Therapeutic delivery can be one of the greatest challenges in treating cancer. No matter how inherently potent, a drug can’t eliminate a tumor if it doesn’t get to it. Injected chemotherapy must bypass filtration in the kidneys and liver, then diffuse out of the blood vessel and into cancer cells. While physically crossing these barriers can be a challenge, most cancers remodel the vascular system to increase their blood supply, actually improving drug delivery. In most cases vascular remodeling is essential for the increased metabolic needs of solid tumors, yet some tumors, particularly pancreatic ductal adenocarcinoma (PDA) grow with reduced vascularization. PDA tumors evade the immune system behind a dense stromal matrix, moreover, chemotherapeutic intervention is ineffective because elevated interstitial fluid pressures (IFP) cause vascular collapse. Thus, for PDA otherwise effective therapies may fail because they never reach the tumor cells. While the decreased perfusion of PDA tumors has been documented, the biophysical mechanism of this phenomenon has remained unclear. Scientists in the Hingorani Lab (Clinical Research Division) described the unique biophysical nature of PDA that is responsible for vascular collapse in an article published in Biophysical Journal and further discussed the underlying theories in a review in Gastroenterology.
Fluid pressures are measured by numerous techniques. Interestingly, when assessing IFP in a murine model of PDA two methods gave very different readings. Researchers found that IFP was approximately four times higher when using a piezoelectric pressure catheter (PC) versus the wick-in-needle (WN) approach. The WN was historically used to measure pressures caused by edema, the accumulation of free-flowing interstitial liquid, where pressures exerted by a gel-fluid phase would be masked. Normally only small amounts of interstitial fluid flow freely, most is bound to large polysaccharide structures called glycosaminoglycans, particularly hyaluronan (HA). These properties were validated in vitro by measuring with the PC and WN simultaneously. WN measurements matched PC values only when the probes were submerged in pure water and HA was layered above. These same pressures transmitted through a layer of HA on the bottom failed to be registered by the WN. Additionally, if HA was mixed into the probe solution the WN consistently registered a lagged response relative to the PC.
The HA gel-fluid like properties not only accounted for the differences between WN and PC measurements, they also explained variability seen among tumor samples. Researchers made two discoveries when measuring IFP in a genetic murine PDA model as well as two allograft and four xenograft settings. First, the PDA model had nearly double the IFP of transplanted tumors, and second, higher IFP correlated to greater HA content. To finally connect these ideas researchers treated tumors with PEGPH20, a version of hyaluronidase, the enzyme that degrades HA. In the PDA model both systemic and intratumor injection of hyaluronidase reduced the IFP measured by PC nearly in half, while a minor increase was observed by the WN. This is consistent with the conversion of gel phase fluid into a more free-fluid state. These findings support the clinical studies currently underway using PEGPH20 to increase chemotherapeutic delivery to PDA tumors. While PDA is the most severe example of tumors with decreased vascularization, other tumors may benefit from hyaluronidase treatment. One of the authors, Dr. Christopher DuFort said, "High levels of HA accumulation are most common in pancreas cancer. Other tumor types expressing elevated HA that could benefit from this type of approach include breast (~56%), prostate (~46%), bladder (~42%), and head and neck squamous cell carcinoma (HNSCC; ~29%). A similar framework could also be used with other cancers to identify sub-populations of patients with high levels of HA and elevated tumoral pressures that would most benefit from this type of treatment. In light of these results, a revisiting of other diseases and the approaches used to treat them is certainly warranted and will be an area of future work."
While this work focused on the restrictive nature of the fluid tumor microenvironment, PDA also builds an extensive extracellular matrix containing high levels of collagen and cancer associated fibroblasts. While this is another physical barrier isolating tumors from therapy it appears it may also prevent further growth and metastasis. Attempts to remove the stromal barrier tend to increase tumor burden, yet there is also evidence that some level of remodeling the extracellular matrix may be beneficial. "Recent studies have shown that perturbations to the tumor extracellular matrix though the ablation of stromal fibroblasts resulted in a worsening of an already near-universally lethal disease. In our recent studies with the Greenberg lab we showed that engineered T cells can bypass the inherent defense mechanisms of PDA to induce stromal remodeling and tumor cell death while preserving the beneficial tumor restraining properties of the stroma. This did occur along with an increase in apoptosis of activated fibroblasts and a decrease in overall collagen content that may be responsible for increased perfusion into the tumor mass, the extent of which is still an active area of investigation. Further remodeling the stroma and decreases to tumoral pressures through the depletion of hyaluronan could enhance these results and increase the efficacy of this approach substantially and is a future direction and natural extension of this work." said Dr. DuFort.
DuFort CC, DelGiorno KE, Carlson MA, Osgood RJ, Zhao C, Huang Z, Thompson CB, Connor RJ, Thanos CD, Scott Brockenbrough J, Provenzano PP, Frost GI, Michael Shepard H, Hingorani SR. 2016. Interstitial Pressure in Pancreatic Ductal Adenocarcinoma Is Dominated by a Gel-Fluid Phase. Biophys J.;110(9):2106-19.
DuFort CC, DelGiorno KE, Hingorani SR. 2016. Mounting Pressure in the Microenvironment: Fluids, Solids, and Cells in Pancreatic Ductal Adenocarcinoma. Gastroenterology. Epub ahead of print.
Funding for this research was provided by the National Cancer Institute, the Lustgarten Foundation, and the Giles W. and Elise G. Mead Foundation.