Investigations into the underlying cellular processes that lead to cancer have revealed a number of interesting findings regarding the biological conditions responsible for tumor development. Individual forms of the disease emerge from mutations and malfunctions of particular genes and the attendant changes in protein complexes and cell structures these mutations give rise to. An understanding of the myriad ways in which certain mutations can affect these agents will hopefully enable scientists to create cancer therapies that precisely target these low-level conditions. These therapies should then result in cancer treatments that are both more effective and less likely to cause serious side effects.

Research into the causes of lung cancer and other epithelial malignancies, such as mesothelioma, has shown that epidermal growth factor receptor (EGFR) is often implicated in the development of some of these cancers. EGFR is the surface receptor for a family of protein ligands known as the epidermal growth factor (EGF) family. When an EGF-member binds to EGFR, the receptor is activated and begins a cascade of downstream signaling activity important for a large number of cellular activities that are necessary for the body to function normally. However, mutations to EGFR can lead to its overexpression and/or its constant activation. When this happens, EGFR is no longer performing within the strict confines of its natural function, but is instead helping to create the conditions of tumor growth by increasing “cell proliferation, motility, and angiogenesis.” EGFR malfunction is also thought to inhibit apoptosis.

Treatments for EGFR have been developed and have shown at least some effectiveness in the treatment of lung cancer, but its effect on the development of mesothelioma, and the disease’s most common forms of pleural mesothelioma and peritoneal mesothelioma, is not really understood. Researchers are also unsure as to the effect that EGFR therapy would have on mesothelioma treatments in general. To answer this question, researchers from Japan enrolled a number of patients with pleural mesothelioma into a study that investigated the relationship between EGFR mutations and overexpression and the development of mesothelioma.

Overview of the Study

The researchers enrolled 25 patients with confirmed pleural mesothelioma into their study. There were 23 male patients and 2 females. The histological sub-types of the tissue samples were as follows: 12 epithelial mesotheliomas, 8 biphasic mesotheliomas, 4 sarcomatous mesotheliomas, and one desmoplastic mesothelioma (this is usually considered to be a sarcomatoid mesothelioma, but the researchers here treat it as a separate entity). A sample of the malignant tissue was taken from each patient and then analyzed for EGFR gene mutation or amplification and then for protein expression levels.

Results

None of the 25 tissue samples evidenced any sign of the 13 mutations of the EGFR kinase domain that have been reported as common in EGFR-implicated lung cancer incidence. Four of the samples demonstrated some form of polysomy, which is a genetic condition where cells possess extra chromosomes, while the other 21 samples demonstrated normal chromosomal structures. When the samples were analyzed for protein expression, 68% (17 of the 25) of them demonstrated some type of membrane immunoreactivity, with eight of the samples (32%) returning scores high enough to be considered positive findings for EGFR overexpression. However, protein overexpression did not correlate with overall survival in any way.

When viewed within the entire data set, none of these findings could be correlated with patient age, gender or tumor pathology. Increased survival was seen in those presenting with epithelial mesothelioma, but this is to be expected as the epithelial subtype always presents with a better prognosis than the other subtypes. Multivariate analysis did not identify any other variable as an independent prognostic factor.

Conclusion

The authors state that although their sample size was small, their results suggest that EGFR mutations only play a role in a small subset of patients with pleural mesothelioma. These findings serve as another identification of the differences between mesothelioma and lung cancer: while specific EGFR mutations have been implicated in the development of lung cancer, this rarely seems the case with mesothelioma development. In terms of gene amplification, four of the cases identified extra chromosomal pairs, indicating anti-EGFR treatment for mesothelioma will only be effective for the subset of patients who present with EGFR polysomy. The authors were unable to correlate EGFR overexpression with any survival data, whether positive or negative, noting that only epithelial mesothelioma demonstrated a survival effect within this dataset.

From the earliest reports of lung disorders in asbestos workers, which date from the early 1900s, to our current time where asbestos has been conclusively shown to cause a number of terrible cancers and diseases—with the various forms of mesothelioma, such as pleural mesothelioma and peritoneal mesothelioma, probably the most feared of them—the mineral’s health effects are as terrible as they are common to those exposed to it. In light of these hazards, most industrialized nations have banned any use of the mineral, but some, such as the United States and Canada, have preferred strict regulations to an actual ban. However, even for those nations that actually have banned the use of asbestos, the extreme latency that is often associated with asbestos-related diseases means that people will continue to develop lung cancer and mesothelioma for years to come.

One of the great questions of asbestos-disease epidemiology is whether or not the underlying changes that lead to the development of these diseases can be identified earlier in an exposed person’s life, before any outward manifestations of the malignancies make themselves known. If these initial changes could be identified and subsequently tracked, then people at risk could possibly begin treatments to counteract, or at least to attempt to slow down, the progression of the biophysical changes whose endpoint is the worker’s premature death. To accomplish this, former asbestos workers would have to undergo regular screening procedures which would track the precise physiological changes being undergone, as well as quantify them to previously identified changes.

A study that completed such a process has recently been released by Austrian researchers, who analyzed many years of screening data among a cohort of former asbestos workers. Their findings definitely indicate the presence of quantifiable changes among the study’s population members.

Overview of the Study

In 1974 a number of workers from an Austrian asbestos cement factory agreed to take part in a long-term study investigating asbestos exposure. Information dating back to 1950 was captured for some of these individuals and new members were possibly added to the cohort until 1981, when the use of unprotected asbestos was banned in Austria. All these workers received regular checkups and their vital statuses were tracked as well. In 1989, additional screening procedures, including clinical examination, lung-function tests and chest x-rays, were made available to the workers. A total of 322 workers took part in these checkups and the study reports on 309 of them.

For each of the study members, a complete asbestos exposure history was available, as were the results of all checkups from 1989 to 2006. Information on each worker’s smoking history was incorporated into the overall analysis as well. The authors analyzed a number of individual factors for their effects on life expectancy and cause of death.

The workers reported on their individual work histories, including the type of work accomplished and the places in the factory in which the work took place. From this information, an analysis of the average asbestos concentrations found in the various locations was conducted and a table developed that grouped these exposures on a scale of 0 to 4, with 0 meaning very low exposure and 4 meaning very high exposure. This table was developed using an exposure scale of fibers/cm³ and was then combined with the number of time the worker spent in this location to determine a worker’s cumulative asbestos exposure, reported in “fiber years.” Chrysotile was the most common asbestos used and most workers were only ever exposed to it, although a subset of workers were exposed to amphibole asbestos fibers, of which, crocidolite was the major form.

Results

The authors report that by the end of their study in 2006, 82 of the original 309 workers had died. Of these 82, 34 died from cancer, 30 from cardiovascular diseases, 6 from respiratory diseases and 10 from other reasons not quantified. Of the 34 cancers, 6 were from lung cancer, 7 were from pleural mesothelioma, 4 were gastric cancers, 9 were digestive cancers, and there were 8 other individual cancers.

The authors found that for those who died of lung cancer, even after controlling for smoking and pure amphibole exposure, cumulative fiber years of asbestos exposure was a significant predictor of lung cancer. This was in contrast to the workers who developed mesothelioma. For this group, fiber years alone was not predictive of pleural mesothelioma onset, but amphibole exposure was highly predictive, as was long latency from first exposure to asbestos. The authors conclude, as have a number of other studies, that any exposure to amphibole asbestos is always a high risk for the future development of pleural mesothelioma.

The authors were surprised to see that higher fiber year figures were significantly predictive of stomach and some digestive track cancers. The worker’s exposures were not implicated in the development of colon or rectal cancer, but the findings linking asbestos exposure to these other cancers are some of the first to clearly show this relationship, so the authors call for more research into this question.

In terms of overall cumulative exposures, workers exposed to asbestos in excess of 70 fiber years saw their life expectancy figure decrease by 25%. Although this was smaller than smoking, it still correlated as a negative prognostic factor.

One of the most significant set of findings was the correlation between reduced lung function and a reduction in life expectancy. Any of the lung function parameters which showed a reduction in an individual worker’s lung efficiency were predictive of a reduced life expectancy. In fact, the authors state that lung function tests were much more predictive of a reduced life expectancy than were x-rays, other clinical examinations or a simple exposure history analysis. This finding should be a clear indication that measurable reduction of lung function for asbestos workers is indicative of potentially serious future medical issues.

Conclusion

The authors conclude their paper by recommending regular screening examinations of former asbestos workers. They state that their findings clearly indicate that screening exams could identify precursor stages of serious illnesses, which could allow patients to start receiving treatments before they present with full-on malignancies. They also recommend that former asbestos workers who are currently smokers immediately stop smoking, as there is an immediate benefit to one’s life expectancy when one quits smoking. All in all, this study goes a long way in showing that even as asbestos workers are prone to the development of a number of difficult malignancies, screening procedures and early treatment for them could be helpful to their future lives.

In mesothelioma research, as in most forms of disease research, the attempt to understand the basic biological features of tumor growth is one of the most active areas of contemporary study. Investigations in this area continue for nearly all forms of cancer with the ultimate goal being the development of therapeutic tools and strategies that allow doctors to precisely target the particular tumor-types involved with an individual’s cancer. If they can develop modalities that target the particular biological processes involved in tumor genesis and progression, they may be able to develop treatment tools that demonstrate both greater therapeutic efficacy and reduced side effects.

In the case of mesothelioma, this category of research has had scientists studying the relationship between a wide variety of tumor structures, malignant and non-malignant cell types and host-induced co-factors for insight into better treatments for mesothelioma. One of the most compelling of the recent research hypotheses has to do with the role that macrophages play in cancer growth. Previous studies have noted some correlation between macrophage density and tumor burden—especially with epithelial cancers, where 80% of the studied cancers demonstrated a correlation between greater macrophage density, tumor stage and poor prognosis. In recognition of those findings, an international team of researchers endeavored to study this relationship further. Their recently-released results do note a significant relationship between a certain type of macrophage and tumor growth and this article is a summary of the findings as presented in their paper.

Overview of the Study

Macrophages are an important type of immune system cell. They are normally responsible for the removal of foreign antigens and invading organisms through a process known as phagocytosis, where they engulf and essentially digest the antigen. They originate from monocytes(one of the immune system’s fundamental cell types) and in the transition from monocyte to macrophage undergo an activation process that is regulated by the presence of specific proteins which then dictate its final form. Two major forms of mature macrophages have been identified:

• M1, known as “classically activated” macrophages; and
• M2, known as “alternatively activated” macrophages.

In most cases, the mature form of the macrophage plays an important role in promoting the health and well-being of the body. However, research into these two phenotypes has shown that the M2 sub-type can also be associated with increased tumor burden in some forms of cancer. These macrophages, called tumor-associated macrophages (TAMs), have been implicated in the development of multiple forms of cancer and mesothelioma, including pleural mesothelioma and peritoneal mesothelioma. Not only have they been shown to have an overall immunosuppressive effect, which allows the growth of tumor tissue to proceed without restraint, they have also been implicated in angiogenesis (the process by which new blood vessels are formed and tumors are supplied with the blood necessary for their growth) and growth-factor overexpression that some feel is responsible for “cell survival, invasion and metastasis.”

In light of these previous findings, the authors of this study were interested in learning what, if any, effect a reduction in the density of TAMs would have on tumor growth and progression. To investigate this, they studied a mouse population with malignant peritoneal mesothelioma and used intraperitoneal injections of liopsome-encapsulated clodronate (CLIP) to trigger apoptosis in the macrophages. Their injections were targeted at just the macrophages and not the actual mesothelioma cells and their testing confirmed this targeting was successful. Their results showed a clear benefit to macrophage depletion.

Results

In nearly all cases of mice treated with CLIP versus the control therapies utilized, CLIP-treated mice showed significant reductions in tumor burden, tumor growth and tumor progression. Whether the test was looking at the future development of tumor tissue after having been exposed to the mesothelioma cell lines or in those mice who exhibited mesothelioma tumor tissue when the testing began, the depletion of TAMs showed a positive effect in the treated mice. The authors report that mice injected with CLIP showed a 4-fold reduction in the number of tumors and a 17-fold reduction in overall tumor burden. They also showed that the metastatic potential for mice treated with CLIP was significantly lowered than in mice for the control groups. For those mice who received CLIP after having already developed mesothelioma tumors, they too showed a marked reduction in tumor size and burden, although they did not show a reduction in metastatic potential as compared to the control groups.

Conclusion

The authors clearly state that their mouse studies show that TAMs are an important factor in the growth of mesothelioma. These macrophages seem to have important effects on growth factor expression, angiogenesis and tumor immunity and their reduction, using intraperitoneal CLIP injections, has been shown to have positive effects on reducing tumor burden and progression. In light of these findings, the authors call for the beginnings of human trials in patients who are unable to tolerate multimodal treatments.

Alfacell Corporation has begun the process of conducting the formal statistical analysis of the results of its Phase IIIb Clinical Trial on the use of ONCONASE® in patients with unresectable malignant mesothelioma. The trial was designed to show that multi-agent use of ONCONASE and doxorubicin for patients with pleural mesothelioma not amenable to surgery is more effective than the use of doxorubicin alone. The statistical analysis is required to complete the final section of ONCONASE’s rolling New Drug Application.

ONCONASE is an anti-cancer compound developed using Alfacell’s proprietary ribonuclease (RNase) technology. Previous laboratory studies have shown that ONCONASE triggers apoptosis in cancer cells while leaving normal cells alone. The mesothelioma trial is the furthest along in the therapeutic use of ONCONASE, but Alfacell is also conducting a Phase I/II trial on the efficacy of ONCONASE for the treatment of non-small cell lung cancer and other solid tumors.