Abstract
Melanomas arising in the mucous membranes are a rare and aggressive subtype. New treatment approaches are needed, yet accumulating sufficient evidence to improve patient outcomes is difficult. Clinical and pathological correlates between human and canine mucosal melanomas are substantial, and the relatively greater incidence of spontaneous naturally occurring mucosal melanoma in dogs represents a promising opportunity for predictive modeling. The genomic landscapes of human and canine mucosal melanoma appear highly diverse and generally lack recurring hotspot mutations associated with cutaneous melanomas. Although much remains to be determined, evidence indicates that Ras/MAPK and/or PI3K/AKT/mTOR signaling pathway activations are common in both species and may represent targets for therapeutic intervention. Sapanisertib, an mTORC1/2 inhibitor, was selected from a PI3K/mTOR inhibitor library to collaborate with MEK inhibition; the latter preclinical efficacy was demonstrated previously for canine mucosal melanoma. Combined inhibition of MEK and mTORC1/2, using trametinib and sapanisertib, produced apoptosis and cell-cycle alteration, synergistically reducing cell survival in canine mucosal melanoma cell lines with varying basal signaling activation levels. Compared with individual inhibitors, a staggered sapanisertib dose, coupled with daily trametinib, was optimal for limiting primary mucosal melanoma xenograft growth in mice, and tumor dissemination in a metastasis model, while minimizing hematologic and renal side effects. Inhibitors downmodulated respective signaling targets and the combination additionally suppressed pathway reciprocal crosstalk. The combination did not significantly change plasma sapanisertib pharmacokinetics; however, trametinib area under the curve was increased in the presence of sapanisertib. Targeting Ras/MAPK and PI3K/AKT/mTOR signal transduction pathways appear rational therapies for canine and human mucosal melanoma.