Increased injection number enhances adenoviral genetic radiotherapy.

Intratumoral injection of an adenoviral vector containing radiation-inducible DNA sequences of the early growth response gene (Egr-1) promoter ligated to a cDNA encoding tumor necrosis factor-alpha (TNF-alpha; Ad.Egr-TNF) increases the radiation killing of a human radioresistant xenograft (SQ-20B). Viral dose-escalation experiments demonstrated that SQ-20B growth inhibition correlated with viral titer. Injection of 5 x 10(8) PFU Ad.Egr-TNF produced regression to a mean volume of 22 +/- 13% of the original tumor volume, 1 x 10(8) PFU to a mean of 62 +/- 24%, and 5 x 10(7) PFU to a mean of 67 +/- 27%. No regression was observed when tumors were injected with 1 x 10(7) PFU Ad.Egr-TNF or with the null viral vector (Ad.null). When two injections of vector (2 x 10(8) PFU Ad.Egr-TNF) were combined with 50 Gy, a significant increase in tumor regression was observed compared with injection of buffer, Ad.Egr-TNF, or 50 Gy. The interactive killing between TNF and radiation was enhanced significantly (P = 0.05) when the number of injections was increased from two to five while maintaining a constant viral titer (2 x 10(8) PFU Ad.Egr-TNF) and a constant radiation dose (50 Gy). Significant TNF-alpha levels were present in irradiated vs. unirradiated tumors following injection with Ad.Egr-TNF. Taken together, these data suggest that the volumetric reduction produced by the combined effects of Ad.Egr-TNF and radiation is enhanced with increasing vector concentration and the number of vector injections.

Radiation can inhibit tumor growth indirectly while depleting circulating leukocytes.

The growth of certain UV-radiation-induced tumors is suppressed by T lymphocytes, but it has been proposed that non-T lymphocytes stimulate the growth of several of these tumors. In this study, the indirect effects of X irradiation on the growth of one such tumor in T-cell-deficient nude mice was tested. Even when the site of tumor cell injection was shielded, whole-body irradiation with 6 Gy before tumor challenge inhibited subsequent tumor growth significantly. The interval during which this indirect inhibition was observed correlated with the depletion of circulating leukocytes, which did not return to normal levels until 12-21 days after irradiation. These data are consistent with earlier results using an antibody to deplete Gr-1+ leukocytes and indicate that radiation can inhibit the growth of certain tumors indirectly without direct effects on the tumor cells or the tumor bed.

Genetic radiotherapy overcomes tumor resistance to cytotoxic agents.

We report that radiation enhances gene therapy of a radioresistant tumor by upregulating the induction of a chimeric gene encoding a radiosensitizing protein, tumor necrosis factor alpha (TNF-alpha). We ligated the radiation-inducible CArG elements of the radiation-inducible Egr-1 promoter/enhancer region upstream to the transcriptional start site of the human TNF cDNA (pE425-TNF). This construct was transfected using cationic liposomes into the variant murine fibrosarcoma cell line, P4L. The P4L cell line was both radioresistant (D0 = 188) and resistant to TNF. After a single intratumoral injection of 10 micrograms of pE425-TNF in cationic liposomes and two 20-Gy doses of irradiation, mean tumor volumes were significantly reduced in P4L tumors as compared to those receiving either pE425-TNF in liposomes or radiation alone (P = 0.01). TNf protein in P4L tumors was induced by radiation as high as 29 times control levels and remained detectable for 14 days. Our data indicate that combined gene therapy using liposomes, together with ionizing radiation to locally activate the induction of a radiosensitizing protein, is successful at overcoming resistance to both TNF and radiation.

Antigenic cancer cells that escape immune destruction are stimulated by host cells.

Cancers induced by UV light in murine skin often regress completely when transplanted into normal syngeneic recipients and grow progressively only in T-cell-deficient hosts. Heritable cancer variants that grow progressively and kill normal mice occasionally evolve in vivo. It is surprising that most of these variants appear to retain their antigenicity and immunogenicity. We have compared three such variants (4102-PRO, 6132A-PRO, and 6134-PRO) with the parental tumors to determine why the variants acquired progressive phenotypes without antigen loss. We found that all three variants grew substantially faster than the parental tumors in T-cell-deficient hosts; one variant, 6132-PRO, also grew faster in vitro. Furthermore, the growth of all of the variants was stimulated by soluble factors released by tumor-induced peritoneal exudate cells, and all attracted more leukocytes than the parental cells. Finally, pretreatment of mice with antigranulocyte antibody reduced the growth of variant but not parental 4102 and 6134A tumor cells. The treatment reduced the growth of both the parental and the variant 6132A lineage cells. We found no evidence for acquired resistance of variant tumors to immune destruction by a host defense mechanism. The parental cells did not grow faster in beige nude mice deficient in natural killer and alpha beta T cells or in SCID mice deficient in B and T cells. The variant parental cells had a similar sensitivity to lysis by polyinosinic-polycytidic acid-induced natural killer cells or thioglycolate- and LPS-induced macrophages. Together, our results are consistent with the notion that these variants escape from immune destruction in vivo by attracting leukocytes that stimulate tumor cell growth.

Spatial and temporal control of gene therapy using ionizing radiation.

Activation of transcription of the Egr-1 gene by X-rays is regulated by the promoter region of this gene. We linked the radiation-inducible promoter region of the Egr-1 gene to the gene encoding the radiosensitizing and tumoricidal cytokine, tumour necrosis factor-alpha (TNF-alpha) and used a replication-deficient adenovirus to deliver the Egr-TNF construct to human tumours growing in nude mice. Combined treatment with Ad5.Egr-TNF and 5,000 cGy (rad) resulted in increased intratumoral TNF-alpha production and increased tumour control compared with treatment with Ad5.Egr-TNF alone or with radiation alone. The increase in tumour control was achieved without an increase in normal tissue damage when compared to tissue injury from radiation alone. Control of gene transcription by ionizing radiation in vivo represents a novel method of spatial and temporal regulation of gene-based medical treatments.

Synergy between T-cell immunity and inhibition of paracrine stimulation causes tumor rejection.

During tumor progression, variants may arise that grow more vigorously. The fate of such variants depends upon the balance between aggressiveness of the variant and the strength of the host immunity. Although enhancing host immunity to cancer is a logical objective, eliminating host factors necessary for aggressive growth of the variant should also be considered. The present study illustrates this concept in the model of a spontaneously occurring, progressively growing variant of an ultraviolet light-induced tumor. The variant produces chemotactic factors that attract host leukocytes and is stimulated in vitro by defined growth factors that can be produced or induced by leukocytes. This study also shows that CD8+ T-cell immunity reduces the rate of tumor growth; however, the variant continues to grow and kills the host. Treatment with a monoclonal anti-granulocyte antibody that counteracts the infiltration of the tumor cell inoculum by non-T-cell leukocytes did not interfere with the CD8+ T-cell-mediated immune response but resulted in rejection of the tumor challenge, indicating a synergy between CD8+ T-cell-mediated immunity and the inhibition of paracrine stimulation.