Abstract B56: Low-dose chemotherapy in combination with intratumoral-regulated IL-12 gene delivery under the control of an oral activator induced a substantial antitumor effect in the murine 4T1 breast tumor model.

Clinical immunotherapy trials are hampered by the tumor's ability to suppress the function of cytotoxic T cells. Low dose chemotherapy has been shown to suppress the function and number of regulatory T cells (Tregs) thereby enhancing immunotherapeutic efficacy. This study explored pairing a potent immunomodulatory cytokine (IL-12) with a bifunctional DNA alkylating agent, Palifosfamide (IPM). We investigated the use of intratumorally administered nonreplicating adenoviral vector, Ad-RTS-mIL-12, expressing IL-12 under control of the Rheoswitch Therapeutic System® (RTS®), inducible expression platform. Expression of IL-12 is regulated by oral administration of a small molecule activator ligand, INXN-1001 (AL). The dose of Ad-RTS-mIL12 (1x1010 vp) was chosen because intratumoral administration of this dose, concomitant with AL at 150 mg/m2 Q1D for 7 days, produced the greatest increase in tumor IL-12; exhibiting a peak tumor level of 283 pg/mg protein on Day 5. Virtually no systemic IL-12 was observed in the serum (0.6 pg/mg protein). The effect of Ad-RTS-mIL12 + AL in combination with IPM was evaluated in a subcutaneous 4T1 syngeneic BALB/c mouse mammary tumor model. IPM alone at 40 – 120 mg/m2 had little effect on tumor growth (< 30%) while increasing doses up to 180 mg/m2 resulted in a dose-related tumor growth inhibition (TGI) when compared to vehicle. In contrast, a single intratumoral injection of Ad-RTS-mIL12 (1x1010 vp) and oral administration of AL (15, 30, 75 or 150 mg/m2) once daily for 5 days led to significant AL dose-related tumor growth inhibition (28, 29, 62, 59%, respectively; p<0.05) on Day 35. Ad-RTS-mIL12 alone had no effect relative to control. Combined treatment with Ad-RTS-mIL12 + AL (30 mg/m2) and IPM (40 to 120 mg/m2 administered IP Q1D for 3 days) had a substantial effect as demonstrated by significantly enhanced tumor growth inhibition (~71-90%, p<0.001). Tumor regression (V/V <1) observed for 18 days at the combination of IPM 120 mg/m2 + AL 30 mg/m2 + AD-RTS-mIL-12 1x1010 vp was concomitant with an increased median survival rate by 14 days when compared to the single agents alone. No overt toxicity as assessed by change in body weight was observed. Plasma exposure of AL increased with increasing dose (AUC0-24h 604 and 4437 ng•h/mL at 30 and 150 mg/m2, respectively). Tumor growth inhibition was correlated with an increase in tumor IL-12 levels and decrease in Tregs. In summary, local intratumoral immunotherapy combined with chemotherapy with IPM offers a distinct advantage over either therapeutic modality alone. These results support our hypothesis that cytotoxic agents at low doses may prime the immune system to enhance immunotherapy, which could potentially be translated into a safe clinical regimen for the treatment of metastatic breast cancer. Based on these results additional nonclinical studies to further explore the immunological mechanism(s) of action with combination therapy with progression into a clinical study are planned. Citation Format: John A. Barrett, Lei Sun, Maria Grigoriadis, Tim Chan, Bill Fogler, Robert A. Morgan, Hagop Youssoufian. Low-dose chemotherapy in combination with intratumoral-regulated IL-12 gene delivery under the control of an oral activator induced a substantial antitumor effect in the murine 4T1 breast tumor model. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr B56.