To evaluate the therapeutic efficacy of both lentiviral pseudotyoped vectors in the invasive xenograft model, vectors expressing the suicide gene HSV1-tk fused to eGFP were injected into established tumors when visible on MRI using the same method as described for the in vivo tropism study. The animals were treated daily with 50 mg/kg ganciclovir for 30 days starting 7 days post vector injection. Tumor growth was measured every 7-14 days by MRI. After 4 weeks of treatment, 7 out of 8 animals in both the LCMV-and the VSV-pseudotype treated groups had a complete remission on MRI ( Figure 6 ). One animal in each group had a stable disease until the end of GC treatment. All animals in the control groups developed large tumors during the treatment period of 30 days ( Figure 6 ). Both, LCMV-and VSV-pseudotype treated animals had a highly significant survival advantage (p,0.001) compared to the control groups ( Figure 7A ). There was no statistically significant difference in survival between the two treatment groups.