Canine oral malignant melanoma (COMM) is the most common malignancy in dogs, accounting for 30–40% of all oral tumors (1, 2). COMM is characterized by extensive local invasion, as well as distant metastasis (2). Melanoma treatment includes radiation therapy and chemotherapy (CT), either alone, as an adjuvant therapy following surgery, or in combination (2). Although radiation therapy provides effective local tumor control, it is only performed in selected facilities (3–5). In addition, radiation therapy may be costly and, hence, beyond the financial means of some owners. Several reports have investigated the efficacy of CT in COMM, but there is no definitive evidence regarding its effectiveness (5,6,7–9). Therefore, there is a need for an alternative treatment for COMM.

Lupeol is a triterpene found in fruits such as olives, mangoes, strawberries, grapes and figs, numerous vegetables and several medicinal plants (10). Previous studies reported that lupeol has antitumor properties (11, 12) and several reports indicated that lupeol inhibits melanoma cell proliferation in vitro and in vivo (13, 14). Recently, Nitta et al (14) reported that systemic lupeol administration inhibited tumor growth in a melanoma-bearing mouse model; however, there is no report evaluating its clinical efficacy in COMM. In this study, we aimed to evaluate the efficacy of subcutaneous lupeol as an adjuvant therapy for spontaneous COMM.

COMM is an extremely malignant tumor, with a high propensity for local invasion and metastasis (1, 2). The treatment options for COMM include surgery, radiation therapy, CT and immunotherapy comprising xenogeneic DNA vaccination. MacEwen et al (20) reported that the median survival time (ST) in dogs with oral malignant melanoma that underwent complete surgical excision was ~17–18 months, 5–6 months and 3 months for stage I, II and III disease, respectively. There may be a potential to prolong ST beyond these reported durations, as all the subjects remained alive during the most recent examination. In stage III cases, the median ST increased in subjects administered adjuvant lupeol treatment (>6 vs. 3 months). The reported median ST for dogs receiving radiotherapy ranges between 5.3 and 11.9 months (3–5, 7, 9); however, as the radiotherapy equipment, treatment protocols and supplementary therapies differ between studies, it is difficult to directly compare previously reported results. Regardless, the results of previous reports suggest that systemic lupeol administration may help prolong the median ST in COMM cases and may prove efficacious as an adjuvant postoperative treatment.

Distant metastasis was not observed in any of the cases, including the 3 stage III cases, which were at highest risk for metastasis. Furthermore, there was no local recurrence for 180 days postoperatively in 7 of the 11 dogs. The median disease-free time (DFT) of dogs undergoing radiation therapy reportedly ranges between 5 and 7.9 months (5, 20). In a study evaluating radiation therapy in 38 dogs with COMM, Theon et al (21) reported a median DFT of 11.3 months in stage T1 cases, 6.0 months in stage T2 cases and 6.7 months in stage T3 cases. The results of the present study suggest that surgery combined with lupeol administration achieves local tumor control equivalent to that of radiation therapy. Saleem et al (13) reported that lupeol inhibits the growth of highly aggressive human metastatic melanoma cells in vitro and in vivo by inducing apoptosis. The results of the present study suggest that, in addition to preventing local progression, systemic postoperative adjuvant lupeol administration may also prevent the development of distant tumor metastasis.

For the local treatment of COMM, aggressive complete surgical excision, such as partial mandibulectomy and maxillectomy is the most effective option (5). Radiation therapy also plays an important role and is an effective treatment for achieving local tumor control; however, radiation therapy is associated with several adverse effects, including dermal desquamation, alopecia and bone necrosis, which may negatively affect the quality of life (7). The subjects must be anesthetized during radiation therapy, which itself is accompanied by serious risks. In addition, therapy is only available at selected facilities and may not be affordable for all owners; therefore, radiation therapy cannot be easily performed in all cases. CT combined with radiation therapy and surgery has been used as an alternative systemic therapy to prevent local recurrence and distant metastasis. Although several reports have evaluated the efficacy of CT in COMM as a sole or adjuvant therapy, there is no clear evidence that CT decreases the risk of local recurrence or metastasis, or that it prolongs median ST (5–9). These limitations highlight the need to develop an effective systemic therapy for COMM.

PHT, PHCT, melphalan and piroxicam were administered in combination, but there is no evidence that these treatments are effective in controlling COMM progression. PHT and PHCT were performed with owner consent after explaining the experimental nature of these treatments. Proulx et al (5) reported that melphalan administered for COMM exerted no beneficial effects. There is limited evidence that non-steroidal anti-inflammatory drugs (NSAIDs) may exert a protective effect against malignant melanoma development in humans (22); however, in a study evaluating NSAID therapy in dogs, there was no significant difference in survival attributed to NSAID administration and NSAID therapy did not provide any survival benefit or achieved an improved therapeutic response in the treated subjects (7). Therefore, these adjuvant therapies were unlikely to confer any beneficial treatment effect in this study.

No severe adverse effects, such as local pain, were observed in any of the dogs during lupeol treatment. There are no known reports of adverse effects associated with lupeol administration, which indicates that lupeol is safe when administered at the dosage recommended in the present study. Notably, this study was only a limited clinical trial and evaluated lupeol as a postoperative adjuvant therapy. Prospective controlled studies are required to assess the clinical efficacy of subcutaneous lupeol administration for the treatment of COMM, as well as in combination with other modalities, such as radiotherapy and CT.

In conclusion, to the best of our knowledge, this study is the first clinical trial to evaluate systemic lupeol therapy for COMM. Postoperative lupeol is easily administered, is not associated with severe adverse effects and was shown to prevent local tumor progression and distant metastasis. Therefore, the results of the present study suggest that lupeol is a potential novel adjuvant treatment option and may be used as systemic therapy for COMM.