ANTITUMOR-ACTIVITY AND REPORTER GENE-TRANSFER INTO RAT-BRAIN NEOPLASMS INOCULATED WITH HERPES-SIMPLEX VIRUS VECTORS DEFECTIVE IN THYMIDINE KINASE OR RIBONUCLEOTIDE REDUCTASE

Herpes simplex virus (HSV) mutants or recombinant vectors might be use ful oncolytic agents. Three general types of HSV vectors can be potent ially used for this purpose: (1) mutants in viral transcription factor s, such as ICP0 and ICP4; (2) mutants in enzymes involved in nucleic a cid metabolism, such as thymidine kinase (TK) and ribonucleotide reduc tase (RR); and (3) mutants in neurovirulence factors, such as gamma(34 .5). We tested the destructive ability of each type against rat 9L gli osarcoma cells in culture. We found that the HSV vectors defective in TK or RR were more efficient at tumor cell lysis in culture than the o ther types of HSV vectors. This increased efficiency provided the rati onale for evaluating the TK and RR mutants in vivo following their ste reotactic inoculation into 9L gliosarcomas implanted in rat brains. We employed the X-gal enzymatic histochemical assay to show that HSV-med iated lacZ gene expression was present in cells within the tumor mass in a relatively selective fashion. Immunoreactive HSV capsid and core antigens were present both in cells within the tumor, as well as in ce lls such as neurons and astrocytes, directly adjacent to the tumor mas s. Long-term survival studies revealed that rats treated with either t he TK or RR mutant lived significantly longer than control rats (p = 0 .014, Kruskal-Wallis one-way analysis of variance). These results indi cate that HSV vectors, defective in enzymes needed in nucleic acid met abolism, can preferentially mediate lacZ gene expression in cells with in the tumor. Furthermore, these vectors can enter into endogenous neu ral cells (accounting for the defection of viral capsid and core antig ens), but probably mediate very low levels of lacZ gene expression, pr esumably due to shut-off of the lacZ gene promoter. The tumoricidal ac tivity of these vectors results in significant prolongation in the sur vival of rats injects with each mutant. A model of HSV vector infectio n and propagation within the tumor and adjacent brain is discussed.