Ribozyme Gene Therapy for Cancer Nears Clinical Trials

New research that is bringing ribozyme therapy closer to clinicaltrials was presented at the recent meeting of the American Associationfor Cancer Research (AACR) by Kevin Scanlon, PhD, director ofBiochemical Pharmacology at the City of Hope National MedicalCenter in Duarte, California, and editor of the journal CancerGene Therapy. The presentation was entitled "TherapeuticApplications of an Anti-Oncogene Ribozyme in Cancer."

The RNA enzyme is very precise and seeks out and cleaves mutatedRNA, halting its ability to launch a complex cascade of eventsleading to cancer. Ribozyme therapy is one of the new gene therapystrategies that holds promise for significantly improving treatmentof solid tumors, such as melanoma, colon, breast, lung, bladderand pancreatic cancer, by targeting specific molecular sites withincancer cells, resulting in more effective treatment and fewerside effects than conventional chemotherapy and radiation therapy.

Until now, ribozyme studies have mostly been confined to tissuecultures and animals; however, within the next year Dr. Scanlonexpects to begin clinical trials at San Diego Regional CancerCenter, in collaboration with Robert E. Sobol, MD, and at theUniversity of California, San Francisco, in collaboration withMohammed Kashani-Sabet, MD.

Ribozymes are a class of small enzymes the normal function ofwhich is to cut out unwanted segments of RNA, a step in the synthesisof protein from RNA. Research in the late 1980s led to the approachof modifying ribozymes to cut oncogene RNA at the mutation sites,thereby disrupting production of the oncogene protein productsbut not affecting normal gene function. Researchers accomplishedthis by designing ribozymes that bind and cleave only the mutatedportion of oncogene RNA. In addition, they attached a molecularstructure--"a hammerhead"--borrowed from plant viroidribozymes. Inside the cancer cell, the modified ribozyme bindsto the mutated RNA; immediately, the hammerhead "snaps"and cleaves the oncogene RNA in two, preventing production ofthe oncogene protein product.

Oncogenes contain mutations and contribute to cancer by expressingmutated proteins. Like other genes, these oncogene proteins areexpressed in two steps: DNA is transcribed into mRNA, which, inturn, controls protein synthesis. Ribozyme gene therapy differsfrom other forms of gene therapy because it does not replace orrepair oncogene DNA, but instead works by inhibiting the secondstep of this process--synthesis of the oncogene protein productfrom mRNA. Describing the basic approach to using ribozymes incancer treatment, Dr. Scanlon notes, "We have to sequencethe oncogene DNA to define the mutation, and then design the ribozymeto specifically cleave the mutation."

From the Lab to the Clinic

In his presentation at the AACR, Dr. Scanlon focused on currrentresearch aimed at making ribozyme therapy more clinically relevant.One of the tasks has been unraveling the complexity of oncogeneinteractions. "Our early assumption was that if a ribozymetargeted to a specific oncogene worked in one type of cancer thatit would work in all types of cancer, and in reality that's nottrue," reports Dr. Scanlon. "Every time we treat a cancer,we have to define which oncogene is most important--is it ras,fos or one of the 50 or more oncogenes identified to date?When we have identified the key oncogene(s), and prevented itsexpression using ribozyme gene therapy, we know we can have animpact on cancer cell growth."

In ribozyme gene therapy, as in other types of gene therapy, oneof the key challenges is delivering the therapeutic agent. "Thefundamental problem is delivery to all cancer cells, and expressionof the ribozyme at a high enough level for a long enough timeto be therapeutic." Currently, Dr. Scanlon's group is studyingthree viral vectors--retrovirus, adenovirus, and adeno-associatedvirus (AAV)--that deliver the ribozyme "package" byinfecting cancer cells. But because the viral vectors can infecthealthy as well as malignant cells, researchers are seeking waysto ensure that the vectors selectively "target" cancercells in order to minimize possible damage to normal cells andto design mechanisms for switching on ribozymes. This approachdepends on the identification of "promoters"--substancesunique to each type of cancer cell. The ribozyme package, whichincludes the nucleic acid sequence of the ribozyme, is designedso that the ribozyme is expressed only when the tissue-specificpromoter is present in the diseased tissue.