Reprofiling using a zebrafish melanoma model reveals drugs cooperating with targeted therapeutics

Phenotype-guided re-profiling of approved drug molecules presents an accelerated route to
developing anticancer therapeutics by bypassing the target-identification bottleneck of target-based
approaches and by sampling drugs already in the clinic. Further, combinations incorporating
targeted therapies can be screened for both efficacy and toxicity. Previously we have developed an
oncogenic-RAS-driven zebrafish melanoma model that we now describe display melanocyte
hyperplasia while still embryos. Having devised a rapid method for quantifying melanocyte burden,
we show that this phenotype can be chemically suppressed by incubating V12RAS transgenic
embryos with potent and selective small molecule inhibitors of either MEK or PI3K/mTOR.
Moreover, we demonstrate that combining MEK inhibitors (MEKi) with dual PI3K/mTOR inhibitors
(PI3K/mTORi) resulted in a super-additive suppression of melanocyte hyperplasia. The robustness
and simplicity of our novel screening assay inspired us to perform a modest screen of FDA approved
compounds for their ability to potentiate MEKi PD184352 or PI3K/mTORi NVPBEZ235 suppression of
V12RAS-driven melanocyte hyperplasia. Through this route, we confirmed Rapamycin as a
compound that could synergize with MEKi and even more so with PI3K/mTORi to suppress
melanoma development, including suppressing the growth of cultured human melanoma cells.
Further, we discovered two additional compounds—Disulfiram and Tanshinone—that also cooperate
with MEKi to suppress the growth of transformed zebrafish melanocytes and showed
activity toward cultured human melanoma cells. In conclusion, we provide proof-of-concept that our
phenotype-guided screen could be used to identify compounds that affect melanoma development and prompt further evaluation of Disulfiram and Tanshinone as possible partners for combination
therapy.