Ex Vivo Drug Sensitivity Screens Identify Personalized Treatment Options for CLL Patients

In the era of targeted therapies, it remains a challenge to determine the optimal treatment for the individual cancer patient. By screening the tumor cells for drug sensitivity ex vivo it is possible to predict to which drugs and drug combinations the cancer will respond. We have developed an ex vivo culture protocol that mimics the chronic lymphocytic leukemia (CLL) tumor microenvironment and allows cell proliferation. Using our unique method, we performed drug sensitivity screens on B cells from 22 CLL patients and 5 matched healthy donors. The drug library consisted of 70 single drugs and 39 drug combinations that are clinically relevant for treatment of CLL, including venetoclax (Bcl-2 antagonist), ibrutinib (Btk inhibitor) and idelalisib (PI3K inhibitor). B cells were cultured in 384-well plates in the presence of drugs covering a concentration range of 5 log10 increments. Drug sensitivity was assessed after three days using the CellTiter-Glo luminescent cell viability assay. The Drug Sensitivity Score (DSS) was then calculated for each treatment based on area under the concentration-response curve. We observed specificity towards CLL cells for drugs including TAK-659 (SYK inhibitor), TGR-1202 and duvelisib (PI3K inhibitors), trametinib (MEK1/2 inhibitor) and venetoclax. Ten combinations with venetoclax were investigated, and all showed high drug sensitivity and specificity towards CLL cells. When ranking the drug combinations from high to low DSS, CLL selectivity and obtained synergy, combinations of venetoclax with trametinib, ibrutinib or duvelisib showed the highest potency. Of interest, similar effects were not observed for venetoclax in combination with other Btk or PI3K inhibitors. In order to understand the mechanisms underlying these differences, signaling analyses by use of phospho flow is currently ongoing. In summary, by applying drug sensitivity screens, we have identified drugs and drug combinations that kill CLL cells. These findings should be investigated further in animal models and in precision medicine clinical trials.