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Bacterial Vectors Show Promise in Cancer Gene Therapy

Bacterial Vectors Show Promise in Cancer Gene Therapy

SAN DIEGO--Genetically engineered bacteria have the potential to deliver anticancer genes directly to a tumor site, according to four presentations of preclinical data at the American Association for Cancer Research (AACR) annual meeting.

The new technology, known as TAPET (tumor amplified protein expression therapy), uses genetically engineered Salmonella bacteria that have been attenuated for virulence and infectivity, while retaining sensitivity to antibiotics. Salmonella bacteria were chosen because they multiply rapidly, can be easily modified genetically, and have been found to grow under both aerobic and anaerobic conditions, such as occur within solid tumors.

These engineered bacterial strains are highly selective for tumor tissue and expand within the tumor to levels 1,000 to 10,000 times greater than found in normal tissue. For example, in one of the scientific presentations at the AACR meeting, a genetically engineered nonpathogenic strain of Salmonella typhimurium injected into tumor-bearing mice rapidly migrated to the tumor site and achieved tumor-to-liver ratios of approximately 10,000:1.

Furthermore, delivery of the modified bacteria, in this and other mouse models, led to significant tumor reduction and prolonged survival.

Tumor localization has been shown using TAPET strains in six tumor models, including melanoma, breast, lung, colon, renal, and liver cancer.

The TAPET research program was initiated at Yale University in 1993 by scientists David Bermudes, PhD, Brooks Low, PhD, and John Pawelek, PhD, and is currently being developed by Vion Pharmaceuticals, Inc. of New Haven, Conn.

"These modified bacteria have lost their ability to cause disease elsewhere in the body but seem to retain it in the tumor and are able to grow to very high numbers in the tumor," Dr. Bermudes said in an interview.

Prodrug-Converting Enzymes

Although the genetically engineered bacteria by themselves have the ability to slow tumor growth, he said, they cannot eliminate the cancer, so Vion is developing a portfolio of TAPET organisms to deliver prodrug-converting enzymes and/or cytokines to tumors.

Dr. Bermudes described research presented at AACR involving a Salmonella strain engineered to express herpes simplex virus thymidine kinase (HSV TK), an enzyme that converts the prodrug ganciclovir (Cytovene) to its toxic phosphorylated form. When introduced into melanoma-bearing mice, HSV TK-expressing Salmonella showed ganciclovir-mediated, dose-dependent suppression of tumor growth and prolonged survival.

In another AACR presentation, Ellen Carmichael, PhD, associate director of biology at Vion, said that TAPET bacteria have been engineered to deliver E coli cytosine deaminase (CD), an enzyme that converts the prodrug 5-fluorocytosine (5-FC) to the toxic 5-fluorouracil (5-FU).

Tumor cells from mice injected with the CD-expressing bacteria showed substantial levels of CD activity, whereas cells from other organs, such as liver, had no detectable CD activity.

Given alone, the modified bacteria led to tumor reduction. Studies are now underway, Dr. Bermudes said, to determine whether co-injection of 5-FC further increases the antitumor effect of the CD-expressing bacteria.

"By converting 5-FC to 5-FU directly in the tumor, rather than administering 5-FU systemically, hopefully, we will be able to reduce the toxicity of 5-FU to other parts of the body," he said.

Addresses Vector Problems

Dr. Bermudes pointed out that the TAPET technology addresses several problems that may be associated with gene therapy of cancer involving viral or liposomal vectors.

First, whereas most viral vectors are delivered locally and thus can only treat tumors locally, TAPET vectors would be delivered systemically and would have the potential to find and destroy clinically undetectable areas of metastatic disease disseminated throughout the body.

At the AACR meeting, Ivan King, PhD, senior director of biology at Vion, showed that attenuated Salmonella typhimurium inhibited the growth of subcutaneously implanted melanoma and lung tumors, as well as lung metastases in mice.

A second problem with gene therapy is the development of a host immune response, which reduces the effectiveness of subsequent treatments. To avoid this problem, Vion is developing TAPET vectors in multiple serotypes.

Third, TAPET has the potential to overcome a major hurdle in the gene therapy of cancer, ie, the inability to deliver adequate numbers of gene copies to cancer cells. This is because TAPET bacterial vectors have been shown to distribute and amplify uniformly within the tumor.

At the AACR meeting, Li Mou Zheng, PhD, associate director of biology at Vion, reported that attenuated strains of Salmonella targeted and amplified within tumors in mice. When examined by light microscopy, Dr. Zheng found that the Salmonella distributed homogeneously within the tumor tissues from the periphery to the center, suggesting that TAPET vectors could potentially deliver the therapeutic proteins throughout the entire tumor.

Fourth, Vion is testing TAPET vectors that may express more than one gene, while viral vectors are generally limited to the expression of a single gene.

Finally, since TAPET vectors remain fully sensitive to antibiotics, they could be cleared from the body in the presence of an antibiotic at any time, allowing greater therapeutic control.

Phase I human clinical trials of the TAPET technology are expected to begin in 1998, according to Vion. "In our preclinical data, we have been able to show efficacy in melanoma, lung cancer, and colon cancer models, and it seems reasonable that the first clinical efforts in humans will be in these cancers," Dr. Bermudes said.

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