Viral-Mediated Antitumor Effects in p53-Negative Tumors
ABSTRACT & COMMENTARY
P53, which acts as a tumor suppressor protein, is a transcription factor that participates in several antiproliferative processes, including cell cycle checkpoints, senescence, and apoptosis. In normal cells, p53 is latent, but it becomes activated in response to stress such as DNA damage (e.g., chemotherapy or radiation) or foreign DNA synthesis (e.g., viral infections). These stimuli can induce proliferation, but when p53 is activated, the cells undergo growth arrest, usually in the G1 phase of the cell cycle, and/or apoptosis. Certain viruses, adenovirus, SV40, and human papilloma virus, encode proteins that inactivate p53; the cell is unable to sense that apoptosis should occur, and the viral infection can transform the normal cells. Nearly all human malignancies have genetic changes that interfere with the function of the G1 checkpoint; the p53 gene itself is mutated in more than half of all human tumors.
In rather elegant work first described in Science,1 Bischoff and colleagues describe the use of a modified adenovirus called ONYX-015 for the infection and ultimate destruction of cells with abnormal p53 expression. ONYX-015 is an adenovirus that has been altered by the deletion of one of its early genes, E1B, which codes for a 55kd protein that binds and inactivates p53. Because p53 function must be blocked in order for viral replication to occur, an adenovirus lacking E1B should be unable to replicate in normal cells, but would be capable of replication in and lysis of cancer cells that lack p53 function. Very clever.
ONYX-015 has been shown to be 100-1000 times less effective at killing normal endothelial or epithelial cells than wild type adenovirus but very effective in killing a variety of p53-deficient tumor cell lines from the brain, breast, cervix, colon, larynx, liver, lung, ovary, and pancreas. Interestingly, seven of 10 cell lines with normal p53 gene sequences were also killed by the attenuated virus; in four instances the cells were p53 deficient because of binding and inactivation of p53 by the E6 protein of the human papilloma virus, whereas in the other three instances, the explanation for killing by ONYX-015 remains unexplained. Thus, ONYX-015 is able to replicate selectively in p53-deficient cancer cells because of a deletion in the E1B gene.
Heise and colleagues recently demonstrated that intratumoral or intravenous injections of ONYX-015 resulted in significant antitumor activity against tumors that were p53 deficient, but not against tumors that expressed wild type p53. Intratumoral injection-induced tumor regression and prolonged survival of ONYX-treated nude mice bearing the appropriate tumor xenografts. Similar findings were observed following intravenous administration of ONYX-015. Heise et al also showed that ONYX-015, combined with 5-fluorouracil and cisplatin, had greater activity against a head and neck tumor xenograft than did either treatment alone.
Emboldened by these preclinical results, Ganly and associates have embarked on phase I trials of the ONYX-015, the early results of which were reported at the annual AACR and ASCO meetings by Dan Von Hoff and colleagues this spring. Sixteen patients with recurrent squamous cell cancer of the head and neck have received 28 courses of intratumoral injection of different doses (from 107 to 3 ´ 109 plaque forming units [PFU]) of ONYX-015. Five patients have had repeat course of treatment, and no significant toxicities have been identified. Four patients have had significant necrosis of their injected lesions. (Heise C, et al. Nature Med 1997;3:639-645; Ganly I, et al. Proc Am Soc Clin Oncol 1997;16:1362a.)
COMMENTARY
Given that p53 abnormalities can be detected in more than 50% of cancers, an effective strategy exploiting the p53 defect would have enormous potential for tumor-specific activity. Elimination of cells with an abnormal p53, or reversal to a wild-type phenotype in malignant cells, are potentially powerful strategies. Selected targeted destruction can be achieved by the "smart bomb cancer virus," as Lowe has described the ONYX-015 in an accompanying editorial,2 or possibly by immune recognition of p53-specific peptides that have been altered by mutation.3 Replacement of the abnormal p53 with a wild-type version of the gene has been accomplished using the adenoviral vector Ad5CMV-p53. In the latter instance, the adenovirus has been modified to include a full-length gene for normal p53. This strategy has also been applied successfully in vitro and in vivo in tumor xenografts in nude mouse models.4,5
Another strategy for restoring the growth suppression function of mutant p53 was recently described.6 Most p53 mutations in human tumors are single amino acid changes in the core domain of the protein that contains the specific DNA-binding activity and leads to mutant p53 proteins that are deficient for specific DNA-binding. Selivanova et al6 have shown that a synthetic 22-mer peptide, corresponding to the carboxy-terminal amino acid residues of p53, can activate specific DNA binding of wild-type p53 in vitro and can restore the transcriptional transactivating function of some mutant proteins in living cells.
Another interesting aspect of the ONYX-015 work is the potential synergy with cytotoxic chemotherapy. Although the mechanism for the enhanced responses is unknown, there are data to suggest that viral E1A expression can increase sensitivity to chemotherapy in a p53-independent manner. Furthermore, since E1A is a potent inducer of p53, p53 levels may increase following infection, thereby increasing the sensitivity of the cell to chemotherapy-induced apoptosis. This would also be true for replacement therapy with the adenovirus containing the wild type p53.
There are obviously many obstacles that need to be overcome before any of these strategies are likely to prove useful for significant numbers of patients. The major obstacle is likely to be delivery of the gene product to the appropriate cells at the appropriate time. Another obstacle is likely to be the immune system. On one hand, a weakened and often ineffective immune system in patients with advanced disease is likely to complicate vaccine strategies designed to eliminate cells with specific p53 mutations, while an immune response to the adenoviral vector could severely limit the ability to treat patients serially with a potentially important viral vector. These problems notwithstanding, molecular approaches to the treatment of cancer offer significant promise for the future.
References
1. Bischoff JR, et al. Science 1996;274:373-376.
2. Lowe S. Nature Med 1997;3:606-608.
3. Yanuck M, et al. Cancer Res 1993;53:3257-3261.
4. Liu TJ, et al. Cancer Res 1994;54:3662-3667.
5. Clayman GL, et al. Cancer Res 1995;55:1-6.
6. Selivanova G, et al Nature Med 1997;3:632-638.
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