UAB 9405 
March 16, 1994 
PAGE - 7 
system to achieve a high level of specificity and to amplify biologic responses. 
Thus, unlike the first two strategies, there is no mandate to achieve genetic 
modification of all cancerous cells. 
The stimulation of the immune system by genetic means to achieve an anti-tumor 
effect has been accomplished in a variety of ways. The use of gene modified 
autologous tumor cell vaccines (48) or genes for tumor-associated antigen 
incorporated into viral vaccines (vaccinia) (49) are current examples. It is also 
recognized that naked plasmid DNA injected into muscle can elicit immunity to 
proteins encoded by the plasmid DNA. This is thought to derive from the unique 
ability of plasmid DNA to be taken up, expressed and to achieve stable 
persistence when delivered in this manner. Whereas gene expression in muscles 
has been accomplished utilizing recombinant retrovirus and adenovirus vectors 
(50,51), naked plasmid DNA offers several unique advantages from the standpoint 
of safety. Firstly, the delivery of heterologous genes by recombinant viral vectors 
is associated with the obligatory co-delivery of viral gene elements. This 
introduces the possibility of recombinational events with other viruses to allow 
derivation of replication-competent recombinant viruses. Naked plasmid DNA 
introduced by polynucleotide vaccination can be designed in a manner to 
eliminate viral sequences thus overcoming this consideration. Additionally, the 
recombinant viruses contain sequences which may allow integration of the 
transferred gene into the host chromosome. This could have consequences from 
the standpoint of insertional mutagenesis or activation of endogenous oncogene 
sequences. Naked plasmid DNA delivered by direct injection into muscle has 
been shown to persist as an unintegrated episome (25,52). This would thus reduce 
the potential consequences of direct gene delivery by minimizing the likelihood of 
an insertional event. Further, preclinical and clinical studies of Nabel (53,54) 
have shown that intratumor plasmid DNA encoding major histocompatibility 
antigen injections in animals and man produces little or no toxicity with elicitation 
of anti-tumor effects. Thus, we believe that polynucleotide vaccines represent an 
advantageous gene therapy vaccine strategy with a favorable toxicity profile. 
( 404 ] 
Recombinant DNA Research, Volume 19 
