Replication-restricted vaccinia as a cytokine gene therapy vector in cancer: Persistent transgene expression despite antibody generation

Background: As antitumoral immunity requires the generation of local immuni ty directed against tissue proteins, we attempted to recreate within tumors the same environment found within tissues affected by autoimmune diseases (i.e., prolonged cytokine expression). Vaccinia virus (VV) has not been wid ely used as a cytokine gene therapy vector because of presumed high immunog enicity that would likely make repeated injections impossible; therefore, w e modified it by inserting the cytokine gene into the thymidine kinase regi on, rendering it replication-restricted. The cytokine chosen was human inte rleukin-2 (IL-2), a molecule with powerful antitumoral effects. Methods: Six patients with the treatment-resistant tumor malignant mesothel ioma received intratumoral (i.t.) VV-IL-2 therapy for 12 weeks by injection of 10(7) plaque-forming units of VV-IL-2 per dose. Serial tumor biopsies, sputum, urine, and blood samples were tested for VV-IL-2 mRNA expression; V V culture and T-cell infiltrates were evaluated by immunohistochemistry. Pa tients and contacts of patients were monitored for changes in VV immunoglob ulin G (Igc) levels and clinical evidence of VV infection. Results: VV-IL-2 was not excreted and was only cultured in one patient from tumor biopsies. A T-cell infiltrate was detected in 50% of tumor biopsies. VV-IL-2 mRNA expression was highest on days 1-3 postinjection and was dete cted for up to 3 weeks after each injection even though VV IgG levels rose in all patients. No significant toxicities, infection of patient contacts, or tumor regressions were observed. Conclusions: I.t. VV-IL-2 administration is safe, is associated with minima l toxicity, and results in i.t. expression of VV-IL-2 for up to 3 weeks pos tinjection regardless of the level of anti-VV Ige titers generated. This su ggests that VV may be a good vector for repeated cytokine gene therapy of s olid human cancer.