RESUMO
Osteosarcoma is one among the most common neoplasms in dogs. Current treatments show limited efficacy and fail to prevent metastasis. Conditionally replicative adenoviruses (CRAd) replicate exclusively in targeted tumor cells and release new virus particles to infect additional cells. We proposed that OC-CAVE1 (CAV2 with the E1A promoter replaced with the osteocalcin promotor) may also enhance existing immunity against tumors by overcoming immune tolerance via exposure of new epitopes and cytokine signaling. Eleven client-owned dogs with spontaneously occurring osteosarcomas were enrolled in a pilot study. All dogs were injected with OC-CAVE1 following amputation of the affected limb or limb-sparing surgery. Dogs were monitored for viremia and viral shedding. There was minimal virus shedding in urine and feces by the 6th day and no virus was present in blood after 4 weeks. CAV-2 antibody-titers increased in all of the patients, post-CRAd injection. Immunological assays were performed to monitor 1) humoral response against tumors, 2) levels of circulatory CD11c + cells, 3) levels of regulatory T cells, and 4) cytotoxic activity of tumor specific T cells against autologous tumor cells between pre-CRAd administration and 4 weeks post-CRAd administration samples. Administration of the CRAd OC-CAVE1 resulted in alteration of some immune response parameters but did not appear to result in increased survival duration. However, 2 dogs in the study achieved survival times in excess of 1 year. Weak replication of OC-CAVE1 in metastatic cells and delay of chemotherapy following CRAd treatment may contribute to the lack of immune response and improvement in survival time of the clinical patients.
RESUMO
Adenoviruses are the most widely used vectors in cancer gene therapy. Adenoviruses vectors are well characterized and are easily manipulated. Adenovirus serotype 5 (Ad5) is the most commonly used human serotype. Ad5 internalization into host cells is a combined effect of binding of Ad5 fiber knob with the coxsackie virus and adenovirus receptor (CAR) and binding of RGD motifs in viral penton to cell surface integrins (αvß3, αvß5). Ad5's wide range of host-cell transduction and lack of integration into the host genome have made it an excellent choice for cancer therapeutics. However, Ad5 has limited ability to transduce cells of hematopoietic origin. It has been previously reported that low or no expression of CAR is a potential obstacle to Ad5 infection in hematopoietic origin cells. In addition, we have previously reported that low levels of cell surface integrins (αvß3, αvß5) may inhibit Ad5 infection in canine lymphoma cell lines. In the current report we have examined the ability of an Ad5 vector to infect human (HEK293) and canine non-cancerous (NCF and PBMC), canine non-hematopoietic origin cancer (CMT28, CML7, and CML10), and canine hematopoietic origin cancer (DH82, 17-71, OSW, MPT-1, and BR) cells. In addition, we have quantified CAR, αvß3 and αvß5 integrin transcript expression in these cells by using quantitative reverse transcriptase PCR (q-RT-PCR). Low levels of integrins were present in MPT1, 17-71, OSW, and PBMC cells in comparison to CMT28, DH82, and BR cells. CAR mRNA levels were comparatively higher in MPT1, 17-71, OSW, and PBMC cells. This report confirms and expands the finding that low or absent expression of cell surface integrins may be the primary reason for the inability of Ad5-based vectors to transduce cells of lymphocytic origin and some myeloid cells but this is not true for all hematopoietic origin cells. For efficient use of Ad5-based therapeutic vectors in cancers of lymphocytic origin, it is important to address the defects in cell surface integrins.