Carcinoid tumors are common neoplasms developing in gastrointestinal and respiratory tract. They are rarely found in the kidney. To date, approximately 40 cases of primary renal carcinoid tumors have been reported, with less of a third of them occurring within a horseshoe kidney. These cases appear to have a better prognosis, even in the presence of distant metastases, compared to those arising in normal kidneys.

Adenocarcinoma of the duodenum comprises 50-70% of duodenal tumors. There is an increase in extracellular matrix metalloproteinases in this disease and it has been suggested that they play an important role in the development and pathology. Therefore, new therapeutic recommendations based on inhibitors of these enzymes, such as doxycycline, are under investigation. The cytotoxic effect of doxycycline was evaluated in the HuTu-80 duodenal adenocarcinoma cell line and its antitumor effect was determined in an immunodeficient murine model. A 10-µM (4.4 µg/ml) concentration of doxycycline was capable of causing apoptosis in 90% of the culture cells. Doxycycline was also responsible for a decrease in tumor growth and an increase in the survival of the mice with HuTu-80-cell tumors. These results suggest that doxycycline is a potential cytotoxic and antitumor agent effective in the treatment of adenocarcinoma of the duodenum.