Liposomes, Gene Therapy Vectors, Monoclonals Too Big For Blood-Vessel Pore Barrier
By David N. Leff
One-third of all malignant tumors are driven to grow and metastasize by their body's own hormones.
Cancers of the breast and prostate are prime examples. "At the beginning stages of tumor growth," pointed out biomedical engineer Rakesh Jain, "all of these malignancies are hormone-dependent. During that phase, one method of treatment is hormone ablation."
Thus, prostate cancer sufferers may undergo orchiectomy — removal of their testicles — to cut off the surge of testosterone, which fuels their tumors.
Jain, a professor at Harvard Medical School, in Boston, is senior author of a paper in today's Proceedings of the National Academy of Sciences (PNAS), dated April 14, 1998. Its title: "Regulation of transport pathways in tumor vessels. Role of tumor type and microenvironment."
Those transport pathways are microscopic pores, or holes, in the nascent arteries, veins and capillaries that supply advancing tumors with blood-borne oxygen and nutrients.
"The natural function of these pores," Jain told BioWorld Today, "is to permit the leakage of such molecules. All of our blood vessels have to have such gateways for allowing things to get out."
But, Jain continued, when it comes to pumping the vectors of anti-cancer gene therapy, liposome microcarrier drug cargoes or therapeutic monoclonal antibodies into these conduits, "if the size of the gene vector is bigger than the size of these pores, they're not going to come out the gateway."
"Gene-therapy vectors have been made for about ten years now," he observed, "and to our knowledge ours are the first measurements of pore sizes [in today's PNAS] in response to any therapy. And the only anti-neoplastic therapy we have reported so far is hormone ablation."
Jain and his co-authors implanted a testosterone-driven mammary carcinoma into male SCID mice, which lacked immune defenses against such foreign proteins. They seeded these malignant cells in skin folds under the backs of the animals "inside glass-windowed chambers," Jain explained, "which allows us to look at the subcutaneous tissue in a live animal in a continuous, non-invasive fashion."
After the tumors had grown for 10 or 12 days to 5-7 millimeters in size, fed by robust blood supplies, half of the test animals were orchiectomized, while a control cohort underwent sham surgery.
"What we showed," Jain recounted, "is that when we carried out androgen ablation treatment, within 48 hours the diameter of the blood-vessel pores went down by an order of magnitude — from 200 nanometers to almost seven. And responding to that cut-off of testosterone, the blood vessels began to die."
Primary, Metastatic Tumor Pores Don't Match
To gauge the effect of tumor metastasis on pore size, the co-authors implanted identical tumor cells into the dorsal (back) window chambers of mice, and into cranial windows exposing to visual inspection blood vessels nourishing regions of their brains.
This simulation of how a malignancy spreads from a primary tumor site to a metastasis showed that "the pore size in primary tumors was not equal to that in metastatic tumors," Jain said. "In the cranial environment they become invariably smaller."
In all, Jain's team tested liposome and latex bead carriers on five tumor types, one human, four murine, in their cranial and dorsal observation chambers. Among their findings and suppositions:
* Tumors had a characteristic pore size, between 380 and 780 nanometers in diameter.
* Pores were highly heterogeneous and variable, not only in size but temporally and spatially.
* Smaller-size particles leaked from larger holes, and diffused further in the space (interstitium) between cancer cells.
* Candidate drugs that work well in vitro may fail to kill tumors in vivo because their molecules are too large to go through the pores.
* Chemotherapies that lose efficacy after a few rounds may, by shrinking tumors, be shrinking blood vessel pores — closing the door behind them.
"These findings," Jain said, "suggest a fundamental change in the approach to designing therapeutic agents. They may need to be smaller and more agile. The public, investors and the government," he went on, "are enamored of the sophisticated new gene therapies, but they don't think: can these new therapies get through the doorway into the tumor?"
Cancer Drugs Battle Moving Targets
He suggested that "drug makers must think about aiming their drugs at a moving target — pores that change size over the course of treatment. Yet drug companies have been taking a one-size-fits-all approach, and gene therapists are designing bigger and more impressive molecules."
Jain frames his take-home message to the drug discovery community in automotive terms: "In addition to making more powerful and highest-tech automobiles, they have to understand what kind of terrain those vehicles are going to travel on. If they make a most-modern car 10 feet wide, but the road is two feet wide, they're not going to go very far.
"So collectively," Jain concluded, "they have to put research investment into this transport-pathway area."
Cancer biologist Mark Dewhirst at Duke University, in Durham, N.C., is familiar with Jain's research.
"We're on a similar track," he told BioWorld Today. "I would admit he's the first one to show that the pore size is very dependant on the tumor type, and where they're located.
"I think one important issue," Dewhirst went on, "and one that Jain has made a number of times, and I agree with, is that the real challenge for gene therapy is getting the genes to the tumor cells. Even though the pore size is varied, there is limited diffusion distance for liposomes away from the blood vessel. They'll leak out of the vessels, then just sit in clusters, not moving very far away."
Therapeutic monoclonal antibodies, he noted, are also problematic. "The pore size is not that important," he said, "but the permeability of the vessels is high. You tend to get leakage of large-weight molecules, such as monclonals, out of vessels. But the interstitial backward pressure tends to push them back in." *