Leah Ann Mitchell,     Fort Collins, Colorado

Amanda Guth,     Fort Collins, Colorado

Steven Dow,     Fort Collins, Colorado

Nonspecific Tumor Immunotherapy

Most cancer immunotherapeutics currently available in veterinary medicine are designed to activate multiple components of the innate immune system nonspecifically to generate antitumor immunity. Natural killer cells, macrophages, and dendritic cells are the primary effector cells responsible for generating antitumor activity following administration of nonspecific immune activators. Release of innate immune cytokines, especially interferon-α (IFN-α) and IFN-γ, also contributes significantly to antitumor activity by suppressing tumor cell growth and by inhibiting tumor angiogenesis. There is generally only a minor T-cell contribution to antitumor immunity following administration of nonspecific tumor immunotherapeutics.

Systemic Tumor Immunotherapy Using Toll-like Receptor and Nod-like Receptor Agonists

Most nonspecific cancer immunotherapeutics stimulate the innate immune system by activating Toll-like receptors (TLRs) or Nod (nucleotide-binding oligomerization domain)–like receptors (NLRs). The TLRs and their activating ligands with greatest relevance to cancer immunotherapy are TLR3 (activated by double-stranded RNA from viruses), TLR4 (activated by lipopolysaccharide), TLR7 and TLR8 (activated by viral nucleic acids and synthetic molecules such as imiquimod), and TLR9 (activated by bacterial DNA). NLRs are expressed primarily in the cytoplasm of immune and nonimmune cells and recognize bacterial cell wall peptides (Nod1 and Nod2) as well as viral nucleic acids (retinoic acid–inducible gene [Rig-I] and nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain containing [NLRP]), and in some cases eukaryotic DNA and uric acid and NACHT-, LRR-, and PYD-containing proteins (NALPs).

Liposome-encapsulated muramyl tripeptide phosphatidylethanolamine (L-MTP-PE) is a Nod2 agonist. L-MTP-PE has been evaluated previously as an immunotherapeutic for prevention of metastases from osteosarcoma and hemangiosarcoma (MacEwen et al, 1999). L-MTP-PE triggers activation of macrophages in the lungs and other tissues and generates production of tumor necrosis factor-α and other innate immune cytokines. Repeated intravenous administration of L-MTP-PE has been shown to prolong survival times significantly in dogs with osteosarcoma and hemangiosarcoma and is roughly equivalent to chemotherapy in overall effectiveness.

Systemic administration of cationic liposomes complexed to bacterial plasmid DNA (CLDC) triggers potent activation of innate immunity in dogs with cancer, presumably primarily by stimulating activation of TLR9. CLDC immunotherapy was shown to activate innate immunity in dogs with metastatic osteosarcoma and significantly improved overall survival times in treated dogs compared with untreated control animals (Dow et al, 2005). In addition, CLDC administration was associated with significant inhibition of tumor angiogenesis. Currently CLDC immunotherapy is being developed in Europe for the veterinary market.

Systemic administration of live attenuated bacteria (primarily engineered Salmonella) also has been evaluated as a novel cancer immunotherapy. The engineered Salmonella localizes to the hypoxic tumor environment following intravenous administration (Thamm et al, 2005). This approach, in some cases, generated local control of tumor growth or tumor regression, presumably mediated by local activation of innate immunity.

Recombinant Cytokine Therapy

Natural killer cells and T cells can be activated by systemic administration of high doses of immune-stimulatory cytokines, primarily interleukin-2 (IL-2) and IFN-α. Immunotherapy with recombinant IL-2 is approved for treatment of melanoma and renal cell carcinoma in humans. Infusions of human IL-2 have been assessed in clinical trials in dogs with advanced cancer and have been reported to produce systemic immune activation (fever) and some antitumor activity. Although human recombinant IFN-α (Roferon, others) is administered systemically at high doses for treatment of melanoma and renal cell carcinoma in humans, the drug rarely is administered at high doses in companion animals for treatment of cancer. However, recombinant IFN-α has been used for treatment of certain oral malignancies in cats, including squamous cell carcinoma.