Pancreatic ductal adenocarcinoma (PDAC), the most common form of pancreatic cancer, is the third most common cause of death from cancer in the United States and the fifth most common in the United Kingdom. Deaths from PDAC outnumber those from breast cancer despite the significant difference in incidence rates.
Late diagnosis and ineffective treatments are the most important reasons for these bleak statistics.
PDAC is an aggressive and difficult malignancy to treat. Until now, the only chance for cure is the complete surgical removing of the tumor. Unfortunately, because PDAC is usually asymptomatic, by the time it is diagnosed 80% to 90% of patients have disease that is surgically incurable. PDAC thus remains one of the main biomedical challenges today due to its low survival rate – just 5% of patients are still alive five years after diagnosis.
However, in recent decades a number of studies have shed light on the molecular mechanisms responsible for the initiation and progression of PDAC. Our recent research has shown that progress toward a cure is possible.
The molecular mechanisms responsible for pancreatic cancer are complex. This is why recent advances in personalized medicine and immunotherapy (which helps the immune system fight cancer) have failed to improve the treatment of pancreatic cancer. This is mainly due to two characteristics:
95% of these tumors are caused by mutations in KRAS oncogenes. Oncogenes are genes that, once mutated, are capable of inducing the transformation of a normal cell into a cancerous cell. KRAS is a gene that acts as an on/off switch. Normally, KRAS controls cell proliferation. When it is mutated, however, the cells start to grow uncontrollably and proliferate – a hallmark of cancer cells. So far, KRAS oncogenes have not been able to be targeted by drugs.
PDACs are surrounded by abundant fibrous connective tissue that grows around some tumor types. In the case of PDAC, this tissue forms a barrier that prevents cells that recognize and attack tumor cells, called cytotoxic T lymphocytes, from reaching the inside of the tumor mass and killing its cells. This renders immunotherapy treatments useless.
For these reasons, PDAC continues to be treated with drugs that destroy cancerous cells but can also destroy healthy ones. Options include Demcitabine, approved in 1997, and Nab-paclitaxel, a new paclitaxel-based formulation. Even if such a treatment is an option, it typically only extends the patients’ lives a few weeks, a marginal improvement at best.
In recent years, however, a number of studies have shed light regarding the molecular mechanisms responsible for the initiation and progression of PDAC.
Today we know that most of these tumors are caused by mutations in the KRAS oncogene. They lead to benign alterations that cause additional mutations in a range of tumor-suppressor genes, which usually repair DNA mistakes, slow down cellular division or tell cells when to die. Mutated cells can grow out of control, and in this context progress to malignant PDAC.
While this process is relatively well known, it has not had an immediate impact on the development of new and more effective treatments.
In search of new strategies
Multiple strategies are currently being studied in an attempt to inhibit the growth of these tumors by blocking the growth of either the tumor cells or their surrounding “shielding” connective tissue. In our laboratory, we focused on blocking the signaling pathways that mediate the oncogenic activity of the initiating KRAS oncogenes.
A decade ago, our lab decided to use genetically engineered mouse-tumor models capable of reproducing the natural history of human PDAC. We did this in order to analyze the therapeutic potential of the main components of the KRAS signaling pathways. These studies have unveiled the reason why the drugs tested so far have intolerable toxic effects, with mice dying within several weeks: they target some proteins that are essential for the dynamic state of equilibrium that is the condition of optimal functioning of the cells. This is called normal homeostasis.
These crucial proteins are mainly kinases, enzymes that are able to modify how other molecules function. They play a critical and complex role in regulating cellular signaling and orchestrate processes such as hormone response and cell division. These results might explain why the KRAS-signaling inhibitors tested so far have failed in clinical trials. On the other hand, the removal of other signaling kinases did not have toxic side effects, but also had no impact on tumor development.
Of the more than 15 kinases involved in the transmission of signals from the KRAS oncogene, only three displayed significant therapeutic benefits without causing unacceptable side effects. These are: RAF1, the epidermal growth factor receptor (EGFR) and CDK4.
It works! (in mice)
In initial studies, we observed that the elimination (via genetic manipulation) of the expression of some of these three kinases prevented the onset of PDAC caused by the KRAS oncogene. However, its elimination in animals with advanced tumors had no significant therapeutic effects. These results caused us to question whether it would be possible to eliminate more than one kinase simultaneously without increasing the toxic effects.
As described in our recent work published in the journal Cancer Cell, the elimination of RAF1 and EGFR expression induced the complete regression of advanced PDACs in 50% of the mice. We are currently studying whether we can increase this by also eliminating CDK4.
The analysis of the pancreas of animals in which we were no longer able to observe tumors by imaging techniques revealed the complete absence of lesions in two of them. Two mice showed some abnormal ducts, probably residual scarring from the tumor. The others had tumor micro-masses of one-thousandth the size of the original tumor. The study of these revealed the presence of tumor cells, in which the expression of the two targets, EGFR and RAF1, had not been completely eliminated, a common technical problem in this type of study.
It is significant that these results were observed not only in mice. The inhibition of the expression of these two proteins in cells derived from nine out of ten human PDACs were also capable of blocking their proliferation in vivo when transplanted into immunosuppressed mice as well as in vitro cultures.
While these results have only been observed in a subset of mice for now, their importance lies in the fact that it is the first time that it has been possible to completely eliminate advanced PDAC tumors by eliminating a pharmacologically directed target.
These observations are clearly important for the development of treatments based on the inhibition of RAF1 and EGFR, but they only represent a first step on a long, hard road ahead.
First, it is important to identify the differences between the PDACs that respond to the combined elimination of RAF1 and EGFR and those that are resistant. As described in our work, the analysis of these two tumor types revealed that they are not active in the same way – more than 2,000 genes are expressed differently.
Identifying additional targets in resistant tumors that do not increase treatment toxicity is not going to be an easy task.
To continue our tests with genetically engineered mice, the immediate but no less difficult task is the development of specific RAF1 inhibitors. Indeed, we only currently have potent drugs against the second target, EGFR. In principle, there are four possible approaches:
Generate selective inhibitors for its kinase activity.
Generate inhibitors for its binding to the KRAS oncogene.
Generate inhibitors for its interaction with effector targets that transmit oncogenic signaling mediated by RAF1.
Degrade the RAF1 protein with drugs.
Designing inhibitors of the RAF1 kinase activity would seem to be the most affordable option, given the experience of the pharmaceutical industry in designing this molecule type.
The problem resides in the fact that there are two other kinases of the same family, ARAF and BRAF, whose catalytic centers (the “active core” of the enzymes) are nearly identical. RAF1 kinase inhibitors are also targeting these other kinases, which causes collateral damage. The ones tested to date have caused high toxicities and the clinical trials had to be stopped.
Continuing to develop effective molecules that are capable of blocking RAF1 activity in patients with PDAC will not be easy. It will surely take more time than we hope, but at least a road map has already been outlined that shows us how to keep moving forward.
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This article was translated from the original Spanish by Sara Crespo, Calamo & Cran.