Cutaneous lymphoma is an uncommon neoplastic skin disease in dogs, with decreased survival length, poor response to chemotherapy with cell proliferation and apoptosis being important factors contributing to poor prognosis. Tumoral resistance is the main cause of therapeutic failure in cancer and the MDR-1 gene is the most studied factor responsible for this phenomenon.

This study aimed to correlate the indexes of cell proliferation, apoptosis and MDR-1 expression to the therapeutic response after lomustine chemotherapy. Fifteen dogs with histopathological diagnosis of cutaneous lymphoma attended at the UNESP Veterinary Hospital (Botucatu - Brazil) were evaluated by immunohistochemistry anti-CD3, anti-CD79a, anti-caspase and anti-MIB-1 and for MDR-1 gene expression by qRT-PCR. After the first evaluation, all the animals were treated with lomustine at a dose of 90 mg/m² every 21 days. Correlation between this association of results and survival time was evaluated. All the animals had T-cell cutaneous lymphoma. Average survival time was 59.3 days. All the animals presented low index of apoptosis, 93.3% presented high index of cell proliferation and 93.3% expressed MDR-1 gene. High indexes of cell proliferation and low indexes of apoptosis and MDR-1 expression were not correlated to clinical response to chemotherapy or survival time of evaluated animals. The immunohistochemical evaluation and the gene expression confirm the aggressive biological behavior and low life expectancy, although there was no correlation between the assessed variables and the animals' survival length.