Apoptosis resistance in leukemias and lymphomas is mediated by aberrant signalling of the NF-_κB pathway. The researchers have demonstrated that cell death of cutaneous T-cell lymphoma cell lines induced by inhibition of the NF-_κB pathway is a result of increased free intracellular iron and reactive oxygen species (ROS). Using T-cells from Sézary patients they show that inhibition of constitutively active NF-_κB causes down-regulation of ferritin heavy chain (FHC) that leads to an increase of free intracellular iron, which, in turn, induces massive generation of ROS. The involvement of FHC was confirmed by direct down-regulation using siRNA.

Importantly, T cells isolated from healthy donors do not display down-regulation of FHC and, therefore, do not show an increase in iron and cell death upon NF-_κB inhibition.

The work, published in the journal Cancer Research, suggests FHC as a novel target for therapeutic intervention in lymphoma.

Inhibition of PKC-βII for Chemoprevention

precancerous adenoma Inappropriate activation of PKC isozymes has been implicated in many forms of cancer and researchers at the Mayo Clinic, Florida Campus, have been elucidating the roles of the isozymes in colon carcinoma using transgenic mice. Their earlier studies, reported in the January 15^th issue of Cancer Research, demonstrated a requirement for PKC-βII in the initiation of colon cancer in mice exposed to a carcinogen. The same study also showed that PKC-ι/λ was required for cancer progression.

Now the scientists have shown that daily administration of the selective PKC-β inhibitor, Enzastaurin, provides a degree of protection to mice administered a carcinogen known to cause colon tumours. Since colon cancer develops over a 10-15 year period in humans, there is a large window of opportunity for the use of chemopreventative drugs. The authors suggest that Enzastaurin could represent a good candidate for clinical study in this setting because it has few side-effects.

Enzastaurin is currently in Phase III clinical trials for the treatment of B-cell lymphoma and high-grade brain gliomas. Although Enzastaurin is selective for PKC-β, it also significantly inhibits other PKC isozymes.

The full study with Enzastaurin is published in the February 15^th issue of Cancer Research.

Two New Potential Biomarkers for Prostate Cancer

Although prostate cancer is one of the most common types of cancer in men, it occurs most frequently after the age of fifty and many men who have the disease will eventually die of other causes without ever being diagnosed. prostate cancer cells Prostate cancer is most often discovered by prostate specific antibody (PSA) tests but, since relatively low levels of antibody are found in some men with prostate cancer and elevated levels can be found in other prostate conditions, the use of screening is controversial. Prostate cancers are, in most cases, slow-growing and the decision whether or not to treat a localised tumour is a trade-off between the expected beneficial and harmful effects of treatment in terms of patient survival and quality of life. In men who are being treated for prostate cancer, however, PSA is invaluable as a ‘tumour marker’ to assess response to therapy and monitor progression of the disease.

Two new biomarkers have recently been described which may improve the diagnosis of prostate cancer and – importantly – distinguish more aggressive forms of the disease so that patients can make more informed decisions about treatment.

The first study, by researchers at the University of Michigan Medical School and published in the February 12^th issue of the journal Nature, showed that levels of sarcosine (N-methyl glycine) were elevated in the urine of men with prostate cancer, particularly those with more aggressive disease. In cell culture experiments, sarcosine was also found to help prostate cancer cells invade surrounding tissues. Along with sarcosine, levels of a number of other metabolites were found to be elevated dramatically during progression from benign tissue to localised cancer to metastatic disease. The team hopes that measuring levels of sarcosine alone, or with other metabolites, could provide a better indicator of likely disease progression.

The second study, described in the February issue of the journal Cancer Epidemiology, Biomarkers & Prevention shows that men with elevated levels of total – and particularly ionised – calcium in their blood have an increased risk of fatal prostate cancer.

The introduction of tests which give a better indication of which individuals will go on to develop an aggressive form of prostate cancer would allow men to make better informed decisions about when, or whether, to begin treatment.

New Insights into DJ-1/PARK7 Oncogenic Mechanism

Loss-of-function mutations of human DJ-1/PARK7 are associated with autosomal recessive, early onset Parkinson’s disease. In contrast, amplification of DJ-1 has been observed in numerous tumours and its expression correlated with poor prognosis. Experimental data suggest that DJ-1 contributes to cell survival by enhancing the phosphorylation of Akt, resulting in inhibition of PTEN function. Further data support a role in protection from oxidative stress – knock-out of DJ-1 enhances hydrogen peroxide mediated cytotoxicity.

Researchers have now identified DJ-1 as an upstream activator of hypoxia inducible factor-1(HIF1) function in cancer cells, conferring resistance to hypoxia-induced apoptosis. HIF1 is a transcription factor that plays a key role in a tumour’s ability to adapt under hypoxic conditions, critical for survival and progression. The authors’ experiments demonstrate that loss of DJ-1 in human cell lines decreases transcription of all HIF1-responsive genes examined and that expression of DJ-1 is critical to the activities of Akt and mTOR that are required to maintain HIF1 stability. In addition, DJ-1 has been shown to regulate the activity of the metabolic sensor, AMPK.

The authors conclude that their results strengthen the case for therapeutic intervention at the level of DJ-1 in cancer cells. It will be necessary, however, to consider the loss-of-function consequences. The full study is reported in the January 27^th issue of PNAS.

Phase III Study for Oxidative Stress Inducer, Elesclomol

Synta Pharmaceuticals has recently announced enrolment of 630 patients for a pivotal Phase III trial of elesclomol (STA-4783) in patients with stage IV metastatic melanoma. Cancer Crab Although melanoma is one of the rarer forms of skin cancer, it causes the majority of skin cancer-related deaths – around 50,000 worldwide each year. Early diagnosis and surgical removal of the lesion whilst it is limited to the uppermost layer of the skin is currently the most effective treatment for melanoma. Prognosis is very poor if the cancer has progressed to the deeper layers of the skin or spread to other tissues: limited treatment options are available and expected survival is only six to nine months.

Elesclomol is a first-in-class oxidative stress inducer that triggers apoptosis (programmed cell death) in cancer cells. Oxidative stress is the result of an accumulation of reactive oxygen species such as superoxide, hydrogen peroxide and hydroxide radicals within the cell. Reactive oxygen species are normal products of cell metabolism and play a role in cell signalling but high levels can lead to oxidative stress, resulting in significant damage to the cell or cell death. Normal cells are able to protect themselves against this damage by using enzymes to remove the reactive species and repair any damage, but many types of cancer cells have a diminished anti-oxidant capacity and typically operate under higher levels of oxidative stress than normal cells. This leaves cancer cells especially vulnerable to increases in oxidative stress. Elesclomol exploits this difference between cancer cells and normal cells and is able to induce apoptosis in cancer cells whilst having little or no effect on cancer cells. Elesclomol showed potent anti-cancer activity against a broad range of cancer cell types and enhanced the efficacy of certain chemotherapy agents with minimal additional toxicity.

In a double-blind, randomised, controlled Phase IIb study in 81 patients with stage IV metastatic melanoma, elesclomol in combination with paclitaxel doubled the median time patients survived without their disease progressing, compared to paclitaxel alone. Elesclomol is being developed and commercialised in partnership with GlaxoSmithKline and is also being investigated as a treatment for other cancers.

Pain Target Linked to Cancer

Blockade of the transient receptor potential vanilloid channel, TRPV1, which is widely expressed in both central and peripheral nervous tissue, has been considered by many groups to be an attractive approach to pain relief. The receptor is activated by a number of endogenous and exogenous stimuli including the endocannabinoid, anandamide; capsaicin, the ‘hot’ component of chilli peppers; low pH; and heat. The sensitivity of TRPV1 to heat has suggested a role in maintenance of body temperature, and clinical trials of at least one TRPV1 antagonist were stopped because of unacceptable levels of hyperthermia.

A new study published in the January 19^th Online First edition of the journal Cancer Research now suggests a link between TRPV1 and the development of cancer. The authors show that TRPV1 interacts with epidermal growth factor receptor (EGFR), a receptor tyrosine kinase that is overexpressed in many human epithelial cancers. Interaction of TRPV1 with EGFR was found to recruit the ubiquitin ligase, Cbl, leading to ubiquitylation and lysosomal degradation of EGFR.

In a further set of experiments, the authors showed that mouse epidermal cells over-expressing TRPV1 were significantly less likely to undergo malignant transformation when stimulated with EGFR, either with or without a tumour promoter. TRPV1 was next shown to be expressed in the skin of wild type mice but not TRPV1 knock-out mice; the knock-out mice also had elevated levels of EGFR protein in the skin. When exposed to the tumour initiator, 7,12-dimethylbenz[a]anthracene (DMBA) and the promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), almost all TRPV1 knock-out mice developed larger and more numerous tumours than age- and sex-matched wild type animals. Pretreatment of all mice with the EGFR antagonist, AG1478, significantly suppressed tumour formation, but the effect was much greater in the TRPV1 knock-out animals.

The authors suggest that channels such as TRPV1 are able have a direct effect which is independent of their function as ion channels; the TRP family of proteins seems to show different levels of expression in cancer tissues, although whether these changes are cause or effect is not known.

Timing Could Be Critical In Chemotherapy

The circadian clock regulates a wide range of physiological activities and, for many years, it has been suggested that the time of day at which cancer patients receive chemotherapy or radiation therapy can influence both the effectiveness and side-effects of the treatment. A lack of understanding of the complex mechanisms underlying this effect – together with logistical considerations – has, however, prevented timing from becoming a major determinant in most treatment centres. A study from scientists at the University of North Carolina at Chapel Hill has now identified a biochemical mechanism which may explain circadian sensitivity to the anti-cancer drug cisplatin.

The results, published in the 21^st January Early Edition of PNAS, suggest that treatment with chemotherapy may be most effective at times of the day when levels of one the DNA repair enzymes are at their lowest. The enzyme is xeroderma pigmentosum A (XPA), the component of the nucleotide excision repair system that repairs bulky lesions in DNA such as those caused by cisplatin. The study showed that the activity of XPA in brain tissue from mice followed a circadian rhythm with peak levels some 5-10-fold higher than the lowest levels. It is not yet known whether the circadian levels of XPA in tumour cells or other cells of the body follow the same oscillations as those in the brain, but the study clearly suggests that circadian changes in levels of XPA should be taken into account when designing chemotherapy regimens. Studies on the effect of timing on treatment with cisplatin in rodents and human patients have so far focussed largely on reducing toxicity, and results are complicated by the fact that rodents are nocturnal. Cisplatin was generally found to be most toxic in both rodents and humans when administered soon after awakening.

The present study could also have implications for the prevention of cancer, for example by allowing people to use extra protection against the damaging effects of ultraviolet irradiation from the sun at those times of day when levels of DNA repair enzyme in the skin are at their lowest.

Mechanism of PPAR-γ Activators in Tumour Suppression

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that play essential roles in the regulation of cellular differentiation, development and metabolism. Since the introduction of the thiazolidinedione class of PPAR-γ activators in the late 1990s to treat type 2 diabetes, the use of PPAR-γ ligands to treat other conditions, including cancer, has been investigated. Researchers at the Mayo Clinic discovered that human anaplastic thyroid tumour cells treated with the PPAR-γ activator, RS5444, express a protein known as p21 that inhibits cell replication and suppresses tumour growth, but the underlying mechanism was not understood.

RS5444

The group has now shown that activation of PPAR-γ with RS5444 (also known as CS-7017) turns on the RhoB tumor suppressor gene, which in turn induces p21 expression, thereby shutting down the cell cycle and blocking tumour growth. The researchers say that it is unusual for a cancer drug to be able to cause re-expression of a suppressed gene in this way, and hope that other cancers in which RhoB is deactivated, such as head and neck, brain, and lung cancers, might respond to RS5444 or to similar drugs. RS5444 is undergoing phase I/II clinical trials (in combination with paclitaxel) in patients with anaplastic thyroid cancer and phase II trials (in combination with carboplatin/paclitaxel) in patients with metastatic non-small cell lung cancer.

Structure of Aromatase Determined

The majority (75-80%) of breast cancers are hormone-sensitive and their growth is stimulated by the hormones estrogen and progesterone. In premenopausal women, most estrogen is produced by the ovaries, and selective estrogen receptor modulators such as tamoxifen are used to block the cancer-promoting properties of estrogen. In post-menopausal women, however, estrogens are produced largely by the action of aromatase on androgens produced by the adrenal glands, and reversible (anastrazole and letrazole) and irreversible (exemestane) inhibitors of aromatase have become widely used as treatments in these women. Now Dr Debashis Ghosh’s group at the Hauptman-Woodward Medical Research Institute have solved the structure of the aromatase cytochrome P450 enzyme from human placenta at 2.9Å resolution. The work is published in the 8 January 2009 issue of the journal Nature.

Unlike the active sites of many microsomal P450s that metabolise drugs and other xenobiotics, the aromatase, which is anchored in the membrane of the endoplasmic reticulum, has an androgen-specific cleft that forms hydrophobic and polar interactions with the substrate, androstenedione. The group hope that the new structural information will pave the way to improved aromatase inhibitors for the treatment of breast cancer. The group has previously solved the structures of two other enzymes involved in estrogen biosynthesis, estrone sulfatase (2003) and 17β-hydroxysteroid dehydrogenase type 1 (1996).

Antisocial Behaviour: A Basis for Cancer

When normal cells meet, in vitro at least, they politely step aside. This process, known as contact inhibition of locomotion, was first recognised more than 50 years ago and involves the cells retracting their protrusions and changing direction on contact. Malignant invasion has been attributed to failure to conform to this orderly conduct, but there has, so far, been no evidence that cells behave this way in vivo. Now a study published in the journal Nature describes the behaviour of neural crest cells, a highly migratory and multipotent embryonic cell population. antisocial behaviour When two migrating neural crest cells meet, either in vitro or in vivo, they stop, collapse their protrusions and change direction. By contrast, if a neural crest cell encounters a different cell type, it fails to demonstrate contact inhibition of locomotion and, instead, invades the other tissue in a way reminiscent of metastatic cancer cells. The authors further showed that inhibition of non-canonical Wnt-signalling abolished both contact inhibition of locomotion and the directionality of neural crest migration. The demonstration of contact inhibition of locomotion in vivo and elucidation of an underlying pathway may lead to new ways to prevent tumour metastasis.