A high-throughput yellow fluorescent protein (YFP) cell-based screen identifies autophagy modulators to increase the effectiveness of radioiodine therapy

New targeted drug strategies are urgently needed to improve radioiodine uptake and efficiently ablate thyroid cancer cells thereby minimising the risk of recurrent disease. High-throughput screening (HTS) offers a promising approach to identify new candidate drugs that will induce sodium iodide symporter (NIS) function to promote iodide uptake. However, significant progress has been limited by a lack of thyroid cell-based assays amenable to HTS. Here, we constructed a thyroid cancer cell reporter consisting of a modified version of the yellow fluorescent protein (YFP) as a biosensor of intracellular iodide. We then screened the Prestwick Chemical Library (1200 drugs; 95% approved; 10 mM dose; n=2) with iodide uptake monitored by quenching of YFP fluorescence. Preliminary results showed that the YFP cell-based assay was sensitive towards iodide uptake (Z-factor=0.82) with cell viability >75% for most drugs (1033/1200; AlamarBlue). Normalization of the primary screen dataset using an interquartile mean well-based method identified 48 hit candidate drugs which increased iodide uptake >2 S.D. above mean. Of particular interest, categorisation of top hits revealed a high proportion of drugs that modulate autophagy (18/48; 37.5%) - a key process for maintaining cellular homeostasis by degrading/recycling intracellular material. Secondary screening confirmed the role of autophagy modulators in enhancing iodide uptake after ranking 73 leading compounds based on their pharmacologic (AUC, EMAX and EC50) and specificity of response (NIS+ve vs. NIS-ve YFP-thyroid cells) at ten different drug doses (0.1–50 μM). Subsequent treatment of primary human thyrocytes with the repurposed drug prestw-138 further demonstrated greater radioiodine (125I) uptake (˜3.5-fold; P<0.05). In summary, we have performed high-throughput screening and identified autophagy modulators as well as other repurposed drugs that induce iodide uptake. We propose that these drugs either alone or in combination with existing therapies might offer new therapeutic strategies to improve the treatment of thyroid cancer.