While normal cells are attending to business, tumor cells multiply bydividing. This divisive behavior sets them apart as self-revealingtargets for cancer chemotherapy.One of the chemical arrows aimed at this self-made bull's-eye ofmalignancy is the enzyme thymidine kinase (TK). When a TK genepenetrates a dividing cell, it turns that cell on to expressing its enzyme.The TK then phosphorylates said cell, rendering it vulnerable togancyclovir (GCV). This is an antiviral that takes its tumor-cell targetfor a viral particle, blocks its DNA replication, and destroys it. (SeeBioWorld Today, Sept. 9, 1993, p. 1.)In normal life, TK is part of a cell's regular nucleic acid synthesismachinery.GCV works because it is a DNA chain terminator. Currently, it findsclinical use in treating cytomegalovirus and herpes virus infections.A number of gene therapists, notably Kenneth Culver at IowaMethodist Medical Center in Des Moines, are gearing up to treathuman brain tumors with direct injections of a viral vector carrying aTK gene. Culver was one of the original trio of NIH clinicians whopioneered gene therapy in 1990, by treating a little girl with SCID _severe combined immunodeficiency disease. (See BioWorld Today,Aug. 18, 1993, p.1.)Culver's approach, now pending imminent clinical trials, is to instilretroviral vectors carrying mouse cells that produce TK genes intoglioblastoma brain tumors.Retroviral vectors (RVV) are the preferred expression vehicles of mostgene therapists. But as Savio Woo of Baylor College of Medicine toldBioWorld Today, "The RV vector system is very good for treatinggenetic diseases, where we want permanent gene expression, becausethe gene is transferred to cells and integrated into their chromosomes"But RV's limitation," Woo added, "is that the viral output, andtherefore transfer efficiency, are very low. This is why Culver andothers must use virus-producing cells rather than the isolated virus.This injection of mouse cell-producing virus into the human brain isanother major drawback."In yesterday's (April 12) Proceedings of the National Academy ofSciences (PNAS), Woo reported switching from retrovirus toadenovirus (ADV), "a much more efficient vector for gene transfer invivo. It can deliver a concentration of therapeutic genes several ordersof magnitude higher than RV vectors."Woo's PNAS paper is titled, "Gene therapy for brain tumors:Regression of experimental gliomas by adenovirus-mediated genetransfer in vivo."He and his team at Baylor, where he heads the Gene Therapy Center,planted 10,000 glioma tumor cells inside the skulls of nude mice. Thetumors spread rapidly in the brain, and spread through the injectionneedle track into the skull/scalp interface.Eight days later, one cohort of 10 animals received the ADV vectorcarrying the TK gene, extracted from the herpesvirus's 174-genegenome. Another got ADV with an irrelevant gene.Twelve hours after the gene injections, two of the treated groups gotabdominal injections of gancyclovir for six days, at either high or lowdoses. The others received a saline solution placebo.Among the animals that got the full treatment _ ADV-delivered TKfollowed by GCV _ mean tumor volume by day 20 averaged 54,000cells. In the control groups, the volume reached 28 million, a greaterthan 500-fold increase. Some of the full-treatment mice survived morethan 50 days after tumor injection, while the primary glioma killed allcontrols by 23 days.Two of the 10 TK/GCV recipient mice had no residual tumor at all.These results of how well the ADV vector system worked, Woo said,"were really stunning to me As I look at them, it's almostunbelievable."Benevolent BystanderBut that's not all. Woo described a "bystander effect" that couldpotentially enhance the treatment tenfold in clinical use. He explained:"Not every cell needs to acquire the TK gene in order to be killed byGCV. Apparently, the toxicity is shared by neighboring cells."He observed that this effect is critical to the outlook for cancer genetherapy. "Imagine," he said, "what if we had to deliver the gene intoeach and every cancer cell." Thanks to the bystander effect, he noted,"But into only 10 percent of the target cells, the criteria are verydifferent."Culver too was pleasantly surprised by Woo's PNAS paper. "Itappears," he told BioWorld Today, "that adenoviral vector of theherpesvirus TK gene has no toxicity in normal brain tissue. That'sexciting." And "What's unique is that nobody has reported use of ADVto deliver HTK gene to the brain before."Culver anticipates FDA approval of his Iowa clinical trial "withinseveral weeks," and hopes to commence actual gene therapy "in thenext month or two." It will combine surgical removal of the gliomawith TK gene therapy for residual cells.Woo and his group have just repeated their animal trial in rats which,unlike nude mice, are immunocompetent. The results, he said, weresimilar, and their report has been submitted to the Journal ofNeurosurgery.Meanwhile, the Baylor team is preparing to move up to subhumanprimates, baboons, to test the safety of the ADV vector itself. "If theseresults are satisfactory," Woo said, "I think Baylor will go forward witha proposal to the RAC for human trials."Glioblastomas are not in themselves very prevalent, he observed, butserve as an ideal model for extending the vector system to otherregionalized tumors. n
-- David N. Leff Science Editor
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