Phenotypic Screening of the Reframe Drug Repurposing Library to Discover New Drugs for Treating Sickle Cell Disease

Stem cell transplantation and genetic therapies offer potential cures for patients with sickle cell disease (SCD) but these options require advanced medical facilities and are expensive. Consequently, these treatments are not available, and will not be available for many years, for the vast majority of the millions of patients suffering from this disease, who live in under-resourced countries. Hydroxyurea is considered standard therapy for patients with SCD and would be even more therapeutic if it increased fetal hemoglobin in all red blood cells, which it does not. What is urgently needed now is an inexpensive oral drug, in addition to hydroxyurea, that inhibits sickle-hemoglobin polymerization in all cells. To this end, we have just completed a phenotypic screen of the 12,657 compounds of the Scripps ReFrame drug repurposing library using our recently developed high-throughput assay to measure sickling times following deoxygenation to 0% oxygen of red cells from individuals with sickle trait (HbAS), the heterozygous condition. The ReFrame library is a very important collection because the compounds are either FDA-approved drugs or have been tested in clinical trials with extensive drug annotation in an open-access format. Compounds, which inhibit sickling in our assay at concentrations that can be achieved in human serum, without side effects that would be deleterious for SCD patients, can be brought to clinical trials with very few or even no pre-clinical studies. From dose-response measurements, we discovered that 106 of the 12,657 compounds are anti-sickling at concentrations ranging from 31 nM to 10 microM. We also showed that compounds that inhibit sickling of trait cells are effective with SCD cells. As many as 21 of the 106 anti-sickling compounds should emerge as drugs. This estimate is based on a comparison of the distribution of inhibitory concentrations in our assay with the known distribution of free concentrations of oral drugs in human serum (see Figure). Moreover, the expected therapeutic effect for each level of inhibition can be predicted from our measurements of sickling times for cells from individuals with sickle syndromes of differing clinical severity. We believe our results represent a breakthrough in drug therapy by serving as a catalyst for us and others to develop one or more of these 106 anti-sickling compounds into effective drugs for treating SCD.

No relevant conflicts of interest to declare.