JERUSALEM - Drug delivery by slow-releasing, biologically compatible nanoparticles loaded with the novel tyrosine kinase blockers, tyrphostins, may be able to prevent coronary restenosis following angioplasty, bypass operations and heart transplantation. Restenosis is the accelerated form of atherosclerosis that starts in response to the injury to the blood vessel wall caused by these procedures.

An international Israeli-German research team headed by Alexander Levitzki, of Hebrew University, in Jerusalem, has developed a new approach to treating restenosis. The group proved tyrphostins prevent vascular smooth muscle cell proliferation and inhibit their migration from the inner layer of the arterial wall (the media) into the lumen. This passage forms a restenosis of the lumen, and an obstruction to the flow of blood.

“The accelerated form of this proliferative response is the primary cause of intervention failure following percutaneous transluminal coronary angioplasty [PTCA], after coronary artery bypass surgery [CABG], and also post-heart transplantation,“ said Shmuel Banai, director of the Interventional Cardiology Laboratory in the cardiology department of Bikur Cholim Hospital, in Jerusalem. He is the senior clinical collaborator on the project.

“No previous therapy has proven effective to date and reclosure of an artery after PTCA typically occurs in 30 percent to 50 percent of all interventions within six months,“ he said. “Diffuse concentric narrowing of the coronary arteries in the transplanted heart occurs in 20 percent to 50 percent of transplanted hearts within five years, and remains the major long-term problem limiting survival after cardiac transplantation.“

For the first time in an animal model, the Levitzki team demonstrated the efficacy of tyrphostins as blockers of restenosis. The paper appears in the May issue of the journal Circulation.

Ex Vivo Test Results Show Promise

Tyrphostin AG 1295 caused a significant reduction in all parameters of outgrowth kinetics of smooth muscle cells from human and porcine arterial tissue specimens in the explant model in rodents and pigs. The accumulation of smooth muscle cells around the explants was reduced by 82 percent to 92 percent in AG 1295-treated specimens, compared with non-treated controls. The time to initiation of smooth muscle cells' outgrowth was markedly delayed in treated explants, and the mean number of cells accumulated 10 days after the initiation of outgrowth was significantly lower in treated explants than controls.

“Using the balloon injury model of the pig, the most faithful model for humans, we showed that controlled local delivery of AG 1295 over a four-week period significantly reduced neointimal formation in the treated arteries versus control,“ Banai said.

Injury to the blood vessel wall represents a pivotal initiator in the pathogenesis of atherosclerosis or arteriosclerosis, both in chronic disease and in accelerated forms such as that which occurs after therapeutic intervention in patients undergoing PTCA, said co-investigator Matthias Karck, professor of thoracic and cardiovascular surgery at Hanover Medical School, in Hanover, Germany.

During the past two years, the Levitzki team has shown that novel tyrosine kinase blockers are selective, nontoxic and reversible inhibitors of smooth muscle cell proliferation but only very mild and non-significant inhibitors of endothelial cells' proliferation - the perfect combination of traits to allow healing but prevent restenosis after injury to the blood vessel.

Smooth muscle cell activation is the principal pathogenetic substrate of vasculopathy and in the process of injury-induced neointimal proliferation and premature coronary narrowing.

“We therefore concluded that the tyrphostin AG 1295 appears to have a marked, selective and nontoxic inhibitory effect on the activation, migration and proliferation of smooth muscle cells, [which are] fundamental elements associated with pathological neointimal formation after arterial injury in vivo,“ Banai said.

Current efforts are directed toward improving the potency and specificity of these agents in vivo and refining the routes of sustained in vivo delivery, added chief chemist collaborator Aviv Gazit, at Hebrew University. He is a long-term collaborator with Levitzki.

“Success of these efforts would point us to a novel, effective therapeutic approach to the still very threatening proliferative process which forms the principal pathogenetic substrate of restenosis following interventions and the other accelerated arteriosclerotic syndromes in man,“ said Levitzki, a professor of biological chemistry.

Applied Research Succeeds

Since Levitzki and colleagues described tyrphostins in 1988, the blockers have been applied to cancer, leukemias, papilloma, psoriasis, inflammatory conditions such as sepsis and now restenosis.

“We showed over the years that we can generate selective blockers for a large variety of tyrosine kinases, each of which is relevant to a different disease. We synthesized selective PDGF receptor kinase blockers, which may become a key factor in preventing atherosclerotic disease etiology,“ said Levitzki.

Encouraging news is backing him up. A scientific team at the Ludwig Institute, in San Diego, with Levitzki's team showed that the tyrphostin, epidermal growth factor receptor (EGFR) kinase inhibitor AG 1478, synergizes with cisplatin to kill human glioma (fatal brain tumor). This study was published in the Proceedings of the National Academy of Sciences, dated May 12, 1998. A patent was filed in advance of the publication.

The cells are driven by a highly expressed and highly active mutated version of the EGFR, appearing in 40 percent of gliomas, and the mutated receptor confers drug resistance. Therefore, these tumors are refractory to cytotoxic chemotherapy.

Inhibition of the receptor by AG 1478 blocks the highly active anti-apoptotic pathways turned on by the mutated EGFR and sensitizes the cells to the drug. This seems to be a general phenomenon, since AG 825 - a selective HER-2 kinase inhibitor - has been shown to sensitize lung cancer cells that overexpress this relative of the EGFR to cisplatin and other cytotoxic drugs.

“We thus believe combinatorial therapy is yet another advance in the war against cancer,“ Levitzki said. *

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