on a functional level (Fig. 1). H460a cells were transduced with 
either LNSX or LNSX-AS-K -ras daily for 4 consecutive days. 
Cells grew for 7 days without selection. The percent reduction 
in the growth fraction of the AS transduced cells reflects the 
efficiency of transduction as growth of a selected population of 
AS transduced cells does not occur during this time period. 
The growth of the unselected AS transduced cells was less than 
20% at 7 days. Thus, the simple manipulation of exposing cells 
to the packaged retrovirus for 4 consecutive days caused a 
striking increase in transduction efficiency. In a subsequent 
experiment H460a cells were transduced daily for 7 consecutive 
days with LNSX-AS-K -ras and then selected for colony formation 
in G418 (Fig. 2). Colonies were compared to H460a cells that 
were not exposed to selective medium. Following selection the 
efficiency of colony formation by the transduced cells was 98%. 
This reinfection strategy is applicable to regional therapy. The 
low acute toxicity of the retroviral constructs should permit 
multiple treatments. Intratracheal and subcutaneous injections 
indicate that up to 3 injections can be tolerated in mice with no 
acute toxicity. It is anticipated that the residual number of 
endobronchial tumor cells can be reduced to <10 7 so that an 
excess ratio of retroviral particles to proliferating tumor cells can 
be achieved. 
Recombinant DNA Research, Volume 16 
[425] 
